Research On 10kV Delta-Connection SVG Based On Cascaded H-Bridges

Nowadays,with the rapid development of social economy,the scale of power system is becoming larger and larger.Because of the direct connection with user equipment,10kV distribution network has more and more kinds of loads.The operation of a large number of inductive,unbalanced and non-linear loads seriously affects the power quality of power grid.As a grid compensation device,Static Var Generator(SVG)can comprehensively compensate reactive,negative sequence and harmonic components in the power grid to improve power quality.Cascaded SVG based on H-bridge inverter is widely used in high-voltage and high-power power power systems because of its easy modularization and low harmonic content.The three-phase cascade type SVG is divided into two types according to the connection mode of the cascade module:a delta connection SVG and a star connection SVG.The star-connected SVG has fewer H-bridges than the delta-connected SVG,but in the case of a large load imbalance,the negative-sequence compensation capability of the delta-connected SVG is much stronger than the star-connected SVG,so the guarantee is guaranteed.From the perspective of power quality,it is of great significance to study the application of delta connection SVG in the distribution network.In order to develop a delta-wired SVG for use in a 10kV distribution network,firstly,the basic principle of SVG is expounded from two aspects:working principle and mathematical model.By comparing the mathematical models of the two connection modes of SVG,the advantages of the three-tier R-shaped connection SVG are revealed.That is,because there is no coupling between the three-phase cascade modules,the mathematical model is the same as the single-phase H-bridge cascade module.Compared with the coupling phenomena existing in the star-connection SVG,the analysis process of the delta-connected SVG is simpler.Then,the control strategy of the delta-connection SVG is studied and compared with the control strategy of the star-connection SVG.Firstly,in the aspect of compensation quantity detection,according to the instantaneous power theory,the reactive power,negative sequence and harmonic component detection algorithms are designed for the two SVG.Secondly,in terms of control,the SVG of both wiring modes adopts the voltage and current double closed loop.Control method,in which the DC side voltage is layeredly controlled,the delta-connected SVG only needs to design the voltage stability of the cascade module itself and the DC-side voltage balance control of the internal H-bridge unit,and the star-connected SVG needs to design the overall DC The side voltage is stable,the phase-to-phase voltage is stable and balanced,and the DC voltage in the phase is stable and balanced by three layers of control.It is concluded by comparison that the SVG control strategy of the delta-connection is simpler than the star-connected SVG.In this thesis,the compensation simulation model of delta-connection SVG to lOkV system is built in MATLAB/Simulink software environment.The simulation results verify the correctness and effectiveness of the compensation detection algorithm and control method,and establish the simulation model of star-connection SVG.For comparison,it is verified that the delta-connection SVG negative sequence compensation ability is stronger when the load imbalance is large.Finally,based on the grid simulator,dSPACE controller and actual sensor and load circuit,a hardware-in-the-loop low-power simulation platform is built to verify the reliability of the delta-connection SVG detection algorithm.It provides theoretical support for the development of triangular wiring SVG in the future.