pls tell me this theoretical questions answer. I have a generator not having a governor. if the load increases what happens to the generator? as I guess you will say it will slow down. but if it slows down it will produce smaller power since i don't touch the fuel(torque) and load is getting bigger. it is a paradoxical situation, since as the load gets bigger the frequency gets smaller as we feel on the main grid. but this situation is out of sense, how can it be that if there is no a governor the speed? so produced power gets smaller as the load is getting bigger.....

 

Your query seems to be a confusing. What I gather is that you would like to know the loading effects on the generator. If the generator is islanded and you add the load to it, the speed would obviously go down and the controller would infuse in higher quantity of fuel to bring back the turbine speed to the normal operating speed. In case of an synchronised condition, the load addition would not make any difference in the speed but the import/export statistic would change to the amount of the load added/removed.

One more thing, what you are mentioning is if the speed goes down and there is no change, the machine would trip since the speed would go down and if the fuel is not increased proportionally then there would be an underspeed trip

 

the energy equation will always balance... energy out will always be equal energy in. No paradox at all.

 

Let's go the other way round. Assuming the generator is on finite bus, and with AVR on manual. if I now increase the driving torque of the generator, will the terminal voltage increase?

There is normally a supposed fall in terminal voltage due to armature reaction, but when does armature reaction kick in? It must be at a certain value of percent loading!

Thanx & Best regards,
Shahvir

 

Electrics, please try to re-think and re-phrase your question. It doesn't make too much sense.

As said above me, if you are in island, load increases, frequency drops, you reach the lower limit for generator breaker closed, breaker opens, speed increases back to 100%. That, if you are lucky. If not, if your gas valves are blocked and gas flow doesn't change, you will go in overspeed and I strongly recommend the hand-break

If you are in a bigger grid (national, for example), the effect is similar but hardly noticeable due to the huge energy buffer involved.

 

Actually, I've a query on the same lines, but the other way round. Let's say I've a Generator hooked up to a 'Finite Bus'. It is feeding a constant purely resistive load. Assume AVR to be on manual ( or Permanent magnet Generator). Now, if I were to increase the input torque of the Generator, the terminal voltage Vt (across load) would increase (albeit by 1%).

But there is also ‘Armature Reaction’ which causes Generator terminal voltage Vt to fall. But these two pheneomenon cannot occur simultaneously. So, what is the percent loading when armature reaction comes into picture? What actually is the behavior of Vt under these different circumstances?

Thanks & Best regards,
Shahvir

 

Are there no takers for my query?

Best regards,
Shahvir

 

I've started a couple of times to try to reply to your theoretical question, and I think it's just too theoretical for me.

Mr. Phil Corso has made available a treatise on armature reaction in the past. My recommendation is to contact him for a copy of his "The Physics of ... Armature Reaction" (I can't remember if there was an exclamation in the title or not.)

Be sure to let us know what you learn and if you find the paper informative.

If you have a synchronous AC generator feeding a finite bus (I'm interpreting that to be similar to what's called an "island" mode where there is likely one prime mover operating in Isochronous mode to control the frequency), if you increase the torque into a generator whose prime mover is operating on droop speed control the Isochronous prime mover will reduce its torque input to its generator to try to maintain a constant frequency.

If the generator you are increasing the torque input to is the isochronous unit, then the frequency of the 'finite bus' will increase.

My own personal experience with armature reaction, and I've been rebuffed for this explanation, is that if one holds excitation of a synchronous generator on an AC grid constant while increasing torque input to the generator (increasing the current flowing in the stator) that the generator terminal voltage will decrease. I deem this to be 'armature reaction', the magnetic field of the stator increasing because of the increasing stator current flow acting to counter or "shrink" the magnetic field of the rotor. Now, whether or not that's exactly what happens, I do know that if excitation is held constant as load (torque) is increased, then generator terminal voltage will decrease.

Other than that, I don't know how to respond any further. I may not even understand the question correctly. I'm interpreting the question to be about AC generators, and synchronous AC generators in particular. If that's not what you're asking about, then someone else will have to answer the question.

 

Dear CSA,

I understand the effects of Armature Reaction on terminal voltage. But from whatever info i've come across, Armature Reaction comes into picture at a certain level (percentage)of loading.

If AVR is assumed to be on manual (excitation constant) and input torque is now increased at light loads, the terminal (busbar) voltage increases (albeit by 1%). My belief is armature reaction only comes into picture at near full load and only then will the terminal voltage start to decrease (as stator current is now significant). I just wanted to verify whether this phenomenon is true.

Thanks & Best regards,
Shahvir

P.S. Plz do not vilify Mr. Corso, he's a very noble and helpful person

 

voltman,

I don't know what kind of generator you are testing or have been observing or reading about. I'm not that familiar with the precise details of what happens in the first 1% of loading a generator, or what happens mathematically when loading a generator, or what happens if the load is 100% resistive or has 20% inductive reactance and 10% capacitive reactance.

I'm speaking in general terms about synchronous generators operating on machines rated 20MW and greater that I have operated, observed, and troubleshot for many years. And, I am only offering my observations without any mathematics or proof (no supporting data). That's all I have to offer.

My experience is that if a machine (prime mover and generator) is loaded from a condition where the generator terminal voltage is exactly equal to the bus voltage (zero VArs; unity power factor) and all that happens is the torque output of the prime mover increases and the generator excitation is held constant, that as the real power increases, the reactive power will start to increase in the leading direction (Lagging VArs and Lagging power factor decrease, or Leading VArs and Leading power factor increase).

