Why do electric transformers explode?

Sixty milliseconds is fast. But sometimes, it's not fast enough. That's the gist of a great explainer by Cassie Rodenberg at Popular Mechanics, which answers the question, "Why do transformers explode?"

Before I link you over there, I want to add a quick reminder of what transformers actually are.

Although giant robots that turn into trucks do also explode from time to time, in this case we are talking about those cylindrical boxes that you see attached to electric poles. (Pesco posted a video of one exploding last night.) To understand what they do, you have to know the basics of the electric grid.

I find that it's easiest to picture the grid like a lazy river at a water park. That's because we aren't just talking about a bunch of wires, here. The grid is a circuit, just like the lazy river. Electricity has to flow along it from the power plant, to the customers, and back around to the power plant again. And, like a lazy river, the grid has to operate within certain limits. The electricity has to move at a constant speed (analogous to what engineers call frequency) and at a constant depth (analogous to voltage). This is where transformers come in.

Although the single circuit is easiest to picture, the grid is actually made up of lots of interconnected and redundant circuits. And those circuits aren't all at the same voltage.

Imagine that the water park has a couple different rivers — one for little kids that's really shallow, and another that's deeper. What if you wanted to take your inner tube directly from one to the other? To do that, you might follow a channel that slowly descends to a greater depth. Then, you could flow from the shallow river to the deep one without getting out of the water.

That's essentially what transformers do.

But when flooded with too much electricity, the sudden surge can cause a transformer explosion. As transformers detect an energy spike, they're programmed to turn off, but it can take up to 60 milliseconds for the shutdown. However fast those milliseconds may seem, they still may be too slow to stop the electrical overload.

A chamber full of several gallons of mineral oil keeps the circuits cool, but given too much electricity, the circuits fry and melt, failing in a shower of sparks and setting the mineral oil aflame. Mineral oil, in turn, combusts explosively and rockets transformer scything into the air.

All it takes is a trigger, a corroded or faulty wire, and the circuits surge will get ahead of the breaker ...

This explainer comes from 2010, but it's describing the same stuff you see in the current videos of transformers exploding all over New York in the wake of Hurricane Sandy.

To go back to the lazy river analogy, if there's a sudden rush of water pouring down the channel, it's likely to overflow. When that happens on the electric grid, what you get is an exploding transformer.

Read the full Popular Mechanics piece

Image: Taken by user georgeweld on Instagram. Found via Buzzfeed.


    1. And when was the last time you saw a pressurised river?
      What happens at greater depth, how does the pressure change? 

      I think depth is a reasonable part of the rivers analogy.

      1. It’s not a pressurized river, but water pressure in a hose that the analogy holds for; and that one is a bit more accurate for the physics of what happens.

    2. When a substation is, say, flooded (or has salt spray from the sea, from the winter salting of roads, or even an errant squirrel), the secondary windings (that’d be the non-power-plant end of the transformer) are connected to ground. The near-zero resistance to ground, means near-infinite current. The circuits/fuses/disconnectors at the substation are supposed to disconnect the transformer before it’s damaged, but sometimes not in time.

      Current (or amperage) is what melts fuses. So, when the ultra-high current surge (equivalent to volume-flow of water) is high and fast enough, it melts the transformers’ wiring, or even vaporizes them.

      Big substation transformers are usually cooled by oil – as in “the transformer is sitting in an oil bath”. Molten/vaporized metal meets oil…the oil explodes. Usually in a very spectacular fashion.

      Because I know you’re asking: If the power-plant-end is connected to ground, it’s the power plant and transmission lines which are connected to ground. These can ‘usually’ survive the trauma, since amperage is lower (y’know, ‘cuz the voltage is much higher). In really bad freezing rain conditions, grounding the power transmission lines is a method used to melt ice off of the lines – and is a procedure that would be performed at a substation.

      1. Got it, thanks. Where does most of the energy come from, electrical resistance heating (arcing) or decomposition of the oil? If it’s the oil, why is the color of the explosion toward blue?

        1. Let’s see if I remember the terminology from high school chemistry:  The oil needs a minimum energy to ignite – which is temperature.  Recall the ignition temperature of paper, from Bradbury’s book.

          The melting point of pure copper is 1,095 degC (or 1,981 degF for those people still living in the Pleistocene Era).  

          If the current is high enough to melt the transformer’s windings/wiring – the molten/vaporized copper will ignite the oil that touches it – assuming that the ignition point of oil is less than 1,095 degC, which is definitely the case. 

