Basic knowledge of reactive power compensation and harmonic control

What are the effects of reactive power

Increase the capacity of the equipment.

Increase equipment and line losses.

The voltage drop of the line and transformer will increase. If it is the impulse reactive power load, the voltage will fluctuate violently, and the power supply quality will be seriously reduced.

What is reactive compensation

A large number of loads in power system are electrical inductive, so we call the load absorbing inductive reactive power as "reactive load", while the equipment absorbing capacitive reactive power is called "reactive power". Reactive power compensation is to absorb or supply moderately variable reactive power to improve the power supply quality of AC power system.

Most network components consume reactive power, and most loads also need to consume reactive power. The reactive power required for network elements and loads must be obtained from somewhere in the network. Obviously, it is unreasonable that these reactive power is provided by the generator and transmitted over a long distance, and it is not usually possible. The reasonable method is to generate reactive power where the reactive power is needed, which is reactive compensation.

There are several common methods of reactive compensation

Synchronous compensation machine

synchronous motor

Synchronous generator

Shunt capacitor

Static var compensator

Static var generator

What are the effects of reactive compensation

Improve power factor of power supply system and load, reduce equipment capacity and reduce power loss.

Stabilize the voltage of the receiving end and the power grid, and improve the power supply quality. The dynamic reactive power compensation device is set in the appropriate place of long distance transmission line, which can improve the stability of transmission system and improve the transmission capacity.

In the case of unbalanced load of three-phase such as EAF steelmaking and electrified railway, the active and reactive load of three phases can be balanced by proper reactive compensation.

What is synchronous compensation machine

Synchronous compensation machine is also called synchronous camera, which is actually a synchronous motor which is running without mechanical load and no-load.

What is synchronous motor

When the synchronous motor is over excited, it will generate reactive power, which is equivalent to reactive power supply; under excitation operation, it absorbs reactive power, which is equivalent to reactive load. The load of the power grid is usually inductive, so the synchronous motor is usually operated under the normal excitation or over excitation condition to improve the power factor of the power grid.

What is synchronous generator

Generator is not only the active power, but also the most basic reactive power supply.

What is a parallel capacitor

Parallel capacitors are widely used to improve the power factor of load, and it is an important reactive power compensation equipment in power system. The main disadvantages of parallel capacitors are: one is low voltage regulation effect, the other is that the reactive power with hysteresis (inductive) can not be adjusted continuously.

What is static reactive power compensation device

The adjustable reactive power compensation device using power electronic technology. Its important characteristic is to adjust the reactive power of the compensation device.

What is static var generator

Adjustable reactive power generator with power electronic technology. It can make it generate reactive power, and it is capacitance, and it can absorb reactive power and present electrical sensitivity. PWM control can make the input current close to sine wave.

There are several ways of reactive compensation

Centralized compensation

Decentralized local compensation

Local compensation of single machine

What is centralized compensation

The compensation capacitor is directly connected to the high voltage or low voltage bus of the substation, and the reactive power within the power supply range of the substation is compensated, which is called the centralized compensation mode.

What is decentralized local compensation

The capacitor is connected to the bus of power box in workshop, and reactive power compensation is made for the motor and other reactive load nearby. Compared with the centralized compensation in the main distribution room, it is called decentralized local compensation, which can be regarded as a local compensation method.

What is the local compensation of single machine

The capacitor is placed near the motor, and the single local compensation method is called the single unit local compensation mode.

How harmonic is produced

Some loads present non linear impedance to AC. At the base frequency, the current is not a sine wave and there is distortion. If it is expressed in Fourier analysis, the current waveform contains some harmonic components. The loads with high harmonic are static power converter (rectifier and inverter), various power sources, arc furnace and SCR AC power controller.

There are several ways to suppress harmonic

Filtering compensation method

The method of transforming harmonic generation source

Try to improve the number of phases of the main harmonic source in power system, namely rectifier.

High power factor rectifier is used.

What is LC filter

LC filter is also a passive filter, which is a traditional and common method to suppress filtering. It is a filter device which is composed of filter capacitor, reactor and resistor, which is used in parallel with harmonic source. This method can compensate harmonic and reactive power, and it has been widely used because of its simple structure. The main disadvantage of this method is that the compensation characteristics are affected by the impedance and operation state of the power grid, which makes the parallel resonance easily with the system, which leads to harmonic amplification, overload and even burn down of LC filter.

What kinds of LC filters are

Single tuning filter

High pass filter

Double tuned filter

What is static reactive power compensation device

Static reactive compensation device (SVC) refers to the reactive power compensation device without moving parts. Because of the excellent performance of the static reactive power compensation device using thyristor, in recent years, its market has been growing rapidly and stably in the world, and has occupied the leading position of static reactive power compensation device. Therefore, SVC is often used to refer to static reactive power compensation devices using thyristors, including thyristor control reactor (TCR), thyristor switching capacitor (TSC), and the hybrid device (TCR + TSC) of both, or the hybrid device of thyristor control reactor and fixed capacitor (TCR + FC), thyristor control reactor and mechanical switching capacitor (TCR + MSc) And so on.

What is static var generator

Static var generator (SVG) is also a kind of power electronic compensation device. Unlike static var compensation device (SVC), it is not necessary to use large capacity reactor, capacitor and other energy storage elements to achieve reactive power compensation. It is generally referred to the dynamic reactive power compensation device of self commutation power semiconductor bridge converter.

What is APF

APF (activepowerfilter) is also a kind of power electronic compensation device. The basic principle is to detect the harmonic current from the compensation object, and the compensation device generates a compensation current equal to the harmonic current and opposite polarity, so that the current of the power grid only contains the basic wave component. The filter can track and compensate the harmonic with the frequency and amplitude change, and the compensation characteristics are not affected by the impedance of the power grid.

What is TCR

TCR (thyristorcontrolledeactor) is a static reactive power compensation device with thyristor switching reactor. Because the TCR can only absorb the reactive power of the inductive, it is often used in combination with the parallel capacitor. After parallel capacitor, the total reactive power is net reactive power after TCR and shunt capacitor reactive power are offset.

Features: the reactive power of the compensation device can be continuously adjusted, and harmonic generation is produced. It is generally used with TSC or FC filter.

What is TSC

TSC (thyristorswitchedcapacitor) is a static reactive compensation device for thyristor switching capacitor. The switch of capacitor is thyristor.

Features: the reactive power of the compensation device can be adjusted intermittently, and no harmonic generation can be generated. It can be used separately or matched with FC filter.

What is SR

Sr (saturedreactor) is a saturable reactor and an early static reactive power compensation device.

What is TCT

TCT (thyristorcon controller dtransformer) is a static reactive power compensation device of thyristor control transformer, and it is a deformation of TCR.

What is MSc

MSc (mechanicalyswitchedcapacitor) is a mechanical switching capacitor, and the switch of capacitor is mechanical circuit breaker.

What is FC

FC (fixedcapacitor) is a fixed capacitor, which is generally used in combination with reactor to form a multi loop LC filter.