D2.3.24 Proposal for condition monitoring method …sgemfinalreport.fi/files/D2.3.24 Proposal for...

D2.3.24 Proposal for condition monitoring method for cable networkPertti Pakonen, Bashir Siddiqui, Antti Hilden, 19 February, 2015

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Contents of the presentation• Goal of condition monitoring tests• Priorization of cables for condition monitoring• Proposed condition monitoring method for cable network

– High priority cables– Medium priority cables– Low priority cables

• Description of the proposed condition monitoring methods• Measurement from primary vs. secondary substation• Measurement from phase conductors vs. ground straps• Examples of condition monitoring measurements in city and

rural networks

2Pertti Pakonen et al.

Condition monitoring of 6…36 kV cables• Goal of condition monitoring

– To reveal potential defects in materials and installation during their warranty period

– To prevent unplanned interruptions caused by deterioration of the cable and cable accessories during operation due to electrical, mechanical and thermal stresses

– To extend the useful life of cables at the end of their lifecycle– To produce condition information for

• Network maintenance• Network operation• Planning of network renewals


Priorisation of the cables• The extent and methods of condition monitoring should be

defined based on the priority of the cable

• The priority depends on, among others, – Criticality of the cable from the customer point of view

• Number and importance of the customers fed by the cable• Harm due to fault (outage costs)

– Criticality of the cable from structural and installation environment point of view

• Cable type and accessories (e.g. resilience to environmental stresses, the amount and degree of difficulty of handcrafting in the installation phase)

• Installation method and soil type (e.g. ploughing in rocky soil is a risk)• Environmental stresses (humidity, salt, impurities of air, temperature,

temperature variations)


Condition monitoring methods for different cable priority

classes


Condition monitoring methods• Proposed condition monitoring method for cable network

– High priority cables• Continuous on-line PD monitoring, disturbance records• Periodic thermal imaging and ultrasonic mapping of terminations• Periodic tan-delta

– Medium priority cables• Periodic on-line PD, disturbance records• Periodic thermal imaging and ultrasonic mapping of terminations

– Low priority cables• Thermal imaging of terminations• Optional periodic on-line PD • Disturbance records


High priority cables• Primary method: continuous on-line PD measurement• Secondary methods: periodic thermal imaging, analysis of

disturbance records (see Task 6.12)• Supporting methods: continuous or periodic ultrasonic monitoring

– discrimination between internal (harmful) and external discharges in terminations

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• Preferred PD measurement method– Inductive PD sensor installed permanently

at the cable terminations above the ground strap (see figure) to measure PD pulses in the phase conductor

– 50 Hz synchronization of the PD data• From MV busbar voltage transformer

secondary (primary substations) or voltage sensor integrated into the cable termination

• From the LV phase voltages (if MV measurement is not available)

Recommeded installation(least susceptible to interference)

Alternative installation(more susceptible to interference)

Pertti Pakonen et al.

Medium priority cables• Periodic inspections for example 12, 20, 60, 120, 180, 240, 270, 300 etc. months

from commissioning (twice during guarantee period, then 5 year intervals and 2.5 year intervals after e.g. 20 years)

• Primary method: periodic on-line PD detection, disturbance records• Supporting methods:

– Thermal imaging: detection of discharges in terminations– Ultrasonic mapping: discrimination between internal (harmful) and external

discharges in terminations• Methods to enable periodic on-line measurement

– Installation of permanent low cost inductive sensors on the cable termination • Preferably above the ground strap (see figure on the previous page) to measure PD

pulses in the phase conductor or alternatively• On the ground strap (more susceptible to disturbances)

– or modification of the cable termination ground strap routing so that PD sensor can be safely installed without supply interruption. Modification could be done e.g. during scheduled secondary substation maintenance or repair outage.


Low priority cables• Periodic inspections for example 20, 60,

240, 360 etc. months from commissioning (once during guarantee period, then 5 year intervals)

• Primary method– Thermal imaging (+ optionally ultrasonic

mapping) of cable terminations conducted in conjunction with regular secondary substation inspections:

• detection of internal and external discharges in terminations

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– Optional periodic on-line PD detection• detection of internal discharges in joints and cable itself

• Secondary method:– Analysis of disturbance records

• detection of e.g. intermittent earth faults in the cablePertti Pakonen et al.

