How Grid tie inverter distributes power on split-phase supply

recepta · 2017-03-24T22:08:41.407Z

I want to add 1 solar panel to cover my vampire loads during the day. I live in US, 2 phase 110v or 220v between phases. My vampire loads range from 80w to 220w depending if the refrigerator is running. Phases are not loaded evenly. Phase A has always more load than the phase B. If connect grid-tie inverter, such one made by Enphase, between the two phases, how it will distribute the power if lets say phase A=40w, phase B=160w. I can’t sent any excess power back to the grid and was looking into power router option with some sort of load to kill the excess power.

Any help is greatly appreciated.

Robert.Wall · 2017-03-24T22:29:33.434Z

I can’t tell you how your inverter will react in detail, but unless it has a neutral connection (that is not just for its control supply) then it must circulate the same current in both legs of the supply. So I can see it is possible that you will import on one leg and export on the other.

I can say that if you have two separate 120 V ‘dump’ loads (say water or space heaters), one on each leg of the supply, and each controlled by a power diverter like Robin Emley’s or MartinR’s, then the diverters will attempt to mop up any power returned to the supply on that leg.
If you had a single diverter and a single 240 V dump load, then you would need to monitor the vector sum of the currents in the two legs, and the diverter would attempt to balance that sum to zero; and again I can see it is possible that you will import on one leg and export on the other.

recepta · 2017-03-24T23:02:50.927Z

You brought up a good point about having dump loads on each of the phases. I did not even think about that. Since I am planning on using only one solar panel at at a max I will need to dump about 120w. I was just thinking to dump it on some high wattage resistors. Robins router is using powerful and quite expensive “relay” to divert the power, since my dump loads will be much lower, I wonder if I could use any relay rated for 120v 120W?

Also, if I will import on one leg and export on the other, would my power meter register any anomalies?

Robert.Wall · 2017-03-24T23:24:09.372Z

You can’t use an electro-mechanical relay, you will need a triac (which is in fact what both those designs use), as the switching needs to be synchronised to the mains frequency (so as to minimise r.f. interference).

recepta:

Also, if I will import on one leg and export on the other, would my power meter register any anomalies?

I think that depends very much on how your meter is set up. You will only know (a) if the supplier will tell you or (b) when you try it, unfortunately.

Bill.Thomson · 2017-03-25T03:08:26.125Z

recepta:

if I will import on one leg and export on the other, would my power meter register any anomalies?

Unless you have a net meter, you’ll end up paying for any energy you export.

recepta · 2017-03-25T04:44:38.061Z

Got it about not using mechanical relays. Since my dump loads will be very small I guess I can use triac rated for low power. Since I need to dump power on 2 phases, will I need 2 voltage sensors, one for each phase?

recepta · 2017-03-25T04:44:59.503Z

Got it thanks.

Robert.Wall · 2017-03-25T13:13:31.461Z

recepta:

will I need 2 voltage sensors, one for each phase?

I don’t think so. Don’t confuse yourself by referring to your supply as two phases. It isn’t. If you look at the diagram near the top of the “Use in N.America” page, your supply comes off a single centre-tapped winding on a transformer. So it is more accurate to call them ‘legs’ of the same phase. As the diverters (I’m talking ours, not any third party’s) look for the nett power to balance at zero, the voltage doesn’t play a large part in the measurement itself, but it is necessary to calculate the real power (what you pay for) in the first place. So provided that the voltage balance between the two legs is good, I don’t see a need for two a.c. adapters.

There’s not much to be gained by derating your triac. It won’t be significantly cheaper, and it’s less likely to survive a fault downstream. Where you will save is, whichever triac you use, it won’t need much of a heatsink.

Still thinking along the lines of currents, you’ll need to be very careful when choosing your c.t(s), because they need to take your full load and remain accurate at well below 1 A so as to get it right as the PV output drops.

recepta · 2017-03-25T21:12:27.258Z

Thanks for the detail explanation. I think, the more I read/think about this, it would make more sense for me to go with the 120v inverter and move most of my vampire loads on one phase.

Robert.Wall · 2017-03-25T21:27:03.015Z

I wouldn’t disagree. It isn’t as if one or two amps will significantly unbalance your supply - I doubt the transformer would even notice. And it will both cheapen and simplify the scheme.

recepta · 2017-03-25T21:48:20.072Z

Yes, I totally agree with you. And looking at my historical data, most of the vampire loads are on Phase B, anyways. I just need to switch one circuit from A to B, what draws about 30-35W 24/7.

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