Conditions for Parallel Operation of Transformers and Electrical Technology

Transformers are the most important electrical equipment in power systems and are generally operated for long periods of time. In order to make the transformer safe and economical to operate and improve the reliability of operation, two or more transformers are generally operated in parallel.

The parallel operation of the transformer is a complicated calculation process, and its essence is to increase the total capacity of the transformer. However, when the transformers are operated in parallel, it is not easy to connect the secondary side to one. Before the transformers are operated in parallel, be sure to carefully check the nameplate of the transformer to see if it meets the basic conditions for parallel operation. The conditions for the parallel operation of the transformer are:
1. The wiring group is the same.
2. The ratio is the same, and the difference does not exceed ±0.5%.
3. The short-circuit voltage is the same, and the difference does not exceed ±10%.
4. The capacity ratio of the two transformers does not exceed 3:1.

Parallel operation of the transformer is a parallel operation of the “power supply”, and its complexity is far greater than the parallel operation of the load. If the transformers do not meet the parallel conditions and are forcibly connected in parallel, there will be a large “circulation” between the two transformers, which not only wastes power, but also brings great safety hazards.

In addition, when the transformers are running in parallel, the total impedance will decrease because their impedances are connected in parallel. The short-circuit current of the system will be greatly increased, and higher requirements are imposed on the selection of upstream switching devices. This issue should have been considered at design time.

If the original system is not running in parallel, for example, most companies’ power systems are single-bus segmented wiring. Under normal circumstances, from the equipment management level, in the case where both incoming switches H2 and H3 are closed, the sectional switch H1 is not allowed to be closed. Generally, it will be clearly stipulated in the power supply and power contract signed by the enterprise and the power supply company. However, from a technical point of view, in the case where both incoming switches H2 and H3 are closed, the sectional switch H1 can be closed, and this operation realizes a bumpless switching of the operating load. However, the time for the three switches to be closed at the same time should be as short as possible. After confirming that H1 is closed successfully, H2 or H3 should be opened as soon as possible.

Of course, in order to make the transformer safe and economical, and to improve the reliability of the power supply, two or more transformers are often used in parallel.