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High Frequency Diagnostics and Engineering Ltd
Technical Assessment of Cable Asset
Management Approaches (TACAMA) Webinar
A review of cable testing and condition assessment techniques
10th November 2020
Presented by: Martin Judd, Technical Director, HFDE Ltd
Overview of TACAMA
Context is the increasing use of AC cables at transmission voltage levels:
▪ SHET owns and operates an extensive network of underground and subsea cables that is
evolving rapidly to service the needs of
i. dispersed sources of renewable generation, and
ii. increasing expectations that transmission routes should utilise underground cables to reduce
visual impact on sensitive landscapes.
▪ Commissioning of new cable circuits in remote locations presents logistical challenges for
transporting HV test equipment.
The TACAMA project reviewed:
▪ alternative voltage withstand test methods for cable circuits rated at ≥ 132 kV,
▪ existing / emerging technologies for testing the operational performance ability of HV cables
operating at ≥ 33 kV, and
▪ methods for condition monitoring or condition assessment of cables that are applicable both
during on-site HV testing or throughout the operational life of cable assets.
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Material to be covered
▪ Information Sources and Review Methods
▪ HV Commissioning Tests for New Transmission Cables
▪ Practical challenges
▪ Potential solutions
▪ XLPE – electrical stress and space charges
▪ Recommendations
▪ Partial Discharge Sensors and Monitoring
▪ Cable Impedance Scanning
▪ Trapped DC Voltage on AC Cables
▪ Summary of Main Outcomes
Presentation will focus on the most significant
themes of TACAMA rather than attempting to
cover all avenues of investigation . . .
Disclaimer & Acknowledgements
Any references to (or images of) products, equipment, suppliers or manufacturers
in this presentation are purely for illustration and do not represent any expression
of endorsement or preference by the TACAMA project team.
Project Team
Scottish Hydro Electric Transmission (SHET)
Project Manager: Matthew Hamilton
David Joy, Ahmed Badawi, Othmane El Mountassir, Gerry Cleary
Contractor (HFDE Ltd)
Project Manager: Martin Judd
Gary Catlin (HV Diagnostix, Auckland, NZ)
The TACAMA project team gratefully acknowledges support for and input to this
study from colleagues within SHET and from external contributors who provided
materials and took part in discussions or surveys as part of this study.
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Technical Scope
▪ AC cables - XLPE insulation
▪ On-site tests - commissioning of new cable installations at 132 kV & above
▪ Dielectric withstand tests
▪ Other diagnostic tests
➔ Define a tailored strategy and associated methodology for deploying the most
promising testing approaches on transmission networks.
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Information Sources and Review Methods
Documents
▪ A review of existing and emerging methods for testing power cables has been carried out covering
scientific literature, standards and technical guidance documents published by leading authorities.
▪
This material is embodied in a structured folder of references associated with the TACAMA project.
▪ Examples of comprehensive reviews of cable test methods are the outputs of an industry-wide
research consortium in North America, the CDFI (Cable Diagnostic Focused Initiative).
▪
CDFI has carried out studies over many years that underpin the IEEE 400 series of standards for cable testing.
▪ IEC standards for cables do not specifically address on-site test conditions.
▪ CIGRE has published many Technical Brochures on cables and testing.
▪ Most interesting in the context of progressing HV test methods for cable commissioning will be the
long anticipated Technical Brochure from CIGRE Working Group B1.38:
▪
“After Laying Tests on AC and DC Cable Systems with New Technology”.
