Research On Power Quality Comprehensive Compensation Device Based On Modular Three-level

With the rapid development of industrialization,the electricity load connected to the low-voltage distribution side of the power system is increasing,which leads to the power quality problem becoming more and more serious.Loads with different characteristics will cause different power quality problems.For example,the connection of inductive loads will cause the transmission line to transmit a large amount of reactive power and reduce the power factor;The connection of non-linear loads will inject harmonic currents into the system;The connection of asymmetric loads will lead to three-phase unbalanced current,etc.At present,the main governance power quality solution is to use various power quality compensation devices.In order to improve the utilization rate and cost performance of the device,it is hoped that the power quality compensation device can be multi-functional,that is,one power quality compensation device can deal with different power quality problems.Therefore,a power quality comprehensive compensation device has been studied in this paper,which adopts the neutral point clamped three-level inverter as the main topology.By changing its control algorithm,multiple functions can be realized,such as compensating reactive power,suppressing harmonic and eliminating three-phase unbalance.In the first chapter,the concept and related research of power quality,commonly used power quality compensation devices and typical three-level topologies have been introduced.In the second chapter,the system structure of the researched power quality comprehensive compensation device has been introduced,and the principles of reactive power compensation,harmonic suppression and three-phase unbalance elimination have been explained,respectively.In the third chapter,taking the neutral point clamped three-level inverter as example,the operating principle,mathematical model,modulation strategy and inherent neutral point voltage unbalance of the three-level inverter have been studied.Aiming at the neutral point voltage unbalance problem,the self-balance characteristics are analyzed emphatically,and a neutral point voltage control method that injects an appropriate compensation voltage into one phase has been proposed.Since the three-phase four-wire system is often used in my country’s low-voltage distribution network,the modulation strategy and the neutral point voltage umbalance discussed in this chapter are all suitable to the three-phase four-wire system.In the fourth chapter,the control system of the power quality comprehensive compensation device has been researched,which is mainly divided into two parts: the current inner loop and the voltage outer loop.The current inner loop can be divided into two parts,the command current extraction of the upper link and the current tracking control of the lower link.In the command current extraction,various components in the load current are extracted based on the double dq transformation method and the orthogonal construction method,and the software phase-locked algorithm with positive and negative sequence separation is used to improve the extraction accuracy.In the current tracking control,PI control and improved deadbeat control are studied.In the voltage outer loop,a control method combining PI controller and limiter is adopted.Finally,according to the design of the main circuit parameters and controller,a comprehensive power quality compensation device suitable for three-phase four-wire system was built in the laboratory.The performance of the two current tracking controls was compared through experiments,and it is verified that the researched power quality comprehensive compensation device can achieve multiple functions,such as compensating reactive power,suppressing harmonic and eliminating three-phase unbalance.