DC Line Protection Research Articles

Flexible DC transmission line fault type identification scheme based on Pearson correlation coefficient

Abstract Faults occurring on the DC side of the line are very harmful to the MMC-MTDC system, and with the popularization of flexible DC transmission technology based on overhead lines, how to discriminate the type of fault is the difficulty of MTDC DC line protection. This paper gives a fault kind identification approach based totally on the Pearson correlation coefficient, which discriminates the fault kind with the aid of calculating the Pearson correlation coefficient of the line voltage at each pole after the fault.

A single-ended high-impedance fault protection scheme for hybrid HVDC transmission lines based on coordination of control objective

Hybrid HVDC transmission technology advancement has led to significant progress. A reliable and effective DC line protection scheme is crucial for DC transmission systems. The traditional protection schemes suffer from insufficient sensitivity and reliability during high-impedance faults on DC lines. To address the issue, this study introduces a hybrid HVDC line single-ended protection scheme based on the coordination of converter control objectives. In the fault-ride-through(FRT) stage, different control strategies and objectives are applied to the converter based on the fault pole and fault direction identification results. Internal and external faults are discerned by calculating measured voltage with the coordination of the converter control objective. This approach maintains heightened sensitivity to high-impedance DC line faults. Moreover, it operates independently of double-ended communication and demands a low sampling rate. Extensive simulation results robustly substantiate the efficacy of the proposed protection scheme.

A Decision Tree Based Ultra-high-speed Protection Scheme for Meshed MMC-MTDC Grids with Hybrid Lines

The reliable operation of modular multi-level converter based multi-terminal high voltage direct current (MMC-MTDC) grids requires high-speed and selective DC line protection. A single-ended protection technique is proposed for MTDC grids with offshore wind farms and hybrid lines that involve overhead lines and submarine cables in series. Positive and negative poles’ currents at one end of each line section are analyzed using discrete wavelet analysis (DWA). To classify fault type and identify fault zone, a fine decision tree is supplied with an energy index and the envelope slop of detail 1 coefficient (D1) obtained from DWA. Only 0.2 ms following the fault inception are needed for calculating the energy index and the envelope slope to perform relay functions. The approach was tested on a three-terminal two-poles ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm$$\\end{document} 400 kV MMC-MTDC model. The simulation results validate the effectiveness of the suggested protection technique under various fault scenarios, even with up to 200 Ω fault resistance. The proposed technique is not only able to detect the faulty line, but also it identifies overhead line faults and submarine cable faults for hybrid type lines. Moreover, the proposed technique is not affected by wind power injection changes, AC faults, or data noise. The simulated model was produced in the MATLAB/Simulink platform.

A Protection Principle of LCC–VSC Three-Terminal HVdc System Based on Instantaneous Boundary Impedance

In the field of hybrid multi-terminal HVdc (high voltage direct current) transmission, an inevitable problem is the short availability of fault information. Namely, only 1-ms complete fault information can be used to identify faults. To overcome this limitation, this study introduces the concept of instantaneous boundary impedance and proposes a dc line protection method. First, a dc transmission line is divided into three types of zones, namely internal, forward-external, and backward-external zones. Then, the instantaneous boundary impedance is analyzed and compared under normal operation and different types of faults. The results indicate that under normal operation, the instantaneous boundary impedance is zero; under a dc line fault, the instantaneous boundary impedance is less than zero; finally, under an external fault, the instantaneous boundary impedance is greater than zero. The proposed principle is validated using Wudongde LCC (line-commuted converter)-VSC (voltage source converter) three-terminal hybrid HVdc RTDS (real-time digital simulation) test system. The experimental results demonstrate that the proposed protection method is effective under different types, locations, and impedances of faults and signal-to-noise ratios within a period of 1 ms. Compared to the existing methods, the proposed protection method can achieve better performance in withstanding fault impedance (600 Ω) and has better robustness against noise interference (10 dB).

