Small wind turbine in parallel with mains
Small wind turbine in parallel with mains
(OP)
May I bother you with a technical query slightly outside my capability? Consider a 1Kva wind turbine being fed on to a common single phase busbar also supplied by the grid via a standard 50Kva tx. Such a situation would be found in a typical domestic installation that has installed the micro wind turbine of the sort now widely available in retail stores. Lets suppose the load is resistive and rarely exceeds 1Kw for long periods. Can you advise if simple proportionality can be applied to assess the current drawn from the two sources (even as a rough guide) or is information required on the internal impedance of the respective sources? The kernel of my query lies in the claims made by the suppliers made about pay back period.
Regards,
Lyledunn






RE: Small wind turbine in parallel with mains
RE: Small wind turbine in parallel with mains
You could talk with one of the manufacturers of such products. Some units are provided with specialized electronic controllers that handle the interconnection and load balancing.
RE: Small wind turbine in parallel with mains
Regards,
Lyledunn
RE: Small wind turbine in parallel with mains
It is possible to install a tie controller that could regulate the turbine output to avoid net power export, but I suspect it won't catch it all.
Also, if you have an induction generator, you may have real power going in one direction and reactive power going in the other.
RE: Small wind turbine in parallel with mains
You should maybe consider running the wind generator into a battery bank and then running a large,(now relatively inexpensive), inverter out. Then you will decouple the turbine from the grid and gain some backup capability. Selectively run appropriate circuits in your house with circuit sized transfer switches.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Small wind turbine in parallel with mains
Regards,
Lyledunn
RE: Small wind turbine in parallel with mains
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Small wind turbine in parallel with mains
RE: Small wind turbine in parallel with mains
If the windmill is producing more than you are using, the power flows back to the utility, regardless of what your contract says.
RE: Small wind turbine in parallel with mains
We are going to run a small diesel generator in parallel with the grid.
First we run it up to rated speed, say 1800 rpm.
Then we match the voltage.
Then we watch the synchroscope and close the breaker at the correct time.
Now the governor/throttle is set to provide enough fuel to maintain the set at 1800 RPM.
The generator is not supplying or drawing any power.
However the generator is now locked to the utility power. If we reduce the fuel the engine will not slow down but the utility will supply enough power to keep the set running at 1800 rpm.
If we increase the fuel feed the set will not speed up but will supply power to the grid.
The power out of a generator in parallel with the grid depends only on the throttle setting. The more power the engine produces the more power will be supplied to the grid.
dpc mentioned phase angle. This is correct. The generator shaft moves a few degrees (or in some cases only part of a degree) ahead of the synchronous position, and when this is converted into electrical degrees, this is the phase angle.
When the set is being motored the generator shaft position is a little behind the synchronous position. When the set is producing power the generator shaft is a little ahead of the synchronous position.
Power out depends only on power in (plus losses).
The exact small number of degrees is academic and is only considered in the rare cases when someone wants to run two synchronous frequency convertors in parallel, run two generators on one shaft and synchronize them or something similar.
What does this mean to a wind generator?
The amount of power depends still on the power in. The harder the wind blows, the more power the wind generator will put out.
An induction generator does not lock on like a synchronous generator does but slips. Now instead of a few degrees of mechanical displacement we are talking about several RPM of mechanical slip. However, the phase displacement of the resulting power will be the similar few degrees of leading phase angle.
The harder the wind blows, the more power your wind machine will produce and a corespondingly less amount of power will flow through your power meter.
Similar to a diesel generator a wind turbine may be motored.
If the wind does not keep the wind turbine above synchronous speed, the utility will supply the power to keep it turning at near synchronous speed. As itsmoked pointed out, you now have a large fan that is quite expensive to run.
itsmoked makes some good points about other issues.
remember, the harder the wind blows the more power your set puts out.
The farther you pull the throttle on a diesel set, the more electrical power it puts out.
respectfully
RE: Small wind turbine in parallel with mains
RE: Small wind turbine in parallel with mains
This describes it in one sentence. If you don't use it in the house then your meter runs backwards. Then, if you use more power than your turbine produces your meter runs forwards again. You should never just unload a wind turbine so you can't just "keep" the power inside your house.
