Video: TWA Standard Series

Video transcript

What the TWA Standard Series measures, how it is connected to each kind of transformer, and how a test is run from the first clamp to the last. Winding resistance finds loose joints, broken strands and worn tap changer contacts. The difficulty is the inductance: a power transformer resists every change of current, so a reading can take minutes to settle, on 6 windings and dozens of taps. The range runs from a 10th of a micro-ohm to 10 kilo-ohms, selected automatically. The test current goes from 5 milliamps to 25 amps of true,

ripple-free direct current, with up to 55 volts available to saturate the core quickly. There are two models. They share the range, the current and the demagnetisation. The TWA40D adds what a full tap changer test needs: all 3 phases at once, a channel for the motor current, and a sampling step of a 10th of a millisecond instead of 4. On the right, the 4 H sockets for the high voltage side and the 4 X sockets for the low voltage side. On the left, the earth stud, the tap changer control socket and the motor current input. And the lamp to learn:

discharge. While it is lit, nothing comes off the transformer. With it come the mains and earth cables, a 5 metre tap changer control cable, the USB cable and the PC software. The winding leads are two sets of 4, H and X, 10 metres as standard with TTA clamps, and the same sets fit the TRT turns ratio testers. The order is part of the safety. The transformer is isolated from line and load, with one end of the winding earthed. The instrument is earthed first, with its own cable.

Leads go to the instrument, then to the transformer, and come off in reverse, the earth last. A three-phase transformer is connected once. The earth first. Then the 4 H leads onto the high voltage bushings, and the 4 X leads onto the low voltage bushings. The current flows through both windings of each limb together, so the core saturates sooner, and all 6 resistances come from this one hookup. An autotransformer takes the same 8 leads, with one difference:

H0 and X0 both go to the common neutral bushing. And when the low voltage side is tested on its own, the H leads are disconnected. A single-phase transformer needs only two leads from each set: H1 and H0 across the high voltage winding, X1 and X0 across the low voltage winding. On a single-phase autotransformer, H0 and X0 share the same terminal. The full setup for a tap changer test. The earth, the H and X leads, the temperature probe on the tank to correct the result to the reference temperature,

the current clamp on the tap changer motor supply, on the TWA40D, and the control cable into the motor drive, so the instrument changes the taps itself. The control cable is simply two dry contacts. Brown is the common. Pressing UP closes yellow to brown, pressing DOWN closes green to brown, in parallel with the raise and lower controls of the drive. So one operator changes taps from the instrument, without walking to the operation box. At the instrument the test is set in the Test menu:

the current, whether there is a tap changer, the test type and the side. Then the vector group, because each configuration has its own algorithm for fast stabilisation. Press START, watch the deviation fall towards zero, and record. The instrument moves on to the next phase by itself. In OLTC mode the current keeps flowing while the tap changer operates, and every change leaves a short dip. Its depth, the transition ripple, and its duration are measured at each tap.

A dip out of line is the one to investigate. The TWA40D records all 3 phases at once, with the motor current. A direct current test leaves the core magnetised, which means inrush at switch-on, false readings and wrong relay operation. So demagnetisation comes last. The instrument reverses a controlled current, each step at 60% of the one before, down to 5 milliamps. Results stay in the instrument, 500 of them, or go out on a flash drive.

The PC software, over USB, controls the test, downloads the memory, corrects the resistance to the reference temperature and writes the report. The dynamic resistance test and the quick YN mode are run from it. Transformer testing, from Amperis.