How does ALFC and AVR maintain grid frequency and voltage in power systems?

[cnh1995]

ALFC and AVR are used to maintain grid frequency and grid voltage constant w.r.t. changes in real and reactive power. Real power has a direct effect on frequency while reactive power has a direct effect on the grid voltage.

Consider a power system running at normal grid volage with rated freauency of 50Hz, with generators equipped with AVR and ALFC. If the load demand increased, the primary ALFC loop will operate (free governor operation) and governor will increase the mechanical input according to its % droop. Let's say the new frequency settles down to 49.7Hz. Then central LFC will send signal to the generator's speed changer and its reference power setting will be increased by a suitable amount, which will bring back the frequency to 50Hz (for the increased demand).

But before the central LFC changes the reference power setting of the generator, the generator is operating at reduced frequency (49.7Hz) for some time (small time).

Does this reduction in frequency affect terminal voltage? Generated emf in the generator is proportional to the speed of rotation. Will the reduction in frequency reduce the terminal voltage? Does AVR respond to this change? Or does it only operate during reactive power changes in the system?

 

[jim hardy]

Does this reduction in frequency affect terminal voltage? Generated emf in the generator is proportional to the speed of rotation. Will the reduction in frequency reduce the terminal voltage? Does AVR respond to this change?

Yes to all. AVR measures machine terminal voltage.

Or does it only operate during reactive power changes in the system?

AVR measures machine current too, so will respond to reactive power.
There's a reactive droop setting on AVR so that you can operate plants in parallel on the grid and they'll share reactive load gracefully.
Ours were set to give ~5% reactive droop.

 

[jim hardy]

Generated emf in the generator is proportional to the speed of rotation. Will the reduction in frequency reduce the terminal voltage?

That's why most AVR's have a volts/hz measurement and a volts/hz limiter .
Volts per hz is flux and overflux can melt the stator iron. That happened to us once, before the machine was synchronized to the line. Something had gone wrong with the voltage indication. Operators noticed it was taking too many field amps to make normal voltage, and there was no way to know which of the meters was wrong. About the time they decided to shut back down and investigate , the machine shut itself down . Molten iron was running out the end.http://www.ccj-online.com/guard-against-over-fluxing-ensure-proper-generator-protection-maintenance/

Missteps conducive to over-fluxing. Almost without exception, over-fluxing occurs when the field is applied, but before the machine is synchronized to the grid. Reason: Once the breaker is closed, increasing field current mainly increases VAr loading, while mildly increasing the terminal voltage.

An old-timer from GE told me it happens someplace in the world about once a year .

 

[cnh1995]

That's why most AVR's have a volts/hz measurement and a volts/hz limiter .
Volts per hz is flux and overflux can melt the stator iron. That happened to us once, before the machine was synchronized to the line. Something had gone wrong with the voltage indication. Operators noticed it was taking too many field amps to make normal voltage, and there was no way to know which of the meters was wrong. About the time they decided to shut back down and investigate , the machine shut itself down . Molten iron was running out the end.http://www.ccj-online.com/guard-against-over-fluxing-ensure-proper-generator-protection-maintenance/An old-timer from GE told me it happens someplace in the world about once a year .

This is really helpful! Especially that link related to the physics of overfluxing. It's really great to learn from a man with so much knowledge and practical experience in EE.
I will study more about AVRs and will post if anything is unclear.

I can't thank you enough @jim hardy!

 

[jim hardy]

No cnh, the Thanks are due you, for tolerating my boring anecdotes.

 

[cnh1995]

for tolerating my boring anecdotes.

Not boring at all. In fact, your anecdotes are always interesting.
Practical experiences like that are very important, especially for a student like me who has worked only on limited lab machinery and some simulations. I have learned much more from you than from my textbooks or lectures.