Strategies for Diagnostic and Preventive Maintenance of Oil Insulated Instrument Transformers in the 110 kV Distribution Network

The expansion of renewable energies and the associated transformation of energy supply networks present significant challenges for energy supply companies. Integrating into existing networks, which often operate with aging technology, requires careful monitoring and maintenance to ensure operational safety. This paper offers insights into network operation practices, particularly within distribution networks, and discusses the importance of diagnostics and systematic, preventive replacement programs. Current transformers (CTs) at the 110 kV voltage level built between 1976 and 2005 were examined, which have recently shown occasional failures during normal operation. The focus of aging diagnosis is on dissipation factor measurements in the frequency domain (FDS) and partial discharge (PD) measurements in the test field after removal from the corresponding substations. This involves addressing aging symptoms that manifest in a frequency-dependent change in capacitance and oil analysis values while being free of partial discharges. In conclusion, an outlook on the application of diagnostic methods to other older types of transformers is provided. The derivation of a cyclical testing program is proposed, which allows for rapid diagnosis with minimal effort. Measurements in the frequency domain up to 200 V between 1 kHz and 100 mHz serve as a method for early detection. Finally, the risk to people and materials posed by operationally aged transformers is highlighted, and an outlook on the application of diagnostic methods to further older instrument transformer types is given.