Environmental Engineering Reference
In-Depth Information
occurring in either can lead to the shutdown or
collapse of the other.
The NPPs and electric transmission grids are
complex engineering facilities, which determine
the persistent economic development of any
countries. When they are connected together in
a controlled, dynamic and distributed network,
further complexity is created. This complexity of
engineered systems is a consequence of several
factors: the sheer size and interconnectivity of
the electric grid, the nuclear safety requirements
imposed on NPPs the need to balance electricity
supply and consumption throughout the grid at
all times, and die nature of electricity - that it is
generated as it is used. Unlike other commodities,
it is difficult to store electricity. This means the
electric grid system requires continual surveillance
and adjustment to ensure supply always matches
demand. Unlike NPPs, the inherent, natural and
passive safety feedback systems based on physical
laws are rather weak. Hence electric grids require
continuous control and balancing actions based
on engineered systems.
Stability in the grid system is maintained by
matching the electricity generation with the ever
changing demand. The electricity from many
power generating stations is “pooled” in the trans-
mission system, and each customer draws from this
pool. Power, entering the system, flows along all
available paths to the distribution systems. This
pooling of electricity also means that power is
provided from a variety of generating stations of
different sizes, including nuclear, coal, oil, natural
gas and renewable energy sources such as wind,
solar, biomass and hydro power, which must all he
synchronized to the same rhythm with millisecond
accuracy. For a power grid to remain stable, the
frequency and phase of all power generation units
must remain synchronous within narrow limits.
A generator that loses synchronism with other
generators but stays connected to the grid will ex-
perience large electrical currents, which will lead
to overheating and large mechanical forces that
will rapidly destroy the generator. So protective
circuit breakers disconnect (trip) a generator from
the grid, when the generator loses synchronism.
The reliability of off-site power is usually
assured by two or more physically independent
transmission circuits to the NPP to minimize the
likelihood of their simultaneous failure. Similarly,
the reliability of on-site power is enhanced by suf-
ficient independence, redundancy and testability
of batteries, diesel generators, gas turbines and the
on-site electric distribution systems to perform
safety' and other functions even if a single failure
occurs. Because of the importance of reliable
off-site power as well as considerations of cost
effectiveness and efficiency, the electric grid is an
important factor in NPP site selection, which must
take into account the plant's position within the
grid as well as its proximity to centres of electric-
ity demand, population density and other factors.
In addition to assuring that the electric grid
will provide reliable off-site power to NPPs, there
are other important factors to consider, when an
NPP will be the first nuclear unit on the grid and,
most likely, the largest unit. If an NPP is too large
for a given grid, the operators of the NPP and the
grid may face several problems.
Off-peak electricity demand might be too low
for a large NPP to be operated in base load mode,
i.e. at constant full power.
There must be enough reserve generating
capacity in the grid to ensure grid stability dur-
ing the NPP's planned outages for refueling and
maintenance.
Any unexpected sudden disconnect of the NPP
from an otherwise stable electric grid could trig-
ger a severe imbalance between power generation
and consumption causing a sudden reduction in
grid life.
The technical issues associated with the inter-
face between NPPs and the electric grid includes
(NUREG-1150, 1989):
•
The magnitude and frequency of load re-
jections and the loss of load to NPPs;
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