Previous in Forum: Terms Used with Electronic Integrated Circuits   Next in Forum: Multicore Armored Cable
Close
Close
Close
14 comments
Anonymous Poster

Generator PF

01/04/2010 6:57 PM

It has been observed that Generator PF always maintain below unity (in Lagging). why ? if we maintain unity then what is effect occurred on generator ?

Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru
Engineering Fields - Environmental Engineering - New Member APIX Pilot Plant Design Project - Member - New Member

Join Date: May 2007
Location: Anywhere Emperor Palpatine assigns me
Posts: 2774
Good Answers: 101
#1

Re: Generator PF

01/04/2010 7:56 PM

Frictional losses in the bearings, electrical resistance, all the usual reasons why only morons believe in over-unity systems.

__________________
If only you knew the power of the Dark Side of the Force
Reply Score 1 for Off Topic
Anonymous Poster
#3
In reply to #1

Re: Generator PF

01/04/2010 10:11 PM

Sorry, I just do not get the connection between power factor and over unity machines - am I missing something here (maybe I am a moron).

Reply
Guru
Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

Join Date: May 2009
Location: Richland, WA, USA
Posts: 21006
Good Answers: 781
#4
In reply to #3

Re: Generator PF

01/04/2010 10:23 PM

Not at all; your question was perfectly sensible. One benefit of power factor correction is that the current (amps) in the conductors leaving the generator will be a bit less, as will the associated heat losses.

__________________
In vino veritas; in cervisia carmen; in aqua E. coli.
Reply
Power-User
New Zealand - Member - Kiwi

Join Date: Oct 2007
Location: Christchurch, New Zealand
Posts: 328
Good Answers: 29
#5
In reply to #3

Re: Generator PF

01/04/2010 11:53 PM

As far as I can see, the OP was referring to a unity power factor, ie a power factor of 1.

One would have to read the post very carelessly indeed to see anything at all to do with over-unity power production.

__________________
paulusgnome
Reply
5
Power-User
New Zealand - Member - Kiwi

Join Date: Oct 2007
Location: Christchurch, New Zealand
Posts: 328
Good Answers: 29
#2

Re: Generator PF

01/04/2010 9:26 PM

The power factor is determined by the characteristics of the load. If, as is usually the case, the load has an inductive component, then the power factor will be lagging, and will (of course) be less than 1.

You can correct the power factor back to close to 1 with capacitors. If the load is constant, the correction can be accomplished with fixed capacitors, but it is more usual to use a bank of capacitors that are switched by a controller in response to changes in load.

It is not a good idea to correct the power factor back to 1. At unity PF, the inductive reactance of the load and the capacitive reactance of the capacitors are equal, and this can create a resonance condition with high circulating currents that will trip circuit breakers. It is common to correct PF back to 0.85, or 0.9. I have read of installations where they correct back to 0.95, but this is less common.

__________________
paulusgnome
Reply Good Answer (Score 5)
Active Contributor

Join Date: Nov 2008
Location: Bangalore
Posts: 22
#6
In reply to #2

Re: Generator PF

01/05/2010 12:00 AM

in general, when entire load gets transfered on to Generator during failure of normal supply, the capacitor bank will get isolated from the loop, why? As a rule we consciously provide control wiring to achieve this. Kindly answer

__________________
Let Noble Thoughts Comes to Us From All The Sides
Reply
Commentator

Join Date: Aug 2009
Posts: 91
#7
In reply to #2

Re: Generator PF

01/05/2010 6:27 AM

Hi all

As i understands if the power factor is 1 this means the voltage and current are in phase .this implies that the reactive power will be zero .As reactive power is related to electric and magnetic fields then these will be zero .S=P+Qj Q=O and the machines requires also reactive power to maintain the fields inside .

__________________
Correct me if i am wrong i am learner
Reply
Power-User

Join Date: Jul 2009
Location: Montreal, Canada
Posts: 284
Good Answers: 18
#14
In reply to #7

Re: Generator PF

01/07/2010 10:09 AM

Hi, ikhanuk.

