Alan Sbravati

Dielectric prototype test of a full-scale 420 kV power transformer HV lead exit insulated with natural ester liquid

A high voltage bushing electrode/lead assembly for a 420 kV power transformer was built considering the same test program as applied previously with mineral oil. The lead exit combined high-quality solid insulation components impregnated with the natural ester Envirotemp™ FR3™ fluid together in a compact geometric turret, keeping the same outside diameter of the original solution applied for mineral oil. The electrical tests were done in accordance with IEC and IEEE test standards at the High Voltage Test Institute of Graz, Austria. Lightning impulse voltage applied to the test specimen reached a BIL level of 1950 kV, switching impulse of 1560 kV, AC tests up to 850 kV maintained a partial discharge level of less than 5 pC. The paper presents testing results of the lead assembly in FR3 fluid. Preparation of the solid and liquid insulation is reviewed in regards to drying, degassing and particles management. Testing limits imposed by the test bushing, dimensions and clearances of the test lab, rather than the specimen in the natural ester FR3 fluid, are discussed. It is concluded that using advanced design concepts and high-quality insulation components in the same turret arrangement as previously tested, FR3 natural ester insulating liquid proved dielectrically as valuable as mineral oil at transformer testing limits according to IEC and IEEE standards. The insulation assembly has been fully approved using FR3 fluid.

Characterization of inhomogeneous field breakdown in natural ester liquid compared to mineral oil

Natural ester liquids as insulation material in power transformers offer many advantages compared to conventional mineral oils. Some have, for example, higher thermal capacity, higher flash/fire points and better environmental sustainability. Despite several studies confirmed the equivalent dielectric capacity for the typical construction elements of transformers, some studies representing extremely inhomogeneous field distributions indicated a lower breakdown voltage for vegetable-based oils. Our contribution illustrates the influence of the electric field distribution on the breakdown voltage of a natural ester liquid compared to mineral oil for different degrees of inhomogeneity. The breakdown voltage tests are performed for AC and lightning impulse excitation voltage. The breakdown voltage of a standard 1.2/50 μs lightning impulse is measured based on the breakdown test defined in ASTM D3300. AC breakdown voltage is determined by a stepwise rise of voltage as described in IEC 60060-1. Different types of electrodes are used in different gap distances, for representing several levels of inhomogeneity of the electric field. Envirotemp™ FR3TM natural ester liquid and Lyra X mineral oil were used in this study. The results of the measurements are compared, taking into account the degree of inhomogeneity. An electrode configuration composed of a needle of 0.06 mm tip radius, despite not representative of any real construction of any transformer, is widely applied for investigation of breakdown phenomena. The use of a needle of very small radius qualifies the D3300 test as a “scale test” as it creates high levels of voltage gradient using relatively low voltage application, allowing researchers to study partial discharge inception behavior and other breakdown characteristics. In mineral oil the partial discharges start to occur at lower voltages than in natural ester, but the difference of voltage between the inception voltage and the disruptive condition is larger, resulting in a higher breakdown voltage. Studying several different configurations from spherical electrodes to needle configuration, allows correlating the degree of inhomogeneity of the electric field with the breakdown characteristics where the equivalency of the breakdown voltage is confirmed. This paper includes also an assessment of the impacts of these differences for the dielectric design of power transformers.

Comparative study on inhomogeneous field breakdown in natural ester liquid and mineral oil

