Characterization of a fast step generator
Lighting impulse measurements are made as a matter of routine in high voltage testing of high-voltage electrical equipment. The test is often decisive for acceptance of the equipment under test, and consequently proper and precise calibration of the measuring system is needed. The present work centres on the need to quantify the errors of reference measuring systems for lightning impulse. Scale factor determination at low frequency (or DC) is the starting point for this determination. The extrapolation from this frequency domain to the domain where microsecond pulses must be faithfully captured requires application either of methods in the frequency domain or in the time domain. Radio frequency measurements are only well defined for coaxial structures and at impedances in the range of 50 O or thereabouts, making them difficult to apply to the large structures of high-voltage measuring systems. The converse method in the time domain is to apply a Dirac impulse to the system and calculate the response to an assumed input signal by convolution. A true Dirac pulse is not readily available and in practice the applied pulse is a step voltage, which is then derived with respect to time and convolved with the applied signal to obtain the response of the measuring system.
The step generator used for this purpose should have very fast front without oscillations. The intent is to achieve a close approximation of an ideal step function, which when derived with respect to time, yields the impulse response of a tested system. A necessary prerequisite is that the step is much steeper than the lightning impulse, and is flat after the step on times much longer than the impulse.
The ideal switch element in such a step generator should have infinite resistance and zero capacitance in the off-state, very fast switching to on-state and very low resistance in on-state. The mercury wetted reed switch has often been used for this purpose since it has good characteristics in all these respects. Few, if any, electronic components exhibit competitive advantages compared to the reed switch. The relative lack of parasitic effects means that it is close to being an ideal device.
Based on earlier experiences by the authors, a new design has been developed with focus on electrical screening and coaxial design in order to realise a step generator that works into a high impedance instrument. Considerable work has been performed to characterise the new device with regard to steepness of step and most importantly, to voltage stability after the step. The most demanding part of this work has been to separate the performance of the switch from that of the oscilloscope. Findings indicate that the step rise-time is less than 0.5 ns, and settling to within 0.5 % within 10 ns.
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