The practical field measurement challenges of transformer FRA and developing the fundamentals for an on-line approach

One of the most effective and powerful tools for the diagnosis of poor mechanical integrity of transformer windings is Frequency Response Analysis (FRA). For large transformers within a power network, which are often critical and expensive, FRA adds enormous value in the diagnosis of winding defects. Which can easily result in the catastrophic failure of a transformer. FRA due to its non-invasive nature and relatively fast assessment, has become an essential tool for the monitoring of transformer winding structural integrity. Enabling utilities to maintain and monitor power transformer windings in a cost-effective manner to minimize the likelihood of an unexpected and costly outage. Transformers are only designed to withstand certain levels of mechanical forces which can be surpassed during short circuit events, transport mishaps or loss of clamping pressure; which may result in winding displacement and deformation. Defects can also occur due to transformer aging, through short circuits and over voltages. To date, FRA has been almost exclusively utilized off-line, which requires a transformer to be completely isolated from the network. However, there is a need to move from off-line to on-line measurement due to the growing importance in the development of smart grid systems. In this study, FRA sensitivities have been investigated through case studies, which inform interpretation of FRA for both off-line and on-line measurements. Real scenarios such as conducting measurements on GSU transformers at power stations, autotransformers at substations and measurements on a rectifier transformer at an Aluminium smelter company have been investigated. This thesis proposes a bushing tap signal injection method using high frequency Current Transformers (CT). The developed technique has the potential to be used for on-line monitoring of power transformers. The technique uses a wide-band signal coupled into the transformer winding via a high frequency CT. The response signal is also measured through another higher frequency CT at a corresponding phase bushing. This non-invasive monitoring tool provides a safe means to access an in-service power transformer, which makes it feasible for permanent onsite monitoring of power transformers.