How to control the power quality of the smart grid?-Dinghong Electric Power

NEWS

How to control the power quality of the smart grid?

Release time:

2021-01-25

As new energy sources such as solar energy, wind energy, and biomass energy are connected to the distribution network in the form of distributed power generation, microgrids, small and medium-sized power stations (including energy storage power stations, electric vehicle charging stations), etc., a smart grid under the new situation 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, power load, power quality compensator, etc. On the one hand, as the core power of new energy access, the massive access of power electronic conversion equipment makes the power quality of the transmission and distribution network present new features and new problems, which need to be solved urgently; on the other hand, the diversity of the load on the power consumption, Non-linearity and impact are becoming more and more serious, and the efficient use of electric energy is imminent. These new problems have brought opportunities and challenges to power quality control technology. As the core of smart grid, microgrid is a nonlinear complex system that couples multiple energy sources. Its internal distributed power has the characteristics of intermittency, complexity, diversity, and instability. Its power quality presents new problems and New features are increasingly prominent. Therefore, in order to ensure the safe and stable operation of the distribution network when the microgrid is connected, one of the key issues that need to be studied and resolved is the power quality problem.

1. Classification of power quality compensators

Power quality compensation control technology can be divided into active control technology and passive governance technology. Figure 2 classifies and introduces the corresponding compensation devices for different power quality problems. Passive treatment technology is to suppress or treat 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 filters (PPF), Source power filter (APF), hybrid active power filter (HAPF), reactive power compensator, dynamic voltage restorer (DVR), power quality integrated regulator (UPQC), etc. Among them, the power quality compensator based on modular multilevel converter (MMC) is becoming a research hotspot and future trend of medium and high voltage power quality control technology due to its low-voltage modular cascade structure. The active control technology is to take into account the power quality management function by changing its own input or output impedance characteristics of electrical equipment or distributed power sources. 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 an additional compensator.

2. Control method of power quality compensator

At present, power quality compensators mostly use voltage source or current source converters. Commonly used compensator current control methods mainly 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 link; the harmonic frequency division compensation method, compared with the traditional full-band compensation method, improves the detection accuracy and compensation accuracy of each harmonic, especially suitable for Various high and low voltage hybrid active filter devices, etc.

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

With the increase in the penetration rate of large-scale distributed power stations (10 kV~35 kV) such as photovoltaics and wind energy, the interaction between the harmonics generated by the distributed power station system mainly composed of multiple inverters and the transmission and distribution system has also increased. It's getting more complicated. The harmonics output by the distributed power station show the characteristics of high frequency and wide frequency domain. Figure 3 shows the relationship between the resonance amplification factor of a typical distributed power station and the harmonic order and transmission distance. When the harmonics propagate in the transmission grid, they are affected by the distributed capacitance in the transmission line and the background harmonic voltage and other factors, which will cause the resonance amplification of current and voltage. There are two solutions to suppress the series-parallel resonance problem of wide-frequency domain harmonics in the transmission network, namely: changing the parameters of the transmission network, and eliminating resonance through parallel reactors; installing high-voltage hybrid active filter devices to reduce the flow into the grid Harmonic current content.