CABLES

CABLESWhat are the purposes of cable trays?a.Avoid sagging of cablesb.Give mechanical supportWhat are the disadvantages of paper insulation?a.Absorbs moisture.b.Cable termination/sealing problems.What are the advantages of XLPE?a.Easy routing at heightsb.Easy maintenancec.Large current (90 C)d.No sheath (no fatigue)e.No paper tape wrapping techniquef.High dielectric strengthg.Very little deformation even at high temp,h.More rated current, overload, short ckt capacityi.Low tan delta and hence suits long routesj.Very lightk.Good mechanical properties. What is the specified cable life?50 years. How armours /sheaths grounded?

1 core cables -- sheaths/shields/armours grounded at one end only, other endinsulated to prevent the circulating current through sheaths.3 core cables -- grounded at both ends but not including core balance CTs, sinceeven small induced current causes 50N operation.Why armouring done?For mechanical strength, protect against damage by impact of an object.How the required conductor size can be reduced by use of HRC fuses?HRC fuse limits the peak amplitude of fault current. HRC fuse melts at prospectivecurrent but not allowWhat are the types of cables used in 6.6 kV and 415 V system voltages?In 6.6 kV system AC (unearthed) grade having stranded aluminium conductorscables are used. Their insulations are as follows.1. XLPE Crossed linked polyethylene insulation.s circuit to pass their high rupturing capacity that is its kA.2. FRLS PVC Fire retardant low smoke insulation with PVC inner and outerinsulation. In RB copper conductor stranded cables are used.3. FS fire survival insulation.4. HR PVC heat resistant insulation.In 415 V system 1100 V grade copper or aluminium stranded cables are used. Theirinsulations are HR PVC and FS type.At what temperature cables are rated?Normally cables are rated for 40CMaximum temp in CShort time temp in C

PVC70160

HRPVC85160

Fire survival90250

Silicon rubber90250

XLPE cable90250

Why 1.1 kV grade cables used for 415V?To take care of the both earthed / unearthed systems.While carrying out cable joints, why should we ensure the continuity of 1) metallisedpaper for PILC, 2) sheath and armour.Continuity of metallised paper ensures less voltage gradient, hence preventingpuncture of insulation.Continuity of sheath / armour ensures that grounding is maintained, so no overvoltage is induced, and easy to detect earthfault in cables.Why bimetallic washers provided in aluminium copper transition joints?To avoid galvanic corrosion failure.What is the type cable used in radiation areas?Mineral insulated (MI) cables.Power and control cables.1.Define conductor.Conductor: A material of low resistance used to transmit electrical energy. Examplewires, cables, bus bars etc.

2.Define unprotected insulated wire.Unprotected insulated wire: Unprotected insulated wire is which the insulation of the wire is not covered by a protective sheathing to prevent it from mechanical damage.

3.Define cable.Cable: One or more insulated conductor enclosed in a protective mechanicalsheathing of either GI wire or GI strip or aluminium to protect the insulation frommechanical damage.

4.Define insulated wire.Insulated wire: A conductor or multi-stranded conductor which has a insulatingmaterial on it is called a insulated wire.

5. Explain briefly about armoring for an under grounded cable.Armoring is required to protect the cores from mechanical damage.6. Explain briefly about grounding of cable trays.Cable trays are grounded because to avoid any shocks to personnel incase of leakage.A grounding wire runs at the side of tray through a parallel groove clamp through outthe length of the cable tray. If trays are one above another we can loop up thegrounding wire to the tray below. This saves extra ground wire.7.What are the differences between the power and control cable?

Power cable: It is used for supplying current to load. It is of larger current carryingcapacity available in single core, 2 cores, 3 cores, 3 | cores, and 4 cores. Single coreis available upto 1000 mm2. Usually power cables are of aluminium. These cablesare graded for higher voltages and possess more cross section area.Control cable: Control cables are used for control purposes for logics, indication orannunciation etc. These are of lower current carrying capacity and voltage grading isalso less. These are of less cross sectional area are available in pairs of 2,5,10,25,50etc.

8.What is the purpose of using corrosion inhibiting compound?

