66kv bushing inspection

Standards Usually followed
• IEC : 76 Parts I-IV
• IS : 2026 Parts I - IV

CLASSIFICATION OF TESTS
• ROUTINE TESTS
• TYPE TESTS
• SPECIAL TESTS

Routine tests:
A. Visual inspection
B. Oil Leakage Tests:
C. Measurement of winding resistance (IEC 60076-1, 10.2)
D. Measurement of voltage ratio and check of phase displace
ment (IEC 60076-1, 10.3)
E. Measurement of short-circuit impedance and load loss (IEC
60076-1, 10.4)
F. Measurement of no-load loss and current (IEC 60076-1, 10.
5)
G. Separate source voltage withstand test (IEC 60076-3)
H. Induced overvoltage withstand test (IEC 60076-3)
I. Test on On-Load Tap Changer, if applicable (IEC 60214)

Type tests:
A. Temperature-rise test (IEC 60076-2)
B. Lightning impulse test (IEC 60076-3)

Special tests:
a) Partial discharge test (IEC 60270 and Annex A of IEC 60076-3)
b) Chopped wave test, if applicable (IEC 60076-3)
c) Determination of capacitances windings-to-earth, and between
windings
d) Determination of transient voltage transfer characteristics
e) Measurement of zero-sequence impedance(s) on three-phase
transformers (IEC 60076-1, 10.7)
f) Short-circuit withstand test (IEC 60076-5)
g) Determination of sound levels (IEC 60076-10)
h) Measurement of the harmonics of the no-load current (IEC 600
76-1, 10.6)
i) Measurement of the power taken by the fan and oil pump mot
ors

Routine tests:
A. Visual inspection:
- Is the transformer conservator type and suitable for outdoor inst
allation?
- Is it ONAN or ONAF cooling?
- If ONAF cooling, are fans covered by protection sheet? And is th
e cooling system fitted with winding temperature relay, Auto-Ma
nual switch and a control box to contain control devices?
- Are fans (if applicable) easily accessible for maintenance?
- Is the magnetic circuit earthed to the clamping structure throug
h one removable core insulation test link placed in an accessible
position beneath an inspection opening in the main tank cover?

Routine tests:
- Check tank thickness against approved specs if possible.
• Is it possible to remove the instrument bulbs from the pockets?
• Is the top cover of tank designed in such a way that water does n
ot accumulate?
• Is the transformer provided with bi-directional rollers to allow lo
ngitudinal and transverse movement on skid rail?

Visual inspection:
Check the cover thickness
Check the base thickness
Routine tests:

Visual inspection:
• Is the tank fitted with pockets for the thermo
meter and the bulb of a winding temperature i
ndicator and an oil temperature indicator?
• Is the thermometer pocket(s) fitted with capti
ve screwed cap to prevent the ingress of dirt a
nd water?
• Are the pocket(s) located in the position of ma
ximum oil temperature at rated power?
Routine tests:

Routine tests:
pockets for the
thermometer and
the bulb

Routine tests:

Routine tests:
- Are radiators hot dip galvanized and painted?
- Check if the design allows any pockets in which water can collect
- Are radiator valves provided on each radiator connection to the t
ank so that any radiator may be removed for repairs without taki
ng the transformer out of service?
- Do radiator valves have the open and closed positions clearly ma
rked?
- Are the radiators fitted with lifting lugs, suitable drain and vent p
lugs so that they can be completely drained and vented?
- (Look for an evidence proving that the source of the radiators fro
m the approved supplier list).

Routine tests:

Routine tests:
Valves
Valves are designed to withstand more
than 25 psi and full vacuum. To prevent
interference with the transformer and
radiator, all parts are contained within the
body. The valve is designed so it can be
locked in the open position. Valves are
painted ANSI 70 gray and shipped
complete with NBR O-ring, shipping cover
plate, four 3/4" x 2-3/4" grade 5 bolts,
nuts and washers. Viton gaskets
are available.