Condition monitoring process

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High priority cables Medium priority cables Low priority cables

Continuous on-line PD monitoring

Periodic on-line PD detection

On-line PD analyzerThermal+ultrasonic-PD location andassesment

Cable repair or replacement,commissioning measurements

Off-line PD, tan delta

Temporarily installedcontinuous on-line PD monitoring

Periodic - thermal imaging + optional ultrasonic mapping- optional on-line PD det.

Provided by a network construction/maintenance contractor

Provided by network operator and/or a service provider

Provided by a service provider


Condition monitoring process

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High priority

Medium priority

Low priority

012

20 60 120 180 240 300 360

0

0 20 60 240 360

420

Continuous monitoring

Measurement times (months from commissioning):

Continuous monitoring if neccessary


Equipment and implementation of PD

measurements


Continuous on-line PD monitoring –equipment and implementation

• Goal: to detect (and locate) partial discharges in – Terminations – Joints – Cable itself

• Measurement equipment:– Permanently installed PD sensors

• Inductive (most commonly used in cable condition monitoring)

• Or capacitive– Automatic PD measuring unit

• Recording at least– PD intensity trend– PRPD pattern

• Preferably also– PD location information


Measurement sensitivity requirements

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Reference: Ossi Bergius, “Implementation of on-line PD measurements in MV cable networks”, MSc thesis, 2012


Potential installation locations for inductive sensors

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Reference: P.C.J.M . Van Der Wielen, J. Veen, P.A.A.F. Wouters, E.F. Steennis, On-line Partial Discharge Detection of MV Cables with Defect Localisation (PDOL) Based on Two Time Synchronised Sensors, 18 thInternational Conference on Electricity Distribution (CIRED), 2005

Potential locations for HFCT, around:1. Ground strap of cable2. Cable sheath3. A single phase conductor

(the recommended method)

4. A single transformer-busbarcable

5. Ground strap of transformer-busbar cable

6. Transformer ground strap


Temporary continuous on-line PD monitoring: an example of low cost equipment

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• 4 Channel• 2 x HFCT sensors, 2 x TEV sensors

Discrimination between cable/switchgear PD• PD magnitude monitoring

• 3G/GPRS data upload to FTP server with 30 min intervals, 6 months local data storage• Alerts or alarms from high PD levels

• PD “criticality” display with 7 color coded LEDs


Periodic on-line PD detector measurement -equipment and implementation

• Goal: to detect partial discharges in – Terminations – Joints – Cable itself

• Quick and simple measurement for surveying a large cable population with equipment that are easy enough to use for any electrician

• Measurement equipment:– Handheld PD detector indicating PD level– Inductive PD sensor (to detect cable PD)– TEV (transient earth voltage) sensor and US ( ultrasonic)

sensor (to detect PD in terminations and switchgear)

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– Handheld detectors available from • at least one manufacturer for cable measurements (also HFCT sensor)• several manufacturers for switchgear measurements (TEV and US sensor)

– Various measurements functionalities: PD level (handheld), PD pattern (advanced)

IPEC (TEV, acoustic)

HVPD (HFCT, TEV, acoustic, tablet application)

EA Technology(TEV, acoustic)

IPEC (HFCT, TEV, acoustic)


Periodic on-line PD analyzer measurement -equipment

• Goal: to detect, locate and evaluate partial discharges in – Terminations – Joints – Cable itself

• Measurement equipment:– Inductive PD sensors for three phases (HFCT or Rogowski)– Oscilloscope capable of sampling 20 ms continuously with a

sampling rate of at least 50 MS/sAt least 1 Mpoints/channel memoryPreferably selectable 20…25 MHz low-pass filtering

(bandwidth limit)– Differential probe for 50 Hz phase voltage signal from

• MV busbar voltage transformer secondary (if possible)• LV mains outlet (note the phase (L1, L2 or L3) where outlet is

connected and the vector group of the MV/LV transformer)


Periodic on-line measurement - procedure1. PD sensor test/calibration, aiming to:

– check that all sensors and cables work and a correctly connected– determine the amplitude correction factor of each sensor– store the calibration pulse waveforms (for use in the interpretation of

the PD measurement results)2. Pulse injection test, aiming to:

– to determine the pulse propagation velocity in the cable if the cable length is known based on the network information system or cable path measurement (or if the propagation velocity is known, to detetrmine the cable length)

– store a reference waveform of the pulse reflections (joints, cable end)3. PD measurement, aiming to:

– to store several (or preferably tens or hundreds of) 50 Hz cycles of 3-phase PD data and 50 Hz phase voltage reference for further analysis


PD sensor test/calibration:• Measurement setup (sensors around the cable termination):

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Fig. 2. On-site demonstration (sensors around the ground straps).

