An Adaptive Low-Voltage Ride-Through Method for Virtual Synchronous Generators During Asymmetrical Grid Faults

Low Voltage Ride Through (LVRT) behaviour of Inverter Based Resources (IBR) during asymmetrical grid faults is dictated by the requirements of Grid Code (GC) at the Point of Common Coupling (PCC) of the renewable energy plants. Earlier GCs demanded only positive sequence current injection, which gave freedom to inverter manufacturers to implement different negative sequence injection strategies, while newer GCs define specific negative sequence current injection requirements. The existence of different negative sequence current control strategies during grid faults creates problems for line protection relays, as fault detection algorithms malfunction for certain angle relationships between the negative sequence voltage and current phasors. This paper analyses different negative sequence injection algorithms of IBRs and demonstrates certain relationships between sequence impedances of the inverter that can be used for improving line protection algorithms. The analytical findings are confirmed via Real Time simulations of a Type 4 wind farm connected to a power system with different network strength characteristics.

In order to meet the grid codes and ensure safe operation of doubly-fed induction generator (DFIG) based wind energy conversion system (WECS) during asymmetrical grid faults, firstly, this paper designs low voltage ride-through (LVRT) control schemes for the back-to-back converters of DFIG-based WECS. Then, due to the converter capacity constraint, the controllable regions of DFIG during asymmetrical grid faults are analyzed by optimal method while the asymmetrical LVRT limitation of DFIG is given, which points out that DFIG-based WECS can achieve LVRT only under mild asymmetrical fault situations. At the last, a typical simulated system of a 9 MW DFIG-based wind farm connected into power system is used for case study. The simulation results validate the accuracy of the analysis for the asymmetrical LVRT capability of DFIG-based WECS.

Synchronous generators (SGs) inherently support grid stability through their physical inertia and capability to tolerate overcurrent conditions during faults. In contrast, inverter-based resources (IBRs), due to the limitations of semiconductor switching devices, lack such overcurrent handling capacity, making them vulnerable during grid disturbances. This paper investigates the low voltage ride-through (LVRT) capability of IBRs employing virtual synchronous generator (VSG) control to enhance their transient response during fault-induced voltage sags. A comprehensive VSG model is developed, incorporating virtual inertia and damping emulation to replicate the dynamic behavior of SGs. The proposed control strategy is validated through time-domain simulations in MATLAB/Simulink under both symmetrical and asymmetrical fault conditions. Simulation results demonstrate that the VSG control effectively enables voltage recovery, sustains stable power output, reduces the rate of change of frequency (RoCoF), and ensures grid code-compliant current injection with acceptable total harmonic distortion (THD). The findings confirm the potential of VSG-based control in fulfilling LVRT requirements and enhancing grid resilience in high-renewable penetration scenarios.

Over the next few decades non-conventional wind energy will be a great contribution to the increased power demand. Hence to provide a good quality and reliable power even after severe short circuit faults, the system should resume its normal state besides ensuring dynamic stability, according to respective wind grid codes of the nation. Wind grid codes include Low Voltage Ride Through (LVRT) behavior and Reactive power demand at fault duration as major features. The present Doubly Fed Induction Generator (DFIG) wind turbine would better achieve its grid codes with Enhanced Flux Oriented Control (EFOC) in Rotor Side Converter (RSC) and enhance its dynamic behavior with integrated Battery Energy Storage System (BESS) at dc link capacitor. The paper aims at improving LVRT and dynamic characteristics of the system and provides a comparison between with and without BESS during symmetrical and asymmetrical grid faults.

This paper proposes an optimum resistive type fault current limiter (OR-FCL) as an efficient solution to achieve maximum fault ride-through (FRT) capability of fixed speed wind turbine (FSWT) during various grid faults. A dedicated control circuit is designed for the OR-FCL in a way that enables it to insert an optimum value of resistance in the FSWT's fault current path. Therefore, the OR-FCL significantly improves transient behavior of the FSWT. It will be shown that, the optimum resistance value depends on fault location and pre-fault active power. So, the control circuit of the proposed OR-FCL is capable to calculate the optimum resistance value for all the pre-fault conditions. In this way, the FSWT achieves its maximum FRT capability during symmetrical and asymmetrical faults even at zero grid voltage, as recommended by some new grid codes. The analytical analysis is provided in detail to highlight the process of calculating the optimum resistance of the OR-FCL. Moreover, the effect of the resistance value of the OR-FCL on the FRT behavior of FSWT is investigated, too. To show the high efficiency of the proposed OR-FCL, its performance during various operation conditions of the FSWT, will be studied. It will be proved that each operation condition needs its own optimum resistance value during the fault to achieve the maximum FRT capability of the FSWT. The comprehensive sets of simulations are carried out in PSCAD/EMTDC software; the results approve efficiency and effectiveness of the proposed approach.

