LVRT-Enabled Virtual Synchronous Generator Control for Grid-Forming Inverter-Based Resources

An enhanced second-order-consensus-based distributed secondary frequency controller of virtual synchronous generators for isolated AC microgrids

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.

This article presents an improved virtual synchronous generator (VSG) control strategy of doubly-fed induction generator (DFIG) during symmetrical voltage fault. Based on VSG control, DFIG can behave like a conventional synchronous generator (SG), which can support frequency and voltage in grid-connected mode. Moreover, when microgrid disconnects from grid, DFIG can provide voltage and power for local loads. However, during symmetrical voltage fault, the transient components in DFIG cannot be controlled properly by VSG. To solve this problem, the air-gap flux feedback is designed in VSG control to accelerate the decay of transient components, and rotor current limiting method is also introduced in the article. In the proposed method, there is no need to switch the control structure or control targets. DFIG always works in the VSG control. Therefore, DFIG can operate in the standalone mode when the connection to grid is cut off. The decay time constant, the voltage control, current limiting control and parameter variation of the improved VSG control are analyzed in detail. Finally, the simulation and experimental results in both normal and fault grid condition are given to validate the effectiveness of proposed VSG control strategy.

This paper develops the transient stability enhancement of the inverter-based resources (IBRs) with the virtual synchronous generator (VSG) control employed. Firstly, based on the large-signal model, the instability phenomenon of the VSG control during voltage dips is fully analyzed. It is found that the frequency deviation Δω in the active power control loop (APCL) of VSG is always positive when the VSG is transiently unstable. Then, for guaranteeing stronger transient stability, an additional integral controller with the frequency deviation is added to the APCL in the VSG control. Moreover, a large-signal model of the proposed strategy is established. Then, the phase portraits with an integral controller in the APCL disabled/enabled are presented. It is seen that the improved control strategy has better transient stability under voltage dips. Finally, the simulation results verify the effectiveness of the proposed method.

With the continuous development of wind power capacity, a large number of wind turbines connected by power electronic devices make the system inertia lower, which leads to the problem of system frequency stability degradation. The virtual synchronous generator (VSG) control can make wind turbines possess inertia and damping. However, the stochastic dynamic behavior of wind generation results in the stochastic changing of operating condition; this paper presents an adaptive subsynchronous oscillation (SSO) damping control method for the wind generation with VSG control. Firstly, the small signal model of the permanent magnet synchronous generator (PMSG) with VSG is built, and the model of state space is derived and built. The active power of PMSG is selected as the variable parameter vector to establish a polytopic linear variable parameter system model. Then, based on the hybrid H2/H∞ control method, each vertex state feedback matrix is solved by linear matrix inequality, and a subsynchronous oscillation adaptive damping controller with polytope is obtained. Finally, the 4-machine 2-area system connected to two PMSGs with VSG control is used as the test system for time domain simulation. The simulation results demonstrate that the LPV based adaptive damping controller could provide enough damping under the circumstances of wider changes of wind power outputs.

For a VSC-HVDC transmission system based on a hybrid topology converter of full-bridge and half-bridge, a kind of virtual synchronous generator (VSG) control strategy which can be applied to modular multilevel converter (MMC) grid-connected structure was researched and proposed. First, based on the conventional VSG control strategy, the energy stored in the equivalent capacitor of MMC power module was used to imitate the rotor inertia of synchronous generator. The characteristics of generator can be simulated during transient frequency fluctuations and it can help relieve the power fluctuations. Secondly, with respect to the structural characteristics of the direct grid connection through the reactor on the AC side of the MMC, which is unlike the microgrid inverter, there are no additional filter capacitors. So, the existing commonly used VSG control strategy of the microgrid inverter and double-closed-loop structure composed of filter capacitor voltage and AC current cannot be directly applied. For this, a method where the given values of inner current loop are calculated by grid impedance matrix was used. So, a double-closed-loop control structure composed of a power outer loop based on VSG control and a current inner loop is obtained. It can effectively improve the current control capability during the transient process. Finally, a hybrid MMC simulation model was built based on PSCAD to verify the correctness and effectiveness of the proposed methods.

