Generator and Reactive Power Output

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This does not help with explaining the function of a generators AVR.

Yes I agree this is one function of the generator AVR. But why do they say reactive power is produced from the generators AVR and real power from the generators prime mover?

Is the AVR adjusted when reactive currents are drawn to enable the generators output voltage to remain constant?

AVR senses the voltage in the main gen winding and controls the excitation to maintain the generator output voltage within the spesified limits, compesating for load, speed, temperature and pf of the generator. 3 phase RMS sensing is employed for superior voltage regulation.

Simply put: The Prime mover does not deal with reactive power. It only knows active power. Therefore, the alternator receives active power that it converts to electric power: Active and reactive.

Here, also, the Alternator delivers power but does not really control active-reactive. The load dictates what it should get from the alternator.Therefore, the total power which is rated in KVA is the total value the prime mover should make available to the alternator.

The Voltage drop due to reactive power Only happens at the point where there is a need for the reactive power. Therefore, the genset AVR only sees the voltage at the Output of the alternator and maintains it to the set value. If you consume more reactive power, the voltage at the output might not change if less active ... and the total KVA was the same.

The AVR role is not just simple to understand by itself. It does not react to reactive power as such but to the whole picture ....

That is correct and thanks for reminding me of that usage of Synchronous generators.

As far as the Op is concerned, he should know that the AVR is not adjusted to produce at the same time active and reactive power: these are dictated by the nature of the load.

Using a Synchronous Alternator to produce VArs is normally done at the end of a transmission line, where the VArs are needed so as to reduce the VArs transmitted down the line, which will cause voltage drop and heating ... This usage is instead of the capacitor banks.

I hope that I did not make any mistakes here, but this is in general what i understood from my old books.

Thanks

Please accept some clarifications to your generalizations.

Simply put: The Prime mover does not deal with reactive power. It only knows active power (Correct). Therefore, the alternator receives active power that it converts to electric power: Active and reactive(Reactive power comes from the magnetic field created by the Rotor Field winding and/or the external system if paralleled with other generation, never from the prime mover) .

Here, also, the Alternator delivers power but does not really control active-reactive (Actually it does, changing the field current will have an effect on the terminal voltage and VAR flow in or out of the generator, it's effects can be quite profound depending on the relative size of the generator in proportion to the overall amount of paralleled generation). The load dictates what it should get from the alternator (Only in the case of an isolated generator/load combo, when paralleled the governor controls the real power output of the generator).Therefore, the total power which is rated in KVA is the total value the prime mover should make available to the alternator (No, the prime mover only provides KW, never KVAR).

The Voltage drop due to reactive power Only happens at the point where there is a need for the reactive power (Not quite, it is distributed across the entire connected system and can in fact be a voltage rise in a lightly loaded and/or capacitive system). Therefore, the genset AVR only sees the voltage at the Output of the alternator and maintains it to the set value. If you consume more reactive power, the voltage at the output might not change if less active ... and the total KVA was the same (Not quite sure what you're trying to say here).

The AVR role is not just simple to understand by itself (Very true! It is possibly the least understood part of the generation system). It does not react to reactive power as such but to the whole picture (The AVR only attempts to hold the Sensing Point voltage constant, regardless of what external system influences; i.e., rising or falling voltages, rapidly changing loads, switching transients, faults, etc., are taking place. Modern excitation systems also have additional setpoint modifiers such as Minimum and Maximum Excitation Limiters as well as Transient Stability Controllers, Reactive Compensators, etc. to react to these events faster and more accurately than any human operator can.) ....

To 'SIMPLY' answer back on some points you commented on: I removed the previous texts to reduce the length here.

'Please accept some clarifications to your generalizations.' {Thank you.}

{This is not an academic issue here(unless the OP is considered to be seeking an answer to a homework or such...) Therefore, we are looking at an industrial power generator feeding an internal grid, and not correcting any power factor issue. In this context, Reactive power is consumed by windings in the load circuits, and therefore, provided by the genset as and when required. The AVR is not instructed to produce any of it as such, but is working in the quadrant sector that is Inductive (lagging not leading). IF the Load has a P.F. of, let us say 0.7, there is nothing that particular Alternator can do about it via its AVR control (normally)!. This is assuming that the genset is not set in the leading P.F. sector whereby it is used to correct the P.F. by producing Reactive power...}

{'Actually it does...': Of course, the field current has an effect on the whole operation including the VARs and kWs produced! But is there any setting/control on the AVR that affects the VARs directly just for the sake of it? I am not lecturing here the OP on the whole topic but trying to guide him ...}

{ The Prime Mover does provide the means to generate the VARs : If your Genset is rated 500 kVA and 400 kW at P.F. of 0.8, does this means that you can load it with 500 kW at 0.8 or is it that you can load it max 400kW at 0.8? In this case, you are generating 400 kW and 300kVAR into the load! The apparent power is 500kVA and this is what the prime mover has to provide: if 500kW is taken out, at 0.8, you will be overloading the prime mover as well as the alternator current capacity.}

