Prevent Unplanned Outages with On-line PD Measurement of MV/HV Power Cables

Cable Partial Discharge Measuring Systems

Ammar Khan Technical Application Specialist – Cables CBM

1 Dated: June 21, 2016

• Partial discharge theory

• Power cable structure

• PD detection and localization

• Why online PD measurement

• Sensors used for online PD measurement

• Qualitrol offerings

• Case Studies

Overview

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PARTIAL DISCHARGE THEORY

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• According to IEC 60270

Partial discharge (PD) is a localized electrical discharge that only partially bridges the insulation

between conductors and which may or may not occur adjacent to a conductor.

• The term ‘partial discharge’ includes a wide group of discharge phenomena – (i) internal discharges occurring in voids or cavities within solid or

liquid dielectrics; – (ii) surface discharges appearing at the boundary of different

insulation materials; – (iii) corona discharges occurring in gaseous dielectrics in the

presence of inhomogeneous fields; – (iv) continuous impact of discharges in solid dielectrics forming

discharge channels (treeing).

What is Partial Discharge?

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PD Phenomenon

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HV across conductor

PD Pulses

Breakdown in cavity

The detection and measurement of discharges is based on the exchange of energy taking place during the discharge. These exchanges are manifested as:

– (i) electrical pulse currents – (ii) dielectric losses – (iii) E.M. radiation (light) – (iv) sound (noise) – (v) increased gas pressure – (vi) chemical reactions

Results of Partial Discharges

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POWER CABLE STRUCTURE

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• Designed life of power cables is approximately 40 years • Cable Types that can be tested using Qualitrol Online PD Technology:

– XLPE (Cross Linked PolyEthylene) – EPR (Ethylene Propylene Rubber) – PE (PolyEthylene) – PILC (Paper Insulated Lead Covered)

Power Cable, its Basic Structure

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A Power Cable Circuit

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Joints

Terminations Cross-bonding Links

Multi point grounding

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Solid grounding

Distributed PD measurement

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Zone 1 Zone 2 Zone 3

PD DETECTION AND LOCALIZATION

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PD Detection into Cables

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Long Cable

Defect

Measurement EndRemote End

PD at Cable Terminations

• PD in cable induces HF signals

• The signals travel in both

directions

• Signals attenuate during

propagation and reflect back at

terminations

• Detection Principle

• Capture the HF signals emitted

by PD at termination point

• Analyse signals to identify PD

fault and differentiate with Noise

or External PD signals

Long Cable

Defect

Measurement EndRemote End

PD in Joints and inside the Cables

PD Fault Localization

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• Time Domain Reflectometry (TDR)

Principle is used to locate PD.

Measure time difference between

main and reflected HF signals (∆T)

• For Longer cables, reflected pulse

may not reach to the measurement

end Requires a signal injector at the

other end to inject same HF signal

• For accurate localisation requires Accurate length of cable

Cable Return Time (Tr)

Long Cable

Defect

Measurement End

Remote End

ΔT

Long Cable

Measurement EndRemote End

Tr

WHY ONLINE PD MEASUREMENT

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• No need to de-energize the cable system • Non destructive testing • Accurate Detection of Incipient Faults under operating conditions • Accurate Localization or Zoning of Incipient Faults • Trending Fault Severity via regular CHECKUPs • Helps in scheduling maintenance and planned outage • Ensure that cable is defect free after repair / workmanship

Why Online PD Monitoring?

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• Most of the incipient faults or defects in power cables generate PD signals well before the actual failure except water trees. • PD monitoring helps in identifying the incipient faults before the break down -> Saves cost of UNPLANNED OUTAGE

SENSORS USED FOR ONLINE PD MEASUREMENT

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• HFCT (High Frequency Current Transformers) – To detect any PD signal coming from Cable insulation – One for each Cable – Installed at termination / joints

• TEV (Transient Earth Voltage) – To detect any external PD signals coming from termination

or nearby Switchgear Panel – One for each Termination – Installed near the openings or vents of terminations

• RF Antenna (Radio Frequency)

