6.4.7.5 Algorithm of the restricted earth-fault protection1. Check if current in the neutral Ineutral (IN) is less than 50% of the base sensitivityIdmin. If yes, only service values are calculated, then REFPDIF algorithm is notused.2. If current in the Ineutral (IN) is more than 50% of Idmin, then determine the biascurrent Ibias.3. Determine the differential (operate) current Idiff as a phasor, and calculate itsmagnitude.4. Check if the point P(Ibias, Idiff) is above the operate-bias characteristic. If yes,increment the trip request counter by 1. If the point P(Ibias, Idiff) is found to bebelow the operate-bias characteristic, then the trip request counter is reset to zero.5. If the trip request counter is still zero, search for an eventual heavy external earthfault. The search is only made if the neutral current is at least 50% of the Idmincurrent. If an external earth fault has been detected, a flag is set which remains setuntil the external fault has been cleared. The external fault flag is reset to zerowhen Ineutral falls below 50% of the base sensitivity Idmin. Any search for anexternal fault is aborted if trip request counter is more than zero.6. As long as the external fault persists, an additional temporary trip condition isintroduced. This means that REFPDIF is temporarily desensitized.7. If point P(Ibias, Idiff) is found to be above the operate-bias characteristic), so thattrip request counter is more than zero, a directional check can be made. Thedirectional check is made only if Iresidual (3Io) is more than 3% of the IBasecurrent. If the result of the check means “external fault”, then the internal triprequest is reset. If the directional check cannot be executed, then direction is nolonger a condition for a trip.8. When neutral current, residual current and bias current are within some windowsand some timing criteria are fulfilled, the ratio of 2nd to fundamental harmonic iscalculated. If it is found to be above 60% the trip request counter is reset and TRIPremains zero.9. Finally, a check is made if the trip request counter is equal to, or higher than 2. Ifyes, and at the same instance of time tREFtrip, the actual bias current at this instanceof time tREFtrip is at least 50% of the highest bias current Ibiasmax (Ibiasmax is thehighest recording of any of the three phase currents measured during thedisturbance), then REFPDIF sets output TRIP to 1. If the counter is less than 2,the TRIP signal remains zero.6.4.8 Technical dataTable 68:REFPDIF technical dataFunction Range or value AccuracyOperate characteristic Adaptable ± 1.0% of Ir for I ≤ Ir± 1.0% of I for I > IrReset ratio >95% -Minimum pickup (4.0-100.0)% of IBase ± 1.0% of IrTable continues on next page1MRK502052-UEN B Section 6Differential protection175Technical manual
