Key Factors For Selling Back To The Utility Grid With an OutBack Power System

Newer hybrid Inverters offer multiple smart modes of operation. However they've become more complex to program. For Grid-Interactive systems, most users want to maximize their sell back to the utility grid. With this in mind there are a few tweaks you can make to your OutBack system for maximizing sell back potential.

Here are a few key factors we must understand before getting into the programming.

Qualifying The AC Source. Multi-mode Inverters that operate in Grid-tied, only produce an AC signal when the grid isn't present. For example; in a grid outage, the Inverter's internal transfer switch will isolate the grid input and switch over to battery backup mode. This will only power the Inverter's AC output or critical loads. (This is also a UL1741 safety requirement.) When the Inverter isn't in backup mode and the grid is present, it will synchronize or phase match the grid's AC wave signal. This allows the Inverter to manipulate the current flow using brute force, or in other words, increase its voltage essentially creating an IR drop. This method is used not just for selling but for battery charging. Because of this synchronization scheme, the AC source must be "clean" and within a very strict frequency/voltage thresholds of 59.3-60.5Hz and 108-127Vac. The Inverter will trigger a fault if the source falls outside of these thresholds.

The MATE3 will display a 5 minute timer that must be counted down during the initial "hand shake" while the inverter is synchronizing. From the MATE3 press the AC INPUT hot key then "Sell Status"

Inverter Charger Timers. One priority for a battery based/Hybrid Inverters is to keep the batteries full before it attempts to sell back to the grid. For this to happen, the Inverter's charger must go through an entire charge cycle. What this means is the Absorb and Float timers must be counted down to 0. For example: If you have your Absorb set to 57.7V for 2 hours and Float set to 54.4V for 1 hour the batteries must maintain the voltage at those targets for the amount of time programmed before the Inverter switches to the sell target.

Watt-Tip: To enter the sell quicker, you can reduce the timers down to zero, this will eliminate the need to wait 'hours'. Although this is optional you may opt into keeping the timers per the battery manufacture recommended settings.

Sell Target Voltage. This is not a charging stage but rather a constant-voltage mode of operation. Once the timers are counted down to zero, the next target becomes the SELL RE set point, therefore the inverter will sell down to this target and as long as excess current comes in usually from renewable sources the selling will continue up until no more RE sources are available.

Excess power is sent first to any loads (wired to the AC OUTPUT) which is known as "offsetting". If the exported power exceeds the loads the reminder is "sold" back to the utility grid through the (AC INPUT) One important point is if the battery voltage falls below the SELL RE set point the inverter will stop selling and enter a "pass through" mode.

MATE3 settings. 

Set your Input priority/type to Grid. PATH: Lock / Settings / Inverter / AC Input and Current limit.

Set your AC input mode to Grid-tie. PATH: Lock / Settings / Inverter / AC input modes and limits.

Adjust the Inverter charging parameters timers to zero. Record what the float voltage is set to. Adjust all ports if you have more than one inverter. PATH: Lock / Settings / Inverter / Battery Charger.

Enable Offset. PATH: Lock / Settings / Inverter / Grid Tie Sell.  

Adjust your SELL RE value to be about 1 volt under your float voltage. 

Confirm or set all charge controllers (FM60/FM80) are set to "Grid-tie Mode" 

PATH: Lock / Settings / Charge controller / Grid tie mode.

Not selling? 

• Actual battery voltage is below SELL RE.
• If HBX was enabled it will conflict with the selling. 
• If using FLEXtime check your AC input modes. 
• System not connecting to the AC Input. (Flashing yellow light) 
• High load demand on the AC output or critical loads. (Loads have priority)
• Wrong input type/priority (Gen).
• Offset set to disable.
• Batteries have not yet reached the Absorb voltage (stuck in bulk).
• Not enough Renewable energy being produce. 
• Reduced selling? The amount of power an inverter can sell is not necessary equal to its specified output wattage. It will vary based on temperature, battery technology, efficiency and other factors. 
• The maximum Sell current set too low. Setting available in the Grid interface protection menu. 

Footnotes: 

• Default SELL RE is set to a value when the batteries are at rest and will not consume any energy when selling. However, if you prefer to Float or maintain the batteries while selling then set the SELL RE to the Float Voltage. Move your actual float voltage to 1 volt above the SELL RE. 
• If you have a FLEXnet DC and prefer to fully charge the batteries before you begin to sell you may enable grid-tie charge control. Check out this video for further information: FLEXnet DC Advance Controls
• These settings apply only to FXR and Radian-A/E models. 
• The Frequency/voltage thresholds mention in this article come from the US standard IEEE. If your country/state have different requirements check out the video: How to Access the Grid Interface Protection Menu

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