Three GTs were operating in parallel with a load of 4, 4 and 10 MW each. GT on Mark V was operating in ISOC mode with 10 MW. One of the GTs with 4 MW tripped. ISOC mode GT picked up the load instantly. But soon it started dropping and the other GT on droop mode began picking up till the limit of it. Frequency by this time dropped from 60 to 58 HZ. Why this has happened is not clear. Are there any ISOC settings to be reviewed? Can anybody share their experience on this?

 

It is quite obvious that when bus frequency drops the GT which is running in droop mode will pick load according to droop setting. It means that your bus was disturbed at that time which should be controlled by ISOC mode GT. so check ISOC mode GT.

 

Do you have some kind of load sharing control system/scheme in place?

I have to qualify everything below with the statement that it specifically applies only to GE-design heavy duty gas turbines with Speedtronic turbine control systems. While the basics of Isoch and droop speed control should be applicable to any turbine control system (governor), the specifics as I describe them below are only directly applicable to a GE Speedtronic turbine control system. Some control system (governor) vendors use load instead of speed as their reference and feedback, or some form of load-biased speed, so the actual details of your system (because we don't what control system(s) are used at your site) will depend on the control system(s) used at your site.

We don't know what the rated outputs of each of the three generators are, only that the total load at the time of the event was apparently 18 MW (4 + 4 + 10). If the remaining droop unit's rated output could accept 18 MW (or more), and something caused it to start loading, that would cause the Isoch unit to start unloading.

Because the only way the Isoch unit *should* load or unload is in response to changes in grid frequency. If the droop unit starts loading and the Isoch unit did nothing, the grid frequency would start to increase. But, the Isoch unit should unload as the droop unit loads up.

If the remaining droop unit started taking load for some reason (as you say it did) the Isoch unit would unload itself to zero power output. If the load was all being held by the droop unit, the frequency would not be maintained (because that's not the main purpose of droop control).

So, I would suggest that something caused the remaining droop unit to start loading itself, which caused the Isoch unit to unload itself until if reached zero power. I could even envision the Isoch unit beginning to be a load on the system if the droop unit continued to try to load up. If the reverse power relay of the Isoch unit didn't open the generator breaker, it could be a load on the system.

We don't know enough about the plant or the control systems in place (not necessarily the turbine control systems, but any other load sharing or power control system or scheme) to make much of an educated guess. But, it's not usually possible to unload Isoch units as one would load or unload a droop unit. Because the setpoint of an Isoch unit (and I'm referring specifically to a GE-design heavy duty gas turbine with a Speedtronic turbine control system) is turbine speed, which is directly proportional to grid frequency. 100% turbine speed should equal 100% grid frequency.

If it were possible to change the Isoch unit's speed setpoint and lower it while it was running, that would cause it to reduce speed and grid frequency. If the Isoch unit were trying to control a lower(ing) grid frequency, the droop machine, presuming nothing was trying to change its speed setpoint (and power output), would start taking load but the grid frequency would continue to drop. (A droop machine running in pure, straight droop speed control will not maintain frequency when the grid frequency begins to drop, but as grid frequency begins to drop it will start to load up, because as the error between the turbine speed reference and the actual turbine speed increases, the fuel will be increased.)

My *guess* is that when the first droop unit tripped, something started loading the other droop unit. And the system that did that wasn't properly configured. But that's just a SWAG.

 

Thanks for the reply.

I would like to clarify a bit more.

4 mw machines are rated for 11 mw and are using HIAC5000 (Hitachi advanced control system. The Mark V machine is 20 mw rated, which was on 10 mw.
We usually keep the 20 mw machine in isoch mode and other machines in droop mode. We have experienced many heavy load start ups and shutdowns with the isoch machine handling well. In this instance the 4mw GT tripped. The isoch machine immediately picked up 4 mw, but speed still dropping, droop machine also picked up load. Then the isoch machine shed some load and was stable for next 10 sec. Then it started dropping continuously till the droop machine reached the limit of 10.8 mw. By this time frequency dropped to 58.5 hz, which caused load shedding (normal frequency 60 hz) from DCS. After the load shedding, system frequency returned to normal.
My concern is that the isoch machine, which is Mark V, behaved the same way earlier also and caused power failure. At that time, the isoch machine was on 10 mw. The droop machine was on 6 mw (only one droop machine). The droop machine tripped and all 6 mw was picked up by the isoch machine initially, but turbine speed dropped to the trip level and breaker opened.

Are there any constants in Mark V to be changed to address this problem? Please help.

 

Iqbal, You haven't directly answered the question of whether or not there is some kind of load sharing and/or load shedding control system in place at the site but you did mention some kind of load shedding by the DCS.

NOTE: Load shedding is *NOT* the same as reducing load with the turbine control system. Load shedding is dropping load from the system by opening breakers or shutting down parts of the plant to reduce load on the system. They are not the same and the terms can't be used interchangeably. If the load of a turbine is being reduced through its governor (control system) that is not the same thing as load shedding by opening breakers or stopping motors or shutting down part(s) of the plant by the DCS or some other control system.

But, from what you are saying, it sounds like the plant operates fine until one or both of the smaller droop units is unavailable. And, then something causes the Isoch unit to not maintain frequency, which doesn't sound like a Control Constant issue. It really almost sounds like the Isoch unit is being switched from Isoch to Droop automatically, or something is driving the speed/frequency setpoint of the Isoch unit down. Or something is loading the Droop units to maximum which is causing the Isoch unit to unload to minimum and not be able to control frequency. But it doesn't sound like Control Constants; not at all, unless someone has mucked with the typical Control Constants which should be set to control frequency to between 99.83% and 100.17%, or +/-0.17%.

