hi all
I am going to work in old plant (John Brown) fram5 it was Mk II upgraded to Mk vi. This plant is not tied with network and it work at the emergency only in summer.

But what I heared they found problem when one unit in droop system assume it is 8 MW the load is one chiller when they need to put the second chiller in service the first chiller take trip by under voltage. But if there is two unit GT in service each one 4 MW or one unit in isochronuse this phenomena did not happened. So what is the reason?

Second thing is: isochronous=constant frequency by automatically adjusting output power. Droop= frequency is automatically adjusted but when power increase the frequency decrease a bit. My question is how this happen logically? If you have document explain in details the difference I will be thanks.

Another thing, in the isochronous mode is the load increased the MW increasing with regardless the set point or it is controlled by the turbine set point even the load increased?

best regards.

 

You seem to be saying that these two units do not synchronize to any larger grid ('network') and are only operated occasionally. Is that correct?

If this is true, then I will tell you that it *is* possible to operate a turbine in droop speed control mode *and* manually maintain frequency *IF* (note that 'if' is in capital letters!) the load is fairly stable. In this situation, it is the operator's responsibility to adjust the turbine output to respond to any changes in load which will tend to change the frequency.

However, if there is a large change in load (8 MW is a pretty big load change even if it doesn't all come on the 'network' instantaneously) it's probably pretty difficult for a person to manually change the load quickly enough to maintain the frequency very well.

But, you said that when one unit was being operated in droop mode at 8MW and another 8MW was added to the 'network' that the first 8MW load tripped because of undervoltage.

The voltage of a generator is also affected by its speed, which is a function of frequency. Also, some exciter regulators don't respond to terminal voltage changes very quickly and we don't know what kind of exciter regulator you have (static, rotating, etc.) or how it's tuned. Also, we don't know very much about the power system and its protective relay scheme to really speculate on that aspect.

Lastly, you say you "heard" this. A lot of things that allegedly happen in a power plant during an event like this are just, well, what someone thought happened *or* some events are mistakenly linked to others. One shouldn't believe everything one hears, particularly in a power plant. Operating data and alarm logs usually clear up a lot of "eye-witness hearsay".

So, there's little we can say about that "rumor."

Now, if the two units were synchronized to each other and supplying a load independent of a grid, neither unit has to be in isochronous mode, but both units cannot be in isochronous mode (unless there is some kind of isochronous load sharing for the two units which is unusual but not unheard of).

It would be possible for two units in this condition to both be operated in droop speed control and to be able to control frequency, if the load were fairly stable and the operators were fairly attentive and responsive.

The ideal situation would be to have one unit in isochronous speed control and one unit in droop speed control. The isochronous unit would respond to load changes to maintain frequency. If the droop unit were loaded to 4 MW and left there (without Pre-Selected Load Control enabled), it would maintain 4 MW as long as the frequency was stable. If the frequency was unstable, the output of the droop unit would be unstable. If the droop unit were loaded to 4MW with Pre-Selected Load Control enabled and active, it would maintain 4 MW regardless of any frequency change.

If the isoch unit were at 4 MW and the droop unit was at 4 MW and an additional 8 MW of load "suddenly" came on the system, then the isoch unit would increase it's output to 12 MW (presuming that the unit was capable of putting out 12 MW!) and the droop unit would stay at 4 MW (whether it was in Part Load or Pre-Selected Load Control). And the frequency would be stable.

The isoch unit would continue to increase its output if more load were added to the 'network', until it reached exhaust temperature control (Base Load). The droop unit would continue to run at 4 MW (we're presuming no is touching the controls of the droop unit).

Now, let's say the 'network' load for these two units was 16 MW, and the isoch unit was being operated at 4 MW and the droop unit was being operated at 12 MW. And 8 MW of load suddenly dropped off the 'network.' The isoch unit would drop its output to 0 MW but couldn't decrease it any further, and the droop output would drop to 8 MW (because that's all the load there is!), *BUT* the 'network' frequency would increase above nominal until an attentive and responsive operator reduced the fuel being admitted to the droop unit to make the frequency come down to nominal.

Let's say the load on the 'network' for one unit operating in isoch mode and one unit operating in droop was 16 MW, and the droop unit was operating at 4 MW and the isoch unit was operating at 12 MW. Now let's say that someone wanted to increase the load on the isoch unit, the *only* way to do this is to reduce the load on the droop unit. As the load on the droop unit were reduced, the load on the isoch unit would increase by an equal amount. So, if the load on the droop unit were reduced by 2 MW the load on the isoch unit would increase by 2 MW, to 14 MW. And the frequency would remain stable.

When a unit is being operated in isoch mode, it's "reference" is frequency, not load. If an operator tries to raise load on the isoch unit by clicking on RAISE SPD/LOAD, what will happen is that the frequency of the 'network' will increase, but the load (which is the lights and pumps and motors and computers connected to the 'network') doesn't change. (Usually, Pre-Selected Load Control is disabled when isoch speed control is active, but even if it weren't, the turbine control would get mighty confused if it were told to maintain a certain load while in isoch mode.)

Or, if someone wanted to increase the load on the droop unit, the effect would be that as the droop unit were loaded the isoch unit would unload by an equal amount. So, if the droop unit load was increased by 2 MW, the isoch load would decrease by 2 MW. And the frequency would remain stable.

In this example, one is just changing the amount of power being produced by each generator and its prime mover. But, even if the generators are capable of 20 MW each, if the 'network' load is 8 MW, the combined output of the two generators (whether they are in droop or isoch or some combination of the two) can only be equal to 8 W if the frequency is to remain at the setpoint and stable.

The thing to remember is: The load of a 'network' is the aggregate sum of all of the lights and motors and computers connected to the 'network.' To maintain the frequency of the 'network', the amount of power being produced by the generators connected to the 'network' cannot be greater or less than the sum of all the lights and motors and computers.

Even if the prime movers of the generators are capable of producing more power, the amount being produced has to be equal to the load to maintain the frequency. If the amount being produced doesn't equal the amount of lights and motors and computers, the frequency will drop.