DC Line Fault Research Articles

Research on adaptive active power coordinated control strategy of Zhangbei Flexible DC Grid project

Zhangbei Flexible DC Grid project with grid characteristics is different from the previous multi-terminal flexible DC project. Zhangbei Flexible DC Grid project uses a DC breaker (DCB) to isolate DC line faults. Limited by the current domestic research and development level of high-voltage DCB, it does not have overload operation capacity. Against this problem that the overload of DCB endangers the safety of equipment and system after a permanent fault of the DC overhead line, the adaptive power coordination control strategy is proposed when the sending end converter station is connected to the new energy station and the AC grid respectively, The feasibility and rationality of this strategy are verified by the RTDS real-time simulation system of Zhangbei Flexible DC Grid project and provides technical reference for the long-term upgrading and transformation of Zhangbei Flexible DC Grid project.

A backup protection scheme for the transmission lines based on the directional traveling wave and waveform similarity

The voltage source converter based high voltage direct current (VSC-HVDC) system is a practical solution to improve renewable energy integration, flexible consumption, and wide-range complementarity. Because the DC line fault must be cleared within a few milliseconds, a reliable and rapid identification of faulty lines and poles is crucial. Unfortunately, the traditional backup protection cannot respond to internal faults fast enough and is sensitive to fault resistance. What is worse is that a double-pole-to-ground fault with unequal positive-pole and negative-pole fault resistances cannot be identified by most of the existing protection methods. The paper proposes a pilot backup protection based on directional traveling wave and waveform similarity for VSC-HVDC transmission lines to compensate for the drawbacks. The fault characteristic of 1-mode directional traveling waves and the waveform similarity of instantaneous currents of DC lines are elaborated theoretically. Based on the analyses, the protection is established, and the four-terminal VSC-HVDC grid is modeled in PSCAD and OPAL-RT. Detailed theoretical analyses and extensive simulation/experiment results have proven that the proposed protection scheme has higher reliability, selectivity, rapidity, and sensitivity. Compared with the existing backup protections, the proposed protection scheme has superiorities in fault resistance and distance tolerance (2,000 Ω and the entire length of DC lines), fault differentiation of double-pole-to-ground fault with unequal positive-pole and negative-pole fault resistances, immunity from signal noise and measurement error interference, and quicker fault judgment (less than 4 ms).© 2017 Elsevier Inc. All rights reserved.

Convolutional neural network based on fast Fourier transform and gramian angle field for fault identification of HVDC transmission line

Flexible DC grid puts forward high-speed dynamic, and high-reliability requirements for DC transmission line protection. How to identify DC line faults ultra-fast and reliably is one of the critical technologies for the further development of the flexible DC grid. To solve this problem, a new DC transmission line fault identification scheme based on the convolutional neural network (CNN) based on fast Fourier transform (FFT) and gramian angular field (GAF) is proposed. The proposed scheme aims to further reduce the identification time of DC transmission line fault types under the requirements of rapidity and accuracy of DC transmission line protection. In this scheme, the obtained feature data are processed by FFT and GAF to obtain the feature images of different fault types corresponding to DC transmission lines. Then the CNN is used to classify the feature images to realize the fault identification of DC transmission lines. Compared with previous methods, this method can apply the mature image classification algorithm in artificial intelligence algorithm to DC transmission line fault identification and realize the rapid identification of DC transmission line fault types. The simulation results show that this method can only use the fault data within 1.5 ms after the fault to achieve 100% accuracy of fault recognition. When the fault data acquisition window is reduced to 1 ms it still has 99.88% classification accuracy, which has certain advantages in the speed and accuracy of DC transmission line fault recognition.

Fault Detection in HVDC System with Gray Wolf Optimization Algorithm Based on Artificial Neural Network

Various methods have been proposed to provide the protection necessitated by the high voltage direct current system. In this field, most of the research is confined to various types of DC and AC line faults and a maximum of two switching converter faults. The main contribution of this study is to use a new method for fault detection in HVDC systems, using the gray wolf optimization method along with artificial neural networks. Under this method, with the help of faulted and non-faulted signals, the features of the voltage and current signals are extracted in a much shorter period of the signal. Subsequently, differences are detected with the help of an artificial neural network. In the studied HVDC system, the behavior of the rectifier, along with its controllers and the required filters are completely modeled. In this study, other methods, such as artificial neural network, radial basis function, learning vector quantization, and self-organizing map, were tested and compared with the proposed method. To demonstrate the performance of the proposed method the accuracy, sensitivity, precision, Jaccard, and F1 score were calculated and obtained as 99.00%, 99.24%, 98.74%, 98.00%, and 98.99%, respectively. Finally, according to the simulation results, it became evident that this method could be a suitable method for fault detection in HVDC systems.

