Feeding solar power back into municipal grid: Issues and finger-pointing

If you apply more volts to a line than what it is carrying what do you think happens? I run machines that use regenerative braking. They draw energy from the line to set things in motion. To slow or stop them the electric motor acts as a generator producing a higher voltage than the grid, forcing power back into the grid. An inverter can do the same thing using solid state circuits. The inverter in my Prius takes DC current from the battery and converts it to whatever voltage and frequency is needed at the time to run the variable frequency AC motor. When slowing down the motor becomes an AC generator and the inverter converts the output to a DC voltage just a bit higher than the battery, pumping charge back into it.

On 4/10/2011 10:02 PM Bruce Richmond spake thus:

Sorry, I don't think you know what you're talking about.

You seem to think that you can "force" or push "voltage" into a line, by using a higher voltage than what's on the line.

That's not at all what's at work here when one has a photovoltaic system and an intertie feeding power back into "the grid".

The intertie and the house's power connection are going to be at pretty much exactly the same voltage. What happens is that the PV system is connected *in parallel* with the grid; it's dumping more *current* into the system, not more voltage.

You do understand the difference between current and voltage, don't you?

David Nebenzahl wrote in news:4da29d48$0$26573 $ snipped-for-privacy@news.adtechcomputers.com:

or

Under the terms of the Ontario microFIT program, you are paid 80 cents per kwh for any electricity your project generates and "makes available to the grid".

For about the first year of the existance of the program, a "behind the meter" connection was allowable, but at some point last year, Measurement Canada (a federal gov't department tasked with regulating commercial scales and other forms of measurement devices) published some sort of guide or position paper stating their disapproval of this method.

"Behind the meter" meant that your house retained the same single hookup to the power mains lines (ie - the grid) and the primary meter be capable of bi-directional current measurement. The meter measuring the power output of your PV system (which also had to be bi-directional) could be connected to the grid through your meter. This is also known as a "series" connection.

Under the new(er) rules, your revenue meter must make a parallel connection to the grid (in parallel with your load meter). One result of this is that you will usually be billed an extra $5 or $10 a month for having a second service connection to the mains grid - even if it's the same physical wires carrying both services to your home.

As a load customer, you are billed based on what your primary meter is reading. As a power generator, you are paid for what your PV meter says you delivered to the grid. This is (and was) the case regardless how the revenue meter was connected.

The whole point of the microFIT program is to encourage home owners to fork out the estimated $35k to $55k to put up 3kw to 10kw PV system on their roof and contractually garantee them a rate of 80 cents per kwh for 20 years. You need approval all up and down several layers of burocracy to get your revenue meter plugged in (the last step of the process) before you start getting paid.

Alternatively, there is nothing stopping you from installing panels on your own home and basically hooking everything up exactly the same way as you would under the microFIT program, except there is no revenue meter. Your load bill would be the net energy you pulled from the grid. The payback you would get from your investment would therefore take much longer.

Unless your invertors were set to operate at a slightly higher output voltage. Even just a few volts differential between the mains voltage and the invertor output would mean that you could push current out into the grid, and by doing that raise the local grid voltage slightly.

This explains a lot about inverter technology, though not whether they use a higher voltage, a leading phase angle or both to force power into the line:

On 4/11/2011 3:57 AM Han spake thus:

1. So what in the world does that have to do with the point I stated? (Rhetorical question. Answer: nothing.)
2. Y'know, if you used a non-brain-damaged news client that didn't mangle long URLs (unlike your Xnews), you wouldn't have to dick around with those tinyurls.

David Nebenzahl wrote in news:4da349f9$0$12552$ snipped-for-privacy@news.adtechcomputers.com:

I was agreeing with your statement of parallel systems, and offered a picture to sort of substantiate.

Sorry, I'm staying with Xnews for a while longer.

Have a wonderful day, David ...

Only problem with that is that many home service panels use breakers with an AIR of only 10kA, not 100kA. (my old house, built in 2000 was

Here's are some modern service panels that come with 10k AIR breakers.

The utility that is being ultra-conservative may have to consider that older homes in their service area may not even support this.

Can you just imagine the hue and cry when some homeowners are told they have to spend a couple hundred bucks to upgrade their service panel because of changes in the utility's distribution?

daestrom

I love how you can state that without any sort of qualification...

You seem to have forgotten about higher end bathroom fans which use an inverter to operate a DC motor which is much quieter than an AC motor...

AND

Light Rail vehicles are operated on DC systems which drive HUGE traction motors with either an overhead wire or third rail using the other rails as a one-way return path through the vehicle chassis...

This is 600VDC and up, and WILL kill you if you make any mistakes around it...

Those two quite common examples seem to refute your absolute determination that ALL rotating electric machinery is operated with AC motors...

~~ Evan

Those two quite common examples seem to refute your absolute determination that ALL rotating electric machinery is operated with AC motors...

