Generator Testing Power Users Conference
Nov. 3-4, 2015
Bert Milano, consultant
[email protected]
Littleton, Colorado
303-973-8479
1
AC and DC Test Procedures
Refer to IEEE standards and
guides for procedures, test
voltages etc.
2
AC Tests
AC High Potential Test
Corona Probe Test
Power Factor
Partial Discharge. To be covered by
others
3
DC Tests
Insulation Resistance (IR)
Polarization Index (PI)
DC High Potential Voltage Withstand
IEEE Timed Voltage Step
DC Ramp
Reverse Energization
4
DC Testing Currents
Leakage
Conduction
Polarization Capacitive
Polarization Absorption ***
molecular orientation very slow!
Decays, hours, asphalt vs. epoxy
Degree of cross linking
5
DC Insulation Resistance
Comments:
Resistance corrected for temperature
Sensitive to moisture, contamination
Temperature correction issues
Instrumentation
1 minuteAbsorption current
6
Polarization Index Test
PI testing stator windings is fairly common
If PI decreases Insulation quality concern
leakage, conduction, water, moisture,
contamination or deterioration
For applied step voltage V:
PI = I1 minute / I10 minute Where I is the total insulation current:
leakage ,conduction, absorption (cap.0)
7
Polarization Index Test
PI = I1 (leakage + conduction + absorption)
I10 (leakage + conduction + absorption)
Leakage and Cross linking
8
Polarization Index Test
Reading obtained at 0.5 minute ***
Every minute to 10 minutes
Ratio, no units, size and temperature fairly
independent
Numbers greater than 2 acceptable
High PI number comments
9
Polarization Index Test
Significant Digits
0.5/0.07 = 7.14. Wrong
7 Correct
Can raise concerns over the years
Commercial test sets. excellent
10
Polarization Index Test
Below 2 winding wet, dirty, contaminated
Instrumentation accuracy
Thermal/PD damage/aging may affect IA
0.5 minute reading
Plot the data
See IEEE standards for further analysis
My paper
11
Absorption Current Response
IA=VCKT-n
Where V is the applied voltage step
C is the capacitance
T is time
K is the insulation constant
n is the absorption exponent
K and n can change with aging/damage
12
n Log10(PI)
IIA
1 10
Time (minutes)
I
IA (per IEEE)
13
PI Test
Why the 0.5 minute
reading?
&
Why plot?
14
PI I1/I10
1
I=IA
10
Time (minutes)
0.1
15
Crystal Bar Log-Log Plot
New bar,11.5 kV (rated13.8 kV), 35 MVA
Epoxy mica insulation, hydro unit
Excellent insulation (new)
Bar never placed in service
5 kV PI, inflection at 1.5 minutes
Error without the plot trending issue
16
Figure 2 Crystal Bar
0.0001
0.001
0.01
0.1
0.1 1 10
Time (minutes)
Cu
rren
t (u
A)
PI = 27.8
K = 1.43E-3
n = 1.44PI = 64
K = 1.46E-3
n = 1.81
0.017 uA
0.0155 uA
17
Crystal Bar Ramp Test Plot
Figure 3: Ramp DC test of a single stator (Crystal Power Plant) bar with and without grading treatment
shorted
Normal Grading Treatment Not Altered Grading Treatment Shorted
18
Insulation Ageing
n & K, related to insulation characteristics
IA=VCKT-n
Damage mainly near line end & front coils
Thermal - chemical changes
Mechanical - radial forces/damage
Voltage - Partial Discharge conductive
byproducts
19
PI Differences Between Phases
New 6.9 kV winding passed:
Acceptable 5 kV PI, passed 16 kV dc and 14 kV ac tests
PI test (2.00, 3.21 and 4.51)
Failed the Ramp DC test with distinction!
Winding rejected per DC ramp test
20
PI Differences Between Phases
Extreme conduction current
2 phases failed
Traced to water in 3 coils brazing
Epoxy is not waterproof, once wet,
delaminates, appears as insufficient
impregnation, and stays delaminated
PI after repairs: 3.25, 2.94, and 3.97
21
Voltage (kV)
Me
as
ure
d C
urr
en
t (
A)
FIGURE : Unacceptable ramped high-voltage DC test results
Scale Change
22
1 Minute High Potential
Withstand Test
Why one minute?
23
High Potential Test
Answer:
That is about how long a test
engineer can hold his breath!
24
AC and DC High Potential
Withstand Testing Go no go tests
No information as to quality
Insulation either passes or fails
Insulation good for 3 to 5 years ! If tested
to the appropriate voltage
25
High Potential Test
Main objection to DC Testing:
It stresses the end winding insulation
And AC high-pot doesnt stress or Test
the end windings
26
Why DC Test
Response:
End winding has same voltage rating!
Why not test the end winding?
Contamination, cracks
Grading Treatment
27
High Potential Test
End winding AC voltage during operation
Ac voltage distribution
Coil insulation capacitance ..high
Air space capacitance low
Thickness and Dielectric constant
Voltage appears across the air gap,
not the insulation
28
29
High Potential Test
End winding DC voltage during operation
1 minute test
Absorption current decay.
IR very high
Ia determines the voltage distribution
Voltage 15-30% of test value better (more
searching) than the AC test
30
DC High Potential Test
Secondary objection:
DC testing is hazardous to insulation 1959
Response: GE promoted 0.1 Hz test set!!
Absorption current to the rescue
1 minute test: conduction current is much
too low to affect the internal stress, BB
31
When Contaminated End Turns
Both tests stress the end windings
End Winding Condition:
Aging minimal
Mechanical minimal
Thermal minimal
Voltage minimal
32
Selecting the Test Voltage
Warning:
Hi-Pot Testing at a very low voltage is an
unconscious decision to run the unit to
failure.
Testing is Insurance
33
AC / DC Test Voltage Ratio
Reclamation (2 or 3 efforts)
EPRI and others
No significant differences between ac & dc
The Best study
Brown Boveri Review, April/May 1968,
Vol. 55, pages 208-214, Direct-Voltage
Testing of the Insulation of Windings for
Rotating Machines
34
BB: AC/DC Test V Ratio 1.56
35
AC / DC Test Voltage Ratio
VDC maintenance test Voltage:
1.7 times 85% = 1.44
Still lower than the 1.56!!
36
AC / DC Test Voltage Ratio
K factors for new insulation are larger, and
as the insulation ages the K factor drops.
Brown Boveri states This may be taken as
evidence that long-term changes (fatigue)
are revealed earlier by a DC test.
37
AC / DC Test Voltage Ratio
So:
Decide if you want your end of life winding
to fail in service or under test
There are other more valuable tests to
assess without the risk of failure
DC Ramp
Corona Probe
38
DC Ramp Testing
39
Ramp DC Testing Synopsis
Valuable non-destructive test
Does not damage insulation
Early warning and early termination
Automated 16.67 V/sec, or 1kV/sec
It is a very low-frequency test
Current plotted against voltage (time)
40
Ramp DC Testing Synopsis
Trend over time
Comparative analysis also of value
Phase-to-phase
Sister units
Excellent repeatability
41
Ramp DC Test Description
Ramp DC Test results provide information on the following current components (see next slide example): Capacitive
Absorption
Leakage/Conduction
The ramp linearizes absorption component
Capacitive component response to ramp is a constant current
Provides ability to analyze individual insulation current components
Thus any non-linear response provides diagnostic value Leakage/Conduction
42
IA ~ 2.6 A
Icon. or IL ~ 0.5 A
IC ~ 18.6 A
Example Ramp Test
43
Capacitive Component
I = C times V
