1. VLF CABLE TESTING INCLUDING PARTIAL DISCHARGE & TANGENT DELTA
7/24/2018
VLF CABLE TESTING INCLUDING PARTIAL DISCHARGE & TANGENT DELTA
Michael T. Peschel
High Voltage, Inc.
Copake, NY. USA
7/24/2018

2. SUBJECTS COVERED Review of DC Issues What Is VLF VLF Applications
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Review of DC Issues
What Is VLF
VLF Applications
IEEE Standards
Lab and Field Results
How To Perform The Test
Myths about VLF
Who Uses VLF
Selecting a VLF Model
Selecting A Cable Test Method
VLF Conclusion
Tan Delta Testing
VLF Partial Discharge Testing
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3. THIS CAN BE PREVENTED
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In-service failures cause great damage to faulted cables and adjacent cables. Not so if failed under a VLF test.
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4. WHY WAS DC USED DC hipots are small, portable, and economical.
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DC hipots are small, portable, and economical.
DC originally used with PILC cable and worked well.
Same techniques used when HMW, XLPE, EPR, and other solid dielectric cables were installed.
Years later DC was found to harm insulation and leakage current measurements are often ineffective for exposing defects. VLF work started.
Until recently, AC field testing of cable was not practical. Now it is practical and economical.
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5. DC USE DISCOURAGED
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Worldwide consensus exists among engineering organizations, utilities, and cable manufacturers that typical DC test voltages of 4 – 5 Vo (4 – 5x line-to-ground operating voltage) damage insulation, lead to failures, and leakage currents are often not indicative of insulation integrity. Using lower voltages renders test meaningless.
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6. Avoid DC Voltage On Service Aged, Solid Dielectric Insulation
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DC Voltage Polarizes Cable.
Water trees trap space charges – leads to failures.
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7. WHY IS DC HARMFUL? WATER TREES
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Tree shaped channels are found within the insulation of operating cables resulting from the presence of moisture and electrical fields.
Prevalent in solid dielectric cables.
Eventually leads to the inception of PD.
Leads to insulation failure.
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8. Water Trees
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9. WHY DC IS DAMAGING
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DC hipot output negatively charges up water tree areas.
These “trapped space charges” remain after test.
When AC is reapplied, there’s a high difference of potential across very little of the insulation. Leads to pd, electrical trees, & cable failure
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10. UTILITIES RESPONSE TO DC CONCERNS
7/24/2018
Typical DC test voltages were/are times V0.
At these high voltages, damage is done.
Some reduce voltage to 15 kVDC on 15 kV cable.
15 kV is only 30% above the peak AC stress in service.
At this voltage, leakage currents are meaningless.
Some do nothing, letting cables fail and then repair.
Many have adopted VLF AC hipoting to expose bad insulation and accessories. VLF is the best splice checker.
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11. VLF – Very Low Frequency AC
IF NOT DC - THEN WHAT?
VLF – Very Low Frequency AC
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Advantages
Light Weight.
Low Cost.
Simple to Use.
Results are certain
Disadvantages
Voltage Waveform in Some Designs – Can’t Use For TD & PD
Cable May Fail Under Test.
Perhaps Multiple Failures In Severely Deteriorated Insulation
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12. WHAT IS VLF?
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13. A VLF HIPOT IS SIMPLY AN AC OUTPUT
INSTRUMENT BUT AT A LOWER FREQUENCY.
THE LOWER THE FREQUENCY OUTPUT, THE LOWER THE CURRENT AND POWER REQUIRED TO TEST HIGH CAPACITANCE LOADS LIKE CABLES.
DON’T OVERCOMPLICATE IT.
IT’S A SIMPLE AC WITHSTAND TEST.
VLF IS THE EASIEST, CHEAPEST, MOST CERTAIN WAY
OF TESTING THE AC INTEGRITY OF A CABLE.
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14. DROP THE FREQUENCY Very Low Frequency: 0.1 Hz and lower.
By decreasing the frequency, it is possible to test
miles of cable with a small and affordable unit.
Output frequencies range from 0.1 – 0.01 Hz.
IEEE400.2 recognizes frequencies as low as 0.01Hz.
