Unsymmetrical Grid Faults Research Articles

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.

Accurate Estimation of Phase Angle for Three-Phase Systems in Presence of Unbalances and Distortions

The amplitude and phase imbalances can occur in three-phase voltage systems due to unbalanced loading and/or unsymmetrical grid faults. In this case, the conventional Clarke transform cannot produce accurate orthogonal signals, and hence a phase-locked loop (PLL) relying on the conventional Clarke transform is not able to produce accurate phase angles of three-phase voltage systems. This article proposes a three-phase PLL relying on an adaptive Clarke transform (ACT) algorithm for tracking the accurate phase angles of grid voltages in the presence of distortions and both the amplitude and phase imbalances. The proposed method includes bandpass filter in each phase to reject dc offset and harmonics. An algorithm is reported to track the unbalanced amplitudes and phase angle deviations of filtered voltages. A set of analytical expressions are presented for the coefficients of ACT matrix to make them adaptive under the amplitude and/or phase unbalanced conditions. A PLL is used to track the phase angle from the orthogonal signals generated by the ACT. A phase correction algorithm is also used to reject the phase error caused by the fixed tuned bandpass filter, which makes the method frequency adaptive without using any frequency feedback loop. The ACT is capable of generating orthogonal signals under unbalanced condition, whereas the conventional Clarke transform fails to do so. When compared with delayed signal cancellation (DSC)-based PLLs, the proposed method produces accurate results regardless of voltage unbalances and distortions. The simulation and experimental results are presented to verify the usefulness of the proposed method.

Transient Stability Augmentation of a Wave Energy Conversion System Using a Water Cycle Algorithm-Based Multiobjective Optimal Control Strategy

This paper introduces a novel optimum design of the proportional–integral (PI) controller in the power converter circuits using the water cycle algorithm (WCA) to augment the transient stability of a grid-connected wave energy conversion (WEC) system. The proposed system relies on the Archimedes wave swing device, which is coupled with a linear permanent magnet synchronous generator (LPMSG). The WEC system is interfaced with the power grid via a generator-side converter (GSC) and a grid-side inverter (GSI). The GSC is used to control both of the d -axis and q -axis current of the LPMSG to minimize its power losses and extract its maximum real power, respectively. The GSI is implemented to control the terminal voltage at the point of common coupling and the dc-link voltage through a complete vector control scheme. The proposed optimal WCA-based PI control strategy is applied to both converters. The optimization process depends on the simulation-based optimization approach. In the proposed approach, the criterion of integral squared error is chosen as a multiobjective function. To validate the proposed WEC system model, the simulation results are compared with the practical results, and their error reaches less than 1%. The effectiveness of the proposed WCA-based PI control strategy is tested and compared with that obtained using the genetic-algorithm-based PI control scheme under symmetrical and unsymmetrical grid fault conditions taking into account a successful and unsuccessful reclosure of circuit breakers. The validity of the proposed control strategy is extensively checked based on simulation studies in the PSCAD/EMTDC environment.

Doubly fed induction generator‐based variable‐speed wind turbine: Proposal of a simplified model under a faulty grid with short‐duration faults

AbstractThe aim of this study is to provide a simplified model of a variable‐speed wind turbine (VSWT) with the technology of a doubly fed induction generator (DFIG), which operates under faulty grid conditions. A simplified model is proposed, which consists of a set of electrical and mechanical equations that can be easily modeled as simplistic electrical circuits. It makes it an excellent tool to achieve fault ride‐through capability of grid‐connected VSWT with DFIGs. Both symmetrical and unsymmetrical grid faults, which cause symmetrical and unsymmetrical voltage sags, have been applied to the system in order to validate the model. The proposed simplified model has been compared with the traditional full‐order model under multiple sags (different durations and depths), and the results reveal that both models present similar accuracy. As the idea is to reduce the computational time required to simulate the machine behavior under faulty grid conditions, the proposed model becomes suitable for that purpose. The analytical study has been validated by simulations carried out with MATLAB.

