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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.
[1]. 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. [2],
similar requirements can be found in International Atomic
Energy Agency (IAEA) Safety standard Ref. [3]. 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. [4] 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. [5] 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
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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. [6] – [9]
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
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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. [10] regarding this type of error and proposed possible
solution.
1) Regulatory requirements
As stated in NRC’s report second level undervoltage
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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
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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
[1]
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).
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Electric Power Systems, 10CFR50 Appendix A, USNRC Washington
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[10] UNITED STATES NUCLEAR REGULATORY COMMISSION,
Branch Technical Positions (PSB) BTP PSB-1 - Adequacy of Station
Electric Distribution System Voltages, USNRC Washington DC USA
(1981).