VU limit pre‐assessment for high‐speed railway considering a grid connection scheme

Single‐phase 25 kV AC traction power supply systems (TPSSs) are widely used in high‐speed railways, leading to an increasingly prominent problem of voltage unbalance (VU) in the power grid. To fairly regulate the system VU level, implementing VU limit pre‐assessment is an urgent need. A grid connection scheme including traction transformer selection and exchange phase connection design is a vital part of TPSS design that obviously affects the VU factor of power systems. A uniform mathematical model is established using Thevenin's theorem, which reveals the impact of multiple grid connection schemes on the VU propagation behaviour of a three‐phase grid. Afterwards, under the requirements of IEC/TR 61000‐3‐13, an improved VU limit pre‐assessment process is proposed, which correlates mapping between the grid connection scheme and VU limit. The unbalance compensation calculation is given, adopting the allocation limit; furthermore, the optimal design of the grid connection scheme with minimised compensation capacity as an object is presented. The correctness of the model and the rationality of the pre‐assessment process are verified by a case study.