Cigré SC A2: Power transformers and reactors – fire safety

July 14th, 2014, Published in Articles: Energize

 

A failure of a transformer does not always cause a fire, however, if this does happen the consequences can be very severe. The extent of the damage may even cascade to the adjacent plant. Cigré Study Committee (SC) A2 initiated a working group (WG A2.33) which looked at transformer fire safety and compiled a technical brochure on the topic.

The scope of the work focused on transformers (and reactors) rated at less than 10 MVA and 66 kV. However, much of the discussion and many of the recommendations will also be applicable for transformers with lower ratings. The main focus is on mineral oil immersed transformers, comprising an electrical grade laminated steel core, with Kraft paper and/or enamel covered copper or aluminium winding conductors where the active parts is contained within an oil filled steel tank. The design and construction of transformers is not discussed in this brochure and it assumes that the reader is familiar with types of transformers which are within its scope. There are other forms of insulating liquids which are used in transformers for fire risk mitigation and/or environmental reasons. Most of these fluids are high flame point fluids which have lower, but not zero, fire risk. Those fluids are discussed in this brochure in the context of fire risk mitigation.

Much of the content of this brochure is equally applicable to industrial and other special purpose oil immersed transformers. However, some of these transformers have special features and installation requirements, which require specific attention and special requirements which may not be covered in this brochure.
The aim of the brochure is to promote good fire safety practices which it endeavours to do by:

  • Presenting typical transformer fire scenarios.
  • Quantifying the probability of transformer fires and tank ruptures.
  • Discussing internal arcing, and possible measures which reduce the risk and consequences of a transformer fire.

The guide presents practical and cost effective strategies for fire prevention and for control and risk mitigation measures which can be applied to both transformers and transformer installations. The brochure is intended for use by transformer users and engineers who specify and design transformers and transformer installation, and help them define and apply best practices in the domain of transformer fire safety. This guide, however, does not replace the relevant national, provincial or local regulations which must be considered, and where mandatory, complied with.

The brochure assumes that the reader has a basic understanding of transformer technology and substation installations. Tank rupture prevention and the containment of oil are considered critical for limiting the consequences of a transformer failure and reducing the risk that a minor transformer fire escalates into a major or catastrophic oil fire. The objective of this brochure therefore includes defining key parameters influencing tank ruptures. Results obtained by model simulation, laboratory testing and experiences during service life are presented to give an overview of the state-of-the-art in that domain.

The work done made distinction between two separate situations, being:

  • where a transformer may catch fire, i. e. causing the fire.
  • where a transformer is a victim to a fire originating elsewhere.

The working group concentrated on the case where a transformer is the origin of the fire and the transformer installation and adjacent assets may become victims of a fire caused by the transformer. The case when a transformer is the victim of the failure is also of concern. If an external fire is given sufficient time to heat up the transformer liquid so much that the liquid is spilled over via the conservator, or other device, then the external fire will be strengthened if the transformer is oil filled or contains other combustible liquid.

The working group endeavoured to do this by preparing technical brochure 537, which covers the following aspects of transformer fire safety:

  • An introduction to transformer fire safety issues with listing of useful standards and guide documents with information on transformer fire safety.
  • Physics of fires and typical transformer fire scenarios.
  • Guidance on the probability of a transformer fire event occurring.
  • Discusses the physics of arcing within transformer tank and gives formulas and examples on how a user might be able to predict the likely range of arcing energy, volume of gas generated and likely pressures which might be developed during an internal arcing event.
  • Guidance on issues to consider when determining what fire protection maybe required and what should be installed at a specific site. Discussing the risk mitigation options available for the transformer.
  • Discussing the risk mitigation options available for the substations and other transformer installations.
  • Advice on planning and the importance of being prepared for a fire event.
  • Conclusions and some recommendation for improvement on standards for improved fire safety on tanks and cable boxes.

The working group concluded that the current IEC standard and most national standards for transformers are deficient in that they do not provide any guidance on the requirements and methods of verification of transformer tank designs and that of cable boxes. Its view was that these standards should have minimum requirements and provide guidance on pressure withstands capability or internal arcing withstand capability with or without venting. To the best of the working group’s knowledge, only Japan has a guide document for design of transformer tanks.

The working group therefore recommended that the IEC extends the existing 60076 series on transformer standards to include prescriptions or guidance for the internal arcing withstand capability of transformer tanks, which should address:

  • Guidance for the purchaser on what information should be provided in the specification or other form of documentation.
  • Guidance for the suppliers on how to document performance of tanks.
  • The requirement for different “classes” of transformers: (distribution transformers, power transformer, gas cushion transformers, and possibly with different requirements for various voltage classes, system fault levels and installed location).
  • Possibly venting type/requirements.
  • Arc energy withstand capability rating of tanks.
  • Verification method(s) for fault energy withstand capability rating of tanks.

It is further recommended that the IEC produces a new standard on cable boxes to address:

  • Guidance for the purchaser on what information should be provided in the specification or other form of documentation for cable boxes.
  • Guidance for the supplier on how to document performance of cable boxes.
  • Classification of different types of cable boxes: oil insulated, air or gas insulated, possible voltage rating, current rating, degree of protection (IP class), venting method.
  • Arc energy withstand capability rating of cable boxes.
  • Verification method(s) for fault energy withstand capability rating.

The brochure is available at www.cigré.org

Contact Rob Stephen, Eskom, Tel 031 204-5693, rob.stephen@eskom.co.za

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