Sepam - HV/MV Protection and control units (ENG) - Trinet

I >I >I >directional discriminationH1H2I > I >1In a looped system, in which faults are fedfrom both ends, it is necessary to use aprotection system that is sensitive to thedirection of the flow of fault current in orderto locate and clear it.Example of the use of directionalprotections:D1 and D2 are equipped with instantaneousdirectional protections; H1 and H2 areequipped with time-delayed overcurrentprotections.In the event of a fault at point 1 , only theprotections on D1 (directional), H1 and H2detect the fault. The protection on D2 doesnot detect it (because of the direction of itsdetection system). D1 breaks. The H2protection de-energizes and H1 breaks.t H1= t H2AIIt D1= t D2t H= t D+ ∆tD1D2way of detectionexample of use of directional protectionssélectivité par protectiondifférentielleIprotectedequipment∆IR shigh impedance differential protectiondiagramIprotectedequipmentI'I'These protections compare the current atthe ends of the monitored section of thesystem. Any difference in amplitude andphase between the currents indicates thepresence of a fault. This is a selfdisciminatingprotection system as it onlyreacts to faults within the area it covers andis insensitive to any faults outside this area.The protected equipment can be: a motor,an AC generator, a transformer, or aconnection (cable or line).This protection is used to :c detect fault currents lower than ratedcurrentc trip instantaneously since discimination isbased on detection and not on time delays.There are two main principles:The high impedance protective device isseries-connected with a stabilizationresistor (1) in the differential circuit.The percentage-based differential protectivedevice is connected separately to the I and I'current circuits.The difference between these currents I - I'is determined in the protective device andthe protection stability (1) is obtained by arestraint related to the measurement oflet-through current I + I' .2(1)The stability of the differential protectivedevice is its capacity to remain dropped outwhen there are no faults within the zonebeing protected, even if a differential currentis detected:v transformer magnetizing current,v line capacitive current,v error current due to saturation of thecurrent sensors.∆Ipercentage-based differential protectiondiagramMerlin Gerin protection guide 19