Ageing assessment of primary transformers under future operational scenarios

• Main Author: Wang, Dongmiao
• Other Authors: Wang, Zhongdong ; Liu, Qiang
• Published: University of Manchester 2017
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.764510

As the UK starts to transit into a low carbon economy, many low carbon technologies will be integrated into power networks. One distribution network operator (DNOs) has proposed some low carbon technologies which aim to manage the demand at peak times, the frequency and the high voltage through the control of the existing 33/11(or 6.6) kV primary transformers in the distribution network. The application of those technologies could change future operation scenarios and might further impact the ageing status of primary transformers. In UK distribution networks, the majority of primary transformers were installed in the 1960s and have been in service for a long time. Hence, this thesis aims to assess the ageing of primary transformers under future operational scenarios caused by the application of low carbon technologies. The work for ageing assessment starts from the management of different types of databases by proposing a general framework. This framework includes database development, database aggregation, database presentation and analysis, as well as techniques and methodologies for ageing assessment. The proposed framework is applied to develop databases for operational and maintenance data, environment data, load data and oil test data. The analysis is then performed on the developed databases, and the results finally applied to transformer ageing assessment. The proposed low carbon technologies involve three techniques: transformer tripping, lowering and staggering tap positions. Out of these techniques, the technique of lowering tap positions could not accelerate the ageing of transformers. Hence, the analysis focuses on the other two. The technique of tripping one transformer could double the current of the other transformer in the substation. The technique of tap staggering could cause overload in the transformer with the lowered tap position. A methodology is proposed to assess the operability of transformers by generating loading guides for the operational scenarios caused by the application of those two techniques. Based on the generated loading guides, ageing assessment is performed through another proposed methodology which incorporates the results of thermal modelling into the methodology of health index and probability of failure. The results of incorporation have shown that the tap staggering technique could only impact the ageing status of no more than 7.5% transformers with the total increase of probability of failure less than 5% in the next five years. Moreover, the impact of the tripping technique is negligible if the loading guide is strictly followed. As an alternative approach, the ageing status of transformers can be also assessed through acidity analysis. The assessment starts from the analysis of the influence of five key factors such as in-service age, manufacturer, manufacture year, load and environment on the acidity by Analysis of Variance (ANOVA). The results of factorial analysis have confirmed the influence of in-service age, manufacturer and manufacture year found in the previous research. However, ambiguity is found in the influence of environment and load. Finally, the short-term influence of the load increase caused by the application of low carbon technologies is excluded through the comparison of oil acidities sampled 24 hours before and after the on-site trial tests.