R=50 ohm

PD calibrator

Oscilloscope

3 x HFCT

Fig. 1. Sensors around the cable terminations.


Pulse injection:

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Pulse generator(10 Vp-p voltage) Oscilloscope

3 x HFCT

HFCT

Fig. 2. On-site demonstration (sensors around the ground straps).Fig. 1. Diagram of the pulse injection setup.


PD measurement:

• Important considerations:– Install the sensors at the cable terminations above the ground strap (see figure) to measure

PD pulses in the phase conductor– make sure that the volts/div setting is high enough so that there are no range overflows in

the measurement data (if some of the pulses are clipped, filtering and data analysis results will be degraded)

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PRPD pattern measured at a rural secondary substation

PD measurement setupat a city primary substation


Documentation of the PD measurements• The following information is proposed to be documented from PD

measurement setup:– Measuring device + sensor(s), firmware/software version– Connection method/place of the sensor– Measurement date/time, temperature and humidity (outdoor locations)– Network configuration, cable load currents

• From PD measurements of each phase:– Background noise level [pC], approximation between the PD pulse trains– Measurement voltage (optional)– PD levels [pC] in each phase - largest repeatedly occurring pulse

(LROP), max pulse– PRPD (phase resolved PD) patterns in each phase– PD location(s) in each phase


Which PD magnitude is harmful?

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In product standards of cables and cable accessories the following requirements are specified for maximum apparent charges • XLPE cables < 5 pC• terminations and joints of XLPE cables < 20 pC

The harmfulness of the discharge depends on, among others: • The dielectric material in or on which the partial discharge takes place• Magnitude and direction of electric field at the discharge location• The gas in which the partial discharge occursPractical apparent charge limits for XLPE and paper-oil insulated cables:


PD measurement system vendors and service providers (underground cable monitoring)

• IPEC Ltd, UK, http://www.ipec.co.uk• HVPD, UK, http://www.hvpd.co.uk/products/• EA technology, UK, http://www.eatechnology.com/partial-discharge/partial-discharge-

instruments• Iris Power LP, Canada, http://www.irispower.com/• Omicron, Austria, http://www.omicron.at/en/products/app/power-transformer/partial-

discharge/• Power Diagnostix Systems GmbH, Germany, http://www.pd-systems.com/• Doble Lemke, Germany, http://www.doble-lemke.eu/• SebaKMT, Germany, http://www.sebakmt.com/cz/products/power-

networks/diagnosis/partial-discharge-measuring/lpd-monitor.html• Dynamic Ratings, Inc., USA, www.dynamicratings.com• DNV GL http://www.dnvkema.com/services/advisory/ope/asset/condition/scg/ (Smart

Cable Guard on-line PD monitoring services)• Emerson Electric Co, USA, www.emersonnetworkpower.com (measurement services)• Prysmian Group (Pry-Cam, continuous and periodic on- line PD monitoring)• Dekra Industrial Oy (off-line PD, tan delta)


Thermal imaging


Thermal imaging• Thermal imaging may detect

– loose contacts e.g. in cable lugs– internal and external partial discharges in cable terminations and

support insulators/metal oxide arresters (MOAs)• the existence of PD should be confirmed by electrical PD measurements• thermal imaging supports PD location based on electrical PD measurement

• Equipment:– Thermal camera

• Resolution: at least 120 x 160

• Documentation of the results:– Thermal camera model and software version, distance btw. lens and terminations– Date and time, temperature and humidity (outdoor locations), cable load currents– Thermal image with locations of the hotspots – Temperatures and locations of the hotspots in numerical form– Average cable termination temperature (+ optionally temperature difference

between the hotspots and average cable termination temperature)27Pertti Pakonen et al.

