Diesel generator sets are increasingly being operated in parallel, often due to the increased demand for power. There is a frequent need for service to connect generator sets in parallel. With the development of modernization, generator sets are more and more commonly used in defense projects, weapon systems, field operations, and other engineering applications. To meet high-load or uninterrupted power supply requirements, it is often necessary to run two or more units in parallel. During parallel operation, uneven power distribution is often observed, and excessive uneven distribution can severely affect the safety and reliability of the power station system, as well as posing serious hazards to the generator sets. This hazard originates from the system's circulating current issue, which is a common and challenging problem in the debugging of parallel stations. Our technical staff, based on some experience with debugging parallel units, has proposed the causes of circulating current, factors affecting the even distribution of power, and solutions to address these issues.
(1) To parallel two units, it is first necessary to adjust the no-load voltages and voltage regulation characteristics of both units to be completely identical. This is the prerequisite for the even distribution of reactive power between the two units and also the foundation for the subsequent equal power distribution adjustment. Once these two adjustments are balanced, it can be ensured that the output terminal voltages of the two units in parallel are equal under any load, while also guaranteeing equal power distribution, which in turn ensures a circulating current of 0 (an ideal state). It indicates that the root cause of the circulating current is that the no-load voltages of the two units are not completely equal or that their voltage regulation characteristics differ, leading to unequal output terminal voltages and the generation of a circulating current.
Two units have equal no-load voltage and voltage regulation characteristics, but their output currents differ, indicating an uneven power distribution among the units. This discrepancy can lead to unequal U1 and U2 values, thereby causing circulating current.
(3) Many factors affect reactive power distribution, such as the characteristics of automatic voltage regulators and the stabilizing effect of voltage equalization lines, which will not be analyzed further.
Dynamic balance analysis of power distribution is exemplified by the parallel operation of two identical power generating sets. According to GB/T12786-2006 General Technical Conditions for Automatic Internal Combustion Engine Power Stations: The difference in active power distribution between 20% and 80% of the rated load shall be ≤ ±10%. The average distribution of active power between the two parallel sets primarily relies on the balance chain between the load distributors of the two sets.
