New Frontiers of DGA Interpretation for Power Transformers and their Accessories Michel Duval

1. New Frontiers of DGA Interpretation for Power Transformers and their Accessories Michel Duval November 16, 2012

2. Risk of failure of transformers The risk of failure of transformers in service, as indicated by dissolved gas analysis (DGA), depends on three main parameters: -the type of fault involved (e.g., arcing or hot spot). -the location of the fault (in oil or in paper). -the amount of gases formed (concentrations and rates).

3. The most dangerous faults: -high-energy arcing D2 in oil and paper. -low-energy arcing D1 in paper. -hot spots of high temperature T3 and T2 in paper.

4. The less-dangerous faults: -low-energy arcing D1 in oil. -hot-spots T3 and T2 in oil. -hot spots T1 in paper.

5. The non-dangerous faults: -hot spots T1 in oil. -corona partial discharges PD (unless very large amounts of hydrogen are formed). -catalytic reactions of free water in drain valves. -aging of paper at low temperatures.

6. Identification of faults: -general faults (PD, D1, D2, T1, T2, T3) can be identified by several methods (Rogers, Dornenburg, Key Gas, IEC ratios, Duval Triangle 1). -faults in paper are more difficult to identify. CO, CO2 and the CO2/CO ratio are generally used for that purpose. However, it has been shown recently at CIGRE that they are not always good indicators.

7. Limitations of CO and CO2: -large amounts of CO (>1000 ppm) and low values of the CO2/CO ratio (<3) are often observed in closed transformers without any fault. -this has been attributed at CIGRE to the low availability of O2, which favours the formation of CO over CO2.

8. Limitations of CO and CO2: -also, localized faults involving small volumes of paper often do not produce detectable amounts of CO and CO2 against the usually large background of these gases in oil. -in many cases, local faults with carbonization of paper are more reliably detected by the formation of the other gases (H2 and hydrocarbon), using for example the Duval Triangles 4 and 5.

9. Duval Triangle 4 (updated 2012) Zones: -S: stray gassing of oil (T<200C). -O: overheating, T<250C. -C: possible carbonization of paper (probability of 80%, not 100%). -PD: corona partial discharges (change C2H6 zone boundary from 1 to 0.6, and verify it with stray gassing tests in the lab). (Graph courtesy: Serveron)

10. Duval Triangle 5 (updated 2012) Zones: -T3 and T2: hot spots in oil only (T>700C and >300C). -C: possible carbonization of paper (probability of 90%, not 100%). -O, S, PD, use rather Triangle 4. (Graph Courtesy: Serveron)

11. Use of Triangles 5 and 4: -Triangles 5 and 4 should be used only after having identified first the general type of fault (D1, D2, T3, T2, T1, PD) with Triangle 1. -Triangles 4 and 5 should never be used in case of electrical faults D1 or D2. 11

12. Use of Triangles 5 and 4: -Triangle 5 should be used only in case of thermal faults T3 or T2, to distinguish between faults with carbonization of paper and faults in oil only. -Triangle 4 should be used only in case of faults PD, T1, T2 or high levels of H2, to make further distinctions between these low-energy faults. - A small algorithm is available for free for Triangles 1 to 7 from duvalm@ireq.c. 12

13. Recommendations of IEC for CO, CO2 -in IEC 60599, presently under revision: -in open transformers, high CO values (>1000 ppm) and low CO2/CO (<3) ratios indicate faults in paper. -in closed transformers, this also applies, but only if other fault gases (Cn, H2) are formed. -high CO2 values (>10,000 ppm) and CO2/CO ratios (>15) indicate accelerated paper aging. 13

14. Transformer aging -the risk of failure of transformers indeed increases with age and load. -however, in a majority of cases, this is because of high moisture content, gas-producing faults, leaking gaskets and membranes, rusting of tank and dielectric problems, not so much because of paper aging.

15. Transformer aging -for instance, a large number of transformers have been reported to operate quite normally with DPs of paper between 200 and 100. -also, no or very few convincing cases have been reported so far, of transformers with a DP of 200 where failure could be clearly attributed to the mechanical weakness of paper.

16. Transformer aging -so, based on present observations, the risk of failure at low DPs of paper appears to be rather low, not very high as generally believed. -transformers with gas-producing faults are more at risk, and should be replaced in priority. -for replacements based on DP only, a limit value of 100 or 150 appears more realistic than 200, based on present observations.

17. Transformer aging -aging paper loses density and thickness, but this can easily be compensated by re-clamping windings. -brittle paper squeezed between windings turns (where it is most needed) will not fall off or tear away, because of the compression forces applied to windings, even if its tensile strength is low. -and it can easily resist compression forces down to very low DPs, without creeping.