It is used for aluminium conductors while crimping to a lug or ferrule. It preventscorrosion of aluminium conductor due to oxidation and due to saline atmosphere.

9.Why aluminium armouring for single core 1000 mm2 is used?Aluminium armouring for single core 1000 mm2 is used so that heating will not take place due to the flux around the conductor, as the aluminium is a non-magnetic material.10. What is resistance of copper compare to the aluminium?Copper is less resistive than aluminium.11. What is applied over the steel tape armour of PILC cable as serving?Bituminous covered jute.12. What is used as insulation for PILC cable?Impregnated paper.13. A small quantity of impurity reduces how much of conductivity of copper?35% of conductivity will be reduced due a small impurity in the conductor.14. Why cast aluminium tri-foil clamp is used in single core cables laying?When three conductors are clamped together the fluxes around the conductors are getcancelled

15.What are the parts of a cable gland?

a.Check nut.b.Nipple.

c.Metal washer.d.Neoprene rubber.

e.Metal washer.f.Compression nut.

16.What are the advantages of PVC insulated cable?

a.Plumbing is not required. Joints can be made easily.

b.As PVC is light the injury caused to it while laying is less.

c.It is corrosion resistant.

d.It has high fire retarding property.

e.It does not break down even if moisture enters.

Miscellaneous (Tests on power cables)1.What are precautions to be taken while doing maintenance or repair work on powercables?

A research organised by EPRI (electric power research institute) on medium voltageXLPE cables found that DC high potential at 80% of the factory value

a.Subsequently reduces the life of the cable and

b.It did not identify significantly weakened cable.

Based on above research recommended maintenance proof test voltage = 60% of the

factory test voltage.

Maintenance and repair:

Before attempting for any corrective maintenance on power cables like replacement

of lugs or jointing of cables following should be taken care

a.IR value should be good and it should be comparable to the previous values.

b.The quality of joints should be such that it dies not add any resistance to thecircuit. Before jointing is commenced it is advisable that IR of both sections ofcable to be jointed be checked.

c.Before jointing a paper insulating cable (for PVC cables this step is not required),the paper insulation should be tested for the presence of moisture by immersion inhot compound for paraffin wax at a temperature between 120C and 140C. thepresence of moisture indicated by the formation of bubbles when a piece of thepaper is immersed in hot compound. Use only single strip of the paper.

d.In case dia of die to be used for crimping is slightly more than dia of cables, thenuse some loose strand before doing crimping of lug. While crimping it should beensured that homogeneity of cramped conductor strands is achieved otherwise itwill add resistance and create over heating.2.What are the tests to be carried out during DC high voltage test on power cables?DC high voltage testing.During DC high voltage testing flow of following currents will take place.

a.Capacitance charging current.

b.Dielectric absorption current.

c.Surface leakage current.

d.Partial discharge current (corona).

e.Volumetric leakage current.

a. Capacitance charging current.

The capacitance charging current is high as the DC high potential is applied and canbe calculated by the formula

ig = E twhere ig capacitance charging current.

rc / RE voltage in kilovolts.

r resistance in mega ohms.c capacitance in micro farads.t time in seconds.The charging current is a function of time and will decrease as the time of theapplication of voltage increases. It is the initial charging current when voltage isapplied and therefore not of any value for test evaluation. Test readings should betaken until this current has decreased to a sufficiently low value.

b.Dielectric absorption current.

The Dielectric absorption current is also high as the test voltage is applied anddecreases as the voltage applicable time increases. This current can be calculated bythe formulaia =VCDT-nWhere ia dielectric absorption current.

V test voltage in kilovolts.

C capacitance in micro farads.

D proportionately constant.

T time in seconds.

n constant.Again time should be allowed before recording test readings so that this current hasdecreased sufficiently.

c.Surface leakage.

The surface leakage current is due to the conduction on the surface of the insulationand not desired in test results and should therefore be eliminated by carefullycleaning the surface.

d.Partial discharge current.

The partial discharge current, also known as corona current is caused b ionization ofair due to high-test voltage. This current is not desirable and same is normallycontrolled by providing semi-conducting tape to separate the conductor frominsulation.

