Parallel Generator and Synchronization With Grid

IEEE Engine Generator Paralleling Concepts

Have you asked your generator supplier for an education?

He'd know best.

You are correct, the governor (throttle control) controls how much of the load the generator carries. The rotating magnetic field from the generator field windings "connects" the generator rotor to the grid. You can think of it as a flexible coupling between the generator engine and the outside world. As the governor setting is increased, the rotor moves slightly ahead of the stator field, delivering more power to the grid and load.

If you have a power meter on your generator and you know the total load, set the governor so that the power meter reads 70% of that. If the load decreases, the power will go to the grid instead.

To answer your question accurately requires more information. The main variables are the size/strength of the grid, the size/type of generator, the size/type of the load, and how they're all connected.

For example, a large steam-turbine generator feeding a relatively small load at the end of a long radial line will have entirely different responses than a small diesel generator feeding a relatively large load all connected to the grid through short transmission lines. Then there's the type of governor, its control modes, if there's any automatic voltage control, transformers with OLTC (On Load Tap Changer), Capacitors, AVR, etc.

It's a good question that requires some good data to yield an engineering response beyond; 1. yes, the governor controls the output power of the generator, 2. the governor will adjust the power output of the generator, and 3. it depends.

Hi Ram,I've been waiting for your answers.
I just want to know the concept of Gas Turbine generator when its parallel with the Grid, act I'm a bit confused how the generator limit its power to the grid and load. I guess my questions were not cleared.
1)Parallel with Grid.
My understanding is , during parallel mode, Generator operated under droop mode, the power export from generator is dictated by the governor setting droop setting, no load frequency and etc), let's say generator governor is set for 15MW at 50hz, and 0W at 54hz at no load, once the generator synchronized with grid, it will produce 15MW (the grid frequency is 50hz) no matter how much the load.Let's say if the load is 12MW, the power output from the generator is still maintained at 15MW, 3MW would be exported to the grid.If the load increased to 18MW, the output power from the generator is still maintained at 15MW, 3MW would be supplied by grid. To avoid the generator from exporting power to the grid during light load, the plan operator must know/forecast the load pattern and adjust the governor setting accordingly(Generator frequency at no load).
Island Mode
There are 2 scenario, the first one before parallel with the grid and the other scenario, during the grid failure.
Scenario 1 (Before Parallel with grid)
Utility grid supplies power to starter motor to start the GTG, GTG runs in isochronous mode at no load. -( I'm not sure whether GTG running in droop mode or isochronous mode, I just assume, please correct me if I'm wrong). GTG geeting signals from Backsyn panel and start synchronize with utility grid, once its synchronized with utility grid, the breaker will closed. How does the Generator switches from iso mode to droop mode?, does it require plant operator to change the setting manually?, what will happen if generator continues to operated in iso mode?
Scenario 2 (Utility Grid Failure)
Generator switches from droop mode to Island mode, same as above question,does it require plant operator to change the setting from droop mode to iso mode manually? what will happen if generator continues to operated in droop mode?

Ok, so you want me to condense a 2 hour "lunch 'n learn" for control operators into a few paragraphs, but first a few assumptions to make it go smoothly. I'm going to assume that; the GTG (Gas Turbine Generator) is one powered by a jet engine whose exhaust powers a free turbine directly attached to the generator shaft, the grid is much larger than the 15MW unit, that the plant is fed by a radial line attached to a single bus that has the GTG and the plant loads connected to it.

1. Modern digital governors have multiple inputs fed from instrumentation transformers providing generator terminal voltage and current from which frequency and real power (MW) can be derived, as well as the traditional mechanical inputs such as shaft speed, etc.. The governor has multiple modes that allow it to operate in either parallel (fixed load, droop, or load following) or standalone (isochronous/blackstart/islanding).

Once the unit is paralleled the governor can be told to either provide a fixed amount of fuel or a fixed amount of MW, but in either case the frequency is set by the system, not by the governor, and the speed of the generator never varies except to follow the minor perturbations as the load on it from both the system and the plant change.

What does change is something called the load or torque angle, which represents the rotor's position (angle) relative to the system and combined loads. Add more fuel and the rotor angle advances relative to the system and more power is pushed out of the generator terminals, reduce the fuel and the angle falls back a little, but it is never allowed to fall behind the system, otherwise it becomes a motor.

