Hi everyone,
My doubt is purely electrical.
When a grid connected synchronous generator load setpoint adjusted (say for example setpoint adjusted from 50MW to 100MW) eventually gas fuel (in GT) or steam flow (in ST) will be increased at the same time excitation current and voltage as well will increase to meet the desired load which we entered as setpoint.

When I’m raising the excitation alone how VAR only raising in the generator. Considering the Lenz law how the rotor rotational speed is not coming down due to raise in field excitation current and voltage. How MVAR alone raising upon increasing the field current?

 

@curiouscat,

Personally I'm not familiar with the Lenz law, but I think it might be possible for me to explain what happens so that you may be able to understand better.

We are talking about an AC (Alternating Current) system of multiple synchronous generators and their prime movers synchronized together. When this is happening all the prime movers and synchronous generators are all acting like ONE (as in one single) synchronous generator supplying one load--a load probably larger than any single synchronous generator and its prime mover could ever hope to power. This means that the frequency of the electrical power at every outlet and driving every load (lights, motors (and motors power pumps and compressors (of many types of refrigeration equipment, for example), and computers and computer monitors and televisions and radios and electric tea kettles is IDENTICAL. This is because all of the synchronous generators are producing electrical power at the SAME frequency because there's no method or system on earth that can aggregate hundreds or even thousands of synchronous generators running at different frequencies and make one single frequency available to power the loads connected to the electrical system. It just can't happen, and has never been able to happen--since the very first days of alternating current power generation and distribution. Remember--the generator(s) and the prime mover(s) at the location where you work have to be SYNCHRONIZED to the grid with other synchronous generators and their prime movers already producing power on the electrical system. That's done by a very rigorous process of matching generator speed (which is directly proportional to generator output frequency!) and phasing and then closing the generator breaker at the proper instant in time to prevent serious damage to equipment at the location and elsewhere on the electrical system.

Every synchronous generator has a speed at which the generator rotor must spin at in order to produce electrical power at a specific frequency. Two-pole synchronous generators have to run at 3600 RPM to produce 60.0 Hz; 50.0 Hz synchronous generators have to run at 3000 RPM to produce 50.0 Hz. Four-pole generators have to run at 1800 RPM to produce 60.0 Hz, and 1500 RPM to produce 50.0 Hz. If you monitor the speed of any synchronous generator synchronized to an electrical power system with other synchronous generators and their prime movers you will see that for all intents and purposed the speeds of the machines are relatively constant (under normal grid operation conditions).

There is a formula that relates synchronous generator rotor speed to generator output frequency: F=(P*N)120, where F is frequency (in Hz), P is the number of poles of the generator, N is the speed of the synchronous generator rotor (in RPM). The number of poles of a synchronous generator is fixed--it can't be changed. This means that there is only variable that will affect frequency: generator rotor speed (the term N). And since it's desired for the frequency available at every place where AC electricity can be used to be constant (usually at 50 Hz or 60 Hz) that means that the speed of ALL of the generator rotors synchronized to an electrical power system to be relatively constant--and they are (as well as the speed of the prime movers driving the synchronous generators). It's all about that little formula: F=(P*N)/120. Very simple. Almost.

Further, the nameplate rated generator output terminal voltage occurs when the generator is running at a relatively constant speed when a relatively constant direct current is being applied to the generator rotor windings. Still further, the basic formula for real electrical power is Watts=Volts multiplied by Amperes, or P=VI, where P is expressed in watts (or kilowatts, or megawatts), V is generator terminal voltage and I is generator stator current (in Amperes). Now, again, if you monitor the voltage of any synchronous generator at your site while it is synchronized to a grid with other generators and their prime movers you will see that for all intents and purposes, under normal operating conditions (stable kW or MW output) the terminal voltage of the generator is relatively constant and stable, and it doesn't every change by very much (most synchronous generator excitation systems are only rated to produce 5% more or 5% less than the generator nameplate voltage rating.

All of this means that the synchronous generators synchronized to the grid are ALL running at a relatively constant speed AND all of the output voltages of the generators are also relatively constant and stable and don't change very much during normal operation. So, that means that if one wants to increase the power produced by a synchronous generator one DOES NOT increase the generator terminal voltage (by increasing the generator field excitation) so the only other adjustment that can be made is to increase the amperes being produced in the generator stator windings.

