PV systems

[studentspark]

Photo voltaic systems

Im not working on one ! just wondering how they work

Would some one please give me a simple break down of the wiring of a PV system at the AC side
Im unsure what is going on in the fuse board.

As I understand it power is generated at the panel
Goes through a DC isolator
Through an inverter
Through a generation meter
through ab AC isolator
Into the consumer unit - where the generated power can be used around the home
Out through the utility meter - where you put electricity 'back in'

Sorry for such basic questions but

How is the PV power wired into the CU

Reading about it I see it goes into a spare way on the consumer unit.

So there fore you have two live feed coming into the board

I don't get this bit

If you plug a generator in you need to use a change over switch. so you are not feeding the grid ?
How can you have any isolation on the supply side?

All I see on the wiring diagrams are isolators for maintenance.


I must have this wrong, or completely misunderstand it.

 

[DetlefSchmitz]

The generated power can be fed into a spare way in the consumer unit, but it much better to feed it (via an MCB) into a service block to the incomer, as this then allows separate monitoring of import and export (using clamps).

Feeding the grid is kind of the idea. You can get paid a small amount for doing this (about 3p a unit).

The inverter has the responsibility of generating a waveform in sync with the grid, and also to detect if the grid is switched off, and to turn itself off quickly if that happens.

 

[JohnW2]

The generated power can be fed into a spare way in the consumer unit ....

How would one do that? - it sounds a bit like a 'hacksaw job'!

Kind Regards, John

 

[DetlefSchmitz]

Mmm... not sure if it's just a joke I'm missing...

Anyway common practice is to feed backwards through an MCB in the consumer unit. I feel it's rather an ugly way to do it, but it is often convenient as it's there.

The other consieration is whether to RCD protect it. It takes a bit of mental contortion to work out what or who is being protected. I prefer not having an RCD (which seem prone to tripping in these cases), and ensuring that the cable back to the inverter is not shallowly buried in a wall.

 

[JohnW2]

Mmm... not sure if it's just a joke I'm missing...

No - just me being poor/slow in thinking

Anyway common practice is to feed backwards through an MCB in the consumer unit. I feel it's rather an ugly way to do it, but it is often convenient as it's there.

I was just being silly, not stopping to think that there is nothing 'directional' about an MCB (hence not twigging the 'backwards') - I was thinking that the feed would have to be into the normal 'supply side' (necessitating the 'hacksaw' on the L bus) with a cable then taking the normal 'load side' back to the L bus!! Not clear thinking at all - so my apologies! You can tell that I've never been involved with this PV stuff!

Kind Regards, John

 

[JohnW2]

The other consieration is whether to RCD protect it. It takes a bit of mental contortion to work out what or who is being protected. I prefer not having an RCD (which seem prone to tripping in these cases), and ensuring that the cable back to the inverter is not shallowly buried in a wall.

Continuing to try to reduce my ignorance concerning this PV stuff ... if I'm now thinking right, presumably the neutral of the inverter output would have to be connected to earth on the inverter side of any RCD for that RCD to serve a useful purpose - otherwise current through the N side of the RCD could not be different from that in the L side (if there were no other possible path back to the inverter's neutral). Is that what is, or would be, done - or am I still thinking totally wrong?!

Kind Regards, John

 

[DetlefSchmitz]

Don't worry - some of it is a bit counter intuitive.

 

[DetlefSchmitz]

Posting this a bit to fast for comfort as I should think first really.

Firstly no - i don't believe you would make such a connection to earth.
Secondly there are two situations - inverter supplies house, or (at night) house supplies inverter (may be real need for monitoring power, or may be just because that is the state at night with the inverter off).

Thinking about it I'm not sure - as I avoided an RCD altogether in my setup. You raise an interesting point.

 

[JohnW2]

Posting this a bit to fast for comfort as I should think first really. ... Firstly no - i don't believe you would make such a connection to earth.

Hmmmm. As I said, in the absence of a connection from the inverter N to earth on the inverter side of an RCD in the feed from the inverter, I can't see how (when inverter was supplying house) the L and N currents through the RCD could ever be different, hence the RCD could not trip.

Secondly there are two situations - inverter supplies house, or (at night) house supplies inverter (may be real need for monitoring power, or may be just because that is the state at night with the inverter off).

Ah, yes, it could/would be different if/when the inverter was a 'load' of the house supply. The need for a N-E connection required to facilitate RCD function would then be on the house side of the RCD, and such a connection would obviously exist somewhere (at the substation with TT or TN-S, at the cutout with TN-C-S).

Kind Regards, John

 

[GeoffTitch]

Mine goes through a ninvertor in the roof. From there it goes to the main board, where it has its own meter and CU (or something with a switch anyway)

 

[studentspark]

Many thanks

I kind off presumed MCB's were directional, but when you think about it, I guess the mechanism will pick up the over current or short circuit.

It all seems a bit rough though!

I am a bit suprised that feeding back on to the grid can be controlled by a bit of electronics, and the DNO don't mind that.

How quickly does this inverter disconnect?
I guess it will be seconds, but how does that effect disconnection times

Could you be in a position where the total load for the house, or for a circuit is on the PV side. though protected via the PV MCB? but disconnection will take longer due to the need for two reactions.

 

[JohnD]

Could you be in a position where the total load for the house is on the PV side. though protected via the PV MCB? but disconnection will take longer due to the need for two reactions.

If the load in the house was exactly equal to the PV supply, yes, it would be possible for the PV to meet all the house's demand. There is not a "PV side"

However, if mains supply is lost, the inverter shuts down, regardless of load.

I don't know what you mean by "two reactions"

 

[studentspark]

I'm not quite sure what I mean


I was wondering if you lost main supply - for a short period the installation would be only on the PV side.

Not sure what you mean by no PV side

 

[JohnD]

no, if you lose mains supply the inverter will turn itself off, so you will have no electricity.

Could you be in a position where the total load for the house, or for a circuit is on the PV side

I don't know what you mean by "on the PV side"

 

[JohnW2]

Could you be in a position where the total load for the house, or for a circuit is on the PV side

I don't know what you mean by "on the PV side"

Nor do I (for sure) but I suspected that what he was trying to say was along the lines of "...where the total load of the house, or of a circuit, could be satisfied by the PV-generated electricity".

Kind Regards, John