Diagnostic Insulation
Testing
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Predictive Maintenance issues
Basis of Insulation Tests
Spot Tests & Polarisation Index
Dielectric Discharge Test
Power factor testing
AVO Range
Maintenance Regimes
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Corrective maintenance
Preventive (Periodic) Maintenance
Predictive Maintenance
(Record & trend data to determine next
action - variable maintenance periods)
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Appropriate regime varies
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Test Techniques - DC
Testing
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Total Current has 3 components
 Capacitive
Current - Charging
 Absorbtion Current -Polarisation
 Leakage Current
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TIME DEPENDENT
Temperature Dependent
Insulation Test Currents
I/uA 100
Charging
Total
10
Leakage
Absorption
1
0,1
1
10
T
Insulation Equivalent Circuit
Leakage
Capacitance
Absorption
Diagnosis from Insulation
Tests
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Record & Trend
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Spot Tests
- Record conditions as well (Temp, RH)
Time resistance tests
- Polarisation Index Test (PI)
Step Voltage Test (SV)
Dielectric Discharge Tests (DD)
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Insulation Test as a
Go/NoGo Test
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Spot Test carried out for given time
(1 minute)
Commissioning
National /Manufacturer’s Standards
Time Resistance Tests
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Independent of equipment size and
temperature
Some information without past results
Options;
 Curves
 Time
- Resistance Ratios
Time ResistanceTest
R
Absorbtion current not seen
t1
t2
Time
Time Resistance Tests
Polarisation Index
(10 min/1 min ratio)
<1
Insulation Condition
unsatisfactory
1,0 - 2,0
dubious
2,0 - 4,0
good
>4,0
very good
Time Resistance Tests
Dielectric Absorption
Ratio (60 s/30 s)
<1
Insulation Condition
unsatisfactory
1.0 - 1.4
dubious
1.4 - 1.6
good
>1.6
very good
Step Voltage Test
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Test voltage is increased in 5 steps
Lower insulation resistance values at
higher voltage indicate presence of
cracks/holes
Step Voltage Test
5000 V
Resistance
2000 V
1000 V
Voltage
Dielectric Discharge Test

Test is run until currents stabilise
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Measurements made on discharge
DD = Current after 1 min
Voltage x Capacitance
Dielectric Discharge Test
+
-
-
-
+
+ +
+
-
-
-
+
+ ++
-
-
+
+
-
+
-
+
+
-
+
-
+
-
+
-
-
+
+
Dielectric Discharge Test
DD Value
>4
2.0 - 4.0
<2.0
Insulation Condition
unsatisfactory
dubious
good
Tan Delta Testing
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Polarisation and capacitive current
dominate
Aging (moisture/dirt) sensitive
Low voltages can be used
Perfect Insulation
I (capacitive)
ITOTAL
IR = 0
V (applied)
Facts about Insulation

In a “REAL insulation system” there
is also a loss current flowing inphase with the voltage
IC
//
IR
=
ITOTAL
REAL Insulation
I (capacitive)
IC
ITOTAL
IR
V (applied)
Facts about Insulation
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In practice NO insulation is perfect
Resistive loss current is always
present
Total current leads the voltage by a
phase angle  and lags capacitive
current by angle 
REAL Insulation
I (capacitive)
IC
ITOTAL


IR
V (applied)
What is Tan Delta?
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Tan Delta is the ratio of in-phase
(resistive) current to the 90-degree
(capacitive) current
Tan Delta equals Dissipation Factor
What is Power Factor?

Power factor is the ratio in-phase
(resistive) current to the total
current.
Tan Delta Testing
I(capacitive)
Tan  =
IC
ITOTAL

Power Factor =

IR
V(applied)
IR
IC
IR
ITOTAL
Tan Delta v. Power Factor

Both values are similar up to 20%
power factor and tan delta; beyond
20% they diverge.
Tan Delta 3 Terminal Test
Low voltage
(L)
High voltage
(H)
CHL
CLG
CHG
Ground
Connection (G)
Tan Delta 3 Terminal Test
Configuration
Measures
GST L-GND
CHL + CHG
GST L-Guard
CHG
UST
CHL
GST H-Guard
CLG
GST H-GND
CHL + CLG
Why perform the
Tan Delta Test?
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Normal insulation resistance testing
limited to 5kV dc
Tan Delta is up to 12kV ac
Most test specimens normally operate
with AC voltage
Much closer resemblance to the normal
operating condition of the insulation
Tan Delta Testing
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Comparison with name -plate values
Very sensitive to small changes
a.c. test at rated voltage
Larger, heavier test equipment
Insulation Test Diagnosis
Test Type
Main Failure Mode
“Spot” Insulation Test Go/no go
Polarisation Index
Moisture/dirt
Step Voltage
Cracks/holes
Dielectric Discharge
Absorbed
moisture/dirt
Loss measurement
Tan-delta
Tests Performed On
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Transformers
Circuit Breakers
Rotating Machinery
Insulating Liquids
High Voltage Bushings
Surge Arrestors
High Voltage Coils
MEGGER BM11D
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Portable, IP54 , large PbAcid battery
Readings to 500 GOhm
Display is large, clear & Easy to Use
Safety features
 Automatic
discharge, locking test leads,
Hazard display
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Integral Timer
MEGGER BM21
All BM11D features plus;
 Readings up to 5TOhm
 Selectable Test Voltage
 Selectable Test Time
 Checks Capacitance at test End
 Can show current as well as resistance
 Burn mode
MEGGER BM25
As BM21 features plus;
 Automated tests
 Polarisation
Index
 Step Voltage
 Dielectric Discharge
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RS232 download results every 5 s
Download software & lead
MEGGER S1-5005
As BM25 with 5 mA test
current.
BM25 has 2 mA test
current.
MEGGER S1-5010
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Heavy Duty - Mains & Battery
Automated Tests
Results Storage
RS 232 communications
Graphical Display
MEGGER S1-5010
MEGGER S1-5010
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Memory for 75 sets of results
Windows PC software
 remote
operation
 results storage
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External output for HV “ON” indicator
Switchable digital filter for noise
rejection
IP54 weatherproofing
S1-5010 and S1-S1 software
S1-5010
S1-S1 software
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Duplicates the S1-5010 on your PC screen.
Control the S1-5010 from your PC via an
RS232 link.
Download tests to your PC
 in real time.
 as a batch at the end of testing.
Annotate the tests results with details of the
test.
Recreate graphs etc. in other software.
BIDDLE Delta 2000
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500 V - 12 kV power factor tester
Automatic bridge operation
High noise immunity
Menu driven display
Measures voltage, current, power (loss),
power factor/tan-delta and capacitance
BIDDLE Delta 2000
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Results storage
Printer for on-site results
Downloading test data
Accessories
Oil test cell
 Bushing connectors
 Bar code wand