At 0.1 Hz, it takes 600 times less power to test a cable, or any other high capacitance load, than at 60 Hz. At 0.01 Hz, 6000 times higher capacitive loads can be tested than at 60 Hz with the same power consumption.
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15. VLF EXPLAINED Xc = 1 2 x pi x f x C
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Xc =
2 x pi x f x C
The lower the frequency, the higher the capacitive reactance, or Xc. The higher Xc , or resistance across the power supply output, the lower the current/power needed to apply a voltage.
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16. 60 Hz vs. 0.1 Hz At 60 Hz. a 1 μF cable has an Xc of 2.65 kOhms.
7/24/2018
At 60 Hz. a 1 μF cable has an Xc of 2.65 kOhms.
At 22 kV peak, it requires 8.3 amps of current to test.
Total power supply rating must be 183 kVA.
At 0.1 Hz, the Xc is 1.59 megohms.
At 22 kV, the current needed is 14 mA.
Total power supply needed is only kVA.
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(22 kV is the typical test voltage for 15 kV cable)

17. 60 Hz. vs. 0.1 Hz. 60 Hertz 0.1 – 0.02 Hertz 50 kVAC @ 3 kVA
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60 Hz. vs. 0.1 Hz.
60 Hertz
0.1 – 0.02 Hertz
50 3 kVA
Can test ~ 50’ of cable
kVA
Can test ~ 5 miles of cable
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APPLICATIONS
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19. IEEE STANDARDS EXIST Power Cable Large Rotating Machinery
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Power Cable
IEEE & IEEE
Large Rotating Machinery
IEEE
Diagnostic Testing:
Tan d
Partial discharge
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20. CAN OTHER LOADS BE VLF TESTED?
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Yes, but no standards exist that define the test.
Most other loads are low in capacitance, permitting
60Hz AC hipots to be used.
Sometimes large insulators are VLF tested if a
powerful enough 60Hz hipot is not available.
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21. WHY TEST CABLES WITH AC VOLTAGE?
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Cables are designed to carry AC voltage. They are factory tested with AC voltage. Cables operate under AC voltage stress.
Cables should be tested with AC voltage. Why would you not use AC if you could?
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22. Can Now AC Stress Test Cable
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With VLF, utilities, testing services, industrials, and others can now AC stress test cables in the field.
Just like with vacuum bottle or rubber glove testing, now a go/no-go AC stress test can be performed on power cable.
If a cable can’t hold 2 – 3 times normal voltage, it’s not healthy. Find the problem, make the repair, and move on.
At the very least, every newly installed and repaired cable should be VLF tested before energizing, since many failures are due to installation damage, faulty workmanship, stress from in-service failures, or over voltage thumping.
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23. Other Methods Don’t Get It Done.
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DC Hipot
5 kVdc “Megger”
DC Hot stick adaptor
24 hour soak
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24. VLF IT!
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25. IEEE STANDARDS
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26. NORTH AMERICAN STANDARDS FOR VLF TESTING
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IEEE overall cable testing standard sanctions VLF testing of cables.
IEEE standard for VLF cable testing
IEEE covers VLF testing for rotating machinery. Now being updated. Standard for smaller motors/gens. under consideration.
Assoc. edison illuminating companies
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27. MOTOR & GENERATOR TESTING
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IEEE Std (1974)
Recommended Practice For Insulation Testing of Large AC Rotating Machinery with High Voltage VLF.
Refers to Large AC Machines 10 MVA/6 kV & above.
Test Waveform: Must be Sinusoidal.
Test Frequency: 0.1 Hz.
Test Voltage: 1.63x 60 Hz RMS Level
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28. CABLE TESTING IEEE400.2-2004 Recommends test voltage of 2 - 3V0
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IEEE
Recommends test voltage of V0
(V0 equals line-to-ground voltage)
Test duration is 15 – 60 minutes
Best recommendation is for 30+ minutes
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29. IEEE400.2 FIELD TEST VOLTAGES
For Shielded Power Cable Systems
Using Sine Wave Output VLF
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Hz Test Voltage
System Voltage
phase to phase
kVrms 5 15 25 35
Installation
phase to ground
kVrms/kVpeak
9/12
18/25
27/38
39/55
Acceptance
phase to ground
kVrms/kVpeak
10/14
20/28
31/44
44/62
Maintenance
phase to ground
kVrms/kVpeak
7/10
16/22
23/33
33/47
Test voltages are generally 2 – 3 time the line-to-ground system voltage.