This is what leads me to believe that armature reaction from the increased current flow in the stator is causing the generator terminal voltage to decrease, because, if one decreases terminal voltage when torque is held constant then the power factor and VAr meters move in the Leading direction.

I have my theory from my recollection of electricity class that an increasing stator field will "reduce" the rotor field, which in turn decreases generator terminal voltage. The speed of the machine should be constant (which is another variable we haven't talked about but which is very important since voltage is a function of both speed and excitation, and for a synchronous generator speed is presumed to be constant).

I'm still curious about the nature of your question. Is this for some thesis or paper for some theoretical class? Are you trying to troubleshoot or understand some physical phenomenon that you have observed or which has been reported to you? Do you have physical data to support your theory and hypothesis?

 

Dear CSA,

Thanx very much for reply. I have presented this query for my own academic interest as i'm very much interested in principles electrical machines and want to understand the happenings inside of them under different conditions. The query is not part of any thesis or project or research but for my own interest.

Okay, to cut a long story short, let me come to the point to avoid further distress. Let's assume I compare two different power systems, one, a Permanent Magnet (PM) DC Generator system and the other a PM AC Generator (1 phase) system connected to 'Finite Bus' respectively. Both are seperate power systms feeding a purely resistive loads.

Now, if I were to increase the input torque of say Generator 'Adc' and that of Generator 'Aac' respectively, the following changes should occur in both the seperate power systems respectively;

DC system:- Rotor speed increases, induced Emf 'EAdc' would increase, which in turn, would increase terminal voltage 'Vt' across load. Gen 'Adc' would take more load and Gen 'Bdc' now would take less load. This would also cause Gen 'Bdc' to speed up (due to less load sharing), but speed of 'Bdc' would still be less than 'Adc' due to armature voltage drop 'IbRb'. Load current 'iL' should increase due to increase in 'Vt'. If input torque of Gen 'Adc' is further increased, load current 'iL' increases. 'Armature Reaction' would now come into picture offsetting increase in 'Vt' which will now gradually start to decrease. This, in turn should decrease 'iL'(as 'iL' proportional to 'Vt' - Ohm's law)

Ac system:- Rotor speed increases, induced Emf 'EAac' would increase, which in turn, would increase terminal voltage 'Vt' across load (albeit by 1%). Gen 'Aac' would take more load and Gen 'Bac' would now take less load. This would also cause Gen 'Bac' to speed up (due to less load sharing) and settle at speed equal to that of Gen 'Aac' due to 'synchronous bond'. Load angle 'delta Aac' would increase and 'delta Bac' would decrease respectively. Load current 'iL' should increase due to increase in 'Vt'. If input torque of Gen 'Adc' is further increased, load current 'iL' increases. ''Armature Reaction' would now come into picture offsetting increase in 'Vt' which will now gradually start to decrease. This in turn should decrease 'iL'(as 'iL' proportional to 'Vt' - Ohm's law)

I just wanted to understand whwether my understanding of both the above mentioned DC and AC power systems correct? I humbly request you to point out any descrepency or flaw in my understanding to help me be on the right track.

Thanks & Best regards,
Shahvir

 

voltman,

You have entirely too many variables in your scenarios for me.

I'm not a theoretician; I can only offer the benefit of my experience tinctured a little bit with some education from a couple of decades ago.

My recommendation is to contact Phil Corso (cepsicon [at] aol.com), who seems to be excellent with the theory aspect of things. He also seems willing to take discussions off-line to provide as much detail and back-and-forth as is required.

He is an excellent resource and I've learned from his explanations as have many people; he's a great asset to control.com.

He's actually very correct in many of his explanations, it's just that it's not the way that most people are taught to think of elecrical phenomena. When you really stop and think about what the components of electrical system do, his definitions of power are technically absolutely correct.

We just differ in our styles, that's all. He and I have agreed to disagree and coexist. We're all good.

Good luck on your journey of discovery! Remember: It's not the destination, it *IS* the journey.

 

Dear CSA,

You would be pleased to know that i'm already benefitting from Mr. Corso external to the forum on several electrical phenomena and in this case too he's trying to make me understand from my perspective.

My only intention to re-posit this query here is to gain a different perspective from other experts. He's even provided me with vector diagrams but sometimes it becomes difficult to relate the theoretical with the practical. Hence, we are stuck somewhere in between.
Thanx anyways for your help.

Best regards,
Shahvir

 

> pls tell me this theoretical questions answer. I have a generator not having a
> governor. if the load increases what happens to the generator?

---- snip ----

There are a lot of books in physics which carefully outlined in details the many conservation laws which describe the grand symmetry of nature. Lenz law for example is a very good topic to read which would likely shed light to your question.

 

> the energy equation will always balance... energy out will always be
> equal energy in. No paradox at all.

Transient load chance may not follow steady state equation. You miss one term in your mathematical equation to make it more generic. The more generic equation is as follows:

Energy in (from 0 to T)-Energy out (From 0 to T) = Incremental energy stored (From 0 to T)

Validity of this equation can be traced back to Newton's 3rd Law of motion that states "The body in motion tends to be in motion, the body at rest tends to be at rest".

When you are trying to understand how power system works be sure you rephrase the energy balance equation correctly. If you miss incremental energy term, then there will be at least one question that you won't be able to find the answer for the rest of your life.