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437

a . : : _ earth insingle core cables.'E cables.

OPEN 9

SHORTFig. Q. 385. Types of faults in power cables .

...... ""'"-'.Jl... Ul" ofLocating Cable Faults.

1.2Capacitance Bridge method1.4Radar method (Reflection or Pulse EchoMethod)

2.2Audio Frequency (Tone Tracing) method2.4ArcReflectionmethod

:::r::=:thod 2.6 Megger Two-End method.me methods oflocating cable fault._ cables is difficult as the measurements are influenced by the type ofcable,

3..:.ortCircuits: (cl)Lowfault Impedance (a2). High Impedance Fault_WOIIIUIIlJLL.. Short Circuits: (bI) Lowfault impedance (b2). High Impedance Fault

~onlinear Faults (High Impedance Fault): : : r : : : ! i l t ) is a conducting path between two separate conductors or between phase

.lIIlIlIfllUlllli~nl I l l J r ' " eutral, The conducting path (short) may be of low impedance or high: " . U I I t " . U , I ! ! i fj ! J l I ! ll l ll n m r r l l D . : : ! !fault)the conductor is physically broken and nocurrent flowsthrough that

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. . . .. .438

Nonlinear fault (high impedance fault) has shown unfaulted C;Ul.ll..o ....UIUIIII:'.shows a short circuit at higher voltage. Nonlinear fault are also known as '

Fault Location Methods. These have two broad categories(1) Terminal Methods and (2) Tracing Methods1. Terminal Methods. These involve measuring some electrical

conductor.Froin one of.the cable.terminals and comparing it with healthy conduClillnrThe terminal methods can be further subdivided into bridge me'

resonance methods. Terminal methods give approximate l o c a t i o r i o f the - - m 1 Wfrom the terminal of the cable but does not pinpoint ex.act fault location.

2. Tracer Methods involve sending electri,cal signal on the faulted caThe change in signal characteristics are traced along the cable length anG.One of the 'following methods : "

1. Tracing current method3. Impulse (thumper) voltage

2. Audio frequency (tone.4. Earth gradient method

~ Q. 387.2. Descrjbe the steps to be taken after occurrence faultvoltage cable.

Strategy to Locate and Repair Faulty Underground Cable,The long underground-buried higb voltage cables are laid between ' > L L I . J l Q l " " ' ' '' ' '. ' '

transformer/receiving substation. A fault may-occur in the cable. The faultas the cable is b'uded. . . "

Locating a fault quickly 'and repairing/replacing cable, must be,donethis a well-practiced strategy is usually evolved by 0 &,M cable crew.

, ! ,~e strategic steps in identifying and locating cable faults are :1. Conduct open circuit voltage tests on three phases and neutral!of fault. (Phase to phase ?'Phas,e to ground? Open circuit ?

",2. Conduct Meggei"Test. (high resistance fault ? Low Resistance Fa3. Locate the fault.4. Analyze the fault,5. Take RepairlReplacement Actions.6. Take Preventive Actions for future.

Q. 387.3 Explain how to check Cable fault by means of a MeggerMegger test is. conducted from each end of thecable as shown in Fig. 3 8 ' ~ , , ; J l j U i lRefer Fig. 387.3(a) Megger test from end B on conductor to earth rn.rJj j I lQI i

(Insulation Resistance) indicating a possible open conductor in that phase.C ON D U C T OR,

, (a) Short circuit ' :UHI!!I IINSULATION

,MEGGtRr- READSOPENCIR"UIT INFIN lTVFAULT

Fig. Q. 387.3(a), Checking by Megger.- - . . . . . -- -- - -- -

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439INSULATION SHEATH (EARTHED)

FZ~~~~~ZZ~~2Z~B , I j "

MEGGERREADS20-200MIl

:::lbeckingby Megger high resistance fault detected from both ends.o check high resistance nonlinear cable fault by VII method.

: 3 . 3 : Iow resistance to high voltage and open circuit to low voltage Megger:.....:::It.pplication ofhigh voltage V drives current I through the fault (high

oflow voltage V does not produce any current through nonlinear fault

(A) HIGH V - = CURRENT I flOWS(B) LOW v = OPEN CIRCUIT-NO CURRENT

!I. Checking high resistance nonlinear cable fault by VII method._.,......,...vLoop Method for locating cable faults.

