Overview of electrical protection requirements for integration of a smart DC node with bidirectional electric vehicle charging stations into existing AC and DC railway grids

Abstract

Distribution network operators (DNOs) and railway traction system operators (RTSOs) who will connect bidirectional electric vehicle charging stations (EVCSs) (treated as both load and source) to their power networks need a unified set of requirements for safe operation. However, such requirements are currently unavailable. Accordingly, the authors in cooperation with the Spanish national RTSO and the most important Spanish DNO have elaborated a unified regulatory framework of requirements for the interconnection protection systems and the earthing arrangement of a DC node used as a reference to feed bidirectional EVCSs, now under construction. This node connects a 0.4-kV AC secondary distribution network (SDN), a 25-kV AC railway traction system (RTS), a 3-kV DC RTS, a local distributed resource (DR) system, and two bidirectional EVCSs. The DR system includes both a photovoltaic PV system as well as backup storage systems (battery and supercapacitor). Thus, the DC node has all the potential interconnections to serve as a reference for requirements regarding the interconnection of bidirectional EVCSs.This unified regulatory framework is the result of a critical review which enabled us to modify, harmonise, and adapt requirements in a wide range of grid-interconnection standards and codes as well as company operation practices to the specific behaviour of bidirectional EVCSs. This is particularly important for RTSs where the interconnection of this reference DC node significantly changes the protection practices of RTSOs and the RTS earthing arrangement. This unified framework is proposed as a technical specification for the companies involved in the implementation of this type of DC node to feed bidirectional EVCSs.