Virtual synchronous generator (VSG) control-based grid-connected converter (GCC) is an attractive solution to improve the stability of more renewable energy-integrated power system. Unfortunately, the inherent power coupling (i.e., the interaction between active and reactive power loop) defect of VSG control severely restricts the power delivery capacity and grid support capability of GCC. The virtual inductor is commonly used to reduce coupling, but its decoupling capability is very limited. And, the power coupling mechanism and its limiting factors are not clear. For this issue, the nature of power coupling in VSG is investigated firstly. The decoupling capability of virtual inductor is studied, and the reason for decoupling effectiveness is revealed. It indicates that the effectiveness of decoupling results from the proper voltage compensation, but this kind of positive effect is limited by the d-axis voltage drop across virtual inductor. Then, a q-axis voltage drop based power decoupling control (QVPDC) is proposed to further reduce power coupling, which does not consider d-axis voltage drop when applying virtual inductor. Compared with the virtual inductor method, the decoupling performance of QVPDC is better, and the computation burden is reduced by half. Finally, the analysis and proposed method are validated by simulation and experiment.

For doubly fed induction generator (DFIG)-based wind turbines, when severe grid faults occur, it is necessary to inject transient and negative-sequence components into the rotor current reference to achieve controllable low-voltage ride through (LVRT) capability. However, the classical proportional-integral (PI) controller cannot provide precise control of the transient and negative-sequence rotor current, because such current references in synchronous reference frame (SRF) are AC components at grid frequency and double grid frequency, respectively. To solve this problem, this paper proposes a feedforward current references control (FCRC) method for rotor side converter (RSC) to improve the system transient control performance. Under the proposed method, the additional feedforward of rotor current references is introduced into the current loop to improve its tracking capability of the aforementioned AC current references, thereby achieving the transient control targets precisely. Moreover, the proposed method will not affect the stability of the original current loop. Furthermore, the method has strong robustness against DFIG parameters deviation and grid frequency variations. In addition, the method is highly simple to implement. Finally, the validity of the proposed FCRC method is demonstrated by simulation and experiment.

Currently, frequency-domain electromagnetic interference (EMI) modeling has become the most commonly used method for EMI prediction and filter design. However, because of the unpredictability of the ringing effect in the present frequency-domain EMI modeling methods, the precision of the predicted EMI interference source is notably limited, which results in the inaccuracy of the system level EMI prediction. In order to achieve an accurate EMI prediction, this study presents a systematical study on the interference source of a 10 kVA three-phase bridge converter. The precise mathematical models of the common-mode and differential-mode interference source are constructed. Then, the characteristics of the multiple ringing effect, which is an important part of the interference source, are intensively analyzed. Based on the investigation of interference source, an improved interference source synthesis method is proposed. It can quickly and precisely predict the CM and DM interference source with multiple ringing effect, which can be used to achieve accurate frequency domain EMI prediction. Moreover, the multiple ringing effect is extended for IGBT parasitic parameter measuring. Instead of complicated device-level measurement, the grounding capacitance and junction capacitance can be effectively extracted through the ringing frequency measurement.

This letter proposes a method for the condition monitoring of submodule capacitors in modular multilevel converters (MMCs) without additional circuitry or computationally heavy algorithm. The proposed method leverages the discharging curve of submodule capacitors in connection with its parallel bleeding resistors. It is independent of the MMC control and modulation schemes. Moreover, it potentially does not require thermal and load related calibration for capacitor degradation monitoring. The principle, case study, and proof-of-concept of the proposed method are presented.

Voltage-controlled three-phase three-leg (TPTL) inverters are widely applied in various occasions. The inverters are generally switched to the current-controlled mode to limit the fault currents when short-circuit faults happen. Under symmetrical fault, the inverter can be controlled as a symmetrical, three phase current source. However, the voltage limiting could happen under asymmetrical fault. As a result, the fault phase currents will be distorted and cannot be completely controlled. The switching devices could be damaged while the setting design and reliable action of the protection devices will be also greatly affected. Considering the voltage limiting is mainly caused by large load impedance of healthy phase under asymmetrical fault, the voltage limiting is avoided by reducing the load impedance by means of the virtual impedance in parallel with the output filter capacitor in this paper. As a result, the current limiting loop is not broken and the fault phase currents can be effectively controlled. In addition, the implementation of virtual impedance in control system and the influence of virtual impedance on the stability of system are also analyzed in detailed. The theoretical results are validated by experiments.

Series-parallel forward dc-dc converter system is a good option in high-voltage high-power applications. A switch fault, in this converter, should be detected as soon as possible, to avoid further damage and save maintenance cost. In this paper, a new switch fault diagnosis method, which treats output inductor voltages as diagnostic criteria, is proposed for this system. With the proposed method, expensive sensors are needless, since diagnostic signal can be easily acquired by adding a small auxiliary winding into inductor magnetic core. Based on the feature of diagnostic signals in such system, a low-cost diagnostic circuit is designed. The failed module caused by open circuit fault or short circuit fault could be detected within one switching period, allowing for an immediate fault-tolerant action. This method also shows generality for different connections and buck-type isolated topologies. Besides, it works reliably under input-voltage variation and load step, and no misdiagnosis will occur during normal dynamic process. Finally, experimental result verifies the effectiveness of this method. The proposed scheme has a good potential to be applied in real applications for its simplicity and robustness.