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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.

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.

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.

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.

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.

It is of great importance for multi-terminal DC (MTDC) system to immediately isolate the DC fault line within several milliseconds after DC fault due to VSC or half-bridge based MMC converters with limited ability to control DC fault current during DC fault. Failing of the DC fault isolation during “ Capacitor Discharging Stage ” may lead to the overall MTDC system out of operation with the fast blocking of IGBTs. Therefore, initial DC fault current analysis is essential for system protection design, especially for fault detection. Accordingly, a transient simplified model is first proposed with only high-frequency components of the fault DC network reserved. As a result, the original complex DC network is largely reduced and the fault DC current with several milliseconds can be clearly expressed based on the proposed model. According to the distinct differences between the current of faulty cable and that of healthy cable, a non-unit DC fault detection method utilizing the transient average value of line current with low computational complexity and high-sensitivity is further proposed. Numerical simulations based on PSCAD/EMTDC have been carried out, which well-demonstrate the accuracy of the proposed transient simplified model for initial DC fault analysis and the effectiveness of the proposed transient average current based fault detection scheme. Compared with the traditional change of rate of current based protection method, the proposed one stands out for relatively low sampling frequency, high fault resistance tolerant ability, and robustness against outside noise and data missing.

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.

Doubly-fed induction generator (DFIG)-based wind farm has the characteristic of transient fault with low voltage ride through (LVRT) capability. A new three-phase fault direction identification method for the outgoing transmission line of the wind farm is presented. The ability of the new directional relay to differentiate between a three-phase fault in one direction or the other is obtained by using the increment of phase angle difference between the memory voltage signal and the fault current signal within a certain time, and using the amplitude variation of the fault current. It can be inferred that the fault current is supplied by the wind farm whether the phase angle differs or the current amplitude varies considerably. Different fault locations at the outgoing transmission line have been simulated by PSCAD/EMTDC to evaluate the reliability and sensitivity of the proposed technique. Results show that the new directional relay is of faster response when a three-phase fault occurs at the outgoing transmission line of a DFIG-based wind farm.

With the expansion of distribution network, the current component of fault point is complex when single-phase to grounding fault occurs. The traditional arc suppression coil compensation method cannot achieve full compensation of fault point current. Therefore, the active arc suppression method using power electronic devices has become a research hotspot. The active arc suppression method is to control the bus fault phase voltage to 0. This method has good arc suppression performance in the case of no load, but the fault point current will surge when the long line is under heavy load, and the arc cannot be extinguished. This paper first introduces the principle of the existing active arc suppression method, and proposes a new control method based on the fault phase current unchanged. This method measures the current before and after the fault phase at the bus outlet, and the current injected into the inverter is consistent. This control method is simple and reliable, and it is still used well in the presence of load.

The distribution network of small current grounding system often has the problem of single-phase grounding fault. The traditional passive arc extinguishment cannot meet the requirement of zero residual current at the fault point. The active arc suppression method uses the zero-sequence current injected by the power electronic equipment to control the voltage of the fault point. This method requires a large-capacity single-phase inverter, which needs high cost in production. This paper introduces the current status of the active grounding arc suppression method of the existing distribution network at first. Then, it proposes a new structure of the active arc suppression device for the distribution network. Also, it analyzes the factors influencing the performance of flexible grounding active arc suppression method. This paper provides a theoretical basis for further improving the control method of the active arc suppression method and enhancing the arc suppression performance.

Aiming at the traditional 100 Hz protection using a long delay strategy without considering the limitation of AC and DC side protection cooperation, an improved 100 Hz protection scheme is proposed. In this paper, according to the transfer characteristics and equivalent impedance of the AC/DC side of the converter, combined with the fault composite sequence network, a calculation method of 100 Hz component fault current is proposed. On this basis, through cooperation with the AC side protection settings and timing, an improved 100 Hz protection scheme with short delay capability is proposed. Simulation verification shows that the protection scheme can effectively identify the asymmetric fault and valve fault on the AC side of the fault, and can be used as remote backup protection for the AC side system.

