Analysis of Control Conflict between UPFC Multiple Control Functions and Their Interaction Indicator

Interactions among multiple control functions of a UPFC installed in a power system have been observed in power system simulation and been reported in authors' previous publications [1,2]. This paper presents new analytical results about these observed interactions and concludes that they are due to the control conflict between the series and shunt part of the UPFC, which are connected through the internal common capacitor inside the UPFC. Investigation in the paper reveals, for the first time as far as the authors are aware of, that the linkage pattern of UPFC series and shunt part decides whether the control functions implemented by the UPFC series and shunt part conflict each other or not. This linkage pattern of UPFC series and shunt part can be described by the flow of active power through the UPFC at steady-state operation of the power system. Hence in order to predict the possible interactions among multiple control functions of the UPFC, an interaction indicator is proposed in the paper which is the direction and amount of active power flow through the internal link of the UPFC series and shunt part at steady-state operation of the power system. This proposed interaction indicator can be calculated from power system load flow solution without having to run simulation of the power system with UPFC controllers installed. By using the indicator, the interactions among multiple control functions of the UPFC caused by badly set controller's parameters are excluded. Therefore the indicator only identifies the possible existence of inherent control conflict of the UPFC.