'(Not quite sure what you're trying to say here)'.{ What you just said in the following paragraph under "The AVR only attempts..."}

{ This, your last entry, summarizes it all. In fact, unless you are a power provider of some size, with the latest technologies, you will have to stick to Power Factor Correction outside the Genset AVR in most cases. You can use a particular genset to produce Capacitive power to correct the power factor at a particular point on the grid.But then this unit is dedicated for the job. Otherwise, the normal setup is production of Active and Inductive reactive power to the requirement of the grid. When you compensate the P.F. downstream, you actually relieve the set from producing the inductive power.}

All the above in good spirit.

Thanks for those responses guys, they are very informative. I am however, struggling to come to grips with how increasing the generators excitation only gives you more reactive power?

If you have a 300kVA inductive load which runs at .8pF, the load consumes 240kW and 180kVAR. If your generator prime mover is rated at 300kW, it will supply the load with the 240kW that it requires. This power comes out at unity pF from the prime mover. The current is then displaced from the voltage by the inductive load and you get real and reactive power present in the system. Where does the exciter come into all of this? If you system requires more real power, which means a higher current draw, doesn't the exciter simply increase to raise the generator voltage to keep it at the nominated level with the higher current draw, hence giving you more real power?

Increasing the excitation current does not just give more reactive power!

There is a confusion here: Single genset OR Genset parallelled to an existing grid.

If you are looking at a single genset supplying its own grid, the excitation does not just increase the reactive power: It increases the voltage output from the alternator. the AVR will tend to control the Output voltage only. The Loaded grid will determine the p.f. and any intended correction will have to be done from ouside the alternator. Now If you Over-compensate the inductive load, then you might bring the p.f. beyound the 1.0, to -0.95 for example. This is not advisable ...

If the Alternator is being parallelled to an existing grid, then increasing the excitation cannot increase the grid voltage (unless the size of this alternator is significantly big compared to the grid power) but it will increase the power sharing of the unit.The power factor seen at this genset will increase towards unity pushing the other gensets to take up the inductive loads, therefore their p.f. will redister lower than unity... Until you over excite this genset and its p.f. becomes leading: This is the point when it will start producing capacitive kVAR.

Anyway, this is quite complicated and requires more detailed knowledge.

All these responses are much appreciated, but they are all very high level. I guess I wasn't quite clear in my OP, but here is what I am really after an explanation of:

Can someone please explain to me:

Why the generator excitation circuit (ie. increasing the strength of the rotors magnetic field), is said to "provide" reactive power to the grid (considering the generator is not paralleled)?

If it is not parallelled, it will not provide reactive power. You will simply have a higher voltage!

ok now im really confused.. you're telling me that if you slap a bunch of inductive loads onto the generator, the exciter plays no role in the generator except to increase the generators output terminal voltage?

OK, Now that you are getting more excited, ans since (remember) you said that you did not want a lot of theory but prefer a practical answer to clarify quickly the concept:

- An Alternator is a Synchronous Machine which can be used as a motor also.

- When used as a generator, it is driven by a prime mover at synchronous speed to produce either 50 or 60 hz current. Its voltage is regulated with an AVR controlling the excitation field current to maintain a constant voltage. It is not used as a reactive power generator and therefore, you cannot overexcite it without being punished severely with a higher than wanted voltage. The load circuit dictates the p.f. and causes some of the power to be reactive (Inductive, lagging).

- When used as a Motor, you supply the power from the grid and excite the field to provide the necessary torque and power to drive the load (mechanical). In thus doing, you can overexcite the field, but then you tend to use the Motor without load, and make it run in the leading p.f. zone. This regime will generate capacitive reactive power that can be used to correct the power factor of the grid at the point of connection.

Warning: Electric moving machines, and particularly Synchronous machines are not simple devices that can be readily understood by everyone. You need to educate yourself in this domain. Otherwise, just accept the information given until you can discuss it profoundly in the theoretical sense.

I Hope I was clear enough without too much theory (Simple enough to grasp the gist of it for the purpose you are contemplating?)

Cheers.

Thank you for that simple explanation, but let me just clarify one thing:

I have a generator here which has had a study performed on its turbine to determine the actual power output of the prime mover, which was determined to be 1565kW. The generator itself is rated at 1800kVA.

I have a network which I have totalled up the power requirement to be 1552kW and 653kVar at .92pf.

If we connect run this generator and supply this network, will the generator be able to supply this network? Based on what I've been reading over the past couple of weeks, every text book I can find states that the prime mover supplies the "real" power and the exciter supplies the "reactive power". My understanding of this lies in the fact that, power comes out of the prime mover with current and voltage in phase. The load "rips" the current out of phase with the voltage and hence the you now have a reactive power component in the system.

