Future proofing transformer tanks: why passive protection against internal arc should be studied?

		Task Force members: Moritz Bengler, Tirdad Boroomand, Samuel Brodeur, Joakim Johansson,  Håkon Ottar Nordhagen, Alvaro Portillo, Mohamed Ryadi, Ewald Taschler,  Peter Zhao, Hanxin Zhu
Jean-Bernard Dastous Convenor	Pascal Müller Chair SC A2

 

Low-impedance arc faults within insulating fluid-filled power transformers and reactors pose a critical threat, marked by a sudden surge in pressure, to the main tank and associated components like bushings and tap-changers. Such faults can lead to the rupture of these integral elements, resulting in catastrophic outcomes such as fires, oil spills, part ejections, substantial downtime, and potential safety hazards for both utility personnel and the public.

 

The situation during an internal arc is very different from the loading conditions for which a transformer is traditionally designed.

 

In recent decades, there has been a growing focus on addressing the issue of internal arcing. This has prompted utilities and manufacturers to devise effective mitigation measures aimed at minimizing or eliminating the adverse effects of internal arcing. Among these, the most effective strategies involve modifying tank and accessory designs to enhance their ability to withstand internal arcing up to a specified fault energy level and to fail safely beyond that point, without relying on additional components not normally part of a transformer.

 

Previous work on this topic

CIGRE Brochure 537 “Guide for Transformer Fire Safety Practices”, published in 2013, delivered several conclusions and recommendations regarding transformer tank design, the most important being:

 

• The probability of a tank rupture (and a possible fire) from an arc will depend on the arcing energy available and the tank pressure withstand capability.
• The effectiveness of venting devices cannot be relied upon to provide adequate and sufficiently fast pressure relief to prevent tank rupture for high energy arcing faults.
• 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.

 

Current work on this topic

CIGRE Brochure 537 and IEEE Std C57.156 provide general guidelines for good transformer fire safety practices but do not provide guidance on the requirements and methods of verification of designs to withstand internal arcing. To address these issues, a CIGRE Task Force under the A2 committee was formed in the fall of 2023, with the objective of generating a position paper on the subject.

 

The main purpose of this paper was to quickly collect the experience of utilities and manufacturers regarding passive arc energy containment, describe the state of the art, and identify possible gaps in current practice. Passive protection was defined by the task force as designing a transformer main tank and its internal and external components to withstand the effects of an internal arc, without using additional external devices.

 

About the Task Force

The task force comprised 11 members from 8 countries, with 4 utility representatives, 5 manufacturer representatives, 1 consultant and 1 representative from a research institute.

 

The scope and objectives of the task force were to:

 

1. Provide an overview of existing approaches, arc specifications, and standards for passive protection against internal arcing faults.
2. Identify and report on the best practices for evaluating the mechanical withstand of transformer tanks and accessories by calculation.
3. Provide an overview of manufacturers' current practices in designing arc-resistant tanks using a passive protection, fail-safe approach.
4. Provide examples of user mitigation strategies.
5. Identify the key elements for developing an IEC standard on the passive protection of power transformers and reactors against internal arcing.

 

After two years of work and several drafts, a comprehensive report was generated, whose content largely exceeded the size of a reference paper. This report comprised the following main sections:

 

1. Internal Arcing: Description of arcing-related phenomena, including arc energy determination and gas generation in mineral oil and alternative insulation fluids.
2. Design Pressure: Description of the main equation and calculation scheme to estimate the design pressure in the main tank.
3. Passive Protection Design: Discussion on the effectiveness of pressure-relief devices and available design modifications to render a tank more resistant to arcing faults.
4. Calculation Methodologies: Description of the available methods to evaluate pressure loads and assess transformer resistance to internal arcing (Figure 1).
5. Testing Methodologies: Description of the available methodologies, and their pros and cons, to assess transformers' arcing resistance: arcing tests, static pressurization tests, and gas injection tests (Figure 2).
6. Implementation Examples of Passive Protection Design: Description of how passive protection design is applied in the design of transformer tanks and components, as well as utilities' responses to arcing phenomena-related challenges.
7. Arc-Resistance Technical Specification Requirements: Discussion of points to consider in establishing such requirements.

 

 

Figure 1: Examples of calculation methodologies to assess pressure loads and tank resistance

 

 

But it doesn’t stop there for the Task Force!

 

Figure 2: Test examples

 

 

Making the transition from a Task Force to the Working Group A2.76

This will contribute to the development of a harmonized approach to power transformer passive protection design, with the goal of eventually lending itself to international standardization.

 

Still a lot of work ahead to:

• Understand the phenomenon of low impedance arcing and how to calculate the corresponding energy released during such events.
• Evaluate the design pressure in the main tank and its internal and external components, such as OLTCs and bushings.
• Understand the concept of passive protection design.
• Understand the available calculation methodologies to assess the strength of a given transformer design.
• Understand the available testing methodologies that can be used to verify experimentally the arc withstand capability of a given transformer design.
• Know the best design practices and calculation parameters to appropriately assess and design an arc-resistant tank.
• Identify the key elements that should be part of a standard on passive protection against internal arcing

 

The main benefit of this work will be to provide the industry with state-of-the-art arc-resistant tank design practices, calculation, and testing methodologies keeping everyone safe working around transformers one component at a time.