Since most of the electrical loads are inductive, the power factor before compensation is lagging. If too many capacitors are put in to compensate the reactive current, the power factor will become ahead, which is called over compensation.

In the case of over compensation, the capacitive reactive current appears in the system, which increases the apparent current, so the loss of the system increases. It is not a good thing to put more capacitors into the system instead of increasing the loss of the system. In addition, because the capacitor will make the voltage rise (the voltage rise here is mainly caused by the inductance of the power supply line and the leakage inductance of the transformer, which has little relationship with the synchronous generator), the voltage will further rise in the case of over compensation, and the impact will be greater in the case of lower load at night and higher grid voltage. So people always don't want compensation to happen.

But things have two sides. Over compensation is not always a bad thing.

Usually, the compensation device is installed in the low voltage side of the transformer, which detects and controls the reactive current of the load, but it can not compensate the reactive current of the transformer itself. It is generally believed that the reactive power of the transformer can only be compensated at the high voltage side. In fact, it is not. The reactive current of the transformer can also be compensated by over compensation at the low voltage side. Because the transformer is an ideal component, the so-called ideal component is the component with no direction of energy transmission. For the same transformer, if the high voltage side is connected to the power supply, the low voltage side to the load is a step-down transformer, if the low voltage side is connected to the power supply, the high voltage side to the load is a step-up transformer. According to this principle, there is no difference between the reactive power compensation on the low voltage side and on the high voltage side.

For the reactive power compensation device installed to reduce the user's power rate tariff (power factor adjustment tariff), if the method of over compensation is not adopted, there may be problems.