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Existing nonunit protection schemes inevitably require setting, which is a serious problem in practical engineering. Faults occurred at different fault zones will result in different equivalent models, therefore, the fault zone can be determined by recognizing which equivalent model the fault fits well with. In this article, this “model recognition” idea is introduced in fault identification and a “setting-less” protection method is proposed. First, the Peterson equivalent circuits when faults occur at backward external zone, internal zone, and forward external zone are presented, respectively. Accordingly, the corresponding three fault voltage expressions are derived, which are defined as three fault modes. Then, the three fault modes are used to approximate the measured fault voltage using Levenberg-Marquardt optimal approximation method. The fault mode that best fits the measured fault voltage is recognized as the final determined fault mode, which is used for fault identification without setting threshold value. Numerous test studies carried out in Power Systems Computer Aided Design/Electromagnetic Transients including DC (PSCAD/EMTDC) and real-time digital simulator have demonstrated that the proposed method can be utilized for different multiterminal dc systems and is effective under different fault locations, different fault types, and high fault impedances. The proposed method does not require high sampling frequency and has good robustness against measuring noise. [ABSTRACT FROM AUTHOR]

To identify the fault area more accurately and improve the reliability and sensitivity of the traditional protection principle on DC lines, an active injection for single-ended protection in the DC grid is proposed. Considering the influence of fault impedance, the effect of frequency-dependent parameters, numerical differential error, and the strength of DC boundary, high controllability of hybrid modular multilevel converter is utilized to inject sinusoidal signals with different frequencies into the line at DC terminal of each converter. Then the single-ended fault identification equation is constructed to preliminarily identify the fault area. According to the amplitude difference of the injection signals in different fault areas, the fault areas of the DC grid can be further distinguished. Simulation results verify the effectiveness of the proposed protection principle, improved reliability and sensitivity of protection, and full length protection of line without DC boundary. [ABSTRACT FROM AUTHOR]

Hybrid AC/DC transmission grids require high-speed operation and high reliability for relay protection. Traditional derivative-based traveling wave protection methods have exposed the problems of low sensitivity and low reliability. In this paper, the fault traveling wave is analyzed considering the attenuation, distortion, and boundary effect of the DC transmission line as well as three converter types. The expressions of the fault initial traveling wave are derived and the conclusions show that 1) the expressions of the fault initial traveling wave at the end of the transmission line have the formation of a(1 − exp(−bt)); 2) the index of the exponential function in the fault initial traveling wave expression is related to the fault location, and is not affected by the fault impedance and line attenuation. Then a Levenberg–Marquart fitting method is proposed to fit the measured zero-mode fault current initial traveling wave and extract the index coefficients. Finally, a novel traveling wave protection method for DC transmission lines is proposed using the obtained index coefficients. The simulation results show that 1) the proposed protection method can complete fault identification within 1 ms, even with high fault impedance; 2) the proposed protection method has high robustness to the influence of the noise. [ABSTRACT FROM AUTHOR]

The fast and selective protection using fast dc circuit breaker to break the fault current and isolate faulty lines satisfies the requirement of fault clearance in HVDC grids. However, traditional derivative-based traveling wave protection suffers from low-sensitivity problem when high-impedance faults occur. Based on the analysis of the fault initial traveling wave, it is concluded that the distortion degree of the fault initial traveling wave is related to the location of the fault, and the amplitude of the fault initial traveling wave is related to the fault impedance. Then a Levenberg–Marquart (LM) optimal approximation method is proposed to extract the fault distance information from zero-sequence fault current initial traveling wave. The optimal approximation parameters respectively reflecting the fault distance and fault impedance can be obtained. A non-unit traveling wave protection scheme is proposed based on the parameters. A ±400 kV modular multilevel converter (MMC) HVDC grid test system is built in PSCAD/EMTDC to verify the proposed method. The simulation results show that the parameters can directly correspond to the fault distance and fault impedance, and the proposed method can fast and reliably identify dc line faults. Finally, the sensitivity of the proposed method is studied by comparing with three different methods. [ABSTRACT FROM AUTHOR]

