So I'm working on a University project regarding a Solar System and my part is to design a circuit that can switch automatically between 2 Power Sources:
Power coming from the Utility Grid (120 AC)
Power coming from the Solar System (From the Output of DC/AC Inverter [120 AC])
The main objective of my part is:
1.* Use the Solar System as a backup power.
2.* Automatically switch to the Solar System in case of any Utility Grid Outage.
3.* Automatically switch back to the Utility Grid when it gets restored.
In order to achieve that, I am using an Arduino UNO R3 along with a 4 Channel DC 5V Relay Module.
I have designed a Rectifier Circuit (Please see the attached Figure) so that the Arduino could measure the 120 AC Voltage. A transformer is used to step down the 120 VAC to 6 VAC which goes through the Rectifier Circuit. Using a Variable Resistor, the voltage would be regulated to 5 VDC. This 5 DC Voltage will go to the Arduino which will in turn measure the AC voltage.
The whole idea of the Rectifier Circuit is allowing the Arduino to detect if a Utility Grid outage happens (When 120 VAC becomes 0 VAC) so that it could send a signal to the Relay Module to switch to the backup Power Source (i.e. The Solar System).
However, I am uncertain if I'm doing the right thing and I'm not sure about the connections/coding of the Relay Module as I just have basic knowledge about Relays.
So could someone please let me know if I'm on the right track and guide me through this project or probably refer me to a similar project so I could benefit from?
You are most likely too young to recall the great argument in the 1970's relating to an "uninteruptable power source" vs a "standby power source". Today when you buy a UPS, it is definitely not a UPS, it is a SPS, a standby power source. It is standby because it has to switch when the utility power is lost.
One or more cycle of power will be missing during the switch and the pseudo AC voltage wave will not be in sync with the old power AC.
The system you are designing will have to switch from one power source to another and this takes time. Big question is will the equipment you are powering be affected by a power loss of one or two cycles?
A true UPS will drive the equipment at all times and will have enough reserve to keep driving it while the power source is switched. Can your inverter operate 24/7 and only have it's source changed from commercial to solar? You will need some storage batteries to buffer the time out periods.
For your circuit, I have done similar, but eliminate the resistor, change the filter cap to something much greater, 50-100UF. Don't need the zener. The circuit will give much less than the maximum an Arduino power voltage. Probably 10-11 volts. If you really need 5 volts, use a 7805 or other regulator in place of the zener. Look up the circuits for 7805. Possible need a couple of small capacitors.
You also have the cart before the horse. If the primary power is lost, the Arduino also loses power. So some type UPS or SPS is needed for it.
What will happen if the day is cloudy and rainy and the solar cannot generate enough power?
Only one major problem overlooked..... No supply authority is going to allow any grid connect system to continue to generate power if the main grid goes down for whatever reason. So essentially a waste of time and effort.
Paul_KD7HB:
You are most likely too young to recall the great argument in the 1970's relating to an "uninteruptable power source" vs a "standby power source". Today when you buy a UPS, it is definitely not a UPS, it is a SPS, a standby power source. It is standby because it has to switch when the utility power is lost.
One or more cycle of power will be missing during the switch and the pseudo AC voltage wave will not be in sync with the old power AC.
The system you are designing will have to switch from one power source to another and this takes time. Big question is will the equipment you are powering be affected by a power loss of one or two cycles?
A true UPS will drive the equipment at all times and will have enough reserve to keep driving it while the power source is switched. Can your inverter operate 24/7 and only have it's source changed from commercial to solar? You will need some storage batteries to buffer the time out periods.
You also have the cart before the horse. If the primary power is lost, the Arduino also loses power. So some type UPS or SPS is needed for it.
What will happen if the day is cloudy and rainy and the solar cannot generate enough power?
You are close, but need a bit more engineering!
Paul
Having sold UPS systems for over a decade I am sorry but I must disagree, we sold Online UPS systems which were true UPS's, we also sold Line Active UPS which switched in the Inverter in less than 4ms.
(smallest UPS sold 700VA, largesest UPS system sold 4.5Mw)
Having said that, I would use a Mains Fail Relay to detect the state of the mains and a set of interlock relays (both electrically and mechanically) to control source of supply. You will have a small break in supply but I am sure cleverer minds will be able to suggest a way to use the Arduino to detect and switch at zero crossing point.
KISS. Use two 120V coil relays, one attached to each system. Connect the NO contacts to inputs on the Arduino. When the 120V power fails, the contacts open, and the input level changes.
I use something similar to power always on items in my house. I have a 120V relay connected to the solar system. with the contacts connected to the solar and the house power (common to devices). When the solar is on, the relay is closed and powering the circuit from solar. When the solar runs out (early moring, snow storm, etc.) the relay opens and connects the devices back to the grid.
bluejets:
Only one major problem overlooked..... No supply authority is going to allow any grid connect system to continue to generate power if the main grid goes down for whatever reason. So essentially a waste of time and effort.
Again I disagree, at no time was there mention of feeding into the Grid, just the load. There are a number of commercial devices that do this already, Google Hybrid systems for a start point. (we often used Solar Controller systems for long run time UPS systems because of the high recharge current available)
Kiwi_Bloke:
Again I disagree, at no time was there mention of feeding into the Grid, just the load. There are a number of commercial devices that do this already, Google Hybrid systems for a start point. (we often used Solar Controller systems for long run time UPS systems because of the high recharge current available)
Check out any supply authority regulations on any grid connect solar supply..... Feeback or not.... No system generation allowed at loss of main supply.
The Solar System is idling in the back ground, when the main grid supply fails, the load is disconnected from the mains, then connected the Solar System Power Inverter.
At no time is the Solar System connected to the Main Grid.
I think Bluejets is talking about anti islanding, which in this case does not apply.
