AC Coupling in Utility-Interactive and Stand-Alone Applications: Page 8 of 16

No-grid–present operation. The battery-based Conext XW inverter/charger continuously monitors the utility-input voltage and frequency. If the voltage or frequency moves outside the acceptable ranges—for example, during a power surge or outage—the XW opens its input relay, disconnecting all the inverters from the grid. As soon as the relay opens, the XW transfers from Charge mode to Invert mode to provide power to the critical loads terminated in the ac subpanel using energy stored in the battery. The grid-direct Conext TX inverter may detect the temporary loss of ac during this transfer and go off-line until it detects a stable ac output from the XW for a minimum of 5 minutes.

During utility failures, the XW serves as a voltage source for the grid-direct TX inverter, providing tightly controlled voltage and frequency on its ac output. The TX inverter qualifies and synchronizes with the ac-voltage reference provided by the XW just as it would if the utility grid was present. The XW’s anti-islanding feature prevents the export of power from its AC1 connection during a utility outage, and the XW and the TX inverter continue to power backup loads. Any excess power from the TX inverter charges the battery bank in a Bulk Only mode.

SYSTEM REGULATION 
When the Conext XW inverter/charger is in Invert mode, electrical current flows through it in either direction. If the Conext TX inverter is providing more power to the ac bus than the loads can consume, current flows back through the XW to charge the battery bank. Unlike in Charge mode, in Invert mode the XW does not regulate charging when power is flowing from its ac output to the battery. During brief grid failures, this is not a problem if the battery is sufficiently discharged. However, if the battery is fully charged and there is not enough load on the ac system, and if the TX inverter continues feeding power to the ac bus, the battery voltage could potentially rise until an overvoltage fault condition (high batt cut out setting) is reached. This causes the XW and the entire system to shut down, including the ac loads terminated at the subpanel, and could result in damage to the battery if high batt cut out is not set appropriately for the installed battery type.

To avoid this mishap, the Conext XW inverter/charger features integrated frequency-phase–shift protection for ac-coupled applications. This strategy varies the line frequency according to a predetermined pattern to prevent the grid-direct Conext TX inverter from overcharging the battery. The XW executes a pattern-generator algorithm that varies the line frequency in a linear manner to avoid overload. The frequency-generation function of the XW changes the ac-coupled grid frequency with a linear rate of change of 0.4 Hz/s. When the charge-bulk voltage is exceeded, the frequency decreases in a linear progression until the TX inverter drops off-line. While the XW and TX inverters are in AC-Coupling mode, the XW changes the frequency only when the charge-bulk voltage setting is exceeded. You can adjust this setting in the custom battery menu.

Once the battery voltage reaches its charge-bulk voltage, the Conext XW shifts its output frequency, causing the TX to disconnect and begin its 5-minute anti-islanding waiting period. No separate control wiring is required. During this period, the ac loads are powered from batteries via the XW only. When the 5-minute waiting period is complete, the TX reconnects to the XW’s ac output and provides power for ac loads and recharging the battery. If the battery is fully charged and the ac critical loads are insufficient to absorb the PV array’s ac output, the TX on-off cycle continues until the grid is restored and the system returns to normal operation.

EQUIPMENT, DESIGN, INSTALLATION AND OPERATION 
Schneider Electric’s Conext XW inverter/chargers and TX inverters have been developed to provide a fully integrated ac-coupled system. However, in some applications, ac-coupled systems can be more complex than their dc-coupled counterparts, and integrators tend to have less experience with ac-coupled systems. Here I address specific equipment specifications and use of Conext inverters in ac-coupled systems, as well as general design details that you should consider related to ac-coupled system architectures.

Article Discussion

Related Articles