Liang Huang

Grid-forming (GFM) control strategies face critical challenges in fault response, protection coordination, and grid code compliance. Conventional voltage-controlled GFM (VC-GFM) and current-controlled GFM (CC-GFM) each possess distinct advantages but individually fail to fully address these challenges. This paper systematically analyzes both VC-GFM and CC-GFM approaches, identifying their complementary strengths and inherent limitations. Investigation into a hybrid control strategy integrating VC-GFM and CC-GFM reveals that reliance on mode switching leads to issues such as switching-induced overvoltage and current-source conflicts. To address these limitations, a novel voltage-source-sustaining GFM (VSS-GFM) control strategy is developed to provide seamless fault ride-through without mode switching. The effectiveness of the proposed control schemes is validated in both synchronous and converter-dominated grids.

With the growing capacity of renewable energy, grid-forming (GFM) inverters are considered as promising solutions for low-inertia power grids. However, the overcurrent problem of GFM inverters during large grid disturbances is still a significant challenge. Generally, the GFM inverter has two types of overcurrent problems: 1) the steady-state overcurrent; and 2) the transient overcurrent. To address the steady-state overcurrent problem, a virtual power angle limiter method has been proposed in existing literature, which can limit steady-state overcurrent and maintain transient stability simultaneously during grid faults. Nevertheless, the transient overcurrent problem has not been fully addressed, which is still a remaining issue. To address the transient overcurrent problem, a hybrid transient virtual resistor (HTVR) method is proposed in this article, which can increase the generating speed of the transient virtual resistor as fast as possible. Thus, the transient overcurrent of the GFM inverter can be restrained below 1.2 or 1.1 p.u. in the grid fault and recovery scenarios by using the HTVR method. Simulation and experiments verify the effectiveness of the proposed method.

The classical phase-locked loop based vector current control scheme has been widely used in grid-following (GFL) inverter systems. However, GFL inverters with this classical control scheme suffer from synchronization instability issues under weak and faulty grid conditions, which has become a major stability challenge for GFL inverters. To address this challenge, a reference frame rematching (RFR) method has been proposed recently, which enables the GFL inverter to ride through severe low-voltage faults (e.g., zero-voltage ride-through) stably under nominal grid frequency conditions. However, when the grid frequency varies, the existing RFR method becomes less effective. To solve this problem, this letter proposes an improved RFR method, which can stabilize the GFL inverter in either grid voltage sag or grid frequency drop case. So, the large-signal stability and robustness of the GFL inverter can be improved. Simulations and experimental results verify the effectiveness of the proposed method.

The classical phase-locked loop (PLL)-based vector current control scheme has been widely used in grid-following (GFL) inverter systems. However, GFL inverters with this classical control scheme tend to be unstable under severe grid fault conditions, especially in weak grid cases. This article reveals that the PLL angle's dynamic is the main reason for instability. Specifically, the control dq frame and the system dq frame are synchronized easily in strong grid cases where the PLL angle's dynamic is relatively small, but the two dq frames have a risk of losing synchronization in weak grid cases where the PLL angle's dynamic is relatively large. To address this problem, this article proposes a reference frame rematching method, which can transform the original control dq frame into a new control dq frame that is almost identical to the system dq frame. So, the losing synchronization issue can be addressed. A significant advantage of the proposed method is that it can make sure that an equilibrium point exists during severe grid faults. Besides, it can achieve a seamless transition between normal and grid fault cases, without the need of grid fault detection. Simulations and experiments verify the effectiveness of the proposed method.

To insure a stable and reliable operation of the existing and future grids, it is important to study the stability of the power electronic based converters’ controls which can replace synchronous generation. Most existing studies use a Thévenin equivalent model of the grid to test the grid following controls. The present article demonstrates the limits of these studies. An alternative setup is proposed and is shown to provide more insight into the underlying mechanisms of the grid following control behaviour in weak grids, using a linearized state space model. These insights are exploited to propose further improvements to the control. These improvements are shown to extend the stability of the control while reducing the interactions among converters.

As the capacity of wind power generation increases, grid-forming (GFM) wind turbine generators are deemed as promising solutions to support the system frequency for future low inertia power grids. So far, the GFM converter with a nearly ideal dc voltage source has been studied thoroughly. However, when the GFM converter is applied to wind power applications, there are some realistic problems and challenges, such as coordinating wind turbine control and GFM control, limited energy to support the grid, variable wind speed, etc. These problems still need more discussion. In this paper, an overview of challenges and potential solutions of GFM converters applied to wind power generation systems are provided, where different energy reserving schemes, GFM control schemes, and overcurrent protection schemes are compared and discussed. Finally, a few perspectives on future trends are shared according to the authors’ knowledge.

As the capacity of renewable energy generation increases, grid-forming (GFM) inverters are deemed as promising solutions for low inertia power grids. However, power-electronic-based inverters have limited overcurrent capability, so additional overcurrent protection schemes are necessary. More importantly, the stability should not be jeopardized by using additional overcurrent protection schemes. However, GFM inverters with the conventional current reference limiting method tend to be unstable under large grid disturbances such as grid frequency drop. To address this problem, this article proposes a virtual power angle limiting method. The main idea of this method is to limit the output power by limiting the power angle instead of directly limiting the current reference. Thus, the power synchronization control law is still satisfied when the overcurrent protection is triggered, so that the stability can be maintained. Besides, after redesigning the power angle limiting value, the proposed method is also suitable for grid voltage sag cases. A significant advantage of the proposed method is that the output current can be limited automatically during grid voltage sag or frequency drop events without the need of fault detection or tuning control structures and parameters. Simulation and experimental results have verified the effectiveness of the proposed method.