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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.

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!

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

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.


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.