Online PD for Cable Network

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Online PD measuring on medium voltage cables

Transcript of Online PD for Cable Network

  • Diagnostic of PD in Cables

    PD activity incipient faults in cables. PDM is best indicator of insulation

    degradation being caused by cavities, electrical trees and other such defects, etc Provides Early Warning insulation fault. XLPE more susceptible to PD than paper

    insulation.

  • PD in Cables can be caused by

    Interfacial tracking in joints stress cones. Surface discharge at cable termination. Discharge in Bulk XLPE insulation. Joints/splices termination can sustain >> 100 pC for weeks. Bulk XLPE does sustain ~ 100 pC in hours BD. Once PD occurs in XLPE insulation, an electrical tree initiates and can progresses very rapidly

  • On-line PD Testing

    Provides quick Look-See tests on large number of feeders in a power network identify locate. Monitors and evaluate PD levels, cumulative activity and provides trends to compare with other and past data for asset management. Considered the most cost effective diagnostic technique that helps to avoid unplanned outages.

  • OLPD System Acquired at KSU is based on 4-phase approach

    Phase 1: On-line PD Screening with handheld PD surveying devices

    Phase 2: On-line PD Testing with PD diagnostic spot testers

    Phase 3: PD Location/Mapping with on-line PD cable mapping

    Phase 4: On-line PD Monitoring with portable on-line PD monitors

  • Long experience on cable network shows 5-20% of cable circuits have high PD in prescreening (Ph-I) quick cheap. PD surveyor: hand held.

    Ultrasonic / acoustic sensor. HFCT BW is 20 MHz. TEV BW is 100 MHz

    Provides LED based PD level Identifies 5 to 10% of cables feeders that may experience failures.

    Phase-I: Surveyor

  • Phase-II: Longshot Spot Tester

    Utilizes HVPD Longshot unit with software. It measures / records PD activity synchronously on four channels fed from the sensors attached on the cables on PC based 400 MHz CRO. Sensors used are:

    HFCTs. TEV RF Antanae.

  • Most prominent challenge in OLPD is to differentiate and isolate PD pulses from high Electromagnetic interference (Noise) prevalent in the field. Noise sources are:

    Frequency converters / thyristor firings. Variable speed drives. Surface discharges on external Insulation. Radio frequency interference. Cross talk from neighboring equipment.

  • PD pulses undergo attenuation and dispersion during their travel in the cable and their rise time / fall time values change.

    To isolate attenuated PD pulses from the noise pulses, state of the art filtering techniques are required.

  • Longshot Unit

  • Long shot + set of filters, it is possible to: Differentiate PD signals from noise. Establish location of PD. Device based on Windows PC + On board

    LAN Port.

  • Online PD Monitoring Sensors

    HFCT To capture PD travelling along the length of cable

    TEV To capture locally induced PD signals inside

    switchgears RF Antenna To capture external noise and interference

  • Online PD Monitoring Sensors

    High Frequency Current Transformer HFCT

    Frequency response of HFCT

    Transient earth voltage sensor

  • Cable PD Monitoring

    15

  • Cable PD Monitoring

    16

  • Earthing Requirements

    There are two prerequisites for conducting a successful online PD measurements.

    There must be independent access to either the earth-strap or the core of the cable at the switchgear/transformer. There must be an insulated gland between the cable earth and the switchgear earth.

  • Earthing Requirements

  • CT Around Earth Strap in a Substation

    Work to continue to understand the usefulness of this system

  • Laboratory Experimental Setup

    Long cable with proper terminations One end connected to HV transformer Other end open Toroid is used at the end to relief electric field stress around sharp edges of conductor. HFCT around core or earth TEV near the termination

  • Laboratory Experimental Setup

    Long Cable

    HV Transformer with primary and secondary circuit

    breakers

    AC Conductor

    Insulation Insulation screen

    Metallic shield

    Sheath Toroid

    Measuring instrument

    C1 C2 Filter with 50 Ohm termination

    Stress cone

    HFCT

  • Laboratory Experimental Setup D

    C

    B

    A

    HFCT

    HFCT

    A

    HFCTD

    HFCT

    B C

    500m500m500m

    JointsStart of PD pulse

    500m

    Joints

    HFCT HFCT

    HFCT

    HFCT

  • Cable PD Source Localization

    Single-ended PD site location method

    PD pulse train as seen from the measurement end

    Location from measurement end (% of Cable

    Length) = 100*(1-T/L)

  • Cable PD Source Localization

  • Cable PD Source Localization

    Segment Waveform

    Time us14131211109876543210

    Volts

    (mV)

    50

    0

    -50 Reflected pulse Main pulse

    Cable Length = 100m Time Difference = 1.2s Defect Location = 0% of the length (approx.)

