Peter Werelius

Dielectric spectroscopy for diagnosis of water tree deterioration in XLPE cables

An HV dielectric spectroscopy system has been developed for diagnostics of water tree deteriorated extruded medium voltage cables. The technique is based on the measurement of nonlinear dielectric response in the frequency domain. Todays commercially available systems are capable of resolving low loss and small variations of permittivity as a function of frequency and voltage. Experience from more than 200 field measurements was combined with laboratory investigations. Small samples were used in an accelerated aging test to elucidate the correlation between water tree growth and dielectric response. Furthermore, field aged cables were investigated in the laboratory. It has been shown that the dielectric response of water tree deteriorated crosslinked polyethylene (XLPE) cables can be recognized and classified into different types of responses related to the aging status and breakdown strength. The influence of termination and artifacts such as surface currents was investigated. The measurement method enables us to separate the response of the cable from the influence of accessories. Finally, two different field studies of the implementation of the diagnostic method are presented. The field studies show that the fault rate decreased significantly when replacement strategy was based on the diagnostic criteria formulated.

Dielectric frequency response measurements and dissipation factor temperature dependence

The condition of the insulation is an essential aspect for the operational reliability of electrical power transformers, generators, cables and other high voltage equipment. Transformers with high moisture content can not without risk sustain higher loads. Bushings and cables with high moisture content at high temperature can explode due to “thermal runaway”. Typically, dissipation factor (DF) or power factor (PF) test at power frequency 50/60 Hz is carried out in the field following well known procedures. DF measured values are then normalized to 20oC for comparisons with guidelines and trending. However, the temperature correction factor for the normalization of the field measurement is questioned because cellulose with different moisture contents as well as oil with different conductivity will have different correction factors. It has been proved that good insulation has less temperature dependent response than the bad insulation. DFR modeling, accurately taking the temperature effect into account, can be used to model the temperature dependence of DF over a wide temperature range based on measurements over a frequency range. Naturally, from such modeling, also temperature correction factors for correcting a measurement values obtained at one temperature, e.g. 32C, to a reference temperature, e.g. 20C, can be calculated. This paper will provide a background of DFR and its modeling specifically applied to model the effect of temperature and calculating temperature correction factors based on actual DFR response. The theoretical basis is backed up with several case studies of measurement on samples as well as on real objects, e.g. bushing and transformers at multiple temperatures.

Frequency domain response of medium voltage XLPE cable terminations and its influence on cable diagnostics

This paper presents dielectric spectroscopy measurements of medium voltage XLPE cable terminations. Different response types are classified and the influence of terminations on cable diagnostics is evaluated and verified with laboratory and field measurements.

Dielectric Frequency Response and temperature dependence of power factor

Modern technology and developments in signal acquisition and analysis techniques have provided new tools for transformer diagnostics. Of particular interest are dielectric response measurements where insulation properties of oil-paper systems can be investigated. Dielectric Frequency Response, DFR (also known as Frequency Domain Spectroscopy, FDS), was introduced more than a decade ago and has been thoroughly evaluated in a number of research projects and field tests with good results. DFR data in combination with mathematical modeling of the oil-paper insulation is proven as an excellent tool for moisture assessment. Since the modeling theory contains influence of temperature, DFR and modeling can be used to calculate the temperature dependence of the insulation system. This paper gives a background to DFR, insulation modeling and how these tools can be utilized to improve understanding of power factor temperature dependence and how this can be used for decisions on maintenance and/or replacement.

Improvements of the transformer insulation XY model including effect of contamination

Dielectric Frequency Response, DFR (also known as Frequency Domain Spectroscopy, FDS), was introduced more than 20 years ago and has been thoroughly evaluated and proven in a number of research projects and field tests with good results. DFR data in combination with mathematical modeling of the oil-paper insulation is proven as an excellent tool for understanding insulation properties e.g. moisture content in cellulose insulation and temperature dependence of the insulation system. The model used to describe the insulation system inside transformers and bushings is called an XY-model. The model works well for most transformers but there are sometimes measurements showing an irregular response and not the expected shape for oil-paper insulation. In those cases, the moisture analysis becomes more difficult and sometimes almost impossible. One example is “frequency hump” phenomena sometimes observed in DFR measurements. A model of the internal creep currents from e.g. contamination has been developed and by applying it in moisture assessment software, better interpretation is obtained. Simulations are complemented with measurements on an actual XY-cell with a conductive layer on the surface of the insulation. Results support the simulation model.

