Question

1. What is the need of a sub-station in the power system ?

2. Why are pole-mounted sub-stations very popular ?

3. Where we erect a terminal sub-station ?

4. Why do we use isolators on both sides of circuit breaker ?

5. What is the utility of instrument transformers in sub-stations ?

Answer 1

A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltages from high to low, or the low to high or perform any of several other important functions. Between the generating station and consumer, electric power may flow through several substations at different voltage levels. A substation may include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages.

Substations may be owned and operated by an electrical utility, or may be owned by a large industrial or commercial customer. Generally substations are unattended, relying on SCADA for remote supervision and control.

The word substation comes from the days before the distribution system became a grid. As central generation stations became larger, smaller generating plants were converted to distribution stations, receiving their energy supply from a larger plant instead of using their own generators. The first substations were connected to only one power station, where the generators were housed, and were subsidiaries of that power station.

Generally generated voltage is about 11to 13.8kV but it can't be transmitted as it is due huge current, to overcome the current losses and conductor dia requirement we reduce the current by increasing voltage and maintaining same power... And at transmission substation be lower the voltages and divide the power to different areas and to different distribution system. In a distribution substation voltages are further lowered and power is broken down to various locations for end consumers.

2.

Distribution sub-station placed overhead on a’ pole. It is the cheapest form of sub-station as it does not involve any building work .Transformer and other equipment are mounted on H-type pole (or 4-pole structure).

he 11 kV line is connected to the transformer (11kV / 400 V) through gang isolator and fuses. The lightning arrest­ers are installed on the H.T. side to protect the sub-station from lightning strokes. The transformer steps down the volt­age to 400V, 3-phase, 4-wire supply. The voltage between any two lines is 400V whereas the voltage betw,een any line and neutral is 230 V. The oil circuit breaker (O.C.B.) installed on the L.T. side automatically isolates the transformer from the consumers in the event of any fault. The pole-mounted sub-stations are generally used for transformer capacity upto 200 kVA. The following points may be noted about pole-mounted sub-stations :

• There should be periodical check-up of the dielectric strength of oil in the transformer and O.C.B.
• In case of repair of transformer or O.C.B., both gang isolator and O.C.B. should be shut off.

Another advantage of the pole mounted transformers is the speed of construction of such substations. Furthermore, these installations have relatively low construction costs and present an element of portability and can undergo easy maintenance.This makes the use of pole mount substation very frequent.

3.

A terminal sub-station is one in which the line supplying to the substation terminates or ends. It may be located at the end of the main line or it may be situated at a point away from main line route. In the latter case, a tapping is taken from the main line to supply to the sub-station.

4.

Isolators are considered to be OFF LOAD switches.this indicates we can only open the line when it's carrying Zero current...We use circuit breaker to open the line but can't use circuit breaker in open position for longer duration as it detoriates the contacts and oil. So when a circuit breaker is open by automatic means or by manual we isolate the line by opening the isolators in both ends of circuit breaker.Even isolating a single isolator on one side is not sufficient due to return current or induction current from the other end.so isolators open makes the line open and safe to operate on circuit breaker if any maintenance is to be done.main purpose is to isolate one portion of the circuit from the other and is not intended to be opened while current is flowing in the line. Isolators are generally used on both ends of the breaker so that repair or replacement of circuit breaker can be done without any danger. Isolators should never be opened ON load.

5.

In any electric circuit it is mandatory to measure the electrical quantities like current, voltage, frequency and many more.Measuring of the quantities is done to ensure the aupply, metering purpose and also protection purposes.it is not practical to measure high current and high voltage by means of conventional metering devices. So use of instrument transformers make the work simpler by lowering the electrical quantities like voltage, current and power ,So we can measure tgese by any of the conventional techniques..A CT (current transformer) ,VT(voltage transformer) also known as PT(Potential transformer) are employed at any substation to quantify the measure the quality of power input and output. Instrument transformers are nothing but CT and PT