To further improve the capability of fault identification for protection and adaptive reclosing, a fault identification scheme based on a control and protection coordination strategy for a hybrid multi-terminal HVDC system is proposed. The protection principle for fault isolation is constructed based on the difference of the polarity characteristic of fault current between the fault line and non-fault line in the uncontrolled fault stage and fault current limiting stage. In the adaptive reclosing stage, an active fault identification scheme based on the amplitude distribution law of the injection signal along the line is proposed. Intensive simulation studies prove the fault identification scheme can identify the fault area in various fault scenarios and identify permanent faults to carry out adaptive reclosing.

In order to solve the problem that the long-time delay of traditional differential protection which has reduced the reliability of high voltage direct current line backup protection, consider the control strategies and response characteristics of the converter controller, a pilot protection for HVDC transmission line based on polarity characteristics of control signal deviation is proposed. By analyzing the fault characteristics of the converter control signal during the fault, drawn the conclusion that when the DC system occurs fault, from the converter dynamic regulation stage to the fault steady-state, the deviation of rectifier constant current control signal and the deviation of inverter constant current control signal will meet the positive polarity and the deviation of inverter constant extinction angle control signal will meet the negative polarity, but these polarity characteristics are not satisfied when the AC system occurs fault, therefore, the protection criterion is constructed. The simulation results show that the proposed backup protection principle has strong anti-transition-resistance ability and high reliability. Compared with the traditional differential protection, the proposed principle can reliably identify the internal fault from the converter dynamic regulation stage to the fault steady stage, which improves the reliability of the traditional HVDC line backup protection.

Considering the advantages and limitations of traditional identification method, combined with the strategy of active detection, the principle of DC grid pilot protection based on active detection is proposed to improve the sensitivity and reliability of hybrid MMC DC grid protection, and to ensure the accurate identification of fault areas in DC grid. By using the DC fault ride-through control strategy of the hybrid sub-module MMC, the fault current at the converter station DC terminal is limited. Based on the high controllability of hybrid MMC, sinusoidal fault detection signals with the same frequency are injected into the line at each converter station. Based on model recognition, the capacitance model condition is satisfied by the detected signals at both ends during external faults whereas not satisfied during internal faults. The Spearman correlation coefficients is then introduced, and the correlation discriminant of capacitance model is constructed to realize fault area discrimination of DC grid. The simulation results show that the active detection protection scheme proposed in this paper can accurately identify the fault area of DC grid, and is not affected by fault impedance and has low sampling rate requirement.

As Type-3 wind turbine (WT) has already become a typical type of highly penetrated power sources in modern power systems, characterizing its properties during grid fault is a basic requirement for system analysis. However, many significant factors have not been considered in the existing analytical methods and results, such as various switching of internal control and protection circuits in low voltage ride through (LVRT) solutions. In this paper, performance of Type-3 WT during LVRT is first illustrated and summarized as the sequential switching characteristic in multi time scales, viz., instant control, ac control, dc voltage control, and rotor speed control time scales. Thus, for magnitude drops in stator voltage, a mathematical model is proposed to understand this characteristic and fault current analysis's features. Then, an analytical method, based on the operational inductance, is proposed so that fault current components, with the sequential switching considered, can be depicted with a unified and simple analytical expression. By identifying magnitude of symmetrical current, the transient and steady-state equivalent circuits of Type-3 WT are proposed to estimate momentary current and electrical power approximately. Finally, analytical results are verified by comparisons with real-time simulation results of a detailed 1.5 MW WT model in RT-LAB.

Fault characteristics analysis is precondition of relay protection. Theoretical derivation of permanent magnet synchronous generator (PMSG) fault current is necessary. In accordance with the theory that grid-side converter of PMSG is a power balance system under control action, an electromagnetic transient model of PMSG was proposed. Based on it, the active and reactive current was deduced and then the expression of three-phase fault current was obtained and its components, frequency, and attenuation speed of every component and other characteristics were analysed comprehensively. Simulation on PSCAD and field fault recording data as practical analysis verifies that the expressions and the characteristics are correct. The results of this paper are not only important to relay protection, but also are references to transient calculation and transient modelling.

