1 Nuclear Power Plant Electrical Power Supply System Requirements Željko Jurković, Krško NPP, [email protected] Abstract—Various regulations and standards require from electrical power system of the NPP to be reliable, redundant, diverse, independent and provide sufficient capacity for all safety related equipment to operate properly and be able to fulfill its intended purpose. This paper will focus on capacity part of requirements, specifically voltage and frequency constraints that must be met during operation of a NPP. Furthermore, in this paper are given examples of adverse effects of number of events that were caused by deviations in voltage and frequency in past. Index Terms—NPP, Electrical power supply requirements, power system degraded condition, I. INTRODUCTION T HE safe and economic operation of a nuclear power plant (NPP) requires the plant to be connected to an electrical grid system that has adequate capacity for exporting the power from the NPP, and for providing a reliable electrical supply to the NPP for safe startup, operation and normal or emergency shutdown of the plant. Important characteristic of nuclear power plant is that after reactor shut down, it continues to generate heat. Residual heat generation can last for days after shutdown and require continuous operation of reactor cooling system for prolonged period of time. Such system must have robust, reliable and diverse sources of electrical power. Extended unavailability of cooling systems can have adverse effects on reactor core and can cause release of radioactivity into environment. There are two basic characteristics of power source/system that determine quality of supply and have great influence on operability of safety related electrical equipment. Those two characteristics are system voltage and system frequency Ref. . Cause and consequences of deviation of mentioned variables from nominal values are explained in chapters III and IV. II. DESIGN BASIS FOR ELECTRICAL POWER SYSTEM During design process of an NPP and its power supply system special care and attention must be given to determine allowable limitations which are then applicable for NPP operation and become part of NPP design. The full load of the auxiliaries of a NPP is typically 5-8% of the NPP rated load, and substantial and thorough analysis must be carried out in order to determine maximal voltage and frequency deviation which guarantee stable and reliable operation of all safety related loads. Limitations that are determined during design of a NPP are stated for highest level of power system (safety related busbars) and must take into account applicable regulations and standards, physical layout of electrical power system and anticipated system consumption. A. Regulations and standards Importance of stable and reliable power supply for safety systems was recognized by regulatory bodies and there are various regulations and standards which provide minimum requirements that AC power supply system must fulfill. In this chapter such regulations and standards shall be investigated. Basic design demand that power supply system must fulfill is given by Nuclear Regulatory Commission (NRC) in 10CFR50 Appendix A. “General Design Criteria” Ref. , similar requirements can be found in International Atomic Energy Agency (IAEA) Safety standard Ref. . Those requirements are: - Reliability - Redundancy - Diversity - Independence - Capacity NRC’s “General Design Criteria” requires from power supply system “...to provide sufficient capacity and capability ...” i.e. to provide sufficient voltage and frequency for all safety related systems to operate at designed level. Most detailed capacity constrains (in regard to voltage and frequency) can be found in IEEE standard Ref.  which states: “The variations of voltage, frequency, and waveform (including the effects of harmonic distortion) in the Class 1E power systems during any mode of plant operation shall not degrade the performance of any safety system load below an acceptable level. Particular attention should be paid to the effects of degraded grid conditions;...“. IAEA Safety Guide Ref.  requires that Emergency Power Supply (EPS) has the ability to detect and disconnect from its power source in event of degradation (overvoltage, undervoltage, overfrequency and underfrequency), and have possibility to connect directly to the alternative source or the standby power source for that division of the EPS. This implies that once deviation of voltage and/or frequency exceeds the levels specified in the design requirements ESP 2 should switch to alternative source of power (“Fast Transfer” function) and then if degradation is still present to standby power source (in most cases Diesel Generator). Limitations regarding degradation of power source depend on various electrical equipment requirements installed throughout plant. There are numerous standards Ref.  –  which define maximum tolerances in regard to voltage and frequency deviation for different components. NPP usually has large distribution network and there are numerous electrical components that must be taken into account during the establishment of system limitations, and beside standards main source of equipment operating limits are user manuals provided by the manufacturer. Fig. 1 shows typical constraints derived from regulations, standards and manufacturers documentation. Figure 2. – Typical Electrical connection of a NPP Figure 1. – Limitations regarding voltage and frequency B. Power system layout and consumption Nuclear power plants have extensive electrical power system, and it is not uncommon for electrical safety related components to be far away from the main safety related busbar. Distance to the components and connecting equipment (cables and transformers and associated voltage drops) must be taken into account as it is one of the most important element which affects the minimum allowable busbar voltage. Determination of proper limitations must take into account also equipment that is used to provide power supply to safety related busbars i.e. feeding transformers and their characteristics. Main issue regarding transformers is whether there is TAP changer installed or not. American based NPP’s normally do not have TAP changer installed and voltage on safety related busbars is directly proportional to voltage on grid. Since it is normal