A Novel Transient Analysis of Multiterminal VSC‐HVDC System Incorporating Superconducting Fault Current Limiter

Power transmission using a voltage source converter‐ (VSC‐) based high‐voltage direct current (HVDC) system offers autonomous control of real and reactive power, constant DC voltage polarity, and bidirectional power flow. This helps to realize the multiterminal VSC‐HVDC system and its integration into renewable energy sources to meet the growing power demand. However, there is a risk of higher voltages and currents during a DC line fault. The barrier to the advancements of VSC‐MTDC systems is the nonavailability of commercial, higher‐rated DC circuit breakers. This necessitates research on alternative methods of DC fault‐clearing schemes with available technologies. In this direction, a superconducting fault current limiter (SCFCL) is an alternative option to mitigate the problems encountered in VSC‐MTDC system operation. Because of this, there are not many VSC‐MTDC systems available worldwide. This paper discusses different issues associated with the transient performance of the VSC‐MTDC system. A representative case involving resistive SCFCL for DC line protection is presented. The simulations are carried out in the PSCAD/EMTDC platform.

Protection scheme for LVDC distribution network based on stepwise blocking of power electronic transformer

AbstractCascaded H‐bridge power electronic transformers (CHB‐PET) can efficiently connect AC/DC distribution networks, realizing power transmission and electric energy conversion. Once a pole‐to‐pole DC fault occurs on the low‐voltage direct current (LVDC) side of CHB‐PET, the fault current has a fast rise and high amplitude, which easily leads to the rapid blocking of power electronic devices and makes it difficult for DC line protection to obtain fault information. A stepwise blocking scheme of PET is proposed for LVDC network protection. When the fault on the LVDC side of PET is detected, cascaded H‐bridge and low‐voltage H‐bridge of dual‐active‐bridge are blocked first, and then high‐voltage H‐bridge of dual‐active‐bridge is blocked after a 5 ms delay. The stepwise blocking can provide fault information for DC line protection. On this basis, wavelet packet transform is employed to extract different frequency components of fault current. The energy of the specific frequency band corresponding to the switching frequency of DAB and the total energy of all frequency bands contained in the fault current is calculated; then their ratio can be utilized to construct the protection scheme. Finally, the PSCAD/EMTDC simulation results verify the feasibility and effectiveness of proposed protection scheme based on stepwise blocking of CHB‐PET.

Non-unit ultra-high-speed DC line protection method for HVDC grids using dynamic fitting algorithm with data reconstruction

Existing non-unit protection schemes are inevitably influenced by the high fault-resistance in MMC-HVDC grid, which is a serious problem affecting the rapidity and reliability of fault recognition. This study proposes a dynamic fitting of data reconstruction-based protection method to improve the accuracy and rapidity of HVDC grid protection. Due to the propagation characteristics of the initial voltage traveling wave being unaffected by the transition resistance, the self-adapting exponential fitting algorithm is used to recognize the fault waveform. Then, sliding data window with the voltage traveling wave fit by the optimal basis exponential fitting algorithm could avoid the influence of initial value selection on fitting convergence. Meanwhile, the original fault traveling wave is reconstructed by “Bayesian compressed sensing algorithm (BCSA)” in a low sampling rate. This method could reduce the operation response time of the proposed method to satisfy the ultra-high-speed requirement of MMC-HVDC grid and also adapt the limitation of sampling rate in existing protection devices. Practical case studies in PSCAD/EMTDC and protection prototype testing have demonstrated that the proposed method is effective under different fault locations, fault types, and high fault impedance.

Fault protection scheme of flexible DC grid based on transient power instantaneous energy difference

The DC line protection is the key to the stable operation of the multi-terminal flexible DC grid, and the current limiting reactor (CLR) as the boundary condition provides great convenience for the design of the flexible DC grid fault protection scheme. Using the boundary condition characteristics of the CLR, the paper proposes a fault protection method for flexible DC grid based on transient power instantaneous energy (IE) difference. First, the equivalent circuit of the DC line of the flexible DC grid under different faults is established. On this basis, the conclusion that the transient power amplitude of internal fault is much larger than the external is deduced from the full-power in the time-domain. The Teager energy operator is used to calculate the maximum IE of the transient power, and then the internal and external faults are identified. Secondly, according to the difference of the transient power amplitude of the CLR on the fault pole and the healthy pole, the ratio of the IE of the positive and negative CLR transient power is used to construct the fault pole selection. The scheme is simple and easy to implement, and does not need to consider the synchronization problem of data at both sides. Finally, the simulation results show that the proposed method can better satisfy the speediness and selectivity requirements of DC grid protection. The scheme can accurately detect faults under different working conditions, and has strong fault resistances capability.

