Power Transformer

1.1. Power Transformer

Power transformer is equipment that changes the voltage and current (step-up or step down) without changing the frequency.

Since a substation is mainly concerned with the changes in the levels of voltages and currents, therefore Power Transformer is an important part of the substation. The capacity of the Power Transformer is given MVA. However the capacity of the transformer is greatly depending on the type of cooling used inside it.

1.1.1.      Transformer Core

The transformer core has a great role in minimizing the losses. Therefore we shall use a high grade, non-ageing and cold-rolled electrical silicon steel. Further it is used in thin laminations and a thin uniform coating is placed over it. In this way the losses are minimized to a great extent.

There are two types of core constructions that are mainly followed:

1.1.1.1.  Core type

 The core is shaped with a hollow square through the center as shown in fig 1.

                                 Fig.1 Core Type

1.1.1.2.  Shell type

It is the most widely used type of core construction.  The core consists of E- and I-shaped sections of metal. These sections are butted together to form the laminations. The laminations are insulated from each other and then pressed together to form the core.

Fig.2 Shell Type

1.1.2.      Windings

There are two types of windings in a transformer based on the type of connections to the load or to the source. The winding that is connected to the source is called primary winding and the one connected to the load is called the secondary winding.

 The windings shall be made from copper and free from scale, burr and silvers. They are insulated from each other. The windings shall be so located that they remain balanced electrostatically and their magnetic centers remain co-incident under all conditions of operation.

1.1.3.      Cooling in transformer

There are two types of cooling in a transformer

                                                          i.          ONAN ( Oil Natural Air Natural)

If the transformer's temperature is being kept at acceptable levels by natural air from a ventilation method then it is called ONAN cooling.

                                                         ii.          ONAF ( Oil Natural Force Natural)

 If the transformer's temperature is being kept at acceptable levels by forced air from a fan then it is called ONAF cooling.

When the cooling system is ONAF, its capacity is much greater than when it is ONAN. Say if it is a 26/20 MVA rated transformer, then it means that it is generating 26MVA when it is cooling system is ONAF and 20MVA when it is ONAN.

1.1.4.      Tapings

The tapings in are provided in the high voltage winding. The center tap (principal tap) gives the rated voltage and below and above taps give lower and higher than the rated voltage, respectively.

The tap changing facility can be either ON-load or OFF-load. It means its taps can be changed either without breaking the circuit (On-load) or after breaking the circuit (off-load).

Grid Station

2.      Grid Station

Grid station or substation is the main part in the high voltage transmission systems. As a general, it is mainly designed to step-up or step-down the voltage in the transmission system. The substations, depending      upon their functions, are mainly classified into

1)      Distribution Substations

2)      Transmission Substations

3)      Switching Substations

1.1. Distribution Substations

Distribution substation is a combination of the switching, controlling and voltage step-down equipments arranged to reduce the sub-transmission voltage to primary distribution voltage for residential and industrial use.

The distribution level voltage in Pakistan is 11kV.

1.2. Transmission Substations

Transmission substation is a combination of the switching, controlling and voltage step-up equipments arranged to increase the generated voltage to a level best suited for its transmission and that minimizes the losses.

The transmission voltage has different levels in different countries. In Pakistan the transmission level is 132kV, 220kV and 500kV.

1.3. Switching Substations

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.

Fig. 1 General Layout of Transmission and Distribution System  

2.      Need for the Grid stations

The power house produces a fixed amount of power that has to be transmitted over a long distance without or small losses in it. The dynamics that take part in transmitting our power are voltage and current and power factor. In some parts we need to increase current and reduce voltage and vice versa.

2.1.  Transmission level:

The generated voltage at the power house is very low and when we transmit power at this voltage level over a long distance we lose it to a great extent, since voltage and current are inversely proportional to each other as shown in  equation below;

P=VI

V=P/I

So a low voltage creates high current, and this high current is what that produces loss in power transfer as can be seen from the power loss equation;

P= I² R

2.2.  Distribution Level:

The power that reaches the distribution level has a very high voltage. This high voltage should be reduced so that it can be properly used for residential and commercial purposes
.3.      Equipments used in the Grid Station
The equipments used in the sub-station are assigned standard specifications according to the IEC (International Electro-technical Commission) standards.

Note: IEC is a non-profit, non governmental international standard organization that prepares and publishes international standards for all electrical, electronics and related technologies.

3.1. Tests on Equipments

There are two types of tests that are carried out on each equipment before it is finally approved to be used in the sub station.

3.1.1.      Type Tests

Type tests are carried out by the manufacturer after the equipment is completely designed and manufactured. These tests generally of normal type and include confirmation of name plate, ratings etc.

3.1.2.      Routine tests

Routine tests are carried out by the independent laboratories that are internationally recognized and obey the IEC standards.

Following are some of the main equipments used in the substation.

1)      Power transformer

2)      Circuit breaker

3)      Current transformer

4)      Voltage transformer

5)      Isolator

6)      Surge arrester

7)      Relays

8)      Bus-bars

9)      Insulators

10)   Pad mounted transformer

11)   Tension and suspension strings