Transformers are the most important electrical equipment in a power system and typically operate for extended periods. To ensure safe and economical operation and improve reliability, two or more transformers are usually operated in parallel.
Parallel operation of transformers involves complex calculations, essentially aiming to increase the total capacity of the transformers. However, parallel operation is not simply a matter of connecting the secondary sides together. Before parallel operation, it is essential to carefully check the transformer nameplate to ensure that the basic conditions for parallel operation are met. The conditions for parallel operation of transformers are:
1. The wiring groups are the same.
2. The ratios are the same, and the difference does not exceed ±0.5%.
3. The short-circuit voltages are the same, and the difference does not exceed ±10%.
4. The capacity ratio of the two transformers shall not exceed 3:1.
The parallel operation of transformers is the parallel operation of "power sources," and its complexity far exceeds that of the parallel operation of loads. If transformers that do not meet the conditions for parallel operation are forcibly connected together, a large "circulating current" will appear between the two transformers, which not only wastes electrical energy but also poses a great safety hazard.
In addition, when transformers are running in parallel, their impedances are connected in parallel, resulting in a decrease in the total impedance and a significant increase in the short-circuit current of the system. This places higher demands on the selection of upstream switchgear, a problem that should have been considered during the design phase.
If the original system is not operating in parallel, such as the single-busbar sectionalized wiring common in most enterprise power systems, generally from an equipment management perspective, the sectionalizing switch H1 is not allowed to be closed when both incoming switches H2 and H3 are closed. This is usually clearly stipulated in the power supply contract signed between the enterprise and the power company. However, from a technical perspective, the sectionalizing switch H1 can be closed when both incoming switches H2 and H3 are closed. This operation achieves a seamless switching of the operating load, but the time for all three switches to close simultaneously should be minimized. After confirming that H1 has successfully closed, H2 or H3 should be opened as soon as possible.
Of course, to ensure the safe and economical operation of transformers and improve the reliability of power supply, two or more transformers are often operated in parallel. The significance of parallel operation of transformers lies in:
1. When a transformer fails, the transformer differential protection will trip the high-voltage and low-voltage side switches of the transformer, disconnecting the transformer. Meanwhile, other transformers operating in parallel can continue to operate to ensure the continuity of power supply for users.
2. When a transformer is undergoing preventive testing or maintenance, it can be disconnected from the parallel system, and the remaining transformers can bear the entire electrical load. This ensures both the planned maintenance of the transformer and the uninterrupted power supply to the load, thereby improving the reliability of the system power supply.
3. Increase grid capacity and facilitate dispatching. We know that electricity load is highly seasonal, generally higher in summer and lower in winter. This allows some parallel transformers to be decommissioned during the winter months when load is lower, reducing no-load losses caused by large-capacity transformers and improving the overall grid efficiency.
For example, a power grid might require a 40,000 kVA transformer during peak load. If a 50,000 kVA transformer is chosen, it must operate normally at all times. A failure in this transformer would disrupt the power supply to all downstream equipment, causing significant losses and even safety hazards for sectors where power outages are unacceptable, such as data storage equipment in a large data center. Furthermore, the transformer would be overpowered during winter when loads are lower. However, if three 25,000 kVA transformers are used in parallel, one transformer can be disconnected during periods of low load, reducing no-load losses and reactive power costs, while significantly improving the reliability of the power system.
