DIY timer / controllable transfer switch contactor box.

[daklein]

I recently finished a DIY transfer switch box to move my grid tied array between the grid and the island/off-grid panels. It is a 4.7kw 'grid-tied' system with microinverters which goes through a generation meter. The motivation is to use the grid-tie system energy in the house directly where possible and when there is very little credit for outflow, but still be able to send it out during weekday peak rates if I can't use it. The 'off-grid' is the island AC1 side of a pair of SMA SI6048 inverters & batteries, in the garage at the far end of the house, with about 7kw of AC and DC coupled solar with FLA forklift batteries (DC Solar trailer parts).

The transfer switch contactors move L1&L2 to backfeed either the grid panel (to the right outside) or off-grid panel (basement). The contactors are 30a (20a continuous) rated, and I swapped out the position sensing 4th contact hardware, so they're 4 poles at 20a, 2 for each L1&L2. The array can potentially put out around 20a.

The transfer switch runs on timers as a default, to connect to the grid during on-peak rates. I set the timers manually if I want to keep the extra solar on the off-grid side; on cloudy days I can use all the solar in the house. In the future, I'll have the other system controls run this also, via a set of 24v dc inputs that override the timers.

The relays and timers run on 24v DC, powered from the off-grid side. In addition to the mechanical interlock between the contactors, the control relays are also wired to prevent energizing both contactors at the same time. If the timers or the override control inputs attempt to enable both contactors, then neither contactor energizes.

Here are pictures of the transfer switch: https://photos.app.goo.gl/UcDfekbC1J3qAJ6J9

 

[daklein]

Funny update about this: My raspberry pi system controller runs the transfer switch, and I had set it turn disconnect completely at night.

My logic for turning it off at night: This saves a little standby load from all the microinverters, load from the contactor coil. Not being connected to the grid reduces lightning risk coming in from the grid, and having the rooftop array disconnected from the house reduces lightning risk coming from the array into the house & off-grid system.

After setting that up, I started getting frequently late bills from the utility. Eventually, I realize they read the meter sometime around midnight. The meter is behind the transfer switch. I don't know when they request the data over the radio network, but at least I think they're looking for the meter to have a value at midnight. I set the controller to connect the switch to the off-grid side, so the meter is powered up. Initially I set it to connect at 11p, and disconnect at 2a, plus or minus 2 days from the day the last bill says they'll do the billing next time. That worked for 2 months, but still have had late bills 2 other times, maybe I need a wider window around midnight.

Having the generation meter turned off sometimes also makes them detect an outage at my house. About half a dozen times, linemen come pounding on the door after dark, with the lights all on in the house and the main meter is still on connected to the grid, 'is your power off'? WTF!?!? I put a sign on the door saying don't knock, the power is on, it's the generation meter detecting an outage. Between that and explaining to them multiple times, the linemen must have put some notes in their system, they don't come out anymore.