Electrical Engineering Association

Electrical Substation

Now days the electrical power demand is increasing very rapidly. For fulfilling these huge power demands the modern time requires creation of bigger and bigger power generating stations. These power generating stations may be hydro-electric, thermal or atomic. Depending upon the availability of resources these stations are constructed different places. These places may not be nearer to load centers where the actual consumption of power takes place.

So it is necessary to transmit these huge power blocks from generating station to their load centers. Long and high voltage transmission networks are needed for this purpose. Power is generated comparatively in low voltage level. It is economical to transmit power at high voltage level. Distribution of electrical power is done at lower voltage levels as specified by consumers. For maintaining these voltage levels and for providing greater stability a number of transformation and switching stations have to be created in between generating station and consumer ends. These transformation and switching stations are generally known as electrical substations. Depending upon the purposes, the substations may be classified as-

✓ Step Up Substation

Step up substations are associated with generating stations. Generation of power is limited to low voltage levels due to limitations of the rotating alternators. These generating voltages must be stepped up for economical transmission of power over long distance. So there must be a step up substation associated with generating station.

✓ Step Down Substation

The stepped up voltages must be stepped down at load centers, to different voltage levels for different purposes. Depending upon these purposes the step down substation are further categorized in different sub categories.

✓ Primary Step Down Substation

The primary step down sub stations are created nearer to load center along the primary transmission lines. Here primary transmission voltages are stepped down to different suitable voltages for secondary transmission purpose.

✓ Secondary Step Down Substation

Along the secondary transmission lines, at load center, the secondary transmission voltages are further stepped down for primary distribution purpose. The stepping down of secondary transmission voltages to primary distribution levels are done at secondary step down substation.

✓ Distribution Substation

Distribution substation are situated where the primary distribution voltages are stepped down to supply voltages for feeding the actual consumers through a distribution network.

✓ Bulk Supply or Industrial Substation

Bulk supply or industrial substation are generally a distribution substation but they are dedicated for one consumer only. An industrial consumer of large or medium supply group may be designated as bulk supply consumer. Individual step down substation is dedicated to these consumers.

✓ Mining Substation

The mining substation are very special type of substation and they need special design construction because of extra precautions for safety needed in the operation of electric supply.

✓ Mobile Substation

The mobile substations are also very special purpose substation temporarily required for construction purpose. For big construction purpose this substation fulfills the temporary power requirement during construction work.

Depending upon the constructional feature categories of substation may be divided into following manner-

✓ Outdoor Type Substation

Outdoor type substation are constructed in open air. Nearly all 132KV, 220KV, 400KV substation are outdoor type substation. Although now days special GIS (Gas insulated substation) are constructed for extra high voltage system which are generally situated under roof.

✓ Indoor Substation

The substations are constructed under roof is called indoor type substation. Generally 11 KV and sometime 33 KV substation are of this type.

✓ Underground Substation

The substation are situated at underground is called underground substation. In congested places where place for constructing distribution substation is difficult to find out, one can go for underground substation scheme.

✓ Pole Mounted Substation

Pole mounted substation are mainly distribution substation constructed on two pole, four pole and sometime six or more poles structures. In these type of substation fuse protected distribution transformer are mounted on poles along with electrical isolator switches.

✓ Converter substation

Converter substations may be associated with HVDC converter plants, traction current, or interconnected non-synchronous networks. These stations contain power electronic devices to change the frequency of current, or else convert from alternating to direct current or the reverse. Formerly rotary converters changed frequency to interconnect two systems; nowadays such substations are rare.

✓ Switching station

A switching station is a substation without transformers and operating only at a single voltage level. Switching stations are sometimes used as collector and distribution stations. Sometimes they are used for switching the current to back-up lines or for parallelizing circuits in case of failure. An example is the switching stations for the HVDC Inga–Shaba transmission line.

A switching station may also be known as a switchyard, and these are commonly located directly adjacent to or nearby a power station. In this case the generators from the power station supply their power into the yard onto the Generator Bus on one side of the yard, and the transmission lines take their power from a Feeder Bus on the other side of the yard.

✓ Collector substation

In distributed generation projects such as a wind farm, a collector substation may be required. It resembles a distribution substation although power flow is in the opposite direction, from many wind turbines up into the transmission grid. 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 can also provide power factor correction if it is needed, metering, and control of the wind farm. In some special cases a collector substation can also contain an HVDC converter station.

Collector substations also exist where multiple thermal or hydroelectric power plants of comparable output power are in proximity. Examples for such substations are Brauweiler in Germany and Hradec in the Czech Republic, where power is collected from nearby lignite-fired power plants. If no transformers are required for increasing the voltage to transmission level, the substation is a switching station.