18.1 Introduction

The introduction of the digital recording system into the high voltage measuring technique has a great influence on the measuring technique and the evaluation procedures. The formerly used analogue recording instruments were developed for high voltage measurements and in particular for impulse measurements under very noisy conditions or, in other words, for high electromagnetic interferences. One of the best factors against electromagnetic interference was the high signal to noise ratio, reached by a high signal voltage level up to 1500 V for impulse voltage measurements. This high signal level also leads to a high deflection level of about 100 V/cm and requires no amplifier within the analogue oscilloscope.

The use of digital recording devices in high voltage measurement requires, in particular, a number of measures against electromagnetic interferences. The recording device should be inside a Farady cage to prevent the influence of the electric field. The high signal level should be kept by using a so-called input divider inside the shielded cabin to reduce the voltage level from hundreds of volts down to 2 V, which is the normally used input level of a digital instrument. This voltage divider could be used at the same time for remote controlled change of the total impulse voltage divider ratio. Furthermore, the requirements on the digital recorder concerning resolution in amplitude and time should be equivalent to or better than those of the analogue oscilloscope.

The digital recording devices offer a number of advantages compared to analogue systems. Owing...