Kurt Feser

Procedures for detecting winding displacements in power transformers by the transfer function method

The paper investigates three different ways of using the transfer function method for detecting mechanical winding displacements in power transformers. The most reliable approach is time-based comparison , which requires finger print data from a previous measurement. Such information is, however, usually not available. For multilegged transformers without zigzag-connected windings the results of separately tested legs can be used as mutual references (construction-based comparison ). A third approach is to compare the transfer functions with those obtained from an identically constructed transformer ( type-based comparison). However, for a transformer with given nominal specification data, the winding design may over time undergo changes which causes changes to the transfer function. It is proposed to solve this problem by calculating tolerance bands using transfer functions from a big group of the same-type transformers. A novel statistical algorithm for this purpose is presented. The approach is demonstrated for a set of 28 specified identically 200-MVA power transformers.

Rejection of narrow-band noise and repetitive pulses in on-site PD measurements

This paper deals with the possibilities of noise suppression in partial discharge (PD) measurements under on-site conditions. After the presentation of previous work, a new concept of designing a digital FIR filter with a fixed order to suppress narrow-band disturbances is presented. The method is compared to a design using adaptive filters. A filter device, which was developed as an extension for a conventional PD detector, is presented. In the second part of the paper, a method for suppressing pulse shaped noise synchronous to the test voltage is discussed. The disturbing pulses are detected automatically in a PD signal by comparing several recorded cycles of the test voltage. There are two possibilities to suppress the pulses. The first one is an off-line method that allows attenuation degrees up to 30 dB, the second one works on line, and can process continuous PD signals. For the latter case a suppression of 14 dB for worst case pulses is possible with the developed hardware. In order to demonstrate the abilities of the filters, several measurements are shown. >

Fast Emission Measurement in Time Domain

According to the applicable standards, electromagnetic compatibility emission measurements are carried out in the frequency domain using a measuring receiver. In this paper, the theory, setup, and algorithms, as well as the practical aspects including the examples of a precompliance measuring system in the time domain, are presented. The advantage of this system is that the measurements can be carried out approximately 10-100 times faster with sufficient accuracy and signal-to-noise ratio in the detector modes peak, quasi-peak, and average.

Sensitive online PD-measurements of onsite oil/paper-insulated devices by means of optimized acoustic emission techniques (AET)

Recently developed monitoring-systems show the need as also the possibilities for online- and offline-onsite-diagnostics. The limit in sensitivity for electrical pd-measurements according to IEC 60270 which is reached using filters for sinusoidal disturbances and compensation for corona impulses is still not satisfactory. The acoustic partial discharge measurement (pd-measurement) is also a well known and reliable method often used by transformer manufacturers in testing. In laboratory setups a gain in sensitivity of the acoustic in comparison to the electrical pd measurements could be established. This is reached by adapting the sensors to the propagation path, a modern but moderately priced acquisition hardware and the introduction of new signal processing algorithms. The developed sophisticated measuring systems show remarkable results in online acoustic pd-measurements onsite.

Distortion-free measurement of high impulse voltages

It is shown that every high voltage divider has a limited bandwidth of some MHz, resulting in a limitation for the measurement of front oscillations or front-chopped lightning impulse voltages. It is pointed out that a voltage divider can be easily calibrated and can be constructed to prevent EMC-problems during impulse voltage measurement. Using a spherical electric field sensor with optoelectronic data transmission, a voltage measurement system with an upper bandwidth of 25 MHz has been constructed. The advantage of this system is the distortion-free measurement of all impulse test voltages in any circuit. After a short review of the principle and characteristics of the sensor, some practical applications are highlighted, showing that it can improve high-voltage measurement. >

Moisture in cellulose insulation of power transformers - statistics

The article presents the results of extensive investigations aimed at determining the moisture content in a large population of power transformers (161 units). The project was carried out during the period of 2003-2011 in Poland, Germany, and Sweden. All results were obtained by means of the Frequency Response Spectroscopy (FDS) method. Among the transformers tested were grid transformers, generator step-up (GSU) transformers, and distribution transformers. According to the classification rules established by IEEE, 30% among the investigated transformers could be classified as “wet” (2-4% moisture content). The observed moisture variation with age was found to depend on transformer design and operating conditions. It is suggested that, based on the presented statistical results and knowledge concerning transformer type and age, one may discriminate between probably dry transformers and transformers that require actual moisture analysis and therefore prioritize further condition assessment.

Risk assessment on defects in GIS based on PD diagnostics

The paper, prepared by CIGRE WG D1.03 (TF 09), presents the guidelines for risk assessment procedure on defects in GIS based on PD diagnostics. The procedure, described in detail in CIGRE Technical Brochure 525, starts with sensitive PD measurement to detect the critical defects and follows with identification of the type of the defect and its location inside the GIS. This information taken together with other essential data from laboratory measurements, manufacturers experience, design aspects and trend analysis of the PD activity, are the base for the estimation of the criticality of the defects. Finally, the risk assessment is performed based on the estimated dielectric failure probability and failure consequences that can be different in case of on-site testing or in service activity.

Universal Adaptive Differential Protection for Regulating Transformers

Since regulating transformers have proved to be efficient in controlling the power flow and regulating the voltage, they are more and more widely used in todays environment of energy production, transmission and distribution. This changing environment challenges protection engineers as well to improve the sensitivity of protection, so that low-current faults could be detected (like turn-to-turn short circuits in transformer windings) and a warning message could be given. Moreover, the idea of an adaptive protection that adjusts the operating characteristics of the relay system in response to changing system conditions has became much more promising. It improves the protection sensitivity and simplifies its conception. This paper presents an adaptive adjustment concept in relation to the position change of the on load tap changer for universal differential protection of regulating transformers; such a concept provides a sensitive and cost-efficient protection for regulating transformers. Various simulations are carried out with the Electro-Magnetic Transients Program/Alternative Transients Program. The simulation results indicate the functional efficiency of the proposed concept under different fault conditions; the protection is sensitive to low level intern faults. The paper concludes by describing the software implementation of the algorithm on a test system based on a digital signal processor.

A new method of emission measurement

According to the standards, emission measurements are carried out in the frequency domain using a test receiver. In this paper the setup and algorithms of a measuring system in the time domain is presented. The advantage of this system is, that measurements can be done approximately 10 to 100 times faster. The emphasis in this paper is set on algorithms to increase the measurement accuracy of narrowband and broadband signals. Furthermore, the choice of the best capture time for the time domain measurement is described.

On-site dielectric testing of GIS: theoretical and practical considerations

After reviewing the primary objectives of on-site testing of GISs, (gas-insulated substations), the authors provide an analysis of practical experience in the use of several alternative test techniques in a large sample of operational GIS installations. It is observed that AC tests thus far have been the most effective. Consideration of the physical background, however, supports the inclusion also of a lightning impulse voltage test, but with the test level strictly limited in order to take account of the associated very fast transients which could arise in the event of test flashover. Recommendations for an amended on-site test approach for GIS are presented, together with comments on the implications for further research on measurement techniques. >