At that power level I take it you're talking about a permanent magnet DC windmill with a grid-tied inverter. That seems to be the standard in the 1kW size.
The biggest determinator in the payback period will be figuring out your on-site wind speeds and power outputs. You typically calculate the (power for that wind speed) times (# of hours of that wind speed per year) to get the kWh of production in a year at each speed. Do it for each wind speed from the cut-in speed to max output speed, ie 2-4m/s, 4-6m/s etc etc and then add up to get the total kWh per year. Subtract some losses for cables and the grid-tied inverter to get it more accurate. Multiply by the going utility rate and you get your $$ per year figure.
RE: Small wind turbine in parallel with mains
It depends on your local regulatory environment.
Some jurisdictions require a 'generator's license' just to be able to hook one up.
RE: Small wind turbine in parallel with mains
RE: Small wind turbine in parallel with mains
RE: Small wind turbine in parallel with mains
The surrounding system sets the general voltage level and frequency, but your generator will raise the local voltage level some. Most voltage control is very distributed, load tap changers on transformers and excitation controls on generators being run in voltage control mode - which a small generator run in parallel with the utility is not - with some supervisory control. Properly set up generator control systems allow generators to run in parallel with each other and in parallel with the outside world without any circulating current. Bad setup will have mismatched power flows and circulating or oscillating VAr flows, but it need not be that way.
RE: Small wind turbine in parallel with mains
When generators are run in parallel the governors control the speed and thus the frequency. A characteristic of governors called droop allows the generators to share the load even if the load is changing.
The excitation setting on one set determines how many VARs the set will produce and to a much smaller extent the system voltage. You adjust the excitation on one set to adjust the power factor of that individual set. You adjust the excitation of all sets to adjust the system voltage.
In the case of a small set feeding into the grid, the set will not be able to affect the voltage of the grid and the excitation adjustment will determine the power factor of the generator only.
Note, with a single generator, the load determines the power factor. The load will demand a number of watts and a number of VARs. The generator will have no choice but to supply both watts and VARs.
When generators are in parallel the watts may be supplied by one generator and the VARs may be supplied by the other generator.
A single generator must run at a power factor determened by the load.
Parallel generators may run at different power factors but the system power factor will still be determined by the load.
In a small plant with generators operated in parallel there is a circuit that is sometimes called a quadrature circuit that automatically balances the excitation so as to balance the VAR output of each governor.
To shift the watts from one generator to the other, adjust the power input by adjusting the governors.
To shift the VARs from one generator to the other adjust the excitation.
Measuring the Watts output of a generator is the accepted way of determining the load.
I agree completely with David's post. David has given a good technical explanation. I have tried to give an explanation from the perspective of the operator on the floor of a small generating plant.
I hope both explanations are helpful.
respectfully
RE: Small wind turbine in parallel with mains
Standalone: The governor will control speed directly and real power indirectly - it takes enough power to maintain speed. The excitation system will control voltage directly and reactive power indirectly - the generator will supply however much reactive power it needs to to maintain voltage.
Parallel with utility: The governor has no control over speed (close enough to true for this discussion) and directly controls the real power output. The excitation system has essentially no control of output voltage and directly controls the reactive power output/input.
Multiple small generators - standalone: Two independent controls balance real power (governors) and reactive power (excitation) using various techniques, droop is the most common.
Multiple small generators - parallel with utility: Same as single small generator parallel with utility.
Big utility power plant generators are run in a a mode very similar to the "multiple small generators - standalone." Here they can have some influence on system frequency (speed) and voltage, but a small generator (say under 20MW, but YMMV) paralleled to the utility will not move the voltage or the frequency.
RE: Small wind turbine in parallel with mains
I don't know much about the workings of the US power transmission grid, but it is my understanding that there are 3 "interconnects" and the frequency within the interconnect is the same at all points within that interconnect. If my understanding is correct, then to change the frequency, the generator would have to output enough power to change raise the frequency of the interconnect. What am I missing? Thanks.
Don
RE: Small wind turbine in parallel with mains