TESTING OF 11 KV CABLE
As per addc,for the incoming cable,vlf test to be done before 4
8 hours of energizing.
For other internal mv cables,
once the IR,vlf and IR with PI values is done then it shall be ter
minated and the cold or hot boots shall be heat shrinked.the t
est will be valid for 6 months as per addc,if energizing not don
e in this duration then boots to be removed and cable to be te
sted again.
The PI value below 2 is not a good indication, shall be above.
As per addc,Sheath test is required for 11 KV cable if it's havin
g graphite coating,usually cables below 400 mm2 does not ha
ve graphite coating so
no sheath test required. Also required for 33 kv cables since th
ey will be having graphite coating.
Sheath test after cable jointing is not recommended since duri
ng the joint insulation is degraded and no graphite coating is d
one during jointing, recommend span wise before jointing.
Testing:
Cable preparation on both sides for termination
Inside 11 KV GIS Panel
Preparation inside T/F terminal box
1. Phase check first
Short each phase with earth,keeping other two phases open and
verify each check first.
2. IR test
5000 VDC for 1 minute
3. Vlf
For vlf test of 11 KV Cable:
19.07 KV peak value
13.5 kv rms
Components for vlf test
a) Generator (Connected to earth pit)
b) Ac automatic voltage regulator
VLF KIT
Connected to earth
Set the RMS voltage to 13.5 KV,30 minutes,0.1 hz.
After vlf and IR, discharge to earth to be done.
Note the leakage current value after 1 min,5,10,15 and 30 minutes (
shall be in Micro ampere).
Capacitance value will also be noted during this test( shall be in
nano farad)
4. IR after vlf
Boot to be provided for the two distribution transformers since
the bushings coming from inside are insulated so the terminat
ed cables are also insulated by boots.
For phase shift transformers,
the busbars coming from inside transformers to the terminal b
ox are naked so boot is not required for terminated cables Also
.
5. Calculate the PI value( For cable:IR value after VLF/IR Value
before VLF),For Transformr:IR value after 10 minutes/IR value
after one minutes
Test report for 11 kv cables from 11 KV panel to distribution
transfer(11 KV/0.415 KV),2000 KVA,hermetically sealed .
How the RMS,Peak values are selected for testing a cable:
For new cables:
Test Voltage (RMS): 3×U0
Where:
o U0 = Rated phase-to-earth voltage of the cable.
o For an 11 kV cable (11 kV is line-to-line, meaning
U0=11/ sqrt{3}= 6.35 kV
Test Voltage Calculation:
3×6.35=19.05 KV(peak value).
Peak Voltage:
Since AC is sinusoidal, the RMS voltage is:
V rms=Vpeak/sgrt(2)=19.05/1.41=13.5 KV
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For old cables:
As per ADDC,For existing old cable ,the applied voltage for
vlf shall be 2 times for vlf,not 3 times as for new cables.
For existing 11 KV cable,the minimum IR value shall be 10 me
ga ohm ,if it's below 100 Mega ohm than vlf test shall be done
to check healthiness( at 2 xU0),if passed then use the
cables.,if above 100 Mega ohm ,then no need for VLF.
the applied high voltage for old cables for VLF is
2XU0=2x6.35=12.7 KV(peak value).
V rms=Vpeak/sgrt(2)=19.05/1.41=9 KV
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