  • Cable PD Source Localization

    Segment Waveform

    Time us14131211109876543210

    Volts

    (mV)

    10

    5

    0

    -5

    -10

    Main pulse Reflected pulse

    Cable Length = 100m Time Difference = 1.19s Defect Location = 0% of the length (approx)

  • Cable PD Source Localization

    Main pulse

    Reflected pulse

    Cable Length = 1500m Time Difference = 19.2s Defect Location = 0% of the length (approx)

  • Cable PD Source Localization

    Segment Waveform

    Time us14131211109876543210

    Volts

    (mV)

    6

    4

    2

    0

    -2

    -4

    -6Main pulse

    Reflected pulse

    Cable Length = 1500m Time Difference = 6.4s Defect Location = 66.67% of the length (approx)

  • PD Pulse Propagation

  • Conventional and Online System

    = 100 m

    Software Noise rejection PD mapping

    Filter

  • Location of Defects on Cable

  • Comparison of HVPD and Conventional System

    a) Knife cut in the insulation at position # 3 b) Mechanical damage-sharp cut extending from cables

    sheath down to its insulation c) Test on a field aged water treed XLPE cable.

    Defect Type

    OLPD Conventional PD System

    PDIV (kVrms)

    qm (pC) PDIV

    (kVrms) qm (pC)

    (a)

    (b)

    (c)

    8.0

    8.7

    13.0

    125

    268

    281

    8.0

    8.7

    13.0

    136

    320

    350

    qm=1025%

  • 5. Using OLPD to Detect PD from Cable Defects (1/6)

    Surface discharge at termination.

    4.1 kV (1150 pC)

    6.5 kV (790 pC)

  • Knife Cut in Insulation (2/6)

    8.5 kV (327 pC)

  • Metallic Protrusion (4/6)

    8 kV (20 pC)

  • Water Tree + ET Degraded Field Aged Cable (5/6)

    13.0 kV (52 pC)

  • Defective Joint (6/6)

    5 kV (123 pC)

  • PD Pulse Shapes

    Surface Discharge

    Segment Waveform

    Time us14131211109876543210

    Volts

    (mV)

    200150100

    500

    -50-100-150-200

    Segment Waveform

    Time us14131211109876543210

    Volts

    (mV)

    201510

    50

    -5-10-15-20

    Cut in Insulation

  • Electrical Tree

    Cavity Discharge

    Segment Waveform

    Time us14131211109876543210

    Volts

    (mV)

    604020

    0-20-40-60

    Segment Waveform

    Time us14131211109876543210

    Volts

    (mV)

    30

    20

    10

    0

    -10

    -20

    -30

  • Surface Discharge

    Tap Charge

    Segment Waveform

    Time us14131211109876543210

    Volts

    (mV)

    600400200

    0-200-400-600

    Segment Waveform

    Time us14131211109876543210

    Volts

    (mV)

    10

    5

    0

    -5

    -10

    Diagnostic of PD in CablesPD in Cables can be caused byOn-line PD TestingOLPD System Acquired at KSU is based on 4-phase approachPhase-I: SurveyorSlide Number 6Phase-II: Longshot Spot TesterSlide Number 8Slide Number 9Slide Number 10Slide Number 11Slide Number 12Online PD Monitoring SensorsOnline PD Monitoring SensorsCable PD MonitoringCable PD MonitoringEarthing RequirementsEarthing RequirementsCT Around Earth Strap in a SubstationLaboratory Experimental SetupLaboratory Experimental SetupLaboratory Experimental SetupCable PD Source LocalizationCable PD Source LocalizationCable PD Source LocalizationCable PD Source LocalizationCable PD Source LocalizationCable PD Source LocalizationPD Pulse PropagationConventional and Online SystemLocation of Defects on CableComparison of HVPD and Conventional System5. Using OLPD to Detect PD from Cable Defects (1/6)Knife Cut in Insulation (2/6)Metallic Protrusion (4/6) Water Tree + ET Degraded Field Aged Cable (5/6)Defective Joint (6/6)PD Pulse ShapesSlide Number 39Slide Number 40