Performance of Parallel Temporary Protective Grounds in High … · 2018-05-14 · 2018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL 1 Performance of Parallel Temporary

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


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


• Introduction• Survey Results• NEETRAC Test Program• Test Results• Modeling• Derating Factors• Conclusions

Outline


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


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


Goal: Determine the Derating Factors

Standards bodies suggest minimum 10% derating per TPG

This project is about getting the derating factors


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

–


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


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


NEETRAC Member Survey


• Understand the most common clamps, ferrules, and cable sizes used by NEETRAC member utilities

Survey Objectives


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


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


Strain Relief

Do you use strain relief (also known as bend restrictor) at the cable-

ferrule connection?

Yes No

90%

10%


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?


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?


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?


Guidelines for Multiple TPGs


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


Test Program Design & Samples


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


• 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


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


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


Test Results


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


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)


Group 1 Laboratory Setup

Multiple return buses


Group 2 Laboratory Setup

Single return bus


• 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)


Single TPG performance verified at H rating

Single TPG Tests


Fault Tests are Challenging…(Video)

Triple TPG. 12 in spacing, 80 kA RMS


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


Mechanical and Thermal Failures (Video)

Source


2/0 AWG TPG Tests

Rated Current = 31 kA RMSTest Currents = 41.9 & 52.7 kA RMS


Survive - 2/0 AWG TPG, 12 in spacing,41.9 kA RMS (Test 5-3 , Trip 2)

Return

Bottom Clamps

Source


Survive - 2/0 AWG TPG, 12 in spacing,52.7 kA RMS (Test 6-8, Trip 2)

ReturnBottom Clamps

Source


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


4/0 AWG TPG Tests

Rated Current = 47 kATest Currents = 63.5 & 80.0 kA


• 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?


Survive - 4/0 AWG TPG, 12 in Spacing,63.5 kA RMS(Test 7-3, Trip 1)

Source


Failure - 4/0 AWG TPG, 12 in Spacing,80 kA RMS (Test 8-5, Trip 1)

Source


Survive - 4/0 AWG TPG, 6 in Spacing,80 kA RMS (Test 8-2, Trip 2)

Source


Survive - 4/0 AWG TPG, 3.5 in Spacing,80 kA RMS (Test 7-2, Trip 2)

Source


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.


Modeling

Expected Current SplitsForces Involved


Simulink Model

Parallel TPGsSimulation models are

useful understanding the sensitivity / probabilistic

aspects of TPG performance

(i.e. Monte Carlo)


Simulated Fault Current Waveform

47 kA RMS Fault, X/R = 30

Total Fault CurrentIndividual TPG currents(small phase differences)


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


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


RMS

Curr

ent [

A]27475.1

25420.3

27088.4

Peak Current

RMS Current


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)


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


Forces on TPG 2 Iin

I1 I2 I3

Source F21 F23

F2 = Forces = F23 – F21 + F2Loop

F2Loop


Forces on TPG 3 Iin

I1 I2 I3

Source F32

F31

F3 = Forces = F3loop – F31 – F32

F3Loop


Spacing Impacts Clamp Rotation

SourceIFault

SourceIFault


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


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


• 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


Measurement of Current Split

P1 Shunt

P2 Shunt

P3 Shunt

TPG 1TPG 2

TPG 3

Path lengths differentAdded self and mutual inductances


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


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


TPG 3TPG 2TPG 1

80000

75000

70000

65000

60000


Curr

ent P

eak

[A]

73908.1

68380.7

72867.9

Predicted Currents – Single Return Bus80 kA RMS, 12 in spacing, X/R = 30


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


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


Curr

ent P

eak

[A]

73908.1

68380.7

72867.9

TPG 3TPG 2TPG 1

80000

75000

70000

65000

60000


Peak

Cur

rent

[A]

75817.9

65307.7

73971.4

Original NowCurrent distributions change significantly

because of TPG 2 resistance


Derating TPGs

Two cannot handle twice as much current as one…


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


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%)


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


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


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


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


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.


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


Thank you for your attention

Questions?

Documents

Performance of Parallel Temporary Protective Grounds in High … · 2018-05-14 · 2018 IEEE JTCM – E4 Meeting, January, 2018, Jacksonville, FL 1 Performance of Parallel Temporary