:;;::::_t common bridge method for locating shunt cable faults. Murray Loop,. iiults (Short circuits between conductor earth or two conductors) and if. able. The fault (short circuit) should be o f medium low resistance beloww applicable for open circuit faults._~dge has 4 arms RaJ Rb, RCJRd. Three arms are known and the 4t h armr__e is balanced by adjusting two known arms RaJRb. The unknown=Rd'Rd. Therefore, Rd = n, (RJ R,)

:iihi:;:ual cable resistance per meter must be known. A conducting jumper of, oe installed at the far end ofthe cable between the good conductor and

JUMPER

:::.op Bridge isnductor C and'"Terminal of then . . . , . The loop of the: nductor form the': e 4 arms of the

"11I"lIiIIIIanlil~:e .ort. (2) Resistance A,",_IIIIr'lIili'-J-.::gth L ami (4) Faultedl U l l . :: : . with fault at distanceulll!!l~ when galvanometer G shows null point ..

Fig. Q. 388. Murray Loop method for locating fault in apower cable.

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440 Operation andBalance is achieved by adjusting ratio of.resistances BfA. When theX = 2XL [(A)/(A +B)]X is the unknown length resistance between the measurement termllmll&llli.,I.B, A, G Testing bridge... A Healthy conductor

'. B Faulty conductor in the same cable.XDistance of fault from the Test TerminalQ. 389. Describe Capacitance Bridge Method for locating ea' ..Application. The capacitance bridge method is applicable for loc~l111I!;':".

Mega-Ohms) shunt faults. CapacitanceBridge Method is useful only(short circuits between conductor earth or two conductors) and if one h e : , , ! ! l 1 t : I I I I \ \ ' I l I I I

The fault {short circuit should be of high resistance above 200Method is aiso applicable for open circuit faults (series faults).

Principle and Procedure.The capacitance bridge technique simply

measures the capacitance from one end of thefaulted cable to ground and compares it in terms.

Ofthe distance with the capacitance ofthe healthy(unfaulted) conductor in the same cable Refer theschematic diagram for a capacitance bridge test. Thecable under test is disconnected from the network fromboth the ends, A typical bridge has 4 arms. Byadjusting two known armsR2, Rl, Capacitance Cs, theunknown capacitance ofthe cable is measured. Thetest has two steps as under:

nov50Hzif'

Fig. Q. 389. Capa..t1llitimllllll~

Test A : Capacitance offaulted cable to ground = C1IlFTest B .:Capacitance of faulted cable to Ground = = C2 I l FCapacitance is proportional to the cable length.The ratio of C1/C2 is equal to XI L. Where X is the distance offinlllllllll '

measurement in test L is the total length of healthy cable under test m . 1 t t 1 w ~ 1 1 ,Q. 390. Descnibe Charging Current Method for locatingca . i f u e : ' I f f l m \ l l ,Charging current method is alternative to the

Capacitance Bridge Method. Charging currents arecompared instead of capacitance. When anunloaded cable is energized by AC high Voltage,continuous AC charging currents flow from supply. to cable insulation. The charging current isProportional to cable length.

The capacitance charging current (IeI) of thefaulty conductor of the cable is compared with thecapacitance charging current of the heal thy(unfaulted) conductor of the same cable. Testvoltage is 50 Hz sinusoidal voltage in the range of. 1 kV to several kVs corresponding to the rated cablevoltage.

VARIAC

A. Test! lWI'i i l l lhl l IIUB. Test:! 1 W I 1 1 l m h l ,

Fig. Q . 390. Cl t3.! ,~~n l ll l :' II l ii l ll l ll l li l l I

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-~ - -- ------~ -----~ ---= --.,.441

s given by the expression L1 = L2 (Ic IIIe2) where Ie 1is the current in:mrent in the unfaulted cable, and L2 is the length of the unfaulted_ . Q l I I : S ofBridge Methods for locating Cable faults.

limitations:he accessible from both ends.

must be of same size and length::a T-off cable circuits.Method (reflection or pulse-echo method) oflocating Cable

IJIb."r.",;hi.nn orpulse-echo method) test-pulse is transmitted along the cablerr:- acted from the open end or fault point with. Both the transmitted2"8 recorded on- .The time taken TRANSMITTED."/' ~ /7""""'"

__ ~~_~ ~~~-L~ __ ~~~I L____OSCILLOGRAPH I OPEN IN TfIE

CABLE

TRAN SMITTEDPULSE SPLICES

REFLECTEDPULSE

Q. 392. Typical waveforms on oscilloscope in Radarmethod(a) Open circuit fault (b) Short circuit fault .

ected pulse is seen on the bottom side ofbase line as shown in Fig. (b).: High resistance fault in which current is obtained only at high-test

raaar method is not directly suitable. However, the nonlinear fault is, fault value for applying the radar method.2:31culatedas follows. The separation of the two signals is measured on

