Study On Methods Of Ultra High Speed Pilot Protection For High Voltage Transmission Lines

High voltage transmission lines are the backbone of power grid and used to transmit the power. When a fault occurred on the transmission lines, the line protection should operate quickly to shorten the fault clearing time and guarantee the transient stability. The traditional power-frequency component based protection does not satisfy the requirement of fast fault clearance due to the principle limitation. The transient component based protection has been widely concerned since it can achieve ultra-high-speed operation. However, the present ultra-high-speed protection generally faces the low reliability problems, which hinders the practical process. Therefore, this thesis focuses on the ultra-high-speed pilot protection, fast fault phase selection and fault location for high voltage transmission line. It mainly aims to solve the problems that the traveling wave based directional element are influenced by weak fault and CVT transient transfer characteristic and the natural frequency differential protection has the dead zone. The main contents of the thesis are as follows.(1) In the aspect of the problem that the traveling wave directional element is affected by weak fault, the thesis has proposed the polarity comparison directional element and amplitude comparison directional element based on S-transform energy relative entropy. The polarity comparison directional element employed the S-transform energy relative entropy between fault voltage and current traveling wave to represent their polarity relationship. The amplitude comparison directional element employed the S-transform energy relative entropy between the forward traveling wave and backward traveling wave to represent the amplitude difference. Then the fault direction can be identified. The two directional element avoid the directly extraction of traveling wave polarity or amplitude by introducing the S-transform energy relative entropy to quantificationally describe the polarity relationship and amplitude difference. Therefore, these two directional elements have good performance even for faults with small fault inception angles and high resistances.(2) In the aspect of the problem that the traveling wave directional element is affected by CVT transient characteristics, the thesis takes the pre-fault voltage and pre-fault current as the new reference for polarity comparison. Then two power-frequency based polarity comparison directional element are proposed. The first one utilizes the polarity relationship between the pre-fault voltage and power-frequency superimposed current to judge the fault direction while the second one employs the polarity relationship between the pre-fault current and power-frequency superimposed current. Both directional elements are independent of transient voltage. Thus, they are immune to CVT transient characteristics. Meanwhile, both directional elements are more capable for weak fault conditions since they represent the polarity relationship by the angle between the signals.(3) In order to eliminate the dead zone of traveling wave natural frequency differential protection, after analyzing the distribution of dead zone under different system impedance, a pilot differential protection method based on natural frequencies of current is proposed. The spectrum characteristic of fault current is analyzed and the natural frequencies are extracted by multiple signal classification algorithm firstly, then the difference between the dominant natural frequencies at two ends of transmission lines is used to identify the internal fault and external fault as the primary criterion. The distinct difference of the secondary natural frequency of the dead zone fault and external fault is utilized to identify the internal fault in dead zone and the external fault as the secondary criterion. The simulation results indicate that the proposed method only makes use of the current and possesses capability to identify the internal faults and external faults accurately. The dead zone of natural frequency differential protection has been solved effectively.(4) The thesis presents a phase selection method based on energy features of current modal fault components. This method utilizes the wavelet decomposition to represent the transient energy of combined current modal fault components. Then the fault phase can be selected by comparing the transient energy magnitudes of different combined current modal fault components. A large amount of simulation results show that the proposed phase selection method is capable to identify the fault phase correctly and quickly under various fault conditions. The proposed phase selection method also has strong ability of enduring fault resistance and noise disturbance.(5) For fault location, the traveling wave natural frequency based fault location method is extended to the fault location of series compensated lines. Taking the influence of series capacitor and its protection on the natural frequency extraction and boundary conditions into account, the fault location algorithm based on traveling wave natural frequency for series compensated lines is studied. The proposed fault location method judges whether the fault occurred after the series capacitor or not using the single ended frequency information. For the fault occurred after the series capacitor, the effects of reflection and refraction at the series capacitor on the algorithm need to be considered. Then the accurate location result is obtained with the relationship among the fault distance, the natural frequency and the boundary conditions. A large number of simulation results indicate that the algorithm is insensitive to the fault types, the fault distance and the grounding resistances with good accuracy and applicability.The theoretical research and simulation results in the thesis demonstrate that the dissertation forms a protection theoretical framework including identification between internal and external faults, fault phase selection and fault location, which improves the reliability of ultra-high-speed pilot protection for high voltage transmission line.