- Check the tank finishing which shall consist of two-pack polyur
ethane paint with a total thickness not less than180µm.
- Check the external surfaces of the radiators which shall be hot-di
p galvanized and minimum thickness shall be 60µm.
- Check that the 30kv and 11kv bushings including neutral bushing
are plug-in type that can be connected with elbow connector or
tee connector.
- Are bushings mounted on the tank wall or on the top cover and s
ealed by cable boxes as per the specs?
- Measure the minimum phase to phase clearance and phase to e
arth clearance that shall not be less than those stated in Table 5
of IEC 60076-3.
Routine tests:

Routine tests:
- Check if the plug-in type bushings installed in a manner to preve
nt ingress of moisture and to facilitate easy connection with heat
shrinkable or pre-molded elbow connectors.
- Check the numbers of HV and LV bushings.
- Check if the HV and LV cable boxes are mounted on opposite sid
es of the tank or on the top cover, and arranged for cables enteri
ng vertically from bottom side.
- Check if all cable boxes are completely sealed and provided with
sealing glands to suit cables and fitted with suitable means to cla
mp the non-armored cables.
- Check if the glands are electrically separated from cable box.

Routine tests:
66kv bushing
Elbow connector bushing for
30kv and 11kv
Check the:
Numbers pf bushings per phase.
The distance between bushings.
Bushing locations
Bushing size
Creepage distance.

Routine tests:
- For 66/11kv transformers measure the distance between HV bushings, and m
easure the creapage distance of the bushings.
• Check if the transformer is provided with overhead conservator tank, connect
ed to the main tank as well as the tap changer tank at the highest point to pr
event the trapping air or gas under the main tank cover.
• Check if Bolted end plate for internal cleaning, filling cap, drain valve with ca
ptive cap, and oil level indicator are provided with the conservator.
• Check if conservator is fitted with Valves to cutoff the oil supply to the transf
ormer tank as well as tap changer tank.
• Check if the conservator is provided with an oil seal type silica gel breather.
• Check if the conservator is provided with an air cell to prevent air contact
with oil. (Look for an evidence proving that the source of the air cell from the
approved supplier list).

Check the if the conservator is fitted with air cell
Routine tests:

Routine tests:
- Check if Oil level indicator is fi
tted to the conservator tank t
o show the oil level at all temp
erature with minimum, maxim
um markings and normal level
marking at 30°C clearly visible
from normal access level.

Routine tests:
- Check if there are two earthing terminals provided near the bott
om of tank at opposite side. The terminal shall be able to connec
t the earthing wire of which cross sectional area between 25mm
2 and 70mm2.

Routine tests:
- Check that the transformer is equipped with all the following fittings:
• (a) Conservator with filling holes and caps and a shut-off valve b
etween the conservator and the main tank
• (b) Oil level gauges marked to indicate normal level at 30°C one
or each side of the conservator
• (c) Silica gel breather
• (d) Pocket on tank cover for oil thermometer.
• (e) Earthing terminals on diagonally opposite corners
• (f) Weatherproof metallic marshalling box to house all terminals,
controls, etc.

Routine tests:
- Check that the transformer is equipped with all the following fittings:
• (g) Bi-directional flanged rollers or skid base suitable for plinth mounti
ng application
• (h) Cable supports
• (i) Ladder
• (j) AVR mounted cabinet
• (k) Air release device
• (l) Jacking pads
• (m) Lifting lugs for tank cover
• (n) Lifting lugs for core and winding assembly
• (o) Lifting lugs for whole tank
• (p) Hauling lugs
• (q) Rating and diagram plate
• (r) Valve schedule plate

Routine tests:
• Check if the transformer is fitted with following valves a
t the following locations for filling, draining, filtering, co
nservator isolation etc.
• (a) Oil filtering valves located near to the bottom and t
he top of the main tank
• (b) Drain and filling valve of conservator tank
• (c) Drain valve for main tank
• (d) Drain valve for OLTC tank

Routine tests:
- Check that the transformer is equipped with all the following va
lves:
• (e) Valve between main tank and conservator tank
on connecting pipe
• (f) Valve between OLTC tank and conservator tank o
n connecting pipe
• (g) Valve between main tank and each radiator
• (h) Drain valve for each radiator
• (i) Oil sampling cocks for main tank and OLTC tank
• (j) Other necessary valves

Routine tests:
• Check if each valve is provided with an indicator to show clearl
y the position of the valve and also a name plate to indicate the
purpose of the valve.
• Check if the valves have machined flanges and equipped with loc
king pins for locking in the closed / open position.
• Check if Blank flange plates or captive screw caps are fitted to all
valves. If possible,
• Check if all valves are mounted such that an observer at ground l
evel can operate them easily.