BIDDLE Delta 2000
BIDDLE 2.5 & 12 kV semiautomatic units
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2.5 kV single unit construction
12 kV control unit and
Measure capacitance, dissipation factor
and Watts
CB-100 Low Voltage Bridge
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28 V test voltage
Safe
Compact
Low cost
Often used on bushings & transformers
Testing with Safety
– How
–
does the electrical risk appears ?
- When an electrical line or conductor is
unexpectedly still live
- When current sources are not properly
isolated
- When power generators are connected to the
network
- When there are atmospheric phenomenons
such as lightning
- Through induced currents
Testing with Safety
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However, it has also been observed that
the most common causes of unexpected
presence of voltage on electrical lines or
installations were due to:
- A mistake in operating a switch
- A mistake in identifying the work to
perform
- A "faulty" switch (one of the links for
example remaining partially engaged)
Testing with Safety
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How does the electrical risk disappear ?
–
1. By observing Simple Electrical Safety Rules
directed by common Sense
(even if Working Practices vary from a country to
another, GOLDEN RULES always apply).
2. By using Simple, Light and Reliable Electrical
Safety Equipment.
Testing with Safety
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1. SEPARATE THE WORK AREA FROM
ALL POSSIBLE SOURCES OF POWER
SUPPLY
by opening circuit-breakers, switches…
AND LOCK THE LINKS IN THE OPEN
POSITION
Testing with Safety
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2. CHECK THE ABSENCE OF VOLTAGE
at source and on site, on each conductor
and apparatus involved.
3. APPLY APPROPRIATE SHORTCIRCUIT AND EARTHING SYSTEMS
in sufficient numbers on the conductors of
your network where operation is taking
place.
Testing with Safety
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4. SHOWN, WARN OTHERS AND
LIMIT THE WORK AREA
even during the most simple temporary
maintenance operation
Testing with Safety
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Discharge Sticks
Slowly discharge capacitive items (cables,
capacitors….)
Discharge to a safe level in 5 - 10 times
RC (time constant)
Discharge Sticks
Voltage
35 kV
50 kV
75 kV
100 kV
150 kV
Length
50 cm
136 cm
156 cm
176 cm
208 cm
Resistance
10 kOhm
100 kOhm
150 kOhm
200 kOhm
300 kOhm
Max. cap.
6 uF
6 uF
4 uF
3 uF
2 uF
Oil Testing
MEGGER Foster Oil Electric
Strength Testers
Why test switchgear oil?
Prevent Transformer failure
Principle problems are:
 humidity
 oxygen
 temperature

Oil Testing Methods
Water content
 Acidity
 Colour
 Flash point
 Gas content
 Oil provides HV insulation
Electric strength is the most important
test
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Test Voltage
Breakdown Testing
60
40
20
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Time
Electric Strength Test
Results
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Average over six tests
Scatter can be large
 Standard
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deviation calculated
Report breakdown values, date,
temperature
New Specification EN60156
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Replaces IEC156 (1963)
Optional stirring
Current limited breakdown for silicone oils
Gives guidance on scatter of results
Withstand Testing
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Test voltage held at preset level for
1 minute
Quick test of level of contamination
Increase to breakdown to find
headroom?
Withstand Testing
Test Voltage
60
50
40
30
20
10
0
0
10
20
30
40
50
Time
60
70
80
90
MEGGER Foster OTSAF/2
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Fully automatic, unattended operation
Wide range of tests including five custom
Results include standard deviation, time, date
Results printed or download via RS232
Stirring options for magnetic or propellor
60 kV, 80 kV and 100 kV versions
MEGGER Foster OTSAF/2
MEGGER Foster OTS60PB
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Portable oil test set for on site measurement
Battery powered for two days continuous use
Fully automatic, unattended operation
Pass/fail levels for go/no go testing
Small and lightweight (19 kg = 42 lb)
Complete with battery charger
Variety of electrodes
RS232 interface plus optional printer
MEGGER Foster OTS60SX
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Low cost, Semi automatic set
60 kV maximum output, 0,1 kV resolution
Simple START, STOP & PAUSE control
Weighs only 17,5 kg
Optional vessels for all specifications
Auto 60 second timer for withstand testing
Operates from 110/230 V power supply
MEGGER KF2000
Karl Fischer moisture in oil test set
 Simple to use
 Quick results
 Low cost
 Battery powered, portable
MEGGER KF2000
AVO INTERNATIONAL
Bringing
Reliability to
Amps
Volts
Ohms