AC Coupling in Utility-Interactive and Stand-Alone Applications: Page 9 of 16
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Grid-direct inverter compatibility. Schneider Electric has developed and tested the integration of its Conext XW and TX products in ac-coupled systems. Although this architecture may work with UL 1741/CSA 107.1-01–compliant inverters from other manufacturers, Schneider Electric has not tested these products in ac-coupled systems, so support for systems that integrate other vendors’ products with Conext inverters may be limited.
Stand-alone, off-grid systems. To maximize battery performance and life, the Conext XW– and TX–based ac-coupling architecture is intended for utility-interactive systems connected to a dependable utility grid. Schneider Electric does not recommend or support its ac-coupled system architecture for use in stand-alone, off-grid applications.
Power ratings for single Conext XW installations. The batterybased XW inverter power rating should match or exceed the grid-direct TX inverter power rating. Accordingly, the XW 6048 (6.0 kW/48 Vdc) is compatible with a single TX 5.0 (5.0 kW) or TX 3.8 (3.8 kW) inverter, or with one or two TX 2.8 inverters (2.8 kW each, 5.6 kW total). The XW 4548 (4.5 kW/48 Vdc) and XW 4024 (4.0 kW/24 Vdc) are compatible with a single TX 3.8 or TX 2.8 inverter.
Power ratings for parallel Conext XW installations. In applications that utilize multiple XW inverter/chargers configured in parallel, the total TX inverter power rating should not exceed the power rating of a single XW inverter deployed in the system. For example, while two TX 2.8 inverters can be connected to a single XW 6048, a stacked pair of XW 6048s would also be limited to two TX 2.8 inverters.
Conext XW firmware. To prevent battery damage in ac-coupled applications, XW inverter/chargers should be updated to the latest firmware that includes the ac-coupling feature. At present, XW-specific firmware (version 1.07) is available for each North American XW model and can be downloaded at schneider-electric.com/conextxw.
Conext XW ac qualification period. The XW and TX products are fully compliant with UL and CSA anti-islanding standards. However, for ac-coupled applications, the XW inverter/charger’s ac-qualification period default setting of 10 seconds must be adjusted to 300 seconds.
Backup generators. Schneider Electric’s ac-coupled system has not been tested with a generator providing the ac reference for TX inverters. If the system includes a backup generator, I recommend installing an “either-or” interlock switch to prevent unintended back-feeding of current from the TX inverter to the generator (see Figure 4, above).
System metering. In ac-coupled applications, the power metering on the Conext XW may not work reliably when the inverter/charger is in Voltage-Source Invert mode and power is flowing back into the batteries.
Terminating ac circuits. The designated Conext XW and TX ac outputs are typically connected in the critical-loads subpanel. Each TX inverter requires its own ac breaker in the subpanel, which is connected to the output of the XW inverter/charger. Although there is space to add breakers for the TX inverters directly into the XW power distribution panel, it is more straightforward to install the TX inverter breakers in the ac subpanel.
Critical load and battery-bank sizing. Critical loads that are terminated in the ac subpanel should be selected based on the customer’s essential safety and lifestyle requirements during utility failures. It is not practical to back up all of a home’s loads. The battery bank should be sized to power the critical loads for a specific time period and to avoid completely discharging the battery bank during utility outages. An occasional 50%–60% maximum discharge may be appropriate. When specifying battery bank Ah capacity for ac-coupled systems, the designer should consider potential excess current that the TX inverter produces while it is powering the critical loads in relation to the battery manufacturer’s charge-current recommendations.
Alternatives to ac coupling. If array-to-battery distance is the primary design driver for an ac-coupled system, you should weigh the potential cost and operational benefits of utilizing a dc-coupled system architecture with a higher voltage dc-charge controller. Schneider Electric manufactures charge controllers rated at 150 Vdc (XW-MPPT60-150) and 600 Vdc (XW-MPPT80-600).
SMA Battery-Based and Grid-Direct Inverters
By Greg Smith, SMA America