Fault Transients Research Articles

Transient current curvature based protection for multi‐terminal flexible DC distribution systems

In a flexible DC distribution system, the converters will be blocked within a few milliseconds for its self-protection during the DC line fault. Using such a short period of the fault transient, it is challenging to identify the correct faulted area. In this study, the characteristics of fault current waveform and sudden change are analysed in detail, and the curvature algorithm is used to extract the current waveform difference between the internal and external faults. Then this study proposes a new scheme for the multi-terminal DC distribution system. This protection is developed based on natural variation characteristics and curvature of DC current under fault transients. Compared with the di/dt protection to correctly identify internal high impedance fault. The proposed method could identify faults with different high resistances quickly, meet the speed and reliability requirement of the DC distribution system. Numerous tests are carried out in both simulation and hardware experiments. Testing results derived under various fault conditions have demonstrated that internal faults with different impedances can be correctly identified by employing the proposed protection scheme.

Continuous demagnetisation assessment for triple redundant nine‐phase fault‐tolerant permanent magnet machine

In this study, the risk of partial irreversible demagnetisation of a triple redundant, nine-phase fault tolerant machine based on permanent-magnet-assisted synchronous reluctance machine topology has been comprehensively assessed under various faults, including the worst case scenario when the applied voltage vector is in the opposite direction of the back electromotive force due to sensing and inverter control failure, by employing a continuous demagnetisation model. The dynamic response during fault transients and the post-demagnetisation performance, such as the demagnetisation distribution, reduction in the back EMF and torque, will also be analysed and compared.

Multi-Bandwidth Mode Manifold for Fault Diagnosis of Rolling Bearings

Variational mode decomposition (VMD) has been widely applied to the field of machinery fault diagnosis due to its good time-frequency decomposition capability. However, the performance of the VMD for extraction of fault transient components is dependent on proper parameters in the method and is not satisfactory under noisy conditions. This paper proposes a new method, termed multi-bandwidth mode manifold (Triple M, TM), to extract the real fault transient components for fault diagnosis of rolling bearings. The new TM method unites multiple fault-related modes with different bandwidths by a nonlinear manifold learning algorithm. The modes with large bandwidths contain more fault-related information while those with small bandwidths contain less fault-unrelated components and noise. The advantages of different fault-related modes are fully used by the proposed method, leading to clear fault transients that facilitates the bearing fault detection. First, an efficiency-improved VMD method named recycling VMD (RVMD) is performed on the bearing signal repeatedly with different parameter values to obtain the fault-related modes with different bandwidths, i.e., the multi-bandwidth modes. Then, the manifold learning algorithm is carried out to extract the intrinsic manifold structure of bearing fault transient components from the multi-bandwidth modes. Finally, the bearing fault type is identified from the extracted feature with clear fault transients. The enhanced performance of the proposed method over the traditional methods is validated by a simulation analysis and two experimental applications of bearing defect identifications.

Backup-Protection Scheme for Multi-Terminal HVDC System Based on Wavelet-Packet-Energy Entropy

DC fault protection is a major issue in multiterminal high-voltage DC current transmission systems. Most studies have focused on primary protection, which considers a different theory. However, the poor sensitivity of primary protection under high-resistance fault and maloperation still affect the protection feasibility to a certain extent. Thus, backup protection is definitely required to enhance the stability of an entire system considering the limit in primary protection. This paper proposes a backup-protection strategy based on wavelet-packet-energy entropy as mitigation for primary protection. The pole-to-ground (PG) fault with high resistance is highlighted because detecting the fault transient of PG fault with high resistance is difficult in the most primary relay. Fault detection and classification are achieved using the proposed backup-protection method based on theoretical study and simulation verification. The proposed method also presents the expected robustness of fault resistance, fault distance and different operating conditions.

Harmonic L2/L1 Norm for Bearing Fault Diagnosis

Optimal frequency band selection is a critical step of envelope analysis for bearing fault diagnosis. In order to select the most informative band, it is necessary to define a criterion for frequency band evaluation. Kurtosis is one of such criteria, and the frequency band with largest spectral kurtosis is usually regarded as the optimal one. However, it is experimentally and mathematically proved in the past year that spectral kurtosis is sensitive to outliers in a signal. Recently, spectral L2/L1 norm is formally defined for characterizing fault transients in the squared envelope of the signal, and it is also found that spectral L2/L1 norm is actually the square root of spectral kurtosis. Thus, spectral L2/L1 norm still suffers from the drawback of spectral kurtosis. In this paper, however, we utilize this drawback and calculate L2/L1 norms of frequency bands defined by the harmonics of bearing characteristic frequency in the squared envelope spectrum. Since the new calculation is closely related to harmonics of bearing characteristic frequency, it is named as harmonic L2/L1 norm in the paper. Harmonic L2/L1 norm is then employed to evaluate frequency bands obtained from a 1/3-binary tree of filter banks. After the optimal band selection, the filtered signal is used to calculate the squared envelope spectrum. The proposed method is compared with the fast kurtogram method in two case studies, which indicate that the proposed method is more effective, especially in the presence of strong non-Gaussian noise.

