The fundamental frequency differential currents are the magnitudes which are appliedin a phase segregated manner to the operate - restrain characteristic of the differentialprotection. The magnitudes of the differential currents can be read as service valuesfrom the function and they are available as outputs IDMAG_A, IDMAG_B,IDMAG_C from the differential protection function block. Thus they can be connectedto the disturbance recorder and automatically recorded during any external or internalfault condition.6.1.3.3 Differential current alarmFundamental frequency differential current level is monitored all the time within thedifferential function. As soon as all three fundamental frequency differential currentsare set above the set alarm level (IDiffAlarm), a threshold defined by setting parameterIDiffAlarm a delay on pickup timer is started. When the pre-set time, defined by settingparameter tAlarmDelay, has expired the differential current alarm is generated andoutput signal IDALARM is set to logical value one.6.1.3.4 Bias currentThe bias current is calculated as the highest current amongst all individual windingcurrent contributions, compensated for eventual power transformer phase shift andtransferred to the power transformer reference side. All individual winding currentcontributions are already referred to the power transformer winding one side (powertransformer HV winding) and therefore they can be compared regarding theirmagnitudes. There are six (or nine in case of three-winding transformer) contributionsto the total fundamental differential currents, which are the candidates for the commonbias current. The highest individual current contribution is taken as a common bias(restrain) current for all three phases. This "maximum principle" makes the differentialprotection more secure, with less risk to operate for external faults and in the same timebrings more meaning to the breakpoint settings of the operate - restrain characteristic.The magnitudes of the common bias (restrain) current expressed in the reference sideamperes can be read as service values from the function. At the same time it isavailable as outputs IBIAS from the differential protection function block. Thus, it canbe connected to the disturbance recorder and automatically recorded during anyexternal or internal fault condition.6.1.3.5 Elimination of zero sequence currentsThe zero sequence currents can be eliminated from the differential bias current on a perwinding basis via a parameter.Elimination of the zero sequence current component is necessary whenever:1MRK 502 043-UUS B Section 6Differential protection89Technical Manual
