Sending power back onto grid...?

nsaspook

Re: Sending power back onto grid...?

ggunn wrote: »

Nope right back atcha. Did you miss the subjunctive in my statement? I said if there were zero resistance there would be no voltage rise, but the current would still flow. Heck, connect the inverter directly onto the output lugs of the medium to low voltage transformer, the current will still flow into the grid (many feed in tariff systems are interconnected that way). In dealing with a voltage source, the magnitude of the current flow is dependent on the magnitude of the voltage differential, but with a current source, the magnitude of the voltage differential (if there is any) has virtually no effect on the amount of current. The water analogy is just an analogy. It's water, not electricity, and the analogy does not hold up in every way of looking at it.

We have had this argument before, and we are just going to have to agree to disagree.

I also have to agree with you.
Looking at current as the source of power for a circuit can cause this confusion. The current flow is a function of the electrons in the wiring but that current flow is really a proxy for the energy contained the the EM fields around the wire. The bottom line is there is no requirement for a voltage differential for power to flow from a current source into a voltage source. When we see a voltage rise from feeding power into a circuit from a current source grid-inverter it's the result of the reduction of I/R line losses due the the load resistance decreasing at that node from a non-ideal voltage source (grid).

http://forums.mikeholt.com/showthread.php?t=137089&page=2&

NorthGuy

Re: Sending power back onto grid...?

westbranch wrote: »

BB, all 3 are only US viewable??? the last one took me back to HULU???
add Dailymotion to the list too

I guess Canadian satellite TV companies are doing their "lobbying" well.

boB

Re: Sending power back onto grid...?

If your wire is a superconductor, will a current source sell electricity back even if that current source has 0.0 volts of compliance ?

BB.

Re: Sending power back onto grid...?

Here is a web page that explains the skit:

http://timberry.bplans.com/2008/09/dessert-topping.html

Hopefully, that is not blocked in Canada too. :roll:

-Bill

inetdog

Re: Sending power back onto grid...?

BB. wrote: »

Here is a web page that explains the skit:

http://timberry.bplans.com/2008/09/dessert-topping.html

Hopefully, that is not blocked in Canada too. :roll:

-Bill

I think it was a spoof on the beer commercials in which people would argue whether the beer was good because of low calories or great taste.

Nope, that was not it at all, IMHO.

Who remembers:

"Certs is a candy mint!"
"Certs is a breath mint!"

ggunn

Re: Sending power back onto grid...?

boB wrote: »

If your wire is a superconductor, will a current source sell electricity back even if that current source has 0.0 volts of compliance ?

If by "compliance" you mean a differential between the voltage at the terminals of the inverter and and the voltage at the service entrance, then yes, of course it will.

If you plot current delivered by a GT inverter against voltage differential due to voltage drop in the conductors, the plot will not pass through the origin. In fact, the slope of the line will be in the opposite direction; the more the differential, the less the current will be because the power that the inverter delivers, in Watts, is voltage times current. If the voltage rise at the terminals of the inverter increases, the current decreases commensurately.

Cariboocoot

Re: Sending power back onto grid...?

boB wrote: »

If your wire is a superconductor, will a current source sell electricity back even if that current source has 0.0 volts of compliance ?

Since resistance exists at the atomic level, no. Pretty hard to get those electrons to overcome the weak atomic force without applying a little pressure (Voltage).

Few people realize that if you probe any two points in a conductor with a sensitive enough meter you will read a Voltage. Electrical power is Voltage and current and does not exist without both.

ggunn

Re: Sending power back onto grid...?

Cariboocoot wrote: »

Since resistance exists at the atomic level, no. Pretty hard to get those electrons to overcome the weak atomic force without applying a little pressure (Voltage).

Few people realize that if you probe any two points in a conductor with a sensitive enough meter you will read a Voltage. Electrical power is Voltage and current and does not exist without both.

I guess that even fewer people realize that in superconductors (which actually do exist) current flows freely with no voltage drop.

Cariboocoot

Re: Sending power back onto grid...?

ggunn wrote: »

I guess that even fewer people realize that in superconductors (which actually do exist) current flows freely with no voltage drop.

Well as soon as superconductors get used in the real world they'll know.

ggunn

Re: Sending power back onto grid...?

Cariboocoot wrote: »

Well as soon as superconductors get used in the real world they'll know.

Superconductors exist and we already know. Whether or not they are in field applications or not is irrelevant.

An electric field caused by a voltage gradient in a conductor will cause electrons to move, which results in an electric current, it is true.

But it is also true that electrons moving through a resistive conductor will generate an electric field resulting in a voltage gradient.

As I said before, if you increase the resistance in the conductors from a GT inverter to an interconnection point, the voltage differential goes up, but the current from the inverter goes down. If it is the voltage differential that is causing the current to flow, how do you account for that? You can plot the current from an inverter as a function of voltage rise and extrapolate what the current flow would be for zero voltage differential (i.e., in a superconductor), and it's not zero; in fact the Y axis intercept is the maximum.

Cariboocoot

Re: Sending power back onto grid...?

Well I can't wait to go to the electrical supply store and buy some superconducting wire. Except none of us will live so long. Therefor it is irrelevant to the discussion.

I think you're confusing Voltage to current conversion with Voltage differential. As with PV's a GTI's Voltage potential is higher than its output needs to be for maximum power. Potential and actual or operating Voltage is not the same thing; Potential Voltage always goes down when work is done with the power available.

If you connect an AAA battery in parallel with an AA battery and the Voltages should, by some miracle, be the same in both batteries no current will flow between them. If the AA battery has a Voltage only a tiny percentage higher than the AAA then a percentage of the maximum current will flow. If the AA is replaced with a D cell and the percentages are the same then the amount of current flowing will be higher.

