How to Protect the Loops in Parallel Operation of Generator

Mar. 15, 2022

Parallel operation means that two or more generator sets with equal rated voltage and consistent phase are combined into the same bus to provide active power and reactive capacity to the device. One generator set is started first, and then the generated voltage is sent to the bus. After the other generator set is started, it is in parallel with the previous generator set at this moment. At the moment of closing, the generator set will not have abnormal impulse current, and the crankshaft will not be impacted suddenly. However, the overall current circuit should be comprehensively considered in the operation steps of parallel operation.


1. Reverse work. The current situation of reverse power is caused by the different speed and voltage of generator units, that is, one generator unit has positive power and the other unit has negative power. In other words, the unit with negative capacity becomes a load at this time (the current situation of low frequency and inconsistent speed of this unit). When the voltage is inconsistent, the unit with high voltage supplies one reactive current and reactive voltage to the unit with low voltage (the positive indication of the ammeter of this unit). It is equivalent to adding a condenser group in the power supply system. At this time, the unit with low voltage becomes a large load and receives a large reactive current to ensure the voltage balance of the two units (the ammeter of this unit indicates in reverse). During monitoring, when the voltage of one unit is increased or the voltage of another unit is increased, one unit has reverse power current, and its behavior current is about 20% of the rated current. Reverse acting relay acts, trips and alarms, but does not shut down.


Perkins generator set


2. Overcurrent. The output capacity of today's generator set is corresponding, and its overload level is very low. Most of them are around 5% of the output capacity. The allowable load time is 15 ~ 30 minutes, up to 60 minutes. Beyond this time, the Perkins generator set will be hot, the insulation of cable will be reduced, and the service life will be reduced. Therefore, if there is no special requirement when setting the overcurrent protection, the overcurrent protection can be set at 110% of the rated current. During on load test, bring the current to 110% of the rated current, and the overcurrent relay acts. Trip, alarm and non shutdown.


3. Overvoltage. When the generator set is used in parallel, it is most afraid of the oscillation of the power supply system. Once the oscillation occurs, the voltage rise of the system is easy to produce the insulation breakdown of the electric facilities and the power supply system, which makes the power supply system and the electric facilities unable to operate normally. Therefore, the generator sets used in parallel are equipped with overvoltage protection, and its preset value of 105% of the rated voltage is better. Short circuit overvoltage relay, trip, shutdown and alarm. It can be seen that the speed (frequency) adjustment of the two units must be consistent before preparing for parallel operation of the generator units. In parallel, the speed of the first machine or the group to be paralleled can be adjusted according to the rotation speed rate of the table in the same period, so that the rotation direction of the table in the same period can be rotated clockwise or counterclockwise. The slower the speed is, the better, but the pointer of the table in the same period must be rotated before it can be side by side. Finally, after side-by-side, check whether the current and capacity of the two units are balanced. If the difference is too large, adjust the speed knob to keep the power

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