Diagnostics of Cable Systems
Field Testing –
Commissioning & Routine
Tests II
Bruce Olson & Nigel Hampton
Diagnostics of Cable Systems
Nigel Hampton
2
Diagnostics of Cable Systems
Outline
• Diagnostic Techniques
• Purpose of Diagnostics
• Accuracies
• Diagnostic Outputs
3
Diagnostics of Cable Systems
Test Types
Testing
Development
– tests made in the development phase & prior to approval tests
Approval
– tests made on a design, before commercial supply,
to show satisfactory performance
Routine
– tests made on every component
Field
– tests made on a complete system
4
Diagnostics of Cable Systems
Field Tests
Field Testing
Commissioning
Diagnostic
Return to Service
Normal Work
After Failure
Diagnostics of Cable Systems
Commissioning
Tests carried out with a known chain of
custody – we know what has
happened to the system
Diagnostics of Cable Systems
Commissioning Tests- HV & EHV
HV – mainly Proof Test with a resonant system only,
some PD
EHV – Proof Test with a resonant system and PD
Diagnostics of Cable Systems
Voltage Time Profile for HV & EHV
Commissioning
8
Diagnostics of Cable Systems
Commissioning Tests - MV
• Soak / System Voltage
• Very Low Frequency (VLF) Proof Tests
• Partial Discharge (PD)
• These do not replicate Factory Tests as, generally, only
PD or Proof tests are done
• Factory Test criteria can be relevant in decision making
Diagnostics of Cable Systems
Bruce Olson
Diagnostics of Cable Systems
Diagnostics
Tests carried out without a known
chain of custody – we really don’t know
what has happened to the system
Diagnostics of Cable Systems
Diagnostic Tests - MV
Technologies currently discussed:
• Time Domain Reflectometry (TDR)
• Simple Dielectric Withstand
• Dielectric Loss (Tan δ & Dielectric Spectroscopy*)
• Online Partial Discharge (PD)
• Offline Partial Discharge (PD)
• Isothermal Relaxation Current (IRC)*
• Recovery Voltage (RV)*
• Damped AC (DAC)*
* - currently not used in North America
Diagnostics of Cable Systems
Time Domain Reflectometry
Diagnostics of Cable Systems
TDR Equipment
Accessed 9/30/2009 http://www.baur.at/en/products/cable-fault-location/irg-2000.html
Diagnostics of Cable Systems
Time Domain Reflectometry (TDR)
Test Description
• Measures changes in the cable impedance as a function of
circuit length by observing the pattern of wave reflections.
• Used to identify locations of accessories, faults, etc.
Field Application
• Offline test that uses a low voltage, high frequency pulse
generator.
• Testing may be performed by a service provider or utility crew.
Diagnostics of Cable Systems
TDR Principles
Far
End
Near End
TDR
Equipment
L
Joint
Joint
Diagnostics of Cable Systems
TDR can be Feature Rich
Diagnostics of Cable Systems
Simple Dielectric Withstand
18
Diagnostics of Cable Systems
VLF MODELS FROM VARIOUS VENDORS
28kV
60kV
34kV
28kV
30kV
60kV
56kV W/PD & TD
34kV
60kV
54kV
60kV
Diagnostics of Cable Systems
VLF Waveforms
HVA30 @5kV RMS with 280feet XLPE Load
8000
6000
4000
Cosine-Rectangular
0
-2000
SEBAKMT VLF40 @5kv RMS with 280feet XLPE Load
-4000
6000
-6000
4000
-8000
2000
Sinusoidal
Voltage (kV)
Voltage (kV)
2000
0
-2000
-4000
-6000
-8000
Diagnostics of Cable Systems
Simple Dielectric Withstand
Test Description
• Application of voltage above normal operating voltage for a
prescribed duration.
• Attempts to drive weakest location(s) within cable segment to
failure while segment is not in service.
Field Application
• Offline test that may use:
–
–
–
–
DC
60 Hz. AC
VLF AC
Damped AC
• Testing may be performed by a service provider or utility crew.
