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Wind energy is a good alternative to fossil fuels, as the use of fossil fuels has seriously exacerbated the emission of greenhouse gases such as carbon dioxide and has greatly affected the environment. Conventional AC wind farms and AC transmission systems inevitably face problems involving reactive currents and overvoltage in the context of large-scale, large-capacity, and long-distance transmission. However, the use of all-DC wind turbines, together with DC convergence and DC transmission systems, has obvious advantages over AC transmission in terms of transmission losses and expandability. Such technology does not require bulky frequency transformers and can well solve the aforementioned problems of reactive currents and overvoltage. This paper proposes a new series–parallel structure for an all-DC wind power generation system. The series end uses a DC/DC converter based on the Cuk circuit to solve the current consistency and power balancing problems of the series wind turbine through current control, whereas the parallel end uses a large-capacity DC/DC converter based on the capacity transfer principle, to solve the problem of voltage consistency at the grid-connected end. The series part is used to increase the voltage level of the system, which can reduce the huge construction costs of offshore platforms, and the parallel part is used to increase the capacity of the system, which enables its incorporation into large-scale wind farms to achieve the replacement of fossil fuel energy.

High voltage DC (HVDC) grids have raised high-speed operation and high reliability requirement for relay protection. This paper aims at proposing a protection scheme with high operation speed, high sensitivity, and high reliability. Firstly, 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 (i) the index of the traveling wave propagation term in case of internal faults is greater than that for the external faults, and (ii) the index is independent of fault impedance. Secondly, the Levenberg-Marquardt (LM) algorithm is introduced to extract the index and an index-based non-unit 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 have 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 has high sensitivity, high operation speed, and high reliability.

Traditional single-ended fault location method is influenced by the fault resistance and the remote in feed, which reflects as the two equations built by fault phase have four unknowns. And in the grid-connected converter system, the weak feedback characteristic further reduces the performance of the method. To solve it, a single-ended fault location method based on control and protection coordination is proposed in this paper. First, using the high controlled-ability of the converter, the control strategy for fault ride-through (FRT) operation is divided into two stages by switching control modes. Then, according to the electrical quantities under two stages, four uncorrelated equations can be established, and the equations still have four unknown parameters. The fault distance can be calculated by solving them. The simulation results show that the proposed method has high accuracy and is immune to fault resistance and fault location, it solves the problem of insufficient equations in principle.

Accurate fault location plays an important role in the fault clearance of the transmission line. However, because of the influence caused by fault resistance and the remote end system, there are four unknowns in the two equations built by the fault phase. Thus, the equations cannot be solved. In this paper, a single-terminal fault location method for transmission lines integrated by the inverter-type source is proposed. For a single-phase to ground fault or phase-to-phase fault, due to the specific control strategy, the equivalent positive and negative-sequence impedances of the converter are unequal in most cases. However, the positive and negative-sequence impedances of other elements are approximately equal, and the boundary condition at the fault branch is definite. Combining the boundary condition and the sequence impedance difference, two uncorrelated equations can be built. Substituting them to the equations built by the fault phase, the fault location can be calculated accurately. And the influence caused by fault resistance and the remote end system is eliminated. The simulation results show that the proposed method can achieve fault location accurately. Moreover, it has higher reliability and is independent of control strategy, fault distance and fault resistance.

When a high-impedance fault (HIF) occurs in a distribution network, the fault current is very weak and can be easily confused with capacitor switching (CS), load switching (LS) or inrush current (IC). This can lead to an inaccurate judgment of current protection. Furthermore, when the external noise is too strong, the above problems become more prominent. To address these problems, this paper, based on the build-up characteristics of HIF current, proposes a novel method combining stochastic resonance (SR) using noise and variational mode decomposition (VMD). First, SR is used to detect the transient zero-sequence current in strong background noise and obtain the output current. Second, VMD is adopted to decompose the output current, and low-frequency and high-frequency intrinsic mode functions (IMFs) are obtained. Then, kurtosis and skewness values are calculated to construct the starting and identification criteria, respectively. Finally, when the kurtosis value is greater than 3.0, the protection device is started, and then, when the skewness value is less than 0, that is, when negative skewness occurs, it is judged as a HIF; conversely, when the skewness value is greater than 0, that is, when positive skewness occurs, it is judged as CS, LS or IC.