Thermal imaging• Important considerations related to thermal imaging:

– Before making any maintenance or repair decisions based on partial discharges detected, verify that

• Detected hot-spots are really due to internal discharges and not, e.g. due to

– External discharges on the surface of the termination, which are usually caused by condensed moisture at the surface of the termination and, according to termination manufacturers, not harmful (if necessary, they can be removed by cleaning and drying the termination surface)

• This may be verified e.g. based on – ultrasonic measurements (internal discharges usually cannot be detected

using the horn or parabolic sensor)– location of the hot-spots in thermal image (internal discharges are usually

located around the end of the insulation screen)– the extent of temperature rise compared to surrounding surfaces (in

internal discharges this is usually relatively small)– Detection of loose contacts requires that there is a considerable (e.g. 50

% of the nominal) load current running through the cable


An example of external discharges in 20 kV heat shrink terminations

• Visible white oxidation caused by tracking discharges over the dry bands

• Not harmful according to the termination manufacturer

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• Discharge sites at varying locations• Relatively high temperature increase

(> 10 °C)

Photo: Seppo Suurinkeroinen, Kymenlaakson Sähköverkko Oy

Thermal image: Seppo Suurinkeroinen, Kymenlaakson Sähköverkko Oy


Examples of external discharges in 20 kV secondary substations

• Surface discharges at the end of a cable termination

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• Surface discharges at the end terminal of a metal oxide surge arrester




Optional wireless measurements connectedto thermal camera

• Many new thermal cameras support wireless sensors, e.g.

– Voltage sensors– Current sensors– Temperature

sensors• The sensor reading

is recorded to the thermal image data

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File Location C:\Fluke\IR000016.IS2Image Time 12/10/2014 7:31:00 PMEmissivity 0.95Background Temp 2.0 °CTransmission 100%Image Range 2.5 °C to 14.8 °CAverage Temp 4.6 °CSeverityCamera model Ti110IR Sensor Size 120 x 160Camera Manufacturer Fluke ThermographyCalibration Range -10.0 °C to 250.0 °CCamera serial number Ti110-14060416Camera Firmware 1.0.64Compass NE I3000 18.2 AACA3000 12.6 AACA3000 15.0 AAC

Thermal image Thermal image data

Optional measurement data from wireless sensors (max. 5 or 10 sensors) at IR imaging instant

Sensor type:I3000 AC current 0-2500 AA3000 AC current 0-400A


Ultrasonic mapping


Ultrasonic mapping• Goal: to discriminate btw. two discharge types visible in thermal images

– Harmful internal discharges e.g. due to problems in stress control at the end of insulation screen (e.g. installation faults)

• Usually not detectable by ultrasonic measurement– Usually less harmful external discharges due to condensed moisture and dry

band forming on the surface of the termination• Detectable by ultrasonic measurement

• Equipment: Ultrasonic detector with• Center frequency f = 40 kHz• Bandwidth (-6 dB) f = ±2 kHz

• Documentation of the results:– Ultrasonic detector model and software version– Date and time, temperature and humidity (outdoor locations), cable load currents– Location of the highest ultrasonic source in each phase, average and maximum

ultrasonic noise voltage in dBµV at those locations, measured at a distance of 1,5 m from the termination


Ultrasonic mapping• Detector types

– Detector with cone form concentrator• Most suitable for measurement

distances of 1-2 m– Detector with parabolic reflector

• Most suitable for measurement distances of 5-50 m

• Detector with parabolic detector– Note that depending on the

construction of the detector there may be a considerable parallax error between the focus points of the laser pointer and parabolic reflector (ultrasound) if the measurement is done closer to the target than recommended (e.g. < 5 m)

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Fig. 1. Ultrasonic detector with cone form concentrator.

Fig. 2. Ultrasonic detector with parabolic reflector.


Analysis of disturbance records• Disturbance recordings made

by feeder terminals or dedicated earth fault indicators may be utilised to detect incipient faults (such as intermittent earth faults) in the network

• This may give an early warning of the developing fault already before it causes an actual tripping

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An example of U0 and I0 measured from a feeder having an intermittent earth fault.


Implementation of PD measurements


Measurement from primary vs. secondary substation

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Primary substation (PS) Secondary substation (SS) - Recommended

+ many cables available for measurement (efficient) + often lower noise level compared to PS

+ MV phase voltages available from busbar PT’s for sync. - no potential transformers for 50 Hz sync of PD meas.