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Cable Diagnostics Focused Initiative (CDFI)
Report of the Cable Diagnostic Focused Initiative, Phase II (2016)
Table of Contents:
1 - Introduction
2 - Medium Voltage Cable System Issues
3 - HV & EHV Cable System Aging and Testing Issues
4 - How to Start
5 - Time Domain Reflectometry (TDR)
6 - Dissipation Factor (Tan δ)
7 - Medium Voltage Cable System Partial Discharge
8 - Partial Discharge in HV and EHV Power Cable Systems
9 - Simple Dielectric Withstand
10 - Monitored Withstand Techniques
11 - Metallic Shield Assessment
12 - Other Diagnostic Techniques
13 - Benefits of Diagnostics
Valuable reference providing comprehensive
explanations and evaluations of cable diagnostic
methods. For example, chapters 7 & 8 dealing
with PD total more than 200 pages.
14 - Glossary
Available as individual chapter PDF files from https://www.neetrac.gatech.edu/cdfi-publications.html
CIGRE Technical Brochures (examples below from 2010 onwards)
TB 476, 2011
TB 490, 2012
TB 493, 2012
TB 496, 2012
TB 502, 2012
TB 722, 2018
TB 728, 2018
TB 751, 2018
TB 773, 2019
TB 794, 2020
TB 815, 2020
TB xxx, 2021 (?)
Cable accessory workmanship on extruded High Voltage cables
Recommendations for Testing of Long AC Submarine Cables with Extruded
Insulation for System Voltage above 30 (36) to 500 (550) kV
Non-destructive water-tree detection in XLPE cable insulation
Recommendations for Testing DC Extruded Cable Systems for Power
Transmission at a Rated Voltage up to 500 kV
High-Voltage On-Site Testing with Partial Discharge Measurement
Recommendations for Additional Testing for Submarine Cables (6 – 60 kV)
On-site Partial Discharge Assessment of HV and EHV Cable Systems
Electrical properties of insulating materials under VLF voltage
Fault location on land and submarine links (AC & DC)
Field grading in electrical insulation systems
Update of service experience of HV underground and submarine cable systems
After Laying Tests on AC and DC Cable Systems with New Technology
Note that all CIGRE documents are available from e-cigre at the link below.
Documents over 3 years old can be downloaded free of charge by non-members.
CIGRE members can download all documents free of charge.
https://e-cigre.org/
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CIGRE Working Group B1.38:
“After laying tests on AC and DC cable
systems with new technology”
Established 2012 to address a topic
recognised as requiring technical clarity.
E.g.: extent to which HV supplies such as
DAC and VLF can be used for commissioning
tests of cables operating at voltages above
33 kV - 66 kV.
Length of deliberations by the Working
Group hints at the challenge involved with
reaching a consensus.
Technical Brochure likely to be published Q1
of 2021. Expected to be highly relevant to
TACAMA.
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Wider engagement
“undertake a worldwide review of existing and emerging methods for testing power cables”
▪ Included interactions with some 30 individuals representing cable test service providers,
15 representing cable users, 6 experts from the R&D community and 4 from cable manufacturers.
▪ Online surveys of test service providers and cable network operators.
▪ Questionnaires to manufacturers of cables and accessories.
▪ International Transmission Operations & Maintenance Study (ITOMS).
▪ Video calls, webinar participation, etc.
(for reasons of confidentiality & data protection, companies / individuals will remain anonymous)
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HV Commissioning Tests for New Transmission Cables
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Voltage withstand testing
National Grid SCT 36: ‘Cable Systems - Schedule of Site Commissioning Tests’
Taking 132 kV cable as an example, the conductor to sheath voltage is normally 132Τ 3 = 76.2 kV rms
The required test voltage of 132 kV rms above represents an overpotential factor of
132
76.2
= 1.732 Uo
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Typical HV resonant test set (RTS) and generator
Challenges: Remote rural locations - Adverse weather
- Sensitive environment
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Important to employ technical solutions
with minimum, time-limited
environmental impact
www.rspb.org.uk/
www.birdguides.com/
getlostmountaineering.co.uk/
www.mountaineering.scot/conservation/policies/hilltracks-statement
revive.scot/
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Cable energisation basics
≈C
𝑄 =𝐶∙𝑉
𝑑𝑄
𝐼=
𝑑𝑡
𝑑𝑉
𝐼=𝐶∙
𝑑𝑡
▪ Generating HV in remote locations is not an issue.
