Load shedding is what electric utilities do when
there is a huge demand for electricity that exceeds the generation available.
The alternative is to have a brown-out where the voltage is reduced.
It costs a lot to have generators standing by just in case there is a surge of demand, and the operators of those generators expect to be paid whether they run the generators or not. An alternative is if there is a large consumer of electricity (say, a factory) that could suddenly turn off all its electricity demand, they could agree to do that on request, and it has the same benefit as adding that amount of generation to the electric grid. In fact, it's better -- as there is less demand on the wires which are often saturated at the same time.
That factory has losses from shutting down its equipment and idling its workers, but if the money it gets paid is enough, then it's worth it. This is an example of load shedding.
There are many other cases where lots of smaller consumers agree to reduce demand on hot summer days, such as by reducing air conditioning or lighting.
Someone who aggregates all these smaller cases can have the same effect as one big generator and so get the same money, and the operators of the electric grid are happy as it effectively solves the peak demand problem.
It costs a lot to have generators standing by just in case there is a surge of demand, and the operators of those generators expect to be paid whether they run the generators or not. An alternative is if there is a large consumer of electricity (say, a factory) that could suddenly turn off all its electricity demand, they could agree to do that on request, and it has the same benefit as adding that amount of generation to the electric grid. In fact, it's better -- as there is less demand on the wires which are often saturated at the same time.
That factory has losses from shutting down its equipment and idling its workers, but if the money it gets paid is enough, then it's worth it. This is an example of load shedding.
There are many other cases where lots of smaller consumers agree to reduce demand on hot summer days, such as by reducing air conditioning or lighting.
Someone who aggregates all these smaller cases can have the same effect as one big generator and so get the same money, and the operators of the electric grid are happy as it effectively solves the peak demand problem.
When there
is a shortfall in the electricity supply, there can be a need to reduce demand
very quickly to an acceptable level, or risk the entire electricity network
becoming unstable and shutting down completely. This is known as a “cascade”
event, and can end in a total or widespread network shutdown affecting very
large areas of a country. Some recent examples include the blackouts in
northeast America and Canada in 2003 and across Italy in the same year and in India
As the
system frequency approaches the normal 60 Hz or 50 Hz ,a frequency relay can be
used to automatically begin the restoration of the load that has been shed. The
amount of load that can be restored is
determined by the ability of the system
to serve it .The criteria is that the
available generation must always
exceed the amount of load
being restored so that the system frequency will continue to recover . Any
serious decrease in system frequency at this point could lead to undesirable
load shedding repetition, which could start a system oscillation between
shedding and restoration. This would be a highly undesirable condition. The
availability of generation, either locally or through system interconnections
, determines whether or not the shed
load can be successfully restored.
Therefore,
a load restoration program usually incorporates time delay, which is related to
the amount of time required to add generation or to close tie- lines during
emergency conditions. Also, both the time delay and the restoration frequency
set points should be staggered so that all of the load is not reconnected at the same time. Reconnecting loads on a distributed basis
also minimizes power swings across the system and thereby minimizes the
possibility of initiating a new disturbance.
The drop in
frequency may endanger generation itself .While a hydro-electric plant is relatively unaffected by even a ten percent
reduction in frequency, a thermal generating plant is quite sensitive to even a five percent reduction. Power output of a thermal plant depends to a
great extent on its motor driven auxiliaries such as boiler
feed water pumps, coal pulverizing and feeding equipment, and draft fans. As
system frequency decreases, the power output to the auxiliaries begins to fall
off rapidly which in
turn
further reduces the energy input to the turbine generator. The situation thus
has a cascading effect with a loss of
frequency leading to a loss of power which can cause the frequency to
deteriorate further and the entire plant is soon in serious trouble. An
additional major concern is the possible damage to the steam turbines due
to prolonged operation at reduced frequency during this severe overload
condition .
Load
shedding normally happens in two ways:
Automatic
Load Shedding
This is a
result of concurrent failures of major element(s) in the national grid
(e.g.co-incidental generator or key transmission line failures), resulting in
protection schemes initiating the automatic isolation of additional parts of
the national grid, to protect the entire grid from cascading to a total
blackout. Automatic load shedding always occurs on the transmission system
level, with the result being large
amounts of
electricity and large blocks of customers taken off supply in a very short
time. Typical load reduction amounts can be in the order of 1000MW – 2000MW,
affecting hundreds of thousands of customers.
Manual
(Selective) Load Shedding
This occurs
where time is available (typically up to 30mins) to make selective choices on
what customers are shed. Selective load shedding often occurs on the
distribution system level, and typically requires medium to small amounts of
electricity to be “shed” in a short time (rolling blackout). Typical load
reduction amounts can be in the order of 50MW – 100MW, affecting tens of
thousands of customers at a time.
In order to
minimise the impact on individual customers and share the burden, rotational
load shedding (rolling blckout) will occur on the low priority feeders if the
load shedding duration extends for several hours. Typically the first group of
customers who were shed will be restored after one or two hours, at the expense
of the next group of customers to be taken off supply. This can continue until
the supply/demand equation is balanced again and load shedding is no longer
required.