ISO mode safe minimum load


Do you mean ISOchronous Speed Control mode?

If not, what do you mean?

And what condition or event is prompting the question?

> Do you mean ISOchronous Speed Control mode?


> And what condition or event is prompting the question?

just to know the effect on the machines, and to know it is good to run or not.

Thank you & take care

When operating in Isoch(ronous) Speed Control mode the <b>OPERATOR</b>--yes, the <b>conscious, human operator</b>--is responsible to ensure the load on the Isoch unit does not fall below the setting of the reverse power relay, or that the load does not exceed the exhaust temperature control limit ("Base Load"). In either case, the ability of the prime mover to maintain frequency is compromised.

How does the operator "control" the load on the Isoch unit? By controlling the load on the Droop Speed Control unit(s). Decreasing the load on a Droop unit will increase the load on the Isoch unit. Increasing the load on a Droop unit will will decrease the load on the Isoch unit. One <b>cannot</b> use the RAISE- or LOWER SPD/LOAD targets on the Isoch unit to change the load on the Isoch unit--because it will change the frequency of the "island".

This is the part about Isoch and islanded operation that most plant managers--and operators--just can't get their heads around: Balancing the load on the Isoch unit MUST be done by a conscious, human operator who understands how a "grid" (however small or large) works. The prime mover control system being operated in Isoch mode can't decrease the power below zero (by very much anyway, before the reverse power relay opens the breaker) or increase the power above exhaust temperature control ("Base Load").

A conscious, human operator must monitor the load on the Isoch unit at all times, and also must be aware of the nature of the load on the island--so that if it's possible, for example, for a large load to suddenly drop off the grid (shutdown of a large pump or compressor motor) which would cause the load on the Isoch unit to drop below zero MW, then the conscious, human operator must decrease the load on a Droop unit to add some load to the Isoch unit so that if a large load suddenly drops off the island the Isoch unit won't drop below zero MW.

The same is true if the Isoch unit is operating very near "Base Load"; if there is a chance that a large load could suddenly be started that would result in the load on the Isoch unit going above "Base Load" then a conscious, human operator must increase the load on a Droop unit to decrease the load on the Isoch unit so that if a large load suddenly started the load on the Isoch unit would not exceed the "Base Load" rating of the Isoch unit.

Most PMS (Power Management Systems) can't do this at all, or can't do it very well if at all. That's because the people programming the PMSs don't really understand how any plant might operate at any given time and so can't anticipate every possible scenario. Yet plant managers and operators falsely believe that the PMS will take care of everything "auto-magic-ally" and when it doesn't work they start blaming every control system for their misunderstanding and false beliefs--even if the salesperson said it would work!

As for the effects on the unit of a low load, well, there aren't any. However, many larger gas turbines don't run very well at light loads (less than, say, 10 MW, and I'm speaking of Frame 9Es and some Frame 7E/EAs, and most all F-class units (7 or 9)). Their power output can oscillate, causing an oscillating frequency in Isoch mode. But, there is no damaging or destructive effects of low load operation on most GE-design heavy duty gas turbines.

I hope the previous paragraph answers your primary question. However, I want to be very clear about where the responsibility lies for "automatic" (auto-magic) operation of an island: It's with conscious, knowledgeable human operators. Not with turbine control systems, or Power Management Systems. Conscious, knowledgeable human operators--who might be working with, but should be smarter than, turbine control- and Power Management Systems. Just to say, "Low load operation doesn't hurt the turbine-generator," isn't the complete answer because someone <b>will be</b> tempted to let the Isoch unit load go down to zero and then the generator breaker will trip because the load decreases further and someone <b>will be</b> complaining that their unit "tripped" at low load in Isoch mode and "It shouldn't have done that!" when a conscious, knowledgeable human operator would know what the risks are when allowing the load on an Isoch unit to hover around zero. (Don't ask how I know this will happen....)

Thank you very much for detailed answer.

Well, you have mentioned clearly it can be run on low load. For example, if 100 MW designed machine is running with 1% load (1MW) for continuous one week.

Assume grid is disconnected and ISO mode operation supplying only auxiliaries load (it vary from 0.8 MW to 1.5 MW).

Will it impact any components in the turbine or generator side?

Is there any condition to follow minimum percentage of load for the GT performance issue?

Thank you

None that I am aware, though I have no experience running a unit at 1% of power for very long. As mentioned, most GE-design heavy duty has turbines of the size you are talking about seem to have some trouble controlling a stable fuel flow at such low loads, resulting in some minor oscillations of speed/frequency.

Best of luck with this. I'm sure the bean-counters will absolutely LOVE paying the fuel bill for no load for so long. I hope you are being handsomely compensated for this. Remember, it takes about 20% of rated fuel flow just to maintain FSNL (Full Speed-No Load) operation.
Dear Sir,

Good Day.

I have same condition, we are running on island with only one unit running at 30 MW, ischronous mode. Our machine was supplied by M/s. BHEL, model : PG6581B, MARK VI control.

If we loaded machine beyond the limit of TTRXB (I mean TTXM is more then TTRXB). Then what happen??

Unit will trip on high exhaust temp OR it will goes automatically in temp. control mode???

Shall we Turn ON temp control mode in Isochronous control unit.


