Analysis and discussion on power quality control technology

　　Preface

　　In recent years, the power quality control technology of distribution system has made great progress. Among them, the most representative and influential ones are the distribution system static VAR compensation device (DSTATCOM), active power filter (APF), timely monitoring of power system data (SCADA), and dynamic voltage regulator (DVR).

　　(I) Distribution System Static VAR Compensation (DSTATCOM)

　　There are many fast impact load currents in the power system, such as medium and high frequency electric furnaces, large capacity motors with high inductive loads suddenly started, which will cause voltage flicker and imbalance of current and voltage in the distribution system. Traditionally, static VAR compensators (SVCs) are used to suppress voltage flicker, but the response speed of SVCs is slow (tens of milliseconds), and the flicker suppression rate is difficult to reach more than 50%. In comparison, the voltage source converter connected in parallel with the power system using PWM control, namely the distribution system static VAR compensation device (DSTATCOM), has the advantages of fast dynamic response speed, compensation current independent of system voltage, strong harmonic suppression capability, good voltage flicker suppression effect, and low active power loss. Therefore, DSTATCOM devices have gradually replaced SVC devices and have been widely used.

　　(2) Active Power Filter (APF)

　　In the power system, uncontrolled harmonic voltage and harmonic current are the main reasons affecting the power quality. First, use accurate RMS measuring instruments to regularly measure the circuit and neutral line to conduct harmonic surveys. Strengthen the measurement and inspection of the power quality of user transmission lines, regularly detect and compare the transmission and distribution systems, especially the medium and low voltage systems, to find out the laws of harmonic generation and development. Harmonic pollution suppression and prevention measures are divided into passive filters and active filters composed of power electronic equipment according to the working principles of the devices. Passive filtering devices are composed of passive components such as capacitors, reactors, and sometimes resistors to form a low-impedance path for a certain harmonic or higher harmonics to achieve the effect of suppressing higher harmonics.

　　(III) Timely monitoring of power system data (SCADA)

　　In the power system, dynamic power quality problems are problems that have been exposed in recent years with the development of high-tech. Research on the sensitivity and irritability of power users to power quality can provide an important basis for selecting appropriate compensation methods, and also provide an important reference for the formulation of power quality standards. The SCADA system that monitors the quality of dynamic data on site is a research direction for distribution management systems. Field data not only includes measurement data such as power, voltage, and current, but also includes operations such as opening and closing, overcurrent, and quick disconnection, as well as event data generated by accidents. When an accident occurs and causes tripping, the fault recording data on site must also be recorded. It can be seen that the amount of data that needs to be communicated is incomparable to general industrial control. Since the field data of the power system changes very quickly, an overcurrent may only last for more than ten milliseconds, and the data is fleeting, so the requirements for data real-time and communication speed are very high.

　　Reliable and efficient communication of the underlying data of the monitoring system is the key to system reliability and the focus of designing monitoring software. Some existing software integrates data communication, processing and monitoring into one software. Although it is intuitive and compact, it is very inconvenient to upgrade and improve the system. A small change requires the entire system to be reorganized. Therefore, a modular structure is a better choice.

　　In the design of a large-scale power monitoring system, the hardware adopts an independent way of separating the communication station and the monitoring station. The software separates the bottom communication software from the monitoring software and works independently in the communication station. The communication station is responsible for collecting the bottom-level real-time data and conducting two-way data communication with the upper-level monitoring station. Due to the independence of the communication station, the tasks of the upper-level monitoring station are greatly reduced, which not only improves the communication rate of the bottom layer, but also speeds up the data refresh speed of the monitoring interface. If there are a large number of instruments at the bottom layer, a multi-channel dual-port RAM intelligent communication card can be used on the communication station and expanded to multiple serial ports to further improve the bottom-level communication speed. In this way, the quality of power transmission can be improved in a timely and effective manner.

　　(IV) Dynamic Voltage Regulator (DVR)

　　DVR is equivalent to a dynamically controlled voltage source connected in series in the power distribution system. By adopting appropriate control methods, the output of the voltage source can offset the adverse effects of power system disturbances on the load voltage, such as voltage drops, voltage imbalance and harmonics. When the energy on the DC side is obtained through system rectification, even if a single-phase fault occurs on the system side, the other two phases can still provide power to maintain the normal operation of the DVR, and it is also possible to compensate for long-term voltage drops. If a battery is connected in parallel at both ends of the DC side capacitor, or a large-capacity capacitor is used for energy storage, the device can also play the role of UPS, that is, it can provide power to the load for a certain period of time when a short-term fault occurs on the system side. By adopting a suitable topological structure, DVR can comprehensively manage dynamic voltage quality problems in the distribution system, such as drops, surges and steady-state voltage quality problems (such as harmonics, fluctuations, and three-phase imbalance). It is a multi-objective voltage quality comprehensive management device.

　　In addition, the power quality control technologies of the distribution system include uninterruptible power supply (UPS), unified power quality controller (UPQC), solid-state transfer switch (SSTS), distributed generation system (DG), etc.

　　Conclusion

　　With the development of high-tech industries and the application of information technology in various fields of society, the issue of power quality has increasingly attracted widespread attention. The development trend of improving power grid quality in the future will be to realize intelligent, information-based, and hierarchical interactive management of the entire process of the power industry, including power generation, transmission, power supply, power consumption, customer power sales, grid hierarchical dispatching, and comprehensive services.