Grid Faults Research Articles

Research on the Control Method of the PV Grid-Connected Inverter under an Asymmetrical Power Grid Fault

The negative-sequence current component and harmonic components generated when an asymmetrical fault occurs in the power grid seriously affect the normal operation of the photovoltaic (PV) grid-connected inverter. In order to suppress the negative-sequence current component and the harmonic component of the grid-connected current, and to meet the normal grid-connected operation requirements of the PV grid-connected inverter when asymmetrical faults occur in the grid, this paper proposes the proportional integral double-resonant (PI-DR) current controller, which consists of the PI controller and double-resonant controllers. The PI-DR current controller can directly control the negative-sequence current component and the grid-connected current harmonic component in the output current in the forward synchronous rotating coordinate system, without decomposing the positive and negative sequence currents. The PI-DR current controller ensures that the PV grid-connected inverter can realize normal grid-connected operation and improves the quality of the power when an asymmetrical fault occurs in the power grid. MATLAB/Simulink experiments show that the PI-DR current controller can improve the dynamic characteristics of the PV grid-connected inverter and improve the operating capability of the system when an asymmetrical fault occurs in the power grid.

Enhanced DC-link voltage control of PMSG-based wind turbine generators by machine-side converter during asymmetrical grid faults

Under asymmetrical grid fault conditions, oscillating active power can occur on the grid-side converter (GSC) of permanent magnet synchronous generator (PMSG)-based wind turbine generators. The oscillating active power flows into the DC-link, causing voltage oscillations that can lead to a shortened lifetime of DC-link capacitors and additional harmonics injected into the grid. While conventional methods can mitigate the oscillating active power through negative sequence current injection by the GSC, this hinders the grid support of the GSC regarding negative sequence reactive current during grid fault. To address this issue, a novel control scheme is proposed that uses the machine-side converter (MSC) to transfer the oscillating power to the motor. Firstly, the active power capacity of the MSC is analyzed and compared with that of GSC, and the results show that MSC has enough capacity to accommodate the oscillating active power. On this basis, a novel MSC control scheme is proposed to transfer the oscillating active power on the DC-link to the motor, which has a much larger inertia to buffer the power oscillation. Compared with conventional methods, the proposed control scheme relieves the GSC from mitigating the power oscillation to support the grid. Simulation studies using DIgSILENT PowerFactory are presented to verify the effectiveness of the proposed control scheme for PMSG-based wind turbine generators.

Collaborative Optimization Scheduling of Resilience and Economic Oriented Islanded Integrated Energy System under Low Carbon Transition

With the development of new energy sources and the increase in the installed scale of energy coupling equipment, the low-carbon transformation of the energy supply of the integrated energy system (IES) has a serious impact on the reliability of the IES supply, and there is an urgent need for a reasonable and accurate assessment and trade-off between the IES resilience and economics. In this regard, this paper models the overall optimization of the resilience and economic configuration and operation scheduling of the IES in the islanded operation mode after grid faults, proposes a two-layer optimization strategy model of resilience and economy, and solves the unit configuration, coupled output characteristics, and optimal scheduling of the islanded IES using the Markov decision-making process and forbearing stratified sequencing method, and evaluates and analyzes the resilience and cost of the various types of IES configuration schemes. Resilience and cost are also evaluated and analyzed. Finally, an example analysis is carried out in an electric-heat-cooling integrated energy system. The results show that the proposed two-tier optimization strategy model can optimize the IES configuration scheme and coordinate the scheduling of each equipment, and the overall annualized cost of the energy system decreases by CNY 45.21 thousand, or a percentage decrease of 5.24%, compared to the same configuration of the conventional strategy. The typical day toughness index improved by 7.33%, 7.56%, and 13.01% in the spring, summer, and autumn, respectively.

Power grid fault event reasoning based on entity extraction model of knowledge map

In the initial construction process of smart grid dispatching control system in power grid dispatching control center, because different subsystems are in decentralized development, independent operation and independent management, it is easy to reduce data interconnection, which leads to difficulties in data sharing and restricts the information level of the system. The data is multi-source, and the data format is inconsistent, resulting in the application problems that the data can not be shared, accessed, managed, analyzed and mined in real time among different subsystems. In order to solve the problems of data sharing and mining, this paper constructs a knowledge map entity extraction model to study the power grid fault events. Based on the knowledge map theory, the structured and unstructured data related to power grid dispatching are processed to improve the application efficiency of data. Cleaning the preprocessed data to obtain the corresponding entity value and attribute value. The knowledge extraction model of power grid fault event reasoning knowledge mapping is constructed, and the power grid fault event reasoning knowledge edge mapping system is designed to extract the relationship between events and complete data storage. The experimental results show that the text prediction degree of the proposed model is high, which can reach more than 95; The accuracy is 96.71%, the recall rate is 94.88%, and the F1 value is 9.27%. This proves the feasibility of this study, in order to provide data and theoretical support for intelligent management and real-time dispatching of power grid.

