The Bassett system features an AC-coupled wind system, lithium-ion energy storage, and the flexibility of a Princeton Power Systems DRI-10 inverter. AC-coupling is when the AC output of a generating source (like a grid-tied PV system, wind-electric system, or even a genset) is connected to the inverter’s AC (load) port. Being bidirectional, this port allows AC to flow either in or out.
Several advanced, battery-based inverters have the ability to simulate a stable utility grid on their load port (AC output)—whether or not the utility grid is connected—so that any attached inverters will sync their outputs. This allows adding batteries without getting rid of existing batteryless grid-tied inverters, and without reconfiguring the PV array with DC charge controllers. The Bassett system AC-couples the wind-electric system rather than the PV system, which is managed on the DC side via the DRI-10’s internal MPPT charge controller.
Care must be taken to make sure that the AC-coupled system’s power is managed well, because unlike the real grid, the load port of such an inverter is not an infinite sink for backfed energy. Some mechanism must be able to curtail energy input from the AC-coupled generating sources, if the batteries become fully charged, the grid is down, and the house loads are not enough to consume all the energy.
If you have a wind generator that must always have a load to avoid turbine overspeed, and thus can’t simply be open-circuited like PV can, a bit more strategy is needed to allow the wind turbine to “sense” the status of the inverter load, grid, and battery ports. We added voltage sense relays (for load and battery; it already sensed the grid) to the Weaver 5 wind controller so that it furls as necessary to curtail turbine output.
The other not-so-common feature of the Bassett system is the 384 V (120 cells times 3.2 V per cell) lithium-iron-phosphate battery attached to the DRI-10 inverter. This inverter can work with batteries ranging from 250 to 600 VDC—very different from the 12, 24, or 48 input voltages commonly used in other battery-based inverters.
While the DRI-10 has a battery management system (BMS) tailored for lead-acid chemistry, it also has an external BMS option in its firmware. We used this option with the BMS supplied by the lithium battery distributor to monitor the voltage and temperature status of all 120 cells and to automatically disconnect the battery from the inverter should the BMS sense an under- or over-voltage in any cell.
We also created a Web interface for both the DRI-10 and the BMS so that the designer, installer, and customer can monitor important system parameters, such as battery, PV, and grid status, at all times.