Good question: the rotating electric machine will indeed draw both real and reactive power; but sticking a PF-correcting device at its feed connections or upstream of it (but still within the production plant) will make it appear from outside the plant (strictly speaking, upstream of the device) that the machine plus the PF-correction device will consume less reactive power.

The machine is an inductive + resistive load, where the inductive part draws reactive power and the resistive draws real power. To counteract the inductive part, you can do something as simple as putting an industrial capacitor in parallel with the machine.

What'll happen then is that the machine will (like before) store and release reactive power in magnetic fields generated inside of it. However, when it will draw and store reactive power, it will draw it from the energy previously stored in the capacitor that's in papallel with it. When the machine will release reactive power, the reactive power will get stored in the capacitor instead of being fed back to the production plant's electricity supplier.

The result: reactive power moves from the capacitor to the machine and vice versa, instead of from the electricity supplier to the machine and vice versa.

Cheers! DZ

__________________
Do unto others. Then run.
Reply Score 1 for Good Answer
7
Associate

Join Date: Aug 2008
Location: TN, India.
Posts: 35
Good Answers: 2
#8
In reply to #2

Re: Generator PF

01/05/2010 9:20 AM

The reason for unity PF as you said is something that I am not heard. See, the PF correction is done by addition of parallel and series capacitors, mainly to reduce the inductive loading and to get Surge Impedance Loading ( SIL ) and in transmission lines at power frequency there is no chances of resonance.

On the other hand in power generation, the power factor of a particular generator can be varied to any value within its limit by means of excitation. The generator is connected to grid so, lowering the excitation is not going to reduce its terminal voltage, but its MVAR export or generation will be lowered. Hence keeping the same MW and lowering the MVAR means a lagging operation. But power factor improvement by generating MVAR in generator is not recommended as it is increasing line loses, so it is advised to be done at near to load centers.

But the actual answer to the original post is that the generator is more stabel in lagging operation or margin of stability is higher in lagging PF. It means, if at all any disturbance occurs in Generator, the rotor may oscillates and reverts back to its original position without reaching its upper limit or out of synchronism. So to keep the upper margin higher for the generator rotor, the machine is operates under lagging condition. And the name of the entity which I am discussed here is nothing but LOAD ANGLE.

__________________
?????????? ????????? ????????,.........??????? ???????????? ???????? ????. ????????????
Reply Good Answer (Score 7)
2
Power-User

Join Date: Jul 2009
Location: Montreal, Canada
Posts: 284
Good Answers: 18
#11
In reply to #2

Re: Generator PF

01/06/2010 9:21 AM

No offense to Paulus, but his comment is absolutely wrong.

First thing first: adjusting the power factor of a generator has nothing to do with the load's power factor. One sets how much effective power (MVA = MW + MVAR) is to be generated by adjusting the generator's excitation, and one sets the generator's power factor by adjusting the phase angle between its rotor and its stator (the power is 'generated' in the stator by the rotating rotor, which is driven by a turbine). The excitation level and the phase angle are both adjusted through a given generator's controls. AS for the load, the generator has no effect on it .. it has its own SEPARATE PF.

Note that there are limits to a given generator's operation PF, that is to the quantity of reactive power that it can generate in proportion to real power. Below a certain power factor (around 0.70 to 0.75, I believe), the magnetic link between the rotor and the stator will break and the generator protection circuits will trip. Think of this as being like when there's too much slippage between an induction motor's rotor and its stator's rotating field ... the link between the two is lost and the motor stalls.

As K. Dhana has said, operating a given generator near PF = 1 makes for unstable operation since the generator's rotor-frequency will tend to oscillate a little, which will lead to arcing in the generator, since the voltage it will generate will become slightly out of phase with the transmission network's, and which will trip the generator's protections.

