Influence of high humidity environment on performance and reliability of power modules used in offshore wind turbines

The world is reliant on growth in renewable energy generation sectors such as offshore wind to meet the ever-increasing demand for energy and facilitate the drive towards a net zero economy. The ambient environment around the power converters used in such applications poses significant reliability concerns due to the high temperatures, high humidity, and presence of corrosion-causing contaminants. Degradation mechanisms for power electronics modules with respect to temperature stress are widely studied, but the other environmental factors have only recently received significant attention. This thesis studies the impact of the high-temperature high-humidity environment on samples representative of the insulation structures found in typical high power devices. The primary encapsulant material chosen for use within high power modules is often silicone gel, a crosslinked polymer providing a strong dielectric barrier between the conductive elements, good thermal conductivity which facilitates proper cooling, and a pliant conformal nature giving way to thermal expansions of the internal components, reducing thermomechanical fatigue. The ability of this material to guard against the ingress of moisture has been called into question in recent years, prompting much research.

The humidity-driven changes in the electrical properties of the gel are measured in a configuration representative of the substrate and die attach layer. The diffusion of moisture within the gel is shown to have a significant influence on its permittivity, resistance, and dielectric loss. The dynamics agree with non-Fickian diffusion under the Langmuir model, and the rate of changes are shown to be a function of the coating thickness. Comparisons are made of the switching performances of high voltage insulated gate bipolar transistors in dry and humid environments. It is revealed that although the gel is significantly influenced by humidity, these changes are not sufficient to alter the switching behaviours in a meaningful way. Partial discharge testing is carried out on gel-encapsulated substrates taken from power modules with fast positive square pulses. Two distinct behaviours of partial discharge are observed, which are categorised by their relative amplitudes. Strong discharges do not vary with humidity and are conceptually attributed to voids within the ceramic of the substrate. Weak discharges are significantly influenced by the ambient humidity and are conceptually attributed to the gel-substrate interface known as the triple interface.