Detection of the single-pole-grounded faulty feeder in ungrounded or high-impedance grounded MVDC distribution systems is a challenging task, because such fault cannot provide persistent and considerable fault current. To solve this problem, zero-mode fault network of radial MVDC system is analyzed and an algorithm based on identification capacitance to ground of the feeder’s zero-mode fault network is proposed. Then on the premise of external fault, the mathematical model of zero-mode network is built for each feeder. The transient zero-mode voltages and currents of each feeder are utilized to calculate the capacitance to ground of zero-mode fault network. For the healthy feeder, the zero-mode fault network is its own zero-mode network and the calculated capacitance to ground is positive. But zero-mode fault network of faulty feeder is an equivalent zero-mode network of its background healthy system and its calculated capacitance to ground is negative. This characteristic is the basic theory of the proposed algorithm. Considering the calculation error, the polarity of the average value and deviation of calculated capacitances and the number of positive calculated capacitances are used to detect the faulty feeder. The simulation results have shown that the proposed algorithm is accurate, reliable and more sensitive to detect high impedance ground faults. Index terms: MVDC distribution grids; single-pole-grounded fault; Faulty feeder detection; parameter identification

To reduce the impact of traveling wave velocity, wave head recognition, and LMD (Local Mean Decomposition) end effect and mode mixing on location accuracy, this paper presents a fault location method based on improved LMD for hybrid three-terminal HVDC transmission lines, which is not affected by wave velocity. Firstly, the improved LMD is used to get the time when the fault traveling wave first arrives at the four measuring terminals. Then the fault branch is identified according to the ratio of the time of the fault traveling wave arriving at the measuring terminal. Finally, the equation is listed according to the principle of traveling wave location and the measurement equation which is not affected by the wave velocity is deduced. A hybrid three-terminal HVDC transmission system model is built by PSCAD to simulate various faults and output fault data under various fault conditions. MATLAB is used to process the fault data and verify the location algorithm. The simulation results show that the location method proposed in this paper is not affected by fault distances, fault types, infeed levels and transition resistances. In addition, it has higher accuracy than LMD based on CSI (Cubic Spline Interpolation) algorithm, wavelet packet location algorithm and double-ended location method of using wave velocity calculation.

Due to the special fault features of the inverter-interfaced system, traditional phase selectors exist adaptability problems. In this paper, a novel voltage phase selector based on a two-stage control strategy is proposed, which is not influenced by the special features of the converter. First, the fault features of the converter during fault ride through (FRT) operation are analyzed. Second, using the high controlled-ability of the converter, the control strategy is divided into two stages by adding disturbances to the power references of the control system at the second control stage. On this basis, a novel superimposed sequence network is defined as the subtraction of the compound sequence networks under the first and the second control stage. Then, the phase-angle differences of sequence voltages in the novel superimposed sequence network are analyzed, which are not influenced by the fault features of the converter and can be used to establish phase selection criteria. The simulation results show that the proposed phase selection scheme is immune to the fault features of the converter system, fault resistance, fault location and the operation modes of the remote end system.

Effective fault identification scheme plays an important role in the fault isolation stage and system recovery stage of multi-terminal DC grid. Therefore, an active detection based single-end DC fault identification scheme is proposed to improve the reliability and sensitivity of fault identification. The control strategy is suggested of the generation of sinusoidal signals at different frequencies by the hybrid modular multilevel converters. Based on the blocking effect of the fault branch on the injection signal, by introducing the improved correlation discrimination algorithm, a single-end fault identification scheme is proposed by using the amplitude difference between the measured and calculated values of injection signals. The proposed scheme can be used for fault isolation and adaptive system recovery. Intensive simulation studies prove the improved sensitivity, and reliability of the proposed scheme for the full length of transmission line without relying on DC boundary. Moreover, the scheme can identify the permanent fault during the system recovery stage.

