kiwi Rodalco, what is a half day brown power dip ? And how does a manual crank-handle tap-changer work ?
Transpower is doing maintenance on the 220 kV lines hence they feeding some grid exit point substation via the 110 kV lines.
Because of higher currents drawn at 110 kV there is obvious voltage drop on the 33 kV bus, so the tapchangers in the local 33/11 kV subs have to tap up from the default tap 5 (33000V) to tap 8 (32250V), this normally occurs during peak periods too for about 1 to 2 hours or so.
The tapchangers are controlled via the AVR (automatic voltage regulator) which senses the 11 kV lines via the 11kV/110 Volts substation VT, in the above two cases we had mechanical problems with the centrifugal switches hence the motors didn't want to start.
On most tapchangers there is a square shaft provided which can be operated manually with a "crankhandle" to step the tapchanger up or down.
It is good practise to use HV gloves class 1, or better, and a rubber mat although this output shaft is at earth potential.
A tapchanger is designed that the drive motor has brought a quite heavy flywheel up to speed prior to commencing a tapchange.
A centrifugal switch then releases a latch so that the tap change can commence.
It is important that the tapchanger NEVER gets stuck between two taps because the temporary shunt resistor will then burn out.
This is a safety measure so that in case power fails during a tapchange operation, enough kinetic energy is in the flywheel to allowe for a complete tapchange.
This resistor provides resistance in series with one of the windings during parallelling the two windings during a tapchange.
Hope that explains it in a nutshel
Regards Raymond RF.
