switching between main AC and sine wave inverter

[GastonRafaelMelo]

Hi to all (again),
I desing a sine wave inverter and its working as suppose to do. Now I want to know how can I do to switch betwwen AC mains and the inverter output. As far as I can see is using AC relays and according to the current use. Is there other method? could you point me to some references?
Regards
GAstón

 

[BradtheRad]

Switching is simple if your generated power only stays in your house.

It's vital to arrange the relay so it never connects your generated power allowing it to go out to the grid... regardless whether a blackout is on. The reason is that it might electrocute linemen working on wires close by or distant.

There are people who sell their power to the electric company, however that immediately makes everything complicated.

 

[GastonRafaelMelo]

Thanks for your reply, using relay I must take care of the phase of my inverter and the grid difference? IF the grid is down I want to connect only vitals in my house, but I will like to use the grid to charge my batteries like the UPS but sadly I can't find any circuit of references.
thanks for your reply
Gastón

 

[KlausST]

Hi,

you talk about switching "vitals" from grid to inverter when grid is down?

If so:
1) When grid is down ... what phase do you have? ;-)
2) a relay is slow in switching ... thus you can´t "synchronize" it to 50Hz/60Hz phase. (in a meaningful and reliable way)

Klaus

 

[GastonRafaelMelo]

1) When grid is down ... what phase do you have? ;-)

OK. So you're saying it is not important to measure phase? What about in case you have reactive loads?
what about surge current in case of switching inverter back to main AC?

Thanks again for your help on this questions.
Gaston

 

[KlausST]

Hi,

I´m saying: If grid is OFF you can´t measure phase.

What about in case you have reactive loads?

Yes - What about it? What´s your concern?

Every time you swith ON an electrical device you do this without caring about phase.
Or do you care about phase when you switch ON a light bulb or a PC or a fan?
That´s the same that your relay does: switching from 0V (grid OFF) to 230V AC (inverter)

what about surge current in case of switching inverter back to main AC?

Again: What´s your concern? You switch from 230V to 230V.

As already mentioned: you never connect your inverter to the grid. Do you?

You have to care about phase when you connect a synchronous generator to the grid...but that´s a complete different story.

Klaus

 

[GastonRafaelMelo]

KlausST thanks again for your reply. So if I use a AC relay with coils connected to main AC, when the main AC is going down the contactor will switch to inverter automatically. I have to over dimension the current in the AC relays?
As far as i know the relay doesn't switch immediately (it takes a couple of ms to perform switch) and at that moment the electronics connected to the GRID is not OFF its working thats why i was asking about phase in case of reactive loads. How a UPS handle this?
The inverter is not going to be connected to the GRID, only power my house but I don't want to burn some sensitive electronics.
Thanks again KlausST
Gastón

 

[KlausST]

Hi,

Show a sketch. It´s not clear how you want to connect everything.

Klaus

 

[GastonRafaelMelo]

KlausST thanks. Something like this (attached picture below).

 

[KlausST]

Hi

I have to over dimension the current in the AC relays?

Why? It´s just a standard situation for the relay. Switching a load ON/OFF.

and at that moment the electronics connected to the GRID is not OFF its working

Focus on the relay contacts:
* Grid is OFF, grid voltage is zero.
* relay contact current is zero. If you don´t think so, please tell us. We don´t know the behaviour
* then it switches over to supply the load from the inverter. This is just a standard situation. Typical load inrush current (Indeed I´d rather expect less inrush current)
If you expect something different, please tell us.

How a UPS handle this?

Again: A standard situation. Load becomes connected to the inverter.

The inverter is not going to be connected to the GRID

Please be sure to use the correct rely to keep on local safety standards. In the EU you need some safety relays with "forced" contacts.

I don't want to burn some sensitive electronics.

Again: I just see a standard situation. Nothing special.

Klaus

 

[GastonRafaelMelo]

thanks Klaus
Gaston

 

[Easy peasy]

Disconnecting the mains from a load and then later connecting your inverter is simple, but,

swapping over fast so that appliances are not affected i.e. essentially do not see the changeover is much more complex

for the latter you need to synchronise the two waveforms and have the amplitudes approx the same too,

then you need a high speed break before make relay ( or SSR's ) to perform the switching

given the nature of you question the latter part may be outside your knowledge at the moment.

 

[BradtheRad]

like to use the grid to charge my batteries like the UPS

Charging 12v batteries can be done by an ordinary car battery charger. Often it has a switch to choose Manual (full Amperes) or Automatic (tapering charge followed by trickle charge).

To assemble my home backup power system, I purchased a combination inverter-and-charger. I did things manually at separate times, rather than hooking it all up for full-time automatic operation. Anyone would say my setup was haphazard.

Our main breaker panel had a robust changeover switch to disconnect mains. Everything I did required that I remember the correct steps to disconnect and reconnect things. There were times I was careless due to fatigue.

Does your building have a changeover switch? If not then you'll need to custom place extension cords to vital appliances.

 

[FvM]

Many low- and medium power systems, e.g. for computers are powering the load through the inverter instead of using of using a changeover switch. This avoids even minimal voltage drops and possible risk of switch shorts. Mains is either supplying the low voltage or high voltage DC bus. Power demand of your application hasn't been mentioned yet. A pump as shown in post #9 has usually no problems to accept a short interruption.