The fault characteristics of the wind power system, which are reflected in the weak feed features, variation of system impedance, frequency deviation, and non-sinusoidal waveform, will cause the inadaptation of traditional phase selectors to a wind power integration system. In this paper, a new phase selection principle based on the waveform correlation of transient voltage is proposed, which is valid for relay protection of intermittent solar or wind power plants. First, the characteristics of transient voltage of various fault types are deduced according to their boundary conditions. And on this basis, the correlations between the fault transient voltages of every two phases can be acquired. With the differentiated waveform correlations of different fault types, the fault phase selection criterion based on correlation coefficient is formed accordingly. Because it depends on the waveform characteristics of fault transient voltages, the new phase selector is not influenced by the wind power system’s fault characteristics and it can select the correct fault phase quickly with high sensitivity. The proposed novel phase selection criterion is verified by the PSCAD-based simulation results and field fault recording data of the wind power integration system.

An analytical method of fault characteristic for the HVDC system based on frequency response characteristics of boundary elements is presented here. The computational formulas of transfer function and input impedance are deduced using the distributed parameter model of HVDC transmission line, and the amplitude-to-frequency-characteristics of the transfer function and input impedance are analyzed. Based on the amplitude-to-frequency difference between internal and external faults, a non-unit protection method for VSC-HVDC transmission line is presented. Using the current ratio of high-to-low-frequency, this protection method can distinguish internal from external fault. The presented algorithm only uses local-end current, has high operation speed, and is easy to implement. Simulations on a ±200 kV VSC-HVDC system are conducted to demonstrate the validity and feasibility of the developed protection method.

The active power fault component is calculated by extracting the voltage fault component and current fault component at the protection installation. According to the difference of the high and low frequency energy of the signals on both sides of the boundary element of the hybrid bipolar high voltage direct current transmission line, and considering the influence of different transition resistance on the result, a protection scheme based on the transient energy ratio of the active power fault component is constructed and specific protection criteria are given. In the protection scheme, the difference value of energy ratio between positive and negative poles is used to distinguish single and bipolar faults; the amplitude of energy sum of positive and negative bands is used to distinguish positive and negative faults and the amplitude of energy ratio is used to identify internal and external faults. The ±500kV hybrid bipolar HVDC transmission system is built by PSCAD/EMTDC electromagnetic simulation software. The active power fault component is decomposed by wavelet packet transform, and the transient energy of each node is obtained, which is verified by the protection criterion. The verification results show that the protection scheme has high resistance to transition resistance and the data time window is only 3ms. Under the different working conditions, it can quickly and accurately identify internal and external faults and perform fault pole selection.

In order to ensure the safe operation of the hybrid double-ended HVDC transmission system, the current similarity between the two ends of the hybrid double-ended DC transmission line is analyzed in this paper.By analyzing the fault characteristics，it is found that the fault current on both-end lines when internal fault occurs is no obvious linear relationship,whereras the fault current on both-end lines when external fault occurs is obvious linear relationship. According to the difference of current fault characteristics on the double-end lines when internal or fault occurs, a pilot protection method for hybrid DC transmission lines based on correlation coefficient is proposed in this paper. The simulation model of LCC-MMC hybrid double-ended DC transmission line was built by PSCAD, which is used to output the fault data under different conditions. And the fault data processed with MATLAB to carry out the algorithm simulation.The simulation results verify the correctness of the proposed protection method. In the practical application, the proposed method does not need the fault data on both ends to be strictly synchronized,and it can identify the fault quickly and reliably with strong practicability.

In order to improve the reliability of DC line protection, it is necessary to study the principle of back-up protection, which can distinguish transient and steady process after fault. Principle of fault whole-time Differential protection based on polarity characteristics of current fault component difference is proposed. By analyzing the characteristics of the current fault components in transient-state (current fault components in the non-response state, the dynamic regulation state) and the steady-state of the fault, during the whole process of the fault, it is found that the fault component difference between rectifier side and inverter side satisfies the positive polarity characteristics when internal fault occurs. But it is not satisfied when external fault occurs. The protection principle criteria satisfying the characteristics of the whole process of the fault is established to realize the protection action. The simulation results show that the principle can operate correctly under various working conditions, and is less affected by the harmonics of fault characteristics of AC system. It has strong resistance to transition resistance and anti-interference ability.

