The IT earthing system (unearthed neutral) in LV

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The principle consists of measuring andcomparing voltages between the (+) polarity andthe earth, on the one hand, and the (-) polarityand the earth on the other. This principle makesauxiliary power sources unnecessary, since thenetwork supplies the PIM directly viameasurement sensors (resistances).This technique applies to two-phase AC and DCnetworks and does not allow live fault tracking.For AC networksThe most commonly used PIM are those withinsulation measurement by DC current injection.Permanent measurement of insulationresistance required use of active systems inplace of the previously used passive systems.This resistance can be measured accurately inDC (see fig. 11 ), which is why the first PIMs,placed between the network and the earth,injected a low DC current which flowed throughthe fault. This simple, reliable technique is stillextensively used today, but does not allow livefault tracking.Note that when these PIMs are used on mixednetworks (containing rectifiers without galvanicinsulation), they may be disturbed or evenVNI PIM321NPEI PIMR BFig. 11 : principle of PIM with current injection.“blinded” if a fault is present on the DC part ofthe network.These were followed by PIMs with AC currentinjection at low frequency (< 10 Hz), operatingon the same principle. Although these PIMsallow live fault tracking, they can be “misled” bycable capacities that are seen as insulationfaults and disturbed by frequency converters(variable speed controllers).For all AC and DC networksFinally, nowadays, given that networks arefrequently of the mixed AC/DC kind as well asvariable frequency, the new PIMs are able tomonitor insulation on all types of networks.c Some use squared wave pulses at very lowfrequency (≈ 1Hz). They allow PIM not to bedisturbed by earth leakage capacities, as theyare then immediately loaded then unloaded bythe next strobe pulse of opposite sign. They areuniversal in use and easily adapted to modernnetworks, in particular to those supplying powerelectronic devices which often deform the ACpulse. However, their response time, dependingon the network’s earth leakage capacity, may beas much as a few minutes and does not allowthe detection of intermittent faults.c In order to compensate the usage restrictionsof these PIMs for very long networks andnetworks with a large number of capacitivefilters, the low frequency AC current injectiontechnique has been improved by means of“synchronous demodulation” (see fig. 12 ): thistype of PIM applies a low frequency AC voltagebetween the network and the earth, measuresthe current flowing back via network insulationimpedance and calculates the voltage-currentshift.It is then possible to determine the resistive andcapacitive components of this current and thusrelate the threshold to the resistive componentonly. This upgrade, the result of digitaltechnology, combines the advantages of DCcurrent and low frequency AC current injectionwithout their disadvantages.I BFBF~mAVI R-BF= U BFU BFR NetworkI C-BFC NetworkI C-BFI BFZ NetworkI R-BFFig. 12 : the low frequency AC current injection technique has been improved by means of “synchronousdemodulation”, which enables the insulation drop (resistive leakage) to be distinguished from capacitive leakage.Cahier Technique Schneider Electric no. 178 / p.12
PIM standardsc The manufacturing standardsStandard IEC 61557-8 is in existence sinceFebruary 1997. It defines the specialspecifications governing insulation monitorsdesigned for permanent monitoring, irrespectiveof the measurement principle, of insulationresistance with respect to earth of unearthed ACand DC IT system networks, and of AC ITsystem networks containing rectifiers suppliedwithout galvanic separation (transformer withseparate windings).It places particular emphasis on three points.v Properly inform specifiers and contractors. Themanufacturer must provide the characteristics ofthe devices he produces and in particular thosethat are dependent on network capacity(response time and threshold values).v Ensure that these devices are properlyintegrated in their electrical environment. Thisrequires compliance with the specifications ofstandards IEC 61326-1 and 61326-10concerning ElectroMagnetic Compatibility(EMC).v Guarantee operating safety for users.The main stipulations are: device operatingtesting must be possible without inserting anadditional impedance between the monitorednetwork and the earth, settings must beprotected to prevent modification by error or byunauthorised users, and impossibility of devicedisconnection (the need to use a tool fordisassembly).c The operating standardsAs concerns PIM setting, standard IEC 60364provides an initial answer: “A PIM designedaccording to… is set at a value less than theminimum value of the insulation resistancedefined for the installation in question”, i.e.greater than or equal to 0.5 MΩ for a circuit witha nominal voltage greater than or equal to 500 V.Guide NF C 15-100 states: “…set at a valueroughly less than 20% of the resistance of theinstallation as a whole…”However, a clear distinction must be madebetween the insulation resistance of theinstallation, which only takes electricaldistribution into account, and the insulation levelwhich is set for overall network monitoring,including the various machines and switchgearconnected to it.In the previous chapter we saw that for faultsgreater than 500 Ω, contact voltage does notexceed 5 V with an earth connection of 10 Ω(see fig. 5). In practice, for a normal industrialinstallation, it is thus reasonable, without takingrisks, to set the lower alarm threshold at a valueof between 500 Ω and 1,000 Ω, ensuringeffective fault tracking (and thus location of thereported insulation fault). To organise preventivetracking, it is useful to have a first level thresholdaround 10 kΩ for example. This threshold mustbe adapted according to installationcharacteristics and operating requirements. Notethat short networks allow a higher preventionthreshold.2.3 Tracking the 1 st insulation faultWhen tracking a fault, although certain operatorsmerely identify the faulty feeder, accuratedetermination of the location of this fault isrecommended (e.g. damaged cable or insulationfault in a device) in order to put it right as quicklyas possible.Tracking by successive de-energisation offeedersThis means of fault tracking is quoted formemory only. It consists of opening the feedersone by one, beginning with the main feeders.When the faulty feeder is opened, the currentinjected by the PIM decreases markedly anddrops below the detection threshold. The audiblealarm normally controlled by the PIM then stops,enabling remote identification of the faultyfeeder.This procedure, which requires interruption ofoperation on each feeder, is contrary to theoperating philosophy of the IT earthing system,which stipulates continuity of supply. Althoughfrequently used in the past, it is graduallydisappearing with the development of the newfault tracking systems which allow live tracking(without power breaking).Live trackingc Detecting the fault currentAs seen above (see fig. 3a), a current I d flowsthrough the first insulation fault at the samefrequency as that of the network (50 Hz or60 Hz), returning to the source via the capacitiesof the other sound phases and via the neutralimpedance if any.An initial live tracking method (withoutinterrupting distribution) consisted of using aclamp-on probe to measure the earth “leakage”current on each feeder. The faulty feeder wasthe one on which the highest value wasmeasured.This method has two drawbacks, namely:v It is not reliable for networks with a largenumber of feeders some of which are highlycapacitive (how can the earth current of a shortfaulty feeder be distinguished from that of a longcapacitive feeder?).Cahier Technique Schneider Electric no. 178 / p.13
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