power factor of syncronous generator

[pbrod]

Hi,

I have a question about the power factor of a 4500kVA (rated power) syncronous generator.

The generator has the following characteristics:
4500kVA rated power
0,8 rated power factor
6,3kV rated voltage
50Hz rated frequency

This installation will be used for power production using a steam turbine.

My question is what will be the power factor of the generator when connected to our factory grid. The PF of the factory is about 0,92. Is this also the PF of the generator when connected to our grid?
If we want to optimise the power output of the generator we need to work at a high power factor to obtain a higher generator efficiency. Is this done by controlling the exitation of the generator or do we need to improve the PF of the factory grid?

Many thanks in advance!!

 

[ScottyUK]

Is the machine synchronised to a larger system - e.g. the utility - or is this an islanded application where the generator supplies all the load without any grid connection? There are significant differences in the behaviour of the machine in the two states.


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If we learn from our mistakes I'm getting a great education!

 

[pbrod]

The machine is syncronised with the grid, in no case there is an island operation mode.

 

[dpc]

The power factor will basically be controlled by the voltage regulator, especially when tied to the grid. Increasing the excitation will increase the vars exported from the generator.

Assuming the grid can provide all your plant's var requirements, you can adjust the generator excitation until the power factor is unity and load it up.

You might want to think about the power factor penalty that the utility may impose if you provide nothing but kW from your generator and import all of the vars from the grid.

Your interconnection agreement may have some restrictions on this type of approach.

 

[waross]

The first choice for economical operation is often to run the generator over excited so that the plant connection to the grid is at unity power factor. You may have to compromise somewhat to stay within the alternator capability curve.

If we want to optimise the power output of the generator we need to work at a high power factor to obtain a higher generator efficiency.

Check out this thread. You will see that improving the power factor of a generator from 0.8 to 1.00 (unity) will gain 1% in efficiency.
Compare the cost of 1% of the fuel for your turbine to 1% of your power bill as a penalty for a 1% drop in PF to the grid. Some utilities use a more punitive penalty charge.

Bill
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"Why not the best?"
Jimmy Carter

 

[pbrod]

Thanks for the feedback!

For the moment we don't pay for the reactive power we take from the grid. But we will take a look to improve the power factor near the electric motors to reduce the energy consumption in the power cables.
And if I'm right this we can't do with the reactive power from our generator.

The fuel for our turbine is free so we want to squize as much as possible out of it. So then we can try running it with a PF of 1.

The interconnection agreement is not yet discussed.

 

[waross]

Wait until you receive the interconnection conditions. Until then you are wasting our time.
The old formula for PF penalties was:
PF of 90% to 100%: No penalty.
PF below 90% a penalty of 1% of the power bill for each percentage that the PF was below 90%.
Loads place two demands on the power supply:
A real power demand (KW)
A reactive power demand (KVAR)
If you set your generator to supply only real power then the grid will have to supply the same amount of reactive power as before. If, for example, your generator is supplying one half of your plant demand, this will drop your power factor to about 72%. Expect at the least a penalty of 18% on your power bill. No utility will supply free VARs at that power factor.
You may have missed this before.
If a generator is producing one Mega Watt at a PF of 0.92, and you raise the PF to unity, the generator will still produce one Mega Watt with a saving of less than 1/2 of one percent of free fuel.
A set rating of 0.8 PF usually implies that the prime mover is sized to develop the rated KW. If the prime mover is oversized you may be able to develop more than rated KW as long as you don't exceed the KVA rating and the capability curve.
If your efforts incur utilities penalties you may expect to lose more than you gain.
By the way, I have tried something similar in the field. We were running two 600 KW sets and some 350 KW sets. Unfortunately the 350 sets were not available. When the load built up on the 600 KW set that was in service the second 600 KW set was added. It seemed like a lot of capacity (1200 Kw) when the load may be only 650 KW.
Our grid load was running at about 80% PF. If we could raise the grid PF then we may be able to get up to 750 KW or KVA. before spending any money, we went to the plant for a "proof of concept" test. We put two generators online. We set one to pick up the entire KVAR load leaving the other set running at unity power factor. Then we advanced the throttle on the set at unity power factor.
Then reality raised its ugly head. At only a few Watts loading past 600 KW the V-12 diesel started to bog down. It had enough power for 600 KW and no more.
Another "good" idea in the trash bin.
?????

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cranky108]

First question I would ask is why do you want to generate power at all?
Is it cheeper, or are you using extra steam, or is it that you want a backup source?

The reason to generate power makes a difference in how you operate the generation. If it is a backup source, you will want to match the load you want to backup. If it is cost, then you don't want to generate vars, some capacitor can do that. If it is because of extra steam, then why worry about var generation, because you probally have extra overhead capacity on your generator anyway (most of the time).