NEWS

With the large-scale integration of new energy sources such as solar energy, wind energy, and biomass energy into the distribution network in the form of distributed generation, microgrids, and small and medium-sized power stations (including energy storage stations and electric vehicle charging stations), the smart grid under the new situation is facing many new problems. Figure 1 illustrates the power quality control structure under the smart grid architecture, which mainly consists of distributed generation, transmission and distribution network, electricity load, power quality compensator, etc. On the one hand, as the core driving force for the integration of new energy, the extensive integration of power electronic transformation equipment has led to new characteristics and problems in the power quality of the transmission and distribution grid, which urgently need to be solved; On the other hand, the increasing diversity, nonlinearity, and impact of electricity load on the power consumption side make efficient utilization of electricity urgent. These new problems bring opportunities and challenges to power quality control technology. As the core of smart grids, microgrids are nonlinear and complex systems coupled with multiple energy sources. The distributed power sources within them have characteristics such as intermittency, complexity, diversity, and instability. The new problems and characteristics of their power quality are becoming increasingly prominent. Therefore, in order to ensure the safe and stable operation of the distribution network under the connection of microgrids, one of the key issues that urgently needs to be studied and solved is the issue of power quality

1. Classification of power quality compensators

Power quality compensation control technology can be divided into active control technology and passive governance technology. Figure 2 categorizes and introduces the corresponding compensation devices for different power quality issues. Passive governance technology suppresses or addresses power quality issues such as harmonics, reactive power, and three-phase imbalance by connecting additional power electronic compensators in parallel or series. Compensation devices mainly include passive power filters (PPF), active power filters (APF), hybrid active power filters (HAPF), reactive power compensators, dynamic voltage restorers (DVR), and power quality comprehensive regulators (UPQC). Among them, the power quality compensator based on modular multilevel converters (MMC) is becoming a research hotspot and future trend in medium and high voltage power quality control technology due to its low-voltage modular cascade structure. Active control technology, on the other hand, refers to the use of electrical equipment or distributed power sources to balance the power quality management function by changing their input or output impedance characteristics. Active control technology for power quality can not only improve power utilization, but also improve the overall power quality of the system without the need for additional compensators.

2. Control method of power quality compensator

At present, power quality compensators mostly use voltage source or current source converters. The commonly used methods for compensating current control include hysteresis control, deadbeat control, model predictive control, proportional integral (PI) control, proportional resonance (PR) control, repetitive control, and nonlinear robust control. In addition, by improving conventional current control, the control performance of a single current control method can be improved. For example, the control method combining conventional PI and vector PI can simplify the harmonic detection process; Compared with traditional full frequency compensation methods, the harmonic division compensation method improves the detection and compensation accuracy of each harmonic, and is particularly suitable for various high and low voltage hybrid active filtering devices.

3. Analysis and Control of Power Quality in Large Distributed Power Plants

With the increasing penetration rate of large-scale distributed power plants such as photovoltaic and wind power (10 kV~35 kV levels), the interaction and coupling between harmonics generated by distributed power plant systems mainly composed of multiple inverters and transmission and distribution systems have become increasingly complex. The harmonics output by distributed power plants exhibit high-frequency and wide frequency domain characteristics. Figure 3 shows the relationship between the resonant amplification coefficient of a typical distributed power plant and the number of harmonics and transmission distance. During the propagation of harmonics in the transmission network, resonance amplification of current and voltage is generated due to factors such as distributed capacitance in the transmission line and background harmonic voltage. There are two governance schemes to suppress the series parallel resonance problem of wide frequency domain harmonics in the transmission network, namely: changing the transmission network parameters and achieving the goal of eliminating resonance through parallel reactors; Install a high-voltage hybrid active filtering device to reduce the harmonic current content flowing into the power grid.