Measurement of High Voltage and High Currents

MEASUREMENT OF HIGH VOLTAGE AND HIGH CURRENTS

INTRODUCTION

Transient measurements have much in common with measurements of steady state quantities but the Short-lived nature of the transients which we are trying to record introduces special problems. Frequently the transient quantity to be measured is not recorded directly because of its large magnitudes e.g. when a shunt is used to measure current, we really measure the voltage across the shunt and then we assume that the voltage is proportional to the current, a fact which should not be taken for granted with transient currents. Often the voltage appearing across the shunt may be insufficient to drive the measuring device it requires amplification. On the other hand, if the voltage to be measured is too large to be measured with the usual meters, it must be attenuated. This suggests an idea of a measuring system rather than a measuring device. Measurements of high voltages and currents involve much more complex problems which a specialist in common electrical measurement does not have to face. The high voltage equipments have large stray capacitances with respect to the grounded structures and hence large voltage gradients are set up. A person handling these equipments and the measuring devices must be protected against these over voltages. For this, large structures are required to control the electrical fields and to avoid flash over between the equipment and the grounded structures. Sometimes, these structures are required to control heat dissipation within the circuits. Therefore, the location and layout of the equipments is very important to avoid these problems. Electromagnetic fields create problems in the measurements of impulse voltages and currents and should be minimized. The chapter is devoted to describing various devices and circuits for measurement of high voltages and currents. The application of the device to the type of voltages and currents is also discussed.