Maik Koch

Aging Performance and Moisture Solubility of Vegetable Oils for Power Transformers

This paper discusses the suitability of vegetable oil as an insulating medium in power transformers. A high flash point and very good environmental compatibility compared to conventional mineral oil are good reasons to use vegetable oil in power transformers. In this paper, the physical and electrical performance and the aging behaviour of three natural vegetable oils are compared to one synthetic and one traditional mineral oil. The breakdown voltage of new, dry, natural vegetable oil is more than 80 kV, which is above the breakdown voltage of mineral oil. Regarding the aging of paper, natural esters are more friendly toward paper than mineral oil. The water solubility clearly exceeds that of mineral oil but the same dependence of dielectric parameters on relative moisture saturation was observed for all liquids. Aging of vegetable oils under the presence of air leads to oxidation and, thus, to a strong increase of viscosity. Therefore, transformers filled with these liquids need to exclude oxygen from air. The conducted experiments show vegetable oil to be applicable for use in power transformers.

Diagnostic Application of Moisture Equilibrium for Power Transformers

This paper aims at providing reliable methods of assessing moisture in oil-paper-insulated power transformers. The traditional method of moisture evaluation, oil sampling with the subsequent application of an equilibrium diagram, suffers from severe errors resulting in an overestimation of water concentration. To improve this, moisture sorption in cellulose (paper, pressboard) and moisture solubility in insulation oils were theoretically described and represented as sorption isotherms and equilibrium diagrams under the special consideration of aging. Several steps lead to the key recommendation of this paper, which is to use water saturation in oil and in cellulose to describe water in power transformers. The practical application and the use of vegetable oils are considered as well.

Robust measurement, monitoring and analysis of partial discharges in transformers and other HV apparatus

This paper describes and practically illustrates modern methods for robust, and at the same time sensitive measurement of partial discharges; applied in the laboratory, onsite and during on-line monitoring of HV apparatus. For conventional, electrical measurements, the principle of dynamic noise gating is illustrated. Above this, amplitude-based and spectrum-based impulse correlations allow for discrimination between noise and partial discharges, but also for separation of several discharge sources. Automatic pattern recognition then classifies the separated sources and provides basic risk assessment. Acoustic waves of partial discharge events serve for PD localization, but, due to their high damping, often require combination with electrical or electro-magnetic methods. Measurements in the UHF range, well-known from gas-insulated equipment, are of particular benefit for noise-immune analysis of oil-filled equipment, too and compared to the traditional, electrical method in this paper.

A new method for on-line monitoring of bushings and partial discharges of power transformers

This article describes a new on-line method for capacitance and dissipation factor measurements of high voltage bushings, for recording grid transients and for partial discharge monitoring at power transformers. Failure statistics identify HV bushings as one of the major reasons for transformer failures, which motivates operators for continuous on-line monitoring of their insulation systems health. The needed accuracy for reliable detection of failures is in the range of 5 pF for capacitance and 0.1 % for dissipation factor. For reaching this goal, a relative comparison of the bushing currents of the three phases, measured at the bushing taps, does not provide sufficient accuracy due to imbalances in the network. Additionally, simultaneous ageing, as it can be anticipated for a three phase transformer, will not be detected. To overcome these shortcomings, this paper introduces an absolute method for measuring capacitance and dissipation factor of HV bushings. Even under field conditions a measurement accuracy similar to accurate off-line power factor tests was reached. Partial discharges are an early breakdown indicator for HV dielectrics. With the measurement on the HV bushing tap, not only failure of the bushings dielectric but also of the transformer insulation system can be detected. As for off-line measurements, the discrimination between noise and true internal discharges is the key for successful diagnosis. Strategies for sensitive partial discharge measurements under disturbed on-line conditions are described, also involving measurements in the UHF range.

Moisture diagnostics of power transformers by a fast and reliable dielectric response method

This contribution introduces a newly developed dielectric response method to measure the dielectric response and calculate the moisture concentration in oil-paper insulated power transformers. This method combines the measurement of charging and discharging currents in time domain (Polarization and Depolarization Currents PDC) and the dielectric spectroscopy in frequency domain (Frequency Domain Spectroscopy FDS). The frequency range from 5 kHz down to 0.1 Hz is measured in frequency domain, whereas the range from 0.1 Hz down to 0.1 mHz or below is measured in time domain. During the measurement the time domain data are continuously transformed into frequency domain. This results into a reduction of the time need of 47–75 % compared to the conventional measurement in frequency domain. Two novel features improve moisture analysis: First, the influence of conductive aging byproducts is compensated, and next, the low frequency data are weighted. Moisture analysis is based on the comparison of transformer measurements to a modeled dielectric response as derived from a data pool. This data pool consists of dielectric responses of new pressboard samples measured at various temperatures and moisture contents. The time dependent properties of oil conductivity are also considered. This follows into a reliable moisture analysis even of aged oil-paper insulations. Examples of onsite moisture determinations compare this new method to other measurement methods like oil and paper sampling with subsequent Karl Fischer Titration.

