Transformer is the most important power equipment in the power system, and it usually runs for a long time. In order to make the transformer safe and economic operation, improve the reliability of operation, generally let two or more than two transformers parallel operation.
Parallel operation of transformer is a complicated calculation process, its essence is to improve the total capacity of transformer. But when the transformer runs in parallel, the secondary side is not simply connected to a piece. Before the parallel operation of the transformer, it is necessary to carefully check and confirm the nameplate of the transformer to see whether it meets the basic conditions of parallel operation. The conditions for parallel operation of transformers are:
1. Wiring groups are the same.
2, the change ratio is the same, the difference is not more than ±0.5%.
3., short circuit voltage is the same, the difference is not more than ±10%.
4. The capacity ratio of two transformers is not more than 3:1.
The parallel operation of transformer is the parallel operation of "power supply", and its complexity is far greater than the parallel operation of load. If the transformer does not meet the parallel conditions and forced parallel together, there will be a great "circulation" between the two transformers, not only waste electric energy, but also bring great security risks.
In addition, transformer parallel operation, because their impedance is parallel, the total impedance
will be reduced, the short-circuit current of the system will greatly increase, the selection of upstream switching equipment has put forward higher requirements, this problem should have been considered in the design.
If the original system is not in parallel, such as most of the enterprises of electric power system is a single busbar section connection, in general, from the equipment management level, switch the two into line under the condition of H2 and H3 are closing, section switch H1 are not allowed to switch, generally in the enterprise of the power supply of electric power supply company will expressly agreed upon in the contract. However, from the technical point of view, segmented switch H1 can be closed when both of the two incoming switches H2 and H3 are closed. This operation realizes no disturbance switching of the operating load, but the time for the simultaneous closing of the three switches should be shortened as much as possible. After confirming the successful closing of H1, switch H2 or H3 as soon as possible.
Of course, in order to make the transformer safe and economic operation, improve the reliability of power supply, often use two or more than two transformers in parallel operation. The significance of parallel operation of transformer lies in:
When a transformer fails, the differential protection action of the transformer will jump the high and low voltage side switch of the transformer and remove the transformer, while other transformers running in parallel can continue to operate to ensure the continuity of power consumption by users.
2. Transformer preventive test or maintenance, can be removed from the parallel system, by the rest of the transformer to bear all the power load, not only to ensure the transformer maintenance plan, but also to ensure the load without interruption of power supply, improve the reliability of the system power supply.
3. Improve the capacity of power grid to facilitate dispatching. As we know, the seasonal load of electricity is very strong. Generally, the load is large in summer and small in winter. In this way, in winter when the load is small, part of the parallel transformer can be withdrawn from operation, which reduces the no-load loss caused by the operation of large capacity transformer and improves the efficiency of the whole grid.
For example, a power supply network needs a transformer with a peak load of 40,000 kVA. If a transformer with a peak load of 50,000 kVA is selected, the transformer needs to be in normal operation at all times. If the transformer fails, it will affect the normal use of electricity for all downstream equipment of the transformer, which will cause huge losses for some power departments that are not allowed to power off, and even cause safety accidents. For example, data storage equipment in big data room. And when the load is reduced in winter, the transformer will appear the phenomenon of big horse pulling small car. If three 25000KVA transformers are used in parallel operation, one transformer can be cut off when the load is small, so as to reduce the transformer's no-load loss and reactive load expenditure, and greatly improve the power supply reliability of the power system.