NPP: Power Grid Mutual Safety Assessment - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 401

• Grid transients causing degraded voltage
and frequency in the power supply of key
safety and operational systems of NPPs;
• A complete loss of of-site power to an
NPP due to grid disturbances;
• An NPP unit trip causing a grid distur-
bance resulting in severe degradation of
the grid voltage and frequency, or even to
the collapse of the power grid.

be re-synchronized to the grid and its production
quickly raised again to full power. This operational
characteristic of the NPP is important, when the
loss of load is expected to last for just a short time.
Degraded Grid Voltage or Frequency. Electric
grids are controlled to assure that a particular fre-
quency, either 50 or 60 Hz, is maintained within a
small tolerance, typically within ± 1%. When the
grid develops an imbalance between generation
and load, the grid frequency tends to 'droop' if the
load exceeds generation and increase if genera-
tion exceeds the load. A reduction in frequency
can be caused by several events, such as insuf-
ficient available generation, a major electrical
disturbance, such as a circuit fault or the trip of
a major generator unit. A small droop in the grid
frequency caused by the loss of generation can be
controlled by quickly activating the grid's avail-
able “spinning reserve,” either automatically or
manually, starting up additional generation capac-
ity, such as gas turbines or hydroelectric power,
and disconnecting selected loads (i.e. customers)
from the grid (load shedding).
Isolating the section of the grid with the NPP
from the rest of the grid ('system islanding') can
also help maintain the proper frequency in the
islanded system. System islanding may reduce
the load on the NPP, requiring that its genera-
tion be reduced accordingly by a quick set-back
to an intermediate power level. Proper islanding
prevents the NPP from tripping because of the
lower frequency, but may further aggravate the
power imbalance in the rest of the grid. A plant
trip including reactor shutdown should be regarded
as a last resort. During a trip the plant is subject to
rapid changes in power, pressure and temperature,
which shorten the lifetime of the plant. Moreover,
if the NPP is immediately disconnected from
the grid, the lost generation will exacerbate the
degraded conditions on the grid.
Any change in the grid frequency affects an
NPP's operation by changing the speed of the
NPP's turbo generator and the speed of pumps
circulating coolants through the reactor and die

Influence of Grid Disturbances
on Nuclear Power Plants

Load Rejection and Complete Loss of Load . A load
rejection is a sudden reduction in the electric power
demanded by the grid. Such a reduction might be
caused by the sudden opening of an interconnec-
tion with another part of the grid that has carried a
large load. An NPP is designed to withstand load
rejections up to a certain limit without tripping
the reactor. An NPP's ability to cope with a load
rejection depends on how fast the reactor power
can be reduced without tripping and then how fast
the reactor power output can be increased hack to
the original level, when the fault is cleared. Load
rejections of up to 50% are accommodated by a
combination of several actions: rapidly running
back the steam turbine to the new lower demand
level, diverting the excess steam from the turbine to
the main steam condenser unit or to the atmosphere
if this is permitted by licensing regulations, and
reducing reactor power via insertion of control
rods without tripping the reactor.
A loss of load is a 100% load rejection that is
the entire external load connected to the power
station is suddenly lost, or the breaker at the
station's generator output is opened. Under this
severe condition, it may still be possible to 'island'
the NPP so that it powers only its own auxiliary
systems. During this 'house-load' operating mode,
the reactor operates at a reduced power level that
is still sufficient to assure enough electricity for its
own needs, typically 5% of full power. Once the
grid disturbance has been eliminated, the NPP can