Semi-conducting tape is used to separate the conductor from the insulation to preventpossible damage of the insulation from the corona and ionization. The voltage maydevelop between stranded conductor and insulation, thereby causing the ionization ofair and breakdown of cable insulation. The application of semi-conducting smoothesthe voltage stress and keeps such voltage stress constant and to a minimum.

e.Volumetric leakage current.

The volumetric leakage current flows through the insulation volume itself. This is thecurrent that is used to evaluate the condition of the insulation under test. Sufficienttime should be allowed for the volumetric current to stabilize before test readings arerecorded.

1. What is good dielectric break down value for insulating oil?60 kV.2. What is the temperature coefficient of insulating materials?Insulators are negative temperature coefficient materials.3. What is gap between the electrodes in transformer oil testing kit?0.1 Inch.4. What is the life insulation if temperature increased by 10C?The life of the machine insulation decreases by half if the temperature of the insulation increases by 10C.5. What is the value of vacuum maintained by vacuum pump in oil filteration machine?27 Hg.6. What is the DC HV test voltage range?1.7* 1.5* rated voltage.7.What do you mean by term insulating resistance? How it is measured?Insulating resistance: insulating resistance is the opposition offered by an insulatingmaterial to the flow of current (electrons) through it when an high potential isapplied across it.Insulating resistance are measured by megger.First the equipment whose resistance is to be measured is disconnected from supply.If the machine is a large one, there may be accumulated static charge on the machine.So we have to discharge it by connecting a wire between the terminals and groundfor 15 minutes. Otherwise megger will give wrong reading.After this we should remove the wire and we have to connect megger terminals (live& earth) to the motor terminal and earth. The rating of the megger should be selectedb properly. Then rotate the megger at rated speed of 160 rpm and take the readings.8. What you mean by dielectric absorption test?Whenever we apply a potential from the megger to test the IR value, initially theneedle of the megger will go to low value of the resistance. This is due thecapacitance effect of the insulation material and after some seconds the needle willstart moving towards the higher value. Because in the insulating material there isstrain on the molecules when the potential is applied. Polarization of the moleculesoccurs and they form a Di pole. The negative charges are attracted to positiveterminal and positive charges are attracted to negative terminal. So there is a strainon the insulation molecules and they align themselves parallel. This aligning maytake more time. This test is done to know the condition of insulating material.If the insulation is good the graph is as shown as B and if there is dirt, moisture thegraph will flatten early as shown in A.After the test terminals to be discharged so that molecules may return to theirunstressed state.I

Time 9. Draw the transformer drying out curve and explain each stage.At this point the heaters areSwitched off.Time in hours When we start the filtering process initially the temperature will be low, as theinsulation value is high. But on temperature increases the IR value starts to decreasebecause the moisture entrapped in the coils are released due to rise in temperatureand this causes the IR value to go down. This is the first stage.Then comes the point where all the moisture is released and then will be no decreasein IR value or rise in the temperature. This is the second stage.At this point the heaters are switched off. Now the moisture is removed by the oilfilters and the IR value goes up and as the heaters are off the temperature decreases.This is the third stage.10. The insulation resistance of a DC motor is observed to be 15 M at a temp. of 70C.what is its value corrected to 40C. the correction factor for 70C is 8.0.Observed resistance at 70C - 15 M.Temperature correction factor - 8.Rm = kt * Rtkt - correction factor.Rm = 8 * 15Rt - resistance measured at +C.Rm = 120 M.Rm - corrected value to 40CThe IR of DC motor corrected to 40C is 120 M.11. The armature of a 600 kW, 0.24 k, 1000 rpm DC generator has an indicated IR toground of 2 M at a temp. of 30C. what is the recommended value of insulation? Isit advisable to put the machine in service? Give reason. Correction factor for 30C is0.5.Data given arekV 0.24Indicated IR 2MTemp. - 30CCorrection factor 0.5Recommended value (Rm) = kV + 1 M= 0.24 + 1= 1.24 MIndicated IR at 30C = 2MCorrection factor 0.5So value corrected to 40C = Rm = kt * Rt= 0.5 * 2= 1MThe generator cannot be put in service because the corrected value is lesser thanrecommended value. It should be sent for IR re-conditioning .