Diagnostics of Cable Systems
Setup
VLF
DUT
22
Diagnostics of Cable Systems
23
Diagnostics of Cable Systems
Withstand Test Process
oltage
The goal is to have
circuit out of service,
test it such that
“imminent” service
failures are made to
occur on the test
and not in service
HOLD
EARLY
Hold Entry
Voltages and Times for VLF
covered in IEEE Std. 400.2
Ramp Entry
t=0
tTest
Time
Diagnostics of Cable Systems
Dielectric Loss (Tan δ)
25
Diagnostics of Cable Systems
TD Assesment
26
Diagnostics of Cable Systems
Tan δ Equipment
Diagnostics of Cable Systems
Dielectric Loss (Tan δ)
Test Description
• Measures total cable system loss (cable, elbows, splices & terminations).
• May be performed at one or more frequencies (dielectric spectroscopy).
• May be performed at multiple voltage levels.
• Monitoring may be conducted for long durations.
Field Application
• Offline test that may use:
– 60 Hz. AC
– VLF AC
– Damped AC
•
•
Testing may be performed by a service provider or utility crew.
Step voltage up to pre determined level with post test analysis
Diagnostics of Cable Systems
Setup
VLF
Tan
Delta
DUT
Data
29
Diagnostics of Cable Systems
Cable System Equivalent
T
C
S
C
T
Diagnostics of Cable Systems
Cable System Equivalent
T
Cable system (cable, splices, and terminations) is
reduced to simple circuit.
C
S
C
T
Tan δ
Diagnostics of Cable Systems
SIMPLIFIED CABLE MODEL AND PHASOR
Tan Delta = IR/IC = 1/(2πfCR)
I
The tangent of this
angle is calculated
IC
δ
IR
IC
IR
= tangent of δ
V
Diagnostics of Cable Systems
Dielectric Loss Test Process
Voltage
Loss measurement
Time
Diagnostics of Cable Systems
Tan δ Ramp Test Data
100
Voltage
[p.u.]
0.5
1.0
1.5
1.7
90
Tan-delta [1e-3]
80
70
Mean
60
Tip Up
50
40
30
Scatter
(represented by
30 Cycles
Standard Deviation - IQR
could be used)
20
10
0
1
2
3
4
5
Time [min]
34
Diagnostics of Cable Systems
Tan δ Monitored Withstand
Diagnostics of Cable Systems
Monitored Withstand Test Protocol
Voltage
Tan δ
Ramp
HOLD
Focus in this section is on the Hold
Phase
Time
Diagnostics of Cable Systems
Ways Not to Pass a Monitored Withstand
Failure – Insulation puncture
OR
High Dielectric Loss
OR
High Instability – Measured by standard deviation in
consecutive measurements at one voltage level
Diagnostics of Cable Systems
Tan δ - MV Test Protocol
Features:
• Stability @ 0.1 Hz & U0
• Tip Up @ 0.1 Hz & between 1.5 U0 & 0.5 U0
• Mean Tan δ @ 0.1 Hz & U0
Frequency
[Hz]
Ramp
Withstand
0.10
0.05
0.02
0.25 0.50
0.75
1.0
1.25
1.50
1.75
2.0
2.25
2.5 Voltage
[U0]
38
Diagnostics of Cable Systems
If this had been a Simple Withstand
No Failures On Test
18 Segments Tested
0
2
4
6
Length Tested (miles)
8
10
Diagnostics of Cable Systems
Monitored Withstand - Stability
18 Segments Tested
Pass - Stable Loss
Pass - Un Stable Loss
30 min test
60 min test
0
2
4
6
Sequence of Lengths Tested (miles)
8
10
Diagnostics of Cable Systems
Offline Partial Discharge
41
Diagnostics of Cable Systems
Offline PD Equipment for MV or HV
Diagnostics of Cable Systems
Offline Partial Discharge
Test Description
• Measurement and interpretation of partial discharge signals
above normal operating voltages.
• Signal reflections (combined with TDR information) allows
location to be identified within cable segment.
