The influence of unified power flow controller on work of distance protection

The short-circuit current in transmission nets (220kV) is from a dozen or so to over 20 times greater that duty current. Therefore when short-circuit occur on the same line there the UPFC exist, this controller has to be by-passed. So it does not influence on first and second zones of distance protection. Short-circuit in nearby power line, when in this side co-supply from other line additionally occur, it can cause, that current in considered line will grow up less than 2 times. Then the UPFC can stay in work. Introduced add voltage and the change of current phase in consider line in relation to predominant current of short-circuit could cause considerable measurement mistakes of short-circuit impedance. This phenomenon will be observed in III zone of distance protection. The example characteristics of measured impedance by distance protection on base derivate equation are presented in this article. The principle of III zone set value selection of this protection is talked over on basis of these characteristics.