Issues Related to the Use of Lockable-Open AC Disconnects with Grid-Tie Battery Backup PV Systems

SDG&E may from time to time, for legal or operational reasons, wish to assure itself that a PV system is disconnected from the grid. A concern of the utility is that disconnecting means be provided to the utility that will not result in loss of service to loads that could result in unwarranted or unanticipated customer inconvenience. For this reason, there is a general policy that the Lockable AC Disconnects on a PV system backfeeding the grid be installed in such a manner that opening the Disconnect will not, by that action, result in customer load being disconnected.

In the most common type of grid connected PV system, there is a distinct separation between inverters backfeeding the utility and any loads, so satisfying this criterion is usually feasible. However in most PV battery backup systems, the inverter's utility-side terminals are used both for receiving power from the grid (used by protected loads or battery charging) and also for backfeeding power from the PV modules to the grid. This means that disconnecting the inverter from the grid will not only disconnect the backfeed, but also disconnect the protected load from the grid. (Most PV grid-tie battery backup inverters contain an internal transfer switch designed to supply the protected load directly from utility power when such power is available and within preset "quality" limits, i.e. voltage, frequency, impedance.)

My understanding from a discussion at SDG&E is that disconnecting a grid-tie battery backup system by placing the lockable AC disconnect in a line feeding the grid-tie inverter may be acceptable, based on the presumption that the system is designed so that, when the inverter is disconnected from the grid, the inverter will continue to supply power to the protected loads. Just as it would have done in the case of a power outage. Hence, opening the lockable AC Disconnect will not result in the protected loads being deprived of AC power.

An issue was raised concerning the possibility that the customer may choose to install an inverter bypass switch so that, if the inverter needs to be removed from the circuit, the loads could be supplied directly by the grid. A suggestion was made that, in the event that an inverter bypass switch were installed, the power supplied through the bypass switch with the inverter disconnected from the circuit, be supplied by a separate lead not disconnected by the Lockable AC Disconnect. The effect of this suggestion would be to permit bypassing the inverter and the Lockable AC Disconnect simultaneously, so that the load could continue to be supplied by the grid while the inverter is disconnected from both AC line and load.

There were remarks affirming the need for clear labeling of all wires used in such an installation, especially in view of the fact that such an installation would be more complex than usual in grid-tie PV installations, creating more of a need for clear identifications. Also possible placarding of the bypass switch (if installed) to clearly indicate its function and operation.

I hope I have correctly summarized the points discussed. I felt the discussion was productive and educational, and wish to give my thanks to all of the participants.

Line Load

No Bypass Switch

Bypass Switch

Line Load

Please let me know your comments.

Thank you.

Jim Easton, PE

4364 Bonita Rd., No. 166

Bonita, CA 91902-1421

Tel: 858-527-0240

Fax: 619-470-8616

www.JimEaston.com

Jim Easton 1 of 2 11/21/05