A Novel Protection Based on Current Correlation for DC Lines in Hybrid Cascaded Multi-Terminal HVDC Transmission System

This article proposes an innovative DC line protection utilizing current correlation coefficient for hybrid cascaded high-voltage direct-current (HVDC) system. First, the equivalent discharge circuit after DC line fault is inferred, and then fault characteristic analysis on the superimposed network reveals that for the transmission lines connecting line commutated converter (LCC) and modular multilevel converters (MMCs), high-frequency currents of faulty line and non-fault line are negatively correlated, their correlation coefficient is theoretically -1. Whereas, high-frequency currents of two non-fault lines show a strong positive correlation, their correlation coefficient equals theoretically 1. Then, cosine correlation coefficient is adopted to describe current correlations and construct the identification criterion. Simulation demonstrates that protection could differentiate the faulty line accurately. Additionally, it has high sensitivity for high-resistance fault. And it is simple and has low computational complexity, and is insensitive to the line boundary element.

A Non-unit transient travelling wave protection scheme for multi-terminal HVDC system

The rapid and accurate dc line protection scheme is of great value in isolating dc line faults and improving the reliability of multi-terminal HVDC systems. Nowadays, the traveling wave (TW) protection is widely used as the primary protection in HVDC transmission systems. However, the traditional TW protection has the problems of poor resistance to high impedance faults and the setting value depends on simulation. For the hybrid three-terminal LCC/MMC HVDC system, this paper provides a fast non-unit protection scheme. Firstly, the time-domain expressions of the backward line-mode voltage TW after different line faults occur are derived respectively. Based on the expressions, a non-unit protection scheme depending on the difference value of the line-mode backward TW is proposed. Finally, the scheme is verified by simulation. The method only needs single-ended data to realize the rapid dc line protection without converter station communication which has a clear theoretical calculation method in setting values and a strong robustness to high-resistance fault.

A protection method for LCC-VSC hybrid HVDC system based on boundary transient power direction

In a line-commuted converter–voltage source converter (LCC-VSC) system, in the hybrid multi-terminal high-voltage direct-current (HVDC) sending end, a line-commuted converter (LCC) converter is used, while the receiving end uses a full-half hybrid modular multi-level converter (MMC), with a fault self-clearing capability. The DC line faults can be self-cleared within 1–2 ms. Therefore, there is an inevitable problem of short availability of faulty information since only 1-ms complete fault data are available to identify faults. Thus, it is necessary to develop a fast and reliable protection scheme for an LCC-VSC system. To address this need, this paper proposes a DC line protection method based on the boundary transient power direction. First, the characteristics of boundary transient power direction of the internal and external faults are analyzed. For an internal fault, the boundary transient power direction is positive, whereas for an external fault, this direction is negative. Therefore, this paper uses the sum of the boundary transient power directions to structure internal- and external-fault identification elements and a pole detection element. The proposed method is validated by real-time digital simulations using Kun-Liu-Long LCC-VSC three-terminal hybrid HVDC–RTDS test system. Numerous tests have demonstrated that the proposed protection method is effective in detecting different fault types under various fault distances, fault transition resistance values, and signal-to-noise ratios in the period of 1 ms. Compared with the existing methods, the proposed protection principle has a better performance in withstanding fault transition resistance (600 Ω) and noise interference (10 dB).

