1.Standard requirement of reactive power for power quality of grid
The national standard GB12325-90 “Allowable Deviation of Power Quality Supply Voltage” has specified the principle of reactive power compensation for each voltage level of power grid. According to the standard, reactive power should be compensated locally as far as possible to reduce reactive power flow on the grid. For distribution lines of 10kV and below, the voltage network of the common distribution transformer can be properly dispersed and installed with 0.4kV low-voltage capacitor banks that can be switched, so that the line can meet the compensation requirements at the maximum load and no reactive power back-off occurs at the minimum load. For allowable deviation of power supply voltage, the sum of absolute value of positive and negative deviation of power supply voltage of 35kV and above shall not exceed 10% of rated voltage, the allowable deviation of three-phase power supply voltage of 10kV and below shall be 7% of rated voltage, and the allowable deviation of 220V single-phase power supply voltage shall be +7% and -10% of rated voltage. Voltage deviation is defined as the difference between the actual voltage and the rated voltage when the voltage change rate is less than 1% per second.
State department standard SD325-89 “electric power system voltage and reactive power technical guideline (try out)” regulation, reactive power and reactive power load of the power system should be used when peak or trough points (voltage) layer and the (electric) area basic balance principle, configuration and operation, to avoid the long lines or multistage transmit reactive power transformer.
2.Reactive power requirements for grid loads
Reactive power is very important to the operation of power supply system and load. Most of the load in the distribution system is inductive, such as for ventilation, air conditioning, water pump asynchronous motor, many fluorescent lamps used for lighting, as well as for communication and computer system rectifier, electronic voltage regulation power supply. Among them, motors, fluorescent lamps and other loads as well as transformers and reactors must consume reactive power to work normally, which is determined by their own electrical characteristics; And frequency converter, rectifier and electronic regulated power supply and other power electronic device usually adopt phased approach, the ac current often lags behind the voltage, they not only should consume large amounts of reactive power, but also produce a large amount of harmonic current, even simple diode rectifier device, although the basic phase ac voltage current, but as a result of current waveform distortion, The large amount of harmonic current generated also consumes reactive power. In order to transmit active power, a certain phase difference between the voltage at the transmitting end and the voltage at the receiving end is required, while in order to transmit reactive power, a difference between the voltage at the two ends is required, which can only be achieved within a very narrow range. Obviously, it would be unreasonable and often impossible for all this reactive power to be supplied by generators and transmitted over long distances.
If the reactive power in the power grid is in short supply, the electrical equipment will not have enough reactive power to establish a normal electromagnetic field, then these electrical equipment can not maintain the rated condition of work, the terminal voltage of the electrical equipment will drop, thus affecting the normal operation of the electrical equipment. For example, the electromagnetic torque of an asynchronous motor running in a power system is proportional to the square of its terminal voltage. When the voltage is reduced by 10%, the electromagnetic torque is reduced by approximately 19%. At this time, if the resistance of the mechanical load driven by the motor is unchanged, the slip rate of the motor will increase, the heating will increase, and the temperature of the winding will increase, which accelerates the aging of the insulating medium and greatly affects its service life.
3.Reactive power quality problems of existing power grid
The influence of reactive power on power grid is mainly in the following aspects.
(1) Increase the capacity of equipment The increase of reactive power will lead to the increase of current and apparent power, so as to increase the capacity of generators, transformers and other electrical equipment and wire capacity. At the same time, the size and specification of power users’ starting and controlling equipment and measuring instruments should also be increased.
(2) Equipment and line loss increase The increase of reactive power, so that the total current increases, so that the loss of equipment and line increases.
(3) the voltage drop in the line and transformer increase If is impact of reactive power load, but also can make the sharp fluctuations in voltage, reduce the power supply quality, in recent years, with the use of the large capacity motor and other perceptual load, the power system reactive power problem there are three new characteristics: one is the increase of demand of reactive power is larger; Second, the fluctuation of reactive power is large; Third, the reactive power problem is often accompanied by harmonic problems. The direct consequence of the traditional reactive power problem to the power system is the reduction of the system voltage. Now the reactive power problem not only brings the voltage reduction problem, but also affects the voltage stability problem. Several large area power outages in recent years in the world are either caused by reactive power problems directly or accompanied by voltage collapse in power outages. In China, the present structure and used to have a lot of different load, the current due to the large increase in the so-called “bad load”, such as air conditioning load as well as the large capacity electric arc furnace and so on, causes the sudden increase to the power grid reactive power demand makes the grid voltage continue to reduce, the traditional reactive compensation devices (parallel capacitor) of reactive power output is greatly reduced, This could drag the grid voltage to the brink of collapse.
In addition, due to the imbalance between the development of power supply and load, the distribution of reactive power compensation capacity in various regions is unreasonable, resulting in excess reactive power compensation in some regions, resulting in high voltage, while in some regions, reactive power compensation is insufficient, the voltage is low, and even high and low voltage coexist in some regions. This is also the result of the current reactive power can not be done in situ compensation, so it is difficult to achieve the reactive power partition, hierarchical balance.
