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There may come a time when your transformer gets close to full load. At this point you have two options.
Replace the transformer with one that has a larger kVA.
Parallel in a new transformer.
It is sometimes more practical to parallel in a new transformer as there is minimal downtime to the operation.
Three Rules And The Truth (For Paralleling)
Three conditions must be met before you can connect transformers in parallel.
- The transformers must have the same primary and secondary voltage ratings.
- If the voltage ratings of the transformers are not the same, large circulating currents will flow in both the primary and secondary windings. Circulating currents are currents that flow between the two transformers but not through the loads. The smaller transformer will act as a load on the larger transformer. Due to the low resistance of the transformer winding, the currents that circulate can end up being quite large and dangerous.
- Even though the voltages induced in the secondaries of the transformers are AC, the same circulating currents flow in each of the secondary windings. Any current flowing in the secondary of the transformer must be matched by a current in the primary so that the proper CEMF is produced in the primary windings. The current in the primary is equal to the secondary current divided by the turns ratio. This means that circulating currents proportional to those in the secondaries will also flow in the primaries.
- When making the connections, you must observe the terminal polarity of the transformers.
- This still allows you to parallel a subtractive-polarity transformer with an additive-polarity transformer if you ensure that the connection terminals have the same instantaneous polarity
- It is possible to replace the secondary windings of the transformer with batteries to analyse what would happen if the proper polarities were not observed. Figure 11 shows two batteries with equal voltages connected improperly in parallel. The batteries act as though they are in series with one another and only the resistance of the windings themselves will limit the current.
- This current will be quite large and will most likely exceed the ratings of the windings and burn out the transformer.
