● Common phase current inputs can be used.● It provides internal CT ratio mismatch compensation. It can match CT ratios up to 1:40 resulting flexibility insubstation design and reduced cost.● Advanced algorithms make the protection secure.With High Impedance REF, there is no bias characteristic, and the trip threshold is set to a constant level. However,the High Impedance differential technique ensures that the impedance of the circuit is sufficiently high such thatthe differential voltage under external fault conditions is lower than the voltage needed to drive differential currentthrough the device. This ensures stability against external fault conditions so the device will operate only for faultsoccurring inside the protected zone.High Impedance REF protection responds to a voltage across the differential junction points. During external faults,even with severe saturation of some of the CTs, the voltage does not rise above certain level, because the otherCTs will provide a lower-impedance path compared with the device input impedance. The principle has been usedfor more than half a century. Some advantages of using High Impedance REF are listed below:● It provides a simple proven algorithm, which is fast, robust and secure.● It is less sensitive to CT saturation.2.4.1 LOW IMPEDANCE REF PRINCIPLELow Impedance REF can be used for either delta windings or star windings in both solidly grounded and resistancegrounded systems. The connection to a modern IED is as follows:IEDV00679I Phase AI Phase BI Phase CI NeutralPhase APhase BPhase CIEDI Phase AI Phase BI Phase CPhase APhase BPhase CConnecting IED to star winding for LowImpedance REFConnecting IED to delta winding for LowImpedance REFFigure 90: Low Impedance REF Connection2.4.1.1 LOW IMPEDANCE BIAS CHARACTERISTICUsually, a triple slope biased characteristic is used as follows:P14x Chapter 7 - Restricted Earth Fault ProtectionP14xEd1-TM-EN-1 167
