Theory And Applications Of Fast Distance Relays Based On Transmission Lines Transient Components

With the development of large-scale new energy and EHV/UHV transmission technology,China has formed the world's largest AC/DC hybrid power system with the highest voltage level.The risk of cascading failure caused by single failure of the AC system is greatly increased.The relay protection of the AC system puts forward higher requirements.The fault transient characteristics of AC/DC hybrid power grid with large-scale new energy are more complex than the traditional AC systems.Therefore,the nominal frequency phasor is difficult to accurately extract and the linear circuit superposition principle has the risk of failure.As a result,the performance of traditional distance relays degrades in the new situation of the power system.Improving the performance of distance relays in AC/DC hybrid power grids with large-scale new energy is an urgent problem to be solved.The distance relays principle based on transient components is an important research direction to solve the problem facing by the nominal frequency phasor based distance relays.With the support of the key project of smart grid technology and equipment of National Key R&D Program of China and the National Natural Science Foundation of China,this dissertation focuses on the principle and application of fast distance relays based on transient components.Aiming at the difficulty in detecting the fault direction under zero-voltage fault conditions of the equal transfer process of transmission lines(ETPTL)based fast distance relay algorithm,a novel fault direction identification element is proposed in this dissertation which is suitable for the traditional AC transmission lines and doubly fed induction generators(DFIGs)based wind farm integration system.The virtual digital capacitor voltage transformer(CVT)design method based on transient equivalent circuit has demerits as online computation burden and custom design requirements.In order to improve the engineering practical value of ETPTL based fast distance relay,a virtual digital CVT generalized design method based on transient characteristic matching is proposed.And the transient characteristics of different CVTs can be accurately matched by parameters tuning.On this basis,an improved fast distance relay scheme is formed.Then,a prototype of the improved fast distance relay is developed and the performance of the proposed fast distance relay scheme is verified by the real-time digital simulator(RTDS)closed-loop test system.Aiming at the difficulty in detecting the fault direction under zero-voltage fault conditions of the ETPTL based fast distance relay algorithm,this dissertation proposes a novel fault direction identification element based on the low frequency components.With the differentiated correlation coefficient between the memory voltage drop and actual voltage drop on equivalent impedance from relay location to back-side and opposite-side equivalent power sources under different fault conditions(forward faults and reverse faults),the fault direction identification criterion is formed.Meanwhile,an outgoing line fault identification element is also presented.By comparing the integration of the absolute values of residual voltage and setting voltage during a short time after the fault,the outgoing fault can be detected rapidly.With the cooperation of directional element and outgoing line fault identification element,the outgoing line fault can be quickly isolated.Then,the fast distance relay scheme based on the ETPTL is improved.Virtual digital CVT transfer method and fault point voltage restructuring method are applying to reduce the CVT transient impact on the outgoing line fault identification element.The influence of equivalent impedance changing on the proposed directional element is also analyzed.And the proposed directional element is not sensitive to the variation of equivalent impedances.Simulation results show that the proposed fast distance relay scheme has fast tripping speed under the outgoing line fault conditions and effectively reduces the CVT transient characteristic impact on distance relays.The main problem of the distance relay used in DFIGs based wind farm integration system is the fault direction identification under the zero-voltage fault conditions.First,the instantaneous value equivalent model of DFIG is established and characteristics of the rotor flux of DFIG under different fault conditions(balanced faults and unbalanced faults)are also analyzed.The characteristics of the transient electromotive force(EMF)of DFIG after the faults are quite different from that of conventional synchronous generators,which shows that the EMF of DFIG has the inertia within a short time after the faults.Then,a directional element in time domain based on the inertia of the EMF of DFIG is presented.On this basis,an improved fast distance relay scheme is formed.Extensive simulation results indicated that the proposed fast distance relay scheme can detect the fault direction quickly with high sensitivity under zero-voltage fault conditions in wind power integration systems and has the good performance under other fault conditions.The design of virtual digital CVT transfer plays a key critical role in the ETPTL based fast distance relays.However,there is not a simple and universal design method of virtual digital CVT.This is the main obstacle for the fast distance relay to be applied in practice.This dissertation proposes a simple and universal design method of virtual digital CVT to promote the fast distance relay application in practice.A cascaded infinite impulse response(IIR)digital filter is designed to match the transient characteristic of CVT.The rules of optimum ordering and pole-zero pairing of the cascade IIR filter are given to ensure the performance of the cascade IIR filter.The proposed virtual digital CVT design method can achieve the standardized design of proposed fast distance relays.The virtual digital CVT design becomes a parameter setting procedure in numerical distance relay devices and it is easy to adapt the different structures of CCVT in practice.The proposed virtual digital CVT design method can greatly promote the fast distance relay application in practice.In order to test and verify the performance of the proposed fast distance relay scheme,a prototype device is developed.The RTDS closed-loop test system is built in the laboratory.Different fault points and fault-types are set to test the performance of the proposed fast distance relay.The RTDS experimental results show that the proposed fast distance relay scheme can reduce the impact of CVT transient to improve the performance of distance relays.It takes no more than 5 ms to send the tripping signal under the zero-voltage forward fault conditions.And it takes no more than 15 ms to send the tripping signal under the internal fault conditions.The proposed fast relay scheme has the high reliability under the external fault conditions.At the end of this dissertation,all the research work and results are summarized and the future study points are given.