A fault identification scheme for protection and adaptive reclosing is proposed for a hybrid multi-terminal HVDC system to increase the reliability of fault isolation and reclosing. By analyzing the "zero passing" characteristic of current at the local end during the converter capacitor discharge stage, the fault identification scheme is proposed. The distributed parameter-based fault location equation, which incorporates fault distance and fault impedance, is developed with the injection signal and the distributed parameter model during the adaptive reclosing stage. The fault distance is determined using a trust region reflection algorithm to identify the permanent fault, and a fault identification scheme for adaptive reclosing is developed. Simulation results show that the proposed scheme is suitable for long-distance transmission lines with strong anti-fault impedance and anti-interference performance. Also, it is less affected by communication delay and DC boundary strength than existing methods.

In order to improve the ability of the modular multi-level converter to quickly clear and isolate faults in the multi-terminal flexible DC grid, this paper analyzes the characteristics of current faults in the multi-terminal flexible direct current grid, introduces the comprehensive distance of current fault waveform, and proposes a line protection method based on the similarity of current fault waveform. This protection method firstly uses Euclid DTW (Dynamic Time Warping) distance and entropy weight method to process electrical quantity signals, calculates the comprehensive distance of current fault waveform of two converter stations, and builds the protection criterion based on the similarity of current fault waveform. Then, the fault identification is realized by using the calculated comprehensive distance, and the fault pole identification is realized by using the ratio of comprehensive distance between positive and negative poles. Finally, PSCAD was used to build a simulation model to output the fault data under different fault conditions, and MATLAB was used to process the fault data to verify the protection algorithm. The simulation results show that the proposed method has good resistance to high resistance, and its rapidity and adaptability are further improved. The research results will provide a theoretical basis for the safe and reliable operation of flexible DC power grid.

Feeder automation is important to improve the reliability of power supply for distribution network. Existing voltage‐time feeder automation scheme exists the problems of long fault processing time and the upstream switches of the fault point need to tolerate fault current twice and the upstream sections need to experience two short time power outage. In this paper, a quick fault location and isolation method for distribution network based on adaptive reclosing is proposed. Three key technologies of the strategy are introduced, including a phase segregated switch of the distribution network based on the permanent magnet operating structure, the phase selector based on transient current characteristics, and the temporary fault and permanent fault identification method based on the power frequency voltage characteristics. The proposed strategy can avoid the second impact on upstream switches and power‐off of the upstream section in case of the permanent fault and can effectively shorten the fault processing time.

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.

At present, the MVDC distribution grid is developing rapidly and relay protection is the key technology to ensure stable operation. Therefore, this paper focuses on the fault line selection method. Firstly, the fault characteristics are analysed when a fault occurs in the line, and a novel single‐pole fault line selection method is proposed based on 0‐mode power. The polarity of current fault component measured at the terminal of each line is used to identify the internal and external faults. And the 0‐mode power is used to select the faulty line. Finally, to verify the correctness of the proposed method, a radial MVDC distribution grid is built in PSCAD to output fault data in different fault conditions, and the fault line selection algorithm is implemented in MATLAB. The simulation results show that the proposed method is fast, effective, reliable and it has a certain degree of tolerance to transition resistance with a strong anti‐data delay and anti‐interference ability. Moreover, the fault line selection method proposed in this paper can still correctly select the fault line in the case of serious faults such as multiple line faults, sudden removal of lines, and sudden changes in photovoltaic output power.

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.

Zero-sequence voltage and current based single-phase grounding (SPG) fault detection and location methods in non-effectively grounded distribution network have limitations in practical use due to lack of zero-sequence CT and PT in some cases. To deal with this problem, the phase current characteristics under SPG fault are analysed and the SPG fault detection method and section location method is proposed. PSCAD-based simulation results and fault recording data show that the proposed method behaves well when SPG fault with different impedances occurs in different sections. The proposed method can select the faulty phase automatically without using voltage signals, which is suitable for all IEDs.

