Research On The New Criteria Of Grid Fault Identification For Stability Control In Modern Power System

The characteristics of AC/DC hybrid,UHV long-distance transmission and large-scale new energy integration in modern power system have brought great changes to the fault characteristics of AC electric quantity in power grid,and the traditional grid fault identification criteria for stability control are facing great challenges.The new criterion of AC line fault trip,DC commutation fault identification,the improved scheme of out-of-step trip criterion based on phase angle principle and the new criterion of non planned island identification for grid-connected PV system are studied.The main work and innovative results are as follows:(1)The influences mechanism of modern power system transient characteristics on traditional trip criteria are revealed.The influences of DC system,long-distance transmission and new energy integration on AC line fault characteristics are analyzed in theory.Combined with traditional fault and no-fault trip criteria used for stability control,it is found that the main factors leading to the incorrect action of traditional fault trip criteria are insufficient rise of effective value of fault phase current,insufficient drop of effective value of voltage and imbalance of positive and negative sequence impedance.The crowbar protection in DFIG wind farm will lead to the misoperation of traditional no-fault trip criterion.The results of theoretical analysis are further verified by the simulation.(2)New criteria of AC line fault trip for modern power system stability control are proposed.The timeliness requirement of stability control for line fault trip criteria is analyzed.It is proved that the AC equivalent network can be used for fault analysis after the DC system short circuit and the crowbar protection are put into operation.Based on the typical characteristics of current sequence components and phase-to-phase cosine voltages during different faults,new criteria of line fault trip for modern power system stability control are proposed.The effectiveness and reliability of the new criteria in AC-DC hybrid,UHV long-distance transmission and large-scale new energy access network are proved by RTDS test.For the half wavelength transmission line,the location of fault point is determined by time difference method,then the electrical quantity at the fault point is calculated and the new criteria of fault trip suitable for half wavelength transmission line are proposed.(3)New criteria of DC commutation failure and multi DC simultaneous commutation failure independent of DC control and protection system are proposed.The relationship between the the currents on both sides of the converter valve in case of DC commutation failure is analyzed.Combined with the power impact of the commutation failure on the AC power grid,new criteria of DC commutation failure and multi DC simultaneous commutation failure for stability control are constructed,which is verified by the hardware in the loop test based on RTDS.The new criteria does not rely on the signal provided by DC control and protection,avoiding the risk of simultaneous failure of the first and second defense lines.(4)An improved method of out-of-step splitting criterion based on phase angle compensation is proposed.The influence mechanism of series compensation capacitor on out-of-step oscillation center and phase angle track is analyzed.It is known that when the series compensation capacitor is near the electric center of the line,the oscillation center will appear on both sides of the compensation capacitor at the same time.The out of step splitting criterion faces the risk of misjudging the direction of oscillation center.It is also found that the small equivalent impedance angle of the line will lead to the failure risk of out-of-step splitting criterion.Based on the fact that the phase angle at the maximum moment of the current is equal to the equivalent impedance angle of the line in the process of oscillation,an improved criterion of out of step splitting based on phase angle compensation is proposed.The dynamic model test shows that the improved out-of-step splitting criterion base on can phase angle be used to distinguish out-of-step oscillation of high series compensation lines.(5)A new criterion of non planned island identification for grid-connected PV system is proposed,which synthesizes power-frequency variation impedance and harmonic variation rate.The variation law of power-frequency equivalent impedance and harmonic voltages and currents of the grid-connected PV system before and after islanding is studied.A new criterion of islanding identification is constructed by using the significant difference of power-frequency variation impedance and harmonic voltages.The new criterion is not affected by the short circuit and harmonic disturbance of the system,and does not affect the power quality in the detection process.The correctness of the new criterion is verified by RTDS test and actual islanding data.