Optimal Power Flow for an HVDC Feeder Solution for AC Railways

Sammanfattning: With today’s increasing railway traffic, the demand for electrical power has increased. However, several railway systems are weak and are not being controlled optimally. Thus, transmission losses are high and the voltage can be significantly lower than the nominal level. One proposal, instead of using an extra HVAC power supply system, is to implement a HVDC supply system. A HVDC supply line would be installed in parallel to the current railway catenary system and power can be exchanged between the HVDC grid and the catenary through converters. This thesis investigates different properties and behaviours of a proposed HVDC feeder solution. An AC/DC unified Optimal Power Flow (OPF) model is developed and presented. Decision variables are utilized to obtain proper control of the converters. The used power flow equations and converter loss function, which are non linear, and the use of binary variables for the unit commitment leads to an optimization problem, that requires Mixed Integer Non-Linear Programing (MINLP) for solving. The optimization problem is formulated in the software GAMS, and is solved by BONMIN. In each case investigated, the objective is to minimize the total active power losses. The results of the investigated cases presented in this thesis, show that the proposed OPF-controlled HVDC solution reduces the losses and provides better voltage profile at the catenary, compared with today’s supply systems.