Unbalanced Grid Faults Research Articles

Using neural network super‐twisting sliding mode to improve power control of a dual‐rotor wind turbine system in normal and unbalanced grid fault modes

SummaryAccording to recent research work, increasing electric power generation is one of the significant advantages of the dual‐rotor wind turbine (DRWT) compared to the other types for the same wind speed. In this research work, a modified super‐twisting sliding mode control (STSMC) based on the neural network (NN) is suggested to regulate the stator powers of a DRWT‐based doubly‐fed induction generator (DFIG) in normal and unbalanced grid fault modes. The design of this strategy involves replacing the gains of conventional STSMC with the NN algorithm to enhance robustness, mitigate the impact of unbalanced grid voltage, and consequently improve the quality of the generated power of DRWT‐based DFIG. This forms the primary contribution of this work. The suggested strategy is compared with vector control (VC) and conventional STSMC in terms of reference tracking, power ripples, response dynamics, harmonic distortion of stator current, and the effect of an unbalanced grid fault. Finally, the utility and effectiveness of the designed controller are confirmed through computer simulations. Furthermore, when the grid is subjected to a 20% voltage drop, the results demonstrate that the suggested strategy reduced the total harmonic distortion (THD) value of the stator current by 12.92% compared to VC and by 9.29% compared to conventional STSMC.

A Model Based LCL-Type Grid Connected Converter Under Balanced and Unbalanced Faults in a Micro-Grid Distributed Generation

This research was conducted to verify the significance of the LCL-filter on the grid current and the impact of variable fault resistance values on the reactive power genereated in a grid-tied inverter. The stability of LCL-type grid connected inverter with capacitor current feedback in active damping state was evaluated in this paper. The effects of balanced and unbalanced grid faults on the active and reactive power was studied through simulation at different fault resistance values of 0.00025Ω and 2.5Ω. The FFT waveforms showed that THD values of 48.56% and 38.45% were achieved for the grid voltage at 0.00025Ω and 2.5Ω fault resistance while THD values of 9.50% and 4.41% were obtained for the grid current at a varied current feedback coefficient (K<SUB>CP</SUB>) of 4.75 and 14.75. Simulation results also showed that a very negligible real and reactive power was gained with a zero grid voltage within the fault zone at 0.00025Ω fault resistance. At a 2.5Ω fault resistance, a voltage sag was produced which accounted for the transient response in the real power generated and reactive power absorbed during the fault period. The result obtained from the root-locus plot showed that the loci for the derived LCL-filter current transfer function intersected at +j 8.734 and -j 8.734 which makes the system marginally stable All simulation procedures were realized in MATLAB/SIMULINK 2015.

Dynamic Voltage Equalization Control of D-STATCOM Under Unbalanced Grid Faults in a Low-Voltage Network

Unbalanced grid faults are very harmful to the network operation. Distributed static synchronous compensator (D-STATCOM) can help to support the grid voltage by injecting the reactive power into the inductive power grid. However, this approach turns to be ineffective for a low-voltage grid, where the network impedance is mainly resistive. In this article, a dynamic voltage equalization control (DVEC) scheme is proposed to suppress the negative sequence voltage, and therefore, reduce the voltage unbalance during the faults in the resistive network. This effect is achieved through transferring proper active power from the negative sequence channel to the positive sequence channel. And the dual-sequence active currents are controlled by DVEC without exchanging extra active power with the grid except the little circuit loss compensation. By flexibly adjusting the control parameter of DVEC, current peak and active power oscillation can be limited within a preset range, respectively, during the process of maximizing the voltage control. As a result, the DVEC scheme can ensure the safe operation of D-STATCOM and the stability of the power grid. Moreover, experimental results are reported to validate the correctness and effectiveness of the proposed scheme.

Inverse Application of Artificial Intelligence for the Control of Power Converters

This article proposes a novel application method, inverse application of artificial intelligence (IAAI) for the control of power electronic converter systems. The proposed method can give the desired control coefficients/references in a simple way because, compared to conventional methods, IAAI only relies on a data-driven process with no need for an optimization process or substantial derivations. Noting that the IAAI approach uses artificial intelligence to provide feasible coefficients/references for the power converter control, rather than building a new controller. After illustrating the IAAI concept, a conventional application method of artificial neural network is discussed, an optimization-based design. Then, a two-source-converter microgrid case is studied to choose the best droop coefficients via the optimization-based approach. After that, the proposed IAAI method is employed for the same microgrid case to quickly find good droop coefficients. Furthermore, the IAAI method is applied to a modular multilevel converter (MMC) case, extending the MMC operation region under unbalanced grid faults. In the MMC case, both simulation and experimental online tests validate the operation, feasibility, and practicality of IAAI.

