Parallel operation of Transformers

Why Parallel Operation of Transformers is required?

Parallel operation of transformers means connecting two or more units so they share one load. It is often cheaper to run several smaller-rated transformers in parallel than one larger-rated electrical power transformers. The next list sets out the main reasons:

1. To maximize electrical power system efficiency:
   Typically, an electrical power transformer is most efficient at full load. By operating multiple transformers in parallel, we can activate only those needed to meet current demand, ensuring they run close to their full load rating. As the demand increases, additional transformer can be switched on one by one to meet the total demand, maintaining optimal efficiency.
2. To maximize electrical power system availability:
   When multiple transformers operate in parallel, any one of them can be shut down for maintenance. The remaining transformers in the system will continue to serve the load, ensuring no interruption of power.
3. To maximize power system reliability:
   If any one of the transformers run in parallel, is tripped due to fault of other parallel transformers is the system will share the load, hence power supply may not be interrupted if the shared loads do not make other transformers over loaded.
4. To maximize electrical power system flexibility:
   There is always a chance of increasing or decreasing future demand of power system. If it is predicted that power demand will be increased in future, there must be a provision of connecting transformers in system in parallel to fulfill the extra demand because, it is not economical from business point of view to install a bigger rated single transformer by forecasting the increased future demand as it is unnecessary investment of money. Again if future demand is decreased, transformers running in parallel can be removed from system to balance the capital investment and its return.

Conditions for Parallel Operation of Transformers

When two or more transformers run in parallel, they must meet the next conditions for satisfactory performance. These are the conditions for parallel operation of transformers.

1. Same voltage ratio of transformer.
2. Same percentage impedance.
3. Same polarity.
4. Same phase sequence.

Same Voltage Ratio

Connecting two transformers with different voltage ratios in parallel on the same primary supply gives different secondary voltages. If those secondaries feed the same bus, a circulating current flows between them. The low internal impedance then drives a large circulating current and extra I²R loss.

Same Percentage Impedance

The current shared by two transformers in parallel should be proportional to their MVA ratings. The current each unit carries is inversely proportional to its internal impedance. Those two facts together mean the impedance of transformers in parallel should be inversely proportional to their MVA ratings. In other words, percentage impedance (or per-unit impedance) should match on every transformer that runs in parallel.

Same Polarity

All transformers operating in parallel must have the same polarity, or a large circulating current will flow even with no load. Polarity is the direction of induced EMF in the secondary. Transformers have opposite polarity if their induced EMFs point in different directions on the same input. They have the same polarity if those EMFs point in the same direction under the same supply.

Same Phase Sequence

The phase sequence, meaning the order in which the phases reach their maximum positive voltage, must match on two parallel transformers. If it does not, each pair of phases will be short-circuited during the cycle.
Those conditions must be followed for parallel operation of transformers. In practice two transformers rarely have exactly the same percentage impedance, so units that run in parallel should have values as close as possible.