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Generator Load Sharing
Generator load sharing

Hi all

I've been researching for days about synchronous generators and cannot get my head around how real power output of generators can be adjusted when paralleled to the grid or to each other.

I'm across the reactive power side of things, increasing terminal voltage adjusts amount of reactive power absorbed/produced by the generator (i.e. terminal voltage via excitation increased implies producing VARs and decreased implies absorbing VARs) and I understand how generators operate when connected on their own (i.e. load placed onto generator creates opposing force to slow down prime mover, reducing generator output frequency, more fuel is added to prime mover then generator output frequency restores to nominal).

What I don't understand is how do you control the amount of real power the generator produces, when connected to the grid or in parallel with another generator.

My understanding so far is follows (correct me if I'm wrong):

Let's start with the generator output CB open. Generator is started, rotor is excited by DC voltage. Fuel is added to the prime mover and the prime mover begins to spin. As the prime mover spins, a magnetic field is induced in the stator windings and a generator terminal voltage is produced (the generator is currently unloaded). Synch checks are done to ensure stator output frequency, phase and voltage are the same as the grid.

Generator CB is then closed onto the grid (which is just a whole bunch of other generators). The grid induces a voltage back onto the rotor and hence rotor will slow down. Fuel is added to the prime mover to bring the rotor back up to nominal speed, the same way it would if a resistive load was placed onto it.

Now the prime mover is receiving enough fuel to spin whilst connected to the grid, but as I understand it no power is delivered to the grid yet.

More fuel is added to the prime mover and this is where I am lost. If we add more fuel to the prime mover, does it not begin to accelerate creating a gap between the stator magnetic field (from the grid) and the rotor magnetic field? If this is the case, then the rotor will be phase shifted from the stator magnetic field? Looking at the two wave forms, you would have the grid waveform at nominal frequency and the rotor induced waveform phase shifted by this 'gap'.

If there is a phase shift, let's assume it is 90deg, as I've read that 90deg equates to full load output of the generator. Then won't the connected load see the grid voltage plus the induced voltage from the rotor? Adding two phase shifted waveforms would result in higher RMS voltage across the load? This is bad?

Sorry I'm probably way off here, but thought I could get some help understanding what is going on as it's driving me crazy.

2 out of 2 members thought this post was helpful...

The thing about AC power generation is that when two or more (or two hundred or two thousand or more) synchronous generators are synchronized together on a grid no generator can spin faster or slower than any other generator based on the construction of the generator. There's this little formula that relates speed and frequency as follows:

     F = (P * N)/120
where F=Frequency (in Hz)
P=Number of poles of generator
N=Speed of generator rotor (in RPM)
You can solve the formula for speed or frequency by rearranging the terms based on algebraic principals. But, basically, the formula related speed and frequency.

When multiple generators are synchronized together, it's the two magnetic fields inside each generator that keep the rotors locked into synchronous speed per the formula above. ALL generators operate at their synchronous speed. For a 50 Hz grid, a two-pole synchronous generator will operate at 3000 RPM; a four-pole generator will operate at 1500 RPM, and so on based on the number of poles.

One, or six, or sixteen or sixty synchronous generators synchronized to a 50 Hz grid with other generators can run at 51.2 Hz, or 49.6 Hz. They all have to run at 50 Hz.

When additional energy flows into the prime mover driving the generator it would seem logical that the prime mover and generator would increase speed--but it can't. The two magnetic fields inside every synchronous generator prevent the generator rotor from spinning any faster or slower than its synchronous speed (based on the number of poles of the generator rotor). And, the generator converts that additional torque into amperes.

And the formula for 3-phase electrical power is:

     P = Vt * Ia * 3^0.5 * PF
where P=Watts
Vt=Generator Terminal Voltage
Ia=Generator Stator Amperes
3^0.5=square root of 3 (1.732)
PF=Power Factor
Generators run at a fairly constant terminal voltage, so that term can be considered to be fixed. And, the square root of 3 never changes, so that term is fixed. And, if for the purposes of this discussion, we consider the PF of a generator output to be 1.0 (resistive), it is also fixed. That means to produce more power the generator stator amperes have to increase. And, that's what a generator does--it converts torque from the prime mover to amperes. In exactly the same way that an electric motor converts amperes into torque. And generator drive motors.

What happens in an electrical generation and distribution system is that a generator converts torque into amperes, which are then transmitted and distributed to various locations via wires, and then motors convert the amperes back into torque. It's as simple as that.

The additional torque being provided to the generator rotor by the generator's prime mover that would tend to increase the generator's speed is converted into amperes because the generator speed can't increase when it is synchronized to a grid with other generators.