Fault Detection Using Backward Propagating Traveling Waves for Bipolar LCC-HVDC Lines

The line commutated converter (LCC)-high voltage direct current (HVDC) transmission lines may have various complementary protection algorithms based on derivatives of line parameters or differential line currents. The DC line fault detection based on distance relaying, traveling wave (TW), artificial neural network, fuzzy logic, and wavelet are a few of the popular schemes. However, the TW-based technique has been reported as one of the fastest techniques to detect and locate faults. The conventional TW techniques face limitations in detecting high resistance DC line faults. Therefore, this paper introduces an improved fault detection, classification, and location algorithm based on backward propagating TWs and fault classifier indices. The proposed algorithm is tested to detect, classify, and locate the faults under different system conditions such as pole-to-ground, pole-to-pole, and rectifier and inverter sides’ external faults. A bipolar LCC-HVDC transmission system of 900 km length is utilized to simulate the various fault conditions. The simulation results indicate fast, accurate, and reliable detection of faults. This scheme uses only single-end data measurement. The simulation study was performed for various fault locations (0–900 km) and transition resistances (0–150 Ω).

Semi AI-based protection element for MMC-MTDC using local-measurements

The multi-terminal HVDC system based on the modular multilevel converter (MMC-MTDC) is a promising technique for flexible power transmissions to multiple regions. As such a system is quite sensitive to DC faults, there is an acute need to propose a protection element that can trip the local DC circuit breaker (CB) within several milliseconds once there is an internal DC line fault. However, the existing main protection scheme faces a dilemma balancing selectivity and sensitivity. To solve this problem, a novel semi artificial-intelligence (AI) based protection element is proposed, including a start-up criterion and a fault-identification criterion. The start-up criterion is based on the propagation characteristics of the initial fault-induced surge. To enhance the real-time performance of the protection element, it will not trip the fault-identification process unless the fault is identified as a forward one. The fault-identification criterion is based on artificial intelligence (AI), and further determines whether the forward fault is internal, which only works if the start-up criterion trips. Simulation results indicate that the proposed protection element has satisfactory speed, sensitivity, and selectivity against internal DC faults and is quite secure under external fault conditions. The impact of disturbances, such as the white noise, abnormal samplings, etc., on the security of the proposed protection element is also discussed.

Single-ended protection method of MMC-HVDC transmission line based on random matrix theory

The physical electrical boundary at both ends of the MMC-HVDC transmission line is composed of smoothing reactors and has a high-frequency blocking effect. This behavior causes the time-domain characteristics of the initial part of the polar line current waveform to be significantly different under the internal and external MMC-HVDC line faults, and can lead to two different fault modes. However, the operation of the DC line traveling wave protection with du/dt as the core can be easily affected in the presence of high resistance (HR). Furthermore, it is difficult to set the criterion setting value. To solve the aforementioned problems, this paper proposes a line single-ended protection method based on the random matrix theory (RMT). First, the difference between the internal and external line current fault modes is analyzed. Second, the physical characteristics of the two modal differences in the RMT are analyzed. Third, the single Ring Law is used to achieve visualization, and mutation energy is used as the starting criterion. The fixed values of the spectral radius and Euclidean distance of the spectral radius are utilized as the identification criteria for the internal and external line faults. Furthermore, the set value of the spectral radius is used as the fault selection criterion. Last, a 77-level bipolar MMC-HVDC system grounded through the grounding electrode lead is built in RTDS to verify the adaptability of the proposed method to HR and noise. The results show that the method can quickly and reliably detect DC line faults and fault poles under different fault locations and different transition resistances, and the RMT protection can be further improved with the increasing number of fault samples.

Fault detection and diagnosis of monopolar configured VSC based high voltage direct current transmission line

The development of HVDC transmission technology using an MMC has been promoted, by overcoming the drawbacks of traditional VSC technology. The extension of flexible transmission to overhead lines, especially the use of HVDC transmission based on an MMC, raises the issue of DC fault. So identification of fault, clearing the dc fault, and design of fast-acting protection system operating against fault becomes significant. This article provides insights on the monopolar structured MMC and operational characteristics, fault analysis, and a fault protection scheme. DC line faults on HVDC lines using MMC-VSC are major issues; isolation of complete system is not a viable option. It is observed that Pole to ground fault is the most common fault, which leads to generous overcurrent in the AC grid and results in converter valves getting damaged. This article analyzes the response of MMC-HVDC under different DC and AC faults conditions for five-level MMC HVDC systems, to better understand systems under fault. Faults also have an impact on the converter stations' performance. The voltages fluctuate in faulty situations. In comparison to the inverter station, the rectifier station has the most impact. Simulation is performed out in PSCAD software. The correctness and effectiveness of DC and AC fault analysis helps to check the capability of locating fault occurring on HVDC transmission lines quickly and accurately

