Measurement challenges in partial discharge diagnostics of high voltage insulation systems Martin Judd Technical Director, High Frequency Diagnostics [email protected] Tuesday 4 th September
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Measurement challenges in partial discharge diagnostics of high voltage insulation systems
Martin Judd
Technical Director, High Frequency Diagnostics
Tuesday 4th September
Outline:
1. Introduction
2. Partial Discharges ( PD ) and Power Transformers
3. Factory and Site Acceptance Tests ( FAT / SAT )
4. Calibration and Practical Application
5. Asset Management Perspective
1. Introduction
Presenter’s background
10/2014 – future Founder & Technical Director, HFDE Ltd, Glasgow
High Voltage – High Frequency – PD – Diagnostics
10/2012 – 10/2014
Professor of High Voltage Technologies, University of Strathclyde
Course Director for Electrical & Mechanical Engineering Electrical Plant and Diagnostics Research Manager of the David Tedford High Voltage Technologies Laboratory Chair of the Universities High Voltage Network (UHVnet) in the UK
4/2004 – 10/2012
Senior Lecturer – Reader, HVT Research Group, University of Strathclyde
UHF PD location techniques for power transformers Partial discharge detection using non-standard techniques Energy harvesting for wireless sensors FDTD modelling in PD research applications
11/1993 – 4/2004
Research Assistant – Senior Research Fellow, Centre for Electrical Power Engineering, University of Strathclyde
PhD: ‘Excitation of UHF signals by PD in GIS’ EPSRC Advanced Research Fellowship UHF sensors for GIS – design and calibration
8/1985 – 11/1993
Senior Engineer, Receiver Protection Group Design & production of gas-filled waveguide RF switches for radar
Engineer, Microwave Systems Development R&D on radar systems and components from X-band to 94 GHz
9/1981 – 6/1985
BSc Electronic Engineering (Telecommunications)
Context of this presentation
I am presently the UK representative on CIGRE Joint Working Group A2/D1.51, Improvements to PD Measurements for FAT & SAT of Power Transformers.
Provide some technical insight into the practicalities of a very specific topic.
Reminder of some physics aspect of the equipment and materials that build the power systems we are so dependent upon.
M D Judd, B M Pryor, S C Kelly and B F Hampton, "Transformer monitoring using the UHF technique", Proc. 11th Int. Symp. on High Voltage Engineering (London), Vol. 5, pp. 362-365, August 1999
1998: First internal UHF transformer sensor designed and installed by Strathclyde University and ScottishPower (Cambuslang, Glasgow)
M Cap, P Drexler and P Fiala, “Partial Discharge Detection and Localization System”, PIERS Proceedings, Guangzhou, China, August 2014
2014: UHF probes on a power transformer at a nuclear power plant (Dukovany, Czech Republic)
0
500
1000
1500
2000
2500
1985-89 1990-94 1995-99 2000-04 2005-09 2010-14 2015-19*
20 69214
572
1320
1970 2030N
um
be
r o
f P
ub
lica
tio
ns
Year of Publication
Scientific publications that include the terms “UHF” and “partial discharge”
Notes: Sampled using Google Scholar on 10th June 2018, search term UHF “partial discharge”, excluding patents and citations. * Data for 2015 – 2019 extrapolated as the result for 2015 – present (mid 2018) scaled by factor ( 5/3.5 ).
Example transformer diagnosis using the ultra-high frequency (UHF) method – Remote on-line PD location
Key points:
Some dangerous PD sources are intermittent, exhibiting long “dormant” periods
Essential to capture PD location signals during times of activity, so monitoring has to be continuous
Remote on-line partial discharge monitoring with location capability
IP66 enclosure
Containing: Tek TDS3054 scope, UPS unit, thermostat,
small heater
From 4 UHF sensors
230 V ac mains supply
At the transformer
In the control room optical fibre
Ethernet link
Weather-proof cable entry points
Internet accessible (remote desktop, FTP, etc)
1 TB USB data store
Compact tower PC/server
UPS 230 V ac power
HV side view of 4-sensor location points
On the HV side, coordinates of interest lie along the line x = 1.4 m, z = 2.1 m
Confirmed during outage, internal visual inspection. Repaired on-site.