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.

The transient behavior of DC-link voltage (DCV) significantly affects the low-voltage ride-through for phase-locked loop (PLL)-based grid-connected doubly-fed induction generator (DFIG) systems. This study investigates the DCV transient behavior of a PLL-based DFIG system under asymmetrical grid faults. First, by considering the coupling characteristics of positive and negative sequence (PNS) components, a nonlinear large-signal model of DCV is developed. Furthermore, the transient characteristics of DCV under varying parameters are analyzed using phase trajectory diagrams. In addition, the transient stability (TS) mechanism of DCV during asymmetrical faults is examined through an energy function approach. The analysis indicates that the transient instability of DCV is primarily associated with the control characteristics of PNS PLLs, while the TS level of DCV is mainly determined by the power coordination control between the rotor side converter and grid side converter. Moreover, a coordinated control strategy is proposed to enhance the TS of DCV under asymmetrical grid faults. Finally, both simulation and experimental results are presented to validate the theoretical analysis and the effectiveness of the proposed strategy.

This paper proposes an extended variable structure control strategy for a PV-Battery Virtual Synchronous Generator (VSG) to enhance Low Voltage Ride-Through (LVRT) capability under asymmetrical fault conditions in low-voltage distribution systems. The method introduces dual-sequence current control to actively inject both positive and negative sequence currents during faults. Two realistic local load types-constant impedance (CZ) and constant power (CP)-are analyzed to assess their impact on generation-load match, voltage support, and fault ridethrough performance. Eight different asymmetrical sag conditions are simulated, and the Voltage Unbalance Factor (VUF) is used to evaluate system response. Results show that CZ loads enable more effective VUF suppression due to their passive nature, while CP loads impose stricter current constraints that limit control flexibility. The proposed strategy is particularly suited for Grid-interactive Efficient Buildings (GEBs), where preserving voltage symmetry and complying with current injection requirements are critical during grid disturbances.

Control of grid-connected power converters is continuously developing to meet the grid codes, according to which the generation units should keep connected to the grid and further provide ancillary services, such as voltage and frequency support, negative sequence current injection, inertia emulation, etc. A virtual admittance controller is proposed in this paper for the objective of voltage support under asymmetrical grid faults. By using independent and selective admittances for positive and negative sequence current injection, the unbalanced voltage can be significantly compensated during asymmetrical faults. The controller is based on the generic control framework of the synchronous power controller (SPC), which is able to control a power converter with emulated and improved synchronous generator characteristics. Simulation and experimental results based on two paralleled 100 kW grid-connected power converters demonstrate the controller to be effective in supporting unbalanced voltage sags.

The increasing penetration of renewable energy resources has introduced challenges such as post-fault temporary overvoltage (TOV) due to plant-level control lagging, which has been widely investigated in existing studies under symmetrical faults. However, TOV analysis under asymmetrical grid faults has often been ignored in existing studies. In particular, negative-sequence fault ride-through (FRT) control, involving additional negative-sequence current injection, results in unclear TOV mechanisms and the lack of corresponding TOV evaluation methods in multi-infeed renewable power systems (MIRPS). To address this gap, this paper investigates the TOV mechanism under asymmetrical faults considering negative-sequence FRT control and establishes an analytical evaluation method applicable to the MIRPS. The effectiveness of the proposed method is verified through EMT simulations in PSCAD/EMTDC.

Conventional fault analysis and calculation methods developed for synchronous-generator-dominated power systems exhibit limited applicability to distribution networks with high penetration of distributed generation (DG). These methods cannot provide a reliable theoretical basis for fault characteristic analysis or protection coordination. Existing studies on this scenario have primarily focused on the integration of grid-following (GFL) inverter-based resources (IBRs). By contrast, research on the integration of grid-forming (GFM) IBRs—particularly the virtual synchronous generator (VSG), which enables stable and sustainable utilization of renewable energy resources as synchronous generators—remains significantly inadequate. Therefore, this paper introduces a concise fault analysis and calculation method tailored to distribution networks with VSG-type DGs. First, the control strategy of the VSG-type DGs is examined, and the active-power response characteristics of VSG-type DGs are analyzed for faults in distribution networks. Equivalent models of a typical distribution network with VSG-type DGs are then established for symmetrical and asymmetrical faults. Subsequently, leveraging the active power–frequency and reactive power–voltage dependencies, a fault calculation method for distribution networks is proposed and its generality is examined. The method is convenient to implement and computationally efficient. It requires no detailed information on internal PI controller parameters or time constants of the control loops. PSCAD/EMTDC simulations are performed to verify the high accuracy and suitability of the proposed method for multi-DG scenarios, which facilitates the integration of VSG-type DGs in distribution networks.