Fault Analysis in a Grid Integrated DFIG Based Wind Energy System with NA CB_P Circuit for Ridethrough Capability and Power Quality Improvement

Abstract In this paper a three phase fault analysis is done on a DFIG based grid integrated wind energy system. A Novel Active Crowbar Protection (NACB_P) system is proposed to enhance the Fault-ride through (FRT) capability of DFIG both for symmetrical as well as unsymmetrical grid faults. Hence improves the power quality of the system. The protection scheme proposed here is designed with a capacitor in series with the resistor unlike the conventional Crowbar (CB) having only resistors. The major function of the capacitor in the protection circuit is to eliminate the ripples generated in the rotor current and to protect the converter as well as the DC-link capacitor. It also compensates reactive power required by the DFIG during fault. Due to these advantages the proposed scheme enhances the FRT capability of the DFIG and also improves the power quality of the whole system. Experimentally the fault analysis is done on a 3hp slip ring induction generator and simulation results are carried out on a 1.7 MVA DFIG based WECS under different types of grid faults in MATLAB/Simulation and functionality of the proposed scheme is verified.

Dual current control strategy to fulfill LVRT requirements in WECS

Purpose – The purpose of this paper is to investigate a control strategy to fulfill low-voltage ride through (LVRT) requirements in wind energy conversion system (WECS). Design/methodology/approach – This paper considers an active front-end converter of a grid connected WECS working under grid fault conditions. Two strategies based on symmetrical components are studied and proposed: the first one considers control only for positive sequence control (PSC); the second one considered a dual controller for positive and negative sequence controller (PNSC). The performance of each strategy is studied on LVRT requirements fulfillment. Findings – This paper shows presents a control strategy based on symmetrical component to keep the operation of grid-connected WECS under unsymmetrical grid fault conditions. Research limitations/implications – This work is being applied to a 2 kVA laboratory prototype. The lab prototype emulates a grid connected WECS. Originality/value – This paper validate the PNSC strategy to LVRT requirements fulfillment by experimental results obtained for a 2 kVA laboratory prototype. PNSC strategy allows constant active power delivery through grid-voltage dips. In addition, the proposed strategy is able to grid-voltage support by injection of reactive power. Additional features are incorporated to PNSC: sequence separation method using delay signal cancellation and grid frequency identification using phase locked loop.

Research on Rotor over-Current of a Doubly-Fed Induction Generator under Power Grid Fault

This paper showed that the doubly-fed induction generator (DFIG) was sensitive to the faults of power grid. Through theoretic analysis for active power characteristic of DFIG, it was recognized that the conventional PI-based vector control strategy was likely to cause the excitation system overregulated due to lack of additional protection technique. On this condition, a DFIG risks damaged to its power converter due to overcurrent or overvoltage. At the basis of prior art, this paper developed a novel control scheme, which focused on solving overregulated problems caused by faults of power grid. The proposed control strategy was tested on an illustration DFIG-based power system modeled by MATLAB/SIMULINK. The results demonstrate that the proposed control scheme can effectively resist overcurrent under the conditions of both symmetrical and unsymmetrical grid faults.

Unified Power Control for PMSG-Based WECS Operating Under Different Grid Conditions

A unified power control strategy is proposed for the permanent magnet synchronous generator-based wind energy conversion system (WECS) operating under different grid conditions. In the strategy, the generator-side converter is used to control the dc-link voltage and the grid-side converter is responsible for the control of power flow injected into the grid. The generator-side controller has inherent damping capability of the torsional oscillations caused by drive-train characteristics. The grid-side control is utilized to satisfy the active and reactive current (power) requirements defined in the grid codes, and at the same time mitigates the current distortions even with unsymmetrical grid fault. During grid faults, the generator-side converter automatically reduces the generator current to maintain the dc voltage and the resultant generator acceleration is counteracted by pitch regulation. Compared with the conventional strategy, the dc chopper, which is intended to assist the fault ride through of the WECS, can be eliminated if the proposed scheme is employed. Compared with the variable-structured control scheme, the proposed strategy has quicker and more precise power responses, which is beneficial to the grid recovery. The simulation results verify the effectiveness of the proposed strategy.