What are the conditions for Parallel Operation of Transformers in Substation?
Discover the essential conditions for the parallel operation of transformers, ensuring optimal performance when two or more transformers are connected in parallel.
For efficient parallel Operation of Transformers, following conditions must be met:
1.Same Voltage and Turns Ratio :
Voltage Rating (Primary and Secondary) of all the Transformers connected in Parallel should be same. Tap Position should be the same. If Transformers with different Voltage Ratios are made parallel, for the same Primary Voltage, there will be different Secondary Voltage, which will result in the flow of Circulating current in the secondary due to unequal induced EMF. Due to this Circulating Current, No Load current will be much higher than the No-Load Current of Independent Transformer. During the Loading, this Circulating current results in unequal Loading on the two Transformers, resulting in heating the Transformer.
2.Same Tap Position:
Tap Position of all the Transformers connected in Parallel should be such that, the Secondary Voltages should be same. Ideally the Transformers in Parallel must have same Voltage Ratio on all Taps and so we can operate the Parallel Transformers on same Tap. To have same Tap Position for Parallel Transformers, we need to use one RTCC as Master and the other RTCC as Follower. While changing the Tap Position for Parallel Transformers, the Taps may get Mismatched temporarily due to Mechanical Delay, which may result in the Flow of Circulating Current and Maloperation of the Protection Relays.
3. Same Percentage Impedance:
If %Z is not the same, the load sharing between the Transformers in parallel, will not be equal. The Transformer having Lower percent Impedance, carries more Load, resulting in Overheating. It may not be possible to have exactly the same %Z and so at least their MVA Ratings should be same.
4. Same MVA Ratings (Optional but recommended):
It is not desirable to have widely different Power Ratings, because, the Load will not be shared in Proportion. Though it can be managed if the Percentage Impedance are the same. It is not recommended to Operate two transformers with difference in their MVA Ratings more than 2:1, permanently in Parallel.
5. Same Polarity: Different Polarities will result in Short Circuit, when connected in Parallel.
6. Same Vector Group and Phase Displacement/Phase Shift
7. Same Phase Sequence
Frequently Asked Questions:
1.When do we say that Transformers are connected in Parallel?
Answer: When the primary windings of two or more transformers are connected to a same Voltage Level and their secondary
windings are connected to a common bus or we can say common load.
2. What is the need for Operating Transformers in Parallel?
Answer: The primary reason for Operating Transformers in Parallel is to have Uninterrupted Power Supply. Lets say two Transformers, TFR 1 & TFR 2 are connected in parallel, and TFR 1 Trips due to some reason. Now, the since the two Transformers are in Parallel, the entire load will be supplied by TFR 2 and there will be no Interruption in Power Supply.
