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Co-phase traction power supply system provides continuous power to traction loads without neutral sections. In order to reduce system unbalance, compensate reactive power and harmonics, a railway power conditioner (RPC) operates together with traction transformer in each substation. In the past study, the RPC is designed to achieve three-phase balance and unity power factor (PF) at the grid side. As a result, its rating is high. According to the power quality tariff plan in China, the penalty for reactive power can be avoided if the PF is higher than 0.9. In this study, a grid-side PF of 0.9 is achieved via different control approaches after analysis. Among these approaches, the rating of the RPC in the worst case is more than twice that in the best case. Hence, selection of a suitable control parameter is necessary. The minimum rating of the RPC is achieved by setting the power angle of phases A and B lagging and the power angle of phase C leading under partial compensation. The rating of the RPC is reduced to 70% by setting PF to 0.95 instead of 1. Simulation and experimental results are provided to show the validity of the modelling, design and control method.

Pulse-width modulations (PWMs) are widely investigated in active power filter (APF) applications. However, there are seldom PWMs proposed in hybrid APF (HAPF) applications because of their non-linear current characteristic. This study proposes a non-linear adaptive hysteresis band PWM controller for HAPFs to reduce the switching loss and keep the total harmonic distortion (THD) at an acceptable level. In contrast to previous studies, as the coupling LC impendence of an HAPF can yield a non-linear compensating current, the quasi-linear and non-linear regions are exploited to obtain a low switching frequency. In addition, an approximated THD index is proposed to assess THD in a simplified way in the control system, which results in a faster system response. Finally, the performance of the non-linear adaptive hysteresis band controller is verified by comparing simulation and experimental results with a state-of-the-art PWM for HAPFs.

Capacitive-coupling inverters (CCIs) have low operation voltage and enhanced reactive power control capability. It is a promising alternative to provide reactive power and voltage regulation in microgrids (MGs). Droop control is one of the most widely used primary controllers under a hierarchical framework in a MG. However, conventional droop control performance needs improvement since the droop curve assumption is not valid in part of CCI's operational area. At the same time, reactive power sharing error due to the uncertainty factor of feeder impedance needs to be minimized. In this paper, virtual-impedance droop control is proposed for CCIs to solve the two issues. A virtual impedance selection method is developed with consideration of CCI's second-order LC coupling branch. First, the virtual impedance is selected for reducing the coupling between the active and reactive power of CCIs. The stability and sensitivity of the system are also evaluated based on a small-signal model to provide guidelines for virtual impedance selection. The proposed control varies the equivalent impedance via a feed-back loop in CCI's control for accurate power control and sharing in MGs under mismatched feeder impedance. The validity of the proposed virtual-impedance droop control is verified through simulation and experimental results.

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