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If a generator is running at 100MW of a gas turbine rated 120MW and a command is issued from Mark V to increase load to 120MW. What happens to generator that it increases load to 120MW?

Isn't it obvious? It does so because it is commanded to do so, you said so yourself. All good generators do as commanded, don't they?

Seriously, though, and in as few words as possible, when an operator clicks on Raise Spd/Ld or increases the Pre-Selected Load Control setpoint when the unit is being operated in Pre-Selected Load Control mode the fuel flow-rate into the turbine is increased, which increases the torque being produced by the turbine, which the generator converts to amps, which results in an increase in load.

For the exact details of what's happening, read on.

If one wants to make something spin, one needs to supply some force to it. That force is usually referred to as torque. The more torque applied to something, the faster it will usually spin. Decrease the torque applied to something, and it will usually slow down.

A gas turbine is a device that produces torque, and the amount of torque being produced can be varied, and is in direct proportion (usually) to the amount of fuel being burned in the combustor(s) of the gas turbine. Increase the fuel flow-rate, and the amount of torque being produced by the turbine will increase. Decrease the fuel flow-rate, and the amount of torque being produced by the turbine will decrease.

The generator (more correctly called an alternator) used with GE-design heavy duty gas turbines is a synchronous generator. It is the nature of synchronous generators being operated in parallel with other synchronous generators on an AC grid to supply a load which is much greater than any single generator that they will all be running at the same speed, and no single generator and its prime mover (the gas turbine in your case) can operate at speeds higher or lower than the other generators with which is it connected, because they are synchronous generators (alternators).

There is a formula that describes the relationship between the frequency of an AC system and the speed and type of synchronous generators connected to the system: F = (P * N)/120, where F is the frequency (in Hertz) of the grid, P is the number of poles of the generators (an even number never less than two, and the number of poles of any generator is usually fixed and can't be changed while the generator is running), and N is the speed of the generator rotor, usually the field (in RPM). The (synchronous) generators used with Frame 9E GE-design heavy duty gas turbines are two-pole generators and are directly connected to the turbines (i.e., there is no reduction gear between the turbine and the generator, and the generator is not connected to a "free" turbine which spins independently of the axial compressor of the turbine), so that's why the 100% speed rating of the turbine is 3000 RPM because the grid the generator is connected to is a 50 Hz system (nominally). (Note there are no approximations in this formula, and note that speed and frequency are directly proportional. Increase the speed of a synchronous generator and it's frequency will increase; decrease the frequency of a grid to which a generator is connected and the generator speed will decrease proportionally; speed and frequency are directly related.

So, we've established that when a two-pole (synchronous) generator is connected to a 50 Hz grid with other generators that it will be operating at 3000 RPM (N = (120 * 50)/2 = 3000 RPM), and when the prime mover (the turbine) is directly coupled to the generator that it, too, will spin at 3000 RPM. Again, this is because the generator rotor is locked into the same speed (in synchronism with) the other generators at a speed that is dictated by the frequency of the grid and the number of poles of the generator (which never changes), and the turbine shaft is directly connected/coupled to the generator rotor.

Further, it is the nature of most AC grids that there are so many generators, of various number of poles, so many all supplying such a large load that any one single generator and its prime mover can't usually have any appreciable effect on the frequency of all the other generators and the speeds of their prime movers (unless the prime mover being loaded or unloaded has a very large power rating/capacity in relation to all the other generators and the load).

When a generator is accelerated to rated speed during starting prior to synchronization, the prime mover is producing just enough torque to keep the generator (and turbine and compressor, in this case) speed equal to the frequency of the grid (in this case, 3000 RPM). When the unit is synchronized, the power "output" of the generator is very low, usually, because during synchronization the turbine speed (and generator frequency) is adjusted to be just slightly higher than a speed which would be equal to grid frequency. This means that there is slightly more torque being produced by the turbine than is required to keep the generator rotor spinning at a frequency equal to grid frequency. When the generator breaker is closed, the speed of the generator rotor, and the turbine and axial compressor, actually slows down. Yes, that's right; the speed decreases because once the generator breaker closes the generator is then locked into synchronous speed which is a speed that is proportional to the grid frequency of all the other generators it's now operating in parallel with.