          The now-ignited oil will burn at a higher-than-its-ignition-point temperature. This will cause the oil molecules right next to the copper-touching oil, to ignite.  Rinse, repeat. 

          This ignited (as in ‘small-explosion’) oil will cause the rest of the oil be turned into a mist.  This increases the surface area of the oil exposed to oxygen, increasing the rate of reaction – Kaboom!

          If you check the video, you can see the explosion happens in a few phases.

          The colour of the light given off by the chemical reaction / explosion tells you what temperatures are achieved – the blue part of the flame in your propane torch is somewhere around 3,000 degK. That substation very likely achieved much higher.

    3.  Best to keep in mind that these are just analogies.  The same phenomenon can admit multiple analogies some of which might be better suited to some purposes than others but all of them break down at some level of analysis.  Whether the “correct” analogy is “voltage as pressure” or “voltage as depth” depends on the context.

      Maggie’s been consistent about “voltage as depth” and the consistency is probably more important from a science journalism point of view than any nitty-gritty details about why physics profs like to use the pressure analogy instead.

    4. Well, neither analogy is completely accurate, since analogies never are, so, whatever analogy helps people understand the situation at hand is useful.

      The “pressure” analogy is probably more useful more often, but this analogy seems sufficient for this model.  Models are not reality, voltage is not pressure — voltage is LIKE pressure, and voltage is LIKE pressure in more ways than voltage is like depth, but, in THIS explanation, “depth” is sufficient.

  1. “Why do electric transformers explode?”

    Why, for our YouTube entertainment of course! You mean those things actually do something else?

  2. Really in terms of physical things like water, I’d think the best correlation would be voltage to flow rate (or pressure) and current to volume.

    Why a transformer fails really involves what is causing it to be overloaded.  If it is a faulty disconnect circuit then the transformer simply has to deal with excess power input or draw leading to heat and breakdown of the mineral oil.  If it is from an external source of power input, like lightening, then it is very possible that the internal wiring becomes shorted and destroys everything by simply becoming the faulty part and drawing excess power.  (Of course the same thing can happen if a high enough voltage becomes present on the lines going to the transformer.)

    The pretty blue arc isn’t the transformer itself failing, as much as it is the excess voltage arcing between the conductors.  (Voltage that the safety system should have disconnected.)

  3. Electric transformers explode due to overcurrent conditions. If the coils heat up too much or and there are hot spots, acetylene will be produced from the transformer oil. The oil will also expand rupturing the case at which time the acetylene explodes and the transformer oil ignites. For this reason, very large transformers have monitoring and protective functions which will trip off circuit breakers that supply the transformers due to faults. In the event that your protective feature fails, or breakers fail to isolate the transformer then you are in big trouble.

    There are a couple of things that bother me about this post. You said

    Electricity has to flow along it from the power plant, to the customers, and back around to the power plant again.

    This isn’t correct. This may seem to be correct if you were talking about a 1 phase grounded system, but it doesn’t apply to 3 phase ungrounded systems that power plants use. And considering that there will be transformers that will electrically isolate and convert from 3 phase ungrounded to 1 phase grounded, it can’t be true.

    Second, your mechanical transformer analogy doesn’t work. It implies that there is an electrical connection between the coils. That is not true, the connection is through magnetic flux. And it doesn’t explain that the power input is almost equal to the power output. I’ve worked in electrical generation for many years. There is no decent mechanical analogy for a transformer that I have heard. Trying to make one work will only confuse people about the key characteristics of a transformer which include electrical isolation and near conservation of power.

    Edit: I meant to say “or there are hot spots”. Most transformers explode due to old age under nominal conditions when hot spots (part of the coil that has heated up far beyond normal due to degradation) produce too much acetylene. Another option which doesn’t occur often is the build up of static charge due to the movement of the transformer oil. One interesting potential failure mode (which would be taken care of due to protective features) is an overcurrent condition caused by solar storms on transformers with a neutral ground. Power plants sometimes have to limit output during solar disturbances due to this additional current.

    1. No analogy is ever going to be perfect. I invariably find that every analogy breaks down at some point, so it will always be a trade-off between the educational value and the accuracy. Ultimately, I try to use the analogy as a crutch – it gets the mindset headed in the right direction, but it will need to be discarded as one approaches a complete understanding of the topic or technology at hand.

