In order to meet the reliability and economy of underground engineering power supply, general underground engineering power stations are equipped with two or more synchronous generator units as the main power source. Two or more diesel generators supplying power to the grid simultaneously is called parallel operation of the generator set, and the process of putting the generator set into parallel operation is called parallel operation.

There are usually two situations that require parallel operation: ① To meet the demand of the power grid load, when the single machine load reaches 80% of the rated capacity and the load may still increase, it is necessary to consider parallel connection with another generator; ② When it is necessary to replace the operating power supply unit with a backup unit, in order to ensure uninterrupted power supply, it is necessary to replace it through parallel operation.

The parallel operation methods of diesel generator sets can be divided into two categories: accurate synchronization method and self synchronization method.

1. Accurate synchronization and parallel operation

Accurate synchronous parallel operation is a commonly used parallel operation method in underground engineering, which requires that the voltage, frequency, and phase of the standby and operating units be adjusted to be very close before the main switch of the standby generator can be closed. The impact current, impact torque, and bus voltage caused by parallel operation using this method are very small, and will not have any adverse effects on the power system; However, if asynchronous parallel connection is caused for some reason, the impulse current is large, and in the most severe case, it can be the same as the three-phase short-circuit current at the machine end. So accurate synchronization and parallel operation require strict and careful operation.

2. Self synchronous parallel operation

Self synchronous parallel operation is more accurate and simple. The operation process is as follows: the prime mover brings the speed of the unexcited generator to close to the synchronous speed, that is, the main switch of the generator is closed, and excitation is immediately applied to the generator. By relying on the self synchronization action between the units, synchronization is pulled in, allowing the generator to operate in parallel with the power system. Due to the capacity limitation of underground engineering power stations, the power system of underground engineering generally does not adopt self synchronous parallel operation. Therefore, self synchronous parallel operation is currently not seen in underground engineering power stations.

At present, the main parallel methods used are: manual quasi synchronous parallel; Coarse synchronous reactor parallel operation; Semi automatic quasi synchronous parallel operation; Automatic quasi synchronous parallel operation.

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