Study On Wide-area Backup Protection Based On Pilot Impedance And Correlation Of Current Waveform

The interconnected power grid is increasingly complex in structure and flexible in operating conditions.Conventional backup protection based on local measurement information is faced with such problems as complicated value setting,long operating time,and misoperation,which makes it difficult to meet the requirements of power grid for safe and stable operation.Wide area backup protection uses wide area measurement information to identify fault elements,which solves the shortages of conventional backup protection.However,in the case of high resistance grounding fault,asynchronous information and abnormal sampling values,the existing wide-area backup protection algorithm is not sensitive enough,and may appear maloperation or miss trip.In order to solve the problems mentioned above,two transmission line fault detection algorithms based on pilot impedance and correlation of current waveform is studied in this paper.Firstly,the common structure of wide-area backup protection system is introduced,and the characteristics of various structures are analyzed.Then the fault area detection method in wide area backup protection algorithm is introduced,which provides the suspected fault lines for the fault detection algorithm proposed in this paper.In this paper,a fault detection algorithm for transmission lines based on pilot impedance is presented.Firstly,the pilot impedance is defined by using the line parameters and the fault component voltages and currents on both sides of the line.The features of pilot impedance in internal and external faults are demonstrated,and the fault judging quantity is defined according to the characteristics of pilot impedance.According to the value of fault judging quantity in internal and external faults,the fault detection criterion is proposed.In the case of power swing and open-phase operation,the blocking measures and open criterions are added.Simulation experiments are carried out in IEEE 39 bus system and two-machine transmission line model.The simulation results verify that the algorithm is not affected by the transition resistance,and the protection criterion setting is simple and has a high margin.Moreover,the algorithm requires little information synchronization and can always detect faulted lines in the case of open-phase operation,power swing or power flow transfer.In this paper,a fault detection algorithm for transmission lines based on the correlation of current waveform is presented,and this algorithm and the existing methods of detecting fault areas constitute a complete wide area backup protection scheme.Firstly,the magnitude of sequence voltage is used to identify the suspected fault lines.Then the sample values of positive sequence fault component current,negative sequence current and zero sequence current of the line are superimposed to obtain the comprehensive current of fault component.According to the inconsistency of the current waveforms on both sides of the internal fault line and the external fault line,two feature sequences are constructed by using the comprehensive current of fault component on both sides.In one cycle data window,along with the sampling time point by point,the Spearman's rank correlation coefficient between feature sequences in the data window is calculated.When the correlation coefficient is greater than the protection threshold,the line is judged to be a fault line.According to the simulation results of IEEE 39 bus system,it is verified that this algorithm has high anti-interference ability and is not affected by transition resistance,power swing and power flow transfer.Moreover,the algorithm can always detect faulted lines under the influence of gaussian white noise,abnormal data and CT saturation and requires little information synchronization.