In this paper, the stability of LC-type voltage source inverter is investigated, with the emphasis focused on the LC resonance. It has been found that the traditional capacitor-voltage inductor-current dual-loop control is unstable when the resonance frequency is higher than one-sixth of the sampling frequency. In addition, due to the coupling between current inner loop and voltage outer loop, it is difficult to design control parameters to ensure the system stability in traditional dual-loop control. In order to develop a systemic design principle, the capacitor-voltage control with inductor-current feedback active damping evolved from traditional dual-loop control is analyzed in detail. The mathematic model is established to reveal the reason of instability, and the stable regions are identified. Moreover, the optimal damping coefficient is also derived to enhance the system damping performance. Finally, the experimental results are provided to verify the effectiveness of the proposed method.

Virtual space vector pulsewidth modulation (VSVPWM) provides a new approach to neutral point voltage (NPV) control and common mode voltage (CMV) suppression. Few values of coefficients configured in the virtual vectors cannot adjust the NPV continuously. An improved VSVPWM (IVSVPWM) with active neutral point voltage control (ANPVC) and CMV suppression is proposed in this article. Continuously variable coefficients configured in the virtual vectors adjust the NPV actively and continuously for any load power factor over the full modulation indexes. Failure of CMV suppression is analyzed, and the undesired vectors with high CMV are avoided by limiting the range of the variable coefficients. The limited variable coefficients and the selected basic vectors with low CMV suppress the CMV effectively. Simulation and experimental results are given to validate the proposed novel IVSVPWM, which can achieve quicker ANPVC and reduce CMV to half of the conventional VSVPWM simultaneously.

Voltage-controlled voltage-source converters (VSCs) are widely applied in islanded microgrids, dynamic voltage restorers, and uninterruptible power supplies, etc. Typically, an LC filter with a large filter capacitance is employed to improve the quality of output voltage and system robustness against external disturbances. Nevertheless, a large capacitance may lead to a low LC-resonant-frequency and cause unstable voltage control of VSCs when a single-loop proportional-resonant (PR) or resonant controller is used. This paper proposes a novel design of single-loop voltage control, where a negative proportional gain is adopted in the PR voltage controller, and it is revealed that the voltage control can be stable even with a low LC -resonant-frequency, e.g., lower than one-third of the system sampling frequency. To tackle the poor dynamic issue introduced by the negative proportional gain, this paper further proposes a closed-loop zeros configuration-based control method, which can flexibly tune the system dynamic response without affecting its stability. Finally, simulation and experimental results are provided to verify the effectiveness of the proposed method.

In voltage-source inverters, it is necessary to add the dead time between gating signals of the upper and lower switches to prevent short circuit in a phase leg. But in current-source inverters (CSI), due to the large series inductor in dc link, it is necessary to add the overlap time in gating signals of switches during the commutation process for avoiding voltage spikes generated by the intermittent dc current. However, the overlap time will cause negative influence in the output current waveform. In this paper, the current commutation process of CSI within a carrier period is investigated, which shows that the overlap time can cause output-current-vector errors. The output-current-vector errors are then analyzed, which are equivalent to a superposition and introducing additional low-order harmonics in the output current waveform. The relationship between harmonic distortion and the overlap time is derived. An overlap-time compensation method is then proposed, which compensates the dwell time of vectors according to the relations of voltages at the ac side of the inverter. The proposed overlap-time compensation method can effectively decrease the current harmonics introduced by the overlap time and improve the output current of the inverter. Simulation and experimental results are finally provided to verify the feasibility and validity of the proposed method.

Series compensations have been widely used in modern power systems to extend the power transfer capacity. Since more and more renewable power generation systems are installed in power systems, the grid-connected inverters (GCIs) used by the renewable power generations may interfere with series-compensated power lines, which introduce subsynchronous resonance (SSR) causing power system failure that has been observed in real cases. The phase-locked loop (PLL) of the GCI's control system is the main factor that can cause the SSR in the series-compensated power grid. To mitigate the impact of the PLL and obtain the stable operation of the power system, a q -axis voltage feedback method has been proposed in literature to damp the harmonic oscillations in the weak power grid where, however, the series compensation is not considered. Because the real q -axis voltage cannot be obtained, the q -axis voltage output from the PLL is used as the feedback variable in literature. However, due to the dynamic response of the PLL, the q -axis voltage output from the PLL is different from the real q -axis voltage, and the direct use of it for the damping of the SSR in the power grid with series compensations is invalid, which is investigated in this article. Besides, an observed q -axis voltage feedback method is proposed in this article to damp the SSR in the series-compensated power grid with GCIs being connected. By putting an observer in the q -axis voltage feedback path, the real q -axis voltage can be estimated and applied to damp the SSR. Simulation and experimental results have been provided to verify the effectiveness of the proposed observed q -axis voltage feedback method in the damping of the SSR.

The backflow power in dual active bridge (DAB) converters can cause power circulation in the H-bridge instead of flowing to the load, which increases the power loss of switching devices and magnetic components and lowers the system efficiency. Hence, a unified optimal modulation strategy (UOMS) is proposed in this article to reduce the backflow power and consequently improve the system efficiency. However, there is a conflict between minimizing the backflow power and achieving the zero-voltage-switching (ZVS) conditions of all the switching devices. Hence, a regulatory factor ‘γ’ is introduced into the control variables to minimize the backflow power on the premise of all ZVS conditions and hence comprehensively improve the system efficiency. Moreover, the regulatory factor ‘γ’ simplifies the calculation form of the control variables, which is easier to be performed in the controller and can effectively shorten the computational time. According to the value ranges of the control variables (D ₀ , D ₁ , D ₂ ) in the proposed UOMS, twelve operating modes are classified and four effective modes are detailed. For the four effective modes, the optimal triple-phase-shift (TPS), optimal primary-dual-phase-shift (PDPS), optimal secondary-dual-phase-shift (SDPS) modulation strategies are derived and their switching relationships are revealed. With various transferred power, the proposed UOMS combines the optimal TPS, PDPS, SDPS and SPS modulation strategies, and can realize smooth and seamless transition switching processes among them in the full operating range. Therefore, the DAB converter can always operate in the optimal states for the full operating range of both the ‘Buck Mode’ ( k >1) and ‘Boost Mode’ ( k <1). Finally, experimental results verify the correctness of the theoretical analysis and the effectiveness of the proposed UOMS.