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30. 7/24/2018
IEEE Test Definitions
Acceptance Test: A test that demonstrates a degree of compliance of a cable and its accessories with the requirements of the purchaser. Highest test voltage
Installation Test: A test made after installation but before a cable system is put into normal operation. The test is intended to show shipping damage or errors in workmanship or damage during installation. Middle test voltage
Maintenance Test: A test made during the operation of a cable system and intended to detect deterioration of the system and to check the entire workmanship so that suitable maintenance procedures can be initiated. Lowest test voltage
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31. Cable Test Voltages Determined 2Vo – 3Vo Depending On Cable Size.
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12.5 kV system = 7,217 V0 or line-to-ground.
x 3
21.7 kV Maintenance test
The peak of the sine wave is the factor driving the inception of partial discharge. Hence, the 22 kV test spec is used for the peak of a sine wave VLF. The test is equivalent to ~ 2x V0 rms. Also, kV/mm insulation varies greatly with cable size. Need at least 2Vo.
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32. WHY 2Vo – 3Vo An abbreviated test or a test too low in
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The IEEE standard says 2 – 3V0 for minutes.
Cables routinely see 2x normal voltage due to
reflected waves, transients, etc. VLF test voltage
must be at least 2x normal line-to-ground.
Must use enough voltage for enough
time to let the VLF do its job of
growing electrical trees to failure.
An abbreviated test or a test too low in
voltage is worse than no test.
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33. XLPE TREE GROWTH IEEE 400-2001 Test voltage factor (V/Vo rms) 2 3 4
Sine wave VLF rapidly grows electrical trees to failure.
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Test voltage factor
(V/Vo rms) 2 3 4
Growth rate at 0.1-Hz sinusoidal test voltage (mm/h)  
A 15kV 133% cable has an insulation thickness of 5.9 mm.
In a 30 minute test, nearly all defects triggered into pd will grow to failure.
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34. AND LABORATORY RESEARCH
RESULTS FROM FIELD USE
AND LABORATORY RESEARCH
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35. VLF TEST RESULTS
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Numerous case studies show that if a cable passes a proper
VLF test, there is a > 95% assurance of no in-service failure in the next few years. Nothing is perfect, but only a few percent possible failure rate post VLF testing is very good. Far better by multiples than with other testing methods.
If a cable can hold 2 – 3 times normal voltage for 30-60
minutes, it’s good for years.
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36. XLPE TESTING STATISTICS
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TNB in Malaysia 35kV 2.1 V0 for 60 min.
17,435 VLF tests performed – 2,179 cable failures
Minutes
to failure Failures % of total
0 - 12
1472
67.62
469
21.54
129
5.93
107
4.92
89.16%
2.78% of tested cables failed later in service. (Many cables were PILC)
Tests conducted 2001 – Continue to buy VLF and test all cables.
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37. XLPE TESTING STATISTICS
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Japan: Furukawa, Chubu & Tokyo Electric
Research done to determine test voltage and duration versus expected life. Results were:
A 33 kV cable tested at 60 kV 0.1 Hz
has a 97% probability of no failure for 3 years.
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38. Germany produced some of the first
VLF products more than 20 years ago.
German standard: 3Vo rms for 60 minutes.
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39. Most US users test at the IEEE recommended voltages for 30 minutes.
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40. The World View Of VLF DC not recommended by cable companies
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DC not recommended by cable companies
for cables > 5 years & in moist environments
IEEE 2 – 3Vo for minutes
30+ 3Vo recommended.
Germany 3Vo for 60 minutes
Japan Vo for 15 minutes
Malaysia 3Vo for 60 minutes
Over 60 countries have purchased the HVI VLF
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41. HOW TO PERFORM A VLF TEST
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42. TEST PROCEDURE VLF testing is easier than DC testing.
