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Insulation Strength of Transformer


Abstract of the project

The power transformer is the most significant component in a power system whose reliability mainly depends on its insulation. The test voltages are of different types viz. Power frequency, lightning impulse and switching impulse. In case of tests with lightning impulse voltage, standard wave shape of 1.2/50 µs is used to test the transformer. While testing large power transformers, it is very difficult to generate impulse voltage with standard 1.5/50 µs waveshapes. Even if it is tested with standard waveshape, parts of the windings undergo stress with non-standard impulse voltage of both unidirectional oscillatory waveshapes. Moreover, during operating conditions, most of the power apparatus in the power system are exposed to impulse voltage of non-standard waveshapes, which are not according to the shape defined in standard IEC 60060. Hence apart from the standard 1.2/50 µs voltage, it is essential to estimate the insulation strength for impulse voltages with non-standard waveshapes also. As the insulation strength is not the same for all the waveshapes, a detailed study on the behaviour of insulation under various types of over voltage is essential to make an optimal design.

In various types of insulation, transformer oil and oil-impregnated paper (OIP) are the most common insulating medium used in the power apparatus. Unavoidable small voids in resin cast transformer may lead to partial discharge. As most of the transformer failures are due to these small insulations, it is a must to assess the breakdown characteristics of air, transformer oil and OIP in small gaps.

The breakdown characteristic of insulation under standard lightning impulse plays a crucial role in the insulation coorrdiantion of the entire power system. Therefore it is essential to adhere to the standard (1.2/50 µs) lightning impulse voltage-time characteristics for any insulation. Experimental voltage-time characteristics of air, oil and OIP are obtained using statistical procedures and a Hyperbolic voltage –time characteristics model is proposed.

To evaluate the insulation strength under unidirectional non-standard impulse voltages, Disruptive Effect method is used. The parameters of the Disruptive Effect method are obtained from the voltage-time characteristics. The variations of Disruptive Effect method parameters (onset voltage and critical disruptive area) with respect to small electrode gap distances are determined. Validation of the method is done using voltages of unidirectional non-standard impulse waveshapes with different frequencies of oscillation.

The strength of these small insulation gaps under bi-directional oscillating impulse voltages is estimated by using unconditionally sequential approach. Bi-directional waveshapes of different frequencies are generated using modified Marx circuit and using transformer windings. Efforts are made to determine the value of the material-dependent parameter (of the unconditionally sequential approach) the positive reinforcement time (tpr). A modification to the unconditionally sequential approach is suggested to take into account the polarity effect in the breakdown due to bid-directional waveshapes.

In general, this project presents the procedure to estimate insulation strength under non-standard impulse voltages with small air, oil and OIP insulation gaps.

Extent of Industry involvement /Interaction

A typical transformer windings and Reactors of 66kV to generate uni-directional and bi-directional The most onerous tap positions in a power transformer of Hackbridge Hewittes Ltd., Chennai was decied based on the developed, unconditionally sequential approach.

Possibility of technology transfer /commercialisation

As the project has obtained the disruptive effect parameters based on extensive experimentation for all complex geometries, the proposed methodology with the experimental data and the mathematical model will be of great use to all high voltage design engineers to achieve a reliable design.

No. Of Research Papers Published in Journal: Two

(i) S.Venkatesan, S.Usa, ‘Impulse volt-time characteristics of oil and OIP insulation “American Journal of Applied Science, Vol2, No.2, pp.591-596,2005.
(ii) S.Venkatesan, S.Usa, ‘Impulse strength of transformer insulation with non standard waveshapes” IEEE Transactions on Power Delivery, October 2007, pp.591-596,2005.

No. of Research papers presented in conference /Seminars : 4

No. of Ph.D/M.Tech thesis emerging out of the project : One Ph.D and Four M.E