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Insulation Monitoring and Earth Fault Detectionin 3 Phase 3 Wire Unearthed Systems

- Ms. Aditi RathiSwitchgear Product Engineering

An accident poses danger for peopleand property. All factors leading to anyaccident are thus a cause of concernand attention. Insulation failure in anelectric circuit is one such veryimportant factor posing a serious safetythreat. An insulation failure or an earthfault may lead to fatal injuries topeople, arcing leading to fire andinterruption of supplies and operations.According to IEC479, a current of only500mA magnitude can cause fatalcardiac arrests to humans. In the eventof an electric shock, the path ofcurrent flow through a human body, isalways through its heart.

There are various factors leading toinsulation deterioration: electricalinfluences like over voltage, overcurrent, lightening, magnetic andinductive influences; mechanicalinfluences like shock, impulse,breakage, puncture due to ingressof nails, vibration; environmentalinfluences like moisture, temperature,pollution, chemicals, ageing; otherinfluences like biting by rodents.

Looking at the severe consequencesof an insulation failure, it becomesimperative to provide protection to menand material. The best protection canbe provided if there can be apro-active approach to forewarn animpending fault. This will allow time totake actions to prevent the occurrenceof a severe fault. This is the leadprinciple followed in all L&T engineeredInsulation Monitoring Systems.

Types of electricaldistribution systems:

Based on the type of earthing, thereare three types of distribution systems:TT, TN and IT. The first letterindicates the condition of earthing ofthe current source. T stands for directearthing of one point of the currentsource. I stands for either isolation ofall active parts from earth orconnection of one point to earth via animpedance. The second letter indicatesthe earthing condition of the body ofthe electrical installation. T means thatthe exposed conductive parts are

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Safety of operating personnelis of utmost importance andinsulation of electrical systemsplays a major role in decidingthe reliability of the system.Deterioration of insulation isnatural as the system ages.Hence, the occurrence of earthfault also is eventual. Whatis important is detection ofearth fault and monitoring ofinsulation.

In different types of electricalsystems i.e. TT, TN and IT; thetypes of earth faults and theirdetection techniques differ. Overa period of time, with theadvent of microprocessor, thesetechniques and the devicesused for insulation monitoringhave undergone a lot ofchanges.

In this issue of L&T CurrentTrends, we are discussingthe various types of electricalsystems, and the insulation faultdetection techniques employed,mainly in a 3 phase 3 wireunearthed system.

earthed, independent of any part ofthe current source that might exist. Nmeans that the exposed conductiveparts are directly connected with the

earthing point of the current source; inAC systems the earth point isgenerally the neutral point.

Hence the TT system (Fig 1) is thesystem with one of the points of thecurrent source solidly earthed.Exposed conductive parts areconnected to the earth independent ofthe earthing at the current source. Itis typically a 3 Phase 4 Wire systemlike the domestic distribution up to ourhouses.

The TN system, typically a 3 Phase 5Wire system, is the system with oneof the points (usually the neutral)solidly grounded. Also there isprotective earth (PE) conductor that isrun from the source. All the exposedconductive parts are connectedseparately to the source earth and theprotective earth.

In both TT and TN systems, an earthfault on any of the phases is a line-to-ground (LG) fault. The protections areprovided via a short circuit protectiondevice (SCPD), an over-currentprotective device (OCPD) or a residualcurrent device or a RCD (consisting ofa core balance current transformer orCBCT that saturates with the zerosequence current and in turn gives atrip to isolate the fault). Based on themagnitude of the current to ground,either SCPD, OCPD or RCD operate.An insulation monitoring device (IMD)may also be provided to forewarnincreasing insulation deterioration. ThisIMD is usually a sensitive device thatcan sense very low leakage currents.

The IT system (Fig 2) is the unearthedsystem unlike the TT and theTN systems. The source is isolatedfrom earth and so are all conductive

parts of thesystem. In theevent of a faultto ground onone of thephases, thecurrent doesnot get a pathof return fromthe groundback into thesystem. Andhence, since

the circuit is not complete there is noleakage to ground. The first earth faultjust converts the unearthed systeminto an earthed system. Hence theSCPD, OCPD or RCD cannot operateto clear the fault. A second earth faulton another phase is equivalent to aLine- Line-Ground fault. Based on themagnitude of the current to ground,either SCPD, OCPD or RCD operate.In such systems, IMD may be usedto indicate the occurrence of the firstearth fault so that a catastrophic failureupon the occurrence of a second earthfault can be prevented. The IMDs forsuch a purpose employ principledifferent from the principle employedby the IMDs for the other two types ofsystems.

