Grid synchronised three phase VSI-based hardware emulator to realise low frequency electromechanical oscillations in power systems and their effects on physical distance relay.

Practical testing of distance relays under power swing conditions is becoming all the more essential. This is in view of the increasing use of power electronic apparatus due to penetration of renewable sources and/or FACTS controllers now-a-days. To perform such practical tests, a hardware platform emulating power system swing occurrences, as caused by electromechanical oscillation of alternators, is necessary. As a first step it is required to model the phenomenon before its implementation. In this paper an analytical small-signal model of the low-frequency electromechanical oscillations in a grid-connected alternator is presented. Off-line simulation-based validation for a Single Machine Infinite Bus (SMIB) system is done. To emulate an open-loop alternator, a prototype of a controlled grid synchronised VSI with AC-side reactor is developed and practically tested. A control scheme is proposed and implemented to emulate the active power swing phenomenon. Effects of machine inertia and initial power angle on the swing characteristics are investigated. In-process estimation of apparent impedances, as seen from the distance relay terminals downstream towards the grid, is done. Principle of estimating the apparent impedance during power swing is presented with corresponding experimental results. Both real-time and off-line simulation results are in close agreement with the experimental results thus establishing the accuracy of the model and the precision of the experiments. [ABSTRACT FROM AUTHOR]