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Revision 1 December 2014 Breakers, Relays, and Disconnects Student Guide GENERAL DISTRIBUTION GENERAL DISTRIBUTION: Copyright © 2014 by the National Academy for Nuclear Training. Not for sale or for commercial use. This document may be used or reproduced by Academy members and participants. Not for public distribution, deli to, or reproduction by any third party without the prior agreement of the Academy. All other rights reserved. NOTICE: This information was prepared in connection with work sponsored by the Institute of Nuclear Power Operations (INPO). Neither INPO, INPO members, INPO participants, nor any person acting on behalf of them (a) makes any warranty or representation, expressed or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this document, or that the use of any information, apparatus, method, or process disclosed in this document may not infringe on privately owned rights, or (b) assumes any liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, method, or process disclosed in this document. ii Table of Contents INTRODUCTION ..................................................................................................................... 1 TLO 1 CIRCUIT PROTECTION ................................................................................................ 2 Overview .......................................................................................................................... 2 ELO 1.1 Circuit Protection .............................................................................................. 3 ELO 1.2 Circuit Interrupting Devices .............................................................................. 8 ELO 1.3 Transfer and Disconnect Switches .................................................................. 14 ELO 1.4 Safety and Equipment Protection .................................................................... 17 ELO 1.5 Electrical Drawings ......................................................................................... 21 ELO 1.6 Automatic Transfer Switches .......................................................................... 33 ELO 1.7 Motor Controllers ............................................................................................ 34 TLO 1 Summary ............................................................................................................ 40 TLO 2 CIRCUIT BREAKERS ................................................................................................. 42 Overview ........................................................................................................................ 42 ELO 2.1 Circuit Breaker Construction and Function .................................................... 43 ELO 2.2 Racking Circuit Breakers ................................................................................ 53 ELO 2.3 Circuit Breaker Indications ............................................................................. 57 ELO 2.4 Circuit Breaker Control Power........................................................................ 62 ELO 2.5 Circuit Breaker Operation ............................................................................... 65 TLO 2 Summary ............................................................................................................ 69 TLO 3 PARALLELING AC SOURCES .................................................................................... 70 Overview ........................................................................................................................ 70 ELO 3.1 Paralleling AC Sources ................................................................................... 70 ELO 3.2 Abnormal Conditions During Paralleling Operations ..................................... 72 TLO 3 Summary ............................................................................................................ 74 BREAKERS, RELAYS, AND DISCONNECTS SUMMARY .......................................................... 74 This page is intentionally blank. iv Breakers, Relays, and Disconnects Revision History Revision Date Version Number Purpose for Revision Performed By 11/6/2014 0 New Module OGF Team 12/10/2014 1 Added signature of OGF Working Group Chair OGF Team Introduction In this module, you will learn about circuit interrupting devices and circuit switching devices. The module discusses the types of devices, describes their design and operation, and gives examples of the appropriate Rev 1 1 applications for each. This module also reviews safety measures for operation of electrical circuits. Understanding of how the different circuit switching and interrupting devices operate is vital to the operator's ability to operate the plant safely. Objectives At the completion of this training session, the trainee will demonstrate mastery of this topic by passing a written exam with a grade of 80 percent or higher on the following Terminal Learning Objectives (TLOs): 1. Explain the purpose, safety precautions, and operation of electrical circuit interrupting and circuit switching devices. 2. Explain the construction, operation, and indications for electrical circuit breakers. 3. Describe the conditions that must be met prior to paralleling two generators including effects of not meeting these conditions. TLO 1 Circuit Protection Overview An operator needs to understand the principles of circuit protection and how the circuit interrupting devices work, and have the ability to read and interpret control drawings to respond to electrical system upsets, and maintain plant safety. When this section is complete, you will be able to read simple electrical diagrams, identify various circuit interrupting and switching devices, explain and follow safety precautions for using circuit these devices, and describe the principles of circuit protection. Objectives Upon completion of this lesson, you will be able to do the following: 1. Explain the principles and applications of circuit protection, including selective tripping. 2. Describe the protection provided by each of the following: a. Fuses b. Protective relays c. Circuit breakers d. Overloads 3. Describe the function of the following types of switches: a. Disconnect switch b. Automatic transfer switch c. Manual transfer switch 4. Describe the personnel safety and equipment protection procedures and precautions associated with circuit interrupting devices and relays. 2 Rev 1 5. Interpret symbols for breakers, relays, and disconnects in a simple one-line diagram, and explain the operation of the control circuit. 6. Explain the purpose and function of normal and power seeking automatic transfer switches. 7. Describe the functions, operation and protective features of motor controllers. ELO 1.1 Circuit Protection Introduction At the completion of this section, the student will be able to explain the principles and applications of circuit protection. Circuit Protection Guidelines Circuit protection schemes must de-energize circuits when necessary to protect the equipment and circuit from electrical system faults. They also maximize the reliability of the system by avoiding unnecessary trips, tripping only those portions of the circuit necessary to isolate the fault. Circuit protection schemes incorporate the following key features in order to accomplish the following goals: Selective tripping — a system that includes tripping devices sized and located to isolate faults at the lowest level in the distribution system. 2. Diverse circuit interrupting devices — a circuit includes different types of devices to interrupt current flow in different places, so one type of component failure does not prevent circuit protection. 3. Diverse fault sensors — a circuit with various sensor types to detect different types of faults to ensure that faults are detected and isolated. 1. Selective Tripping Selective tripping is a term used to describe a method for protecting electrical distribution systems by incorporating circuit breakers, fuses and other protective devices at locations such that the protective device closest to a fault will operate to remove the fault from the system and still maintain the largest possible portion of the system energized. The following figure depicts a simple example of selective tripping. Figure: Selective Tripping Rev 1 3 In the figure above, the fuses, which are the protective devices closest to the load, have 50 ampere (amp) ratings. If some sort of electrical fault (short or ground) on the load causes current flow to the load to exceed 50 amps, the fuses will blow first, stopping current flow to the load and isolating whatever faulted condition resulted in the excess current flow from the rest of the system. The output breaker for the generator is set to trip at 500 amps. In the fault scenario described above, the output breaker for the generator would remain shut and the generator would remain connected to the distribution system in order to power other loads. Circuit Breakers Circuit breakers are the most versatile device for interrupting current flow. They are the primary means of normal electrical switching, and serve as the primary current interrupting device for circuit protection. Design criteria and construction materials of circuit breakers are such that they have the capability to interrupt the maximum fault current they may encounter to isolate faulty equipment, without failing themselves. Circuit breakers do not sense the fault themselves. They rely on protective relays or overload devices to sense the fault and send a signal to open the circuit breaker. Circuit breaker cabinets often include protective relays with the breaker they will open, but the protective relay location need not be in the same cabinet. The protective relay senses the fault condition and sends a signal to the appropriate circuit breaker or breakers, which then open to isolate the fault. Different designs of the circuit breaker control circuit allow for any of a number of sensed conditions to trip the circuit breaker. Different fault sensor relays (discussed below) react to problem conditions in the circuit. If a problem develops while the circuit breaker is closed, the relay will close its associated contact, energizing the breaker tripping coil, and the circuit breaker trips. Fuses Fuses are much simpler devices than circuit breakers because they do not have the capability to provide switching. Fuses alone are for overcurrent protection. The fuse is both the sensing element and a sacrificial interrupting element. A fuse has a current carrying strip sized to carry normal current. If and when a circuit fault causes an overcurrent condition, then the current carrying strip in the fuse element melts and breaks the circuit. Because fuses do not rely on signals from other components, they are highly reliable circuit interrupters. 4 Rev 1 Overloads Overload devices monitor and if necessary trip single loads as part of the selective tripping scheme. Like fuses, overload devices are both the sensing element and the interrupting element. Thermal Thermal interrupting devices use a bimetallic strip. Normal current generates some deflection in the bimetallic strip, but not enough to activate the linkage that trips the breaker. An excessive current causes more deflection in the bimetallic strip, engaging a linkage that trips the breaker. Since the strip requires time to heat up, an inherent time delay exists with all thermal overloads. Magnetic Magnetic interrupting devices use a coil and a tripping armature. After overload calibration, a preset current flow picks up the armature and trips the breaker. Fault Sensor Relays Various types of protective relays react automatically to fault conditions to protect equipment from damage and protect personnel from injury. By using relays that sense different conditions, the circuit protection scheme reacts more promptly to system faults. Common relay types include the following: Undervoltage — generally set to trip a breaker when voltage drops to 60 to 70 percent of normal value, and the setpoint is adjustable. Underfrequency — trips a breaker when frequency drops below a preset value to protect loads on a system that cannot tolerate a significant change in frequency. Adjustable reverse power relay — senses a change in the normal direction of current indicating an abnormal condition, and if the current reversal is greater than an adjustable limit, energized the relay. These devices protect electrical generators from damage due to motoring. The relay trips the generator output breaker (antimotoring). Lockout relay (also known as an 86 Device) — senses a trip signal, built-in solid-state contact closes and sends a signal that it is closed. Once tripped, lights warn against resetting. Overload relay devices (also known as an overcurrent) a. Short-term — allows starting currents to decay. b. Long-term — light overload conditions, after a time delay. c. Instantaneous — high-current due to short-circuits, with no time delay. Rev 1 5 Selective Tripping Example A power plant internal electrical system must be set up to protect equipment against the expected range of faults, and also to keep the equipment available for operation so the power plant can stay in operation and reliably generate power. In order to meet both goals, the power plant uses selective tripping, diverse fault sensors, and diverse interrupting elements. Selective tripping is a design that arranges protective devices so that the device closest to a fault will trip to remove the fault from the circuit but not interrupt the rest of the circuit. Consider an example power plant with four condensate booster pumps. Full power operation requires all four pumps, but the plant can operate indefinitely at 85 percent power with only three condensate booster pumps available. The plant can continue operating at 60 percent power with two condensate booster pumps available. While these pumps and motors are reliable, there may be instances when a failure occurs, rendering one or more pumps inoperable. A failure could occur at the individual pump, or at the power supply level. During design of the plant electrical systems careful selection and placement of selective tripping devices should minimize the impact of any of these possible faults. If a fault occurs in an individual condensate booster pump or motor, we need to de-energize the pump motor quickly to minimize damage to the pump and motor. Depending on the