AC Coupling in Utility-Interactive and Stand-Alone Applications: Page 4 of 16
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OutBack Power Technologies designs and manufactures a full range of products, including stand-alone and utilityinteractive battery-based inverter/chargers that can be utilized in both dc- and ac-coupled systems, and high-efficiency MPPT dc-charge controllers. While ac coupling seems like a simple concept, ac-coupled systems have nearly endless ramifications, implications and subtleties. The following insights and guidelines, based on my experience supporting the integration of OutBack products in ac-coupled systems, will assist you in the development of optimized and reliable ac-coupled installations and help you make informed and accurate choices when deciding between ac-coupled and dc-coupled system designs.
SYSTEM DESIGN AND OPERATION
Grid-direct inverters are current-source inverters. They convert power generated by a PV array from dc to ac, but rely upon an external ac source to operate because they cannot create an independent ac-voltage waveform. In contrast, multi-mode battery-based inverter/chargers are voltagesource inverters. In Stand-Alone mode, they generate an ac voltage and frequency supply independent of any external ac power source. Some models can also operate in a Utility- Interactive mode that exports excess PV array generation to the utility in a similar fashion to grid-direct inverter systems.
Battery-based voltage-source inverters can provide a stable ac voltage and frequency reference that allows griddirect current-source inverters to operate when the grid is not present. In this operational mode, ac PV generation from the string inverters is synchronized with the batterybased inverter output via a critical-loads subpanel and is consumed first by local loads, including battery charging. If loads exceed PV production, the system meets the deficit with energy pulled from the batteries.
In most ac-coupled systems, the battery-based inverter must reliably process the entire output of the connected ac PV generation from the string inverters under all conditions. My experience indicates that the battery-based inverter must be sized to more than 125% of the power rating of the connected ac-coupled array. Multiple OutBack Radian inverters can be stacked in parallel at 120/240 Vac. Our standard FX and VFX inverter models can be stacked in both series and parallel configurations to increase capacity as well. If a single FX inverter is ac-coupled with a 240 Vac grid-direct inverter, an OutBack autotransformer can be used to step down the grid-direct inverter’s output voltage to 120 Vac (see Figure 3 , p. 80). If multiple OutBack inverters are stacked to increase capacity in ac-coupled systems, the Power Save function must be defeated by setting the master-power save level to 1 on the master inverter and the slave-power save levels to 1 on all slave inverters. In addition, there should be at least 100 Ah of battery capacity at 48 Vdc nominal per 1 kW of array power to ensure that the battery capacity is sufficient to absorb the PV output without excessive heating, which can damage batteries or shorten their service life.
One aspect of ac-coupled systems you should consider is that the battery-based inverter must be able to provide a stable ac supply to meet the default IEEE regulatory limits of the grid-direct inverter. Battery-based inverters used in ac-coupled systems must have good voltage regulation and must be sized to support the largest expected motor-starting surge without allowing voltage to sag or spike. Any instability in the ac supply due to poor regulation during power surges caused by local loads or excessive voltage drop in the ac wiring results in unstable operation and reduced PV output due to the grid-direct inverter dropping off-line. In addition, integrators have learned—sometimes the hard way—that some grid-direct inverters are more sensitive to changes in ac supply than others and that several string inverter manufacturers do not support the use of their equipment in ac-coupled applications.
Another complicating factor is the integration of motordriven ac generators with ac-coupled systems. For backupgenerator– based system charging, the battery-based inverter must shift its frequency to synchronize with the new ac input source when the generator is running. Once connected, the generator becomes the new ac voltage and frequency supply for the system, including the ac-coupled string inverter. The initial inverter/generator synchronization often results in the grid-direct inverter disconnecting from the system. Few motor-driven generators can provide sufficient voltage and frequency regulation to meet the IEEE regulatory requirements for grid-direct inverters to ensure stable operation. More important, generator electronics can be damaged if they are subjected to back-fed current from the string inverters. Therefore, ac-coupled systems are often designed to prevent input from ac-coupled string inverters and generators at the same time. If this is the case, the generator must be sized to not only charge the batteries, but also power all ac loads to make up for the PV generation that is lost because the string inverters are off-line. Frequently, this scenario ends up requiring a larger generator than would be necessary if the PV array was simply dc coupled.