Fault Analysis Of Valve Side Winding Of Converter Transformer In VSC-HVDC System

High voltage direct current transmission technology(HVDC)has the advantages of less power loss,high transmission efficiency,fast and reliable adjustment without synchronous stability problems.It is widely used in large-scale interconnected grids,cable power transmission,and communication between AC systems operating at different frequencies or non-synchronous at the same frequency.Converter transformer is an important equipment and link connecting AC and DC systems which is used to provide and regulate AC voltage,exchange energy and realize AC and DC isolation.Internal faults of converter transformer would impact on AC power grid and DC power grid,and even threaten the safe and stable operation of AC and DC power grid.At present,the fault analysis of converter transformer is mostly focused on the line commutated converter based high voltage direct current(LCC-HVDC)system,and there is little research on the fault analysis of converter transformer in the voltage source converter based high voltage direct current(VSC-HVDC)system,the characteristics of internal winding faults are still unclear.Therefore,analyzing the internal fault of converter transformer winding and finding the fault characteristics in VSC-HVDC system can provide a theoretical basis for operation performance analysis,new principle research and scheme design of relay protection device,which is of great significance and application value to ensure the security and stable operation of ACDC power system and reduce economic losses.This thesis analyzes the valve side winding fault of converter transformer in VSCHVDC system.Firstly,based on the pseudo bipolar structure of three-phase two-level voltage source converter in VSC-HVDC system,a classification method of valve side working state of converter transformer under normal operation is proposed.According to the 8 conduction state of the converter valve,the valve side working state of the converter transformer in case of non fault is defined as loop state,grid charging state and charging-loop compound state,which can be used as the classification basis of valve side winding fault.On this foundation,according to the different working states of the valve side of the converter transformer corresponding to the different fault time,the faults of the valve side winding are classified and analyzed.In the process of fault analysis,Thevenin equivalence,superposition principle and method of time division and situation division are adopted.The loop equation is written according to the equivalent circuit corresponding to the fault current path,and the fault characteristics during the period when the valve is not locked are obtained based on the actual engineering parameters.The VSC-HVDC system simulation model is built on PSCAD/ EMTDC.The characteristics of valve side winding fault of converter transformer analyzed in this thesis are verified.