severity of the fault, isolating it quickly limits the damage and repair time required. We also want to isolate this fault in such a manner that we only remove the faulty condensate booster pump from service, which keeps the other three condensate booster pumps and all other plant equipment available for use. This requires the plant selective tripping design to have fault sensors and circuit interrupting devices installed on the motor for each condensate booster pump. Induction motors have high initial currents when started. Therefore, whatever overcurrent protection we choose for an induction motor, it must satisfy the following requirements: Allow the starting current surge for several seconds. Interrupt the current flow quickly enough to prevent or minimize damage to the component. Interrupt current flow quickly enough to contain any damage to that component alone, as well as protect the rest of the electrical system. Choices include fuses, overloads, or a protective relay to trip the motor circuit breaker. Fuses are simple and reliable, but a fuse must be large enough to allow high starting currents to pass. Fuses sized this large may fail to interrupt a smaller fault. Other options would be a relay and circuit breaker, or a magnetic overload with a time delay to allow starting currents. For a large motor that is important to plant operation, an overcurrent relay 6 Rev 1 with a time delay to allow starting current to decay is frequently the alternative chosen. If the fault occurs at the power supply level at a switchgear that supplies two of the four condensate booster pumps, and other important plant equipment, the selective tripping scheme should isolate this switchgear to de-energize the fault while maintaining power to the rest of the plant electrical system. The selective trip design would include overcurrent relays monitoring the switchgear and tripping the power supply breaker coming into the switchgear. Therefore, a fault at the switchgear could de-energize the switchgear, causing the loss of two condensate booster pumps, but the plant could still operate at 65 percent power while isolating the electrical fault to prevent or minimize damage to important plant equipment. Such tripping schemes must consider all plant equipment in the same way that we have considered condensate booster pumps here, and must consider the impact of each to continued plant operation. However, a farsighted tripping scheme design provides maximum plant equipment protection from electrical faults while keeping the plant in operation. Knowledge Check Select all that are correct. Selective tripping... A. uses only circuit breakers to interrupt circuits, because fuses cannot receive trip signals from outside sources. B. uses diverse fault sensors to ensure faults are detected and isolated promptly. C. isolates a fault using the circuit interrupting device closest to the fault to keep most of the electrical system in operation. D. isolates a fault using multiple circuit interrupting devices simultaneously to ensure that the fault is quickly and completely isolated. Knowledge Check Which one of the following breaker trip signals will trip the associated motor breaker if a motor bearing seizes while the motor is running? A. Rev 1 Time-delayed overcurrent 7 B. Undervoltage C. Underfrequency D. Instantaneous overcurrent ELO 1.2 Circuit Interrupting Devices Introduction Several different types of devices provide circuit protection. Fuses, relays, circuit breakers, and overload protection devices all play a part in a comprehensive circuit protection scheme. Fuses A fuse is a device that protects a circuit only from an overcurrent condition. A fuse has a fusible link directly heated and destroyed by the current passing through it. A fuse contains a current-carrying element sized so that the heat generated by the flow of normal current through it does not cause the element to melt; however, when an overcurrent or short-circuit current flows through the fuse, the fusible link melts and opens the circuit. There are several types of fuses in use; the figure below shows common fuse types. Figure: Typical Types of Fuses The plug fuse is a fuse that consists of a zinc or alloy strip, a fusible element enclosed in porcelain or borosilicate glass housing, and a screw base. 8 Rev 1 Circuits rated at 125 volts or less to ground normally use this type of fuse, which has a maximum continuous current-carrying capacity of 30 amps. The cartridge fuse consists of a zinc or alloy fusible element enclosed in a cylindrical fiber tube with the element ends attached to a metallic contact piece at the ends of the tube. Circuits rated from 250 volts to 600 volts normally use this type of fuse, which has a maximum continuous currentcarrying capacity of 600 amps. Fuses provide protection against circuit faults that lead to overcurrent conditions only. They do not provide protection against undervoltage, overvoltage, or underfrequency failures unless those failures lead to an overcurrent condition. Relays Various types of protective relays detect fault conditions then send signals to trip one or more circuit breakers to isolate the fault, protecting equipment from damage and personnel from injury. Sensors monitor different parameters to provide prompt response to a fault condition while avoiding unnecessary system interruptions. Overcurrent These are the most common relay protection devices in use. Different levels of overcurrent protection are frequently included in power systems to provide the necessary protection without causing unnecessary system interruptions. Instantaneous Overcurrent An overcurrent relay can be set with a high-current rating, without a time delay, as an instantaneous overcurrent protection. If a large short or ground occurs, the current on the system would be extremely high, and could potentially cause large-scale damage to the electrical system. The instantaneous overcurrent relay signals one or more circuit breakers to open and isolate this fault. In this instance, opening with no time delay is necessary to prevent or minimize damage. Time Delay Overcurrent An overcurrent relay can also be set to monitor current and allow a specific value of current to exist for a short time, but if the current remains above the preset value longer, the relay would signal circuit breakers to open and isolate the component. Power plant and industrial systems need overcurrent delay relays to allow for starting large loads. When a large induction motor starts, it draws a starting current that is several times larger than its normal running current. This starting current persists until the motor is up to its normal running speed. High initial starting current is unavoidable; it is Rev 1 9 required to start the motor. If the high initial starting current persists for more than a few seconds, the motor overheats and damages the motor. A time delay overcurrent device can monitor the current, and allow the starting current to exist for a preset time, allowing the motor to come up to speed. At the end of this time, the current should have decreased to normal running current, below the overcurrent relay setting. If, after the time delay, the current is still too high, the relay will initiate trip signals to isolate the motor and prevent damage. An overcurrent relay with time delay can be set with a still lower current value and longer time delay to protect against smaller but more persistent overcurrent condition caused by smaller fault conditions, such as bearing failures or other mechanical interferences causing more resistance and drawing more current. Undervoltage Large power systems use undervoltage relays because an undervoltage condition can damage the components of the entire system. Most large systems have many induction motors. Induction motors draw more current to produce the same power if voltage falls below the motor's design voltage. When one of these motors draws more current, the increased current causes excessive heat and challenges the entire power system. The undervoltage relay can isolate the portion of the system that is causing the undervoltage and protect the system from increased current flow and potential damage. Underfrequency and Overfrequency Frequency should be stable on a large grid. If it varies beyond specified limits, the underfrequency and overfrequency relays will trip circuit breakers to protect the equipment. Lockout Relay Lockouts are relays that prevent quick re-energizing of a system after detection of a fault condition. When a relay detects a fault, circuit breakers open to isolate the fault, and the relay that detected the fault may be isolated from the fault. If that occurs, the relay no longer senses the fault condition, and system conditions would appear to be normal (to the relay). If the relay were to automatically reset, and the circuit breaker to close, the fault would be reconnected to the system. Lockout relays do not allow a signal to reset until the operator has investigated the fault condition and manually resets the lockout. 10 Rev 1 Reverse-Power Relay This device senses a change in the normal direction of current indicating a change in the direction of power flow coming from the breaker (out of the generator) to power flowing into the breaker (into the generator). This device protects a generator from damage from motoring by tripping the generator output breakers. Circuit Breakers Circuit breakers interrupt circuits, both for circuit protection in the event of faults and for switching during normal operation of the electrical system. Circuit breakers do not sense faults, but relays or overloads that sense them are often contained in the same cabinet as the circuit breaker that they signal to trip. A later section will cover circuit breakers in detail. Overloads Thermal Thermal overloads, shown in the figure below, consist of a heat sensitive element and an overload heater connected in series with motor load circuit. When motor current is excessive and sustained, heat from the breaker thermal heater causes a bimetallic strip to deflect, activating a linkage that opens the motor break or motor line contacts. Since it takes time for the heat to build up, the thermal overloads (OL) have an inherent time delay. Figure: Three-Phase Magnetic Controller with Thermal Overloads Rev 1 11 Magnetic Magnetic overloads consist of a coil connected in series with motor load circuit. If motor current exceeds a preset value, the magnetic field induced in the coil will move the armature and open the overload device contact, tripping the motor breaker. Some magnetic overloads operate instantly when motor current exceeds the tripping current, and some have short time delays. After activation, it is necessary to reset an overload device to resume motor operation. An operator can reset magnetic overload devices immediately after tripping as directed by station procedures. Thermal overload relays must cool before resetting. Resetting Overload Devices There are three ways to reset an overload: 1. Manual reset — manual resets are often located in controller enclosure that contains the overload device. The reset usually has a handoperated rod, lever, or button that returns the device-tripping mechanism to its original position and resets interlocks. 2. Automatic reset — automatic resets typically use a spring or gravity operated device to reset the overload device without operator action, only after the condition that caused overload has cleared. 3. Electrical reset — an electromagnet controlled by a push button actuates an electrical reset. Overload devices are necessary to operate the device remotely. In designing the protective features for an electrical system, there are several different options to consider. The options for overcurrent protection are overloads, fuses and overcurrent relays that signal circuit breakers to trip. The option chosen depends on the nature and importance of the component. Fuses and overloads protect only against overcurrent. They have no capability to detect frequency, voltage, or direction of power flow. However, overcurrent is the protection needed in most cases. Frequency and voltage are systemwide parameters. As a result, most frequency and voltage monitoring occurs at systemwide levels. Reverse power is only monitored on generator output breakers. Therefore, most loads only require overcurrent protection. Induction motors draw high initial currents when started. Therefore, whatever overcurrent we choose for an induction motor, it must satisfy the following requirements: 12 Allow the starting current surge for several seconds. Interrupt the current flow quickly enough to prevent or minimize damage to the component. Rev 1 Interrupt the current flow quickly enough to contain any damage to that component alone, and protect the rest of the electrical system. Fuses and thermal overloads have an inherent time delay while they heat up, but the delay varies. Time delay relays and magnetic overloads can have precisely set time delays, so large induction motors often use these for protection. Smaller motors and other components frequently use fuses, because they are relatively inexpensive, require a low level of maintenance, and are reliable. No signal is required from another component to trip a fuse. No complex linkage is required. It has a fusible link, which melts if too much current passes through it, making it inherently reliable. Many circuit breakers have overloads installed, making them an easy and inexpensive choice. Knowledge Check Fuses will detect and isolate which of the following fault conditions: A. Underfrequency B. Overcurrent C. Undervoltage D. Phase imbalance Knowledge Check Which of the following protective relays would sense and isolate a light but persistent overload condition? Rev 1 A. Overcurrent—long time delay relay B. Undervoltage relay C. Instantaneous overcurrent relay D. Overcurrent—short time delay relay 13 ELO 1.3 Transfer and Disconnect Switches Introduction Transfer and disconnect switches provide flexibility within an electrical distribution system. They can change the lineup of the system or change the power source for the loads within the system. They provide direct visual indication that the circuit is broken. Transfer and Disconnect Switches Disconnect switches, also referred to as disconnects, are two-position switches used for the isolation of power supplies from one or more loads or motor control centers. When used in pairs, they facilitate the transfer of power supply from