UPFC converter-level control system using internally calculated system quantities for decoupling

The paper presents a Unified power flow controller (UPFC) converter-level control system which enables the device to track the changes in reference values of the active and reactive power flow through the transmission line, and of the reactive power exchange with the shunt branch. First, a co-ordinated control of both branches of the device is presented, where all quantities being controlled are also dynamically mutually decoupled. This is possible only if on-line values of input quantities are available in the controller. Under practical operating conditions, the input values are filtered and processed with certain delay, which means that they cannot be directly used for decoupling too. To overcome this problem, a modified control structure was formulated. It is based on the calculation of system quantities in the controller by using the mathematical model of the device. The decoupled UPFC control can significantly reduce interactions with other controls in the system. The derived control was tested with digital simulation. (C) 2003 Elsevier Science Ltd. All rights reserved.