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VLF testing is easier than DC testing.
Isolate cable ends like with DC testing, although
no cleaning and bagging is necessary.
Remove all arrestors, capacitors, transformers, etc.
Connect VLF HV lead to conductor & ground to shield.
After selecting appropriate test frequency, apply voltage
There are no leakage currents to read. Test is go/no-go
If cable holds, test is over. Cable is good for years.
If cable fails, make repairs and repeat test, or replace.
If second insulation failure occurs, maybe stop testing and replace.
Most models can test all three phases at once, saving time.
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43. VLF Test Hookup Per IEEE400.2
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Some VLF units have no return wire. HV output and ground cables only.
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44. CABLES INCLUDED WITH TWO PIECE MODEL
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CABLES INCLUDED WITH TWO PIECE MODEL
Ground hook
HV output
from tank
2 test leads for capacitance measurement
Cable connectors
hook or clamp
Phase jumpers
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Interconnect
with grounds
Scope bnc

45. 7/24/2018
VLF-4022CM
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46. OPERATING INSTRUCTIONS
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OPERATING INSTRUCTIONS
1. Select Frequency
3. Rotate to zero
(Zero start intl’k)
6. Rotate to
raise voltage
5. Press HV On
4. Press Main Power
2. Turn to mA
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47. WHAT IS CABLE FAILURE INDICATION?
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Thermal overload on panel will trip
Voltmeter will indicate breakdown
Current will spike
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48. FAULT LOCATING NECESSARY
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When VLF testing, cable failures will occur.
Someone has to find them.
Testing contractors will need to use fault locators and should want to for increased billing.
Thumpers, or VLF/thumpers must be purchased.
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49. 7/24/2018
COMMON VLF MYTHS
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50. VLF IS DESTRUCTIVE TO INSULATION.
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NOT TRUE. Cable is factory tested at voltages far higher than field VLF levels. VLF is destructive to existing defects that are severe enough to be triggered into partial discharge during the test. That’s the point of the test – light up defects and let them grow to failure. Minor defects and good insulation are not effected.
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51. VLF AGGRAVATES DEFECTS THAT FAIL LATER IN SERVICE
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Only if the test is not properly performed, like any testing method. The proper VLF voltage must be applied for a long enough time to permit defects triggered into pd to grow to failure. An improper test is worse than no test at all. The same can be said for 60Hz hipoting, pd testing, and other tests. If done wrong, problems could result. If done correctly - positive results.
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52. VLF IS REALLY ALTERNATING DC
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Not true. At 0.1 Hz there are polarity reversals every 5 seconds. Even at 0.01Hz there are reversals every 50 seconds. That does not compare at all to a 15 minute, constantly negative output DC hipot test. Space charges and traveling waves are not developed in insulation during a VLF test. The IEEE recognizes frequencies down to .01Hz as ok.
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53. VLF EFFECTIVENESS IS UNPROVEN
7/24/2018
Very wrong. Tell that to the hundreds of users over more than 15 years that have tested tens of thousands of cables with great success. Tell that to the IEEE, CEA, EPRI, cable companies, and many international organizations that have all endorsed VLF.
A VLF instrument is just a low frequency AC hipot. Simple product – simple test – simple physics. There is nothing to not work.
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54. ONLY 0.1 HZ IS A VALID TEST
7/24/2018
The IEEE standard recognizes frequencies down to 0.01Hz. While
it is true that most of the worldwide research over the last 20 years
into testing levels and durations has been done at 0.1Hz, that does
not mean lower frequencies are ineffective.
It’s a tough argument to make that 60 Hz is ok, DC is ok for a lot of things including new cable, 0.1Hz is ok, but 0.05Hz is not?
.1Hz
60Hz
.05Hz DC
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55. ONE WAVEFORM OUTPUT IS BETTER THAN ANOTHER
7/24/2018
Of the three major VLF vendors, two output waveforms
are offered: sine wave & cosine-rectangular (trapezoidal).
Both work well to VLF hipot cable, although the sine wave output grows electrical trees faster - see IEEE400.