1II!;~:a1'atl.onx and multiplied by the scope calibration constant k to give the

refieeted wave (L) can be expressed in terms of the amplitude of!l..o.""T'ClUtonstants as follows:

' :J lJ lf fi ll iH I ~ , w ' l l l I f t l t l l l i m m n l l l _~._ l l In\U l lmI lUJ f u l J l l l t ; : j [ l l n n l l l l i W I I l T l f ! t 1

l II fu \ l\ II II I~ w m I I 1 Il '( 1 I

R-Zt,= R + zItline conductor upto the faultthe line conductor up to the fault.

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. . . . . _

442 Operation and Maintenance of ElPolarity of Reflected Wave.(a) If the line is open circuited, R = 00 and the reflected wave is + I;=+L:Therefore the reflected. wave is of equal magnitude and same polarity as the(b) Ifthe line is short circuited, R = 0 and the reflected wave is I; = - It,Therefore the reflected wave is ofequal magnitude and opposite polarity as tffi::;;

wave,Types of Pulses. Pulses employed are either (1) Short Duration Pulse or (2) ~~, The short duration pulse method is more commonly used for 'locating faul

The pulse duration is short in comparison to time t.The pulse width should be adequate enough for observing on the oscillos

method employs long step pulses as compared to the transit time ofsignal fromto the faultand back.

Q ', 393, Explain the Resonance Method of locating faults in PowerResonance Method of locating faults in Power Cable

'Applic;ltion. This method can be used on all types of cables and workssystems: It is not very suitable for:nonlinear faults,

Prin,ciple. The resonance technique is based on the principleof wavethevoltage changes rapidly from normal frequency voltage for nonresonance C0I1CJ:~111l IIfrequency indicates the length of cable between the terminal and the fault: I ~ _ thea frequency generator (or oscillator) is connected to the end of the faulty cablefrequency is varied until resonance occurs.

Under Resonant Condition, the voltage decreases for open circuit faultsAlternative 1Open Circuit Fault LocationN = Number ofhalf wavelength. Normally half waveresonance is used for'

Faults, in which case N = 2, The voltage changes rapidly from voltage at the nOI1re!!11IIIAlternative 2. Short circuit condition. 'Under Resonance Condition, the voltage increases for short-circuit faults,'_Calculation. The resonant frequency is inversely proportional to the W Q ! v e . i l! l lf l ll , w I

D ofthe fault can be determined by the expressionD =Vllfr.N]

where, N = number of quarter of half wavelength,N = 2 for Open circuit faults and N = 4 for Short Circuit Faults,f r , = Resonance frequency in Mz; K = Dielectric constant of cable,D = Distance to fault, V = Propagation velocity

The minimum frequency required is determined by the cable length,frequency is determined by the distance to the nearest point at which a fault ~cable the phase shift governs the speed ofthe transmitted and reflected wave __The phase shift is a (unction of the dielectric constant of the insulating rnapropagation of bare conductors, which is 107 mlus, the velocity for cable wiMzus; Di'electric' constant of insulation system in the cable must be known.

Q. 394. State the difference between Tracing (Tracking) Method a . . i . . l 1 > " " , ' " " " " ' "or Radar Methods., _

I: _ _ TONE OBTArNEDr ----A... _I-- ._.. / - - -I TRACKrNG - - - - = - - -~START ' MI!.GI>iETIC FI:~~~TAL

FJg" Q , 3;6. Fault ira~ki~: ba:Pacltor-D' ' .,.. .' ,g Y. '" IScharge 'TIL1 l J I I I 1 I ! ! ' -vd a Dc ..', ,I.:~lUI1Icliarged,' ,rn.:The' .: c a p a C I t o rI . . . , pulse creat ''I"QJlg 1S 't ., '_ .es an...". III ermlttent a"I..... 'Ugh th" . . ue to,reIe !ault arc T ' L ", .. 11eenerg

. "In Fault Tracing techniques current/voltage is injected into the cable, '] I)IU' ncable differs from, healthy cable. ' .-

In Terminal Techniques, the parameter offaultedc~bl~ are m~asured fromThe faulty cabie readings are compared with healthy cable readings.