Best practices for Dielectric Frequency Response measurements and analysis in real-world substation environment

Modern technology and developments in signal acquisition and analysis techniques have provided new tools for transformer and bushing diagnostics. Of particular interest are dielectric response measurements where insulation properties of oil-paper systems can be investigated. Dielectric Frequency Response, DFR (also known as Frequency Domain Spectroscopy, FDS), was introduced more than 20 years and has been thoroughly evaluated in a number of research projects and field tests with good results. DFR data in combination with mathematical modeling of the oil-paper insulation is proven as an excellent tool for moisture and oil conductivity assessment of power transformers. Dielectric response measurements are usually performed at a much lower voltage level than traditional power frequency tan-delta measurements. Due to this, the signal-to-noise ratio may sometimes be extremely low, especially at low frequencies and when measuring low capacitance objects e.g. bushings and instrument transformers. The interference suppression capability of the test set thus becomes an important parameter when considering different methods and instruments. In this paper investigation of the electromagnetic interference in terms of AC hum currents, induced DC currents and low frequency interference in AC as well as HVDC substations are presented. Solutions to handle these different types and levels of interference are investigated and examples of measurements under those conditions are presented. The paper also covers DFR response analysis using the XY-model. Numerical analysis using COMSOL has been performed and compared with the simplified analytical XY-model to investigate how non-ideal conditions influence geometry parameters and results.

Advanced Technique for Moisture Condition Assessment in Power Transformers

The presence of moisture in a transformer needs to be monitored throughout its service life. Moisture deteriorates transformer insulation by decreasing both electrical and mechanical strength. High moisture content accelerates solid insulation aging, reduces the breakdown strength and makes the transformer vulnerable to the overload conditions due to high temperature spot bubbling. In addition to that, partial discharge can occur in a high voltage region because of the moisture disturbance. Traditional indirect estimation of the moisture concentration in the solid insulation of power transformers includes testing the oil samples as well as measurement of the insulation resistance and loss tangent (50/60 Hz) of the transformer. However, these methods usually give limited information and may lead to wrong conclusions. Direct measurement is not viable and may not be representative to take a paper sample from the surface insulation because moisture distribution is not homogeneous along the insulation geometry. Dielectric Frequency Response, DFR was introduced more than 20 years ago and has been thoroughly evaluated and proven. Several documents have been published summarizing the research work and field tests all over the world. DFR is a practical non-intrusive and non-destructive technique for moisture condition assessment in power transformers, a breakthrough compared with traditional methods. Scientists, researchers and utility operators have shown great interest in the development and application of DFR technique. In this paper, the limitation of traditional methods is presented at first. Later, a comprehensive review of DFR method demonstrates its advantages over traditional methods. Finally, latest research about the mathematical model, temperature correction and test voltage is included to answer common questions regarding the application of DFR method.

Continuous monitoring of power transformer solid insulation dry-out process — Application of dielectric frequency response

Power and distribution transformers are some of the most important components of the energy system, and the aging of these devices during normal operation is inevitable. From the beginning of the manufacturing process, a strict control over the quality of materials used for the construction and assembly of transformers is implemented. Special attention is given to the solid insulation components in order to minimize exposure to the environment where they might be contaminated. After completion of assembly of the active part of the transformer, the windings and core are subjected to different dry-out processes to dry the solid insulation without affecting the life expectancy parameters defined by the degree of polymerization of the cellulose. Throughout the life of the transformer and due to the normal aging effect, several byproducts will evolve within the insulation system, and field operators will encounter the challenge of re-conditioning the insulation system of the transformer removing as much as possible these aging byproducts on-line or off-line. A variety of methods are available; selecting the right method should be the first decision. The dryout process must later be evaluated, and remaining moisture in the insulating system requires accurate estimation before the transformer is put back on service. In this work, factory and field dry-out processes are continuously monitored using Dielectric Frequency Response (DFR), analysis and validation is carried out comparing results against other available methods. On-line DFR application is demonstrated to optimize the dry-out process, providing valuable information regarding process efficiency.