for voltage to sway during day so will voltage on safety related busbars change. On the other hand in the Europe transformers usually have TAP changer installed and voltages on safety related busbars are thus stable and do not change or sway. In addition to the distance of a component main impact to limitations has combination of system consumption and current grid state. Voltage conditions on the safety busbars must be determined for various modes of plant operation, and worst case scenarios should be determined on the basis of which calculations are performed. Following scenarios must be taken into account during limitations determination: - NPP power system high consumption during low grid voltages. - NPP power system low consumption during high grid voltages. - Power source supplying NPP electrical power system (feeding transformer). - Concurrent motor start or large motor start Additional considerations must take into account external grid events such as sudden loss of large power producers (i.e. trip of nearby NPP or other large power plant) or loss of large power consumer since such events influence the system frequency and may cause serious threat to stable and safe NPP operation. One other important factor that must be taken into account is modifications carried out during plant operation. Generally NPP’s operate for longer than 40 years and modifications made to the NPP can have important impact on power supply system (i.e. increase of the consumption or rearrange power supply system) therefore special attention must be given in order not to bring power supply system beyond design requirements. These activities cannot be taken into account during the NPP design process, but rather demand that the design basis constraints are recalculated on project basis. During the design process and determination of system limits possible fault root causes (i.e. initiating events) and their outcome must be investigated for purpose of validation of the calculated constraints. III. VOLTAGE CONSTRAINTS It is normal for grid voltage to sway, as grid voltage is dependent of total system production and consumption of reactive power, and in the case of absence of the feeding 3 transformer TAP changer voltage present at the safety busbars will also change. There are two distinctive voltage events that can occur with different causes and effects, and are therefore separately considered. A. Undervoltage/degaded voltage All components and equipment within NPP electrical distribution system have their own protection and it is considered that the undervoltage event can be only caused by events initiated outside the safety related electrical system (i.e. prior to safety related busbars). Degraded voltage condition is considered to be special type of undervoltage event where voltage has lowered enough to cause equipment failure and/or damage if this state is present long enough. Undervoltage event is the most likely event to occur among considered events. 1) Causes and effects of undervoltage Undervoltage state at safety busbars can be result of following events: Degradation of high voltage (grid voltage) - Either imbalance of production/consumption of reactive power within system or loss of one or more phases can cause voltage to decrease significantly and have negative impact on the electrical safety related equipment of a NPP. Error in transformer TAP changer operation – It is possible for transformer TAP changer to malfunction and to bring down the voltage on the safety related busbars below allowable level. Consequences of degraded voltage condition are loss of multiple electrical components, trip of motors due to thermal protection actuation (Thermal overload) and inability of electrical (such as auxiliary relays) components to actuate or stay actuated, thus limiting the ability of the safety (and non safety) related equipment to operate and perform its intended function. 2) Protection As stated earlier during the design process various events and equipment requirements must be taken into account. Undervoltage protection is usually divided into two groups, first being loss of voltage protection second degraded voltage protection. Loss of voltage protection is usually set at 70% of nominal voltage with short time delay (3 sec), while degraded voltage protection requires more elaborate approach and is discussed in detail in section V. B. Overvoltage Since grid voltage level depends on production and consumption of reactive power present on grid it is unlikely for production to be greater than consumption and thus occurrence of this type of event is rare. 1) Causes and effects of overvoltage Overvoltage can be caused by following events: Error in operation of transformer TAP changer - It is possible for transformer TAP changer to malfunction and to raise the voltage on the safety related busbars above allowable level. Loss of offsite power (LOOP) – In case that NPP substation should for any number of reasons loose connection with grid (i.e. island operation of NPP), sudden loss of burden would cause rise in voltage and frequency. High grid voltages with concurrent low consumption – In case that there is no transformer TAP changer, there is high grid voltage present and NPP electrical distribution system is lightly loaded (during refueling), a chance exists for voltage to rise above design limits. Error in Automatic Voltage Regulator (AVR) of Diesel Generator – In case of failure of AVR of diesel generator voltages produced by generator will automatically rise to maximum level which can be greater than 130% of nominal voltage. Overvoltage as consequence has arc flash which has capability to destroy equipment and render them useless. 2) Protection Occurrence of overvoltage event is rare and basic overvoltage alarm is sufficient for alarming purposes and NPP generators usually have installed overvoltage protection and equipment for LOOP detection. IV. FREQUENCY CONSTRAINTS In normal operation system frequency is determined by ratio of active power production and consumption, and it is not expected for system frequency to vary. 1) Cause and effects of frequency deviation Beside LOOP event discussed in chapter III.b source of frequency deviation can be wide gird (system) disturbances. This implies loosing either large number of power plants within system (which would lower the system frequency) or loosing large number of system burdens (which would increase the system frequency). Increase or decrease in frequency results in increase or decrease in motor speed which then translates into deviation in pump rotation speed which ultimately results in increase or decrease in flow. Therefore the final result of decrease in the electrical system frequency would be decrease of flow in the reactor coolant system and reactor temperature would start to increase. Similarly increase in the electrical system frequency would increase reactor cooing system flow, resulting in decrease of temperature in reactor. 