High voltage measurement

This article is devoted to the measurement of high DC, AC and transient voltages.

Measuring voltages of the order of a few kilovolts to a few hundred kilovolts involves the same techniques as those used to measure voltages commonly encountered in electronics and electrical engineering, and the measuring equipment itself is identical: oscilloscopes and voltmeters.

However, there are fundamental differences when it comes to implementation.

• First of all, as soon as the threshold of a few kilovolts is exceeded, the voltage falls outside the measuring range of the instruments. It is therefore necessary to use voltage reducers, the aim of which is to withstand most of the applied voltage at their terminals, while presenting the measuring device with a usable signal.

• Secondly, any high-voltage phenomenon is likely to include spectral components in the high-frequency range. High-frequency components appear either predictably, as is the case when test shock waves are applied, or accidentally, in the event of insulation breakdown. Measurement circuits should be designed to measure high-frequency phenomena.

• Finally, in most cases, voltage measurement requires that the electrodes between which the measurement is made are visible; and if these electrodes are at high potential with respect to earth, safety measures must be implemented to protect people in the vicinity.

High-voltage measurement applications fall into two main categories: measurement to check the correct operation of an installation (electrical distribution substation, etc.), and measurement applied to on-board equipment (X-ray generators, microwave generators, etc.). While sophisticated and expensive equipment can be used for the former, in the latter case, manufacturers often have to turn to rustic solutions, such as resistive dividers combined with RC filters.

This article begins with an analysis of the traditional measurement chain, with its dividers, transmission cables and the influence of interference. These principles are then applied to the measurement of transients on the oscilloscope, with resistive and capacitive dividers. This is followed by the measurement of average and rms values, using framed or digital devices, then peak and surge voltage measurements and dielectric testing.

The principles of high-voltage measurement of a test generator and an X-ray generator are studied.