Video: TWA Advanced Series

Video transcript

What the TWA Advanced Series adds to a winding resistance test, how it is connected, and how a fully automatic tap changer test is run. The Advanced Series measures all 6 windings from one connection, and analyses the on-load tap changer while it operates: dynamic resistance, motor current and vibration, recorded together, with the whole sweep run from the touch screen. Resistance from a 10th of a micro-ohm to 10 kilo-ohms. Test current up to 25 amps of direct current,

and up to 40 during the synchronisation check. Dynamic resistance sampled every 10th of a millisecond, and 32 gigabytes of internal memory. Against the Standard Series, the Advanced one adds a touch screen with stored test templates, a fully automatic test run from the instrument itself, a vibration channel, 32 gigabytes of memory and Ethernet. And both of its models record the dynamic resistance at a 10th of a millisecond. The two models differ in little more than the screen, 10.1 or 7 inches,

and the built-in thermal printer, which only the TWA400 offers. The measurement is the same on both. The touch screen in the lid runs the whole test, and the flash drive port sits beside it. On the panel: the H sockets for the high voltage leads, the X sockets for the low voltage leads, the earth stud, the discharge lamp and the emergency stop. With it come mains and earth cables, the 5 metre tap changer control cable, USB and Ethernet cables and the PC software.

The H and X lead sets are the same as the TRT turns ratio testers use, so both can share one connection. Before any lead goes on: the transformer isolated from line and load, one end of the winding earthed, and the instrument earthed with its own cable. Leads go to the instrument first, and come off in reverse, with the earth last. On a three-phase transformer everything is connected once: the earth, 4 H leads onto the high voltage

bushings and 4 X leads onto the low voltage side. The current flows through both windings of each limb together, which saturates the core faster and shortens the test. On an autotransformer the same 8 leads are used, and both neutral leads, H0 and X0, go to the common neutral bushing. When only the low voltage side is tested, the H leads come off. A single-phase transformer takes two leads from each set: H1 and H0 across the high voltage winding, X1 and X0 across the low voltage winding.

The leads that are not used stay unconnected. The tap changer control cable is two dry contacts. Brown is the common, yellow closes to it for UP and green for DOWN, in parallel with the raise and lower controls of the drive. That is how the instrument changes the taps during the automatic test. Two more channels turn the resistance test into a tap changer diagnosis. The current clamp goes on the supply of the drive motor, and the accelerometer on the outside of the tap changer tank, held by a magnet, a screw or wax.

The motor start can also trigger the recording, which is the method for reactor-type tap changers. In the fully automatic test the instrument does the whole sweep: it measures a tap, decides from the deviation when the reading is stable, records it, operates the drive through the control cable, records the transition, and moves on. The motor operating time and the stability criterion are the settings that matter. On a YN winding the 3 phases can be recorded together:

up to 40 amps in total, divided through the phases and back through the neutral lead. A phase delayed by a mechanical problem shows on the graph. This mode records graphs only. Each tap change leaves a short dip in the test current. Its depth, the transition ripple, and its duration are calculated for every change. They should be consistent from tap to tap; a dip out of line, or a phase that lags the other two, is the one to investigate.

A direct current test leaves the core magnetised, and so can taking a transformer out of service. The instrument removes the remanence by reversing a controlled current in decreasing steps down to zero, on 3 phases or one. Results stay in the instrument's memory or go out on a USB stick. The PC software is optional: it prepares tests, reads the transition time and ripple of every tap change, compares one test with an earlier one, and produces the report.

Transformer testing, from Amperis.