droop mode of twin spool machine


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Droop mode operation of twin spool machine is different from single spool machine?

Can you explain if different droop setting machines are running parallel & load needs to be shared,How they wiil react?
Please state some example also.

I'm making the assumption that LM6000 is a GE frame 6 combustion turbine... twin spool being a 2 shaft machine, CT being connected to the generator shaft thru a reduction gear (in my case FSNL rpm of CT is 5190 RPM, reduction gear reducing this speed to 3600 RPM for a 2 pole generator).

Single spool machine would be the prime mover - (steam turbine or diesel engine) - being directly connected to generator shaft. The operating speed of the prime mover would be 3600 RPM for a 2 pole generator, 1800 RPM for a 4 pole, 720 RPM for a 10 pole.

There are two modes of operation:
isochronous (also known as "zero droop")
synchronous (also known as "droop")

A unit set to run in isochronous, or zero droop mode will maintain the system frequency (60 hz nominal) from no load to full load and will absorb or shed the changes in load (system demand). If you are providing prime power, e.g., no interconnection with a utility grid, your zero droop unit should perform as above.

If you are connected to a utility grid, do not run your unit on zero droop (utility grid is providing the frequency and functions as the "zero droop unit")... putting your unit into "zero droop" with a utility grid will probably result in swinging and hunting load as these forces fight each other, eventually either your unit would trip or the utility interconnection breaker will open. (Same scenario would result if running independent of the utility grid and you had more than 1 unit on line with "zero droop".)

If you have more than 1 unit on-line and are not connected to the grid, only 1 unit should have the zero droop, it will maintain the frequency and take all load changes. Load is shifted between units by raising or lowering the governor control of the non-zero droop unit. Raising or lowering the governor control of the zero droop unit will change the system frequency. I have seen some interesting gyrations by people who did not fully understand this... (e.g. "we must have a governor problem!")

Having more than 1 machine on with identical speed droop settings (2%, 4%, etc.) should result in both machines equally splitting system load changes and a drop in frequency as the system demand increases (or a increase in frequency as demand drops).

Does this help? I can continue if I'm on the right track...

What do you mean by "sharing load?"

Droop speed control is Droop speed control is Droop speed control--regardless of the number of SHAFTS a prime move has: one, two, or three (Intermediate Pressure). The Droop speed control reference will be for the shaft which is connected to/driving the alternator (AC synchronous generator).

If a machine has 4% Droop and is being operated on a grid which is at rated frequency, then it will pick up 25% of its rated load for each 1% increase in speed reference setpoint--up to rated load. If a machine has 5% Droop and is being operated on a grid which is at rated frequency, it will pick up 20% of its rated load for each 1% increase in speed reference setpoint--up to rated load.

Another way to say this is: When the Droop speed control reference (setpoint) is equal to the Droop setting (plus the rated speed), the machine will be at its rated power output.

Yet another way to say this is: When the difference between the Droop speed control reference (plus rated speed) and the actual speed of the machine is equal to the Droop setting the unit will be at its rated power output.

Now, if a machine is NOT at rated output, and the grid frequency drops by 1%, then the difference between the Droop speed control reference and the actual prime mover speed increases by 1%--and this will cause the prime mover to pick load in proportion to its Droop setpoint. A machine with 5% droop would pick up 20% of its rated load; a machine with 4% droop would pick up 25% of its rated load--UP TO RATED OUTPUT. In other words, once the difference between the Droop speed control reference (setpoint) and the actual speed reaches the Droop rating, the machine will be at rated output.

If a machine with 4% Droop had it's Droop speed control reference at 103% and the actual speed of the turbine was at 99%, the difference is equal to the Droop setting and the unit would be at rated power output.

If a machine is already at rated power output and the grid feqeucny drops, it cannot pick up any more load--it's already at maximum. Machines which are at or near rated load will not pick up much if any load if the grid frequency drops by much.

