First, the concept of power

power is the amount of work done per unit time or the amount of energy converted.

1, apparent power: apparent power refers to the total power emitted by the generator, which can be divided into active part and reactive part.

2, active power: active power is the electrical power required to maintain the normal operation of electrical equipment, that is, the electrical power to convert electrical energy into other forms of energy (mechanical energy, light energy, heat energy).

3, reactive power: is used for the exchange of electric and magnetic fields in the circuit, and used to establish and maintain magnetic fields in electrical equipment, it does not work externally, but is converted into other forms of energy. All electrical equipment with electromagnetic coils, to establish a magnetic field, reactive power must be consumed. Reactive power does not work, but to ensure the transmission of active power must first meet the reactive power of the grid.

Second, the need for reactive power compensation reasons

Under normal circumstances, electrical equipment not only needs to obtain active power from the power supply, but also needs to obtain reactive power from the power supply. If the reactive power supply in the power grid is in short supply, the electrical equipment does not have enough reactive power to establish a normal electromagnetic field, and these electrical equipment can not maintain the rated condition, and the terminal voltage of the electrical equipment will drop, thus affecting the normal operation of the electrical equipment.

However, the reactive power supplied from the generator and high-voltage transmission line is far from meeting the needs of the load, so some reactive power compensation devices should be set up in the grid to supplement the reactive power to ensure that the user needs the reactive power, so that the electrical equipment can work at the rated voltage. Reactive power compensation is to connect the device with capacitive power load and inductive power load in the same circuit, and the energy is exchanged between the two loads, so that the reactive power required by the inductive load can be compensated by the reactive power output of the capacitive load.

Third, the general method of reactive power compensation

There are three main methods for reactive power compensation: low pressure individual compensation, low pressure concentrated compensation and high pressure concentrated compensation. The following briefly introduces the scope of application of the three compensation methods and the advantages and disadvantages of using this compensation method.

1, low pressure individual compensation

Low voltage individual compensation is based on the demand of individual electrical equipment for reactive power will be a single or more low-voltage capacitor bank dispersed and electrical equipment and connected, it and electrical equipment share a set of circuit breakers, through the control, protection device and the motor at the same time switching. Random compensation is suitable for compensating the reactive power consumption of individual large capacity and connected operation (such as large and medium-sized asynchronous motors), mainly compensating the reactive power of excitation. The advantages of low-voltage individual compensation are: when the electrical equipment is running, the reactive power compensation is invested, and when the electrical equipment is stopped, the compensation equipment is also withdrawn, so it will not cause reactive power backward, with less investment, small space occupation, easy installation, convenient and flexible configuration, simple maintenance, and low accident rate.

2, low pressure concentrated compensation

Low voltage centralized compensation refers to the low voltage capacitor connected to the low voltage bus side of the distribution transformer through the low voltage switch, and the reactive power compensation switching device is used as the control and protection device, and the switching of the capacitor is directly controlled according to the reactive power load on the low voltage bus. The switching of the capacitor is carried out in a whole group, and smooth adjustment cannot be achieved. The advantages of low voltage compensation: simple wiring, small operation and maintenance workload, so that reactive power in place balance, so as to improve the utilization rate of distribution, reduce network loss, with high economy, is currently one of the commonly used means of reactive power compensation.

3, high pressure concentrated compensation

High voltage centralized compensation refers to the compensation method that the shunt capacitor bank is directly installed on the 6 ~ 10KV high voltage bus of the substation. When the user is far away from the substation at the end of the power supply line and the user has a certain high voltage load, the reactive power consumption of the power system can be reduced and can play a certain compensation role; The compensation device automatically switches according to the size of the load, and the compensation benefit is high.

Iv. Classification of reactive power compensation devices

Compensation can be divided into load compensation and line compensation from the range of compensation, and can be divided into inductive compensation and capacitive compensation from the nature of compensation. The methods of shunt capacitive compensation are listed below:

1.synchronous adjustment camera

The basic principle of the modulation camera is no different from that of the synchronous generator, which only outputs reactive current. Because it does not generate electricity, it does not need to drag the prime mover, and the adjusting camera without starting the motor has no shaft extension, which is essentially equivalent to a synchronous generator idling in the grid.

The adjusting camera is the earliest reactive power compensation device used in the power grid. When the excitation current is increased, its output capacitive reactive current increases. When the excitation current is reduced, the output capacitive reactive current is reduced. When the exciting electric field is reduced to a certain extent, the output reactive current is zero, and only a small active current is used to make up for the loss of the adjusting camera. When the exciting current is further reduced, the inductive reactive current is output.

The adjustable camera has a large capacity, is not sensitive to harmonics, and automatically increases the output reactive current when the grid voltage drops, so the adjustable camera plays an irreplaceable role in the reactive power safety of the grid.

Due to the high price, low efficiency and high operating cost of the adjusting camera, it has been gradually replaced by shunt capacitors. However, in recent years, due to the importance of reactive power safety in the power grid, some people advocate the re-use of the adjusting camera.

2, parallel capacitor

Shunt capacitor is the main reactive power compensation method at present. Its main characteristics are low price, high efficiency, low operating costs, and high reliability in the case of perfect protection.

In high-voltage and medium-voltage systems, fixed-connected shunt capacitor banks are mainly used, while in low-voltage distribution systems, automatic reactive power compensation devices that automatically control capacitor switching are mainly used. The structure of automatic reactive power compensation device is varied and suitable for a variety of different load conditions. The low voltage automatic reactive power compensation device will be described in detail.

