Solar Inverter Basics: How Power is Drawn and Distributed

[Wambat]

TL;DR

How does your House Appliances decide WHERE to draw their power from?

OK.. I understand about the sync. Not a problem, but...
My understanding is that the Inverter is wired in parallel with the incoming mains supply.
So what happens when you switch on your toaster? What decides WHERE the current is drawn from?
and assuming it comes from the Inverter, what decides that?

This has been puzzling me for ages, so thanks in advance for your learned replies :-)
Wambat

 

[Baluncore]

My understanding is that the Inverter is wired in parallel with the incoming mains supply.

The junction is made on your side of the meter. So it does not matter where the actual current comes from. If you are generating power at that time, it will come from your generator. Otherwise it would cancel the incoming current.

 

[Rive]

and assuming it comes from the Inverter, what decides that?

To push energy back into the 'line' your inverter will produce an output waveform with a bit higher voltage than the 'line'. So if you switch on a load then the load will 'pick' the source with the higher voltage - your inverter.
Well, more or less.

 

[Wambat]

To push energy back into the 'line' your inverter will produce an output waveform with a bit higher voltage than the 'line'. So if you switch on a load then the load will 'pick' the source with the higher voltage - your inverter.
Well, more or less.

Sounds like a fairy tale to me...
If I apply Ohms Law, your "bit higher voltage" isn't going to make ALL the current to the appliance come from the inverter.
Maybe I'm missing the point.. can you explain it a bit differently?

Regards,
Wambat

 

[Wambat]

The junction is made on your side of the meter. So it does not matter where the actual current comes from. If you are generating power at that time, it will come from your generator. Otherwise it would cancel the incoming current.

Yeh.. so the connection is in Parallel on your side of the meter.
So you said "it will come from your generator"... but WHY??
IF it was two identical DC Batteries.. ie Mains 1 battery and generator the other, the current would be shared equally.
Why is it not the case with AC??

Regards,
Wambat

 

[Rive]

If I apply Ohms Law, your "bit higher voltage" isn't going to make ALL the current to the appliance come from the inverter.

The inverter is designed to deliver a calculated amount of power to the 'line' and sets the output voltage difference accordingly (don't treat the inverter as a voltage source! It's a power source - the output power is, what will be constant). In such setup what will make up Ohm is the internal resistance of your load and the internal resistance of the 'line' (which is both a source and sink).

Try to draw it up and do some calculations. It'll work out nicely.

 

[anorlunda]

It is analogous to how the water "decides" on which side of the islands to flow. All the water doesn't flow on one side. It divides in proportion. The word "decide" is not the best choice in this context.

In that sense, some of the power from Niagara Falls reaches New York City via Miami, Florida. And some fraction of the power from Indian Point (just north of New York City), goes to other cities in other states. A small fraction to be sure, but nonzero.

By the way, to pick a nit: It isn't voltage difference in AC power, it is phase difference that drives power flow.

 

[Baluncore]

Maybe I'm missing the point.. can you explain it a bit differently?

There is an active node in your switchboard. Connected to that one terminal block is;
(1) your inverter;
(2) your side of the supply meter, and;
(3) all your loads.

Your loads will consume current.
Your inverter will generate some current.
The difference will be made up of current flowing through the meter to/from the grid.

You cannot follow the individual electrons.
You only know that the total of all currents on a node must sum to zero.

 

[Rive]

It isn't voltage difference in AC power, it is phase difference that drives power flow.

I do know that, but it makes way too difficult to explain something simple that way. Voltage will do more good at this point I think.

BTW rivers are also good

 

[Wambat]

There is an active node in your switchboard. Connected to that one terminal block is;
(1) your inverter;
(2) your side of the supply meter, and;
(3) all your loads.

Your loads will consume current.
Your inverter will generate some current.
The difference will be made up of current flowing through the meter to/from the grid.

You cannot follow the individual electrons.
You only know that the total of all currents on a node must sum to zero.

All connected to one Terminal block.. so in parallel.. OK.
"Your loads will consume current." Yes...
"Your inverter will generate some current." Well.. how about we say it will DELIVER the current required by the load.
"The difference" .. let's make it easy and say there is no "difference". Say it's a 2KW inverter and my LOAD is drawing 1KW. ... I'd be a bit cranky if ANY of the power came from the Mains! No point putting is solar if that was the case.
"You cannot follow the individual electrons." Why not?
"You only know that the total of all currents on a node must sum to zero." Sorry, I don't follow that?

 

[Wambat]

The inverter is designed to deliver a calculated amount of power to the 'line' and sets the output voltage difference accordingly (don't treat the inverter as a voltage source! It's a power source - the output power is, what will be constant). In such setup what will make up Ohm is the internal resistance of your load and the internal resistance of the 'line' (which is both a source and sink).

Try to draw it up and do some calculations. It'll work out nicely.

Sorry, I don't get any of that really...
Although the inverter is "Rated" is 2Kw, that's not constant at all. It's the load that determines the amount or current required.
I could "draw it up", but I would get that calculations very wrong :-(

 

[Rive]

Although the inverter is "Rated" is 2Kw, that's not constant at all. It's the load that determines the amount or current required.

In a classic solar inverter setup the inverter will 'try' to push out the maximal available power of the solar array (to the grid). There is no need for any (local) load to be present.

I could "draw it up", but I would get that calculations very wrong :-(

It'll make thing easier anyway. We can always point out what's wrong.

 

[Baluncore]

"You only know that the total of all currents on a node must sum to zero." Sorry, I don't follow that?

