TYPES AND FUNCTIONS OF SUB-STATION

Types of Sub-station

Substations are of three types. They are:

>Transmission Substation

>Distribution Substation

>Collector Substation

Transmission Substation

A transmission substation connects two or more transmission lines. The simplest case is where all transmission lines have the same voltage. In such cases, the substation contains high-voltage switches that allow lines to be connected or isolated for fault clearance or maintenance. A transmission station may have transformers to convert the voltage from voltage level to other, voltage control devices such as capacitors, reactors or Static VAR Compensators and equipment such as phase shifting transformers to control power flow between two adjacent power systems. The largest transmission substations can cover a large area (several acres/hectares) with multiple voltage levels, many circuit breakers and a large amount of protection and control equipment (voltage and current Transformers, relays and SCADA systems). Modern substations may be implemented using International Standards such as IEC61850.

Distribution Substation

A distribution substation transfers power from the transmission system to the distribution system of an area. It is uneconomical to directly connect electricity

consumers to the high-voltage main transmission network, unless they use large amounts of power. So the distribution station reduces voltage to a value suitable for local distribution. The input for a distribution substation is typically at least two transmission or sub transmission lines. Input voltage may be, for example, 400KV or whatever is common in the area. Distribution voltages are typically medium voltage, between 33 and 66 kV depending on the size of the area served and the practices of the local utility. Besides changing the voltage, the job of the distribution substation is to isolate faults in either the transmission or distribution systems. Distribution substations may also be the points of voltage regulation, although on long distribution circuits (several km/miles), voltage regulation equipment may also be installed along the line.

Complicated distribution substations can be found in the downtown areas of large cities, with high-voltage switching and, switching and backup systems on the low-voltage side. Most of the typical distribution substations have a switch, one transformer, and minimal facilities on the low-voltage side.

 Collector substation

In distributed generation projects such as a wind farm, a collector substation may be required. It somewhat resembles a distribution substation although power flow is in the opposite direction. Usually for economy of construction the collector system operates around 35 KV, and the collector substation steps up voltage to a transmission voltage for the grid. The collector substation also provides power factor correction, metering and control of the wind farm.

Functions of the substation

a. To Change voltage from one level to another.

b. To Regulate voltage to compensate for system voltage changes.

c. To Switch transmission and distribution circuits into and out of the grid system.

d. To Measure electric power quantity flowing in the circuits.

e. To Connect communication signals to the circuits.

f. To Eliminate lightning and other electrical surges from the           system.

g. To Connect electric generation plants to the system.

h. To Make interconnections between the electric systems of more than one utility. 

Substation Transformer Type

Further, transmission substations are mainly classified into two types depending on changes made to the voltage level. They are:

a. Step-Up Transmission Substations.

b. Step-Down Transmission Substations.

a. Step-Up Transmission Substation

A step-up transmission substation receives electric power from a nearby generating facility and uses a large power transformer to increase the voltage for transmission to distant locations.

There can also be a tap on the incoming power feed from the generation plant to provide electric power to operate equipment in the generation plant.

b. Step-Down Transmission Substation

Step-down transmission substations are located at switching points in an electrical grid. They connect different parts of a grid and are a source for sub transmission lines or distribution lines.

Layout

a. Principle of Substation Layouts

Substation layout consists essentially in arranging a number of switchgear components in an ordered pattern governed by their function and rules of spatial separation.

b. Special Separation

i. Earth Clearance: This is the clearance between live parts and earthed structures, walls, screens and ground.

ii. Phase Clearance: This is the clearance between live parts of different phases.

iii. Isolating Distance: This is the clearance between the terminals of an isolator and the connections.

iv. Section Clearance: This is the clearance between live parts and the terminals of a work section. The limits of this work section, or maintenance zone, may be the ground or a platform from which the man works. 

c. Separation of maintenance zones

Two methods are available for separating equipment in a maintenance zone that has been isolated and made dead.

i. The provision of a section clearance

ii. Use of an intervening earthed barrier The choice between the two methods depends on the voltage and whether horizontal or vertical clearances are involved.

i. A section clearance is composed of the reach of a man taken as 8 feet plus an Earth clearance.

ii. For the voltage at which the earth clearance is 8 feet the space required will be the same whether a section clearance or an earthed barrier is used.

Maintenances

Maintenance plays a major role in increasing the efficiency and decreasing the breakdown. The rules and basic principle are discussed.

Separation by earthed barrier = Earth Clearance + 50mm for barrier + Earth Clearance Separation by section clearance = 2.44m + Earth clearance

i. For vertical clearances it is necessary to take into account the space occupied by the equipment and the need for an access platform at higher voltages.

ii. The height of the platform is taken as 1.37m below the highest point of work.

Maintenance is done through two ways:

a. By Establishing Maintenance Zones.

b. By Electrical Separations.

a. Establishing Maintenance Zones

Some maintenance zones are easily defined and the need for them is self evident as in the case of a circuit breaker. There should be a means of isolation on each side of the circuit breaker, and to separate it from adjacent live parts when isolated either by section clearances or earth barriers.

b. Electrical Separations

Together with maintenance zoning, the separation, by isolating distance and phase clearances, of the substation components and of the conductors interconnecting them constitute the main basis of substation layouts. There are at least three such electrical separations per phase that are needed in a circuit:

i. Between the terminals of the bus bar isolator and their connections.

ii. Between the terminals of the circuit breaker and their connections.

iii. Between the terminals of the feeder isolator and their connections.

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