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Gas Turbine Generators - Loading Rate

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RRaghunath

Electrical
Aug 19, 2002
1,729
Normally, the loading ramp for GTGs is stipulated by the manufacturers, such as 20 minutes from no load to full load.

My doubt is when a machine is functioning as "Spinning Reserve" in the power system, the machine is expected to pick up load pretty quick in case of tripping of one of the loaded generators (otherwise the grid may be in jeopardy) and with ramp rate stipulation as above, how is this achieved?

Could some one throw light on the subject. Thanks in anticipation.
 
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If you were in an "islanded" situation, say 2 generating units running to supply power to an industrial facility and not connected to the grid, then tripping of one unit would necessitate extremely fast load pickup from the other unit. However, this is not a normal case. Usually a CTG which is lightly loaded and designated "spinning reserve" is one of many on the grid. When a unit trip occurs, all of the units are affected to some degree. Those electrically closer to the tripped unit are affected more than the others. But most of those units are running in droop mode and will not significantly change load automatically. The reason they pick up any load is that the system frequency was reduced very slightly due to the trip event. All of the droop units saw an error signal, a change in the difference between the frequency setpoint and their current operating point.

If a unit was designated as spinning reserve, then it is almost certainly part of a control area and the operator of the unit will receive a telephone call to change his setpoints to pick up more load or, if the unit has AGC (automatic generation control), then the area control person or computer will introduce a setpoint change from the central dispatch facility, leaving the local operator out of it. Some of these units are at unmanned sites anyway and there is no local operator to intervene.

When the setpoint change is received, the spinning reserve unit will ramp up to the new setpoint and the other units on the system which picked up an incremental load can back down. Frequency is restored to "normal" or "desirable" level, the load has been redistributed as desired, and the system is happy.
 
Thanks for the attention.

"when the set point change is received, the spinning reserve unit will ramp up to the new set point" - Could you give an idea of the time that would be involved in ramping up. This is important to ensure that the ramp up takes place positively before the load shedding schemes in the power system operate.
 
It would be better if you were more explicit about your actual configuration.
Island, small grid (your units contribute a significant % of the total load - e.g. 10%), large grid.
There are several operating modes of a GTG:
1. Isochronous
2. Pure Droop - No load selected(speed control)
3. Preselected Load (droop)
4. Max Load - temperature control (droop)

Hopefully your "spinning reserve" would be drooping to the grid with no load selection.

In case you select isochronous control, the unit will load itself very fast depending on the frequency change, tipically the allowable deadband is 99.99 to 100.01% of the rated frecuency (expressed as percentage as your local frequency is unknown)
Isochronous would be the fastest reacting operating mode for the unit...but has a drawback...if the frequency goes up...you may trip the breaker on reverse power. typically isoch units are running @ 70~80% load.

In droop, if the unit is in "pure droop" that is load is "floating" and is a result of the droop built into the speed control, the unit's speed reference is constant. and the load depends on the error between the frequency and the speed reference. This is also fast (although not as fast as isochronous control) and the ramp will depend upon the speed control loop gain and time constant.
For a step change in the error, how fast will the fuel control system compensate to maintain the speed of the unit: this is the ramp you have.

In preselected load: DO NOT USE THIS OPERATING MODE!!! In preselected load the unit will OPPOSE the results required by the grid as soon as the load goes outside the preselected load dead-band limits.

Max load (temperature control) the unit is basically irresponsive to frequency changes.

In case a new setpoint is sent by an external source to the control system there are two alternatives:
1. change the load setpoint in the unit with the automatic ramp rate as usual
2. use an emergency ramp rate that should be built into the control system.

What values to use for each one?
That is really to be determined by the OEM's Controls Engineering department.
The normal (auto)load rate could be as low as to load the unit in 40 min due to constraints such as heat recovery steam generator (HRSG) requirements or as high as 4 min (fast start) or 2 min (emergency start)... all this depends as well upon what is the combustion system (standard? wet nox control? dry nox control?).
Without a complete system assessment it is impossible to recommend a ramp rate, specially for emergency conditions.

Also, when confronted with such sudden load changes the excitation system must be evaluated regarding its ability to cope with such step changes, i.e. if the excitation does not respond fast enough... you may hit the under excitation limit and either trip the excitation (and the unit) or damage the generator (out of step).

The OEM should be capable of studying what your best alternative is, and supply the technical services required to implement such alternative.

Hope this helps.
saludos.
a.

 
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