Field Application
• Offline test that may use:
– 60 Hz. AC
– VLF AC
– Damped AC
service provider
utility crew
utility crew
• Step voltage up to pre determined level with post test analysis
Diagnostics of Cable Systems
PD Setup and Calibration
•
Cable Map?
•
TDR
•
–
Length
–
Splices
–
Propagation velocity
Calibration
–
Inject 0.5, 1, or 2 nC signal
–
Verify TDR info
Jicable’11, 19 - 23 June 2011 - Versailles France
Diagnostics of Cable Systems
PD Interpretation
–
Voltage
–
PD Level
–
Frequency
–
Location
–
Discharge Pattern (phase resolved)
–
Phase to phase
–
Year to year
Diagnostics of Cable Systems
Setup
VLF
FILTER
Data
DUT
PD
SENSORS
46
Diagnostics of Cable Systems
47
Diagnostics of Cable Systems
Differences for PD Tests
Laboratory
Calibrated – how many
mV = pC
Sensitivity Check – can a
PD like signal be seen
PD Technique
Criteria
Field
X
X
Defined
IEC
Variable
Wide
Bandwidth
Defined
Depends
upon
situation
Diagnostics of Cable Systems
Offline PD Test Process
oltage
Discharge measurement
Time
Diagnostics of Cable Systems
PD Pulse
Diagnostics of Cable Systems
Offline PD (60 & 0.1Hz) Outcome Sequences
A
B
C
No PD
PD
Diagnostics of Cable Systems
Online Partial Discharge
52
Diagnostics of Cable Systems
Setup
DUT
Data
PD
SENSORS
53
Diagnostics of Cable Systems
Online PD Equipment
http://www.utilx.com/pdfs/CableWISE_Generation_Sample_Report.pdf
Diagnostics of Cable Systems
Online Partial Discharge
Test Description
• Measurement and interpretation of discharge and signals on
cable segments and/or accessories.
• Signals captured over minutes / hours.
• Monitoring may be conducted for long durations.
Field Application
• Online test that does not require external voltage supply (no
customer outage required)
• Testing performed by a service provider.
• Assessment criteria are unique to each embodiment of the
technology
• Measurements require sensor placement at multiple locations
along cable circuit
Diagnostics of Cable Systems
Online PD Test Process
oltage
Continuous measurement
U0
Time
Diagnostics of Cable Systems
Distribution of PD along Lengths
•
5000 ft. portion of sample feeder
•
Mixture of different PD levels for different sections and accessories.
Cable Section
No PD
PD
Accessory
Diagnostics of Cable Systems
Nigel Hampton
58
Diagnostics of Cable Systems
Current Use of Diagnostics
59
Diagnostics of Cable Systems
Safe
60
Diagnostics of Cable Systems
Effective
61
Diagnostics of Cable Systems
Useful
62
Diagnostics of Cable Systems
Valuable
63
Diagnostics of Cable Systems
Utility Use of MV Diagnostics
VLF
DC
Tan Delta
PD On
PD Off
TDR
IRC
DA C
Category
No Use
Occasional
Standard
Testing
Diagnostics of Cable Systems
Lengths Tested
PD
10.0
Tan D
Median 814 ft
Median 485 ft
7.5
5.0
Percent
2.5
0.0
VLF Withstand
100
1000
10000
100000
10.0
Panel variable: Technique
Median 3500 ft
7.5
Based on diagnostic
data supplied to CDFI
5.0
2.5
0.0
65
100
1000
10000
100000
Measurements made
with TDR
Cable Length - log (ft)
TDR
Diagnostics of Cable Systems
Distribution of Causes for MV Failures
Unknown
1.1%
Terminations
5.6%
Splices
37.1%
Cable
56.2%
Diagnostics of Cable Systems
Locations of Failures differ between Utilities
39
Utility
C1
55
21
8
5
18
12
9
22
3
17
6
10
20
1
2
11
24
19
15
7
13
23
4
14
16
Cable
Unk nown Source
A ccessories
0
20
40
Percentage
60
80
100
Diagnostics of Cable Systems
Accuracy
68
Diagnostics of Cable Systems
Objective of Diagnostic Tests
The target population contains both “Good” and “Bad” components
– “Good” – Will not fail within diagnostic time horizon
– “Bad” – Will fail within diagnostic time horizon
“Bad” Components
Target Population
“Good” Components
Diagnostics of Cable Systems
Diagnostic Operation
