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.
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
