Why are power compensation capacitors installed in the distribution room?
The core purpose of installing power compensation capacitors in the distribution room is to **improve the
power factor of the power system** and optimize the power quality and operation efficiency. The following
are the specific reasons and working principles:
### I. Why is it necessary to improve the power factor?
1. **Definition of power factor**
The power factor ($\cos\phi$) is the ratio of **active power (P)** to **apparent power (S)**:
\[
\cos\phi = \frac{P}{S} = \frac{P}{\sqrt{P^2 + Q^2}}
\]
Among them, **reactive power (Q)** is the energy required for inductive loads (such as motors and
transformers) to establish a magnetic field. It does not directly do work but affects the magnitude of the current.
### II. Hazards of a low power factor
1. **Increase in line losses**
The current formula is: $I = \frac{P}{\sqrt{3}U\cos\phi}$. The lower the $\cos\phi$, the larger the current,
and the line losses ($I^2R$) increase proportionally.
2. **Waste of equipment capacity**
The capacity of transformers and generators is determined by the apparent power (S). If $\cos\phi = 0.7$,
a 1000kVA device can only provide 700kW of active power, and 300kVA of capacity is occupied by reactive power.
3. **Decrease in voltage stability**
Insufficient reactive power will lead to an increase in the line voltage drop, affecting the power supply
quality of end devices.
4. **Electricity bill penalty**
In China, **power factor adjustment electricity bills** are implemented. When $\cos\phi < 0.9$, additional
electricity bills will be charged (such as for industrial users).
### III. Working principle of power compensation capacitors
1. **Capacitive reactive power compensates for inductive reactive power**
- Inductive loads (motors) consume **inductive reactive power ($Q_L$)**, and the current lags behind the
voltage by 90°.
- Capacitors provide **capacitive reactive power ($Q_C$)**, and the current leads the voltage by 90°.
- After the two are connected in parallel, **$Q_L - Q_C$** reduces the total reactive power, and the power
factor approaches 1 (as shown in the figure below).
2. **Calculation formula**
The reactive power capacity to be compensated:
\[
Q_C = P \left( \tan\phi_1 - \tan\phi_2 \right)
\]
Among them, $\phi_1$ is the phase angle before compensation, and $\phi_2$ is the phase angle after compensation.
### IV. Actual benefits of compensation capacitors
| Benefit dimension | Specific performance |
| Reduce losses | The current decreases, and the line losses are reduced by about \(1 - \cos^2\phi\) times (for example, when \(\cos\phi\) changes from 0.7 to 0.9, the losses are reduced by 39%) |
| Release capacity | Under the same active power, the apparent power decreases, and the transformer can carry more loads |
| Stabilize voltage | Reduce the voltage drop caused by the line reactance and improve the voltage quality at the end |
| Save electricity bills | Avoid power factor penalties, and some regions offer rewards to users with a high power factor |
### V. Configuration methods of compensation capacitors in the distribution room
1. **Centralized compensation**
Capacitor banks are centrally installed on the low-voltage bus side of the distribution room, which is suitable
for scenarios with concentrated loads (such as factories).
2. **Group compensation**
Compensation is carried out in groups according to the distribution branches to balance the local reactive
power demand.
3. **On-site compensation**
It is directly connected in parallel beside large equipment (such as motors), which is suitable for high-voltage
and long-distance equipment.
### Summary
Installing compensation capacitors in the distribution room is to **offset inductive reactive power with
capacitive reactive power**, thereby:
✅ Improving the power factor, reducing line losses and equipment capacity requirements;
✅ Improving voltage stability and avoiding electricity bill penalties;
✅ Being an important measure for the energy conservation and economic operation of the power system.