RF “spark detection” is by no means a recent development . . .
http://ethw.org/Milestones:First_Generation_and_Experimental_Proof_of_Electromagnetic_Waves,_1886-1888
Photograph taken by Hertz of his electromagnetic wave apparatus Sculpture, Karlsruhe Institute of Technology
2. Partial Discharges (PD) and Power Transformers
Partial discharge
PD is both a cause and indicator of insulation degradation, which, if left unchecked, may lead to failure of HV assets
cable joints and terminations
bushings
gas insulated switchgear
power transformers
instrument transformers
Failure may be costly to the DNO / TNO
high replacement costs with long and manufacturing lead times (transformers)
regulatory penalties for the consequent outages
ecmweb.com/test-measurement/partial-discharge-and-asset-management
Electrical tracking on a barrier board
Partial discharge
Standard defining partial discharge testing is IEC 60270:2000+AMD1:2015 “High-voltage test techniques - Partial discharge measurements.” *webstore.iec.ch+
Practical explanation:
Small sparks eroding and chemically degrading materials
Damaging surfaces and interfaces between materials
Releasing contaminants or particulates into the insulation system
Damage caused by PD may eventually become so serious that insulation is bridged or (more likely) the insulation withstand capability is reduced making it more prone to flashover during transient over-voltages.
PD measurement showing ‘phase-resolved’ pulse pattern
Electric field {∝ V} exerts physical pressure on its “conduits” – Electric fields are force fields . . .
deterioration of materials
joints – interfaces between components
Feeder capacity leads to flooding
Electrical insulation is intended to prevent the uncontrolled release of energy passing through equipment . . .
electrical-engineering-portal.com/substation-fire-protection
www.graphicproducts.com/articles/arc-flash-accident-reports/
Typical sites initiating electrical stress:
Void or inclusion in solid insulation
Corona discharges at sharp edges
Surface discharges between busbars on an insulator
Partial discharges in overstressed gaps (unshielded
cable in conduit)
after I. Cotton, A. Nelms, M. Husband, Higher voltage aircraft power systems, IEEE Aerospace and Electronic Systems Magazine, Vol. 23, No. 2, pp. 25-32, Feb 2008
Asset Management: Consequences of failure
Safety and environmental
Degraded power quality
Loss of public trust
Loss of revenue
Increased costs
Staff dissatisfaction
Loss of reputation
Business failure
Prosecution for criminal offences
Sometimes the stresses placed on insulating materials may not be as intended . . .
PD: Electron avalanche
+ + + + + +
- - - - - -
Electron acceleration and velocity
vector
+
Positive ions
electrons
+ +
+ +
+
Electric fields in a solid insulator with a spherical void
close-up view showing equipotential lines and the electric field vector arrows
HV electrode, +10 kV
earth, 0 V
Volume of scholarly works that include “partial discharge” in metadata:
0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000
1986 – 1995
1996 – 2005
2006 – 2015
2017 only
Totals
IEEExplore Google ScholarDate of web search: 1 Sept 2018
Typical structure of HV power transformer
Typical 3-phase power transformer active part
Grid transformer in service
New CIGRE Technical Brochure 735
3. Factory and Site Acceptance Tests ( FAT / SAT )
Factory and Site Acceptance Tests ( FAT / SAT )
Present level of interest in UHF PD detection techniques for transformers is indicated by the membership of CIGRE JWG A2/D1.51, Improvement to Partial Discharge Measurements for Factory and Site Acceptance Tests of Power Transformers:
Focusing largely on the use of UHF sensors to enhance quality control as an addition to the conventional electrical PD measurements and DGA.
IEC 60270 PD testing and criteria will remain for the FAT, but it is well known that most commissioning sites are far too noisy for conventional PD measurements.
Also, while DGA is eminently suitable for identifying PD in service, the time lag is too great for it to be of use on the timescale of a normal SAT.
Factory and Site Acceptance Tests ( FAT / SAT )
UHF sensors can support the FAT / SAT procedures in several ways, including:
1. Taking snapshots (“fingerprinting”) of UHF PD signals (or the absence of them) for a transformer that passes the FAT and then repeating these on-site during the SAT to see if anything has changed.
2. Where a transformer fails the FAT due to PD, the UHF sensors could be used to speed up the processes of PD location and determining remedial action.
3. Over time, experience of the combined IEC / UHF measurements will itself contribute to better diagnosis and rectification of PD issues.
