Frequency response analysis for transformer winding condition monitoring

Abstract

The large power transformer is one of the most expensive assets in a power system network. Special attention needs to be taken to monitor this expensive asset. Among the most critical aspect of a transformer that needs to be monitored is the mechanical condition of the windings and core. One of the best approaches to achieve this is by performing the Frequency Response Analysis (FRA) test on the transformer. The test measures the transfer function response of the transformer winding. If any physical changes occur, it will affect the original response, which can be used to detect any abnormality. However, the critical challenge in this technique is to correctly interpret the measured response in determining the transformer status. Although various investigations have focussed on this issue, the interpretation aspect of FRA is still not fully established. In order to contribute to the improvement of a FRA interpretation scheme, this thesis investigates the sensitivity of FRA measurement on several common winding deformations and explores a new potential diagnostic scheme of FRA. A wide range of power transformers have been used throughout this study ranging from a small sized prototype laboratory transformer to a 30 MVA power transformer installed at a substation. Initially, a mathematical model is established to simulate the frequency response of a power transformer. This is achieved by comparing three models, which are available in th e literature. These models are compared in terms of their accuracies to simulate the response and their applicability to studying winding deformation. The comparison shows that the multi -conductor transmission line model is the best approach due to its abi lity to model each turn of a winding. With the developed model, the sensitivity of the winding response is investigated. This study shows that a minor change to the winding geometrical parameters could cause a considerable change on the response. On a different issue, it is found that a similar winding failure mode may cause a different response variation depending on the winding type. A study based on measurement is also conducted to investigate the influence of winding s from other phases to the tested winding. It reveals that the end to end open circuit response is susceptible to the condition of an adjacent winding. Additionally, investigation on the winding response sensitivity due to the tap changer setting is also carried out. This thesis studies several winding deformations, which includes tilting and bending of conductors and inter-disc fault. These three faults are examined in terms of their severity of damage and location of the fault. Statistical analysis is applied to determine the overall condition of the winding. On the other hand, transfer function based analysis is proposed to extract further iii information if the winding is found to be faulty. This includes using the pole plot and Nyquist plot, in which the latter proved to be useful for all wind ing failure modes. The transfer function is achieved by applying vector fitting algorithm. Several case studies are presented in this thesis based on the measured responses in the university substation and also provided by various power utilities. The proposed analysis uses statistical indicators and the Nyquist plot. Additionally, analysis from the proposed method is also compared with two other interpretation schemes available in the literature. These two interpretation schemes are known as relative facto r analysis and a analysis for determining transformer overall condition and winding failure modes respectively. The former is found to agree with most of the results of the proposed methodology while the latter is found to be inapplicable to most of the ca ses. Finally, the influence of the non-mechanical aspect of a transformer on the frequency response is investigated. Based on laboratory experiments conducted on accelerated ageing of transformer insulation, both FRA and Frequency Domain Spectroscopy (FDS) tests are conducted. Two analyses are proposed from the FRA measurement for observing the increase in moisture content in the insulation and for computing the inter -winding capacitance. Comparison with the results from the FDS test proved the applicabilit y of the proposed methodologies. Overall, the findings from this thesis could be very useful in improving the understanding of various factors which may influence FRA measurement and subsequently in examining the frequency responses using the proposed approaches.