The Decoupling Control Study Of Unified Power Flow Controller

As the most flexible controller of the Flexible AC Transmission System(FACTS),an Unified Power Controller (UPFC) can control concurrently or selectively the transmission voltage, impedance and angle or alternatively,the real and reactive power flow in the line. This paper introduces the principle,dynamic model, performance and basic operating modes of the UPFC.UPFC is a multi-input and multi-output(MIMO) multivariable controller and dynamic interactions are found among its multiple functional UPFC, which may result in poor control performance and sometimes even closed loop system instability. Focus of study in this paper is to investigate two possibilities causing UPFC control interactions: (1) interactions are caused by impropriate setting of UPFC controller's parameters; (2) interactions are irrelevant with setting of UPFC controller's parameters, the case of inherent interactions of UPFC control. Results presented in the paper show that in most cases UPFC control interactions are caused by impropriate setting of its controller's parameters. However, there are cases that the interactions are inherent when they do not disappear no matter how UPFC controller's parameters are tuned.The work presented in this paper also reveals that with variations of power system and UPFC operating conditions, UPFC controller's performance can change greatly, because controllable parameter region of UPFC controllers moves in its parameter space with the variations of operating conditions. Typically, at two different operating conditions, there could be no union of the corresponding controllable parameter regions. That means variable-parameter control scheme should be considered for UPFC controller's design to ensure UPFC's robustness to variations of operating conditions. Hence in this paper, a novel variable-parameter control for UPFC based on Artificial Neural Network (ANN) is proposed, in conjunction with a decoupling design of multi-variable UPFC control that is also based on an ANN decoupling control design.