Study on two-stage control for Unified Power Flow Controller to damp tie-line low frequency oscillation

Abstract

This paper proposes a novel two-stage control scheme of UPFC for damping tie-line low frequency oscillation. The first-stage control is damping control: positive and negative states of the angular speed difference between the COIs (center of inertias) are used to control charge and discharge of the DC capacitor, therefore real power of tie lines is decreased and increased correspondingly, the damping of the system is intensified and the oscillation of the system is quickly suppressed. Moreover a method to estimate the angular speed difference between the COIs using locally measurable quantity at the UPFC location is proposed, and the estimation result is quite accurate. The second-stage control is coordinated control: After the first-stage control comes to a close, the system operates at a new steady state operating point. Under the steady-state condition the Relative Gain Array (RGA) method in control theory is used to determine the best control scheme, under which the interactions among UPFC control channels are the most weak. Therefore it is easy to implement the coordinated control among UPFC power flow controller, AC and DC voltage regulators and the system can operates fast at the expected steady-state operating point. Simulation results show that the two-stage control for UPFC is effective and feasible in damping area mode low frequency oscillation.