An Offline and Online Approach to the OLTC Condition Monitoring: a Review

Firas B. Ismail*, Maisarah Mazwan, Hussein Al-Faiz, Marayati Marsadek, Hasril Hasini, Ammar Al-Bazi, Young Zaidey Yang Ghazali

*Corresponding author for this work

Research output: Contribution to journalReview articlepeer-review


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.

Original languageEnglish
Article number6435
Issue number17
Publication statusPublished - 2 Sept 2022

Bibliographical note

Funding Information:
The authors would like to acknowledge the financial support provided by Tenaga Nasional Berhad Seeding Fund with the project code: U-TD-RD-21-07, that is managed by UNITEN R&D Sdn. Bhd.

Funding Information:
The authors would like to acknowledge the publication support through TNB Seeding Fund provided by Tenaga Nasional Berhad (TNB), Project No. U-TD-RD-21-07.

Publisher 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 (


  • condition monitoring
  • diagnostic methods
  • electricity distribution
  • energy
  • incipient fault
  • on-load tap changer (OLTC)
  • transformer


Dive into the research topics of 'An Offline and Online Approach to the OLTC Condition Monitoring: a Review'. Together they form a unique fingerprint.

Cite this