My query is that when the generator is synchronized with the grid, just after closing the circuit breakers, it enters into a stage called as floating generator.

In this stage, the voltage difference across the circuit breaker is ZERO. Also the frequency and the phase difference is ZERO.

When there is no potential difference across the breaker, the current can not start flowing in any direction.

For the current to flow from generator towards grid, the generator voltage is needed to be higher than the grid voltage. How then the current starts flowing which in turn introduces the armature reaction.

Please throw light onto this.

 

A generator is a device for converting torque into amps. Those amps can be transmitted over wires to many different locations, where they can be applied to motors which are devices for converting amps into torque. (Amps can also produce heat, which can be in the form of light, and they can power computers, what I like to call "virtual torque").

The voltage potential, or lack thereof, has nothing at all to do with real current flowing into or out of an electric machine. It has everything to do with the amount of torque that's being applied or produced.

The amount of torque applied to an electric machine operating as a generator is directly proportional to the amount of amps flowing in the stator (of a synchronous generator, or, alternator).

The amount of amps flowing in the stator of an electric motor (an AC induction motor, for example) is directly proportional to the amount of amps flowing in the stator.

The "floating stage" you are referring to is most likely the point at which the amount of torque being applied to the generator by the prime mover driving the generator is exactly equal to the amount of torque required to keep the generator rotor spinning at the speed that's proportional to the grid frequency (because speed and frequency are directly proportional).

The speed and frequency of a synchronous AC machine are related by the formula:

F = (P * N) / 120

F = Frequency, in Hertz
P = Number of poles of the generator rotor
N = Speed of the generator rotor (in RPM)

Synchronous speed is defined as the RPM that corresponds to the grid frequency; for a 50 Hz grid, a two-pole synchronous generator must spin at 3000 RPM. Or, the synchronous speed of a 50 Hz, two-pole synchronous generator is 3000 RPM.

If the amount of torque being applied by the prime mover to the generator rotor increases, the tendency is for the speed to increase, but if the generator is being operated in parallel with other generators on a grid with a stable frequency, then the speed of the generator cannot increase and the increased torque will be converted by the generator into amps. More torque, more amps. Less torque, less amps.

If the amount of torque being applied to the generator by the prime mover is less than that required to keep the generator rotor spinning at synchronous speed, and if the generator is being operated on a grid with a stable frequency in parallel with other generators, the tendency would be for the speed to decrease. But the speed cannot decrease and amps begin to flow into the generator and it actually becomes a motor to keep the prime mover spinning at synchronous speed. (This is called "reverse power", or "motorizing the generator", and it's usually not good for most prime movers and can sometimes result in catastrophic failure of some prime movers.)

It's all about torque. If there isn't any more torque being applied than the exact amount required to keep the generator rotor spinning at synchronous speed, then there won't be any amps flowing in the stator. More torque means amps will be flowing "out" of the stator. Less torque, means amps may be flowing "in" to the stator.

Voltage is important, but not for the amount of amps flowing through the stator or for the direction of amps flowing in the stator.

The voltage level controls the amount and direction of VArs, or reactive power, not amps which is real power.

Go ahead. The next time the generator is "floating", crank up the voltage and see what happens. Not much. Not much will happen until you crank up the torque from the prime mover, which means increasing the fuel flow, or the steam flow, or the water flow, or the wind flow (depending on the type of prime mover).

And, remember: Electricity is just about transmitting torque from one location to other locations, over wires. So, the torque is converted to amps in the generator, and converted back into torque in the motors that are connected to the wires coming out of the generator(s).

Hope this helps!

 

Actually:

The amount of amps flowing in the stator of an electric motor (an AC induction motor, for example) is directly proportional to the amount of torque being produced by the motor.

And, when a generator breaker is closed, connecting the generator to the grid in parallel with other generators, there is virtually no difference in potential across the breaker contacts at any time the breaker is closed. So, it's definitely not potential across the generator breaker contacts that causes current to "flow out" of a generator.

If we were to follow your logic, there would never be any current flow across any closed switch, because the potential across a closed switch (and generator breaker contacts are just a big switch) is zero.