Using natural ester liquids as insulation material in power transformers offers many advantages compared to conventional mineral oils. There are, for example, higher flash and fire point, greater water solubility, and better environmental sustainability. Despite several studies confirmed the equivalent dielectric capacity for the typical construction elements of transformers, some studies representing extremely inhomogeneous field distributions have indicated a lower breakdown voltage for vegetable-based oils. This investigation illustrates the influence of the electric field distribution on the breakdown voltage of vegetable-based oils compared to mineral oils. The breakdown voltage tests are performed using different experimental setups and different types of excitation voltage. The breakdown voltage of a standard 1.2 / 50 μs lightning impulse is measured based on the breakdown test defined in ASTM D3300. Different types of electrodes are used in different gap distances, for representing several levels of inhomogeneity of the electric field. The less inhomogeneous field distribution is achieved using two sphere electrodes at low sparking distance. For the extremely inhomogeneous field distribution, one sphere electrode is substituted by a needle electrode having a defined tip radius. Oils used are Envirotemp™ FR3™ fluid as natural ester liquid and Lyra X as mineral oil. Both oils are dried and degassed to a relative moisture content of <; 15 % saturation before starting tests. The result of the breakdown voltage obtained from a slightly inhomogeneous electric field distribution show a comparable dielectric strength for both oils. However, in extremely inhomogeneous electric fields, a decrease of breakdown voltage of natural ester liquid compared to mineral oil occurs. This is due to differences in the chemical structure and discharge mechanisms in natural ester in comparison to mineral oil. The main aim of study is to determine the limit of inhomogeneity for keeping the equivalency of dielectric behavior.

Electrical conductivity of oil and oil-impregnated pressboard dependent on aging byproducts

The estimation of material parameters is an essential step during the design of high voltage direct current (HVDC) insulation systems. The key role of insulation coordination in power transformers is to provide the required insulation components among different voltage classes to prevent dielectric breakdown. The design of a feasible insulation for transformers requires understanding of electrical field distributions. The DC field stresses are determined by means of insulating material conductivity which is normally a combination of dielectric liquid, paper and pressboard. One of the parameters which has an influence on the electrical conductivity is the aging of solid and liquid insulations. During converter transformer operation time, there is a normal degradation of all insulation materials. Aging byproducts like various acids increases the electrical conductivity of the system. This can lead to a variation of the field distribution over the operation time of the converter transformer. In this contribution, the influence of different carboxylic acids on electrical conductivity of oil and impregnated pressboard is investigated. The results compare the effects of adding different amounts of hydrochloric acid to mineral oil and natural ester liquid during electrical conductivity measurement. The conclusion illustrates different behavior of electrical conductivity comparing short and long-chain acids content of specimens.

Validation of long term performance of natural ester and Kraft paper as high temperature insulation system

In early 2003, CPFL Energia, a Brazilian utility, was interested on evaluating the performance of natural ester filled transformers, especially regarding the highly increased overloading capability. As a field test, in association with the transformer manufacturer “Itaipu Transformadores”, they built two prototype transformers. Based on original overhead distribution line three phases transformers of 15kV/45kVA, after small adjustments in the windings and cooling system, keeping same core and tank, associated with the use of natural ester based dielectric liquid, and increasing the average winding temperature rise limit from 55°C to 85°C and a hotspot temperature rise limit of 100°C, the transformer nominal rating was shifted to 88kVA.

Study on moisture influence on electrical conductivity of natural ester fluid and mineral oil

HVDC equipment is normally stressed with AC, DC and super-imposed stresses during its operation time. Contrary to AC field distribution, DC field stresses are determined by the conductivity of the insulating materials, which consist of insulating liquid and cellulose-based transformer board. As a result, a deep knowledge of the permittivity and conductivity of barrier-oil insulation materials are necessary for construction of the reliable insulation systems. To be considered as viable insulating fluids for HVDC equipment, the electrical oil conductivity of alternative dielectric liquids should thoroughly be investigated dependent on different parameters, which affect the behavior of oil conductivity. One of the parameters influencing electrical conductivity is moisture content of oil. Water appears in transformers as an unwanted substance affecting transformer life dramatically and leading to a decrease of dielectric strength of insulation. Besides this effect of the moisture, it is also essential to know if the moisture content of insulating liquid has an influence on its electrical conductivity. In this contribution, a fundamental study of electrical conductivity of Envirotemp™ FR3™ fluid is presented in term of water content variation of oil samples. The measurements are established in comparison with mineral oil used as conventional insulating liquid in HVDC converters. The effect of moisture during conductivity measurement is studied at different field strengths and two different measurement temperatures. The measurement results show that the rate of field strength dependency for oil conductivity is strongly dependent on level of moisture content for both investigated liquids.