Study On Control Strategy And Semi-Physical Simulation Of Cascaded H-Bridge Transformer-Less SSSC

The rapid development of China’s economy and science and technology has put forward higher requirements for power quality.Static Synchronous Series Compensator can flexibly control the power flow of the grid,quickly conduct reactive power compensation and suppress sub-synchronous oscillation.It is of great significance to improve the dynamic and static characteristics of the power system and the stability of the power system.Therefore,it has attracted more and more attention.Most of the traditional SSSC devices adopt the mid-point clamp topological structure to access the transformer system.The transformer parameters design is complex,bulky and the loss is huge,which makes the device response speed slow and hinders the development and market investment of SSSC.In view of a series of problems caused by the use of transformers,a transformer-less SSSC based on cascaded H-bridge topology is studied in this paper.Firstly,the paper starts with several common multilevel topologies,analyzes their working principle and device usage,and then explains the reason why the cascaded H-bridge multilevel topology is chosen as the main circuit topology of SSSC device.At the same time,the advantages of transformer-less SSSC based on cascaded H-bridge multilevel topology are analyzed in detail,and the feasibility of the proposed method is investigated according to the current situation of engineering application.Secondly,this paper establishes the mathematical model of cascaded H-bridge transformer-less SSSC in the abc coordinate system,and the equation of state in the dq coordinate system is constructed.The system is decoupled by state feedback precise linear decoupling.On the basis of the above,the control strategy of SSSC is presented.Considering that the problems of pulse delay and switching loss in cascaded H-bridge multilevel topology will lead to the instability and imbalance of DC capacitance voltage,a two-stage DC capacitance voltage control strategy is proposed: The DC side voltage general stability control is designed to balance the energy exchange between the SSSC device and the system through control.The DC side voltage equalization control equalizes the energy of all H-bridge units by adding active power correction.Finally,based on the above theory,it was verified on Matlab/Simulink simulation platform and RT-LAB semi-physical simulation platform.The detailed parameters,system architecture and experimental waveform are given in the simulation.The relevant experimental results show that cascading H-bridge transformer-less SSSC model built in this paper has good reactive power compensation capability in both inductive and capacitive compensation states.By controlling related parameters,the transmission power of the line can be changed effectively,and the dynamic response speed is fast,and the output waveform quality is good.The two-stage DC voltage control strategy proposed in this paper can effectively control the DC voltage and ensure the safe and reliable operation of the SSSC device.