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