Development of a measurement setup for dielectric characterization of SF6-free alternative gases
T.B. Plantfeber (TU Delft - Electrical Engineering, Mathematics and Computer Science)
P. Vaessen – Mentor (TU Delft - High Voltage Technology Group)
D. van der Born – Mentor (TU Delft - High Voltage Technology Group)
A.G.A. Lathouwers – Graduation committee member (TU Delft - High Voltage Technology Group)
Sebastian Rivera – Graduation committee member (TU Delft - DC systems, Energy conversion & Storage)
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Abstract
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.