Generator Excitation & Power Factor
Generator Excitation & Power Factor
(OP)
I'm trying to visualize the relationship between the exciter field in a synchronous generator and the generator's power factor.
Double check me on this: When synchronizing the generator, increasing the field current will increase the flux density, which increases the voltage induced in the stator.
When the generator is paralleled with the utility, you have an infinite bus and cannot adjust stator voltage so the generator becomes over-excited. But when over-excited, I just can't visualize what mechanism of the increased field strength is causing the power factor in the stator to lag.
I'm am trying to find an analogy or description to help me understand why this should be so. Any suggestions? Thanks.
-John
Double check me on this: When synchronizing the generator, increasing the field current will increase the flux density, which increases the voltage induced in the stator.
When the generator is paralleled with the utility, you have an infinite bus and cannot adjust stator voltage so the generator becomes over-excited. But when over-excited, I just can't visualize what mechanism of the increased field strength is causing the power factor in the stator to lag.
I'm am trying to find an analogy or description to help me understand why this should be so. Any suggestions? Thanks.
-John





RE: Generator Excitation & Power Factor
If a paralleled generator is badly under-excited it will input inductive VARs from the other generators to maintain excitation. When the generator is over excited it is exporting inductive VARs and supplying more than its share of VARs into the system relative to the other generators.
A synchronous condenser is a special synchronous machine which is started like a motor but which does not drive a load. Some do not have a shaft extension. As the excitation is increased, the machine exports VARs to offset load VARs.
Some synchronous motor applications use an oversized motor and over excite the motor to supply VARs to the system.
A synchronous motor may theoretically be used as a synchronous condenser, but in practice the high excitation levels may result in field currents above the current rating of the field windings and over-heating of the field windings. It depends.
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator Excitation & Power Factor
The rest sounds good though :)
RE: Generator Excitation & Power Factor
I'd draw it for you but in a text-only forum it is quite difficult. Google synchronous machine vector diagram. This interactive one might be of interest: http://d
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If we learn from our mistakes I'm getting a great education!
RE: Generator Excitation & Power Factor
You stated in another thread that you are not an engineer and I suspected that when I read the original post.
Your question can be answered, but you have asked a group of (mostly) engineers or if not engineers then people well versed in this field so in order to try to help you (which I think most engineers are willing to do) I would ask your background so that they can couch their answers in less complicated terms that will aid your quest for knowledge.
And, that said, in the interest of full disclosure, I am a ME who works in the power generation world so sometimes I have to have things explained to me in "not so in depth" EE terms. I learn a lot here.
I don't understand your statement about not being able to adjust the stator voltage. To my EE friends, is that an accurate statement? Once synchronized, isn't the raising of the stator voltage the way power is pushed out onto the grid? Or is it only by the machine trying to "speed up" the grid at synchronous frequency?
BigJohn's assertion has confused me here.
rmw
RE: Generator Excitation & Power Factor
This does not change the MW output, but only the reactive component, the VARS. (or MVARS) Only the governor of the prime mover can change the MW.
The Power factor will change becoming "lagging" as seen by the Power factor meter, as the Generator is seeing a "lagging" load requiring VARS. There is some confusion on PF meters, some old ones having a 360 degree scale where the pointer can do a full circle.
hope this is clear...
rasevskii
RE: Generator Excitation & Power Factor
Using the motor convention (suitable for all loads), VArs in is lagging while VArs out is leading. Any load that requires VArs is therefore lagging and any load that supplies VArs is leading. Generally the only leading load is something capacitive, although anything with an active front end (power electronics) can be leading if so desired.
Using the generator convention (suitable only for sources) VArs out is lagging while VArs in is leading. A lagging source supplies a lagging load. If current into the load lags the voltage then current out of the source also has to lag the voltage.
Instead of worrying about supplying VArs (leading loads and lagging generators) or absorbing VArs (lagging loads or leading generators), I find it much easier to just consider VAr direction in relation to Watt direction. In all cases, if something absorbs both Watts and VArs or supplies both Watts and VArs it is lagging, whether a load or a source. If something absorbs one while supplying the other, it is leading, also whether a load or a source.
RE: Generator Excitation & Power Factor
I'm a utility electrician for a bunch of generating stations, I'm just looking for a non-mathematical explanation for this. I think visually, so I'm trying to visualize what's actually happening to the stator when excitation is changed while the unit is paralleled.
When a paralleled stator is over-excited, it may technically change the voltage measured at the generator leads, but my experience is that value is very small because of the ability of the grid to hold the generator at grid voltage, that's what I meant when I spoke of "infinite bus." I'm not too concerned with this part.
What does change significantly with excitation is the PF reading and that's the part I'm trying to understand.
Here's a question that may help me a bit: If the load on the generator were almost purely resistive (ignoring line reactance, etc.) and the generator was over-excited, what would the power factor meter read? My understanding is that generator PF is determined purely by the load. So what would the increased excitation do to the generator under that circumstance?
Thanks so far.
-John
RE: Generator Excitation & Power Factor
It would be unusual in practice for a large grid to be so close to unity.
On the other hand, The small islanded system that I was associated with was generating with a mix of 600 KW and 350 KW diesel generators. Machines were added or dropped off as the load changed. We seldom went over two x 600 KW.
On a system that small, you could get a lot of interaction between the machines if the excitation was not correctly adjusted. Raising the excitation would often result in a combination of higher voltage on the grid and a shift in power factor and uneven VAR sharing.
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator Excitation & Power Factor
About the question of generator power factor being leading or lagging...I had to think through several times bacause some of the post were at the same time convincing and confusing. I wonder if this isn't the same type if question as deciding whether current flows from a positive terminal to a negative terminal or the other way.
I say that the power factor of whatever you are looking at is determined by measuring the phase relationship between voltage and current at the terminals of the device. If the current is lagging the voltage at the terminals then the power factor is considered to be lagging.
To picture this in a simple way, picture a single generator (source) connected to a single load. The source voltage and current must be the same as the load. If the load is inductive and the current lags the voltage (lagging power factor) then the source current must be the same. Meters installed at the termanals of the source must read the same voltage, the same current, and the same lagging power factor as meters instaled at the load.
For ideas of what happens when paralleling, refer to waross's post.