Synchronous Generator Power Factor


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What are the significant aspects of synchronous generator designed for 0.85 power factor rating compared to generator of 0.8 PF rated? Any issues/impacts to be addressed when operating the said generators in parallel with grid? What is the recommended design PF for lower capacity (10-40 MW) and higher capacity generators?
Synchronous generator ratings are "maximum capacity" ratings, that is, they are for the maximum amount of active and reactive current which can be safely generated simultaneously. The rating does NOT mean that the generator will operate at that power factor/kW at all times when synchronized to a grid with other generators.

The capacity of a generator to produce power--active and reactive--is a function of the ability of the generator cooling system to remove the heat developed when power--active and/or reactive--is being generated. (This presumes the prime mover is capable of producing the torque required for the active power, and the exciter ("AVR") is capable of producing the required excitation for the active power.

When power--active and/or reactive--is being produced current--amperes and reactive current--is flowing in the generator stator windings and the generator rotor windings. Current flowing in a conductor produces heat. Heat, left unremoved, can damage insulation and lead to arcs and sparks--in either or both of the generator stator and the generator rotor.

Refer to the reactive capability curve (the "D" curve) of any generator to see how the power--active and reactive--of a generator is a function of the cold gas (air or hydrogen) temperature.

A synchronous generator rated for the same kW but a pf of 0.80 can produce more reactive power (lagging VArs) than a generator rated for 0.85 pf when producing the same kW.

There is no recommended design pf for any synchronous generator--it depends on the application and site requirements. Most synchronous generators are rated for either 0.80 or 0.85k pf, as a general rule--and this usually refers to the LAGGING pf, not the leading pf. Again, for the details of any particular synchronous generator refer to the manufacturer's reactive capability curve ("D" curve).

Operating any synchronous generator on any grid requires that the operators refer to the reactive capability curve to ensure they are operating the generator within it's capability--the capability of the generator's cooling system to remove heat to ensure long life, and to keep the smoke and the arcs and sparks inside the windings. Because, when smoke and arcs and sparks get outside the windings then that means downtime and lots of lost generation and revenue (money).

Hope this helps!
Oh, and in case it wasn't clear: Synchronous generators don't ONLY operate at their rating. The active (real) power, kW, is a function of the torque applied by the prime mover and is controlled by controlling the energy flow-rate into the prime mover with the prime mover's governor.

The reactive power (VArs) is a function of the excitation applied to the synchronous generator rotor and the excitation is varied using the exciter ("AVR").

The rated power factor is that point in a conventional capability chart where the armature limit curve intersects the rotor overheating curve. So, if someone wants to operate a 0.85 pf lag rated machine in 0.8 pf lag condition, he can of course operate it. but its achievable reactive generation will be affected by field overheating limit. What i mean is that in comparison to lower pf rated (0.8) machine, this machine will take a higher proportion of field current to maintain the same 0.8 pf lag condition.

On the other hand, operation of 0.85 pf rated machine in 0.9 pf region will be decided by stator MVA limit or armature limit.

As for the under exciting region, stator core end overheating locus and practical stability limit has to be looked after.
Thanks for your explanation.

However, when the client specify for an STG, which one he has to opt for,0.8 PF or 0.85 PF?On what grounds?

From manufacturer's point of view, what may be the design changes that are required between the two alternatives?

In order to be able to decrease the power factor to run at higher lagging reactive current it's necessary to increase the current flowing in the generator rotor. That means more rotor winding material and better insulation as well as a design and construction to allow better circulation of the cooling gas (air or hydrogen).

The location on the grid where the machine will be synchronized and the nature of the grid loads in that area determine whether or not it will be necessary to have the capability to run at lower power factors/higher reactive currents. And/or, if it might be necessary to run in "island" mode (isolated from a grid) to power a local load, such as refinery or cement plant, then the nature of that load must also be considered when choosing a generator.

Usually a power system study is performed before specifying equipment and this is where the particulars of the site and the nature of the expected loads is considered. I say "usually before," because in some parts of the world a power system study is never done, or it's not done until after its realized that the new generation equipment (generator; exciter; transformer) is not working very well in the system it's being applied in.

Many times the grid operator/regulator has input or requirements for generators being connected to their grid to help with maintaining or enhancing stability in a particular region.

Hope this helps!