one source to another. Disconnects differ from breakers because disconnects are manually operated and are not usually designed to be opened under load. Breakers are designed to be opened under load, thereby protecting the electrical load. The disconnect switch design does not usually include arc chutes or any other means to extinguish the arc drawn when opened. Before opening a disconnect switch, verify that all electrical loads fed by the disconnect switch are off, or not operating. The figure below shows front and side views of a typical disconnect switch. Figure: Typical Disconnect Switch Opening a disconnect switch under load can result in damage to the disconnect switch and injury to personnel. The figure below shows a 480 volt disconnect switch fault. Disconnects may contain fuses which provide overcurrent protection for the loads supplied by the disconnect switch. Disconnect switches that are not equipped with fuses provide isolation for the circuit only. Separate fuses or breakers would be required elsewhere in 14 Rev 1 the circuit to provide protection for loads equipped with unfused disconnects. Figure: 480 Volt Disconnect Switch Fault Low voltage, considered 600 volts alternating current (AC) and below, switches that are enclosed and may be locked in the off position are sometimes referred to as safety switches. These types of disconnects are often used as isolation points for electrical maintenance. Transfer switches connect and disconnect electrical circuits in order to provide smooth power transfer from one power source to another. Manual transfer switches are similar to disconnect switches, except that they have three positions that allow transfer of the power supply for an electrical component from one source to another. Like disconnects, manual transfer switches may contain internal fuse protection. Knowledge Check Which one of the following statements describes the use of high-voltage disconnects? Rev 1 A. Disconnects must be closed with caution when under load because of possible arcing. B. Disconnects may be used to isolate transformers in an unloaded network. C. Disconnects trip open like circuit breakers, but must be manually closed. D. Disconnects should be limited to normal load current interruption. 15 Knowledge Check Refer to the simplified drawing below of an electrical distribution system showing 7.2 kilovolt (KV) switchgear, step-down transformers, and 480 volt motor control centers (MCCs). The high-voltage side of each step-down transformer has a remote-operated disconnect. The control circuit for each disconnect is positioninterlocked with the associated MCC feeder breaker. Which one of the following describes the interlock operating scheme that will provide the greatest protection for the disconnect? A. Permits opening the disconnect only if the feeder breaker is open. B. Permits opening the disconnect only if the feeder breaker is closed. C. Permits opening the feeder breaker only if the disconnect is closed. D. Permits opening the feeder breaker only if the disconnect is open. Knowledge Check A 480 volt AC motor control center supplies a load through a breaker and a manual disconnect. Which one of the following sequences will provide the greatest level of personnel safety when de-energizing the load for maintenance and when re-energizing the load after the maintenance? 16 Rev 1 A. Open breaker first (de-energizing); shut disconnect first (re-energizing) B. Open disconnect first (de-energizing); shut breaker first (re-energizing) C. Open breaker first (de-energizing); shut breaker first (reenergizing) D. Open disconnect first (de-energizing); shut disconnect first (re-energizing) ELO 1.4 Safety and Equipment Protection Introduction After completing this section, the student will be able to describe the personnel safety and equipment protection procedures and precautions associated with circuit interrupting devices and relays. Safety and Equipment Protection Guidelines Personnel should observe the following safety measures during the operation of protective devices, circuit breakers, and switches: Do not open a disconnect switch under load. Before opening a disconnect switch, verify that all of the equipment the disconnect feeds is off. Disconnects should not be used to start and stop equipment. Follow all precautions for working on energized equipment when checking voltages on breakers, relays, and switches with test equipment. Each power station will have procedures governing the operation of circuit interrupting devices, including required protective clothing and equipment. Ensure that you learn and follow the procedures for operating circuit-interrupting devices at your facility. Grounding in electrical distribution systems helps prevent accidents to personnel and damage to property caused by: a. Fire triggered by lightning. b. A breakdown between primary and secondary windings of transformers. c. Accidental contact of high-voltage wires and low-voltage wires. If some point on the circuit connects to ground, lightning striking the wires will conduct into the ground, and breakdown between the primary and secondary windings of a transformer will cause the primary transformer fuses to blow. Rev 1 17 Do not remove or replace any fuse under load. Never replace a fuse with one that has a different voltage or current rating than that of the intended circuit. Perform the following before racking out circuit breakers: a. Ensure the circuit breaker is open. b. Ensure control power is off when applicable. c. Tag or lockout applicable electrical sources. Always strip loads prior to re-energizing a dead bus. Restart loads one at a time to avoid starting currents for all loads concurrently. Safety and Equipment Protection Example When preparing to work on or around energized electrical equipment, there are required precautions to make the work area safer for all concerned. Consider a job that will require workers to work in an open switchgear cubicle, with energized circuits inside the cubicle, in a high-traffic area of the plant. Consider the following hazards, and the precautions taken for each: Hazards A worker who has nothing to do with the job comes through the area and distracts the workers. The worker in the cubicle temporarily loses his balance and contacts an energized circuit. A noise in the area startles the worker in the cubicle and he jumps, contacting an energized circuit. The worker drops a tool that contacts an energized circuit, creates a short with sparks, causing equipment damage. The worker mistakenly touches an energized circuit. Precautions Some of the hazards discussed above are spontaneous spur of the moment, individual reactions to stimuli that do not give time to prevent them; therefore, they dictate the nature of our precautions. We have to prevent the stimulus, prevent the response, or ensure that the response does not cause damage or injury. Since response to the stimulus is likely to be instinctive and without time for thought, precautions that address preventing the response are not likely to be successful. We must concentrate on preventing the stimulus, or ensuring that the response does not cause damage or injury. The precautions below anticipate the expected human response and ensure the response does not cause damage or injury: 1. Restrict access to the work area and to the surroundings. This has multiple effects. By restricting access, we limit the likelihood that someone outside the immediate job will distract the workers. If 18 Rev 1 2. 3. 4. 5. 6. 7. something does go wrong, we reduced the number of people in the immediate area. Cover as much of the work area and uninvolved devices as possible in rubber matting or other insulating material; including floors, the cubicle door and frame, and uninvolved parts of the cubicle. If rubber matting covers something, the matting will prevent touching it, and that item(s) will not be part of a current path. Know the energized parts of the work area and mark them clearly. Use insulated tools. The insulation of the tool provides one more barrier between you and a closed circuit, whether the tool is in your hand or you drop it inadvertently. Use appropriate protective clothing. Some of the protective clothing prevents forming the circuit and getting you shocked. Other parts protect against the effect of an accident if one does occur. Follow work practices. While working with only one hand is more difficult and time consuming, it avoids a complete a circuit through your body. Working with both hands can inadvertently form a closed circuit that travels through you, and the electrical current will flow there. If the job requires dexterity and you cannot do it with bulky gloves, keep one bulky glove on the non-working hand to prevent the instinctive reaction of grabbing and completing a circuit. Plan, know exactly how to de-energize the equipment, and be prepared to do it, if needed. Have a qualified co-worker standing by as an observer, safety monitor to help or to de-energize the equipment. Safety and Equipment Protection Example Consider the activity of re-energizing a bus. Any time you are about to reenergize an electrical system, you should approach the job carefully. This should only be done by someone qualified and authorized to do so by station procedures. The following questions should help you determine the appropriate action: 1. Are there plant procedures for re-energizing this equipment? In many cases, there will be, because re-energizing must follow specific procedures to prevent damaging the equipment. 2. Are there plant procedures or reference documents to determine what equipment will experience power upon re-energizing the bus, and how the equipment will respond? 3. Is there any equipment that can be isolated and re-energized later? 4. Which equipment should be isolated and re-energized later, for electrical system reasons or for equipment control reasons? 5. How will the equipment respond when re-energized, and what will we need to do to control it? 6. What equipment damage could occur? 7. Will starting currents for all of the equipment be excessive? After asking the above questions, you can develop a strategy to ensure that no damage occurs, the plant is appropriately controlled, and the evolution Rev 1 19 goes as planned. If you cannot come up with that strategy, do not reenergize the equipment—get help. Knowledge Check Closing the output breaker of a three-phase generator onto a de-energized bus can result in... A. a large reactive current in the generator. B. a reverse power trip of the generator circuit breaker if generator frequency is low. C. an overcurrent condition on the generator if the bus was not first unloaded. D. an overvoltage condition on the bus. Knowledge Check Which one of the following is an unsafe practice if performed when working on or near energized electrical equipment? A. Attach a metal strap from your body to a nearby neutral ground to ensure that you are grounded. B. Use insulated tools to prevent inadvertent contact with adjacent equipment. C. Cover exposed energized circuits with insulating material to prevent inadvertent contact. D. Have a person standing by with the ability to remove you from the equipment in the event of an emergency. Knowledge Check A 480 volt AC motor is supplied power via an electrical disconnect in series with a circuit breaker. Which one of the following describes the proper operations to isolate power to the motor? 20 Rev 1 A. Open the disconnect first, then the breaker. B. Sequence is not important as long as the motor is operating. C. Open the device that is closest to the power source first. D. Open the breaker first, then the disconnect switch. Knowledge Check The primary reason for isolating emergency electrical loads from their power supply bus prior to energizing the bus via the emergency diesel generator is to prevent an... A. underfrequency condition on the loads. B. overcurrent condition on the generator. C. underfrequency condition on the generator. D. overcurrent condition on the loads. ELO 1.5 Electrical Drawings Introduction After completion of this section, you will be able to interpret symbols for breakers, relays, and disconnects in a simple one-line diagram, as well as explain the operation of the control circuit. Rev 1 21 Type Symbol Breakers Trip Coil Closing Coil Open (A) Contact Closed (B) Contact Fuses Indicating Lights Overloads Rectifier Bridge 22 Rev 1 Type Symbol Relays Switches Transformer When using drawings, there is usually a legend on the first sheet of the drawing series that provides a guide to symbols. Each supplier has differences in their conventions, so the operator should review the drawing legend when in doubt about the meaning of symbols. Recall that A contacts are open when the controlled circuit breaker is open, and closed when it is closed. B contacts are the opposite. B contacts are open when the breaker is closed, and closed when the breaker is open. Typically, contacts in valve control circuits will be labeled, but if they are not, then A contacts are open when de-energized, and closed when energized. B contacts are closed when de-energized and open when energized. The above drawing representation shows contacts in their deenergized state for both A and B contacts. Rev 1 23 Circuit Breaker Control To operate circuit breakers from a remote location, use an electrical control circuit, such as the one shown below in the figure. Figure: Breaker Control Circuit Control power for the breaker is AC taken from the line side and then rectified to supply direct current (DC) control power. Note that the control power circuit is fused. The major components of the control circuit are: Rectifier unit Closing relay Closing coil Tripping coil Auxiliary contacts Circuit breaker control switch To close the circuit breaker the operator takes the control switch turned to CLOSED position. This provides a current path to energize the closing relay (CR), which closes the auxiliary contact designated on the schematic. Closing this auxiliary contact energizes the closing coil (CC), which closes the circuit breaker. Once the breaker is closed, it latches in the closed position. The (b) contact associated with the closing relay opens de-energizing the closing relay. This prevents repeated closing attempts if the breaker trips open (antipumping feature). When the breaker closes, the A contact closes and 24 Rev 1 enables the trip circuit. The circuit breaker control switch returns to a neutral position when released. To open the