The bidirectional ac/dc converter is widely used to realize the power conversion between ac and dc microgrid, but the faults of switch devices and unbalanced grid voltages may lead to the decline of power quality and affect normal operation of the converter. The four-switch three-phase (FSTP) fault-tolerant structure is reconstructed from a six-switch three-phase structure with switch device fault. In order to reduce harmonic currents and output power fluctuations under unbalanced grid voltages, finite states model predictive direct power control (MPDPC) with power compensation method is proposed for FSTP structure and predictive power model of the bidirectional FSTP ac/dc converter is established. The power compensation values are expressed by grid voltages and their quadrature signals that lagging 90 electrical degrees in the αβ stationary coordinate system. Compared with the conventional method, phase-locked loop, pulse width modulation, and complex positive-/negative-sequence extraction of grid voltage are not required. Ripples of active power or reactive power under unbalanced grid voltages are eliminated. The proposed fault-tolerant MPDPC with power compensation method ensures the continuous and reliable operation of the bidirectional ac/dc converter with high power quality. Simulation and experimental results are presented to validate the proposed control strategy under symmetrical unbalanced grid voltages with switch device faults.

This paper proposes a simple and cost-effective current measurement technique for four-phase switched reluctance motor (SRM) control, by splitting the dual bus line of the converter, without pulse injection and voltage penalty. Only two Hall-effect sensors are utilized, where one is installed in the upper bus to measure two phase currents, and the other is placed in the lower bus to measure other two phase currents. In order to realize independent current measurement in the whole turn-on region, switching functions are redesigned so that upper switches of two phases act as the choppers, while lower switches of the other two phases are employed as the choppers. Compared to traditional drives, the developed system requires only two Hall-effect sensors in the dual bus line, without a need for individual phase sensors or additional devices, which reduces the cost and volume for SRM drives. Furthermore, compared to the single-sensor based current measurement scheme, the proposed method has no need to implement pulse injection and will not cause any voltage penalty and current distortion, which also improve the current measurement accuracy and system performance. Simulation and experiments carried out on a 150-W four-phase 8/6 SRM confirm the effectiveness of the proposed technique.

The modular multilevel converter-based battery energy storage system (MMC-BESS) can help the power grid to run safely and stably, which plays an important role in the modern power grid. The inconsistency of cells in BESS may lead to overcharge or overdischarge of cells, which may decrease the operation life of battery packs and cause safety problems. Normally, before the MMC-BESS is connected to the grid, it is necessary to prebalance the system batteries in an offline state. This article analyzes the topology, the mathematical model, and the equalization control model in offline MMC-BESS. According to the analysis, an offline MMC-BESS stepwise balancing control strategy considering the state of charge, state of health, and state of function, is proposed. In this strategy, the state of dischargeable energy is set as the criterion of offline equalization of the system. The balancing of submodules in MMC-BESS would be achieved by controlling the circulating currents and voltages of arms. Finally, the feasibility of the offline equalization control strategy is verified by simulation and experiment.

A new definition of power factor, which is suitable for an arbitrary waveform of voltage and current in a single‐phase system, is presented according to similarity between voltage and current waveforms. The similarity is described in infinite dimensions real vector space, and the power factor is defined as the multiplication of rootmean‐ square (RMS) value factor λ _RMS and phase factor λ_ϕ. The new definition is compatible with IEEE Trial‐Use Standard 1459–2000, and can be a useful reference for power factor measurement, power factor correction and power compensation.