PV System Isolation xfmr - bonding?

[winslowfam]

Hi...our client has a new PV system with 2 inverters, whose outputs are connected to a new PV System Disc. Sw., and then to an isolation transformer. The transformer is wired 480/277V WYE primary/480V DELTA secondary, with the secondary is wired to a new overall system AC Disc. Sw. This switch is then wired to the existing plant power system. Proper grounding and bonding was designed all the way through to the transformer, per code. But, the transformer secondary (DELTA - so, no neutral) was not bonded to the new AC Disc. Sw. This new switch, though, is bonded to the plant system ground via an EGC. So, the overall PV system is grounded WYE to the transformer primary, then ungrounded 480V from transformer secondary to the AC Disc. Sw., but then is back to a grounded 480V from the AC Disc. Sw. to where it connects to the plant system. Is this correct? Shouldn't the transformer secondary be bonded to the plant ground system somehow instead of being ungrounded?

 

[Smart $]

... Is this correct? Shouldn't the transformer secondary be bonded to the plant ground system somehow instead of being ungrounded?

If the plant system is system is grounded, the grounding would be through the main bonding jumper at the service disconnect.

 

[winslowfam]

Yes, the plant system is grounded at the service...and the new PV system is grounded on its end, as well. But, the final connection between systems (secondary of the isolation transformer to the new disconnect switch) is ungrounded, 480V Delta, and only for a few feet. Shouldn't the two ground systems be bonded between these two points, xfmr secondary & new disconnect switch?

 

[Smart $]

Yes, the plant system is grounded at the service...and the new PV system is grounded on its end, as well. But, the final connection between systems (secondary of the isolation transformer to the new disconnect switch) is ungrounded, 480V Delta, and only for a few feet. Shouldn't the two ground systems be bonded between these two points, xfmr secondary & new disconnect switch?

There should be a supply-side bonding jumper (read EGC on the line side of a system disconnect), but nothing connected to circuit conductors.

Assuming this is a grid-interactive PV system when the switch is open, the inverter is required to automatically disconnect. Otherwise, a ground detector would be required when switch is open.

 

[winslowfam]

I've been reviewing 250.30(B) regarding grounding of separately derived AC system for an ungrounded system. The isolation transformer on the output of the PV AC system is essentially a separately derived system with an ungrounded secondary...does that sound right? This code article states that the equipment of an ungrounded separately derived system shall be grounded and bonded as specified in 250.30(B)(1) through (B)(3). So, I think I should recommend installing a bonding conductor between the isolation xfmr. secondary and the Disc. Sw. it feeds. What do you think?

 

[Smart $]

I've been reviewing 250.30(B) regarding grounding of separately derived AC system for an ungrounded system. The isolation transformer on the output of the PV AC system is essentially a separately derived system with an ungrounded secondary...does that sound right? This code article states that the equipment of an ungrounded separately derived system shall be grounded and bonded as specified in 250.30(B)(1) through (B)(3). So, I think I should recommend installing a bonding conductor between the isolation xfmr. secondary and the Disc. Sw. it feeds. What do you think?

A supply-side bonding jumper is required regardless of whether the secondary is a grounded system or not. In the case of "not", see 250.30(B)(3), which refers you back to 250.30(A)(2). This bonding jumper connects only to the equipment and not any circuit conductor.