November 23, 2017

Study of transformer explosion prevention with bushing turret protection

Abstract Liquid-filled power transformers typically contain thousands of liters of flammable insulation. When this insulation breaks down, there is a high risk of transformer failure...

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Abstract

Liquid-filled power transformers typically contain thousands of liters of flammable insulation. When this insulation breaks down, there is a high risk of transformer failure that would endanger human life, generate environmental hazards, and destroy valuable assets. Because live tests involving arcing are expensive and potentially dangerous, numerical simulations are a useful alternative to study faults over a wide range of transformers. Since bushings are common sources of transformer failure, we examine the role of deploying a transformer fast depressurization system on two simulated transformer designs, with protections localized in the bushing turret region, to ensure that the transformer is robust to internal arcs.

Keywords: transformer explosion, explosion prevention, NFPA 850

1. Introduction

Liquid-filled power transformers typically contain thousands of liters of flammable dielectric insulation. When this insulation breaks down, the resulting short circuit triggers a chain of chemical reactions that produce a mixture of combustible gases such as acetylene and hydrogen. The sudden gas formation can quickly pressurize the transformer tank beyond its withstand capacity to conditions where catastrophic structural failure is imminent. The explosion of a transformer not only costs the power industry substantial financial losses, but also can endanger human life and environmental safety.

A 2015 survey report on transformer reliability by CIGRE found that annual transformer failure rates are on average about 1%, with rates over 1.3% per year for high risk classes of power transformers [1].

Among the various root causes reported, failures related to bushings are frequent sources of transformer fire and explosion. The same 2015 CIGRE document concluded from a survey of 675 major failures of transformers with voltage classes of at least 100 kV that bushings were the source of failure for 48.5 % of cases resulting in explosion or fire, for which the failure origin is known.

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