Simplified rail power conditioner based on a half-bridge indirect AC/DC/AC Modular Multilevel Converter and a V/V power transformer

Abstract

This paper presents a comprehensive study about a Simplified Rail Power Conditioner (SRPC) based on a half-bridge indirect AC/DC/AC Modular Multilevel Converter (MMC) and a V/V traction power transformer. The proposed system with a half-bridge MMC can decrease the costs, reduce the control complexity, and require less hardware devices in comparison with the rail power conditioner based on a full-bridge indirect AC/DC/AC MMC. Moreover, the SRPC with a half-bridge MMC is able to compensate current harmonics, reactive power, and the Negative Sequence Components (NSCs) of currents, which are caused by the unbalance loads between power grid phases. This paper explains the system architecture and its control algorithms based on a pulse-width modulation and a proportional-integral controller, which is used to control the compensation currents. The simulation results of the SRPC show the submodule voltage balancing control and the DC-bus voltage control in order to verify its effectiveness. The compensation strategy based on the NSCs detection is described and evaluated through simulation results.

Power quality phenomena in electrified railways: Conventional and new trends in power quality improvement toward public power systems

“…Electric locomotives are normally using the AC/DC/AC power converters to drive the traction motors. However, these converters, especially the old half-controlled ones, are causing different harmonic content that flow into the PPS [4], [5]. There will be a series of harmonic components that have the double, third, fourth, etc., of the fundamental frequency.…”

“…Nowadays, the new locomotives contain power electronics converters controlled by a pulse width modulation technique and they produce zero reactive power [3]. Regarding the TPS, reactive power is required to compensate the NSCs of currents, hence, a contradiction between the NSCs compensation and the reactive power compensation may occur [4]. Therefore, the NSCs in the single-phase TPS must be in antiphase with the three-phase voltages/currents of the PPS [3].…”

“…Woodbridge power transformer normally requires two additional autotransformers for the two-phase railway traction power supply [6], [16]. The unbalanced V/V power transformer has a very simple construction comparing to the balanced power transformers and it can be overloaded [4]. Therefore, it is widely used in the railway applications.…”

“…The SVC is branded nowadays as a low-cost compensator because it may reduce the power factor of the system when it is designed for NSCs compensation. In other words, there is a tradeoff between power factor correction and NSCs compensation when using the SVC [4]. If the system carries reactive power, then the SVC cannot be a useful compensator.…”

“…Therefore, the RPC also compensates the reactive power, where one of the RPC converters provides a capacitive reactive power while the other converter compensates an inductive reactive power [21]. Since the load power is mainly provided by the PPS through a V/V or a Scott power transformer, unlike the SFC, these devices cannot perform a frequency conversion but, on the other hand, the load sections can be overloaded as an advantage of using the V/V [4]. In addition, problem of the NSs can be omitted by using a co-phase RPC system [3].…”