Generators are electric machines that convert torque (provided by the prime mover) into amperes. Motors are electric machines that convert amperes into torque (to power pumps and other electrical loads). The reason that electric power systems exist is to transmit torque from a place where it is abundant (where it is made) to many places where torque is required (think factories, air conditioner systems, elevators, water pumps (fresh water and sewage!)) and tea kettles (which convert amperes into heat) and now things like televisions and computers and computer monitors. THAT'S WHY electricity is used in our world today--because it's relatively simple to produce in various locations and its even simpler to transmit that electricity using wires (and transformers) to even more locations where it can be used to do work (of many different kinds). If the world never had electricity and only had hydraulic systems, it would be hydraulic fluid that is pumped through pipes and hoses from places where it is plentiful (large tanks/reservoirs of hydraulic fluid) to places where it is used (to drive motors in factories and other applications) to do work. Electricity is simply a transmission medium--a way to get torque from one place (or many places actually) to even more places where it can be used to do work.

For combustion turbines to produce torque they have to burn some sort of fuel--natural gas, diesel fuel, crude oil, naphtha, propane, butane, etc. To increase the torque produced by the combustion turbine one has to increase the amount of fuel flowing into the combustion section of the turbine which increases the amount of torque being applied to the generator the combustion turbine is driving. For steam turbines to produce torque steam (usually at a constant pressure--but not so for many combined cycle steam turbines) is introduced into the turbine to make it spin and increasing the flow-rate of steam increases the amount of torque produced and applied to the synchronous generator it is driving.

Every synchronous generator has two magnetic fields inside the generator that interact with each other. One is produced using direct current applied to the generator rotor. The other results from the flow of alternating current in the stator windings--that alternating current is supplying electrical power to the system it is synchronized to. And we all know how strong even small magnets can be because we've all tried to push like poles of two magnets together only to have them fly apart when we released them, and we have all separated small magnets from each other when their opposite poles were attracted to them. We can all agree magnetism is and can be a very strong force.

The magnetic field produced in the generator stator windings by the flow of alternating current in those windings appears to rotate around the stator. And the North magnetic pole of the generator rotor is VERY STRONGLY attracted to the South magnetic pole created by the flow of alternating current in the generator stator windings--VERY STRONGLY. SO STRONGLY, in fact, that under normal operating conditions the prime mover driving the generator rotor CANNOT make the generator spin any faster the speed which corresponds to the frequency of the grid the synchronous generator and its prime mover are synchronized to. Not no how. Not no way. The speed of the generator rotor is locked into a speed that is directly proportional to the system frequency of the electrical system it is synchronized to and because of the strength of the two magnetic fields which are "locked" together inside the generator it isn't going to spin any faster (or any slower) ever, under normal conditions.

THIS is what keeps the machines (the synchronous generators and the prime movers driving them) constant. NOT the governor (control system) of the prime. Not the excitation system of the generator. The frequency of the electrical system the synchronous generator LOCKS the generator rotor into a speed that is determine by F=(P*N)/120 (or, if you wish, you can rearrange the equation to solve for N: N=(120*F)/P.

As we already agreed the way to increase the power produced by a synchronous generator is to increase the torque being applied to it--not by increasing the excitation current being applied to the generator rotor. Again, that's because the basic formula for electrical power is: P (in Watts)=V*I, and under normal conditions the generator terminal voltage doesn't ever vary by more than plus or minus 5%. That means for a generator rated at 13,800 Volts the generator terminal voltage can increase or decrease by more than plus or minus 690 Volts, which is a pretty small amount when we're talking about MW (Megawatts). So, the only effective way to have a large operating range of power for a synchronous generator is to change the amount of torque being applied to the generator rotor. And, for a combustion turbine that means increasing the fuel flow rate to the machine; for a steam turbine that means increase the steam flow-rate (and pressure and temperature for a combined-cycle steam turbine).

Again, I want to go back to what it is that a generator does. It converts torque from the prime mover into amperes flowing in the generator stator windings (when the generator breaker is closed). And, when a synchronous generator and its prime mover are synchronized to a grid with other synchronous generators and their prime movers we have agreed that the speed of the generator rotor--and of the prime mover driving the generator rotor--can't change; they are fixed by the frequency of the electric power system the machine is synchronized to. So, what happens because the synchronous generator rotor speed CAN'T increase when the amount of torque being applied to the synchronous generator rotor increases that "extra" torque gets converted by the synchronous generator into amperes flowing in the synchronous generator stator windings. Again, the speed of the synchronous generator rotor can't be changed when the generator breaker is closed (under normal operating conditions and as long as the electric power system frequency is stable and relatively constant), so the synchronous generator does what it does and converts that extra torque (from the increased energy flow-rate into the prime mover) into amperes flowing in the synchronous generator stator windings which causes the MW of the machine to increase.