▪ Current I is the problem when C is large.
▪ If we double cable length, we must supply twice the current.
▪ If we double the voltage, we must supply twice the current.
▪ We can either try to compensate the reactive power with an inductor or reduce dV/dt.
Importance of stressing cable insulation at commissioning
% of cables in which PD occurred,
relative to the total that exhibited PD
% of cables in which PD inception
has occurred, relative to the total
that exhibited PD after 60 minutes
After 10 minutes, only about 50 %
of the potentially defective cables
have begun to show PD activity.
PD inception voltage (PDIV) relative to U0
Duration of HV commissioning test at 1.7U0 ( minutes )
CIGRE Technical Brochure 728, “On-site partial discharge assessment of HV and EHV cable systems”, May 2018
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n
HV test waveforms
Frequency
Type
Alternating current
50/60 Hz
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Waveform
AC (system power frequency)
AC (resonance)
nce) Alternating current
20 to 300 Hz
idal) Very low frequency
0.1 Hz
Very low frequency (VLF, sinusoidal)
)
0.1 Hz
Cosine rectangular (CR) VLF
Very low frequency
Damped alternating voltage/
oscillating wave
20 to 1,000 Hz
Damped AC (DAC)
A Eigner and K Rethmeier, “An Overview on the Current Status of Partial Discharge Measurements on AC High Voltage Cable Accessories”,
IEEE Insulation Magazine, Vol. 32, No. 2, pp. 48-55, March/April 2016
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VLF supplies rated for 200 kV peak:
Mohaupt
DRT unit
High Voltage Inc (US)
“provides a 200 kV AC peak output voltage suitable for
performing VLF hipot tests on 138 kV cable and as a voltage
source for tan delta and PD testing on 160 kV cable”
b2 HVA200
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HVA200 VLF unit being
prepared for voltage withstand
testing of 132 kV cables
DAC test during preparation for PD testing of 132 kV cables
HV source
HV inductor
HV divider and
PD detector
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Cosine Rectangular VLF (low input power requirement)
release
swing
Hold
catch
swing
Theoretical electric field distribution in a screened HV cable
R
r
Instantaneous
potential = V
XLPE
HV
Conductor
Screen interface (XLPE-semicon)
Potential = 0 V
Conductor interface (XLPE-semicon)
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Utilisation of dielectric strength at increasing cable voltage ratings
‘routine test’ = factory testing of the individual components
‘test after installation’ = commissioning, which in this diagram corresponds to:
2Uo for cables operating up to 110 kV
1.7Uo for cables operating at 132 kV and above
HIGHVOLT Web Talk, ‘New trends for on-site testing of cables’ https://www.youtube.com/watch?v=k6c9JgcMjDg&feature=youtu.be accessed 1 July 2020
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Injection of space charges: conduction in XLPE
▪ Current density increases as a
function of electric field:
▪ Note increase in steepness of the
current density trend above a
certain electric field strength.
▪ This reflects increased mobility for
electric charges within the material.
G Mazzanti and M Marzinotto, “Extruded Cables for High-Voltage Direct-Current Transmission: Advances in Research and Development”,
IEEE Press Series on Power Engineering (published by Wiley), 2013
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Electron motion (power frequency)
conductor
semicon
XLPE
semicon
screen
●● ●
●● ●
●● ●●●●
● ●● ● ●●
● ●
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Electron motion (VLF)
-V 0V
conductor
semicon
● ●● ● ●
●● ●● ●
-
x
XLPE
E(x)
semicon
E0
screen
0
d
Influence of space charge injection / migration on field distribution in XLPE cable
cable segment
cross-section
cable segment
cross-section
Electric field ( kV/mm )
AC XLPE
Electric field ( kV/mm )
DC XLPE
outer semicon
layer
inner semicon
layer
In a cable with insulation specially formulated for DC
conditions, the field distribution soon settles to a
fairly uniform value across the bulk of the insulation.
outer semicon
layer
inner semicon
layer
In contrast, an AC cable (where the design relies
on capacitive field grading) accumulates space
charge that causes a highly non-uniform field to
develop, which places excessive electrical stress
on the insulation near to the inner semicon layer.
