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ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL
Understanding the Performance of Parallel Temporary Protective Grounds
Thomas Lancaster, Shashi Patel, Josh Perkel, & Anil Poda
NEETRAC
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• Introduction• NEETRAC Test Program• Test Results• Modeling• De-Rating Factors• Conclusions
Outline
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• Available fault currents on the order of 80 kA exist on utility systems– Grade 7H assemblies are rated at 68 kA
• Need ability to reliably protect linemen• Limited information on the performance of
parallel TPGs– How much to de-rate each TPG?– How many TPGs to use?– What spacing to use?
• NEETRAC undertook large study to try and provide information on these questions
This project does not answer all questions about parallel TPGs
Results apply to tested TPG assemblies
Project Purpose / Scope
4ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL
ASTM F855 Symmetric & Asymmetric Fault Tests
Table 1X/R ≤ 1.8
Table 2X/R = 30
Extracted from ASTM F855-2015: Standard Specifications for Temporary Protective Grounds to be Used on De-Energized Electric Power Lines and Equipment
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Standards bodies suggest minimum 10% de-rating per TPG
This project was about getting the de-rating factors
Goal: Determine the De-Rating Factors
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• ASTM F855 Standard Specifications for Temporary Protective Grounds to Be Used on De-energized Electric Power Lines and Equipment– “The thermal withstand rating of each TPG used in the multiple assembly set should
be reduced by at least 10% to account for unequal current division”• IEEE 1246 - Guide for Temporary Protective Grounding Systems Used in Substations
– “To account for unequal current division, reduce the thermal current rating (4.6.2) by at least 10% of each TPG used in the multiple assembly set”
• U.S. Bureau of Reclamation Facilities Instructions, Standards, and Techniques – Personal Protective Grounding for Electric Power Facilities and Power Lines– “In grounding applications where a single personal protective ground cable does not
have the necessary withstand current rating, or would require an unacceptably large conductor, identical ground cables may be connected in parallel. To account for unequal current division between parallel grounds, derating multipliers should be applied as follows.”
De-Rating – What is out there?
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• Review conducted of all TPG fault testing performed by NEETRAC and supportive members– 14 projects in total– Covers projects 1996 – 2013– 352 separate fault tests– Many different hardware and configurations tested– Tests performed according to ASTM F855– 73% of tests utilized asymmetric fault currents
Historical TPG Tests
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5.1%Unknown
4.1%3
27.8%2
63.1%1
Multiple TPG Tests 17 Tests at 80 kA RMS
Historical data is not able to answer de-rating question
Historical Data – Single and Multiple TPGs
ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL
Test Program Design & Samples
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Level(s)
FixedParameters
Current Waveform X/R 30(Peak multiplier = 2.69)
Clamp Spacing 3.5, 6, or 12 inCable Length [ft] 20
Clamp Orientation Perpendicular to bus(ASTM F855)
Mounting Hardware None
Factors
TPGs in parallel [#] 23
Cable Sizes 2/0 (133 kcmil)4/0 (212 kcmil)
Current (RMS)2/0 AWG (Grade 3H) Irated = 31 kA4/0 AWG (Grade 5H) Irated = 47 kA
2/0 – 41.9 and 52.7 kA4/0 – 63.5 or 80 kA
Design Full Factorial Design 4 Replicates
Test Program Design
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• Replicates were used to verify observed performance
• TPGs are not inexpensive devices– Tests required 2 or 3 samples per test
Test Program Design - Complications
Replicate Tests[#]
Probability of all Tests Passing at Random
[%]1 502 253 134 6
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Final TPG SamplesProperty Specification
Cable Size 2/0 AWG 4/0 AWG
Ferrule Threaded w/shroud Threaded w/shroud
Strain Relief Yes Yes
Top Clamp Grade 3 C‐Clamp Grade 5 C‐Clamp
Bottom Clamp Grade 5 – Flat Face Grade 5 C‐Clamp
Cable Length 20 ft 20 ft
Install torque Manufacturer specified
ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL
Test Results
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• Group 1 tests completed at NEETRAC NJCL in September, 2016 – Single TPG performance verified before proceeding with
parallel tests– Setup developed to capture current split (i.e. current flowing in
each individual TPG)o First time this was doneo Setup likely impacted Pass/Fail results of tests due to
unequal impedance paths– 24 tests completed (8 at 80 kA)
• Group 2 tested in June, 2017– 29 tests Completed (21 at 80 kA, includes single TPGs)
Tests Completed
29 Tests at 80 kA
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Multiple return buses
Group 1 Laboratory Setup
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Single return bus
Group 2 Laboratory Setup
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• All samples new and unused• Spacing is center-to-center and equal for all TPGs in a
particular test• All clamps torqued to manufacturer recommendations
using calibrated torque wrenches• Any set screws also torqued to manufacturer
recommendations• Each TPG hung freely off the ground
Sample Installation (Both Groups)
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Triple TPG 12 in spacing, 80 kA RMS
Fault Tests are Challenging… (Video)
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Source
Mechanical and Thermal Failure (Video)
ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL
4/0 AWG TPG Tests
Rated Current = 47 kATest Currents = 63.5 & 80.0 kA
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Source
Survive – 4/0 AWG TPG
12 in Spacing, 63.5 kA RMS(Test 7-3, Trip 1)
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Source
Failure – 4/0 AWG TPG
12 in Spacing, 80 kA RMS (Test 8-5, Trip 1)
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Source
Survive – 4/0 AWG TPG
6 in Spacing, 80 kA RMS (Test 8-2, Trip 2)
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Source
Survive – 4/0 AWG TPG
3.5 in Spacing, 80 kA RMS (Test 7-2, Trip 2)
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CableSize
TPGs[#]
ClampSpacing
[in]
Restraint[Yes/No]
ReturnBus
Config.
