Identifying faults in oil-filled transformers is mainly done by applying the dissolved gas analysis (DGA) method. In 2019 CIGRE published a technical bulletin TB 771 , which explains advanced techniques for DGA interpretation this method takes into account the type and location of the faults to establish acceptable and dangerous levels of gas present in the transformer.
To accurately determine the location of arcing faults, (in oil or paper), TB 771 recommends the use of acoustic levels, CO2/CO ratios as well as the furan content of the oil. Acoustic tests might however be unavailable, too expensive, or might pose a risk due to the arcing.
The CO2/CO ratio can be misleading in many instances as it is influenced by various factors such as the oxidation of oil in sealed units , normal aging of paper, and to a very small degree by the arcing.
Thirdly, arcing faults might not produce a high concentration of furans to be detectable. 
Cases of transformer arcing were studied as per the available databases at IEC  and CIGRE  to see if there is the production of specific gases in the fault zones D1 and D2 of the Duval Pentagon 1 or 2. The method described below is a new diagnostic tool made available by Michel Duval and Jerzy Buchacz and forms part of the diagnostic toolbox provided by the Duval Pentagons. This method provides an early warning of a serious fault involving arcing in the paper.
Arcing faults D1 and D2 in the paper insulation of transformers
Fig 1: Paper failure of the transformer core due to electrical fault.
Nobody wants this to be their transformer.
Table 1 is actual data taken from the IEC TC 10  and CIGRE WG 47  database of cases of low energy arcing faults, D1 in the paper insulation of transformers.
In Figure 2 the inspected cases of high-energy arcing faults D2 in the paper insulation are plotted as per IEC TC .
Arcing faults D1 and D2 in the Oil of Transformers
Cases of low-energy arcing faults D1 in the oil of transformers in DGA are plotted in Figure 3, based on data from the IEC TC 10  database.
Cases of high-energy arcing faults, D2 in oil of transformers are plotted in Figure 4, this is based on data from the IEC TC 10  database.
The cases of low-energy arcing faults, D1 in the oil, in the DGA database of CIGRE WG 47, occur in the same area as Zone D1 as in Figure 3. However, they are not included here.
Arc-Switching in the Oil of On-Load Tap Changers
Under normal operating conditions, oil-type on-load tap changers (OLTCs) will involve arc-switching. Therefore they can serve as good examples of arcing taking place in oil because there is no paper present in the on-load tap changer compartment. Figure 5 represents the cases as identified "normal operation" for the OLTCs of the compartment type, data from the IEC TC 10 of 2002  were used.
New Sub-Zones of Pentagons 1 and 2 for Arcing Faults D1 and D2 in the Paper Insulation of Transformers.
When we compare Figures 1 to 5, it is clear that the arcing faults in the paper insulation and in the oil occur in different sub-zones of Pentagons 1 and 2. These new sub-zones are indicated in Figure 6.
The symbol (-H) was used in Pentagon 2  to high-temperature faults, T3, in oil only thus (T3-H). (-H) was assigned to Figure 6 to indicate arcing faults - D1 and D2 in oil, thus (D1-H) and (D2-H), the continuation of the same symbol ensures consistency and ease of interpretation. To indicate arcing faults in Paper, the symbol (-P) was used. This is then indicated as (D1-P) and (D2-P) on Pentagon 2. If the gas formation indicates an area in sub-zone D1-P or D2-P of Figure 6, it might be very possible that the arcing fault is in the paper, but this is not 100% certain, it might be that a few arcing faults present in oil might show up in these areas.
Arcing faults in paper will never show up in D1-H and D2-H, no false negatives were reported for these regions. It was noticed that the on-load tap changers of the in-tank type (MR) might often produce gases in the D2-H sub-zone.
It is advised that the arcing faults in the paper should be confirmed by other observations, like acoustic tests and CO2/CO rations. As mentioned previously, the CO2/CO ratio might be misleading because of the influence of other factors apart from arcing. Acoustic tests are more efficient but less sensitive to arcing faults inside the coil due to the high attenuation of acoustic signals by the winding and insulation barriers.
It is quite important to differentiate between an arcing fault in the oil and one in the paper due to the practical implications. It has been shown by CIGRE  that the dangerous (pre-failure) concentration value of C2H2 ( Acetylene) in the case of arcing faults D1 in oil is 1400 ppm, whereas it is only 45 ppm for arcing faults D1 in paper. This can cause a major failure if the diagnostician does not differentiate correctly between the two scenarios.
As per Figure 6, new sub-zones have been identified in the Duval Pentagons. This amendment allows for the identification of arcing faults D1 and D2, in the paper or in the oil as this differentiation is crucial to ensure accurate diagnostic results to the customer. This was previously only possible through expensive acoustic testing, or somewhat unreliable CO2/Co ratios.
This enables maintenance efforts to concentrate on the units that are identified as critical due to arcing in the paper. This scenario is in the minority but potentially extremely dangerous. Arcing in oil is much more common and not as concerning from an asset reliability viewpoint, depending on the location of the fault and the gas production rate. In some cases arcing in oil might be of concern and would require further investigation.
 CIGRE D1/A2.47, "Advances in DGA interpretation." CIGRE Technical Brochure # 771, Paris, France: CIGRE, Jul 2019
 Mineral Oil-Impregnated Electrical Equipment in Service Guide to the Interpretation of Dissolved and Free Gases Analysis, IEC Standard 60599, 2015.
 M. Duval and A. de Pablo, " Interpretation of gas-in-oil analysis using new IEC publication 60599 and IEC TC 10 databases." IEEE Electr. Insul. Mag., Vol 17, no 2, pp 31-41, https://ieeexplore.ieee.org/document/917529 , 2001.
 Guide for the Interpretation of Gases Generated in Mineral Oil-Immersed Transformers, IEEE Standard C57.104-2019.
M Duval and L. Lamarre, "The Duval Pentagon - A new complementary tool for the interpretation tool for the interpretation of dissolved gas analysis in transformers," IEEE Electr. Insul. Mag., vol 30, no. 6, pp 9-12, https://ieeexplore.ieee.org/document/6943428 , 2014
 M. Duval, "Use of Duval Pentagons and Triangles for the interpretation of DGA in electrical equipment." presented at the TechCon North America Conf., Albuquerque, NM, 2016.
 M. Duval, " A review of faults detectable by gas-in-oil analysis in transformers." IEEE Electr. Insul. Mag, vol 18, no 3, pp 8-17. https://ieeexplore.ieee.org/document/1014963