Parallel Operation of Transformers; Conditions for Parallel operation of Transformers

Transformers are backbone of electrical systems in any industry. There is always advisable to use Transformers in parallel. There are following reasons why transformers are advisable to use Transformers in Parallel:-
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1.       To improve electrical System availability:-

If numbers of Transformers are connected in parallel then power supply from other transformers can be restored even if one no. transformer get faulty. Also it becomes easy to do maintenance transformers by taking out one by one without interruption of power supply, So overall availability of electrical system increases.

2.       To improve efficiency of Electrical system:-

Transformer efficiency is always maximum at full load. When numbers of transformers are connected in parallel then load can be switched to only Transformers which will cater the total load by running them nearer to full load.  When load increases further other parallel connected transformers can be connected in line. By this electrical system efficiency can be improved.

3.       To provide flexibility in electrical system:-

If in future load demand increases or decrease it becomes easy to add and remove Transformers instead of changing whole electrical system transformer. This way it provides more flexibility to electrical systems if transformers are connected in parallel.

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4.       To improve system economy:-

If load increases in system then it becomes economical to add a Transformer as per requirements in parallel with previous transformer instead of replacing whole transformer, As bigger Transformer will add to additional cost and also non-utilization of previous transformer.

As you have seen that there is advisable to use Transformers in parallel. There are certains parameters which must be kept in mind for parallel operation of Transformers.

Conditions for parallel operation of Transformers are:-

1.       Transformers should have same Voltage ratio

2.       Transformers should have same  impedance and X/R ratio.

3.       Transformers which are to be connected in Parallel should have same polarity.

4.       Transformers undergoing parallel operation should have same phase sequence.

1. Transformers should have same voltage ratio:-

Transformers undergoing parallel operation should have same voltage ratios. Now let’s take an example where transformers under parallel operation have same primary voltages but have different secondary currents, when they are connected in parallel there will be circulating currents in secondary and which will reflects in primary also. As Transformer internal impedance is very small this will leads to sufficiently high circulating currents and leads to high I²R losses.

2.   Transformers should have same Impedance and X/R ratio:-

     When Transformers same impedance then they share load according to their ratings. But if there is different X/R ratio then there will be difference in phase angle of current carried by Transformers which will leads to one Transformer working at Higher power factor then other. This will leads to unequal sharing of load of Transformers.

Thus we can easily say from above that Impedance is inversely proportion to Transformer rating. This means if transformer has same percentage impedance or per unit impedance then parallel operation will be done.

In case of single phase Transformers in Three phase banks it becomes very difficult of match Transformer impedances, so it is always good practice to match X/R ratio also for parallel operation of Transformers.

3. Transformers which are to be connected in Parallel should have same polarity.

Transformers which are undergoing parallel operation should have same voltage polarity at its output. If Transformers induced voltages have opposite polarity the this will leads to very high circulating currents and may even leads to short circuit at its output.

4. Transformers undergoing parallel operation should have same phase sequence.

For parallel operation Transformers should have same phase sequence. If this not done then this will leads to short circuits. This condition is of utmost importance for parallel operation of Transformers.

Thus we have seen the conditions for parallel operation of Transformers. 

For Tan-delta, Loss angle test visit link:-

http://electrialstandards.blogspot.com/2015/10/tan-delta-test-loss-angle-test.html