An Adaptive Overcurrent Limiting Strategy for VSGs Considering Transient Angle Stability

This paper investigates transient angle stability of parallel system of two virtual synchronous generators (VSGs) and hybrid system comprised by VSG and diesel generator (DG). Compared to single grid-connected VSG, the paralleled VSGs system and hybrid system are facing larger challenges on stability aspect during serious faults. Although there exist several publications on transient stability, the transient instability mechanism is rather ambiguous, especially for paralleled systems. Besides, swing equations of paralleled VSGs system and paralleled DG-VSG system are established to discuss the dynamic characteristics of the system during short-circuit fault. The increase of power angle between DG and VSG is caused by the different angular accelerations, which deteriorates the transient angle stability of paralleled DG-VSG system. Thus, paralleled DG-VSG system is more prone to instability than paralleled VSGs system, which can be illustrated by power angle (P-δ) characteristic curves. Finally, simulation results validate the correctness of theoretical analyses.

Since the wide application of virtual synchronous generators (VSGs), the power grid faces great challenges in the safe and stable operation due to their limited thermal capacity and weak anti-disturbance ability. During transient period, for example, a fault occurs in the transmission line, the VSG may lose the transient angle stability and provoke the current hard limit. Even if the fault is cleared by tripping of line, it still faces the problem of instability and voltage dips. To address this problem, in this paper, the post-fault large-signal model of VSG is derived first via the travelling waves based fault information acquisition. Subsequently, with the effect of both active and reactive power loops taken into account, a two-stage simultaneous control scheme is proposed for improving the transient stability of VSG, while considering the current limitation during fault state and voltage support after fault clearance. This method is fulfilled by mode switching and an additional feedback control based on the fault signal. Finally, the effectiveness of the proposed method under both symmetrical and asymmetrical faults is verified. Moreover, the application of the proposed method in a multiple VSGs system is also verified. Besides, the robustness to parameter mismatch and the feasible operating region for the method are discussed.

Virtual synchronous generator (VSG) can simulate synchronous generator turning and damping characteristics, which can further improve the penetration rate of renewable energy in the grid. However, the output impedance of virtual synchronous generator in medium and low voltage microgrids is dominated by resistance, which leads to the coupling of active/reactive power and affects the grid integration control effect, and the phase of VSG and grid voltage will be biased in the absence of a pre-synchronization link. For this purpose, a strategy of grid-connected control of VSG with virtual impedance is proposed. Firstly, the VSG mathematical model is established and virtual impedance is introduced into the VSG electrical portion to improve the grid-connected inverter output characteristics. Secondly, a secondary frequency and voltage regulation strategy to improve VSG control is proposed to restore the frequency and voltage of the VSG, and the SOGI phase-locked loop collects the grid voltage phase to complete the grid-connected pre-synchronization design. Finally, the effectiveness and correctness of the proposed strategy were verified by the simulation results based on MATLAB/ Simulink.

Virtual synchronous generator (VSG) control algorithm can simulate the characteristics of the traditional synchronous generator, inverter with control can provide the voltage and frequency support for large power grid, and the corresponding inertia and damping characteristics, improving the stability of large power grid. Aiming at the problem of multi inverter parallel circulation suppression, the island micro grid under the virtual synchronous generator (VSG) based on double closed-loop control strategy of virtual impedance, VSG parallel system simulation model built two units of the same capacity, the virtual impedance to suppress the circulation, the simulation results show that the proposed control strategy of circulation.

The transient angle instability and overcurrent of the virtual synchronous generator (VSG) under severe fault are risks for the stable and safe operation. Existing works have proposed some efficient methods for the transient stability enhancement or current limitation of VSG. Yet, the consideration for both of them is rarely studied. Moreover, the reasonable parameter design method for the control is usually lacked. In this paper, with the large signal model of VSG, a dual-loop PQ control scheme is introduced. With the proposed method, both the transient angle stability and overcurrent can be controlled simultaneously. In addition, how to design the reasonable coefficients of the proposed control scheme with additional requirement of output active power is also presented. The effect of the proposed method can be achieved with the coefficients in the allowable range. Finally, this method is validated by simulation results.

Attributed to the ability of inertia provision and good regulation performance, the virtual synchronous generator (VSG) has been proven as a promising solution to the problems introduced by using converter interfaced generation. Although the small-signal stability control of the VSG has been widely analyzed, the transient characteristics still needs more study, which belongs to a large-signal problem. In this article, the transient angle stability control of the VSG is investigated. Two possible VSG emulation methods, i.e., the power-emulation and the torque-emulation, are compared from the perspective of transient characteristics. Then, the transient of the VSG internal voltage and its impact on the transient angle stability are quantitatively studied in details. Thereafter, an enhanced VSG controller is proposed, where a large-signal analysis is also presented to evaluate its influence on the acceleration and deceleration areas. Finally, the analysis and effectiveness of the enhanced VSG method are validated by the experimental results.

By applying virtual synchronous generator (VSG) method in high penetration level microgrids, current limiting issue under fault condition has become an important challenge. The main objective of this paper is to propose a novel current limiting strategy which has the capability of voltage tracking error reduction to avoid voltage controller saturation. This purpose is obtained by adaptive virtual impedance-based reference voltage correction during fault condition. Furthermore, positive and negative peak detection-based current limiting method implemented in double loop control system prepares soft transition ability. Moreover, this current limiting method generates the adaptive term of virtual impedance part. In contrast to conventional VSG methods, the proposed approach provides high quality voltage and current for microgrid in both grid-connected and islanded modes during fault conditions. The offline and hardware in the loop (HIL) simulation results confirm that the peak current of the inverter is restricted to the threshold value under symmetrical and asymmetrical faults for two operating modes of microgrid.