Hybrid HVDC consisting of line commutated converter (LCC) on rectifier side and modular multilevel converter (MMC) on inverter side is the favourable future trend of HVDC projects utilising the advantages of both converter technologies. However, fault clearing and adaptive restart capabilities of such hybrid HVDC are not much investigated in the literature. In this paper fault clearing of hybrid HVDC transmission line is investigated using mechanical DC breaker at MMC terminal and forced firing angle retardation at LCC terminal. Then, the characteristics of the line voltages are analysed during the fault blocking states of converters to clear the transient fault. Characteristics of the line voltages of LLC rectifier have shown prominent difference under permanent and temporary fault states, and also dependent on the severity of fault. Therefore, based on line voltage characteristics, a restart strategy is proposed for hybrid HVDC which is adaptive to the fault characteristics. Several PSCAD/EMTDC based simulation studies are conducted to validate the proposed fault recovery strategy. Simulation results validate the effectiveness of the proposed strategy for hybrid HVDC under DC line faults.

In hybrid bipolar DC transmission systems with different types of converters at each pole, the transient high‐frequency component of the voltage signal under a single‐pole grounding fault and an inter‐pole fault is significantly different for internal and external faults because of smooth‐wave reactors on both sides of the DC line. Based on these characteristics, a single‐ended electrical quantity protection scheme based on transient energy is proposed. First, the voltage fault component is extracted and then processed by using a wavelet packet transform to obtain the transient energy in each frequency band. Second, the protection criterion is determined based on the ratio between low‐frequency energy and the sum of high‐frequency energy. After the setting principle is given, the influence of the protection scheme under high transition resistance is analysed. The protection scheme is implemented in MATLAB and tested based on fault data obtained from a hybrid bipolar HVDC transmission model built in PSCAD under different operating conditions. The effectiveness of the proposed protection method is verified by simulation tests under different fault types at different fault distances. The proposed method can provide strong tolerance to high transient resistance, accurately identify internal/external faults and automatically identify fault poles.

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.

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.

The multi‐terminal hybrid line commutated converter (LCC)/modular multilevel converter (MMC)‐based high voltage dc (HVDC) has been applied to solve the simultaneous commutation failure problem in East China. Here, the propagation of the travelling wave is characterised by transfer functions and the characteristic of the fault current travelling wave in the multi‐terminal hybrid LCC/MMC HVDC system is analysed. Then a directional pilot protection for the multi‐terminal hybrid LCC/MMC HVDC system is proposed. The proposed method is implemented to the designed HCM5000 dc control and protection device and verified in the Kun–Liu–Long three‐terminal hybrid LCC/MMC HVDC real‐time digital simulator (RTDS) test system. The test results show that the proposed method meets the engineering requirement of multi‐terminal hybrid LCC/MMC HVDC system for line protection.

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.

As a new highly penetrated power source of modern power systems, doubly fed induction generator (DFIG)-based wind turbine (WT) has a different fault current contribution when compared with a synchronous generator. DFIG-based WT's fault current has significant impacts on distance relay's performances and may result in maloperation. In this study, the impacts of DFIG-based WT's fault current on the dynamic performances of two typical distance protection elements, mho and polarisation, are studied in terms of EMTP simulations and fault current expressions in symmetrical fault scenarios. It is discovered that the zone-1 distance relays of the lines connecting to DFIG-based wind power plants tend to overreach. Also, the following analysis indicates that the transient components of DFIG-based WT's fault current and the phasor estimation unit are the underlying reasons. With those understanding, this study is expected to help design new setting methods and for distance protection in power systems integrating wind power plants to prevent this kind of possible overreach.

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.

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.

Difficulties in identification of the fault property, for a fault on a single outgoing line of windfarm, arise due to rapid decrease in the electromagnetic energy of the transmission line after the tripping of three-phase breaker. An active solution to such problem is proposed based on low current injection from parallel connected static synchronous compensator (STATCOM). In the reclose sequence after wind turbines’ cut off, the circuit breaker near wind farm and grid-connected circuit breaker of STATCOM are reclosed first. Therefore, based on selected switching operation of power electronic devices of STATCOM converter, a short duration discharge of dc capacitance to the faulty ac line is suggested. Correlation coefficient of resulting current waveform is utilized as an identification criterion for fault presence. Finally, a three-phase adaptive auto-reclosing scheme is proposed for the single outgoing line of 110 kV wind farm. The effectiveness and robustness of the proposed scheme are verified by PSCAD/EMTDC simulation.

