NPP: Power Grid Mutual Safety Assessment - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 402

secondary coolant circuits. The main reactor
circulating pumps, steam generator feed water
pumps and long term decay heat removal systems
rely on stable electric power to function properly.
The speed of the reactor's main coolant pumps
is directly proportional to the frequency of the
electric power supply. Therefore, if the frequency
of the power from the grid drops far enough, the
pumps will slow, which will lead to inadequate
core cooling, and the reactor w ill trip.
Other AC motors in the NPP may also trip
due to rising currents and consequent overheating
caused by reduced frequency. The performance
of AC motors is directly affected by the voltage
and frequency of their power supplies. If electric
grid voltages are not sufficient, motors cannot
develop sufficient motor torque to start, and if the
frequency drops below a certain value, the start
and operation of AC motors would require higher
operating voltages. If the voltage is insufficient,
it results in excessive current being drawn by the
motor that in return would lead to overheating and
the opening of protective breakers.
The frequency and voltage ranges, in which
large AC motors can operate, are relatively nar-
row. Thus, in severely abnormal conditions, safety
systems in nuclear power plants are required to
take protective actions such as tripping the reactor
and turbine, separating the plant electrical systems
from the degraded conditions present on the grid,
and switching to on-site emergency power sources
until the grid voltage and frequency are restored to
acceptable values. These actions protect the NPP
by safely shutting it down and keeping it cooled.
However, any sudden automatic shutdown of a
large baseload nuclear unit during periods, where
there is already a mismatch between generation
and load on the grid can only further degrade the
grid's condition, potentially leading to a partial
or full collapse.
Loss of Off-Site Power. Any loss of off-site
power would he caused by external events beyond
the NPP's switchyard, such as transmission line
faults and weather effects like lightning strikes,

ice storms and hurricanes. A loss of off-site power
interrupts power to all in-plant loads, such as
pumps and motors, and to the NPP's safety systems.
As a protective action, safety systems will trigger
multiple commands for reactor protective trips
(e.g. turbine and generator trip, low coolant flow
trip, and loss of feedwater flow trip). The reactor
protection system will also attempt to switch to an
alternate off-site power source to remove residual
heat from the reactor core. If this fails, in-plant
electrical loads must be temporarily powered by
batteries and stand-by diesel generators until off-
site power is restored. However, diesel generators
may not be as reliable as off-site power from
the grid in normal conditions. Diesel generators
may fail to start or run 1% of the time. However,
the probability of failure can be significantly re-
duced by installing independent trains of diesel
generators. Batteries can provide power only for
a limited time.

Influence of NPP
Disturbances on the Grid

Trip of an NPP Causing Degraded Grid Frequency
and Voltage. Even at steady state conditions, when
the generation and loads on a grid are in balance, if
a large NPP (e.g. 10% of the grid's total generating
capacity) trips unexpectedly, the result can be a
significant mismatch between generation and load
on the grid. Unless additional power sources are
quickly connected to the grid, this can degrade the
grid's voltage and frequency and, thus the off-site
power supply to the NPP. The degraded voltage
and frequency on the grid can potentially result
in the NPP protection system disconnecting the
degraded off-site power to the NPP. This will force
the NPP to switch to on-site emergency power to
run safety and core cooling systems until off-site
power is restored. This should be done as soon as
possible for safety reasons: the possible concur-
rent failure of the NPP's on-site power system and
delayed recovery of off site electric power would
make it nearly impossible in most NPPs to cool