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Paper RSE 14005598

Requirements for increasing safety and reliability: new design of CurrentTransformers and experience on multiple stress test on composite bushings


Cigre' Session 45 24-30 , Agosto-2014.

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A. Di Giulio (TERNA S.p.A), D. Falorini (TERNA S.p.A), A. Fraioli (TERNA S.p.A), M. Gullo (STRI AB), M. De Nigris (RSE SpA) , V. Iuliani (TERNA S.p.A), M. Svanberg (ABB), P. Cardano (ALSTOM Power), G. Pirovano (RSE SpA)

T&D 2014 - Transmission and distribution of the electric energy

The paper describes the reasons and the advantages that led TERNA to the adoption of new design of Current Transformers with polymeric insulation and SF6 gas filled. The report illustrates also the construction charactertics of these components and the multiple stress tests on bushing samples carried out at RSE and STRI laboratories.

Some failures of HV equipment and transformers that occurred a few years ago, led Terna to devote the maximum effort to improve the safety of substations. Terna therefore carried out an extensive campaign in search of solutions to reduce the risk linked with the potential explosion of porcelain insulated equipment, the projection of equipment pieces, and to increase at the same time the overall reliability of the substations.

As for the risk of explosion, the research was directed toward the possibility to replace oil with SF6 in equipment internal insulation while, for the risk of projection of pieces of equipment (including the porcelain housing) in case of explosion, it was decided to adopt equipment housings made of polymeric materials.

Terna has started to install exclusively SF6 type HV circuit breakers since the early 80’s and, since 2003, has equipped circuit breakers with the overpressure valves and polymeric external insulation, to further improve their safety characteristics.

As far as voltage transformers (VTs) are concerned, it was not possible to reduce dramatically the risk of explosion, as data transmission requires the use of capacitive VTs and as the present constructive technology of voltage dividers is based on the use of dielectric oil. The porcelain external insulation of VTs, has systematically been replaced with polymeric insulation. CTs with SF6 insulation were introduced in Terna since 1998 but, at that time, insulating oil was preferred because of the risk of possible leakage of SF6 equipment, triggering the necessity of putting out of service an entire substation bay to avoid the risk of failure of the component. The project discussed in this paper was carried out in view of eliminating the drawbacks related to the use of oil and porcelain insulation in CTs, thus enhancing their safety and reliability. The activity was conducted involving two major European equipment manufacturers.

Transformer bushings are purchased since 2010 only using external polymeric bushing and the RIP technology.Before proceeding to a large-scale deployment, Terna carried out a series of tests with STRI and RSE to assess the effectiveness of the solutions envisaged and to verify if the improvement introduced, in addition to ensuring greater safety in case of failure, did not reduce the performances of the equipment in terms of operation and reliability, with special reference to its behavior under different environmental conditions.

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