T Yamanaka, S Maruyama and T Tanaka, “The development of DC+/-500 kV XLPE cable in consideration of the space charge accumulation”,
Proc. 7th Int. Conf. on Properties and Applications of Dielectric Materials, (Nagoya, Japan), Vol. 2, pp. 689-694, June 2003
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132 kV cable subject to AC at system voltage
R = 35 mm
r = 19 mm
semicon
interfaces
conductor
XLPE
screen
Limits on electric field at the interfaces originate in
IEC 60840, which defines 8 kV/mm and 4 kV/mm at
the conductor and insulation screens, respectively.
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132 kV cable withstand test using VLF at 0.1 Hz
2.8 s
1.1
20
3.9
10
10
0
1
2
3
4
5
6
7
8
10
20
9
10
Emax ( kV/mm )
Emax ( kV/mm )
20
3.6 s
0.7
4.3
10
10
0
1
2
3
4
5
6
7
8
10
20
time ( s )
1.7 Uo (183 kV peak phase to screen)
time ( s )
2.5 Uo (270 kV peak phase to screen)
9
10
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132 kV cable withstand test using DAC
Charging phase of the operating cycle
3
1 1000
10
Cable voltage ( kV )
30 ms
0.3 s
3s
183 kV peak for 1.7 Uo
183
116 kV for threshold on E
116
100
10
0.01
0.1
1
time after initiating cable charging (s)
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Calculations based on:
▪ capacitance of 0.2 uF/km
▪ 100 mA charging current
Compact RTS configuration for offshore 66 kV cable commissioning
▪33
In series: Capable of
energising 7 – 10 km of
132 kV cable at 1.7 Uo
(3.7 t)
Image courtesy of JDR Cable Systems
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Series connection load range (within 20 – 300 Hz limits)
Load range: DE 5000/80-38 (3.7t) in series connection of 2 reactors
Corresponds to 1.7 Uo for 132 kV
cable. Max testable length would
typically be in the range 7 – 10 km
based on capacitance of a single phase
Information courtesy of HIGHVOLT UK
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Estimate of lowest test frequency at limit of 132 kV testable length
Load range: DE 5000/80-38 (3.7t) in series connection of 2 reactors
Test frequency for 1800 nF max
capacitance of 132 kV cable at
1.7 Uo would be about 25 Hz.
Information courtesy of HIGHVOLT UK
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Partial Discharge (PD) Sensors and Monitoring
Monitored Voltage Withstand Testing: PD detection
National Grid SCT 36: ‘Cable Systems - Schedule of Site Commissioning Tests’
▪ Terminology is subjective – not presently amenable to control by standards
▪ Monitored test is better than unmonitored to an extent that depends on monitoring effectiveness
▪ Requires cooperative approach and technical appreciation of capabilities and limitations
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Phase-resolved partial discharge detection:
PD amplitude
PD pulses are
plotted at the phase
position where they
occurred
Voltage divider with
PD sensing [a]
0
10
20
30
cycle number
(1 sec total data)
360 
40
50 0 
180 
phase position on
the HV sinusoid
High Frequency Current
Transformer (HFCT) [b]
[a] www.onsitehv.com/technology/en/products/CableDiagnostics/DAC-HV200.html
[b] www.ipec.co.uk
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PD sensors at HV cable terminations
132 kV cable
termination
Earth
lead
132 kV
cable
HFCTs with weatherproof covers installed on
earth bonding leads at cable terminations
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HFCT
transfer impedance ( mV / mA )
PD
antenna
TEV
sensor
7
6
5
4
3
2
1
0
0.1
1
10
100
frequency ( MHz )
(a)
(b)
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HFCT PD sensors at joints
On the cross-bonding leads in a
transmission cable tunnel [a].
