Research On HVDC Transformers That Are Beneficial For Suppressing Commutation Failures

In recent years,LCC-HVDC technology based on grid-commutated converters has become increasingly mature and widely applied in China.However,the characteristics of the thyristors used in the converter make commutation failure unavoidable,which seriously affects the stability and reliability of the HVDC system.In addition,the converter transformer,as an important piece of large electrical equipment in HVDC,has a long operation time of more than 5 seconds for tapchanging and cannot achieve rapid voltage regulation.It also has a high failure rate and is one of the bottlenecks restricting the development of LCC-HVDC.Therefore,this article focuses on the technical difficulties of the slow operation time,high failure rate of the converter transformer,and commutation failure.A new type of converter transformer is proposed,which fully leverages the advantages of power electronics technology to achieve fast and precise voltage regulation.This solution utilizes the active voltage regulation technology,replaces the tap-changing of the converter transformer with an active voltage regulator,and forms an active voltage transformer(AVT).It is of great significance to achieve accurate pressure regulation,maintain stability of the valve-side voltage,and effectively suppress commutation failureFirst,this article introduces the main circuit topology and basic operating principles of LCC-HVDC,and respectively describes its control and operation characteristics on the rectifier side and inverter side.On this basis,a simulation model of the Lingzhou-Shaoxing ±800kV ultra-high voltage DC transmission system is constructed in the PSCAD/EMTDC simulation platform to simulate its steady-state and transient operation.Secondly,the mechanism and causes of commutation failure are analyzed,and the principle of suppressing commutation failure with an AVT is studied,along with its basic control strategy.On the basis of a detailed understanding of the PLL working principle,a dual-loop control strategy of voltage outer loop and current inner loop is adopted and verified in the CIGRE HVDC model.The simulation results show that this solution can achieve voltage regulation,demonstrate the function of the tapchanger,and achieve fast and accurate voltage regulation.Finally,the AVT is applied to the Lingzhou-Shaoxing model,considering different fault conditions to verify its ability to withstand commutation failure.On this basis,the mutual influence between the AVT and CFPREV in HVDC is studied.The simulation results in PSCAD/EMTDC show that the AVT can replace the tap-changer to achieve fast voltage regulation,and can cooperate with CFPREV to some extent to suppress commutation failure.