With a large number of new energy sources such as solar energy, wind energy and biomass energy being connected to the distribution network in the form of distributed generation, microgrid, small and medium-sized power stations (including energy storage power stations and electric vehicle charging stations), the smart grid under the new situation is facing many new problems. Figure 1 describes the power quality control structure under the smart grid architecture, which is mainly composed of distributed power generation, transmission and distribution network, electricity load, power quality compensator, etc. On the one hand, as the core power of new energy access, a large number of power electronic conversion equipment is connected, which makes the power quality of transmission and distribution network present new characteristics and new problems to be solved; on the other hand, the diversity, nonlinearity and impact of power side load are increasing day by day, and the efficient use of power is imminent. These new problems bring opportunities and challenges to power quality control technology. As the core of the smart grid, the microgrid is a nonlinear complex system coupled with a variety of energy sources, and its internal distributed generation has the characteristics of intermittency, complexity, diversity and instability, and its power quality presents new problems and new features are becoming increasingly prominent. Therefore, in order to ensure the safe and stable operation of the distribution network under the condition of microgrid access, one of the key problems that need to be studied and solved is the power quality problem.

1. Classification of power quality compensator

Power quality compensation control technology can be divided into active control technology and passive control technology. Figure 2 for different power quality problems, the corresponding compensation device classification introduction. Passive control technology is to suppress or control power quality problems such as harmonics, reactive power and three-phase imbalance by connecting additional power electronic compensators in parallel or in series. The compensation devices mainly include passive power filter (PPF), active power filter (APF), hybrid active power filter (HAPF), reactive power compensator, dynamic voltage restorer (DVR), power quality comprehensive regulator (UPQC), etc. Among them, the power quality compensator based on modular multilevel converter (MMC) is becoming the research hotspot and future trend of medium and high voltage power quality management technology because of its low voltage modular cascade structure. Active control technology is the power equipment or distributed power supply by changing its own input or output impedance characteristics to take into account the power quality management function. The active power quality control technology can not only improve the power utilization rate, but also improve the overall power quality of the system without adding additional compensators.

2, power quality compensator control method.

At present, the power quality compensator uses voltage source or current source converter. Commonly used compensator current control methods are: hysteresis control, dead beat control, model predictive control, proportional integral (PI) control, proportional resonance (PR) control, repetitive control and nonlinear robust control. In addition, by improving the conventional current control, the control performance of the single current control mode can be improved. For example, the control method combining conventional PI and vector PI can simplify the harmonic detection link, and the harmonic frequency division compensation method, compared with the traditional full-band compensation method, improves the detection accuracy and compensation accuracy of each harmonic, and is especially suitable for various high and low voltage hybrid active filter devices.

3. Power quality analysis and control of large distributed power plants

With the increase of the permeability of photovoltaic, wind power and other large-scale distributed power stations (10 kV ~ 35 kV), the interaction coupling between the harmonics generated by the distributed power station system mainly composed of multiple inverters and the transmission and distribution system is becoming more and more complex. The harmonic output of distributed power station shows the characteristics of high frequency and wide frequency domain. Figure 3 shows the relationship between the resonant amplification factor of a typical distributed power station and the number of harmonics and the transmission distance. During the propagation of harmonics in the transmission network, the resonant amplification of current and voltage will be generated due to the influence of factors such as the distributed capacitance in the transmission line and the background harmonic voltage. There are two governance schemes to suppress the series-parallel resonance of broadband domain harmonics in the transmission network, namely: changing the transmission network parameters to eliminate resonance through parallel reactors; installing high-voltage hybrid active filter devices to reduce the harmonic current content flowing into the grid.