I know what impedance voltage is. I will state it just make sure we are talking about the same thing. Impedance voltage is relative short circuit voltage, when secondary of a transformer is short circuited, and we are slowly raising the primary voltage "Vpr" until the current at the secondary reaches its nominal value. The ratio of the "Vpr" and nominal voltage of the transformer is called impedance voltage. It usually goes from 3% to 20%. Now, what does this parameter tell me about the transformer? How is this useful? Is it better to have a lets say 20 ohm or 200 ohm impedance of transformer? I know its important for 2 transformers when working in parallel to have matched impedance voltages, to prevent one transformer "hugging" more power than the other. Additional question: When we have a "rush" and a lot of power is taken from transformer, its voltage at secondary tends to fall. Correct? To prevent this, regulation transformers slip one ring up, and change the transfer ratio of the transformer, so we can get more voltage at the secondary. Is this correct?