Generator Overload Question


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we have 1 gas turbine GE model MS 60001B 38 MW. we connected with grid. what happen if plant islanding condition and internal load is 40 MW over capacity generator 2 MW? How much frequency drop (normal 50 hz) or plant will blackout by frequency low? We run at iso mode if case islanding and droop mode when connected grid.

please tell me.

This is a very difficult question to answer, because when the speed of the axial compressor of a single-shaft heavy duty gas turbine drops below 100%--as it will when the frequency decreases--the air flow through the machine will decrease. If the machine is operating at Base Load when this happens the decrease in air flow will cause the exhaust temperature to increase and the CPD to decrease--both of which will cause the Speedtronic turbine control panel to decrease the fuel flow to keep the exhaust temperature from climbing too high and to keep from exceeding the design firing temperature. This will also cause the torque to decrease will contribute even further to a drop in frequency. So, it's a vicious circle, and it's very likely the unit will trip on under-frequency.

My estimate of the speed/frequency decrease <b>to begin with</b> is as follows. 2 MW of a 38 MW rating is approximately 5.26% of 38 MW, so I would say the speed would initially decrease by at least 5.26%, which result in a frequency drop of the same 5.26%, from 50 Hz to 47.37 Hz. <b>BUT,</b> remember than when the frequency and speed start dropping the power output of the gas turbine will also decrease even further, making the frequency and speed drop even more.

Hope this helps!

What most sites do in cases like this is they have some kind of "load-shedding" scheme or operating procedure that drops, in this example, at least 2 MW as soon as the grid tie breaker opens in order to keep the gas turbine from being overloaded.

If the unit has Peak Load capability, then it might be possible to switch exhaust temperature control from Base Load to Peak Load when operating in island mode. This might buy you some time in order to be able to lose a couple of MW to get load down where the unit is operating closer to the design firing temperature.
Thank you very much for your answer (CSA). We will prepare function load shedding for protect plant blackout.

best regards
We have 4 LM6000 PC gas turbines and the islanding mode has recently been added. normally our machines run on droop mode when connected to grid. we have done some special arrangement. Now when the system blacks out, Our 2 machines survive (their load drops to house load or auxiliary load) while other two come on FSNL. This way the plant doesn't trips. The machines that come on house load change from droop mode to isochronous mode. The frequency increases as their is no load in their grid. Now as we start to supply the load to system, the frequency can drop rapidly and must be controlled.

That's how your system survives black out and energizes the grid again.

Hope that clears your query. You can contact me on [email protected] for further details.

Either I didn't understand your post, or something seems amiss.
I suspect the units have Woodward MicroNet controls, and there is some "external" power management system that is handling the island operation, something like a Woodward DSLC or DSLC-2. And, that you are being told the units are in Isochronous speed control, but they're really in Droop speed control receiving signals from the DSLC.

Isochronous speed control should be capable of holding frequency regardless of load, and even if there is no load or the load is very low. And, as load is added Isochronous speed control should also be capable of maintaining frequency. So, it's very odd that you say that frequency is high and then drops as load is added and that you have to pay special attention as load is added--because that's not how Isochronous speed control should work.

On an AC power system when frequency is stable and load is stable if a large motor is suddenly started and allowed to stabilize for a couple of seconds the immediate effect is for the frequency to drop. But Isochronous speed control senses the drop in frequency and immediately adjusts the energy flow-rate into the to return the frequency to near normal and maintain it there. If a large motor were suddenly stopped the frequency would tend to increase but Isochronous speed control would decrease the energy flow-rate into the generator's prime mover to reduce the frequency to near normal and maintain rated frequency.

So, what you are reporting--that operators seem to have to pay attention to frequency and load when islanded--is pretty strange because a properly tuned "power management system" (which is what DSLCs and similar systems are sometimes called) should not require any manual operator intervention, and they should be much more capable of controlling frequency when operating independent of the grid.

What happens when there is no external "power management system" coordinating multiple turbines operating in island mode independent of a grid is that one unit is usually being operated in Isochronous speed control mode. Other units might also be supplying power to the island and they would be in Droop mode. As load is added to the system the Isochronous machine will increase it's load to maintain frequency but the load on the Droop machines will NOT change. As the load on the Isochronous machine increases to near the machine's rated output the operators have to increase the load on one or more of the Droop machines--which will reduce the load on the Isochronous machine by the exact same amount. If the Isochronous machine is allowed to be loaded to its rated output and the load is increased even further the grid frequency will start to drop because the Isochronous machine can't increase it's output and the Droop machines aren't trying to control frequency because that's the Isochronous machine's job.

So, operators in situations like this have to be very aware of load swings on the island and make sure that the maximum load of the Isochronous machine isn't reached--and the only way they can reduce the load on the Isochronous machine is by increasing the load on the Droop machine(s). Similarly, if the load on the system drops such that the load on the Isochronous machine is near zero and the load continues to decrease the Isochronous machine will trip on reverse power--unless the operators decrease the load on one or more Droop machines to keep the load on the Isochronous machine above it's reverse power trip setpoint.

Many people mistakenly believe that in such a scenario with multiple machines that the Isochronous machine will automatically adjusts its load for all operating conditions in order to maintain frequency--but it has limits. It can't go below zero, nor above it's maximum. And operators also mistakenly believe that they can control load on the Isochronous machine using the Isochronous machine's governor/control system--but they can't. They can only change the frequency setpoint, not the load. To change the load on an Isochronous machine in such a situation the load can only be changed by increasing or decreasing the load on one or more Droop machines.

Power management systems, including the DSLC, can be very simple or very ... "sophisticated" (a polite way of saying very complicated). In either case, they require some tuning to make them work correctly, and unfortunately sometimes during the tuning there can be inadvertent plant black-outs and turbine trips and so the tuning is usually cut very short or completely abandoned and the power management systems don't operate like they're supposed to or are capable of.

Again, unless I misunderstood the post it sure doesn't seem like the system as described is working as it should. It may be better than before it was implemented, but it certainly doesn't seem like it's working as well as it could if the frequency changes as load changed when operating independent of the grid.