As the second part of two series paper, the protection scheme is proposed. First of all theoretical analysis and threshold calculation methods are introduced. Aiming to the problem that relay needs to send alert signal instead of trip signal in non-effectively grounded system when single pole to ground fault occurs, a fault type identification method is introduced based on the ratio of absolute values of two poles voltage derivatives. Then 1 mode voltage based protection scheme is developed and the feasibility of design of criterion based on pole voltages, impact of sampling frequency, fault types, resistance and branch on the proposed criterion are discussed comprehensively. MMC based two-terminal and radial and VSC based four-terminal dc distribution systems are simulated in PSCAD. Relay behaviors are tested in three systems under different locations, resistances and types of faults. Results show that the proposed methods can be applied to both MMC and VSC based dc distribution systems and are immune to the fault type, fault location and fault resistance.

DC distribution network has a lot of advantages over traditional ac distribution network and it is considered as one of the best power supply modes in the future. However, protection relay design for dc distribution network is a major challenge due to the requirement of high speed fault detection for the vulnerability of converters. In this paper, it is proposed that the boundary protection is very suitable for dc distribution network since it can protect the whole line only based on local measurement and does not need to coordinate with neighboring relays. However, it needs to find suitable line boundary for dc distribution network. Impedance features of magnetic ring is tested and difference in steepness and peak time of voltage travelling wave in case of internal and external faults when magnetic ring installed at both ends of dc distribution line is analyzed. Results show that magnetic ring is feasible to be used as line boundary. Besides, a wide band model of magnetic ring is proposed based on vector fitting and circuit synthesis method. It paves the way of analysis on features of internal and external faults at dc distribution line and validation of proposed fault detection method.

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.

This study presents a single-ended fault location method that does not need the information of the remote-end system. As the foundation of the proposed method, the fault location equations under different fault types are derived based on the fault loops. The fault distance, fault impedance, and equivalent resistance and inductance of the remote-end system are considered as the unknowns of the fault location equation. Then the equation can be solved using the Gauss-Newton method, by which the four unknowns can be obtained. Therefore, the proposed method is not sensitive to the fault impedance and the system parameters. A two-terminal 500 kV ac transmission line test system is built in PSCAD to verify the proposed method. Numerous simulation data of faults of different fault distances, fault impedances, and fault types verify the accuracy and robustness of the proposed method. The proposed method is a time-domain method that can locate the fault using 5 ms data after the faults.

Modular multilevel converter (MMC) shows prominent advantages on supplying passive networks, which are usually classified by power supply shortage and relatively weak structure. In order to reduce the unnecessary outage time and improve the system stability, an adaptive ac auto-reclosing scheme for passive networks is proposed based on characteristic signal injection. Additional MMC controller is designed to inject the characteristic signals into the healthy line, which in turn induces corresponding signals to the faulty line. Then fault properties are assessed by the travelling wave based fault location method. Finally, an adaptive auto-reclosing scheme is proposed to shorten the reclosing time and avoid the second fault shock to a large extent. The proposed scheme could be applied for both single-circuit and multi-circuit transmission lines. The simulation results obtained from PSCAD/EMTDC verify the effectiveness of the proposed method.

After the isolation of a single pole-to-ground fault, the adaptive auto-reclosing concept in high-voltage dc (HVDC) systems is difficult to be applied due to little coupling between the poles. With fault ride through capability, the hybrid modular mult-ilevel converter (MMC) can be advantageous for future HVDC projects. This research proposes an adaptive auto-reclosing operation for hybrid MMC-based bipolar HVDC systems, based on fault identification by characteristic signal injection. The proposed method is based on voltage controllability of healthy pole MMC to inject dc voltage perturbations. Healthy line perturbations result in the induced characteristic signals to the faulty line, which contain information about the fault property. An additional control strategy for hybrid MMC, to inject signals with suitable amplitude and frequency, is also discussed in detail. Then, fault identification is realized by wavelet transform of traveling waves caused by the signal injection. Finally, a dc adaptive single-pole auto-reclosing scheme is proposed to shorten the reclose time and to distinguish between temporary faults and permanent faults. Simulation results verify the effectiveness of the proposed method in PSCAD/EMTDC software.

