Video: TWR – Connection guide

Video transcript

What comes in the case, what every cable is for, and how the TWR is connected to each kind of test object. What ships with it. The current clamp is the one people do not expect: it is powered from the instrument itself, so there is no battery to find. The panel splits cleanly in two. On the left, the 4 H sockets for the high voltage side. On the right, the 4 X sockets for the low voltage side. Colour coded, and that colour is the phase.

And the rest. The discharge lamp is the one to learn: while it is lit, the magnetic circuit is still giving back the energy that was put into it, and nothing comes off the transformer. The leads are two sets of 4, H and X, 10 metres as standard with TTA clamps, and the same sets fit the turns ratio testers. A three-phase transformer is connected once and completely. Earth first. Then 3 or 4 H cables onto the high voltage winding, and 3 or 4 X cables onto the low voltage winding.

All 6 or 8 terminals are on at the same time, and on a large transformer that is the difference between a morning and a day. An autotransformer takes 4 H and 4 X cables, and there is one difference that matters: both neutral cables go to the same neutral bushing. A single-phase transformer needs two H cables and two X cables. The instrument is three-phase but it does not have to be used that way. A current transformer reverses what you would expect.

Almost all the turns are on the secondary and only one or two on the primary, so the H cables go on the secondary. And when there are no primary terminals, you make the primary: the X cables go through the core and are shorted together on the far side. One turn. And on a large unit with the bushings up in the air, the picture is the same: the leads run up to the bushings, the instrument stays on the ground, and nothing is moved between the tests. Now what the connection buys you.

Winding resistance at up to 25 amps of true direct current, with no ripple, and with the instrument regulating both the injection and the discharge. And this is the test that finds a tap changer going wrong. The current is held through the winding while the tap changer operates, and the dip it makes is recorded. Motor current and vibration are recorded alongside it, so a mechanical problem and an electrical one can be told apart. And after any direct current test the core is left magnetised,

so demagnetisation is part of the job rather than an extra. The tap changer control unit is built in here as well, and with it the instrument can run the whole sweep by itself. And the results, which never have to be copied by hand. Printed at the transformer, exported to a stick, or taken to the PC. Transformer testing, from Amperis.