Fault Recovery Analysis of Grid-Forming Inverters With Priority-Based Current Limiters

Grid-forming (GFM) inverters are required to operate robustly against grid faults. However, due to the limited over-current capability of inverters, current-limiting controls are usually applied to protect these semiconductor devices, which may prevent GFM inverters from a successful fault recovery. To understand this phenomenon, this study analyzes the fault recovery process of a GFM inverter with a priority-based current limiter. According to whether the GFM inverter can ensure transient stability and exit the current-limiting mode after fault clearance, three post-fault scenarios are identified, including <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">normal operation, current limitation, and oscillations</i> . Further, the impacts of the short-circuit ratio and control parameters on the post-fault behavior of GFM inverters are demonstrated. To illustrate the implications of these theoretical results, typical numerical examples are presented. Finally, the theoretical findings are validated through experimental tests.

Analytical Dual-Sequence Current Injections Feasible Region of Weak-Grid Connected VSC Under Asymmetric Grid Faults

For phase-locked-loop synchronization (PLL-Syn) based Weak-Grid connected Voltage Source Converter (WG-VSC) system under asymmetric grid faults, published works mainly focus on existing of equilibrium point and small signal stability, which cannot address PLL-Syn transient stability problems. For the first time, we have proposed an analytical solution to the question of “ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Whether the WG-VSC system can reach to a new available equilibrium point (EP) of fault-on state from original EP of pre-fault state without loss of PLL-Syn transient stability under asymmetric faults?</i> ”. Dual-sequence current injections feasible region (CIFR) of WG-VSCs in positive and negative sequence current injection space is defined firstly. Then, by considering <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PLL-Syn transient stability constraint, Maximum available current constraint, and PCC Voltage constraint</i> , an analytical expression of CIFR has been obtained. Finally, experiments results based on detailed switching model in RT-BOX hardware-in-loop platform have been provided to verify the CIFR constructed in this paper. It should be noted that the work of this paper can not only address PLL-Syn transient stability problems of WG-VSC system under asymmetric faults, but also can pave the way for asymmetric fault ride-through control considering transient stability constraints.

Modeling of Asymmetrical Grid Faults for P-HIL Accuracy and Stability Analysis in LVRT Tests

Modeling of Asymmetrical Grid Faults for P-HIL Accuracy and Stability Analysis in LVRT Tests

Hardware in the Loop-Based Testing of Three Schemes for Mitigation the Effect of Unsymmetrical Grid Faults on DFIG

This paper presents three-proposed schemes to mitigate the effect of unsymmetrical voltage sag fault on a wind-driven grid-connected Double Fed Induction Generator (DFIG). The first tested scheme comprises a static compensator (STATCOM) connected to the DFIG stator, while a three-phase parallel RL external impedance is connected to the rotor circuit in the second scheme. The STATCOM and the added rotor impedance are connected simultaneously in the third scheme. The effect of applying the three schemes on the responses of the stator and rotor voltages and currents, the dc-link voltage and current, the electrical torque, and the rotor speed during an unsymmetrical voltage sag are presented and compared at sub-and super-synchronous speeds. All systems were emulated, implemented, and tested through an OPAL RT-4510 Digital Real-Time Simulator (DRTS) in a Hardware-In-the-Loop (HIL) application. The internal Field-Programmable Gate Array (FPGA) chip assisted in using this platform as a Rapid Control Prototyping (RCP) for virtual mitigation control and testing. The Matlab/ Simulink RT-lab software packages combination helped in the RT development environment. All real-time waveforms of parameters for the proposed scenarios were monitored through the HIL-controller and data acquisition interface and then compared with the simulated results. The results reveal that the simulation waveforms and the real time waveforms are congruent. Results prove the better performance of the DFIG during unsymmetrical voltage sag for sub-synchronous speed when applying both protection schemes, while best results are obtained when using only the rotor impedance at super-synchronous speed operation of the DFIG.

