Research On Commutation Failure Detection Method Based On Temporal Features Of Three-Phase AC Minimum Current On The Valve Side

Line-commutated converter-based high voltage direct current(LCC-HVDC)systems is very sensitive to voltage.Therefore,AC fault can induce commutation failure of LCC-HVDC.Commutation failure will increase the current of the converter,cause the converter to heat,shorten the lifetime of the converter,and endanger the normal operation of the equipment.The occurrence of commutation failures will cause DC power fluctuations and even DC blocking,threatening the safe and stable operation of the power grid.In order to maintain the stable operation of the converter,DC engineering is allocated with specialized converter protection.However,DC blocking which is caused by the commutation failure often occurs.Consequently,there are hysteresis,mismatch problem,maloperation and other issues in protection which respond to commutation failure.In order to solve this problem,a new commutation failure detection scheme is constructed based on the valve state.The details are summarized as follows:Considering the defects of the protection responding to commutation failure in converter protection.The protection principle,detection performance and valve state are analyzed,and the root cause of protection mismatch is obtained.The analysis shows that the protection adopts the principle of balance current and selects the valve area differential current to detect commutation failure.Due to the mismatch between the amplitude characteristic of valve area differential current and the valve state,the protection has the problem of mismatch.Therefore,the protection performance is affected differently by AC and DC system operating conditions,fault conditions and other factors.In order to solve the problem that amplitude characteristics of valve area differential current cannot be distinguished the valve status.In this paper,a minimum current temporal characteristic which can trace commutation process is proposed.The verification shows that the non-zero values of this novel characteristic indicate that the valve is in the process of commutation,which realizes the tracking of commutation process.This characteristic is periodic discontinuous spike wave in normal operation and no periodic spike wave with different shapes after fault.The valve state can be extracted by using the difference of amplitude characteristic and temporal features.The commutation failure is the most extreme case of abnormal valve state.Based on the valve state,a single threshold commutation failure detection method based on the minimum current temporal characteristic is proposed.The detection criterion is constructed based on the significant difference between the amplitude characteristic and the temporal features of the minimum current temporal characteristic before and after commutation failure.Then,the reasonable threshold selection is analyzed.Finally,the proposed detection scheme is verified and analyzed through simulations on the PSCAD/EMTDC platform of the actual HVDC project.Compared with the protection mentioned above,the new method has better perform in rapidity and sensitivity,and is less affected by DC power,AC system strength and fault factors.The single threshold commutation failure detection method has complex threshold setting of amplitude characteristics and temporal features.At the same time,when building a new principle of commutation failure protection in the future,it is necessary to consider the bypass strategy to locate the fault phase.In order to solve those problem,the minimum current temporal characteristic based on single commutation process is proposed.Then,the optimization strategy of detecting amplitude characteristics with high threshold and temporal features with low threshold is proposed.Naturally,a dual threshold commutation failure detection method based on phase separation of the minimum current temporal characteristic is established.Finally,the correctness and validity of detection method has been verified in actual HVDC PSCAD/EMTDC.In this paper,the minimum current temporal characteristic constructed by valve side AC current is proposed to realize the tracking of commutation process and the extraction of valve state,which solves the problem of mismatch between valve area differential current amplitude characteristic and valve state.On this basis,a single threshold commutation failure detection method based on the minimum current temporal characteristic is proposed.To further improve it,a dual threshold commutation failure detection method based on phase separation of the minimum current temporal characteristic is proposed.The new detection has better detection perform in rapidity and sensitivity,and is less affected by DC power,AC system strength and fault factors.The correctness and validity of some research results have been verified in actual LCC-HVDC PSCAD/EMTDC control.This work has been funded by Joint Fund Program of the National Nature Science Fund Program of China(U1766213)、National Natural Science Foundation of China(51677073).