      I would suggest, however, that there are good transformer analogies if look at air powered gas booster pumps — http://www.hydratron.co.uk/media/html/gas_pumps.htm — or air amplifiers — http://sprague.cwfc.com/Products/spokes/Amplifiers.htm

      In each case, these convert one pressure/volume to another pressure/volume in part by trading off one for the other. In the case of the gas booster pumps, it does it without actually connecting the “primary” gas supply with the “secondary” boosted gas. There is still a mechanical connection, but there is no connection of the “working fluid” on each side of the device, in much the same way that a properly operating transformer does not create regular electrical connections between the primary and secondary windings.

      The transformer does not create a mechanical connection, true. But this is an analogy, and the gas booster pumps don’t create a magnetic connection because they don’t work in the electromagnetic domain. I will note that most transformers do not have an active mechanical connection, but they do have some form of mechanical structure to ensure they are correctly constructed and aligned.

      The step-down side exists as well. The mixing version (an impure analogy, I agree) would be the Dyson ring fans, or a locomotive stack blower (had to look that one up), both a form of venturi blower. There are simple fan blowers than use high-pressure air jets to power the spinning blades. If you want the non-mixing aspects, then there air motor powered blowers — the blower discharge is separate from the air motor discharge.

  4. A gear box is a good analogy for a transformer. Converting high voltage low current (high rpm low torque) to higher current lower voltage (higher torque lower rpm).

    1. A gearbox still requires mechanical connection between the gears, while a transformer does not require electrical connection between the primary and secondary coils. Really, the best analogy that I know of is a torque converter. But since a torque converter is a far more complicated device to explain than a transformer, it is sort of a wasted analogy. Also, most people want to continue using the differential pressure or head equals voltage and flow equals current analogy, which requires more complicated mechanical components. The explanation of a transformer itself is easier than building these more and more complex analogies. And if you are clever during your description of a transformer and don’t analogize it, it is an easy segue to synchronous motors and generators (let the primary be the field and the secondary be the stator and let the movement of the rotor take care of the changing magnetic field)!

  5. Because the ones that didn’t combust turned out to cause cancer or hurt endangered species when they leaked.

  6. I get the impression that these explosions happen far more often in the US and southern / central America than in Europe. This can be deducted from highly accurate subjective observations of Youtube posting frequencies.

    Jokes aside, is this really just a skewed world view or are European installations less likely to fail so spectacularly? If so, what is different in the European and American transformer and fuse designs? This would probably make a very informative comparison.

    1. My guess would be simply the smaller distances involved in Europe.  I bet our electrical grids are just simply physically larger, leading to many more points of potential failure.

  7. The distribution voltage is high to reduce current. Loss is a function of current squared. Normally that voltage stays within a certain range and your local transformer provides a split phase output of 120/240. The lower for house outlets and the higher for stoves and water heaters etc.. If enough street load is lost while your circuit remains hot there can be a sudden surge. The pole transformers are often run beyond their ratings. The can is usually painted gray but parts of it will look reddish from continuous overheating.

  8. Ugh!  Reading everything I think more dark than light is spread about.

    First off: The oil in transformers is used to insulate the high voltage coils (they’re just bare wire) and cool them by conducting heat to the metal case via convection.  They used to use Polychlorinated biphenyl PCB’s because they are relatively flame resistant and a good insulator.  Trouble is some PCBs are horribly toxic.  So now they use other somewhat more expensive stuff.

    Second: Line Transformers all have a thermal circuit breaker and a fusible link to protect the transformer if it’s over loaded (drawing too much current).  If you short the secondary side of the transformer or there is a phat short circuit inside then the breaker trips, failing that hopefully the fusible link melts.  Lot of outages are due to breakers tripping.  In which case the lineman climbs the pole and resets the thing and suddenly you have access to p0rn again.

    Third: What happens when a transformer blows up is a arc caused by a voltage surge, age, whatever, etc between a set of coils.  In that case the magnetic field induces massive current through the shorted coils.  They heat and they melt, the oil inside carbonizes and no longer insulates.  It’s possible for this to go on for a long while as power is dumped inside the transformer via arcing* instead of being transferred through it. As long as the amount of current being drawn isn’t too much the breaker won’t trip.

    *If you’ve ever seen an arc welder in action, that’s what’s going on.

    Forth: Circuit Breakers.  They all have a minimum current at which they will trip.  and a maximum current that they can interrupt.  Higher than that and they usually fail.

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