In all these systems, foot earthing orsafety earthing should invariably beprovided to the equipment.

Advantages of an IT system:

First of all, since the tripping devicesdo not operate upon the first earth faultthere is nointerruptionof supply.The use ofan IMD toindicate theoccurrence ofan impendingearth faultcan allowtime to takeactions toclear the first

earth fault and prevent a catastrophicfailure upon occurrence of a secondearth fault. Primarily due to this, ITsystems are employed for criticalplatforms like ships, submarines, offshore oil and gas platforms, nuclearreactors, etc. Also, since there is noconnection of the IT system to theearth, there is no leakage to earth evenwhen a human body lies on the pathof the fault to ground. Hence IT systemensures greater operator safety. Thereare a lot of international standards likeIEC, NES, etc. specifying regulationsfor electrical safety onboard suchplatforms.

Theoretically, on the occurrence of thefirst earth fault, there is no leakage toearth as there is no return path for theleakage current from the ground backinto the system. However practicallyevery system has a leakagecapacitance to earth owing to thecabling, the filter capacitors at the inputof various electronic equipment, etc.This leakage capacitance providesreturn path for the leakage current inthe event of the first earth fault. Usuallythe leakage capacitance is very lowleading to very high capacitiveimpedance and hence the value ofleakage current is very low. However ifthe system is very old and has a highvalue of leakage capacitance, thisleakage current value goes up.

Methods of InsulationMonitoring in IT systems -Passive Measurement:

Classically, passive methods likethree-voltmeter method or three

Fig 2

Fig 1

lamp method were used for ACsystems (Fig 3). In these methods,voltmeters or lamps respectively wereconnected across each of the threephases and the protective earth (eg.nearest hull point in case of a ship).An earth fault on any one of thephases led to reduced voltage withrespect to earth on that phase andincreased voltage with respect to earthon the other two phases. This wouldbe visible as a zero in the voltmeter ora reduced glow in the lamp of thephase with fault. The detection of earthfault by these methods depended uponthe perception and discretion of theoperator. Also, these methods couldnot give alarms and so the earth faultwould be detected only when theoperator would check for it.

Similar were the drawbacks with thevoltage shift method used for DCsystems (Fig 4). In these methods,voltmeters used to be connectedbetween both positive and negativelines and ground. In fault-freecondition, the voltage with respect toearth on both the voltmeters would benearly identical. In case of fault to earthon any one of the lines, the voltage inthat line with respect to earth wouldfall to nearly zero. Due to this, thesepassive-measuring principles for DCsystems failed to detect symmetricalor balanced faults. Hence the use ofpassive measuring principles wasbanned by the IEC. Instead, activemeasuring principles wererecommended.

Methods of InsulationMonitoring in IT systems -Active Measurement:

Active measuring methods aredesigned to detect even high ohmic

insulation faults(i.e. very lowleakage toground) andsymmet r i ca lf a u l t s .M o r e o v e r ,u n l i k econvent ionalp a s s i v esystems, theynot only allow

insulation monitoring but also allowinsulation measurement. Earlier,

megger was the only option forinsulation measurement. Unlikemegger, active measuring methodsallow online measurement of insulationi.e. insulation measurement with thesystem powered. To understand it refer(Fig 5).

The conventional method of activemeasurement inAC systemsinvolved a DCv o l t a g ei m p r e s s e dbetween themain electricalsystem andthe earth. Ami l l iammeterwould beconnected inseries with thisvoltage source.In a fault-free situation, there would beno return path from the IT system tothe ground for the current injected bythis source. But in case of an earthfault, the circuit would get completethrough the fault resulting in adeflection in the milliammeter.Thecurrent in the milliammeter would beinversely proportional to the insulationresistance of the primary electrical

system connected. However, inmodern electrical systems, impressionof DC voltage is susceptible to effectslike extraneous DC voltages in thesystem, excessive capacitive leakageto earth, variable low frequencies andharmonics. These effects distort theresults of this method. Hence idealactive measurements should employvoltage sources immune to suchdisturbances.

The new age technique involvesmicroprocessor-based devices

that work on anA d a p t i v eMeasurementPrinciple (AMP).The voltagei m p r e s s e dbetween thesystem and theearth is not asimple DCvoltage but anadaptive pulsethat is immune

to all such disturbances.

