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, micro-grid, small and medium-sized power stations (including energy storage power stations, electric vehicle charging stations), etc., the smart grid under the new situation Faced with many new problems. Figure 1 depicts the power quality control structure under the smart grid architecture, which is mainly composed of distributed generation, transmission and distribution networks, power load, and power quality compensator. 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 transmission and distribution network present new features and new problems, which need to be solved urgently; on the other hand, the diversity of power-side load, Nonlinearity, impact, etc. are increasing, and efficient use of electric energy 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 with multiple energy sources. The internal distributed power supply has the characteristics of intermittent, complexity, diversity and instability, and its power quality presents new problems. New features are 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 issues that need to be studied and solved is the power quality problem.

Figure 1 Power quality control structure diagram under the smart grid framework

Power quality compensation control technology can be divided into active control technology and passive management technology. Figure 2 classifies the corresponding compensation devices for different power quality problems. Passive management technology suppresses or manages power quality problems such as harmonics, reactive power, and three-phase unbalance by connecting or connecting additional power electronic compensators. 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 use power equipment or distributed power to change the power input quality control function by changing its input or output impedance characteristics. Power quality active control technology not only improves energy efficiency, but also improves overall system power quality without the need for additional compensators.

Figure 2 Classification diagram of power quality compensation controller

At present, power quality compensators mostly use voltage source type or current source type converters. Commonly used compensator current control methods are: hysteresis control, deadbeat control, model predictive control, proportional integral (PI) control, proportional resonance (PR) control, repetitive control and nonlinear robust control. In addition, the control performance of a single current control mode can be improved by improving conventional current control. For example, the combination of conventional PI and vector PI can simplify the harmonic detection link; the harmonic frequency division compensation method improves the detection accuracy and compensation accuracy of each harmonic compared with the traditional full-band compensation method, and is particularly suitable for Various high and low voltage hybrid active filter devices, etc.

With the increase of the permeability of large-scale distributed power plants (10 kV~35 kV grade) such as photovoltaic and wind energy, the interaction between the harmonics generated by the distributed power station system composed of multiple inverters and the transmission and distribution system is also More complicated. The harmonics outputted by the distributed power station exhibit high frequency and wide frequency domain characteristics. Figure 3 shows the relationship between the resonant amplification factor of a typical distributed power station and the harmonic order and transmission distance. Harmonics in the process of transmission in the transmission network, due to the distribution capacitance in the transmission line and the background harmonic voltage and other factors, will generate resonance amplification of current and voltage. There are two kinds of treatment schemes to suppress the series-parallel resonance problem of wide-frequency domain harmonics in the transmission network, namely: changing the parameters of the transmission network, eliminating the resonance by the shunt reactor; installing the high-voltage hybrid active filter device to reduce the flow into the power grid. Harmonic current content.

Fig. 3 Relationship between resonant amplification factor of distributed power station and harmonic order and transmission distance

Distributed power, energy storage, and load access to the microgrid through power electronic converters are one of the main forms of new energy distributed generation. There are two modes of operation in the microgrid: islanding and grid-connected. The power quality problems of microgrid have both commonalities and their own characteristics. The commonality is concentrated on how to effectively solve the wide frequency domain and high frequency harmonic current generated by the cluster distributed inverter.

The new characteristics and control technology of microgrid power quality problems are mainly reflected in two aspects.

1) Internal loop suppression method of microgrid: Due to different control factors, filtering parameters and other factors, the inverter's equivalent output impedance of the port on the common connection point is different, resulting in the fundamental wave between the inverters. The generation of harmonic circulation affects the power sharing effect between the inverters and the stability of the microgrid voltage. The virtual impedance technology can be used to change the amplitude-frequency characteristics of the inverter's own output impedance, so that the equivalent output impedance of each inverter is the same, which can effectively reduce the circulating current between the inverters and achieve power sharing.

2) Resonance analysis and suppression method in microgrid grid-connected operation mode: in the grid-connected operation mode of microgrid, the closed loop equivalent impedance of the parallel inverter, the closed loop equivalent impedance of the inverter and the equivalent impedance of the distribution network There is coupling between each other. As the number of parallel stages increases, it is easy to cause low-order harmonic resonance, which causes amplification of specific sub-harmonic currents, by adopting active passive damping and active damping methods, or passive active filtering methods. It can solve the problem of coupling resonance caused by inverters, but related research needs further study.

5. Development trends and prospects of power quality control technology

New energy or distributed energy is connected to the power grid in the form of microgrid/power station/distributed power generation. It poses new challenges to the analysis and solution of power quality problems. The research on power quality analysis and control technology for smart grids has a long way to go. High standards and new requirements have been put forward for power quality compensators. At the same time, the promotion of wide bandgap devices such as silicon carbide and the development of new converter topologies will also promote the development of power quality management devices. In addition, the coordinated control of distributed power and power quality compensators and the advanced power electronic converters with active power quality control functions are also expected to be further studied and popularized.

Fiber Optic Adaptor

Fiber Optic Adaptor,Fiber Adaptor,Fiber Optic Adapter Kit,Fiber Connector Adapters

Huizhou Fibercan Industrial Co.Ltd , https://www.fibercannetworks.com