circuit breaker, turn the control switch to the TRIP position. This provides a current path to energize the trip coil (TC), which releases the latching mechanism, and the circuit breaker trips open. When the breaker opens, the A contact opens, de-energizing the trip coil. The (b) contact closes, allowing the next remote closure when turning the control switch to the close position. The control circuit can incorporate protective features. Those commonly included are: Overcurrent Underfrequency Undervoltage Protective relays sense each of these fault conditions, which close a contact to energize the breakers trip coil, tripping the breaker. Breakers with multiple trips will have multiple contacts in parallel, any of which can energize the trip coil and trip the breaker. Valve Control Circuits When the valve is fully open or fully shut, either one of the open or shut lights will light while the other will be off. When the valve is midposition, both lamps will light. This differentiates between mid-position and a loss of control power. With loss of control power, both lamps will be off. The figure below shows a valve open circuit. The (a) contacts are open when the relay is de-energized. The (b) contacts are closed when the relay is deenergized. Figure: Valve Open Circuit Rev 1 25 Note that the valve open limit switch contact is closed, so the A1 relay is energized. The (a) contact from the A1 relay will be energized, and closed. The (b) contact from the A1 relay will be energized and open. Therefore, the energized A1 relay provides a current path to light the open light, but not the closed light. The valve shut limit switch contact is open, because the valve is open, so the A2 relay is de-energized. The A2 relay’s (a) contact is open, and its (b) contact is closed. This also provides a current path to light the open lamp, and does not light the closed lamp. When the valve is in the intermediate position, neither the open nor the shut limit switches will trigger, so the contacts for relays A1 and A2 will be open, and neither relay will energize. The figure below shows a valve midposition circuit. Figure: Valve Midposition Circuit Since neither the A1 nor the A2 relay is energized, the B contacts for both will be closed, and both the open and closed indicating lamps will be illuminated. When the valve is closed, the limit switch (LS) for the closed valve contact will trigger, energizing relay A2. The open limit switch contact will be open, so relay A1 will be de-energized. This results in the valve shut indicator lighted and the valve open light being off. The figure below shows a valve closed circuit. 26 Rev 1 Figure: Valve Closed Circuit Electrical Drawings Example Consider the following drawing: Figure: Valve Control Circuit Opening and Closing the Valve As you can see from the figure above, energizing the K1 relay opens the valve, and de-energizing the K1 relay closes the valve. To manually open the valve, depress push button 2 (PB2), which energizes the K3 relay and closes the K3 contact, then energizes the K1 relay. There is no other means to open the valve through the control circuit. After the valve opens, the K1 relay has a seal in K1 contact that maintains power to the K1 relay and keeps the valve open. To close the valve, depress push button 1 (PB1), which interrupts power to the K1 relay. This also causes the K1 seal in contact to open upon de-energizing the K1 relay, so the valve closes and stays closed. There is also no other means to close the valve through the control circuitry. This is a manually opened and closed valve with no automatic features. Rev 1 27 Valve Response to Loss of Control Power The K1 relay must remain energized for the valve to stay open. If the control circuit loses power, the effect is the same as pushing PB1—an interruption of power to the K1 relay and the valve closes. The K1 seal in contact opens, so upon power restoration, the valve stays closed. Alarm Light Function As shown in the figure below, the alarm lights when the K2 time delay relay picks up and closes the K2 contact. This occurs after energizing the K2 relay for a 10-second period. Time delay relays of this type energize, timeout, and then cause their respective contacts to close. If the relays deenergize before the time delay completes, they reset to zero. The K2 time delay relay is energized when PB2 is pushed to open the valve. This energizes the K3 relay and closes the K3 contact, energizing the K1 relay and closing the K1 seal in contact. This is important, because the K3 relay receives energy only momentarily, when PB2 is depressed. If the K1 seal in relay is not closed, the K1 or K2 relays remain de-energized. Power is supplied to the K2 time delay relay only if LS1 (limit switch) is made up, which occurs when the valve is fully closed. The alarm illuminates when the button is pushed to open the valve, then 10 seconds later, with an open signal still applied, the valve is fully closed. If the valve is partially or fully open, LS1 will be open and the alarm will not sound. If the operator depresses PB1 to close the valve, the K1 relay will de-energize and its seal in contact will open, so the alarm will not sound. Electrical Drawings Example Consider the following drawing: 28 Rev 1 Figure: Control Power Circuit Trace the Figure Refer to the control power circuit above and identify the different ways the motor could be de-energized. The control power source is from one phase of line current at termination L1; it returns to a different phase at termination L2. Following the control power loop from termination L1, the first component that can interrupt control power is the stop pushbutton. If the stop push button is depressed, the maintain contact is de-energized and the line contacts will de-energize, stopping power supply to the motor. Continuing through the maintain contact, the next components that could de-energize the motor are the overloads. They sense current on two phases. If either phase senses an overcurrent condition, the overload relay will then open the line contacts, de-energizing the motor. Analyze the Control Circuit Control power circuit and identify the different ways the motor could be deenergized. The control power source is from one phase of line current at termination L1; it returns to a different phase at termination L2. Following the control power loop from termination L1, the first component that can interrupt control power is the stop pushbutton. If the stop push button is depressed, the maintain contact is de-energized and the line contacts will de-energize, stopping power supply to the motor. Continuing through the maintain contact, the next components that could de-energize the motor are the overloads. They sense current on two phases, Rev 1 29 and if either phase senses an overcurrent condition, the overload relay will open the line contacts, de-energizing the motor. Knowledge Check Refer to the drawing of a typical valve control circuit below. What is the purpose of depressing the S1 push button? A. To de-energize the K3 relay after the initiating condition has cleared. B. To maintain the K3 relay energized after the initiating condition has cleared. C. To prevent energizing the K3 relay when the initiating condition occurs. D. To manually energize the K3 relay in the absence of the initiating condition. Knowledge Check Refer to the drawing of a valve control circuit below. Note that limit switch (LS) contacts are shown open regardless of valve position, but relay contacts are shown open/closed according to the standard convention for control circuit drawings. Which one of the following describes the purpose of the alarm? 30 Rev 1 A. Alert the operator when the valve motor circuit has been energized for 10 seconds after push button 2 (PB2) is depressed. B. Alert the operator that the valve is opening by sounding the alarm for 10 seconds after PB2 is depressed. C. Alert the operator when the valve has not moved off its closed seat within 10 seconds of depressing push button PB2. D. Alert the operator if the valve has not reached full open within 10 seconds of depressing push button PB2. Knowledge Check Refer to the drawing of a valve motor control circuit below for a valve that is currently fully open and has a 10-second stroke time. Limit switch LS2 has failed open. Note that limit switch (LS) contacts are shown open regardless of valve position, but relay contacts are shown open/closed according to the standard convention for control circuit drawings. Which one of the following describes the valve response if the control switch is taken to the closed position for 2 seconds and then released? Rev 1 31 A. The valve will begin to close and then stop moving. B. The valve will not move. C. The valve will begin to close and then open fully. D. The valve will close fully. Knowledge Check Refer to the drawing of a motor and its control circuit below. Note that relay contacts are shown open and closed, according to the standard convention for control circuit drawings. The motor has been operating for several hours when it is decided to stop the motor. What is the status of the starting resistors before and after the motor STOP push button is depressed? 32 A. Initially bypassed; bypass is removed immediately after the STOP push button is depressed. B. Initially inserted in the motor circuit; bypassed immediately after the STOP push button is depressed. Rev 1 C. Initially bypassed; bypass is removed following a preset time delay after the STOP push button is depressed. D. Initially inserted in the motor circuit; bypassed following a preset time delay after the STOP push button is depressed. ELO 1.6 Automatic Transfer Switches Introduction After completing this section, the student will be able to explain the purpose and function of normal and power seeking automatic transfer switches. Automatic Transfer Switches Electrical distribution systems use automatic transfer switches (ATS) to quickly disconnect a de-energized electrical load from one power supply and connect it to a backup power supply such as an emergency diesel generator. These switches ensure that a source of power is available to essential electrical loads at all times. There are two categories of automatic transfer switches, based on how they operate. They are classified as power-seeking or normal-seeking depending on whether they are designed to prefer a particular source of power to another. Normal-Seeking Automatic Transfer Switches Normal-seeking automatic transfer switches (ATS) prefer one power source (normal source) to another. This means that the ATS will monitor its normal source at all times and prefers to have its electrical loads connected to the normal source of power. If for some reason the normal source of power is lost, the ATS automatically shifts its electrical loads to an alternate source of power. As soon as restoration of the normal source of power occurs, the ATS automatically shifts its electrical loads back to the normal source. Many normal-seeking ATSs are equipped with a time delay that prevents them from shifting back to the normal source of power until the normal power source demonstrates stability for a preset period, for example, five seconds. This time delay ensures that the ATS will not shift its electrical loads back to an unreliable source of power, upon restoration of the normal source. Rev 1 33 Power-Seeking Automatic Transfer Switches Power-seeking automatic transfer switches do not make a distinction between power sources. If power is lost to the source supplying the switch, the ATS will automatically shift to another available power supply. This type of ATS will not automatically shift back to the original source of power once it returns. It will stay connected to the new source of power until that source of power is lost or until manually shifting the ATS back to the original source of power. Knowledge Check The normal power source for an automatic transfer switch (ATS) has been de-energized, and the ATS has switched to the alternate power source, which is currently supplying the loads. Select all of the following statements that are true about the ATS response. A. A normal-seeking ATS will switch back to the normal source when power is restored. B. A normal-seeking ATS will switch back to the normal source if power is lost to the alternate source, even if power is not restored to the normal source. C. A power-seeking ATS will switch back to the normal source if the normal source is restored, and power is subsequently lost to the alternate source. D. A power-seeking ATS will switch back to the normal source when power is restored. ELO 1.7 Motor Controllers Introduction After completing this section, the student will be able to describe the functions, operation, and protective features of motor controllers. Motor Controllers Motor controllers range from a simple toggle switch to a complex system using solenoids, relays, and timers. The basic functions of a motor controller are to control and protect the operation of a motor. This includes starting and stopping the motor, and protecting the motor from overcurrent, undervoltage, and overheating conditions that would damage the motor. There are two basic categories of motor controllers: the manual controller and the magnetic controller. 