AC Coupling in Utility-Interactive and Stand-Alone Applications: Page 3 of 16
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Since this is not the normal path for incoming current, the battery-based inverter/charger cannot regulate the current or control the battery voltage. This brings up two important points. First, the Magnum battery-based inverter must be rated to handle the full power output of the PV array. The maximum ac-output power of the PV/string inverter system must be no greater than 90% of the continuous power rating of the Magnum inverter system. Second, the system must include a means to regulate the battery voltage to prevent an overcharge condition. You can best accomplish this with a two-step regulation approach that combines frequency shift with diversion control.
STRING INVERTER REGULATION
Some Magnum battery-based inverters include a feature that allows the ac-output frequency to shift when the battery voltage rises to a predetermined level. Magnum’s MS-PAE Series inverter/chargers (revision 4.1 and higher) include an AC-Coupled Support mode. When activated, this causes the inverter-output frequency to shift to 60.6 Hz. This mode is enabled using an optional Magnum remote that allows the battery type setting to be set to custom. It activates when the battery voltage increases by 2 V ( for 24 Vdc nominal inverters) or 4 V ( for 48 Vdc nominal inverters) above the absorb voltage setting. The frequency returns to 60.0 Hz when the battery voltage falls 2 V or 4 V below the absorb voltage setting for 24 Vdc and 48 Vdc nominal inverters, respectively. This frequency shift protects the battery against overcharging by dropping the ac-coupled grid-direct inverter off-line. Activating the AC-Coupled Support mode is one method of regulating ac-coupled string inverters. However, this battery-protection approach does not enable three-stage battery charging. We therefore recommend that frequency shift be viewed as a secondary approach to a primary diversionbased battery-management system that provides more advanced battery-charging functionality.
In most systems, dc or ac diversion loads should be added in parallel with the battery to reduce the on/off cycling of the grid-direct string inverter that frequency shift would otherwise control. Relying solely on the frequency-shift method is rather crude because it is essentially a bang-bang controller— off or on—and once the frequency-shift happens, the string inverter attempts to reconnect every 5 minutes. While this approach is technically sufficient to prevent battery overcharging, is it optimal to have the battery voltage repeatedly swinging around by several volts? A more sophisticated option is to employ a diversion-based battery-management system. These regulation systems can utilize a dc diversion controller with dc resistive loads and/or ac resistive loads driven by dc-controlled relays. In a dc or ac diversion system, three-stage battery charging is maintained and the surplus energy from the string inverters can be put to work rather than just taking the PV system off-line.
To better support ac-coupled systems, Magnum is developing the AC Diversion Controller, which is optimized for use with Magnum MS-PAE Series inverter/chargers. This controller maximizes the use of on-site–generated PV power by diverting excess energy to ac loads such as domestic waterheater tanks. It includes complete three-stage battery charging functionality and also supports inverter frequency shift as a fail-safe protection against battery overcharging. The AC Diversion Controller communicates with Magnum MS-PAE Series inverter/chargers and automatically diverts excess current into specified diversion loads. When a diversion load such as an electric water heater reaches its regulation setpoint, excess current is diverted to a resistor bank. Alpha versions of the system are currently operating very well with SMA and Solectria Renewables string inverters, and additional testing is under way. Magnum Energy has started the listing process for the AC Diversion Controller system with ETL. The product is currently scheduled for release in Q4 2012.
OutBack Power Technologies Battery-Based Inverters
By Phil Undercuffler, OutBack Power Technologies
For many PV system designers, a watershed moment comes when they realize that they can connect a grid-direct inverter to the output of a battery-based inverter. The flexibility and scalability of these ac-coupled system architectures provide benefits over dc-coupled systems in some cases. In the retrofit market, ac coupling may be a good solution when customers with existing grid-direct systems suddenly realize that their substantial investment in solar power is unusable during a power outage. Off-grid projects with large daytime ac loads and sites with multiple buildings or significant array-to-battery distances may also be good candidates for an ac-coupled design approach. However, today’s higher voltage dc-charge controllers have offset some of the commonly perceived advantages of ac coupling. In many applications, dc coupling can provide more reliable and stable operation than can ac coupling and ultimately makes more sense.