Understanding the moisture temperature and ageing dependency of power transformer winding clamping pressure

Abstract

Progressive loss of winding clamping pressure is a failure mode that reduces the short circuit withstand capability of power transformers. Short circuit faults exert strong electromagnetic forces on the transformer winding structure, which could deform and displace its conductors. Failure to maintain the structural integrity of the winding structure during short circuit faults often leads to a catastrophic transformer failure, which could cause a disastrous network failure. On the other hand, due to grid expansion both short circuit current magnitude and the frequency of fault occurrences have been increasing. Early identification of loose clamping conditions of field aged power transformers through non-invasive measurements is therefore, an attractive solution to overcome the above challenges. However, the accuracy of existing clamping pressure estimation techniques is low, and hence are rarely applied for clamping pressure detection of substation transformers. To estimate loose clamping conditions of substation transformers accurately, available knowledge on the changes in power transformer winding clamping pressure under different operating conditions is insufficient.To fulfil the above knowledge gap, in this thesis, a set of comprehensive laboratory investigations were conducted to measure the through-thickness compression behaviour of pressboard under various transformer-operating conditions. In addition, the influence of moisture, temperature and ageing dependency of pressboard mechanical properties on a static clamping system was investigated. It was found that through-thickness compressive stress-strain behaviour of pressboard increases with increasing temperature, moisture content and ageing levels. Further, it was found that moisture absorption and ageing intensifies the thermal softening of pressboard. Moreover, it was found that clamping pressure applied on a pressboard stack through a static clamping system increases with increasing temperature and moisture absorption. On the other hand, ageing of pressboard has the inverse effect.In order to investigate the effect of moisture, temperature and ageing dependent pressboard mechanical properties on clamping pressure of a practical transformer winding, Finite Element Modelling (FEM) based simulations were conducted. Dynamic behaviour of the winding clamping pressure of a 100 kVA disc type test transformer under typical transformer operating conditions was simulated. A gasket model based material model which can be characterised using experimental compressive stress-strain data was proposed for simulating pressboard in FEM simulations. The pressboard material model, FEM simulation method and FE model of the test transformer winding was validated through laboratory experiments. FEM simulation results revealed that temperature, moisture and ageing dependency of pressboard mechanical properties has a significant impact on transformer winding clamping pressure. It was further found that progressive loss of winding clamping pressure could be overshadowed by moisture ingress and temperature rise. Moreover, temperature dependency of winding clamping pressure was found to be ageing, moisture content and winding pre-stress level dependent, such behaviour has not been reported before.The applicability of vibro-acoustic condition monitoring techniques to detect loose clamping conditions was also investigated. Axial vibration characteristics of a disc type transformer winding were modelled using numerical methods. Experimental compressive stress-strain curves of pressboard were coupled with an axial vibration model of the transformer winding. Through the results of the above analysis, moisture, temperature, solid insulation ageing and winding pre-stress level dependency of the transformer axial vibration characteristics were investigated. Further, vibration spectrum of a three-phase transformer winding was measured under different temperature and winding clamping pressure levels. It was found that vibration characteristics of a transformer winding are sensitive not only to clamping pressure but also to pressboard condition. Finally, knowledge gained through laboratory investigations and numerical modelling was used to interpret the tank vibration signals captured from several field aged substation transformers.