34 Rev 1 A manual controller, illustrated by the figure below, is a controller with a manually operated contact assembly. A toggle-type handle or a push button arrangement provides the means to operate a mechanical linkage. The manual controller includes thermal and direct-acting overload units to protect the motor from overload conditions. The manual controller is an ON-OFF switch with overload protection. Small loads such as machine tools, fans, blowers, pumps, and compressors normally use manual controllers. These types of controllers are simple, and they provide quiet operation. To close the contacts, move the handle to the ON position or push the START button. They remain closed until manually moving the handle to the OFF position or pushing the STOP button. The contacts also open if the thermal overload trips. Manual controllers do NOT provide low-voltage protection or low-voltage release. When power fails, the manual controller contacts remain in their prepower failure condition, and if closed, the motor will restart on power restoration. This feature is highly desirable for small loads because operator action is not required to restart the small loads in a facility; however, it is undesirable for larger loads because an automatic restart could result in equipment and personnel hazards. Figure: Manual Motor Controller Magnetic Controller A large percentage of controller applications require that the controller be operated from a remote location or operate automatically in response to control signals. Manual controllers cannot provide this type of control; therefore, magnetic controllers are necessary. Magnetic contactors contained within the controller perform basic magnetic controller operations, such as the closing of switches or contacts. Rev 1 35 A magnetic controller is one that will automatically perform all operations in the proper sequence after the closure of a master switch. Frequently, the master switch (for example, float switch, pressure switch, or thermostat) operates automatically. Manually operated master switches for these types of controllers include push buttons, drum switches, or knife switches. The following figure shows a typical magnetic controller and its component parts. Figure: Three-Phase Magnetic Controller With Thermal Overloads Magnetic Contactor A magnetic contactor in the following figure is operated by an electromagnet. The magnetic contactor consists of an electromagnet and a movable iron armature on which movable and stationary contacts are mounted. When there is no current flow through the electromagnetic coil, a spring holds the armature away from the magnet. When the coil is energized, the electromagnet attracts the armature and closes the electrical contacts. 36 Rev 1 Figure: Magnetic Contactor Assembly Some magnetic controllers incorporate overload devices into the controller. These overload devices protect the motor from damaging overcurrent conditions. Motor Controller Types and Operation Within the two basic categories of motor controllers, there are three major types of AC across-the-board controllers in use today, including lowvoltage protection (LVP), low-voltage release (LVR), and low-voltage release effect (LVRE) controllers. Low Voltage Protection (LVP) The main purpose of an LVP controller is to de-energize the motor or load during low voltage condition and keep it from re-starting automatically upon return of normal voltage. The figure below shows a LVP controller. LVP Controller Operation Pushing the START button energizes the contactor coil M, closing the M and Ma contacts. Releasing the START button does not affect the circuit, because the Ma contact remains closed, shunting the open start push button switch. When a low voltage condition occurs, the M coil will drop out at some predetermined value of voltage (usually 70 to 80 percent of rated voltage), and the M and Ma contacts will open. The START button must then be pushed to restart the motor. Depressing the STOP button de-energizes the M coil, which opens the M and Ma contacts, stopping the motor. Rev 1 37 Figure: LVP Controller Low Voltage Release (LVR) The purpose of the LVR controller is to de-energize the motor in a low voltage condition and restart the motor upon restoration of normal voltage. Small and/or critical loads primarily use this type of controller (for example, cooling water pumps required for safety-related equipment). The figure below shows a LVR controller. LVR Controller Operation Placing the START switch in RUN energizes the M coil, closing the M contacts and starting the motor. When a low-voltage condition occurs, the M coil drops out, opening the M contacts and de-energizing the motor. On restoration of normal voltage, the M coil re-energizes, closing the M contacts and restarting the motor. 38 Rev 1 Figure: LVR Controller Low Voltage Release Effect (LVRE) The LVRE controller maintains the motor across the line at all times. Small loads that must start automatically upon restoration of voltage often use this type of controller, and is operated manually. A LVRE controller may or may not contain overloads. If overloads are used, they will be in the lines to the load. The figure below shows a LVRE controller. Figure: Low Voltage Release Effect (LVRE) Controller Knowledge Check Refer to the drawing of a typical valve control circuit for a 480 volt AC motor-operated valve below. The valve is currently open with the contact configuration as shown. If the S1 push button is depressed, the valve will ____________ and when the S1 push button is subsequently released, the valve will ____________. Rev 1 39 A. remain open; close B. remain open; remain open C. close; open D. close; remain closed TLO 1 Summary During this lesson, you learned about circuit protection principles including: interrupting devices, protection methods, simple electric diagrams, and safety precautions. The information below provides a summary of information in this TLO. 1. Explain the principles and applications of circuit protection, including selective tripping. o Selective tripping is arranging circuit breakers, fuses, and other protective devices in an electrical distribution system so the device closes to a faulted component operates first, in order to keep the largest portion of the distribution system energized. 2. Describe the protection provided by each of the following: o Fuse — protects a component from overcurrent. o Protective relays — designed to protect generating equipment and electrical circuits from any undesirable condition, such as undervoltage, and underfrequency. Circuit breaker — the purpose of a circuit breaker provides a means for connecting and disconnecting 40 Rev 1 circuits of relatively high capacities without damaging them. The three most commonly used automatic trip features for a circuit breaker are: Overcurrent Underfrequency Undervoltage 3. Describe the function of the following types of switches:: o Disconnect switches — two-position switches used for the isolation of power supplies from one or more loads or motor control centers. o Safety switches — low voltage (600 volts AC and below) disconnect switches that are enclosed and may be locked in the off position. These types of disconnects are often used as isolation points for electrical maintenance. 4. Describe personnel safety and equipment protection precautions associated with circuit interrupting devices and relays. 5. Interpret symbols for breakers, relays and disconnects in a simple one-line diagram, and explain the operation of the control circuit. 6. Explain the purpose and function of normal and power seeking automatic transfer switches o Automatic transfer switches (ATS) — used in electrical distribution systems to quickly disconnect a de-energized electrical load from one power supply and connect it to a backup power supply such as an emergency diesel generator. o Normal-seeking ATS — constantly monitors the availability of the preferred normal electrical source. On loss of the normal power supply, ATS automatically shifts to a backup power supply. When normal power returns, ATS automatically shifts back to the preferred normal power supply. o Power-seeking ABT — an ATS that does not prefer a particular source of power. On normal power supply loss, the ATS automatically shifts to backup power supply. Upon return of normal power supply, ATS remains on backup power supply and must be manually shifted back to normal power supply. 7. Describe the functions, operation and protective features of motor controllers. o Motor controller protects a motor o Starts o Stops (controls) 8. A controller's protective features include the following: 9. Fuses 10. Thermal overloads 11. Magnetic overloads 12. Controller types include the following: Rev 1 41 o LVP — de-energizes motor on low-voltage and keeps it from automatically restarting. o LVR — de-energizes motor on low-voltage and restarts upon restoration of voltage to normal. o LVRE — maintains motor across the line at all times. Now that you have completed this lesson, you should be able to describe circuit interrupting and switching devices as well as identify appropriate applications for each. You should also be familiar with required safety precautions in operating electrical circuits. 1. Explain the principles and applications of circuit protection, including selective tripping. 2. Describe the protection provided by each of the following: a. Fuses b. Protective relays c. Circuit breakers d. Overloads 3. Describe the function of the following types of switches: a. Disconnect switch b. Automatic transfer switch c. Manual transfer switch 4. Describe the personnel safety and equipment protection procedures and precautions associated with circuit interrupting devices and relays. 5. Interpret symbols for breakers, relays and disconnects in a simple oneline diagram, and explain the operation of the control circuit. 6. Explain the purpose and function of normal and power seeking automatic transfer switches. 7. Describe the functions, operation, and protective features of motor controllers. TLO 2 Circuit Breakers Overview Circuit breakers are the primary means of switching and circuit interruption in power systems. Understanding how the construction, operation, and indications for circuit breakers is essential to plant operators. Objectives Upon completion of this lesson, you will be able to do the following: 1. Explain the construction and functions of circuit breakers, the different types of circuit breakers and their applications, and the protective features incorporated into circuit breakers. 2. Describe the following associated with racking out circuit breakers: a. Purpose for racking out circuit breakers b. Effect of racking out breakers on control and indicating circuits c. Removal of control power on breaker operation 3. Describe the indications provided for each of the following: 42 Rev 1 a. Local circuit breaker position indications b. Control room circuit breaker status indications c. Circuit breaker and protective relay trip indications 4. Describe the effects of losing circuit breaker control power on breaker operation and indications. 5. Explain the operation of circuit breakers. ELO 2.1 Circuit Breaker Construction and Function Introduction On completion of this section, you will be able to explain the construction and functions of circuit breakers, the different types of circuit breakers and their applications, and the protective features incorporated into circuit breakers. Each facility has specific types and models of circuit breakers that are unique to their location, plant specific vendor information, operating procedures, and in some cases task qualification are required to operate and rack out these breakers. We will review characteristics and operation of some common breakers used in many nuclear plant facilities. There is significant operating experience where both breaker failure and improper operation have contributed to station events and personnel injury, up to and including fatalities. Circuit Breaker Description A circuit breaker is a device that has three fundamental purposes: first, providing circuit protection; second, performing normal switching operations, third, isolating power from a circuit to allow safe maintenance or repair. Circuit breaker design is flexible because designs allow any undesirable condition to actuate a circuit breaker. For example, a circuit breaker can automatically disconnect a circuit completely when any abnormal condition exists. The circuit breaker opens the circuit and stops the current flow when the abnormal condition exceeds a predetermined value, without damaging the circuit or the circuit breaker. Circuit breakers commonly replace fuses and sometimes function as switches. A circuit breaker differs from a fuse because it trips to break the circuit and may be reset, while a fuse melts and must be replaced. Air circuit breakers are breakers where interruption of the breaker’s contacts take place in an air environment. Oil circuit breakers use oil to quench the arc drawn when the breaker contacts open. Higher voltage and current demands led to the development of magnetic air circuit breakers, compressed air circuit breakers, vacuum circuit breakers, SF6 circuit breakers, and other types. All circuit breakers perform the same three basic functions, but higher voltage and current demands require larger and more expensive breakers. Rev 1 43 Voltage in distribution systems falls into one of three groups: high-voltage, intermediate-voltage, or low-voltage. Circuit breaker classifications use the same three groups. High-voltage is voltage that is above 15,000 volts. Intermediate- or medium-voltage is voltage between 15,000 volts and 600 volts. 3. Low-voltage is voltage at 600 volts or less. 1. 