Improved reliability by simplification of a sub-transmission protection system

Based on protection failure characteristics and failure causes, a new simplified protection scheme has been introduced for sub-transmission (150 and 110 kV) systems in the Netherlands. The fact that a redundant 2 Mb/s Wide Area Network for telecommunication is implemented, supported the proposition to replace the old mechanical and electronic distance relays by single multifunctional digital relays. Single advanced relays in stead of duplicated old technology relays will give large improvements in terms of reliability and availability, whereas security is treated more in balance with respect to dependability. The new policy of simplification has also lead to reconsidering the application and the schematics of Breaker Failure Protection and Busbar Protection: reluctant in its application and, with some exceptions, non-busbar-selective. So far, the service experience is promising, even during the project of replacement that is almost finalized.