This is where I am now lost as to whether your real power output of the generator decreases, as there is now reactive power present.

For the scenario above, if the system was at unity pf with 1552kW, then the generator can supply 1565kW and we do not have a problem (although the spare capacity is a little concerning). Now that we have the current out of phase with the voltage, and hence 653kVAR present required by the system, does the exciter just increase the terminal voltage to sustain the generator terminal voltage at a constant level when dealing with the extra current draw now required to sustain 1552kW with 653kVAR ?

If this is the case, then if the system is running at unity pf and we increase the generators excitation, whats to say that you do not get more kW output of the generator? (ie. if the prime mover is putting out 1500kW and we increase excitation, the terminal voltage is increased, product of current * higher terminal voltage = more power?)

Your prime mover, the Turbine, you say has been checked and can give a maximum of 1565 kW. If this is correct, then it can only give 1565 kVA: that means it can make the turbine produce 1565 kW at p.f. = 1.0

Now. you say that the network connected to the generator requires 1552 kW at p.f.=0.92 (which reflects the 653 kVAR ). This status requires 1686.95 kVA( 1552/0.92)

Therefore, Your Turbine will be Overloaded even though the Alternator is capable to take the load since it is rated at 1800 kVA

You need a Turbine rated > 1800 kW for that Alternator, or at least > 1687 kVA for your network. Your 1565 kW turbine will only give 1440 kW at 0.92 p.f.

The Exciter does not generate any power to supplement the network. If anything, the exciter consumes power to maintain the alternator output voltage, and therefore uses some of the output from the turbine to be able to do the job. The whole system works together. There is no such thing as exciter producing the kVARs required. You have a misconception of the whole operation that is going on in the Generator+ Prime mover.

As I explained before, someone is confusing between Synchronous Motor operation (regime) and Synchronous Alternator (Allmost all alternators used in normal power plants are Synchronous machines).

I hope this has helped you make up your mind about the system you have.

Let us try to understand what is reactive power? This is all about how the load current affects the field created by the excitation winding of the rotor. At no load, there is no current in the stator and the magnetic flux produced produces the voltage in the stator windings. The moment generator is loaded, there is current in the stator winding which produces its own magnetic flux. How this magnetic flux produced by stator affects the original flux produced by the rotor?. This phenomenon is called armature reaction. The effect caused by armature reaction depends on the power factor of the load. If the power factor is zero lagging then the flux produced by armature current opposes the rotor flux (it is then called demagnetizing effect), when the power factor is zero leading, then armature flux is in line with the rotor flux and adds to it (it is then called magnetizing effect) and lastly if the power factor is unity, then the armature flux is at 90 degree to the rotor flux (it is called cross magnetizing effect). One can draw a vector diagram taking main flux as horizontal line and take armature flux at various angles (depending on the power factor) to get resultant effective flux that would be responsible for voltage at the generator terminals. The AVR would sense the voltage at the terminals of the generator, and would take corrective action by way of increasing the excitation (if the power factor is lagging) or decreasing the excitation (if the power factor is leading) so as to produce again the same resultant magnetic flux that was at no load.

Now coming to deliberately changing the field current, then the armature current would draw current at such a power factor that would maintain the same resultant flux what is needed to produce rated (or the value set on the AVR) voltage. In other words if excitation is reduced, then armature current would be leading (absorbing reactive power, magnetizing armature reaction) and if the excitation is increased then armature current would be lagging (supplying reactive power to the system, demagnetizing armature reaction).

I hope function of AVR would be clear with above explanation

Hi all,

I have been following this thread and I am stuck with the same question polerz is stuck with. For last few months I have been reading all the available articles on this topic and nothing seems to answer what I have been trying to understand.

As far as I can apply my knowledge of electrical engineering, the turbine has the only function of changing the magnetic flux by rotating the rotor at constant speed. The voltage induced in the stator terminal is completely determined by the strength of the magnetic field (provided by the exciter current) and the speed at which the flux changes (determined by the speed of the turbine) and the current drawn from the stator terminal depends on the load. The total power{apparent power (reactive power+ active power)} is the result of the strength of the magnetic field by the rotor and the rate of change in magnetic flux density. The generator produces the apparent power, not the active and reactive power. It is up to the load what fraction of that total power be used for active power and what fraction be used for reactive power. So the AVR has no knowledge of active/reactive power but reacts to the total power. Be the power be reactive or active, genset sees total power being consumed and AVR reacts to it by increasing or decreasing the excitation current to maintain stator terminal voltage.

Now if I am right until here, when the current flows through the transmission line, how does the active current flow in transmission line with phase and reactive current flow in the same transmission line with leading or lagging the phase voltage? How does the genset realize that the current being drawn is for active or reactive?

I actively believe that generator produces the total power and the total power is the resultant of the strength of magnetic field and the change in magnetic flux. I am completely against the concept that turbine produces active power and excitation current produces VAR.