There is no rule for grounding the neutral. It depends on what you are trying to do since unground and solidly grounded are all going to do different things to the utility distribution system under fault conditions. Unless you are running single phase loads or using single phase inverter topology the grounding will not effect the PV system.
If you leave it ungrounded then you cannot have an imbalance in the phase currents on either side and a ground fault on the utility side could result in an over voltage condition. If you solidly ground it then it becomes a ground fault current source and could desensitize the utility protection on the distribution system.
The distribution system may look like an ungrounded 3 wire system but when designing MV distribution systems the earth is considered the grounded neutral conductor. For smaller branch circuits sometimes there is only one conductor and the earth is used as the return. Look up single wire earth return distribution systems.
Also see this thread: http://forums.mikeholt.com/showthread.php?t=138605
If you leave it ungrounded then you cannot have an imbalance in the phase currents on either side and a ground fault on the utility side could result in an over voltage condition. If you solidly ground it then it becomes a ground fault current source and could desensitize the utility protection on the distribution system.
The distribution system may look like an ungrounded 3 wire system but when designing MV distribution systems the earth is considered the grounded neutral conductor. For smaller branch circuits sometimes there is only one conductor and the earth is used as the return. Look up single wire earth return distribution systems.
Also see this thread: http://forums.mikeholt.com/showthread.php?t=138605
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