High-speed electric trains have recently gained significant attention over other modes of transportation due to their lower emissions, reduced initial and maintenance costs, and high efficiency. Traction power supply systems (TPSSs), which supply electric trains, typically include specialized traction transformers that convert three-phase systems into single-phase ones. To mitigate power quality issues in these systems, railway power conditioners (RPCs) are employed alongside traction transformers. Despite extensive re- search on power quality improvement, the challenge of energizing traction transformers has been largely overlooked in the literature. When a trans- former is energized, a high-magnitude asymmetrical current, known as in- rush current, can flow into the energized winding while the other winding is open-circuited. This can lead to the malfunction of differential protective relays and generate axial and radial forces that deform transformer wind- ings, potentially causing turn-to-turn faults. These issues not only shorten the transformer’s lifespan but also create disturbances in the power grid sup- plying the transformer. Therefore, this research project seeks to develop methods to reduce or eliminate inrush currents during the energization of traction transformers. It is essential to address inrush currents without de- pending on the transformer’s design information or initial status.
New energization methods are proposed to address the challenges asso- ciated with energizing traction transformers, specifically focusing on single- phase and V/V traction transformers, which are the most common types in- stalled in TPSSs. Compared to existing energization techniques, these meth- ods offer several advantages: 1) They do not require knowledge of the initial residual flux density, 2) They are fast, even for high-power transformers, 3) They account for the closing operation of circuit breakers (CBs) and all re- lated uncertainties, minimizing their impact on transformer energization, 4) If they rely on measurement data, they are designed to be immune to er- rors in the outputs from measurement devices, and 5) They aim to mitigate inrush current (keeping it below the transformer’s nominal current) using ex- isting infrastructure or by adding a small device that incurs minimal initial and maintenance costs. To achieve these goals, three energization methods are proposed for single-phase traction transformers, designed to effectively address inrush current problems while being fast, immune to measurement errors, less sensitive to CB closing operations, and requiring only a small device for flux adjustment. Additionally, two methods are introduced for V/V transformers: one for transformers energized with independent-pole- operated CBs, and the other for those energized with gang-operated CBs, both of which rely on RPCs for flux adjustment.
To assess the proposed methods’ performance and compare them with ex- isting energization methods, they are implemented in PSCAD/EMTDC and thoroughly tested under different conditions. Furthermore, a hardware-in- the-loop (HIL) test system is developed, which is composed of transformers, a Typhoon 604 HIL device, and an instantaneous turn-on solid-state relay.
This setup facilitates the practical application and validation of the methods on real transformers. The results consistently demonstrate that the proposed methods significantly reduce inrush currents compared to existing energiz- ation methods. Specifically, they limit the current magnitude to 70% of the transformer’s nominal current (equivalent to more than a 98% reduction compared to the peak inrush current of the worst-case energization scenario), while other methods can result in current magnitudes reaching 200%–300% of the nominal value.