+ warm and dry environment - usually only 1-4 cables available for measurement

- often higher noise level compared to SS - usually outdoor environment especially at rural SS

Fig. 2. Measurement from secondary substation.Fig. 1. Measurement from primary substation.

Secondary substationPrimary substation

PS - PD site =120 mPD site

PD site – SS =1160 m

Reflections from primary substation

Direct pulse from PD siteReflections from secondary substation

Direct pulse from PD site


Measurement from phase conductor vs. ground strap

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Phase conductor - Recommended Ground strap

+ less phase to phase crosstalk and disturbances compared to ground strap

+ lower 50 Hz current

- higher 50 Hz current -> HFCT saturation + usually accessible

- not accessible in plug-in terminations - more susceptible to phase-to-phase crosstalk and disturbances compared to phase conductor

Fig. 1. Measurement from phase conductor, no phase to phase cross-talk.

Fig. 2. Measurement from ground strap, same PD appears in all three phases.


Measurement from phase conductor vs. ground strap

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Fig. 1. Full 50 Hz cycle. Fig. 2. Single PD pulse and its reflections.The pulses measured from ground strap are equal to those measured from phase conductor, but opposite in polarity (measurement location: a rural secondary substation).

Ground strapmeasurement

Phase conductormeasurement

IdenticalHFCT’s in both.


50 Hz synchronization for PD measurement from switchgear bay capacitive voltage outputs

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• Comparison for the 50 Hz synchronization for PD measurement– capacitive voltage detector outputs of

primary substation switchgear– busbar phase voltage transformers

• Both connected to oscilloscope using LeCroy AP031 differential probes

• Results:– Phase difference between the

measurements approx. 4.8 degrees (bay capacitive voltage detector output leading)

– Waveform excellent in both measurements


Examples of condition monitoring measurements in

city and rural networks


Examples of PD in city network cables (primary substation measurement)

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Instrument LeCroy WaveRunner LT354 ML

Cable 1682 m PILC + 893 m XLPE

Phase Noise [pC] LROP [pC] Max [pC]

L1 1800 7800 11000

L2 2500 8500 10600

L3 1900 8500 10600

Instrument LeCroy WaveRunner LT354 ML

Cable 881 m PILC + 1296 m XLPE


L1 260 - -

L2 60 90 105

L3 260 - -

Corona in switchgear (phase L2)Void discharges in cable junction + noise


Examples of internal discharges in cable terminations of a rural secondary substation

PD-measurement before repair

PD-measurement after repair

• Discharge sites at the ends of insulation screens

• Very small temperature increase -approx. 1 °C

partial discharges

Thermal image before repair

20 ms


An example of the summary documentation of a periodic on-line condition monitoring measurement of a rural secondary substation

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File Location C:\Mxxx.IS2Image Time 12/10/2014 10:04:50 PMEmissivity 0.94Background Temp 2.0 °CTransmission 100%Image Range 3.5 °C to 16.5 °CAverage Temp 5.0 °CSeverity NoneCamera model Fluke Ti32IR Sensor Size 320 x 240Camera Manufacturer Fluke ThermographyCalibration Range -10.0 °C to 600.0 °CCamera serial number Ti32-10120423 (9Hz)DSP Version 1.1.59OCA Version 1.1.59

L1568 pC

/div

PRPD patterns

Thermal image Thermal image data

PD calibration: 1000 pC = 88 mV

Instrument LeCroy WaveSurfer 44 MXs-B

Time 09:59:08 PM


L1 95 422 977

L2 78 355 888

L3 89 266 373

LROP = largest repeatedly occurring pulse

PD levels

Instrument Fluke Ti 32 SDT 170 MD

Time 10:04:50 PM 10:15:00 PM

Phase d [cm] T [deg C] d [cm] L [dBuV]

L1 5 13.2 10 43

L2 0 14.2 0 53

L3 11 14.6 10 48

d

Hot spot ormax. ultra-sound

L2568 pC

/div

L3227 pC

/div

Date 10-12-2014

Secondary substation ID of the secondary substation

Bay, network configuration Bay 4

Ultrasoundlevel


Conclusions: proposed condition monitoring methods for cables of different priority

x = mandatory, o = optional, c = conditional, to monitor or to locate and evaluate the harmfulness of PD detected earlier by other means 1) If PD is detected by continuous or periodic monitoring, on-line PD analyzer measurement (or off-line PD

measurement) is used to locate and assess the harmfulness of the discharge2) If cable is repaired, these measurements are conducted before re-energizing the cable, otherwise optional (may be

used as an alternative to PD analyzer measurements in noisy environments, requires a supply interruption)

