F. Vahidi

Electrical conductivity measurement and determination of ion mobility in insulating oil

The insulation system of power transformers is a key topic which has to be taken into account in order to achieve an optimized design and safe longterm operation. The design of feasible insulation for high voltage direct current transformers requires the understanding of field distributions in insulating materials in both AC and DC operation because the transient stresses are dependent on both conductivities and permittivities and geometry. The steady state DC field stresses are determined by conductivity of insulating material which is normally a combination of oil and paper. Therefore knowledge about the conductivity of the oil is of great importance for design and safe operation in DC case. Moreover the space and surface charges will occur in field space and also will influence the electric field. If such charges are present in a noteworthy amount, models depending on material properties only will probably be an approximation. The conductivity of insulating material is dependent on several parameters, e.g. time, temperature and electrical field strength and the geometry of the arrangement. In this contribution, a new measuring cell with plate type electrodes is presented for analysis of oil conductivity. The measurement accuracy of the set-up is investigated both analytically and experimentally. Consequently, measurements are performed for investigating the time behavior of conductivity. The results show that the determination of ion mobility using polarity reversal test represents to a suitable approach to investigate the transit times more precisely.

Space charge formation in insulating liquids under DC stresses

The occurrence of space charges has to be understood, in order to study the field distortion which is due to space charges. Hence, the investigation of space charge behavior and its origin in insulating liquids is necessary for design of optimal insulation system. It is important to know about the type of charge carriers present in insulating gap. Applying different DC stresses with various polarities can be helpful to determine the thickness of space charge regions near the electrodes and consequently, to achieve knowledge of space charge distribution both in insulating liquid and along the electrode surfaces. If an electrostatic field is applied onto the insulation between two electrodes, the charge carriers will begin to move towards the electrodes and neutralize themselves on metal electrodes. To determine the velocity of charge carriers during the measurements, a polarity reversal test is recommended which can be performed after long-duration conductivity measurements. In this investigation, the polarization current is measured using a plate-shaped test cell with stainless steel electrodes which allows a variation of gap between 1 to 5mm. In this case, the measurement set-up is based on low level current measurement with high sensitive ammeters because the current flowing through an insulation system is some pAs only. Finally, all these input parameters are used to describe the physical phenomena which occur during the conduction process.

Influence of electrode material on conductivity measurements under DC stresses

The optimization of design and operation of converter transformers during test and operation needs understanding of insulation system behavior under AC, DC and mixed stress. In order to calculate the electrical field under all kinds of stresses, the material properties have to be known. The common metals exposed to noteworthy electric stress inside power transformers are copper (conductors) and aluminum (shields). Stainless steel is a material which is often used in dielectric material test arrangements. In typical arrangements bare metals as well as metals covered with insulators are found. In this contribution the influence of different bare electrode materials onto the conductivity values of mineral oil and onto their function of time is investigated. The behavior of each electrode material is also investigated at different measuring temperatures which shall be representative for the conditions of insulation during transformer operation.

Standardized survey of transformer reliability: On behalf of CIGRE WG A2.37

There is limited literature available in the public domain discussing failure statistics of transformers. This contribution presents the methodology for a standardized failure data acquisition developed by the Cigré Working Group A2.37. The working group collected 964 major failures which occurred in the period 1996 to 2010, within a total population of 167,459 transformer years, contributed by 58 utilities from 21 countries. The overall failure rates of substation transformer were all within 1%. For three groups of substation transformers, detailed population data were collected enabling the calculation of hazard curves. All populations show a low hazard rate and no distinct bathtub curve character. An increasing probability of failure after a particular age, which would justify an exchange of the transformer, cannot be derived from the available data. Windings, tap changer and bushing related failures were the major contributors, followed by lead exit related failures, irrespective of application or manufacturing period. Dielectric mode failures were the most prominent, followed by mechanical and electrical type failures, for substation transformers, whereas GSU transformers had higher contributions of thermal and dielectric mode failures.

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.

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.