PD Testing Considerations for MV Plant Cables Cable Testing Philosophy Repair Mistake Aging Damage Damage Manu- Trans- In- Operation Power frequency facturing portation stallation 50/60 Hz Power frequency 50/60 Hz Factory Factory Testing Acceptance Acceptance Testing Maintenance Maintenance Testing Cable Aging Management- SLIDE 2 Various Manufacturing Defects 1. Cavity at shield 5. Protrusions 2. Cavities due to shrinkage or 6. Splinters gas-formation in insulation 7. Fibers 3. Defects in the insulation 8. Contaminants in the shield insulation shield 4. Loosely bound solid particles Cable Aging Management- SLIDE 3 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 Partial Discharge Theory 1 E() x = R x ln r 3ε E = E() x void 1+ 2ε 3ε 1 = + ε R 1 2 x ln r © Kinectrics Inc., 2011 5Cable Aging Management- SLIDE 5 Partial Discharge Theory 6 Electrical Field Conditions 4 Voltage/Electric Field Voltage/Electric 2 Phase Angle 0 0 45 90 135 180 225 270 315 360 -2 -4 Appl. Voltage [kV] Background Field [kV/mm] Void Breakdown Strength Void Field [kV/mm] Cable Aging Management- SLIDE 6 -6 Partial Discharge Theory 20 Electrical Field Conditions 15 10 Return Voltage/Electric Field Voltage/Electric Initial Discharge Second Discharge Discharge 5 Phase Angle 0 0 45 90 135 180 225 270 315 360 -5 -10 Appl. Voltage [kV] -15 Background Field [kV/mm] Void Breakdown Strength Void Field [kV/mm] -20 Cable Aging Management- SLIDE 7 40 Partial Discharge Theory Electrical Field Conditions 30 20 Voltage/Electric Field Voltage/Electric 10 Phase Angle 0 0 45 90 135 180 225 270 315 360 -10 -20 Appl. Voltage [kV] -30 Background Field [kV/mm] Void Breakdown Strength Void Field [kV/mm] Cable Aging Management- SLIDE 8 -40 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 400.3 is a guide) • Should be performed by independent third party Cable Aging Management- SLIDE 9 © Kinectrics Electrical Trees (XLPE) © Kinectrics Inc., 2008 Cable Aging Management- SLIDE 10 PD Theory: Apparent Charge Apparent Charge at discharge location VPD Measured Apparent R Charge Attenuation Attenuation C iPD VPD iPD VPD R Measured Apparent R Charge Cable Aging Management- SLIDE 11 © Kinectrics TDR on Tape Field Cable Cable Aging Management- SLIDE 12 Cable Testing Philosophy Repair Mistake Aging Damage Damage Manu- Trans- In- Operation Power frequency facturing portation stallation 50/60 Hz Power frequency 50/60 Hz Factory Factory Testing Acceptance Acceptance Testing Maintenance Maintenance Testing Cable Aging Management- SLIDE 13 Acceptance & Maintenance Tests Types of Tests Standards • Soaking • IEC 60502, IEC 60840, IEC 62067 • DC HiPot Testing Guides • AC HiPot Testing • IEEE 400 • VLF Testing – 400.1 DC Testing • Damped AC – 400.2 VLF Testing • Partial Discharge – 400.3 PD Testing – 400.4 Damped AC Testing – 400.5 AC Withstand • Cigre 182 • Cigre 279 Cable Aging Management- SLIDE 14 VLF Testing After-laying field tests Pros Cons • Inexpensive compared to AC • Still discussion on test levels versus HiPot Non-Resonant testing test time • Effective in finding watertree’d • Issues with charge injection and cable for field aged, distribution trapped charge when testing at class cables frequencies less than 0.1 Hz • Not as effective in finding weak • Easy to perform spots in the insulation as a • Non-expert test conventional AC HiPot. • IEEE Std 400.2 • 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., 2011 15Cable Aging Management- SLIDE 15 VLF vs. AC Testing After-laying field tests 1 0.1HZ 0.5 0 0 1 2 3 4 5 6 7 8 9 10 -0.5 -1 1 20HZ 0.5 0 0 1 2 3 4 5 6 7 8 9 10 -0.5 -1 1 60HZ 0.5 0 0 1 2 3 4 5 6 7 8 9 10 -0.5 -1 © Kinectrics Inc., 2011 16Cable Aging Management- SLIDE 16 VLF vs. AC Testing Withstand Voltage of XLPE Cables © Kinectrics Inc., 2011 17Cable Aging Management- SLIDE 17 VLF vs. AC Testing Withstand Voltage of XLPE Cables VLF AC Withstand Cable System Installation Acceptance Maintenance Acceptance Maintenance Waveform Rating ( φ-φ) (φ-∅) ((φ-∅)) (φ-∅) (φ-∅) (φ-∅) [kV] [kV rms] [kV rms] [kVrms] [kVrms] [kVrms] 