Connecting an Unsychronised Diesel Genset to the National Grid

this question may be off the mark a little but what are the parameters needed to force electricity back onto a grid whether using a generator or Solar/Wind Inverters. Does the voltage have to be higher than the grid ??? Thanks John

You actually have to excite the generator to increase voltage, but while connected to the grid, you cannot just increase the grid's voltage with a single generator as its simply too big, but what you actually do is that you take more load. Or supply more current, depending on how you want to look at it!!

Its that simple.

To Andy German

to be specific I am using this generator discussion to try to understand the EE principles necessary for the Solar Inverters or Wind Inverters to push the current back onto the grid. I have not been able to find a good explanation from the people that sell these Inverters. They use a PMM algorithm to optimize the Voltage Current from the solar panels but the is no explanation on this power is put back on the grid. Here is an except from a patent indicating it can be complicated

"Grid connected Ac inverters generally behave as a current source that injects a controlled AC wine wave current into the utility line. The controlled AC current is generated in sync with the observed utility zero crossings and may be exactly in phase, generating at unity power factor where upon real power only is exported. it is also possible to generate a variable amount out of phase-at other than unity power factor where upon real and reactive power is exported to the grid. An effective change in reactive power output can be made by either phase shifting the output current waveform with respect to voltage or by creating an asymmetric distortion to the output current wave form"

Am I making this too complicated??

Nope, it is complicated, but what do you want to know? CR4 will always try and help.

The electronics to put power back on the grid incorporate many safety locks, are complicated and expensive......Personally I feel that it is not worthwhile!!!

Supplying your own needs will save you a bundle.....but whether or not there is a payback in 5, 10 or even 20 years, is another question......I somehow doubt it!!!

You may also get agro from neighbors too with windmills and the like......

thanks Andy for the rating!

jemclau, the voltage should be the same, but the frequency should be "pushing" the grid frequency faster; earlier stated, that cannot happen, but trying would draw current, and the harder you push, the more current is fed into the grid, and the meter going faster backwards. Large, gridconnected windmills are started up with power from the grid, and the generator or auxillary motor gets the blades to rotate in the wind up to "grid speed" (the gearbox between blade and generator sets blades to rotate at say 20 RPM, generator 750 or 1500 RPM, 50 hz). Then the mill automatically connects in phase. When the wind blows faster than idle, that means to just overcome air-, gearbox- and bearing-resistance, the mill is not rotating faster (yes, a little due to iron-(eddy current) and copper-(resistance)losses), but trying to push the grid frequency faster. Thats why large windmills dont go faster than rated, despite the windspeed is increasing. They cant push the grid faster, because the grid is strongest. But more wind gives more "push" and therefor more current. It takes some sophisticated electronics to feed DC-current from solar and small windmills into the grid = expensive. And there is some loss in that conversion, but I dont know the precise efficiency-factor. My guess would be like a switched mode supply, say 94%? An AC-generator, like a motor, has an efficiency, I think, around 80-85 %?

regards, moe

Moe

My Danish friend I call Blue Tooth tells me that there is a lot of Wind Generation in the Land of the Vikings

I am specifically trying to understand how the Solar or Wind inverters can feed current back on the Grid. Here is an excerpt for a patent I am looking at that makes me think it is more complicated than the Inverters sales people know.

"Grid connected AS inverters generally behave as current source that injects a controlled AC sine wave current into the utility line. The controlled AC current is generated in sync with the observed utility zero crossings, and may be exactly in phase, generating at unity power factor where upon real power only is exported. It is also possible to generate a variable amount out of phase-at other than unity power factor where upon real and reactive power is exported to the grid. an effective change in reactive poser output can be made by either phase shifting the output current wave form with respect to voltage or by creating an asymmetric distortion to the output current waveform."

I quess with this info I am asking if there are controls on the Inverter that regulates the power going back to the grid?

Thanks John

Hi jemclau, I dont know if you're familiar with the term "power factor", but its (the short version) a term used to describe the angle of which the current is out of phase with voltage. At resistive loads, power factor is 1, V and A is in phase. However, when the load is reactive, and that can be both capacitive or inductive, the power factor goes lover than 1. If we connect a transformer to 50 hz AC, it has an inductive load+a resistive load, powerfactor 0,9. If we then put a capacitor across the transformer, the power factor becomes 1 again, that is if dimensions are right. The power companies likes resistive loads, because reactive loads increases current in the generator, making it hotter and you dont pay for the higher current x same voltage=more watt. They can specify loads to be better than, say "power factor 0,95".

The term "unity power factor" must be, when power factor is 1.

And the term "excitation" must be when genset is trying to go faster than grid, please help, Andy??

I dont think, there are controls on the inverter to regulate how much power is fed into the grid. That depends on DC voltage at the generating unit or battery. But there could be some sort of adjustment relative to max DC current, minimum DC voltage threshold and so on. And a grid-tie inverter automatically adjusts power out from input DC voltage.

The info you mention is for very specific grid loads specified by grid company.

regards moe

Generally an AC Generator has a small DC Generator to provide a DC voltage for the AC Rotor.......if you follow that!!!

By adjusting the amount of DC current flowing through the rotor, you increase or decrease the output voltage (excitation) or the AC Generator......

Though when an AC Generator is connected to the grid, increasing excitation does not increase voltage, the grid is too large for that, but it causes that Generator to supply more current into the grid.....that is how load is taken or shed.....

Moe thanks for the info Yes I know power factor well since here in the States the electric company charges you extra for a poor power factor and why they use capacitance in form of lighting and capacitors for boosting the power factor . However I did not know that the reactive load increased current in the generator. I will have look into this further. Back to my main question the solar and wind companies that sell inverters do not have any explanation of any technology needed to feed current back to the grid so it must be a transparent process that they easily take care of. One of the companies SMA has sent me a CD so I will see if they explain in the CD. Thanks again

I can't tell you the exact method any manufacturer uses, but here is the basics. A synchronising transformer is connected to the mains. The isolated extra low voltage signal is used to control the phasing of the inverter. It is not especially complicated and a similar method of phase control is used in larger DC packages to synchronise the switching of the power semis typically thyristors in that application. For the control of an inverter that is connected to mains, synchronisation is probably the simplest task. Zero crossing is easy to detect and a sample and hold indicates which side of zero the voltage has come from (and by default where it must go next) which takes care of the 180deg out of phase condition. Other considerations are current control, available exportable power, and supply failure, some sort of PID with multiple summing / error detectors is expected for this. Mains failure lockout in this application is as important as synchronisation because the low impedance of a de-energised grid is effectively a dead short.

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