When the diesel generator sets are running in parallel, the power distribution is divided into two aspects: active power and reactive power. The so-called even distribution means that the active power and reactive power of each generator running in parallel should be proportional to their rated power. Guangxi Dingbo Power Equipment Manufacturing Co., Ltd. pointed out that at present, there are clear requirements for the parallel power distribution of diesel generator sets. For stable operation, its power distribution error should meet the following requirements:
1. The difference between the active power actually undertaken by each generator and the calculated value distributed in proportion to the rated power. When the rated power of the generator is the same, it should not exceed ±10% of the rated active power of the generator. When the rated power of the generator is different, it should not exceed 110% of the rated active power of the large generator and ±20% of the rated active power of the small generator.
2. The difference between the reactive power actually undertaken by each generator and the calculated value allocated according to the ratio of the rated power of the generator should not exceed ±10% of the rated reactive power of the large generator.
If the power distribution of the units running in parallel is greatly unbalanced, it will not only affect the efficiency and economical operation of the diesel generator set, but also cause the failure of the entire power station in serious cases. Let's take a look at the consequences of uneven power distribution when diesel generator sets are running in parallel.
1. The active power distribution of parallel generator sets is seriously unbalanced.
(1) When the total load power is large, one generator set is often fully loaded or overloaded, while the other generator is still in a light load state, so that the capacity of the generator set cannot be fully utilized and the performance of the entire power station can be exerted. The overload of diesel generator sets will not only bring serious harm to the diesel engine or generator, but also cause the protection equipment to work and affect the operation of the entire power station.
(2) Conversely, when the total power of the load is very small, the imbalance of active power will often cause active circulation between the generator sets, causing some sets to switch to the motor state, which is also not allowed for diesel engines. When the diesel generator set has reverse power protection, the reverse power phenomenon will cause the reverse power to trip and cause a fault.
For diesel power plants with frequent and large load changes, the above situation requires special attention.
2. The reactive power distribution of parallel generator sets is uneven.
(1) Although this will not seriously affect the diesel engine, the rated value of the generator power is determined according to the apparent power. The unbalanced reactive power will inevitably cause the unbalance of the apparent power of the generator, which will also limit the capacity of the generator set. Especially for the power station with low power factor, the load capacity of the unit does not depend on the rated power of the diesel engine, but on the rated current value of the generator.
(2) The unbalanced reactive load can also be regarded as a circulating current between the two generators. Sometimes this circulating current will exceed the load current of the generator, resulting in additional losses inside the generator, even overloading the generator. The reactive overload current will also trip the main switch of the generator, causing a failure.
(3) In addition, the magnitude of the generator's reactive current is consistent with its excitation current, and the unbalance of the reactive current often occurs at the same time as the unbalance of the excitation current, which makes the excitation system overload or even cause a fault. Such as the generator rotor winding is burned, the rectifier diode of the phase compound excitation device is damaged, etc., the consequences of this fault are often quite serious.
It can be seen from the above analysis that the uniform distribution of reactive load and the uniform distribution of active load of units operating in parallel have the same meaning.
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