Public Utilities

What is load shedding and why is it carried out?

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2010-03-07 10:02:28

When the supplying company receives more demand for electrical

power than its generating or transmission or installed capacity can

deliver, the company has to resort to rationing of the available

electricity to its customers. This act is called load shedding.

Load shedding can also be referred to as Demand Side Management

or Load Management


Demand Controller devices are used to shed loads when a pre-set

Kw reading has been reached.

These devices are tied in to circuits supplying electric heat,

stoves, dryers, hot tubs--anything that tends to use a lot of

power. When set properly, Demand Controllers can be very effective

at reducing your energy bill.


This question may be referring to the kind of load shedding

which often occurs in places where the total electrical power load

which can be taken by consumers greatly exceeds the available

amount of energy which can be generated by the local power station

or national network of power stations. This is a situation which is

common in many developing countries. As soon as total power

demanded exceeds a certain percentage - usually 98% - of the

maximum possible power that can be generated, parts of the

distribution network have to be disconnected. Such disconnections

are known as "load shedding".

If loadshedding was not done the generating equipment's overload

breakers would automatically shut down the whole power station to

protect its alternators (electrical generators) from very severe

damage. Such damage would be extremely expensive to repair and

would take a lot of time to do.

So in practice, to keep the power stations running 24/7 under

such conditions, loadshedding is applied to different parts of the

distribution network at various set times throughout a regular

"power availability" period of, usually, a week. For example, parts

of the network supplying homes and small business offices may only

get power for two or three hours at a time every day or every other

day, whilst important places - such as hospitals, major factories

and, typically, government offices - may get power almost 24/7.


Load shedding, normally used in industrial, large commercial,

and utility operations, is monitoring electric usage continuously

(usually by automated instrumentation) and shutting down certain

pre-arranged electric loads or devices if a certain upper threshold

of electric usage is approached. there are two reasons for doing

it, both of them financially motivated.

Power companies sometimes set up an industrial customer or a

school with an electric billing rate in steps, i.e. if you are

pulling less than this amount of electric current during certain

times you get billed at one rate for the electricity you use. If

you are pulling MORE than this amount of electric current during

that time, you get billed at a higher rate, even if you use the

same amount of electricity overall. The highest current you draw

during the time period in question is your "peak demand". The power

company has to have generating capacity built and in place to

generate the sum of all of the "peak demands" of its customers

during the highest peak demand period. Say that period is 10 am to

12 am during weekdays. If the highest peak demand exceeds the

capacity of the power company to generate, they either have to

build another power station to cover that 2 hour period each day,

which is a waste of money, or they have to buy power from some

other power company during those 2 hours which is also expensive.

So power companies look for ways to cut down on the highest peak

demand. They encourage their large customers to cut back on peak

demand during those "highest peak demand periods" by charging them

more if they exceed a certain peak demand. Customers can stay below

this set peak demand limit by monitoring their electric demand and

cutting off unnecessary electric loads if they get too close to

their demand limit. That is one form of load shedding.

The other way is for the power companies to ask their small

customers for permission to install a piece of equipment in their

home or business and wire one or two appliances to the equipment

which will shut down the appliances based on a radio signal from

the power company. The appliances are usually electric water

heaters. The power company gives a discount to these customers.

Then, if the power company sees that it's demand is coming close to

its generating capacity, it sends out a signal and cuts off all

these appliances. That is also called load shedding.

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