DC Grids: A Thing of the Past or the Future?

[John Galt]

I live in the USA and I was wondering, if I plug a gasoline generator capable of generating power into the wall outlet in my home and generate more current than I'm using to power my house, will the excess current being generated force the meter to spin the opposite way in proportion to the to the excess current being generated by the generator?

What is the maximum amount of current and voltage that could flow from the house into the grid past the meter without damaging any system components between the generator and the grid?

Just scratching my noggin at this one after some reading.

 

[russ_watters]

You aren't allowed to do it without permission, but if you choose to generate power and sell it to the power company, the power company is generally required by law to buy it from you.

But since the current is a/c, I'd be surprised if the meter would spin backwards.

 

[Danger]

Russ, this isn't an area of expertise for me by any means, but I do know that the meter does reverse itself when you're feeding power into the grid rather than tapping it. I've read several different accounts of the situation, and all agreed. Usually, the amount generated just offsets part of the regular power bill, but someone with a good generator capacity actually ends up with a negative amount due.

 

[John Galt]

If you use AC in your house to power appliances, then why wouldn't AC be able to flow past the meter? Is there a rectifier between the grid and the meter and an inverter between the meter and household load?

 

[russ_watters]

Russ, this isn't an area of expertise for me by any means, but I do know that the meter does reverse itself when you're feeding power into the grid rather than tapping it. I've read several different accounts of the situation, and all agreed. Usually, the amount generated just offsets part of the regular power bill, but someone with a good generator capacity actually ends up with a negative amount due.

Ya know, I seem to remember hearing that before. I don't understand why that would be, though.

 

[russ_watters]

If you use AC in your house to power appliances, then why wouldn't AC be able to flow past the meter? Is there a rectifier between the grid and the meter and an inverter between the meter and household load?

I'm not following - all of the power we're talking about here is AC, and your electric meter is not in your house circuit, it uses induction to (essentially) passively measure the current.

 

[NoTime]

I'm not following - all of the power we're talking about here is AC, and your electric meter is not in your house circuit, it uses induction to (essentially) passively measure the current.

The meter is a watt meter.
Not a current meter.

 

[Manchot]

The meter is a watt meter.
Not a current meter.

Well, the amplitude of the voltage wave is fixed, so if you know the current, you know the power.

On a side note, suppose that you have a generator capable of generating a sine wave in phase with the power grid and with the same amplitude. How would you go about connecting it to the grid? Could you just plug it in?

 

[NoTime]

Well, the amplitude of the voltage wave is fixed, so if you know the current, you know the power.

No, you don't. You have to know the phase as well.

On a side note, suppose that you have a generator capable of generating a sine wave in phase with the power grid and with the same amplitude. How would you go about connecting it to the grid? Could you just plug it in?

It needs a slightly greater amplitued.

 

[russ_watters]

Yes, wattmeters have voltage taps. It wasn't relevant to what I was talking about.

 

[NoTime]

Well, you said you would be surprised if the meter would spin backwards.
So why would you be?
It has to when reactance is involved.

 

[TuviaDaCat]

Well, you said you would be surprised if the meter would spin backwards.
So why would you be?
It has to when reactance is involved.

there is not "opposite direction" to ac... its a sine wave, it changes its direction every 110 times in one second(here we are given 55 hrz).
though you could make a different phase, though that won't helf you anyway with that watt meter...

 

[Farsight]

This is an interesting thread.

It's got me thinking about negative energy, which reminds me of money - my debit is my bank's credit. And if my electricity bill is negative, that means I used negative energy. But there was no negative energy, just positive energy going from B to A instead of A to B. Only the energy seems as real as the money in my bank, which I can't touch or smell or see, regardless of whether I'm in debit or credit. Sure I can touch cash, but that's not really money, it's just a bit of paper that says its money. Money makes the world go round, but does it actually exist in its own right? And where does that leave energy? Hmmmn.

Sorry, I was rambling there. This wikipedia write-up of electricity meters seems pretty good:

http://en.wikipedia.org/wiki/Electricity_meter

 

[NoTime]

there is not "opposite direction" to ac... its a sine wave, it changes its direction every 110 times in one second(here we are given 55 hrz).
though you could make a different phase, though that won't helf you anyway with that watt meter...

While that is more or less true there remains a distinct difference between power source and power load.
When you have reactance what is the source and what is the load switch places. For a perfect reactance no power is consumed, but in the real world it is. Large users are charged for reactive loads.
In any event the old meters would run backwards, don't know it that's still true.