The increased penetration of wind power has made its low voltage ride through (LVRT) capability more crucial than ever. Currently, the LVRT performance of wind generators is verified using experimental voltage sags with different magnitudes and durations. The actual fault, however, also causes the phase angle jump (PAJ) of the grid voltage. In this paper, the impact of PAJ on the LVRT capability of doubly fed inductor generator (DFIG) under three-phase fault is analyzed. To achieve this, detailed analytical calculations and experimental verifications are conducted. The results demonstrate that the PAJ imposes more challenges on the LVRT by intensifying the rotor overcurrent through the natural stator flux. As a result, DFIGs that have passed the existing LVRT test may still fail to comply with the grid code requirements in practice. To tackle this issue, an improved test procedure is recommended. The new procedure considers the impedance angle of the target system as an additional test condition, thus can reveal the LVRT performance of DFIG under the potential PAJ.

This paper aims to develop a coordinated controller to enhance the low voltage ride through (LVRT) performance of a permanent magnet synchronous generator (PMSG) based variable speed wind energy conversion system (WECS). This coordinated control comprises of a pitch angle control, a flux weakening control and a static synchronous compensator (STATCOM) control. The pitch angle control is adjusted to smooth the WECS output power fluctuations or to enhance the LVRT performance. The flux weakening controller is used to reduce the overvoltage of DC-link capacitor, and the STATCOM control is used to ensure continuous operation of WECS during symmetrical and asymmetrical grid faults. The proposed coordinated control method is compared with the conventional LVRT strategy based on the braking chopper (BC) via time series simulation results and effectiveness of the proposed control is verified.

For doubly-fed induction generator (DFIG)-based wind power systems, when crowbar-less control methods are used for low voltage ride through (LVRT), the LVRT capabilities are different depending on the specific method. However, because of the limited voltage output of rotor side converter (RSC), there is a limit for crowbar-less control methods. In order to analyze the limit under asymmetrical voltage dips, an optimal crowbar-less control method is proposed to minimize the rotor current during faults. Based on the proposed method, the control limit under asymmetrical faults can be obtained, and then the LVRT capabilities for asymmetrical faults, with crowbar-less control methods, can be evaluated. Finally, a case study of a typical 1.5 MW DFIG-based wind power system is carried out, and from the results, it can be seen that among the three types of asymmetrical faults, the phase-to-phase faults are the most difficult to ride through, and the single-phase-to-ground faults are the easiest to ride through.

The focus of documented research in this paper is the Low Voltage Ride Through (LVRT) capability of electric grid connected Wind Energy Conversion System (WECS) using Takagie-Sugeon (TS) type fuzzy controller. The increased deployment of Wind Turbine technology in modern power systems makes the LVRT capability of WECS one of the important research topics. Consequently, to avoid power network instability, power utilities defined technical specifications for modern WECS. These specifications are known as Grid Codes and are further categorized as static and dynamic requirements. The LVRT capability falls under the category of dynamic requirements of Gird Codes. The most frequently and extensively used generator for wind turbines is the Doubly Fed Induction Generator (DFIG) due to its lower cost, less power loss, variable speed, and four-quadrant operation. However, DFIG based WECS are highly sensitive to electric network disturbances because the stator of the DFIG is directly connected to the power system. Therefore, in this research study, the main attention is drawn towards the control of power electronic interface i.e., grid side converter of DFIG based WECS. The simulation results of symmetrical and asymmetrical faults, with different fault injection locations, are analyzed in order to provide a comprehensive understanding of the impact of electric grid disturbances on WECS. The results of the TS-type fuzzy controller are compared with the PI controller to illustrate the effectiveness of the fuzzy controlled DFIG based WECS. The aforementioned comparison of simulation results demonstrated that the transient stability of WECS is considerably improved by using TS type fuzzy controller. Thus, it is concluded that the TS-type fuzzy controller can be an efficient and promising solution for modern DFIG based WECS.