Except for the benefits of mimicking synchronous generators, virtual synchronous generators (VSGs) still face particular small- and large-disturbance stability problems in practical applications. Therein, the large-disturbance stability is a much more dramatic and intractable problem, such as a short circuit or line tripping. Accordingly, this article explores this challenging problem from the perspective of output impedance optimization for VSGs. First, focusing on the potential risk of overcurrent during the electromagnetic process, the dynamic performances of different virtual impedance control structures used in VSGs are comparatively analyzed, and the output impedance boundary for suppressing the transient current is derived. Then, the output impedance boundaries for maintaining electromechanical transient stability are discussed in depth by using Lyapunov stability theory, which provides novel virtual impedance optimal design guidelines exclusively for the large-disturbance stability. Furthermore, a large-disturbance stability enhancement strategy based on adaptive virtual impedance control is proposed, which can limit the electromagnetic transient current and improves the electromechanical transient stability simultaneously. Finally, experiments validate the accuracy of the theoretical analysis and the effectiveness of the proposed strategy.

To reduce the frequency deviation and the rate of change of frequency (RoCoF) in a low inertia power system, some converters are required to provide the frequency response (FR) power normally associated with the frequency deviation and/or the RoCoF, by droop/inertia/PD control. In this paper, a rapid power compensation (RPC) based FR strategy is developed to optimize the ability to compensate grid imbalance power, by fully exploiting the converter idle capacity. To this end, first, mathematical proof demonstrated the improved performance of the RPC strategy in terms of frequency deviation suppression versus droop control, and in terms of RoCoF suppression versus inertia control, with identical converter capacity limit. Moreover, it is proved that the RPC strategy can achieve consistent FR performance w.r.t. the optimal PD control, i.e. it can maximize the suppression of frequency deviation and RoCoF simultaneously, yet avoiding the limitations due to unknown grid parameters. Finally, by analyzing the operation modes and identifying the pertinent switching logic, detailed implementation of the proposed RPC strategy is developed. Its superb FR performance is verified by experiment results in a 2-converter low-inertia system, and simulation results in an IEEE 4-machine 2-area system.

In wireless power transfer system (WPTS), the control technique of the active rectifier (AR) is vital for improving the transfer efficiency of resonant network, expanding operating range of load and increasing power density. An indispensable component of such control technique is the accurate and reliable phase-locked method for the resonant current. However, the traditional phase-locked method based on DSP controller tends to lose driver pulses, which might cause damage to the safe operation of WPTS. This paper illustrates the essential reason why the driver pulses lose and proposes a phase-locked loop combined with the chained trigger mode (PLL-CTM) which can lock the phase of resonant current accurately and produce the driver pulses reliably. With the proposed PLL-CTM applied, the reliability of WPTS can be enhanced tremendously. Furthermore, based on PLL-CTM, this paper also presents a double-side phase shift control (DPSC) strategy to achieve CC/CV charging and minimize the power loss of resonant network simultaneously. Finally, a 500W WPT prototype is built to verify the feasibility of DPSC with PLL-CTM. Benefiting from DPSC with PLL-CTM, the WPTS not only obtains a high accuracy in steady state, but also achieves a good dynamic performance with the step change of $R_\text{L}$ and $R_\text{Eref}$ .

Dual active bridge (DAB) converters either adopt single-degree-of-freedom (SDOF) modulation or multiple-degree-of-freedom modulation. The SDOF single phase shift (SPS) modulation has low power transmission efficiency under light load, and the multiple-degree-of-freedom modulation has high requirements for model accuracy and controller design. Focusing on these limitations, this paper proposes an SDOF hybrid modulation method to optimize transmission efficiency, switching device current stress, and zero-voltage-switching (ZVS) implementation under the light-load condition. To this end, first, the SDOF SPS, primary-side internal phase shift (PS-IPS), and secondary-side internal phase shift (SS-IPS) modulation methods are comprehensively analyzed in terms of current stress, ZVS feature, and transmission power. Second, by comparing the current stress, ZVS, and loss features of SDOF modulation methods under the light-load condition, the case-dependent optimal modulation method is determined. On this basis, an SDOF hybrid modulation strategy within the light-load range is developed by resorting to a proper switching rule, which judges the voltage ratio between the two ends of the inductor and the size of the transmission power. Finally, a 600 W experimental prototype is built, and the results show that the proposed strategy effectively improves the transmission efficiency of the converter under the light-load condition.

Droop control, inertia control, and PD control are three typical frequency response (FR) strategies of grid-tied inverters interfaced renewable energy sources (RES) to restrain the power system frequency indicators, i.e., frequency deviation ( $\Delta \omega $ ) and the rate of change of frequency (RoCoF). Due to the highly limited inverter capacity, the FR performance in power systems with high RES penetration should be optimized. To this end, this paper first analyzes the impact of high-penetration renewable energy injection on power system frequency stability, and derives the system frequency expressions in the absence of an FR scheme, and in the presence of droop/inertia/PD controls that are representative for various virtual synchronous generator-based FR schemes. Under the same constraint of available FR capacity, control parameter ranges of these FR schemes are obtained, and their optimal conditions for restraining the maxima of $\Delta \omega $ and RoCoF are identified and compared analytically. Comparison conclusions are finally proved by the provided experiment and simulation results.

In a two-stage photovoltaic (PV) system, batteries are generally connected to the DC-link via a converter for buffering the power imbalance induced by the grid supportive services of grid-side inverter and the maximum power point tracking (MPPT) of PV source. Considering the limited battery capacity, the MPPT operation is easily compromised to avoid over-energizing the batteries in the conventional virtual synchronous generator (VSG) control, degrading the energy efficiency and extending the payback period. To concurrently achieve grid supporting and maximum PV power harvesting without increasing batteries, a coordinated VSG control for the photovoltaic/battery (PV/Bat) system is proposed in this paper. In the proposed strategy, the DC-link voltage level is segmented to differentiate the operations of converters. When the DC-link voltage is within the safety margin, the PV source operates at MPPT mode and the inverter output frequency is programmed proportionally to the DC-link voltage deviation. Meanwhile, droop control is implemented to associate the battery power with the DC-link voltage. When the DC-link voltage goes beyond the safety margin, PV de-loading and inverter rectification will be adaptively activated to restore DC-link voltage within the predefined range. The effectiveness and advantages of the proposed strategy are eventually verified by simulations and experiments.