 

[Danger]

Conspiracy Theory Alert!
Power companies are shifting to new meters? Maybe ones that won't run backwards so they won't have to pay for the electricity that all of these young whippersnappers are backfeeding to them? :uhh:

 

[russ_watters]

Well, you said you would be surprised if the meter would spin backwards.
So why would you be?
It has to when reactance is involved.

How does a power meter know if the power factor of the load is lead or lag except to measure it? Would they assume it is always lag and flip the meter if it is backwards? Since the primary purpose of a meter is to measure real power (and real power is what makes it spin), why would reactive power change its direction? What happens if you have a capacitive load (rare, but it happens)?

Googling turns up mixed results. Some sites say that most existing meters will spin backwards, others say they need a special meter. Ie:

Can I use my existing meter to take advantage of net metering?
The standard kilowatt-hour meter used for residential customers will be replaced with a "net meter", which accurately registers the flow of electricity in either direction. This means the 'netting' process associated with net metering happens automatically-the meter spins forward (in the normal direction) when the consumer needs more electricity than is being produced, and spins backward when the consumer is producing more electricity than is needed in the house or building.

http://www.lipower.org/cei/solar.metering.html

 

[John Galt]

In California, solar power companies install mostly photovoltaic cells in many locations. A big marketing incentive for consumers to buy is that if you aren't using power and your cells are generating it, the meter spins backwards and the power company may even owe you $$ per kwh at market rate if you come out generating more power than you are accepting from the grid.

I do know that if this is to be done the power companies need to know the size and location of the installation so they can account for the extra electricity flowing into the grid as part of their load management operations. I know that licensed professionals are the only ones who can do this type of installation but I don't know if extra gear is added besides a line splice. I just thought somebody may know a definitive answer, but we all do live in different places with different legal codes and electric grid structure.

 

[NoTime]

How does a power meter know if the power factor of the load is lead or lag except to measure it? Would they assume it is always lag and flip the meter if it is backwards? Since the primary purpose of a meter is to measure real power (and real power is what makes it spin), why would reactive power change its direction? What happens if you have a capacitive load (rare, but it happens)?

Googling turns up mixed results. Some sites say that most existing meters will spin backwards, others say they need a special meter. Ie: http://www.lipower.org/cei/solar.metering.html

It's built just like a bench watt meter without the return spring. Two coils. One passes any current the other is acrost the two lines for voltage. If there is real power a force is generated in the disk that moves it.
If there is a reactance (lead or lag) there is always some resistance in the circuit that consumes real power. When the phase rotates the energy stored in the reactance feeds back onto the grid (also consuming real power) with the phasing such that the disk will move in the opposit direction.
Kind of a two step forward one step back kind of thing.
In the case of a bench meter the needle vibrates.
Oh! They probably don't have meters with pointers any more.

 

[denni89627]

I don't understand how you could simply feed power back into the grid. I understand AC as a sine wave but wouldn't your feed be impossible to phase correctly with the incoming feed? I would imagine fluctuations would turn your generator into something that sounds like a jet engine and then something really bad (or really cool if it's not yours) would happen.

 

[denni89627]

I just thought of something else, but awesome. I was talking to an electrician yesterday that told me something pretty interesting. The garment district in Manhattan still has many buildings fed off of the old DC grid set up at the turn of the century. They should be reading this thread!

 

[Danger]

DC grid?

 

[Slayer26M]

Well first of all if you plug a generator into a wall outlet you should shut off your main breaker. In the event of a power outage you'd end up backfeeding the city transformer, and energizing the primary side which could kill someone working on the line. Anything that you power up will only draw the current that it needs, if there is excess current from the generator it's because there are more devices up the line drawing it. I have seen a rotary meter run backwards on a 3 phase system after installing the CT's backwards. To answer manchot's question, no you cannot just plug it in, you'd need to vary the reactive power, and the frequency by adjusting the rpm of the generator, and using ossciliscopes and the 2 lamp method synchronizing your sine waves, then throw the switch when both sources are in phase at the same frequency, otherwise you get a loud bang when the generator tries to pull itself into synch.

 

[denni89627]

DC grid?

Crazy right. I actually did a little research and I found this:

"In January, 2005, Consolidated Edison announced that it would cut off DC service to its remaining 1600 customers (all in Manhattan) by the end of the year."

So I guess my electrician friend is a little outdaded, or this info I found on Wiki is wrong. It's still pretty wild that it was around up until a few months ago. Mainly because the old elevators couldn't run of AC I hear.