This paper presents a single-ended fault location method for MMC-HVDC line. The method generates an active signal through converter control during fault ride through process. The fault location algorithm is based on parameter identification and distributed parameter line model. Simulation results has proved the high reliability and accuracy of the proposed method. The location can be calculated repeatedly and the accuracy of the location result is independent of higher sampling frequency. The impact of transition resistance can be eliminated with DC current control.

In AC/DC hybrid system, unsuccessful auto-reclosing in single-phase faults in AC side may cause continuous commutation failure, an adaptive single-phase reclosure scheme is proposed using the impedance property of the fault loop in transmission lines. The impedance is determined by the terminal-voltage of the fault phase and current of the ground capacitance. In transient faults with extinguished arc, the impedance is mainly determined by the ground capacitance of the fault phase. In permanent faults, it is mainly determined by the ground capacitance of the fault phase and transition resistance, which distincts from transient faults. The results show that the scheme is unaffected by fault location, line compensation and transition resistance and it is suitable for transmission lines with shunt reactors at one or both terminals.

In order to clarify the coupling relationship of fault information transmission characteristics in AC/DC hybrid system, a simplified model of DC system with electrical quantity transmission characteristics is established. Based on the switching function transfer theory, the law of fault information transmission on AC and DC sides is analyzed. On this basis, the dynamic phasor theory is introduced. Considering the steady-state, symmetrical and asymmetrical fault conditions, a simplified model of DC system is established based on the equivalent impedance of DC system and the control system. Combined with the simulation verification, the accuracy of the simplified model is verified by comparing with the detailed model. According to the general law of fault information transmission characteristics, the theoretical basis is provided for the follow-up research on the backup protection principle of AC/DC hybrid system.

Current differential protection based on Kirchhoff's law has sensitivity problem due to the long-distance DC transmission lines with distributed parameter effect. In this paper, the charge of the DC transmission line under the situations of external and internal faults is analyzed. The feature is that the charge will be injected into the fault point from both sides of the DC line when the internal faults occur, and if the external faults occur, the distributed capacitance of the DC line will be discharged. Then a pilot protection principle based on charge balance is proposed using the polarity of the sum of the charge in a time period. The proposed principle is simple and independent of the parameters of the DC line, and it theoretically suits for both transient process and steady-state process. Finally, an MMC-HVDC transmission system is built in PSCAD/EMTDC, and the simulation results show that the method is reliable and not affected by fault resistance.

The control system is significant for steady and dynamic response of voltage source converter multi-terminal DC (VSC-MTDC) system. The VSC-MTDC system is first built and small signal models of different control modes are deduced. Then the converters in VSC-MTDC system are classified into two types. As for VSCs connected to active networks, design procedures for PI control parameters in outer loop and inner loop are given respectively. Design procedure is also given for VSCs connected to passive networks when AC voltage control is adopted. Simulation results, carried on PSCAD\EMTDC, are presented under various operating states.

When the current differential protection is applied to very long transmission lines such as half-wavelength transmission lines, it suffers from the distributed capacitive current as well as from low sensitivity in case of a high-resistance fault. In case of an external fault, the variation of the line storage energy, the distributed resistance energy loss, and the energy injected from both ends of the transmission line satisfy the energy balance equation, and in this case the energy balance is independent of the line length. However, in case of an internal fault, the energy dissipated by the fault resistance breaks the energy balance. Based on the theory of energy balances, this paper proposes a novel pilot protection principle. Compared with the existing long-distance transmission line protection principles, the proposed pilot protection principle has the advantage of not being influenced by the distributed parameters of the long line, and in addition it is suitable for identification of high-resistance grounded faults. A simulation based on EMTDC/PSCAD illustrates the benefits of the proposed approach.