Self-adapting PI controller for grid-connected DFIG wind turbines based on recurrent neural network optimization control under unbalanced grid faults

• Establish the MPPT with optimal speed ratio combined with hidden layer recurrent neural network (HLRNN) to obtain faster convergence. • Optimize the PI controller gains by using a hidden layer recurrent neural network. • Investigate the active power behavior in the presence of unbalanced grid faults. • Amelioration of the response time of the produced power compared with some other existing neural network schemes. This paper suggests the Hidden Layer Recurrent Neural Network (HLRNN) for controlling a wind power generation system based on a dual-fed induction generator. The generator's stator is connected directly to the electrical grid, while the rotor is linked through bidirectional converters. A PI controller-based Indirect Vector Control (IVC) scheme has been established to pilot the system. The PI regulator allows linear systems to perform well, but when subjected to physical variation conditions, the system's response becomes unstable, making the PI controller insufficient. This paper aims to ensure the PI controller gains self-adaptation regardless of the severity of the circumstances. The effectiveness of the proposed control is demonstrated by taking into account critical conditions such as wind speed changes, generator parameter variations, and asymmetrical faults in the grid. Furthermore, it is confirmed by the enhanced performance and the reduced oscillations during a voltage dip. The simulation results, achieved using MATLAB/Simulink, demonstrate that the response time is reduced to 1.8 (m s), the static error is minimized to 0.16%, and the overshoot is improved to 0.24% when compared to the IVC based on the PI controller and some existing neural network schemes.

Flexible control of grid-connected renewable energy systems inverters under unbalanced grid faults

The sustainable energy opportunity of the future lies in the renewable energy-based distributed Power Generation systems (DPGSs). However, there are different concerns and integration issues that surround them. To increase the rate at which renewable energy can be used, this paper focuses on an integrated utility grid system. A novel strategy to control the grid side inverter is developed for a DPGS when considering an unbalanced grid. First, an analysis of the consequences of the occurrence of a fault on the grid is performed. Secondly, a control strategy is proposed to ensure the operation of the DPGS connected to the grid even if the latter is unbalanced. The proposed approach consists in developing control loops for each of the three symmetrical sequences in a specific reference frame to ensure the operation of the grid-connected DPGS even during asymmetrical grid voltage fault. The found results are promising in terms of the guarantee of service continuity during faulty conditions.

Flexible Power Regulation and Limitation of Voltage Source Inverters under Unbalanced Grid Faults

This paper develops a flexible power regulation and limitation strategy of voltage source inverters (VSIs) under unbalanced grid faults. When the classical power theory is used under unbalanced grid faults, the power oscillations and current distortions are inevitable. In the proposed strategy, the extended power theory is introduced to compute the power feedbacks together with the classical power theory. Based on the combination of the classical and extended power theory, the proposed strategy can achieve the sinusoidal current provision and the flexible regulation between three common targets, i.e., constant active power, balanced current, and constant reactive power. Meanwhile, the proposed strategy is associated with a power limiter, which is capable to keep the currents under the pre-defined threshold and to compute the maximum apparent power for better utilization of the inverter capacity. With this power limiter, the rated inverter capacity is fully used for both the active and reactive power provisions under unbalanced grid faults. Using the proposed power regulation and limitation, the VSI can avoid overcurrent tripping and flexibly regulate its power under unbalanced grid faults. All the conclusions are verified by the real-time hardware-in-loop tests.

Improved Current Reference Calculation for MMCs Internal Energy Balancing Control

The paper addresses an improved inner current reference calculation to be employed in the control of modular multilevel converters operating during either balanced or unbalanced conditions. The suggested reference calculation is derived based on the AC and DC additive and differential voltage components applied to the upper and lower arms of the converter. In addition, the impacts caused not only by the AC network’s impedances but also by the MMC’s arm impedances are also considered during the derivation of the AC additive current reference expressions. Another issue discussed in this article regards that singular voltage conditions, where the positive-sequence component is equal to the negative one, may occur not only in the AC network but also internally (within the converter’s applied voltages). Several different inner current reference calculation methods are compared and their applicability during the former fault conditions is analyzed. The paper presents a detailed formulation of the inner current reference calculation and applies it to different unbalanced AC grid faults where it is shown that the presented approach can be potentially used to maintain the internal energy of the converter balanced during normal and fault conditions.