2. Low-Voltage Air Circuit Breakers A low-voltage circuit breaker is one that is suited for circuits rated at 600 volts or lower. One of the most commonly used low-voltage air circuit breakers is the molded case circuit breaker below. Figure: Molded Case Circuit Breaker The figure below shows a cutaway view of the molded case circuit breaker. Figure: Cutaway View of Molded Case Circuit Breaker 44 Rev 1 Operating a Molded Case Circuit Breaker Manually moving the operating handle to the ON or OFF position connects or disconnects a circuit using a circuit breaker. All breakers, with the exception of some small ones, have a linkage between the operating handle and the electrical contacts that allows for a quick make or quick break contact action, regardless of the movement speed of the operating handle. In a short-circuit or overload condition, design precludes holding the handle shut. If the circuit breaker opens under one of these conditions, the handle will go to the trip-free position. The trip-free position is midway between the ON and OFF positions. In this condition, the circuit breaker moves to a midposition and cannot be closed without moving the handle to the OFF position to reset the trip latch, and then to the ON position to close the breaker. Automatic Tripping A circuit breaker automatically trips when the current through it exceeds a predetermined value. In lower current ratings, thermal tripping devices provide the means of automatic tripping of the circuit breaker. Thermal Tripping Elements Thermal trip elements consist of a bimetallic element calibrated so that the heat from normal current through it does not cause it to deflect. An abnormally high current, possibly caused by a short-circuit or overload condition, will cause the element to deflect and trip the linkage that holds the circuit breaker shut. Spring action will open the circuit breaker. The bimetallic element, which is responsive to the heat produced by current flowing through it, has an inverse-time characteristic. If an extremely high current is developed, the circuit breaker will trip rapidly. For moderate overload currents, it trips more slowly. Rev 1 45 Figure: Thermal Tripping Element Magnetic Tripping Elements Molded case breakers with larger current ratings also have a magnetic trip element to supplement the thermal trip element. The magnetic unit utilizes the magnetic force surrounding the conductor to operate the circuit breaker tripping linkage. Arc Chutes When the separable contacts of an air circuit breaker open, an arc develops between the contacts. Different manufacturers use many designs and arrangements of contacts and their surrounding chambers. The most common design places the moving contacts inside an arc chute. The construction of this arc chute allows the arc chute magnetically draw the arc formed as the contacts open. When the arc enters the arc chute, it divides into small segments, and cools. This action extinguishes the arc rapidly, which minimizes the chance of a fire and minimizes damage to the breaker contacts. Ratings Molded case circuit breakers come in a wide range of sizes and current ratings. There are six frame sizes available: 100, 225, 400, 600, 800, and 2,000 amps. The size, contact rating, and current interrupting ratings are the same for all circuit breakers of a given frame size. The trip element rating governs the continuous current rating of a breaker. Breakers are 46 Rev 1 available in voltages from 120 to 600 volts, and interrupting capacity ranges as high as 100,000 amps. Large Air Circuit Breakers Large commercial and industrial distribution systems require larger air circuit breakers. The circuit breakers in the figure below are available in higher continuous current and interrupting ratings than the molded case circuit breaker. Large air circuit breakers have current ratings as high as 4,000 amps, and interrupting ratings as high as 150,000 amps. Figure: Large Air Breaker Front Operation Most large air circuit breakers use a closing device, known as a stored energy mechanism, for fast, positive closing action. Energy is stored by compressing large powerful coil springs attached to the contact assembly of a circuit breaker. Once these springs compress, operating a latch releases the springs, and spring pressure shuts the circuit breaker. Two means of compressing circuit breaker closing springs are manually or using a small electric motor. Classification of this type of circuit breaker depends on its spring compression method: either a manually or electrically operated circuit breaker. When a large air circuit breaker is closed, the latch secures its operating mechanism. Closing the circuit breaker compresses a set of tripping springs or coils, and a trip latch serves as the trip for the circuit breaker. The trip latch mechanism is operated by two methods: manually or remotely using a solenoid trip coil. Designers choose electrically operated circuit breakers when circuit breakers operate at frequent intervals or when remote operation is required. Rev 1 47 On tripping the electrically operated stored energy circuit breaker, the spring charging motor recharges the spring so that the breaker is ready for the next closing operation. Manually operated circuit breakers require manually compressing the closing springs (usually with a hand crank) before operating the breaker. Pistons normally puff air across the contacts to help extinguish the arc during breaker operation. The following figure shows a large manually operated air circuit breaker that is a stored energy circuit breaker. Pulling downward on the large operating handle on the front of the breaker compresses the closing springs. To close this circuit breaker, manually depress the small closing lever. To trip this circuit breaker, operate the tripping lever, located at the bottom front of the breaker. Figure: Large Air Circuit Breaker High-Voltage Circuit Breakers Circuits with voltage ratings higher than 600 volts use high-voltage circuit breakers, including breakers rated at intermediate voltage. Standard voltage ratings for these circuit breakers are from 4,160 volts to 765,000 volts with three-phase interrupting ratings of 50,000 kilo Volts Amperes (kVA) to 50,000,000 kVA. 48 Rev 1 In the early stages of electrical system development, the majority of highvoltage circuit breakers were oil circuit breakers. However, circuit breaker development has also resulted in magnetic and compressed air types; all three types are in use today. Magnetic Air Circuit Breaker Ratings for magnetic air circuit breakers go up to 750,000 kVA at 13,800 volts. This type of circuit breaker interrupts in air between two separable contacts with the aid of magnetic blowout coils. As the current-carrying contacts separate during a fault condition shown below, magnets draw the arc out horizontally and transfer it to a set of arcing contacts. Simultaneously, the blowout coil provides a magnetic field to draw the arc upward into the arc chutes. The arc, aided by the blowout coil magnetic field and thermal effects, accelerates upward into the arc chute, where it is elongated and divided into many small segments. Figure: Magnetic Air Circuit Breaker While the construction of this type of circuit breaker is similar to that of a large air circuit breaker used for low-voltage applications, electricity operates the magnetic air circuit breakers. Compressed-Air Circuit Breakers Compressed-air circuit breakers, or air-blast circuit breakers, depend on a stream of compressed air directed toward the separable contacts of the breaker to interrupt the arc formed when opening the breaker. Recent developments produced air-blast circuit breakers used in extra high-voltage applications with standard ratings up to 765,000 volts. Rev 1 49 Figure: Compressed Air Arc Shute Oil Circuit Breakers Oil circuit breakers, shown in the figure below, are circuit breakers that have their contacts immersed in oil. Current interruption takes place in oil, which cools the arc developed and thereby quenches the arc. The poles of small oil circuit breakers can be in one oil tank; however, the large highvoltage circuit breakers have each pole in a separate oil tank. The oil tanks in oil circuit breakers are normally sealed. The electrical connections between the contacts and external circuits are through porcelain bushings. Figure: Oil Circuit Breaker GE Magne-Blast Breaker The GE Magne-Blast breaker, below, is a medium voltage breaker that is widely used in power plant switchgear application. The early designs were an air circuit breaker, with a solenoid-operated mechanism; while the latest designs were outfitted with vacuum type contacts, which can be retrofitted to the earlier design breakers. 50 Rev 1 A mechanical counter, visible on the breaker’s front window and primarily used for maintenance, indicates the number of open and closed cycles on the breaker. The breaker manually closes or trips at the switchgear. Figure: GE Magne-Blast Breaker The Magne-Blast Breakers are also manufactured in a horizontal draw out design. The chassis contains the primary contact assembly and bushings, interlocks, and ground strap. The primary contact assembly is the main current-carrying part of the breaker. The assembly consists of all the barriers, arc chutes, and air puffer system. The charging motor charges the closing spring(s). The charging motor is located below the operating mechanism on the front left side and connected to a drive fitting and levers to the ratchet wheel. Some breaker designs include a manual bar to charge the closing springs locally at the breaker. The spring discharge interlock discharges the closing springs if they are charged when the breaker is rolled in or out of the switchgear cubicle. Protective Relays Design of a circuit breaker control circuit incorporates any of a number of protective features. The list below includes the most commonly used automatic trip features for a circuit breaker. If any one of the conditions exists while the circuit breaker is closed, it closes its associated contact and energizes the tripping coil, which, in turn, trips the circuit breaker. Undervoltage relay — an adjustable device generally set to trip the breaker when voltage drops to 60 to 70 percent of normal value. Underfrequency relay — an adjustable device that will trip the breaker when frequency drops below a preset value to protect the loads on a system that cannot tolerate a significant change in frequency. Rev 1 51 Reverse-Power relay — this device senses a change in the normal direction of current indicating an abnormal condition in the system. This is actually a change in the direction of power flow from power coming out of the breaker to power flowing into the breaker. This device protects a generator from damage from motoring by tripping the generator output breaker. Trip relay — this device releases the breaker operating mechanism and allows it to open when the appropriate conditions exist or upon receiving either an automatic or manual signal. This device is normally de-energized but requires energizing to activate the trip latch. Overload relay devices (also known as overcurrent) — a circuit breaker must be able to detect and react to above normal currents as well as short-circuit currents. By providing the breaker with three over current tripping devices, the breaker can sense and act on three values of current. This provides the breaker with long-time, shorttime, and instantaneous tripping abilities. The three trips all sense the same current; however, they each react differently to this current. Long-Time trip — device reacts to light overloads to trip the breaker after a time. Short-Time trip — device reacts to a slightly higher current and trips the breaker in less time than a long-time trip. Instantaneous trip — device quickly reacts to trip the breaker due to high-currents produced by short-circuits. Knowledge Check A typical 120 volt AC manual circuit breaker has tripped due to overload. To close the circuit breaker, move the breaker handle from the... 52 A. midposition to the OFF position to reset the trip latch, and then to the ON position. B. OFF position directly to the ON position; trip latch reset is not required. C. midposition directly to the ON position; trip latch reset is not required. D. OFF position to the midposition to reset the trip latch, and then to the ON position. Rev 1 ELO 2.2 Racking Circuit Breakers Introduction After completing this section you will be able to describe the following associated with racking circuit breakers: Purpose of racking out circuit breakers Effect of racking out breakers on control and indicating circuits Effect of removing control power on breaker operation Circuit Breaker Racking It is possible to rack circuit breakers to three different positions, each of which provides specific features needed to support power plant operation and maintenance. The three positions are racked in, racked out, and racked to the test position. Racked In Racked in (connected) is the normal position. In this position, there are two breaker options. The first option closed supplies power to the load. The second option opened isolates the load. Essentially, in the racked in position, the circuit breaker is performing its design function. Racked Out (Disconnected) When in the racked out position, the circuit breaker will not supply power to the load. Set a circuit breaker to the racked out position to de-energize the load, allowing safe working conditions. Note that racking out alone may not completely de-energize the load. Usually, control power must also be isolated. Racked to Test When in the racked to test position, the breaker will not supply the load, but receives control power, which allows opening and closing the breaker to test the breaker itself. To de-energize an electrical component as well as its associated control and indication circuits completely, rack out the component breaker and remove control power fuses. There are many types of circuit breakers. The following circuit breaker racking discussion is general in nature. Draw Out You can disconnect circuit breakers by moving the breakers physically away from the bus, see below figure. Perform this with a racking tool that is manually connected to the breaker. Rev 1 53 Figure: Draw Out Breaker The racking tool may be a manual device, but some plants have breaker racking tools that move the breaker with the operator at a distance, see below figure. The breaker moves from the CONNECT position to the TEST position, and then to the DISCONNECT position. Plants usually provide interlocks to prevent operation of the circuit breaker at any point between these three discrete positions. Figure: Breaker Racking Tool The interlocks also mechanically control the breaker tripping mechanism. That is, when the racking operation begins, a mechanism trips the breaker mechanically, preventing closing of the breaker. Movement of the breaker releases the trip mechanism. Ensure that plant procedures and required personal protective equipment (PPE) are used during breaker racking operation. Accidents when racking breakers can be fatal! 54 Rev 1 Figure: Racking Arc Blast A separate connecting device supplies control power to the circuit breaker. This device usually consists of a sliding contact strip with one part mounted on the back of the cubicle at the floor and the other part mounted on the circuit breaker. When racking the breaker back in, contact is made between the two parts when the circuit breaker is moved to the TEST position (from DISCONNECT) and continues as the breaker is moved to the CONNECT position. Consequently, the status light indication observed in the control room or on the front of the breaker is not a conclusive indication of breaker position or operability. To perform a verification of breaker position and operability, verify all four of the following items: Indication on the floor of the circuit breaker housing corresponds to the markings on the circuit breaker. Racking release lever is fully in the CONNECT position (extreme counterclockwise position). The closing spring motor toggle switch is in the ON position and the closing spring is charged. The control power breaker is closed and/or control power fuses are in place ensuring power is available. Rev 1 55 Figure: Breaker Racked Out Knowledge Check To completely de-energize an electrical component and its associated control and indication circuits, the component breaker should be... A. open with the control switch tagged in the open position. B. racked out with control power fuses removed. C. open with the control switch in pull-to-lock. D. racked out and tagged in racked-out position. Knowledge Check When a typical 4,160-volt breaker is racked to the test position, control power is __________ the breaker and the breaker is __________ the load. 56 Rev 1 A. removed from; isolated from B. available to; isolated from C. removed from; connected to D. available to; connected to ELO 2.3 Circuit Breaker Indications Introduction After completing this section, the student will be able to describe the indications provided for each of the following: 1. Local circuit breaker position indications 2. Control room circuit breaker status indications 3. Circuit breaker and protective relay trip indications Circuit Breaker Position Verification A number of means can determine circuit breaker status and physical position: 1. Breaker mechanical indicator’s status of OPEN or CLOSED. 