Public defence in Measurement Science and Technology, M.Sc.(Tech.) Jussi Havunen

Public defence from the Aalto University School of Electrical Engineering, Department of Information and Communications Engineering
Suurjännitepulssien mittaamiseen käytettävä jännitteenjakaja.

The title of the thesis: Application of Signal Processing Methods for Precision Impulse Voltage and Partial Discharge Measurements

Doctoral student: M.Sc. Jussi Havunen
Opponent: Dr. Joni Klüss, RISE Research Institutes of Sweden
Custos: Prof. Erkki Ikonen, Aalto University School of Electrical Engineering, Department of Information and Communications Engineering

Modern world relies on electricity so failure in power grid is considered as a significant risk for society. Failure can occur due to the breakdown of grid components, such as transformers and cables. High-quality grid components can reduce the probability of failure. To ensure the quality of the components, they need to be tested after manufacturing. Some of the tests are performed with impulse voltages to simulate the stresses caused by lightning strikes or switching events in the grid. By measuring electric discharges (partial discharge) during voltage stress, imperfections in insulation material, which can lead to preliminary ageing, can be detected. Important part of high-voltage testing is the used measuring systems and their measurement uncertainty. More accurate measuring systems allow to improve the detection of partial discharges during testing. Accurate measurements allow to develop cost-efficient products that withstand the required voltage stresses without an excessive amount of insulation. Cost-effectiveness of grid components is important since the electricity transmission prices have increased significantly during the last years. 

This thesis demonstrated the use of signal processing methods to analyze and correct the dynamic behavior of measuring systems used for impulse voltage measurements. Corrections are based on measurable properties of the instruments, and their effectiveness was verified against traceable reference measuring systems. After the corrections, the performance of the measuring systems could be improved significantly. The demonstrated methods allow the national metrology institutes to improve their measuring uncertainty but can also be used to improve the performance of commercial measuring systems used in testing. 

Additionally, a calibration method for small charge impulses was developed. The measuring system is based on charge amplifiers, and it can be used to support the development of more sensitive instruments used for measuring partial discharges. State-of-the-art measurement uncertainty was achieved with small charge levels using this calibration system. This calibration method is currently used by several national metrology institutes in Europe and is planned to be included in the next revision of the related partial discharge standard. 

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