Maximum Asymmetrical Support in Parallel-Operated Grid-Interactive Smart Inverters

Parallel operation of grid-interactive inverters has been continuously gaining attraction, and their contribution to sustaining the host grid stability has become strongly demanded. This paper investigates and discusses the trends in the most recent interconnection grid codes. Grid codes call for simultaneous requirements for the low-voltage ride-through capabilities of parallel operation of grid-interactive inverters as well as their effective asymmetrical voltage support by advanced active/reactive bi-sequence power provision under unbalanced grid faults. Addressing these demands in an optimized way becomes very challenging. This paper thus proposes a new methodology to simultaneously achieve three main objectives: (1) coordination of the asymmetrical ride-through and voltage support capabilities of parallel-operated inverters, (2) maximizing the utilization of each unit and their collective contribution in boosting the positive-sequence voltage and reduction of the negative sequence voltage subject to the constraints from the inverters and the grid, and (3) minimizing the impact of the support on the active power injections. Simulation and experimental results illustrate the effectiveness of the proposed algorithm.

Emulation of an Induction Machine for Unbalanced Grid Faults

A laboratory power level 5.0 hp induction machine emulation with grid interface for various asymmetric fault transients is implemented in this article. It is done in real time with power-hardware-in-loop (PHIL) technology. The PHIL emulation procedure permits the testing of complex control strategies of an ac microgrid while determining the machine behavior during grid faults. Since asymmetric grid faults generate opposite sequence components in the individual harmonic currents, a novel emulator control strategy with combination of proportional-integral (PI) and proportional resonant (PR) controllers is proposed to mimic the behavior of the real machine under grid faults. A step-by-step procedure for designing the PI and PR controllers is given in detail. An improved induction machine mathematical model with trapezoidal integration is also suggested for precise tracking of the emulator. An emulator laboratory setup is developed with Opal-Rt as the real-time controller and linear amplifier as the emulating device. Simulation and experimental results are provided to validate the tracking capability of the proposed emulator. The results confirm that with the proposed control, emulator currents track the real machine currents.

Online current limiting-based control to improve fault ride-through capability of grid-feeding inverters

Grid-feeding inverters require much better operation monitoring and improved performance for their safe and efficient integration into the power system. Specifically, these inverters must participate in grid stability enhancement under faulty conditions and without any disconnection. In this regard, a fault ride-through capability is proposed for an inductive filtered grid-feeding inverter, which is controlled in both positive and negative sequences to ensure dual current control. Grid voltage dips are tackled by injecting a reactive power according to grid codes requirements. Enhanced active power control is introduced by suppressing oscillations, which are twice the grid-frequency. In order to avoid the inverter and semi-conductor components tripping under grid faults, a new current limiting strategy is proposed. It is based on real-time active power reduction using a closed-loop current magnitude controller. The effectiveness and flexibility of the proposed control strategy are verified by numerical simulations in both balanced and unbalanced grid fault cases.

Active power improvement of three-phase grid-connected inverter under unbalanced grid faults

Unbalanced grid voltage causes fault-phase over-current, affects the life span of devices and even leads to control system instability. The conventional current limiting strategy reduces the output power of grid-connected inverter. In order to increase the active power injected by the grid-connected inverter into the grid, this paper proposes a new current limiting strategy. On the premise of ensuring that the DC side voltage fluctuation and grid-connected current harmonics meet the grid standards, the fault-phase current and the normal phase current are respectively limited. The simulation results verify the proposed strategy can effectively improves the active power output of the grid-connected inverter.

An augmented state observer-based sensorless control of grid-connected inverters under grid faults

The increasing integration of grid-connected renewable energy systems impose major challenges of reliability and power quality within power grid system. Grid-connected systems must ensure grid stability, accommodate low voltage ride-through capability, and comply with grid codes under unbalanced voltage dips to avoid any grid-disconnection. In this context, this paper proposes a novel control strategy to improve grid-connected inverter control under unbalanced grid voltage conditions. Positive and negative sequence components of grid voltage are estimated using an augmented state and disturbance observer. The estimated values of grid voltage and phase-angle are used to protect the inverter from any over-current tripping even at unbalanced grid voltage dips. This can be achieved by reducing active power and suppressing its fluctuations with new current references, which are obtained in the synchronous reference frame according to grid codes. The effectiveness and flexibility of the proposed power control strategy have been demonstrated by numerical simulation and validated by experiments in both balanced and unbalanced grid fault conditions.