Single-ended DC line protection scheme based on first peak time of current-limiting reactor voltage

Fast and sensitive DC line protection is an important guarantee for the security of the flexible DC grid. However, the reliability and sensitivity of the DC line protection are greatly affected by the high fault resistance. To overcome this problem, this paper proposes a novel single-ended line protection scheme for multi-terminal flexible DC grid. Firstly, the propagation characteristics of traveling wave after the internal and external faults are analyzed. Furthermore, a single-ended protection criterion based on the first peak time of current-limiting reactor transient voltage is proposed to distinguish the internal and forward external faults, which is independent of simulation. Then, based on the change characteristics of fault component current, a fault direction criterion and fault pole identification criterion are constructed to identify the backward fault and fault pole, respectively. Finally, extensive simulations and experiments show that the proposed protection scheme can identify the fault quickly and sensitively even for the internal remote-end fault with high fault resistance.

Dynamic State Estimation Based Protection for Flexible DC Grid

Fast and reliable dc line protection is one of the key techniques for flexible dc grid with the development of flexible dc transmission technology. However, the serious overcurrent caused by dc faults has a prominent contradiction with the weak overcurrent capability of power electronics. In this article, the high-fidelity dynamic model of the transmission line is first established based on the frequency-dependent characteristic of line parameters. Then, a new dynamic state estimation based protection (DSEBP) scheme is proposed, which uses the redundancy feature of state estimation to improve the dependability of protection. Finally, the actual measurements and the estimated states are compared with the similarity to identify the fault by the chi-square distribution. The proposed method's effectiveness is verified through simulations power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) and experiments, and the factors affecting the performance of DSEBP are analyzed. Compared with the existing protection methods, the proposed method is more dependable, more sensitive, and available for actual projects.

Research on DC Line Protection Method of Large PV Plant Based on Transient Direction

Due to the complex fault transients associated with DC convergence in photovoltaic (PV) plants and the large number of power electronics used in systems, the protection device is unable to determine the location of faults, and thus the protection is inadequate. This paper proposes a protection scheme for the DC system of a large PV plant. The accurate model of the PV plant is built in consideration of the DC system, including the current limiter. The differential voltage and current are utilized to realize the location of the fault section. The voltage transients are used to locate the fault within the section accurately. Study cases in PSCAD/EMTDC show the proposed protection scheme is effective and practically useful for PV plants.

A Pilot Directional Protection Scheme for LCC-HVDC Lines Based on Grounding Resistance

In LCC-HVDC system, existing DC line protection methods have poor immunity to the fault resistance in internal fault cases, and easily mal-operate in external fault cases. In view of this problem, a new pilot directional protection scheme for HVDC lines based on grounding resistance is proposed. First, the impedance characteristic equations of the converter considering different commutation failure scenes are established. Then, by combining the impedance characteristics of different components, the fault transient network of LCC-HVDC system is built. On this basis, the frequency band where the fault component network is inductive is derived, and the equation of grounding resistance is constructed using the voltages and currents on two line ends in the inductive network in different frequency bands. And then, a protection criterion is formed according to the consistency between the grounding resistance and the actual fault resistance. Finally, simulation tests on the RT-LAB platform verify that, the proposed method can correctly identify different types of internal and external faults, unaffected by the variations in the operating status of the converter and the operation mode of AC system. It is simple, reliable and strongly immune to the fault resistance. Besides, it does not require the interaction of synchronous information, thus it is easy to implement.

Multiterminal Hybrid DC Line Protection Based on Intrinsic Mode Energy Entropy

In view of the UHV multiterminal hybrid DC transmission system, the DC line protection with universal applicability, fast response speed, stability, and reliability is particularly important for its safe and economic operation. In this article, the boundary frequency characteristics of the UHV multiterminal hybrid DC transmission system are analyzed by considering the DC boundary of the rectifier side and inverter side, line frequency variation, and other factors. According to the fact that the boundary of the fault line has a strong attenuation effect on the high-frequency component of the transient current, a protection method based on the intrinsic mode energy entropy is proposed to distinguish the faults inside and outside. The criterion to initiate the protection is constructed by using the amplitude of the transient power; the fault direction criterion is constructed by using the positive and negative characteristics of the transient energy of the fault power detected by power direction elements on both sides of the T-zone; the fault pole selection criterion is constructed by using the ratio of low-frequency transient power changes of positive and negative poles. Finally, the Kun–Liu–Long line UHV three-terminal hybrid DC transmission system model is built on the PSCAD/EMTDC simulation platform, and a large number of simulation examples verify that the protection can operate reliably under different fault poles, fault positions, and transition resistances.

