No abstract

The time of checking the registers of smart meters of accuracy class 0.2 S for indirect measurement of active electricity via instrument transformers, during the first, extraordinary or periodic control and verification has been significantly increased according to the latest Rules on meters of active electrical energy of accuracy class 0.2 S from December 23, 2016. This is especially pronounced when the meter is set to show the measured value of electrical energy in kWh on the secondary side of instrument transformers with the often used meter of active electrical energy of accuracy class 0.2 S, rated phase voltage 110 / √3 V and for the rated current 1 A. In that way, the total time as well as the costs of testing such meters has increased a lot. In addition, with electricity meters set in this way with a resolution of three decimal places and a unit in kWh, there is an additional error when reading the measured value of active electrical energy and especially when calculating the energy loss of active electrical energy. A more acceptable approach to setting such meters will be considered.

A specially developed algorithm used in the program in MATLAB in a simplified model of a large power transformer will determine the locations of partial discharges using four UHF sensors by simulating ten real waveforms from a source of partial discharges. Subsequent interpolation of the recorded signals will be performed, in order to obtain the closest possible shapes to the actual waveforms. Actual waveforms were obtained by experimenting with a single UHF sensor in a real large power transformer. The ten signals of the actual shape are mutually different heights of the initial several peaks and different shapes, but still have similar maximum amplitude, frequency spectrum and the prevailing frequency of the most prominent partial discharges. It was important to determine where the actual signal begins, for which a unique calculation procedure was made, and later to accurately determine the differences between the occurrences of the first (reference) peaks of the actual signals at individual sensors. The most favourable threshold value was taken into account when determining the differences in signal arrival times by the method of the first acceptable peak. The deviations in the calculated positions of the sources of partial discharges obtained for 120 randomly selected points in the volume of the transformer tank model will be analysed.

This paper provides a detailed analysis of the online detected partial discharge (PD) in the predominantly VHF range in the power transformer during normal operation in the thermal power plant. A standard UHF drain valve sensor is used with the ability to also capture VHF frequencies of received signals. If PD detection is performed during off-line testing, there are excessive costs proportional to the time during which the power transformers are disconnected from the network. For this economic reason, online PD detection techniques are more convenient. The UHF technique has a higher signal-to-noise ratio compared to IEC 60270 and the acoustic method. To accurately determine the strength and waveform of the PD signal, especially if the source position is far from the UHF sensor or if the signal is weak, it is necessary to properly separate the useful part of the recorded signal from the background noise. The criterion for this is that there are no time shifts of the first peaks of the most prominent PD. For that reason, the beginning and the approximate end of PD signal has to be determined. The results show some obvious similarities of PDs in the recorded signals, such as frequency range, duration, repetition rate and the same dominant frequency, which sufficiently indicates that it is the same type of PD.

Partial discharges caused by initial weaknesses in the insulation system of the power transformer cannot be completely ignored, because they can warn in advance of possible serious deficiencies, which in the worst cases could cause irreversible failure of the power transformer. Monitoring of partial discharge signals using UHF sensors during power transformer operation enables their processing in order to determine the main properties of recorded partial discharge signals (e.g. amplitude, repetition rate, frequency range) to prevent power transformer failure. Using simulations in ANSYS HFSS, this paper investigates the waveforms and delays of UHF electromagnetic signals on UHF sensors mounted at different locations of a small power transformer tank. Electromagnetic waves are emitted by a specially designed model of the source of partial discharges in the insulation of the power transformer. Full-wave electromagnetic simulations are performed on the model of a small power transformer of core construction. The effect of reflections of electromagnetic UHF waves from the walls of the tank is taken into account, then the diffractions of waves around and the reflections of waves from the elements of the three-phase magnetic core and three-phase primary and secondary windings. The simulation is based on finite element method. It is a numerical method that is applied to differential equations with limit values in order to obtain an approximate solution. The source and receiver sensors in the computer simulation are designed with transmitting and receiving UHF antennas, respectively. The advantages and disadvantages of this computer simulation will be described.