How so?

Vaughn

On 4/11/2011 2:42 PM Evan spake thus:

... especially when it's pure BS. But that's our Harry.

Welll, since this is a.h.r, and since you're picking a nit, let me pick yours. I've installed several "higher-end" vent fans (Panasonic), all of which use AC induction motors which are very quiet. Which bath fans use the setup you described? (Besides which, why in the world would you need an "inverter" to run a DC motor from an AC supply? Perhaps you meant "rectifier"?)

To "push" power back into the pipe the generator's phase leads the line's, so in a sense the generator's voltage is greater than the lines (as someone pointed out Kirchhoff didn't lie).

Look deeper in the motor. It's all AC on the inside. ;-)

Exactly!

Vaughn

Here's are some modern service panels that come with 10k AIR breakers.

The utility that is being ultra-conservative may have to consider that older homes in their service area may not even support this.

Can you just imagine the hue and cry when some homeowners are told they have to spend a couple hundred bucks to upgrade their service panel because of changes in the utility's distribution?

daestrom

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Well that situation would be unfortunate and impossible to regulate as `legal, not conforming`

This is not a problem here as 10kA hasn`t been passed for many years. I believe any Canuck panels have to to have the class `R`or rejection fuse holders so that only the nasty electricians can force an old `code` fuse into the holder. The 100kA fuses have been promoted for a few decades with the seriousness getting more severe in later years. I thought they were actually not allowed, here, anymore. This may be incorrect. More research would be required to verify.

Either way the 10kA doesn't take much more impedance to drop it down with a few added factors mentioned in my previous text. Most of our service feeds are (ACSR) aluminum conductor, steel reinforced, and present higher impedances. I don`t see some small cogen circuit at the end of a few hundred feet of grid (mine plus yours) being a fault capacity concern in a residenial environment.

Having said that I guy up the street is just finishing installing 200kW or more of PV panels. Wait until they produce nothing all winter as they were snowed in most of last winter being at a low slope. The hook up wasn't`t completed so he won`t find out until next winter...LOL See how much harmonic crap we get on the street when they go online.

mike

I think you are pushing it....the brushes on a dc motor "guide" the dc to different windings.....it is still dc...

In an ac motor the windings are generally in parallel...all the ac is applied at one time....

I think I got that right....is a long time since I covered motor theory.....the ac is not "chopped up"....or guided anywhere....

You could say that all electric motors operate the same....as they all depend on magnetism (all generally used motors...there are some operate on static electricity, etc) have fun...sno

Our regional and municipal electricity distributors are not pointing the finger at the capability or specs of residential service panels or neighborhood distribution / stepdown transformers as the reason why they won't let small-scale (less than 10kw) roof-top PV systems to connect to the grid.

They are saying that the local sub-station doesn't have the "capability" to allow for a handful (or perhaps even a single) small-scale PV systems to be hooked up and they would need to "upgrade" the sub-station in some way.

For more about this, see here:

- our electrical service runs underground - not on poles in our neighborhood).

There may not yet even be a single residential PV system that's been connected to the grid for the area being served by the sub-station in question.

Here's the problem:

Many of the load devices you find in a typical home (primarily electric motors that run cooling systems, air conditioners, fridges and freezers) are not capable of regulating their input voltage.

So when a secondary electricity source comes on-line (like a small PV system) then in order to push it's current into the local grid it will have to *try* to raise it's output voltage in order to see some current flow. It might only be a few volts, maybe less.

But does that mean there will be a measurable net reduction in the current being supplied by the high-voltage substation for that corner of the city?

Not if your typical load device in homes surround the PV system will simply operate at a higher wattage.

The only sort of load that can effectively be regulated by a slight increase in local grid voltage are electric heaters. When you raise their input voltage slightly, they will put out more BTU of heat, and if their heat output set-point doesn't change, then their operational duty cycle will change slightly.

But in the case of an AC compressor, the fact that it might be getting a slightly higher input voltage because a neighboring house is feeding PV power into the local grid won't mean that the AC compressor will reduce it's current consumption from the municipal utility supplier because of the extra current coming from a neighbor's roof-top solar array. It just means the motor will use BOTH sources of current and (I suppose) run a little hotter but in the end not do any extra cooling work in the process (it's rotational speed won't change).

Same theory would hold true for lighting (incandescent especially). If you raise the input voltage, you'll get more light output - the bulb will simply consume all the juice it would normally get from the utility in addition to that being supplied by the neighborhood PV system.

The only way that a neighborhood PV system can actually suppliment municipal utility power is when the PV system is wired up as a dedicated sole supply source for a few select branch circuits. The way I see it, you have to feed certain select loads 100% from a PV system (ie - disconnect them from the municipal energy source) if you're going to make a meaningful contribution to the supply-side of a municipal or city-wide grid.