Transformer

The assembly of the apparatus to change some characteristics of the electric power supply is called substation. The present-day electrical power system is A.C. i.e. electrical power is generated, transmitted, and distributed in the form of the alternating current. The electric power is produced at power plant stations which are located at favorable places generally quite away from the consumers. It is delivered to the consumers through a large network of transmission and distribution.

A transformer is a device that transfers electric current from one circuit to another, usually by the principle of mutual induction. During this process, the frequency remains constant whereas the voltage can be increased or decreased according to the need. This transfer of electricity occurs with the help of two coils. One of which is known as the Primary Coil, which is connected to a source of alternating current. The other is known as the Secondary Coil, and it is connected to an external circuit.

Substations maybe owned and operated by an electrical utility, or may be owned by a large industrial or commercial customers.

 

1.1 Classification of Substation

     Ã˜  Transformer Substation

Ø  Step-Up Substation

Ø  Step-Down Substation

Ø  Primary Grid Substation

Ø  Secondary Grid Substation

Ø  Distribution Substation

Fig1.1: Transformer


1.2 Transformer Substation

They are known as transformer substations because the transformer is the main component employed to change the voltage level, depending upon the purposed served transformer substations. A transmission substation connects two or more transmission lines. The simplest case is where all transmission lines have the same voltage. In such cases, substation contains high-voltage switches that allow lines to be connected or isolated for fault clearance or maintenance of transformer. A transmission station may have transformers to convert between two transmission voltages, voltage control/power factor correction devices such as capacitors, reactors.

 

1.3 Step-Up Substation

The generation voltage is stepped up to high voltage to affect the economy in the transmission of electric power. These are generally located in the powerhouses and are of the outdoor type. Such types of substations generate low voltage like 3.3, 6.6, 11, or 33kV. This voltage is stepped up by the help of a step-up transformer for transmitting the power over large distances. It is located near the generating substation.

 

1.4 Step-Down Substation

This the substation is placed near the load Centre where the primary distribution is stepped down for sub-transmission. The secondary distribution transformer feeds the consumer through the service line.

 

1.5 Primary Grid Substation

This substation lowered the value of primary stepped-up voltages. The output of the primary grid substation acts as the input of the secondary substations. The secondary substation is used for stepping down the input voltage to more lowered for further transmission.

 

1.6 Secondary Grid Substation

At a secondary substation, the voltage is further stepped down to 11KV. The 11KV lines run along the important road of the city. The secondary substations are also of the outdoor type.

 

1.7 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 sub-transmission 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).

Distribution system


1.8 Working principle of Transformer

Transformer works on the principle of Faraday’s Law of Mutual Induction. This principle states that the rate of change of flux is directly proportional to the induced electromagnetic flux.

Similarly, in a transformer, when an alternating current flow through one of the coils, it creates a magnetic field around it, which constantly produces a changing magnetic flux and so, when another coil is brought near it, some of the EMF is also induced in the secondary coil as well. Since the secondary coil forms a closed loop, the EMF produces the current in it as well. These windings are usually made on an iron core to make the magnetic field stronger, and laminated afterward so that the flux does not weaken due to air, which is a perfect insulator. But still, some power losses are observed such as Eddy Current losses and Hysteresis loss.

 

Fig 1.2: Working Principle of Transformer

 

1.9 Power Transformer

The power transformer is an electric device that is used to step up or step down the voltage level of the supply fed to its primary winding. The stepping up or down depends upon the number of turns of the primary and secondary winding. If the number of turns on both the windings is the same, and the losses of the transformer are negligible, we may conclude that the voltage across each of the windings is the same. In this case, the transformer is just utilized in isolating two electrical circuits.

Generally, the power transformer is used in stepping up the voltage of the supply in order to decrease the transmission losses and then stepping down is done for the distribution purpose at the load centers.


 

Fig 1.3: Power Transformer


 

Item

Rating

Item

Rating

Company Name

TOSHIBA

OPTG. Pressure of PRV

10 PSI

Transformer Type

OUTDOOR

Standard

ANSI C57.12.10&C57.12.90

Type of Cooling

ONAN/ONAF

Frequency

 

HZ

50

Rated MVA

5/6.25

Insulation Level

HV

kV

LI 200 AC 70

Rated kV

HV

33

LV

LI 110 AC 34

LV

11.55

LVN

LI 110 AC 34

Rated Line Current (Amps.)

HV

87.5/109.3

Core & Coil

KG

6800

LV

249.9/312.4

Tanks & Fittings

3600

Vector Group

Dyn 1

Mass Of Oil

3400

Number of Phases

3

Total Mass

13800

% Impedance HV/LV

6

Transport Mass

10800

Temp rise ˚C

Top Oil

60

Volume of Oil

L

3542

AVG WDG

65

Tank Strength

 +10 PSI-8 PSI

Year of MFG.

Sep-11

Maker's Reference No

 

Diagram DRG No.

P6.25M-104

Polarity

SUBTRACTIVE

Maker's Serial No

157

 

                         Table: three-phase transformer rating 





Item

Rating

Item

Rating

Company Name

TOSHIBA

Consumer LOI. No.

830/EVL/2003

KVA

3333/4166

Type of Cooling

OA/FA

Volts at No Load

H.V.

33000

Frequency

HZ

50

L.V.

11550/1.732

Impedance Voltage %

5.87

Amperes

H.V.

101/126.3

Polarity

SUBTRACTIVE

L.V.

500/625

Total Weight

KG

7300

Phase

H.V.

1

Weight of Coil

1380

L.V.

1

Type of Oil

MINERAL OIL

Out Line DRG. No.

3 OG 7952

Conductor Material

H. V

COPPER

Serial No.

624603

L. V.

COPPER

OP. Pressure

 +10, -8 PSI

Year of MFG

2003

H. V. BIL

200 kV

Temp. Rise Oil/ Winding Above Ambient Air Temp. of

˚C

60/65

L. V. BIL

110 kV

40
                      Table: Rating of Single-Phase transformer


 

 

1.10 Step up and Step-down Transformer

The step-up transformer transforms a low voltage, high current AC into a high voltage, low current AC system. In this type of transformer, the number of terns in the secondary winding is greater than the number of turns in the primary side winding which as shown in figure 4.10(a).


On the other hand, a step-down transformer transforms a high voltage, low current AC into a low voltage, high current AC system. In this type of transformer, the number of terns in the secondary winding is lower than the number of turns in the primary side winding which as shown in figure 4.10(b).
Fig 1.4: Step up (a) and Step-Down (b) TransformerAdd caption

 


1.11 Instrument Transformer

Instrument transformer is a special transformer that is used in the electrical power system for stepping down currents and voltages of the system for measuring and protection purposes. The primary winding of the transformer is connected to the high voltage or high current circuit, and the meter or relay is connected to the secondary circuit. Instrument transformers may also be used as an isolation transformer so that secondary quantities may be used in phase-shifting without affecting another primary connected device.

Fig 1.4: Instrument TransformerAdd caption

1.13 Potential Transformer

The potential transformer may be defined as an instrument transformer used for the transformation of voltage from a higher value to the lower value. This transformer step down the voltage to a safe limit value which can be easily measured by the ordinary low voltage instrument like a voltmeter, wattmeter and watt-hour meters, etc.