The increasing penetration of inverter-based resources (IBRs) into electric power systems and the retirement of synchronous generators (SGs) both reduce system inertia and challenge frequency stability. This is particularly evident in terms of the rate of change of frequency (ROCOF) measured over short, practically measurable time windows (P-ROCOF). Conventional frequency-constrained unit commitment (FCUC) uses a theoretically idealized ROCOF (T-ROCOF) constraint that underestimates the effect of the fast frequency response (FFR) provided by virtual synchronous generators (VSGs) on P-ROCOF, leading to over-provisioning of generator capacity and higher operating costs. This paper presents a neural network-assisted FCUC (NN-UC) framework that formulates a mixed-integer ROCOF constraint based on an NNtrained to accurately predict P-ROCOF, and introduces a post-solution compensation procedure to correct NN prediction errors.We conducted one-year scenario studies on modified IEEE 9-bus and Japanese EAST 10-machine systems. Results show that NN-UC reduces both total operating cost and committed SG capacity compared to the conventional UC, which uses the T-ROCOF constraint. Even after worst-case compensation for NN’s ROCOF prediction errors, NN-UC still achieves net SG capacity reductions in most scenarios. These findings demonstrate that explicitly modeling the VSG’s FFR in FCUC yields both economic and operational benefits, while the compensation procedure ensures reliable operation.

In an islanded microgrid supplied by virtual synchronous generator (VSG) based inverter-based resources (IBRs), inter-IBR oscillations pose a serious threat to frequency stability and power-sharing accuracy. While VSG control enables gridforming inverters (GFMs) to emulate inertia and improve frequency dynamics, it also introduces poorly damped modes. This paper proposes a distributed cooperative control strategy that enhances damping of inter-IBR oscillations without compromising key frequency performance metrics such as the rate of change of frequency (RoCoF) and frequency nadir. The proposed control introduces a dynamic state variable that adjusts the auxiliary frequency reference in real-time based on both local frequency error and a distributed feedback term derived from the mismatch in normalized active power among neighboring inverters, thereby enhancing damping and actively suppressing inter-IBR oscillations. In addition, the control incorporates normalized active power mismatch to achieve proportional load sharing, implemented using a sparse communication topology among neighboring IBRs. An eigenvalue analysis based on small-signal model is conducted for appropriate selection of control gains to achieve system stability. Simulation results under various IBR configurations demonstrate that the proposed control method effectively suppresses inter-IBR oscillations, ensures proportional power sharing, and achieves robustness. Accordingly, the applicability of the proposed control design is validated for both homogeneous and heterogeneous VSG systems.

Analyzing low voltage ride through capability of doubly fed induction generator based wind turbine

Existing low-voltage ride-through (LVRT) methods of virtual synchronous generators (VSGs) are facing the problem of inverter damage due to prolonged overcurrent caused by the delay in fault detection. Additionally, the newly updated grid codes (GCs) mandate that inverter-based resources (IBRs) should inject negative-sequence current during asymmetrical faults. To address this issue while complying with the new GCs, this article proposes an adaptive LVRT method. At the onset of the fault, the proposed method compensates for the voltage difference between the reference and feedback values of voltage control loop, while simultaneously adjusting the angle output of the active power loop based on the phase change of the point of connection (POC) voltage and the active power output of the VSG. Thus, it enables a seamless transition between normal and LVRT conditions. Furthermore, it complies with the new GCs and guarantees the maximum utilization of the phase current. The method offers two notable advantages. First, it can achieve fast fault current limitation without the need for fault detection. Second, it eliminates the necessity for parameter design, making it straightforward to implement. The simulation results in PSCAD confirm the effectiveness of the proposed method under different asymmetrical fault conditions.

