fuse fail when closing the local breaker when the line is already energized from theother end. When the remote breaker closes the voltage will return except in thephase that has a persistent fuse fail. Since the local breaker is open there is nocurrent and the dead phase indication will persist in the phase with the blown fuse.When the local breaker closes the current will start to flow and the function detectsthe fuse failure situation. But due to the 200 ms drop off timer the output BLKZwill not be activated until after 200 ms. This means that distance functions are notblocked and due to the “no voltage but current” situation might issue a trip.The operation mode selector OpMode has been introduced for better adaptation tosystem requirements. The mode selector enables selecting interactions between thenegative sequence and zero sequence algorithm. In normal applications, theOpMode is set to either UNsINs for selecting negative sequence algorithm orUZsIZs for zero sequence based algorithm. If system studies or field experiencesshows that there is a risk that the fuse failure function will not be activated due tothe system conditions, the dependability of the fuse failure function can beincreased if the OpMode is set to UZsIZs OR UNsINs or OptimZsNs. In modeUZsIZs OR UNsINs both negative and zero sequence based algorithms areactivated and working in an OR-condition. Also in mode OptimZsNs both negativeand zero sequence algorithms are activated and the one that has the highestmagnitude of measured negative or zero sequence current will operate. If there is arequirement to increase the security of the fuse failure function OpMode can beselected to UZsIZs AND UNsINs which gives that both negative and zero sequencealgorithms are activated and working in an AND-condition, that is, both algorithmsmust give condition for block in order to activate the output signals BLKU orBLKZ.10.2.3.3 Negative sequence based M13683-17 v9The relay setting value 3U2> is given in percentage of the base voltage UBase andshould not be set lower than the value that is calculated according to equation 40.1003223 UBaseUUEQUATION1519 V5 EN-US (Equation 40)where:U2 is the maximal negative sequence voltage during normal operation conditions, plus a marginof 10...20%UBase is the base voltage for the function according to the settingGlobalBaseSelThe setting of the current limit 3I2< is in percentage of parameter IBase. Thesetting of 3I2< must be higher than the normal unbalance current that might existin the system and can be calculated according to equation 41.Section 10 1MRK 504 169-UEN ASecondary system supervision172 Transformer protection RET650 2.2 IECApplication manual
