This paper is devoted to an accurate investigation on the various dielectric and physicochemical properties of two kinds of transformer oils, which are submitted to accelerated thermal aging in presence of copper and presspahn. The first one is a non-treated new oil while the second one namely reclaimed oil, is extracted from transformer commissioned in 1989. The used oil is reclaimed in laboratory by adsorption process, in order to restore its quality by removing contaminants and restoring its insulating properties. The two kinds of oils are submitted to accelerated thermal aging under two temperatures 100°C and 120°C, in order to assess the stability of both oils. The investigated parameters are dissipation factor, acidity, hydrogen gas, viscosity, resistivity and index color, according to specifics standards. It’s shown that reclaimed oil degrades more quickly during thermal aging than new oil in presence of copper and presspahn. The results reveal instability of reclaimed used oil after accelerated thermal aging process.
Global warming has been a burning issue since the 1980s, and SF6 has been one of the most potent global-warming gases used by the power industry. SF6 is used in high-voltage circuit breakers. They are prone to leakage into the atmosphere, creating adverse effects on the environment. Replacing SF6 with supercritical CO2 as the arc quenching medium in circuit breakers can be a much better alternative for battling global warming. The prototype of a 72 kV, 20 kA (peak) supercritical CO2-based circuit breaker is being developed and will be tested using a synthetic test circuit. 20 kA fault current will be passed through the test breaker. The high current is generated using a capacitor bank and an inductor (resonant circuit). COMSOL simulation was used to obtain the required inductance and ensure that no flashover occurred between the turns of the inductor.
The residual life of a transformer is commonly associated with the estimated life of the insulation system, namely the solid insulation. While the temperature is the most relevant parameter for the calculation of the solid insulation life consumption, the influence of the water and oxygen content must be taken in consideration. The traditional paper life curves, commonly called "Arrhenius Curves" are traditionally focused on the hydrolytically catalyzed thermal degradation of the paper, assuming water content of less than 0.5% in the paper. Only recently, when the latest version of the IEC loading guide was published, charts presenting life expectancy curves for different levels of moisture and oxygen were introduced. The inclusion of such parameters into the calculation affects the largely, reducing the calculated lifespan of transformers to as much as two thirds of that predicted by the traditional curves. However, the challenge lays on how to estimate the actual water content in the paper in different regions of the windings, since there is currently no viable way to measure such values. This article presents an approach for continuously and iteratively estimate water content in different regions of the windings, based on the known initial conditions and the effective loading and ambient temperatures. The results from the model are then compared with the information obtained from online moisture and temperature sensors, and an artificial intelligence model is used to minimize the deviations. Thus, the proposed methodology allows assessing paper degradation rate more accurately. This information is of paramount relevance for asset management and for strategic loading decisions, allowing the inference of a failure risk associated both with the paper degradation and risk of bubbling, i.e., microbubbles formation.
HV insulation components work in a very harsh environment. These elements, depending on their application, are exposed to high voltage and high temperature (>100C), outdoor work, dielectric gases, and many other conditions. For many years such elements have been produced mainly from thermoset materials with filler, in particular epoxy resin. In general, the current material and its processing lead to a labor-intensive and complex manufacturing process. Additionally, epoxy material is very problematic to recycle thus, it would be desirable to find a material which is easier to process and more ecofriendly. Nowadays, we see more and better polymers like thermoplastics that seem to be good alternative. But their implementation is not an easy task and requires performing a lot of tests which are very challenging. The paper consists of selected HV insulation requirements and examples of tests that must be performed to find the best material candidates. Performed analyses and tests for different thermoplastic materials show its potential for implementation as alternative to epoxy resin. This way seems to be the right one as thermoplastic material offer a lot of benefits especially in terms of sustainability and circular economy.
This paper presents a high-frequency equivalent circuit model applicable for partial discharge (PD) location in an inductive test of a 6.6 kV-VT cast resin transformer (CRTr), in which a voltage is applied between the high-voltage terminals. Proposed equivalent circuit consists of the winding inductance, and the capacitance between adjacent inter-windings and that between the winding and ground of CRTr. The proposed circuit allows to simulate PD phenomena occurring in the windings. The estimation of the dominant frequency f r for PD detection can be achieved through the frequency characteristics of the impedance Z(f) measurement of CRTr and the calculation of its high-frequency equivalent circuit. This allows for the identification of PD and noise in the field, leading to improved defect detection sensitivity. Measurements of PD current waveforms were also made in a 6.6 kV VT with the inductive test circuit. Measured results were compared with waveforms calculated using the equivalent circuit. As a result, it was found that the polarity of the rising part of the current and dominant frequency components of simulated PD current waveforms were almost consistent with experimental ones.