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Condition monitoring method (P)eriodic/(C)ontinuous, On-/off-line

Cable priorityHigh Medium Low

PD monitor c – on x c -PD detector p – on - x oPD analyzer p – on c 1) c 1) c 1)

Tan delta (+ off-line PD) p – off o 2) - -Thermal imaging p – on x x xUltrasonic mapping p – on x x oVisual inspection/camera p – on x x xAnalysis of disturbance records c – on o o o


Acknowledgements:We would like to thank the following distribution network operators for offering us a possibility to make measurements in their networks:• Helen Sähköverkko Oy

– Measurements of several city network cables at three primary substations

• Tampereen Sähköverkko Oy– Measurements of a city network underground cable from primary and

secondary substation ends• Kymenlaakson Sähköverkko Oy

– Measurements of rural network cable terminations (external discharges)

• Kouvolan Seudun Sähköverkko Oy– Measurements of rural network cable terminations (internal

discharges)


References:• P.C.J.M. van der Vielen, On-line detection and location of partial discharges in medium

voltage power cables, Ph.D. Thesis, Eindhoven, Netherlands 2005, Eindhoven University of Technology. 207 p.

• A. Gerstner, A. Borlinghaus, C. Goy, Integral cable condition assesment, 21st International Conference on Electricity Distribution (CIRED), Frankfurt, 6-9 June 2011.

• C. Eastham, C. Smith, F-C Chen, Detection and Location of PD in MV cables in electrically noisy industrial environments, 21st International Conference on Electricity Distribution (CIRED), Frankfurt, 6-9 June 2011.

• B. Lanz, Assuring Critical Cable System Reliability with Effective Diagnostic, ICC Subcommittee F working group F10 meeting March 22, 2010.

• HVPD Technical Guide for PD Levels in MV and HV Cables and Joints, HVPD Ltd. Manchester, UK, 2009.

• P. Nevalainen, P. Pakonen, M. Takala, P. Verho, Inspection of MV underground cables using partial discharge, tan and sheath integrity measurements, Nordic Insulation Symposium, Gothenburg, Sweden, June 15-17, 2009.

• O. Bergius, “Implementation of on-line PD measurements in MV cable networks”, MSc thesis, 2012.

• J. Vepsäläinen, Mittaavan kunnossapidon hyödyntäminen keskijänniteverkon häiriöidenvähentämisessä ja elinkaarihallinnassa, Diplomityö, 2014.


References:• O. Vuorinen, Using process data in condition based maintenance, MSc thesis, 2011.• H. Kuisti, J. Altonen, Intermittent earth faults challenge conventional protection schemes,

15th International Conference on Electricity Distribution (CIRED), Nice, Italy 1999.• P.C.J.M . Van Der Wielen, J. Veen, P.A.A.F. Wouters, E.F. Steennis, On-line Partial

Discharge Detection of MV Cables with Defect Localisation (PDOL) Based on Two Time Synchronised Sensors, 18 th International Conference on Electricity Distribution (CIRED), 2005

• R.R. Macinlay, M. Michel, C.W. Walton, Tools for partial discharge testing MV cables and plant, 18 th International Conference on Electricity Distribution (CIRED), 2005.

• S.M. Gargari, P.A.A.F. Wouters, P.C.J.M. Van Der Wielen, E.F. Steennis, Practical Experiences with On-line PD Monitoring and Interpretation for MV Cable Systems, 2010 International Conference on Solid Dielectrics, Potsdam, Germany, July 4-9, 2010

• H.M. Pereira, M. Marques, M.B. Pinheiro, R. Palhares, T. Raczy, E.F. Steennis, P.M.F. Almeida, Partial Discharge On-line Monitoring in MV Underground Power Cables as Part of Condition Based Maintenance Strategy, 22nd International Conference on Electricity Distribution (CIRED), 2013

• M. de Witte, Y. Tits, M. Arens, A. Francois, M.Van Den Berg, J. Van Slycken, Partial Discharge Monitoring on MV Switchgear, 22nd International Conference on Electricity Distribution (CIRED), 2013.


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