Parallel Grid and Self Generation System


Thread Starter


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Our customer uses a low of 900kWh and a peak of 1.2MWh every day. We would like to provide a continuous base load of 800kWh 24/365 and have the grid supply the rest of their electric needs. What do we need to do to make sure our base load is always used by the main BUS? Do we need to make sure our waveform is synced with the grid and have our phase angle slightly ahead of the grid's phase angle? Is their a way to make sure our electric is primary and the grid secondary without modifying our customers electric panel?
Is the generator connected to the Customer's MAIN bus?

Or is the generator connected separately to the grid, and the Customer's MAIN bus is also connected separately to the grid?
We need to be connected to the customer's MAIN bus. We do not go through the grid's transmission lines, transformers or meter. We also cannot got back into the grid. They will not take our electric even if free. They do not like us! Coming out of our Cummins generator backend we will have shut off switch gear array, then a separate feed-in breaker going directly into the customer's Main bus. I was thinking that since our natural gas to electric generator is directly on-site our resistance will be less than the grid and thus be the first power to be used. If, for some reason the customer drops below our 800kW input our generator senses the load drop and matches the needed load. this way we do not push excess (export) electric back into the grid. Also, if we are synced then we will not have an over voltage which would be additionally needed to push back into the grid. I just want to know how the customer's MAIN bus knows to take our 800kW first then what is left (+/- 300kW) it gets from the grid. Or is something additional require to accomplish this feat?
The grid will supply whatever is not supplied by the generator in the configuration you describe. Your generator needs to operate in Droop speed control mode--because it will be paralleled with the grid, and as such you will need synchronization circuits and protection, as well as other generator protection (differential relays; overvoltage; reverse power; under-/over-frequency; overcurrent; etc.).

But there isn't any special control or "diode" or anything like that required; it will all happen "automatically."
Wow, that actually makes sense. We have had meetings with major electrical contractors and the major global alt/gen set supplier. Neither has been able to answer that question in a way I could understand. Thank you.

We will be starting commercialization (Phase III) in six months then manufacturing to follow. I would like to know further about your background. I believe my email address was included in the beginning post. If not let me know since I am a novice on this board. I would like to discuss a possible consulting relationship.


Michael Lee
here is what another company just relayed to me on this subject, do you agree?

Here is a little explanation on the generator control, and how it works when connected to a utility or another source.

Once the generator is connected to the utility the speed control (Governor) cannot change the frequency of the generator, it is locked to 60 Hz.
There should be a contact to the generator from the utility breaker so the generator controller knows when it is parallel with the utility.
When the generator controller see that it is paralleled it should go into a base load control mode.
In base load mode the generator controller will raise the speed bias to the (Governor) to achieve the base load set point which your case is 800 kW. This will add fuel to the (Governor), since the generator is locked to 60 Hz any added fuel will result in Power (kW).

Phase angle or Power factor (Reactive Power) of the generator is of course another control loop.
The voltage regulator must have a droop CT installed to for the generator parallel with another voltage source, like the utility.
Droop basically is a voltage vs load curve. The more power the generator is producing the lower the voltage from the regulator.
The voltage is also locked to the grid voltage when the generator is in parallel with the utility.
The Phase angle or Power factor (Reactive Power) is controlled by the controller raising the voltage set point to the regulator until the Power factor set point is achieved.
A dry contact must be installed short out the droop CT when it is not paralleled with the utility.

This will work fine unless you cannot export power. If the load decrease to less the 800 kW an you cannot export you will need another controller monitoring the utility power and commanding the generator controller to reduce its baseload set point .

I hope helps.

Nothing too glaringly wrong with this simple explanation. The thing to remember is synchronous generators are really pretty dumb--they just convert torque to amperes (in the same way that motors convert amperes to torque). It's the governor of the prime mover (the TMAT?) that has to have all the "brains" to be able to control speed (frequency) when disconnected from the grid, and to be able to stably control the amount of torque being produced and transmitted to the synchronous generator so the power being "produced" by the generator is also stable.

If you search for "droop speed control" you will find MANY threads about what it is and how it works. It goes back to the very beginnings of AC electrical power generation before there were transmitters and transducers and electronics--there was only speed. And most prime movers used some form of fly-ball governor. Well, amazingly enough, the electrical power generation system still uses that model today--though with lots more "intelligence." But, they still have to interact on the electrical grid with other synchronous generators and their prime movers--and droop speed control is how that's done. And, the generator doesn't care what mode the governor is in (Droop or Isochronous)--it just converts torque to amperes. Any torque provided that would try to make the generator spin faster than synchronous speed gets converted to amperes. If there's not enough torque to maintain synchronous speed the synchronous generator becomes a synchronous motor--because it's always going to spin at synchronous speed when synchronized (connected) to a grid with other prime movers and generators.

As far as how the synchronous generator is loaded and to what load setpoint it is loaded to, well, that is, again, handled by the prime mover governor. The rate at which the prime mover can be loaded is determined by the type of prime mover, and the torque it can provide (measured by the output of the synchronous generator) is limited by the prime mover construction and controlled by the governor, which is responsible for keeping the prime mover from destroying itself trying to produce too much torque, as well as stably controlling the energy flow-rate into the prime mover.

Droop speed control is basic proportional control, if that helps at all. The prime mover speed, once the generator is synchronized to the grid, is fixed by the frequency of the grid. When synchronizing the generator to the grid the prime mover speed reference is such that the generator frequency is equal to or slightly greater than grid frequency (say, 100.2% or 100.3%). Once the generator breaker closes, synchronizing the generator to the grid, the speed drops immediately to 100.0% (presuming the grid is at 60.00 Hz) and the difference--the 0.2% or 0.3%--which was driving the generator slightly faster than grid frequency, gets converted into amperes. And increasing the prime mover speed reference just increases the error between the speed reference and the actual speed--which increases the energy flow-rate into the prime mover which increases the torque produced by the prime mover. BUT, the synchronous generator and prime mover are locked into synchronous speed (100%) and they can't go any faster, so the generator converts the additional torque (resulting from increasing the prime mover speed reference) into amperes, which is load.

And, for reactive power control, it's very similar. Again, once the synchronous generator is synchronized to the grid, changing the excitation applied to the generator rotor will attempt to change the generator terminal voltage (it may, in fact, have a slight effect on the actual generator terminal voltage--but on large grids, not usually very much). The additional energy that's being applied to the generator rotor (the generator's "field") to try to increase the generator terminal voltage gets converted into reactive current--VArs, specifically lagging VArs (from the generator's perspective).

So, real power is controlled by the prime mover governor, and reactive power is controlled by the excitation system (the "exciter" or commonly called the "AVR" (Automatic Voltage Regulator)).