Video: TRT – Connection guide

Video transcript

What comes in the case, what every cable is for, and how the TRT is connected to each kind of transformer. Start with what is in the case, because two of these are easy to leave behind and both are needed on the first job. On the panel, the two blocks on the right are the whole measurement. Four H sockets for the high voltage winding, 4 X sockets for the low voltage winding, colour coded so a lead can only sensibly go in one place. And the rest of the panel: power, the earth stud, the tap changer control connector,

the flash drive and the ports for the PC, and the emergency stop. The leads come as two sets: an H winding set and an X winding set, 4 cables each, 10 metres as standard, terminated in TTA clamps. 5, 15 and 20 metre sets are available, and extension sets on top of those. A three-phase transformer is the ordinary case, and the point of the instrument is that it is connected only once. The earth first. Then the 4 H cables onto the high voltage bushings, and the 4 X cables onto the low voltage side.

Every vector group is then tested without touching the hookup again. A single-phase transformer uses part of the same set: two H cables and two X cables. Nothing else is needed, and nothing else is connected. A phase-shifting transformer is the one that looks intimidating and is not. It is a series transformer and a shunt transformer in one tank, and the instrument is connected to both sets of bushings at once, exactly as it would be on any other three-phase unit.

And a capacitive voltage transformer, which needs more voltage than the instrument makes on its own. The CVT 20 sits between them and takes the test voltage up to 5 kilovolts. The handheld version is connected the same way, with clamps instead of connectors, and it is the one that goes up the pole. Here it is on a three-phase distribution transformer. A current transformer is the case people get wrong, and it is worth saying slowly. ACT has almost all its turns on the secondary and only one or two on the primary.

So the X cables go to the primary, and the H cables go to the secondary. And when there are no primary terminals at all, which is most of them once installed, the X cables are slid through the core and short-circuited together. That single pass is the primary: one turn. One more hookup worth knowing, because it saves a whole second visit. The turns ratio tester and the winding ohmmeter share the same test leads, so both can be connected to the transformer at the same time and the two tests run one after the other without anybody touching a clamp.

The tap changer control unit is built in, and it is what turns a two-person job into a one-person job. The 5 metre control cable goes from the panel to the tap changer, and the instrument drives it. And the test itself, once everything is on. Enter the nameplate, pick the vector diagram, run, and read the deviation rather than doing the arithmetic yourself. On the screen the sweep comes out as a chart rather than

a column of numbers, with the tolerance band drawn on it. A tap that steps outside the band is visible without reading a single figure. Transformer testing, from Amperis.