the method and significance of reactive power compensation and power factor

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Time of issue:2021-10-22 09:41
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Reactive power refers to the bias caused by long-term use of two-phase (lighting and use of electric welding machine), that is, one or two phases do not work or do little work. When the reactive power is too high or too low, it will cause high-voltage incoming line current bias, line overheating, electric fuse blown, resulting in loss of phase and damage to the running motor. Reactive power is too high or too low to be fined is a means of the electric power department to control users to use electricity correctly!

The usual remedy is to install a group of reactive power compensation cabinets in the power distribution room. When using one-phase and two-phase power (usually when using multiple welding machines), measure the current and exchange the power connector to make the output voltage uniform. .

Active power is the electrical power required for the normal operation of electrical equipment. Reactive power is by no means useless power. It is very useful. The motor needs to establish and maintain a rotating magnetic field to make the rotor rotate, thereby driving the mechanical movement. The rotor magnetic field of the motor is established by obtaining reactive power from the power supply. The transformer also needs reactive power to make the primary coil of the transformer generate a magnetic field and induce a voltage in the secondary coil. Why is the power of the device being fined for low power: 100 units of power are delivered to the device. However, because most electrical systems have inherent reactive power losses, you can only use 70 units of power. Unfortunately, although only 70 units are used, 100 units have to be paid. In this example, the power factor is 0.7 (if the power factor of most equipment is less than 0.9, it will be fined), this kind of reactive power loss mainly exists in the electrical equipment (also called inductive load). Power factor is a measure of motor performance standard.

This article explains the method and significance of improving power factor, as well as the method of improving power factor in both natural power factor and artificial compensation.

I. Overview
In addition to the active power supply and the reactive power supply in the power supply system, both are indispensable. When the inductive load is too much, the power factor is low, which affects the economic operation of the line and the distribution transformer. Work power compensation equipment to improve the power factor of the system, so as to achieve the purpose of saving energy and reducing losses.

　　1. The relationship between power factor and reactive power
　　 The cosine of the phase difference (φ) between the voltage and the current is called the power factor, which is represented by the symbol cosφ. In numerical terms, the power factor is the ratio of active power to apparent power, that is, cosφ=P/S.
　　 P2+Q2=S2
　　In the operation of the power grid, the power factor reflects the extent to which the apparent power output by the power supply is effectively utilized. What we hope is that the larger the power factor, the better. In this way, the reactive power in the circuit can be reduced to a minimum.
　

　 2. The relationship between active power loss and power factor of transmission and distribution lines
　　 Due to the resistance of the wire, when the current flows through the line, the line itself generates active power loss, and the active power loss is proportional to the square of the current. Therefore, when the line transmits a certain amount of active power, the active power loss generated by the line itself is inversely proportional to the square of the power factor, and increasing the power factor can reduce the active power loss of the line.

　　3. The relationship between the copper loss of the transformer and the power factor
When the transformer is in operation, when a certain amount of active power is output, its copper loss is proportional to the square of the apparent power of the load carried by the transformer, and the apparent power is inversely proportional to the power factor of the transformer. Therefore, increasing the power factor can make the transformer the copper loss decreased.
　　

4. The relationship between the required capacity of the transformer and the power factor
　 Because when the transformer outputs a certain amount of active power, its apparent power is inversely proportional to the power factor of the transformer, so when the transformer outputs a certain amount of active power, the increase of the power factor can reduce the required capacity of the transformer, thereby increasing the power supply capacity of the transformer.
　　