In response to the issues of transient power angle instability and fault current (FC) over-limit under severe grid three-phase symmetrical faults for grid-forming voltage source converters (GFM-VSCs), this article proposes a novel joint control method combining second-order sliding mode control (SOSMC) with voltage adaptive regulation. Initially, an improved integral sliding surface is introduced into the active power-frequency loop of the GFM-VSC, and a Lyapunov function is constructed, which pertains to a large-signal problem. Based on this stability criterion, an output power control law is designed to strictly limit the fluctuation of the power angle. Subsequently, a reactive power–voltage loop switching control is employed to provide stable reactive power and support grid voltage, while ensuring that the steady-state current (SSC) is maintained within a safe threshold. Additionally, during faults, a virtual impedance is engaged, and the transient current (TC) can be suppressed by adjusting the ratio of resistance-to-inductance (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i>/<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i>) and recovery resistance. This method achieves power angle stability (PAS) and fault current suppression (FCS) simultaneously, even during grid voltage sags accompanied by phase changes. Finally, the correctness and effectiveness of the proposed method are validated on the real-time test platform.

A circulating current suppression strategy of VSG based on adaptive virtual complex impedance

With the increasing application of renewable energy sources (RESs), more and more grid‐connected voltage‐source converters (VSCs) in the grid need to operate in a “grid‐forming” manner. However, when large disturbances occur in the grid, grid‐connected converters may lose synchronization and cause the current to exceed set limits. To ensure the stable and safe operation of VSC during large disturbances and to improve the power supply capacity during voltage dips of RES, this paper proposes an emergency control method that takes into account the virtual synchronous generator (VSG) current limit and transient stability based on a large‐signal mathematical model of the VSG during fault and normal states. The method divides the operation area of the VSG into non‐emergency area and emergency area. If the operating point is in the non‐emergency region after a fault, only the active power reference value is changed to avoid current exceeding the limit and transient instability; if the operating point is in the emergency region, the current exceeding the limit and transient instability are avoided by changing the active power reference value and adding in the virtual impedance. Finally, the effectiveness of the proposed control strategy is verified by simulation.

The virtual synchronous generator (VSG) allows power electronics interfaced distributed generators (DG) actively taking part in the power regulation of distribution network (DN), providing adjustable inertia and damping as well as frequency and voltage support, which is meaningful to facilitate the safe and stable operation of DN. However, the dynamic characteristics of VSG have not yet been discussed when the frequency and amplitude of grid voltage are disturbed. To evaluate the power regulation capabilities of VSG, it is thus necessary to unmask the dynamic characteristics of VSG, especially when it is connected to weak grid. To address this issue, a small signal model of VSG controlled three-phase converter is first deduced. On this basis, the relationships of power responses and grid voltage disturbances are revealed. In order to improve the dynamic performances and reduce coupling effect between active and reactive power regulation, the influence of main parameters involving inertia, damping, voltage regulation coefficient and virtual impedance on dynamic response is analyzed by root locus and the parameter design method is presented. Finally, the Matlab/Simulink simulation and experimental results show the correctness and feasibility of the theoretical analysis presented above.

In recent years, countries around the world have been vigorously developing new energy and electrifying new energy sources. As a result an increasing number of power electronic devices appear in the grid which makes the power grid develop toward low inertia and low damping. The Virtual Synchronous Generator (VSG) control strategy can simulate the working state of the synchronous generator and provide the grid with inertia and damping, which has become a research hotspot for scholars. Due to the unbalanced load distribution and the random change of power load, the grid voltage may become unbalanced during the actual operation process. The three-phase output current of the VSG would be unbalanced and a fluctuation of output power appears. To solve this problem, a new current and power quality control strategy based on VSG is proposed in this paper, in which the current balancing or constant power is achieved without changing the basic characteristics of the inverters using VSG control strategy.

The virtual synchronous generator (VSG) was proposed to emulate a synchronous machine's dynamics when integrating power electronic converter-based distributed energy resources to the power grid. However, the VSG's synchronization stability during grid faults is not fully explored. The underlying mechanism of loss of synchronization still needs to be further revealed due to VSG's nonlinear characteristics. In this article, a step-by-step analytical method based on combining the linear and nonlinear models is proposed to analyze VSG's dynamic behaviors during a large disturbance. The relationship between the linear and nonlinear models is first brought to light, showing that the linear model is suitable for qualitative analysis to give an intuitive physical insight. Simultaneously, the latter is adopted for quantitative analysis to assess stability after a grid fault. Moreover, to avoid the conflict of the synchronization stability and the inertia response, a transient damping method is added in the active power control loop, which can simultaneously improve the synchronization stability and frequency stability. Design guidelines are also proposed to identify the parameter of the transient damping with different inertia requirements. Finally, the experimental results verify the analytical method and the theoretical analysis.

A comparative study of single-loop VSG and multi-loop VSG

In recent years, the increasing penetration of distributed generation in microgrids challenges the control and coordination of energy resources. Especially in microgrids with virtual synchronous generator (VSG)-controlled converters and conventional synchronous generators (SG), the inherent inertia difference (i.e., the VSG and SG) results in a poor transient performance when the VSG and/or loads are cut in/out. Thus, this paper explores the transient performance of microgrids with parallel VSG and SG systems. More importantly, a novel pre-synchronization control method is proposed to eliminate the phase jump while meeting the requirements in case of closures or re-closures of generation units. A small-signal dynamic model is presented, and accordingly, the VSG inertia and its damping can be designed considering the capacity ratio of VSG and SG units. In addition, with the power angle stability analysis, an active power provision strategy is introduced to suppress the transient power oscillation due to the inertia difference. Finally, the feasibility of the proposed methods is verified by simulations on a microgrid consisting of parallel VSG and SG units.