PD Testing Considerations for MV Plant Cables

Transcription

1 PD Testing Considerations for MV Plant Cables

2 Cable Testing Philosophy Damage Mistake Aging Repair Manufacturing Transportation Installation Operation Power frequency 50/60 Hz Power frequency 50/60 Hz Factory Testing Acceptance Testing Maintenance Testing Cable Aging Management- SLIDE 2

3 Various Manufacturing Defects 1. Cavity at shield 2. Cavities due to shrinkage or gas-formation in insulation 3. Defects in the insulation shield 4. Loosely bound solid particles 5. Protrusions 6. Splinters 7. Fibers 8. Contaminants in the insulation shield Cable Aging Management- SLIDE 3

4 Electrical Trees (XLPE) Often begins near Protrusions at the shield Protrusions at the conductor Near impurities At water trees Two Types Branch type trees Bush type trees Cable Aging Management- SLIDE 4

5 Partial Discharge Theory 1 E( x) = R x ln r 3ε Evoid = E( x) 1+ 2ε 3ε 1 = 1+ 2ε R x ln r Kinectrics Inc., Cable Aging Management- SLIDE 5

6 6 4 2 Voltage/Electric Field Partial Discharge Theory Electrical Field Conditions 0 Phase Angle Appl. Voltage [kv] Background Field [kv/mm] Void Breakdown Strength Void Field [kv/mm] Cable Aging Management- SLIDE 6

7 Voltage/Electric Field Partial Discharge Theory Electrical Field Conditions Initial Discharge Return Discharge Second Discharge 0 Phase Angle Appl. Voltage [kv] Background Field [kv/mm] Void Breakdown Strength Void Field [kv/mm] Cable Aging Management- SLIDE 7

8 Voltage/Electric Field Partial Discharge Theory Electrical Field Conditions 10 0 Phase Angle Appl. Voltage [kv] Background Field [kv/mm] Void Breakdown Strength Void Field [kv/mm] Cable Aging Management- SLIDE 8

9 PD Testing Diagnostic Test Can be performed on-line or off-line With respect to commissioning testing often performed either during a HiPot test or after HiPot test Should preferably be done at.. At near power frequency Elevated voltages to identify problem prior to in-service failure Most PD monitors require experts for operation and data interpretation expert test No standard for field PD testing (IEEE is a guide) Should be performed by independent third party Cable Aging Management- SLIDE 9 Kinectrics

10 Electrical Trees (XLPE) Kinectrics Inc., 2008 Cable Aging Management- SLIDE 10

11 PD Theory: Apparent Charge Measured Apparent Charge Apparent Charge at discharge location V PD R Attenuation Attenuation C i PD V PD i PD R V PD Measured Apparent Charge R Cable Aging Management- SLIDE 11 Kinectrics

12 TDR on Tape Field Cable Cable Aging Management- SLIDE 12

13 Cable Testing Philosophy Damage Mistake Aging Repair Manufacturing Transportation Installation Operation Power frequency 50/60 Hz Power frequency 50/60 Hz Factory Testing Acceptance Testing Maintenance Testing Cable Aging Management- SLIDE 13

14 Acceptance & Maintenance Tests Types of Tests Soaking DC HiPot Testing AC HiPot Testing VLF Testing Damped AC Partial Discharge Testing Standards IEC 60502, IEC 60840, IEC Guides IEEE DC Testing VLF Testing PD Testing Damped AC AC Withstand Cigre 182 Cigre 279 Cable Aging Management- SLIDE 14

15 VLF Testing After-laying field tests Pros Inexpensive compared to AC HiPot Non-Resonant testing Effective in finding watertree d cable for field aged, distribution class cables Easy to perform Non-expert test IEEE Std Cons Still discussion on test levels versus test time Issues with charge injection and trapped charge when testing at frequencies less than 0.1 Hz Not as effective in finding weak spots in the insulation as a conventional AC HiPot. Voltage wave may not be sinusoidal (depends on test kit). Not straight forward to interpret PD data acquired during VLF conditions Research still to be done on the effectiveness in detecting, for instance, interfacial phenomena Kinectrics Inc., Cable Aging Management- SLIDE 15

