Parallel Operation of Transformer, Why Transformer Use in Parallel

• When the load current increases above the capacity of one transformer, we can connect one more in parallel instead of replacing the existing transformer with a bigger one.
• Following are some of the reasons for connecting the transformers in parallel :

       1. Higher reliability.
       2. Better maintenance and repairs.
       3. It is easy to make addition in the existing setup.

• If a single large transformer is used then its failure will shut off the installation. The parallel transformers solve this problem and improve the reliability.
• It is possible to repair or maintain the transformers in parallel connection one by one without shutting off the plant.
• With increase in the load current we can easily add a new transformer to the existing parallel unit.

Parallel Transformers on No Load

• Figure shows the parallel connection of two transformers on no load. The transformers are paralleled on both sides with proper polarities. The primary voltages V₁ and V₂ are equal. If the voltage ratios of the transformers are not equal, then the induced secondary voltages E₁ and E₂ will be in phase with each other but will not be equal to each other. Hence the voltage across switch S will be equal to (E₁
  – E₂).

• When the switch S is closed, a circulating current will start flowing due to this potential difference (E₁ – E₂). The circulating current depends on the total leakage impedance of the two transformers as well as on the voltage ratios of the two transformers.
• In order to control the leakage current to a safe value, the voltage ratios should be as close to each other as possible. We can tolerate only a small difference in voltage ratios for this reason.