Low-voltage switchgear Capacitor compensation is indispensable in such systems; so what is the function of capacitor compensation? Its significance lies in its role as a capacitor. Let us first examine the principle of capacitor compensation: during compensation, the capacitor is connected in parallel with the load, effectively acting as an energy storage reservoir. When the load increases, the power supply has internal impedance, which causes the output voltage to drop.

1. Low-voltage switchgear Principle of Power Capacitor Compensation

In principle, a capacitor functions like a generator that produces soluble reactive current. The principle of reactive power compensation is to connect equipment with capacitive reactive load in parallel with inductive reactive load across the same capacitor, allowing energy to be exchanged between the two loads. This reduces the loading on transformers and transmission lines in the power grid and increases the system’s active power output. With a given level of active power output, losses in the power supply system are thereby reduced. In comparison, using capacitors is a relatively simple and cost-effective method for reducing the reactive load on transformers, the power supply system, and industrial distribution networks. Consequently, capacitors are indispensable for reactive power compensation in power systems. At present, the use of shunt capacitors as reactive power compensation devices has become highly widespread.

2. Compensation Characteristics of Power Capacitors in Low-Voltage Switchgear

Advantages

The reactive power compensation device for power capacitors is easy to install, and the installation location can be readily adjusted as needed. It incurs minimal losses (only about 0.4% of the rated capacity). The construction period is short, investment is low, there are no rotating parts, and operation and maintenance are simple. Moreover, if an individual capacitor bank is damaged, it does not affect the operation of the entire capacitor bank.

Disadvantages

The drawbacks of shunt power-factor correction devices employing power capacitors include the inability to provide continuous, smooth adjustment—only discrete steps are possible; poor ventilation can lead to overheating and bulging of the capacitors when their operating temperature exceeds 70°C; suboptimal voltage characteristics and inadequate short-circuit withstand capability; residual charge remaining after disconnection; low precision in reactive-power compensation, which can compromise compensation performance; challenging operational management of compensation capacitors; and insufficient attention to ensuring their safe operation.

3. Reactive Power Compensation Methods

High-Pressure Dispersion Compensation

High-voltage dispersion compensation is essentially a reactive compensation capacitor installed on the high-voltage side of a single transformer, designed to improve power supply voltage quality. It is primarily used in urban high-voltage distribution systems.

High-Voltage Centralized Compensation

High-voltage centralized compensation is a compensation method in which capacitors are installed on the 6 kV to 10 kV high-voltage busbars of substations or user-owned step-down substations. Capacitors may also be installed on the low-voltage busbars of the user’s main distribution room, making this approach suitable for applications with concentrated loads located close to the distribution busbars and requiring large compensation capacities. When the user itself has a certain amount of high-voltage load, this method can reduce reactive power consumption in the power system and thereby provide compensation. Its advantages include convenient automatic switching, effective improvement of the user’s power factor, high utilization efficiency, low investment, easy maintenance, flexible adjustment, prevention of over-compensation, and enhanced voltage quality; however, the economic benefits of this compensation method tend to decline.

Low-Pressure Dispersion Compensation

Distributed compensation in low-voltage switchgear involves installing one or more low-voltage capacitor banks near individual electrical equipment to meet their specific reactive power requirements, thereby compensating the reactive power of all upstream high-voltage circuits and transformers. The advantages include: reactive power compensation is engaged only when the associated electrical equipment is in operation, and disengaged when the low-voltage switchgear is de-energized, which reduces reactive power flow in the distribution network and transformers and minimizes reactive power losses in the grid. It also allows for smaller conductor cross-sections and reduced transformer ratings, resulting in a more compact footprint. However, the drawbacks are low utilization efficiency, high capital investment, and unsuitability for applications involving variable-speed operation, forward rotation, jog starting, stall conditions, or semi-regenerative braking motors.

Low-voltage switchgear Centralized Compensation

Centralized compensation in low-voltage switchgear involves connecting low-voltage capacitors to the low-voltage busbar of a distribution transformer via low-voltage switches, with a reactive-power compensation switching device serving as the control and protection element. The switching of the capacitors is directly controlled based on the reactive power on the low-voltage busbar. Capacitor switching is performed on a group-by-group basis and does not allow for continuous, stepless adjustment. The advantages of low-voltage switchgear compensation include simple wiring, minimal operation and maintenance, effective reactive-power balancing, improved utilization of distribution capacity, reduced network losses, and high cost-effectiveness; it is currently one of the most commonly used methods for reactive-power compensation.