IEEE- ICC Fall 2000 DIDACTICS PARTIAL DISCHARGE TESTING ON-LINE TECHNIQUE

1. IEEE- ICC Fall 2000 DIDACTICS PARTIAL DISCHARGE TESTING ON-LINE TECHNIQUE Nagu Srinivas and Nezar Ahmed Cable|WiseDTE Energy Technologies Affiliate of Detroit Edison

2. On-Line PD Testing • No need to de-energize the cable system • Testing cable at system’s operating conditions • Non-destructive (will not activate dormant PD sites)

3. PD has to be sustained at system operating voltage to fail the insulation The time frame for PD during transient over-voltages is too short to fail the insulation Transient over-voltages may be required to initiate PD in some cavities PD at Operating Voltage

4. PD Testing Over-voltages are also utilized in PD cable testing techniques: Is it necessary ? • PDIV versus PDEV (residue voltage, statistical time lag) • Sub mm defects

5. Testing Technique • Higher sensitivity is needed for on-line testing compared to off-line testing • Higher sensitivity can be accomplished with appropriate PD coupling techniques and noise rejection

6. DTE On-Line System • High frequency sensors • PD signals are analyzed in the frequency domain • Noise rejection is made through signal processing in the frequency domain • System is capable of measuring very low PD • PD frequencies up to 300 MHz can be detected • Not limited by cable length, operating voltage, insulation type, construction or branching

7. Detection System • PD induces a current in the cable conductor and shield • These currents are out of balance: • Concentric • Taped Shield • Lead Sheath

8. PD Sensors • PD pulses are detected with special sensors (inductive/capacitive coupling) • U-shape couplers are available with different size openings (1.5 to 6 inch) • Sensitivity range from 100 kHz to 300 MHz • 100 kHz to 100 MHz inductive coupling • Over 100 MHz capacitive coupling 0.0 DTE Energy PD Sensor Relative Coupling Efficiency -20.0 Commercially Available sensor -40.0 300 0.0 100 Frequency (MHz)

9. Detection Mechanism Pulse Magnitude Time (ns) Pulse Width PD site Distance A B C D E F A B PD (mV) PD (mV) 0 Frequency (MHz) 100 0 Frequency (MHz) 100 D C PD (mV) PD (mV) 0 Frequency (MHz) 100 0 Frequency (MHz) 100 F E PD (mV) PD (mV) 0 Frequency (MHz) 100 0 Frequency (MHz) 100

10. A =Energy (area under the curve) PD (mV) T1 T2 Time (ns) A PD (mV) 0 Frequency (MHz) 100 B PD (mV) 0 Frequency (MHz) 100 C PD (mV) F0 = 1/T0 0 Frequency (MHz) 100 Detection Mechanism Frequency conversion of an individual PD pulse A: digital conversion B: Analog conversion with setting 1 C: Analog conversion with setting 2.

11. A PD (mV) 0 Frequency (MHz) 100 B PD (mV) 0 Frequency (MHz) 100 PD C A PD envelop F1 F0 F2 0 Frequency (MHz) 100 Several sweeps of the same PD signal using the ultimate setting and increasing sweep time Detection Mechanism

12. A PD envelope PD (mV) F1 F0 F2 0 Frequency (MHz) 100 Detection Mechanism • A = Average energy of detected pulses as measured at the testing point • SW = Minimum sweep time to generate the PD envelope • FO = Resonant frequency of the PD pulse • F = F1 – F2 = Fequency bandwith of the PD envelop The sweep time is adjusted to yield a PD envelope

13. 6.0 (a) PD (mV) 0.0 F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11 F12 F13 F14 200.0 0.0 Frequency (MHz) Frequency sweep from a cable showing multiple PD sites 6.0 (b) PD (mV) 0.0 0.0 Time (ms) 50.0 4.0 (c) PD (mV) 0.0 Time (ms) 50.0 6.0 (d) PD (mV) 0.0 Time (ms) 50.0 Zero-span of PD sites

14. 1.0 0.5 Cable Correlation factor EPR Insulation Polyethylene Insulation Paper Insulation 0.0 Frequency (MHz) 1.0 0.5 Cable Correlation factor 500 kcmil 750 kcmil 1000 kcmil 0.0 Frequency (MHz) Calibration • Select correction factor () ( is a function of distance, cable insulation, cable size, rated voltage and cable construction) •  takes into consideration both the magnitude attenuation and the change in PD pulse width •  is generated either from lab or field data Effect of cable insulation on the correlation factors for a single conductor, 500 kcmil, taped shield, 25 kV rated. Effect of cable size on the correlation factor for a single conductor, EPR insulated, taped shield, 25 kV rated.