2. Breaker position indication lights: a. Red lamp illuminated indicates closed. b. Green lamp illuminated indicates open. c. Amber lamp illuminated (if supplied), indicates a mismatch between breaker position and the last position placement of the breaker control switch. If the last manual positioning of the control switch was to close the breaker, and the breaker is now open, this indicates a mismatch (probably a protective trip of the breaker), and the amber lamp should be illuminated. The breaker control switch has red and green flag indicators to indicate the last position placement of the switch. The red flag indicates that the last breaker position was CLOSED. The green flag indicates that the last breaker position was OPEN. d. All lamps out indicates loss of control power to breaker. Load-Side Voltage and Current Voltage indicated on the load side verifies that the breaker is closed. No voltage indicates the breaker is open, but double-check that the voltmeter is reading accurately. Current indication on the load side indicates breaker is closed, but no current could indicate that breaker is in the closed position with no load, or the breaker is open. No current is an inconclusive indication. Rev 1 57 Circuit Breaker Trip Flags Personnel may observe an automatic circuit breaker trip by a local device (relay) protection flag indication. Once a circuit breaker protective trip occurs, the trip coil energizes causing the circuit breaker to trip open. A local mechanical flag indicates actuation of a circuit breaker protective trip device. A mechanical flag for each individual device indicates multiple protective device trips. Conditions such as overcurrent, undervoltage, underfrequency, and reverse power usually have protective devices. If activation of any of these protective devices causes the circuit breaker to trip, personnel must reset the associated local mechanical trip flags manually once the condition clears. Operator Responsibilities An operator should use all available indications to determine the actual condition of a circuit breaker. The best indications to use are those that are solely dependent on breaker position like load side voltmeter readings and the local OPEN/CLOSED mechanical flags. Protective devices (relays) are less conclusive indicators of breaker position indication because a breaker may indicate tripped by a protective trip mechanical flag but may actually be closed. It is necessary to reset a protective relay manually once the condition that caused it is clear. If not reset, the trip flag may still indicate tripped from the previous trip, yet the circuit breaker is closed. Circuit breaker position indicating lights are also less conclusive indicators of the breaker’s actual position. For example, without breaker control power, a breaker may indicate open (red light not illuminated) when in reality it is closed. A burned-out indicating lamp may indicate a condition that is incorrect. Likewise, load side ammeter readings are not a conclusive indication of breaker position, because the breaker can be in the closed position with no load (zero [0] amps) indicated. If load side amps are greater than zero (0), it is an indication that the breaker is in the closed position and loaded. Using all of these indicators together allows the operator to establish breaker position accurately. Knowledge Check Which one of the following describes the normal operation of a local breaker overcurrent trip flag indicator? A. 58 Actuates when a breaker overcurrent trip has occurred; can be manually reset when the overcurrent condition Rev 1 clears. B. Actuates when a breaker has failed to trip on an overcurrent condition; can be manually reset when the overcurrent condition clears. C. Actuates to cause a breaker trip when the overcurrent trip setpoint is reached; can be remotely reset when the overcurrent condition clears. D. Actuates when no lockout is present; satisfies an electrical interlock to remotely close a breaker. Knowledge Check Breaker local overcurrent trip flag indicators, when actuated, indicate that... A. the associated breaker has failed to trip open during an overcurrent condition. B. a breaker overcurrent condition is responsible for a breaker trip. C. an overcurrent condition has cleared and the breaker can be closed. D. a breaker trip will occur unless current is reduced. Knowledge Check Given the following indications for an open 4,160 volt AC breaker: The local OPEN/CLOSED mechanical flag indicates open. A breaker overcurrent trip flag is actuated on one phase. The line-side voltmeter indicates 4,160 volt AC. The load-side voltmeter indicates zero (0) volts. Assuming no operator actions were taken since the breaker opened, which one of the following could have caused the breaker to open? A. Rev 1 An operator opened the breaker locally. 59 B. An operator opened the breaker from a remote location. C. A ground fault caused an automatic breaker trip. D. A loss of control power caused an automatic breaker trip. Knowledge Check The following remote indications are observed for a 480 volt AC load center supply breaker. (The breaker is normally open.) Red indicating light is on. Green indicating light is off. Load center voltage indicates zero (0) volts. Breaker incoming voltage indicates 480 volts. What is the condition of the breaker? A. Open and racked in B. Closed and racked to test position C. Open and racked to test position D. Closed and racked in Knowledge Check The following indications are observed for a motor breaker in the control room: Red position indicating light is off. Green position indicating light is off. Load amps indicate normal load current. Assuming one of the indicating lights is burned out, what is the condition of the breaker? 60 A. Open and racked in B. Closed and racked to test position C. Open and racked to test position D. Closed and racked in Rev 1 Knowledge Check The following indications are observed in the control room for a normally-open breaker that directly starts/stops a 480 volt AC motor: Red position indicating light is on. Green position indicating light is off. Load current indicates 50 amps. Supply voltage indicates 480 volts. What is the condition of the breaker? A. Closed and racked in B. Open and racked to test position C. Open and racked in D. Closed and racked to test position Knowledge Check While remotely investigating the condition of a normally-open 480 volt AC motor control center (MCC) feeder breaker, an operator observes the following indications: Green breaker position indicating light is out. Red breaker position indicating light is illuminated. MCC voltmeter indicates 480 volt AC. MCC ammeter indicates zero (0) amperes. Based on these indications, the operator should report that the feeder breaker is __________ and racked __________. Rev 1 A. closed; to the test position B. closed; in C. open; in D. open; to the test position 61 ELO 2.4 Circuit Breaker Control Power Introduction After completing this section, the student will be able to describe the effects of losing circuit breaker control power, including circuit breaker indicator lights and the ability to open and close a circuit breaker remotely. Circuit Breaker Control Power An electrical control circuit must be incorporated to operate circuit breakers remotely, or by using the control switch at the breaker. Any source can supply control power, but the typical source is AC supplied from the breaker line side, rectified to DC for the control circuit. The power source is usually fused and sometimes has switches in the circuit to isolate control power. The figure below shows a typical breaker control circuit: Figure: Breaker Control Circuit Major Components of a Control Circuit The major components of the control circuit are: 1. 2. 3. 4. 5. 6. 62 Rectifier unit Closing relay Closing coil Trip coil Auxiliary contacts Breaker control switch Rev 1 Control circuits include their own protective features. The circuit shown has contacts driven by underfrequency and undervoltage relays. If either of these relays energizes, its contact in this control circuit will close, energizing the trip coil, and tripping the circuit breaker. To close the breaker, position the control switch to CLOSED. This energizes the closing relay, which closes a contact energizing the closing coil, and shuts the breaker. When the breaker closes, the b contact opens, de-energizing the closing relay, which then de-energizes the closing coil. The a contact closes, enabling the trip coil to energize, either through the control switch, or through protective relay contacts. To open the breaker, position the control switch to TRIP. This energizes the trip coil, releasing the latch and allowing the breaker to open. After the breaker opens, the a contact opens, de-energizing the trip coil. The b contact closes, allowing the control switch to energize the closing relay, preparing for the next closure. When breaker control power is lost, the following capabilities are lost: 1. Local and remote breaker indication lamps are out. 2. Remote breaker control (open/close) is lost. 3. Breaker closing springs will not recharge after a local closing of the breaker because the charging motor does not have power. 4. Capability to trip open on a protective relay trip is lost. Note that overloads or fuses, if provided, are still functional in an overload condition. They are not dependent on control power. Knowledge Check Loss of breaker control power will cause... A. the remote breaker position to indicate open regardless of actual breaker position. B. breaker line voltage to indicate zero regardless of actual breaker position. C. failure of the closing spring to charge following local closing of the breaker. D. inability to operate the breaker locally and remotely. Knowledge Check Which one of the following would cause a loss of ability to remotely trip a circuit breaker and a loss of remote Rev 1 63 breaker position indication? A. Racking the breaker to the test position B. Loss of control power for the breaker C. Failure of the breaker control switch D. Mechanical binding of the breaker tripping bar Knowledge Check Which one of the following will cause a loss of indication from the remote breaker position indicating lights associated with a typical 480 VAC load supply breaker? A. Removing the breaker control power fuses B. Locally opening the breaker C. Burnout of the local breaker position indicating lights D. Loss of breaker line voltage Knowledge Check The following indications exist for an open 4,160 volt AC breaker: All phase overcurrent trip flags are reset. The control power fuses indicate blown. The line-side voltmeter indicates 4,160 volt AC. The load-side voltmeter indicates zero (0) volts. Assuming no operator actions occurred since the breaker opened, which one of the following could have caused the breaker to open? 64 A. A ground fault caused an automatic breaker trip. B. An operator tripped the breaker manually at the breaker cabinet. C. An operator tripped the breaker manually from a remote Rev 1 location. D. A loss of control power caused an automatic breaker trip. ELO 2.5 Circuit Breaker Operation Introduction On completion of this section, the student will be able to explain the operation of circuit breakers. Circuit Breaker Operation Guidelines There are two operation modes for circuit breakers: manual or electrical. In addition to the operating modes, circuit breaker locations can be local or remote. With the variety of types and sizes, one would expect a variety of operating mechanisms. Manual and local operation is typical for smaller breakers serving smaller loads and infrequently used breakers. Larger breakers serving larger loads, frequently operated breakers, and breakers that require protective relay protection typically have electrical operation, usually remotely. Operating a Molded Case Circuit Breaker Manually moving the operating handle of a molded case circuit breaker to the ON or OFF position connects or disconnects the breaker. All breakers, with the exception of some small ones, have a linkage between the operating handle as well as the electrical contacts that allows for quick make as well as quick break contact action, regardless of the movement speed of the operating handle. Large Air Circuit Breaker Operation Most large air circuit breakers use a closing device, known as a stored energy mechanism, for fast, positive closing action. Energy is stored by compressing large powerful coil springs attached to the contact assembly of a circuit breaker. Once these springs are compressed, operating the latch releases the springs, whose pressure quickly shut the circuit breaker. The two means of closing circuit breaker springs are manually or by a small electric motor. There are also two ways of operating this type of circuit breaker, either manually or electrically. When a large air circuit breaker is closed, a latch secures the operating mechanism. As the circuit breaker closes, a set of tripping springs or coils are compressed, and a trip latch trips the circuit breaker. The two means of operating the trip latch mechanism are manually or remotely by means of a solenoid trip coil. Circuit breakers operated at frequent intervals or requiring remote operation often use electrically operated circuit breakers. Upon tripping the Rev 1 65 electrically operated stored energy circuit breaker, the spring charging motor recharges the spring so that the breaker is ready for the next closing operation. Pulling downward on the large operating handle on the front of the breaker compresses the closing springs. Depressing the small closing lever manually closes this circuit breaker. The tripping lever, located at the bottom front of the breaker, trips this circuit breaker. Local Breaker Operation Design of some circuit breakers is for local rather than remote operation. There are two operation methods to open or close locally operated breakers: electrical and manual. Electrically Operated Breakers Electrically opened and closed locally operated breakers have a local breaker control switch. The control switch uses breaker control power to operate the circuit breaker (electrically) that is normally located in the associated switchgear or motor control center (MCC). The control switch has three positions including, TRIP, midposition, and CLOSED. When taken to TRIP, the breaker opens. Midposition is the normal resting position of the control switch. After operation and release, a spring returns the breaker switch to the midposition. When taken to CLOSED, the breaker closes, depending on satisfaction of interlock(s) conditions. Electrically operated circuit breakers have logic circuitry, which automatically checks that breaker interlocks are satisfied before allowing the circuit breaker to close. On a loss of control power, this type of breaker will fail as is. The breaker control switch may also have a PULL OUT position that prevents the automatic closing of the breaker. The PULL OUT feature is standard with this type of control switch and does not imply that the breaker has any automatic closing features. Electrically operated breakers also have two different position indicators, trip and close. The figure below shows a breaker control switch. 