Synchronous Generators

[johl04]

Hello everybody,

Can you please explain me why synchronous generators operate normally run at a lagging power factor?

Thank you in advance

 

[davidbeach]

They don't, at least not on their own. When a generator is in load following mode (think standby generator with load and no other sources) the generator will be run to produce what the load needs, and most loads are lagging, so the output of the generator will be lagging. If the load were leading, the output of the generator would be leading.

In generator reference, supplying VArs is lagging and absorbing VArs is leading, just the opposite of what it is in a load reference. I find it better to think of it as when Watts and VArs are in the same direction it is lagging and when Watts and VArs are in the opposite direction it is leading, then you don't need to worry about generator reference or load reference.

Even when the generator is connected in parallel with other sources, the system generally needs VArs, so the generator is run to produce VArs, in other words a lagging power factor.

But the generator itself has no particular preference.

 

[viktorel]

Hi.
Pls. don’t mix with same type of motors. Generators must work possibly close to cos=1, and in base loading is inductive.

 

[ScottyUK]

The generator does have some preference: they have limited capability to operate at a leading power factor because the machine is under-excited and the field is weaker. A capability curve will show the stability limit which is a practical restriction on the machine to ensure that the generator does not pull out of synchronism under fault conditions on the system.


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Sometimes I only open my mouth to swap feet...

 

[dpc]

You may just be getting twisted around some terminology.

A generator operating at a lagging power factor is producing vars, not consuming them. Since most loads are lagging, the generator runs at a lagging power factor to provide the vars.

 

[waross]

When a gen set is operated by itself, or in "Island Mode" the field strength sets the voltage and the characteristics of the load determine the power factor.
When a set is connected to the grid, the combination of all the generation in the system determines the system voltage. Any one small gen-set is makes very little contribution to the system voltage. Now the field strength determines the production of VARs. As the field strength on one machine is increased, the machine will supply more VARs to the system and the other machines making up the grid will supply correspondingly less. Total VARs consumed by the loads will remain the same. Smaller sets that are connected to the grid are often run in PF control mode so as not to overload the set with reactive current if the grid voltage drops and the small set tries to produce more reactive current than it's windings can handle safely.

 

[SASHmash]

To simply answer your question johl04, it’s because the power companies are supplying inductive loads. A synchronous generator is doubly fed (prime mover and field excitation).
The excitation voltage applied to the generator will determine the phase angle of the generated apparent power (S). Hence the utilities can exercise a level of control over the load they supply.