Transformers in parallel

Parallel operation of transformers, the purpose of parallel operation, single-phase transformer parallel operation, three-phase transformer parallel operation, polarity, power-sharing, and various related issues are highlighted.

Objectives of parallel operation: -

1. It is more dependent to operate two transformers in parallel than one large transformer. Because, even if there is a problem with the same transformer, the supply can be ensured with another.
2. To provide a three-phase supply with the help of a few single-phase transformers.
3.Load to guarantee to share.
4. To supply power at a load greater than the rated load of a transformer.
5. To facilitate the rest of each transformer.
6. Transformers often have to be maintained and repaired during operation, if there are two or more transformers, one of them can be switched off and the supply can be continued with the other.
7. Transformer efficiency is more available.

parallel operation of single-phase transformer (condition for parallel operation of the transformer)

1. All side transformer high side and low side voltage ratings must be the same The transformation ratio must be the same.
2. Transformers should be connected according to polarity That is, high side with high side and low side with low side.
3. Each transformer must have its own equivalent resistance and reactivity ratio. That will be Re: Xe.
4. The equivalent impedance of each transformer must be inversely proportional to its own KVA rating.

The following conditions must be met for the proper operation of two or more three-phase transformers in parallel.

parallel operation of the three-phase transformer (Conditions of Parallel Operation)

3 phase polarity test/3 phase transformer test

The conditions described in the case of a single-phase transformer will be applicable in the case of a three-phase. Also, the following conditions have to be fulfilled.

1. The phase displacement between primary and secondary voltage must be the same.
2. The phase must be the same.
3. In the case of three-phase transformer banking, all the transformers' core or cell structures should be the same.

what is the polarity test?

The purpose of the polarity test:-
1. When two or more transformers are operated in parallel, the polarity is observed and connected.
2. When a three-phase supply is given by banking by three single-phase transformers, the connection is made by looking at the polarity.
3. The connection has to be made knowing the polarity, otherwise, there may be a short circuit in the transformer in unequal polarity.

If the parallel operation is performed without knowing the polarity, the following problems will occur: -
1. Short circuit conditions will occur.
2. Circulating current will flow among themselves.
3. Will does not supply any load current from outside.
4. If the KVA of the transformer is not equal then it may be damaged due to circulating current flow.

Subtractive polarity

After connecting one terminal on the high side to the low side terminal on the opposite side, a voltmeter is connected to the other high side terminal and the rest on the low side terminal. If the voltmeter text shows less than the applied voltage, there will be subtractive polarity.

In this case, since the direction of the induced voltage of both coils is the same, the voltmeter reading will show less than the applied voltage as the subtraction of Ep - Es.

Additive polarity

After connecting one terminal on the opposite side to the low side terminal, a voltmeter is connected to the other high side terminal and the rest on the lower side terminal.

In this case, the induced voltage of both coils is reversed so the voltmeter reading will show more than the applied voltage as a sum of Ep + Es.

Importance of polarity test of transformer

Description of the test required to determine the polarity of the transformer

1. The high side and low side of the transformer should be identified first with the help of an Avometer or DC supply.
2. The 2 terminals on the high side must be marked with the letter H1
3. Now one end of the high side should be connected to any other head of the low side and the other end of the high side and the other end of the low side should be shortened by a voltmeter.
4. Now you have to connect the high side voltmeter and give a low voltage supply
5. If the voltmeter connected to the ends of the two coils shows more than the supply voltage, the additive polarity will be.
6. If the voltmeter connected to the ends of the two coils shows less than the supply voltage, there will be subtractive polarity.

Parallel operation of equal voltage ratio transformer

Load operation:
When two transformers are connected for parallel operation while the ratios remain the same, the total load current is divided by the inverse of the impedance of the transformer.



Since, `I_1Z_{e1}` and `I_2Z_{e2}` Both `\frac{V_P}a-V_S` Is equal to. So it can be written



When two KVA ratings are connected in parallel with different KVA ratings, they share the total load according to their own KVA rating and will share the load accordingly when their equivalent impedance is inversely proportional to their own rating.

Parallel operation at a voltage equal to the ideal condition of the transformer



`\Rightarrow Z_{AB}=\frac{Z_AZ_B}{Z_A+Z_B}`---------(1)


`\Rightarrow I_AZ_A=I_BZ_B=IZ_{AB}`

`\Rightarrow I_A=I\frac{Z_{AB}}{Z_A}`-----------(2)

`\Rightarrow I_B=I\frac{Z_{AB}}{Z_B}`---------(3)

`Z_{AB}` Putting the value in Equation 2,

`\Rightarrow I_A=I\times\frac{Z_AZ_B}{Z_A+Z_B}\times\frac1{Z_A}`

`\Rightarrow I_A=I\times\frac{Z_B}{Z_A+Z_B}`-------(4)

`\Rightarrow I_B=I\times\frac{Z_A}{Z_A+Z_B}`-------(5)


On both sides of the equation No. 4 above `\frac V{1000}` We get by multiplying by

`\Rightarrow I_A\times\frac V{1000}=I\times\frac{Z_B}{Z_A+Z_B}\times\frac V{1000}`


`\Rightarrow Q_A(KVA)=Q\times\frac{Z_B}{Z_A+Z_B}`[Q Total power KVA ]


`\Rightarrow Q_B(KVA)=Q\times\frac{Z_A}{Z_A+Z_B}`

Why is it necessary to know for sure the polarity for transformer parallel connection?

Polarity is very important during operation and banking in parallel with two or more transformers. When two or more transformers operate in parallel without correctly determining the polarity, they begin to exchange power by creating local circuits among themselves without providing electrical power outside.
As a result, the main purpose of the parallel operation is disrupted and the transformer suffers considerable damage If this condition is allowed to continue for a while then the current or succulent current will burn the transformer winding. So the polarity of the transformer must be seen before the operation.

Why should the transformation ratio be the same?

Parallel operation What should be the transformation ratio of both transformers
 If it is operated in parallel when it is not the same, it will work in parallel operation, but the circulating current of welding will flow due to the difference of secondary induced emf.

This will result in the tendency of the transformer to carry the unequal load in the load condition and in this case, the full KVA output from the transformer will not be available but the transformer will overheat.

The ratio of its own equivalent reactance and resistance during parallel operation.

The ratio of reactance and resistance should be the same during parallel operation. If the translation of reactance and resistance is not the same, then the load factor will not be the same for both the transformers while supplying power to the load, so the transformers will not be able to operate satisfactorily.

What is the circulating current?

Although both transformers' transformation ratio is not the same during parallel operation, there is an inequality in the induced secondary EMF of the transformer and its phase is not correct.
As a result, some current flows in both the windings of the transformer in the form of rotation even under load and even under no load. This flowing current is the circulating current.
Due to recirculating current, the transformer tends to carry unequal load Also KVA output is not fully available after the transformer and the transformers become overheated.

FAQ In Parallel Operation Of Transformers

Q1.Why is the parallel operation done?

Ans:-To supply more power than the rated load of the transformer.

Q2.What happens as a result of the current flowing in the transformer?

Ans:- The current flow of the transformer tends to carry an unequal load Full KVA output is not available and the transformer overheats.

Q3.Why is the circulating current flowing?

 Ans:- Circulating current is carried out during parallel operation if the transformation ratio of both transformers is not the same.

Q4.What is the polarity of the transformer?

Ans:- The polarity of a transformer is generally of two types: -

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