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Load sharing on droop and isochronous mode of Mark-IV control system
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Posted by Ritesh on 16 May, 2007 - 12:03 am
We have two Gas turbines based on Mark-IV control Speedtronics system. At a time we have only one of them in operation and the second serves as a cold stand by. At the time of changeover of the GTG we faced the following problem.

While doing De-synchronization of the GTG we faced the problem that none of the machines could take the load and which lead to the trip of both the machines. Both the machines were on Droop mode. is it a right practice or not to keep botht the machines in droop mode while doing the change over of the GTG.

We tried to change the operating mode of the machine from Droop to Isochronous but could not succeed. Pls let us know what could be the possible reason behind it.


Posted by markvguy on 17 May, 2007 - 1:06 am
When only one unit is in operation, is it supplying all the load--and is it in Isochronous mode?

We just don't have enough information to be able to really speak directly to your situation.

If one gas turbine is supplying all the load (sometimes referred to as a "power island") it should be in Isochronous mode to automatically control the frequency for load changes. When the second unit is synchronized, it should be in Droop mode, and as the Droop unit is loaded the load on the Isochronous unit automatically unload to maintain the frequency. When the load on the Isochronous unit reaches 0 MW, the breaker can be opened manually and the Droop unit will have all the load; it can then probably be switched to Isochronous mode.

In Mk IV, the typical name of the logic signal which indicates Isochronous mode has been selected and is active is L83ISOK. You need to find this signal and work backwards in the sequencing to find what the permissives are for selecting Isochronous mode.

There may even be some logic which disables Isochronous mode under certain conditions. Your Mk IV Speedtronic elementary has all the details of your unit's configuration.

It is NOT desirable to have two units in Isochronous mode at the same time--without some kind of Isochronous Load Sharing scheme, it's ugly and usually results in wild frequency fluctuations and even trips. So, it's possible that one of the permissives is that two units can't be in Isochronous mode at the same time.

markvguy


Posted by Ritesh Vyas on 18 May, 2007 - 12:47 am
Sir,

Thank you very much for you valuable reply. I think I could not make the situation very clear in the previous post. So I am writing them again. Let me call them GT1 and GT2.

GT2 was running on approx 18MW. it was to be stopped due to 125 VDC earth fault detection. GT1 was in stop condition.

GT1 was started. Both GT1 and GT2 were synchronised and GT1 had 16MW load GT2 had 2 MW load. both machine running in droop mode. At the time of desynchronization, operator tried to take GT1 on ISO mode but it did not go to ISO mode. However the LED of ISO button on membrane keypad was blinking after the ISO was pressed. Which is normal thing. After that operator pressed execute key but mode was not changed.

LED of DROOP on the keypad was on.
Operator observed the frequency going down.
Operator tried to raise the speed of GT1 by pressing the button speed/MW raise, but freq. kept on going down.
They tried to synchronise GT2, but by that time
power had gone.
Operators observed that at that time printer was also not working.

After the power got restored we took print of history and the first alarm which we saw there was C0046 Excitation Breaker Open.

There is no alarm of low freq.
As the AC power was not available, back up fan could not start so GT1 tripped after 3 mins as per its timer.

Could you pls give me your e-mail ID so that I can mail you the PDF file of alarms?

Thanking you,
Ritesh Vyas


Posted by markvguy on 19 May, 2007 - 12:47 am
This helps--a little. Are these units operating independently of an electrical grid--or are they operating in parallel with other generators on an electrical grid?

When one presses one of the membrane buttons on the <OPM> keypad, it will blink for a few seconds to indicate the operator has "armed" the action, and if EXECUTE is not pressed within approximately five (5) seconds, the flashing will stop and the LED will be extinguished. If the action is accepted when EXECUTE is pressed, the LED will be continuously lit.

If some permissive in the Speedtronic is blocking the enabling of the command (in this case the selection of Isochronous Control Mode), then the blinking will stop because the command cannot be accepted. The blinking will also stop if EXECUTE is not pressed within the specified period of time.

That's why you need to look in the Mk IV Speedtronic elementary to find the signal mentioned before and see what the permissive are to enable Isochronous control.

Or, you can work "forward" from the <OPM> buttons, usually shown on the 01 sheets of the Speedtronic elementary.