Lastly, why generators are run so as to have lagging power factors rather than leading ones: it's because that generating in lag mode GENERATES MVARs that a load in lag mode CONSUMES. Conceptually, think of transmission lines as partly-plugged, leaky water hoses (the analogy isn't exact, but the general idea is). Water-pressure gets generated atthe head of a hose when water is 'pushed' into one, just like electrical pressure gets created when electricital presuure (voltage) gets 'pushed' into transmission lines. Pressure gets lost in the hose because of friction between the moving water and the hose's inner surface; this is similar to real voltage's dropping in transmission lines because of the lines' electrical resistance. Pressure is also lost in the water because of partial plugging within it (similar to a line's inductance) and because the hose leaks (similar to a line's capacitance in relation to parallel lines, and ground). The water-pressure at the hose's tail end equals the one at the head end's minus the pressure lost to plugs and leaks; similarly, the voltage at a transmission line's end is voltage at the head end minus real and MVAR losses along the way. The difference lies in the total MVAR losses being 90 degrees out of phase with the real power. As I said, the analogy's not exact, but one gets the idea ... a generator has to generate enough real power AND MVARs to overcome lines losses and to feed an end-load.

Cheers! DZ

__________________
Do unto others. Then run.
Reply Good Answer (Score 2)
Power-User
New Zealand - Member - Kiwi

Join Date: Oct 2007
Location: Christchurch, New Zealand
Posts: 328
Good Answers: 29
#12
In reply to #11

Re: Generator PF

01/06/2010 2:31 PM

I stand corrected, and no offence taken.

Damn, its hard to be right all the time!

__________________
paulusgnome
Reply Off Topic (Score 5)
Active Contributor

Join Date: May 2008
Posts: 17
Good Answers: 2
#9

Re: Generator PF

01/05/2010 11:09 PM

Dear Engineers,

After seeing all comments, I would like add, What ever may be, Load coneectec to grid or Geneartor line,Load side charector remain same, we have to maintain Nearly unity for get full utilisation of KW capacity available.In case of Low PF , you can load only load below capacity(under utilsation (KW=KVA*PF)), Ig Pf is more than unit(leading) it affects the excitation ,

Reply
Guru
Panama - Member - New Member Hobbies - CNC - New Member Engineering Fields - Marine Engineering - New Member Engineering Fields - Retired Engineers / Mentors - New Member

Join Date: Dec 2006
Location: Panama
Posts: 4273
Good Answers: 213
#10

Re: Generator PF

01/05/2010 11:24 PM

Every generator of any size that I have encountered is generally rated for a certain kVA at a power factor of 0.8. Per manufacturers' literature, generally, if the power factor is higher than this, then one must reduce the power rating of the unit (i.e., lower the load). When dealing with grid power, one is more concerned about power factor, because the utilities have figured out how to charge for the reactive power your system demands, and you pay a penalty. For a generator, this is not a concern. The amount of fuel you burn is a function of the kW, not the kVA- I have never been able to measure any difference between fuel consumption when the plants were operating at a PF of 0.96 or 0.8. I have noticed that the units tend to run hotter with a higher power factor...

Reply Score 1 for Good Answer
Power-User

Join Date: Nov 2009
Location: Nnewi,Anambra,Nigeria
Posts: 146
#13

Re: Generator PF

01/07/2010 9:51 AM

The generator has its internal active and inductive(lagging) components.

If u maintain unity pf,yr generator apparent power(KVA) will be equal to its active power(KW).

Patrick Whowha

__________________
Patrick Whowha
Reply
Reply to Forum Thread 14 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

Anonymous Poster (1); cwarner7_11 (1); DreadZontar (2); DVader1000 (1); ikhanuk (1); k.dhana (1); keshi123 (1); Patrick Whowha (1); Paulusgnome (3); Tornado (1); vinukumar (1)

Previous in Forum: Terms Used with Electronic Integrated Circuits   Next in Forum: Multicore Armored Cable

Advertisement