It's not clear how this is the same "problem" as G.Rajesh, but here goes anyway.

How do you change the load in the machine when it's running in Isochronous mode? (That is a real question.)

If the load on an Isochronous unit increases to the point that TTTRX and TTXM are equal the unit will automatically be on CPD-biased exhaust temperature control--it will be at Base Load.

If load increases any further then the frequency of the unit and load will drop.

It should not be possible to select BASE LOAD when the unit is in Isochronous mode.
Dear Sir,

Thank you for your reply.

> How do you change the load in the machine when it's running in Isochronous
> mode? (That is a real question.)
----------We are not connected with Grid. We are supplying power to our local network (local industrial area) with only one unit is running (Isochronous mode).

Our local network demand is increasing then automatically ISO unit was take care of load demand.<pre>
Now at 35MW. TTXM : 975.79 deg. F
TTRXB : 1000.83 deg. F
FSR : 58.72 %</pre>
If ttxm and ttrxb are same unit will go in TEMP CONTROL MODE, Whenever unit is running on Isochronous mode ??? it is possible

In Droop mode it goes in temp. control but same will happen with ISOCHRONOUS unit also.

If it is possible in ISO unit then it will goes to under frequency trip on further load demand...

Please guide me b'coz i have no idea to run ISO unit neck to neck.

> Why load will increase in this case when big compressor or motor is stopped, in Island mode?

--------------big motor stopped then load reduced and when it will start unit will takecare of that load.
If the load on an Isoch machine was at or near zero MW, and a large load stopped then the Isoch machine would try to decrease it's load below zero MW and could actuate the reverse power protective function, opening the generator breaker thereby removing the frequency control unit from the island.

However, if there was at least one other machine, operating in Droop Speed Control, in parallel with the Isoch machine then by decreasing the load on the Droop machine the load on the Isoch machine would increase above zero MW, and THEN if the compressor stopped hopefully the Isoch machine's load would not decrease below zero MW, and would not actuate the reverse power protective function.

Yes; if the load on the Isoch unit causes TTXM to be equal to TTRX the Speedtronic will prevent further increases to load.

If the load on the Isoch unit were to continue to increase then very bad things will happen. Yes, the frequency will begin to decrease because the turbine can't burn any more fuel because it's reached exhaust temperature control. <b>AND,</b> because the speed drops below rated when frequency decreases the air flow through the axial compressor decreases--which means that even less fuel can be burned so load decreases even more. This in turn leads to an even lower frequency--and turbine speed and axial compressor speed--which means even less fuel can be burned which means that load and frequency and speed will continue to decrease. And so on and so on and so on. It's a very nasty spiral.

If only gas turbine is running in Isoch mode with no other generator(s) and their prime mover(s) to help with load, it is very important to make sure that the load doesn't go above the point at which TTXM is equal to TTRX--because if it does, the lights will usually go out (black-out) and you will be reaching for your torch (flashlight).

Hope this helps!

(TTRXB does NOT mean Base Load Exhaust Temp Control Reference, as is commonly and incorrectly thought to mean. TTRXB means Speed-Biased Exhaust Temperature Control, and when there is no speed bias used then, yes, TTRX and TTRXB will be equal at all times. And a speed bias is almost never used on single-shaft GE-design heavy duty gas turbines.)

I hope you question is, in "ISO" mode if more load taken, will it trip the unit or won't unit permit to place more load?

I think, it will trip on over frequency.

But in "Droop" mode, it won't permit to place more load (temp. control).

Take care

When load exceeds the ability of the unit to maintain rated speed and frequency, then the frequency will decrease.

The unit would not trip on over-frequency, but rather the generator breaker would open (be "tripped") on under-frequency if the load continued to increase without stopping. If the turbine speed was less than L14HS dropout (TNK14HS2), usually about 94.5% TNH or 94% TNH, then the unit would go on fired shutdown automatically.
Sorry to jump to the topic,

It might be at your site has more than one unit of turbine and all running in ISOCH. this was because you are on island mode condition.

If more than one governor on a grid is in Isochronous mode then one of two things has happened. Either there is some means of Isochronous Load-Sharing employed by all the governors operating in Isochronous mode, OR, the Isochronous governors have all been de-tuned, likely to a great extent. Isochronous Load Sharing involves some means of communicating between those governors or some higher level control system coordinating those governors.

I think one thing people forget to remember is that when units are synchronized together on a grid there is virtually <b>NO</b> communication between the governors. That's one of the wonders of Isochronous- and Droop Speed Control. They all work together without any means of communication--no discrete or analogue or otherwise signals. The "communication" is all done by means of speed sensing and control, because prime mover speed and grid frequency are directly proportional.

When conscious, knowledgeable human operators are properly over-seeing the grid the governors all work together without any kind of communication--other than speed sensing and control.

I never cease to be amazed at how simple Isochronous- and Droop Speed Control are--and yet how many aspects of power generation control and -operation they touch. And how they work seamlessly together without any direct communication or higher level control or involvement. It's what makes describing and explaining Droop- and Isochronous Speed Control so difficult.

But, still: Multiple units operating in Isochronous on a grid of ANY size means there is either some higher-level control or communication between those governors or the Isochronous control PI controllers of those governors have been de-tuned to prevent oscillations which would cause grid frequency oscillations, maybe worse.