I am working on a simple solar system for heating water. The idea is to use sun power and if not enough, use the power grid at night. I have a solar panel and a MPTT boost converter which collects the power from the panel and sends it to the boiler. I am using a timer controlled mechanical SPDT relay to switch between the solar and the power grid - grid at night, solar at day. My problem is that my controller sometimes burns out during the switching of the SPDT relay. What I suspect is that there is a shoot through during the switching time, which places 220V AC from the grid on the MPTT controller output. The MPTT controller is designed for 100V DC max. I noticed also that this does not happen always. It probably depends on the point at which the relay cuts the 220V sine wave, because the relay sometimes sparks, sometimes not. The shoot through is always accompanied with a spark. Is it possible, that the spark (when it happens) crosses the air gap of the relay contact? Is there a simple way to solve the issue? (The overkill for me would be to build my own time delayed spdt relay)
Your application seems unsafe for a couple reasons:
- There is no guaranteed isolation between the contacts even if they configured to break-before-make. In other words, from safety point of view, there is no guarantee that the MPPT controller will be isolated from the AC network. From a product/earth safety perspective this solution is dangerous.
- Even if the AC network is actually disconnected from the network, the boiler and connected wires have their own internal inductance, and once you open the relay, it will cause a spark (depending when the current crosses zero).
- If solar grid is wired in a SELV network, you are causing an unwanted risk by sharing AC with SELV on the same contacts of the relay.
The solution would be either:
- Use an isolating transformer from AC to SELV to power the boiler
- Use an inverter next to the MPPT controller
- Use a bolier that is designed for the purpose
As already pointed out, since the power to the boiler resistance is V^2/R, you will get an heating effect that is much lower (and maybe negligible).
There is less chance of a "shoot through" with the properly selected "break before make" relay that has a sufficient voltage standoff rating. Since most relays are designed this way I suggest that there is a different cause for the controller getting fried. The boiler load probably has a large inductive component to it. Whenever the load is actively drawing current and then the relay suddenly opens this inductive part of the load can cause a huge spike of voltage that can easily fry the semiconductor conductor components in the controller. This spike of voltage can also be a contributor to the sparking you see across the relay contacts.
For DC switching the solution for this voltage spike is to add a reversed biased diode (a.k.a flyback clamping diode) across the inductive load to clamp the over voltage and provide a path to dump the current that wants to flow due to the collapsing field in the inductive load.
The same diode scheme does not work for AC switching of inductive loads because of the ever changing polarity of the voltage. In this case there are a class of circuits called snubber circuits that can be installed across the load. In their simplest form the snubber can consist of a series resistor and capacitor. The component values are selected based upon the AC frequency and the inductive characteristics of the load.
Since you have this somewhat non-conventional idea to switch between an AC source and a DC source it will be more difficult to find a voltage spike clamping solution that works well for both cases. So the recommendation that I suggest is to first solve the safety problem as pointed out by others and then figure out the inductive load problem.
Some may suggest putting a DC to AC converter in line with the output of your MPTT controller which provides an isolated AC output to deal with the safety problem. Since such converters are not 100% efficient a certain part of the energy produced by your solar panel will be lost as heat in the DC to AC unit. For this reason I suggest that instead you get a suitable AC to DC switching power supply that has an isolated output and use that on the AC mains side of the circuit. From the mains side there is less concern regarding the conversion losses and with modern component technology it is possible get AC to DC switching power supplies with better than 90% efficiency. With isolated DC now being supplied from the line side the relay is switching between two DC sources and any inductive voltage spike clamping can use the simple and effective diode method.
One has to guess that your boiler is already able to operate on a DC voltage due to that is how you have shown in your diagram. If by chance the boiler is using any motors that are designed to run on an AC voltage source only then you would have to change those out for motors designed to run on DC voltage.