2) Protection There are usually frequency protection relays installed in electrical distribution system for protection against such events. V. DEGRADED VOLTAGE CONSIDERATION AND CASE STUDY As stated in chapter III.a Degraded voltage condition is most common of all considered events and once consequences of such event are taken into consideration it is of great importance to adequately determine limitations of operation which will ensure reliable operation of the electrical distribution system of a NPP. NRC has issued a special report Ref.  regarding this type of error and proposed possible solution. 1) Regulatory requirements As stated in NRC’s report second level undervoltage 4 protection must satisfy following criteria: a) The selection of undervoltage and time delay setpoints shall be determined from an analysis of the voltage requirements of the Class 1E loads at all onsite system distribution levels b) Two separate time delays shall be selected for the second level of undervoltage protection based on the following conditions: 1) The first time delay should be of a duration that established the existence of a sustained degraded voltage condition (i.e., something longer than a motor starting transient). Following this delay, an alarm in the control room should alert the operator to the degraded condition. The subsequent occurrence of a safety injection actuation signal (SIAS) should immediately separate the Class 1E distribution system from the offsite power system. 2) The second time delay should be of a limited duration such that the permanently connected Class 1E loads will not be damaged. Following this delay, if the operator has failed to restore adequate voltages, the Class 1E distribution system should be automatically separated from the offsite power system. Bases and justification must be provided in support of the actual delay chosen. Furthermore once all calculations are carried out NRC requires that the analytical techniques and assumptions used in the voltage analyses must be verified by actual measurement. The verification and test should be performed prior to initial full-power reactor operation on all sources of offsite power. 2) Case study- NEK second level undervoltage protection As a result of Periodic Safety Review (PSR-1), NEK has decided to install additional equipment in order to provide for degraded voltage protection. Extensive review of NEK’s auxiliary power supply system was carried out with following conclusions: All NEK’s transformers that connect safety related busbars with transmission system have TAP changer, and it is considered that a TAP changer malfunction is the most likely cause for voltage degradation. Since TAP changer operation is very slow (1-2% per minute) it was determined that NRC scheme is partially applicable and decision to depart from the NRC requirements was made. In order to accommodate anticipated slow operation of the TAP changer additional alarm level with its own time delay (15 sec) was installed at 6,3 kV safety busbars. Second level of undervoltage is installed with only one time delay. Following this time delay automatic class 1E distribution system will be automatically separated from offsite power sources and Black-out sequence is started. Time delay was determined after extensive consideration and recording of various motor starts. Largest motor (Reactor Coolant Pump, RCP) starts were recorded and time delay was selected in order to accommodate anticipated voltage drops that can occur. Recordings are shown in Fig. 3 - 6 Figure 3. – NEK RCP#1 bump start recording Figure 4. – NEK RCP#2 bump start recording As it can be seen from Figures 3 to 6, maximum duration of start of RCP pump can be 15 seconds, so in order to accommodate any transients and to prevent any spurious trips time delay of 20 seconds was chosen. Figure 5. – NEK RCP#1 start recording Figure 6. – NEK RCP#2 start with RCP#1 running 5 Once alarm level and second level of undervoltage protection are taken into account following figure is obtained: calculation and determination. Form this paper it can be easily concluded that defining proper and corresponding limits of operation for the NPP electrical distribution system is demanding task and may different known and unknown variables must be taken into account. REFERENCES  Figure 7. – Voltage protection and alarm at NPP Krško safety related busbars Calculation carried out in order to determine lowest allowable voltage that can be present at 6,3 kV safety busbars revealed that most susceptible safety related motor is located aprox. 300m away form safety related busbars. When mentioned motor is at lowest allowable voltage voltage at safety busbars was calculated to be 5748V or 91,3% of nominal voltage. Figure 8. – Voltage protection and alarm at NPP Krško safety related busbars From mentioned calculation it can be easily concluded that even in case when voltage on 6,3 kV level is within ±10% of nominal value there exist possibility that some components can be in or near degraded voltage condition. VI. CONCLUSION In this paper overview of applicable standards and possible solutions were presented. Also a case study was included to give the reader a better overview of process of setpoints INTERNATIONAL ATOMIC ENERGY AGENCY, Electric Grid Reliability and Interface with Nuclear Power Plants, IAEA Nuclear Energy Series No. NG-T-3.8, IAEA, Vienna (2012).  UNITED STATES NUCLEAR REGULATORY COMMISSION, General Design Criteria for Nuclear Power Plants — Criterion 17 — Electric Power Systems, 10CFR50 Appendix A, USNRC Washington DC USA.  INTERNATIONAL ATOMIC ENERGY AGENCY, Safety of Nuclear Power Plants Design, IAEA Safety Standards Series No. SSR-2/1, IAEA, Vienna (2012).  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