Study of Resistive-Type Superconducting Fault Current Limiters for a Hybrid High Voltage Direct Current System.

In this paper, a hybrid high voltage direct current transmission system containing a line commutated converter and a voltage source converter is developed. To enhance the robustness of the hybrid transmission system against direct current short-circuit faults, resistive-type superconducting fault current limiters are applied, and the effectiveness of this approach is assessed. Related mathematical models are built, and the theoretical functions of the proposed approach are expounded. According to the transient simulations in MATLAB software, the results demonstrate that: (i) The superconducting fault current limiter at the voltage source converter station enables to very efficiently mitigate the fault transients, and owns an enhanced current-limiting ability for handling the short-line faults. (ii) The superconducting fault current limiter at the line commutated converter station is able to mildly limit the fault current and alleviate the voltage drop, and its working performance has a low sensitivity to the fault location. At the end of the study, a brief scheme design of the resistive-type superconducting fault current limiters is achieved. In conclusion, the application feasibility of the proposed approach is well confirmed.

Research on application of FDNE based on RTDS and comparison analysis of fault transient of Shanghai Sijing power grid

As a most widely applied real-time digital simulation system, real time digital simulation (RTDS) is however quite limited on scale of simulation by the number of hardware purchased. Through frequency-dependent network equivalent (FDNE), scale of simulation of RTDS could be largely increased by frequency equivalent on limited hardware. Based on basic principles of FDNE, function limitations and problem are analysed, taking the Shanghai Sijing district power grid as an example; detailed simulation model and FDNE equivalent module are built, transient characteristics under faults before and after FDNE equivalent are analysed. Furthermore, the fault characteristics of high-voltage direct current (HVDC)-Sijing power grid is also studied, which provides a realistic means for real-time digital simulation of AC–HVDC hybrid power grid with limited hardware of RTDS.

A method for tachometer-free and resampling-free bearing fault diagnostics under time-varying speed conditions

Bearings often operate under time-varying rotational speed conditions. Processing the signal in the time-frequency domain and extracting the Instantaneous Fault Characteristic Frequency (IFCF) and the Instantaneous Shaft Rotational Frequency (ISRF) are important approaches for bearing fault diagnosis under time-varying speed conditions without signal resampling and without using a tachometer. However, there are two problems: (1) the collected bearing signal is often contaminated by random noise and interferences transmitted from other components, which affects the accuracy of the extracted IFCF, and (2) the ISRF cannot always be found in the Time-Frequency Representation (TFR) of the extracted bearing fault transients, which impacts the accuracy of fault identification. Therefore, a new tachometer-free and resampling-free method is proposed for bearing fault diagnosis under time-varying speed conditions which consists of three main steps: (1) bearing fault signature extraction via Oscillatory Behavior-based Signal Decomposition (OBSD) to suppress the influence of random noise and interferences, (2) IFCF and ISRF estimation via applying a IFCF&ISRF search algorithm to the TFR of the decomposed signal, and (3) automatic bearing fault identification based on the average curve-to-curve ratios of the searched IFCF and ISRF. The IFCF&ISRF search algorithm is proposed based on the analysis of the frequency characteristics of bearing vibration signals. The algorithm can estimate the ISFR even if it is not present in the extracted bearing fault signature. The effectiveness of the proposed method is validated using both simulated signals and experimental data.

A Novel Fault Location Method for a Cross-Bonded HV Cable System Based on Sheath Current Monitoring.

In order to improve the practice in the operation and maintenance of high voltage (HV) cables, this paper proposes a fault location method based on the monitoring of cable sheath currents for use in cross-bonded HV cable systems. This method first analyzes the power–frequency component of the sheath current, which can be acquired at cable terminals and cable link boxes, using a Fast Fourier Transform (FFT). The cable segment where a fault occurs can be localized by the phase difference between the sheath currents at the two ends of the cable segment, because current would flow in the opposite direction towards the two ends of the cable segment with fault. Conversely, in other healthy cable segments of the same circuit, sheath currents would flow in the same direction. The exact fault position can then be located via electromagnetic time reversal (EMTR) analysis of the fault transients of the sheath current. The sheath currents have been simulated and analyzed by assuming a single-phase short-circuit fault to occur in every cable segment of a selected cross-bonded high voltage cable circuit. The sheath current monitoring system has been implemented in a 110 kV cable circuit in China. Results indicate that the proposed method is feasible and effective in location of HV cable short circuit faults.

