The electrical power is generated at generating station (such as hydro electric power plant, thermal power plant and nuclear power pant) which is located far away from the load centre. Electrical power is transmitted through transmission lines and distributed through distribution lines. For economical reasons, voltage is transmitted at high voltage and distributed at low voltage 400/230 volts. These voltage transformations are carried out at substation, located at suitable places. Thus, it form the most important part of power system. In this article, we will study about the its various types and various function.
It is an assembly of various apparatus (such as, circuit breaker, protective relays etc) which are installed to control transmission and distribution of the electric power.
The various function are given below
It is used to transformation of voltage level from higher level to lower level or vice-versa.
The function of substations is switch ON and OFF the power lines.
In some cases these required to convert DC power into AC power or vice-versa. For some electric traction system DC convert AC power into DC power. In electrolysis process DC power is used.
We can change the frequency from higher level or lower level to higher at the substations.
To improve power factor by installing capacitor banks at the receiving end of the line.
CLASSIFICATION OF SUBSTATION
These are classified in several ways. However, the most important ways of classifying the substations are given below
- Service requirement
- Operating voltage
ACCORDING TO THE SERVICE REQUIREMENT
The substations where voltage level is changed is called transformer substation. These substations receive electric power at some voltage level and deliver it at some other voltage. The main component in this substation is transformer. Maximum number of substations in power system is this type of substations.
In switching substation voltage level is not changed. It means incoming line voltage and outgoing line voltage is same. It performs only switching operation connect or disconnect the power lines.
POWER FACTOR CORRECTING SUBSTATION
In these types of substation power factor is improved. Such substation is located at the receiving end of the transmission lines. These substations are equipped with synchronous condensers and capacitor banks. But usually synchronous condenser is used.
In these types of substations are installed to change characteristics of the electrical power (such as converting ac into dc or vice versa). We know that electrical energy is generated in ac form. But most of application dc power is required such in electric traction system, in electrolysis processes. These substations receive ac power and converted into dc power. In these types of substations frequency level is also changed. We can change frequency from higher level to lower level and vice versa.
CLASSIFICATION ACCORDING TO THE OPERATING VOLTAGE
HIGH VOLTAGE SUBSTATION
These are used for the voltage from 11kv to 66kv. These types of substations are basically located in industries or near the consumers.
EXTRA HIGH VOLTAGE SUBSTATION
The operating voltage in this substation is varies from 132kv to 400kv. The electric power is generated at low voltage but transmitted at high voltage to reduce transmission losses. But electric power sends to consumer at low voltage. The of voltage is changed in EHV substations. These substations also located at generating station.
ACCORDING TO THE DESIGN
In this type of substation, the apparatus is installed with in the substation building, so that why these substations are called indoor substation. These substations are designed for voltage upto 11kv but can be erected for 33kv. These are located where the surrounding atmosphere is contaminated with such impurities which may damage the equipment. These substations are installed in industries. The extension is not possible. This is the limitation of indoor substations.
The auxiliaries of the indoor type of substations are
Storage batteries are used to operate the relays in substations. Batteries are heart of the substations.
Emergency lighting in substations in case of failure of supply.
For the voltage above 66kv, outdoor substation is used. In outdoor substations, equipment is installed outdoor. It is because for high voltages, the spacing between conductors and the space required for the switches, circuit breakers and other equipment becomes so great that it is not economical to install the equipment indoor. The outdoor substations are divided into two types.
Pole Mounted Substation
Foundation Mounted Substation
POLE MOUNTED SUBSTATION
These types of substations are erected for mounting distribution transformer of capacity about upto 200KVA. The substation is located near the consumers and are cheapest, simple and small in size. All equipment are mounted on the supporting structures and these equipment are outdoor types. The H type pole is used for supporting structure. In early days, the supporting structure is made from wood but now RCC structure is used. Triple pole mechanically operated (TPMO) switch is used for switch ON and OFF of HT transmission line. HT fuse is installed for protection purposes. Lighting arrester are also used to protect the transformer against the surge. Earthling is used to protect the maintenance cost of such transformer is low.
FOUNDATION MOUNTED SUBSTATION
These substations are built in open place and in this type of substations all the equipment is assemble into one unit enclosed by a fence from the point of view of safety. These substations on above 200KVA. To select the site for these substations, there should be good access for heavy transportation. The extension is possible in foundation mounted substations.
ADVANTAGES OF OUTDOOR SUBSTATION OVER INDOOR SUBSTATION
Fault can be located easier than indoor substations.
The extension of installation is easier, if required.
Building materials required for the outdoor substation is smaller than indoor substation.
Outdoor substation can be easily repaired.
The construction work required is comparatively smaller and cost of the switch gear installation is low.
DISADVANTAGES OF OUTDOOR OVER INDOOR SUBSTATION
The space required for the outdoor substation is more.
In case of outdoor substations more protection devices are required for the protection against lightning surges.
The length of control cables required is more.
In sophisticated areas or thickly populated cities, there is scarcity of land as well as the prices of land are very high these are installed. The equipment are placed underground. The design of requires more careful considerations as compared to other types of substations.
EQUIPMENT IN A TRANSFORMER SUBSTATION
Bus Bar is a conductor carrying an electric current to which many connections may be made. It is a bar of conducting material such as Aluminum, copper etc. The Bus Bars are usually of Aluminum.
The insulators performed two functions, first function is to support the conductor such as bus bar and other function is to confine the current to the conductors. The most commonly used material for the manufacture of insulators is porcelain. The insulators may be pin type insulator, suspension type insulators, strain type insulators etc.
Isolators are the switch which used to disconnect a part of system for maintenance and repair. The isolator is designed to open circuit at no load.
A circuit breaker is a equipment which can open or close a circuit under normal as well as abnormal conditions. It is designed so those which can open the circuit manually under normal conditions and automatically under fault conditions. Circuit breaker may be air blast circuit breaker, oil circuit breaker vacuum circuit breaker and SF6 circuit breaker.
Transformer is a static device which transfer electrical energy from one circuit to another at different voltage level, frequency remains same. A transformer used in a substation to step up or step down the voltage.
The large current and high voltage cannot be measured with ordinary instruments. To measure these quantities at this level transformer are connected with these instruments. These transformers are called instrument transformer. The transformer connected with ammeter to extend the range of ammeter is called current transformer (CT) and transformer which is connected with voltmeter to extend the range of voltmeter is called potential transformer (PT).