**1. Introduction**

The basic components of an oil-filled electrical transformer are copper windings and an iron core. The windings contain copper coil turns bundled together and wrapped with insulating paper. These components are entirely immersed in a mineral insulating oil in the transformer tank [1]. Overheating of the insulating oil in-service can occur due to increased current load, oxidation, and corrosion deposits on windings, or unexpected electrical and mechanical faults resulting in the formation of partial discharge (corona) and arcing phenomena, which in turn can generate flammable gases. The energy in these phenomena can be very high and develop temperatures in hundreds of degrees, though for a short time, causing fire and explosion accidents [2–5]. The transformer fires that contain several tons of mineral insulating oil can not only impact on the delivery of energy, but also on workplace health and the surrounding environment [6,7]. In [8], statistical data analysis of the failure rate of European substation transformers between 2000 and 2010 showed that around 9.5% of the total failures caused fire accidents and 3.3% caused explosion accidents. Another source of failure rates from 2015 [9] showed a significant increase in fire and explosion accidents in USA. These accidents occurred regardless of using the regular maintenance strategy during the transformer's useful life [2,10].

**Citation:** Jadim, R.; Kans, M.; Schulte, J.; Alhattab, M.; Alhendi, M.; Bushehry, A. On Approaching Relevant Cost-Effective Sustainable Maintenance of Mineral Oil-Filled Electrical Transformers. *Energies* **2021**, *14*, 3670. https://doi.org/10.3390/ en14123670

Academic Editors: T M Indra Mahlia and Islam Md Rizwanul Fattah

Received: 20 May 2021 Accepted: 18 June 2021 Published: 20 June 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The common regular maintenance strategy used for high voltage substation power transformers is condition-based maintenance (CBM) [3,11] which is defined according to EN 13306:2017 [12] as "preventive maintenance, which includes assessment of physical conditions, analysis and the possible ensuing maintenance actions". Oil analysis technology is commonly used in the condition monitoring (CM) parameters to detect faults, such as partial discharge, arcing, oxidation, and corrosion [2,13–15]. Fault detection is based on the concentration value level of the measurable variables, such as hydrogen gas, acetylene gas, acidity, hydrogen sulfide gas, toluene, and more, related to the standard caution limit (the maximum value that indicates fault incidence). The corrective action is carried out once the measured value of a measurable variable exceeds its caution limit [13–17]. Onsite electrical testing technology is also used in the maintenance strategy to identify the type and location of the fault. However, this technology is usually recommended at the transformer shutdown time for scheduled maintenance or when a fault is suspected [3,18]. Assessment of the transformer's condition, based on CM parameters, is a crucial key in the CBM. In this context, several mathematical models are utilized in modelling the deterioration status in order to assess the overall condition of transformers [11,19,20]. The drawbacks of these models are modelling the deterioration with a single path without considering the complexity of the deterioration in transformers [21], and the assessment of overall condition without providing information about the root cause of the fault [20].

Sustainable maintenance of transformers is considered a new challenge that requires an understanding of the process of sustainable development and the integration between social and environmental aspects [11,22,23]. The vital objectives of sustainable maintenance are extending the lifetime of the transformers and reducing the negative impacts in a way that contributes to the organization's own competitiveness [24]. Sustainable maintenance is defined according to [22] as "proactive maintenance operations striving for providing balance in social, environmental, and financial dimensions". Nowadays, investigations spotlight three technical methods for achieving sustainable maintenance. The first method [25], is selecting energy-efficient green transformers from manufacturers that perform best with sustainability aspects. The concept of "green" according to [26] is related to a consideration of the environmental aspect as well as the economic benefit. An example of this method is provided by qualified raw resources such as amorphous materials used in the core design, yielding a core with higher hardness compared to current common iron core (silicon steel laminations), resulting in reduced energy loss and greenhouse gas emissions [25]. The main shortcoming of this method is its availability only for the newly manufactured transformers. The second method [27–29] is replacing the mineral insulating oil with natural ester oil. The strength in using natural ester oil is the excellent specifications, such as higher cooling efficiency, lower inflammation risk due to high flash point value, and lower toxicity than mineral oil. The disadvantages of this oil are the low electrical resistivity, which reduces the insulation capability of the oil, and the high investment costs comparing with mineral oil [28]. The last method [30] is the waste oil recycle process to remove the hazard contamination. One of the vital techniques in this process is using the reduction procedure to remove the toxic chemical, such as polychlorinated biphenyl (PCB) from oil waste, which has negative impacts on human and environmental health [31]. In general, the most critical challenge for achieving sustainable maintenance is the investment costs of these available technical methods and the return on investment [25,27,30]. Therefore, it is crucial and necessary to establish a relevant sustainable maintenance with regard to minimized costs, i.e., relevant cost-effective sustainable maintenance. The purpose of this paper is to integrate sustainability perspective into the maintenance strategy.

Sustainability assessment in many organizations is still unclear for stakeholders [22,24]. The main reason for this challenge is the complications of carrying out the assessment due to the lack of a well-organized procedure and lack of competence in the integration of sustainability in the maintenance strategy. However, the sustainability assessment can be addressed by utilizing ABCD procedure of the Framework for Strategic Sustainable

Development (FSSD) [32–34]. The outcome of applying this procedure in a power plant was creating a model for early fault diagnosis to achieve the prioritized action for improving the maintenance strategy toward sustainability.
