| HVDC transmission project plays an important role in China power grid,but its inherent characteristics,which are different from AC transmission also seriously threaten the safe and stable operation of power grid.Usually,it is easy to recover from commutation failure under the protection of DC control system.But if the two ends of the system are weak AC,continuous commutation failure may occur,which may cause the DC converter station lockout and have serious impact on AC systems on both sides.Therefore,it is extremely urgent to study the suppression of commutation failure in HVDC transmission engineering,so as to reduce or even completely avoid the chain failure of the power grid,which is of far-reaching significance to effectively protect the power grid.In this paper,the basic structure,control strategy and control characteristic curve of the HVDC transmission system are studied,and the commutation failure process,commutation failure mechanism,influence factors of commutation failure and the fault recovery process of the HVDC system are analyzed in detail.It is found that the static recovery characteristics of voltage dependent current order limiter(VDCOL)and the method of obtaining compensation voltage affect the continuous commutation failure.On the basis of analyzing and summarizing the inadequacy of existing strategies to suppress commutation failure,the function of VDCOL was improved,the fault severity coefficient m based on commutation area was introduced,and the mathematical model of conventional VDCOL regulation curve was changed,and the dynamic variable slope VDCOL was designed.Compared with conventional VDCOL,this control strategy can improve the sensitivity of DC current instruction,improve the recovery speed and shorten the recovery time of the system.At the same time,the dc current change rate is introduced to change the offset voltage,and then change the VDCOL input voltage,the control strategy considering the failure with dc current fluctuation characteristics of fault recovery process,changed the original offset voltage method,overcome the dynamic variable slope VDCOL breakdown at inverter side,can’t enabled in a timely manner,Control and adjustment of the system this disadvantage.After the comprehensive analysis of the advantages and disadvantages of the two control strategies,in order to complement the advantages and disadvantages of the two control methods,the combined application of the two control methods is considered to reduce the probability of continuous commutation failure.In the PSCAD simulation software,combined with the operational parameters of the actual project,a simplified model of Xiangjiaba-Shanghai ultra-high voltage direct current(UHVDC)transmission system is built to carry out the simulation test.Based on the real time digital simulator(RTDS)simulation platform,combined with the specific operation scheme of Xiangjiaba-Shanghai UHVDC project,the relevant steady-state and fault tests are carried out.The correctness and effectiveness of the combined control strategy for suppressing commutation failure and promoting rapid recovery of system voltage and transmitted power are verified.The joint control strategy provides another direction for the current research on restraining commutation failure and is of great significance for maintaining the safety and stable operation of power grid. |
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