AC Coupling in Utility-Interactive and Stand-Alone Applications: Page 6 of 16
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Integrators would undoubtedly welcome microinverters with full warranties and support for use in ac-coupled systems. However, the reality is that the potential market for these products in ac-coupled systems pales in comparison to the size of the grid-direct market that microinverter and ac-module manufacturers are focused on. While only a few of these manufacturers have devoted resources toward the testing and support of their products in ac-coupled systems, it is likely that additional manufacturers will follow suit as ac-coupled systems become more common.
CONTROLLING EXCESS GENERATION
Determining the best approach for regulating excess PV generation is a primary challenge when designing an ac-coupled system for stand-alone or utility-interactive applications. This is further complicated when systems utilize equipment from different manufacturers. In dc-coupled systems, traditional methods of controlling PV power involve some variation of pulse-width modulation (PWM) to regulate the output of the PV array and optimize battery charging. However, PWM regulation approaches do not apply to ac-output regulation for a grid-direct inverter. Therefore, the typical methods available for regulating the energy balance in ac-coupled systems are to either knock the grid-direct inverter off-line using a blackout relay or frequency-phase shift, or absorb excess generation using diversion loads.
The ideal regulation strategy for a specific application varies based on the amount of time the system is expected to operate in Off-Grid mode. In grid-tied, ac-coupled systems, the string inverter spends most of its time in Grid-Direct mode, where it is synchronized to the grid’s voltage and frequency reference. Assuming a relatively stable utility supply, the string inverters are ac coupled with the battery-based inverter’s output during infrequent, short-term outages only. In this case, it is acceptable to rely on a simple control method such as blackout relays or the frequency-shift functionality provided by some batterybased inverters to drop the string inverters off-line. If the system is off-grid or the utility-power source to the site is unstable, a more sophisticated regulation approach is recommended.
Knocking the PV generation off-line using frequency shift is perhaps the easiest, least expensive way to control excess generation. However, when the system is operating within IEEE regulatory limits, this approach is an all-or-nothing solution. Once the frequency exceeds a window of 59.3 Hz to 60.5 Hz, the grid-direct inverter disconnects and ceases to export power. In addition, once the inverter is dropped off-line, it remains off-line for 5 minutes, and the battery must be able to support the full ac load for the duration of the string inverter’s waiting period. This is often referred to as the 5-minute sledgehammer. Using a blackout relay to drop the string inverter off-line results in a similar system operation.
If an ac-coupled system includes multiple string inverters, a more sophisticated approach is to utilize multiple staged voltage-controlled relays to regulate each inverter’s output. This method provides more granular regulation by shedding PV generation in smaller increments. In this configuration, as the batteries approach full charge, a portion of the PV array can be knocked off-line, leaving the remainder operating to support the ac load.
Compared to frequency-shift or relay-based regulation approaches, using diversion loads to control excess generation provides more stable and reliable operation, as well as more sophisticated battery-charging functionality. A diversion controller regulates in seconds or milliseconds and provides much finer resolution than a 5-minute array/string inverter disconnect. PWM diversion controllers shunt excess power to a dc load and provide a tapered, temperature-compensated charge to the battery. Depending on the size of the array, one possible limitation is that large dc-diversion loads may not be common or readily available. However, ac-diversion loads such as space heaters and water heaters are inexpensive and commonly available, and can put excess PV generation to good use. When using dc- or ac-diversion loads, remember that they must always remain available and should be sized to absorb the maximum PV generation expected. To protect against battery overcharging, you should build some redundancy into diversion-load systems.
All OutBack FX and VFX inverter models include a 12 Vdc programmable auxiliary output with adjustable standard algorithms that can drive an external relay or contactor to control excess generation. OutBack’s Radian inverter includes a 12 Vdc auxiliary output as well as a 10 A 250 Vac/30 Vdc dry-contact relay that can independently control diversion loads or supplemental relays. One counterintuitive aspect of the OutBack inverter auxiliary programming is that the DC Divert function is preferred when systems are ac coupled.