63.5 kA(1.35 Irated)
80.0 kA(1.7 Irated)
4/0 AWG
2 12No Split 3/4 0/4
Yes* Single Not Tested 0/4
3
12
No Split 4/4 0/4
Yes*
Single Not Tested
3/4
No
0/3
6 1/4
3.5 4/4
* Restraints were not installed deliberately. Support frame acted as restraint unintentionally.
Overall Results – 4/0 AWG Tests
ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL
Modeling
Expected Current SplitsForces Involved
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Parallel TPGs Simulation models are useful understanding the sensitivity
& probabilistic aspects of TPG performance
(i.e. Monte Carlo)
Simulink Model
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10
01
02
03
04
05
06
07
08
0011 0021 0031 0041 0051 0061 0071 008
E
]%[ selp
maS GPT
]Ωµ[ ecnatsiseR detamits
2
elbaCeziS
0/40/
Primary sample dependent parameter:Resistance
(all meet ASTM F2249)
Randomly generate a resistance for each TPG in Monte Carlo simulation
TPG Resistance Distributions
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Impacts force TPGs experienceF ∝ Ij Ik
Impacts heat generated during faultT ∝ I
TPG 3TPG 2TPG 1
80000
75000
70000
65000
60000
TPG (TPG 1 closest to source bus)
Curr
ent P
eak
[A]
73908.1
68380.7
72867.9
TPG 3TPG 2TPG 1
30000
28000
26000
24000
22000
20000
TPG (TPG 1 closest to source bus)
RMS
Curr
ent [
A]
27475.1
25420.3
27088.4
Peak Current
RMS Current
Monte Carlo Predicted Current SplitsThree 4/0 TPGs, 80 kA, 12 in Spacing, X/R = 30
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• ASTM F855 Grade 5H TPGs are designed to withstand 47 kA for 15 cycles.
• Based on current magnitude alone, 3 TPG’s should survive 80 kA RMS fault.
• BUT…High power tests show that 3 TPGs survive at 3.5 in and not at 12 in. TPG 3TPG 2TPG 1
50000
45000
40000
35000
30000
25000
20000
TPG (TPG 1 closest to source bus)
RMS
Curr
ent [
A]
27475.1
25420.327088.4
Grade 5H Single TPG Withstand Level
De-Rating
Forces between TPGs are not equal(and they are large)
Why do we need to de-rate?
