Research On A Novel Power Quality Compensator Based On T-type Three-level Topology

In recent years,with the expansion of capacity of EAF and rolling mill in industrial steelmaking,the wide application of power electronic technology and the rapid development of railway electrification,power quality problems such as voltage fluctuation,imbalance and harmonics become increasingly serious.Improving power quality has practical significance for improving people's livelihood and economic development.Common power quality compensation devices,such as static var generator,switching capacitor banks and active power filter,have some defects while realizing the functions of reactive power compensation,harmonic suppression and load balancing.Therefore,another kind of power quality compensation topology is proposed by scholars.The topology has simple control strategy.It naturally isolates harmonic and unbalanced components of the load current to the power system,and keeps the power factor close to 1.It has been applied in three-phase three-wire system.However,when applied in three-phase four-wire system,the compensation effect for harmonic and unbalanced current components is not good and zero sequence current components cannot be compensated either.To solve these problems,this paper proposes a novel power quality compensator based on T-type three-level topology and an improved 3d-svpwm modulation strategy for the compensator.In Chapter 1,the concept and the current situation of power quality are introduced,as well as the national standards of power quality in our country.In chapter 2,the basic principles and measures of reactive compensation,load balancing and harmonic suppression are introduced.Common power quality compensation devices such as static var generator,switching capacitor banks and active filter are introduced,as well as their defects.The power quality compensation topology used in this paper is introduced.It has simple control strategy and better compensation effects compared with the common device topology.However,it has drawbacks such as incapability of compensating zero sequence current.In chapter 3,a novel power quality compensator based on T-type three-level topology is proposed.The neutral point of DC side of the inverter is grounded to compensate the zero sequence current,and the shunt filter capacitors are connected on both the system side and the load side to improve the voltage quality.An improved 3d-svpwm modulation strategy is proposed as well.Chapter 3 analyzes the working principle of reactive compensation,harmonic suppression and load balancing of the compensator,as well as its DC-link voltage control strategy and parameter selection.The performance of the compensator is simulated and verified.In Chapter 4,the harmonic and unbalanced current components on the system side caused by the traditional 3d-svpwm modulation scheme are analyzed in detail.An improved 3d-svpwm modulation strategy and positive and negative balancing control strategy are proposed,which can not only improve the current quality on the system side,but also improve the positive and negative voltage balancing situation of the DC side.Finally,the effect of the improved modulation strategy and balancing control strategy is verified by simulation.