Mositure ingress in free breathing transformers

This paper determines the moisture ingress in free breathing transformers by analysis of on-line monitoring data and by laboratory measurements. A grid transformer, a transformer equipped with a heat recovery system and a generator step-up unit served as example transformers. Approximately 1,6 kg of water enter the large grid transformer having moderate temperature changes. The transformer with the heat recovery system suffers from rapid temperature changes, thus approximately around 16 kg/a increase its moisture concentration. The constant load of the generator step-up transformer let nearly no water contaminate it. Laboratory investigations on dehydrating breathers proofed the drying capability of these systems. They achieve at average volume flows a drying efficiency of 99 %. To conclude, the water mass entering transformers strictly depends on the operation mode and breather condition of the specific transformer.

A fast and reliable dielectric diagnostic method to determine moisture in power transformers

This paper describes a novel dielectric diagnostic method ldquoDIRANArdquo which features a fast measurement technique and an improved analysis algorithm for determining moisture in pressboard and paper of oil-paper-insulated transformers. Dielectric response methods provide substantial advantages compared to moisture determination by oil sampling and conventional equilibrium diagrams, for example the influence of aging can be compensated and it is unnecessary to wait for moisture equilibrium. For the new instrument, the novel combination of time and frequency domain measurements substantially shortens the measurement time. The data analysis algorithm compares the measured dielectric properties of the actual power transformer to modeled dielectric properties. The software features a compensation for the influences of conductive aging by-products, which have similar dielectric properties as water. Without compensation the analyzed moisture content will be too high and may lead to unnecessary actions like drying. After moisture analysis, the new software categorizes the moisture concentration according to IEC 60422.

Emission and propagation mechanisms of PD pulses for UHF and traditional electrical measurements

Partial Discharge (PD) measurements are considered as a powerful technique for testing and monitoring the condition of high-voltage (HV) insulations. PD diagnostics is generally accepted as an early breakdown indicator, which is also reflected in numerous standards. In the present contribution, the authors describe the fundamentally different propagation mechanisms of electrical impulses in the conventional (according IEC 60270) and the ultra-high frequency (UHF) range. The low-frequency components of the PD signals propagate mainly via the conductor, whereas the high-frequency components (UHF signals) are radiated as electromagnetic waves. The sensitivity and spectrum of the measurement depends strongly on the geometric position of the PD, damping and resonances of the propagation path, and the frequency response of the receiver. Field simulation has been used as an appropriate tool for calculating PD signal propagation in HV apparatus.

On-site methods for reliable moisture determination in power transformers

This paper discusses present day and new on-site approaches to determine moisture in power transformers: dielectric response methods and equilibrium diagrams. In recent years dielectric diagnostic methods have been developed, which derive moisture concentration of the solid insulation from its dielectric properties. This paper presents a new method, which combines measurements in frequency and time domain, and which makes reliable diagnostics possible even for heavily aged insulations. The second part of this paper deals with equilibrium diagrams applied for moisture determination. Since the conventional application of these diagrams leads to erroneous results for aged insulations, a new representation of equilibrium diagrams based on moisture saturation in oil is introduced. Examples of onsite moisture determinations compare the new methods to conventional approaches and describe the assessment of moisture analysis results.

New tools for diagnostic measurements on power transformers

With the advancing age of power transformers, a regular check of the operating conditions becomes more and more important. The Dissolved Gas Analysis (DGA) is a proven and meaningful method such that if increased proportions of H2 and hydrocarbon gases are found in the oil, the fault must be located as soon as possible. In order to find out the reason for high gas values, further tests have to be performed for the transformer. Common test methods are: Winding resistance measurement, On-Load Tap Changer (OLTC) test, turns ratio and excitation current measurement, measurement of leakage reactance, Frequency Response of Stray Losses (FRSL), Frequency Response Analysis (FRA), Capacitance and Dissipation factor measurement and Partial Discharge (PD) measurement