Cable testing

MV

The details of Site Tests required for newly installed and old in service cable after repair, alteration are explained here.

L.V. POWER CABLE:

a) New LV Power Cables will be subjected at Manufacturers work to all routine tests in

accordance to IEC 245.2, 540, 332-1.

b) After laying underground or in cable trenches, the new cables should be tested by 1000 Volts

Megger each phase separately as IEC 540 and 227.

c) New LV Cables are tested by 2kV A.C. 60/50Hz for one minute each core separately while the

other cores grounded along with their sheather armour. If no collapse of voltage occur during

the test period then cables are considered passed the test.

d) L.V. Cable insulation resistance test by 2kV megger is also useful after A.C. H.V. Test.

The insulation resistance more than 100 M Ohms is acceptable.

However, actual insulation resistance cab be estimated approximately by following formula:

IR = (36.7/L) * (Log 10D) at 20oC

IR = Insulation Resistance in Megga Ohms

L = Length of Cable in Meters

D = Nominal Outer. Diameter of insulation

e) For old control and Power Cables 500V Megger Tet is enough and IR Value > 10 Megga

Ohms is acceptable.

f) Note the Insulation Resistance Value depends upon many factors like Temperature, Humidity,

Length of Cable,Type ofinsulating material and thickness of insulation. So, it varies from

cable to cable and time to time.

PILOT CABLE:

These cables are used for unit Protection of 33kV and 132kV underground Power Cables and

For Telecommunication and data transfer purpose for Substations to SCADA SYSTEM.

These cables are layed alongwith the respective power cables in the same trench. These cables are

required to be tested after jointing is completed and termination into the end boxes. So, following

tests are necessary to perform for newly layed Pilot Cables:

a) LOOP RESISTANCE TEST AND SHEATH CONTINUITY TEST:

Loop resistance must be checked for all pairs (usually 12 pairs).

Normally two sizes are used , one with resistance 19 Ohm/Km Loop and the

other with 58 Ohm/Km Loop at 20oC ambient temperature.

However, 5% variation in Loop R is acceptable. Actual Resistance/Km Loop should be referred

to Manufacturers data Sheet.

Sheath armour continuity check. Should also be made at same time.

b) CROSS TALK/COUPLING TEST:

Each pair in the Pilot Cable should be continued with its two wires coupled together by twisting

all along the route. If one of these two wires split in a joint by jointing mistake and go into other

pair. Then the two mixed pairs would get interference from each other as undesirable

phenomenon. So, cross talk or coupling test is performed as following:

This test is made by telephone set installed at both ends of same pair. If the voice from both end

alternatively is very celar and shift at one end the telephone set to other pairs, no voices are

listened from any otherpair except the right ones, then the pair has no split anywhere and no cross

talk.

If any weak talk comes in other pairs, there is split in pairs of jointing places. It should be located

and remove. The test on other pairs should be continued same way.

c) MEGGER TEST:

For newly installed Pilot Cables 5kV Megger should be used as under:

I - All core to ground with other cores grounded.

II - Between cores of pairs.

III - By 1kV megger check sheath armour to gound after disconnecting it from ground.

IV - The Insulation Resistance value in case I and II should be more than 1000 Mega Ohms

and in case of III it should be more than 2 Mega Ohms

V - For old repaired Pilot Cables 1kV Megger should be used and the IR value more than 2 to

10 Megga Ohms acceptable for putting into service.

VI - D.C. high voltage test is not recommended at Site for new or old in any case. However,

H.V. Tests are required to be performed at manufacturers work place in accordance to

IEC 189-1.

MEDIUM VOLTAGEPOWER CABLES:

a) New cables will be subjected to Manufacturers works all Routine Testsin accordance to IEC

502 – for XLPE and IEC 55 – 1 for paper insulated power cables and other relevant

Specifications.

b) D.C. High Voltage Tests on new and old Power Cable are not preferred. However, in case

of absence of other sets reduced D.C. H.V. test can be performed for shorter time.

c) Instead of D.C. High Voltage test it is recommended to perform H.V. Test at Site for new

and old cables with (VLF) very low frequency (0.1 Hz) approximately sine wave. Zero to

Peak of the voltage wave is considered as test voltage level.

d) VLF (0.1 Hz) should be used specially for XLPE insulated cables at Site. Following are the

tests to be performed on new and old Cables.

CONTINUITY TEST:

Continuity on all phases not including sheath armoring case of newly installed cable should be

checked after laying Underground and Old Cables after repair.

PHASE SEQUENCE CHECK:

At both ends of termination points R, Y, B phases should match to the R, Y, B phase sequence of

Switchgear. For information when standing at back side of Switchgear panel your right hand is

Red phase, Middle Y phase and your left hand is the Blue phase. The cable both end phases

should match the Switchgear sequence at S/S ends.

