Abstract
Abstract This paper presents a partial compensation scheme for V/v transformer cophase traction power supply in high-speed railway systems. The scheme compensates variable traction load current, and controls the current phase at the secondary side of the V/v transformer for power factor correction and negative sequence current reduction. To achieve this, the grid side current phase angles are optimized while satisfying the grid code on the power factor and voltage unbalance limits. The optimized phase angles are then used to design control references under varying load conditions. The compensation control action is updated regularly based on real-time measurements of the traction load, and the required currents are controlled by a 25-level single-phase back-to-back MMC power conditioner to achieve the compensation target. Static and dynamic load compensation performances are verified based on the simulation studies.
Highlights
Three electrified railway traction power supply solutions are widely used, namely the 16 2/3 Hz 15 kV ac system, 50 Hz 25 kV ac system and 3 kV dc system
The compensation control action is updated regularly based on real-time measurements of the traction load, and the required currents are controlled by a 25-level single-phase back-to-back modular multilevel converter (MMC) power conditioner to achieve the compensation target
Using the phase angles of three-phase current as the control objectives, the cophase controller can dynamically compensate the unbalanced current arising from the traction load such that the negative sequence component and the grid side power factor are corrected to the desired limits with the least resistive losses
Summary
Three electrified railway traction power supply solutions are widely used, namely the 16 2/3 Hz 15 kV ac system, 50 Hz 25 kV ac system and 3 kV dc system. To step down the voltages, V/v (V/x) transformers with autotransformer connection are widely used in high-speed railway systems [1] These transformers generate two singlephase voltage sources for two traction supply arms, which inevitably introduces power unbalance and low power factor current into the grid, causing power quality issues. These transformers lack the ability to suppress harmonic currents produced by variable traction loads. The existence of a neutral section blocks the energy sharing between the two supply arms and introduces a no-power zone where the train has to rely on inertia and on-board energy sources without external supply Another issue is the inevitable negative sequence current, no matter how balanced the traction loads are in each supply arm.
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