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Abstract: Secondary control cable is one of the important parts for protection device to get the accurate fault generated current travelling waves. This paper studies current travelling wave transmission characteristics in secondary control cable based on Alternative Transient Program (ATP). Valuable transmission characteristics of control cable have been got from the simulation results: current travelling waves which pass the control cable will be distorted and some interfering current travelling waves will be generated; duration of interfering current travelling waves depends on the length of the control cable; the original fault generated current travelling wave wave-front is delayed and smoothed by the control cable; magnitude of fault generated current travelling wave wave-front has been varied by the control cable. And these transmission characteristic of control cable provide useful references to the protection algorithms based on fault generated current travelling waves. [Copyright &y& Elsevier]

• Cascading fault in hybrid AC/DC grids is defined. • Mechanism and process of cascading faults are analyzed. • Some solutions to prevent cascading faults are pointed out. With the increasing use of DC transmission in existing AC networks, the possibility of AC and DC circuits running parallel to each other and sharing the same right-of-way or even the same tower is increasing. Cascading fault is a newly emerging fault type in hybrid AC/DC grids which severely threatens the power quality and safe operation of the network. Cascading fault is defined as an event which initiated from a fault in the inverter AC system or an abnormal operation of the DC system which leads to the multiple fault incidents in hybrid AC/DC power grids. In this paper, first, the mechanism and process of cascading fault in hybrid AC/DC grids are described. Subsequently, based on the analysis of several fault cases which may result in cascading faults in AC power grids, the typical characteristics of hybrid AC/DC power grids are demonstrated. The results from the simulation case studies in PSCAD/EMTDC indicate that the cascading fault is initiated from an inverter AC fault which results in a long-lasting communication failure, leading to a DC system block. Afterward, the power flow transfers from the blocked HVDC line to the paralleled AC lines which results in cascading trips in AC lines. Finally, some feasible solutions to prevent the cascading faults in hybrid AC/DC grids are pointed out. [ABSTRACT FROM AUTHOR]

The high-speed and selective dc line protection method is required for the safe operation of HVDC grids. In this paper, the expressions of the line-mode fault component voltage at the line end as well as its first peak time (FPT) when different internal and external faults occur are derived respectively. The conclusion proves that the FPT of line-mode fault component voltage for internal faults is smaller than that for backward and forward external faults, which is independent of fault impedance and fault type as well. Then a FPT-based non-unit ultra-high-speed dc line protection method for HVDC grids is proposed. The algorithm is verified by a four terminal ±400 kV MMC-HVDC grid PSCAD test model as well as implemented in the developed protection device and validated by the Zhangjiakou-Beijing MMC-HVDC grid RTDS test system. The test results show that i) the FPT-based protection algorithm is independent of fault impedance and fault type, ii) the proposed method can fast and correctly identify dc line faults in the HVDC grids, even with 500 Ω fault impedance, and iii) the sensitivity and fault identification time of the proposed method meet the engineering requirement of MMC-HVDC grids for dc line protection. [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]

High voltage dc grids are raised high-speed operation and high reliability requirement for relay protection. This article aims at proposing a protection scheme with high operation speed, high sensitivity, and high reliability. First, the symbolic expressions of the line-mode fault component traveling wave in case of the internal and external faults are derived, which consist of the traveling wave propagation term and the dc line boundary term. It is theoretically demonstrated that the index of the traveling wave propagation term in case of internal faults is greater than that for the external faults, and the index is independent of fault impedance. Second, the Levenberg–Marquardt algorithm is introduced to extract the index and an index-based nonunit protection method as well as the protection scheme is proposed. The proposed method is theoretically robust and independent of fault impedance and fault type. In the validation, the fitting results are indicated that the symbolic expressions are correct and the extracted indices are effective to characterize the location of the faults. Numerous fault cases of different fault distances, fault impedances, and fault types have demonstrated that the proposed protection scheme is of high sensitivity, high operation speed, and high reliability. [ABSTRACT FROM AUTHOR]

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]

