Single-phase Inverters Connected to Three-phase Grid
Single-phase Inverters Connected to Three-phase Grid
Yes, you can. You would connect the single phase inverters (ideally a number of them divisible by three for balancing) phase to phase (A-B, B-C, and C-A) or phase to neutral (A-N, B-N, and C-N). As jben says, it's not as common as it used to be when there were a lot fewer 3 phase inverters. My first commercial system was 72 SMA 7000's phase to neutral on a 480V service.
This isn't entirely correct. If single-phase inverters are connected to a three-phase grid topology, the system must comply with Articles 705.42 and 705.100. Conditions exist (e.g. 240 volt delta) where one phase could open, creating an "open delta". Single-phase inverters connected in such a configuration won't sense the lose of one phase, and will continue to export power.
To remedy this condition requires the use of external sensing and control hardware to open all three phases should one phase open. For those not familiar with an open delta, the voltage across the "open" phase will still measure essentially the same as for the other two phases. Inverters connected to the "open" phase will not sense the loss of that phase, and will continue to export power. Depending on the wattage of the solar array connected across an open phase, it's entirely possible to overload the transformer, since an open delta is rated for 57.7% of a full delta transformer.
There is also a risk of phase imbalance if part of an array should cease to export power. This could occur if a breaker in that array should trip, and the other two arrays continue in full operation.
Single-phase inverters *can* be used in a three-phase topology if all requirements in Article 705 are met. This requires multiple points of sensing, multiple types of sensing, and three-pole relays or contactors of appropriate ratings to open all three phases with the loss of any one phase in the system. Grid-interactive inverters will cease exporting power with loss of the grid as required in UL 1741, UL 1741SA, and UL 62109. IEEE 1547 contains the details of the required shutdown conditions, and is a subset of the UL Standards just mentioned.
Since inverters are a source, not a load, traditional methods of sensing an open phase may not be effective in such cases. Moreover, solar inverters are a current-limited current-source, making over-current in the event of the loss of a phase an impossibility, as would be the case such as for a large three-phase motor.
Lacking the protections just referenced, a Code violation exists when single-phase inverters are connected to a three-phase grid because inverters on the unaffected phases won't cease to export power in the event of the loss of a phase.