Conversely, when the generator prime mover produces less torque and the generator rotor would tend to slow down--but it can't when it's synchronized to a grid with other generators--causes the generator to produce fewer amperes, which means the electrical power output of the generator decreases.

These are AC power generation fundamentals. When synchronous generators are synchronized to a grid with other synchronous generators, all the generator spin at speeds which are proportional to their construction (number of generator rotor poles). And, when the torque being provided to a generator rotor increases--which would tend to increase the generator rotor speed--the generator, which can't speed up (or slow down) converts the torque to amperes. More torque means more amperes; less torque means less amperes.

If the amount of torque being provided to the generator rotor by the prime mover is not sufficient to keep the generator rotor spinning at its synchronous speed then amperes flow into the generator from the grid to keep it spinning at its synchronous speed. That's called reverse power, and most generator rotor prime movers don't like to be spun by the generator, so protective relays open the generator breaker to protect the prime mover.

Other than the above, most of your understanding is basically okay (except for the part about the generator slowing after initial synchronization and the prime mover having to be sped up). Once the breaker closes the speed of the generator rotor--and the prime mover driving the generator rotor--is fixed by the frequency of the grid with which the generator is synchronized. Period. Full stop. It's can't go faster or slower than its synchronous speed. Period. Full stop.

Generators convert torque to amperes.

Motors convert amperes to torque.

Wires are used to transmit torque from generators to motors.

And the speed of synchronous generator rotors is directly proportional to the frequency of the grid they are synchronized to.

Think about it. On a properly regulated grid with stable frequency, every device connected to the grid sees the same frequency--both loads (motors, etc.) and generators. It has to be. If generators could spin at any speed, why would it be necessary to synchronize them with such sophisticated equipment? Why not just connect the generator to the grid at any speed?

Hope this helps!

3 out of 4 members thought this post was helpful...

So, as has been proven and said many times in the past, I'm NOT the best proof-reader of my own writing.

>>CORRECTON<<
One, or six, or sixteen or sixty synchronous generators synchronized to a 50 Hz grid with other generators can run at 51.2 Hz, or 49.6 Hz.

The above sentence SHOULD HAVE BEEN WRITTEN TO SAY:

One, or six, or sixteen or sixty synchronous generators synchronized to a 50 Hz grid with other generators CANNOT run at 51.2 Hz, or 49.6 Hz.

My sincere apologies for any confusion.

All generators run at their synchronous speeds (based on the number of poles of the generator rotor) when synchronized to a grid with other generators. The two magnetic fields inside each generator FORCE them to act as ONE SINGLE LARGE generator, supplying one single large load (the total of all the motors and televisions and tea kettles and lights and computers and computer monitors). There can only be ONE frequency for all the generators, and for all the load(s).

It is patently false for textbooks and references to say that synchronous generators slow down as load increases. It just doesn't happen in the real world. And by load increasing, I'm referring to the amount of power being produced by a generator and its prime mover.

Watch the speed (and frequency) of the synchronous generator(s) at your site or ship as it(they) are loaded or unloaded after they are synchronized to a grid with other generators. Unless the grid is small, you will not see any appreciable change in speed (or frequency) unless you have a highly accurate tachometer and/or frequency meter. And, on a well-regulated grid the frequency should stay relatively constant--because AC power is transmitted best when the frequency is at or near rated. And devices work best when the frequency of the grid they are connected to is at or near rated.

By G.A.abobaker on 20 November, 2018 - 5:03 pm

Hi Mr. CSA,

The statement was a little bit confusing but it was Ok. in fact i have another question. which is, if i need to start a isolated (independent) power plant (say 50 MVA seam turbine) using (temporarily) the national grid (infinite bus), and then to put all the loads of that isolated system on the steam generator. how can i share the load between these two power sources? and how can i increase the loads of the steam turbine to eventually take the all loads and to get rid of the other source (national grid) and finally isolate it?

what would make the loads being withdrew from steam generator while the grid still exist?

many thanks for your detailed explanation.

2 out of 3 members thought this post was helpful...

G.A.abobaker,

I can say the same thing; your question is a little bit confusing but I will try to answer as best I can.

Here's what I think you're trying to ask. You have a transmission and distribution system that is capable of powering a load independently of an infinite grid at some point in time. The load is being powered by the national grid when the steam turbine-generator power plant is being started, and the auxiliary loads of the power plant are also being powered by the national grid when the power plant is being started.

