The transition away from SF6-based insulation in high-voltage equipment has accelerated interest in alternative gas mixtures with lower environmental impact. One such mixture is CO2/O2 (70%/30%), which is used as a carrier gas in C4-FN based mixtures but is also being explored for stand-alone insulation applications. To enable detailed characterisation of partial discharges (PDs) in such alternative gases, a high-bandwidth measurement setup was developed. The setup is based on a custom designed test compartment that uses a needle-to-plane electrode configuration to generate corona discharges and applies a high-frequency current transformer (HFCT) as a measurement impedance.
Initial measurements using a conventional IEC 60270 configuration revealed a resonance frequency of 3.5 MHz, which limited the effective bandwidth of the system. By eliminating the traditional coupling capacitor and instead using the inherent capacitance between the conductor in the bushing and the grounded enclosure as a coupling path, the resonance frequency was shifted to 144 MHz. Aside from the significant increase in bandwidth, this also improved sensitivity. The frequency response of each segment of the measurement circuit was characterised, allowing the derivation of the PD current from the measured voltage and enabling calibration-free charge estimation. The setup was used to study the PD behaviour in a CO2/O2 (70%/30%) mixture at pressures ranging from 0.2 to 0.4 MPa and voltages up to 1.5 times the PD inception voltage. A novel phenomenon was observed: certain PDs were rapidly followed by another discharge, after which a longer interval was required for the next event.
Multiple charge estimation methods were adapted to suit the measurement circuit. After evaluation, the modified frequency domain method demonstrated the strongest correlation with peak current, especially at low discharge magnitudes. These results demonstrate that the developed measurement setup is suitable for detailed PD analysis in alternative gases and that it is able to offer new insight into the behaviour of alternative gases.

This thesis investigated the AC insulation performance of a Fluoronitrile gas mixture with a varying amount of humidity in the gas. Also the AC insulation performance of a humid Fluoronitrile gas mixture at different operating pressures is investigated. Furthermore, the partial discharge (PD) behaviour of corona in a Fluoronitrile gas mixture is investigated. This is achieved by comparing the Partial Discharge Inception Voltage (PDIV), the Phase Resolved Partial Discharge (PRPD) pattern and some of the PD characteristics (e.g. rise time, fall time) of corona in a dry and humid Fluoronitrile gas mixture.

The fresults from this research confirm that humidity influences the electrical breakdown behaviour of a Fluoronitrile gas mixture. An increase in the amount of humidity in the gas results in a decrease of the AC insulation performance. This also applies to higher operating pressures, although the influence of humidity on the AC insulation performance decreases as the operating pressure increases. The PD behaviour of corona in a Fluoronitrile gas mixture is also affected by the presence of humidity in the gas. The PRPD pattern and PD characteristics of corona in a Fluoronitrile gas mixture differ with the presence of much humidity in the gas, as the charge of the corona discharges in a humid Fluoronitrile gas mixture are larger. The PDIV however did not change with the presence of much humidity in the gas.