• Energy Research

In high-voltage equipment, it is vital to detect any failure in advance. To do this, a determination of the partial discharges occurring at different voltage types as well as at different electrode configurations is essential for observing the oil condition. In this study, an experimental setup consisting of a needle–semi-sphere electrode configuration immersed in mineral oil is prepared for laboratory experiment. In such a way, a non-uniform electric field is created and the leakage currents are monitored from the grounded electrode. A total of six different electrode configurations are analyzed during the tests by the use of hemispheres of different diameters as grounded electrodes and copper and steel pointed (medical) needle high-voltage electrodes. In the experiments, the partial discharges occurring at four different voltage levels between 5.4 and 10.8 kV are measured and recorded. The effect of the different electrode configurations and voltage levels on the harmonic distortion are noted and discussed. It is experimentally confirmed that it is possible to measure the leakage current caused by the partial discharges of the corona type in oil at the different metal points, creating high-voltage electrodes and different electric field distributions based on the proposed non-invasive measurement technique. The studies showed that there is a significant rise of even harmonic components in the leakage current during the increase in the partial discharge intensity with the 5th harmonic as dominant.

This article deals with the influence of temperature on the lightning impulse breakdown voltage (LI BDV) of mineral oil in a non-uniform electric field. The experiments were performed in point–plane electrode arrangements under a standard lightning impulse voltage of negative polarity. The LI BDV for the temperatures of 20, 40, 60, 80, 100 and 110 °C was measured using a step method with registration of the light signals emitted during pre-breakdown and breakdown phases. The average propagation velocity was also determined. The results confirm that the mineral oil tested under LI voltage behaves similarly to AC voltage-based stress. The LI BDV increases with temperature up to 100 °C and then tends to decrease. The results demonstrated that viscosity dependent processes are responsible for increasing the LI BDV with temperature up to the 100 °C; and liquid vapor formation processes are responsible for decreasing the LI BDV at temperatures equal to and higher than 100 °C.

This article is focused on considerations based on experimental studies concerning changes of selected parameters of identical compact fluorescent lamps (CFLs) intended for use in buildings during their operation. The studies constituted a long-term experiment whose goal was an evaluation of selected operating parameters of the CFLs in terms of meeting the requirements set out in the specified regulations as well as the issue of marking the lamps with the energy efficiency class. The measurements were performed with the authors’ experimental setup consisting of original equipment designed and made especially for the purpose of the measurements. The studies covered registration of the luminous flux as well as selected electrical parameters such as active power, current and the power factor during the so-called “start-up time” and operation time equal to 100 h, 500 h, 1000 h, 2000 h, etc. with a 1000 h step. The studies were finished with the moment of natural burnout of the CFLs tested. The results showed that the biggest drawback of CFLs is lack of preservation of the required time to reach 60% of the stabilized luminous flux just after short time of lamp operation. Similarly when assessing the conformity of the parameters declared by the manufacturer that have been verified, it can be stated that they are true only at the initial stage of lamp operation.

The article describes a case study when the voltage collapse during lightning impulse tests of new power transformers was noticed and when the repeated tests finished with a positive result. The step-by-step process of reaching the conclusion on the basis of dissolved gas analysis (DGA) as a key method of the investigations was presented. The considerations on the possible source of the analysis showed that the Duval triangle method, used in the analysis of the concentration of gases dissolved in oil samples taken from bushings, more reliably and unambiguously than the ratio method recommended in the IEC 60599 Standard, indicated a phenomenon which was identified in the insulation structure of bushings analyzed. Additionally, the results from DGA were found to be consistent with an internal inspection of bushings, which showed a visible trace of discharge on the inside part of the epoxy housing, as a result of the lightning induced breakdown.

The process of building and modernizing the high voltage substations generate the problems concerning the determination of the optimal technological solutions, which should be friendly to the environment, meet the criteria of sustainable development and be cost effective. Since many institutions are involved in the decision making process to reach an optimal solution for the planned substation, consensus is sometimes not possible without the commitment of the instruments which support this decision. Hence, in this paper the authors proposed a new approach based on the Analytic Hierarchy Process (AHP) method in relation to supporting the decision of choice of high voltage substation technology between two commonly applied solutions: Air Insulated Substation (AIS) and Gas Insulated Substation (GIS). On the basis of applied regulations as well as expert opinions a survey was prepared and then analyzed in terms of the elaborated criteria. The results show that the AHP method is well suited to the issue under consideration, indicating clearly that GIS technology is strongly preferred. Hence, this opens the discussion for the use of the AHP method in a wider form in the decision support process in the specific field, which high-voltage engineering is.

Liquid dielectrics are different from each other, but are used to perform the same tasks in high-voltage electrical equipment, especially transformers. In similar conditions, the insulation performance of transformer oils under different types of voltage will provide dielectric resistance. In this study, three different dielectric liquids applied in transformers, namely mineral oil, natural ester and synthetic ester, were tested. Tests under AC and negative DC voltage were performed at electrode gaps of 2.5 mm, 2 mm and 1 mm using disk and VDE type electrodes as per ASTM D1816-84A and ASTM D877-87 standards, respectively. In turn, the impulse voltage tests were performed under an electrode configuration suggested by the IEC 60897 standard. The current data of 500 ms prior to breakdown under AC electrical field stress was decomposed using the empirical mode decomposition (EMD) and variational mode decomposition (VMD) methods. These analyses were conducted before the full electrical breakdown. Although synthetic ester has the highest dielectric strength under AC and negative DC electrical field stress, mineral oil has been assessed to be the most resistant liquid dielectric at lightning impulse voltages. In addition, stabilization of mineral oil under AC and negative DC voltage was also seen to be good with the help of calculated standard deviation values. However, synthetic ester has a significant advantage, especially in terms of dielectric performance, over mineral oil in spite of the stability of mineral oil. This indicates that liquid dielectric selection for transformers must be carried out as a combined evaluation of multiple parameters.

Synthetic esters have become more and more popular in last few decades, explaining the increasing number of units filled with this liquid year by year. They have been investigated under different aspects, both from the fundamental point of view and breakdown mechanisms, well as from the application point of view. However, their use in high voltage equipment is always a challenge and deeper knowledge of the various aspects that can be encountered in their exploitation is needed. The intent of this review paper is to present the recent research progress on synthetic ester liquid in relation to the selected issues, most important for ester development in the authors’ opinion. The described issues are the breakdown performance of synthetic esters, lightning impulse strength and pre-breakdown phenomena of synthetic esters, synthetic esters-based nanofluids, combined paper-synthetic ester based insulating systems, application of synthetic ester for retro-filling and drying of mineral oil-immersed transformers, DGA(dissolved gas analysis)-based diagnosis of synthetic esters filled transformers as well as static electrification of synthetic esters. The different sections are based both on the data available in the literature, but above all on the authors’ own experience from their research work on synthetic ester liquids for electrical application purposes.