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On-line PD testing & Diagnosis of MV & HV Cables.

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Presentation on theme: "On-line PD testing & Diagnosis of MV & HV Cables."— Presentation transcript:

1 On-line PD testing & Diagnosis of MV & HV Cables

2 Contents Introduction to PD damage to cables On-line PD Measurement Motivations Sensors and Measurement Location and Field Applications Monitoring Case Studies Conclusions

3 Cables Paper (PILC, MIND etc.) XLPE EPR PVC 3 Core and Single Core Cables Mixed cables with transition joints Right: 33kV PILC 3-core cable Left: 11kV PILC ‘Belted; cable 33kV XLPE 3-core cable 33kV XLPE single-core armoured cable

4 PD Detection – Energies for Different Points in Cable System

5 PD Damage – Incorrect Fitting of Stress Cone Tracking on 115kV Termination (before failure) Failed 115kV Termination (same type)

6 PD Damage to Cables – Water Tree Conversion to Electrical Tree in XLPE Cable Water trees turn into electrical trees immediately before failure PD testing will not detect early stages of water tree growth Bow-tie water trees Electrical trees

7 On-line PD Spot-Testing Quick to perform Overview of PD activity MV and HV Cables, all insulation types Advanced diagnostic techniques for noise- reduction

8 Continuous Monitoring Detect load varying and intermittent PD (mostly on PILC) Detect incipient faults through changes in pattern Temporary and permanent applications

9 PD Detection Theory

10 HV Capacitor –Placed in parallel with cable at termination –C = 500pF – 1nF (typically) –C is high impedance to low freq (i.e. 0.1 – 400 Hz) high voltage applied to cable –C is short circuit to high frequency PD signals (i.e. kHz/MHz range) –Off-line only Cable under test Coupling Capacitor Measurement Impedance

11 Sensors SensorAttachment PointPD Detection Method HFCT Power cable earth strap/drain wire or power cable with earth strap/drain wire brought back through sensor. Current impulses from PD in cables, cable terminations and plant/switchgear cables are terminated into. TEV Metal-clad plant housing close to vents/seams/gaskets. Electromagnetic radiation from PD sites in plant that is induced onto the plant metal housing. Airborne Acoustic Over vents in plant housing with line of site to PD source. Airborne acoustic (ultrasonic) radiation from corona and surface discharges in the plant.

12 HFCT Sensor Attachment Temporary Outside Cable Box 2 1

13 HFCT Attachment at Cross-bond Points

14 Circuit Name Cable Type Voltage (kV) Length (m) Return Time (μs) Return Propagation Speed (m/μs) Test DateTest method Hotel Maluri Inn - Taman Maluri - Y Phase 3c XLPE 112322.98027/07/08OWTS, PD at 1.3U 0 HFCT OWTS Supply with inbuilt HV Capacitor Cable under test OWTS HV Capacitor HFCT

15 On-line PD Detection Equipment

16 PD Detection Equipment and Methodology Detect cable and local PD Cable PD PD signals from power cable components HFCT detection Local PD PD signals from nearby sources: cable terminations, switchgear, transformer etc HFCT/TEV/Acoustic detection

17 Wideband Capture of PD signals, BW, 50-400MHz Capture all of the PD energy detected by sensor Capture simultaneously with power cycle and extract impulsive signals Separation of PD and noise signals based on wave shape Continuous (RF, etc) noises removed with hardware notch/high pass filters

18 HVPD On-line PD Technology PD Detection HVPD-Longshot™ with PDGold© Software Wideband PD test system PD Location HVPD Longshot™ with PDMap© Software Transponder Installed at far cable end PD Monitoring HVPD Longshot™ (up to 48 hours) HVPD Mini™

19 On-line PD Signal Measurement Cable PD Pulse: 16mV, 1160pCLocal PD Pulse: 15mV, 24dBNoise Pulse: 6mV 20 ms 15 us One power cycle of raw data from HFCT sensor

