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Diagnostics of Oil-Impregnated Paper Insulation Systems by Utilizing Lightning and Switching Transients
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Development of the power gridtowards a more reliable and smarter system requires frequent on-line monitoring of the power components. Power transformers and their bushings are particularly important components in a power transmission system and their insulation degradation may lead to catastrophic failures. Time consuming and costly replacement of these components raise the importance of their frequent monitoring. A fault in a power transformer bushing can also involve in the failure of the transformer. Therefore, on-line diagnostics of power transformers and their bushings is of great interest. Several methods exist for diagnostics of these components. However, some of them can only be done off-line in maintenance periods, and the existing on-line methods generally provide less information, especially on the internal solid insulation parts.

In this project, a new technique for on-line diagnostics of the power transformer and the bushing insulation is proposed. In this technique, natural transients happening in the power system such as lightning and switching surges can be used as stimuli for on-line dielectric response measurements. This technique can provide information on insulation close to what Dielectric Spectroscopy offers in off-line measurements. The wide-ranging frequency content of power system transients is their advantage for being usedas stimuli when measuring the Dielectric Response. The response can have particular signatures due to different types of defects in the insulation varying with frequency.

Oil-impregnated paper as a major insulation component in power transformer and its bushing has been investigated in this project. Moisture content and temperature, as two important degradation factors in this type of insulation, have been studied to evaluate the performance of the proposed technique in the diagnostics of the oil-impregnated paper. The results are verified with the dielectric response obtained through commercial instruments. The results show that the proposed technique has the ability to track the changes in dielectric response due to the moisture content and temperature. Measurements were done at both highvoltage (40kV) and low voltage (10V) levels, and the corresponding circuit models to achieve reasonable accuracy for the results are discussed.

Moving on from the material samples, a further study was done on three service-aged 150 kV bushings to investigate the feasibility of the technique on the diagnostics of power transformer bushings. Their dielectric response measured by the transient stimuli showed good agreement with their response obtained by the commercial instruments. The effect of the transformer winding on the transient response of the bushing is a further aspect of the real conditions for on-line diagnostics. This has been investigated through the simulation of transient models for transformers and bushings, and possible solutions for distinguishing the responses are presented.

The proposed new on-line diagnostics technique by utilizing natural transients can provide information about the insulation system in a certain range of frequency without interrupting the operation or requiring an external voltage source. However, the validity range of the results depends on the bandwidth of the applied transients and other measurement considerations. This approach can be valuable in frequent monitoring of dielectric properties of the power transformers and their bushings as a complement to the other available on-line techniques.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. , x, 56 p.
Series
TRITA-EE, ISSN 1653-5146 ; 2014:018
Keyword [en]
Capacitance, Dielectric Response, Loss tangent, Moisture content, Oil - impregnated paper, Temperature, Transformer Bushing, Transients
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-144092ISBN: 978-91-7595-097-6 (print)OAI: oai:DiVA.org:kth-144092DiVA: diva2:710964
Presentation
2014-04-29, H21, Teknikringen 33, KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20140409

Available from: 2014-04-09 Created: 2014-04-08 Last updated: 2014-04-09Bibliographically approved
List of papers
1. Insulation Condition Diagnostics of Oil-impregnated Paper by Utilizing Power System Transients
Open this publication in new window or tab >>Insulation Condition Diagnostics of Oil-impregnated Paper by Utilizing Power System Transients
Show others...
2013 (English)Conference paper, Published paper (Refereed)
Abstract [en]

Transients of varied spectral content, such as lightning and switching impulses, are inevitable phenomena that happen in power systems. These natural transients can be used as the stimuli for online dielectric spectroscopy for monitoring power components. The wide range of frequency in those impulses makes it possible to evaluate the dielectric response of their insulation. This paper presents a new technique for online diagnostic of oil-impregnated paper by utilizing those natural transients. Paper with different moisture levels was used to investigate the effectiveness of the proposed technique.The presence of water in the impregnated paper can be seen within those range of frequencies existed in high voltage transients. However, the validity of the results depends on the bandwidth of the applied impulse. Change in frequency response due to different moisture content in oil impregnated paper can be detected by this method. The results have been verified with results from low-voltage frequency-domain dielectric spectroscopy (FDS).

National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-144089 (URN)
Conference
18th International Symposium on High Voltage Engineering
Note

QC 20140409

Available from: 2014-04-08 Created: 2014-04-08 Last updated: 2015-02-06Bibliographically approved
2. Dielectric Response of Oil-impregnated Paper by Utilizing Lightning and SwitchingTransients
Open this publication in new window or tab >>Dielectric Response of Oil-impregnated Paper by Utilizing Lightning and SwitchingTransients
2015 (English)In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 22, no 1, 335-344 p., 07033403Article in journal (Refereed) Published
Abstract [en]

