Transformers in parallel operation, the ideal operating condition is: when transformers are already in parallel without load, each transformer is like when operating alone with no load, only having no-load current, and there is no circulating current between transformers; when load is applied, each transformer can proportionally distribute the load according to its capacity, that is, large-capacity transformers share more load, and small-capacity transformers share less load. To achieve the above ideal operating condition, transformers in parallel operation must meet the following conditions: 1. Transformers should have the same connection group label; 2. The voltage ratios of transformers should be equal, with a maximum allowable difference of ±0.5%; 3. The impedance voltage percentages of transformers should be equal, with an allowable difference not exceeding ±10%; 4. The capacity ratio of transformers should not exceed 3:1.
Improve the reliability of power supply. When a transformer fails, other transformers in parallel operation can still maintain power supply. 2. Improve the economy of operation. Dispatch can switch some transformers on or off according to load changes to reduce energy loss, improve operating efficiency, and thus enhance the economy of operation. 3. Transformers can be installed in stages. For example, if the load of a substation gradually increases, parallel transformers can be installed accordingly year by year as needed, thereby reducing initial investment and fully utilizing the economic benefits of funds. 4. It is beneficial for arranging transformer maintenance.
Transformers in parallel operation, in addition to meeting the parallel operation conditions, should also pay attention to safe operation. Generally, the following aspects should be considered: 1. For newly commissioned and overhauled transformers, before paralleling, phase verification should be carried out first, and after trial paralleling in the no-load state, formal parallel operation with load can be carried out. 2. The paralleling of transformers must consider the economy of parallel operation. Transformers that are not economical are not allowed to operate in parallel. At the same time, attention should be paid to avoiding frequent operations. 3. When performing paralleling or de-paralleling operations of transformers, the use of disconnecting switches and drop-out fuses is not allowed. The paralleling and de-paralleling operations should ensure correct operation, and reverse power transmission through transformers is not allowed. 4. For transformers that need to operate in parallel, before parallel operation, the distribution of transformer load current should be estimated based on actual conditions. After paralleling, immediately check whether the operating current distribution of the two transformers is reasonable. When it is necessary to de-parallel a transformer or shut down one transformer, it should be estimated based on actual load conditions whether it is possible to cause overload of one transformer. Also, the actual load current should be checked. In the case where transformer overload may be caused, transformers are not allowed to perform de-paralleling operations.
