substations types

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Substations Types

The main purposes for establishment of a substation are:

Need for connecting a generator to the transmission system: We have already discussedthatgenerators generate electricityat a low voltage mostly below 22 kV. But bulk quantity of

power can be transmitted to very long distance only at high or extra high voltage. So generator

transformers (GT) are used at the power station to step up the voltage to transmission level.

Here the substation is not a separate entity rather it is switchyard of the Power Station. These

substations are quite complex. One can find here more numbers oftransformersin comparison

to other types of substations.

To inter-connect two or more transmission lines at two or more different voltage levels: In atransmission system it is often planned or it is required to facilitate power flow between

transmission lines at different voltage levels. These are transmission substations.Inter

connecting Transformers(ICT) are used for this purpose. The transmission substations operate

at Extra High Voltage (EHV). See Figure below.

To step down the voltage to a lower level for further transmission (sub-transmission) at aneconomic rate: Sometimes for shorter distance transmission of comparatively less power, the

transmission company may feel that it is economical to transmitat a voltage level in-between

the standard transmission and distribution level. Hence the requirement of establishing the

substation.

To step down the voltage to a lower level for feeding a high voltage distributionnetwork: These are the substations where the voltage is stepped down to a voltage level

suitable for forming the major distribution network for a township.

Step down to low voltage for supply to consumers: These are area substations, where thevoltage is again stepped down to a level suitable for house holds consumption.

HVDC substation: There are two main purposes of a HVDC substation. 1) Two dissimilar systemswith different frequency can be connected by HVDC link to facilitate power transfer from one

side to the other side. 2) Direct current long distance bulk power transmission. HVDC

substations have many advantages.

Switching substation: These are the substations meant for switching purposes only. TheSwitching substation does not have a transformer. In a power system these are few in numbers.

The switching substations are meant for disconnecting and connecting a part of network and

facilitating maintenance works.
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Here I reproduce the SLD of abasic imaginary power systemwhere we can see few types of substations

and their relative configuration.

A substation may not be exclusively for one of the above purposes. A substation may be established

primarily for the purpose of bulk power transmission but at the same time may have another

transformer installed for stepping down the voltage to a lower level and distribute locally. Sometimes

any combination from the above is adopted depending on the requirements.

The substations can be Air Insulated Substation (AIS) or Gas Insulated substation (GIS). It can be over

ground or Under ground.

The other important major elements usually found in the substation are Capacitor bank and Reactor.

A substation has switching arrangements to connect or disconnect these high voltage equipments or

elements to the system.

Some important functions that are carried out in the substation are controlling the flow of energy, VAR

compensation, voltage control, power factor improvement and measurement and recording of system

parameters and initiating proper action.
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The switching arrangement is chosen as per the requirement.

A substation has many equipments, some of these arecircuit breaker, isolator, current transformer,

potential transformer, surge absorber, measuring and recording instruments, relay and protective

devices, batteries, battery chargers, fire fighting equipments etc.

The substation transformers can be connected several ways. We have already discussed

abouttransformer connection and Vector Group. We will discuss about all types of substations in future.

We have already discussed in brief about differenttypes of electrical substations. Now we will discuss

about the main factors influencing selection of substation. The discussion is mainly for transmission

substation which requires many factors to be considered before selecting the site, size etc. The smaller

or distribution substation has fewer requirements so only some of the points are required to be

considered.

A substation is like a junction point in a power system. Manytransmission linesat different voltage

levels terminate at a substation. Shut down of the substation means shutdown of all the lines

terminating at the substation. Clearly a substation is a very important unit of power system.

A transmission substation may be comprised of one or more of the following main sections depending

on its size and importance. These are Switchyards, Control rooms, Office building and Colony for

employees. See the figure at the bottom. The substation planning requires several factors to be

considered. A substation which is not properly planned or Ill designed may face several problems, in

particular if future expansion or renovation of the switchyard is required. Proper design of substation

also enables it to operate in normal state for maximum possible time so avoiding overloading to themaximum. It is true that many substations undergo expansion and renovation with time. So for

substation planning future expansion must be considered.

Some important primary factors in the design of substation are operational flexibility, supply reliability,

secutity and short circuit withstand capability etc.

One important factor to be considered first is the site selection. Substation design and some equipment

selection depends on site selection. Hence it has a bigger influence on the cost of substation. The

orientation of the lines to be terminated at the substation also decides the substation orientation and

equipment arrangement.

The site should be near the load center keeping in view the future load growth.

Some general factors to be considered are listed. It should be remembered that some of the factors are

actually interdependent.
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Access road to the site for smooth movement of construction machines, equipments andtransformers. Good Roadways to construction site and shorter distance to rail head are desired.

The site should be chosen to avoid soil filling, earth removal etc. The requirement of soil fillingand earth removal takes time and increases total cost of substation

Historical data of worst flood is taken into account to avoid water logging of the substation incase of possibility of flood. Flood plains and wetlands are avoided.

Atmospheric conditions like salt and suspended chemical contaminants influence selection ofequipments and maintenance requirements.

Interference with communication signals. The construction company have to take permissionfrom the appropriate authority.

Electric and magnetic field strength are of particular concern especially for Ultra High Voltage(UHV) systems at 765 kV,1200 kV or above. Research organisations has shown the impact of

strong Electric/magnetic fields due to UHV substations and lines on human health. Such new

concerns are also required to be addressed properly

Forest land, sanctuaries and national parks are avoided. Almost all governments has laidstringent rules to comply for approval of forest land and wild life sanctuary. The usual process

takes time to get approval from the concerned authorities. This process delays the construction

activities.

Approval is also required from aviation authority. Substation should be away from airport anddefence establishments.

Water supply and sewage system are the two most important facilities to be given dueconsideration.

Some other factors related to the general public:

The substation should be located far from the crowded places. Efforts are always made to locatetransmission substations outside the city areas.

The locals should be made aware of the upcoming substation. To avoid public resentment it isbetter to involve the local people in the process. If required they should be educated and

trained. Many times the local people also plays an important role to check vandalism and theft.

Heritage sites and tourist spots are avoided. Electric substation is a source of noise. While charged transformers, reactors and EHV lines are

sources of continuous hissing noise, operation of different equipment also emit sudden noise.

The design should be adopted to tackle the issues by complying to the standards set by the

appropriate authority for reduction of noise pollution and avoid public resentments.


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Landscaping should be done to keep the substation out of direct view of common people.

Finally it is safety of man and machine. The safety of the personnel involved both in construction phase,

operational safety and safety of public is to be followed as per the safety rules and regulations framed

by international bodies and local authorities. These rules are not to be compromised. Moreover themanufacturer instruction manuals, safety procedures and other documents for equipment and

machines are to be referred.

A substation should have adequate arrangement of fire fighting system. It is the ultimate safety measure

of any substation. The fire fighting system should be suitable for dealing with fire due to electrical arc.

Usually costly equipments like transformer should have Nitrogen gas or high speed water jet as

fire fighting system.

In the figure below is a conceptual substation

substations types

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