Dennis Albert

The paper is devoted to the modeling of a three-phase, three-legged transformer under no-load and inrush current conditions. To simplify the study, transformer simulations are carried out in the ATPDraw environment, which now incorporates a dynamic hysteresis model (DHM). The main distinction of the DHM-based transformer models proposed from existing analogues lies in their ability to replicate transformer behavior over an extended range of excitation voltages. In addition to the use of the DHM, this improvement is achieved by reproducing a variable nature of equivalent air gaps at core joints. Two model versions with variable core gaps differ in whether they account for or disregard magnetic anisotropy of the core steel. Both these experiment-oriented models were fitted accurately to the same measured data by using individual flux–current characteristics of their gaps. In such a way, we have demonstrated the difficulties in distinguishing between effects caused by the core gaps and magnetic anisotropy in practical transformer modeling. By extending the method of variable core gaps to inrush currents modeling, residual fluxes in the core and current peaks were replicated accurately. The results simulated with the topological models presented are in a good agreement with measurements performed on a 300 kVA Yyn 11.350/0.235 kV transformer.

Incorporating the magnetic core material in a lumped‐element transformer model generally results in a nonlinear network with hysteresis. Such a network can be solved subsequently in the time domain using time‐stepping methods. The transient response reveals the inrush, whose decay time is generally enormous compared to the time constant of the excitation period (e.g., 20 ms for a voltage source with a frequency of 50 Hz). To directly obtain the steady‐state solution of the lumped‐element transformer model, the harmonic balance method is superior to such an analysis, because it is a frequency domain approach inherently yielding the steady‐state solution. The great advantage of harmonic balance is that it can handle nonlinearities by evaluating them in the time domain and using their spectral components in the frequency domain. This paper theoretically derives the harmonic balance algorithm and further solves a lumped‐element single‐phase transformer model based on a mutual and leakage flux approach utilizing Hopkinson 's law. The nonlinear network element is represented by an energy‐based dry friction‐like hysteresis model depicting the core material of the transformer. The hysteresis parameters are fitted to no‐load current measurements, and the no‐load case, the short‐circuit case, and a chosen load case are simulated and compared to measurements. The algorithm's efficiency in terms of iterations and computational time is demonstrated by simulating with several excitation frequencies and for several load cases and comparing it to a transient analysis revealing the transformer's inrush.

The work proposes a method of reproducing the no-load losses and currents of large five-legged transformers with the use of their transient models. This aim is achieved by employing topological transformer models based on a dynamic hysteresis model (DHM) and taking into account equivalent transformer capacitances. It is proposed to reproduce initially the measured total losses by using DHM means, and then to determine an equivalent per phase capacitance value that provides a best possible coincidence of calculated and measured current waveforms.

Preserving the stable operation and proper functionality of the electric power grid is of utmost importance. Integral grid components such as power transformers are negatively affected by occurring disturbances. In particular, the time course and distribution on the different phases of direct currents within the transmission grid, among them geomagnetically induced currents, is of great interest. In this paper, we report on the development of a fiber optic current sensor exploiting the Faraday effect intended to observe these direct currents on all three phases of a high-voltage power grid. The proposed low-cost sensor is constructed with an adjustable phase retarder and a single-mode sensing fiber. A linear current relationship for direct currents down to 1A was observed. From the sensor signal, the presence of a direct current bias superimposed on an alternating current can be identified. Simulations based on a Jones formalism model provided application limits of the single-mode sensing fiber. Furthermore, we developed a calibration procedure to counteract temperature-induced variations of the sensor scale factor within the fiber-optic coil and achieve a maximum current error of 0.5% down to a rated current of 5A in a range from 15-65°C.

Power transformers are key components in the electrical energy transmission and distribution. Therefore, special attention is paid to the condition of the transformers, using electrical, chemical, acoustical, and mechanical techniques to assess the transformer health status. However, many diagnostic methods require the transformer to be taken out of service, which can be difficult due to operational restrictions. This paper deals with the detection of distorted excitation voltage of transformers utilizing vibration measurements on the transformer tank. We investigate the applicability of vibrations measurements on the transformer tank to assess changes in the electrical excitation of the transformer. We are using standard vibration measurement equipment for on-load tap changer diagnostic to detect voltage harmonics and dc bias via vibration measurements on the transformer tank. We found that small dc bias in the supply voltage and harmonics with amplitudes below the power quality limits can be detected. These results may be useful to investigate increased transformer sound on-site and to detect other transformer faults with minimum effort on-site.

Geomagnetically induced currents (GICs) are a consequence of space weather activity that can affect power grid operation and stability worldwide. GICs manifest as quasi-direct currents flowing between the power grids and the conductive earth, and are often measured with a Hall sensor placed at the transformer neutral. Globally, the number of power grid substations with GIC measurements has grown quickly in recent years, but Austria remains one of the few countries with a dataset of long-term GIC observations. GIC measurements in substations in the Austrian power grid have been carried out since 2016, with a maximum of seven concurrent substation measurements, providing a unique opportunity for GIC measurement analysis.In this study, we present an analysis of the last six years of GIC measurements in Austria. Seven custom-built stand-alone GIC measurement systems have been installed in the 220 and 380 kV transmission levels, measuring currents up to 25 A. We identify recurrent geomagnetic activity in the measurements, and also find man-made sources of low frequency currents using frequency analysis. We focus on two geomagnetic storms from September 2017 and May 2021 to discuss the effects of GICs on a mid-latitude power transmission grid. In conclusion, we find that there is a daily level of noise in the data and that, even during the largest events when 14 A were measured, transformer heating remains unlikely.

Purpose: The purpose of this paper is to investigate the applicability of a direct current (DC) hysteresis measurement on power transformer terminals for the subsequent hysteresis model parametrization in transformer grey box topology models. Design/methodology/approach: Two transformer topology models with two different hysteresis models are used together with a DC hysteresis measurement via the power transformer terminals to parameterize the hysteresis models by means of an optimization. The calculated current waveform with the derived model in the transformer no-load condition is compared to the measured no-load current waveforms to validate the model. Findings: The proposed DC hysteresis measurement via the power transformer terminals is suitable to parametrize two hysteresis models implemented in transformer topology models to calculate the no-load current waveforms. Originality/value: Different approaches for the measurement and utilization of transformer terminal measurements for the hysteresis model parametrization are discussed in literature. The transformer topology models, derived with the presented approach, are able to reproduce the transformer no-load current waveform with acceptable accuracy.