**1. Introduction**

Oil-submerged cellulose paper is extensively used as a solid dielectric material in electrical power transformers [1]. During the operational lifetime of the power transformer, cellulose paper progressively decomposes on account of the multi-pronged stresses it is susceptible to [2]. Considering that the insulation condition is a crucial factor to ascertain the reliable operation of power transformers, assessment of the insulation condition has obtained considerable attention from various researchers in the power industry [3]. The effective residual duration of the operation of a power transformer is determined in keeping with the condition of its paper insulation by measuring the degree of polymerization (DP) [4]. According to [5], the earliest DP value of newly manufactured cellulose paper is in the range between 1000 and 1200, which plunges to about 200 to 300 once the cellulose paper attains the end of its lifecycle. Whereas the DP is the most artful indication to examine the decomposition of a cellulose paper insulation, it is seldomly employed by the manufacturing industry due to the intricacy of harvesting the physical paper samples, particularly from hotspot regions of the unit in-service. Intrinsically, considerable research attempts have been performed to associate the DP value with various measurable dissolved-in oil indicators [6–10]. Cellulose is a straight polymer of glucose molecules inextricably linked

**Citation:** Thango, B.A. Feedforward Artificial Neural Network (FFANN) Application in Solid Insulation Evaluation Methods for the Prediction of Loss of Life in Oil-Submerged Transformers. *Energies* **2022**, *15*, 8548. https:// doi.org/10.3390/en15228548

Academic Editors: Najib El Ouanjli, Saad Motahhir and Mustapha Errouha

Received: 14 October 2022 Accepted: 13 November 2022 Published: 15 November 2022

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to glycosidic bonds [5,6]. The cause of towering thermal stresses, hydrogen bonds have the propensity to disintegrate, which is caused by the reduced cellulose molecular chain and the formation of chemical derivatives that are then suspended in the dielectric oil. Furanic compounds and moisture are the principal derivatives of cellulose decomposition [7,8].

In recent times, several DP and LOL models for power transformers have been presented, however, no model has been universally accepted. It is critical to identify the major impediments. Previous reviews in this context have concentrated on a single solution based on specific case studies and alternative routes that can be taken by the manufacturers based on data availability have not been addressed. To corroborate the contribution to the current research, a summary of recent research works is tabulated in Table 1.


**Table 1.** A summary of recent research works.

It can be observed from the most recent works that the methodologies proposed herein have not been reported. It is therefore vital to disseminate the proposed methods to support transformer manufacturers and dielectric laboratories with reliable alternatives to these methods. In this work, the remnant loss of life of the cellulose paper insulation was predicted by proposing two FFANN approaches. The first model is based on predicting the DP when only the 2-furaldehyde (2FAL) concentration measured from the oil samples is available for new and existing transformers. The second FFANN model proposed is based on predicting the transformer LOL when the 2FAL and DP are available to the utility owner.