Around the conductors in a link box [b]. In this case, the HFCTs
would need to be rated to handle both the conductor current and
the power frequency electric field. In addition, they decrease
clearances between the cross-bonding links, so installations of
this kind may need type testing for AC and impulse voltage
withstand as well as short circuit current and internal arcing.
[a] W Koltunowicz, U Broniecki, D Gebhardt, O Krause, “Experience in the Monitoring of HV Cable Systems”, Omicron Article, May 2019
[b] “PD measurement of prefabricated cable joints and cable terminations in HVAC XLPE cable systems”, Svenska kraftnät Technical Guideline TR14-04-2E, Ed. 1, 13 December 2019
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PD sensing arrangements
Single-ended:
▪ Simple
▪ PD sensitivity decreases
with increasing distance
into cable
▪ Short cables
Open
terminals
Test
terminals
HV supply
Coupling
capacitor
PD signal
Joint 1
0
Joint 2
Distance into cable, x
▪ More complex
▪ PD detection sensitivity
variation is controlled
▪ Long cables
L
Open
terminals
Test
terminals
Distributed:
Joint N
HV supply
Blocking
impedance
PD signal
Joint 1
0
Joint 2
Distance into cable, x
Joint N
L
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PD monitoring of long cables with cross-bonded joints
HFCT
60 kHz - 80 MHz
Remote
unit
Remote
unit
Remote
unit
Image courtesy of IPEC Ltd
Remote
unit
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Installation of HFCT PD sensors at cable terminations
Sensor on the earth
bonding lead
Sensor on the cable sheath
with earth lead routed back
through the aperture
Sensor enclosing cable
and screen: Cancellation
of PD current signal
Sensor above cable
screen: DANGEROUS!
HFCT
HFCT
HFCT
HFCT
Images courtesy of IPEC Ltd
Example: Designing out a potential means of attaching PD sensors
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Example: Building in a facility to apply PD sensors
HV cable
Switchgear earth
Insulated bushing
Cable
termination box
Modified earth
HFCT PD Sensor
Image courtesy of IPEC Ltd
PD recommendations
▪ ‘Future proofing’: consideration should be given during cable design and construction to
building in provision to retrofit PD sensors for future uses, either in fault investigations or for
continuous monitoring.
▪ Attachment points for HFCT PD sensors at cable terminations can often provide a convenient
means of monitoring connected equipment within a substation (such as transformers,
switchgear, surge arresters) that might itself have no facilities for attaching PD sensors.
▪ Utility should consider nominating an engineer to specialise in dealing with contractors on the
topic of PD detection both for the commissioning of new cables and diagnostics on cables that
have been in service.
▪ In the absence of standards for PD detection in the field, it is important to have concrete
evidence from the test service provider about the effectiveness of the test procedure and the
validation of its operation prior to testing.
▪ The contractor monitoring for PD during commissioning tests should be asked to provide
documented evidence of the absence of PD when a cable has passed the voltage withstand test
(including relevant supporting information such as the background noise levels, any interference
present and the duty cycle of monitoring).
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Cable Impedance Scanning
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TDR (Time domain reflectometry)
Responds to impedance changes along the length
of the cable. Impedance changes can be expected
at open circuits and joints and from positions of
unexpected damage to the cable system.
▪ Low-voltage test with clip-on attachment
An example cable TDR trace of reflected signal
plotted against time, showing a ‘ripple’ on the
trace that was caused by moisture ingress.
NEETRAC, Report of the Cable Diagnostic Focused Initiative, Phase II, released February 2016.
