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Transformer Diagnostic Measurements • Tran Transf sfor orme merr Over Overvi view ew • Life Life Spa Span n and and Oper Operat ating ing Condi Conditio tions ns
• Cigre Guide for Transformer Maintenance • Oil Tes Tests ts – Dissol Dissolve ved d Gas Gas Analy Analysi sis, s, Degre Degree e of Poly Polyme meris risat atio ion n (DP) & Furan Analysis • Overvie Overview w of CPC100 CPC100 and and Operat Operating ing Soft Softwar ware e (Tools (Toolset) et) • Elec Electr tric ical al Me Meas asur urem emen ents ts • Dielec Dielectri tric c Dissi Dissipat pation ion Fact Factor or – Tan Tan Del Delta ta • Dielec Dielectri tric c Diss Dissipa ipati tion on Fac Facto torr - Moi Moist sture ure Conte Content nt • Pa Part rtia iall Disc Discha harg rge e • Swee Sweep p Fre Frequ quen ency cy Resp Respon onse se • Test Testing ing Curre Current nt Tran Transf sform ormer ers s • Summary
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Topics – Guide for Transformer Transformer Maintenance Maintenance
> What is Cigre? > Over Overvi view ew of of Gui Guide de > Mainte Maintenan nance ce Strate Strategy gy > Mainte Maintenan nance ce Proces Process s > Transfor Transformer mer Component Component Selection Selection and Maintenance > Mainte Maintenan nance ce Actio Action n Cat Catalo alogue gue > Major Major Work Work – Transfo ransforme rmerr Repa Repair ir > Appendix
© OMICRON OMICRON
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Topics – Guide for Transformer Transformer Maintenance Maintenance
> What is Cigre? > Over Overvi view ew of of Gui Guide de > Mainte Maintenan nance ce Strate Strategy gy > Mainte Maintenan nance ce Proces Process s > Transfor Transformer mer Component Component Selection Selection and Maintenance > Mainte Maintenan nance ce Actio Action n Cat Catalo alogue gue > Major Major Work Work – Transfo ransforme rmerr Repa Repair ir > Appendix
© OMICRON OMICRON
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What is Cigre? CIGRE (International Council on Large Electric Systems) is one of the leading worldwide Organizations on Electric Power Systems, covering technical, economic, environmental, organisational and regulatory aspects. A permanent, non-governmental and non-profit International Association, based in France, CIGRE was founded in 1921 and aims to: Facilitate the exchange of information between engineering personnel and specialists in all countries and develop knowledge in power systems. Add value to the knowledge and information exchanged by synthesizing state-of-the-art world practices. Make managers, decision-makers decision-makers and regulators aware of the synthesis of CIGRE's work, in the area of electric power.
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What is Cigre? Through the years, CIGRE has become a meeting place for senio r engineers in the field of electrical energy: researchers, solution providers, utilities, regulators and normalisation institutions, and generally all electrical e ngineers involved with electrical po wer. wer.
Internal Organisation The work at CIGRE is divided in study committees : A1 Rotating Electrical Machines
A2 Transformers
A3 High Voltage Equipment
-----------------------------------------------------------------------------------------------------------------------------------------B1 Insulated Cables
B2 Overhead Lines
B4 HVDC and Power Electronics
B5 Protection and Automation
B3 Substations
-----------------------------------------------------------------------------------------------------------------------------------------C1 S Sys yste tem m Deve Develo lopm pmen entt and and Econ Econom omic ics s
C2 S Sys yste tem m Oper Operat atio ion n and and Cont Contro roll
C3 Syste System m Env Envir iron onme ment ntal al Perf Perfor orma manc nce e
C4 Syst Systema ema Techn echnica icall Per Perfor forma manc nce e
C5 Ele Elect ctri rici city ty Mar Marke kets ts and and Reg Regul ulat atio ion n
C6 Dis Distr trib ibut utio ion n Syst System ems s and and Disp Disper erse sed d Gene Genera rati tion on
-----------------------------------------------------------------------------------------------------------------------------------------D1 Materials and Emerging Emerging Test Test Techniq Techniques ues © OMICRON OMICRON
D2 Information Information Systems Systems and Telecomm Telecommunicat unications ions Page 4
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What is Cigre? More specifically, issues related to planning and operation of power systems, as well as design, construction, maintenance and disposal of HV equipment and plants are at the core of CIGRE's mission.
Problems related to protection of power systems, telecontrol, telecommunication equipment and information systems are also part of CIGRE's area of concern © OMICRON
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Topics – Guide for Transformer Maintenance > What is Cigre?