The main disadvantage of shunt capacitors is their sensitivity to harmonics. When the power grid contains harmonics, the current of the capacitor will increase sharply, and it will resonate with the inductive components in the power grid to amplify the harmonics. In addition, the shunt capacitor is a constant impedance component, and its output reactive power decreases when the power grid voltage drops, so it is not conducive to the reactive power safety of the power grid.

3. SVC

The full name of SVC is static reactive power compensation device, and the two words static are corresponding to the rotation of the synchronous adjustment camera.

The International Big Grid Conference defines SVC as seven subcategories:

a, mechanical switching capacitor (MSC)

b, mechanical switching reactor (MSR)

c, self-saturated reactor (SR)

d, thyristor control reactor (TCR)

e, thyristor switching capacitor (TCR)

f, Thyristor (TSC)

g, self-commutation or grid commutation converter (SCC/LCC)

According to the above subcategories, we can see: except for the adjustable camera, the reactive power compensation devices with inductors or capacitors are almost defined as SVC. Therefore, at present, a large number of materials or advertisements appear in the word "SVC", the reason is no more than two: one is that the author does not understand the definition of SVC, and the other is to confuse the letter combination that ordinary people do not understand.

At present, the main products of SVC advertised in the domestic market are thyristor control reactor (TCR) and thyristor switching capacitor (TSC). As for TSC, we will discuss it in another article, and only briefly introduce the thyristor controlled reactor (TCR) here.

The basic structure of TCR consists of a set of capacitors connected in parallel in the line and a set of thyristor controlled reactors connected in parallel in the line, and the capacity of the reactor is usually designed to be the same as that of the capacitor. Since the reactor is controlled by a thyristor, its inductive reactive current can change. When the thyristor is turned off, the reactor has no current, while the capacitor is fixed, so the compensation amount of the whole device is the largest. When the conduction Angle of the thyristor is adjusted, the inductive current of the reactor will offset part of the capacitor current, so the compensation amount is reduced. The smaller the compensation amount, when the thyristor is fully passed, the reactor current will cancel out the capacitor current, and the compensation amount is 0. The fixed capacitor bank must be designed as a filter or equipped with another filter.

In summary, it can be seen that the structure of TCR is complex and the loss is large, but it has the characteristics of continuous adjustable compensation amount, and it is also applied in high pressure systems.

4. STATCOM

STATCOM is a device that uses fully controlled high-speed power electronic devices such as IGBT, GTO or SIT as switches to control current. Its basic working principle is as follows:

By detecting the electrical parameters of the system, a sinusoidal current waveform with appropriate amplitude and the same phase as the power supply voltage is predicted. When the instantaneous current of the system is greater than the predicted current, the STATCOM will absorb the part greater than the predicted current and store it in the internal energy storage capacitor. When the instantaneous current of the system is less than the predicted current, STATCOM will release the energy stored in the capacitor to fill the part that is less than the predicted current, so that the compensated current becomes a sine wave in phase with the voltage.

According to the working principle of STATCOM, theoretically STATCOM can realize real dynamic compensation, which can be applied not only in inductive load situations, but also in capacitive load situations. And can be harmonic filter, play the role of filter. However, the actual STATCOM cannot meet the theoretical requirements due to technical reasons, and because the switch operating frequency is not high enough, it will also output harmonics to the power grid.

The structure of STATCOM is very complex, expensive, poor reliability, and large loss. At present, STATCOM is still in the research and trial stage, and has no practical application value. The compensation of reactor (TSR) can be divided into series compensation and parallel compensation.

5. The advantages of reactive power compensation

1.According to the power factor of the electrical equipment, the power loss of the transmission line can be calculated. Through the field technical transformation, the power factor below the standard requirements can be reached and the purpose of power saving can be realized.

2.the use of reactive power compensation technology to improve the power factor of low-voltage power grid and electrical equipment is an important measure to save power.

3.Reactive power compensation, it is the use of reactive power compensation equipment jacquard necessary reactive power, in order to improve the power factor of the system, reduce energy consumption, improve the power grid voltage quality, stable equipment operation.

4.Reduce power loss, general factory power wiring according to different lines and load conditions, the power loss of about 20%-30%, the use of capacitors to improve the power factor, the total wave is reduced, can reduce the power supply flow and power loss.

5.Improve the quality of power supply, improve the power factor, reduce the total load current and voltage drop, the secondary side of the transformer installed capacitance can improve the power factor to improve the secondary side voltage.

6.Extend the life of the equipment, improve the power factor after the total line current is reduced, so that close to the saturation of transformers, switches and other machinery and equipment and line capacity load is reduced, so you can reduce the temperature rise to increase the life (every 10 degrees Celsius temperature reduction, life can be extended by 1 times).

7.Finally meet the monitoring requirements of the power system for reactive power compensation, eliminate the penalty caused by too low power factor.

8.Reactive power compensation can improve power quality, reduce power loss, tap the potential of power generation equipment, reactive power compensation to reduce the user's electricity expenses, is a less investment, quick effect of energy-saving measures.

9.The influence of reactive power compensation technology on the low-voltage distribution network of electric units and the economic and social benefits brought by improving the power factor, determining the compensation capacity of reactive power, ensuring that the compensation technology is economic, reasonable, safe and reliable, and achieving the purpose of saving electric energy.