Solutions for Over-Compensation by Low-Voltage Capacitors

Capacitor compensation principle: During capacitor compensation, the capacitor and the load are connected in parallel. Just like an electrical reservoir, when the load increases, due to the internal resistance of the power supply, the output voltage of the power supply will decrease. Because the capacitor needs to maintain the original voltage at both ends, that is, the charge within the capacitor needs to release a part, it delays the downward trend of the voltage, which is the capacitor compensation principle.

 

For the power grid, regardless of whether the load is inductive or capacitive, reactive power will be generated, and there will also be active power loss. If the compensation capacitor is too large, and the power network is capacitive, and since the power supply network is generally inductive, resonance will occur, resulting in resonance overvoltage and overcurrent. In severe cases, it may cause the power grid to separate or damage the electrical equipment. Therefore, over-compensation in power supply and consumption is not allowed. An excessively large capacitance will have certain adverse effects on both the power supply end and the load end.

For ordinary electricity users, a power factor of 0.9 is sufficient. However, at this point, there is still approximately half of the reactive power equivalent to the active power in the power supply network. At this time, the power supply network needs to supply an electric current 1.25 times the active current to enable the load to operate normally. This will increase the line loss of the power supply network by 56%. Even if the power factor is compensated to 0.95, the reactive power still accounts for 31% of the active power. Moreover, when compensating, measures such as series reactors are taken, there will be no resonance or amplification of voltage and current phenomena.

Therefore, we often say that reactive power compensation must be carried out precisely to the right extent. If the compensation is insufficient or compensation is excessive, it will lead to a decrease in power factor. Because over-compensation essentially means too much capacitive reactive power. Since it is excessive reactive power (whether capacitive or inductive), it is still reactive power. Therefore, whether when installing new compensation devices or when modifying the reactive power compensation capacitor cabinets, it is necessary to seek professional teams. Based on the actual situation on site, a compensation plan should be tailored according to local conditions to achieve the purpose of improving the power factor and saving electricity bills.

 

Long-Term Solutions

(1) Controller Optimization

● Set target power factor to 0.92~0.95 (lagging)-avoid near unity (1.0).

● Increase switching delay (e.g., 5~10 mins) to prevent rapid cycling.

● Enable overvoltage protection (e.g., auto-cutoff at 105% rated voltage).

(2) Capacitor Bank Reconfiguration

● Split large banks into smaller units (e.g., 3×50kVar instead of 1×150kVar) for finer control.

● Dynamic compensation: Install SVG (Static Var Generator) for stepless adjustment (ideal for fluctuating loads).

(3) Harmonic Mitigation

● Test for harmonics (e.g., 5th/7th order)-over-compensation can worsen resonance.

● Solution: Add detuning reactors (e.g., 7% impedance to block 5th harmonics).