Ⅱ. Improve power factor
　　 One main factor affecting power factor
　　1. A large number of inductive equipment, such as asynchronous motors, induction furnaces and other equipment are the main consumers of reactive power. Among all the reactive power consumed by industrial and mining enterprises, the reactive power consumption of asynchronous motors accounts for 60% to 70%; and the reactive power consumed when asynchronous motors are at no load accounts for 60% of the total reactive power consumption of the motors. ~70%. Therefore, to improve the power factor of the asynchronous motor, it is necessary to prevent the no-load operation of the motor and increase the load rate as much as possible.
2. The reactive power consumed by the transformer is generally about 10% to 15% of its rated capacity, and its no-load reactive power is about 1/3 of the full load. Therefore, in order to improve the power factor of the power system and the enterprise, the transformer is not Should be no-load operation or long-term low-load operation state.
　　3. The power supply voltage exceeding the specified range will also have a great impact on the power factor. When the supply voltage is higher than 10% of the rated value, the reactive power will increase rapidly due to the saturation of the magnetic circuit. When the supply voltage is 110% of the rated value, the general reactive power will increase by about 35%. When the supply voltage is lower than the rated value, the reactive power is also reduced correspondingly to increase their power factor. However, a decrease in the supply voltage will affect the normal operation of electrical equipment.
　　 Two general methods of reactive power compensation
　　1. Take appropriate measures to try to improve the natural power factor of the system
　　Improving the natural power factor does not require any compensation equipment investment, and only adopts various management or technical measures to reduce the reactive power consumed by various electrical equipment. This is the most economical way to increase the power factor:
　　1) Reasonably select the capacity of the motor to make it close to full load operation.
　　2) For the motor whose actual load does not exceed 40% of the rated capacity, it should be replaced with a small-capacity motor.
　　3) Reasonably arrange and adjust the process flow, improve the operation mode of electrical equipment, and limit the no-load operation of the induction motor.
　　4) Choose the correct transformer to increase the load factor of the transformer (generally 75% to 80% is more appropriate). For transformers with a load rate of less than 30%, they should be replaced.
　　5) For the wound rotor asynchronous motor with a load rate of 60% to 90%, it can be synchronized if necessary, and the motor can output reactive power to the power system at this time.
　　2. Manual compensation of power factor
　　 The user's power factor simply cannot meet the requirements by increasing the natural power factor. Therefore, manual compensation must be performed. There are three main methods for reactive power compensation: low-voltage individual compensation, low-voltage centralized compensation, and high-voltage centralized compensation.
　　1) Low-voltage individual compensation
　　 Low-voltage individual compensation is to connect single or multiple low-voltage capacitor banks to the electrical equipment in parallel according to the reactive power requirements of individual electrical equipment. It shares a set of circuit breakers with the electrical equipment. Simultaneous switching with the motor through the control and protection device. Random compensation is suitable for compensating the reactive power consumption of individual large-capacity and continuous operation (such as large and medium-sized asynchronous motors), mainly by compensating the reactive power of excitation. The advantages of low-voltage individual compensation are: reactive power compensation is input when the power-consuming equipment is running, and when the power-consuming equipment is out of operation, the compensation equipment is also launched, so it will not cause reactive power backwards. It has the advantages of low investment, small space occupation, easy installation, convenient and flexible configuration, simple maintenance and low accident rate.
　　2) Low-voltage centralized compensation
　　 Low-voltage centralized compensation refers to connecting low-voltage capacitors to the low-voltage bus side of the distribution transformer through a low-voltage switch, using reactive power compensation switching devices as the control and protection device, and directly controlling the switching of the capacitors according to the reactive load on the low-voltage bus. The switching of the capacitor is carried out in the whole group, and smooth adjustment is not possible. The advantages of low-voltage compensation: simple wiring, small operation and maintenance workload, reactive power balance on the spot, thereby improving the utilization rate of the distribution transformer.
　　3) High-voltage centralized compensation
　　 High-voltage centralized compensation refers to the compensation method in which the parallel capacitor bank is directly installed on the 6-10kV high-voltage bus of the substation. It is suitable for users who are far away from the substation or at the end of the power supply line, and the users themselves have a certain compensation effect; the compensation device is automatically switched on and off according to the size of the load, thereby reasonably improving the power factor of the user and avoiding the reduction of the power factor and the electricity bill. Increase.
　　3. reactive power supply
　　 In addition to the synchronous motor, the reactive power of the power system also has electrostatic capacitors, static var compensators and static var generators. In addition to capacitors, the other types can absorb both capacitive and inductive reactive power.
　　1) Synchronous motor
　　 There are three types of synchronous motors: generators, electric motors and tuned cameras. Synchronous generator is the only active power source and the most basic reactive power source. It mainly operates with lagging power factor and provides reactive power to the system. However, if necessary, the excitation current can also be reduced to make the power factor lead. The so-called "phase advance operation" to absorb the excess reactive power of the system. The synchronous motor is a no-load synchronous motor. The advantage is that it can absorb or supply reactive power to the system under the condition of under-excitation or over-excitation. A synchronous motor equipped with a self-excited device can smoothly adjust the input or output voltage according to the voltage. Work power. However, it has large active power loss, complex operation and maintenance, slow impact, and has gradually withdrawn from the power grid.
　　2) Parallel capacitor
Parallel capacitor compensation is currently the most widely used reactive power source. Since the alternating current passing through the capacitor is just ahead of the voltage on the capacitor plate in phase, in contrast to the lag in the inductance, the parallel capacitor itself consumes very little power. Flexible installation, saving investment; it can improve the power factor by providing reactive power to the system and reduce the reactive power provided by the generator.
　　3) Static var compensator
　　 The static var compensator is composed of a switching reactor and a capacitor controlled by a thyristor. Because the thyristor responds very quickly to the control signal, and the number of on-offs can also be uncontrolled. When the voltage changes, the static compensator can quickly and smoothly adjust to meet the needs of dynamic reactive power compensation, and at the same time, it can also achieve split-phase compensation; it is effective and strong for three-phase unbalanced load and shock load; but because of the thyristor High-order harmonics will be generated during the switching process of the control reactor. For this, a special filter is required.
　　4) Static var generator
Its main body is a voltage source inverter, which is properly switched on and off by the switchable thyristor, which converts the DC voltage on the capacitor into a three-phase AC voltage synchronized with the voltage of the power system, and then connects to the grid through a reactor and a transformer in parallel . Appropriate control of the inverter's output voltage can flexibly change its operating conditions to make it in a capacitive, inductive or zero-load state. Compared with the static var compensator, the generator has a faster corresponding speed, less harmonic current, and can still inject larger reactive power into the system when the system voltage is low.
　
In summary, improving the power factor plays a role in promoting the country’s energy utilization and the economic benefits of enterprises. It is an indispensable condition for ensuring the power quality and voltage quality of the power system, reducing network losses, and safe operation. It should be adopted according to different situations. Corresponding measures are taken to improve the power factor, reduce reactive power loss, and thereby improve economic benefits.