Gulf Island Off-Grid System - Part 6 (Solar Expansion & Intertie)

As we’ve described in our earlier posts, we have two solar systems, a small 250 amp-hour system for the cabin where we live (Part 1, Part 2), and a larger 1000 amp-hour system for the guest house and workshop (Part 3, Part 4, Part 5). These two systems are connected together manually, so that we can transfer power between them as needed.

The next phase of building out our power system involves the following additions and changes:

1. Adding another inverter and string to the larger system to capture the afternoon/evening sun (our existing array only captures sun from the morning through 2 P.M.)
2. Allowing the transfer of power between the two systems to be performed automatically based on SOC levels and weather.

Solar Expansion

Our current 5 KW array is in the sun from 7 A.M. to 2 P.M. However, over the last year, we have observed a significant number of days where the clouds from the night’s rains do not burn off until the afternoon. We also tend to use the shop during the late afternoon, which results in us starting to draw down our battery even before the sun has set.

Fortunately, we anticipated this, and when we connected the two systems together, we designed the buried cable run so that we could add an inverter close by to a west-facing location. Specifically, we need to build a new fence for our garden to keep the deer out, so why not build a solar fence?

This lends itself to a simple triangular mounting structure, where panels are mounted on a 45 degree angle onto the fence structure. We can then put chicken wire along the back of the fence to allow chickens to run outdoors under the panels.

We have a Sunny Boy 3000TL-US inverter, and we will be connecting either 10 Hanwa Q series 250 watt panels in a single string or 15 of the same panels across two stings. A dual-string DC disconnect will be mounted on the solar fence, provided by an SMA CU 1000-US-10 disconnecting combiner, with a buried high voltage cable run back to the inverter.

The inverter will be connected into the larger system using the same design we used for our other two inverters.


Automated Power Transfer

Transferring power from the larger system to the smaller system currently involves throwing a switch that connects the AC input to the Magnum inverter to the AC output of the SMA Sunny Islands. This bypasses the Magnum, putting the cabin loads onto the larger system, and activating the battery charger in the Magnum.

Transferring power from the smaller system to the larger system currently involves throwing several switches to turn on a high-voltage DC power supply, which is connected to an SMA Windy Boy. The Windy Boy regulates the current flow (up to the preset limit in the DC power supply) based on the amount of power that the Sunny Islands (and other loads) can accept. So, for example, if we wished to limit the amount of power transferred to 200 watts, we would program a corresponding current limit (at 500 VDC, that would be 400 milliamps).

Currently, any transfer of power needs to be manually initiated and monitored. This doesn’t work too well when we need to transfer power when we’re asleep, or when we’re not home. So, our goal is to make it so that the transfer can be initiated automatically, based on algorithmic control (for example, instead of just dumping power to our diversion load), or remotely.

Here is the one-line diagram of what we have today:



Here is the one-line diagram of our preliminary design for the enhanced system:



Not shown in the above diagram are the wiring for the inverter and panels, low-voltage control wiring to the PLC, wiring for the front panel indicators and network (RS-485 and Ethernet) wiring.

Another potential addition would be to connect both AC busses to another Square D TVSS.

The front panel of the main box that holds the above equipment will look like the below mock-up:



Over the next few months we'll be working on building the panel using the equipment we already have on hand (it turned out we only need a couple of extra parts to put this together), and preparing to install the solar panels. We'll keep everyone updated as this project progresses, and of course, answer any questions that come up.