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In this article we look at a solar DC combiner box that we used on a 100 kW ground mount project at Mitiamo Water authority.
This system also included a 404 kWh energy storage system in addition to the PV array.
For this site it was stipulated that 316 stainless steel was used and we decided to go with a 2mm wall thickness as opposed to the 1.6mm standard for extra durability.
The enclosure dimensions are 600 x 400 x 200mm and has an IP67 rating and was mounted to the ground piling via unistrut after creating some threads in the 5mm wall thickness.
The enclosure houses 8 x DC isolators and we have 2 x strings per isolator. Two strings enter the isolator and then one feed exits and connects to the matched SPD ( Surge Protective Device) and then a 50 metre run to the inverter station.
The SPD’s take a positive and a negative feed from the DC isolators and then there is the cable run to the inverters. The SPD’s are designed to protect the circuit from any transient surges that may damage the inverter and other equipment. They monitor what’s going on within the cable.
For example if a lightning strike hits the array and sends a current back through the cable it would automatically divert into the SPD but this is not specific lightning protection.
We have created a pathway to earth via 25mm2 earth cable. Effectively we are creating an alternative pathway; a path of least resistance.
Surge protection as standard for ground mount systems?
For this particular site we did run a lightning protection study in accordance with the applicable Australian standards and they have a spreadsheet that asks a series of questions and based on the supplied answers recommends yes or no to lightning specific protection.
There is also a lightning map that can be used as a quick reference.
Soil resistivity and earthing
In certain areas the geology of the ground will determine how the system is earthed. In areas of high basalt concentration earthing can be a problematic exercise. For example in the Port fairy area, South West coast of Victoria, Australia obtaining correct earth is a real issue.
With this project not much of a problem and we are connecting earth to the pilings which are driven 2.34 metres into the ground and using 25mm2 cable just to make sure.
This project also included a weather station and comms cable was run from the ground mount array back to the inverter station. So as well as the 8 x 6 mm2 solar DC cables we had to work out how to protect the comms cable.
Our earth and solar DC cable was run in 63mm conduit for ease of installation and also for future proofing. The comms cable was placed separately in 40 mm conduit which was not a necessity in regards to EMF ( Electromagnetic interference, frequency) but made for easy installation.
In this case the DC combiner box was there to house the DC isolators and the Surge Protection Devices and the 8 x strings connected to the inverter but DC combiner boxes can get more intricate with the inclusion of solar DC string fusing and parallel strings.
Hope this helps when assessing the needs of your next commercial solar project.
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