4. The availability of UHF sensors (or sensor facilities) on transformers in service will
provide an opportunity for periodic PD surveys to be performed, and
enable convenient retrofitting of continuous PD monitoring when there is a known PD problem, or the transformer is being closely monitored for a period prior to refurbishment or replacement.
Generator step-up transformer at final test equipped with UHF sensors during FAT. As well as the 3 UHF sensors visible here on the LV side, sensor No. 1 is located on the HV side of the tank opposite to sensor No. 3.
Example of UHF PD signals recorded during FAT, which the transformer passed . . .
Amplified (×20 voltage gain) UHF signals recorded during overpotential testing of C-phase at 200% with a measured PD level in the region of 5 – 10 pC.
0 10 20 30 40 50 60 70 80 90 100
time ( ns )
Am
plit
ude
( 1
0 m
V /
div
)
Frequency spectrum of the largest signal from UHF sensor No. 1
200 400 600 800 1000 1200 1400 1600 1800 20000
0.5
1
frequency ( MHz )
rela
tive
sp
ect
ral d
ensi
ty
Amplified (×20 voltage gain) UHF signals recorded during overpotential testing of B-phase at 160% with a measured PD level in the region of 20 – 30 pC.
0 10 20 30 40 50 60 70 80 90 100
time ( ns )
Am
plit
ude
( 1
0 m
V /
div
)
FAT SAT
IEC PD UHF
PD Factory action:
UHF
PD Site action:
No No Great – good quality
transformer!
N The perfect experience!
Y Something new has appeared . . .
What do we advise?
Yes Yes Apply usual IEC test
criteria. UHF diagnosis
tools if decision is fail.
N Any problem was fixed.
Y Is it worse than any level that was accepted at
FAT (fingerprint comparison)?
Yes No Apply usual IEC test
criteria.
N Well the UHF was useless anyway!!
Y Something new has appeared . . .
What do we advise?
No Yes Should the transformer
ever fail test in this case?
N Any problem was fixed.
Y Is it worse than any level that was accepted at
FAT (fingerprint comparison)?
Areas in which the CIGRE JWG needs to give practical advice (pink shaded):
4. Calibration and Practical Application
Standard for PD measurement (conventional electrical method) 3 Definitions
For the purpose of this International Standard, the
following definitions apply.
3.1
partial discharge (PD)
localized electrical discharge that only partially bridges
the insulation between conductors and which can or
can not occur adjacent to a conductor
NOTE 1 Partial discharges are in general a consequence of local
electrical stress concentrations in the insulation or on the surface of the
insulation. Generally, such discharges appear as pulses having a
duration of much less than 1 μs. More continuous forms can, however,
occur, such as the so-called pulse-less discharges in gaseous
dielectrics. This kind of discharge will normally not be detected by the
measurement methods described in this standard.
NOTE 2 "Corona" is a form of partial discharge that occurs in
gaseous media around conductors which are remote from solid or
liquid insulation. "Corona" should not be used as a general term for all
forms of PD.
NOTE 3 Partial discharges are often accompanied by emission of
sound, light, heat, and chemical reactions.
The vocabulary of metrology:
www.bipm.org/utils/common/documents/jcgm/JCGM_200_2012.pdf
2.39
calibration
operation that, under specified conditions, in a first step,
establishes a relation between the quantity values with
measurement uncertainties provided by measurement
standards and corresponding indications with associated
measurement uncertainties and, in a second step, uses this
information to establish a relation for obtaining a
measurement result from an indication
NOTE 1
A calibration may be expressed by a statement, calibration
function, calibration diagram, calibration curve, or calibration
table. In some cases, it may consist of an additive or
multiplicative correction of the indication with associated
measurement uncertainty.
NOTE 2
Calibration should not be confused with adjustment of a
measuring system, often mistakenly called “self-calibration”,
nor with verification of calibration.
