12018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Performance of Parallel Temporary Protective Grounds in High Fault Current
Scenarios
Thomas Lancaster, Shashi Patel, Josh Perkel, & Anil Poda
NEETRAC
22018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
The information contained herein is, to our knowledge, accurate and reliable at the date of publication.
Neither GTRC nor The Georgia Institute of Technology nor NEETRAC shall be responsible for any injury to or death of persons or damage to or destruction of property or for any other loss, damage or injury of any kind whatsoever resulting from the use of the project results and/or data.
GTRC, GIT and NEETRAC disclaim any and all warranties, both express and implied, with respect to analysis or research or results contained in this report.
It is the user's responsibility to conduct the necessary assessments in order to satisfy themselves as to the suitability of the products or recommendations for the user's particular purpose. No statement herein shall be construed as an endorsement of any product, process or provider.
Notice
32018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
• Introduction• Survey Results• NEETRAC Test Program• Test Results• Modeling• Derating Factors• Conclusions
Outline
42018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Project Purpose / ScopeSurvey the member utility TPG work practices / available fault currents and perform testing to understand the following issues for multiple TPGs:
• Spacing of multiple TPGs (i.e., close together or far apart)
• Current split• Electromechanical forces on TPGs• Clamp mounting (if practical)
Data will be gathered for presentationto industry groups to enhance the standardsin their next revisions.
This project does not answer all questions about parallel TPGs
Results apply to tested configurations
52018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
ASTM F855 Symmetric & Asymmetric Fault Tests
Table 1X/R = 1
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
62018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Goal: Determine the Derating Factors
Standards bodies suggest minimum 10% derating per TPG
This project is about getting the derating factors
72018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Derating – What is out there?• ASTM F855 Standard Specifications for Temporary Protective Grounds
to Be Used on De-energized Electric Power Lines and Equipment–
• IEEE 1246 - Guide for Temporary Protective Grounding Systems Used in Substations
–
• U.S. Bureau of Reclamation Facilities Instructions, Standards, and Techniques – Personal Protective Grounding for Electric Power Facilities and Power Lines
–
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Historical TPG Tests
• 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
92018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
5.1%Unknown
4.1%3
27.8%2
63.1%1
Multiple TPG Tests
Historical Data - Single and Multiple TPGs
17 Tests at 80 kA RMS
Historical data is not able to answer derating question
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NEETRAC Member Survey
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• Understand the most common clamps, ferrules, and cable sizes used by NEETRAC member utilities
Survey Objectives
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TPG Clamps
0
2
4
6
8
10
12
14C
Cla
mp
C C
lam
p w
ithha
nger
Sprin
g lo
aded
Duc
k B
ill
Tow
er &
Flat
-Fa
ce C
lam
p
All-
Ang
leC
lam
p
What types of TPG clamps do you use?
Bronze Aluminum
132018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
0.0%
10.0%
20.0%
30.0%
40.0%
50.0%
60.0%
70.0%
80.0%
What grades of TPG assembly do you use?
0
5
10
15
20
25
What types of ferrules do you use?
Copper AluminumUnshrouded Shrouded
Grades and Ferrules
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Strain Relief
Do you use strain relief (also known as bend restrictor) at the cable-
ferrule connection?
Yes No
90%
10%
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TPG Cables
0%
20%
40%
60%
80%
100%
#2 1/0 2/0 3/0 4/0 250kcmils
350kcmils
Which cable sizes do you mostly use?
162018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
TPG Assemblies
02468
10121416
#2 1/0 2/0 3/0 4/0 250kcmil
350kcmil
Which TPG assemblies (clamp-ferrule -cable) do you use?
TPG assembled in houseTPG assembled by Manufacturer
0.0%10.0%20.0%30.0%40.0%50.0%60.0%70.0%80.0%90.0%
What cable lengths do you use?
172018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
TPG Connections & Work Rules
0.0%
20.0%
40.0%
60.0%
80.0%
100.0%
120.0%
When using TPGs, what do you usually connect the grounds to?