Single terminal coordination-protection scheme for hybrid three-terminal DC line based on station terminal current characteristics

Grounding electrode is an essential component in hybrid multi-terminal HVDC transmission system, By connecting converter station to the bus bar as a main station, this work analyzes the fault current component characteristics of grounding electrode line under various fault conditions for a hybrid three terminal DC transmission system. It is concluded that under single pole line grounding fault, current at the measuring ends of grounding, fault and sound electrodes satisfies the Kirchhoff current theorem, while considering converter station as a node. However, for reverse fault of line protection, current at above mentioned electrodes doesn’t follow the Kirchhoff law. Under various DC line faults, fault line is selected according to the mutation direction of current at the back-side of bus bar. Essentially, the current obtained by grounding electrode’s measuring point is a zero mode current, hence, it can sense the fault current precisely and has directivity. A comprehensive single terminal coordination-protection scheme for DC lines in hybrid multi-terminal transmission system is constructed by using currents’ characteristics of station terminal. The proposed method does not rely on boundary elements, and has an ability to quickly identify internal and external faults, select the fault pole, and withstand high resistance. The developed algorithm for protection scheme is simple, requires low sampling rate, and ensures high reliability for protection scheme.

A Power Allocation Strategy for DC Line Fault in Serial Hybrid LCC-MMC HVDC System

The serial hybrid line commutated converter-modular multilevel converter (LCC-MMC) high voltage direct current (HVDC) system has structural features that cause the inverter side three MMCs to form a closed-loop after a DC line fault. If the steady-state control strategy of the MMCs is still used during the phase shift, there will be a severe overcurrent problem. In addition, during the power recovery process, the rectifier side and the main converter MMC simultaneously deliver active power to the constant active power converter, resulting in the loss of control of the constant active power converter. Based on this, an MMC active power allocation strategy is designed in this paper. In this strategy, during DC line faults, the constant DC voltage converter is switched to constant active power control, and the other two converters are switched to voltage droop control. During the phase shift, the active powers of the two droop converters are allocated according to the droop coefficients, and the active powers of the MMCs are dynamically and smoothly reduced to solve the uncontrol problem of the two converters during the power recovery process. Finally, a serial hybrid LCC-MMC HVDC system model is built in PSCAD/ETMDC based on engineering design parameters to verify the accuracy and validity of the proposed control strategy. Comprehensive simulations show that the proposed strategy can reasonably allocate the powers of the MMCs in case of DC line fault. Besides, it still shows great adaptability with different power distribution, fault locations, and fault resistances.

Frequency Domain Characteristic Analysis of Fault Electrical Quantities in HVDC Transmission System

The transient frequency domain characteristics of different fault electrical quantities in HVDC transmission system are studied in this paper firstly using analytical calculation. Firstly, the equivalent model of LCC converter suitable for DC system fault characteristic analysis and the equivalent model of DC line considering parameter frequency variation effect are established. On this basis, the DC system fault equivalent circuit is constructed. The frequency domain expressions of relevant electrical quantities under different DC line fault conditions are obtained according to the solution of equivalent circuit. On the basis of the frequency domain expression of fault electrical quantity, the frequency component diagram of fault electrical quantity is obtained. Finally, the frequency spectrum of fault voltage under different fault conditions is analyzed and compared.

Reclosing Current Limiting for DC Line Faults in VSC-HVDC Systems

The problem of reclosing current limiting in voltage source converter based high-voltage direct current (VSC-HVDC) systems is becoming more and more serious. A soft reclosing scheme for DC permanent faults is presented in this paper. Because the converter voltages of stations at both terminals of the disconnected faulty line may be different, the choice of which terminal to reclose first will affect the reclosing overcurrent. A method for selecting the terminal to reclose first is inves-tigated to achieve a minimum peak overcurrent during the reclosing process. In order to ensure that the hybrid DC circuit breaker (HDCCB) adapts to the needs of the reclosing process better, the traditional HDCCB is improved by adding a soft reclosing module (SRM). The energy dissipated in the arresters is significantly reduced when using the improved HDCCB. The improved HDCCB will be able to reclose multiple times safely and thus increase the possibility of successful reclosing. Moreover, the recovery time after the HDCCB is successfully reclosed is very short with the improved HDCCB and its control principles. Simulation results show that this proposed scheme is capable of limiting the reclosing overcurrent when the fault still exists.

