Grid-connected inverters (GCIs) operating in grid-following (GFL) mode may be unstable under weak grids with low short-circuit ratio (SCR). Improved GFL controls enhance the small-signal stability of GCIs under weak grids but result in a decrease in dynamic performance and remain unstable under very weak grids. While the small-signal stability of GCIs operating in grid-forming (GFM) mode is just the opposite: stable under weak or even very weak grids, unstable under strong grids. Both GFL and GFM modes are difficult to meet the stability requirements under large fluctuations in SCR. Therefore, this article proposes a hybrid-mode control strategy for GCIs that can adapt to large fluctuations in SCR. This control strategy weights the modulation signals output from GFL mode and GFM mode, and removes the phase-locked loop of GFL mode, improving the small-signal stability of GCIs under a wide range of SCR. The sequence impedance model of the hybrid-mode GCIs is established, and the small-signal stability is analyzed in this article. The experimental results verify the effectiveness of the proposed strategy.

The stable operation of grid-connected inverters (GCIs) with traditional current source mode (CSM) control is affected by the large fluctuations of short-circuit ratio (SCR) under weak grids. Improved CSM control enhances the stability of GCIs under weak grids but remains unstable for very weak grids, while the stability of GCIs with voltage source mode (VSM) is just the opposite. Therefore, previous research works have proposed the impedance adaptive dual-mode control strategy: when the SCR is large, the GCI is controlled by CSM, and when the SCR is small, it is controlled by VSM. However, the existing literature still lacks the analysis of the parameter stability region and mode switching boundary of GCI in different modes. Therefore, based on the D-partition method, this article derives the parameter stability region of GCI in different modes under the constraints of multiple performance indexes such as phase and gain margin, current-loop bandwidth and phase-locked loop bandwidth, and the switching boundaries of CSM and VSM are also proposed. Finally, based on the grid impedance identification algorithm, the impedance adaptive dual-mode control is realized, which effectively improves the stability of GCI when SCR fluctuates greatly, and the experimental results verify the correctness of the above analysis.

The grid-connected inverter is the vital energy conversion device in renewable energy power generation. With the increasing installed capacity of renewable energy, the grid presents characteristics of weak grids with large grid impedance. Generally, the inverter often obtains grid synchronization information by the phase-locked loop (PLL), and to suppress the background harmonic and amplitude disturbance of grid voltage, the grid voltage feedforward (GVF) control is also needed. However, previous researches revealed that the PLL and the GVF will shape the quadrature-axis (q-axis) output impedance of the inverter into a negative resistance in the low-frequency band, and this would be responsible for the instability of the inverter under a weak grid. To resolve this instability issue, this paper proposes an impedance controller for reshaping the qaxis impedance into a positive resistance in the low-frequency band, which eliminates the negative effect introduced by the PLL and the GVF, so the inverter will operate under very weak grids. Moreover, the proposed control strategy is only for q-axis impedance reshaping, it does not affect the output characteristics of d-axis, thus ensuring the ability of the inverter to suppress background harmonics and amplitude disturbance of the grid voltage. The conclusions are verified by experimental results.

With the development of power electronics technology, power modules delivery considerably greater power density, which makes junction temperature an important parameter for reliable operation. Existing thermal models often ignore the influence of the thermal boundary conditions on the thermal parameter and cannot accurately predict junction temperature in various operating conditions. This research proposes a three-dimensional (3-D) compact thermal network model that considers the thermal boundary conditions and can be obtained from the finite-element method (FEM). A novel two-step method for thermal parameter extraction is presented and the effects of boundary conditions on thermal network parameters are discussed in detail. The experimental results and FEM simulation show that the proposed thermal network model can accurately estimate the junction temperature in different thermal boundary conditions.

This paper investigates the use of a virtual synchronous generator (VSG) to improve frequency stability in an autonomous photovoltaic-diesel microgrid with energy storage. VSG control is designed to emulate inertial response and damping power via power injection from/to the energy storage system. The effect of a VSG with constant parameters (CP-VSG) on the system frequency is analyzed. Based on the case study, self-tuning algorithms are used to search for optimal parameters during the operation of the VSG in order to minimize the amplitude and rate of change of the frequency variations. The performances of the proposed self-tuning (ST)-VSG, the frequency droop method, and the CP-VSG are evaluated by comparing their effects on attenuating frequency variations under load variations. For both simulated and experimental cases, the ST-VSG was found to be more efficient than the other two methods in improving frequency stability.

High frequency (HF) signal injection methods have shown good candidate for permanent magnet synchronous motor (PMSM) sensorless control at zero or low speed. However, conventional methods have some shortcomings, several improved methods have been put forward by scholars. In order to analyze the essential relationships among these methods, a unified graphical model is proposed based on the analysis of mechanism of conventional HF signal injection methods. According to the unified graphical model, the signal modulation modes can be categorized into phase-modulated based HF injection (PMHI) and amplitude-modulated based HF injection (AMHI), meanwhile the demodulation modes can be classified into open loop calculation and close loop convergence. Besides, novel methods are easily deduced from the unified graphical model. Furthermore, the effects of adverse factors, including resistance, digital control delay, inverter nonlinearity and cross-saturation on PMHI and AMHI are analyzed, respectively. Finally, experimental results prove the validity of the theoretical analysis.