When the steam turbine-generator reaches rated speed, it would then need to be synchronized to the national grid. AND, at some point the auxiliary power supply to the steam turbine-generator power plant has to be switched from the national grid to the steam turbine-generator. There would likely be a momentary interruption of power as the switching of the auxiliary power is made from the national grid to the steam turbine-generator output depending on how much time the transfer requires (though it could be done in less than one second with the proper switchgear), and this is going to require some dedicated switchgear and protection to accomplish. (There will have to be a "tap" off the steam turbine-generator output--likely before the generator breaker--that would be used to provide the auxiliary power for the power plant AFTER the auxiliary power supply from the national grid to the power plant was opened.)

There will need to be some kind of method of determining, approximately, the amount of load which is to be supplied by the steam turbine-generator power plant when it is separated from the national grid. This amount will have to include the auxiliary load of the steam turbine-generator power plant. The steam turbine-generator will have to be loaded up to equal this amount of power while still synchronized to the national grid in anticipation of separating from the national grid.

Once the steam turbine-generator is loaded up to the amount of the load to be powered by the steam turbine-generator independent of the national grid PLUS the amount of the auxiliary load of the steam turbine-generator power plant, the power plant AND the load to be powered by the plant independent of the national grid must be separated from the national grid (again through dedicated switchgear).

Now, the steam turbine-generator will be powering the load to be powered independently of the national grid PLUS it's own auxiliary power load, and at approximately the right level (MW) and at approximately the correct frequency. At this point, the steam turbine governor should be switched from Droop Speed Control to Isochronous Speed Control--so that any changes in load (either the load being powered independently of the national grid OR the auxiliary load of the power plant) will be handled automatically by the steam turbine generator WHILE maintaining the desired frequency.

That's about it--if I understand the question correctly.

Now, if the steam turbine-generator power plant needs to be shut down for any reason WITHOUT interrupting the power to the load being powered by the plant, it will first be necessary to re-synchronize the steam turbine-generator and it's load(s) to the national grid. (Usually, the steam turbine governor would be switched back to Droop Speed Control immediately prior to re-synchronizing the unit with the national grid--or it would have to be very quickly switched back to Droop Speed Control immediately after re-synchronizing to the national grid to avoid instability of the steam turbine-generator output (load)).

Once re-synchronized to the national grid the load on the steam turbine-generator can be reduced to approximately zero, and then the generator breaker can be opened. The national grid will be providing the power to the load which was being powered by the steam turbine-generator independently of the national grid, BUT the steam turbine generator will still be supplying the auxiliary load of the steam turbine-generator power plant.

At this point, the steam turbine-generator power plant auxiliary power supply will have to be switched over to the national grid (by opening the steam turbine-generator auxiliary power supply breaker and closing the national grid auxiliary power supply breaker--which will cause a momentary interruption in power to the power plant auxiliaries depending on how long the transfer requires). At that point, the power plant can then be shut down using national grid power.

Now, without understanding exactly how your "plant" is powered and connected to the national grid it's extremely difficult to say any more. It would take some "special," dedicated switchgear to accomplish the above, and probably even more to accomplish something other than the above. And, that switchgear would likely have to include multiple synchronization circuits (one to synchronize the steam turbine generator to national grid, and one to re-synchronize the steam turbine-generator back to the national grid to avoid blacking-out the load being powered independently of the national grid when trying to take the steam turbine-generator and its power plant off-line in a controlled and orderly fashion without causing disruption to the load).

I hope I've understood the question correctly, and if I haven't I hope you will read and re-read the information and apply it to your situation because the circumstances you described were incomplete at best and it would probably take a LOT of back-and-forth to answer your specific question or situation (if there even is a real situation and this isn't just a what-if scenario which hasn't really been properly thought-through). I does the best I can with what I was given to work with--which wasn't very much in the way of detail and seems to have made several assumptions which I've had to try to fill in.

It's also made more difficult because what you should be providing is what's called a one-line diagram of the electrical system at the "plant" which would make it much easier to provide specific details about which breakers to open and close when.

Hope this helps! I'm sure it's not the exact details you were expecting, but then there wasn't enough specific information to go on.

0 out of 1 members thought this post was helpful...

Many thanks Mr. CSA about explanation.

>Hope this helps! I'm sure it's not the exact details you
>were expecting, but then there wasn't enough specific
>information to go on.

You are right. i didn't give you enough details.

The idea is to start up a power plant supplying chemical plant. the island power system consists of Gas turbine and steam turbine. gas turbine originally intended for starting up. unfortunately gas turbine under emergency maintenance. to start up the steam turbine separate power source needed to start up steam boiler. solution is to connect national grid to that power island (in the same switch gear). then to start the steam turbine, synchronized it with national grid and then put loads on steam turbine. laterally to isolated the national grid.