4.Methods of reactive power compensation
The role of reactive power compensation is:
(1) improve the power factor of power supply and electricity system and load, reduce the capacity of equipment, reduce power loss;
(2) Stabilize the voltage of the receiving end and the power grid, improve the quality of power supply, and set the dynamic reactive power compensation device at the appropriate place in the long-distance transmission line can also improve the stability of the transmission system, improve the transmission capacity;
(3) In the three-phase unbalanced situation, through the appropriate reactive power compensation can balance the three-phase active power and reactive power load.
The early reactive power compensation devices are electrostatic capacitors and synchronous compensators, which are mainly used in the high voltage side of the system for centralized compensation. Shunt capacitor compensation is still one of the main compensation methods and has a wide range of applications. The essence of synchronous compensator is synchronous motor. When the excitation current changes, the motor can change the magnitude and direction of the output reactive current smoothly, which is very beneficial to the stable operation of the power system. However, the application of synchronous compensator is limited due to its high cost, complex installation and difficult maintenance. At present, the static reactive power compensator combined with power electronic technology is widely used. In the reactive power compensation technology, the local compensation is the most ideal one, but the load has the characteristics of large dispersion and large quantity. The reactive power compensation device is required to be small in size, low in cost, easy to operate, easy to maintain and install, and must be capable of dynamic compensation. Limited by technology, dynamic compensation has not been well developed in the early stage of reactive power compensation. In the past 30 years, with the development of power semiconductor devices and power electronics application technology, this problem has been solved. Thyristor switching capacitor (TSC) and thyristor controlled reactor (TCR) are the two most widely used devices at present.
Reactive power compensation and power electronics technology are combined in three ways. One is as a switching capacitor. Because of the short response time of power semiconductor switch, it can accurately choose the Angle of capacitor switching, realize zero voltage conduction, avoid the generation of inrush current, and improve the reliability of capacitor use and the stability of power system. The second is as a reactive power output regulation switch. Because of the high switching frequency of power electronic devices, it is convenient to control the conduction Angle of capacitor current, so as to realize the continuous regulation of reactive power and fast track the change of load reactive power. The third is the introduction of power electronic current converter technology, which uses the converter as a reactive power source to compensate the reactive power, such as static switching camera (STATCON) and active filtering (APF).
At present, the commonly used reactive compensation device for static var generator (SVG), its basic principle is to phase bridge type circuit through the reactor or directly on the grid in parallel, properly adjust the ac output voltage of bridge type circuit, phase and amplitude or direct control of the ac current, can make the circuit absorption or meet the requirements of reactive current, Realize the purpose of dynamic reactive power compensation. The essence is to use the voltage difference between the output voltage of the inverter and the voltage of the grid on the output reactance of the inverter to form the required reactive compensation current. By controlling the amplitude of the output voltage of the inverter and its phase relative to the voltage of the grid, the nature and magnitude of the reactive power absorbed by SVG can be controlled. The specific control methods for SVG to generate required reactive current by adjusting the reference value of reactive current can be divided into indirect control and direct control. The former indirectly controls the AC side current of SVG by controlling the phase and amplitude of the AC voltage fundamental wave generated by the SVG converter. The latter is the use of tracking PWM control technology on the inverter output current waveform instantaneous value feedback control, its response speed and control accuracy than indirect control method has been greatly improved. The connection mode between inverter and load can be divided into series compensation and parallel compensation. The former uses the transformer to compensate the series voltage, which is actually equivalent to phase-shifting control voltage source. The latter, in turn, acts as a current source, absorbing capacitive (or inductive) reactive current from the system. At present, the parallel stationary reactive power generator is widely used, and its advantage is easy to control. In addition, multilevel technology is widely used in stationary phase modulators, which greatly increases the capacity of reactive compensators.
As the instantaneous reactive power theory is put forward, there is a used for dynamic suppress harmonic and compensate reactive power of the new power electronic devices – active power filter (APF), its structure and SVG is similar, but the detection signal decomposition and the way of processing, can achieve at the same time for harmonic elimination and reactive power compensation, and both can be realized respectively. Its advantages are that it has a very fast response to the change of the compensation object, can be adjusted continuously and does not need redundant energy storage components, and will not occur resonance problem. However, there are still some problems such as high cost, low compensation capacity, complicated realization and difficult elimination of high harmonics. With the development of power semiconductor devices towards large capacity and high frequency, this kind of devices which can compensate both harmonic and reactive power must have a good development prospect. In addition, the integrated flow controller (UPFC) is a kind of more novel power regulator, it use converter changing string concatenated transformer and superimposed on the phase voltage transmission line voltage, make its amplitude and phase Angle can change continuously, so as to realize on a line of active and reactive power adjustment accurately, and can increase transmission capacity and dampening system oscillation, It has the advantage of flexible operation.