Routine tests:
Check if the transformer is fitted with all the following Pr
otection devices
• (a) Buchholz relay with alarm and trip contacts
• (b) Oil temperature indicator with alarm and trip co
ntacts
• (c) Winding temperature indicator with maximum p
ointer and alarm / trip contacts
• (d) Pressure relief device with trip contact
• (e) Buchholz relay for OLTC with trip contact

Pressure relief valve with tripping contacts
Routine tests:

Routine tests:
• Check the OLTC type, and the IP of the control box.
• Check if there is Visible means to de-energize the control
and short the current transformer prior to control testing
or removal.
• Check if all leads in the control enclosure are color coded
and labeled for easy identification.
• Check if the terminal strips of the control back panel cons
ist of clamp-style quick connectors for ease of access.
• Check if all printed circuit boards are conformal coated fo
r fungi and moisture protection.
• Check if there is a locking device at outside of the box.

Routine tests:
• Check that the control comprises the following devices and functions as minimum:
• (a) Tap position indicator which tracks the movement of the tap changer motor
• (b) A six-digit electronic operations counter, which counts every tap changes
• (c) A AUTO / MANUAL switch, which allows automatic or manual operation of the OLT
C.
• (d) A RAISE / LOWER switch, which shall only be active when the control switch is in th
e MANUAL position
• (e) Adjustable time delay
• (f) Adjustable set voltage bandwidth
• (g) Terminal block which have symbol marking to connect control cables with control c
ircuits, current transformers and voltage transformers
• (h) Automatic voltage regulator with resistance / reactance and voltage control device
s
• (i) A single phase 230V power outlet for lamp
• (j) Heater to prevent condensation
• (k) All necessary meters, relays, signal lamps and switches for automatic OLTC operati
on

Routine tests:
B) Oil Leakage Tests:
• All tanks and oil filled compartments shall be tes
ted for oil tightness by being completely filled wi
th air/oil of a viscosity not greater than that of i
nsulating oil conforming to the latest edition of I
S: 335 at an ambient temperature and subjected
to a pressure equal to the normal pressure plus
0.35 Kg/cm2 measured at the base for a period
not less than 12 hours for oil and I hour for air d
uring which time no leakage shall occur.

C. Winding resistance measurement
 Use 4 wire method
 Inductive effect to be considered
 Effect of core magnetization
 Provide sufficient stabilizing time
 All other windings to be open circuited
 High current for very low resistance
 Discharge circuit to be arranged
Routine tests:

Winding
Resistance
Testing
ndb Technologies
WRT-10
Routine tests:

Routine tests:

Winding resistance measurements in transformers / generators & large
motors are of fundamental importance for the following purposes:
• Manufacturers will use the test to determine “copper losses” which are
calculated using the formula I2*R
• At the time of commissioning, a winding resistance test can be used to
detect loose connections, typically at the connections to bushings, and
abnormalities with the tap-changer as a result of poor workmanship
or shipping damage.
• Calculation of winding temperature at the end of a temperature test
cycle.
Routine tests:

• The voltage across an inductor is proportional to the time
rate of change of the current through it.
• The DC current source must be extremely stable. Refer to
formula for DC voltage across a transformer below: v = I * R
+ (L di/dt) where:
• vdc = voltage across transformer winding
• I = DC current through transformer winding
• R = resistance of the transformer winding
• L = inductance of the transformer winding
• di/dt = changing value of current (ripple)
Routine tests:

• Selecting the Proper Test Current Range
• Transformer manufacturers typically recommend
that the current output selected should not exceed
10% of the rated winding current. This could cause
erroneous readings due to heating of the winding.
Test current ranges are typically from 0.01% to
maximum of 10 % of rated winding current.
Routine tests:

Stabilised resistance measurements
• On large transformers with high inductance windings, it could take a few
minutes for readings to stabilize. The time required for readings to
stabilize will vary based on the rating of the transformer, the winding
configuration, the instruments output current selected.
• Readings on a Star/Wye-configured transformer should typically
stabilize in 10 to 30 seconds
• For large transformers with delta configuration, magnetization and
getting stable readings can take significantly longer time, sometimes as
long as 30-60 minutes.
Routine tests:

Stabilised resistance measurements - B
By connecting the primary and secondary windings in series (using a
Jumper cable), the speed of saturation and stability of the readings is
increased because there are more amp-turns contributing to the flux in
the core. The resistance measurement of both windings is made at the
same time on channels A and B of the WRT-10.
Delta Windings
If possible, always inject test current to HV & LV simultaneously. Correct
measurements values can be obtained approximately 7 times faster
when injecting in both HV & LV in comparison to measuring LV only
Routine tests:

Indicates if the voltage reading (and displayed resistance reading) has
stabilized.
WRT-10 Reading Stability indicator
Routine tests:

WRT-10 Reading Stability indicator
- Gray when the WRT-10 is idle, when it is dumping current,
or when measuring has begun but is not yet stabilized.
- Yellow when approaching stability (1% or less variation in
the value for ten consecutive readings).
- Green once the nominal current value is reached and the
resistance measurement is almost the same each time (within
a 0.25% tolerance for ten readings in a row).
Routine tests:

Windings resistance measurements can reveal a
great deal of information.
- Faulted winding (open winding or shorted turn)
- Integrity of numerous welded and mechanical
connections
- Poor Joints
- RA switch, LTC (Diverter switch, Tap Selector switch)
damage
Diagnostics
Routine tests:

Winding resistance measurements for one of
GECOL’s 30/11kv 20MVA transformers
Routine tests:
Temp: 28.3oC average
Tap position Tap voltage 1U-1V 1V-1W 1W-1U
1 0.16848 0.16992 0.16998 0.16946
2 0.1667 0.16737 0.16696
3 0.16452 0.16504 0.1648
4 0.16187 0.16248 0.16312
5 0.15913 0.15974 0.16052
6 0.15768 0.15816 0.15803
7 0.15553 0.15565 0.15567
8 0.15278 0.15355 0.1533
9 0.14954 0.15015 0.14989 0.14986
10 0.15313 0.15383 0.15355
11 0.15549 0.15596 0.15624
12 0.15751 0.15792 0.1591
13 0.15915 0.15995 0.16035
14 0.16214 0.16284 0.16288
15 0.1646 0.16482 0.16487
16 0.16686 0.16733 0.16712
17 0.16857 0.1688 0.16903 0.1688

Winding resistance at temperature 75oC
at tap 1 RHV at 23 oC = 0.16946 RHV at 75 oC = 0.199516
RHV at 23 oC = 0.017741 RLV at 75 oC = 0.017856
Routine tests:

D: MEASUREMENT OF RATIO and Vector Group
 TO ENSURE :
 CORRECTNESS OF DESIGN
 CORRECTNESS OF CONNECTION TO TAP CHANGER
 PRESENCE OF ANY SHORT CIRCUITED TURN
Routine tests:

Ratio measurement -Key points
 Single phase measurement
 Use 4 wire measurement
 Use wires of adequate cross section
 Use high impedance voltmeter
 Make simultaneous measurement
 Leave all other windings in open condition
 Preferably use ratio meter
Routine tests:

METHOD OF MEASUREMENT
 BY VOLTMETER
 BY RATIOMETER
Routine tests:

Why shall we test vector groupt
 ACCEPTANCE TEST
 IMPORTANT FOR PARALLEL OPERATION
 IMPORTANT FOR INTERCONNECTED SYSTEM
Routine tests:

•The vector group of transformer is an essential property for
successful parallel operation of transformers. Hence every electrical
power transformer must undergo through vector group test of
transformer at factory site for ensuring the customer specified
vector group of transformer.
 IMPORTANT FOR PARALLEL OPERATION
 IMPORTANT FOR INTERCONNECTED SYSTEM
Why shall we test vector group:
Routine tests:

Examples for different vector groups
GROUP Connection Connection
0
(0o
)
Yy0 Dd0
Routine tests:

1
( 30o
)
Yd1 Dy1
Routine tests:

6
( 180o
)
Yy6 Dd6
Routine tests:

11
( - 30o
)
Yd11 Dy11
Routine tests:

Procedure of Vector Group Test of Transformer
Let’s have a Dyn11 transformer (GECOL transformers).
- Join phase C of HV and n of LV together.
- Apply 400 V, three phase supply to HV terminals.
- measure: B-b, A-c, and 1W-2n and note that:
1U-2n < 1V-2n < 1W-2n
Routine tests:

