I. Power of power grid

(I) Classification and definition

1. Reactive power

Reactive power is a kind of power that can neither do active work nor cause loss in the power grid, and it is also indispensable. In the actual power system, asynchronous motors, as the traditional main load, make the power grid generate inductive reactive current; most power electronic devices have a very low power factor, resulting in a large amount of reactive current in the power grid. Reactive current generates reactive power, which brings additional burden to the power grid and affects the power supply quality. Therefore, reactive power compensation (hereinafter referred to as reactive compensation) has become one of the main means to maintain the high-quality operation of the power grid.

2. Active power

Active power is the electrical power required to maintain the normal operation of electrical equipment, that is, the electrical power that converts electrical energy into other forms of energy (mechanical energy, light energy, thermal energy).

(II) The role of reactive power

Under normal circumstances, electrical equipment not only needs to obtain active power from the power supply, but also reactive power from the power supply. If the reactive power in the power grid is insufficient to meet the demand, the electrical equipment will not have enough reactive power to establish a normal electromagnetic field. These electrical equipment cannot maintain the rated working conditions, and the terminal voltage of the electrical equipment will drop, thus affecting the normal operation of the electrical equipment.     However, the reactive power supplied by the generator and the high-voltage transmission line is far from meeting the needs of the load, so some reactive compensation devices should be set up in the power grid to supplement the reactive power to ensure the user's needs for reactive power, so that the electrical equipment can work under the rated voltage.

1. Principle

Connect the device with capacitive power load and the inductive power load in parallel in the same circuit, and the energy is exchanged between the two loads. In this way, the reactive power required by the inductive load can be compensated by the reactive power output by the capacitive load.     However, when determining the reactive compensation capacity, it should be noted that over-compensation should be avoided when the load is light, and the reverse transmission of reactive power will cause an increase in power loss; in addition, the higher the power factor, the smaller the effect of the compensation capacity in reducing the loss will be. Under normal circumstances, increasing the power factor to 0.95 is a reasonable compensation.

2. Principle Analysis

The actual active current is: IR;

The inductive current before compensation is: IL0;

The total line current is: I0;

After connecting the capacitor bank in parallel, the capacitive current is: Ic;

The inductive current of the line after compensation is reduced to: IL;

The total line current after compensation is: I;

If the power factor is to be increased from cosφ1 to cosφ2, the required capacitor bank current is: Ic= IL0 - IL = IR (tgφ1-tgφ2) That is: Q=P (tgφ1-tgφ2)

(II) Compensation methods for reactive power compensation

1. Centralized compensation

The capacitor bank  is centrally installed on the primary or secondary busbar of the substation, and an automatic control device is installed to enable it to be automatically switched with the change of load. When the capacitor bank is connected to the primary side of the transformer, the line loss can be reduced and the primary bus voltage can be increased, but there is no compensation effect on the transformer and its secondary side, and the installation cost is high; when the capacitor bank is installed on the secondary side of the transformer, the transformer can increase its output, increase the secondary voltage, expand the compensation range, and the installation, operation and maintenance costs are low.

Advantages: The utilization rate of the capacitor bank is high, the management is convenient, and it can reduce the reactive load of the power line and the main transformer of the substation.

Disadvantages: It cannot reduce the reactive load of the low-voltage network and the high-voltage distribution line, and a special room needs to be built separately. Industrial and mining enterprises currently use centralized compensation.

2. Group compensation

All capacitor banks are installed on the high-voltage side bus of each distribution user with a low power factor, and can be put into or removed at the same time as the change of part of the load.    When group compensation is used, the compensated reactive power is no longer transmitted through the lines above the trunk line, thereby reducing the reactive loss on the distribution transformer and the trunk line. Therefore, group compensation has a significant benefit in reducing losses and saving electricity than centralized compensation. This compensation method has a wider compensation range and better effect, but the equipment investment is large and the utilization rate is not high. It is generally suitable for places with small compensation capacity, many and scattered electrical equipment, and some places with large compensation capacity.

Advantages: The utilization rate of capacitor banks is higher than that of single on-site compensation, which can reduce the reactive load in high-voltage power supply lines and transformers.

Disadvantages: It cannot reduce the reactive load of trunk lines and branch lines, the operation is not convenient, and the initial investment is large.

3. Individual compensation

That is, individual compensation for large-capacity electrical equipment with particularly poor power factor and loads with large reactive compensation capacity, or electrical equipment powered by longer lines. The capacitor bank is directly installed in the same electrical circuit of the electrical equipment and switched on and off at the same time as the electrical equipment. The motor in the figure is also a discharge device for the capacitor bank.

The non-functional on-site compensation consumed by the electrical equipment can balance the reactive current on-site, but the utilization rate of the capacitor bank is low. It is generally suitable for compensation of electrical equipment such as high- and low-voltage motors with large capacity.

Advantages: The compensation effect is the best.

Disadvantages: The capacitor bank will work and stop with the electrical equipment, so the utilization rate is low, the investment is large, and the management is inconvenient.

4. Principles for configuring compensation capacity: comprehensive planning, reasonable layout, hierarchical compensation, and local balance.