Offshore AC Fault Protection of Diode Rectifier Unit-Based HVdc System for Wind Energy Transmission

Offshore ac fault protection of wind turbines (WTs) connecting with diode rectifier unit-based high-voltage dc (DRU-HVdc) system is investigated in this paper. A voltage-error-dependent fault current injection is proposed to regulate the WT current during offshore ac fault transients and quickly provide fault current for fault detection. Considering different fault locations, the fault characteristics during symmetrical and asymmetrical faults are presented and the requirements for fault detection are addressed. A simple and effective offshore ac fault protection solution, combining both overcurrent protection and differential protection, is proposed by utilizing the developed fast fault current providing control. To improve system availability, reduced dc voltage of the DRU-HVdc system is investigated, where one of the series-connected DRUs is disconnected and the onshore modular multilevel converter actively reduces dc voltage to resume wind power transmission. The proposed scheme is robust to various offshore ac faults and can automatically restore normal operation. Simulation results confirm the proposed fault protection strategy.

Metrology requirements of state‐of‐the‐art protection schemes for DC microgrids

Environmental incentives to combat climate change are providing the motivation to improve the energy efficiency of power distribution systems and integrate state-of-the-art renewable technologies. Examples include wind/photovoltaic resources, energy storage systems, and electric vehicles integrated via efficient power electronic converters (PEC). Subsequently, DC microgrids (MGs) and distribution systems are receiving considerable attention in the literature because they offer a simple, yet flexible, interface between these modern resources and consumers. However, many technical challenges relating to the design and standardisation of DC protection devices still exist that must be overcome prior to widespread adoption. For example, many protection schemes tailored for DC MGs have been proposed, but few of them have considered the metrology requirements for practical implementation. This study will first review the key features of DC-side fault transients simulated on a DC MG model in MATLAB/Simulink, and analyse the disruptive impact on PEC components. Secondly, a review of newly published DC protection schemes is performed. These protection schemes are classified by their fundamental operating principles and mathematically derived metrology requirements are given.

Impact of an inverter‐based DG on a double‐ended fault location method

This paper describes the influence of Distributed Generation (DG) on a double ended fault location based on measuring the high frequency fault transients. The additional non-fundamental frequency current components from DG will influence the accuracy of an impedance based fault location technique based on non-fundamental frequencies. A double-ended impedance based fault location technique that utilizes the high frequency content (up to 5 kHz) is studied. The study showed that double-ended method is still able to locate a fault with a maximum error of 4% compared to the case without DG which showed a percentage error up to 2%.

Fault Analysis and Protection of a Series-DC Collection and Transmission System

Series-dc collection and transmission is suggested to be competitive for remote offshore wind farms. However, severe overcurrent, overvoltage, and other impacts that arise from faults cause challenges on system protection, which have not been fully studied yet. In this paper, simplified equivalent circuits are developed for fault analysis, based on which analytical models for fault transients are derived. The expressions can be used for stress estimation, as well as provide insight in understanding the mechanism of fault propagation. A cost-effective protection scheme characterized by a breakerless design is proposed. Parameter selection for stress mitigation is also discussed. Simulation results verify that the proposed fault protection and stress mitigation design is effective for such a series-dc system.

Adaptive Control of Microgrid Security

The low inertia of inverter-based distributed energy resources (IBDERs) can threaten the dynamic security of microgrids (μGs) and make μGs more susceptible to incipient fault transients. This letter proposes a control approach to improve the transient response of IBDER and maintain a reasonable dynamic security margin for μGs. The IBDER is controlled to mimic the inertial response and voltage support characteristic of synchronous generators subjected to severe disturbances. The proposed method adapts to various μG conditions and maintains the coherency of IBDER with synchronous generator-based distributed energy resources.

Fault characteristics and protection adaptability analysis in VSC‐HVDC‐connected offshore wind farm integration system

Due to different fault ride through controls of both the wind farm and the voltage-source converter high-voltage alternating current (VSC-HVDC) converters, the offshore wind farms that are connected to the power system through VSC-HVDCs have unique fault transients. During a fault on AC side transmission lines, the wind farm and the VSC-HVDC together provide faulted current, whose amplitude and phase will be decided by their different control strategies collectively. The fault transients are different from those in the traditional AC grid. Thus, the performance of traditional protection will be affected. In this study, the AC fault current expression is deduced, while the influencing factors on the current's amplitude and phase are analysed in detail. Based on these characteristics, the performance of distance relays under the grid side fault of the VSC-HVDC-connected offshore wind farm integration system is evaluated. Simulations based on PSCAD have verified the theoretical analysis.

