- Slides: 29
Internal Arc testing of paper-oil insulated transformers Igor Žiger, univ. spec. transf. IEEE Transformer Committee meeting Atlanta, Georgia, 2016.
Test purpose and background § To discover, prevent or mitigate any major fault connected with an instrument transformer in service. § To demonstrate conformance of the transformer to a certain level of safety Major failure definition: Failure of equipment which causes the cessation of one or more of its fundamental functions. A major failure will result in an immediate change in the system operating conditions. Failures that caused fire or explosion A distinct subgroup of major failure category.
Information on transformer reliability § A Total of 3 Cigre trasnformer reliability surveys were performed I. Technical Brochure 057 (1970 -1986) II. Technical Brochure 394 (1985 -1995) III. Technical Brochure 512 (2004 -2007) § Technical brochure 512 general facts: - 1290335 Instrument transformer years - 25 Countries, 73 Utilities - 690 Major Failures - Service experience: MVT CT CCVT 13, 5 % 18, 7 % 64, 2 % 3, 5 %
Instrument transformer major failure rates § Major failure rates: Type of IT CT MVT Combined Failures per 100 IT-years 0, 077 0, 104 0, 107 0, 038 § Fire and explosion failure rates: Type of IT CT MVT Combined Failures per 100 IT-years 0, 0183 0, 0113 0, 0064 0, 0090
Major failure breakdown across transformer components § Major failure :
Major failure breakdown across transformer components § Fire and explosion failure :
Internal arc test definition § Introduced internal arc withstand capability testing. § IEC 61869 – Clauses 6. 9 and 7. 4. 6 § IEEE C 57. 13. 5 – Clauses 3, 4. 9 and 12. 2 § Test conditions: To simulate a complete breach of insulation accompanied by an internal arc 100 % of rated short-time thermal current with assymetry applied (usually 1. 7 times) Duration of the test: 0, 1 to 0, 5 seconds Location of arc inception: – The area of the maximum dielectric stress – Recommendation is to use either upper part of the insulation (for top core tranformers) or the bottom part (for hairpin current or closed-core voltage transformers)
Internal arc test definition § Test requirements: Iternal arc protection Class 1 – Permits the fracture of the transformer housing, accompanied by fire, but all debris must be contained within a designated area Iternal arc protection Class 2 – No external effect other than the operation of a suitable pressure relief device Both classes allow burn-through and/or transformer fire !
Reality check Class I example:
Reality check Class II example:
Reality check Extreme example: No insulator fracture
Issues with proposed testing Internal arc testing is inapplicable to paper-oil insulated tranformers: § Major failure origin and location analysed without proper context § Unrealistic scenario for oil-immersed capacitively graded insulation transformers with § Does not exclude all transformer explosions, only under certain conditions § Cost and logistics § Does not deal with failure cause, rather with failure consequence
The context for major failure occurence Service conditions when major faults occured Fire and Explosion Failures: Normal service - no switching command in the substation 6% Fault clearing operation in the substation 2%2% Normal service - during switching command in the substation During or immediately after testing / maintenance 86% Invalid response De-energised - available for service
The context for major failure occurence Service conditions when major faults occured Major Failures: Normal service - no switching command in the substation Fault clearing operation in the substation 3%1% 4% 2% Normal service - during switching command in the substation 14% During or immediately after testing / maintenance 76% Invalid response De-energised - available for service
The context for major failure occurence How the faults were discovered Planned visual inspection and maintenance System trip 4% 1% Monitoring system alarm 8% 11% 13% Other methods 63% Failure of similar equiment Unplanned site inspection 80 – 90 % of major transformer failures are slow by nature
Unrealistic scenario for oil-immered transformers § Very low probabilty of a sudden and complete breach of capacitively graded insulation § Arc inception location is inadequately suggested for oil immersed transformers § Covers only failures located within the transformer housing (either top or bottom)
Unrealistic scenario for oil-immered transformers Arc inception location IEC Reality
Unrealistic scenario for oil-immered transformers Arc inception location
Explosion safety under certain conditions § According to the experience of EDF – very low success rate when full short-circuit current is applied (recommendation for 80 % of the rms value) § Porcelain insulator fracutre can occur after the application of the current because of thermal shock § Transformer oil fire is permitted
What is the alternative ? - Explosion safe design -
Explosion safe design defintion Explosion-safe design entails the following § Stricter Partial Discharge Criteria § Additional special tests Multiple chopped (Endurance chopped wave) test Lifetime simulation tests § Regular transformer checks and continuous monitoring
Explosion safe design Stricter partial discharge criteria § Partial discharge < 10 p. C at Power Frequency Withstand Voltage for every routine test § Partial discharge < 10 p. C at Power Frequency Withstand Voltage after impulse testing (either routine or type test sequence) Example: reference measurement for PD extinction at 275 k. V instead of at 126 k. V
Explosion safe design PF withstand voltage Rated Voltage 20 yrs PD Free transformers @ PF withstand voltage for 1 min can be assumed to be PD free at rated voltage during time much higher than their entire proposed lifetime
Explosion safe design Lifetime simulation test on 123 k. V CT type AGU Insulation breakdown point
Explosion safe design Lifetime simulation test on 123 k. V CT type AGU This test confirmed theoretical background on most probable fault location.
Explosion safe design Regular transformer checks continuous monitoring § Regular visual inspection and insulation PF measurements § DGA testing of transformer oil (once in a few years) Continuous monitoring § 80 – 90% failures are slow by nature, and accompanied by pressure rise § By installing a simple overpressure switch, the user can get a timely alarm signal or even a trip signal for the protection scheme
Explosion safe design Transformer monitoring - examples Inductive Voltage Transformers Power Voltage Tranformers Combined Transformers Current Transformers
Conclusion § Internal arc testing is not a guarantee that the transfrmer will not have a fire and explosion failure § Worst-case scenario is not considered § Very item specific and is not a testament of safety on a routine test level § There are other more effective ways of ensuring a maximal transformer in-service security § Very costly destructive test