Power Factor Variation

L

Thread Starter

lakmalp

When a generator is over excited, power factor drops from unity towards lagging direction (as far as I know). Could someone let me know how it can be theoretically explained?
 
I thought of updating the post with more details since I didn't get any reply :)

Generator is synchronized with grid and the power factor is negative. From what I have read in control.com posts, it is a sign of under excitation and we (others in the company) found that actually it is the case.

They have rectified the issue and now the PF is lagging (as I heard).

1. What I wanted to know is that is there any 'rule' or something which tells us that we should maintain lagging PF?

2. Why shouldn't we maintain it at unity (after it is synchronized)?

Hopefully, I'll be explained by someone this time :)

Lakmal
 
lakmalp,

Can we agree that motors convert electric current into torque--motors drive pumps and compressors and elevators and conveyors and such. (By the way, the largest use for electric power is pumping water--fresh water; grey water; sewage water; etc.)--and electric motors drive those pumps.

And, can we agree that generators drive motors? And that current is what varies between generators and motors (electric voltages vary--but that's done via transformers and to reduce losses during transmission and distribution)? The formula for three-phase electric power is:<pre>P = Vt * Ia * (3^0.5) * pf

where P = Power (Watts)
Vt = Generator Terminal Voltage
Ia = Generator Armature (Stator) Current
pf = Power Factor</pre>Under normal operating conditions, generator terminal voltage remains relatively constant. The square root of three is a constant. And, presuming the power factor remains relatively constant (it should stay pretty close to 1.0, and not much less than 0.8 positive or negative), the only variable in the equation is armature (stator) current.

And prime movers drive generators--hydro turbines in your case. The only quantity the hydro turbine can produce is torque, which is directly applied to the generator which the generator then converts into amperes in the generator stator, which are distributed to motors (loads) via wires, which motors re-convert into torque.

Nobody ever seems to have a problem with the fact that motors convert torque into amperes--but EVERYbody seems to have a problem with generators converting torque into amperes. Torque is the only parameter applied by prime movers to generators. And, everyone agrees that generators are needed to drive motors.

If you want the maths and physics and vectors and load angles and such for how that is all <b><i>proven</b></i>--you can find it all over the World Wide Web with your preferred World Wide Web browser. All of the maths and formulas and vectors and load angles just prove what is happening inside the generator--that's all. If maths and formulas and vectors and load angles and back-emfs and armature reactions help you to understand all of that--there are hundreds, probably thousands, of World Wide Web sites and YouTube videos to peruse and study. We can't put graphics or drawings in our responses on control.com--but we shouldn't have to, because it's all readily available in lots of different forms and explanations.

As for why speed doesn't change--that's because there are very great magnetic forces at work inside the generator. Two of them: one a result of the DC current applied to the generator rotor (the "field") and one a result of the current flowing in the armature (stator) when supplying a load or loads. You know what happens when you try to separate two magnets with their North and South poles touching each other. And, you know what happens when you try to force the North poles of two magnets to touch each other. Well, that's exactly what happens in a synchronous generator. And the North pole of the generator rotor is locked into step with the apparently rotating South pole of the generator armature, and the South pole of the generator rotor is locked into step with the apparently rotating North pole of the generator stator. And, the prime mover is trying to separate the poles--but the magnetic attraction is SO strong they can't be separated.

That's another thing for which there are thousands of articles and videos about on the World Wide Web--how AC motors and AC generators (specifically synchronous generators) work. How the alternating current flowing in the armature (stator) of both motors and generators induces a magnetic field that appears to rotate around the stator. And that magnetic field captures the opposite poles of the generator rotor's magnetic field and keeps it locked in <b>synchronism</b> with the speed of the apparently rotating stator magnetic field.

Again, this is the place to start. Agreeing that motors convert torque to amps. And generators convert amps to torque. The prime movers driving the generators are actually doing the work of the motors connected to the generators--the prime mover torque is transmitted from where it is produced to motors (and other loads) where is needed and useful by converting it to amperes (in the generator) and then converting it back into torque in the motors.

And, the basic physical principles of AC synchronous generators and AC electric motors rely on magnetism--it's all about magnetism. And we all know how magnets work and attract and repel each other.

So, if we can't agree on this and start here--and then you can find the proofs (used by machine designers to build machines and predict their power input/output) to help with your understanding, then, well, there's not much more to be done. It's not rocket science--obviously--it was developed before rockets started being used to put objects into space/orbit. It's very basic, and it all starts with magnetism. That's how voltage is developed, and it's how torque is converted into amperes and how amperes are converted into torque. Electricity is the way for transmitting torque (power) from a place where it is plentiful or can be produced (think hydro turbines!) to places where is can be useful or is needed (think homes and businesses and factories located long distances from the water used to power the hydro turbines which drive the generators). The hydro turbines are actually doing the work of all the loads at the other end of the wires connected to the hydro turbine's generators.

We gotta start somewhere--and let's get this all sorted before we start directing you to maths and formulas and vectors and load angles and emfs and back emfs and counter-emfs and armature reactions--all of which are just proofs of what's happening to be used to design machines and apply them. So, ask for clarifications now--before we get into the numbers and angles--most of which can't ever be visualized or measured on running generators and motors. They're intangible to the operator--no one can measure back emf or load angle--they're just calculated values used to prove what's happening, because man needs to have formulas and proofs of what he sees happening in order to feel in control of his environment.

All of which is really interesting when you consider that we (thinking men) don't know what electricity is. That's right--we don't know if electricity is a mass with a charge, or a charge with a mass. But, we do know how to use it, and we do know how to quantify it and produce it and how to predict what will happen and build machines to use it--<b>to transmit torque from an area where is plentiful to areas where it is useful or necessary.</b> Your hydro turbine plant is probably located some distance away from houses and factories and businesses, but there is head (water pressure/flow) available at the place where the hydro turbine-generators are located, and they can be easily connected using wires to loads (electric motors and lights and computers and computer monitors) where the homes and businesses and factories are located.
 
Generators supply VAR to the system if they are overexcited. Overexcited means that the excitation current is more than the value corresponding to Unity Power Factor value. Generators are normally operated near to their rated power factor but this is no rule. We can operate them near to unity factor or leading power factor (but with in capability of the generator) also but the decision rests with the user as it should not affect their system stability. Leading Power Factor means the Generator is now absorbing VAR from the system. Or you can say that the excitation current of generator is not sufficient enough to maintain the voltage greater than the GRID system voltage. There is only one harm in operating near to unity power factor or leading factor and that is the system stability. If due to any reason you desynchronize from the GRID in under excited condition, you system may trip on under voltage and the probability of system trip on under voltage becomes high if the AVR is on VAR control mode.
 
Dear CSA,

Thanks for the reply. But, I wonder have you answered my question. Did you mistakenly answer my post?

I am asking this because, I cannot relate your answer to my post. Thanks.
 
lakmalp,

You are 1,000% correct. I posted this reply to the wrong thread. I would never try to answer your questions in this post.

For help with your questions in this post, please contact [email protected]. You will need to supply your affiliation (employer; or name of school (if a student)), and you will be treated to the best response ever.

Best of luck!

And my apology for any confusion. I meant to post my response above to:

http://www.control.com/thread/1453805211
 
Top