Now, when you are loading and unloading a large synchronous machine (changing the MW output of the synchronous generator) something happens to the strengths of the two magnetic fields inside the generator. The strength of the field produced by the flow of amperes in the synchronous generator's stator windings increases, and if nothing is done to change the strength of the field produced by the flow of direct current in the synchronous generator rotor windings the terminal voltage of the synchronous generator will decrease. So, it's normal to have to increase the excitation a little as the machine is being loaded in order for the generator stator windings to remain relatively constant.

The opposite happens when the machine is being unloaded. The strength of the magnetic field of the synchronous generator stator windings weakens as the amperes flowing in the generator stator decrease. Which means that unless something is done the generator terminal voltage will start to increase. So, it's normal for the excitation being applied to the synchronous generator rotor windings to be decreased to keep the generator terminal voltage constant. This is normal.

The real power of a synchronous generator--the Watts, or the kW, or the MW--is a function of the amperes flowing in the generator stator windings. The reactive current flowing in the synchronous generator stator windings is a function of the generator terminal voltage--which is varied by changing the amount of excitation (direct current) being applied to the synchronous generator rotor windings. When a machine is operating at a stable real power output (MW), meaning the amount of fuel flowing into a combustion turbine is stable and relatively constant, and it's desirable or necessary to change the reactive current flowing in the synchronous generator stator windings one does so by changing the amount of excitation being applied to the synchronous generator rotor windings. It also has the effect of slightly increasing or decreasing the generator terminal voltage by a slight amount, but in your case it's being used to change the VAr flow into/out of the machine and you're not really interested in generator terminal voltage--you're interested in the magnitude (and direction) of VArs, both of which can be affected by changing the generator terminal voltage. WITHOUT any appreciable change in real power (Watts; kW; MW).

This is how synchronous generators work. Generators convert torque into amperes (so motors have amperes to convert into torque to do work for us humans). This all happens at a relatively constant frequency--which means the generator rotors spin at a relatively constant speed--and at a relatively constant generator terminal voltage (of course there are transformers to increase and decrease voltage throughout an electrical power system, and even at the places where electrical power is consumed). Varying the generator terminal voltage is one way of controlling the flow of VArs into or out of synchronous generators and of helping the electrical power system to remain stable and "healthy" under many conditions. Changing the real power produced by a synchronous generator involves changing the energy flow-rate into the prime mover of the synchronous generator.

And, yes, when the generator breaker of a synchronous generator is open (such as during start-up or shutdown or synchronization) things operate very differently than when the machine is synchronized to an electric power system with other machines. But, we're talking about synchronized operation. It should be clear that synchronous and synchronized operation mean very specific things--and that magnetism has a LOT to do with synchronized operation. Kind of cool, isn't it? One type of electric machine converts torque into amperes; another (very similar) type of electric machine converts amperes into torque. And wires are how that torque is transmitted from a place where is is produced to many places where it can be used for many purposes. ALL the synchronous machines synchronized to an electric power system run at the same frequency--and therefore at speeds proportional to that frequency per the formula F=(P*N)/120. THIS is VERY difficult to understand for most people (including power plant operators, technicians, supervisors, power plant managers, power plant owners--even many textbook authors and teachers and professors. Once that generator breaker is closed and the synchronous generator is synchronized with other machines they are ALL operating at constant speeds which are controlled by the frequency of the electric power system they are synchronized to. They are all operating as one giant generator producing electric power at a relatively constant frequency (50 Hz, or 60 Hz). When the electric power system frequency deviates from nominal ALL THE SPEEDS OF THE SYNCHRONOUS GENERATORS CONNECTED TO THE ELECTRIC POWER SYSTEM GO UP AND/OR DOWN AS THE ELECTRIC POWER SYSTEM FREQUENCY GOES UP AND DOWN. Which also generally causes the electric power output of the machines to go up and down as the system frequency goes up and down. No amount of wishful thinking can cause one power plant's power output and frequency to be stable as long as its connected to an electric power system whose frequency is unstable. Full stop. Period. It just can't happen. It's inherent in the fundamentals of alternating power generation and transmission, and again, in today's world it's just not going to change any time soon.

So, it seems like a lot--and it is in the beginning. It's just not explained very well anywhere. There are all manner of formulae and physics and power triangles and things which can be used to help predict power system operation and troubleshoot abnormal operation. And very little of that is useful when trying to explain these things to people who aren't mathematicians or physicists to begin with. Don't think you're going to read this and immediately understand it. Re-read it many times, and think it over, and observe the operation at your power plant. And, try to understand the basic fundamentals of AC power systems.