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Iin
I1 I2 I3
Source F12
F13
F1 = ∑Forces = F12 + F13 + F1Loop
F1Loop
Force Types1. TPG to TPG2. Bus on TPG
Forces on TPG 1
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Iin
I1 I2 I3
Source F21 F23
F2 = ∑Forces = F23 – F21 + F2Loop
F2Loop
Forces on TPG 2
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Iin
I1 I2 I3
Source F32
F31
F3 = ∑Forces = F3loop – F31 – F32
F3Loop
Forces on TPG 3
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SourceIFault
SourceIFault
Spacing Impacts Clamp Rotation
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12 in Spacing Max Difference0.1 in Spacing Max Difference
14000
12000
10000
8000
6000
4000
2000
0
Max
Diff
eren
ce in
Pea
k Cu
rren
t [A]
4854.57
5822.33
1 kA difference in peak amplitude predicted for 0.1 in and 12 in spacing
Modeling Example 1Impact of Spacing on Current Split
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TPG 3TPG 2TPG 1
1450
1400
1350
1300
1250
1200
1150
1100
Resi
stan
ce [m
icro
-Ohm
]
1212.49
1349.59
1209.59
Simulated TPG at ASTM F2249 limit
TPG 3TPG 2TPG 1
80000
75000
70000
65000
60000
TPG (TPG 1 closest to source bus)
Curr
ent P
eak
[A]
73908.1
68380.7
72867.9
TPG 3TPG 2TPG 1
80000
75000
70000
65000
60000
TPG (TPG 1 closest to source bus)
Peak
Cur
rent
[A]
75817.9
65307.7
73971.4
Original Now
Current distributions change significantly because of TPG 2 resistance
Modeling Example 2Mixed TPG Groups (Old & New)
ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL
De-Rating TPGs
Two cannot handle twice as much current as one…
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CableSize
TPGs[#]
ClampSpacing
[in]
Restraint[Yes/No]
ReturnBus
Config.1.35 Irated 1.7 Irated
2/0Irated = 31 kA
212 No Single
2/2 0/1
3 Not Tested 3/3
4/0Irated = 47 kA
2 12No Split 3/4 0/4
Yes* Single Not Tested 0/4
312
No Split 4/4 0/4Yes*
Single Not Tested
3/4**
No0/3
6 1/43.5 4/4
* Restraints were not installed deliberately. Support frame acted as restraint unintentionally.** One test excluded since cable shorted to setup bus work after releasing from bus during test
Overall Results
39ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL
• For 2 TPG cases:Base Current = 2 Irated (De-Rating Factor = 0)
Example Applied Current = 1.70 Irated
Difference = (2 – 1.70) Irated
= 0.30 Irated
De-Rating Factor per TPG = 0.30 / 2 = 0.15 (15%)• For 3 TPG cases:
Base Current = 3 Irated (De-Rating Factor = 0)Example Applied Current = 1.70 Irated
Difference = (3 – 1.70) Irated
= 1.3 Irated
De-Rating Factor per TPG = 1.30 / 3 = 0.43 (43%)
How to Calculate De-Rating Factors
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TPG Cable Size
[AWG]
RMS TestCurrent
[kA]
Test Current[x Irated]
TPGs[#]
De-Rating Factor
[% per TPG]
2/0
31.0 1.0 1 0
41.9 1.352 323 55
52.7 1.702 153 43
4/0
47.0 1.00 1 0
63.5 1.352 323 55
80.0 1.702 153 43
Tested De-Rating Factors
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ASTM allows pass rate on QA test of 75% or higher
De-Rating Factors for Test Program (New TPGs)
TPG Size
TPGs[#]
O.C. Spacing
[in]
De-Rating Factor≥ 75%
Survival Rate[% per TPG]
100%Survival Rate[% per TPG]
2/0 AWG
2 12.0 32 32
3 12.0 43 43
Unrestrained Only
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TPG Size
TPGs[#]
O.C. Spacing
[in]
De-Rating Factor≥ 75%
Survival Rate[% per TPG]
100%Survival Rate[% per TPG]
2/0 AWG
2 12.0 32 32
3 12.0 43 43
4/0 AWG
2 12.0 32 > 32
3
3.5 43 43
6.0 > 43 >> 43
12.0 55 55
Unrestrained Only
De-Rating Factors for Test Program (New TPGs)
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• Performance of parallel TPGs during faults are impacted by:– Relative positions / spacing– Degree of difference in current paths (additional/unequal
impedance)– Absolute current magnitude (not relative magnitude)
o 80 kA for 4/0 AWG is more difficult than 53 kA for 2/0 AWG– Restraining the cables to a structure
• Interaction of TPGs with fault current as well as themselves is complicated to predict but may be modeled to some extent– Indicates that large changes in currents can result for small
changes in setup
Conclusions
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• For the samples and configurations used in this project:– De-rating factors for unrestrained TPGs mounted directly to buswork are
clearly higher than the 10% commonly discussed in industry standardso De-rating factor is greater than 30% o Other grounding methods may need to be considered
• De-rating factors for other TPG assemblies or mounting techniques may be different.
Conclusions
45ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL
Suggestions for F18 Committee to Consider• De-rating factors
– Current standards suggest 10% minimum but the minimum part is generally ignored by utilities
– Should provide reasonable de-rating factors based on actual performance of TPG assemblies
• Spacing – Clearly, clamps should be installed as close together as possible– 1 ft may be too much
• Guidance for Design / Qualification Tests– How many samples?– Current F855 specifies considers 2/2 OR 3/4 pass rate acceptable
for QA tests. Is a 75% passing rate high enough?
46ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL
• Effect of different mounting orientation– Degree of performance improvement (i.e. decrease in de-rating
factor)• Unequal spacing for 3-TPG setups• Other clamp styles (duckbill, all-angle, etc.)• Multi-cable clamps• Restraint
– Tying all cables together at different locations– Securing to structures
• Mechanically stronger clamp & ferrule combinations• Mixtures of new and slightly used TPGs (increase of inequality in
current split)
Still Many Unanswered Questions
ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL
Thank you for your attention
Questions?
ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL 48ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL
4/02/0
2000
1900
1800
1700
1600
1500
1400
1300
1200
1100
Cable Size [AWG]
Estim
ated
Res
istan
ce [µΩ
]
1349
1956
1208.04
1770.06
ASTM F2249 2/0 Maximum (not to exceed)
ASTM F2249 4/0 Maximum (not to exceed)
TPG Samples - Resistances
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Single TPG performance verified at H rating
Single TPG Tests
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Two primary failure mechanisms:• Mechanical – High electromechanical forces exerted on clamps as
cables move during the fault– What happens?
o Clamp or ferrule breako Clamp loosens and detaches from structure
– When?o During first few cycles of fault application
• Thermal – Melting of clamp, ferrule, or cable that occurs because of Ohmic heat generation – What happens? - Clamp or ferrule melts– When? - During last few cycles of fault application
How does a TPG fail during a fault test?
ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL
2/0 AWG TPG Tests
Rated Current = 31 kA RMSTest Currents = 41.9 & 52.7 kA RMS
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Return
Bottom Clamps
Source
Survive – 2/0 AWG TPG
12 in spacing, 41.9 kA RMS (Test 5-3 , Trip 2)
ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL 53ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL
Return
Bottom Clamps
Source
Survive – 2/0 AWG TPG
12 in spacing, 52.7 kA RMS (Test 6-8, Trip 2)
ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL 54ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL
Overall Results – 2/0 AWG Tests
CableSize
TPGs[#]
ClampSpacing
[in]
Restraint[Yes/No]
ReturnBus
Config.
41.9 kA(1.35 Irated)
52.7 kA(1.7 Irated)
2/0 AWG2
12 No Single2/2 0/1
3 Not Tested 3/3
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47 kA RMS Fault, X/R = 30
Total Fault CurrentIndividual TPG currents
(small phase differences)
Simulated Fault Current Waveform
ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL 56ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL
• Current predictions do not match measured values nor are they as expected
TPG 3TPG 2TPG 1
80000
75000
70000
65000
60000
TPG (TPG 1 closest to source bus)
Curr
ent P
eak
[A]
73908.1
68380.7
72867.9
TPG 3TPG 2TPG 1
80000
75000
70000
65000
60000
TPG (TPG 1 closest to source)
Peak
Cur
rent
[A]
Measured Currents Predicted Currents
Is the model right?
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• Added impedance also identified by examining the movement of the TPG cables.
• Cables should meet near TPG 2 during a 3-TPG test.
• Current split in Trip 1 indicates additional impedance imbalance
3
2
1
Trip 2, 80 kA3
2
1
Trip 1, 80 kA
Cables meet at different positions
Challenges of Measuring Current Split
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P1 Shunt
TPG 1 TPG 2TPG 3
Path lengths differentAdded self and mutual inductances
P3 Shunt
P2 Shunt
Measurement of Current Split
ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL 59ASTM International F18 – F855 Meeting, October, 2018, Orlando, FL
Actual Waveform Simulation Waveform
Inductance Added:L1=6µHL2=3.5µHL3=4.5µH
Resistance Added:R1=601µΩR2=383µΩR3=470µΩ
ADD TO THE MODEL
Tweak the Model with Series Impedance
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TPGType
321PredictedMeasuredPredictedMeasuredPredictedMeasured
80000
75000
70000
65000
60000
Curr
ent [
A]
Measured = Tests completed at NJCLPredicted = Monte Carlo simulation
Desired model can be made by removing required additional series impedance
Current Split Model - Verification(includes Measurement Bus Work)
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TPG 3TPG 2TPG 1
80000
75000
70000
65000
60000
TPG (TPG 1 closest to source bus)
Curr
ent P
eak
[A]
73908.1
68380.7
72867.9
80 kA RMS, 12 in spacing, X/R = 30
Predicted Currents – Single Bus Return