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Net import/export registers on line side tap system

Net import/export registers on line side tap system

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On a backfed PV installation (that is, an installation where excess solar production is fed back through a breaker on the main distribution panel to the utility) it is relatively easy to see the net import and export of power. The CTs installed on the main utility feeds are in effect net CTs, as they see power imported from and exported to the utility. Any register created using those CTs will thus change polarity when production exceeds consumption, and change back when consumption exceeds production.

However, a line side tap PV installation (where the solar is tapped between the main utility feed CTs and the utility meter itself) only tracks the consumption of the main distribution panel by default. A register using the main utility feed CTs will only see the consumption associated with that panel, regardless of how much production is being fed back to the utility. In this case, a comparison between the main utility CTs (“Grid” register) and the production CTs (“Solar” register) can show how much power is being imported or exported.

To track this information, two formula registers must be created as follows:

Sample Net Import/Export register setup

Net In = min($”Solar” – $”Grid”,0)

Net Out = max($”Solar” – $”Grid”,0)

Note that this assumes two registers, one named “Grid” which tracks only consumption on the main distribution panel, and one name “Solar” which tracks total solar production. If needed multiple registers can be combined.