Download Protective Relays

Chapter 7
SYSTEM PROTECTION
Types of Protection
• There are two types of protection referred to in electric
power systems.
1. System Protection
2. Personal Protection
• The first is system protection, having to do with
protective relays, fault currents, effective grounding,
circuit breakers, fuses, and so on.
• The second is personal protection, having to do with
rubber gloves, insulating blankets, grounding jumpers,
switching platforms, tagging, and so on.
System Protection
• The protection of power system equipment is
accomplished by protective relaying equipment that is used
to trip circuit breakers, reclosers, motorized disconnect
switches, and self-contained protection devices.
• The objective of system protection is to remove faulted
equipment from the energized power system before it
further damages other equipment or becomes harmful to the
public or employees.
• System protection protects power system equipment from
damage due to power faults and/or lightning.
• System protection uses solid-state and electromechanical
protective relays to monitor the power system’s electrical
characteristics and trip circuit breakers under abnormal
conditions.
System Protection……
• Another means for providing equipment protection is
proper grounding.
• Effective or proper grounding can minimize damage to
equipment, cause protective relays to operate faster.
System Protection Equipment and Concepts
• System protection, often called protective relaying, is
composed of relay devices in substations that monitor the
power system’s voltages and currents through the CTs and
PTs and are programmed to initiate “trip” or “close” signals
to circuit breakers if the thresholds are exceeded.
• The relays, trip signals, circuit breaker control systems,
and the system control equipment are all battery powered.
Protective Relays
• A protective relay is a device that monitors system
conditions (amps, volts, etc., using CTs and PTs) and reacts
to the detection of abnormal conditions.
• The relay compares the real-time actual quantities against
preset programmable threshold values and sends dc
electrical control signals to trip circuit breakers or other
opening devices in an effort to clear an abnormal condition
on the equipment it is protecting.
• When system problems are detected and breakers are
tripped, alarm indications are sent to system control and
sometimes other protection operations are initiated.
• As a result, equipment may be deenergized, taken off line,
and consumers will be out of power with minimal
equipment damage.
Protective Relays……
Protective relays are of two types:
1. Electromechanical
2. Solid state
• Electromechanical relays are composed of coils of
wire, magnets, spinning disks and moving electrical
switch contacts, and are very mechanical in nature.
• Solid-state relays are electronic and have no moving
parts.
The basic differences are listed below.
Solid State
• Advantages: Multiple functionality, small space
requirements, easy to set up and test, self-testing, remote
access capability, and they provide fault location
information.
Protective Relays……
Disadvantages: External power required, software can be
complex, and may have many “functional eggs” all in one
basket.
Electromechanical Relays
• Advantages: Usually self-powered, simple and singlefunction design.
• Disadvantages: Normally one relay per phase, difficult
to set up and adjust, and require more frequent testing.
Protective Relays……
Inverse Current–Time Concept
• Typically, protective relays are designed to follow
the inverse current–time curve as shown in Figure 7-3.
• In other words, the time to trip a circuit breaker
shortens as the amount of fault current increases.
• Therefore, a relay sensing a fault located near a
substation would initiate a trip of the breaker faster
than if the fault were located down the line because
less current flows due to the additional resistance of
the wire.
• Between these two points, the relay engineer
adjusts the shape of the curve to meet various system
protection coordination objectives.
Inverse Current–Time Concept……
Upper limit
Lower limit
Inverse Current–Time Concept……
• Relay coordination is the term used to create a
situation in which the most downstream clearing
device from the source clears the fault first.
• The coordination of all the protective relays in
the transmission and distribution systems, or even
a single power line is a very special art and
science.
One-Line Diagrams
• A one-line diagram (also referred to as the single-line
diagram) is a simplified drawing of the system or a
portion of the system that shows the electrical placement
of all major equipment.
• Extra information is added to give the engineer or
systems operator the full picture of the electrical system,
including the system protection schemes.
• One-line diagrams are very useful for planning
maintenance activities, rerouting power after a fault,
switching orders to change system configurations, and to
view the relationships between smaller sections of the
power system and the overall system.
One-Line Diagrams……
One-Line Diagrams……
Some of the most common uses of one line diagram are
discussed below:
1. Line Crews refer to one-line diagrams to know what
protective relays are used on the power line being
worked, to identify disconnect switch locations for
load transfer operations, and to see the relationship to
other nearby lines or equipment that are part of the
system in question.
2. System Operators use one-line diagrams to identify
the electrical placement of breakers, air switches,
transformers, regulators, and so on in substations that
may indicate alarms and/or needs corrective action.
They use one-line diagrams to figure out how to
switch the system equipment to restore power.
One-Line Diagrams……
3. Electrical Engineers use one-line diagrams to
understand system behavior and to make changes to
the power system to improve performance.
4. Consumers use one-line diagrams to identify their
electrical equipment, circuits, and protection
apparatus.
Assignment 3
•
•
What are the major factor involved in load shedding in
Pakistan. Discuss briefly.
How electric power system can be improved in
Pakistan. Discuss it in term efficiency (minimum
losses), voltage and frequency stability.
Note: Assignment will on maximum of 3 pages.
Due date: 29th November