-However -
A sine wave VLF is better suited for use as a voltage source for tan delta and partial discharge testing, both desirable add-ons to VLF testing. Also, a sine wave is necessary for motor/generator testing per IEEE433.
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56. WHO USES VLF
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57. UTILITIES TESTING SERVICES INDUSTRIALS CONTRACTORS
Other methods of cable testing have their place, but VLF is embraced worldwide as the easiest, most effective, most economical method of cable testing.
Over 800 VLF units have been shipped from High Voltage, Inc. over the last eight years, delivered to 54 countries and many US locations. Other VLF products have shipped from other vendors for even longer.
7/24/2018
The following slides are just a few examples of the worldwide acceptance of VLF. Previous slides showed the IEEE, EPRI, CEA, and the European acceptance, via their recommendations and/or standards for VLF testing.
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58. COUNTRIES USING HVI VLF
7/24/2018
HVI has shipped over 800 VLF products to the following:
USA
Canada
Taiwan
Indonesia
So. Korea
Australia
Saudi Arabia
England
Costa Rica
Bolivia
Israel
So. Africa
Singapore
Hong Kong
Belgium
Puerto Rico
Holland
New Zealand
Japan
Malaysia
Russia
Spain
Slovak Republic
Czech Republic
China
UAE
Vietnam
Panama
Jordan
Germany
Cyprus
Honduras
Lithuania
Thailand
Mexico
India
Bulgaria
Fiji
Venezuela
Finland
Greece
Qatar
Argentina
Norway
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59. SELECTING A VLF MODEL
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60. The above chart only covers ratings to 35kV. VLF
WHAT SIZE CABLES ARE TO BE TESTED? Do you do Install, Acceptance & Maintenance testing?
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From IEEE400.2
System Voltage
phase to phase
kVrms 5 15 25 35
Installation
phase to ground
kVrms/kVpeak
9/12
18/25
27/38
39/55
Acceptance
phase to ground
kVrms/kVpeak
10/14
20/28
31/44
44/62
Maintenance
phase to ground
kVrms/kVpeak
7/10
16/22
23/33
33/47
The above chart only covers ratings to 35kV. VLF
testers exist that can test cables rated up to 230kV.
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61. WHAT CABLE LENGTHS ARE TO BE TESTED?
VLF units are rated by the uF of load they can test.
7/24/2018
Example: 15 kV XLPE 750mcm 220 mils ~ .1uf/1000’
Model VLF-25CM
kV Peak
μf Load
Can test up to 4000’ of 15 kV
cable depending on size.
Perfect NETA model
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62. POPULAR MODEL USED BY TESTING CONTRACTORS.
Can test cables rated to 25 kV.
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Model VLF-4022CM
kV Peak
μf Load
μf Load
μf Load
70 lbs lbs
Example: 15 kV XLPE 750mcm 220 mils ~ .1uf/1000’
At 0.1 Hz can test ~ 2 miles. At 0.02 Hz can test ~ 10 miles.
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63. MODEL SIZES AVAILABLE (all vendors included)
7/24/2018
Voltage ratings from 20 kV – 200 kV
Load ratings from 0.4 uF – 50 uF
For a 15 kV cable, 0.4 uF can test ~ 4000’
Some models can test > 30 miles of cable
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64. WILL IT BE USED FOR TAN DELTA
& PD TESTING?
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Sine wave output is needed to use VLF
as a voltage source for td and pd testing.
IEEE433 for VLF testing rotating
machinery requires a sine wave output.
Leave all your future testing options
open by buying a sine wave VLF.
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65. Other Models 90 kV peak Up to 2.75 uF 120 kV peak up to 5.5 uF
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90 kV peak
Up to 2.75 uF
120 kV peak up to 5.5 uF
60 kV peak up to 5.5 uF
200 kV soon available
65 kV peak up to 22 uF
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66. Complete URD Cable Care System Combination VLF AC Hipot
VLF THUMPER
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Complete URD Cable Care System
Combination VLF AC Hipot
and Thumper
VLF Output: kVAC peak
Load Rating: Hz ~ 1.5 miles of 15 kV cable
Discharge: – J
VLF Cable Burning Mode
Radar Ready
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67. SELECTING A CABLE TEST METHOD
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SELECTING A CABLE TEST METHOD
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68. CABLE TEST METHODS AC Power Frequency. AC Series & Parallel Resonant.
