**4. Discussion**

The purpose of applying ABCD procedure in the PSMD power plant was to assess the sustainability level of the transformers and find the gaps to approach the vision of providing sustainable power with regard to minimized costs. The outcomes of this investigation can be summarized as follows:

	- Figure 3a represents the generation of hydrogen and acetylene gases before exceeding the caution limits (CLs) of 150 and 20 ppm, respectively. The daily trend after one month was >0.33%, which indicates that the arcing fault is in progress. Accordingly, carrying out arcing measurement was recommended to find the source of overheating. In the arcing measurement, a high-frequency current transducer clamping on the transformer's grounding cable was used to detect the source and site of the arcing [48].
	- Figure 3b represents the generation of hydrogen gas before exceeding the CL of 150 ppm. The daily trend after one month was <0.33%, which indicates that the partial discharge fault is not in progress. However, if there is an indication of a partial discharge progression, the recommended corrective action could be to carry out a partial discharge test [49,50], utilizing infrared thermal imaging technology [51] to find the source of overheating and the site of the expected flashpoint in the transformer.
	- Figure 3c represents the formation of acidity before exceeding the CL of 0.15 mgKOH/g. The daily trend after one month was >0.33%, which indicates that the oxidation fault is in progress. Accordingly, adding an antioxidant additive in the insulating oil was recommended to suppress the oxidation reaction [52].
	- Figure 3d represents the generation of hydrogen sulfide gas and toluene compound before exceeding the CLs of 1 and 2 ppm, respectively. The daily trend after one month was <0.33%, which indicates that the corrosion fault is not in progress. However, if there is an indication of a corrosion progression, the recommended corrective action could be adding an anticorrosion additive in the insulating oil to suppress the corrosion reaction [53].

As the measured value of the hydrogen was 125 ppm in the example, which is below caution limit (CL= 150 ppm) but above the alarm limit (AL= 120 ppm) (see Table 2), the relative fault detection value (RFDV) of hydrogen gas value should be calculated according to Equation (4),

$$\text{RFDV} = (w1 - \text{WL})/\text{WL} = (125 - 75)/75 = 0.675$$

The REDV was >RAT (0.60), so it was recommended to take another oil sample after one month (30 days) for analysis of the second measured value (*w*2). The value of *w*2 was found 138 ppm. Then the daily trend should be calculated according to Equation (5),

DT = ((*w*2 − *w*1)/*w*1) ∗ 100)/nd = ((138 − 125)/125) ∗ 100)/30 = 0.35%

As the DT of hydrogen gas value was >0.33%, there is a strong indication of starting a partial discharge fault. Accordingly, it is recommended to start an investigation to solve the problem before fault's progression to a risky level. This example shows that the corrective action could be carried out before a measured value exceeds its caution limit. The flow diagram in Figure 4 elucidates the process steps of using the numerical method in the maintenance strategy.

**Figure 4.** Flow diagram of the numerical method. *w*1 = First measured value, *w*2 = Second measured value CL = Caution Limit, WL = Warning Limit, and RFDV = Relative Fault Detection Value.

The flow diagram starts with oil analysis of the first measured value (*w*1) representing any related measurable variable. There are three alternative evaluation outcomes for the *w*1. The first is *w*1 less than WL, which reveals good condition of the transformer, hence a new oil sample could be recommended annually according to [15] or according to the organization maintenance plan. The second is *w*1 more than CL, which indicates a fault in progress, i.e., fault has already occurred and may lead to fire and explosion accidents. The last is *w*1 more than WL and below CL, which reveals the probability of having a fault in the initial stage. In this last alternative, the numerical method could be applied to assess the fault level and carry out appropriate corrective action to prevent fault incidence. If the fault is not in the risky level, the oil sample could be recommended for analysis with three months interval according to [2], in order to gather sufficient data for an accurate evaluation of the fault's progression. The generic procedure described in Figure 4 provides a structured process for CBM purposes using any measurable variable at any caution limit. The main advantage of applying the numerical method is the possibility of early faults detection before progression to a risky level. The disadvantage is the requirement of historical data to track the fault level.


**Table 3.** Matrix of capabilities and shortcomings in the technical methods of sustainable maintenance.


6. For further verification and validation, the model will be applied for tracking the fault type "copper corrosion" on 84 transformers at the PSMD power plant. For future work, the application of the EFD model can be extended to another field, such as any asset containing oil in production manufacturing. Future work could also involve investigations to find a solution to the insufficient historical oil analysis data of transformers to be used in the EFD model.
