Safe operation of transformers in parallel must meet the following conditions. If the conditions are not met, parallel operation may cause safety accidents. When no load is required, there should be no circulation between the parallel coils. When loaded, the load of each transformer should be distributed in proportion to the capacity, so that the capacity can be fully utilized.
1. The same connection group
If two transformers of different wiring groups are connected in parallel, there will be a considerable voltage difference in the secondary circuit. Because the internal resistance of the transformer is very small, it will produce several times the rated current, which will burn the transformer.
2, the voltage ratio is equal.
If two transformers with different ratios are connected in parallel, the secondary side will generate circulation, increasing loss and occupying capacity. It can run in parallel without overloading either one. In order to make the parallel transformer safe operation, China stipulates that the voltage difference of the parallel transformer should not exceed ±0.5% (refers to the case that the tap-changer is placed in the same gear).
3, impedance voltage percentage equal
If the impedance voltage (short circuit voltage) of the two transformers is not equal, the load carried by the transformer cannot be distributed in proportion to the transformer capacity. For example, if a transformer with a large percentage of voltage is fully loaded, a transformer with a small percentage of voltage will be overloaded. Only when any transformer running in parallel is not loaded, it can run in parallel.
It is generally believed that the short-circuit impedance difference of a parallel transformer should not exceed ±10%. Generally, we should try to increase the voltage of the secondary winding of the transformer with large short circuit impedance or change the tap position of the transformer to adjust the short circuit impedance of the transformer, so that the capacity of the transformer running in parallel can be fully utilized.
4. Capacity ratio shall not exceed 3:1
This limits the transformer short circuit voltage difference is too large. Due to the large difference in impedance values of transformers with different capacities, the load distribution is unbalanced, and from the operation point of view, the small-capacity transformer can not play a standby role, so the capacity ratio should not exceed 3:1. However, when both transformers do not exceed the rated load operation, the capacity ratio can be greater than 3:1.