Video: E3

Video transcript

What a power transformer needs watched, and one monitor built to watch all of it. 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, and takes the transformer with it. This is the answer to all of it in one device. Temperatures, load, voltage, tap position and the state of every cooling contactor arrive at it. Cooling, alarms, trips, tap changer control and the control room are what it drives. Cooling is one of the things it does, not the whole of it. It is built for the large machine:

the one with several windings, banks of fans and pumps, a tap changer under load, and a control room that wants to know about all of it. The measurement that governs the life of the unit is the winding hot spot, and it cannot be reached with a probe in the oil. It is calculated, and the monitor calculates it continuously from 3 things it already measures. And the cost of getting it wrong is not linear. Under the IEC loading guide, every 6 degrees

above 98 doubles the rate at which the paper ages. That is the whole reason the temperature deserves a monitor rather than a gauge. Protection on top of it is graded rather than binary. There is an alarm, and then 3 kinds of trip: slow, fast and instantaneous, each with its own timer, and an alarm that warns you the timer has started. Now the cooling, and here is what makes this one different. A conventional monitor starts the cooling when the winding reaches a set point:

by then the heat is already in the paper. This one also calculates where the temperature is heading if every cooler were running, and starts before the transformer gets there. The cooling control itself is unusually deep. Eight groups, 4 stages, pumps duty-cycled so the same one is not always first to run, variable speed drives, soft starters, and a lockout that keeps the pumps off when the oil is too cold to move safely. The tap changer is measured by its work rather than by its count:

how many joules the motor used for each tap change, and how long it took. A tap change that suddenly needs more energy, or less, is a mechanism that has changed. Through faults are one of the leading causes of transformer failure, and their damage accumulates. This keeps the whole history: every fault, when it happened, on which phase, and how much energy went through the windings. With the alternating current cards fitted it becomes an electrical instrument as

well, and that is where two features live that very few transformer monitors have. Harmonics matter more than they used to. A geomagnetic disturbance drives direct current into the neutral, part-saturates the core, and the transformer starts generating harmonics that can destabilise the network. The tell-tale is even harmonics rising above the odd ones, and the monitor alarms on exactly that. And moisture, which is the slowest and most expensive problem a transformer has.

Water in the paper cannot be measured online at all, so it is estimated: from the moisture in the oil, the temperature, and how the two move against each other over time. It also gathers what other instruments already know. A dissolved gas analyser, a fibre optic temperature system, a moisture probe: they become inputs rather than yet another screen in the control room. Now how it is installed, and one thing to expect first. No two of these are identical.

Each one is programmed for the transformer it is going on, so the drawings supplied with your unit, not a generic manual, are the authority on what is in it. The cabling is specified, not left to judgement, and the specification exists because these signals are small and the cabinet is full of motor circuits. The current transformers deserve their own warning, and it is the serious kind. The secondary of a bushing current transformer must never be opened while the transformer is energised. Short it at the terminal block first. Every time.

The supply and the ratings. Everything is specified against the insulation coordination standard, which matters when someone asks you what the wiring has to withstand. The settings protect each other. There is always a margin between cooling on, alarm and trip. Lower one of the upper settings and the monitor lowers the ones underneath to keep that margin. Raise it, and it will not raise them for you.

And the cooling has to be commissioned as machinery, not just as a setting. The pumps are duty-cycled, the cold lockout has to be right, and the test schedule is what proves the fans still turn. Then the commissioning tests, in the order they have to happen. The last two only make sense with the transformer energised, and everything before them exists so that moment is uneventful. Day to day it is web pages and files, and knowing where the files are saves a support call.

Transformer monitoring, from Amperis.