solarprofessional.com | S o l a r pr o 19

Aproject we are designing with a mechanical, electrical and plumb-

ing (MEP) firm includes both a backup generator and a grid-tied PV system. Where should the PV system tie into the ac service? What will ensure the best per-formance and reliability for the equipment and for the backup power supply?

The question of grid-tied PV inverter interaction with backup

generators is an interesting one. It is not uncommon for large facilities—especially data centers, hospitals and schools—to have backup generators on-site. Increasingly, high end homes also include automatic generator backup. When both a backup gen-erator and a grid-tied PV system are present, the question many electrical engineers raise is, “Why not put this expensive PV system to work if and when the grid fails by allowing the inverter to synchronize with the gen-erator during a utility outage?” This is certainly an option, but only if specific criteria are met.

In general, however, the most straightforward method is to con-nect the grid-tied PV system ahead of the generator transfer switch and the subpanel feeding the critical loads. In this configuration, when the grid goes down, the PV system goes down as well. The transfer switch disconnects the generator backed up circuits from the building’s main ac power system circuit, and the generator starts up to run the critical loads.

If the generator is sized and inter-connected such that it backs up all of a building’s loads, it is still possible to isolate the PV system from the build-ing circuits. The PV system just needs to connect on the utility side of the transfer switch on the customer’s side of the revenue meter. This is the most

conservative approach for connecting the PV system, and in most cases this is the best approach. Unless utility power goes down frequently—for a sig-nificant number of hours per month or for many days per year—there will be very little energy production loss if a PV system is not operating during the few hours that the utility is off line.

But what if an electrical engineer wants to go in a totally different direc-tion? Can the PV system connect to the backup circuit? Will the grid-tied inverter synchronize with the gen-erator output and help support the backup loads? The answers to these questions are complex and entirely dependent on the quality of the gen-erator’s ac output.

It is possible for a PV inverter to interconnect with the stable out-put of a high quality generator. The inverter monitors the generator’s voltage, frequency and waveform. If the ac waveform is grid quality, the inverter will attempt to synchronize with the generator. If the ac input to the inverter does not dip, sag or surge when the PV system comes on line and starts delivering power to the system, the inverter will remain on line. If the building loads and the generator output are both large rela-tive to the output of the PV system, everything will work satisfactorily as long as the generator does not go out of UL voltage and frequency. However, if the generator voltage or frequency increases beyond the UL 1741 limits, even for a short time as might happen when the PV system comes on line or with a large load shift in the building, then the PV inverter will go off line.

But what if the instantaneous power output for the PV system is larger than the building loads? If there is more power available from the PV

system than the total load can con-sume, the inverter will attempt to put the extra power somewhere, raising the ac line voltage, for example. In this case, when the PV system comes on line, as soon as the voltage or fre-quency reaches the maximum UL 1741 limit, the inverter will trip off. Unless the total load demand exceeds the instantaneous power output of the PV system at all times, the grid-tied PV system has no chance of staying con-nected to the generator output.

In summary, the most conservative connection point for a grid-tied PV system is on the utility side of the gen-erator and transfer switch. If the total building load will always be larger than the PV system output and if the generator is a large, good quality unit with a fast governor, then a grid-tied PV system can work well with a con-nection on the generator side of the transfer switch.

Whether there is any net benefit to this less conservative approach is questionable, however. As a rule, a generator does not consume four times more fuel when it is running at an 80% load as opposed to a 20% load. So even if the circuit works as expected, there may be little to no net benefit to the customer. Also, keep in mind that any time a grid-tied PV sys-tem or a backup generator is involved, there will be additional questions and concerns during plan review and inspection. Before choosing your tie-in points and before proceeding from planning to implementation, have your plans reviewed by the local AHJ; it may have its own recommenda-tions, requirements or interpretations of the NEC.

—James Worden / Solectria Renewables / Lawrence, MA / solren.com

Interaction of Grid-Direct Inverters with Backup Generators