The multi-terminal hybrid LCC/MMC HVDC has been applied to overcome the simultaneous commutation failure problem caused by the multi-infeed LCC-HVDCs in the East China. The high-speed and selectivity line protection method is required for the multi-terminal hybrid HVDC system. In this paper, the reflection and refraction of the fault traveling waves at different kinds of line terminals are characterized by transfer functions. Then the fault voltage as well as its first peak time at different line terminals when different internal and external faults occur are derived respectively. The conclusion proves that the first peak time of line-mode fault component voltage (FPTV) for internal faults is smaller than that for external faults, which is independent of fault impedance and fault type as well. Therefore, a FPTV-based non-unit ultra-high-speed dc line protection method is proposed. The algorithm is implemented in the developed protection prototype and validated in the Kun-Liu-Long three-terminal hybrid HVDC RTDS test system. The test results show that the proposed method can fast and correctly identify line faults in the multi-terminal hybrid HVDC system, even with 500 Ω fault impedance. Comparing with the traveling wave protection method of ABB, the proposed method improves the sensitivity when high-impedance faults occur. [ABSTRACT FROM AUTHOR]

Commutation failure is a serious malfunction in line-commutated high voltage direct current (HVdc) converters which is mainly caused by the inverter ac faults, and results in a temporary interruption of transmitted power and damage to the converter equipment. In this article, a controllable commutation failure inhibitor (CCFI) is developed which obviates the main drawbacks of the existing power electronic based and fault current limiting based strategies. Under normal circumstances, the developed CCFI improves the steady-state stability and the power transfer capability of the inverter ac lines, while it does not cause excessive voltage stress on the converter valves. In addition, it would reduce the risk of commutation failure occurrence, since it does not lead to any voltage drop in the commutation circuit. When a fault occurs at one of the inverter ac systems, its corresponding CCFI limits the fault current depending on the reduced extinction angle. This would not only inhibit the successive commutation failures on the HVdc converter, but also extend the lifetime of components in the inverter ac systems. The practical feasibility of the developed CCFI is assessed through laboratory testing, using a real-time Opal-RT hardware prototyping platform. The obtained results indicate that the developed CCFI can reliably inhibit the commutation failures during various types of faults. [ABSTRACT FROM AUTHOR]

• A new strategy is presented for determining optimal placement of fault detectors. • An new online algorithm is proposed to determine the accurate fault location. • To extract the traveling wave from postfault signals the MED filter has been used. • Using linear equations increase accuracy and decrease complexity of the algorithm. This paper presents a new minimal placement strategy of fault detectors for fault-location on the transmission network. The proposed methodology includes two offline and online algorithms. First, the topology of a transmission network is analyzed, and the proposed offline algorithm is employed to determine the minimum number and location of detectors. Based on the arrival time of initial traveling wave-front acquired by the installed detectors, the online algorithm is introduced to identify the fault occurrence time as well as fault location. In order to extract the traveling wave from post-fault signals and construct the online algorithm, morphological edge detection (MED) filter is adopted. Extensive simulation studies based on the electromagnetic transients program and MATLAB Software have been carried out to evaluate the accuracy and reliability of the proposed method. [ABSTRACT FROM AUTHOR]

The performance of the traditional derivative-based non-unit traveling wave protection (TWP) in high voltage dc (HVDC) grids shows the problem of low-sensitivity when high-impedance faults occur. In this article, the characteristics of the fault initial traveling wave are analyzed first, two conclusions can be drawn: i) the distortion degree of the fault initial traveling wave is related to the fault location, ii) the amplitude of the fault initial traveling wave is related to the fault impedance. Then a zero-sequence current fitting method is proposed to obtain parameters respectively reflecting fault impedance and fault distance. When a fault occurs, a fault information extraction method based on the fitting parameters will give the estimation interval of the fault impedance, which provide reference for the adaptive threshold value selection of TWP. In this way, the influence of the fault impedance on TWP can be eliminated. A ±400 kV modular multilevel converter (MMC) HVDC grid is built in PSCAD/EMTDC. The verification of the proposed method considers different installation positions of the current limit inductor. The simulation results show that the proposed method can fast and correctly identify the dc line faults and effectively improves the protection sensitivity in detecting high-impedance faults. [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]