182018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Guidelines for Multiple TPGs
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Commonly Used TPG Hardware– Clamps - C-Clamps, Ball Socket, and Flat Face– Cable - 2/0 & 4/0 are most common cable sizes– Ferrules – Compression & Threaded– Cable lengths 5 – 40 ft (ASTM suggests 10 ft)
• Hardware data implies focus on Grades 3/3H and 5/5H assemblies.
• Recommendations for parallel TPGs reported as based largely on engineering judgement.
Important Survey Observations
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Test Program Design & Samples
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Test Program DesignLevel(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
222018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
• 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
232018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Alternative Test Programs Considered
Design DescriptionPhase 11 TPG
[#]
Phase 22 TPG
[#]
Phase 33 TPG
[#]
Current SplitCheck
[#]
Min TPGs Required
[#]
1 1 & 2 TPG6 Factors 32 128 -- 2 162
21, 2, & 3
TPGs6 Factors
32 128 192 5 357
31, 2, & 3
TPGs2 Factors
8 32 48 5 93
242018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Final TPG Samples
Property 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
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Test Results
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4/02/0
2000
1900
1800
1700
1600
1500
1400
1300
1200
1100
Cable Size [AWG]
Estim
ated
Res
istan
ce [μ
]
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
272018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Tests Completed
• 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)
First time this was doneSetup likely impacted Pass/Fail results of tests
– 24 tests completed (8 at 80 kA)• Group 2 tested in June, 2017
– 29 tests Completed (21 at 80 kA, includes single TPGs)
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Group 1 Laboratory Setup
Multiple return buses
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Group 2 Laboratory Setup
Single return bus
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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 (as in ASTM F855)
Sample Installation (Both Groups)
312018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Single TPG performance verified at H rating
Single TPG Tests
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Fault Tests are Challenging…(Video)
Triple TPG. 12 in spacing, 80 kA RMS
332018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
How does a TPG fail during a fault test?Two primary failure mechanisms:• Mechanical – High electromechanical forces exerted on
clamps as cables move during the fault– What happens?
Clamp or ferrule breakClamp loosens and detaches from structure
– When?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
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Mechanical and Thermal Failures (Video)
Source
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2/0 AWG TPG Tests
Rated Current = 31 kA RMSTest Currents = 41.9 & 52.7 kA RMS
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Survive - 2/0 AWG TPG, 12 in spacing,41.9 kA RMS (Test 5-3 , Trip 2)
Return
Bottom Clamps
Source
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Survive - 2/0 AWG TPG, 12 in spacing,52.7 kA RMS (Test 6-8, Trip 2)
ReturnBottom Clamps
Source
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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
392018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
4/0 AWG TPG Tests
Rated Current = 47 kATest Currents = 63.5 & 80.0 kA
402018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
• Trip 1 setup utilized individual return bus work for each TPG– Added different impedances to each TPG– Forced current away from TPG 1 and onto TPG 2– Represents “worse than expected” condition
• Trip 2 utilized single return bus and this changed the survival performance
• Impedance issue shows importance of maintaining as equal a return path as possible
– Same TPG lengths– Same mounting points
Multiple Trips?
412018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Survive - 4/0 AWG TPG, 12 in Spacing,63.5 kA RMS(Test 7-3, Trip 1)
Source
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Failure - 4/0 AWG TPG, 12 in Spacing,80 kA RMS (Test 8-5, Trip 1)
Source
432018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Survive - 4/0 AWG TPG, 6 in Spacing,80 kA RMS (Test 8-2, Trip 2)
Source
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Survive - 4/0 AWG TPG, 3.5 in Spacing,80 kA RMS (Test 7-2, Trip 2)
Source
452018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Overall Results – 4/0 AWG Tests
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
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.