During synchronization, the fuel flow-rate, on the other hand, is held constant once the desired speed is achieved, which means that the torque is being held constant. During synchronization, when the turbine (and generator) speed is increased to be just slightly above grid frequency there is more torque being produced than is required to keep the rotor spinning at a speed that is equal to grid frequency. When the generator breaker closes and the generator speed slows down, which slows down the turbine, the torque remains the same because the fuel was not changed. That extra torque is converted into amps by the generator, and more amps equals more load. That's what a generator is: a device for converting torque into amps. (A motor is a device for converting amps into torque. Actually, the only difference between a motor and a generator is the "direction" of current- and torque flow into or out of the machine.)

The basic formula for power (Watts) is: P = V * I, where P is power (in Watts), V is generator terminal voltage (in Volts), and I is armature current (the alternating current flowing in the stator of the generator). (For a three-phase generator the entire formula is P = V* I * (3^(0.5)) * PF, where, 3^(0.5) is the square root of three (a fixed value, 1.732, I think), and PF is the power factor of the generator (which is a number never greater than 1.0, and which we will presume to be 1.0 for the purposes of our discussion). Coincidentally, the terminal voltage of most synchronous generators is almost a fixed value, as well, and doesn't usually vary by more than approximately +/- 5.0%, which on an 11,0000 Volt generator is only about 550 Volts (out of 11,000).

So, since one of the terms of the three-phase power formula *is* a fixed value (the square root of three), and we are presuming one of the terms (PF) to be fixed and equal to 1.0, and the generator terminal voltage is, for all intents and purposes, a fixed value, the way that a generator produces more power is to increase the number of amps flowing in the stator. The way that amps are increased in the generator stator is by providing more torque from the turbine into the generator; more torque equals more amps. Less torque equals less amps. (We are presuming that the prime mover is always producing at least sufficient torque to keep the generator rotor spinning at synchronous speed. When it doesn't, the generator actually becomes a motor and keeps spinning at synchronous speed and draws current from other generators on the grid. This is what's known as "reverse power" or, "motorizing the generator." It's *very* bad for steam turbines and reciprocating engines even for very short periods of time; not so injurious for gas turbines for short periods of times, especially single-shaft gas turbines like GE-design heavy duty Frame 9E gas turbines.)

Now some people are going to say that when a unit with a GE Speedtronic turbine control system is automatically synchronized to the grid that it loads up to "Spinning Reserve", and they are correct. There is some logic that, once the generator breaker is closed during auto synch'ing, increases the amount of fuel being put into the turbine which increases the torque being produced by the turbine and since the turbine speed can't increase (because it's directly coupled to the (synchronous) generator which is now connected to the grid and it's speed is being controlled by the frequency of the grid to which it's connected) the extra torque that's being produced by the addition of the fuel gets converted into amps.

So, it should be clear that torque, amps, and load are proportional to each other. An increase in fuel results in an increase in torque which results in an increase in load; a decrease in torque results in a decrease in amps which results in a decrease in load.

Now. for a GE-design heavy duty gas turbine to be operated in parallel with other generators on a grid, it is operated in Droop speed control mode. This is one of two governing modes for most prime movers (Isochronous being the other one) and is the mode that allows the turbine and generator to smoothly and stably participate in powering a large load while paralleled with other generators. (Some people refer to this as "sharing load", and while that's technically correct the same term is also used in another description of Droop speed control and this double usage causes lots of problems for most people. So, we're going to refer Droop speed control mode as the governor (control system) mode that allows the stable and smooth production of power by a prime mover and generator when connected to a grid with other generators.)

We discussed what happens during synchronization, when the turbine speed is increased to make the generator frequency just slightly higher than the grid frequency (this is generally referred to as speed matching). And to increase the speed requires an increase in torque, which comes from increasing the fuel flow-rate to the turbine. When the generator breaker is closed, the turbine speed can't change and any attempt to increase the turbine speed will just cause additional torque to be produced, and the generator converts the torque to amps, which becomes load.

When a GE-design heavy duty gas turbine with a Speedtronic turbine control panel is being operated in Droop speed control mode, and the operator wants to raise or lower the load, what happens is that the turbine speed reference is increased or decreased, which causes the fuel flow-rate to be increased or decreased, in an attempt to make the actual turbine speed increase or decrease by increasing or decreasing the amount of torque being produced by the turbine. But, since the actual turbine speed can't increase or decrease, any torque increase results in an increase in amps which results in an increase in load, and any torque decrease results in a decrease in amps which results in a decrease in load.

