- Parallel Operation Defined: Parallel operation of transformers is when multiple transformers are connected to increase system reliability, efficiency, and flexibility.
- Maximizing Efficiency: Efficient parallel operation is achieved by activating only the transformers necessary to meet current demands, optimizing energy use.
- Maintenance and Reliability: Parallel operation allows for maintenance without service interruptions and enhances reliability by providing backup capacity.
- Conditions for Operation: Key conditions for the parallel operation of transformers include identical voltage ratios, impedance, polarity, and phase sequence.
- Future-Proofing: This setup allows for easy adjustment to changes in power demand, either increasing or decreasing capacity as needed.
Why Parallel Operation of Transformers is required?
It is more economical to install multiple smaller-rated transformers in parallel than one larger-rated electrical power transformers. This approach offers several key advantages:
- 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. - 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. - 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. - 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 satisfy the following conditions for satisfactory performance. These are the conditions for parallel operation of transformers.
- Same voltage ratio of transformer.
- Same percentage impedance.
- Same polarity.
- Same phase sequence.
Same Voltage Ratio
Connecting two transformers with different voltage ratios in parallel under the same primary supply results in different secondary voltages. If their secondaries are connected to the same bus, it causes a circulating current between them, leading to high circulating current and unnecessary I2R losses due to the low internal impedance of the transformers.
Same Percentage Impedance
The current shared by two transformers running in parallel should be proportional to their MVA ratings. Again, current carried by these transformers are inversely proportional to their internal impedance. From these two statements it can be said that, impedance of transformers running in parallel are inversely proportional to their MVA ratings. In other words, percentage impedance or per unit values of impedance should be identical for all the transformers that run in parallel.
Same Polarity
All transformers operating in parallel must have the same polarity to prevent large circulating currents without load output. Polarity refers to the direction of induced EMF in the secondary. Transformers are considered to have opposite polarity if the directions of their induced EMFs differ when supplied with the same input power. Conversely, they have the same polarity if their EMFs move in the same direction under identical power conditions.
Same Phase Sequence
The phase sequence or the order in which the phases reach their maximum positive voltage, must be identical for two parallel transformers. Otherwise, during the cycle, each pair of phases will be short circuited.
The above said conditions must be strictly followed for parallel operation of transformers but totally identical percentage impedance of two different transformers is difficult to achieve practically, that is why the transformers run in parallel may not have exactly same percentage impedance but the values would be as nearer as possible.
