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 ( ) II. Technical Brochure 394 ( ) III. Technical Brochure 512 ( )  Technical brochure 512 general facts: Instrument transformer years - 25 Countries, 73 Utilities Major Failures - Service experience: MVTCVTCTCCVT 13,5 %18,7 %64,2 %3,5 %

Instrument transformer major failure rates  Major failure rates:  Fire and explosion failure rates: Type of ITCTMVTCVTCombined Failures per 100 IT-years 0,0770,1040,1070,038 Type of ITCTMVTCVTCombined Failures per 100 IT-years 0,01830,01130,00640,0090

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 – Clauses 6.9 and  IEEE C – 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 transformers with capacitively graded insulation  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:

The context for major failure occurence Service conditions when major faults occured Major Failures:

The context for major failure occurence How the faults were discovered 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 IEC Reality Arc inception location

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 the 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 pC at Power Frequency Withstand Voltage for every routine test  Partial discharge < 10 pC at Power Frequency Withstand Voltage after impulse testing (either routine or type test sequence) Example: reference measurement for PD extinction at 275 kV instead of at 126 kV

Explosion safe design PD Free 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 20 yrs Rated Voltage PF withstand voltage

Explosion safe design Lifetime simulation test on 123 kV CT type AGU Insulation breakdown point

Explosion safe design Lifetime simulation test on 123 kV CT type AGU This test confirmed the 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 Combined Transformers Current Transformers Inductive Voltage Transformers Power Voltage Tranformers

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