THREE PHASE TRANSFORMER CONSTRUCTION,WORKING AND ITS CONNECTIONS

THREE PHASE TRANSFORMER CONSTRUCTION,WORKING AND ITS CONNECTIONS

INTRODUCTION:-

In present times 3 phase transformers are use widely in the world in distribution and transmission level

Mostly the power generators generates power at 11.5 kv that voltage is step up upto 500 kv for transmission system by the help of three phase transformer and then at distribution level this voltage is step down by help of three phase transformer.

so it means three phase transformer has great importance in modern world It is found that generation, transmission and distribution of electrical power are more economical in three phase system than single phase system. For three phase system three single phase transformers are required. Three phase transformation can be done in two ways, by using single three phase transformer or by using a bank of three single phase transformers. Both are having some advantages over other. Single 3 phase transformer costs around 15 % less than bank of three single phase transformers. Again former occupies less space than later. For very big transformer, it is impossible to transport large three phase transformer to the site and it is easier to transport three single phase transformers which is erected separately to form a three phase unit. Another advantage of using bank of three single phase transformers is that, if one unit of the bank becomes out of order, then the bank can be run as open delta

WORKING:-

As equal load connected on all three phases so net flux will be equal to zero because they are balance with each other some three phase transformers are given below

CONNECTION OF THREE PHASE TRANSFORMER

STAR-STAR CONNECTION

Primary and Secondary windings are connected in star connection, this type of connection are  suitable for constant load ,high voltage and low current required areas 

As phase voltage is less so insulation required will also be less , its 3 phase 4 wire system the 4 wire is neutral 

the main disadvantage is if load increased so problem can be occur as floating neutral,harmonics also generated 

Advantages 

1. Due to star connection, phase voltages is (1/√3) times the line voltage. Hence less number of turns are required. Also the stress on insulation is less. This makes the connection economical for small high voltage purposes.

1. Due to star connection, phase current is same as line current. Hence windings have to carry high currents. This makes cross section of the windings high. Thus the windings are mechanically strong and windings can bear heavy loads and short circuit.

1. There is no phase shift between the primary and secondary voltages.

1. As neutral is available, it is suitable for three phase, four wire system.

Disadvantages

1. If the load on the secondary side unbalanced then the performance of this connection is not satisfactory then the shifting  of neutral point is possible. To prevent this, star point of the primary is required to be connected to the star point of the generator.

1. Eventhough the star or neutral point of the primary is earthed, the third harmonic present in the alternator voltage may appear on the secondary side. This causes distortion in the secondary phase voltage

DELTA - DELTA CONNECTION

it is widely used on low voltage system

Advantages 

1. in order to get secondary voltage as sinusoidal, the magnetizing current of transformer must contain a third harmonic component. The delta connection provides a closed path for circulation of third harmonic component of current. The flux remains sinusoidal which results in sinusoidal voltages.
2. Even if the load is unbalanced the three phase voltages remains constant. Thus it allows unbalanced loading also.
3. The important advantage with this type of connection is that if there is bank of single phase transformers connected in delta-delta fashion and if one of the transformers is disabled then the supply can be continued with remaining tow transformers of course with reduced efficiency.
4. There is no distortion in the secondary voltages.
5. Due to delta connection, phase voltage is same as line voltage hence winding have more number of turns. But phase current is (1/√3) times the line current. Hence the cross-section of the windings is very less. This makes the connection economical for low voltages transformers. 

Disadvantages 

1. Due to the absence of neutral point it is not suitable for three phase four wire system.

Key point : The connection is commonly used for large low voltage transformers.

STAR-DELTA CONNECTION

       This type of connection is commonly employed at the substation end of the transmission line. The main use with this connection is to step down the voltage. The neutral available on the primary side is grounded. It can be seen that there is phase difference of  30° between primary and secondary line voltages.

Key point : The delta connection on secondary side allows third harmonic current to flow which provides a sinusoidal flux. 

Advantages 

1. The primary side is star connected. Hence fewer number of turns are required. This makes the connection economical for large high voltage step down power transformers.
2. The neutral available on the primary can be earthed to avoid distortion.
3. Large unbalanced loads can be handled satisfactory. 

Disadvantages 

In this type of connection, the secondary voltage is not in phase with the primary. Hence it is not possible to operate this connection in parallel with star-star or delta-delta connected transforme

DELTA-STAR CONNECTION

The main use of this connection is to step up the voltage i.e. at the begining of high tension transmission system. It can be noted that there is a phase shift of 30° between primary line voltage and secondary line voltage as leading.

The advantages and disadvantages with this type of connection are given below.

Advantages

1. On primary side due to delta connection winding cross-section required is less.
2. On secondary side, neutral is available, due to which it can be used for 3-phase, 4 wire supply system.
3. There is no distortion due to third harmonic componenets.
4. The windings connected on star makes it economical due to saving in cost of insulation.
5. Large unbalanced loads can be handled without any difficulty.

Disadvantages 

Due to phase shift between primary and secondary voltages, the limitation of Star-Delta connection continues for this type of connection as well.