> Overview of Guide > Maintenance Strategy > Maintenance Process > Transformer Component Selection and Maintenance > Maintenance Action Catalogue > Major Work – Transformer Repair > Appendix
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Overview of Guide The guide is a condensed documentation of all activities associated with transformer maintenance. It is intended to help transformer users define and apply best practice for transformer maintence. The document was prepared with a focus on liquid immersed transformers rated 69kV and above and larger than 25 MVA – can be applied wider. Not intended to mandate levels of maintenance. Required maintenance documentation should be supplied with new transformers.
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Transformer Operation and Maintenance Cycle
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Time Based Maintenance (TBM) Maintenance performed at predetermined intervals to reduce the likelyhood of equipment failing in service. Includes routine maintenance actions ( lubrication, oil change etc.) Actioned at fixed time intervals. Provides a high level of risk management if manufacturers recomendations are followed. Easiest to plan but most expensive, particularly when an outage is required.
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Time Based Condition Monitoring (TBCM) Condition of equipment is evaluated by visual checks, measurements and tests at regular intervals.
Often performed in conjunction with minor routine maintenance that requires an outage.
Results of TBCM set the extent of future maintenance programs.
Information limited to “snap shots’
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Condition Based Maintenance (CBM) Scheduled according to equipment condition. Also called “Conditional Preventive Maintenance”. Based on assessing the actual physical condition taking into account its usage, occurrence of events, possible wear of moving parts and performance of similar equipment. Use TBCM, OLCM and continuous monitoring to determine plant condition. CBM is used when technical conditions can be assessed. Goal to reduce costs, requires more complicated planning.
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On Line Condition Monitoring (OLCM)
Techniques, method or measurement performed while the transformer is in operation that provide information about its condition.
May include oil sampling for DGA, infra-red thermal scanning, load and temperature monitoring or making visual observations e.g. oil levels or oil leaks.
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Continuous On-Line Monitoring (COLCM) Refinement of OLCM – measurements are continuously recorded.
The monitoring device can initiate an alarm or message when an abnormal condition arises.
Should be able to reliably predict a likely failure or fault developing before the event.
Can form the basis for Condition Based Maintenance.
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Reliability Centred Maintenance (RCM) An optimised strategy that accounts for operation time, technical condition, its location, operational importance together with safety and environmental factors.
The transformer is assigned a criticality index value.
The interval, content and priority of maintenance tasks are determined by the index.
Assets in risky or critical positions are maintained in a more intensive manner.
Usually used inconjunction with a health index.
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Standards and Cigre References.
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Topics – Guide for Transformer Maintenance > What is Cigre? > Overview of Guide
> Maintenance Strategy > Maintenance Process > Transformer Component Selection and Maintenance > Maintenance Action Catalogue > Major Work – Transformer Repair > Appendix
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Maintenance Strategy For the lifetime of a transformer (>40 years) the maintenance strategy should aim to provide:
Highest Reliability + Minimimal Maintenance
This requires strategies to deal with a variety of technologies, each needing different levels and scope of maintenance.
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Maintenance Strategy - Importance of Maintenance Possible impacts of a lack of maintenance as a function of age. Many degradation or failure mechanisms can be detacted and eliminated or minimised by appropriate maintenance. Good commissioning procedures and tests are essential to build a foundation for a long service life.
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Maintenance Strategy – Early Life Good Commissioning practices are essential for a sound foundation of early life maintenance.
Should focus on:
Oil quality.
Functionality of auxiliaries (cooling and tap changer).
Baseline measurements and tests for future comparisions and diagnostics.
Communications, alarms and monoitoring functions.
Little maintenace should be required in the first 5 years except for possibly OLTC and DGA ( before and after energisation and short intervals – weekly – until there is obviously no problems)
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Maintenance Strategy - Condition Monitoring
A cost effective maintenance strategy relies on information showing the condition of the transformer. Condition monitoring must be technically and economically feasible.
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Maintenance Strategy - Typical Maintenance Intervals
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Oil Tests – an example of Condition Based Maintenance
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Maintenance Strategy – Evaluating DGA Results
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Maintenance Strategy - Continuous On-Line Monitoring On-Line Periodic data collection monitoring the transformer condition while it is in service under normal operation conditions e.g commonly DGA samples or in special cases Partial Discharge.
Continuous On-Line
Ongoing monitoring to
generate early warnings,
develop a basis for setting maintenance periods
Closely monitor known faults for early warning
Act as a ‘Black Box’ when a fault occurs with before and after data.