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.

An adaptive DC line fault recovery strategy for LCC-HVDC system based on voltage gradient

Due to the lack of characteristic variables to determine whether the fault point persists during DC-line fault recovery in a Line-commutated-converter High Voltage Direct Current (LCC-HVDC) system, it is difficult to distinguish transient and permanent faults. Concerning this problem, an adaptive DC-line fault recovery strategy based on voltage gradient is proposed. Firstly, the variation of the topology of DC system after phase shift is analyzed, and the calculation model of voltage at the outlet of inverter station is built, thus the expression of voltage at the outlet of inverter station containing parameters of fault location, fault resistance and fault duration is obtained. On this basis, characteristics of the gradient of voltage at the outlet of the inverter station are analyzed, i.e. its value monotonically decreases and quickly attenuates to 0 in the case of permanent fault, and abruptly drops below 0 the moment fault disappears in the case of transient fault. According to the characteristics of the voltage gradient, the criterion for the identification of transient and permanent faults is constructed, thus adaptive DC-line fault recovery is realized. Finally, simulation tests in RT-LAB verify that, the proposed method can correctly identify the fault nature of permanent and transient faults on the DC line and realize adaptive fault recovery, unaffected by the fault location, fault resistance and fault duration.

A pilot line protection for MT-HVDC grids using similarity of traveling waveforms

Protection of dc line faults is a major challenge for the operation of multi-terminal high-voltage dc (MT-HVDC) grids. To avoid blocking converter and system shutdown, ultra-fast protection is required to detect and isolate line fault with the application of hybrid dc circuit breaker. This paper proposes a pilot protection scheme based on improved Hausdorff distance to detect the overhead line fault for MT-HVDC grid. Compared with literature, this protection method owns higher withstanding ability of noise interference, start-up delay and data anomaly. Moreover, the fault segment can be correctly identified as independent of fault location, and protection sensitivity is maintained even under internal ground fault with high impedance of up to 1000 Ω. The issue of dead zone fault recognition is considered and solved in this paper with reasonable sacrifice in the sampling frequency requirement. Based on Zhangbei HVDC grid in China, a four-terminal dc grid model is developed in PSCAD/EMTDC environment for fault simulation, and the data processing is performed in MATLAB platform. The feasibility and effectiveness of this protection scheme are verified by a simulation study that considers different scenarios.

Adaptive fault recovery strategy of LCC‐MMC based hybrid HVDC

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.

DC Fault Protection Algorithms of MMC-HVDC Grids: Fault Analysis, Methodologies, Experimental Validations, and Future Trends

To protect the converters and minimize the power transmission interruption during dc line faults, it is necessary to detect the faults at ultra high speed for the modular multilevel converter (MMC)-based HVdc grids. This article reviews the state-of-the-art of dc fault protection methods of MMC-HVdc grids, and summarizes the underlying principles of each method. On this basis, the dc fault characteristics analysis in terms of modal-domain, time-domain, and frequency-domain are analyzed, which direct the protection design. Typical boundary protection and nonboundary protection schemes are reviewed. The advantages and disadvantages of existing fault protection methods are compared. A two-terminal MMC-HVdc prototype is developed to test the effectiveness of three fault protection methods. Comprehensive quantitative assessments of the protection methods discussed above are carried out in a four-terminal MMC-HVdc grid. Finally, the future trends of the protection schemes are discussed and the findings are concluded.

Pilot Directional Protection Scheme for LCC-HVDC Transmission Lines Based on the Voltage difference Between Positive and Negative Poles

Currently available DC line protection methods for AC/DC hybrid systems present poor immunity to fault resistance for in-zone faults and mal-operation problems for out-of-zone faults. This paper proposes a new pilot directional protection scheme based on the voltage difference between positive and negative poles (VDPN). The proposed method is not affected by the fault location, the fault resistance, or lightning interference. By analyzing the conducting states of the converter in different time intervals, the differential expressions of the injected current from AC system to DC system are derived. Calculation models of VDPN in different conducting states are built. Then, according to the consistency between the calculated value and actual value of voltage difference, the protection criterion for DC line fault is constructed. Finally, simulation tests in RT-LAB verify that the proposed scheme can correctly identify in-zone and out-of-zone faults in different fault cases. Besides, the proposed scheme has low demand on data synchronicity, thus it is simple and reliable with a good operation speed.

Setting-Less Nonunit Protection Method for DC Line Faults in VSC-MTdc Systems

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.

考虑直流线路和换流器故障的交直流配电网弹性分析

考虑直流线路和换流器故障的交直流配电网弹性分析