You must understand Kirchoff's current law before you twist my statements into confusion.
https://en.wikipedia.org/wiki/Kirchhoff's_circuit_laws#Kirchhoff's_current_law

 

[Wambat]

You must understand Kirchoff's current law before you twist my statements into confusion.
https://en.wikipedia.org/wiki/Kirchhoff's_circuit_laws#Kirchhoff's_current_law

Oh.. yes.. sorry.. Kirchhoff's Law stating the obvious... I mean.. well, the "node" would be consuming power otherwise wouldn't it.

 

[Wambat]

In a classic solar inverter setup the inverter will 'try' to push out the maximal available power of the solar array (to the grid). There is no need for any (local) load to be present.It'll make thing easier anyway. We can always point out what's wrong.

HEY! Now this is interesting... So the Inverter is running "flat out" all the time (when the sun is shining). What is not used by the "house" goes into the grid anyway.
I'll need to sleep on this.

 

[Baluncore]

All connected to one Terminal block.. so in parallel.. OK.
"Your loads will consume current." Yes...
"Your inverter will generate some current." Well.. how about we say it will DELIVER the current required by the load.

NO.
Your inverter will always deliver a current determined by solar illumination, and that current is quite independent of the current drawn by your load.

The load current is determined by what your house demands at that time, and that demand is quite independent of solar illumination, or current delivered by the inverter.

If the difference of those two independent currents was not made up exactly by the grid, through the meter, then where would the unbalanced charges go?

 

[Rive]

. So the Inverter is running "flat out" all the time

Just depending on the irradiation. You might find it working out with a dozen or so watts from a full moon

 

[sophiecentaur]

IF it was two identical DC Batteries.. ie Mains 1 battery and generator the other, the current would be shared equally.
Why is it not the case with AC??

If you take two non-identical batteries and connect them in parallel to feed a load, one of the batteries can easily be supplying virtually all the current to the load and, after the volts have adjusted themselves, it may even charge the other battery.

There is no essential difference when discussing Ac, although the process can be described differently. Two AC sources are nearly always not identical and the same thing can apply - particularly when the frequencies are not exactly the same. (Only when connected to the same turbine shaft and with identical windings and cores. If the electrics are not identical you can get current perpetually sloshing from one to the other and back again) When there is a phase difference, the effect will also be of current flowing from the source with the instantaneously higher V_modulusand will push (or pull) current in and out of the other source. For a small phase difference, the volts across the load will be fractionally lower (Cosine of phase angle) When the frequencies are different, the leading source will be dragging the other along by pouring power into it and, ultimately, driving the other alternator around.

 

[sophiecentaur]

HEY! Now this is interesting... So the Inverter is running "flat out" all the time (when the sun is shining). What is not used by the "house" goes into the grid anyway.
I'll need to sleep on this.

An analogy. (Not the usual naff water analogy)
Imagine you lived somewhere where it rains a lot and you store the water in a small tank for home use (or even no tank at all). Also, you have a pipe to the local water company and that (unrealistically) they have a massive tank at exactly the same altitude as your tank. You mostly get water from your tank and have enough left over to flow down the pipe to their massive tank and they pay you per litre. When the rain stops for long enough, your tank empties quickly and water stops flowing to the water company; water flows from them to you. The water company tank is maintained at the standard level to allow this.

Sometimes, you could be taking a shower when it's raining slightly and some water would be coming from your local rain and some from the water company. So even then, you wouldn't be paying for all your shower water

To make this system work well, you'd need taps and pumps to control the levels and that's the equivalent of what happens with your solar electric system to give or take mains power. It can finely control the Volts from your home system so that you can top up from or dump current into the mains, as appropriate. It has to be smart to do this. Most domestic systems do not store any electric power (unless you live remotely`)

 

[Wambat]

NO.
Your inverter will always deliver a current determined by solar illumination, and that current is quite independent of the current drawn by your load.

Sorry, I don't get this... If there was no Load, there would be no current, regardless of how much sunshine there is. Yes?

 

[Wambat]

Sometimes, you could be taking a shower when it's raining slightly and some water would be coming from your local rain and some from the water company. So even then, you wouldn't be paying for all your shower water

Hmmm... I would expect ALL the water to come from my supply until it run out. Why should I be paying for water when I have Free water in my tank?

 

[berkeman]

Sorry, I don't get this... If there was no Load, there would be no current, regardless of how much sunshine there is. Yes?

It seems that you already answered this question...

HEY! Now this is interesting... So the Inverter is running "flat out" all the time (when the sun is shining). What is not used by the "house" goes into the grid anyway.
I'll need to sleep on this.

 

[hutchphd]

It seems that you already answered this question...

As did @Baluncore!

If the difference of those two independent currents was not made up exactly by the grid, through the meter, then where would the unbalanced charges go?

 

[sophiecentaur]

Hmmm... I would expect ALL the water to come from my supply until it run out. Why should I be paying for water when I have Free water in my tank?

It's up to you. If you want to restrict you rate of use of water to what's available from the sky at anyone time (or more than your small buffer tank can hold) then you can choose to buy it from the company. You only have 'so much' free water in your tank and in a dry period, you could need a pretty vast tank. That's basic Engineering philosophy for you.

In the case of Electrical Energy, the cost of a lot of storage capacity has to be compared with the low cost of buying it from the mains (they are doing your storing for you). It's the overall deal that needs to be considered but the principle of sharing your local resource with the national resource is identical with Electricity and Water.