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Topics – Guide for Transformer Maintenance > What is Cigre? > Overview of Guide > Maintenance Strategy
> Maintenance Process > Transformer Component Selection and Maintenance > Maintenance Action Catalogue > Major Work – Transformer Repair > Appendix
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Maintenance Processes Maintenance Process Good planning requires guidelines to specify “What”, “When” and “How”. Organisation answers ‘Who”, “When”, “How Long”, “Access” and “Resources” Execution can be either Internal or External resources. Optimisation by refining processes based on quality data and developments in diagnostic techniques.
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Topics – Guide for Transformer Maintenance > What is Cigre? > Overview of Guide > Maintenance Strategy > Maintenance Process
> Transformer Component Selection and Maintenance > Maintenance Action Catalogue > Major Work – Transformer Repair > Appendix
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Transformer Component Selection and Maintenance Bushings
Oil Impregnated Paper (OIP)
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Transformer Component Selection and Maintenance
Resin Bonded Paper ( RBP)
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Transformer Component Selection and Maintenance
Resin Impregnated Paper (RIP)
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Transformer Component Selection and Maintenance Oil Systems
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Pressure Regulated Gas Blanket
Conservator Systems Systems
Sealed Tank
Free Breathing
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Transformer Component Selection and Maintenance Cooling Systems – Pumps, fans, radiators, heat exchangers, gaskets.
Gauges, Indicators and Relays –
Oil Level Indicators
Buchholz Relay
Pressure Relief Device
Rapid Pressure Relay
Oil Flow Indicators
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Transformer Component Selection and Maintenance Controls
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Wiring and connections
CT terminations, sensors, fuses, contactors and relays
Filters, heaters, ventilation
Local status indications
Monitoring Devices
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Transformer Component Selection and Maintenance On Load Tap Changer (OLTC) Typical Maintenance issues with Drive Mechanism
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Transformer Component Selection and Maintenance OLTC Typical Manufacture Recomended Maintenance Periods
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Transformer Component Selection and Maintenance Duty Cycle of an OLTC
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Transformer Component Selection and Maintenance Transformer Active Parts
Windings
No routine maintenance required. Older transformers may require the clamping pressures checked especially after a drying process.
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Transformer Component Selection and Maintenance Transformer Oil
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Transformer Component Selection and Maintenance Transformer Oil
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Transformer Component Selection and Maintenance Transformer Oil
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Transformer Component Selection and Maintenance Transformer Oil
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Topics – Guide for Transformer Maintenance > What is Cigre? > Overview of Guide > Maintenance Strategy > Maintenance Process > Transformer Component Selection and Maintenance
> Maintenance Action Catalogue > Major Work – Transformer Repair > Appendix
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Maintenance Action Catalogue Electrical Measurements
Reasons for Tests
© OMICRON
Fingerprint measurement for future reference
Routine tests for determiing trends
Early detect of problems through monitoring
Fault location and troubleshooting
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Maintenance Action Catalogue Electrical Measurements Main focus of on-site tests
Limit the aging effects on insulation oil and insulation material by heat and dielectric stress.
Identification of failures as thermal or dielectric
Measurement and location of Partial Discharge.
Control of mechanical changes thrugh internal forces.
Trending of moisture content
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Maintenance Action Catalogue Electrical Measurements
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Maintenance Action Catalogue Electrical Measurements – Basic Measurements - Winding Ratio
Detects Shorted Turns or Winding Sections. Indication:
Buchholz Trip, OLTC Repair
Reference:
Intial Test or Nameplate Details
Interpretation:
+/- 0.5% of Nameplate
Comments:
Some test equipment may allow magnetising current to be measured concurrently. Avoid performing resistance test before ratio measurement. Residual flux could impact the measurement. Normal after any works that disturbed connections.
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Maintenance Action Catalogue Electrical Measurements – Basic Measurements – Winding Resistance
Detects Contact problems on OLTC, diverter switch, broken conductors or strands, shorted layer or poor bushing connections. Indication:
High internal temperatures, DGA results
Reference:
Factory tests, commissioning tests
Interpretation:
Within 1% of factory resultstemperature corrected. Differences between phases 2- 3%. Comparison between HV and LV approximately square of the ratio.
Comments:
HV resistance much higher than LV, there identification of contact problems more difficult. Need to stablise current before reading which can take tens of minutes.
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Maintenance Action Catalogue Electrical Measurements – Basic Measurements - Magnetising Current
Detects core faults (shorted laminations) or shorted turns or winding parts. Indication:
Buchholz trip or DGA.
Reference:
Comparison of phases, factory or commissioning tests.
Interpetation:
Outer phases within 5%, middle phase up to 30% lower.
