Fundamentals of condenser bushings

Abstract This article focuses on the concept of capacitance-graded, i.e. condenser bushings – both oil-impregnated and resin-impregnated paper bushings – for extra-high voltage transformers. After...

byK.K. MURTY


Abstract

This article focuses on the concept of capacitance-graded, i.e. condenser bushings – both oil-impregnated and resin-impregnated paper bushings – for extra-high voltage transformers. After a description of the dielectric loss, the article goes on to describe the selection, testing, transportation, storage and installation of these bushings, so as to avoid a bushing failure which may lead to a catastrophic failure of transformers.

Keywords: oil-impregnated paper bushings, resin-impregnated paper bushings, dielectric loss

  1. Introduction

The bushing is a device enabling one or several conductors to pass through a partition such as a wall or a tank, insulating the conductor from it. In this case the tank refers to the tank of the EHV transformer [1].

Various type of bushings for different applications are defined in IEC-60137 [1] and IS: 2099 [9]. However, for extra-high voltage (EHV) power transformer applications (66 kV and above), different types of external media matter are used with the lower end dipped in oil inside the transformer main tank.

There are three types of bushings generally used in EHV transformer applications:

  • Oil-to-air type – used for installations in outdoor substations, i.e. air-insulated substations (AIS),
  • Oil-to-SF6 gas type – used in gas-insulated indoor substations (GIS) and in hybrid substations,
  • Oil-to-oil type – used for oil-filled cable boxes to receive cables.

The condenser-type bushings include oil-impregnated paper (OIP) bushings and resin-impregnated paper (RIP) bushings. They have proved their superiority over the synthetic resin-bonded paper (SRBP or RBP) bushings by meeting stringent partial discharge (PD) requirement as stipulated in IEC-60137 for the bushings for EHV transformers [1].

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