Typical conventional electrical PD measurement circuit:
High frequency blocking impedance
HV AC supply
Capacitance of test object (e.g., cable)
Coupling capacitor
Measurement impedance
voltage pulse to the calibrated PD measurement system
Typical conventional electrical PD measurement circuit:
Capacitance of test object (e.g., cable)
Coupling capacitor
Measurement impedance
Injection terminals for a standard calibration pulse, e.g., 100 pC
IEC 60270 measurement of a test object
HV transformer
100 pF coupling capacitor
HV bushing
Variability of UHF signal amplitude with PD position
M. Siegel, S. Tenbohlen, Comparison between Electrical and UHF PD Measurement concerning Calibration and Sensitivity for Power Transformers, Proc. Int. Conf. on Condition Monitoring and Diagnosis (Jeju, Korea), 2014
UHF signal amplitude variation with PD source vertical position
Variability of IEC60270 apparent charge magnitude with PD position
M. Siegel, S. Tenbohlen, Comparison between Electrical and UHF PD Measurement concerning Calibration and Sensitivity for Power Transformers, Proc. Int. Conf. on Condition Monitoring and Diagnosis (Jeju, Korea), 2014
Broadband IEC60270 measurement variation with PD source vertical position
Measuring sound level with a calibrated sensor
How loud is the sound now?
There is no simple traffic light representation for PD severity without further contextual understanding!
Should we be concerned about this PD or not?
Drain valves for monitoring PD
Images courtesy of David Walker, Lead Engineer, SP Energy Networks
Drain valves for monitoring PD
Images courtesy of David Walker, Lead Engineer, SP Energy Networks
Installing a UHF probe sensor
(1) Valve must be of a suitable type
(2) Fit a short extension with air bleed valve
(3) Fit the probe in its retracted position
(4) Determine the required insertion length
Installing a UHF probe sensor
(5) Open the air bleed valve (6) Carefully open the main valve allowing air to be dis-
placed through the bleed valve
(7) Once oil appears at the bleed valve, close it and fully
open the main valve
Installing a UHF probe sensor
(8) Insert probe to the required depth (this view would not
normally be available!)
(9) Lock off the probe stem to hold it at the required depth
(10) Connect UHF signal cable and carry out PD measurements
5. Asset Management Perspective
“Asset Management Excellence”
Asset management excellence is many things, done well1:
– plant performing to design standards
– equipment operating smoothly when called upon
– maintenance costs remaining within budget
– assets delivering high levels of service and output quality
1Asset Management Excellence: Optimizing Equipment Life-Cycle Decisions, John D. Campbell (Editor), Andrew K.S. Jardine (Editor), Joel McGlynn (Editor). 2nd Edition, CRC Press, 2011.
“Asset Management Excellence”
A robust asset management system does not necessarily prevent problems, although it should contribute significantly to reducing their likelihood.
However, enables organisation to respond more effectively to the unforeseen through procedures and systems capable of:
– minimising adverse impacts, and
– allowing the organisation to learn and adapt in response
Pareto principle ( 80 : 20 rule )
The Pareto principle was an empirical observation:
A paper in 1896 showing that approximately 80% of the land in Italy was owned by 20% of the people.
Similar phenomena noted in naturally occurring situations, such as observing that 20% of the peapods in his garden contain 80% of the peas.
Vilfredo Pareto
Use Pareto principle to identify “quick wins”
option 2 option 3 option 9
option 6 option 8 option 4
option 1 option 7 option 5
Effort & Expense
LOW
HIGH
LOW HIGH
Benefit
Comparing alternative investment options
Increase maintenance
Refurbish
Replace
Upgrade
Increase capacity
Add redundancy
outage penalty
risk mitigation
cost
value
- £
£
££
£££
££££
££££
-
To conclude – CIGRE activity associated with the topics covered:
1. WG A2.27, Recommendations for condition monitoring and condition assessment facilities for transformers, Technical Brochure 343, 2008
2. WG D1.33, Guidelines for Unconventional Partial Discharge Measurements, Technical Brochure 444, 2010
3. WG D1.25, UHF partial discharge detection system for GIS: Application guide for sensitivity verification, Technical Brochure 654, 2016
4. WG D1.29, Partial discharges in transformers, Technical Brochure 676, 2017
5. JWG A2/D1.51, Improvement to Partial Discharge Measurements for Factory and Site Acceptance Tests of Power Transformers, Technical Brochure and Tutorials expected 2019/2020.
High Frequency Diagnostics and Engineering Ltd
Clydeway House, 813 South Street, Glasgow G14 0BX, UK
www.hfde.co.uk
High Frequency Diagnostics and Engineering Ltd is a company registered in Scotland - SC482878 : VAT Reg. No. 191367491
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