The harmonics in the power grid are mainly generated by various large-capacity power converters and other nonlinear loads. The main harmonic sources are various power electronic devices, such as rectifiers and AC voltage regulators. Among them, rectification Devices account for a large proportion. Almost all of them use diode uncontrolled rectification with capacitor filtering or thyristor phase-controlled rectification. The harmonic pollution and reactive power consumed by them are well known; in addition to rectifier devices, chopping and There are many applications of inverter devices, and the input DC power source also comes from the rectifier device, so the harmonic problem is also very serious, especially the chopper and inverter devices powered by a DC voltage source, whose DC voltage source is mostly diodes. After uncontrolled rectification, it is obtained by capacitor filtering, and the harmonic pollution of such devices to the power grid is becoming increasingly prominent.
In order to suppress harmonics in the power grid and reduce the harm of harmonics, while strengthening scientific and legal management, active and effective technical measures must be taken to minimize the harmonic content of power electronic equipment and install effective filtering devices.
1. Take active measures to reduce the harmonic content of power electronic equipment
The reduction of harmonic content is mainly based on the converter device itself, through the structure design of the converter device and the addition of auxiliary control strategies to reduce or eliminate harmonics. The current technologies mainly include:
1) Multi-pulse converter technology For high-power power electronic devices, the original 6-pulse converter is often designed as a 12-pulse or 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 switching moment of the PWM output waveform to ensure the symmetry of the quarter waveform. The amplitude of the harmonic to be eliminated is zero, and the amplitude of the fundamental wave is a given amount, so as to achieve the purpose of eliminating the specified harmonic and controlling the amplitude of the fundamental wave.
3) Multi-level converter technology adopts phase-shifting multiple methods, sequence control and asymmetric control multiple methods for various power electronic converters to superimpose square wave currents or voltages so that the converters are generated on the AC grid side The current or voltage is a step wave close to sinusoidal, 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 the use of capacitors and reactors to form an LC tuning circuit, which can provide a parallel low-resistance path for harmonics in the system and play a filtering role; at the same time, the use of capacitors can also compensate for reactive power and improve the grid The power factor. Due to its simple structure, low cost, low operating loss and low technical requirements, PPF has become a common equipment for improving power quality. However, due to structural and theoretical reasons, the use of passive filter devices to solve the harmonic problem also has some insurmountable shortcomings, such as: it can only filter out specific sub-harmonics, the harmonic compensation band is narrow, and the overload capacity is small. The system impedance and frequency change have poor adaptability, poor stability, large volume, large loss, etc.
(2) Active power filter
It detects the harmonic current in the power grid, and then controls the inverter circuit to generate the corresponding compensation current component and inject it into the power grid to achieve the purpose of eliminating harmonics. APF can be divided into series type, parallel type and series-parallel hybrid type according to the connection mode with the system. Parallel type APF is mainly suitable for harmonic compensation of inductive current source loads. Series type APF is mainly used to eliminate the influence of voltage type harmonic source loads such as diode rectifier circuits with capacitors on the system. Series-parallel type APF has both series and parallel. The function of APF.
The increasingly serious harmonic pollution has attracted great attention from all aspects. With the further understanding of harmonic phenomena, more effective methods will be found to suppress and eliminate harmonics, and it will also help to formulate more reasonable harmonic management standards. In order to better achieve the effect of suppressing harmonics, filter devices with corresponding structures should be used for different harmonic source loads, such as cascaded high-power APF, DSP-based intelligent APF, etc. The research indicates low loss and large Power, high frequency, intelligent APF is its development direction.