Numerical analysis of partial discharge ignition in H2 bubbles floating in dielectric oils, for High-Voltage Solid State Transformer applications

TL;DR

This paper presents a detailed numerical analysis of partial discharge ignition in hydrogen bubbles within dielectric oils, revealing how bubble size, protrusions, and operational conditions influence PD inception voltages relevant for high-voltage solid state transformers.

Contribution

It provides the first comprehensive numerical study on PD ignition in H2 bubbles in dielectric oils, considering various bubble configurations and their impact on insulation performance in SST applications.

Findings

Smaller bubbles have lower inception voltages.

Protrusions slightly affect inception voltage unless in contact with the bubble.

Larger bubbles and sharp protrusions significantly lower the inception voltage.

Abstract

We report on a self-consistent numerical analysis campaign of partial discharge (PD) ignition in H2 bubbles floating in biobased dielectric oils. We investigate various configurations (bubble sizes, bubble position, existence of protrusion) on a cylinder-to-cylinder setup that emulates a specific SST module (from SSTAR Horizon Europe project) under transient overvoltage as well as in its design operational conditions (VRMS = 66 kV, AC excitation of 50 Hz). Our results on electrical characteristics and plasma dynamics leading to the PD ignition, indicate that under transient overvoltage and for mm size bubbles (diameter 1 -4.5 mm), the smaller the bubble the less the inception voltage, while the peak inception voltage is higher than 70 kV. The existence of metallic protrusion can affect the inception voltage of a remote floating bubble only slightly and when this is close to the sharp tip. The extreme scenario of a protrusion in contact (inside) a gas bubble severely affects the insulation properties and drops the PD inception voltage remarkably. The larger the bubble and the sharper the tip of the protrusion the lower the inception peak voltage, that can reach values well below 40 kV. On the contrary and under design operation, larger bubbles increase the severity and probability of PD events, leading to lower instantaneous inception voltages. Current pulses produced in bubbles can quickly transit to intense streamer discharges (which can also transit to catastrophic arcing) if the operational frequency is reduced and/or under transient, HF overvoltage.