Tchau!

 

Hi WTF,
I agreed most of your theories and explanations but I can’t take this statement merely that is “prime mover torque alone causes the stator amperage increase and in turn increases the power output.
I still believe the increase in power output can be achieved by the combination of torque plus excitation.

And this is how I understood how a synchronous motor works since it is purely withdrawing the power from the grid, we can say that the RMF (Ns) in the stator winding is equal to the supply frequency. And the rotor will be locked magnetically with stator (by giving the DC supply to the rotor) as long as the load angle is within limits.

When the load torque goes high to keep the load angle (from lagging to center axis) within limits the DC supply to the rotor is increased consequently the load angle maintained in the synchronous motor.

The reason why I’m writing about the synchronous motor here is I think it’s better to talk/compare synchronous motor over normal (induction) motor regarding current converted to torque in a synchronous motor and torque converted to current in a synchronous generator.

If I’m slightly over exciting the synchronous rotor even after attaining the rotor and stator center axis the load angle definitely not going to be forward since the rotor speed can’t beat the stator speed.

Similarly I tried to implement the same in synchronous generator if prime mover torque increases (as you said once the synchronous generator connected with grid stator and rotor coupled/locked together magnetically) as far as the synchronous generator concerns the load angle will lead from it’s center axis if it keep on increasing pull out torque may occur.
So as to keep the load angle within limits we need to keep the magnetic coupling stronger than before against the torque. The more stronger the rotor magnetic field the flux cutting in the stator winding is more which in turn maximize the stator output.
But the same philosophy not makes sense while I’m implementing when the synchronous generator is at stable load and stable frequency and I’m increasing the rotor field current and voltage, this time also I’m magnetizing the rotor but instead of real power here I’m getting reactive power.

Kindly note I’m not writing these statements to prove that I’m smart. May be someone understands my struggle in my understanding and eventually someone will help me regarding this.

And a very biggggg thanks to you for taking the time to write such a very big thread…
TBH, I received some useful other informations from your answers.

 

THIS is VERY difficult to understand for most people (including power plant operators, technicians, supervisors, power plant managers, power plant owners--even many textbook authors and teachers and professors.

And you were 100% right, no one explained this doubt so far with clear picture…

 

@curiouscat,

Try this exercise (if your management will let you--I'm betting they won't, nor will the Control Room Supervisor or the board operator(s)--they're all too scared of tripping the machine and possibly losing their job(s)).

The next time the load is increased from 50 MW to 100 MW DO NOT adjust the generator excitation while the load is being raised. You can do this when increasing from 50 MW to 75 MW, or even from25 MW to 50 MW. Ideally it would be best to have a trend running which is recording TNH, DVAR, DV, DPF, and MW at a minimum before increasing load. If you want you can use Pre-Selected Load Control to perform the load raise, and then just cancel Pre-selected Load Control by clicking once on either RAISE- or LOWER SPEED/LOAD. I believe you will see that the generator terminal voltage will decrease as load is increased (by increasing the fuel flow-rate), and DVAR will move in the "negative" direction (towards a LEADING power factor). I would suggest that you start the test by setting the generator power factor to a large (magnitude) LAGGING power factor--not excessive, but just say+10 MVAr. During the test as the generator terminal voltage decreases (as the generator stator current increases) the magnitude of the power factor will decrease as it approaches 1.0 (unity power factor), and may, in fact, cross over into the LEADING and the magnitude will start to increase again.

Having a trend recording of the data will provide all the information you need to review your hypothesis and decide if you're correct or not. If you share this information with INTERESTED others at the plant you can help them understand why they do what they do when they do it. But, there are many operators, and even technicians and supervisors, who know what they know and don't want to have to think about how this information might affect what they know. They may have been doing what they do for a long time and have great success (in keeping their jobs) over that time and can be quite risk-averse. (Wait till you try to do the above suggested test--which won't damage anything or trip the machine--but you'll NEVER convince anyone else otherwise. I can pretty much guarantee it. You might have a friendly operator and/or supervisor on a quiet weekend or weeknight after the evening peak has subsided, but you will be a very lucky person if that happens.)

I have seen a synchronous generator's electric power input visibly change (but by only a small amount--which can be disputed as to what is actually causing the change) when it is running at a low load (say between 3- and 10 MW, depending on the size of the prime mover) by changing ONLY the generator excitation. But it usually takes a pretty good change of excitation to see even a small change in the MW output of the synchronous generator. Yes; excitation has an effect--because it is one of the two basic variables in the Power equation (P=V*I). I have demonstrated several times (okay; more than several) that when the generator terminal voltage remains constant during loading and unloading (which some synchronous generator exciters are better at doing than others when in AUTO mode) the only thing that affects electrical power output is a change in fuel flow (or steam flow) to a turbine (combustion or steam, respectively).