It should be seen that in addition to its MAIN purpose of permitting multiple electrical generators to be operated in parallel on an electrical grid--in other words, smooth and stable power output from all the generators connected to the electrical grid--another benefit of Droop speed control is that FOR UNITS WHICH ARE OPERATING IN DROOP SPEED CONTROL MODE AND ARE NOT OPERATING AT RATED LOAD that should the grid frequency experience a decrease or an increase the partially loaded machines will actually increase or decrease their power output AUTOMATICALLY without any operator intervention in order to try to help maintain the load on the grid. This occurs simply because as the grid frequency changes, the difference between the actual speed of the machine and its Droop speed control setpoint changes!

Some OEM instruction books refer to picking up load during a frequency disturbance as "sharing load" under Droop Speed Control. And during such an event if every machine had the same Droop setpoint rating, every machine would accept proportionally the same amount of ITS RATED LOAD in response to a change in grid frequency.

To this author, "sharing load" means that a unit can be parallelled with other units on an electrical grid and operate smoothly and stably. The opposite of this is trying to parallel two or more machines operating in Isochronous speed control on the same electrical grid--one hasn't really "lived" until they've experienced this, and seen the lights go out! And had to come back from a black plant....

Droop speed control is the governor control mode which allows many generators to be connected to a large electrical grid with a load larger than any one single machine could provide and SHARE that load with other machines smoothly stably.

Many texts attempt to describe Droop speed control by suggesting that when a machine operating in Droop speed control mode is loaded that its speed will decrease, and that if the machine has 4% Droop its speed will decrease by 4%. THIS IS ONLY TRUE IF THE UNIT IS NOT CONNECTED TO A STABLE ELECTRICAL GRID, or is operating by itself unparallelled with other electrical generators supplying a load.

When a synchronous AC generator (alternator) is connected to an electrical grid, its speed is governed by the formula: F = (P * N )/120, where F=Frequency (in Hz), P=the number of poles of the generator, N=the speed of the rotor (in RPM). The speed of the alternator--and its prime mover! (since they are usually either directly coupled or coupled through a non-slip reduction gear)--is fixed by the frequency of the grid to which it is connected.

So, describing Droop speed control by saying that the machine speed (frequency) will decrease by an amount equal to its Droop rating when it is loaded WITHOUT SAYING THAT THIS WILL _ONLY_ HAPPEN IF THE UNIT IS LOADED WHILE IT IS _NOT_ CONNECTED TO AN ELECTRICAL GRID is just flat WRONG. And CONFUSING. And doesn't have a thing to do with "sharing load."


Now, can we give this subject a much-deserved rest? It is unfortunate that so many people operate electrical generators and their prime movers without understanding this most elemental and germain concept--but it is also a great credit to the inventors and perfectors of the concept that it works, and works extremely well without many people understanding how or why. Again, it is fervently hoped that this subject will be developed on wikipedia.com and that a usable and understandable definition will be developed with the benefit and experience of many knowledgeable and experienced people.

markvguy (who's exceeded his Droop speed control limit!)
An LM6000 is a GE aero-derivative combustion turbine rated at approximately 40 MW. It is a multi-shaft machine, with the LP shaft directly connected to a 2-pole generator.

There are several LM combustion turbines produced and packaged by GE: LM1000, LM2500, LM5000, and LM6000. These LM "engines" are used on naval ships, cruise ships, in natural gas pumping stations, and power plants (both simple- and combined cycle).

Regardless of whether or not the prime mover of a generator is a hydro turbine, or a diesel engine, or a steam turbine, or an aeroderivative combustion turbine, or a heavy-duty gas turbine--Droop speed control is used to allow the generators to be connected in parallel with other generators on an electrical grid. Not every control system manufacturer implements Droop speed control in exactly the same way--but the concept and the end-result is the same.

I have been truly fascinated by the replies posted by the markvguy. It shows his tremendous knowledge and experience. The way he explains with apt examples, only a master can do. Hats off to him.