The existing Control Constants would have be reviewed to see if they are typical and/or applicable in light of the configuration of the facility and how it needs to be operated.

In my experience with Speedtronic turbine control panels being operated in Isoc mode, they will support frequency and load extremely well if properly configured in a system to do so. In my experience, the operation of an "island" power grid (multiple units operating in parallel and separated from a larger grid) really requires some very trained operators, OR a very well-configured control system which can oversee the sharing of loads and the shedding of loads and which is periodically reviewed to make sure it is configured properly as grid operation changes (more or less load; differeing loads; etc.). Island operation is not something which can be left to a control system which has not been properly configured unless it has been properly tested and observed to behave as desired in all conditions.

Without knowing what the values of the pertinent Control Constants are and being unable to understand exactly how your plant and system are configured, it's really impossible to say if Control Constants need to be or should be modified to achieve desired behavior.

Is there some breaker status contact(s) or some external control system (Load Sharing or Load Shedding or DCS or BOP or ???) that is or could be sending a signal to switch the Isoch unit from Isoch to Droop? Or that could be sending a signal to reduce the speed/frequency setpoint of the Isoch unit? I've seen one case where a plant thought they could reduce load on an Isoch unit by clicking on LOWER SPD/LOAD, when in fact they were actually reducing the speed/frequency setpoint. Load shedding is the act of dropping load (motors, lights, etc.) from the system, not to be confused with lowering load by reducing the turbine governor speed/load setpoint.

Again, from the original set of circumstances, you said the remaining Droop unit started taking on load when the other droop unit went offline. When that happens, an Isoch unit will reduce its load by the same amount to try to maintain frequency if it's maintained in Isoch mode and nothing is changing the speed/frequency setpoint. In your reply you seem to be describing a different set of circumstances, and it really sounds like either:

1) the Isoch unit is being overloaded (which it can be if the system load exceeds the rated output of the Isoch machine), or,

2) something is reducing the speed/frequency setpoint while it's in Isoch mode, or,

3) something is automatically switching the Isoch unit to Droop, in which case as load was added to the unit it would not maintain frequency, or,

4) there is some Isoch load limit in place or some kind of runback scheme on the Isoch unit which is not typical of most sites..

So, if the unit is operating fine most of the time and responding to large load swings with no problems, and only exhibits problems during certain conditions, then I would say the Control Constants are fine and the "certain conditions" need to be investigated and understood to be sure the system is configured to operate as necessary and desired to meet the needs of the system. Because, it doesn't sound like the system is so configured.

There are any number of competent control system integrators and suppliers who could analyze your system and make some analysis and recommendations, though I would recommend some who's familiar with Mark V control systems and operation to observe and review operations and analyze the software and inputs and outputs and the overall system configuration in light of how you would like it to operate and then make recommendations for achieving desired operation.

It would be great if we could follow your troubleshooting and problem resolution process. Could you please let us how this turns out?

 

CSA,
There is a load shedding control in place by our DCS (central control system) if the running GT trips. This is defferent from the load sharing by GT's.

Now coming back to our problem, I have reviewed some control constants. What I could find is the Auto lOad rate of the Isoch machine is only 0.33 %/ min where as the droop machine rate is 0.8 %/ min.

I believe this must have caused the isoch machine drop the speed even after it picked up load. And of course frequency drop caused the droop machine to pick up load faster since its auto load rate is higher.

Shall I change the Auto load rate of the ISOCH machine which is Hitachi H25 to 0.8 %/ min, same as the other two machines which are Frame 5 LA?

Should I refer this to OEM before making changes?

Please advice.

 

I don't believe the auto (or the manual) load rate is active when a machine is in Isoch mode.

Yes; should consult the packager before making changes. Or, make the changes and observe the results the next time the situation occurs.

Load shedding, unloading, and load sharing are *NOT* synonymous.

Load shedding refers to stopping motors or opening the breakers of loads (motors, lights, etc.) to reduce the load on the system.

Load sharing, well, that means many different things, and is a very confusing term unless it's context is properly defined, and it only refers to droop machines experiencing frequency excursions.

Unloading (and loading) refers to reducing the torque being produced (or increasing the torque being produced when loading) which causes the unit to not carry so much load (or increasing the load carried by the unit when loading). It has to do with changing the fuel of the Isoch machine in order to try to maintain frequency, or for a droop machine, changing the fuel to increase or decrease the load being carried by the droop machine.

In your case, we're not clear what is happening first when the one of the droop machines is tripped. It seems the Isoch unit increases its power output to maintain frequency, but then something starts loading the remaining droop machine which causes the Isoch unit to unload. And if the Isoch unit is unloaded until it reaches zero load, then it can't do anything to maintain frequency.

I still think you need to have someone visit the site, analyze the power system and the DCS controls that are active when the condition you are trying to describe occurs, and make some recommendations. Or, call the packager or GE to analyze the situation (but, that's going to lead to a lot more questions than answers; it is almost guaranteed).

 

Thanks CSA. I will post the outcome after our investigation.

 

We made an investigation with the support of GE control engineer. It has been found that the isoch control constant FSKRN3 value in our system is only 0.966%/% whereas this value is 3.0%/%, auto load rate TNKRN1_4 is only 0.3% whereas it is 1.0% normal and a new factor FSRLLB (FSR bias) which is set 3% above FSR. This FSRLLB blocks the increase in FSR by only 3%. This is a new factor put by Hitachi. After consultation with Hitachi, we changed these factors. FSRLLB is changed to 5% (corresponds to 5 MW load increase). Now it is under observation. We cannot do any test as of now.

Thanks.