Fault detection and classification using deep learning method and neuro‐fuzzy algorithm in a smart distribution grid

AbstractThis article proposes a deep learning (DL) model made of Long Short Term Memory (LSTM) and Adaptive Neuro Fuzzy Inference System (ANFIS) to detect fault in smart distribution grid assisted by communication systems using smart meter data. In smart grid, data analysis for fault identification and detection is crucial for grid monitoring. Nowadays, there are several DL techniques developed for smart grid data analysis applications. To solve this problem, a novel data analysis model based on deep learning and Neuro‐fuzzy algorithm is developed for fault location in a smart power grid. First, the LSTM is applied for training the data samples extracted from the smart meters. Then, an ANFIS algorithm is implemented for fault detection and identification from the trained data. Finally, faults are located with the higher accuracy. With this intelligent method proposed, single‐phase, two‐phase and three‐phase faults can be identified using a restricted amount of data. The novelty of the proposed method compared with other methods is the capability of fast training and testing even with large amount of data. To verify the effectiveness of our methodology, an intelligent model of the IEEE 13‐node network is used. The effectiveness and robustness of the proposed model are evaluated using several parameters such as accuracy, precision‐recall, F1‐score, Receiver Operating Characteristic (ROC) curve and complexity time. The obtained results indicate that the proposed deep learning model outperforms existing deep learning methods in the literature for fault detection and classification with 99.99% accuracy.

An ANFIS-based approach to locate arc fault in smart grid using phasor measurement units (PMU)

An ANFIS-based approach to locate arc fault in smart grid using phasor measurement units (PMU)

Transient virtual inertia optimization strategy for virtual synchronous generator based on equilibrium point state assessment

Virtual inertia enables the virtual synchronous generator (VSG) to have a good frequency support capability, but improper virtual inertia will deteriorate the transient synchronization characteristics of the VSG during grid faults, and may even lead to transient instability of the VSG. Aiming at this problem, this paper first analyzes the influence of virtual inertia on the transient stability of the VSG under different equilibrium point states. The research shows that reducing the virtual inertia can improve the transient stability when the VSG has equilibrium points, and increasing the virtual inertia can reduce the risk of transient instability when the VSG has no equilibrium points. Based on the above analysis, it is considered to adjust the virtual inertia adaptively according to the equilibrium point state to enhance the transient synchronous operation ability of the VSG. But directly determining the equilibrium point state depends on the measurement of grid parameters, which is difficult to achieve quickly and accurately in engineering. To this end, a transient virtual inertial optimization strategy based on equilibrium point state assessment is proposed. This strategy indirectly assesses the equilibrium point state according to characteristic parameters, flexibly adjusts the virtual inertia, and enhances the transient stability. Finally, simulation and experiment results verify the correctness of the theoretical analysis and the effectiveness of the proposed optimization strategy.

Zero-Voltage Ride-Through Scheme of PMSG Wind Power System Based on NLESO and GFTSMC

In this paper, a scheme for zero-voltage ride through of a permanent magnet synchronous generator wind power system is proposed. Maintaining the stability of DC-link voltage is the key to realizing zero-voltage ride through. A braking chopper is used to dissipate the active power to restrain the rise of DC-link voltage during a grid fault. However, the braking chopper-caused disturbance to the double closed-loop control of the grid side converter makes the control effect on DC-link voltage worse. Therefore, a robust current feedforward control, based on a nonlinear extended state observer and global fast terminal sliding mode control is proposed. A nonlinear extended state observer estimates the total disturbance in the system and compensates for the control law. Global fast terminal sliding mode control enables the system to reach the sliding mode surface in a finite time, and its control law is continuous without switching terms, thereby eliminating the chattering phenomenon. A nonlinear extended state observer and global fast terminal sliding mode control change the current feedforward control into a nonlinear robust current feedforward control. The control effect is improved, and the use of current transformers is reduced in practical applications, thereby reducing costs. The validity of this scheme has been verified by simulation.

