The parallel connection of diesel generator sets shall first meet the requirements of uniform power distribution. Uniform power distribution includes two aspects: active power and reactive power. The so-called uniform distribution refers to the fact that the active and reactive power borne by each generator should be proportional to their rated power. There are clear requirements for the power distribution of parallel operating generator sets in China, especially when AC generator sets are operating 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 borne by each generator and the calculated value distributed according to the rated power ratio should not exceed ± 10% of the rated active power of the generator when the rated power of the generator is the same.
2. When the rated power of the generator is different, it should not exceed 110% of the maximum rated active power of the generator and ± 20% of the minimum rated active power of the generator.
3. The difference between the actual reactive power borne by each generator and the calculated value distributed in proportion to the rated power of the generator should not exceed ± 10% of the maximum rated reactive power of the generator.
If there is a significant imbalance in power distribution, whether it is active or reactive power, it will not only affect the efficiency and economy of unit operation, but also even cause faults in the entire power plant.
If the active power distribution of parallel generator sets is severely unbalanced, when the total power of the load is large, it is often because one generator set is fully loaded or overloaded, while the other generator is still in a light load state, which cannot fully utilize the capacity of the unit and exert the efficiency of the entire power station. Overloading the unit not only poses a threat to the diesel engine or generator, but also causes protective equipment to operate, affecting the operation of the entire power plant. On the contrary, when the total power of the load is very small, the imbalance of active power often causes active circulation between generator sets, causing some units to switch to the motor state, which is also not allowed for diesel engines. When the unit has reverse power protection, the reverse power phenomenon will cause reverse power tripping and cause faults. For diesel electromechanical stations with frequent and significant load changes, the above situation should be given more attention.
For example, in the diesel electromechanical station of a certain ship's electric propulsion device, due to uneven distribution of active load from parallel generators, when the ship is sailing at full speed, the total load power is large, often causing overload of the heavy-duty generator, causing the switch to trip. As a result, the originally lightly loaded generator also suddenly overloaded, causing the switch to trip and causing serious power outage failure. When the ship docks at the dock, due to the small total load, the lightly loaded generator enters a reverse power state, causing a reverse power trip, affecting the reliability of the power station and even the entire ship's operation. The above situation is sufficient to illustrate that the uniform distribution of power between parallel operating generator sets is not only a problem of economic operation of generators, but also an important factor in ensuring the reliability of power supply for the entire diesel electromechanical station.
Uneven distribution of reactive power may not have a serious impact on diesel engines, but the rated value of generator power is determined based on apparent power. Unbalanced reactive power will inevitably cause an imbalance in apparent power of the generator, which will also limit the capacity of the generator set. Especially for power plants with low power factors, the load carrying capacity of the unit depends not on the rated power of the diesel engine, but on the magnitude of the rated current value of the generator.
The imbalance of reactive power load can also be seen as a loop between two generators, which sometimes exceeds the load current of the generator, causing additional losses inside the generator and even overloading the generator. Reactive overload current can also cause the main switch of the generator to trip, causing faults. In addition, the size of the generator reactive current is consistent with its excitation current. The imbalance of reactive current often occurs at the same time as the imbalance of excitation current, leading to overload of the excitation system and even failure. For example, the generator rotor winding is burned, and the rectifier diode of the phase compound excitation device is damaged. The consequences of such failure are often quite serious. It can be seen that the uniform distribution of reactive load and active load have the same significance. As for the factors and adjustment methods that affect uniform load distribution, they will be discussed in detail in the future.
For details of characteristics and precautions of parallel operation of diesel generator sets, installation, commissioning, maintenance and management of diesel generator sets, please pay attention to Jiangsu Starlight Electricity Equipments Co.,Ltd. for generator technical support.
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