How to consider explosion load of a transformer?
Abstract
When oil filled transformer is installed indoors, the explosion load becomes relevant for the building design.
Introduction
In a common setup, dry type transformers are installed indoors while oil-filled transformers stand outdoors. Dry transformer can be located outdoors as well if the enclosure and overall design support that. In a rare case, the oil-filled transformer is placed indoors. Such installation requires some additional safety considerations. One of them is the explosion load of the transformer.
What is transformer explosion load?
A transformer “explosion load” refers to the mechanical blast pressure exerted on its tank or surrounding structure when an internal fault causes a rapid pressure rise in the oil. This is a safety‑critical topic that needs to be taken very seriously. In this blog we address the engineering factors that influence it, and how it is assessed in practice.
What creates an explosion load
Step 1 (explosion itself)
The root cause is typically an internal transformer fault, such as short circuit or electric arc, leading to evaporation of insulating oil and generation of gases.
The process leads to rapid gas generation and pressure spike inside the tank. If the tank cannot relieve this pressure fast enough, it can rupture and create a blast wave. This mechanism is described as a rapid pressure rise caused by arc‑induced oil vaporization.
Step 2 (oil burn)
The insulation oil is heavily leaking from the ruptured tank. As the oil is now outside, there is plenty of oxygen available for combustion. If the ignition temperature is reached, the oil starts burning. Extensive burning quickly heats up the ambient air that increases the pressure inside the room.
Factors determining the magnitude of explosion load
There are a few factors having a direct impact on the magnitude of explosion load. We will briefly mention the most important ones.
- Location and severity of internal fault
- Rate of pressure rise
- Tank construction
- Transformer size and volume of liquid
- Type of insulating liquid
- Building design
- Protection system and its reaction
ad 1. Location and severity of internal fault
The energy of an electric arc depends on arc voltage and current and on the duration of the arc. The higher the energy the more gas is generated and the higher pressure inside the tank. Besides that, it depends on the location of the fault as well. Fault closer to the tank wall may produce higher local stress (during the transient the pressure inside the tank is not distributed uniformly).
ad 2. Rate of pressure rise
It is not only about the absolute pressure reached during the fault but also about how quickly the pressure rises. Rapid rise of pressure is challenging as the protection relief devices may not react fast enough.
ad 3. Tank construction
Next factor is the robustness of the transformer tank. It depends on the thickness of the steel sheet and geometry/shape of the tank (most transformer tanks are rectangular but oval shapes exist as well).
ad 4. Transformer size and volume of liquid
Larger transformer contain larger volume of liquid (thus more stored energy). They also feature higher power density.
ad 5. Type of insulating liquid
The properties of the insulating liquid influence the gas generation. Most transformers use mineral oil. However, there is a variety of alternative liquids, such as e.g. synthetic esters or natural esters. Those alternative liquids have higher flash point thus increasing a fire safety. However, the type of liquid also influences the degree of gasification in case of fault.
→ Read article about bio-based insulating liquids for transformers
ad 6. Building design
Next point is the building itself. The design determines how much overpressure is possible without structural damage. Is it a brick building? Does it have concrete walls? Is there any pressure relief system? If so, what size? How much free space is inside the building?
ad 7. Protection system and its reaction
The choice of protection system determines how fast is the internal fault detected and eliminated. In case of short-circuit or electric arc, the factor of time is crucial with respect to the amount of dissipated energy and the generated gas.
→ Read more about protection of VFD transformers
Naturally, occurrence of transformer explosion as a consequence of severe faults shall be minimized as much as possible. That is done by suitable system design and reliable protection system. Still, the building shall be designed to withstand such worst case.
Although the explosion is a highly transient event, the approach for a building design is to use an equivalent static load. How much is such a load? Purchase our premium subscription and learn more.
References
[1] Arc resistant design: Introduction, MB Drive Services, September 2021, available online, https://mb-drive-services.com/arc-resistant-design-introduction/
[2] Transformer bio-based insulating liquids, MB Drive Services, November 2021, available online, https://mb-drive-services.com/transformer-bio-based-insulating-liquids/
[3] Protection of VFD transformers, MB Drive Services, March 2020, available online, https://mb-drive-services.com/protection_of_vfd_transformers/
[4] SERGI – Transformer protection, https://sergi-energy.com/risks-protection/transformer-explosion-risk/
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