Why is it important to know? It can be bidirectional which is common in the power industry.
Or the power flows in one direction, and VARs can flow in the other.
It can be important if you have voltage regulation in that you need to know what you are regulating. But other wise it just matches two voltage levels.
cunin,
Step-down transformer typically has secondary rated voltage about 5% over the system nominal voltage to compensate the transformer regulation while on-load. It will be the case on HV side, with the step-up transformer.
This way a step-up transformer can be distinguished from step-down transformer transformer just by looking at the nameplate of the transformer.
raghun,
I would argue the other way. In my experience, a transformer supplying a system designated as 480 volts is normally rated 480 volts. The utilization equipment is commonly rated at a lower voltage to allow for voltage drop (e.g. 460 volts for motors).
The tapchanger is almost always on the high-voltage side because the winding current values are lower. This leads to cheaper construction costs.
Since most xmfrs we see step DOWN the voltage, this is normally called the 'source side' but should more properly be called the 'primary side'
In a step-up xmfr, the primary side is now the load side.
I agree with alehman in that utilities try to provide NOMINAL supply voltages (480V in USA, 600V in Canada, etc) but purchase xmfrs with tapchangers with a + - rating to compensate for source voltage fluctuations in order to provide the NOMINAL voltages.
How about we all just forget about "primary" and "secondary" and just talk about "high-side" and "low-side". Power flow then becomes completely irrelevant. The transformer certainly doesn't care about direction of power flow.
There is difference between IEC and IEEE, when it comes to definition of secondary rated voltage transformers. In IEC, the nameplate refers to no-load voltage whereas IEEE refers it to load voltage (which means on no-load the seconadry voltage will be more than the voltage indicated on the nameplate).
Just to share.
raghun,
I live in the ANSI/IEEE world and I'm not familiar with the IEC standards. With all due respect, in my world transformers give rated voltage out with rated voltage and no load (and nominal tap position if present). The nameplate voltage ratio = turns ratio. To my knowledge there is no load adjustment factor. At full load the output will drop by approximately the % impedance of the transformer.
