Introduction
Internal arcing in liquid immersed transformers can vaporize the surrounding liquid and increase the tank pressure due to the sudden increase in volume. The sudden pressure relay (SPR) will trip the transformer and sound an alarm should a rapid increase in tank pressure occur.
The SPR is designed in such a way as to be able to distinguish between a slow and sudden tank pressure increase. SPRs detect only large sudden pressure fluctuations. Note that a Buchholz relay (if fitted) will also detect a large fault and trip the transformer due to the rapid flow of liquid from the main tank to the conservator tank; so there can be effectively two protection devices for one type fault type.
How SPRs Function
Modern sudden pressure relays consist of three bellows (see image below). Transformer pressure fluctuations deflect the main sensing bellow. Pressure is then transferred to two control bellows arranged like a balance beam ('see saw'), one on each side of a central pivot point. One bellow senses pressure changes through a variable small orifice whilst the other uses a fixed larger orifice. The smaller orifice bellow has a delay feature when registering pressure changes due to the small orifice. The smaller orifice diameter is automatically regulated by a bimetallic strip which adjusts with normal temperature fluctuations in the insulating liquid. The larger orifice bellow responds to immediate pressure changes and is affected much more quickly. Pressure differences within the tank will cause the balance beam to tilt and this activates a switch with a resultant alarm and shutdown. The SPR automatically resets when the two bellows again reach pressure equilibrium.
Sudden Pressure Relay
Older style SPRs have only one bellow. A sudden excessive pressure within the transformer tank exerts pressure directly on the bellow, which moves a spring loaded operating pin. The pin operates a switch which provides an alarm and trip.
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Additional Resources
https://www.wecc.org/Reliability/Transformer%20Protection%20Sudden%20Pressure%20Relays.pdf
https://globecore.com/4-6-sudden-pressure-relay
https://netaworldjournal.org/functional-testing-for-a-sudden-pressure-relay