Available as separate chapter files from https://www.neetrac.gatech.edu/cdfi-publications.html
▪ Equipment is small and inexpensive
▪ Test results can be compared to historical data
▪ Can be used to compare distances to joints and
overall lengths of companion phases
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LIRA (Line impedance resonance analysis )
signature signal magnitude (dB)
Example of LIRA scans from both ends of a
15.6 km, 34 kV subsea cable at different
dates
The peak at 12300 m on the red trace (or
3350 m from the opposite end, yellow
trace) is a consequence of a mechanical
damage caused by an anchor.
distance along cable (m)
P F Fantoni, “Advancements in Wire Condition Monitoring Using Line Impedance Resonance Analysis (LIRA)”, Paper No. 166, Int. Conf. on Condition Monitoring,
Diagnosis and Maintenance (CMDM), Bucharest, Romania, October 2015
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Comparative assessment of TDR & LIRA
Initial
implementation
very short
cables not tested
TDR
scan
LIRA
fingerprint
< 2 km
joint
Trials / optimisation / experience
≥ 2 km
2 or more joints
Eventual
practice
short
cables not tested
TDR
scan?
increasing cable complexity
Note that prior use of these impedance measurement techniques cannot
diminish the requirement for a monitored voltage withstand test.
LIRA
fingerprint
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Trapped DC Voltage on AC Cables
Remnant DC on AC cables after switching
Cable charging times cause concern for cable manufacturers - likened to a quasi-DC condition:
Field grading in AC cable accessories is capacitive, whereas in DC cables the resistivity of the insulation is
important for ensuring the correct distribution of electric stress.
However:
Remnant DC voltages can remain trapped on cables after switching – how can this be acceptable if there
is concern about testing cables / accessories using any HV supply that might ‘look like’ DC?
Questions arising:
▪ Should this be a concern given the reported ‘ageing’ sensitivity of cable systems to much shorter DC
exposure times during non-conventional HV testing? *
▪ Is this a significant issue, and should mitigations be implemented, such as ensuring that a wound VT
is always used?
▪ What about the potential for rapid transient if switching back in at the reverse polarity peak?
▪ Could this be an area for a practical academic research project to clarify the matter?
* Note that in part this might be due to the fact that remnant DC is unlikely to exceed the peak of operational voltage, whereas a
commissioning test would reach a significantly higher voltage.
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Summary of Main Outcomes
▪
▪
▪
HV Supplies for Cable Stress Testing at Commissioning
▪
Trial use of compact offshore RTS units with reactors in series for withstand testing of new 132 kV cable systems
▪
Intended method of voltage withstand testing should be part of the design stage: e.g., if requirement is to use VLF, make this
part of the tendering requirement – “cable & accessories must be suitable for VLF withstand test at X kV for Y minutes”
▪
Trial use of VLF and DAC for testing of service-aged cables taking into account the findings of CIGRE Working Group B1.28
‘After Laying Tests on AC and DC Cable Systems with New Technology’ once published in 2021
PD Sensors and Monitoring
▪
Importance of effective (appropriately distributed) sensing along the cable during withstand tests
▪
Avoid ‘designing out’ capability to install PD sensors through minor adjustments to cable layout
▪
Optical fibres and energy harvesting examples of enabling technologies for PD and other types of monitoring
Cable Impedance Scanning Methods
▪
▪
▪
Introduce a structured approach to testing and evaluating the use of LIRA and TDR based on cable length and complexity
Research
▪
Practical studies: Influence of trapped DC voltages on AC cables
▪
Fundamental: Significance of space charges as a cause of XLPE insulation ageing under non-conventional HV supply regimes
Dissemination
▪
Importance of sharing experience, publication, involvement with Working Groups and influencing standards
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High Frequency Diagnostics and Engineering Ltd
Thanks for your participation
For further information about the TACAMA project please contact:
Matthew Hamilton
Innovation Project Manager
Matthew.Hamilton@sse.com