18. Gas limits in service -once the type and location of fault have been identified, the next parameter to look at is the amount of gases formed. -gas limits in the IEEE Gas Guide C57.104 are presently under revision. The most significant change will be much higher values of CO and CO2 (750 and 7500 ppm) for condition 1. Present values are often a cause of false alarms. 18

19. Gas limits in service -limits for gas concentrations and rates above IEC typical (condition 1) values have also been recommended by CIGRE (in TB # 443, 2010). -CIGRE (WG47) is now investigating the effect of type and location of faults on these gas limits. 19

20. Effect of type of faults on gas limits -the effect on typical (condition 1) values is obtained by calculating 90% percentile values for each type of fault. -the effect on pre-failure (condition 4) values is deduced from inspected fault cases where high levels of gases are observed without failure or just before failure. -specific types of faults are identified first with Triangles 1, 4 and 5. 20

21. CIGRE condition 1 values vs. type of faults: 21

22. Occurrence of types of faults in service: 22

23. Examples of high gas formation without failure current transformer with 6400 ppm C2H4! power transformer with a fault D1 in oil (on a bakelite plate, with 480 ppm C2H2! 23

24. Examples of the effect of fault type on conditions 1 and 4 24

25. Effect of fault type on CIGRE condition 4 -to confirm these values, other examples of the following fault cases would be needed: -arcing faults D1/D2 in oil with high levels of C2H2 and no failure, -arcing faults D1/D2 in paper just before failure, -thermal faults T1, T2 and T3 in paper with high levels of C2H4, C2H6 or CH4 and no failure. -please email such examples to duvalm@ireq.ca. 25

26. Application of DGA to LTCs -a significant percentage (~30%) of failures of transformers in service is due to faults in LTCs. -dissolved gas analysis (DGA) is therefore increasingly used to detect such faults in LTCs.

27. Application of DGA to LTCs -what makes the interpretation of DGA in LTCs more complex than in transformers is that their normal operation involves several different LTC components, depending on their type and operating conditions. -normal gas formation resulting from this normal operation of LTCs must be identified first as precisely as possible in service.

28. Normal gas formation/ operation of LTCs May involve: -arc-breaking-in-oil between contacts, switching of selectors and valves, with the formation of arcing gases D1(C2H2). -current dissipation in transition resistors, increasing their temperature and producing thermal gases in oil (C2H4/ CH4/ C2H6).

29. Normal gas formation/ operation of LTCs -depending on the specific type and design of LTC used (in oil or vacuum, of the compartment type or in-tank type) and its operating conditions (at low or high powers or currents), different mixtures of these gases may be formed during normal operation of the LTC.

30. Faulty or abnormal gas formation/ operation of LTCs May involve, among others: -an increase in the resistance and temperature of arc-breaking contacts, resulting in carbonization of oil, coking of contacts and formation of thermal gases T3 or T2. -abnormal arcing on metal plates or components, producing arcing gases D2 or D1. -abnormal hot spots on LTC enclosures or vacuum bottles, producing thermal gases.

31. Identification of faults in LTCs by DGA Two main methods are used: -the C2H4/C2H2 ratio of IEEE. -the Duval Triangles 2, allowing to: -follow visually the evolution of faults in LTCs, -detect faulty operation in some specific types of LTCs, thanks to the use of a third gas (CH4).

32. The Duval Triangle 2 for LTCs of type I -the original Duval Triangle 2 applies to LTCs of type I, representing a large majority of LTCs presently in service: -examples of LTCs of type I include those of Westinghouse, ABB (except some UZBs, which are of type II), General Electric, McGraw Edison, Siemens, MR-Reinhausen (RM, T and V), Federal Pacific, Ferranti, Hyundai, Cooper, etc.

33. The Duval Triangle 2 for LTCs of type I

34. Example of normal and faulty operation of LTCs of type I

35. Example of normal and faulty operation of LTCs of type I

36. Example of normal and faulty operation of LTCs of type I

37. Example of normal and faulty operation of LTCs of type I

38. Example of normal and faulty operation of LTCs of type I

39. Example of normal and faulty operation of LTCs of type I

40. Example of normal and faulty operation of LTCs of type I

41. The Duval Triangles 2a-2e for LTCs of type II -in LTCs of type II, representing a minority of LTCs in service, gas formation may appear in zone N or in another part of Triangle 2, depending on the specific type and operating conditions of the LTC. -the Duval Triangles 2a-2e have been developed for these LTCs of type II.

42. The Duval Triangles 2a-e for LTCs of type II

43. The Duval Triangles 2a-e for LTCs of type II -Triangle 2a applies to LTCs such as OilTaps M and D. -normal gas formation appears either in zone N or zone N1 depending on operating conditions, and should be determined in service first. -faulty or abnormal operation is indicated by gas formation in zones X3 or T3 (from zone N), or in zones T3 or T2 (from zone N1).

44. Example of normal and faulty operation of LTCs of type II (2a)

45. The Duval Triangles 2a-e for LTCs of type II -Triangle 2b applies to LTCs such as VacuTaps VR. Normal gas formation appears in zone N2 (not in zone N so far). -Triangle 2c applies to LTCs such as VacuTaps VV. Normal gas formation appears either in zone N or zone N3 (at high powers), and should be determined in service first. Faulty or abnormal operation is indicated by gas formation in zones X3 or T3 (from zone N), or in zone T3 (from zone N3).

46. VacuTaps VR (2b): Normal operation in zone T2 is probably due to operation of selectors and resistors. Ref: R.Frotscher (Orlando)

47. VacuTaps VV (2c): During high power tests, normal (in blue) and faulty operation (in red) of resistors. In service, normal operation of selectors Ref: R.Frotscher, CIGRE

48. The Duval Triangles 2a-e for LTCs of type II -Triangle 2d applies to LTCs such as OilTaps R and V. Normal gas formation appears either in zone N or zone N4 (mild overheating). -Triangle 2e applies to LTCs such as OilTaps G (operating at high currents), or in a few resistive UZBs of ABB, depending on operating conditions. Normal operation appears either in zone N or zone N5, and should be determined in service first.

49. Oil Taps R (2d):

50. Oil Tap G (2e) (OilTaps G are high-currents application models)