Synhronous generators - changing excitation

[nForce]

When Synhronous generator operates on the grid, why when we over excitate the generator the grid sees the generator as a capacitor? And the generator sees the grid as an inductor?

Can someone explain to a beginner nice and slow.

Thanks.

[David Hess]

Start with the Wikipedia article on synchronous condensers.

[nForce]

Yes, I have read this, but I don't understand why is always the opposite. So the generator sees the grid as an inductor and the grid sees the generator as a capacitor. Can someone give an example?

[David Hess]

This is not really an explanation but the synchronous condenser does not "see" anything.  Somewhere on the grid is a load, usually inductive, where the current lags the voltage so during part of the power line cycle it is drawing more current and during the rest of the power line cycle it is returning current.  This extra current is not doing any work and instead is just heating up the power line conductors representing an extra loss.

The synchronous condenser does the same thing but it can be tuned with its excitation field strength to act with a specific value of inductance or capacitance.  If it is placed close to the unfriendly load, then it can cancel out the reactance at the load so that the power lines only see a resistive load and no additional power is lost through the circulating currents.  The reactive circulating currents now travel back and forth only between the load and the synchronous condenser.

[nForce]

Oh, thanks. By what the grid sees (inductor or capacitor) I meant is the power factor leading or laging.

I think I know, why is the power factor leading or lagging differently by the position of our measurement. (If we measure from the grid to the generator or vice versa).

It's because if we plot sin(x) and cos(x) on one graph at the start cos(x) signal is the first one, but if we look at the end the cos(x) is the last. Sort of, right?

[nForce]

I was thinking about this, can someone just confirm if my thinking is correct:

If we overexcite the rotor winding that means increase the current which results in more magnetic flux which increases the stator voltage. So because stator voltage is higher then the voltage of the grid, because of that the stator current flows away from generator into the grid. So we are feeding the system with more VArs, and vice versa if we underexcite the rotor, we decrease the stator voltage, and the current flows into the generator, so we are sucking the VArs from the system.

Do I understand correctly? Thanks.