Online detection of windings distortion in power transformers by direct vibration measurement using a thin fiber optics sensor

Abstract

Winding distortions can be mechanically induced during transportation, by large transient currents caused by lightning events or short-circuits, or as a result of gradual clamping looseness as the transformers age, after many loading and unloading cycles. Winding distortions lead to increased electromagnetic forces between the windings and the core, giving rise to changes in vibration signatures that could be useful as online condition monitoring tool. Unfortunately, this method is difficult when using electrical sensors mounted on the tank walls, i.e., vibration signals are highly dependent on mounting positions, and contaminated by noise from oil pumps, cooling fans, or the oil tank itself. For the first time, a thin fiber optics sensor has been developed to fit the 2 mm gap between the winding and the slot for direct winding vibration measurements. This sensor is an extension of the Fabry-Perot fiber optics vibration sensor - The VibroFibre - developed by QPS 5 years ago and already deployed around the world for monitoring Stator End Winding (SEW) vibration. The same hermetic junction plate developed to isolate the hydrogen cooled generator from the outside ambient is now conveniently adapted to become the feedthrough into the transformer tank, thus preventing any oil leaks to the environment. This paper discusses the successful showcase developed between QPS and Xian E-Sys Automation Engineering Co., Ltd in China. The proposed sensor, which is 2 mm thick, 50 mm long, and 20 mm wide, can measure vibration between 20 Hz to 1000 Hz. This work has been inspired by an extensive research program at University of Western Australia performing a rigorous finite element model simulating the magnetic forces interaction and vibration inside the transformer. We will validate all their simulation results showing how it is possible to detect significant winding distortions like buckling and bulging, open and short circuits, and clamp looseness. Missing inter-winding separation can also be identified as we trend the vibration amplitudes at twice the line signal frequency. The special sensor is also capable of measuring temperature to achieve a cost effective monitoring solution to extend the life of the transformers while providing good diagnostics.