If you're not familiar with the Speedtronic elementary, someone on your site should be able to help you look up the signals. The only way to be sure exactly HOW your unit operates is to review the Speedtronic elementary. The Mk IV Speedtronic elementary is one of the finest elementaries every produced--it depicted every input and every output and all the software in between, in a very understandable and logical format using relay ladder diagram (RLD) representation. Most--if not all--of the signal names should be described on the B sheets in the front of the elementary. There is "mapping" in the elementary to track where signals and contacts are used.

Sorry, but it's still not clear how your units are being operated. Are they supplying load independently of an electrical grid and other generators?

Desynchronization is not a term this author is familiar with; it is presumed you mean you are unloading the unit and opening the generator breaker in order to stop the unit. Is this correct?

If you have two generators supplying a load independently of other generators, it is customary--but NOT required--that one of the units is in Isochronous Control. There are many different versions (and perversions) of Droop Control which will permit some limited frequency control, but they are not common--but without being able to look at your Speedtronic elementary it's impossible to say exactly how your units are configured. If Isochronous mode is not used and there is no "special" Droop mode active, then the operators will have to adjust the governor to maintain a stable frequency.

The alarm printout, while it may be helpful, is not going to always list every condition which can trip the unit. If the exciter power supply breaker opened while the unit was supplying load, there is usually an underexcitation (or loss of excitation) relay which will open the generator breaker, at a minimum, and may even trip the turbine.

If the load was increasing or exceeded the power output of the unit--which is what would normally cause the frequency to decrease--that would explain why the frequency to decrease.

Again, it's just extremely difficult to comment specifically on your situation without being able to understand how your units are being operated, what sequencing is present in the Speedtronic turbine control panel, and exactly what the conditions were at the time of the event.

This author is also not familiar with a "fan" which must be running in three minutes in order to start a unit. What kind of fan is this?

There really are just too many unknowns to be able to comment, and it sounds like this is something of an unusual configuration, also.

As an aside, several Mk IV owners have purchased regular personal computer UPSs (Uninterruptible Power Supplies) for use in powering the CRT and printer in the event of loss of AC power. One just needs to choose one with enough VA and amp-hours to be able to power them for the desired time. Then can be plugged into one of the outlets providing power to the printer or monitor, or even the winder, if present, and then the printer and/or monitor can be plugged into the UPS. This will provide CRT and printer operation for a few minutes during a "blackout"; some help, but most people find it very helpful if they experience even one or two loss of AC events a year!

markvguy


Posted by Anonymous on 17 May, 2007 - 11:04 pm
On 14th May, 07 at 1:30 PM approx., both our Gas Turbines tripped while desynchronization of turbine and following are events as per process operator's feed back:

We have MARK IV control system having two GE Frame 5 machine and running at 49.5 Hz and tripping at 47.5 Hz with 2 MW preset load.
GTG1 started to check the DC125 V under voltage alarm in GTG2 (GND_IMP was 1 to 0 Kohm).
GTG1 started on NG and load shifted on GTG1 up to 14.2 MW after synchronization and on GTG2 was at 2.6 MW.
GTG 2 desynchronized and frequency of GTG1 started dropping.
To avoid further dropping of the frequency, GTG1 tried to take on Isochronous from DROOP mode but could not be taken (as per operator feedback system was not allowing to do so).
GTG1 tripped and process person further tried to increase the load on GTG1 before tripping the GTG1.
Meanwhile GTG2 was also tried to take on load but being desynchronized GTG2 also tripped.
Printout also could not be taken as process dept. feels that MarkIV system was hanged and GTG1 tripped because of malfunction of MarkIV.
However the Printer supply is from Electrical dept. at 110 Volt AC and no emergency supply was available for some time after both GTG tripped because of some problem of Emergency Generators which are required to operate the Emergency systems /DC operated motors and Motorized Valves also.

Have you or GE ever faced the Hangover of MarkIV system? And what were the consequences? Could you please help us to find out root cause to establish the reason of happening this event also to avoid in future?

Please confirm for us if we take GTG2 on Isochronous from DROOP after a gap from de-synchronization may also trip the machine.

Could you please explain that which machine should be in Isochronous or in DROOP mode and when?
After that we run the GTG1 stand alone mode for normalization.
After rectifying the DC ground fault in GTG2 we again shifted the full load to GTG2. And now GTG1 in stopped condition.

Your extended help and expert view on the above subject would be highly appreciated.


Posted by I.sanjeev kumar on 2 October, 2007 - 12:46 am
You should always keep one machine in Droop mode and another one in isochronous mode, preferably the one which has higher capacity. It is better to keep a selector switch for droop and isochronous mode. When ever the Tgs are desynchronized, they should automatically change to isochronous mode.

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