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443(Tracking) Methods.

the path or line taken by the fault current Tracking or Tracing iseng the length of line step by step.:urrent tracing methods fault current is injected faulty conductor andved to selected manholes along cable length for measurement. Thethe measurement, traced (tracked) current at selected menhole

. " . " j, . ,. .. J L L . .. ( . . . . ., ' " fault resistance is low or the voltage of the test set is high. through the faulted conduct-or. This method is.suitable for cablesintermediate manholes. Afterlocatingthe faulty section, the entireconsecutive manholes must be replaced. The return current flowsand not though the neutral wire.

"""" ......ULLof Fault Tracking by Magnetic Probe.current is injected in the faulty cable and the ground return.

e conductor has associated magnetic field, encircling the conductorthe current flow. The magnetic field is in air and ground. The magneticcan be sensed by a simple magnetic probeantenna coil moved over the

_".'.~'~' orientations.of the magnetic field is maximum above the cable and changes

, : " . . . u t location. As the probe is moved along the length of the faultedfor exploring the sudden change in the magnetic field. The fault

:- the sudden change in the horizontal component of magnetic fieldzhe vicinity ofthe fault.

1 I I I 1 ! n . , " _ . " t . o " . , stie change suddenly at the fault beca~se current does not flow.:;-rizontally polarized antenna output fall off rapidly at fault location.sf the magnetic field does not change significantly .. of magnetic field is minimum above the cable and does not changefault location. ~t the location offault and remains almost constant

R F MAGNETIC FIELDABOVE EARTH

AULTYCABLECPNDUCTORUNDERGHOUND(LOAD 51 DEDISCONNECTED)

CHANG EIN TONETONE OBTAINED r"'---. HEAD PHONE_ _ _ _ _ _ . 1 0 . . . ~ ~ /' DETECTOR....... __.. ....... -.. ---. b dRACKING -- -y----/

NO TONE.DE1'"EcTED -RbBE; FOR HOO!ZONTA.L.P' T MA,Gf>lETIC FI;ELo

]i".g Q. 396. Fault tracking by magnetic probe method. .__ .,amcito,r_ Discharge ';rhump-Surge Detection Method .

" I t' !I I II I Il l' J I! :d l ll lm l l r ! a DC chargedcapacitor injects a high energy pulse into the faulted. The 'pulse: creates an arc through the fault and emits heats. the

rr . I1 : u :g _ : , intermittent due to repeated charging and discharging of charge QI l f f i m m m ' " the fault arc. The energy is released into the arc in audible thumps.

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444 Operation and Maintenance

. .. . .The fault located b y detecting the acoustic thump by Audio. Micro ~ n n lmagnetic thump by means of magnetic probe. The microphone detector errmoved over the cable for tracking the.fault.

CHARGINGDC PRE-CHARGED DC

/ ENERGY STO RAGE CAPACITOR~y"RCU" 6RmER' CLOSER

INTERMITTENTDISCHARGE CAUS IMIIliAU0 10 i.e MAG 'IttTHUMP"TRACKINGALONG

CABLE LENGTHFig. Q. 397. Capacitor Discharge Thump-Surge Tracking Me 15,

Q. 398. State applications ofHVDC Cables and describe their ._'u.....,.._HVDC cables are necessary for high power long distance transmissio- HVDC submarine under-sea cables between generating end and'- HVDC under-sea cable for interconnection between two Nationalor lake (e.g. HVDC Interconnection between France and UK)- HVDC under-see cable between offshore ocean energy power plant.Dlllu

Conventional long high voltage AC power cables are not suitable in ananlWlIl!,,"overheating by continuous high charging currents. HVDC'cables have no ~ l . J I'WW . . i l l l l lConstruction of an HVDC Cable:7

8 2 1 Conductor (copper)9 3 3 Insulation-oil impregnated4 Screen (shield)10 6 Plastic sheath(, 8 Galvanised steel tape" 5 10Galvanised steel wire12Corrosion protection and i i l e l l l l W n l l l l l12

Fig ..Q . 398. Cross section ofan HVDC cable.1. Conductor is made up of high conductivity electrolytic copper. 1 i ' f ' 1 I I I I ! ) H n l msector shaped with a hollow duct at the centre.2. The stranded conductor is provided with external conductive layer3. The insulation is of impregnated paper layers.4. Screen (shield) is provided over the insulation is of metal tape. T~r:.m l / m l : ::electric field uniformly.5. Lead sheath over the screen gives mechanical protection to the ins m i l l i i u u c 11116. Plastic sheath over the lead sheath gives protection against corros!:lI!lll,ilUllltll7. Cotton tapes over (6) give flexible cushion between plastic sheath callwarmour.8. Galuanised steel armour gives mechanical protection which is fleribJllwll"9. External serving gives protection to the armour and prevents wati&l'ililrulliOllll

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