So if the plant load is 12MW and 15MW of fuel is being consumed, the excess goes out to the grid; increase the plant load to 18MW and the remainder flows in from the grid. Assuming that the system is massive relative to the GTG (one of the definitions of the mythical infinite bus) there will be no change in the frequency, just a shift in the relative position of the torque angle of the rotor relative to the combined load that it sees. The load split isn't so exact because things like the relative impedances of loads, lines, and transformers affect the load division

If you want the unit to maintain a certain load relative to the system, then the action of the governor is biased by the change in load that it sees and adjusts the fuel flow accordingly.

You're right about the daily load swings though, they can be severe and require operator intervention to prevent undesired operation during these periods. In my experience GTGs were used exclusively for peaking, they either ran flat-out or were taken offline as the system load decreased and they were no longer economic to run.

Scenario 1 is kind of correct, the GTGs we used were specifically black start capable so they had either batteries or a supply of high pressure gas to spin the starters, and they used a frequency generator as a setpoint to get near synchronous speed. Once it was reached speed control was set to match the incoming system frequency, and the autosynchronizing system would pulse the fuel to get the speed and phase to match, as well as pulse the AVR to get the voltage to match. When all three were within the proper limits the breaker was closed.

At that point the control system automatically switched to automatic/parallel and ramped up the fuel to get to the desired load, and the AVR setpoint moved as necessary to keep the voltage or VAR output at the desired point.

In Scenario 2 the changeover from Parallel to Isochronous/Island mode is automatically done by sensing the state of the connection to the grid, which is usually determined automatically by the protective relaying system. The actual mechanics depend upon the operating philosophy you put in place.

Some utilities insist that local generation be immediately cut free from the grid leaving the unit to run at idle as local loads are put back on line under their direction, others let the unit run to feed the islanded portion, but such things are usually preplanned/preprogrammed into the control system strategy and/or under contract with the utility system operator(s).

Droop mode should be automatically defeated when the unit is in isochronous mode to prevent the undesirable action of the unit reducing its output as more load is added. All this is based upon the assumption that there is only one unit in service per island.

Here's a good technical paper that deals specifically with controlling a GTG during various operating conditions. It assumes a decent background in power engineering so your best bet is to Google every time you come to a phrase or utility-speak that you may be unfamiliar with.

Class dismissed

Controlling the Captive power generator output so that Watt & VAr export/import to/from grid are maintained within desired limits is a requirement of many industries and is routinely implemented through PLC or some other type of controller.

The key is monitoring the grid interconnector power (W&VAr) flows and generating corresponding signals to raise/lower generator set Governor & AVR.

You can google on the net for articles on the subject.

There is no mention about synchronizing the the 60 Hz frequency so that both the generator and the grid are exactly in phase with each other. Without some form of electronic control, there could be more than just exhaust smoke from the generator,please correct me if I am wrong here. My solar inverter will not come back on line during a power failure, when I start the backup generator because the generator frequency varies by .2 Hertz either side of 60 Hertz. This protects both the solar inverter and the generator. I would like to know if there is a way to work around this safely ?

Why are you posting anonymously?

You need special, certified equipment to allow grid connection of the type you describe.

This equipment is not cheap, its usually the financial "breaker" for many who wish to do this.

Also, it is possible (here it happens occasionally) that the utility cables are simply not thick enough to allow such "back feeding" as you describe, so before you invest any money, get every signed up and agreed.....the problem happens when many in the area already "feed back" (usually Solar), but "locally" the load is small.....it depends o how "picky" your utility are....

Also, most utility companies give very poor payback for the power you supply them......cents on the dollar often.....watch out for that!!!

The simplest method is NOT to have the expensive equipment for back feeding, but have a (relatively) simple changeover switch that allows your house to take power from the grid, or from your alternator, but not both together.....or split the hose load via two such switches if that is very important for you personally.....

The best option is often the simplest. Forget parallel power supplies. As Andy Germany suggested, split the load. Put your beer refrigerator, man-cave TV, microwave, and other necessities on one power supply. Put the rest of the load on the other power supply. Install a switch so you can run everything off the grid, if needed.