Parallel connection of diesel generator sets should first meet the requirements of even power distribution. The even distribution of power includes both active power and reactive power. The so-called uniform distribution means that the active power and reactive power of each generator should be proportional to their rated power. There are clear requirements for the power distribution of parallel-operated generator sets in China, when the alternator sets are operated in parallel. When the load changes within the range of 20% to 100% of the total rated power, it should be able to operate stably, and its power distribution error should meet the following requirements.
1. The difference between the actual active power of each generator and the calculated value according to the rated power ratio shall not exceed ±10% of the rated active power of the generator when the rated power of the generator is the same; when the rated power of the generator is different, Should not exceed 110% of the maximum rated active power of the generator, ±20% of the rated active power of the minimum generator.
2. The difference between the actual reactive power of each generator and the calculated value according to the ratio of rated power of the generator not exceeding the maximum generator amount
If there is a large imbalance in power distribution, whether it is active power or reactive power, it will not only affect the efficiency and economy of the genset operation, but also cause the failure of the entire power station.
If the active power distribution of the parallel generator set is seriously unbalanced, when the total load power is large, it is often that one generator set is fully loaded or overloaded, and the other generator is still under light load, so that it cannot be fully utilized. The capacity of the genset is used to make the performance of the entire power station. The overload of the genset will not only bring harm to the diesel engine or the generator, but also cause the protection equipment to operate and affect the operation of the entire power station. Conversely, when the total load power is small, the imbalance of active power tends to cause active circulation between the generator sets, causing some generator set to switch to the motor state, which is also not allowed for diesel engines. When the genset has reverse power protection, the reverse power phenomenon will cause the reverse power to trip causing a malfunction.
For diesel power plans with frequent load changes and large variation, the above situation should be paid more attention. For example, in a diesel power station of a ship’s electric propulsion device, due to the uneven distribution of active load of the parallel generator, when the ship is sailing at full speed, the total load power is large, often overloading the heavy-duty generator, causing the switch then trips, causing a serious power failure. When the ship stops at the dock, the light load generator enters the reverse power state due to the small total load, causing the reverse power trip, which affects the reliability of the operation of the power station and even the entire ship. The above situation is sufficient to show that the even distribution of power between generators in parallel operation is not only a problem of economic operation of a generator, but also an important factor to ensure the reliability of power supply of the entire diesel engine power station.
The reactive power distribution is uneven, although it will not seriously affect the diesel engine, but the rated value of the generator power is determined according to the apparent power, and the reactive power imbalance will inevitably cause the unbalanced apparent power of the generator. It will limit the capacity of the generator set, especially for power plants with lower power factor, the load capacity of the genset does not depend on the rated power of the diesel engine, but depends on the rated current value of the generator.
The imbalance of the reactive load can also be regarded as a kind of ring between the two generators. The flow sometimes exceeds the load current of the generator and generates additional losses inside the generator. And even overload the generator. The reactive overcurrent also trips the generator’s main switch, causing a malfunction.
In addition, the magnitude of the reactive current of the generator is consistent with its excitation current. The imbalance of the reactive current tends to occur at the same time as the excitation current imbalance, which causes the excitation system of overload or even cause failure, such as the rotor winding of the generator is burned/ the rectifier diode of the compound excitation device is damaged, etc., and the consequences of such failure are often quite serious. It can be seen that the uniform distribution of the reactive load and the uniform distribution of the active load have the same meaning.