I. Fofana

Challenge of mixed insulating liquids for use in high-voltage transformers.1. Investigation of mixed liquids

The aim of this work is to present results of investigations into mixtures of two insulating liquids, recently proposed as alternatives to mineral oil. The mixtures are a combination of the widely available mineral oil and a specific amount of ester liquid, which has similar electrical properties combined with fewer environmental risks but high hygroscopicity. The water saturation limit of esters is more than 40 times larger than that of mineral oils. Esters absorb water vapor from the air in larger quantities than mineral oil, and this hygroscopicity reduces the moisture content in solid insulation due to diffusion from the solid into the liquid, while the dielectric properties of ester liquids are only slightly changed . Although the life of an oil in service depends primarily on its initial quality, service conditions need to be considered also. The investigations have therefore been carried out on unaged mixed liquids as well as on specimens under severe ageing conditions. Pure liquids have also been investigated to provide baseline data for comparison purposes. The first part of the investigation compares the properties of the mixed liquids with those of pure liquids. The second part of the investigation, will evaluate the compatibility of the mixed liquids with insulating papers used in high-voltage transformers.

Dielectric spectroscopic measurements on transformer oil-paper insulation under controlled laboratory conditions

For reliable operation of power transformers, the condition of the insulation system is essential. This paper reports on a detailed study of the effect of ageing, temperature and moisture on frequency and time domain spectroscopic measurements carried out on oil-impregnated pressboard samples as well as on a distribution transformer under controlled laboratory conditions. Because field measurements are generally performed after de-energizing the transformer, extreme care is required in interpreting the results due to inherent temperature instabilities. To avoid large thermal variations that may affect the results, a customized adiabatic room was built around the transformer for measurements above the ambient. Capacitance ratio and direct current conductivity deduced from the spectroscopic measurements, helped to interpret the data. Because, low frequency measurements techniques are time consuming, alternative to a transfer of time domain data into frequency domain data was investigated.

50 years in the development of insulating liquids

The role of electrical insulation is critical for the proper operation of electrical equipment. Power equipment cannot operate without energy losses, which lead to rises in temperature. It is therefore essential to dissipate the heat generated by the energy losses, especially under high load conditions. Failing to do so results in premature aging, and ultimately to failure of the equipment. Heat dissipation can be achieved by circulating certain liquids, which also ensure electrical insulation of energized conductors. The insulating-fluids market is therefore likely to be dominated by liquids, leaving to gases (such as compressed air and SF6) limited applications in power equipment such as circuit breakers and switchgear [1]-[3]. Several billion liters of insulating liquids are used worldwide in power equipment such as transformers (power, rectifier, distribution, traction, furnace, potential, current) [4], resistors [5], reactors [6], capacitors [7], cables [8], bushings [9], circuit breakers [10], tap changers [11], thyristor cooling in power electronics, etc. [12]. In addition to their main functions of protecting solid insulation, quenching arc discharges, and dissipating heat, insulating liquids can also act as acoustic dampening media in power equipment such as transformers. More importantly, they provide a convenient means of routine evaluation of the condition of electrical equipment over its service life. Indeed, liquids play a vital role in maintaining the equipment in good condition (like blood in the human body). In particular they are responsible for the functional serviceability of the dielectric (insulation) system, the condition of which can be a decisive factor in determining the life span of the equipment [13]. Testing the physicochemical and electrical properties of the liquids can provide information on incipient electrical and mechanical failures. In some equipment, liquid samples can be obtained without service interruption.

On the frequency domain dielectric response of oil-paper insulation at low temperatures

Results of Frequency Domain Spectroscopy measurements are known to be largely influenced by environmental conditions, such as the temperature. Because field measurements, last hours after de-energizing the transformer, the ambient temperature may affect the results. Especially in cold regions of the world, extreme care are required to interpret the results when performing tests at relatively low surrounding temperatures. A better understanding and analysis of the dielectric test results are therefore only possible with a clear understanding of the physical behavior of the insulation system in response to the ambient conditions. In this contribution, the dielectric behavior of a composite oil paper insulation system has been explained from the properties of Debye basic model. A series of experiments have been performed under controlled laboratory conditions with preset moisture content inside the insulation. The equivalent circuit parameters of a laboratory made oil paper condenser bushing model were obtained using a non-linear optimization procedure. Since the dielectric parameters values are geometry dependent, poles, calculated from resistances and capacitances, were used as they are independent of the geometry. It was shown that the poles can be regarded as parameters able to be used for insulation condition assessment.

