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Capacitor-voltage feedforward with full delay compensation to improve weak grids adaptability of LCL-filtered grid-connected converters for distributed generation systems. IEEE Transactions on Power Electronics, 33(1), 749-764.

When faced with distorted grid voltage, more harmonics will appear in the output currents of the grid-connected inverters. The grid-voltage feedforward strategy, as the most direct solution to compensate the harmonics, however, is seriously affected by the errors in the grid-voltage feedforward loop, such as delays. This issue is more significant for high-power inverters, where the switching frequency is relatively low (<5 kHz), and the grid-interface inductance is small (<0.5 mH). The errors mainly include the signal distortion caused by the conditioning circuits, the control delay of the digital controller, and the zero-order hold (ZOH) characteristic of pulse width modulation (PWM). In this paper, several improvements have been made to reduce the signal distortion and compensate the delays. A second-order Butterworth low-pass filter in the conditioning circuit is carefully designed with the maximum flat magnitude response and the almost linear phase response to avoid distorting the measured grid voltage. Furthermore, based on the conventional repetitive predictor, an open-loop simplified repetitive predictor is proposed to compensate the delays in the grid-voltage feedforward loop. Three predictive steps are achieved by the open-loop simplified repetitive predictor to compensate the delays: one step for the delay caused by the conditioning circuit, the second step for the control delay of the digital controller, and the third step for the ZOH characteristic of PWM. The effectiveness of the improved grid-voltage feedforward strategy are experimentally validated on a 250-kVA solar power generation system, where the current harmonics are effectively attenuated. In addition, the inverter starting current is suppressed.

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This paper deduces a unified mathematical model of open-circuit fault of six-phase induction machine, and purposes a fault-tolerence sensorless strategy based on the sliding mode observer(SMO) to estimate the rotor flux and rotor speed. In the proposed control scheme, the sliding mode observer is improved to achieve the rotor speed and rotor flux identification under healthy and arbitrary open circuit faults. The stability of the observer is proved by the Lyapunov function. The quasi proportional resonance (QPR) controller is introduced in the harmonic plane to suppress the harmonic current after the fault. The simulation and experiment results show that the proposed sensorless fault-tolerant control strategy based on sliding mode observer can accurately and effectively identify the rotor speed and rotor flux of the machine under normal and fault conditions.

The droop controlled grid-connecting inverter (DC-GCI) has been widely used in microgrid (MG). However, the power flow of the droop control is very sensitive to the fluctuation of grid frequency and voltage magnitude, which will result in a very fast inrush current of DC-GCI. Owing to the linear relationship of P-F, current limiters are required to prevent DC-GCI output power from exceeding maximum power. However, the impact of grid frequency fluctuation on DC-GCI cannot be mitigated by the current limiter for the reason that the fluctuation will lead to the current limiter saturation, which will cause the error between the angular frequency of the P-F droop output and the angular frequency of the grid voltage, and the system becomes unstable. It is a challenge to limit the output current, meanwhile ensuring the stable operation of the DC-GCI. To solve these problems, a modified droop control strategy is proposed to mitigate the impacts of the fluctuation of grid frequency and voltage magnitude on the system stability. In addition, the proposed control strategy in this paper realizes the stable operation of the DC-GCI through the cooperation between the modified droop control and the current limiter, and the current limiter saturation is avoided. The current limiter is used to restrain the transient current to compensate for the slow bandwidth of the droop loop, and the modified droop loop is used to stabilize output power and restrain the steady-state output current. The simulation and experimental results from a scaled-down laboratory prototype have been presented to verify the major contribution of this paper.

General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/userguides/explore-bristol-research/ebr-terms/ This work is licensed under a Creative Commons Attribution 3.0 License. For more information, see Abstract-The adoption of silicon carbide (SiC)MOSFETs and SiC Schottky diodes in power converters promises a further improvement of the attainable power density and system efficiency, while it is restricted by several issues caused by the ultra-fast switching, such as phase-leg shoot-through ('crosstalk' effect), high turn-on losses, electromagnetic interference (EMI), etc. This paper presents a split output converter which can overcome the limitations of the standard two-level voltage source converters when employing the fast-switching SiC devices. A mathematical model of the split output converter has been proposed to reveal how the split inductors can mitigate the crosstalk effect caused by the high switching speed. The improved switching performance (e.g. lower turn-on losses) and EMI benefit have been demonstrated experimentally. The current freewheeling problem, the current pulses and voltage spikes of the split inductors, and the disappeared synchronous rectification are explained in detail both experimentally and analytically. The results show that, the split output converter can have lower power device losses compared with the standard two-level converter at high switching frequencies. However, the extra losses in the split inductors may impair the efficiency of the split output converter, which is verified by experiments in the continuous operating mode. A 95.91% efficiency has been achieved by the split output converter at the switching frequency of 100kHz with suppressed crosstalk, lower turnon losses, and reduced EMI.Index Terms-Silicon carbide (SiC), split output converters, crosstalk, efficiency, electromagnetic interference (EMI).