Example of vector group test for
Dyn11 transformer
A
a
n
C b B
c
3ph 400V
As seen from the
vector diagram, in
order to be Dyn 11
group:
A.c > AB > B.b
has to realized.
Routine tests:

1U,2U
2V
2W
415 V, 3Ø
AC supply
1W 1V
Test circuit for vector group checking of transformers with independent
Windings (HV/LV)
For Yd11 (1V-2V) = (1V-2W) = (1W-2W) < (1W-2V)
1N
Routine tests:
Another example Yd11 transformer

E: LOAD LOSS MEASUREMENT and short-circuit impedance
• IR ² LOSSES IN THE WINDINGS
• STRAY LOSSES IN WINDINGS,TANK,PATRS ETC,
COMPONENTS OF LOAD LOSSES
 The short-circuit loss and the short-circuit voltage
show the performance of the transformer.
 These values are recorded and guaranteed to the
customer and important for operational economy.
 transformers. The short-circuit loss is a data which is
also used in the heat test.
Test significance:
Routine tests:

Test significance
IMPEDANCE VOLTAGE
Voltage required on one of the windings in a pair to pass rated
current through the windings with the other winding in short
circuited condition
 The short-circuit voltage show the performance of the
transformer, and important attribute in calculating
fault current.
 The short-circuit voltage is an important criteria
especially during parallel operations of the
transformers.
Routine tests:

 Load loss measurement
Test with a current 25 % to 80 % or 100%
. preferably not less than 50 %
Loss correction as standard when tested
. with a current as above
Loss measured at ambient and reported at
. reference temp. of 75 ° C.by calculation
Routine tests:

Measurement circuit and performing the test:
VT
Power
Source and
intermediate
Transformer
CT
Power
Analyser
Tested
Transformer
Routine tests:

Before beginnig to measure, the transformer winding/oil
temperature has to be stabilised and the winding/oil
temperature and winding resistances have to be measured.
In general, the HV windings of the transformer are supplied
while the LV windings are short-circuited.
 During measurement, the current has to be at the value of
IN or close to this value as far as possible.
V, I and short-circuit losses of each phase should be
measured during measurement.
 In order to avoid increasing the winding temperature by the
applied current, the measurement has to be completed in a
short time and the measuring current has to be kept
between 25%...100% of the rated current. In this way, the
measurement errors due to winding temperature increase
will be minimised.
Routine tests:

 The losses have to be corrected based on reference
temperature (e.g. 75oC ) stated in the standards and
evaluated.
 The short-circuit voltage Ukm and losses (Pkm ) which are
found at the temperature which the measurement was
made, have to be corrected according to this reference
temperature.
 The direct-current/DC losses on the winding resistances,
while the resistance values are RHV and RLV (phase to
phase measured resistances) are as follows ;
Direct-current loss = at measuring temperature tm:
PDC = 1,5.( I1^2.RHV + I2^2.RLV ).
AC / Additional losses = at measuring temperature tm
Pac = Pkm- Pdc.
Routine tests:

Where:
Routine tests:

“ohmic/DC ” component
“inductive /AC” component
Routine tests:

Errors to be considered
Since the circuit forming the measurement in high power transformers and
reactors are inductive, the power factor (Cos Ø) will be very small (Cos Ø : 0,01
...... 0,003, or angle = 1deg. ..... 10 minutes). For this reason, the errors in
measurement current and voltage transformers will be very high. In this case,
the measurement results have to be corrected by a multiplier.
Routine tests:

Routine tests:

If the measurement current is different than rated current “ IN ”, the short-
circuit voltage and short-circuit losses for the rated current value are calculated as
follows;
Routine tests:

If the transformer short-circuit losses and the voltage are measured at a
frequency which is different than the rated frequency, correction has to be
made to according to below equations:
Routine tests:

Example of measurements of load losses and short-circuit
impedance related to 30/11kv 20MVA transformer:
Routine tests:

COMPONENT OF NO LOAD LOSS
 Hysterisis loss
 Eddy current loss
Routine tests:
F: No-LOAD LOSS MEASUREMENT

