23. Types of Substation

SUBSTATION
The assembly of apparatus used to change some characteristics ( i.e voltage, a.c to d.c, frequency, power factor, etc., ) of electric supply is called a sub station .

Factors Considered:

•          Located at proper site

•          Provide safe and reliable arrangement

•          Easily operated and maintained

•          Minimum capital cost

TYPICAL SUBSTATION:

Types of Substation

 •          Step-up Transmission Substation

•          Step-down Transmission Substation

•          Distribution Substation

•          Underground Distribution Substation

   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.

•           A transmission bus is used to distribute electric power to one or more transmission lines. 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.

•          A substation can have circuit breakers that are used to switch generation and transmission circuits in and out of service as needed or for emergencies requiring shut-down of power to a circuit or redirection of power.
The specific voltages leaving a step-up transmission substation are determined by the customer needs of the utility supplying power and to the requirements of any connections to regional grids.

•          Typical voltages are:
 High voltage (HV) ac:69 kV, 115 kV, 138 kV, 161 kV, 230 kVExtra-high voltage (EHV) ac:345 kV, 500 kV, 765 kVUltra-high voltage (UHV) ac:1100 kV, 1500 kV ,Direct-current high voltage (dc HV): ±250 kV, ±400 kV, ±500 kV

STEP DOWN TRANSMISSION SUBSTATION:

•          Step-down Transmission Substation are located at switching points in an electrical grid.

•          They connect different parts of a grid and are a source for subtransmission lines or distribution lines.

•          The step-down substation can change the transmission voltage to a subtransmission voltage, usually 69 kV.

•          The subtransmission voltage lines can then serve as a source to distribution substations.

•          Sometimes, power is tapped from the subtransmission line for use in an industrial facility along the way. Otherwise, the power goes to a distribution substation

DISTRIBUTION SUBSTATION

A distribution substation transfers power from the transmission system to the distribution system of an area.

The input for a distribution substation is typically at least two transmission or subtransmission lines.

Distribution voltages are typically medium voltage, between 2.4 and 33 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.

COLLECTOR SUBSTATION

In distributed generation projects such as a wind farm, a collector substation may be required, which is similar to a distribution substation although power flows in the opposite direction, from many wind turbines up into the transmission grid.

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 can also provide power factor correction if it is needed, metering and control of the wind farm.

Collector substations also exist where multiple thermal or hydroelectric power plants of comparable output power are in proximity

SWITCHING SUBSTATION

 A switching substation is a substation which does not contain transformers and operates only at a single voltage level. Switching substations are sometimes used as collector and distribution stations. Sometimes they are used for switching the current to back-up lines or for paralellizing circuits in case of failure.

UNDERGROUND DISTRIBUTION SUBSTATION

•          Underground Distribution Substation are also located near to the end-users. Distribution substation transformers change the sub transmission voltage to lower levels for use by end-users. Typical distribution voltages vary from 34,500Y/19,920 volts to 4,160Y/2400 volts.

 An underground system may consist of these parts: 

•          Conduits

•          Duct Runs

•          Manholes

•          High-Voltage Underground Cables

•          Transformer Vault

•          Riser

•          Transformers

From here the power is distributed to industrial, commercial, and residential customers.

SUBSTATION FUNCTIONS

•          Change voltage from one level to another

•          Regulate voltage to compensate for system voltage changes

•          Switch transmission and distribution circuits into and out of the grid system

•          Measure electric power qualities flowing in the circuits

•          Connect communication signals to the circuits

•          Eliminate lightning and other electrical surges from the system

•          Connect electric generation plants to the system 

•          Make interconnections between the electric systems of more than one utility

•          Control reactive kilovolt-amperes supplied to and the flow of reactive kilovolt-amperes in the circuits