Suppression strategy for the inrush current of a solid-state transformer caused by the reclosing process

The automatic reclosing strategy is an effective measure to improve the reliability of a distribution network. It can quickly clear instantaneous faults in the grid. The traditional transformer has proven to be reliable and robust during the reclosing process. However, the influence of the reclosing process on the operational characteristics and reliability of solid-state transformers (SST) is still unclear. The reclosing action may generate a huge inrush current, resulting in shutdown and even damage of the SST. To address this problem, this paper proposes an inrush current suppression strategy. First, the operational performance of the SST under a reclosing process is discussed, and the inrush current generation mechanism is analyzed in detail. Then, considering the controllability of distributed generation (DG), a novel DG-supported inrush current suppression strategy is proposed. The suppression ability of the DG on inrush current in different initial conditions is analyzed. Finally, the effectiveness of the proposed strategy is verified by simulation and experiment. These show that the proposed strategy can help to enhance the FRT capability of the SST, as well as support the SST to maintain continuous power supply and physical integrity during grid faults.

Low-voltage ride-through capability in a DFIG using FO-PID and RCO techniques under symmetrical and asymmetrical faults

The power grid faults study is crucial for maintaining grid reliability and stability. Understanding these faults enables rapid detection, prevention, and mitigation, ensuring uninterrupted electricity supply, safeguarding equipment, and preventing potential cascading failures, ultimately supporting the efficient functioning of modern society. This paper delves into the intricate challenge of ensuring the robust operation of wind turbines (WTs) in the face of fault conditions, a matter of substantial concern for power system experts. To navigate this challenge effectively, the implementation of symmetrical fault ride-through (SFRT) and asymmetrical fault ride-through (AFRT) control techniques becomes imperative, as these techniques play a pivotal role in upholding the stability and dependability of the power system during adverse scenarios. This study addresses this formidable challenge by introducing an innovative SFRT–AFRT control methodology based on rotor components optimization called RCO tailored for the rotor side converter (RSC) within a doubly-fed induction generator (DFIG) utilized in wind turbine systems. The proposed control strategy encompasses a two-fold approach: firstly, the attenuation of both positive and negative components is achieved through the strategic application of boundary constraints and the establishment of reference values. Subsequently, the optimization of the control characteristic ‘beta’ is accomplished through the utilization of a particle swarm optimization (PSO) algorithm integrated within an optimization loop. This intricate interplay of mechanisms aims to optimize the performance of the RSC under fault conditions. To measure the efficacy of the proposed control technique, a comparative analysis is conducted. Fractional-order (FO) proportional–integral–derivative (PID) controllers are employed as an additional method to complement the novel approach. By systematically juxtaposing the performance of the proposed SFRT–AFRT control technique with the FO-PID controllers, a comprehensive evaluation of the proposed approach's effectiveness is attained. This comparative assessment lends valuable insights into the potential advantages and limitations of the novel control technique, thereby contributing to the advancement of fault mitigation strategies in WT systems. Finally, the paper highlights the economic viability of the proposed control method, suggesting its suitability for addressing broader power network issues, such as power quality, in future wind farm research.

A New Short-Circuit Current Calculation and Fault Analysis Method Suitable for Doubly Fed Wind Farm Groups.

The transient characteristics of wind farms in groups are quite different; in addition, there is a strong coupling between the wind farms and the grid, and these factors make the fault analysis of the grid with wind farm groups complicated. In order to solve this problem, a mathematical model of the converter is established based on the input-output external characteristics of the converter, and a transient model of a doubly fed wind turbine (DFIG) is presented considering the influence of the low-voltage ride-through control (LVRT) of the converter, and the effect mechanism of the LVRT strategy on the short-circuit current is analyzed. Finally, a short-circuit current calculation model of a doubly fed wind turbine with low-voltage crossing control is established. The interaction mechanism between wind farms during the fault is analyzed, and a short-circuit current calculation method of doubly fed wind farm groups is proposed. RTDS is used to verify the accuracy of the proposed short-circuit current calculation method for doubly fed field groups. On this basis, a method of power grid fault analysis after doubly fed field group access is discussed and analyzed.

Protection of Line Faults in HVDC Grids Through Convexity Detection in Backward Traveling Wave Voltages

Short-circuit faults in high-voltage DC (HVDC) grids must be timely isolated. Conventional fault isolation schemes for HVDC grids typically rely on numerical simulations or analytical calculations for threshold setting, which complicate engineering implementation. To solve this issue, this paper first introduces the convex point of backward traveling wave (BTW) voltage as the reference for the faulty line isolation in HVDC grids, which is interpreted as the singular feature in the second derivative (SD) of BTW voltage. The singularity in the SD of BTW voltage is independent of system or fault parameters, having good generality in HVDC grids. Through recognizing the unique pattern in the wavelet transform modulus maxima of the singular feature, the proposed protection method can identify faulty lines in HVDC grids in the initial phase of faults. Numerical studies carried out with PSCAD/EMTDC and MATLAB/Simulink demonstrate the effectiveness, robustness and applicability of the proposed method in detecting and isolating various single- and double-pole faults in HVDC grids.