Comments:
Magnetising current at rated voltage is only 0.1 to 0.3 % of rated current. Voltage dependancy of magnetising current requires testing at same voltage and tap position. Avoid winding resistance test before magnetising current measurement. Residual flux could influence measurement.
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Maintenance Action Catalogue Electrical Measurements – Basic Tests – Capacitance and Dissipation Factor
Detects – Capacitance – winding change in geometry. - core to ground mechanical damage. - Dielectric Dissapation Factor (Tan Delta) – deterioration or contamination of insulation materials. Indication
Moisture, Routine test
Reference
Initial tests, trend analysis, comparision to idential units, nameplate details.
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Maintenance Action Catalogue Electrical Measurements – Basic Tests – Capacitance and Dissipation Factor What is Dissipation Factor?
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Maintenance Action Catalogue Electrical Measurements – Basic Tests – Capacitance and Dissipation Factor
Interpretation
For windings 0.5% normal, above 1% questionable For Bushings
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Maintenance Action Catalogue Electrical Measurements – Basic Tests – Capacitance and Dissipation Factor Additional Information – Permissible change varies with make and type. A doubling of initial values warrants investiagtion or replacement. A 10% change in capacitance also warrants replacement. Temperature can affect the Dissipation Factor.
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Maintenance Action Catalogue Electrical Measurements – Basic Tests – Leakage Reactance
Detects deformation of windings Indication
High through currents, high inrush current, Buchholz trip, protection trip.
Reference
Initial tests, comparision to identical units.
Interpretation
Within 2% of factory tests, differences between phases <2%. Changes above 1% should be investigated with other tests e.g. FRA. Care to be taken when comparing singal phase to three phase tests.
Comments
If measured from LV side results to be corrected by square of winding ratio.
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Maintenance Action Catalogue Electrical Measurements – Basic Tests – Insulation Resistance
Detects insulation deterioration or failures in the winding to core Indication
Buchholz trip, DGA
Reference
Initial tests, comparions to identical units
Interpretation
Transformers > 69kV, > 1 Gohm, < 69KV, >500 Mohm @ 20 deg C
Comments
An additional measurement with IR is the Polarisation Index PI. PI is the IR after 1 minute compared to the reading after 10 minutes. Initially used on motors. Can be misinterpretated in complex oil-cellulous insulations. FDS/PDC measurements preferred. Can influence FDS/PDC measurements if done prior.
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Maintenance Action Catalogue Electrical Measurements – Basic Tests - Core Grounding
Detects multiple core grounding faults. Indication
High internal temperatures shown via DGA.
Reference
Initial tests
Interpretation
Normally >10Mohm, less than 100Kohm indactes problem. Sometimes core lamination packets are separated by resistors of approximately several Kohm.
Comments
© OMICRON
In service current from core to tank is mA’s. If a fault develops the current is several amps or higher.
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Maintenance Action Catalogue Electrical Measurements – Special Tests – Frequency Response Analysis (FRA) Detects shorted turns, core faults, winding deformations and displacement, faulty core grounding,faulty screen connections, damage during transport. Indication
High through fault currents, Buchholz trip, DGA, Impact recorder results.
Reference
Time based – comparing to previous results Type based – comparing to equivalent type Phase comparision
Interpretation
No defined limits, comparisions indicate changes.
Comments
For transport checks measure before departure and on arrival Winding resistance measurements can influence results.
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Maintenance Action Catalogue Electrical Measurements – Advanced Tests – Dissipation Factor Polaristation Depolarisation (PDC) Method
Detects water content in solid insulation of oil paper insulation, insulation failures, ageing of insulation, contamination of insulation liquids, partial break down of capacitive bushing layers. Indication
High moisture in oil, general assessment.
Reference
Moisture determination by modelling, comparison to fingerprints.
Interpretation
Moisture determination by comparision to a modelled dielectric response. An algorithm rearranges the modelled dielectric response and determines moisture content and oil conductivity.
Comments
Also considers the effect of aged pressboard to compensate for the influence of aging.
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Maintenance Action Catalogue Electrical Measurements – Advanced Tests – Frequency Domain Spectroscopy (FDS) Detects water content in solid cellulous insulation, insulation failures, contaminations of oil. Indication
High moisture in oil, general assessment.
Reference
Moisture determination by modelling, comparison to fingerprints. Moisture limit in IEC 60422.
Interpretation
Moisture determination by comparision to a modelled dielectric response. An algorithm rearranges the modelled dielectric response and determines moisture content and oil conductivity.
Comments
The combination of PDC and FDS reduces the test duration.