16 VLF vs. AC Testing After-laying field tests HZ HZ HZ Kinectrics Inc., Cable Aging Management- SLIDE 16

17 VLF vs. AC Testing Withstand Voltage of XLPE Cables Kinectrics Inc., Cable Aging Management- SLIDE 17

18 VLF vs. AC Testing Withstand Voltage of XLPE Cables Waveform Sinusoidal Cable System Rating ( φ-φ) Installation (φ- ) VLF Acceptance ((φ- )) Maintenance (φ- ) AC Withstand Acceptance (φ- ) Maintenance (φ- ) [kv] [kv rms ] [kv rms] [kv rms ] [kv rms ] [kv rms ] Cable Aging Management- SLIDE 18

19 AC HiPot Testing Stipulated in IEC (MV), IEC (HV) and IEC (EHV) as well as most cable testing standards Testing at power frequency from 20 Hz to 300 Hz Recommended high potential test level for acceptance testing ranges from 1.25U 0 to 2U 0 but typically 1.7U 0. High potential voltage applied for 60 minutes AC HiPot testing is gaining ground in Europe and North- America Proven test methodology for identifying installation related defects. Kinectrics Inc., Cable Aging Management- SLIDE 19

20 AC HiPot Testing Pros Proven test methodology; test levels and duration well established Based on IEC standards Reliable equipment available. Decades of experience with tests Can perform PD testing during AC HiPot tests Gets soft failure so meaningful forensics can be performed Cons Not a diagnostic test Requires heavy equipment Not necessarily cheap Kinectrics Inc., Cable Aging Management- SLIDE 20

21 The Phenomenon of Resonance L R i C C Cable i R i L Cable Aging Management- SLIDE 21

22 MV Test Spec (Plant Cables) Equipment Power Cable Rated Voltage (kv) Test Level (kv) Hipot Test Time (mins) AC Testing Partial Discharge Test Extinction Allowable Pulse Level (kv) Mag. (pc) Time (mins) Test Level (kv) DC Testing Time (minutes) Unshielded: 4.16 N/A N/A N/A N/A N/A 25 (35) 5 Shielded: (including cable bus) Bus Duct (13) 13 (18) 19 (27) (4.5) 7.5 (7.5) 13 (13) 3 (3) 3 (3) 3 (3) (35) 35 (45) 55 (70) Isolated Phase Bus (60) Metal-Enclosed Rigid (36) (10.5) 100 (100) 2 38 (50) 1 Switchgear (19) (3.5) 100 (100) 2 N/A N/A (36) (10.5) 100 (100) 2 N/A N/A Cable Aging Management- SLIDE 22

23 PD Interpretation Criteria Comm. Testing (Off-Line) Measure PDIV, PDEV Measure PD Level & Intensity Identify signature partial discharge pattern Identify location of partial discharge source Guiding criterion is that the cable system should be PD free at the specified test voltage at commissioning fro HV &EHV systems Less then 3pC or 3pC for MV systems using conventional systems Maintenance Testing (On-Line) Assess individual PD sources separately Identify location of partial discharge source Assess impact of individual PD sources (insulation materials, experience) Develop individual assessment/ranking of joints and cable sections Kinectrics Inc., Cable Aging Management- SLIDE 23

24 PD Testing Pros Can help identify insulation defects which does not cause failure during an AC HiPot test Can use external, non-invasive sensors Can be performed continuously at each accessory using daisy chained PD systems Cons No standard or acceptance criteria For long lengths of cable a distributed PD measurement should be performed (at each accessory) Requires an expert to perform the test and evaluate the data Cannot compare factory PD tests to field tests May be practically difficult to perform (manhole access, traffic control) Cable Aging Management- SLIDE 24

25 Off-Line vs. On-Line Off-Line Requires cable disconnected and cable conductors exposed. Requires external power source Done at elevated voltage Can perform meaningful sensitivity assessment Can measure PDIV, PDEV For cross-bonded cable systems, SVLs must be shorted and, preferably, the cross links removed thus creating a continuous sheath On-Line Non-invasive test Done at rated voltage PDIV and PDEV cannot be measured Often, multiple PD sources from various types of HV equipment are detected Can only detect sources present at the time of testing! Can be performed continuous over long lengths of time thus characterizing the dynamic PD behaviou Cable Aging Management- SLIDE 25 Kinectrics

