Heya all,

I'm hoping to do a PV based system for a uni project but i've got a couple of questions, including hooking up to the mains.

Here in the UK we can get solar cells put on the roof of our homes and sell the electricity produced back to the national grid. I'm hoping to make a system to monitor the electricity outputted to the grid (and hence how much money you will have earned).

I know that PV cells produce DC but the mains is AC. Using current systems, does the DC output get inverted to 230v AC or simply outputted as it is generated?

If it is converted to AC is there any issue with getting the phases of the mains and inverted sine waves to match up or can you output it without this concern?

Matt

 

It appears that you have answered your own questions.
Yes, the DC must be turned into AC.
Yes, the AC must be in phase with the grid.

 

Before discussing implementation, please keep in mind that any circuit connected to the mains, without proper electrical isolation (ie a transformer), is off-limits and its thread will be closed.

 

In order to hook it up to the national grid it will have to be inverted. I believe this is done in your home. But for the sake of monitoring usage you would have to separate what you are using from what "they" are using. You could monitor the power used my monitoring the current (the DC current prior to inversion).

 

Thankyou gentlemen,

haha don't worry, this is only going to be simulated or at a maximum made into a scale model on a lab bench, I'm not actually playing with the mains!

 

Sensing the output current should be quite easy. You need to monitor the current and voltage, and if the current is positive when the voltage is positive you are using power and in the other case you are producing power. Power factors could make this a bit more tricky but I think it should work anyway. You just need to integrate the instanteneous power to see which way the current goes.

 

This might help, wasn't exactly what I was looking for on Google, but describes what you want, gives you something to search more on.

http://www.grid-tie.com/

Apparently, grid-tie system are very popular, since there aren't batteries to buy, maintain, replace. No manual switching over, if you run out of stored energy.

 

There is another way of doing this without a conventional analog DC to AC inverter and that is to use digital control circuitry. Instead of using an inverter at line frequency (50Hz in your case), you use power hexfets that switch in phase with the grid. The voltage output of the hexfets are driven from D/A converters to match the mains sine wave or to produce the desired current into the grid.