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Maintenance Action Catalogue Electrical Measurements – Advanced Tests – Dissipation Factor at Different frequencies. Measurement of Dissipation Factor at low frequencies, between 20 and 400 Hz, enables the detection of moisture in bushing with high sensitivity.
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Maintenance Action Catalogue Electrical Measurements – Advanced Tests – Dissipation Factor at Different frequencies.
Indicative limits for Dissipation Factor in new and aged bushings at 15Hz, 50/60 Hz and 400 Hz.
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Maintenance Action Catalogue Electrical Measurements- Advanced Tests – Partial Discharge
Detects partial discharge, single insulation faults. Indication
DGA
Reference
Factory results, trend analysis
Interpretation
Not able to define limits for on-site tests. Used inconjunction with acoustic methods to localise PD.
Comments
PD is important to detect and locate a weakness in the insulation.The long term effect of PD is distructive. A significant increase in PD can provide early warning for developing defects. PD generate electromagnetic waves, acoustic signals, chemicals reaction, heat and light.
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Maintenance Action Catalogue Electrical Measurements – Advanced Tests – Frequency Response of Stray Losses (FRSL) Detects local overheating due to excessive eddy currents losses, short circuits between parallel strands of Continuously Transposed Conductors (CPC). Indication
High temperature indentified by DGA.
Reference
Comparision of three phases, factory / commissioning tests
Indications
Reactance – less than 2.5% between phases. Resistance – less than 15% between phases.
Comments
© OMICRON
Can be performed on LV if corrected by square of winding ratio.
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Maintenance Action Catalogue Electrical Measurements – Advanced Tests – Recovery Voltage Method
Detects moisture or aging in oil-paper insulation. Indication
High moisture in oil.
Reference
Fingerprint, trend analysis
Interpretation
“RVM in its present form can only be used for trend indication”
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Maintenance Action Catalogue Dissolved Gas Annalysis DGA is a very useful diagnostic tool and is universally applied. Normal gases measured are Hydrogen H2, Methane CH4, Ethane C2H6, Ethlyene C2H4, Acetlene C2H2, Carbon Monoxide CO, Oxygen O and Nitrogen N2. No consus on absolute maximum levels. The ratio of gases present can indicate particular problems whithin the transformer. The ratio method is discussed in detail in IEC60599. The Duval Triangle is also a popular tool to assist with the interpretation.
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Maintenance Action Catalogue Oil Tests
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Maintenance Action Catalogue On–Load Tap Changer
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Maintenance Action Catalogue Dynamic Resistance Measurement
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Maintenance Action Catalogue Dissolved Gas Analysis of Tap Changer Oil
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Maintenance Action Catalogue Additional OLTC Measurements
Motor Torque Measurement
Vibro-Acoustic Signature
OLTC Oil Tests
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Maintenance Inspection Tasks > Thermography > Main Tank and Conservator > Cooling System Oil Pumps Radiators and Fans Forced Oil-Air Coolers
> Accessories (TI’s, Level Indicators, Rapid Pressure Rise Relay, Pressure Relief Valves etc.) > Cabinets > OLTC > Buchholz Relay
© OMICRON
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Insulation Drying Risks and Consequences of Not Drying Insulation
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Insulation Drying Moisture is generated by: Remaining moisture when manufactured Humid air during transport and installation Humid air through breather Moisture ingress through gaskets Chemical decomposition of cellulose During maintenance Top up oil has moisture
To dry from 3% to 1 %
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Insulation Drying
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Topics – Guide for Transformer Maintenance > What is Cigre? > Overview of Guide > Maintenance Strategy > Maintenance Process > Transformer Component Selection and Maintenance > Maintenance Action Catalogue
> Major Work – Transformer Repair > Appendix
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Major Work – Transformer Repairs On-Site Repair Minor – replace bushings or OLTC, repair connections Major Exchange windings, repair core, upgrade
Advantages Avoid transport problems and risks Transport costs high Reduced down time depending on location.
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Major Work – Transformer Repairs Transformers are classed in 4 catagories to determine appropriate tests
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Major Work – Transformer Repairs Recomended Tests per Category
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Topics – Guide for Transformer Maintenance > What is Cigre? > Overview of Guide > Maintenance Strategy > Maintenance Process > Transformer Component Selection and Maintenance > Maintenance Action Catalogue > Major Work – Transformer Repair
> Appendix > Summary of Diagnostic Tests and Criteria
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Appendix 1 Survey results of Systematic Preventive Maintenance Practices
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Appendix 2 Summary of SF6 Transformer Particularities
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Appendix 3 Operation and Maintenance Manual Content
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Appendix 4 Maintenance Recording
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