bias currentin per unitZero-sequencediff. currentin per unit012345IdMinSection 1OperateconditionallyI operateslope = ----------------* 100 %I restrainSec. 2 Section 3The characteristiccan be movedup and down(vertically)by changingthe setting IdMinEnd Section 1End Section 2IdMin range: 0.04 –1.00 IBaseReset Ratio in all sections:0.95 (a constant)Slope 70%RestrainSlope 100%Characteristic ifIdMin = 1.0 puIEC98000017-5-en.vsdDD0 1 2 3 4 5 6IEC98000017-5 V1 ENFigure 47: Operate - bias characteristic of the Restricted earth-fault protection,low impedance REFPDIF6.2.7.3 Calculation of differential current and bias currentThe differential current (operate current), as a fundamental frequency phasor, iscalculated as (with designations as in figure 45 and figure 46):0Idiff = IN + 3IIECEQUATION2417 V1 EN (Equation 26)where:IN current in the power transformer neutral as a fundamental frequency phasor,3I0 residual current of the power transformer line (terminal) currents as a phasor.The bias current is a measure (expressed internally as a true fundamental frequencycurrent in Amperes) of how difficult the conditions are under which the instrumentcurrent transformers operate. Dependent on the magnitude of the bias current, thecorresponding zone (section) of the operate-bias characteristic is applied, whendeciding whether to trip, or not to trip. In general, the higher the bias current, the higherthe differential current required to produce a trip.The bias current is the highest current of all separate input currents to REFPDIF, thatis, of current in phase L1, phase L2, phase L3, and the current in the neutral point(designated as IN in figure 45 and in figure 46).Section 6 1MRK 504 135-UEN ADifferential protection116Technical manual