Second‐order sliding mode control of wind turbines to enhance the fault‐ride through capability under unbalanced grid faults

AbstractThe integration of wind generation to the grid is growing rapidly across the world. As a result, grid operators have introduced the so‐called grid codes (GC), which nowadays include a range of technical conditions and requirements, which wind generators must fulfill. Among these, the low‐voltage ride through (LVRT) is a requirement for wind turbines to stay connected to the grid and continue to operate during the disturbance. In this study, a control structure, combining inertial kinetic energy storage with a crowbar circuit, is proposed to enhance the ride through capability of a wind turbine generator (WTG) based on a wound‐field synchronous generator (WFSG) under unsymmetrical voltage dips. For the grid‐side converter (GSC), a decoupled double synchronous reference frame (DDSRF) d‐q current controller is used. Furthermore, a second‐order sliding mode controller (SOSMC) with super‐twisting (ST) algorithm is proposed for the GSC and the machine‐side converter (MSC) to improve the response speed and achieve an accurate regulation of the dq‐axis current components simultaneously. The main objectives of the GSC are to achieve a balanced, sinusoidal current and smooth the real and reactive powers to reduce the influence of the negative‐sequence voltage. A series of simulations are presented to demonstrate the effectiveness of the proposed control scheme in improving the LVRT capability of the WFSG‐driven wind turbine and the power quality of the system under unbalanced grid voltage conditions.

Protection collaborative fault control for power electronic-based power plants during unbalanced grid faults

The controlled fault characteristics of power electronic-based power plant during unbalanced grid faults can heavily affect the proper operation of distance relay. In order to solve this issue, a fault control scheme is developed for the power electronic-based power plant, which is compliant with grid protection during unbalanced grid faults. Firstly, we analyze the essential problem of the distance relay on the transmission line connecting the power electronic-based power plant. Subsequently, we build the equivalent fault models of power electronic-based power plant in the sequence systems based on the reactive power supporting requirements defined in the modern fault-ride-through rules and the protection demands. From the analytic equation of apparent impedance in different fault loops, a novel protection collaborative fault control scheme to determine the proper current command angle of the power electronic-based power plant during unbalanced grid faults is deduced. Simulation results prove that the proposed fault control of power electronic-based power plant can improve the accuracy of reactance measurements in distance relay, which in turn reduces the malfunction risk of the relay.

A Dual-Layer Back-Stepping Control Method for Lyapunov Stability in Modular Multilevel Converter Based STATCOM

With the high penetration of the power electronic loads in the grid, the stability of static synchronous compensator (STATCOM) devices is greatly challenged. However, the conventional control methods for the modular multilevel converter (MMC) based STATCOM only consider the stability with small signal disturbances. This article proposes a novel dual-layer back-stepping control (BSC) for the MMC-based STATCOM. In the first layer, the BSC aims to regulate the sum of the capacitor energy and the reactive output current. In the second layer, the BSC aims to control the circulating current. Therefore, the proposed method possesses a fast dynamic response and accurate tracking with the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Lyapunov</i> stability of the MMC-based STATCOM. Compared with the arm-control-based BSC for MMC-based inverters, the proposed method has a simplified structure and a reduced computation burden. Moreover, the proposed method realizes the independent control between the output current and the circulating current. The simulation and experimental results verify the effectiveness of the proposed method. In addition, its robustness toward different circuit parameters and the operation ability under unbalanced grid fault is also verified.

Design and implementation of DVR as fault current limiter in DFIG during grid faults

The wind grid code requirements for low voltage ride through of various countries demand the DFIG to remain connected to the grid during the event of occurrence of voltage dip. Dynamic voltage restorer supports the grid and enables the uninterrupted operation of DFIGs during partial voltage sag. The increased depth of voltage sag causes rise in stator current and eventual increases in rotor current and dc link capacitance voltage of DFIG. This paper focuses on an enhanced topology of fault current limiting dynamic voltage restorer for supporting the grid and DFIG during fault currents caused by severe voltage dips. The optimal parameters design of fault current limiting dynamic voltage restorer for DFIG systems are discussed in detail. The novelty of the proposed control strategy of fault current limiting dynamic voltage restorer is justified by comparing its performance with the existing methods of fault current mitigations popular with wind farms such as crowbar protection and dc chopper methods. Analytical results of fault current limiting dynamic voltage restorer with DFIG in time domain prove to have the combined effect of both crowbar protection and dc chopper-controlled rotor protection during severe unbalanced grid faults.

