May I ask how to put transformers operating in parallel into service?

I. The wiring groups are the same； II. The short-circuit impedance is the same, with a permissible error of plus or minus ten percent ； III. The transformation ratio is the same, with an allowable error of plus or minus 0.5 percent ； IV. The capacity ratio should not exceed 3:1; there are 3 conditions: 1. The transformation ratios of each transformer must be the same. 2. The impedance voltage of each transformer is the same. 3. The connection groups of all transformers are the same.

It represents the magnitude of the transformer’s impedance. The testing method involves short-circuiting the load side of the transformer and gradually increasing the voltage on the power side; the ratio of the voltage applied on the power side to the rated voltage is what is obtained when a rated current flows through the transformer.

It should be considered in terms of the circumstances under which power delivery is required Generally, such operations are carried out only after a power outage occurs on the 35KV side, whether it is due to a request made by the user itself or as part of planned maintenance by the higher-level power supply authorities. In such cases, the electrical system is usually scheduled for maintenance and repair; this means that the high-voltage switchgear for both main transformers undergoes switching operations. During maintenance, the grounding switches of the high-voltage switchgear for both main transformers should be in the grounded position. Of course, if electrical safety procedures are followed strictly, no problems will arise, but accidents can still happen. Imagine this scenario: if the high-voltage switchgear on the 6KV side of Transformer No. 2 is still in the maintenance mode (with the grounding switch connected to ground), and after Transformer No. 1 has been repaired and the system connected to power is ready, an operator makes a mistake and supplies power to Transformer No. 1 – causing the 6KV side connected in parallel to become energized – which in turn leads to a three-phase short circuit and grounding on the 6KV side of Transformer No. 2. Let me tell you clearly: the system will become very unstable, and the consequences will be severe! Therefore, in terms of the operational steps, power should be supplied first and then paralleling should be carried out; even if a fault occurs at some point during the power supply process, the impact will be minimal.

In addition to the conditions for parallel operation of transformers, this marine engineer should also pay attention to the following points: 1. For transformers that are newly put into operation or have been overhauled, it is necessary to check their phase alignment before enabling parallel operation. Only after testing parallel operation under no-load conditions and confirming that there are no issues can they be officially connected in parallel to carry a load； 2. When transformers are operated in parallel, the economic efficiency of such parallel operation must be taken into account; transformers that are not economical to operate in parallel should not be used in this manner. At the same time, it should also be noted that frequent operations should be avoided ； 3. Isolating switches and drop-out fuses are not allowed to be used when performing paralleling or disconnecting operations on transformers. To ensure proper operation of the transformer, it is not allowed to feed power into the transformer in reverse ； 4. For transformers that need to operate in parallel, it is necessary to estimate the distribution of load current across the transformers based on the actual conditions prior to putting them into parallel operation; after they are connected in parallel, it is important to check whether the distribution of operating current between the two transformers is reasonable.

I have a few additional personal suggestions: before the two transformers are operated in parallel, each transformer should be equipped with a device similar to an oscilloscope to monitor the shape of the sine waves. There should also be a paralleling switchgear similar to a high-capacity circuit breaker, which will automatically close the connection when it detects that the sine wave shapes of the two transformers are identical. The operating mechanism and process are the same as those used for connecting to the power grid in large power plants. If not, one can only rely on experience.

6kV incoming switches should be installed on the 6K side of both Unit 1# and Unit 2# main transformers; generally, there are no situations without such 6K incoming switches. When the high-voltage switches on the 35KV side are closed, the 6K bus becomes energized directly. If that’s the case, it’s too unsafe. During maintenance, one transformer can remain in operation, with the bus coupler switch being closed to supply power to the other bus bar. For transformers that are under maintenance, a ground wire is connected on the 6KV incoming switch side; once the maintenance is complete, the ground wire is removed and power supply is restored. After closing the 6KV switch for that transformer, the bus coupler switch is turned off, thereby ensuring uninterrupted power supply. However, the two-ticket system must be strictly followed.