Shumei Chi

Cascaded H-bridge-based grid-forming converters (GFMCs) are capable of providing voltage and frequency support. However, under symmetrical and asymmetrical voltage sags, GFMCs tend to produce significant transient inrush currents that may exceed the converter design limit, posing risks to device reliability. Additionally, new grid codes (GCs) require GFMC to inject both positive- and negative-sequence reactive currents during asymmetric voltage sags. To address this issue while complying with the new GCs, a transient current limiting method with dynamic dual-sequence reactive power support is proposed. First, the mechanisms causing existing methods to fail in limiting transient inrush currents under symmetrical and asymmetrical voltage sags are analyzed. Neglecting the influence of negative-sequence currents is the culprit causing the unreliability of existing methods. Then, a flexible transient inrush current limiting method is proposed by considering the dynamic variation of the negative-sequence current. Compared to existing methods, the proposed method meets the new GC requirements for dual-sequence reactive power and can successfully limit transient inrush currents under symmetrical and asymmetrical voltage sags without relying on grid impedance or grid voltage information. The current limit can be set flexibly while maintaining the behavior of the voltage source and remaining effective during grid voltage restoration. Simulation and experimental results demonstrate the effectiveness of the proposed method.

Fault-tolerant control (FTC) can ensure the three-level T-type inverter (3LT²I) maintains reliable operation when switch is faulty. However, existing FTC strategies exhibit poor output performance and limited neutral-point voltage control (NPVC) ability within the full-range of modulation index (MI) and power factor (PF). To this end, this paper proposes an FTC and NPVC strategy for 3LT²I within full MI and PF range based on modulation signal clamping and decomposition, which improves the system reliability and output performance simultaneously. First, a fundamental cycle is divided into six regions according to the operating conditions when switch fault occurs for independent modulation. Second, the modulation signal is clamped to realize FTC in partial regions and increase NPVC ability in other regions. Furthermore, to realize FTC over the full regions and maximize NPVC ability, the modulation signal is decomposed by selecting the optimal decomposed phase and calculating an accurate decomposition value. The proposed strategy is simple to implement and superior to existing strategies in NPVC ability, output quality and power losses under the full-range of MI and PF. Finally, the effectiveness and superiority of the proposed strategy are validated by experimental results.

Parallel inverters system with modified LCL filters may trigger zero-sequence LC (ZSLC) resonances. Especially in high-power applications, lower switching frequencies are usually adopted to reduce switching losses. In this situation, conventional inverter-side circulating current feedback (ICCF) control cannot behave as expected and introduces new stability issues due to high control delays. To deal with this situation, a passivity-based ZSLC resonance damping method is proposed. First, the equivalent admittance model of ICCF control is established for the first time, which is decomposed into passive component and active component. The results show that the system stability is determined by the active component and the inverter output admittance ratio. Second, by analyzing the frequency-domain characteristics of active components, it is found that the control delay introduces a negative conductance that affects the stability of the ICCF control. Accordingly, a controllable admittance component is added to correspondingly reshape the inverter output admittance characteristics under high control delay conditions, which can mitigate the detrimental impact of control delay in the active component. Thus, the proposed control strategy can enhance the system stability and the circulating resonance current can be effectively attenuated in high-power applications. Finally, the feasibility and correctness of the proposed scheme are verified by the experiments.

Grid-forming inverters (GFMIs) are increasingly being integrated into grid-connected systems dominated by grid-following inverters (GFLIs) to enhance the stability of weak grids. However, the capability of traditional grid-forming control to enhance system stability remains limited. Specifically, when the output power of GFLI increases, the hybrid system consisting of GFMI and GFLI remains susceptible to oscillations and potential instability. To address these limitations, the interaction stability of the hybrid system is comprehensive analyzed and a passivity-enhanced oscillation suppression method is proposed. First, a decomposition impedance model is established to analyze the interaction mechanisms between GFLI and GFMI, as well as the impact of varying power levels. The analysis reveals that increasing the output power of GFMI does not inherently improve its ability to suppress system oscillations. Subsequently, a coupling term responsible for oscillations in the hybrid system is identified and a novel power-adaptive active damping is introduced into the GFMI control to enhance system passivity. In this case, the oscillation of hybrid system can be effectively suppressed under varying power levels and grid impedances. Finally, the experimental results verify the effectiveness of the proposed method.

Grid-forming (GFM) inverters have the advantages of voltage and frequency support in weak grid. However, unlike grid-following inverters, GFM inverters present a more challenging picture during asymmetrical voltage sag. The voltage-controlled virtual synchronous generator (VVSG) is the typical control method of GFM inverter. However, the problem of large inrush, slow response speed, and negative-sequence current control need to be carefully considered. In the existing low-voltage ride-through (LVRT) control strategies, these issues cannot be decently coped with while maintaining the ability to support the grid. In this situation, system instability and deterioration of power quality may occur. To overcome these limitations, an asymmetrical LVRT control strategy is proposed. First, the positive and negative-sequence models are established and the asymmetrical instantaneous currents are calculated. Based on this, the influences of the VVSG control method on the instantaneous negative-sequence currents are analyzed, which reveals the role of negative-sequence virtual voltage amplitude, grid phase angle, and response speed. Then, the double closed-loop control based on VVSG control method is further improved. The negative-sequence virtual voltage is compensated in the outer-loop to achieve control of negative-sequence current and inrush current. A novel passivity-based control (PBC) strategy is extended into innerloop which has fast transient response and strong robustness. Furthermore, through the transient power angle stability analysis, the improvement of the transient margin is proved. Finally, simulation and experimental results verify the effectiveness of the proposed method.

This article addresses the common-mode (CM) resonance problem in parallel inverters with separate dc inputs and improved LCL filters. Unlike single inverter system, parallel system presents a more challenging picture. Especially when the grid impedance and parasitic capacitance are considered, the parallel inverter interactions introduce more complex coupling relationships. In this situation, intricate CM resonances are excited at various frequencies, which is different from the resonance characteristics of a single inverter. Therefore, the control methods based on the single inverter system is insufficient in the parallel system and cannot behave as expected. To solve this issue, the CM resonance suppression problem in parallel system with separate inputs and parasitic parameters is being considered and analyzed for the first time. Firstly, the Thevenin's CM equivalent model of the parallel system is established. Compared with a single inverter model, the increase of CM loops triggers another two CM resonance peaks, which further deteriorates the system stability. Then, the internal and external CM excitations between each inverter are analyzed. The results show that the CM current of each inverter is coupled with the CM excitation sources of all inverters and the resonance characteristics are affected by the number of inverters. Furthermore, the influence of grid impedance and parasitic capacitance on the CM resonance characteristics of the system is analyzed. Accordingly, a CM resonance controller and a control parameter design method are proposed. By damping the CM excitation sources, the CM resonance suppression and CM current attenuation can be achieved simultaneously.