So, when an operator clicks on Raise Spd/Ld or increases the Pre-Selected Load Setpoint when the unit is being operated in Pre-Selected Load Control, what's really happening is that the turbine speed reference is increasing, which results in more fuel being admitted to the turbine, which results in extra torque which can't result in increased speed so the generator converts the torque to amps, which results in an increase in load.

And all good generators do as commanded, especially when it's at the behest of a Mark V Speedtronic turbine control system.

That is great explanation CSA.

I'd like to extend this story on steam turbines. I believe that basic logic is the same: what is fuel with gas turbines, that is steam with steam turbines. If generator's circuit breaker is open, then more steam supplied to the turbine will cause turbine to rotate faster, but after immediately after synchronization, more steam will produce more torque and because, speed is constant, there will be more amps and hence, more power output to the grid.

However, there are some things that are not clear to me. Generator's output power can be increased by adding more steam to the turbine, say, by opening turbine's control valves (I assume turbine has only high pressure and low pressure stage). I think that turbine can increase its torque also by not opening control valves, but increasing steam pressure which is equivalent.

I wonder what approach is used more frequently.
I also believe it has a lot of in common with so called "exploatation concept" (turbine or boiler leading).

In turbine leading mode, turbine's controller is controlling power output by adjusting control valves. That, inevitably, leads to steam's pressure changing. In such case, boiler controller is increasing or decreasing coal feeders speed. More speed means more coal in the boiler. More coal in the boiler means more heat, more heat means higher pressure (or more steam!?!).

Am I on the right track?

Thanks.

In essence, you are increasing the torque applied on the generator shaft (power, but speed is constant) And this increases the angle between Eo (induced voltage in stator) and the voltage at output terminals (theoretical U and f, constants, and connected to an infinite power net). This angle is torque-dependant (called torque angle) more angle > more power delivered > more power drained from driving machine. If the generator is working as a motor, this angle is negative. If the machine is under no load condition (open circuit) this angle is 0.

It's true that more fuel to prime mover increases the torque, which increases the load angle in the generator. It's also true that magnetic field strength of rotor increases as load angle increases, which causes to increase the current through stator windings thus the MW. But what causes the rotor current to increase, i.e. excitation current to increase?

Rotor field strength is a function of excitation current from the synchronous generator exciter, sometimes referred to as the "AVR" (Automatic Voltage Regulator). The AVR, when operating in automatic mode, is attempting to keep the generator terminal voltage equal to the generator terminal voltage setpoint by varying the excitation current applied to the generator rotor which is directly proportional to the rotor field strength.

As the current in the stator windings increases, the tendency is for the generator terminal voltage to decrease (because of what's called "armature reaction"), but the AVR will increase the excitation current which increases rotor field strength which overcomes armature reaction to keep the generator terminal voltage constant.

So, I take exception to your statement that it's true "that magnetic field strength of rotor increases as load angle increases, which causes to increase the current through stator windings thus the MW". One cannot appreciably change the load (MW) of a synchronous generator by changing the rotor field strength; it's done by changing the torque being applied to the generator.

The increase in rotor field strength is the *result* of increased stator amps which are tending to decrease generator terminal voltage, not the cause of increased stator amps. If the stator amps are increased (by increasing the torque applied to the generator rotor) and nothing was done to change the rotor field strength the generator terminal voltage would decrease and the reactive power output of the generator would change.

Remember the formula for the power produced by a three-phase synchronous generator: P = V * I * (3^(0.5) * PF. Look at the rating of the generator's terminal voltage; it's usually plus or minus 5% of some nominal value, say 11 KV, or 13.8 KV. So, about the maximum effect one can have on the output of the generator with the rotor field strength is to change it by plus or minus 5%, which is very little, and that's only under theoretically ideal conditions.

So, one does not usually change the load (MW) of a synchronous generator by changing rotor field strength; that's done by changing the amount of torque being applied to the generator rotor. If the torque is increased, the amperes flowing in the generator stator windings increase, which increases the armature reaction which tends to decrease the generator terminal voltage. So, to maintain generator terminal voltage, the exciter regulator output is increased which causes more rotor field strength.

If you really want to understand armature reaction, you can send your name, the name of the company you are employed by (or he'll probably take the name of your school if you're a student), and your email address to cepsicon@aol.com and request a copy of his armature reaction document.

What is rated to 120MW, the turbine or generator?