A COST EFFECTIVE, NUMERIC
TECHNIQUE
FOR PROJECTING QUALITY
OF INSULATION
AND IMPENDING FAILURES
Marcus O. Durham
THEWAY Corp
Robert A. Durham
RADCo Consulting
Abstract
 Hi
Pot
– Cable & Machines
– Numeric technique
– Predicts impending failure
DC Limitations
 No
correlation between AC strength & DC test
Experience is used
 Aged
insulation => AC Use => DC test
Life 5 times longer if not tested
 Flashover
transient weakens insulation
Reflected wave = 2 times peak
DC Limitations
 Higher
test V for DC than AC = Space Charge
Etotal = Espace + Einsulation
 Insulation
= 5000 V, 90 mil
 EAC = 55 volts / mil
 EDC = 10 X
 Result = Deterioration of insulation
To Tree or Not To Tree
 Problems
are recognized in polyethylene
 May
be in rubber as well
 5000
Volt insulation ???
Testers
Despite Limitations - DC Still Preferred
VOM (volt-ohmmeter)
 IR (insulation resistance)
 Hi Pot (high potential DC)

– More info about quality than other
– 60,000 V field
– 200,000 V lab
120.01
VAC
Ohms
V DC
A AC
OFF
A DC
Elevated Voltage
 Can
cause any insulation to fail
 Difficult
 Most
to interpret w/o destruction
valuable w/ historical data
 Experience,
skill, knowledge of local conditions major aids to determining suitability
Resistance vs. Current
Georg’s Law
 Vtest
= Rinsulation * Ileakage
 Resistance decreases as length increases
 Non-linear, parallel R
 I exponential as length Increases
 Conductivity often called leakage current
A/( k V * k ft ) => ( A/k V) / k ft
Leakage Conductance
 One
number is futile
 Length, diameter, insulation, geometry, voltage
 Bulk resistivity
 D
r =K log  
d
- EPDM 20,000 M W * k ft
- Polyethylene 50,000 M W * k ft
Leakage Conductance
EPDM

K = 20,000 M W * k ft
 G
= 0 .05  A / k V
* k ft

Derate for temp, moisture, oil

Lower values may still be good
Suitability for Reuse
Visual & 5 minute DC withstand
 Lack of Consensus
 75 mil EPDM
 11,000 - 25,000

Comparison Methods
Overpotential Test
Specified leakage current
 Leakage conductance
 Leakage w/ ratio of 3 to 1
 No consistent guidelines

Leakage Current Components
Capacitance Charging
 Between
conductor & ground
 Starts extremely high, decrease exponentially
 Drops to zero in few seconds
0.4
0.3
0.2
0.1
0
1
2
3
4
5
6
Leakage Current Components
Absorption
Dielectric insulation
 Result of Polarization
 Starts high, decreases slowly
 Stabilize in 5 minutes
 Reasonable in 2 minutes

0.25
0.2
0.15
0.1
0.05
0
1
2
3
4
5
6
Leakage Current Components
Conductance
 Steady
state value
 Over, under, around, through insulation
 Corona contributes
 Low value is good
0.5
0.4
0.3
0.2
0.1
0
1
2
3
4
5
6
Leakage Current -vs- Time
i = F + (I - F) e -t/RC
F = constant, property of material
I = initial
1/RC = time constant
Evaluation
More Thorough
1 - Apply increasing voltage
2 - Plot leakage current -vs- applied voltage
3 - If R = ideal, V does not affect I
4 - Increasing I => insulation weakened
A Technique
Not a Number
 Forecast
failure levels
 Normalize
–
–
–
–
p
I
V
L
Megohm - k ft
Microamps
Kilovolt
thousand feet
Step 1
Calculate Bulk Conductance
G=
1000 l
r
Contrast with measured
G = Ileak / Vtest
Step 2
Plot  A -vs- kV
microAmps
I=GV
Becomes upper boundary for Itest
KiloVolts
Step 3
Hi-pot Test
Perform hi pot test in steps
Step 4
Calculate Slope
Calculate derivative at each test point
d(i )
(i2 - i1)
m=
=
d(v ) (v2 - v1)
Step 5
Compare
Compare slope to
bulk conductance
M > G => impending failure
i = F (1 - e
av
)
microAmps
Step 6
Plot Forecast of Failure Point
KiloVolts
a=
F =
( ln i2 - ln i1 ) +( ln v1 - ln v 2 )
( v 2 - v1 )
i
1- e
av
Step 7
For Next Test Point
Recalculate
steps 3 - 6
to refine forecast current
Step 8
Calculate Forecast Voltage
Bulk
i =Gv
Test
i = F (1 - e av )
Intersection
 1   -G 
v =   ln

 a   aF 
Step 9
Calculate Comparative Quality
cq = forecast V / rated test V
Changes between tests show deterioration
Ratio < 40% indicates marginal quality
microAmps
Figure 2 - Results
Bulk
Conductance
kiloVolts
Test Data
Summary
Testing
Insulation for reuse - controversial
 DC Hi Pot common - despite problems
 Measure leakage I at test V
 Math technique to compare quality

Summary
Mathematical Technique
 Calculate
bulk conductance (G) - limit
 Calculate rate of change of leakage G
 Compare derivatives (slope)
 Calculate coefficients of leakage curve
 Determine forecast V at intersection
Questions ?
Questions ?