Improving lightning impulse measurement system of Tampere University: Hardware development and uncertainty evaluation

Electrical grids’ components are tested with standardized lightning impulses to ensure their capability to withstand stresses caused by lightning phenomena to ensure a strong network. Currently the trends in transmission networks are towards higher system and test voltages which generates the need for better calibration capabilities. This thesis focuses on development and characterization of Tampere University’s lightning impulse measurement system. The system is studied because it is needed to be prepared for an European intercomparison campaign, where reference level uncertainties are necessary.

The old system was altered by adding a new low voltage arm to the damped capacitive high voltage divider as well as a new measuring cable. Thorough initial characterizations were carried out for the existing measuring system. It was found out that the HV arm’s elements exhibit unidealities that result in high oscillations in the divider’s response. The high voltage arm’s capacitance also exhibits frequency dependency which is compensated in the new low voltage arm by adding a compensation branch to the printed circuit board. Together with simulations and step response testing, analysis of different improvements are carried out in order to improve divider’s dynamic performance. The compensation is also tuned with step response tests. The uncertainty contributions are studied with different testing setups or by estimation.

The different standard uncertainties for the voltage divider are examined and uncertainty analysis performed using a reference measurement system method. The peak voltage and time parameter uncertainties are presented and compared for three different measurement systems. Accuracy of the time to half value is successfully improved by the added compensation.