persist in the phase with the blown fuse. When the local breaker closes the current will startto flow and the function detects the fuse failure situation. But due to the 200 ms drop offtimer the output BLKZ will not be activated until after 200 ms. This means that distancefunctions are not blocked and due to the “no voltage but current” situation might issue atrip.The operation mode selector OpModeSel has been introduced for better adaptation tosystem requirements. The mode selector enables selecting interactions between thenegative sequence and zero sequence algorithm. In normal applications, the OpModeSelis set to either V2I2 for selecting negative sequence algorithm or V0I0 for zero sequencebased algorithm. If system studies or field experiences shows that there is a risk that thefuse failure function will not be activated due to the system conditions, the dependabilityof the fuse failure function can be increased if the OpModeSel is set to V0I0 OR V2I2 orOptimZsNs. In mode V0I0 OR V2I2 both negative and zero sequence based algorithms areactivated and working in an OR-condition. Also in mode OptimZsNs both negative andzero sequence algorithms are activated and the one that has the highest magnitude ofmeasured negative or zero sequence current will operate. If there is a requirement toincrease the security of the fuse failure function OpModeSel can be selected to V0I0 ANDV2I2 which gives that both negative and zero sequence algorithms are activated andworking in an AND-condition, that is, both algorithms must give condition for block inorder to activate the output signals BLKV or BLKZ.11.1.3.3 Negative sequence basedThe relay setting value 3V2PU is given in percentage of the base voltage VBase and shouldnot be set lower than the value that is calculated according to equation 88.1003223 VBaseVPUVEQUATION1757-ANSI V4 EN (Equation 88)where:V2PU is the maximal negative sequence voltage during normal operation conditions, plus a margin of10...20%VBase is the base voltage for the function according to the settingGlobalBaseSelThe setting of the current limit 3I2PU is in percentage of parameter IBase. The setting of3I2PU must be higher than the normal unbalance current that might exist in the system andcan be calculated according to equation 89.1MRK 505 337-UUS - Section 11Secondary system supervision275Application manual
