One of the likely candidates for the recent explosion at the Sayano Shushenskaya hydro-electric plant in Russia is a transformer. But what is a transformer, and why might one explode?

Transformers are an ubiquitous part of our electricity generation and distribution network. Their job is essentially to transform an electricity supply from one voltage to another.  (Why would we want to do that? – to transmit power long distances without losing much of it on the way, we need to work at high voltage – but high voltage isn’t always what’s easy to generate, and isn’t what you want to feed into your home. So there needs to be a method of changing it).

They do this change by exploiting the interaction between moving electric and magnetic fields. A time-varying electric field generates a time-varying magnetic field, and vice versa. A transformer basically consists of two coils of wire on the same magnetic core. Current in the first coil generates a magnetic field in the core; this magnetic field then generates a voltage in the second coil. The clever thing is that by controlling the number of turns on each coil, we can control the voltage that comes out. If the second coil has twice the number of turns as the first, we will get double the voltage out (but only half the current – we don’t get energy from nothing).  A transformer needs alternating current (a.c.) to work – a d irect current (d.c.) in the first coil won’t generate a voltage in the second; this is why our domestic electricity is a.c.

 With a large transformer, as used in power stations, there will be a huge magnetic field generated in the core. This can place stress on the core. Imagine talking a simple bar magnet and cutting it in two, lengthwise, so you have two magnets parallel to each other – their north poles together and their south poles together. You will feel the force between them. Let them go and they fly apart. Anything that is magnetized will have this kind of magnetic stress – if the material is weak the stress may be enough to break it in two. Transformers, especially big ones, have to be designed very carefully to prevent this kind of effect happening. 

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