Measurement of no-load loss and current
 It’s very much related to the operational performance of a transformer.
 As long as the transformer is operated, these losses occur, so it’s important
for operational economy.
 No-load losses are also used in the heating test.
 No-load loss and current measurements of a transformer are made while
one of the windings (usually the HV winding) is kept open and the other
winding is supplied at the rated voltage and frequency.
 During this test the no-load current (Io) and the no-load losses (Po) are
measured.
 The measured losses depend heavily on the applied voltage waveform and
frequency. For this reason, the waveform of the voltage should be very
sinusoidal and at rated frequency.
 Normally, the measurements are made while the supply voltage is
increased at equal intervals from 90% to 115% of the transformer rated
voltage ( UN ) and this way the values at the rated voltage can also be
found.
Routine tests:

No-load losses and currents:
The no-load losses of a transformer are grouped in three
main topics:
1) iron losses at the core of the transformer,
2) dielectric losses at the insulating material and
3) the copper losses due to no-load current.
The last two of them are very small in value and can be
ignored. So, only the iron losses are considered in
determining the no-load losses.
Routine tests:

No-load losses and currents measuring circuit:
Power
Source and
intermediate
Transformer
CT
VT
Tr.
Under
test
Analyser
Routine tests:

 According to the standards, if there is less than 3% difference between
the effective (U) value and the average (U’) value of the supply voltage,
the shape of the wave is considered as appropriate for measurements.
 If the supply voltage is different than sinusoid, the measured no-load
losses have to be corrected by a calculation. In this case, the effective
(r.m.s.) value and the average (mean) value of the voltage are different. If
the readings of both voltmeter are equal, there is no need for correction.
During measurements, the supply voltage U´ is supplied to the transformer
by the average value voltmeter. In this way, the foreseen induction is
formed and as a result of this, the hysteresis losses are measured correctly.
The eddy-current losses should be corrected according to equation below.
Routine tests:

Example of measurements of no-load losses related to
30/11kv 20MVA transformer:
Routine tests:

Dielectric test of transformer
• The dielectric test of transformer is generally
performed in two different steps, likewise,
separate source voltage withstand test and
induced voltage withstand test of transformer,
which we have discussed one by one below.
Routine tests:
G: Dielectric tests

Separate Source Voltage Withstand Test of Transformer
This dielectric test is intended to check the ability of main insulation
to earth and between winding.
• Procedure
• All three line terminals of the winding to be tested are connected
together.
• Other winding terminals which are not under test and also tank of
the transformer should be connected to earth.
• Then a single-phase power frequency voltage of shape
approximately sinusoidal is applied for 60 seconds to the
terminals of the winding under test.
• The test shall be performed on all the windings one by one.
• The test is successful if no break down in the dielectric of the
insulation occurs during test.
Routine tests:
G: Dielectric tests

Routine tests:
G: Dielectric tests

Example of measurements of separate-source voltage
test for 30/11kv 20MVA transformer:
Routine tests:
G: Dielectric tests

INDUCED OVER VOLTAGE TEST
Dielectric test to verify dielectric strength between
Phases
Turns
Windings
All of above and earth
Routine tests:
G: Dielectric tests

Induced Voltage Test of Transformer
Routine tests:
G: Dielectric tests

Induced Voltage Test of Transformer
 The induced voltage test of transformer is intended to
check the inter turn and line end insulation as well as main
insulation to earth and between windings-
 Keep the primary winding of transformer open circuited.
 Apply three phase voltage to the secondary winding. The
applied voltage should be twice of rated voltage of
secondary winding in magnitude and frequency.
 The duration of the test shall be 60 second.
 The test shall start with a voltage lower than 1/3 the full test
voltage, and it shall be quickly increased up to desired
value.
 The test is successful if no break down occurs at full test
voltage during test.
Routine tests:
G: Dielectric tests

 PD SOURCES
 Voids in insulation
 Sharp edges
 Moisture
 Inadequacy in design
 Loose contacts
 Floating objects
 Inadequate processing
 Insufficient shielding of live objects
 Badly made contacts and joints, etc.