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AC Power Frequency.
AC Series & Parallel Resonant.
DC Hipot.
Very Low Frequency (VLF) AC Hipot.
Tan Delta, Partial Discharge, And Other Diagnostic Methods.
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69. SEVERAL METHODS – WHAT TO USE?
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Ideally, VLF, Tan Delta, and Partial Discharge should all be used.
However, there are real world factors affecting the decision.
1. Type of test results desired
2. Ease of use
3. Cost of equipment
There is no one perfect method that does it all. A variety of
approaches is needed to meet all the requirements encountered.
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70. WHAT’S THE GOAL? Verify new installation? Verify repaired cable?
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Verify new installation?
Verify repaired cable?
Verify critical cable?
Help to prioritize cable replacement or injection?
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71. WHAT’S THE SITUATION? Direct buried – must pinpoint problem to repair
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Direct buried – must pinpoint problem to repair
Cable in conduit or trench - replaceable
Cable in raceways – visible and easily replaceable
How old is cable
What is the failure history
How easy is it to repair
Is there alternate feed should failure occur during test
Is fault location and repair available?
How much downtime can be tolerated?
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72. EXAMPLES OF SITUATION vs. METHOD
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New Install: Diagnostic test not appropriate, the insulation is good. VLF it to make sure there are no installation flaws.
Old suspect cable: There may be many defects, don’t VLF. Use TD to see how degraded cable is. If modest degradation, then PD possibly followed by VLF.
Critical cable in conduit: PD test to expose location and severity of defects. If no PD tester available, VLF.
Direct buried, hard to repair: TD test to evaluate condition to prioritize cable injection or replacment.
Prioritization for replacement or injection: If a comparative condition assessment test is desired, tan delta is the easiest method.
Post repair test: VLF repaired cable to make sure it holds – no further damage caused by over voltage thumping.
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73. NO SHORT AND EASY ANSWER.
SO, WHAT TO DO?
NO SHORT AND EASY ANSWER.
Depends on money and available people.
A combination of methods is needed: some easy and economical (VLF) and some more complicated and more expensive (TD & PD), and each yielding different but important data about your cable.
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74. IT ALL STARTS WITH A VLF
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Fact: As more industrial customers, utilities, and specifying engineering companies spec VLF testing, per IEEE400.2, you will need a VLF.
Fact: In addition to performing VLF AC stress tests, a VLF hipot is the foundation for other tests, like partial discharge and tan delta. You need VLF for pd and td.
To learn the most about the health of your cable, all three tests, if economically and logistically feasible, are recommended. If not all possible, VLF should be the fallback, or nothing in old cable systems.
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75. 7/24/2018
SUMMARY &
CONCLUSION
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76. VLF TESTING SUMMARY
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Has the virtues of DC test equipment (low cost, small size, light weight, easy to use) but is AC.
Does not have the negative consequences of DC.
Requires 1/600 – 1/6000 of the kVA of power frequency.
Can be used for breakdown tests and predictive tests such as Tan Delta and Partial Discharge. (sine wave VLF units)
Internationally accepted Standards exist and more are in the process of issuance.
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77. CONCLUSION
The surest way to verify the AC integrity of any load is perform an AC stress test.
It’s easy and certain. The load holds voltage or fails.
VLF testing is easily performed with minimal training.
Worldwide standards and decades of history exist.
Some VLF models are very portable and affordable.
VLF makes Tan Delta and Partial Discharge diagnostic testing easier and more economical.
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78. CONCLUSION VLF is suitable for use on cables and rotating machinery.
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VLF is suitable for use on cables and rotating machinery.
IEEE, VDE, CEA, EPRI, other country’s engineering organizations, cable manufactures, and major utilities all recommend VLF.
Hundreds of users worldwide have embraced VLF
VLF It! It’s economical, easy, and sure.
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79. OTHER CABLE TEST METHODS USING VLF
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80. YOU HAVE YOUR VLF. WHAT ELSE CAN YOU DO?