Applying the diagnostic will separate the population into:
• No Action Required group
• Action Required group
No Action Required
Action Required
Diagnostics of Cable Systems
Accuracies
Variable time horizons of 1-8 years
No Action Accuracy
Action Accuracy
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 177 188 19 20 21
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Diagnostic Accuracy
100
80
60
40
20
0
Dataset
Diagnostics of Cable Systems
Group Sizes – Act vs. Not Act
100
Tests [% of Total]
80
60
40
20
0
Act
Not Act
Diagnostics of Cable Systems
All Accuracies
100
100
92.5
98.9
Diagnostic Accuracy
80
60
49.7
40
20
10
0
0.5
No Action Accuracy
Action Accuracy
Overall Accuracy
Diagnostics of Cable Systems
Using Results from
Diagnostics
74
Diagnostics of Cable Systems
Outputs from Diagnostics
Numeric
Time Reflectometry (TDR)
Classification
Pass / Fail
Good / Bad
1, 2, 3, 4, 5
NA, FS, AR
Replace / Defer
X
Simple Dielectric Withstand
Dielectric Loss (Tan δ)
Survival
X
X
X
X
Online Partial Discharge (PD)
X
Offline Partial Discharge (PD)
X
Monitored Withstand
Characteristics
X
Transparent
Repeatable
Updatable
X
X
Unambiguous
Opaque
Not Repeatable
Not Updateable
75
Diagnostics of Cable Systems
Route to Use Diagnostics
• Numbers
• Pass / Fail
• Classes
Data
Information
• Field Tools
• Asset Health
Test
Data
Feature
Extraction
Knowledge
Rules
Experience
Library
• Diagnostic
Features
• System Data
76
Diagnostics of Cable Systems
Knowledge Rule - Age
1975
1979
25.0%
28.1%
71.9%
75.0%
1982
Good
Poor
100.0%
77
Diagnostics of Cable Systems
Snohomish PUD Cable System
• Two areas tested
– Glenwood Mobile Homes (1976 & 1981 vintage)
– Evergreen Shopping Center (1986 vintage)
• Homogeneous cable system
–
–
–
–
–
–
78
15 kV voltage class (operated at 7.2 kV)
XLPE insulation
1/0 conductor
Unjacketed concentric neutral
Installed in conduit – no splices
Terminated with elbows
Diagnostics of Cable Systems
Testing Sites
Glenwood Mobile Homes
Evergreen Shopping Center
79
Diagnostics of Cable Systems
Glenwood Mobile Estates
80
Diagnostics of Cable Systems
Knowledge Rule
Field Tool
Tan δ Criteria for PE-Based Insulation
Condition
Assessment
VLF-TD Stability
(standard deviation)
at U0
[10-3]
No
Action
Required
(Best 80 %)
<0.05
Further
Study
0.05
to
0.5
Action
Required
(Worst 5 %)
>0.5
Mean
VLFTD
at U0
[10-3]
Differential TD
TD 1.5 U0 – TD 0.5
U0
[10-3]
&
Or
<5
5
to
80
>80
&
Or
<4
4
to
50
>50
81
Diagnostics of Cable Systems
Tan δ Map (Tip Up & Mean Tan δ)
Tip Up (1.5Uo - 0.5Uo) [E-3]
1000.00
4
50
Evergreen Shopping Center
4
50
Glenwood Mobile Homes
100.00
80
10.00
5
1.00
0.10
0.01
0.1
1.0
10.0
100.00.1
TD @ Uo [E-3]
1.0
10.0
100.0
82
Diagnostics of Cable Systems
Basic Assessment Summary
Evergreen
12 Ckts
1986 Vintage
Glenwood
15 Ckts
1976 & 1981 Vintage
Action Required
13.3%
Further Study
26.7%
No Action Required
100.0%
No Action Required
60.0%
83
Diagnostics of Cable Systems
Example - No Action Required
1000
Test
Voltage
[Uo]
0.5
1.0
1.5
Tan δ Stability
Tip Up
Mean Tan δ
Tan Delta [E-3]
100
10
1
0
5
10
15
20
Measurement Sequence [#]
25
30
84
Diagnostics of Cable Systems
Example - Further Study
1000
Test
Voltage
[Uo]
0.5
1.0
1.5
Tan δ Stability
Tip Up
Mean Tan δ
Tan Delta [E-3]
100
10
1
0
5
10
15
20
Measurement Sequence [#]
25
30
85
Diagnostics of Cable Systems
Wet Circuits
86
Diagnostics of Cable Systems
Knowledge Rule
Field Tool
Monitored Withstand Criteria (PE)
Condition
Assessment
Change in
Tan Delta
between 0
and 10 mins
(E-3)
VLF-TD Stability
(standard
deviation) at
Maintenance
Level [10-3]
Reduce to 15 Mins
<0.25
and
<0.25
and
<5
Extend to 60 Mins
>17
or
>6
or
>45
Mean VLF-TD
at
Maintenance
Level [10-3]
87
Diagnostics of Cable Systems
Monitored Withstand with Water?
