Control of a unified power flow controller in fault recovery and with harmonic filter

This paper addresses two issues: enhancement of fault recovery and controller design accounting for filter dynamics. Performance of the unified power flow controller (UPFC) is improved by a proposed modification of the UPFC into a double static synchronous series compensator (SSSC) in order to double its capability in fault recovery mode. A battery is required as an energy source in the DC link of the double SSSC during the recovery. The battery will be charged during normal operation of the UPFC. The size of the battery for the SSSC has been investigated. It was found that the double SSSC in fault recovery mode has almost twice the performance of the UPFC and the size of battery for the UPFC is reasonable for grid scale use. One of the important parts of the UPFC is the filter at the output of the inverter. The function of the UPFC filter is to attenuate the harmonics generated by the inverter. In this paper, a harmonic trap filter, together with a low-pass filter, are applied to the UPFC and investigated. Single-phase models are used to design cut-off frequencies whilst the d-q model is included in the UPFC controller design. It was found that if the filter dynamics are neglected in the control design, as is often seen in simulation studies, then the controller´s response will be different. The test system used in this work is a one-machine, infinite bus bar system. All of the simulations were implemented using the Electromagnetic Transients Program (EMTP) which is a powerful simulation tool for power system simulation. In additional, the Matlab and Simulink programming environment were used to provide transfer function simulation for the controller design. The simulation results from both programs are compared