Abstract
Correct operation of the protection system is of vital importance for the reliability and security of supply. Information of protection system faults is therefore important for different purposes such as quality of supply regulation and as input data to security and reliability analysis. This paper provides a comparative review of the Norwegian and Finnish fault statistics for line and transformer protection systems at the voltage levels 132-400 kV for the period 1999-2004. Unwanted operation is the major fault type, particularly for transformer protection. Human causes dominate by roughly 50%, technical faults count for 20-30% while there are large portions of faults where the cause is not identified. In Norway 20% of the total energy not supplied at these voltage levels are caused by the protection. The corresponding portion in Finland is only 4%