ONLINE/OFF-LINE TAP CHANGER

No where near enough info to even chance an answer.....sorry.

Try re-reading what you wrote as some one who does not have a clue about what you are talking about!! Thread tapping, transformer tapping, water taps???????

Read "Electrical Transformers and Power Equipment" by author Anthony J. Pansini. ISBN 0-13-247602-9. Brief explanation: Mechanically changing the tranformer primary winding turns to regulate the secondary voltage. TCUL type, Tap Changing Under Load. System uses a series of switches and relays to connect and disconnect sections of windings to create the desired voltage. Much more complicated than this explanation. Read, read, read!!!!!

If you want to choose between ONLOAD (not ONLINE, I guess) tap changer and OFFLOAD one, also to determine number of steps needed for your plant/network, you have to concentrate on 3 points:

1. The maximum variation of the supply voltage.
2. The maximum acceptable tolerance (variation) of the voltage from your load's point of view.
3. The distance between the transformer and the geographical center of the load (substation-switchgear).

However, by default a power transformer has an OFFLOAD tap changer with a limited number of steps, and this is fair enough for a system with stable supply voltage, normal loads, and normal distance between loads and transformer (200-300 mt. I guess).

If the supply voltage varies dramatically, if you have special loads with very narrow voltage tolerances, or if the loads are far enough from the transformer such that load variation may deteriorate the voltage regulation at the customer point, you may need a power transformer with ONLOAD tap changer, and a highr number of steps for a highr qualty of voltage stability at the customer point.

This may simply explain the broad lines on the map of this subject.

Regards... SAMAK

It would appear that you have lost interest in the possible answers to your unclear question, so I am off to look at better blogs than this, byeeeeeeee!