When two or more transformers are operating in parallel, the following three basic conditions must be met:
1. The rated primary and secondary voltages of parallel transformers must be matched equally; The voltage ratio of parallel transformers must be the same, and the allowable difference should not exceed 5%. If the voltage ratio of parallel transformers is different, the secondary winding of the parallel transformer will have a circulating current in the circuit, that is, the winding with higher secondary voltage will supply current to the winding with lower secondary voltage, causing the winding to overheat or even burn out.
2. The impedance voltage (i.e. short-circuit voltage) of parallel transformers must be equal; Due to the fact that the load of parallel running transformers is distributed inversely according to the starting impedance voltage value, if the impedance voltage difference is large, it may cause overloading of transformers with low impedance voltage. Therefore, it is required that the impedance voltage of parallel transformers must be equal, and the allowable difference should not exceed 10%.
When two or more transformers are operating in parallel, the following three basic conditions must be met:
3. The connection groups of parallel transformers must be the same; This is also because the phase sequence and phase of the primary and secondary voltages of all parallel transformers must correspond to the same, otherwise they cannot operate in parallel. Assuming two transformers operate in parallel, one connected to Yyn0 and the other connected to Dyn11, their secondary voltage will experience a 30 ° phase difference, resulting in a potential difference △ U between the secondary windings of the two transformers. This △ U will generate a large circulating current on the secondary side of the two transformers, which may cause the transformer windings to burn out.
In addition, transformers operating in parallel should have the same or similar capacities as much as possible, and the ratio of their maximum capacity to their minimum capacity should generally not exceed 3:1. If there is a significant difference in capacity, not only is it inconvenient to operate, but also when there is a slight difference in transformer characteristics, the circulating current between transformers will be quite significant, especially for transformers with small capacity that are prone to overload or burnout.