Measurement Techniques for Impulse Puncture Testing in Air

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Sähkötekniikan korkeakoulu | Master's thesis
Electrical Systems
Degree programme
Elektroniikka ja sähkötekniikka
Impulse voltage puncture testing (IPT) in air, as per IEC-61211, is used to assess the withstand strength of class B ceramic/glass insulators primarily against very fast front voltage transients (VFFTs) in power systems. This method uses short HV impulses of front times as low as 100 ns, therefore, adequate execution and reproducible results are not ensured because no standard impulse shape exists for VFFTs, no calibration service exists for such systems below 0.84 μs and the measurement system gets affected by many factors e.g. proximity effects, interferences, clearances and stray capacitances and inductances. This study focuses on investigating the testing practices and measurement techniques, for IPT in air as per IEC-61211, to find suitable testing methodologies. Based on the results of a research survey done to know practices and capabilities of HV labs in this regard, testing was performed on a cap and pin insulator, mounted on a metal plate using ball-socket and stressed with steep front HV impulses. A 500 mm wide copper sheet was used for grounding. Measurement system comprised of a fast resistive divider, a 50 tri-axial cable, a 10:1 attenuator of 50 input resistance and a 200 MHz, 8 bit, 2.5G S/s digitizer. The dependency of results on factors like divider-insulator distances, extra cable shielding, various target test voltages, HV damping resistor and different generation circuits was studied. No puncture of insulation happened. The divider showed high overshoot in step response and low bandwidth in impulse results. Due to these reasons, along with its damage during testing, a new divider design is proposed for 500-600 kV that is modular in structure and aims to solve the problem of low bandwidth by allowing its HV arm to be placed in direct contact with insulator. A novel algorithm is also proposed to analyse the linearity of front chopped impulses. It also revealed that following exact definition of Tc in IEC-60060-1 could result in it’s wrong determination from measurement software.
Lehtonen, Matti
Thesis advisor
Hällström, Jari
impulse voltage puncture testing in air, ceramic or glass insulators, measurement techniques of very fast front HV impulses, chopped impulses, fast and small resistive impulse voltage divider
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