On-site partial discharge diagnostics of cast-resin transformers

On-site partial discharge diagnostics of cast-resin transformers

Special Edition - Dry-type Transformers
  1. Introduction

The use of dry-type transformers has proven to be very successful in a range of different applications. Such equipment is often used in medium-voltage power grids in industrial plants and in the marine and civil sectors, such as in subway networks. Deciding factors for the use of dry-type transformers include low-maintenance operation and reduced fire risk.

The latter can be of significance for installation in building complexes and tunnels. Furthermore, the absence of mineral oil as an insulating medium eliminates the need for oil sumps. This makes dry-type transformers particularly advantageous for use in wind turbines, as they are often installed in areas with high groundwater protection requirements or at sea.

In the context of wind turbines, the transformers, which are connected directly to the turbine generator, can be considered as generator step-up units (GSU). Diagnostic measurements and the condition assessments of these units are of corresponding significance. For liquid-filled transformers, periodic oil analyses provide a solid base for a comprehensive health assessment. Based on the oil results, further diagnostic measurements can be performed like dissipation factor, partial discharge (PD), SFRA or winding resistance measurements which can then trigger corrective maintenance measures if necessary. For dry-type transformers, especially cast-resin insulated units, the application of the established diagnostic tools is limited. One of the main differences between oil-filled and dry-type transformers is the insulation system. Instead of mineral oil that acts both as an insulating and cooling medium, the windings of dry-type transformers are, in most cases, cast in epoxy resin. Since no oil analysis is possible, a substitute tool for insulation diagnostics has to be found.

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