Security Region Modeling and Fault Ride-Through Coordinative Control of VSC-HVDC Under Grid Fault

Security Region Modeling and Fault Ride-Through Coordinative Control of VSC-HVDC Under Grid Fault

A Voltage Support Scheme for Distributed Generation With Minimal Phase Current Under Asymmetrical Grid Faults

In the event of grid faults, the disconnection of a large capacity distributed generation (DG) may compromise grid stability. To address this challenge, grid codes in many countries have incorporated fault ride-through capability that requires the DG to remain connected to the grid for a stipulated time and regulate the voltage at the Point of Common Coupling (PCC). A voltage support scheme (VSS) aims at restoring the PCC voltage within stipulated limits by injecting suitable currents. To restore the voltage under asymmetrical faults, a conventional VSS requires unequal current injection. As the phase current of the highest amplitude hits the rating of the inverter, the voltage support capability of the DG is compromised. The VSS proposed in this paper aims to minimize the highest phase current while maintaining the same voltage support at PCC. The reduction in the highest phase current, in turn, provides reserve voltage support capability. The proposed scheme works well for all types of grid impedances, and the optimum points are found to be independent of the grid impedance. This independence of the optimum points simplifies the implementation of the proposed VSS. The simulation and experimental results demonstrate the effectiveness of the proposed VSS.

Robust Adaptive Active Islanding Detection With $\alpha$-Axis Disturbance Injection Under High Impedance Fault, Unbalanced Loading, and Phase Failure in Grid Tied PV System

Power generation with photovoltaic technology is growing expeditiously. With the high penetration of solar-based generation, a concern of islanding detection is always by the distributors due to grid side faults and abnormality. It is the demand of the situation to equip the PV-connected inverter with an islanding detection mechanism to avoid any catastrophe. In the present work, an active islanding detection technique utilizing a smaller magnitude current injection through <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\alpha$</tex-math></inline-formula> -axis current controller of grid side voltage source converter (VSC) is proposed. Due to injection of current through VSC, point of common coupling (PCC) voltage is having different harmonic component in the grid connected and islanded mode. In the proposed detection method, the sensed PCC voltage is used to derive the three signals i.e. AVVM(absolute <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$v_{mag}$</tex-math></inline-formula> variation mean), AMSVV(absolute mean of square variation of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$v_{mag}$</tex-math></inline-formula> ), & FAVVM(Frequency of absolute <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$v_{mag}$</tex-math></inline-formula> variation mean). These three derived signals is used to take the decision for islanding detection. Further, Complex Co-efficient Filter-Second Order Frequency Locked Loop (CCF-SOFLL) is proposed for three phase grid synchronization and frequency estimation. The proposed Strategy is tested under high impedance fault, phase failure and unbalanced loading conditions. It is found that the proposed work clearly differentiates the islanding and faults condition. The Non- detection zone (NDZ) of the proposed strategy is verified under different quality factors ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Q_{f}$</tex-math></inline-formula> ) of the load. To evaluate the effectiveness of the suggested strategy, MATLAB Simulation and experimental validation is performed. The obtained islanding detection time is within the IEEE-1547 standard.

Enhanced Excitation Converter With Parallel/Series DC-Link Based on TAB for DFIG to Improve the LVRT Capability Under Severe Grid Faults

The low-voltage ride-through (LVRT) capability of the doubly-fed induction generator (DFIG) based on the conventional excitation converter is insufficient under severe grid faults. The reason is that the dynamic rotor electromotive forces (EMF) will far exceed the dc-link voltage. To solve this issue, this letter proposes an enhanced excitation converter with parallel/series dc-link based on the triple active bridge (TAB) for DFIG. The dc-link of the enhanced excitation converter consists of the TAB and switching circuit, replacing the capacitive-type dc-link. Based on this topology, the dc-link voltage can increase instantaneously during severe grid faults to resist rotor EMF and improve the LVRT capability of DFIG. Compared with existing LVRT schemes, the enhanced excitation converter enables DFIG to achieve LVRT and inject the reactive current requirements of the grid code into the grid under severe grid faults. Finally, experiments verified the effectiveness of the proposed enhanced excitation converter and its control strategy.