5 9 10 7 13 9 8 11 13 10 15 19 21 16 26 23 25 291 32 241 Sinusoidal 28 32 361 27 42 36 35 39 44 33 46 51 57 43 67 58 69 75 84 63 72 Cable Aging Management- SLIDE 18 AC HiPot Testing • Stipulated in IEC 60502 (MV), IEC 60840 (HV) and IEC 62067 (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 1.25U0 to 2U0 but typically 1.7U0. • 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., 2011 19Cable Aging Management- SLIDE 19 AC HiPot Testing Pros Cons • Proven test methodology; • Not a diagnostic test test levels and duration well • Requires heavy equipment established • Not necessarily cheap • 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 © Kinectrics Inc., 2011 20Cable Aging Management- SLIDE 20 The Phenomenon of Resonance i L R C iR CCable iL Cable Aging Management- SLIDE 21 MV Test Spec (Plant Cables) Rated AC Testing DC Testing Voltage Hipot Test Partial Discharge Test Equipment (kV) Test Level Time Extinction Allowable Pulse Time Test Level Time (kV) (mins) Level (kV) Mag. (pC) (mins) (kV) (minutes) Power Cable Unshielded: 4.16 N/A N/A N/A N/A N/A 25 (35) 5 Shielded: 4.16 9 (13) 5 4.5 (4.5) 3 (3) 2 25 (35) 15 (including cable bus) 8 13 (18) 5 7.5 (7.5) 3 (3) 2 35 (45) 15 13.8 19 (27) 5 13 (13) 3 (3) 2 55 (70) 15 Bus Duct Isolated Phase Bus 25 45 (60) 1 19 100 2 64 1 Metal-Enclosed Rigid 13.8 27 (36) 1 10.5 (10.5) 100 (100) 2 38 (50) 1 Switchgear 4.16 14 (19) 1 3.5 (3.5) 100 (100) 2 N/A N/A 13.8 27 (36) 1 10.5 (10.5) 100 (100) 2 N/A N/A Cable Aging Management- SLIDE 22 PD Interpretation Criteria Comm. Testing (Off-Line) Maintenance Testing (On-Line) • Measure PDIV, PDEV • Assess individual PD sources • Measure PD Level & Intensity separately • Identify signature partial • Identify location of partial discharge pattern discharge source • Identify location of partial • Assess impact of individual PD discharge source sources (insulation materials, ⇒ Guiding criterion is that the experience) cable system should be PD ⇒ Develop individual free at the specified test assessment/ranking of joints voltage at commissioning fro and cable sections HV &EHV systems ⇒ Less then 3pC or 3pC for MV systems using conventional systems © Kinectrics Inc., 2011 23Cable Aging Management- SLIDE 23 PD Testing Pros Cons • Can help identify insulation • No standard or acceptance defects which does not cause criteria failure during an AC HiPot test • For long lengths of cable a • Can use external, non-invasive distributed PD measurement sensors should be performed (at each • Can be performed continuously accessory) at each accessory using daisy • Requires an expert to perform chained PD systems 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 Off-Line vs. On-Line Off-Line On-Line • Requires cable disconnected and • Non-invasive test cable conductors exposed. • Done at rated voltage • Requires external power source • PDIV and PDEV cannot be • Done at elevated voltage measured • Can perform meaningful • Often, multiple PD sources from sensitivity assessment various types of HV equipment • Can measure PDIV, PDEV are detected • For cross-bonded cable systems, • Can only detect sources present SVLs must be shorted and, at the time of testing! preferably, the cross links • Can be performed continuous removed thus creating a over long lengths of time thus continuous sheath characterizing the dynamic PD behaviou Cable Aging Management- SLIDE 25 © Kinectrics PD Monitors Measurement Parameters Types of Monitors • Magnitude • Traditional PD • Phase Angle Monitors ⇒ Derive pulse count • Conventional PD rate Monitors • Signal Shape • Modern Partial Discharge Monitors Cable Aging Management- SLIDE 26 © Kinectrics AC HiPot & Conv. PD Testing Cable Aging Management- SLIDE 27 Broad Band PD Measurements Cable Aging Management- SLIDE 28 Sensor Connections HFCT For Voltage Reference LFCT For Voltage Reference Cable Aging Management- SLIDE 29 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 • Off-line Hi-Pots (ac, VLF) Dielectric loss measurements Partial Discharge testing Cable Aging