The focus of the paper is the difficulty of high impedance fault (HIF) detection in distribution network, and its ease to be confused with capacitor switching (CS) and load switching (LS). Based on the intermittent reignition and extinction characteristics of HIF current, this paper proposes a novel HIF detection method, which combines variational mode decomposition (VMD) and Teager–Kaiser energy operators (TKEOs). The HIF detection method is as follows: First, perform the VMD on transient zero sequence currents to obtain the intrinsic mode functions (IMFs) and select the IMFs with the largest kurtosis value as the characteristic IMFs. Second, calculate the characteristic IMFs to obtain TKEOs and divide into subintervals of TKEOs waveform to calculate the time entropy values. Finally, construct HIF detection criterion as follows: when time entropy value is 0, it is judged as CS or LS. When the entropy value is not 0, it is judged as HIF. A large number of simulations and field data tests show that the method is accurate and stable, and under the interference of 1 dB strong noise, it can accurately judge. Compared with other methods, the method has higher feature extraction accuracy, less calculation time, and better judgment accuracy.

The result of single-ended traveling wave fault location method based on wavelet transform will be affected by false wavelet transform modulus maxima (WTMM) points. In this paper, fault distance/impedance information contained in the wave front of the traveling wave is analyzed first. Then, the fitting method based on generalized logistic function is proposed to extract the fault information. Based on the fitting results, a predictor–corrector fault location method is proposed, which includes the following content. First, the fitting coefficients of the zero-mode fault current wave front are used to estimate fault distance and fault impedance interval. Second, the arrival time of the line-mode fault current reflected wave can be calibrated and the distance calculation formula can be selected according to the fault distance estimation interval. Last, the fault distance will be corrected to be within the fault distance estimation interval, if the first reflected wave is undetected. A model of ±400 kV DC transmission project is built in PSCAD/EMTDC. The simulation results show that the fitting coefficients can directly correspond to fault distance/impedance. The proposed method can effectively eliminate the influence of the false WTMM points, and give the accurate fault location results as well as fault impedance interval.

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.

Detailed model (DM) of modular multilevel converter (MMC) brings about a high computational burden for electromagnetic transient (EMT) simulation. Therefore averaged value model (AVM) of MMC is proposed for fast simulation of large-scale multi-terminal HVDC system. No direct electrical contact exists between AC and DC system in conventional AVM and therefore the coupling relationship among electric quantities in fault analysis could not be accurately modelled. In order to solve the aforementioned problem, an improved AVM (IAVM) of half-bridge (SB) MMC topology is proposed. All capacitor voltages of AVM are constant and an arm is directly equivalent to a controlled voltage source. The proposed AVM can be applied for the DC faults and blocking conditions by introducing a group of switches connecting AC and DC sides. Two extra controlled voltage sources are introduced for correct DC voltage offset in AC voltages under DC line-to-ground (LTG) faults. Simulation results verify the accuracy and efficiency of the IAVM. IAVM could be implemented in commonly used software packages and is suitable for power system fault analysis.

Isolation of the fault dc lines during the initial short period after fault is a major challenge in the multi-terminal dc (MTDC) system based on a voltage-source converter (VSC). Existing protection schemes are mainly designed based on the numerical simulations, which lacks the theoretical analysis. In this paper, a high-frequency equivalent model of a VSC-based MTDC system is first proposed for the fault current calculation. Based on the dc fault analysis, it is validated that the high-frequency fault current components flow in the fault line and then dramatically decay in the healthy line. Accordingly, a novel dc fault detection method for the VSC-based MTDC system is proposed. In the proposed approach, the primary protection utilizes the transient high-frequency energy of line current and the fault line can be identified quickly without communication. In addition, the influence of the transient traveling-wave appearing on the long-transmission line on the proposed protection scheme is also investigated. Numerous simulation studies carried out in PSCAD/EMTDC have demonstrated that the proposed high-frequency equivalent model can be utilized for the initial dc fault analysis of the VSC-based MTDC system and the proposed protection scheme is effective in different time windows, different fault locations, and high fault resistances. Compared with the existing rate of change of current-based protection scheme, the proposed primary protection requires relatively low sampling frequency, is insensitive to the parameter changes, and has high robustness with respect to the outside noises and data missing.