*3.3. Early Fault Diagnosis (EFD) Model—Concept and Verification*

The suggested early fault diagnosis (EFD) model is established to approach relevant cost-effective sustainable maintenance based on a fault trend chart that shows the faults' progression during the useful life of the transformers. This chart is created based on the

outcome of the numerical method for early fault detection. In order to verify the outcome of the numerical method throughout the fault trend chart, four transformers with suspected failure propagation, of the PSMD power plant from the category "Start Fault" were selected based on covering all fault types to follow up their progression. The measurable variables that used in this chart were hydrogen gas, acetylene gas, acidity, hydrogen sulfide gas, and toluene to detect anticipated arcing, partial discharge, oxidation, and corrosion faults. The standard methods used for analysis of these measurable variables are demonstrated in Table 1. The uncertainty (Uexp.) of five repeated measurements of each measurable variable; hydrogen gas, acetylene gas, hydrogen sulfide gas, toluene, and acidity, was ±3.6 of mean value 104.4 ppm, ±0.37 of mean value 10.1 ppm, ±0.035 of mean value 0.51 ppm, ±0.063 of mean value 1.51 ppm, and ±0.031 of mean value 0.10 mgKOH/g, respectively. Furthermore, the relative standard deviation (RSD%) for same measurements was 3.2%, 3.8%, 3.1%, 3.6% and 3,2%, respectively. The historical data of these four transformers has been gathered since December 2018. Measured values for each transformer were recorded with three months interval, in March, June, September, and middle of November and December 2019 (see Figure 3).

**Figure 3.** Fault trend chart of the four transformers with suspected failure propagation. CL= Caution Limit; WL= Warning Limit; *w*1 = First measured value; *w*2 = Second measured value. (**a**) Chart of "Arcing Fault" for the generation of the gases hydrogen and acetylene in the first transformer registered serial number 107772 (Power rate = 20 MVA, HV = 33 kV); (**b**) Chart of "Partial Discharge Fault" for the generation of the hydrogen gas in the second transformer registered serial number 20081807 (30 MVA, 132 kV); (**c**) Chart of "Oxidation Deposits Fault" for the generation of the acidity in the third transformer registered serial number 3247PG1548 (300 MVA, 275 kV); (**d**) Chart of "Corrosion Deposits Fault" for the generation of the hydrogen sulfide gas and toluene compound in the fourth transformer registered serial number 07MD970101 (30 MVA, 132 kV).
