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High-speed trains are more and more widely used around the world. They are commonly installed in interconnected power systems by connecting each traction substation directly in different nodes among the network. The single-phase loads of high-speed railway systems do not produce excessive dangerous phase to phase imbalances in these robust networks. However, if high-speed trains are to be used in small isolated power systems, such as islands or non-interconnected power systems, problems of unacceptable phase to phase imbalances may appear. This is due to the small size of the system and its relatively weak network that lead to a low short-circuit power in their nodes. This paper studies the impact of high-speed trains in small isolated power systems. Two different connections proposed by the infrastructure manager are analyzed. These connections try to reduce the impact of the railway network in power system network imbalances. Both connection schemes have been tested in a real small isolated power system in which a high-speed railway system is expected to be installed by the year 2018. Representative peak and valley scenarios for this year have been used in the analysis. The results obtained evidence that small isolated power systems are very sensitive to the incorporation of high-speed trains in their weak networks since high imbalances are obtained. Moreover, the results allow the selection of the connection that is more capable of reducing power system imbalances.

This study presents a thermo-electric model to implement the energy management system (EMS) of an off-grid system. The EMS is formulated as a unit commitment problem where thermal and electrical components are dispatched simultaneously to minimise system operation costs. Particular attention is paid to the operation of heat pumps. The model contemplates a variable coefficient of performance (COP) of heat pumps. COP is different for the cold water and hot water circuit and depends on the output temperatures of the heat pump and the heat pump operation mode. Furthermore, the heat pumps' ability to switch operation modes is also considered as well as the limitation of heat pumps to operate when hot water thermal energy storage is nearly full. The developed model has been successfully applied to an existing off-grid system at Norvento Enerxia's headquarters in Spain by simulating its operation during a winter week. The impact of modelling multi-mode operation of heat pumps with variable COP has been compared with single-mode operations with either fixed or variable COP. The proposed integrated approach of solving the electrical and thermal dispatch simultaneously has also been compared with a sequential resolution of first the thermal and then the electrical dispatch.