TIME SEQUENCE OF APLLICATION
30 MINUTES
5 MINUTES
5 SEC.
U1
U2
V
O
L
T
A
G
E

A TYPICAL IOV TEST CIRCUIT WITH PD MONITORING SYSTEM
LV HV
Bg .and test tap cap.
A.C
T
E
S
T
V
O
L
T
A
G
E
Measuring impedance and tuning
system
PD meter
E
N

What is the insulation resistance
• It is the surface and volume
resistivity of the insulations
involved
D. Insulation resistance measurement
Routine tests:

Factors affecting IR value
• Temperature
• dryness of insulation
• Cleanliness
• Humidity
• Condition of oil
Routine tests:

APPROXIMATE MINIMUM REQUIREMENT OF IR VALUE
R 20 =( C * E) / SQRT(kVA)
C = 0.8 FOR OIL FILLED TRANSFORMER
= 16 FOR DRY TRANSFORMERS
E = Rated voltage (volt )
kVA = Rated kVA
Routine tests:

PI VALUE= IR 10 Min. ÷ IR 1 Min.
PI value Condition
< 0.1 Dangerous
1.0 to 1.1 Poor
1.1 to 1.25 Questionable
1.25 to 2.0 Fair
> 2.0 Good
Routine tests:

Temperature Rise Test
Objective
Confirm whether the cooling
equipments provided are
adequate to attain oil and
winding temperature rise with in
guaranteed limits and hot spot
temperature is not exceeded

TOTAL LOSS FEEDING
NO LOAD LOSS (100 %)
+
MAXIMUM LOAD LOSS

ACTIVITIES INVOLVED IN TEMPERATURE RISE
TEST
 Cold resistance measurement
 Total loss feeding
 Reduced cooling
 Measurement of ambient temperature
 Measurement of top oil temperature
 Measurement of cooler gradient
 Measurement of hot resistance
 Test objectives calculations

Type of cooling
 ONAN
 ONAF
 OFAF OR ODAF
 ODWF

Correction factors for oil rise and
winding gradient
Top oil rise = ( total loss ) x
( applied loss ) ,
where x = 0.8 for natural air cooling
x= 1.0 for directed oil cooling
Winding gradient = ( rated current ) y
( applied current ) ,
where y = 1.6 for natural forced air cooling
y= 2.0 for directed oil cooling

WHY IMPULSE TESTING ?
• Transformer as a system element subjected to
switching and lightning surges
• The characteristics nature of transformer to
the above disturbances is different from
system oscillation at power frequency.

Vp
1.0
0.9
0.5
0.3
T
T1=1.67 T Time in m sec.
T2
Lightning impulse voltage wave form

Impluse Wave shaping Metering circuit Transformer Calibration &
generator circuit under test chopping circuit
Rs1 Rs2 X1 PD HV LV Sphere
Cg CRO1 CRO2 Shunt Chopping Rod
Essentials of lightning impulse voltage testing

Vp
1.0
Time in m sec.
Tc Chopping time
Lightning impulse voltage ( Chopped wave application )

U
1.0
0.9
Td
Tp
T t T
Switching impulse voltage wave form

LV HV
Rs2 Xl
Rs1
N
Cg Rd Cl CRO1 CRO2 shunt
sphere
Impulse Wave shaping Metering circuit Transformer
Essentials of switching impulse voltage testing
Genarator circuit circuit under test
Calibration
circuit

SOURCE TRANSFORMER TRANSFROMER UNDER TEST
TANK
HV LV PVV
A.C.
TEST CIRCUIT FOR SEPARATE SOURCE
VOLTAGE WITHSTAND TEST
E
P
V
V

Transformer
under
test
CRO SHUNT
CT
S
U
P
P
L
Y
I
N
P
U
T
Harmonic content in exciting current measurement

HV CHL LV
CH CL
E E
Equivalent circuit of capacitance of two winding transformer

LOSSES IN DIELECTRICS
• Imperfection in structure
• Surface leakage
• Contaminants
• External agency causing (1) & (2)

TAN DELTA
• Indication of insulation quality
• <1% for trfr. and <0.7% for bushings.
• vary with temperature
• should not vary with test voltage abnormally

MEASUREMENT OF TAN DELTA
• Bushing terminals to be shorted
• Tank and parts to be earthed
• Porcelain can be thoroughly cleaned
• Humidity shall be favoring
• Electrostatic interference to be shielded
• Temperature
• Shielded cable

ZERO SEQUENCE IMPEDANCE
• For system fault calculation
• Depends on the characteristics design of
winding and core.
Z0 per phase = 3 V/I ohms
% Z0 = Z0 in ohm x kVA
10 x (kV)2

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