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Accessorize. Add on a
Tan Delta measurement instrument.
Partial Discharge detection instrument.
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81. Dissipation Factor or Loss Angle Measurement For Power Cables
TANGENT DELTA or TAN δ
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Dissipation Factor or Loss Angle Measurement For Power Cables
Non Destructive Testing To Determine Insulation Quality
(Similar to Power Factor Testing)
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82. TAN DELTA MEASUREMENT Using VLF @ 0.1 Hz
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Evaluates overall condition of cable (rather than localized problems as with PD measurement)
Absolute values, variations, and trending of values are of interest for predicting insulation integrity
Tan Delta is more easily measured at VLF (magnitude increases as frequency decreases)
Requires VLF sinusoidal applied test voltage
Excellent way to evaluate Water Trees
Useful to help prioritize replacement, injection and helps to determine what additional test may be useful

83. CHARACTERISTICS OF WATER TREES
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Addition of a parallel R-C Component
Water trees increase resistive current through insulation
The R component is voltage dependent (non–linear I = V/R)
Tan Delta values increase with increasing voltage

84. Simplified Cable Model and Phasor Drawing Tan Delta = IR/ IC - measured in radians
7/24/2018
With perfect insulation, a cable is a near perfect capacitor, with a 90° phase shift between voltage and current. Less than 90° indicates insulation degradation. Cables can be rated good, marginal, or bad. Not extremely precise but valuable for prioritizing cable replacement or injection. Also helps to determine what further tests may be worthwhile.

85. TAN DELTA VS. VOLTAGE NEW AND AGED XLPE CABLES
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Aged cable non-linear tan delta #s versus voltage
New cable linear tan delta #s versus voltage

86. TAN DELTA NUMBERS VERSUS INCREASING VOLTAGE
& RATIO OF 2Vo/Vo
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Dissipation Factor – Tan Delta From IEEE Std Field experience shows these number to be too low.
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88. TAN DELTA LOSS ANALYZER
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60kV model
Used with VLF as voltage source
HV Divider Signal analyzer/controller
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89. TAN DELTA TESTING Advantages Disadvantages
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Advantages
Less destructive than VLF, DC, or 60 Hz testing
Helps to prioritize cable replacement and/or injection
Easier to use and interpret than other diagnostic methods
Disadvantages
Can be destructive if cable very degraded
Gives overall condition of cable, not singularities
Not best for mixed type cable runs
More useful with historical data
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90. VLF PARTIAL DISCHARGE TESTING
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91. “SEE” UNDERGROUND CABLE FAULTS
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A PD detection system can show you the location and severity of insulation and accessory defects.
Using a VLF as the voltage generator to apply voltages up to 2Vo, long cables can be tested with portable equipment.
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92. OFF-LINE VS. ON-LINE PD TESTING
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Off-line testing using a VLF permits over voltage analysis. Can measure PDIV and PDEV up to 2Vo.
On-line pd testing can only detect pd at operating voltage, missing defects with a PDIV of 101% and above of operating voltage, yet cables routinely see twice normal voltage due to reflected waves, transients, switching surges, etc.
Off-line testing may be less convenient, but the information gained is far more valuable.
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93. A 100 lb VLF can do the job of a 2000 lb 60Hz series resonant unit.
0.1Hz vs. 60Hz PD TESTING
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Both frequencies initiate pd at defect locations. Both can measure PDIV, PDEV, magnitude, and location.
The benefit of VLF is smaller size, lower weight, far lower cost, and can test longer cables.
A 100 lb VLF can do the job of a 2000 lb 60Hz series resonant unit.
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94. THE BEST OF EVERYTHING
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Start with a VLF tester to perform AC withstand testing.
Add a Tan Delta accessory for evaluating the overall condition
of a cable to help prioritize replacement, injection, or to
determine what other tests might be helpful.
Add a Partial Discharge accessory to map the location
and severity of pd in the insulation and splices, or to
Perform all three tests to get the most complete profile possible
of your cable.
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95. Mike Peschel - High Voltage, Inc.
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Thank You
Mike Peschel - High Voltage, Inc.
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