Lastly, I presumed you were talking about AC generators, more correctly called alternators.

Think of a generator and a motor like this. Suppose you have a source of mechanical energy in one location and you need that mechanical energy in another location. But, you can't pick up the mechanical energy and move it to where it's needed, and neither can you pick up the need and move it to the source.

So, you connect the source of mechanical energy to a generator (or alternator) and install a motor at the place where the mechanical energy is needed. You then use some wire to connect the generator to the motor.

The generator converts the mechanical energy to electrical energy, that electrical energy is transmitted over wires to the motor, where it's converted back into mechanical energy. All generators are just like this. Plain and simple.

In fact, it's kind of a fallacy that current flows out of a generator. It really just kinds of runs in a continual loop through the loads and back to the generator. Kind of like a hydraulic system; there's a hydraulic pump, circulating oil at higher pressure, and that oil does work at a different location than the pump and recirculates through the pump.

That's all electrical energy is really about. Moving mechanical energy easily over great distances using wires. The mechanical energy is converted to electrical energy and then converted back into mechanical energy. Nothing more and nothing less.

The current won't flow unless there is a load to flow through. And if the mechanical energy into the generator exceeds the mechanical energy being used at the other end, then the speed of the generator increases. And, on an AC grid, that's not desirable. Conversely, if the mechanical energy being put into the generator is less than the load at the other end then the speed of the generator will decrease, and on an AC grid that's not desirable either.

Speed is directly proportional to frequency. And current is directly proportional to torque.

Hope this helps, and sorry for the error in the first response.

 

You are right, I am talking about synchronous generator only.
Also, with your first reply, I was in disagreement, but did not want to be impolite to you because I notice that you are taking a lots of pain to answer the questions. Your style of explanation is wonderful!!
I thank you very much for your sincere efforts to reply to this question as well as to so many other questions..

 

It would only be considered impolite to be in disagreement with a response if you expressed doubt. If you need further clarification or take exception to some point(s), that is perfectly acceptable.

Please, help us to help you. If you require further clarification of some point(s) or need further assistance, let us know.

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Thank you for the feedback, and, again, if you need more information or have some issue with the responses provided just let us know.

 

Mr. CSA.....

On this week end I thought a lot about what should happen when we increase power input to the prime mover.

Assuming that just after synchronization of the generator with grid, we do not disturb excitation.But we do increase power input to the prime mover. This increased power input would try to increase the speed of the rotor and momentarily this should happen also that means for a short period the speed of generator rotor should certainly increase. This increase in speed would mean an increase in the magnetic flux linkage with stator windings (because the same pole now will pass by a stator slot at faster speed). This should, for a short time, lead to increase in the generation frequency and an increase in the generation voltage also(this may be verified with the basic flux equation and its derivative). Here, I now invoke the Lenz's law; the increased voltage should appear in such a way that it produces a current in such a direction that it is able to counter the speed rise in the rotor. So the additional amount of emf produced in the stator winding would lead to flow of current in the stator winding (the incremental current) and this current would impose a counter torque on the rotor. This will be materialised by generation of currents in the damper windings on the poles. This counter torque on poles will restore the equilibrium position where-in rotor will again be rotating at the same speed at which the flux from the stator is rotating( the synchronous speed). When this equilibrium will be established, the stator winding will now be carrying increased current and the other synchronous parameters. This is how the increased torque or power from the prime mover gets converted into amps on the stator.

I request you to please go through the explanation that I have given above and point out if I made any mistakes. One thing that I know that I am missing is the phase shift(torque angle)..I could not think about how this phase shift should creep in.

I really need a validation of this from some expert. Your comments are eagerly awaited.

 

You should send your name and affiliation along with your theories to Phil Corso, at cepsicon [at] aol [dot] com. He can help you much more than I will ever be able to.

As an operator or technician, I'm only interested in the net effect of changing torque input to the generator. I don't care if the acceleration increases so fast and for such a short period of time as to be imperceptible or that the load angle increases or that there is counter-emf. It doesn't change the fact that for all intents and purposes when the torque input to the generator increase the amps in the stator windings increase. And the AVR should be able to adjust excitation to keep the generator terminal voltage relatively constant without too much operator intervention.

