ANALYSIS OF BUBBLE FORMATION MECHANISM WITHIN TRANSFORMER INSULATION: EFFECT OF LOADING AND MATERIAL

Abstract

High temperature in transformers is the source of much trouble, causing the transformer to undergo ageing processes which reduce its insulation lifetime, as well as giving rise to potential failures through fires, explosion, and from the formation of bubbles within the insulation. Temperatures within the existing transformers are purported to reach higher and last longer once electrification of energy occurs as part of a move toward low carbon power networks. Analysis of transformer temperatures by simulation of potential future loading and calculation based on the IEC thermal model provides a picture of the changes in thermal behaviour of transformers. Further analysis comparing transformer temperatures to an insulation condition specific bubble inception temperature demonstrates how the risk of inception is increased compared to cases where no electrified heating load is present. Using a bespoke set up developed within this work, tests showed how the liquid only insulation rarely presents a bubbling (and therefore a failure) risk during high temperature overload periods of a transformer. Despite several ‘worsened’ conditions (including gas saturated, and high particulate content liquid), overloads causing temperatures as high as 180°C did not evolve bubbles. However, in extremely wet conditions (≥78% relative saturation), hydrocarbon insulating liquids can cause bubbling (though synthetic esters generated no bubbles, even when completely saturated). Testing of transformer solid insulation generated bubbles in wet paper. Key findings were that thermally upgraded paper resisted bubble formation (i.e. needed a longer time and higher temperature) than non thermally upgraded paper; increasing the rate at which the load is applied reduced the bubble inception temperature and time to bubble inception; and when comparing the energy input to the bubble inception temperature, different shapes of load profile performed similarly. The mechanism of bubble formation is further understood from this work. As well as being linked to temperature and load, the moisture dynamics between the solid and liquid insulation, and between solid insulation layers, where the competing drivers of temperature and concentration difference lead to a build up of moisture between layers which allows the moisture bubble to form and later release from the overlapping paper edges of the outer most layer. In addition to the factors of load, moisture content, and insulation type, the ageing condition of the solid insulation is an important factor. This information is incorporated into the formula used for calculating bubble inception temperatures, as well as forming part of the development of an ab initio formula. Basing the calculation of bubble inception temperatures on the enthalpy of desorption rather than on isotherm behaviour showed improvements, and by accounting for the degree of polymerisation of the solid insulation, a more accurate bubble inception temperature can be established for transformers, given their actual condition. The work from this thesis is of use to transformer designers and operators in allowing them to maximise the operational capacity of the asset while protecting them from bubble induced failure.

Date of Award	31 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Zhongdong Wang (Supervisor) & Qiang Liu (Supervisor)