Cascading fault is a newly emerging challenge in hybrid ac/dc grids including current source converters (CSCs) which commences from a dc or a long-lasting severe inverter ac fault and ultimately provokes a total blackout at the inverter ac side. Nevertheless, since the cascading faults emanating from dc faults are not accompanied by commutation failure, the prevailing commutation failure elimination strategies are unable to inhibit these contingencies. To obviate such a challenge, this paper proposes a coordinated power flow control strategy which can preclude the blackouts caused by cascading faults initiated from either dc or inverter ac faults, thereby improving the reliability of hybrid ac/dc power grids. To achieve this, a back-to-back voltage source converter (B2B-VSC) is employed which is responsible for controlling both the inverter ac bus voltage and the active power flow of its connected ac line. This would not only mitigate the probability of commutation failure due to the reinforced inverter ac bus voltage, but also enhance the power flow flexibility of the power grid during both normal and fault conditions. In addition, the cost associated with implementation of the proposed strategy is much less than installing several backup parallel lines which are useless under no-fault circumstances. Finally, the practical viability of the proposed strategy is scrutinized through laboratory testing using co-simulation of RTDS and Opal-RT simulators. The obtained results show that the developed strategy can effectively manage the power flow of B2B-VSC and CSC-HVDC systems during different possible fault cases so that the power outage at the inverter ac side is minimized. • A power flow control strategy to preclude blackouts caused by cascading faults. • Reliability Enhancement of hybrid ac/dc power grids. • Mitigation of Commutation failure probability in LCC-HVDC Systems. [ABSTRACT FROM AUTHOR]

This article proposes a novel fault diagnosis method to detect the fault of high-voltage capacitor (HVC) for dc filter (DCF) in high-voltage direct current (HVDC) systems. The proposed diagnosis principle is developed through identifying the change of HVC capacitance, which is calculated in real time using the measured voltage and current of DCF. Compared with the traditional methods, the proposed approach is superior in detecting successive faults and symmetrical faults since any HVC fault will change the capacitance parameter of HVC essentially. Simulation and experimental results verify the performance and superiority of the presented method. [ABSTRACT FROM AUTHOR]

Line Commutated Converter (LCC) based High-Voltage Direct Current (HVDC) technology has been in operation with a high level reliability and little maintenance requirements for more than thirty years. The current-source based or classical LCC-HVDC systems are being considered for buried cable transmission as well as overhead transmission. The fault analysis and protection of LCC-HVDC system is a very important aspect in terms of power system stability. This paper proposes a novel protection scheme for LCC-HVDC systems, in which the difference between propagation processes of traveling wave under internal and external faults is used as a criterion for detection of fault incidents in HVDC transmission lines. In order to quantify and intensify this difference, Teager Energy Operator (TEO) is used which has the ability to reflect the instantaneous energy of a signal. The main feature of the proposed scheme in comparison with the existing ones is that it operates faster, since it does not require to extract any harmonic or high frequency component; moreover, a 2-ms sampling window is sufficient for its algorithm which only deals with simple calculations. In order to validate the effectiveness of the proposed protection scheme, several fault events under different fault resistances and fault locations were simulated on a test network using PSCAD/EMTDC software. Also, the performance of the proposed scheme under real fault events was tested using four field data cases. Both simulation data and field data test results indicated that the proposed protection strategy has the ability to accurately discriminate between internal and external faults and detect the faulted pole in the bipolar systems even under high-impedance fault conditions. [ABSTRACT FROM AUTHOR]

Due to the widespread proliferation of distributed generation resources and the current market situation, ensuring the security and reliability of power grids against fault events has become a more challenging task. The aim of this paper is to compare different power flow techniques for power grid vulnerability assessment against symmetrical fault incidents using bus impedance matrix. In this study, first, the relationship of the post-fault voltage phasor at each bus with the pre-fault voltage phasors at that bus and the faulted bus, impedance matrix elements, and fault impedance is investigated through power system analysis under pre-fault and post-fault circumstances. Subsequently, the accuracy of different iterative and non-iterative power flow algorithms, i.e. Newton Raphson (NR), Fast Decoupled (FD), and Direct Current (DC) methods, for the power grid vulnerability assessment is compared. To achieve this, the fault analysis at each bus is performed commencing with a very large fault impedance and ending with the fault impedance at which one of the buses reaches the low voltage violation limit. Finally, to appraise the proposed strategy, several simulations have been undertaken on IEEE 14 bus system using MATLAB software. The simulation results indicate that the power grid vulnerability against symmetrical faults is highly influenced by the type of applied power flow technique. • Extraction of post-fault bus voltage equation using bus impedance matrix. • Power grid vulnerability assessment against symmetrical fault incidents. • Comparison among various power flow methods for power grid vulnerability assessment. [ABSTRACT FROM AUTHOR]