VLF CABLE TESTING INCLUDING PARTIAL DISCHARGE & TANGENT DELTA

1. VLF CABLE TESTING INCLUDING PARTIAL DISCHARGE & TANGENT DELTA Michael T. Peschel High Voltage, Inc. Copake, NY. USA www.hvinc.com

2. SUBJECTS COVERED 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

3. THIS CAN BE PREVENTED In-service failures cause great damage to faulted cables and adjacent cables. Not so if failed under a VLF test.

4. WHY WAS DC USED 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.

5. DC USE DISCOURAGED 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.

6. Avoid DC Voltage On Service Aged, Solid Dielectric Insulation DC Voltage Polarizes Cable. Water trees trap space charges – leads to failures.

7. WHY IS DC HARMFUL? WATER TREES • 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.

8. Water Trees

9. WHY DC IS DAMAGING 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

10. UTILITIES RESPONSE TO DC CONCERNS • Typical DC test voltages were/are 4 - 5 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.

11. IF NOT DC - THEN WHAT? VLF – Very Low Frequency AC 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

12. WHAT IS VLF?

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.

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.

15. VLF EXPLAINED Xc = 1 2 x pi x fx 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.

16. 60 Hz vs. 0.1 Hz At 60 Hz. a 1 μF cable has an Xc of 2.65 kOhms. At 22 kV peak, it requires 8.3amps 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 .304 kVA. (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 Can test ~ 50’ of cable 40 kVAC @ 1.2 kVA Can test ~ 5 miles of cable

18. APPLICATIONS

19. IEEE STANDARDS EXIST • Power Cable IEEE 400-2001 & IEEE 400.2-2004 • Large Rotating Machinery IEEE 433-1974 • Diagnostic Testing: Tan d Partial discharge

20. CAN OTHER LOADS BE VLF TESTED? 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.

21. WHY TEST CABLES WITH AC VOLTAGE? 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?

22. Can Now AC Stress Test Cable • 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.

23. OtherMethods Don’t Get It Done. DC Hipot 5 kVdc “Megger” DC Hot stick adaptor 24 hour soak

24. VLF IT!

25. IEEE STANDARDS

26. NORTH AMERICAN STANDARDS FOR VLF TESTING IEEE 400-2001 overall cable testing standard sanctions VLF testing of cables. IEEE 400.2-2004 standard for VLF cable testing IEEE 433-1974 covers VLF testing for rotating machinery. Now being updated. Standard for smaller motors/gens. under consideration.

27. MOTOR & GENERATOR TESTING • IEEE Std. 433-1974 (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

28. CABLE TESTING IEEE400.2-2004 • Recommends test voltage of 2 - 3V0 (V0 equals line-to-ground voltage) • Test duration is 15 – 60 minutes • Best recommendation is for 30+ minutes

29. IEEE400.2 FIELD TEST VOLTAGES For Shielded Power Cable Systems Using Sine Wave Output VLF ---------------------- 0.1 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.

30. 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

31. Cable Test Voltages Determined2Vo – 3Vo Depending On Cable Size. 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.

32. WHY 2Vo – 3Vo The IEEE standard says 2 – 3V0 for 15 - 60 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.

33. XLPE TREE GROWTH IEEE 400-2001 Sine wave VLF rapidly grows electrical trees to failure. Growth rate at 0.1-Hzsinusoidal test voltage(mm/h) 2.310.9-12.658.3-64.2 Test voltage factor (V/Vo rms) 2 3 4 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.

34. RESULTS FROM FIELD USE AND LABORATORY RESEARCH

35. VLF TEST RESULTS 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.

36. XLPE TESTING STATISTICS TNB in Malaysia 35kV Cable @ 2.1 V0 for 60 min. 17,435 VLF tests performed – 2,179 cable failures Minutes to failure Failures % of total 89.16% 2.78% of tested cables failed later in service. (Many cables were PILC) Tests conducted 2001 – 2002. Continue to buy VLF and test all cables.

37. XLPE TESTING STATISTICS 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 peak @ 0.1 Hz has a 97% probability of no failure for 3 years.

38. Germany produced some of the first VLF products more than 20 years ago. German standard: 3Vo rms for 60 minutes.

39. Most US users test at the IEEE recommended voltages for 30 minutes.

40. The World View Of VLF DC not recommended by cable companies for cables > 5 years & in moist environments IEEE 2 – 3Vo for 15-60 minutes 30+ minutes @ 3Vo recommended. Germany 3Vo for 60 minutes Japan 3Vo for 15 minutes Malaysia 3Vo for 60 minutes Over 60 countries have purchased the HVI VLF

41. HOW TO PERFORM A VLF TEST

42. TEST PROCEDURE • 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.

43. VLF Test Hookup Per IEEE400.2 Some VLF units have no return wire. HV output and ground cables only.

44. 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 Scope bnc Interconnect with grounds

45. VLF-4022CM

46. 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

47. WHAT IS CABLE FAILURE INDICATION? Thermal overload on panel will trip Voltmeter will indicate breakdown Current will spike

48. FAULT LOCATING NECESSARY 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.

49. COMMON VLF MYTHS

50. VLF IS DESTRUCTIVE TO INSULATION. 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.