Stacked Auto-Encoder Based Fault Location in VSC-HVDC

This paper presents an end-to-end approach for locating faults on high-voltage dc (HVDC) transmission lines. Different from traditional methods which rely on communications between different measuring units or feature extraction of post fault transients, the proposed algorithm takes the raw data of locally detected traveling current surges as the only-input and outputs the fault locations directly. Especially, the stacked auto-encoder (SAE) is utilized to model the relationship between fault currents and fault locations. The SAE-based method is performed in time domain and tested with a simulated HVDC transmission line modeled in PSCAD/EMTDC. The simulation results show that this method is effective in locating faulted points and robust against attenuation, overlapping of traveling surges, and various ground resistances.

Sparse voltage amplitude measurement based fault location in large-scale photovoltaic power plants

Large-scale photovoltaic (PV) power plants contain numerous transmission line branches and laterals inside. When a fault occurs conventional fault location methods face challenges due to the complex system structure and the diversity of PV inverter controls. Most of the published fault location methods cannot be directly used in the PV power plant due to the following issues: (1) Most of the fault location methods consider the PV inverter as a constant voltage source while the actual inverters have varied controls during faults. Without analysis of the unique fault transients of the PV, the fault location will suffer from errors. (2) In a complicated large-scale PV power plant with massive quantity of nodes, the synchronised measurements from all the nodes are almost impossible. A method with sparse un-synchronized measurements is required. Therefore, a new negative-sequence voltage amplitude sparse measurement based fault location method is proposed for unbalanced faults. The improved Bayesian compressive sensing algorithm is used to recover the sparse fault current vector and then determine the faulted node. Both the field testing and the simulation results indicate that the proposed method can locate the faulted nodes accurately and effectively without synchronizing measurement requirements from all the nodes. This method also presents a good performance for various unbalanced fault types, fault resistances, inverter controls and signal noise. All these factors make the propose method feasible for industrial applications.

A Non-unit Protection Scheme for DC Microgrid Based on Local Measurements

This paper proposes a nonunit protection scheme for DC microgrids (DC MGs) utilizing only local measurements. This scheme is developed based on the natural characteristics of DC current and its first and second derivatives under fault transients. Since it is based on local measurements, the problems associated with the communication delay are avoided. The selected protection scheme detects and discriminates the faults within a few microseconds of its inception. In this paper, a method to calculate the thresholds, used for the protection scheme, is also discussed. The proposed scheme is validated on a ring-type DC MG architecture under different fault scenarios and tested through MATLAB/Simulink simulations.

The broadband frequency characteristics of major equipment in the ±800 kV converter station and their influences to the short-circuit fault transient

Due to the wide application of high-order resonant and nonlinear electric equipment in the UHVDC converter station, the electromagnetic transient process is a complex issue during the short-circuit fault transient of the system caused by lightning strike or other reasons. In this study, firstly, an operating ±800kV DC line is artificially short-circuited to simulate the flashover fault process. The instantaneous voltages and currents and the electrical fields are measured by advanced sensors and high sampling rate recording devices at different points in the converter station, in order to comprehensively study the high-frequency characteristics of the UHVDC system. Then, the portal impedances of the major UHVDC equipment as well as the stray parameters are systematically measured in an actual ±800kV station. Based on the data measured, the accurate high-frequency characteristics of the ±800kV converter station are analyzed. The results of the study can be used to guide the EMC and relay protection designs of the UHVDC converter station, so that the stability of the system can be improved.

Revision of TRV requirements for the application of generator circuit-breakers

The requirements imposed on generator circuit-breakers greatly differ from the requirements imposed on other transmission and distribution circuit-breakers. Due to the location of installation between the generator and the associated step-up transformer, a generator circuit-breaker must meet high technical requirements with respect to rated normal currents, short-circuit currents, fault currents due to out-of-phase conditions and transient recovery voltages. The question whether the transient recovery voltage requirements laid down in IEEE Std C37.013-1997 (R2008) and in IEC/IEEE 62271-37-013 are adequate for the application of generator circuit-breakers in modern power stations is considered in the present work.In order to quantify the transient recovery voltage requirements for the application of generator circuit-breakers a comprehensive survey of different fault conditions occurring in several power stations has been performed. The fault transients’ simulations have been performed by means of the Electromagnetic Transients Program (EMTP).