MEGGER TEST:

a) 5kV megger should be used for each Phase to grund and Phase to Phase also.

b) Insulation Resistance Value on megger should be noted carefully after certain time when

the value of IR becomes constant usually 5 to 10 minutes for longer cables are sufficient.

The cable should be earthed same time at least before and after megger test.

c) Megger test should be performed before and after each High Voltage test, the value of IR

should be recorded carefully.

HIGH VOLTAGE TEST:

a) Perform High Voltage test with Very Low frequency 0.1 Hz Test set on each phase

separately with other twophase grounded along with armour

b) Peak voltage level of 0.1 Hz wave is considered as test voltage level and it should be for

new cables 3U.0 for 15 Minutes

c) For old repaired cales same VLF (0.1 Hz) Test voltage is used at Ö3 U.o for 10 Minutes.

d) Safety rules should be followed before any H.V. Test start and cable should be

discharged and ground before and after each high voltage test.

e) For judgement to a good cable the Megger value should be > 1000 M Ohms and no

collapse of voltage due to insulation failure inside cable should occur during the test

period.

f) Some new cable are jointed with old cables, the test voltage level to be used should be the

same as old cable and the IR value > 300 M is acceptable.

g) V. Ts, power transformers and other unwanted equipment should be disconnected from

the cable circuit before starting of VLF H.V. Test.

HIGH VOLTAGE POWER CABLES & ACCESSORIES 132KV AND ABOVE

RATINGS:

Following test are required to be performed after laying and jointing.

OIL FILLED CABLES:

a) Oil flow test.

b) Impregnation test.

c) Conductor Resistance test and phasing check.

d) Capacitance test.

e) Sheath Insulation Resistance (IR) Test.

f) Varification of Cross bonding System.

g) Sheath contact resistance for joint boxes (JB).

h) Test on sheath voltage limiter (SVL)

i) High voltage test.

XLPE CABLES:

a) Conductor resistance test and phasing check.

b) Capacitance test.

c) Sheath IR Test.

d) Variation of cross bonding System.

e) Contact resistance of Links in JB.

f) SVL. IR Test

g) High Voltage test.

SITE TESTS ON OLD/REPAIRED OIL FITTED/XLPE CABLES:

a) Sheath IR Test and phasing checks.

b) Contact resistance of links in JB.

c) Oil Flow Test in case of Oil Filled only.

d) Reduced High Voltage test alongwith Megger Test.

3.4-4 BRIEF DESCRITION OF:

a) Oil Flow Test:

This test is performed on each oil section afte jointings have been completed. Oil is fed at

one end of oil section through oil tanks and the other end is keep open to flow oil out. At

feeding end a constant pressure is developed by oil tank and cable itself. A stabilized oil

guage valis noted at feeding and same time oil flow meter installed at feeding end shows

the actual flow rate liters/sec.

The theoretical value of oil flow rate Q can be calculated as under:

Q = Pr /25.5 *n*  L    Liters/Second

Q= Rate of flow Liters/Second

P= Total pressure difference on the Section (bar).

r= Internal radius of cable pipe (mm) (ALUMINUM SHEATH)

n= Viscosity of oil at the Test Temperature (Centipoise).

L= Oil Cable section length (meters).

Compare the observed value and calculated value of Q, there should be no extra ordinary

difference for good cable without obstruction in oil flow path.

IMPREGNATION TEST:

Purpose of this test is to know that paper insulation mentioned inside pipe is fully impregnated by

oil or not. For this purpose a measured quantity of oil is drawn from the Cable System (with feed

tank shut off) and consequent drop in pressure noted.

The impregnation co-efficient (K) is calculated as under:

K = dv/v * 1/dp

K = Impregnation Co-efficient.

dv = The volume of oil withdraw (Liter)

dp = drop in pressure (bar)

v = Volume of oil in Cable Section (Liter)

The calculated value of K should not be more than 4.5 x 10^-4 for a good judgement.

CONDUCTOR, RESISTANCE, CAPACITANCE AND PHASING TEST:

Prior to put loop for end in two phases phasing should be confirmed by simple contnuity check by

grounding each phase in turn. Then thick loop is inserted between Ph-Ph at one end and

resistance Measured by RLC Bridge at other end each Phase to Phase in turn.

For capacitance loop should be taken out and capacitance of each phase to sheath measured by

RLC Bridge.

SHEATH ‘IR’ TEST:

a) For new cables apply 4kV D.C. for 1 minute to cable ALUMIUM SHEATH with ground after

opening all grounding links in link boxes.

No collapse of voltage should occur during the test period. If test is repeated then 5kV should

used for one minute.

b) For old cables the test should be done by 5kV megger and IR Value more than 2 Megga

Ohms is acceptable

TO BE CONTINUED SOON