66 Rev 1 Figure: Breaker Control Switch Circuit Breaker Position Indicating Lights Illuminated red indicates closed Illuminated green indicates open Circuit Breaker Position Indicating Flags Red flag — indicates the breaker was last manually positioned closed Green flag — indicates the breaker was last manually positioned open Manually Operated Breakers Some MCC or load center circuit breakers do not have the capability of electrical operation. The TRIP and a CLOSE push buttons located on the front of the breaker allow manual operation of these breakers. These breakers do not have circuit breaker control switches or position indicating lights like the electrically operated breakers, nor do they have logic circuits to check for interlocks. Remotely Operated Breakers Some remotely operated breakers may include a control power transfer switch, which allows switching breaker operation between LOCAL and REMOTE. Selecting LOCAL restricts the breaker to only local operation. Selecting REMOTE enables operation of the breaker from a remote location such as the control room. In both positions, control power is available for automatic protective trips. Circuit Breaker Operation Example When a circuit breaker is closed, it transits from a position where the contacts are far enough apart so that no current passes between them, to a position where they are in physical contact and the resistance to current flow is small by design. Upon opening the circuit breaker, the transition reverses. In both cases, the circuit breaker must go through the intermediate position when the contacts are close enough that the voltage causes an arc Rev 1 67 between them with high resistance, but enough current flow to damage the contact surfaces if the breaker remains in this position too long. For example, consider what would take place if an operator closing a circuit breaker manipulated the switch part way, became distracted, and stopped in midposition. If the switch directly operated the breaker, this could result in breaker contacts being close, but still apart for an extended period. This condition could damage the breaker contacts and possibly make the plant equipment supplied by the breaker unavailable. To provide positive action, larger breakers use closing and tripping coils. On moving the switch far enough to energize the closing relay, the springs then close the breaker quickly, regardless of the manipulation speed of the switch. The breaker opens in a similar fashion, independent of the manipulation speed of the trip switch. Knowledge Check Describe the effect on operation of a typical breaker when the associated breaker control power transfer switch is placed in the local position? A. Control power will be removed from both the open and close circuits, and the breaker can be electrically operated only from the control room. B. Control power will be available to provide protective trips, and the breaker can be electrically operated only from the control room. C. Control power will be available to provide protective trips, and the breaker can be electrically operated only from the breaker cabinet. D. Control power will be removed from both the open and close circuits, and the breaker can be electrically operated only from the breaker cabinet. Knowledge Check Which one of the following would cause a loss of ability to remotely trip a circuit breaker and a loss of remote breaker position indication? 68 A. Failure of the breaker control switch B. Mechanical binding of the breaker tripping bar Rev 1 C. Loss of control power for the breaker D. Racking the breaker to the test position Knowledge Check Loss of breaker control power will cause... A. inability to operate the breaker locally and remotely. B. failure of the closing spring to charge following local closing of the breaker. C. breaker line voltage to indicate zero regardless of actual breaker position. D. the remote breaker position to indicate open regardless of actual breaker position. TLO 2 Summary In this section, you learned the functions of circuit breakers, how to operate them, their indications, and the effects of losing control power. Circuit breakers are multi-use components. They provide circuit protection, as well as the normal means of switching and for isolation of components for maintenance. Now that you have completed this lesson, you should be able to explain the operation modes of circuit breakers, determine the position of a breaker from the indications available, and predict the impact of losing control power or racking a circuit breaker to different positions. Specifically you should be able to do the following: 1. Explain the construction and functions of circuit breakers, the different types of circuit breakers and their applications, and the protective features incorporated into circuit breakers. 2. Describe the following associated with racking out circuit breakers: a. Purpose for racking out circuit breakers b. Effect of racking out breakers on control and indicating circuits c. Removal of control power on breaker operation 3. Describe the indications provided for each of the following: a. Local circuit breaker position indications b. Control room circuit breaker status indications c. Circuit breaker and protective relay trip indications Rev 1 69 4. Describe the effects of losing circuit breaker control power on breaker operation and indications. 5. Explain the operation of circuit breakers. TLO 3 Paralleling AC Sources Overview Describe the required conditions prior to paralleling two generators, including the effects of not meeting these conditions. Objectives Upon completion of this lesson, you will be able to do the following: 1. Describe the conditions required to properly parallel two AC power sources, including the following: a. Voltage b. Frequency c. Phase 2. Describe the effects of paralleling two AC sources under the following conditions: a. Current out of phase b. Frequencies not matched c. High-voltage differential d. Low-current or too much load ELO 3.1 Paralleling AC Sources Introduction Most electrical power grids and distribution systems have more than one AC generator operating at one time. Normally, plants operate two or more generators in parallel to increase the available power. Understanding the proper method of paralleling AC sources is important to proper and sustained plant operation. Closing a disconnect switch or overriding and closing a breaker without first isolating electrical loads or proper synchronization can result in equipment damage and pose an extreme hazard to personnel. After completing this section, the student will be able to describe the conditions required to parallel two AC power sources properly, including the following three items, voltage, frequency, and phase. Paralleling AC Sources Generally, there are three required prior to paralleling or synchronizing multiple AC sources. 70 Terminal voltages must be almost equal. Frequencies must be almost equal. Rev 1 Output voltages must be in phase. Knowledge Check A main generator is about to be connected to an infinite power grid. If personnel close the generator output breaker with generator and grid voltages matched, but with generator frequency 0.1 Hertz (Hz) higher than grid frequency, this will initially result in the generator... A. picking up a portion of the grid real load. B. experiencing overspeed conditions. C. experiencing reverse power conditions. D. picking up a portion of the grid reactive load. Knowledge Check A main generator is being paralleled to the power grid. Generator voltage has been properly adjusted and the synchroscope is rotating slowly in the clockwise direction. The generator breaker must be closed just as the synchroscope pointer reaches the 12 o'clock position to prevent... Rev 1 A. motoring of the generator, due to unequal frequencies. B. excessive megawatt (MW) load transfer to the generator, due to unequal frequencies. C. excessive MW load transfer to the generator, due to outof-phase voltages. D. excessive arcing within the generator output breaker, due to out-of-phase voltages. 71 ELO 3.2 Abnormal Conditions During Paralleling Operations Introduction to Abnormal Conditions During Paralleling Operations After completing this section, you will be able to describe the effects of paralleling two AC sources under the following conditions: 1. 2. 3. 4. Current out of phase Frequencies not matched High-voltage differential Low current or too much load Remember there are three required conditions prior to paralleling or synchronizing AC sources. Failure to meet these conditions can result in adverse equipment operation and continuity of operation. Equal Terminal Voltages If the voltages of the two AC generators are not close to equal when the breaker is closed, one of the AC generators will act as a reactive load to the other AC generator. This causes high-current exchange between the two machines, possibly causing generator or distribution system damage. There is a reactive load (VAR) transfer from the generator with a higher voltage (lagging power factor). There is a reactive load (VAR) transfer to the generator with a lower voltage (leading power factor). Equal Frequencies A mismatch in frequencies of the two AC generators often causes the generator with the lower frequency to transfer its real load (watts) to the generator operating at the higher frequency and act as a load on the other generator, creating a condition referred to as motoring or reverse powering. The amount of real load (watts) transfer is relative to the frequency difference between the generators. Different electrical frequencies can overload the generators as well as the distribution system. Output Voltages in Phase A mismatch in the phases causes development of large opposing voltages in the two sources. The worst case mismatch would be 180 degrees out of phase, resulting in an opposing voltage between the two generators of twice the output voltage. This high-voltage can cause damage to the distribution system due to extremely high-currents and large mechanical torque exerted on both of the generators. The greater the phase mismatch, the greater the damage that will occur to the generator output breaker because of excessive arcing when the circuit breaker is closed. 72 Rev 1 Powering a De-energized Bus If a bus is not unloaded prior to closing the output breaker can produce an overcurrent condition on the generator. Instantaneous flow of the starting current for all loads that were running when power was lost can exceed overcurrent setpoints. This condition is resolved by using timers to safely sequence loads onto the bus. Knowledge Check Consider paralleling a main generator to the power grid. Assume that generator voltage equals the grid voltage and the synchroscope is rotating slowly in the clockwise direction. Close the generator breaker just as the synchroscope pointer reaches the 12 o'clock position to prevent... A. excessive megawatt (MW) load transfer to the generator, due to out-of-phase voltages. B. excessive arcing within the generator output breaker, due to out-of-phase voltages. C. motoring of the generator, due to unequal frequencies. D. excessive MW load transfer to the generator, due to unequal frequencies. Knowledge Check Closing the output breaker of a three-phase generator onto a de-energized bus can... Rev 1 A. produce an overcurrent condition on the generator if the bus was not first unloaded. B. produce an overvoltage condition on the bus. C. result in a reverse power trip of the generator circuit breaker if generator frequency is low. D. result in large reactive currents in the generator. 73 TLO 3 Summary Now that you have completed this lesson, you should be able to do the following: 1. Describe the conditions required to properly parallel two AC power sources, including the following: a. Voltage b. Frequency c. Phase 2. Describe the effects of paralleling two AC sources under the following conditions: a. Current out of phase b. Frequencies not matched c. High-voltage differential d. Low-current or too much load Breakers, Relays, and Disconnects Summary This module covered circuit protection devices including breakers, relays, and disconnects, their applications and advantages, proper methods for operation, and system responses. After completing this training session, the trainee will demonstrate mastery of this topic by passing a written exam with a grade of 80 percent or higher on the following TLOs: 1. Explain the purpose, safety precautions, and operation of electrical circuit interrupting and circuit switching devices. 2. Explain the construction, operation, and indications for electrical circuit breakers. 3. Describe the conditions that must be met prior to paralleling two generators including effects of not meeting these conditions. 74 Rev 1