On-line monitoring of insulation systems in power components such as power transformer bushings has significant value for their reliability. Dielectric spectroscopy is known as a powerful tool in condition assessment of power components' solid insulation. However, it is difficult to apply it on-line. Therefore, a new approach of intended use for on-line diagnosis of oil-impregnated paper, as a major insulation component of power transformers and bushings, is presented. In this technique, natural transients such as lightning and switching impulses are used as stimuli for on-line dielectric response measurements. The wide frequency range of these transients is their advantage as stimuli for dielectric spectroscopy. The influence of moisture content and temperature on the dielectric response of oil-impregnated paper has been investigated for performance evaluation of the proposed technique. It is shown that this technique can track the changes caused by moisture content and temperature in the dielectric response of oil-impregnated paper. The results of both low and high voltage study cases have been verified by comparison with Frequency Domain Spectroscopy (FDS). According to the well-matched results with FDS, the proposed technique can offer a valuable approach for frequent monitoring of dielectric properties of oil-impregnated paper insulated systems such as those commonly encountered in power transformer bushings. However, the validity range of the results depends on the bandwidth of the applied transients and other discussed measurement considerations.

Keyword
Dielectric response, high voltage impulses, moisture content, oilimpregnated paper, temperature, transients
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-144093 (URN)10.1109/TDEI.2014.004664 (DOI)000349776900040 ()2-s2.0-84923222442 (Scopus ID)
Funder
SweGRIDS - Swedish Centre for Smart Grids and Energy Storage
Note

QC 20150331

Available from: 2014-04-09 Created: 2014-04-09 Last updated: 2017-12-05Bibliographically approved
3. Dielectric Response of Aged Transformer Bushings Utilizing Power System Transients
Open this publication in new window or tab >>Dielectric Response of Aged Transformer Bushings Utilizing Power System Transients
2013 (English)In: 2013 4th IEEE/PES Innovative Smart Grid Technologies Europe, ISGT Europe 2013, IEEE , 2013, 6695398- p.Conference paper, Published paper (Refereed)
Abstract [en]

The reliability of power transformer bushings is of great importance. Failures may lead to severe consequences. Frequent diagnostics by off-line methods is not feasible as the interruption is costly. Therefore it is of interest to have the possibility to perform on-line diagnostics. Transformer bushings can become aged during their operation periods. Several phenomena such as partial discharges, thermal effects and humidity may change the strength of the insulation in power transformer bushings. There are several methods which provide diagnostics of such components, but some of them can only be done offline in maintenance periods and some online methods would lack useful information about the insulation characteristics. In this paper, a new method is investigated, which uses natural transients such as switching and lightning impulses as stimuli to get the on-line dielectric response of the insulation system. Also, laboratory investigations are reported on three in-service aged 132 kV power-transformer bushings. The dielectric response measured under excitation by transient voltage is compared with results from low-voltage dielectric spectroscopy.

Place, publisher, year, edition, pages
IEEE, 2013
Series
IEEE PES Innovative Smart Grid Technologies Conference Europe, ISSN 2165-4816
Keyword
Complex Capacitance, Dielectric Response, Frequency Range, Transformer Bushing, Transients
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-142586 (URN)10.1109/ISGTEurope.2013.6695398 (DOI)000330939800164 ()2-s2.0-84893529868 (Scopus ID)978-1-4799-2984-9 (ISBN)
Conference
2013 4th IEEE/PES Innovative Smart Grid Technologies Europe, ISGT Europe 2013; Lyngby; Denmark; 6 October 2013 through 9 October 2013
Note

QC 20140312

Available from: 2014-03-12 Created: 2014-03-07 Last updated: 2014-04-09Bibliographically approved
4. Effect of transformer winding on the transient response of power transformer bushings
Open this publication in new window or tab >>Effect of transformer winding on the transient response of power transformer bushings
2012 (English)In: Lightning Protection (ICLP), 2012 International Conference on, IEEE , 2012, 6344346- p.Conference paper, Published paper (Refereed)
Abstract [en]

This paper deals with the response of power transformer bushings under the effect of transformer windings during electromagnetic transients such as lightning. In transient domain, investigating the characteristic of each power component separately in the network wouldn't be adequate. Each component can make an influence on the transient current and voltage that may be measured from another one. This paper proposes to find the effect of transformer windings on the transient response of the bushing, which contains valuable information on bushing's condition. It may result helpful in diagnostics of such an important power component to avoid catastrophic failure. Natural transients would be helpful in monitoring of the bushing by utilizing them as stimuli for online dielectric response measurement. Transient response of the lumped model of the bushing in EMTP is compared with its finite element model (FEM) in COMSOL Multiphysics to validate the robustness of modeling approach. According to the frequency component of the lightning, transient electromagnetic wave module of COMSOL is applied. Then, the effect of the transformer winding on the transient response of the bushing is investigated in EMTP.

Place, publisher, year, edition, pages
IEEE, 2012
Keyword
bushing, lightning response, transformer winding
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-114318 (URN)10.1109/ICLP.2012.6344346 (DOI)2-s2.0-84870678262 (Scopus ID)978-146731897-6 (ISBN)
Conference
2012 31st International Conference on Lightning Protection, ICLP 2012, 2 September 2012 through 7 September 2012, Vienna
Note

QC 20130115

Available from: 2013-01-15 Created: 2013-01-15 Last updated: 2014-04-09Bibliographically approved

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