Electrical technology of running side by side with transformers

 

Transformer is the most important power equipment in power system, and it usually runs for a long time. In order to make transformers operate safely and economically, and improve the reliability of operation, two or more transformers are usually run side by side.
Parallel operation of transformers is a complicated calculation process. Its essence is to increase the total capacity of transformers. But when the transformer runs in parallel, it is not simple to connect the secondary side to one piece. Before parallel operation of transformers, we must carefully check and confirm the nameplate of transformers to see whether it meets the basic conditions of parallel operation.

The conditions for parallel operation of transformers are as follows:

1. The connection groups are the same.
2. The variation ratio is the same, and the difference does not exceed (+0.5%).
3. The short circuit voltage is the same, and the difference is not more than 10%.
4. The capacity ratio of two transformers does not exceed 3:1.

The parallel operation of transformer is the parallel operation of “power supply”, and its complexity is far greater than that of load. If the transformer does not meet the parallel conditions and is forced to be connected in parallel, there will be a great “circulation” between the two transformers, which not only wastes power, but also brings great potential safety hazards.
In addition, when transformers are running in parallel, because their impedance is parallel, the total impedance will be reduced, and the short circuit current of the system will be greatly increased, which puts forward higher requirements for the selection of upstream switching equipment. This problem should be considered in the design.

If the original system is not running side by side, for example, most of the power systems in enterprises are single-bus sectional wiring, as shown in the following figure, in general, from the equipment management level, when two incoming switches H2 and H3 are closed, the sectional switch H1 is not allowed to close, generally signed by the enterprise and the power supply company. The contract for power supply and consumption will be clearly agreed upon. But technically, when two incoming switches H2 and H3 are switched on, the sectional switch H1 can be switched on. This kind of operation realizes the undisturbed switching of operation load, but the time of switching on three switches at the same time should be shortened as far as possible. After confirming the success of switching on H1, H2 or H3 should be switched on as soon as possible.

Of course, in order to make transformers operate safely and economically and improve the reliability of power supply, two or more transformers are often used in parallel. The significance of transformer parallel operation lies in:

1. When a transformer fails, the differential protection of the transformer will jump the high and low voltage side switch of the transformer to remove the transformer, while other transformers running in parallel can continue to operate in order to ensure the continuity of power consumption for users.
2. Transformers can be removed from the parallel system during preventive tests or overhauls. The rest of the transformers bear all the power load. This ensures not only the planned overhaul of transformers, but also the uninterrupted power supply of loads, and improves the reliability of power supply.
3. Enhance the capacity of the power grid to facilitate dispatching. We know that the seasonal load of electricity is very strong, usually in summer when the load is larger, but in winter when the load is smaller. In this way, when the winter load is small, the parallel transformer can be withdrawn from partial operation, which reduces the no-load loss caused by the operation of large-capacity transformer and improves the efficiency of the whole power grid.

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