The real-time monitoring of the grid-connected renewable energy generations is of great significance to the operation and control of active distribution systems (ADS). Considering the detailed photovoltaic (PV) system model and ambient influencing factors, this paper develops a state estimation framework for the ADS with integrated PV power plants. Firstly, suitable models of PV arrays and power converters for steady-state state estimation are developed. Based on these models, the state variables of a PV generation system are selected, and the measurement equations are derived. Finally, combined with the state estimation model of distribution networks, a framework for state estimation and bad data processing of an ADS with PV power plants is presented. A parameter estimation approach for PV power plants based on multiple measurement scans is also developed in order to address the practical situation of unknown model parameters. Simulation results show that the proposed framework can effectively capture the operating state of the distribution network and PV power plants. In addition, it enhances the estimation accuracy and bad data processing capability compared to conventional state estimation frameworks.

A high efficiency dual active bridge with coupled inductor (DABCI) for fuel cell vehicles is proposed in this article and its operation principle is analyzed to reveal the optimal operation modes of this converter, which provides the basis for circuit parameters design. Compared with dual active bridge, the DABCI is more suitable for high voltage gain applications because its low voltage bridge works like interleaved boost circuit and it is easy to get high voltage gain by adjusting the duty cycle D. In order to reveal the best operation mode of the DABCI, an optimal current stress modulation strategy is proposed, which can realize zero voltage switching operation in wide load range for all switches. Furthermore, the input current ripple is suppressed by interleaving two boost circuits in low voltage bridge, which can extend the life span of fuel cell vehicles. In addition, advanced magnetic integration technology is adopted to improve the system power density. Finally, a 4-kW rated prototype is made and experimental results are presented to verify the previous analysis.

Interleaving is widely used in paralleled converters to enhance the output current quality, but subsequent circulating currents could lead to harmonic distortion and extra power losses. This article proposes an improved interleaved discontinuous pulsewidth modulation (IIDPWM) method to reduce zero-sequence circulating currents (ZSCC) in paralleled converters with common dc and ac bus. For two-paralleled converters, optimal vector sequences are selected considering minimal common-mode voltage (CMV) differences, the NTVs principle, and power losses. Furthermore, a novel carrier-based scheme is presented to implement the proposed strategy in a general case with any number of paralleled units. The 180 $^\circ$ carrier phase shift between two unclamped phase legs in discontinuous PWM is first applied for ZSCC reduction. Compared to existing works, the proposed strategy shows a practical value achieving good overall performance in ZSCC, output currents, switching losses, ease of use and scalability. Simulation and experimental results verified the effectiveness of the proposed IIDPWM strategy.

In recent years, fault-tolerant control of multiphase drives against open-phase faults has been an interesting research topic. However, in contrast, few research efforts have been devoted to fault-tolerant control against open-switch faults in which only one controllable power semiconductor device in a converter leg is open, but its antiparallel diode operates normally. In this article, a fault-tolerant control strategy against open-phase/open-switch faults based on dc current injection is proposed. In open-switch phases, dc currents, whose amplitudes are equal to the amplitudes of the corresponding fundamental phase currents, are injected to eliminate the nonlinearity brought by faults. Therefore, the fundamental currents in open-switch phases can be precisely regulated. To achieve torque-ripple-free postfault operation, minimize the stator winding losses and maximize the postfault torque production, stator winding losses associated with fundamental currents and injected dc currents are optimized over the full torque production range on the premise that no dc current is mapped in the torque-producing subspace. Simulation results on different kinds of multiphase drives and experimental results on a nine-phase induction drive proved the reduced stator winding losses, extended torque operation range, and reduced torque ripple compared with the existing methods.

Recently, multiple static synchronous compensator (STATCOM) units have been adopted in power transmission systems in order to obtain a better voltage regulation and to share loads. However, they could possibly interact in a negative way instead of helping each other, due to the improper design of the STATCOM controllers. To analyze this problem, a d-q frame impedance-based stability analysis was used to explore the instability with the presence of STATCOMs, where previous stability-related findings are not applicable directly because of some unique features of STATCOMs. This paper identified the frequency range of interactions in a viewpoint of d-q frame impedances and pinpointed that the ac voltage regulation was the main reason of instability, masking the effects of phase-locked loop on power transmission systems. In addition, due to the high impedance of STATCOMs around the frequency range of interactions, the number of connected STATCOMs was the main contributor to stability instead of the topology of power systems or the locations of STATCOMs. A scaled-down 2-STATCOM power grid was built to verify the conclusions experimentally. This paper is accompanied by a video showing instability between STATCOMs in the experiment.

This letter discusses the reactive components and the power factor of dual-active-bridge converters under triple-phase-shift modulation. Different from previous methods based on backflow power integral or Fourier series expansion, this letter presents a straightforward analysis and categories of reactive current through time domain analysis, and then gives a unified time domain definition of reactive current, reactive power, and power factor. Compared with all the previous definitions, the expressions of reactive components and power factor are much more concise and of a profound physical meaning. On this basis, specific calculations of reactive power and power factor are derived for all practical voltage patterns and an optimal global power factor phase shift modulation is proposed to improve the power factor on both bridges. At last, the effectiveness of analysis and proposed modulation were validated using a 500-W laboratory prototype. The results showed efficiency improvements compared to other optimal power factor modulation strategies.