– For Noise gating – One for each Termination – Installed near to termination (Either TEV or RF Antenna)

Sensors used for Online PD Measurement

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• High Frequency Current Transformers – Inductive type sensor – Split core to install sensor easily – Different variants to fit the purpose (current (up to 1000A), diameter

(up to 150mm internal diameter)) – Sensitivity: Up to 10mV / mA – Frequency Response: 200kHz – 15MHz

• Installation requires – Individual cable earth – Cable insulation gland – Installation is done while the cable is energised

Sensors: HFCT, Options for Installation

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• Transient Earth Voltage Sensor – Capacitive type sensor – Integrated magnets help to attach it to

metallic body – Sensitivity: Up to 100mV / V

• Installation requires:

– Metal Switchgear should not be double skinned / walled

• RF Antenna – Can be used in place of TEV – Used to perform noise gating

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Earthed metal clad

TEV

HFCT

Vent

Sensors: TEV and RF Antenna Installation

Sensors Positioning

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HFCT

Sensors Positioning

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Online Cable PD Measurement Topology

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Sensors (HFCT, TEV)

Termination

Portable System / Permanent Installation

Sensors cables

QUALITROL OFFERINGS ON CABLE PD MEASUREMENT

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Field Service

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• Field Service

•Sit Survey and feasibility of Online PD measurement

•Risk Assessment

•Method statement document shared with the customer

•Testing of 6-9 cables per day

•Detailed report with further recommendations within 5 days as standard or

more days depending on the number of days of testing

Portable System

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•Portable System (Periodic Measurement)

•Main Unit (4 simultaneous channels)

•Accessory Box

•Sensors

•Cables

Sensors

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•High Frequency Current Transformers (HFCT)

•Transient Earth Voltage Sensor (TEV)

HFCT-150 1000A

3mV/mA

HFCT-100 500A, 1000A 3-5mV/mA

HFCT-60 50A, 500A, 5-10mV/mA

HFCT-30 50A

10mV/mA

Continuous Monitoring Solution

Continuous PD Monitoring Software

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Continuous PD Monitoring Software

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CASE STUDIES HIGH NOISE ENVIRONMENTS

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Case Study 1: Lab Verification of Technology

Maximum magnitude phase resolved distribution of mixed signals including noise, PD, interference were applied together to test the system and separated out by the system

Case Study 2: GIS Substation

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Cable Termination

GIS termination

220kV, XLPE

Around 500m length

Cable Termination

Remote end

Terminating to Overhead lines

Case Study 2: High Noise Environment

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Before performing any filtering on Channel A single cycle data, high noise level and too much interference can be seen. Not sure which is PD or which is noise. Also not sure that there is any PD?

After filtering, white noise level is decreased. Interference signals can still be seen at the same locations. However, some more pulses appeared in the middle. These pulses in the middle may or may not be PD

Case Study 2: Extracted Noise Pulses

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Channel A noise pulse shape, its maximum magnitude phase resolved distribution and pulse count phase resolved distribution

Case Study 2: Extracted PD Pulses

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Channel A PD pulse shape, its maximum magnitude phase resolved distribution and pulse count phase resolved distribution By TDR, it states that PD is located at remote end.

Case Study 3: Metalclad Switchgear

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Cable Termination

Metal clad

10kV, XLPE

Around 1367m length

One HFCT on the earth shield of cable

Other two beneath the metal housing as earth shield was available there

Case Study 3: Interference and PD

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Before performing any filtering on Channel A single cycle data, low noise level but too much interference can be seen. Not sure which is PD or which is noise. Also not sure that there is any PD?

Case Study 3: Extracted Noise Signals

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Noise pulse shape, its maximum magnitude phase resolved distribution and pulse count phase resolved distribution

Case Study 3: Extracted PD Signals

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Channel A PD pulse shape, its maximum magnitude phase resolved distribution and pulse count phase resolved distribution. By TDR, it states that PD is located at remote end.

Case Study 3: Extracted PD Signals

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Channel C PD pulse shape, its maximum magnitude phase resolved distribution and pulse count phase resolved distribution

THANK YOU

End of Session

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