With an increasing number of distributed energy resources (DERs) integrated to the power system, inverters need to take corresponding responsibility for the security and stability of the system. Virtual synchronous generators (VSGs) are proposed to mimic dynamic characteristics of traditional rotational synchronous generators (RSGs) to compensate for the loss of inertia and reserve capacity. Similar to RSGs, VSGs will experience transient angle instability under certain conditions, which probably threatens the system security. In this paper, transient angle stability of a VSG is investigated by Lyapunov’s direct method. The deteriorative effect of reactive power control loop on transient angle stability is first analyzed and then voltage variation is incorporated into an approximate Lyapunov’s direct method. In this method, the inverter internal voltage is treated as a parameter rather than a state variable. Moreover, the influence of different parameters on transient angle stability is studied. Finally, an enhanced control strategy is presented to improve the transient angle stability by adjusting the reference power. Numerical simulation results are presented to validate the effectiveness of the proposed method and the enhanced control.

With the development of virtual synchronous generator (VSG) techniques, parallel operations of synchronous generators (SGs) and VSGs become increasingly common in a microgrid. The differences between paralleled systems will affect transient stability of the system, which probably threatens stable operation of the system, especially under fault conditions. In this paper, transient angle stability of a paralleled synchronous and virtual synchronous generators (SG-VSG) system is investigated by compared with that of paralleled VSGs system. It is observed that the paralleled SG-VSG system is more prone to transient instability due to the differences between their speed governors. Then, a control method is proposed to improve the transient stability of paralleled SG-VSG system. Furthermore, a Lyapunov method is employed to establish the nonlinear model of islanded microgrid, by which the attraction domain of paralleled system is quantified. The hardware-in-loop experiment is performed to validate the theoretical analysis.

With development of power electronic technology, the paralleled current-controlled voltage source converters (CCSs) and virtual synchronous generators (VSGs) system has advantages in providing power and voltage/frequency regulation at the same time in rural area or remote island. However, the paralleled system faces great challenges in safe and stable operation due to their limited thermal capacity and weak anti-disturbance ability, especially during fault periods. This paper focuses on transient stability and stability-oriented control design of the paralleled CCS-VSG system. First, mathematical model of the paralleled system is established and then the effect of two kinds of CCSs’ current injection angle (active current and reactive current) on VSG’s transient stability has been revealed through extended equal area criterion. Based on the theoretical analysis results, transient stability improvement control is put forward by controlling the CCS to track the VSG’s frame. Compared with the conditions that only active-or reactive current is provided by the CCS, the system can achieve the best transient performance when the proposed control is adopted. Moreover, the effect of CCS’s capacity on transient stability of the VSG has also been discussed. Finally, both Lyapunov’s method and simulation/experimental results are provided to validate the correctness of theoretical analysis.

Parallel operation of synchronous generators (SGs) and virtual synchronous generators (VSGs) is an important way to guarantee the safe operation of the microgrid in remote islands or rural areas. However, the system may experience transient voltage instability when a high penetration level of induction motor (IM) loads is connected. This paper considers the dynamics of SG and VSG to investigate the transient voltage stability of paralleled SG and VSG system with IM load. The effects of SG’s and VSG’s active power loops (APLs) are analyzed in detail due to the low inertia of microgrids and the power coupling introduced by the network impedance, which has not been well-explored in the prior research. The results show that the interaction of APLs between SG and VSG would deteriorate the transient voltage stability of the system. Moreover, critical clearance time (CCT) is employed to estimate the system stability. Based on the theoretical analysis, a control method is proposed to enhance the transient voltage stability of the system. The hardware-in-loop experiment is performed to verify the correctness of the theoretical analysis.

The safety of grid-connected voltage-controlled inverter is threatened by transient angle instability due to fault. The impact of different active power control (APC) and reactive power control (RPC) on the inverter's transient angle stability hasn't been fully investigated. To fill this gap, firstly, a unified model of the voltage-controlled inverter considering different APC and RPC is put forward. Based on the proposed unified model, the effect of different APC and RPC loops on the inverter's transient angle stability are qualitatively analyzed by using equal area criterion (EAC). Then, the TS multimodeling method which is based on the Lyapunov function is applied to evaluate the effect of control parameters on the inverter's transient angle stability quantificationally. Finally, the above analysis is verified based on an experiment. It is found that lowering inertia improves the inverter's global stability but decreases the inverter's first swing stability. Transient stability of the inverter can be improved by increasing damping component of the inverter. Transient angle stability of the inverter is influenced by the RPC loop. With the increase of the output voltage of the RPC loop, reference reactive power, and output voltage of the inverter, transient angle stability of the inverter can be improved.

Electric synchronous machine (ESM) control is a power synchronization strategy that utilizes the measurement of dc-link voltage to generate a frequency reference. By using ESM, the grid-connected inverters with indeterminant renewable energies can work in grid-forming mode, and realizes a closed-loop control of the dc-link voltage. However, under weak grid conditions, the ESM provides a sluggish performance for the regulation of dc-link voltage and is prone to the transient instability issues, including dc-voltage collapse and synchronization instability, which hinders the application of ESM. This paper reveals the instability mechanism of dc-link voltage under large disturbances, and proposes a passivity-based control for the transient stabilization of ESM. Firstly, the transient stability of ESM is analyzed from the view of port-Hamilton system. The corresponding energy function comprises the energy stored in dc-link capacitor and potential energy in ac side. Then, a 2-degree-of-fredom controller is proposed to stabilize the dc-link voltage by enhancing the energy dissipation. It combines a phase-shift controller and supplementary voltage controller, fully utilizing the control flexibility of the inverters. Dissipation analysis proves the large-signal stability of the proposed control. Simulations and hardware-in-the-loop (HIL) experiments validate its transient performance in a single-inverter-infinite-bus (SIIB) system and a multi-inverter test system.