Effect of Individual Sulfide on the Stability against Oxidation of Transformer Oil and Its Electrical Parameters

This study is devoted to the improvement of the transformer oil quality, using a new inhibiting additive, since the extensively used inhibitory additive ionol is sensitive to organosulfur compounds of sulfur oils. From the earlier works, the organosulfur compounds are known to affect ambiguously the transformer oils characteristics. Therefore, it was of interest to study the individual sulfides as the inhibitory additives. The goal was achieved by the researches with the use of the model mixtures consisting of the selectively purified transformer oil (free from ionol) along with the individual sulfides, decylcyclohexylsulfide and decylfenylsulfide, at concentration of 0.5 %, Their effect on stability against oxidation of the selectively purified oil, on its electric strength and tangent of the dielectric losses’ angle was studied. The primary results obtained were, firstly, the rate slowing down of water formation in oil and of water-soluble acids formation in oil upon its oxidation in the presence of the individual sulfides, and, secondly, an increase in the electric strength and decrease in the dielectric losses compared to the transformer oil, containing ionol. It was established that decylcyclohexylsulfide inhibitory properties are stronger compared to those of decylfenylsulfide, which was testified by less quantities (by 1.8 times) of the water formed and of water-soluble acids (by 2.22 times). The introduction to oil of 0.5 % decylcyclohexylsulfide and decylfenylsulfide increased the oil electric strength, correspondingly, by 2.6 and by 5.5 times, upon water concentration in oil equal to 15·10-2 g/kg. The significance of the results obtained is in improving the transformer oil quality produced from the sulfur oils with the use of a novel additive.

Development of the Vacuum Arc Chute for a 110 kV Single-Break Vacuum Circuit Breaker

Requirements for high-voltage vacuum circuit breakers for networks with a grounded neutral, which are stemming from the prospects of using vacuum circuit breakers in networks with the solidly grounded neutral are formulated. The arch chute design for a 110 kV single-break vacuum circuit breaker that takes into account the occurrence of repeated breakdowns is developed. The shielding system equalizes the voltage over the chute casing, decreases the electric field intensity at the “triple point”, protects the casing dielectric from becoming dusted with metallic contact parts erosion products, prevents the occurrence of cascade breakdown between the contact parts and central shield, and eliminates a possible decrease of the chute electric strength as it gradually works out its switching life. It is shown that the chute design includes measures to prevent the possibility of repeated breakdowns during disconnection up to class С1 according to GOST 52565-2006. It has been proven that for vacuum arc chutes for class С2 (for switching a capacitive load) it is necessary to develop a chute with two breaks between the contact parts or to connect two vacuum chutes in series. The shielding system adopted in the chute rules out the possibility of ceramic casing to become metallized, which decreases the chute electric strength as it gradually works out its switching life, and the vacuum arc transfer to beyond the inter-contact gap boundaries onto the central shield, the burnout of which results in that the vacuum arc chute loses its tightness and fully loses its functional properties.

Method of calculating the electric strength of oil channels of the main insulation of power transformers

The most damage-sensitive unit of power transformers is the main insulation of the oil barrier type. The breakdown of such insulation occurs as a result of the breakdown of the oil channel near the high voltage winding. In accordance with traditional methods of calculating the dielectric strength of insulation, the value of the breakdown strength is determined by empirical formulas depending on the selected width of the oil channel. The existing methods do not consider the influence of the oil channel volume, of the electric strength the statistical characteristics of the oil, the design features of the insulation of power transformers, and do not contain recommendations for creating design models. Thus, to improve the calculation accuracy, it is relevant to develop the evaluation method of dielectric strength of the main insulation of power transformers taking into account the volume and parameters of the breakdown voltage distribution of transformer oil, design features. The research results of the breakdown tension in oil channels with different volumes of transformer oil were used. To improve the accuracy of the calculation and taking into account the design features, the model of the main insulation of power transformers was made in the ANSYS program. Boundary data and assumption of linear stress distribution of transformer coils were considered. A method for calculating the dielectric strength of oil channels of the main insulation of power transformers, considering the volume and parameters of the breakdown voltage distribution of transformer oil was proposed. Unlike the existing methods, when calculating the minimum breakdown strength in the model of the main insulation, the design features of power transformers are taken into account and assumptions are justified to improve the accuracy of the calculation. In accordance with the methodology, the parameters of the dielectric strength of the transformer oil in the oil channel of the high voltage winding of the transformer were calculated. It was concluded that with increase of relative value of breakdown tension, dielectric strength of oil channel is decreasing, and it corresponds to physical sense of breakdown. The method for calculating the dielectric strength of transformer oil can be used when choosing the main insulation of power transformers in design.

Parameters of the electric strength of air in a surface antenna during the emission of an ultrahigh-frequency pulse with a trapezoidal envelope

On the basis of the breakdown criterion and the equation of continuity of electrons in air, the amplitude and energy parameters of the electric strength of air in the surface antenna of a powerful microwave relativistic generator are determined when pulses are emitted with a trapezoidal envelope. Triangular and rectangular envelopes were considered as boundary cases of a trapezoidal envelope. The dependence of the parameters of electric strength on the shape of the envelope has been established. The calculation of the dependences of the breakdown field and the maximum permissible energy in a flat aperture on the pulse duration in the range of realizable durations of powerful relativistic microwave generators is carried out. For the same duration, the largest breakdown field has a pulse with a triangular envelope, and the smallest a pulse with a rectangular envelope. Wherein a pulse with a triangular envelope has the lowest maximum permissible energy, and a rectangular one has the highest. The relationships between the maximum permissible energy and the breakdown field for the pulses under consideration are determined. With the same maximum permissible peak amplitude, the highest energy has a pulse with a triangular envelope, and the smallest a pulse with a rectangular envelope.

Electrical Strength of Natural Esters Doped by Iron Nanopowder in a Hydrophobic Carbon Shell

The paper presents the results of measurements of electrical strength of Midel 1204 natural ester doped with iron nanopowder in a hydrophobic carbon shell. The research was conducted for different concentrations of the dopant. The samples were prepared in the High Voltage Technique Laboratory. After mixing, they were tightly closed, and the first measurements were taken after 5 weeks of dissolution of the dopant in liquid. The tests were repeated after another 2 weeks and 3 weeks of dissolution of nanoparticles. An increase in both mean and maximum breakdown voltage was shown for the tested liquid mixtures. The concentration for which the value of electrical strength begins to decrease was indicated. It was also shown that a longer time of dissolution of nanoparticles causes an increase in the electric strength value for the tested samples.

Influence of Field Grading in Setup for Electric Breakdown Testing of Polyethylene Films

High dielectric strength of solid electrical insulation materials for high voltage applications is essential for high reliability and long-term performance.The IEC 60243 and ASTM D149 both describe very similar test methods to determine the short-term electric strength of solid insulation materials. A test sample, usually thin plaque, is placed in an electrode system with surrounding insulating oil. The voltage is then steadily increased until an electric breakdown occurs. Despite the relatively simple test setup, testing materials with high electric strength can be difficult. The breakdown channel is often located outside the active testing area of the electrodes and found at the electrode edge at the triple point between the electrode, test object, and surrounding oil.In this study, we have investigated different possibilities to control the electrical field enhancement in the vicinity of the electrode edges using silicone rubber, field grading silicone rubber, and a high permittivity oil. The testing was performed with semi-spherical electrodes and electrodes as recommended in IEC 60243 on polyethylene films. Electrodes covered with the field grading rubber increased the short-term breakdown strength compared to standard testing without modification. The high permittivity oil and silicone rubber seemed to have limited effect on the breakdown strength.