To maintain the reliability of power transmission it is important to detect the incipient fault of power transformer as early as possible. If the fault of power transformer is not detected promptly, it can evolve resulting in high repair costs or even failure of power transformer. The most commonly used method for power transformer fault detection is the dissolved gas analysis (DGA) of transformer oil. Various methods have been developed to interpret the data of dissolved gas analysis, but not many are applicable for detection of incipient fault. In the framework of this research two standards were used: IEEE C57.104-2008 “Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers” and Standard of Latvian Electrotechnical Committee LEK 118 “Transformer Oil Inspection Standards”. Detection of the incipient fault of power transformer is included in both, however, approaches recommended in these standards vary to a great degree – one approach mostly indicates that transformers are in good condition, but the second approach mostly results in warning about incipient fault, which must be confirmed by additional evaluation. The objective of this paper is to determine the most suitable approach to detect the incipient fault of a power transformer. For this purpose DGA data of 48 oil-filled power transformers have been analysed. These transformers are with rated voltage 110/20 kV and rated power within the range from 6.3 MVA to 40 MVA; all of them have active on-load-tap-changer (OLTC). The DGA data has been provided by Latvian Transmission System Operator (Augstspriegumatīkls, JSC). Current DGA data interpretation approach described innational standard LEK118 states that the limits of fault gas concentration are divided into 3 levels: normal operation of a transformer; detection of incipient fault; detection of existing fault, and that the interpretation of DGA data is carried out by evaluating the concentrations of dissolved gases in oil. However, the case study indicate that only for 10 of 48 transformers condition is indexed as a normal operation, while the other transformers have been identified either with incipient fault or an existing fault. The obtained results conflict with the real maintenance situation since additional tests or repair works were not appointed based on DGA measurement results. Different multi-step approach based on ideas of standard IEEE C57.104 is proposed, where the first step is evaluation of value of total dissolved combustible gas (TDCG), followed by evaluation of individual fault gases in the second step, and additional analysis of acetylene concentration due to large proportion of faults in OLTC and trend of TDCG generation rate in the final step. The proposed approach allowed decreasing the number of transformers that were mistakenly diagnosed with existing fault, although their operation is normal. Study showed the necessity to adjust TDCG values given in national standard, therefore several variations of TDCG concentrations are proposed and tested in this paper as case studies.