Development and performance analysis of intelligent fault ride through control scheme in the dynamic behaviour of grid connected DFIG based wind systems

As the penetration of wind power increases, the fault ride through requirements imposed by grid code is important in grid connected wind systems. The wind systems should remain connected to the grid during grid faults satisfying the grid code which ensures grid stability during fault and post fault conditions. This paper proposes an intelligent fault ride through strategy for Doubly Fed Induction Generators (DFIG) based Wind Energy Conversion Systems (WECS) to achieve real and reactive power control during grid faults. The transient behaviour of the system is investigated under normal conditions and during grid faults. A fuzzy based wind speed estimation method in Maximum Power Point Tracking (MPPT) mode under normal conditions and coordinated Genetic Algorithm based Real-Reactive (GA-PQ) controller with DC chopper in Fault Ride Through (FRT) mode during grid faults is developed in this work. The proposed control scheme provides smooth operation of DFIG during grid faults by controlling the rotor and grid side converters, providing reactive power support to the grid and relieving stress on power converters thereby achieving system stability. The proposed strategy maintains the system parameters during the grid faults by suppressing the rotor and stator over current, dc link voltage overshoot, power oscillations and support the grid voltage under both balanced and unbalanced grid fault conditions with different voltage dips at PCC. Computer simulations in time-domain are performed by verifying the MPPT and FRT capability of the proposed strategy for 6.5MW grid connected DFIG based WECS in MATLAB/SIMULINK 2018b. The proposed coordinated intelligent PQ and DC chopper FRT strategy is compared against existing FRT schemes like crowbar, SDBR, DC Chopper, PQ controller and its hybrid combinations which yields satisfactory results. The proposed scheme is also validated in Opal-RT hardware for LLG fault at 75% dip in voltage. The simulation and real time results observed in the work station demonstrates the effectiveness of the proposed strategy in enhancing the FRT capability of the system.

Distributed and Asynchronous Active Fault Management for Networked Microgrids

A distributed and asynchronous active fault management (DA-AFM) method is developed to manage networked microgrids' (NMs) performance under balanced or unbalanced grid faults. The DA-AFM aims to (1) enable NMs' fast fault ride-through capabilities, (2) limit the total fault contributions by coordinating heterogeneous microgrids in the NM system, and (3) deploy software-defined networks (SDN) to ensure highly resilient AFM. The problem is formulated in an optimization form that can incorporate various fault management objectives and constraints in a programmable and flexible fashion. The scalability and resiliency of the DA-AFM system are guaranteed by adopting an SDN-enabled distributed and asynchronous surrogate Lagrangian relaxation (DA-SLR) algorithm, which avoids single point of failure, preserves privacy, and eliminates the idle waiting of other subproblems. Case studies are performed on a six-microgrid NM system to validate the effectiveness and efficacy of DA-AFM. Testing results show that DA-AFM has excellent convergence performance, supports plug-and-play, is resilient to communication delay and failures, meets real-time requirements and is scalable.

Voltage support control strategy of grid‐connected inverter system under unbalanced grid faults to meet fault ride through requirements

Grid-connected inverter (GCI) has become the main interface for integrating modern power units, such as distributed energy resources, electric vehicles, microgrids and high voltage direct-current transmission systems. To proceed in this direction, this study presents a novel voltage support control strategy to enhance the reliability and stability of the GCI during unbalanced grid fault conditions. The proposed control strategy simultaneously achieves multiple objectives during grid faults; regulating phase voltages magnitude within pre-defined safety limits, increasing the difference between positive sequence (PS) and negative sequence (NS) voltages, eliminating both active–reactive power oscillations and the DC-link voltage oscillations. Reducing active power oscillations ensure an adjustable control for the DC-link voltage oscillations which result in third-order current harmonic component at the grid side. One of the main contributions to previous studies, reference for reactive power is computed online based on resistive–inductive grid impedance model and reference voltage sequences for the grid support. Another important contribution to existing studies is to supply both the active and reactive powers to the utility grid and load. Detailed mathematical analyses are performed to theoretically describe the behaviour of the proposed control strategy. A comprehensive set of results is presented to confirm the theoretical solutions.