Refer (Fig 6). The superimposedextraneous DC (U

DC) forces the

impressed measuring voltage (UM) toshift around the zero point. This offsetis eliminated by injection of analternating pulse, sampling of bothnegative and positive halves in the

cycle and then subtracting themeasurements to eliminate U

DC and

get UM.

UM is an alternating pulsating voltagewith a continuously adaptive frequencythat adapts itself to charge the systemleakage capacitance (and dischargeduring the negative half of the cycle)first and then gives a faithful indicationof the leakage due to insulation failure

Fig 4

Fig 5

Fig 3

For further details on this subject, please contact:EBG - Electrical Systems & Equipment, Larsen & Toubro Limited, Saki-Vihar Road, P.O. Box 8901, Powai, Mumbai 400 072

Fax: 022-6705 1553 * E-mail: kotnisnd@Intebg.com

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ABCI

or earth fault. Typically the capacitivecharging time is τ = [RI II RE] x CE.Theoretically, the charging timeallotted to each pulse (phase A andphase B each) is τ x 5. Actually, thepulse keeps adapting its magnitudeand frequency to constantly gauge thecorrect system capacitance to earth.Disturbances due to variable lowfrequencies and harmonics areeliminated by use of dedicated internallow pass and digital filters.

The same AMP is adapted forinsulation measurement andmonitoring in 1Ph AC, 3Ph AC andDC systems. The microprocessor-based equipment also allows featureslike continuous monitoring, adjustablesettings and configurable alarms. Likeall other IMDs, the IMDs working onAMP also can monitor only theelectrical system galvanicallyconnected to it. Hence, for eachgalvanically isolated section, oneIMD is required. For interconnectedsystems as in ships, themicroprocessor based IMDs on theconnected sections can digitallyinterlock with each other to accept onlyone of them as the master.

Locating the fault:

The same AMP is extended to givean advanced method of fault location.The conventional method with thepassive methods of insulationmonitoring was to switch off the various

connected feeders one by one andobserve the change in the insulationmonitor. The insulation fault indicationwould go off when the faulty feederwould be disconnected. But thismethod required disconnection ofsupplies to various feeders includingeven the critical loads. While themodern technique of AMP allows aprovision of fault location withoutdisconnection i.e. a provision for onlinefault location. Once an insulation failureis detected with the help of the specialvoltage pulses generated by the IMD,the same IMD generates specialcurrent pulses IM proportional to theinsulation resistance and injects theminto the connected system. Since theearth is kept electrically continuous,these pulses find a return path to theirgenerator’s earth through the locationof earth fault.

In IT systems, on first earth fault, thecurrent will be largely decided by theseries resistance of the IMD ( R

I )

and

REIIC

E. IMD being a very high

impedance device, the fault currentwould be too small to be detected bya normal CBCT. Hence, the nature andmagnitude of IM is such that it iselectrically distinguishable from thesystem current and the fault current.Special CBCT kind of devices areemployed that saturate with only IMand thus are used to give indication atthe location where IM is detected i.e.,at the location of fault. These CBCTsneed to be clamped around cables of

all the phases feeding the connectedload to prevent from being saturatedby phase current. Moreover, since thecable armor is always earthed fromconsiderations of interference, theseCBCTs will always indicate an earthfault if encircled around an armoredcable. Hence, these special CBCTsshould always be used roundunarmored cables. The cables areusually unarmored at the entrance andexit of panels.

These special CBCT devices areavailable as both hand-held tongs andfixed units for permanent mounting.The operator can move around thepower distribution system with thehand-held tongs to locate the faultmanually. The fault location can bemade automatic by employing anetwork of fixed units communicatingwith a centralized unit that cancentrally display the location of thefault.

Conclusion:

Depending upon the complexity andimportance of the electrical systemto be monitored, three levels ofsolutions employing AMP can beoffered to detect the earth fault in ITsystems: continuous onlinemonitoring, continuous onlinemonitoring with manual fault locationand continuous online monitoring withautomatic fault location. In all modernelectrical IT systems for importantplatforms like ships, submarines,offshore oil and gas platforms, nuclearreactors, etc., fault location is highlyrecommended by various internationalstandards. If insulation faults are notlocated and removed, the insulationwill deteriorate further with time. Alow level of insulation may causeinterruption of supplies to criticalsystems onboard, may causeincreased costs and even accidents.Hence, for IT systems, it isrecommended to remove the very firstearth fault at the earliest.

Fig 6