Snopud Segment 6912
Test
Voltage
[Uo]
0.5
1.0
1.5
Stability@Uo = 0
Tip Up = 13.4
Tip Up on Tip Up= 12.8
TD@Uo=1.3
Tan Delta [E-3]
10
Stability@1.5Uo=2.7E-3
1
-5
0
5
10
15
Test Time [min]
20
25
30
88
Diagnostics of Cable Systems
0 to 10 mins
Snopud Segment 6912
Change
in Tan
Delta
between
0 and 10
mins
(E-3)
VLF-TD
Stability
(standard
deviation)
at
Maintenance
Level
[10-3]
Reduce to
15 Mins
<0.25
<0.25
<5
Extend to
60 Mins
>17
>6
>45
Difference between
10 & 0 mins
= -4.5
Tan Delta [E-3]
10
Test
Voltage
[Uo]
0.5
1.0
Mean VLF1.5 TD at
2.2
Maintenance
Level [10-3]
1
-5
0
5
10
15
Test Time [min]
20
25
30
89
Diagnostics of Cable Systems
0 to 30 mins
Snopud 6912
Test
Voltage
[Uo]
0.5
1.0
1.5
2.2
Tan Delta [E-3]
10
1
-5
0
5
10
15
Test Time [min]
20
25
30
90
Diagnostics of Cable Systems
Final Classification - Retest
1000
Test
Voltage
[Uo]_2
0.5
1.0
1.5
Tan δ Stability
Tip Up
Mean Tan δ
Tan Delta [E-3]
100
10
1
0
5
10
15
20
Measurement Sequence [#]
25
30
91
Diagnostics of Cable Systems
Example – Action Required
1000
Test
Voltage
[Uo]
0.5
1.0
1.5
Tan δ Stability
Tip Up
Mean Tan δ
Tan Delta [E-3]
100
10
1
0
5
10
15
20
Measurement Sequence [#]
25
30
92
Diagnostics of Cable Systems
Something for another day – Health Indices
Stability 0
Tip Up
614
Tan Delta 2.3
50
0.5
0
1
2
Condition
Assessment
VLF-TD Stability
(standard deviation)
at U0
[10-3]
No
Action
Required
<0.05
Further
Study
0.05
to
0.5
Action
Required
>0.5
Mean
VLF-TD
at U0
[10-3]
What is the classification ?
Differential TD
TD 1.5 U0 – TD 0.5 U0
[10-3]
&
<5
Or
5
to
80
>80
&
<4
Or
4
to
50
>50
93
Diagnostics of Cable Systems
Nigel Hampton
Bruce Olson
94
Diagnostics of Cable Systems
SUMMARY
• Have the correct expectation
– Diagnostics are useful
– Diagnostics are not perfect
• There is no “magic bullet” or “one technique”
• Today the main goal of diagnostics is to indicate which
members of a target population are likely to have “good”
performance
• A lot of work continues in this area
95