High voltage dc (HVDC) transmission, especially voltage source converter (VSC)‐based HVDC transmission requires high‐speed operation for relay protection. However, traditional derivative‐based non‐unit travelling wave‐based protection (TWP) methods are sensitive to the fault impedance, resulting in low protection sensitivity in case of the high‐impedance fault. First, the fault information containing in the fault travelling wavefront is analysed based on the frequency‐dependent parameters of the dc transmission line and the broadband characteristic of the fault‐point initial travelling wave. It can be concluded that the distortion of the fault travelling wavefront depends on the fault distance and the amplitude of the fault travelling wavefront depends on the fault impedance. Then, a principle of non‐unit TWP for dc transmission line based on waveform correlation calculation is proposed. The proposed method is verified in both thyristor‐based HVDC and VSC HVDC PSCAD test systems. The simulation results verify the high‐speed, high fault‐impedance tolerability and correctness of the proposed protection method. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC) is widely used in hybrid AC/DC power transmission system. Strong control capability of MMC provides a new way of the adaptive autoreclosing scheme in such systems to enhance the power supply reliability and system stability. A novel fault identification method is proposed in this study. It is based on characteristic signal injection utilising the control strategy of MMC and line coupling features. A characteristic signal from MMC can be injected to healthy lines which in turn induce characteristic signal to the faulty phase/line. Induced signals have different performances under normal or fault conditions. The proposed method is independent of electrical characteristics of recovery voltage and secondary arc, and multiple independent injections could be implemented. Finally, the adaptive autoreclosing scheme based on injection method is proposed. Simulation studies are carried out under the single line to ground and symmetrical faults, which strongly verify the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

Adaptive reclosing of AC line discriminates temporary faults and permanent faults by the coupling relationship between fault phase and the sound phase. While in the DC network, the electrical quantity of the DC system keeps constant after the DC line fault is cleared, and there is no coupling relationship between the sound and fault pole. This paper presents a fault identification method by injecting characteristic signals into a DC fault line. The electrode coupling characteristics of HVDC transmission lines, the injection method, and the selection of characteristic signal are given in detail. Then, the characteristic signals are injected into the sound pole through the additional control strategy of the converter station. As of electrode coupling characteristics of transmission lines, the fault line induces the coupling characteristic signal. This characteristic signal on the fault line is used to distinguish the fault property by the differences of the propagation feature between the two states, the fault existence and disappearance states. The simulation results verify the effectiveness of the method here. [ABSTRACT FROM AUTHOR]

The half bridge modular multilevel converter (MMC) coordinated with dc circuit breakers (DCCBs) is one of the most promising solutions for the future multi-terminal HVdc system protection. After the isolation of dc line fault, the electrical quantities of the dc system remain stable and no coupling relationship exists between healthy and faulty poles. Thus, ac adaptive reclose scheme is difficult to be applied in dc transmission systems since the information of the adjacent lines cannot be used to assess whether the line fault is cleared or not. An innovative adaptive reclose scheme applied to the MMC-HVdc system is proposed in this paper by injecting voltage pulses to the faulty line. When the breaker is tripped, the converter and dc faulty line are connected by the main breaker of the hybrid HVdc breaker. On the basis of switching control of solid-state devices in the main breaker, the voltage pulse injection to the dc faulty line is realized. Then, the fault characteristic is identified based on the traveling-wave-based fault location algorithm. A fast restart scheme for the MMC-based bipolar HVdc transmission system is also achieved by this method. The simulation results verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Estimating phasors as quickly as possible while guaranteeing accuracy is important to ensure reliable fault detection and isolation in power systems. Window length is an important factor for phasor estimation. The conventional fixed window length cannot handle well the many and diverse fault voltage and current signals. By analysing the relationship between accuracy and window length of a phasor estimation using the matrix pencil method, a criterion is established to determine whether or not the estimated phasor is credible. Then, an adaptive algorithm to estimate the phasors using dynamic window length is proposed. Analytical and experimental investigations show that the proposed algorithm can accurately and timely estimate phasors and can enable reliable and quick relay operation. [ABSTRACT FROM AUTHOR]

A single-phase earth fault section location method based on fault component phase current in non-solidly earthed distribution networks is proposed. During a single-phase earth fault, the fault component currents of three phases obtained from the healthy lines and the sections at the back of the fault point are of the same amplitude and waveform, whereas those differ a lot in the sections between the fault point and the bus. On the basis of this characteristic, a fault section location method is put forward: earth fault detection devices are installed at every section, and each one measures and compares its three phase fault component currents independently, thus the sections between the fault point and the bus are identified, and the fault section location is achieved combining with the network topology. The detailed flow chart and criteria are also given. Simulation results prove the validity and reliability of the method proposed. This novel method requires only current signals, involves a simple principle and a bootstrap characteristic, and is expected in practical application in distribution networks. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]