20 Normal Distribution of All Events in a Single Power Cycle 1 Power Cycle

21 Noise and Interferences in On-line PD Measurements Noise can be problem on-line Cables/plant not isolated from network Sensors often attached to ground Noise sources Continuous: Radio/TV stations, communications Impulsive - Periodic Power electronics, e.g. inverter power supplies, UPS Rotating machine excitation Impulsive – Random Switching Events Corona PD on other cables/plant

22 Noise Source Summary

23 Noise Discrimination – Hardware Filters Preferred over software filtering Remove interferences in HFCT pass-band Use in difficult measurement situations

24 Noise Discrimination – Hardware Filters Single Frequency RF Noise Reduction Power Cycle PD and noise detail

25 Noise Discrimination – Event Recognition Power Electronic Switching Continuous amplitude across power cycle No phase pattern Pulses appear as chain 16kHz repetition frequency

26 PD Test and Location Applications

27 PD location at RMU Signals propagate between cables with little attenuation on two feeder RMUs Reduced detection points on network – no need to test every RMU Synchronous capture necessary to determine source feeder Measured PD pulse Zoom of pulse start

28 Example Test Configuration for PD Location on Circuit with RMUs 11kV RMU (Normally closed) RMU (Normally closed) Transponder Section of Cable Undergoing Mapping Test RMU (Normally open) HVPD-Longshot

29 Cable Return Time, L for On-line Measurements Use from Off-line Tests Estimate from cable length and average propagation speed for cable type Measure with on-line TDR (requires impedance change at far end of cable) PD Site Location,

30 Cables with Cross-Bonding

31 PD Propagation in Cross-bonded Cable Systems Cross-bond point is seen as change in impedance in cable system Some of pulse propagates into cross-bond point and onto all cables Some of pulse is reflected back down source cable Long-cross bond leads/coaxial cross-bond cables are high impedance to HF PD signals

32 Cross-bond Propagation Example Pulse Injection Test Set-up Input signal, L1 Reflection from CB Pulse on outgoing side of CB, L1 Outgoing scale 50% of Injection scale

33 Procedure for Testing Cross-bonded Cable System Detection Sequentially test at terminations and all CB-points N = 2 + number of joints Test 1Test 2 Test 3 Test N

34 Continuous PD Monitoring

35 Continuous PD Monitoring Aspects Detect cyclic changes in activity Load varying activity on PILC cables Humidity related activity from surface discharges Detect changes that relate to incipient faults Gradual rise Sudden rise Sudden drop Carried out on: key circuits, circuits with suspected cyclic PD changes, circuits with high spot-test results

36 PD and Load Relations Although PD incepted by voltage, load can have effect Mostly on PILC cables Load variations –Movement of oil/impregnant –Expansion of conductors S S M T W T F

37 Examples of Monitoring Simultaneous change in PD with monitors on 11kV network Bemish Road Lower Richmond

38 On-line PD Severity Evaluation

39 PD Level Guidelines To determine the true severity the PD is posing to the cable, the following should be taken into account –Cable accessories (joints and terminations) have a higher tolerance to PD than the cable insulation –Mixed circuits: Paper/PILC has a much higher tolerance to PD than XLPE

40 PD Level Guidelines

41 Example of Normalised Distribution of Off-line PD Levels Measured in 33kV Paper and XLPE Cables Mackinlay, R. & Walton, C. Some advances in PD monitoring for high voltage cables MV Paper Cables: Asset or Liability?(Digest No. 1998/290), IEE Colloquium on, 1998

42 Case Study: On-line PDMapping TNB-D PPU – Central Spectrum May 2006

43 PDGold Test: Central Spectrum

44 Install transponder at Substation B due to lower noise level

45 Transponder at Central Spectrum Injection HFCT Detection HFCT Trigger Unit Pulse Generator

46 Transponder Waveform Measured at PPU Direct Pulse Reflected Pulse Transponder Injected Pulse Transponder time delay

47 PDMap Waveform Result: 3x PD Sites found on Cable

48 Conclusions

49 HFCT sensors allow sensitive measurements to be made at multiple points in the cable system On-line PD location can relatively quickly identify weak points for timely remedial action Solutions available for complex circuits such as with cross-bonding

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