462018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Modeling
Expected Current SplitsForces Involved
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Simulink Model
Parallel TPGsSimulation models are
useful understanding the sensitivity / probabilistic
aspects of TPG performance
(i.e. Monte Carlo)
482018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Simulated Fault Current Waveform
47 kA RMS Fault, X/R = 30
Total Fault CurrentIndividual TPG currents(small phase differences)
492018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
10
01
02
03
04
05
06
07
08
0011 0021 0031 0041 0051 0061 0071 008
E
]%[ selp
maS GPT
]μ[ ecnatsiseR detamits
2
elbaCeziS
0/40/
TPG Resistance Distributions
Primary sample dependent parameter:Resistance
Randomly generate a resistance for each TPG in Monte Carlo simulation
502018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Monte Carlo Predicted Current Split(3 4/0 TPGs, 80 kA, 12 in Spacing, X/R = 30)
Impacts force TPGs experienceF Ij Ik
Impacts heat generated during faultT
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
512018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Why do we need to derate?
• 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.
TPG 3TPG 2TPG 1
50000
45000
40000
35000
30000
25000
20000
TPG (TPG 1 closest to source bus)RM
S Cu
rren
t [A]
27475.1
25420.327088.4
Grade 5H Single TPG Withstand Level
Derating
• BUT…High power tests show that 3 TPGs survive at 3.5 in and not at 12 in.
Forces between TPGs are not equal(and they are large)
522018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Forces on TPG 1 Iin
I1 I2 I3
Source F12
F13
F1 = Forces = F12 + F13 + F1Loop
F1Loop
Force Types1. TPG to TPG2. Bus on TPG
532018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Forces on TPG 2 Iin
I1 I2 I3
Source F21 F23
F2 = Forces = F23 – F21 + F2Loop
F2Loop
542018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Forces on TPG 3 Iin
I1 I2 I3
Source F32
F31
F3 = Forces = F3loop – F31 – F32
F3Loop
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Spacing Impacts Clamp Rotation
SourceIFault
SourceIFault
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Is the Model Right?
• 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
572018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
• 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
Challenges of Measuring Current Split
Cables meet at different positions
582018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Measurement of Current Split
P1 Shunt
P2 Shunt
P3 Shunt
TPG 1TPG 2
TPG 3
Path lengths differentAdded self and mutual inductances
592018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Tweak the Model with Series Impedance
Actual Waveform Simulation Waveform
Inductance Added:L1=6μHL2=3.5μHL3=4.5μH
Resistance Added:R1=601μR2=383μR3=470μ
ADD TO THE MODEL
602018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Complete Current Split Model - Verification(includes Measurement Bus Work)
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
612018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
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
Predicted Currents – Single Return Bus80 kA RMS, 12 in spacing, X/R = 30
622018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Modeling Example 1Impact of Spacing on Current Split
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
632018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
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
Modeling Example 2Mixed TPG Groups (Old & New)
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 NowCurrent distributions change significantly
because of TPG 2 resistance
642018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Derating TPGs
Two cannot handle twice as much current as one…
652018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Overall Results
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 0/5
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
662018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
How to Calculate Derating Factors?• For 2 TPG cases:
Base Current = 2 Irated (Derating 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 (Derating 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%)
672018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
TPG Cable Size
[AWG]
RMS TestCurrent
[kA]
Test Current[x Irated]
TPGs[#]
Derating 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 Derating Factors
682018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Derating Factors for Test Program(New TPGs Only)
TPG Size
TPGs[#]
O.C. Spacing
[in]
Derating 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
ASTM allows pass rate on QA test of 75% or higher
692018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Derating Factors for Test Program(New TPGs Only)
TPG Size
TPGs[#]
O.C. Spacing
[in]
Derating 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 55Unrestrained Only
702018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Conclusions (1/2)• 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)
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
712018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Conclusions (2/2)• For the samples and configurations used in this project:
– Derating factors for unrestrained TPGs mounted directly to buswork are clearly higher than the 10% commonly discussed in industry standards
Derating factor is greater than 30% Other grounding methods may need to be considered
• Derating factors for other TPG assemblies or mounting techniques may be different.
722018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Still Many Unanswered Questions• Effect of different mounting orientation
– Degree of performance improvement (i.e. decrease in derating 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
732018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL
Thank you for your attention
Questions?