The harmonic in power grid is mainly composed of large capacity power converter and other nonlinear loads, the main harmonic sources is all kinds of power electronic devices, such as rectifier device, ac voltage regulation device, among them, the proportion of large rectifier device, it is almost always used with a capacitor filter diode rectifier control and not thyristor phased rectifier, Their harmonic pollution and reactive power consumption are well known. In addition to the rectification device, chopping and the application of inverter device is very much also, and the input dc power also comes from the rectifying device, thus the harmonic problem is very serious, especially by the dc voltage source power chopper and inverter device, the dc voltage source is mostly by the diode is not controlled rectifier and capacitor filter, this kind of device for grid harmonic pollution becomes more and more prominent.
In order to suppress harmonics in power grid and reduce the harm of harmonics, while strengthening scientific and legal management, active and effective technical measures should be taken to reduce harmonics content of power electronic equipment as much as possible, and effective filtering devices should be installed.
1. Take active measures to reduce harmonic content of power electronic equipment
The reduction of harmonic content is mainly from the converter itself, through the structural design of the converter and the addition of auxiliary control strategy to reduce or eliminate harmonic. The main technologies currently used are:
1) Multi-pulse converter Technology For high-power power electronic devices, the original 6-pulse converter is often designed as a 12-pulse converter or a 24-pulse converter to reduce the harmonic current content on the AC side.
2) The basic idea of pulse width modulation technology is to control each conversion moment of PWM output waveform to ensure the symmetry of a quarter of the waveform. The harmonic amplitude to be eliminated is zero and the fundamental amplitude is the given quantity, so as to eliminate the designated harmonic and control the fundamental amplitude.
3) Multi-level converter technology for various power electronic converters adopts multiple phase-shifting methods, sequential control and asymmetric control methods to multiply square wave current or voltage, so that the current or voltage generated by the converter on the AC power grid side is nearly sinusoidal step wave, and maintains a certain phase relationship with the power supply voltage.
2. Install power filters to improve filtering performance
(1) Passive power filter
Passive power filter (PPF) is composed of LC tuning circuit by capacitor and reactor, which can provide parallel low resistance path for harmonics in the system and play a role of filtering. At the same time, the reactive power can be compensated and the power factor can be improved. Due to its simple structure, low cost, low operating loss and low technical requirements, PPF has become a common device to improve power quality. However, due to the reasons of structure and principle, passive filter device to solve the harmonic problem also has some difficult to overcome the shortcomings, such as: can only filter specific harmonic, harmonic compensation band is narrow, overload capacity is small, the system impedance and frequency changes of poor adaptability, poor stability, large volume, large loss and so on.
(2) Active power filter
It can eliminate harmonics by detecting harmonic current in the power grid, and then controlling the inverter circuit to generate the corresponding compensating current component and inject it into the power grid. APF can be divided into series type, parallel type and series-parallel hybrid type according to the different connection mode with the system. Parallel APF is mainly suitable for harmonic compensation of inductive current source load, series APF is mainly used to eliminate the influence of voltage type harmonic source load such as diode rectifier circuit with capacitor on the system, series-parallel APF has the functions of series and parallel APF.
The increasingly serious harmonic pollution has aroused great attention from all sides. With the further understanding of harmonics, more effective methods will be found to suppress and eliminate harmonics, and more reasonable harmonics management standards will be established. In order to achieve better harmonic suppression effect, different harmonic source load should adopt the corresponding structure of the filter device, such as cascade high power APF, intelligent APF based on DSP and other research marks that low loss, high power, high frequency, intelligent APF is its development direction.