Partial Discharge in the Context of Distribution Cable Testing

1. Partial Discharge in the Context of Distribution Cable Testing Steven Boggs

2. What is PD? • A gas discharge which does not bridge the system electrodes • Discharge in a cavity • Corona off an electrode • Tracking discharge along an interface • Discharge from electrical tree growth

3. Not All PD is Dangerous • PD between floating metallic components • PD between a metallic component and semicon • PD between concentric neutral wires and semicon rendered nonconducting by solvents • PD involving PD-resistant insulation

4. What Do We Detect? • The discharge causes a transient increase in the “capacitance” between the electrodes by shorting out some of the field • The increase in capacitance results in a reduction in the voltage across the electrodes • The power supply recharges the electrodes

5. Circuit Prior to Discharge

6. Circuit During Discharge During Discharge to Discharge

7. Cavity Before and After Discharge

8. PD Inception and Extinction • Can have peak field in cavity prior to inception • Can have peak-to-peak field in cavity after inception • Thus PD inception can require up to twice the voltage of PD extinction although 1.5 to 1.7 pu is more common in practice • This is the basis for PD testing to 2 to 2.5 pu

9. Typical Discharge Pattern Applied Voltage Field in Cavity

10. Phase-Resolved PD Analysis Provides Greater Specificity

11. Discharge Magnitudes for Spherical Cavities

12. Discharge Magnitudes for Long, Thin Structures PD Magnitude Depends Mainly on Length in the Direction of the Field

13. PD Inception Depends on a free Electron • Most free electrons come from cosmic rays • 3 e-/cm3-s in air, ~ 5 minutes waiting for a 1 mm3 cavity • After the first PD, charge is stored on the cavity wall • XLPE has a “negative” electron affinity • Free electrons will go from the polymer to a vacuum • For small cavities – much below 1 mm, PD is often not self-sustaining • Charge may disappear by surface conduction between PD’s

14. PD Detection Sensitivity • Can detect cavities in the range of 1 mm dia • Can detect electrical trees in the range of 1 mm long • Can detect tracking along dielectric interfaces of some mm. • Cannot detect water trees unless they convert to electrical trees

15. Tracking Along an Interface

16. Tracking Along an Interface

17. Time Domain Cable PD Testing • Off-Line PD Detection • Testing at 1.5 to 2.5 pu to initiate PD which can sustain at normal operating voltage • Locate PD by reflected pulse • Must trigger on first pulse above noise • Sensitivity could be increased by about 10 if could implement on-line DSP treatment of data stream • Can use DSP to enhance detection of 2nd pulse • Energize with AC, LF, damped oscillation, etc.

18. Frequency Domain PD Testing • Use spectrum analyzer to find region where noise is small and compare background to signal during test • Can use spectrum analyzer in zero span mode (as bandpass filter) to correlate signal with operating voltage • Sensitive enough for in-service PD testing • Spectrum analyzer averages to improve S/N

19. Considerations for Time Domain Field PD Testing of Cable? • Sufficient Bandwidth for PD Location • Sufficient Dynamic Range for PD Detection in Noise • Effect of Far End Termination on Reflected Pulse • Detection of “Second Pulse” in Noise

20. Considerations for Frequency Domain PD Testing • PD vs other power frequency correlated noise (SCR’s, etc.) • Ability to locate PD sources – mostly based on frequency content which depends on cable attenuation and varies with cable type • “Calibration” of PD measurement is problematic

21. Assumptions for In-Service Testing • The system has been operating for a long time • If PD could initiate, it would initiate – from surges on the system, etc. • Once initiated, PD is likely to continue as the PDEV is much less (50 to 70%) of the PDIV • Thus if PD can occur on the system, it is likely to be present during an in-service test

22. PD Propagation in a Cable Unlimited Bandwidth With Effect of 5-pole, 15 MHz Input Filter

23. Frequency Domain Detection

24. What Do You Want From PD Testing? • An indication of where to spend your money • Location of defective components where systematic installation errors have been made • Location of defective installed components • Location of cable nearing end of life

25. But What Do You Do When Someone Recommends You Spend $1M? • How certain do you have to be? • What level of “proof” do you demand in terms of evidence on your system and evidence of past success? • What kind of data do you want out of your PD tests?

26. Assumptions for In-Service Testing • The system has been operating for a long time – if PD could incept, it has incepted • Testing will take place without outages which extinguish PD • A correlation with power frequency in the zero span mode indicates PD • Tests can be carried out at frequent intervals along the cable, if necessary, to locate PD sites

27. Can Water Trees Be Detected? • Not directly – Water trees do not generate PD • Water trees can be detected if they cause inception of an electrical tree • But will the electrical tree grow to failure?

28. So What Makes Sense? • Specify detailed report with data and written analysis thereof • Require a detailed explanation of what the test crew will do and on what they base their analysis • Require advanced knowledge of what the test crew needs to know about your system • Location and, if possible, model of splices • Type of cable, XLPE, TR-XLPE, EPR (brand), PILC

29. Do Routine PD Tests Make Sense? • If we had routine pregnancy testing of the population, we would probably have 100,000 pregnant males – false positives! • Many tests only make sense when applied to a “suspect” population – depends on rate of false positives and false negatives

30. What are the Risks – Off-Line? • Polymer high field degradation has a distinct threshold above which the rate of degradation increases rapidly • A few minutes at 2.5 pu could be equal to a long time at normal operating voltage • Risk of causing a water tree to convert to an electrical tree which grows to failure – either during ac or during the “impulse” which results from a breakdown • Can’t really quantify risks – only time will tell • But risks can be minimized by testing at no more than 2 to 2.5 pu. The rational for testing at >2 pu is not clear

31. What are the Risks – In-Service • No risk from elevated voltage • Risk of misdiagnosing power frequency correlated interference from power electronics (e.g., SCR switching) • Risk that PD source cannot be located with sufficient accuracy or PD from accessories cannot be distinguished from cable PD

32. Conclusion • The technology is not yet mature • Primary value today is probably investigating known problem areas rather than routine testing • Time-domain off-line testing provides good PD location accuracy, but testing at elevated voltage is required as are outages. • In-service testing has no risk of causing failures, but little has been published which quantifies efficacy