15. 20.0 10.0 Equipment Correlation Factor (dB) 0.0 Frequency (MHz) PD Magnitude • PD magnitude (qm) is normally expressed in units of (mV/s) • This value is only relevant to this testing method • This value can be converted to pC multiplying (qm) by the equipment conversion factor a1 = gain ratio of inductive sensor a2 = attenuation factor of the instrumentation cables a3 = gain of preamplifier a4 = insertion loss of the filtration system

16. PD Location 200 EPR Insulation Polyethylene Insulation Paper Insulation Resonance Frequency (MHz) 100 0.0 Distance Resonance frequency as a function of distance for selected cable insulations. All three cables are single conductor, 25 kV, 500 kcmil, taped shield 200 500 kcmil cable 750 kcmil cable 1000 kcmil cable 100 Resonance Frequency (MHz) 0.0 Distance Resonance frequency as a function of distance for selected cable sizes. All three cables are single conductor, 25 kV EPR, taped shield

17. PD 50 TV Signal (a) FM Radio Signals PD (mV) 25 Corona 0.0 60.0 100 20.0 Frequency (MHz) 50 (b) PD (mV) 25 0.0 100 20.0 60.0 Frequency (MHz) 50 (C) 25 PD (mV) 0.0 100 60.0 20.0 Frequency (MHz) Noise filtration is achieved through signal processing in the frequency domain

18. Severity Assessment • The severity of a PD condition depends on: • insulating material • environment • type of defect • Severity assessment can be accomplished by one of the following approaches: • statistical approach • trending • pattern recognition

19. Severity Level • Level 1: System is free of partial discharge. No action needed • Level 2: Small level of PD, re-testing in two years is recommended • Level 3: Low probability of failure in 2 years, consider re-testing yearly • Level 4: Medium probability of failure in 2 years, considering replacement or other remedial action • Level 5: High probability of failure in 2 years, cable replacement is recommended

20. Summary of Cable Tests • Over 25 million feet of cable tested in the last 5 years • Cable length tested to date grouped by insulation, installation and rating Insulation Material XLPE > 10 million feet EPR > 4 million feet PILC > 10 million feet Butyl Rubber ~ 0.75 million feet HMW PE ~ 1 million feet Others ~ 0.25 million feet Cable Installation Duct System > 15 million feet Direct Buried > 9 million feet Open trays > 2 million feet Submarine < 0.25 million feet • Cable Rating • 15 kV > 9 million feet • 25 kV > 8 million feet • 40 kV > 8 million feet • 69/120/138 kV ~ 1 million feet

21. Field Data

22. Level 1: Out of 25,000,000 feet of cable tested, no failures have been reported Level 2: Out of 400,000 Level 2 PD sites found, no failures have been reported Level 3: Out of 250,000 Level 3 PD sites found, seven lead to cable failure within 12 months and about 10 others have failed after 2 years of testing Level 4: Out of 1,000 Level 4 PD sites found, seven lead to cable failures after 12 month and 20 more after 2 years of testing Level 5: Out of 100 Level 5 PD sites found, 14 failures were reported after 12 months and 23 more after 2 years of testing Customer Feedback

23. Case History- URD Subdivision • Sixty three single conductor- XLPE cables • Experienced 7 cable failures over 2 years • ON-Line PD testing results revealed 20 % of the system required replacement • Since replacement no cable failure has occurred in the last 3 years. • Laboratory measurements of the replaced sections confirmed field data.

24. Verification of Field Data • 20 year old, 24 kV XLPE direct buried systems from two different utilities exhibited significant PD level coming from splices • 12 of these joints were removed for laboratory evaluation • The splices exhibited PD levels 2 through 5 in the field measurements • Laboratory data confirmed field readings • Ac breakdown values correlated with PD data

25. 20 (A) PD (mV) 10 0.0 800 400 0.0 Distance (ft) 20 (B) PD (mV) 10 0.0 800 400 0.0 Distance (ft) Case History: 69 kV XLPE Cable • PD testing of a 69 kV rated, XLPE insulated, 2,250 kcmil/ Al conductor, PVC jacketed, 3,500 ft long single conductor • The circuit was tested at both ends • PD testing showed 15-20 PD sites in the first 700 ft of all three phases • PD patterns suggested water-tree type discharge • Circuit had failed six months prior to testing and the fault was confirmed to be caused by water trees • PD testing was repeated after six months • A 3-fold increase in PD severity was found at one location while other sites exhibited little or no increase at all • While the customer was planning the replacement of the bad cable section, lightening hit the substation and failed the cable in the previously identified location (200 ft from the substation). • PD data of the first 800 ft of phase C • (A) July 1999 testing • (B) January 2000 testing

26. 12.0 (B) PD (mV) 0.0 0.0 Time (ms) 50.0 1.0 (B) PD (mV) 0.0 0.0 Time (ms) 50.0 Zero spanA: Splice with PD level 5 B: Splice with PD level 2 Case History: 138 kV Splices • A 3.5 mile long newly installed XLPE cable circuit with eight splices per phase was tested for PD • Although both the cable and terminations were PD free, a large number of the 24 splices showed elevated levels of PD • Immediate replacement was advised on one splice showing Level 5 • Six other joints showed Level 4 • Three joints had moderate PD • Rest of joints show PD level 2 or were completely PD free • The joint with PD level 5 removed for examination had several large voids

27. Conclusions • Cable system reliability is of utmost importance for line operations • Early identification of weak system components (cable and accessories) is essential to maintain system reliability • Cost savings can be realized by prioritizing the replacement of weak sections of a circuit • On-Line PD technique is cost effective and reliable • PD can be detected at line voltage by improving detector sensitivity and noise rejection • No need to use over-voltages to detect PD • On-Line PD testing should be utilized to check the integrity of all T&D equipment