And torque is power, and volts is VARs.

As an operator or technician, which is all I am (and for not much longer, either) all of that physics stuff was hopefully done for me by the generator designers. As long I don't exceed the reactive capability limits of the generator, what goes on inside the generator is of no interest to me. And hasn't been since I completed my theoretical studies in university nearly three decades ago. I have not put any of that stuff to use in the real world, because I'm not designing generators. I'm just operating and troubleshooting the prime movers that provide the torque to them.

They do their thing, and they do it without any mathematics from me, thank you very much. If I try to explain anybody's law of physics when describing how a turbine-generator operates they just look at me like I'm crazy. They don't care about that; because none of that is in their control or their job description. It's like asking someone if they know how the transmission in the car they drive is designed to see if they're qualified to drive the car. Or how the derailleur was designed on the bicycle they pedal before they can ride it to work.

I've learned over a couple of decades, that if I'm asked to explain VArs that I don't start with vector diagrams and Pythagorean theorems. Hell, I don't even finish with them. I just try to explain them in terms they might understand, and vector diagrams and mathematics don't help most operators and technicians (even though they're good things to know as a technician, or even as an operator; they're not critical to being a good operator or technician).

Best of luck in your theoretical studies. Sorry I couldn't be of more help.

 

Please somebody do reply to my querry.

 

When a alternator is Synchronized with a grid (incoming feeder), just after closing the circuit breakers, it enters into a stage called as floating generator.

It is a fact that the voltage-frequency difference is zero across the circuit breaker. when a generator is synchronized with grid or the utility power source, the entire system acts like single source of power, and power does not have to flow from generator to the grid or vice versa, which is a forbidden action during parallel operation of two independent sources. The synchronized power system will force the current to flow through the other loads connected to the out going feeders, which will in turn introduces the armature reaction.

 

But there is potential difference between your generator and system loads. When you vary the terminal voltage of your generator, theoretically you alter the flow reactive power. Your generator may draw of supply lagging current depends upon you under excite or over excite your generator terminal, respectively. In reality the terminal load will be altered a bit due to mainly I2R loss that may be altered by a small fraction.

If you increase the output of your prime mover by says 10MW, you are changing the torque angle of your generator which is operating in parallel with the other generators. It produces more torque to accelerate the rest of the system inertial energy of rotating mass. If you are assuming your generator produces net positive load that is not consumed by any load, and we ignore the governor speed droop and AGC -constant frequency responses, your generator will accelerate the rest of system inertial energy of rotating mass until the system frequency reaches to a point that additional damping is equal to 10MW your generator had produced.....

I have more to add. It may take time for me to compose it. When I'm done I will continue from here on.

 

if there is a phase difference between different generators and the breaker is closed, there is for a brief moment a short circuit to the value of the voltage on each side of the breaker. ever sat at night and the lights flicker for just a moment, one explanation for this is another unit coming on line, just a fraction of a degree out of sync with the rest of the grid. the unit shudders a bit but must lock in with the frequency of the grid.

 

HB... damage will not result if the phase-displacement is "just a fraction of a degree!"

In fact, Syncronizer HW or SW, tolerate several degrees. Do you need additional information or details?

Regards, Phil Corso

 

If you are in Canada or the US or anyplace where the grid is "large", flickering lights are almost always the result of local line protection operation (or voltage regulation if you're served by a long distribution feeder). Synchronizing a generator does not produce any effect you would notice in your house lights. Same is true even when a large amount of generation (500 - 1000 MW or more) trips off the system.

Utilities avoid unsynchronized breaker closes whenever possible, especially generator breakers.

 

Dear Mr. Phil Corso,

I have some questions regarding the current the topic, but i don't have any account on this site. Please send me your personal email address on the following address, so i can be able to contact you.

Thanking you and with best regards,

Atta

[email protected]

 

Atta...

It is [email protected]

Phil Corso