“Different types of transformers used in

Generating station”

PRESENTED BY :R.Vasanthan – 14E113V.Sai Krishna - 14E95

S.Padmanathan – 14E82P.Vinith – 14E123

Power Transmission and Distribution

Field Visit – Mettur Hydro Power Plant

Field Visit – Mettur Hydro Power Plant… 4x12.5MW

Types of Transformer

1) Generator Transformer

2) Station Transformer

3) Distribution Transformer

4) Unit auxiliary Transformer

5) Auxiliary Transformer

6) Instrument Transformer

7) Rectifier Transformer

8) Neutral ground Transformer

Functions

This is a step up transformer of high voltage capacity.

This is connected after the generator to step up voltage from 11kv

to 400kv.

The main function of this transformer is to step up voltage so as

to reduce transmission losses, line drop and to increase

Transmission efficiency .

Rating of the generation transformers will be almost equal to the

rating of the generator (500MW generating unit will have

generating transformer rating about 588MVA).

Functions In a generating plant for every generating unit one

generating transformer is required

Rated voltage on LV side corresponds to the rated generating voltage

Rated voltage on the HV side corresponds to rated voltage of the HV bus

Usually these transformers are outdoor type

LV terminals are connected to the generating terminals via isolated phase bus duct

Isolated Phase Bus

Isolated Phase Bus – LV lines Each phase current is carried on a separate conductor,

enclosed in a separate grounded metal housing.

Conductors are usually hollow Aluminum tubes or Aluminum bars, supported within the housing on porcelain or polymer insulators.

Isolated-phase bus is made in ratings from 3000 amperes to 45,000 amperes

Rated for voltages from 5000 volts up to about 35,000 volts

• HV terminals are connected to the outdoor busbars by flexible ACSR conductors via overhead flexible bus

ACSR conductors – HV lines

Functions

Transformer at the generating station is required for starting of the generating units when they are either newly constructed.

These Power transformers are called station transformer, which receives power from the grid and feed power-to-power station distribution system.

Functions

In general station service transformer is used for supplying power to auxiliary equipment in the power plant when the plant is not generating any power.

Rated HV voltage corresponds to the rated voltage of the outer busbars.

Rated LV voltage corresponds to the auxiliary bus voltage

Rated KVA corresponds to the load of common auxiliaries of the station.

These transformers are Outdoor type.

Functions• The Purpose of Unit auxiliary Transformer is to feed power to

generator auxiliaries of that unit

• These transformers are connected to generators and are used as

stepping down transformers. The HV side transformer voltage

corresponds to the voltage of the generating unit and the LV side

voltage is stepped down as per the need.

•Usually these transformers are outdoor transformers

• One Unit auxiliary transformer is present for every generating unit.

Auxiliary Transformers These transformers are employed in the power plants

for delivering power to low voltage loads (voltage below 1kV).

These transformers connects between HV distribution buses and LV distribution buses of the plant. Their rating will be around 1 to 5MVA.

Natural oil cooling or air cooled transformers are used.

•A Rectiformer is a rectifier and

transformer designed and built as a

single entity for converting alternating

current into direct current.

• Rectiformers are used for supplying

power to different field of ESP

(electrostatic precipitator),metal

production,refining processes and DC

traction systems

For metering and protection to maintain instrument accuracy.

For sensing H.V. side current at the secondary side

Used in protection relay, trip coils and pilot wires.

Used in current power measurement.

Current Transformer (CT) Potential Transformer (PT)

Distribution transformer

This is step down transformer is connected according

to applications such as boiler ,turbine , station service

board.

The main function of this transformer is to provide

convenient amount of power in the various boards.

NGT The subject of grounding covers the problems relating

to the conduction of electric current to the earth and through the ground. The earth rarely serves as a part of the return circuit, being used mainly for fixing the potential of circuit neutrals.

The ground connection improves service continuity and protects lives and equipment.

(a) The neutral grounding stabilizes the neutral point.

(b) By employing resistance or reactance in earth connection, the earth fault current

can be controlled.

(c) The over voltage surge due to lightening are discharged to earth hence less damages

to the equipment. Useful amount of earth fault current is required to operate earth fault

relay.

(d) Greater safety to personnel and equipment due to operation of fuses or relay on

earth fault and limitation of voltages. Hence it is economical to ground neutral point.

Advantages of Neutral Grounding Transformer

Cooling system is required for transformer to take away the heat generated in transformer due to losses and maintaining insulation healthy.

Cooling system of transformer is provided by considering design and KVA rating of a transformer. Cooling system of transformer is as follows.

i. Air natural cooling system.

ii. Oil natural cooling system with or without radiator.

iii. Cooling of transformer improved by force airflow by blowers/Air blast by fans

iv. Oil forced natural cooling system (OFN): - circulation of oil by pump to radiator, which has natural cooling.

v. OFB : Forced oil circulation of oil by pump to radiator, which has force, cooled by air blower.

vi. OFW : Forced circulation of Oil through oil cooler and water-cooling is by water.

Generally Oil temp is maintained in between 45 to 55 Dc. & Winding temp in between 55to 80 °c.(depends upon insulation class)

Types of Cooling

ONAN ONAF

OFAF OFWF

Maintenance

Transformer Oil Level Checking

Breather condition monitoring

Oil testing

Oil Filtration

Bushing Cleaning

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