Parallel Operation Impulse Current Calculation Method 2

This article continues to introduce the calculation method of impulse current for parallel operation of diesel generators.

The expression of the transient inrush current during the no - load connection of a doubly - fed induction generator is derived. The influence of the voltage amplitude difference and the phase difference on the inrush current is analyzed, and corresponding measures to suppress the inrush current are proposed. When a three - phase synchronous generator operates symmetrically, the effect of a single - phase can be equivalently considered. Through the finite - element modeling of a single - phase generator, it is verified that the frequency difference has little influence on the inrush current. The closing operation for paralleling does not select the moment when the frequency between the generator and the system is the same. A slight frequency difference makes it easier for the main generator to pull the generator to be paralleled into synchronization. Therefore, the main factors affecting the magnitude of the inrush current are the voltage amplitude difference and the angular difference.

Starting from the analysis of the causes and impacts of the inrush current during the paralleling process of a synchronous generator, the calculation formula of the inrush current when there are both voltage amplitude differences and phase differences is derived. Through the calculation of an example of generator set paralleling, a reasonable range of parameter differences is given, providing a theoretical basis for the parameter setting of the synchronization locking device for generator paralleling.

Regarding the causes and impacts of the inrush current during the paralleling process of a generator, the Laplace transform is used to give the calculation formula of the generator inrush current when there are voltage amplitude differences and phase differences. Taking the actual paralleling of a diesel generator set as an example, a reasonable range of parameter differences is calculated. Through the above analysis and calculation, the following conclusions can be drawn:

1.The magnitude of the inrush current during the parallel operation of a three - phase synchronous generator is mainly determined by the comprehensive effect of the voltage difference and the phase difference at the moment of closing for paralleling. The frequency difference between the generator sets at the moment of paralleling has little influence on the magnitude of the inrush current. An appropriate frequency difference can enable the generator sets to quickly enter the synchronous operation state, but an excessive frequency difference will cause the generator sets to experience long - term oscillations and may even lead to disconnection.

2.When operating the parallel operation of generator sets of any capacity, the voltage amplitudes on both sides of the system to be paralleled are first adjusted to be equal. However, in actual operation, when the parallel circuit breaker is closed, there are always both voltage amplitude differences and angular differences. Therefore, the inrush current is caused by the combined effect of the voltage amplitude difference and the angular difference.

3.Setting a reasonable parameter range for the parallel circuit breaker and the synchronization device can reduce the inrush current caused by the voltage amplitude difference and the angular difference, improve the success rate of parallel operation, speed up the paralleling process, and ensure the safe and stable operation of the system.

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