IMPULSE CURRENT MEASURING SYSTEMS CALIBRATED BY COMPARISON MEASUREMENT
The importance of current measurement and current measuring systems in high voltage has been increasing in the past years. The traceable measurements of impulse shaped currents are necessary for research and development but also for type tests and routine tests of final products. As we know, in the different lines of industry (e.g. telecommunications, power supply) the most used impulse current wave shape is characterized by time parameters T1/T2 8/20 µs.
Typically surge protection devices (SPD), metal oxide arresters (MOA) and instruments with their application (e.g. vacuum tap changer of power transformers) are tested with impulse current 8/20 µs. To meet the purpose of actual measuring requirements, HIGHVOLT applied for an enhancement of the accreditation range for new measurand and impulse current with wave shape 8/20 µs as the calibration procedure for impulse current allows calibrating the current monitor and measurement systems up to 25 kA for shunts or up to 200 kA for inductive current monitors (like Rogowski coils etc).
Additionally for the impulse current tests at MOA's and SPD's the physical quantities charge Q (in C) and specific energy W/R in J/O (often also written as I2t in A2s) are important. These values have to be calculated from the wave shape. For such kind of the calculation the time behavior and time parameters T1 / T2 of the current shape are essentially important.
The calibration of impulse current measuring systems has been usually done by the low voltage components calibration method. This article describes the reference standard and the calibration method for calibration with impulse current wave shape based on comparison measurement against the traceable impulse current standard. The explained method reduces the measurement uncertainty of the calibration object significantly. As explained above, the time behavior and current impulse parameters need to be determined with low uncertainty. Therefore a simplified method for the correction of the calculated values Q and W/R based on the measured characteristic values (current peak value Î, T1 and T2) of the wave shape will be introduced.