Now, because many exciters (even GE exciters) don't do a really good job of maintaining generator terminal voltage as the machine is loaded and/or unloaded--without some assistance from a trained and experienced operator who will be watching the generator terminal voltage (and/or the VAr indication and/or the power factor indication) and make manual adjustments to help keep the generator terminal voltage (or VAr or power factor) at some desired setpoint. (I'm presuming that neither VAr no power factor control is enabled and active.) And, I maintain that the reason the excitation has to be changed is because as the strength of the stator magnetic field increases or decreases it will have the opposite effect on the magnetic field of the generator rotor which affects the generator terminal voltage slightly. (My teaching and reading referred to this phenomenon as "armature reaction" (the armature magnetic field--the generator stator windings--affects the generator rotor magnetic field strength which has an affect on the generator terminal voltage). In my experience, it can be very difficult to see on a GE Mark* turbine control system because DV is usually scaled in kV with only one or two decimal places. (Some GE exciters which communicate on the UDH with the Mark* can have a little better resolution, sometimes three decimal places, which can make it easier to see the effects of changing load while not changing excitation current/voltage.)

I didn't say the change in electric power output of a synchronous generator is completely and totally the result of the change in energy flow-rate into the prime mover, because there is that I component (synchronous generator stator current, or armatuve current) in the P=V*I equation. It's just that because most synchronous generators run at a relatively constant generator terminal voltage, and even if the terminal voltage changes it doesn't usually change by more than plus-or-minus 5% of nameplate rated synchronous generator terminal voltage--so trying to affect a change in MW by simply changing excitation voltage/current isn't going to have much of an effect. To my mind, generators are typically operated in something of a small range of terminal voltage--like I wrote, 5% of 13,000 volts (or 13.8 kV) is 690 volts. 690 volts out of 13,800 volts. And, while that is +690 volts or -690 volts, they are rarely operated at the extreme ends of that range (13,110 volts or 14,490 vplts) except under extreme circumstances. So to me, that means the V term of the power equation (P=V*I) is relatively constant, or, it doesn't usually change by much (no more than plus or minus 5% of rated

Now, where the power plant is located on the electrical power system (proximity to other generators, primarily large generators) the impedance between the generators, and the stability of the electric power system voltage (at any point in the system) all can have an affect on how much excitation can change MW output. [NOTE: I'm not including any tap changers (load or non-load tap changers) in this discussion. I'm also speaking about typical GE-design synchronous generators provided with their prime movers for basic power generation, including most combined-cycle applications. Tap changers can introduce an entirely different mode of operation and application, though they are simple devices the ways they are used and the effects of using them can be very different and are typically only used where there are other issues on the electric power system that need to have some mitigation.]

Anyway, you need to continue to mull this over in your mind. When I graduated university after having some serious electrical training, I knew absolutely ZERO about Droop Speed Control and exactly why a synchronous generator synchronized to an electric power system with other synchronous generators and their prime movers won't change speed as fuel is changed--because, certainly, during start-up and shutdown speed changes when fuel flow changes. Most power plant operators never really give any though to what speed the machine is running at after it is synchronized--they just assume that the speed does change. My university professor taught me the same drivel from textbooks that says as a synchronous machine is loaded its speed will decrease (which ONLY happens under a certain condition, and that condition is NEVER stated or described in the same textbook, AND synchronous machines rarely--if ever!--run under that certain condition). I have purposely read almost everything I can get my hands on about synchronous machine operation, including Droop Speed Control, during my 40 years in the power generation business and it took me many long years (as in a couple of decades) to finally develop a proper understanding of it and come to know that the drivel that written in textbooks and reference books is hopelessly inadequate, and quite wrong, so it's no wonder many people simply can't understand it. And it's all very closely related to this topic you started. (There's been enough written about Droop Speed Control in Control.com that sometimes I think it should be called DroopSpeedControl.com.)

You seem to have some pretty good critical thinking skills. Put them to use in your discovery. Because, as another contributor to Control.com said several times, "Learning is finding out what you already knew." Think about that. When some concept or principle that you are trying to understand suddenly becomes clear to you, one of the things we usually say to ourselves shortly after it becomes clear is, "YEAH! I knew that!" It just took some new wrinkles in your brain to process the available information so that it reinforces what you already knew! And, that's just such a good feeling to see people experience--and to experience for one's self.