High degrees of series capacitors in bulk power transmission systems need special protection principles

The paper describes relaying problems in transmission systems with a high degree of series compensation. The usability of and limits for the use of distance protection will be discussed and experiences gained from real-time tests are presented. The possibilities offered by modern relays, so called two-in-one relays, consisting of communication based differential protection combined with backup distance protection will be shown. By the use of these modern relays, very low fault clearing times like required in protection codes for highly loaded transmission systems can be fulfilled.