Reactive and Active Power grid tie

Re: Reactive and Active Power grid tie

quique wrote: »

Hi

How does reactive power affect anything when connecting a GridTied inverter/central to the grid?

1. The GTI will not in general produce any reactive power output. Its current output is by design exactly in phase with the applied voltage from the grid.
2. A generator or standalone inverter on the other hand will produce a (sinusoidal) voltage output and will try to deliver whatever current is necessary to maintain that intended voltage output.

Therefore if you have loads with a non-zero power factor, you cannot drive them solely from a GTI. You need to have another source (like the grid) connected to handle the reactive portion of the load.
So, if you have just enough GTI output to handle the real power of your load, the reactive portion of the current will come and go through your POCO meter, not totaling up any net power consumption.

There will be I²R losses in the wires corresponding to the reactive current, just as there would be without the GTI in the picture.

When you are dealing with an off grid PV system or generator, you have the opposite problem. All of current must come from the same source and so the reactive current will cause heating in the output circuitry of the standalone inverter or the windings of the generator, limiting the output power actually available for the real part of the load to less than the full nominal output of the power source. For a well designed inverter, the current drawn from the battery will not be increased too much by the reactive power requirement of the load though.

Re: Reactive power vs Active Power

Thank you Coot.
I got a little confusing as I was replying while you were merging.

Re: Reactive power vs Active Power

quique wrote: »

Yes I did, I was looking for how to erase it from here but couldn't find it, sorry.

And thanks for your answer but could I bother you for an "even simpler" explanation? If thats even possible

Thanks

A GTI cannot, by design, supply any Reactive VoltAmps (VAR).
So the VAR portion of the load must come from POCO.
The GTI will supply what it can of the real power (Watts) and together the two will come up with the apparent power (VA) which goes into the load.

I can try to clarify any particular subtopics of that if you like, but it may take awhile.

Re: Reactive power vs Active Power

quique wrote: »

Yes I did, I was looking for how to erase it from here but couldn't find it, sorry.

AFAIK you cannot delete the thread once anyone has replied, but you can delete the original post using the Edit function.
Go into Edit mode, find and hit the Delete button, then check the Delete radio button and hit the new Delete button that appeared when you first Deleted.
Simple but not necessarily intuitive.

Re: Reactive power vs Active Power

quique wrote: »

Inetdog,

So Im looking at a table of power consumption of a company and it reads:

KWARH = 105,258 (which would be Active Power, what you really need)
KVARH = 43,436 (which would be Reactive Power, the offset)
Demand = 565 (apparent Power, what you end up pulling)

Analogies of marbles, beers and horse-pulled cars are great but tying them into whats actually happening is a bit more complex.

Can you shed light on these numbers :-)

Well, those numbers do not make any sense to me at all unless they are putting both apples and oranges into the same table.

Based on my assumption that the first two numbers are energy consumed over some period of time (or a quantity with the same dimensional units where non-energy components like VAR come in) and the third number is a power consumption, for those users whose utility bill includes a per month charge based on the largest 15 minute consumption during that month. I would write it out as follows:

Total real power consumption per month = 105,258 kWh (kilowatt-hours = units of energy.)
Total Reactive power per month (which you do not get billed for at all) = 42,436 kVARh (same dimensional units as energy but does not represent any net energy at all)
Total apparent power consumption per month (again, not what you get billed for) = 148,694 kVAh [which is exactly the sum of the first two] This line was not in their table!
Peak power demand over the course of one month (which you may pay a per month fee for if a commercial user) = 565kW (not KVA, although some POCOs may charge for peak KVA rather than peak kW) Note that this is not directly related to any of the previous three numbers.

If I had to guess, based on the bizarre mix of units etc, I would say the table came from the advertising brochure of a company selling Power Factor Correction snake oil. Or else from someone who did not understand the concepts involved.

Re: Reactive power vs Active Power

boB wrote: »

There has been talk of this but is done on large scale GTI's now, already I believe.

The old Trace SW inverter was a voltage source GTI as well as the Outback GFX inverter...
They only produce low current distortion when driving a low distortion voltage grid source.

So, I think they will actually do this to a certain extent, unless something has changed in
past years.

It kind of makes sense that this should be an option because it can help to fix up the grid voltage
when it is ugly. It's just a matter of feedback technique in the GTI and SMOP (Simple Matter
Of Programming)

boB

Well, it may be a simple matter of programming once the GTI has the needed input information. To be able to supply reactive power as needed, the GTI would have to be interfaced to a current transformer which reads the current to local loads. Not the output current of the GTI or the current to/from POCO.

Kind of like not being able to control End Absorb current correctly unless you have a shunt on the battery.

Otherwise the best the programming would do is to try its best to "correct" any observed distortion in the sine wave voltage applied to its terminals. And that in turn would have to assume that were is not for local loads the voltage from POCO would be distortion free. If the POCO voltage with no local loads is distorted, the GTI will waste output current trying to bail out POCO and will not get paid anything for the pure reactive power component.
And even delivering the reactive component of the local load would increase the IR losses in the GTI output just to reduce a drain on POCO that you are not being charged for.

Utility scale GTI installations might well get paid a bonus by POCO for helping with the reactive power demand. Or at least might reduce the low power factor penalty charge that some commercial users pay for local loads' consumption.