Lets analyse phase-to-earth faults in a single-incomer power systemPostedOCT 17 2014byEDVARDinENERGY AND POWER,PROTECTIONwith5 COMMENTS

Lets analyse phase-to-earth faults in a single-incomer power systemDetect and isolatePower-system protection shoulddetect faultsandisolate the faulty partsof the power system while keeping the fault-free parts in operation.Protection units are chosen according to the power-system configuration (parallel operation of generators or transformers, loop or radial power system, neutral earthing arrangement).Consideration must be given to:1. Earth fault protection, linked to the neutral earthing arrangement:-Resistance earthing on the transformer-Resistance-earthed neutral at busbars-Reactance-earthed neutral-Isolated neutral-Solidly earthed neutral-Compensated neutral2. Phase-to-phase fault protectionOk, this time Ill analyse earth fault protection in a single-incomer power system

Resistance earthing on the transformerEarth fault protection units (ANSI 51N) are installed on the feeders, incomer andneutral earthing connection.

Figure 1 Phase-to-earth fault protection (resistance-earthed neutral at transformer)

Time-based discrimination is used between the different protection units.These units are necessarily different from phase fault protection units since the faultcurrents are in a different range.Thefeeder protection unitsare set selectively in relation to the incomer protectionunit, which is itself set selectively in relation to the neutral earthing protection unit(in accordance with discrimination intervals).The fault current flows through the capacitances of the fault-free feedersand the earthing resistance.All the fault-free feeder sensors detect capacitive current.To prevent inadvertent tripping, the protection unit on each feeder is set higher thanthe feeders capacitive current. fault at 3:the D1 circuit breaker is tripped by the protection unit linked to it, fault at 4:the A circuit breaker is tripped by the incomer protection unit, fault at 5:the protection unit on the neutral earthing connection trips circuit breaker Hon the transformer primary circuit. (fig. 1).The protection unit atDmust be selective in relation to the downstream protection units:if the delay required for protectionAis too long, logic discrimination should be used.The neutral earthing protection unit atHacts as back-up should the incomerprotection unit atAfail to trip.The incomer protection unit atAacts as back-up should a feeder protection unitatDfail to trip.Go back to Index

Resistance-earthed neutral at busbarsA zero sequence generator is used for resistance-earthing.Earth fault protection units(ANSI 51G) are installed on the feeders, incomer and zerosequence generator.

Figure 2 Phase-to-earth fault protection (resistance-earthed neutral at busbars)

Time-based discriminationis used between the different protection units.The feeder protection units and incomer protection unit are set selectively in relationto the earthing impedance protection unit. As in the previous case, the protection uniton each feeder is set higher than the feeders capacitive current.In the event of a fault onfeeder 1, only theD1feeder circuit breaker trips.In the event of fault on thebusbars 2, only the protection unit on the earthingconnection detects the fault. It tripscircuit breaker A.In the event of fault on thetransformer secondary circuit 3, the incomer protectionunits detects the fault. It tripscircuit breaker H.NOTE //when circuit breaker A is open, the transformer secondary circuit neutralis isolated. It may be necessary to protect it by a neutral voltage displacementmeasurement (ANSI 59N).The zero sequence generator protection unit acts as back-up should the incomerprotection unit atAor a feeder protection unit atDfail to trip.If the conditionIsD > 1.3 Iccannot be satisfied for a feeder, a directional earth faultprotection unit may be used to discriminate between fault current and capacitivecurrent.Go back to Index

Neutral earthing arrangementReactance-earthed neutralThe same procedure is used as for resistance-earthing at the transformer or busbars.Go back to Index

Isolated neutralA fault, regardless of its location,produces current which flows through the capacitanceof the fault-free feeders.In industrial power systems, this current is generally weak (a few amperes), allowingoperations to carry on while the fault is being tracked.

Figure 3 Phase-to-earth fault protection (isolated neutral)

Time-based discrimination is used between the different protection units.The fault is detected by an insulation monitoring device or a neutral voltagedisplacement protection unit (ANSI 59N).When the total capacitive current of a power system is high (in the range of tenamperes), additional measures must be taken to quickly clear the fault.Directional earth fault protection can be used to selectively trip the fault feeder.Go back to Index

Solidly earthed neutralThis is similar to resistance-earthing at the transformer, but thecapacitive currentsare negligible compared to the fault current, so the protection function is simplerto implement.Go back to Index

Compensated neutralThe power system is earthed at the transformer.Faults are detected by a specificdirectional earth fault protection unit (ANSI 67NC), which monitors the active residualcurrent and recognizes faults during their initial transient phase.