What you have described is the same as you get when an external Voltage pulls a PV above its Vmp point; the current decreases. A GTI is specifically designed to match a small range of expected Voltage on its output. As this Voltage goes up (for whatever reason) the current must go down because the power input from the panels would be fixed (except for the insolation variances of course).

If you can give a real world example of your current flow with zero Voltage differential by all means do so.

Cariboocoot

Re: Sending power back onto grid...?

*face palm smack*

Wait a minute ggunn. The smug remark by a certain engineer just got me wondering if someone was enjoying our discussion at our expense and why that might be.

Dumb old me should have realized this earlier. Man I must be losing a lot of brain cells in my old age.

Connect current source (which allows Voltage to vary while trying to maintain current) to Voltage source (which allows current to vary while trying to maintain current) and you get the Voltage source fixing the Voltage for both; essentially setting the Vmp for the current source. Since the Voltage source can allow any variation in current, the current source can feed all its got to it.

That's why he said we're both right.

Darn analogies let me down. At least I wasn't trying to compare it to water (think about it: really falls apart).

I apologize to you, ggunn, and to everyone else for dragging this out so long when I should have been able to explain it right in one go. Probably still haven't.

Anyway, you see what the situation is? You need the Voltage difference because of the real world wiring resistance. If you could eliminate that resistance as in your theoretical example, the difference between the two power sources would be current/Voltage (instead of Voltage/Voltage or current/current) which allows the electrons to flow.

boB is probably now laughing his head off. Oh well; at least I can amuse even if I can't inform.

boB

Re: Sending power back onto grid...?

Cariboocoot wrote: »

boB is probably now laughing his head off. Oh well; at least I can amuse even if I can't inform.

No, just enjoying it. You see, when everyone has some truth to their argument, I have to stay out of it !

Now I'm going to lick my floor !

boB

ggunn

Re: Sending power back onto grid...?

Cariboocoot wrote: »

Connect current source (which allows Voltage to vary while trying to maintain current) to Voltage source (which allows current to vary while trying to maintain current) and you get the Voltage source fixing the Voltage for both; essentially setting the Vmp for the current source. Since the Voltage source can allow any variation in current, the current source can feed all its got to it.

Perzactly.

The grid is effectively a voltage source and a GT inverter is a current source.

That's why grid administrators with lots of wind and/or solar on their lines have an added headache; in addition to loading considerations they have to constantly adjust the amount of generating capacity they have online to compensate for the RE components' varying output due to weather conditions.

niel

Re: Sending power back onto grid...?

ggunn wrote: »

Perzactly.

The grid is effectively a voltage source and a GT inverter is a current source.

That's why grid administrators with lots of wind and/or solar on their lines have an added headache; in addition to loading considerations they have to constantly adjust the amount of generating capacity they have online to compensate for the RE components' varying output due to weather conditions.

that would be no different than constantly compensating for wide variances in the loads from the customers. as far as the generation point goes it does not know if there was a megawatt of loads being provided for by re sources as it only sees the net result of the draw on their own generating systems.

nsaspook

Re: Sending power back onto grid...?

niel wrote: »

that would be no different than constantly compensating for wide variances in the loads from the customers. as far as the generation point goes it does not know if there was a megawatt of loads being provided for by re sources as it only sees the net result of the draw on their own generating systems.

The difference is in the amount of energy stored in each power generation source. For large spinning generators there is a huge amount of inerial energy stored in the mass of the generator that can buffer wide varation in loads with little change in voltage as the sudden energy draw changes the rotational output frequency slowly as the governor corrects. Solar and most wind generators have very little inerial energy (under full power) to provide for grid frequency/voltage source stability so you need to have spinning reserves or some kind of energy storage online to match almost every watt they generate when RE weather conditions are changing quickly.

http://www.nrel.gov/electricity/transmission/frequency_response.html

ggunn

Re: Sending power back onto grid...?

niel wrote: »

that would be no different than constantly compensating for wide variances in the loads from the customers. as far as the generation point goes it does not know if there was a megawatt of loads being provided for by re sources as it only sees the net result of the draw on their own generating systems.

Yes, that's why I said in addition to loading considerations. But it is different; load variances tend to be more predictable, and individual loads turning on and off are rarely as impactful as a big solar or wind resource starting up or shutting down. At a Solar Austin meeting a couple of years ago a rep from ERCOT (the grid here in Texas) spoke at length about all the sophisticated computer controls they have to use to balance the grid due to the wind farms out in West Texas and all the PV that is coming on line. It's not a trivial task.

inetdog

Re: Sending power back onto grid...?

ggunn wrote: »

... all the sophisticated computer controls they have to use to balance the grid due to the wind farms out in West Texas and all the PV that is coming on line. It's not a trivial task.

As in many areas, diversity helps. If wind and PV resources are spread over a wide area, the chances of them all fluctuating so closely in time that new sources cannot be spun up in time are decreased.
But since Texas is its own separate Grid, it has less flexibility to achieve that kind of diversity.

Of course Texas does have a couple of DC interties, but I suspect that they are somewhat contractually limited in terms of sudden changes in demand or supply.

ggunn

Re: Sending power back onto grid...?

inetdog wrote: »

As in many areas, diversity helps. If wind and PV resources are spread over a wide area, the chances of them all fluctuating so closely in time that new sources cannot be spun up in time are decreased.
But since Texas is its own separate Grid, it has less flexibility to achieve that kind of diversity.

Of course Texas does have a couple of DC interties, but I suspect that they are somewhat contractually limited in terms of sudden changes in demand or supply.

All true. The rep who spoke to us was mainly talking about the challenges of dealing with the wind farms out in West Texas, which are all in the same area and tend to go online and offline more or less together. When a blue norther sweeps in from Colorado, the grid has to be ready for it.