26 PD Monitors Measurement Parameters Magnitude Phase Angle Derive pulse count rate Signal Shape Types of Monitors Traditional PD Monitors Conventional PD Monitors Modern Partial Discharge Monitors Cable Aging Management- SLIDE 26 Kinectrics

27 AC HiPot & Conv. PD Testing Cable Aging Management- SLIDE 27

28 Broad Band PD Measurements Cable Aging Management- SLIDE 28

29 Sensor Connections HFCT For Voltage Reference LFCT For Voltage Reference Cable Aging Management- SLIDE 29

30 AC HiPot & Unconv. PD Testing The frequency content of a partial discharge pulse depends on Duration Strength of the electrical field Length of the partial discharge it self Pulse propagation path different PD sources have different frequency content Cable Aging Management- SLIDE 30

31 Off-line Hi-Pots (ac, VLF) Dielectric loss measurements Partial Discharge testing Cable Aging Management- SLIDE 31

32 Summary of Partial Discharge Data Cable ID (Phase) AC Hipot DIV (kv) DEV (kv) 13 kv (pc) 2ENA02NC1-AA/AG (Red) 2ENA02NC1-AA/AG (Black) 2ENA02NC1-AA/AG (Blue) Passed Passed Passed ENA02NC1-AB/AH (Red) 2ENA02NC1-AB/AH (Black) 2ENA02NC1-AB/AH (Blue) Passed Passed Passed ENA02NC1-AC/AI (Red) 2ENA02NC1-AC/AI (Black) 2ENA02NC1-AC/AI (Blue) Passed Passed Passed 18.9 > > ENA02NC1-AD/AJ (Red) 2ENA02NC1-AD/AJ (Black) 2ENA02NC1-AD/AJ (Blue) Passed Passed Passed ENA02NC1-AE/AK (Red) 2ENA02NC1-AE/AK (Black) 2ENA02NC1-AE/AK (Blue) Passed Passed Passed >19 >19 >19 >19 >19 > ENA02NC1-AF/AL (Red) 2ENA02NC1-AF/AL (Black) 2ENA02NC1-AF/AL (Blue) Passed Passed Passed > > Cable Aging Management- SLIDE 32

33 Example 1: On-Line PD 142 circuits of single conductor 5 kv EPR 19 years to 21 years old All cables feeding 4.2 kv induction motors Cable Aging Management- SLIDE 33

34 Example 1: On-Line PD (5kV) Partial discharge activity detected through out the system However, of 142 cables, only 7 subjected to partial discharge activity from the cable system it self In one case, partial discharge activity was detected from a cable splice The remaining six cases related to tracking along cable termination surfaces The remaining discharge activity detected originated from the 4.2 kv induction motors, the bus system or other connected HV equipment Cable Aging Management- SLIDE 34

35 Example 2: On-Line PD (27.5) Industrial client 42 cable sections, 27.5 kv XLPE 12 years old Cable circuit constitute single supply line to the plant crucial to avoid unforeseen failure On-line partial discharge measurements performed routinely as a maintenance test Cable Aging Management- SLIDE 35

36 Example 2: On-Line PD (27.5) Cable Aging Management- SLIDE 36

37 Ex 3: Periodic/Remote Monitoring Custom made sensor Cable length Cable specifications Non-invasive install, can be performed live Connected to termination box Cable Aging Management- SLIDE 37

38 Ex 3: Periodic/Remote Monitoring Cable Aging Management- SLIDE 38

39 Ex 3: Periodic/Remote Monitoring Permanent installation at substation 1 day installation Suspected faulty equipment PD measured from vacuum bottle Verified during subsequent off-line testing PROCESSING Cable Aging Management- SLIDE 39

40 Remote Monitoring Setup Example 1 Cable Aging Management- SLIDE 40

41 Cable Testing also involves Being nice to stray doggies at work sites - even while you re busy! The greatness of a nation and its moral progress can be judged by the way its animals are treated.. I hold that, the more helpless a creature, the more entitled it is to protection by man from the cruelty of man" - Mahatma Gandhi ( ) Cable Aging Management- SLIDE 41

42 Contact information Terry Herrmann Structural Integrity Associates (303) Rick Easterling Kinectrics (804) Steve Biagiotti Structural Integrity Associates (303) Mark Fenger Kinectrics (416) Howard Sedding Kinectrics (416) x Cable Aging Management- SLIDE 42