The increasing penetration of renewable energy, such as photovoltaic generation, makes it essential to enhance power system dynamic performance through improved grid-forming control strategies. In the grid-forming control system, the virtual synchronous generator control (VSG) is currently widely used. However, the inertia (J) and damping (D) in the traditional VSG are fixed values, which can result in large overshoots and long adjustment times when dealing with disturbances such as load switching. To address these issues, this paper proposes an adaptive virtual synchronous generator (VSG) control strategy for grid-side inverters, which is accomplished by adaptively adjusting the VSG’s inertia and damping. Firstly, we established a photovoltaic-storage VSG grid-forming system; here, the photovoltaic power is boosted through a DC-DC converter, and the energy storage is connected to the common DC bus through a bidirectional DC-DC converter. We analyzed how J and D shape the system’s output characteristics. Based on the power-angle characteristic curve, the tanh function was introduced to design the control function, and a JD collaborative adaptive control (ACL) strategy was proposed. Finally, simulation experiments were conducted under common working conditions, such as load switching and grid-side voltage disturbance, to verify the results. From the results shown, the proposed strategy can effectively improve the response speed of the system, suppress system overshoot and oscillation, and, to a certain extent, improve the dynamic performance of the system.

Pole–zero placement based feed-forward damping control for virtual synchronous generators in power systems with inverter-based resources

The rapid expansion of offshore wind energy and its integration into weak power grids have introduced significant challenges to system stability, particularly under fault conditions that cause voltage sags. Ensuring adequate Low Voltage Ride Through has become essential for maintaining grid reliability and preventing large-scale disconnections of wind farms.To address voltage sags caused by short-circuit faults during the grid integration of large-capacity offshore wind farms via weak networks, this paper establishes a simulation model of a permanent magnet synchronous generator (PMSG)-based offshore wind farm in the PSAT platform. Taking the Critical Clearing Time (CCT) as the core evaluation index, the Low Voltage Ride-Through (LVRT) capability is systematically assessed. By simulating short-circuit disturbances, the transient voltage response at the point of common coupling is analyzed, followed by a comparative evaluation of the voltage support effects of three dynamic reactive power compensation devices: Static Var Compensator (SVC), Static Synchronous Compensator (STATCOM), and Unified Power Flow Controller (UPFC). Results indicate that the rational configuration of dynamic reactive power compensation effectively mitigates voltage drops, significantly extends the CCT, and enhances transient stability margins. The distinct response characteristics and support efficacies of different devices are clarified, providing a theoretical foundation for optimizing LVRT strategies and selecting compensation equipment in offshore wind integration.

The hybrid AC/DC microgrid gains a great attention in recent years as it can improve the energy efficiency and ensure the reliability of the system. This paper makes an investigation on the operation mode switching performance of an AC/DC combined hybrid microgrid based on a real system at Griffith University, Queensland, Australia. An improved virtual synchronous generator (VSG) control strategy is developed to optimize the switching performance from the grid-tied mode to the islanding mode. The basic VSG principle is described and the improved VSG model is introduced. Based on the improved model, a pre-synchronization method is proposed. Comparison of the microgrid performance between conventional droop control and the improved VSG control is carried out by simulation on Matlab/SIMULINK environment. Simulation results show the advantage of VSG control strategy and the validity of the pre-synchronization method.

Inverters controlled in virtual synchronous generator(VSG) method is able to operate in both grid-connected mode and islanding mode. Due to this characteristic, this paper put forwards a low voltage ride-through(LVRT) strategy which is based on VSG control. If the grid voltage sags, VSG can limit the output current amplitude and meanwhile inject reactive power to the grid. Once the voltage drop time exceeds the maximum ride-through time, VSG would smoothly switch to islanding mode and continues to carry loads independently. The proposed strategy achieves a control algorithm unity between normal operating and grid voltage sag status in grid-connected mode. Simulations can support the validity of the strategy.