Power Factor
Power Factor
(OP)
In our plant, we operate a turbine-generator in over-excitation such that we generate a capacitive 8 MVARS.
Also, in parallel, we purchase from our utility 7.4 MWATTS. We have a power factor of 0.8 on this incoming line. Inductive load. So, Apparent power is 9.25MVA. Consequently, reactive power for this incoming line is 5.55 MVARS.
What is the TOTAL REACTIVE power of our system ?
8MVARS + 5.55MVARS = 13.55 MVARS ? or ( 8 MVARS - 5.55 MVARS ) = 2.45 MVARS capacitive
Thanks
Dan






RE: Power Factor
From the utility' point of view (POV), you are furnishing total net recative power of 2.45MVAR while consuming 7.4MW of real power, so you have a leading power factor of ~0.95, usually denoted as -0.95
RE: Power Factor
RE: Power Factor
Thanks a lot for your replies. However, I still have the same problem since some think that here the total MVARS will be 13.55, and some others think that total MVARS will be 2.55.
What I would believe is the following :
Assuming that S = P -jQ for capacitive and S = P + jQ on the inductive side, what I get, is the following.
From our first source of power, we have
S = 16.56 MVA
P = 14.5 MWATTS
Q = 8MVARS (capacitive since our generators runs over-excited)
Power factor = 0.875
From our second source of power (the incoming line from our utility).
We purchase P = 7.4MWATTS
Power factor = 0.8
S = 9.25 MVA
Q = 5.55 MVARS
Load of our mill is inductive
So, having
S1 = 14.5 - J8
S2 = 7.4 + J5.55
I would believe that
S TOTAL = 21.9MWATTS - J2.45
Consequentely, S TOTAL would be equal to 22.03 MVA
Overall Power factor = 0.994
Am I right to think that, or are some of my assumptions wrong ?
Please help
Thank in advance
Dan
RE: Power Factor
Regards
Marmite
RE: Power Factor
With 13.55 Mvar, the plant power factor is 85% lagging, which is reasonable. With -2.45 Mvar, the plant power factor is 99.3% leading, which is not reasonable unless you have a bunch of capacitors in the plant.
RE: Power Factor
Your plant is at 13.55MVAR reactive and the generator is supplying 8MVAR of this which makes the incoming hydro system see the remaining 13.55MVAR - 8MVAR = 5.55MVAR of this reactive plant load. That would be the answer.
Adding capacitors to your system will remove some of the inductive load that the hydro system sees. You have to add the plant capacitor size to the inductive VARs of the hydro metering to get your plant VARs without the capacitor. In other words - take away plant capacitors and hydro meter reads a worse inductive power factor.
If your plant was at 2.45MVAR capacitive and the generator was supplying 8MVAR of inductive power then the utility would see -2.45MVAR + 8MVAR = 5.55MVAR inductive load. The big burning question though would be why run the generator at 8MVAR when running at 2.45MVAR would be perfect. Anyways, you said the generator is supplying capacitive VARs which immediately makes this wrong.
RE: Power Factor
To be honest with you, I may miss some experience in the heavy industry, so I have been confused by some readings where it was written that kind of things :
SOURCE : WIKIPEDIA
"It is also possible to effect power factor correction with an unloaded synchronous motor connected across the supply. The power factor of the motor is varied by adjusting the field excitation and can be made to behave like a capacitor when over excited."
SOURCE : ABB
"Shunt capacitors, lightly loaded transmission lines and over-excited synchronous machines are capacitive loads and act as sources for reactive power resulting in a leading power factor"
Another source
"At very low power levels the synchronous motor can be made to look capactivie and can be used as a continually adjustable power factor corrector."
Then, with those readings, I mad a misinterpration and I thought that the MVARS produced by an over-excited generator was CAPACITIVE. But, this is not a good way to see the problem.
In fact, I understand now that in overexcitation, the generator will DELIVER VARS instead of consuming (in under-exitation).
So I must then add this VARS to the VARS delivered by the utility, and finally you are right, 13.55 MVARS would be the total MVARS delivered in the case I submitted to you.
Thank a million to all !
Best Regards,
Dan
RE: Power Factor
Please pay attention on ABB and Wikpedia source:
synchronous motor ,not generator.
It's very popular in plants with big syn motors, used them as sync compensator and correct power factor.
More from this, I know one place with 3 big syn motors, them all times work in OE and utillities pay to this plant for export of MVAR's ( it's some week line).
Regards and Good Luck.
Slava
RE: Power Factor
RE: Power Factor
The above said by a ME, now all you electrical guys jump in there and straighten me out.
rmw
RE: Power Factor
Not all weakness is mechanical. A line could be considered electrically weak if it has large watt or var losses.
RE: Power Factor
Syn compensators and capasitor banks used for "generated" MVAR's in long transmission line for:
1. Save losses ( as you said)
2. Increase voltage in "middle" of line.
or near to power station for save MVAR's generation in the power station
or as said jghrist : line electrically weak ( this is my case)
Regards.
Slava