7.9.7 Operation principleThe negative sequence time overcurrent protection for machines (NS2PTOC)function directly measures the amplitude of the negative phase sequencecomponent of the measured current. NS2PTOC sets the START, ST1 or ST2outputs active and starts to count trip time only when the measured negativesequence current value rises above the set value of parameters I2-1> or I2-2>respectively.To avoid oscillation in the output signals, a certain hysteresis has been included.For both steps, the reset ratio is 0.97.Step 1 of NS2PTOC can operate in the Definite Time (DT) or Inverse Time(IDMT) mode depending on the selected value for the CurveType1 parameter. IfCurveType1= Definite, NS2PTOC operates with a Definite Time Delaycharacteristic and if CurveType1 = Inverse, NS2PTOC operates with an InverseTime Delay characteristic. Step 2 can only operate in the Definite Time (DT)mode. The characteristic defines the time period between the moment whenmeasured negative sequence current exceeds the set start levels in parameter I2-1>or I2-2> until the trip signal is initiated.Definite time delay is not dependent on the magnitude of measured negativesequence current. Once the measured negative sequence current exceeds the setlevel, the settable definite timer t1 or t2 respectively, starts to count and thecorresponding trip signal gets activated after the pre-set definite time delay haselapsed. Reset time in definite time mode is determined by the setting parameterstResetDef1 or tResetDef2 respectively. If NS2PTOC has already started but nottripped and measured negative sequence current goes below the start value, thestart outputs remains active for the time defined by the resetting parameters.A BLOCK input signal resets NS2PTOC momentarily.When the parameter CurveType1 is set to Inverse, an inverse curve is selectedaccording to selected value for parameter K1. The minimum trip time setting ofparameter t1Min and reset time parameter ResetMultip1 also influence stepoperation. However, to match the heating characteristics of the generator, the resettime is depending on the setting of parameter K1, which must be set according tothe generators negative sequence current capacity.22K I t=EQUATION2112 V1 ENWhere:I2 is negative sequence current expressed in per unit of the rated generator currentt is operating time in secondsK is a constant [s], which depends on generator size and design1MRK 502 034-UEN - Section 7Current protection207Technical Manual
