The Research On The Protection And Fault Location Of AC Side Under Commutation Failure Condition

Commutation failure caused by the fault on AC side is one of the main reasons for causing cascading failure in AC/DC hybrid system,which is also a typical fault of AC/DC hybrid power grid in China.Fast and reliable implementation of AC side relay protection and fault location are significant for the safe and stable operation of AC/DC hybrid system,because it can avoid the large-scale transfer of power flow,and block cascading failure.After commutation failure occurs,the DC system is equivalent to a short circuit,which can be equivalent to a harmonic current source with time-frequency characteristics,and injects multiple harmonics into the AC system.From the perspective of the order of the influence the commutation failure has on the AC system,first of all,the harmonics flow through the converter transformer(hereinafter referred to as commutation transformer).If an internal fault of converter transformer occurs during this period,traditional differential protection block and refuse to trip,which will cause the damage of converter transformer,the blocking of DC system,the unstable of the whole systems and other hidden dangers.With the continuous injection of harmonics,the electrical quantity of the AC line changes significantly.On the one hand,the relay of the healthy line have to keep no operation;on the other hand,the relay of the faulty line has to trip quickly in order to reduce the interaction of the AC and DC systems.Meanwhile,as the converter station can be either rectifier station or inverter station,the fault detection method has to be applicable to the transmission sending and receiving end detection,thereby placing higher requirements on the selectivity,sensitivity,and reliability of the line fault detection.When the continuous commutation failure occurs,the power of DC system cannot be transferred,causing power impact on the sending and receiving end line,and the AC system operating environment has static to dynamic.For the faulty line,it is necessary to ensure the accuracy of fault location under dynamic conditions,so as to quickly locate the fault in time,restore the power supply,and reduce the loss.Aiming at the problems of AC side relay protection and fault location under commutation failure,researching fast and reliable protection and location algorithm is an important requirement to ensure the safe and stable operation of AC/DC hybrid system.This thesis framework is organized according to the order of the commutation failure influence.To achieve the final gold that improves the reliability of protection and the accuracy of fault location under the commutation failure,a protection and fault location system of thehybrid system with “reliable protection,accurate fault location” is constructed.It also means to realize the fast isolation,restore supply quickly,avoid further expansion of the impact of fault,and prevent the occurrence of cascading failure.The main works of the thesis are concluded as follows.1.After the commutation failure occurs,the converter transformer is first affected.Therefore,in solving the problem of refusing to trip under internal fault with the condition of commutation failure,a new criterion is proposed in this paper.Based on the fault component network,the direction of fault current on the primary and secondary side have been analyzed under different fault conditions.The ratio of wavelet energy entropy(RWEE)of the current fault component on the primary to that on the secondary side is employed to establish the new criterion.Once an internal fault occurs,the energy entropy(WEE)on the primary side is much larger than that on the secondary side,so a large RWEE is obtained.Otherwise,the WEE of both sides are similar,and RWEE is around one.Combined with the traditional protection,the improved scheme has been presented.Simulation results show that the proposed criterion can perform well under inrush current,CT saturation,and DC bias conditions,and can discriminate internal faults from external faults.Most importantly,it avoids the failure of tripping when an internal fault occurs.2.With the further expansion of the impact of commutation failure,the fluctuation of the electrical quantity of the AC line causes protection misoperation.Therefore,an improved adaptive accumulation fault detection algorithm is proposed.It means to solve the problems of fault misdetection and weak adaptability of the end detection,even in the case of AC power transmission line commutation failure in AC/DC hybrid system.In order to ensure the reliability of the algorithm at the receiving end,the characteristics of the current signal are analyzed.The difference between the calculated current phasor value of the half cycle before the sampling and the threshold value is taken as the receiving end detection index and enlarged point by point.Combined with the original method,the improved scheme has been presented.Comparing the values of three continuous output indices,whether the fault occurs or not can be judged.Both results of theoretical analysis and simulation show that applying the proposed method to fault detection,the failure of fault detection can be effectively avoided,besides the proposed method is fast,safe and reliable,and it is not affected by such factors as load,transition resistance and the moment the fault occurs.3.On the basis of reliable protection,in order to avoid the effect of commutation failure hason the transmission line fault location,a new fault location scheme which is suitable to inverter side transmission line is proposed in this paper.Based on the improved dynamic synchrophasor measurement algorithm,the effect of deviation frequency has on the parameters of line have been analyzed.Hence,the deviation frequency is introduced to correct the the parameters of line,the new line equivalent impedance/admittance estimation algorithm have been proposed,the errors caused by the deviation frequency and the change of voltage and current has been compensated and corrected.Based on the voltage and current sychrophasor of both ends,the voltage of fault point has been estimated.According to the New iteration method,an iteration equation has been constructed,and then the fault distance can be obtained.The simulation verified the performance under commutation failure and non-commutation failure condition,it shows that the proposed scheme is independent of the fault resistance,fault distance,and fault type.In most cases,it ensures a higher precision of fault location in comparison with the conventional fault location algorithm.4.Considering the power swing will be caused by the continuous commutation failure,the accuracy series-compensated line fault location is focused on.On the basis of solving the fault location problem of AC transmission line under the failure of commutation,in order to improve the accuracy of fault location for the series-compensated(SC)transmission line under dynamic conditions,a novel fault-location algorithm for a series capacitor compensated transmission lines under power oscillation conditions is proposed.Synchronous voltage and current phasors from both terminals of the transmission line are employed to estimate the fault location while considering the distributed parameter line model.An iterative method is proposed to obtain the voltage across the SC under power oscillation conditions.The fault-location algorithm includes three stages.Firstly,the distributed parameters of the transmission line are estimated by the dynamic parameters estimate method.Secondly,the voltage of the SC can be solved by the iterative method based on the phasor measurements and the transmission line parameters.Finally,the fault location can be attained via the Newton iteration method.The performance of the proposed algorithm under various power oscillation situations has been evaluated,it shows that the proposed method is independent of fault types,fault resistance and fault distances under power oscillation conditions.The accuracy of the proposed algorithm is far higher than that of the conventional fault location algorithms in most cases.In general,on the basis of the Line-Commutated Converter-based HVDC,the paper focuses on the typical fault that DC system commutation failure,comprehensively considers itsimpact on AC side protection and fault location,and proposes corresponding improvement schemes to improve the reliability of the original algorithms.The proposed protection and fault location algorithm is more suitable for AC/DC hybrid systems.