Video: C50

Video transcript

What a power transformer needs watched, and one monitor that watches the temperature, runs the cooling, drives the tap changer and listens to the bushings. Start with the asset, not the instrument. Ask which is the most important part of a power transformer and there is no single answer: it is a collection of sub-systems, and a failure in any one of them can take the unit off line. Its condition is only as good as its worst-performing sub-system.

And each sub-system fails in its own way, on its own timescale. Some of it is slow and thermal. Some of it is sudden: a bushing failure is sudden, and about half the time it starts a fire that destroys the transformer with it. In practice a large transformer ends up with 4 or 5 separate boxes bolted to it. A temperature monitor, a cooling controller, a voltage regulator, and if anyone thought of it, something watching the bushings. They do not talk to each other, and none of them sees the whole asset.

This does all of it from one chassis. The same instrument that calculates the hot spot decides when the fans run, counts the tap changes and measures the current coming out of the bushing test taps. Cooling is one of the jobs, not the whole of it. It starts with the thermal model, because that is what decides how fast the transformer is ageing. The hot spot is calculated for every winding, and the highest one runs the cooling. Every 6 degrees above 98 doubles the rate at which the paper ages.

Then the cooling, which it controls rather than merely watches. Two stages, started on temperature or on load, and every contactor, breaker and motor current watched so that a fan that has quietly stopped does not go unnoticed for a year. The tap changer is the part of a transformer that actually moves, so it is the part most likely to wear out. Four different measurements watch it. And it can drive that tap changer as well, not just watch it.

Automatic voltage control, with line drop compensation, and 3 separate schemes for running transformers in parallel. And then the part most monitors leave out. Bushings are one of the few things on a transformer that fail suddenly and violently, and almost nobody is watching them. It works because of the way a condenser bushing is built. Foils inside it divide the voltage evenly, and the outermost one is brought out to a test tap.

That tap is normally bolted to earth. The monitor takes over that connection. And what it looks for is simply balance. Three healthy bushings on 3 phases produce 3 currents that cancel. When one bushing starts to change, its current changes with it, and the sum stops being zero. With a partial discharge module added, the same bushing connection becomes a listening post for the discharges inside the tank.

Now how it is built and set up. There are two pieces: a control unit, which is a card cage that lives in the marshalling cabinet, and an interface unit, which is the screen and the keypad. They are joined by a single network cable that carries the display data, the keys and the power. These are the cards, and the choice of them is the specification of the monitor. Two of them exist for nothing else but the tap changer and the bushings. Setting up the bushings needs two numbers from the bushing itself:

its nameplate capacitance and its nameplate power factor. Take them from the latest test report, not from memory. Everything the monitor reports afterwards is a change from those two numbers. The rest of the configuration starts with a tool rather than a page. The initial configurator asks what the transformer is and what you want the monitor to do, and builds the starting point from the answers. One configuration detail catches people out, and it is worth saying plainly. The tap position usually arrives as a 4 to 20 milliamp signal,

and the input range has to be set wider than the signal, or the monitor will never read the two extreme taps. Cooling is configured the same way as every thermostat you have ever met, with one addition: both the top oil and the winding hot spot have to fall below their margins before the cooling stops. Whichever one is still hot keeps the fans running. And then the part that turns cooling control into cooling monitoring:

calibrate the motor currents. Until you do, the monitor knows the fans were asked to run, but not whether they did. A word about inhibits, because they are deliberately hard to switch off. Some status inputs do not only raise an alarm: they block the monitor from operating the cooling or the tap changer at all. That is not a limitation. It is there to stop the fans starting while somebody is working on them. Commissioning is done in two halves, and the first half happens on a bench,

before any of it goes near an energised transformer. Day to day, everything is on the web pages, including a virtual copy of the screen that is on the transformer, so you can see exactly what the technician in the yard is seeing. Transformer monitoring, from Amperis.