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This study analyzes and evaluates the feasibility of a harmonic suppression traction transformer (HSTT) for harmonic reduction in railway systems. This new type of transformer can improve the power quality of railway systems by preventing high-frequency harmonic currents from injecting into the traction grid. As the physical size of available space in high-speed trains is strictly limited, low space-occupying filtering techniques are needed. Therefore, an HSTT with integrated filtering inductors (IFIs) capable of being implemented in regular trains is proposed. Taking advantage of the HSTT, a specially constructed inductive-capacitive-inductive (LCL)-type filter is used for harmonic suppression instead of a regular LCL-type filter. The proposed filter is composed of an integrated inductor, leakage inductor of the traction transformer, and an external filter capacitor. In this paper, we analyze the topology of the proposed system, construct a mathematical model to reveal the magnetic decoupling theory of IFIs, and discuss the design and calculation procedures of the HSTT with IFIs. The field circuit coupling simulation of the HSTT with IFIs is performed to validate the effectiveness of the proposed system. Finally, the practical operation based on a 10 kVA prototype shows that the proposed scheme can not only suppress the high-order frequency harmonics but also decrease the installed space of filter devices.

This study proposes to incorporate the non-linear factors, such as dead time and phase-locked loop (PLL), into the impedance model of large-scale grid-connected photovoltaic (PV) system and investigates their influence on the system power quality and stability considering the variation of PV capacity. The fundamental component of dead-time voltage can influence the inverter output impedance. The harmonics also have a vital effect on system power quality, especially when the inverter output admittance matches with the grid admittance. The increase of PV capacity can lower the influence of dead time on system phase margin, but may cause serious magnification of certain dead-time harmonics. PLL can make the system asymmetric and brings about the frequency coupling effect. The complex vectors and the transfer function matrix are used to convert the model of PLL from dq-to αβ-axis. The generalised Nyquist stability criterion is utilised to analyse the system stability. The variations of controller parameters in PLL can make the system oscillate. The increase of PV capacity may intensify this effect. Even the stable controller parameters are adopted, large PV capacity can trigger the system oscillation. Finally, the correctness of the theory is verified by simulation analysis and experiment.

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This paper presented an integrated hybrid railway power quality control system (HRPQC) to deal with the power quality problems inthe electrical railway supply system. The prominent advantage of HRPQC is that, in the premise of adopting hybrid compensating scheme for better performance, it can integrate the active system with the main transformer without other expensive low-frequency high-power auxiliary transformers. HRPQC's mathematical model, control idea, and transforming relationship in harmonic frequency are first presented. Secondly, the harmonic suppressingmechanism and the resonance characteristic are analyzed in detail. Finally, simulating and experimental results verify the effectiveness of the proposed compensating system.

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