Citation: Ismail, F.B.; Mazwan, M.; Al-Faiz, H.; Marsadek, M.; Hasini, H.; Al-Bazi, A.; Yang Ghazali, Y.Z. An Offline and Online Approach to the OLTC Condition Monitoring: A Review. Energies 2022, 15, 6435. https://doi.org/10.3390/en15176435 Academic Editors: Albert Smalcerz and Marcin Blachnik Received: 12 July 2022 Accepted: 16 August 2022 Published: 2 September 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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/). energies Review An Offline and Online Approach to the OLTC Condition Monitoring: A Review Firas B. Ismail 1, * , Maisarah Mazwan 1 , Hussein Al-Faiz 1 , Marayati Marsadek 1 , Hasril Hasini 1 , Ammar Al-Bazi 2 and Young Zaidey Yang Ghazali 3 1 Power Generation Unit, Institute of Power Engineering (IPE), Universiti Tenaga Nasional, Kajang 43000, Malaysia 2 School of Mechanical, Aerospace and Automotive Engineering, Coventry University, Coventry CV1 5FB, UK 3 Asset Management Department, Tenaga Nasional Berhad, Kuala Lumpur 59200, Malaysia * Correspondence: firas@uniten.edu.my Abstract: Transformer failures have a significant cost impact on the operation of an electrical network. In many utilities, transformers have been operating for many years past their expected usable life. As power demand has surged, transformers in some areas are being loaded beyond their rated capacity to meet the demand. One of the vital components in a transformer is the on-load tap changer (OLTC), which regulates the voltage in the distribution network. This study aims to review several condition-monitoring techniques (online and offline) that can monitor the health of the OLTC and assure the safety of the transformer’s OLTC from irreparable damage by detecting the defect at an earlier stage, which is preceded by the specification of typical faults. This paper also discussed the common faults of the OLTC and the root causes of these faults. The OLTC is prone to mechanical faults due to its frequently changing mechanism in the tap operation. The OLTC are also prone to oil as well as thermal faults. As a result, it is critical to monitor OLTC conditions while they are in use. Proper management of condition monitoring (CM) for the OLTC is useful and necessary to increase availability and achieve optimised operating. Condition monitoring (CM) and diagnostics methods (DM) have been developing since the 1950s. CM and DM have been implemented to diagnose and detect an incipient fault, especially for the OLTC. Many techniques, online and offline, are being used to monitor the condition of the OLTC to prevent failure and minimize outages. These DM and CM will prolong the operational cycle and avoid a major disaster for the OLTC, which is an unfavorable scenario. Keywords: on-load tap changer (OLTC); condition monitoring; diagnostic methods; incipient fault; transformer; energy; electricity distribution 1. Introduction of Condition Monitoring for the OLTC An on-load tap changer (OLTC) is a critical component as it adjusts the tap positions to regulate the transformer’s output voltage in response to changes in power generation and consumption. When the OLTC transitions from one tap position to another during operation, the switching contacts are subjected to a high load current, which can eventually lead to a considerable decrease in the conductivity of the contacts and may cause damage to these contacts. Damage to switching contacts and other components can cause the OLTC to malfunction and fail, reducing the transformer’s reliability. Several surveys have found that OLTC faults account for a significant portion of transformer failures [1–5]. As a result, utilities would benefit substantially from establishing robust OLTC CM strategies. Studies, as in Figure 1, show that about 30% of substation power transformer reported outages are related to the ageing effects on OLTCs. Due to this high failure rate, closely monitoring the power transformer’s OLTC condition is critical. Unlike other more static components in a transformer, the OLTC consists of numerous moving parts. Manufacturers typically recommend a maintenance cycle that mostly depends on the total number of Energies 2022, 15, 6435. https://doi.org/10.3390/en15176435 https://www.mdpi.com/journal/energies