Dynamic modeling of DC arc discharge on ice surfaces

A dynamic model for predicting DC arc behavior and critical flashover voltage of ice-covered insulating surfaces is presented. The model takes into consideration insulating geometry, pre-contamination level, and characteristics of ice layers. Assuming arc behavior as a time dependant impedance, it is possible to determine various arc characteristics such as time histories of leakage currents, potential gradient, channel radius, trajectory, propagation velocity and the energy injected into the zones free of ice (also called air gaps). The simulated results provided by the model are in agreement with those obtained experimentally using a simplified ice-covered cylinder as well as a short string of five IEEE standard porcelain suspension units covered with artificial ice.

A predictive model of the positive discharge in long air gaps under pure and oscillating impulse shapes

This paper presents a general model allowing the prediction of the behaviour of given gap submitted to a given voltage. The only input data are the electrode geometry, the applied voltage waveform and the atmospheric pressure and temperature. The model also allows for various conditions for arrest or instability of the discharge. By assuming the discharge channel to be a long conductor and using a typical single LCR conductor line (L, C and R being respectively the line inductance, capacitance and resistance), we derive a predictive model for a positive discharge in long air gaps. This allows one to describe the evolution of the entire discharge, with the parameters L, C and R varying with time according to the channel characteristics and discharge geometry. The model allows us to determine the time histories of the current both during the leader and during the return stroke, the charge, the potential gradient in the leader channel, the power and energy injected into the gap and the channels thermal radius. It also permits the simulation of an image converter working in streak or frame mode and the leader propagation velocities. Furthermore, it allows the trajectory of the discharge, which is obtained from a probability distribution, to be plotted in real time. Good agreement between computed and experimental results was obtained for various test configurations.

Intelligent agent-based system using dissolved gas analysis to detect incipient faults in power transformers

Condition monitoring and software-based diagnosis tools are central to the implementation of efficient maintenance management strategies for many engineering applications including power transformers.

Dielectric spectroscopy techniques as quality control tool: a feasibility study

In this article, the feasibility of using time- and frequency-domain dielectric spectroscopy measurements to monitor the condition of oil-impregnated paper (OIP) condenser bushings is discussed.

Fundamental investigations on some transformer liquids under various outdoor conditions

This paper deals with humidity uptake of some transformer liquids and its influence on electrical breakdown. The investigations were performed under some selective outdoor conditions on different insulating liquids, e.g. mineral oil, esters and silicones used in HV transformers. The temperature was set at 23 or 60 /spl deg/C while the relative air humidity was varied between 20 and 90% to simulate climatic variation. The subject is important in HV transformers with an air breathing system since, even in such transformers where normally the air passes through a dehydrating apparatus with silica gel, a bad run can induce an accident. The scenario considered corresponds to such a defect in the breathing filter system, so that the insulating liquid in the transformer comes into contact with atmospheric humidity. The work presented is in two parts. The first part relates the evolution of the breakdown voltage in terms of the insulating liquid humidity content. The second part describes the evolution of insulating liquid humidity content in terms of surrounding relative air humidity. This evolution is observed during 30 days and the results are discussed regarding the ac electric strength according to International Electrotechnical Commission (IEC) or Verband der Elektrotechnik Elektronik Informationstechnik e.V. (VDE) standards. A correlation between the surrounding relative air humidity, temperature and humidity uptake rate is derived.

Experimental study and analysis of corona discharge parameters on an ice surface

The objective of this investigation is to understand the inception of ice surface corona streamers. A validated onset criterion that is useful for practical electrode geometries is presented. High-speed streak photography techniques were used to observe the streamers propagating over an ice surface. The effects of several experimental parameters such as freezing water conductivity and HV electrode radius on the streamer inception parameters of an ice surface have been examined. Empirical models have been proposed, based on elements derived from the inception parameters to account for corona streamer propagation velocity and inception voltage/field on an ice surface. Since the propagation velocity is a power function of the electric field, its value has been expressed in terms of the inception voltage.