Hello,
I'd like to get a better understanding of the ability of a grid-tied system to produce power when the grid is down.
I know that normally in a grid-tied system, it is not possible to receive power from the array when the utility is not operating. However, I know that some inverters come with AC switches that allow for this option. For instance, this Outback Inverter data sheet indicates that a transfer switch automatically disconnects your loads from the utility grid and powers them from the inverter in the event of an outage, allowing you to continue using your solar and battery back-up power, unlike traditional grid-tie systems.
However, the linked Outback inverter is designed for grid-tied systems with battery back-up. The efficiency of this battery back-up inverter is 91%, rather low compared to standard grid-tied inverters (95.5% for an SMA Sunny Boy Inverter).
I see that there are separate transfer switches available (such as the IOTA Engineering ITS30R 30A Transfer Switch) but these seem to be designed exclusively for RV applications.
What options are there available for grid-tied arrays to continue to generate power even in the event of a utility power outage?
Thank you.
Comments
Brian,
The modern grid-tied inverter is very tightly engineered to backfeed power to a specific voltage and frequency of AC grid (utility power). The electronics are matched and the voltage (both AC and DC) are optimized for efficiency. Grid-tied inverters are also carefully designed to stop working immediately if the grid fails (blackout) or fluctuates (brownout). This is mainly to prevent injury to anyone arriving to repair the local grid - the utility doesn't want some separate source of gereration to come online unexpectedly. If you want to dive in deeper to this, google UL 1741.
So, in order to keep a grid-tied inverter working during an outage, something has to fake it into thinking there is a healthy grid present. There are really only two theoretical ways to do this and only one of the methods is practical.
Sunny Island or other Inverter/Battery Charger combination. The Sunny Island is an inverter/charger form SMA. It runs off a service panel and charges batteries while communicating with an SMA Sunnyboy grid-tied inverter. When the power goes out, it disconnects the grid and creates a micro-grid between itself and the Sunnyboy, thereby conforming with UL 1741 and keeping the grid tied inverter online.
Generator - a generator can also run on the service panel that a grid-tied inverter is connected to. If a transfer switch isolates the service panel from the grid in an outage, then the grid is protected and the inverter could sync with generator. This is possible, but no grid tied inverter is designed to run with a generator and either the generator or the inverter will loose a battle to push power into the other - with costly results.
The outback inverter is a battery based inverter - with a solar charge controller. It can sync with the grid, but it isn't connected directly to the solar array. The solar array charges batteries and the inverter then puts power into the grid after converting power from the battery power. This is why it appears inefficient - but 90% for a "battery voltage" inverter is actually very high.
Thanks for your response! Immensely helpful.
The Sol-Ark inverter will allow a generator and PV's and batteries to work all together in island mode.
Thank you for your thought, and it is accurate, the Sol-Ark Inverter does allow operation as they state but design considerations of load and supply need to be considered so as to not cause issues.
There is another way. Disconnect the solar panels at the inverters. Feed the PV outputs directly into a resistance heating element, such as an electric water heater. This has its dangers. The water heater thermostats are not rated for DC voltage control. This could lead to a fire. But conceptually, if things were really an emergency, you can make use of this idea.