Protection scheme for VSC-MTDC based on low-frequency reactive power

Multi-terminal flexible DC power grid has become the mainstream of power construction and plays an increasingly important role in the power system. However, the DC line fault results in the shutting down of the converters, which can greatly endanger the security of the power grid. So it is essential to find a fast fault identification strategy for DC line protection technology. While the existing backup protection has low reliability due to the existence of distributed capacitance current. Therefore, this paper proposes a novel protection scheme using low-frequency transient reactive power. Through analyzing the characters and relationship between the reactive power of the DC line and the low-frequency fault information during a fault., the fault is identified with the difference of low-frequency reactive power. Then a novel algorithm for calculating low-frequency reactive power using Wavelet transform and Hilbert transform is introduced in this paper. Furthermore, the protection scheme is not affected by the distributed capacitor current in the high-frequency band because of the application of the low-frequency fault information. Finally, the effectiveness and superiority of the protection scheme are proved by simulation of the VSC-MTDC power grid.

A new pilot directional protection method for HVDC line based on the polarity of inductive energy

Concerning the poor immunity of DC line protection to fault resistance in the case of internal fault and its mal-operation proneness in the case of external fault, a new pilot directional protection scheme for HVDC line in AC/DC hybrid system based on the polarity of inductive energy is proposed. First, by analysing the conducting state of converter at different time sections, the impedance characteristic equations of converter in different commutation failure scenes are derived. On this basis, by combining the impedance characteristics of other components in DC system, the fault component network of an AC/DC hybrid system consisting of inductive impedances is established. Then, according to the difference in the flowing direction of the inductive energy in the fault component networks of internal and external faults, a protection criterion for identifying internal and external faults is constructed. Finally, simulation tests in RT-LAB verify that, the proposed method can accurately identify internal and external faults in different fault cases, unaffected by commutation failure and lightning interference and highly immune to the fault resistance. Besides, the proposed method does not involve real-time interaction of synchronous information, thus it is simple and reliable.

A Novel Identification Scheme of Lightning Disturbance in HVDC Transmission Lines Based on CEEMD-HHT

For high-voltage direct current (HVDC) transmission systems that use overhead transmission lines, the lightning disturbance is the main reason for the line protection mal-operation. To avoid the impact of lightning disturbance on the safety and stability of DC line protection, the model of HVDC transmission system including lightning strokes is constructed. On this basis, the complementary ensemble empirical mode decomposition (CEEMD) and Hilbert transform are combined as a time-frequency analysis method to calculate the energy distribution of the signals. Then the low-to-high-frequency energy ratio of the 1-mode voltage signal in a 3 ms data window after the protection startup is used as the protection criterion for identifying lightning disturbance, and that of the current signal in 1 ms is used as the criterion to further distinguish lightning and non-lightning faults. Verified through simulation tests, the scheme appears good adaptability and accuracy in different situations.

Improved Hausdorff Distance Based Pilot Protection for Multi-terminal HVDC system

In order to overcome the weakness of dc line protection under high impedance fault, a traveling wave similarity based pilot protection method is proposed using improved Hausdorff Distance algorithm. The issue of asynchronous data, noise and anomalies are solved with the improved Hausdorff Distance algorithm. The instantaneous frequency based wave head detection method is used to overcome the problem of delay in protection start-up. Based on this protection scheme, external fault can be distinguished from internal faults regardless of fault impedance, fault location and fault type. Moreover, the faulty pole and fault direction are recognized via ratio of transient energy at line terminals. Taking the Zhangbei high voltage dc transmission project as an example, a four-terminal ring type dc grid model is developed in PSCAD/EMTDC platform for fault simulation and the data processing is performed in MATLAB platform. Based on the theoretical analysis and simulation study, the proposed pilot protection is verified with excellent fault impedance withstanding ability and robustness against noise, abnormal data and asynchronous data.