Browsing by Author "Ikonen, Erkki, Prof., Aalto University, Department of Information and Communications Engineering, Finland"

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Application of Signal Processing Methods for Precision Impulse Voltage and Partial Discharge Measurements
(2023) Havunen, Jussi
School of Electrical Engineering | Doctoral dissertation (article-based)
High-quality grid components are the basis for reliable power grid. Different high-voltage tests are applied to grid components according to international standards after manufacturing to ensure the claimed performance. Accuracy of the used measuring systems is crucial when more cost-efficient and better-quality products are being developed. Accuracy and traceability to SI system can be verified by calibration services provided by national metrology institutes, like VTT MIKES in Finland. This thesis presents impulse voltage and partial discharge measuring systems whose performance has been evaluated and improved using signal processing methods at VTT MIKES. Impulse voltage tests are performed to test how a test object withstands lightning strike or switching overvoltage in the grid. Test voltage is generated by an impulse generator and is measured across the test object using a measuring system based on a voltage divider. Approved measuring systems used for testing need to be periodically calibrated traceable to national or international measurement standards. Commercial systems are primarily designed to withstand the stresses of the industrial environment with sufficient measurement uncertainty for testing. Therefore, their performance is not sufficient for references used in calibration. This thesis presents three implemented methods to improve the performance of an impulse voltage measuring system. The first method is based on deconvolution, and it corrects the measured signal in frequency domain by utilizing the measured step response of the system. Time parameter errors of 1 % were improved to be circa 0.1 %. The second method is a time domain correction of the drooping response of a damped-capacitive voltage divider. Correction is based on the measured time constant of the low-voltage arm improving the used system from not-approved to low-uncertainty reference. The third method reduces the distortion caused by the signal cable of a resistive voltage divider by shortening the signal cable or by matching the signal cable only on the divider end. Insulations used in high-voltage systems may have imperfections. Under high-voltage stress, these can cause a local electric field enhancement so that the intrinsic field strength is exceeded causing a localized electrical discharge. Partial discharge can cause premature ageing of the insulation or lead to unrecoverable damage. To enable accurate detection of partial discharge, each test setup must be separately calibrated using a partial discharge calibrator. The lowest charges of calibrators have been difficult to calibrate with reasonable measurement uncertainty using the traditional calibration methods. This thesis presents a calibration method based on charge-sensitive preamplifiers which allows to measure small charges more accurately. New calibration services introduced, lowest calibration limit is extended from 1 pC down to 0.01 pC, with typical uncertainty of 1 %.