Non-unit protection of double circuit lines based on fault generated high frequency signals

Double circuit lines are widely used on the transmission system due to their often significant economic and also environmental advantages over single circuit lines. However, due to the mutual coupling effect, the high frequency signals induced on the healthy circuit due to a fault on one circuit can be quite significant in comparison with those at power frequency. The latter can thus manifest themselves into causing a sympathy trip of the healthy circuit due to protection relay instability. It is thus vitally important to examine the response of the healthy circuit relay. This paper is concerned with the application of the new technique, based on the detection of fault generated high frequency signals, to a typical 400 kV double circuit line configuration of the type encountered in the UK. Particular attention is paid to the aforementioned potential relay instability problem on the healthy circuit, with a view to refining the digital signal processing unit if necessary, and the responses are examined for a number of practically encountered different system and fault conditions. The transient fault studies are based on the well known EMTP software and the system simulation also includes a very realistic nonlinear fault arc model