PARTIAL DISCHARGE LOCATION Selected Topics

Transcription

1 PARTIAL DISCHARGE LOCATION Selected Topics M.S. Mashikian IMCORP ICC Meeting - Fall 2000

2 TOPICS TESTING PHILOSOPHY Excitation Voltage Test Voltage Level METHOD OF REFLECTOMETRY METHOD OF ARRIVAL TIME TESTING Branched Circuits On-line - Discussion of ON and OFF-Line Major Concerns - Answers 1

3 TESTING PHILOSOPHY PURPOSE: Reduce Service Failures - Improve Reliability Reduce Cost or Delay Capital Expenditure TEST GOALS: Identify defects that may cause near-term service failures Characterize severity of defects to help prioritize corrective actions Constraint: Minimize likelihood of test-produced damage Other 2

4 TESTING PHILOSOPHY EXCITATION VOLTAGE Use 50/60Hz produced by resonant test set Ramp voltage slowly until PD is detected or max. test voltage is reached Dwell approx. 2 seconds (will be reduced to ~ 10 cycles). Reduce voltage to zero very rapidly. 3

5 RESONANT TRANSFORMER Inductor L 1. Variable air-gap inductor 2. Fixed inductor C Cable U 1 1. U 1 is 50/60 Hz 2. U 1 is variable frequency Resonance: ωl = 1/ωC L ω 4

6 PARTIAL WAVE - 50/60HZ 1/4 or 1/2-cycle 50/60Hz dc PD is detectable only in the quadrants shown 5

7 OSCILLATING WAVE (OSW) dc Frequency dictated by cable capacitance Attenuation depends on cable 6

8 APDAC 50/60 Hz + dc - ALTERNATING POLARITY DC-BIASED AC 7

9 TEST VOLTAGE LEVEL Decide after discussing with client Minimum: 2U 0 recommended Maximum: U 0 Rationale Consider maximum transient voltage that may exist on system: geographic location and system protection. 8

10 CUMULATIVE PD PROBABILITY, 2500km, EXTRUDED CABLE Cumulative PD Probability, % PDIV Range, U DATA BASED ON 960 PD EVENTS IN CABLES At 1.0 U 0, less than 3% of PD events can be identified. 9

11 TEST VOLTAGE LEVEL PDIV varies with type of excitation voltage: With sinusoidal VLF, PDIV may be as high as 2.5 times that at 50/60Hz. Where PD is detected with 50/60Hz voltage, no PD may seem to exist at the same voltage level in PD tests with other types of excitation voltage. 10

12 PRINCIPLES OF PD TESTING PD Site Location Reflectometry (off-line) Arrival-Time Non-branched (2-termination) system Off-Line On-Line Branched system: Off-Line On-Line Frequency-Domain Testing IMCORP Technologies 11

13 REFLECTOMETRY 25 Amplitude, mv t 2 A C B ~ x t 1 PD Site Estimator AC = t 1 t 2 x AB Time, µs 12

14 TEST REPORT Joint and PD Locations Joint PD 13

15 TEST REPORT Joint and PD Locations Joint PD 14

16 PHASE RESOLVED PD DISPLAY PD in Termination 15

17 ARRIVAL-TIME NON-BRANCHED CABLE sensor 1 Principle x sensor 2 t 1 x L-x t 2 x = 1/2[v(t1-t2)+L] ~ Multi- Channel Estimator Sensor 1 Signal Conditioner Actual From SDCT Sensor 2 SDCT SDCT = Signal detection - conditioning - transmission 16

18 ARRIVAL-TIME BRANCHED CABLES Remote #2 Test Transformer Sensor Branched Cable SDCT Remote #3 Signal conditioner Estimator Multi-Channel From #1,2,3 SDCT Remote #1 SDCT 17

19 TESTING 4-TERMINAL CIRCUIT 18

20 PD-LOCATION 19

21 PD TESTING - Major Concerns Is PD test able to locate water trees? All trees? Some trees? See McBride et al. Does PD testing unduly deteriorate or fail Good cables? Defective cables? - Effect of high dc or thumping How effective is ON-LINE testing? How effective is PD testing of PILC cables? Are PD test results obtained by different methods identical? 20

22 McBride et al. (Georgia Power Res., and EPRI) - IEEE Trans. on Power Del. 4/94 70mils 70mils 35x48-mil void 21

23 McBride et al. (Georgia Power Res., and EPRI) - IEEE Trans. on Power Del. 4/94 Inspection of these wafers revealed a large void (35 mils wide and 48 mils high) in the center of the (70 mils diameter and 70 mils high) water tree. There was significant evidence of PD within the interior of the water tree, with darkened areas near the tips of the tree. This was clearly the site of the PD activity. Cable failures near large water trees have occurred in ac breakdown tests 22

24 McBride et al. (Georgia Power Res., and EPRI) - IEEE Trans. on Power Del. 4/94 Seven cables failed while active PD sites were monitored (at higher voltage than PDIV). With the exception of two samples (out of the remaining 7 samples with PD) all other cable failures occurred at the PD inception voltage while (PD site) locations were being performed Failures were associated with the PD sites. The duration of sustained PD ranged from 2 to 80 minutes prior to failure. The theory that PD above operating voltage results in failure at operating voltage is not supported by this data. 23

25 McBride et al. (Georgia Power Res., and EPRI) - IEEE Trans. on Power Del. 4/94 Very few cables contained active PD sites near operating voltage. No PD above 1-3pC was detected in any of the test cables (61 samples) when energized at operating voltage. (PDIV = 8-35kV). Most PD sites were detected at twice operating voltage or higher. The (pc) level of the PD at a site could not be correlated with the likelihood of a failure at or near the site. 24

26 BREAKDOWN AT PD SITE THROUGH VENTED WATER TREE 25

27 IMPROVED RELIABILITY Peter Daly, San Francisco, Jan. 20, BEFORE UPTI TESTING AFTER UPTI TESTING Feeder Cables 0.40 SAIFI Feeder Cables Year NOTE: Only 40% of Recommended Repair Was Completed 26