AC Coupling in Utility-Interactive and Stand-Alone Applications: Page 2 of 16

Magnum Energy Battery-Based Inverters

By Gary Baxter and Brian Faley, Magnum Energy

Magnum Energy designs and manufactures battery-based inverters for use in stand-alone applications and gridconnected systems that require battery storage to provide uninterrupted power during utility-grid failures. In both applications, Magnum Energy permits and supports ac-coupled system designs that synchronize the ac output of utilityinteractive string inverters from various manufacturers with its battery-based inverter/chargers. We have made specific software upgrades to some of our inverter models to better support ac-coupled systems, and we have additional related products in development.

Recommending the addition of a battery-based inverter system to your clients with grid-direct string inverter systems can be a profitable up-sell. Even when customers have been clearly informed that their grid-direct PV system will not produce power during utility outages, they may have an after-thefact realization that they do want a backup system after they experience frequent or extended utility-grid failures.

With the addition of a battery bank, a critical-loads subpanel and a diversion control and load, Magnum battery-based inverter/chargers can be ac-coupled with existing grid-direct inverter systems. The dc side of the existing string inverter system does not need to be rewired. The ac input and output circuits of Magnum MS-PAE inverter/chargers can be connected in parallel with a home’s ac wiring without damage to the inverter. These models can operate as a stand-alone inverter/ charger in Standby mode, which allows battery charging from the grid with ac transfer to connected loads. Magnum MS-PAE inverters are listed to UL 1741 as stand-alone inverters, not as utility-interactive inverters. These products are not designed to export power to the utility grid. Therefore, the ac output of the inverter should not be connected directly to the utility power-distribution circuits. These inverters can operate in parallel with the ac-wiring circuits only when the utility power is connected to the ac input of the inverter and the inverter is operating in Standby mode.

SYSTEM OPERATION 

When working with ac-coupled systems, it is beneficial to understand how a battery-based inverter operates before and during a power outage. During normal operation (see Figure 1), the utility grid provides the ac voltage and frequency reference required for the synchronization and operation of the string inverter. When the Magnum battery-based inverter/ charger operates in Standby mode, the inverter utilizes the utility grid and the ac output of the string inverter to maintain the battery bank’s state of charge and passes power through to the circuits in the critical-loads subpanel. In such a system, the Magnum inverter/charger is connected as a backup inverter and supplies inverted ac power from the battery to the critical loads only when the grid is down. The grid-direct string inverters are connected on the load side of the Magnum inverter, in parallel with the reference grid power when available or with the inverter power during a grid failure.

During a utility-power interruption (see Figure 2), both the battery-based inverter/charger and the utility-interactive inverter automatically disconnect from the grid. Once this occurs, the battery-based inverter begins inverting and initially uses energy stored in the battery to power the critical loads connected to the ac subpanel. Because the ac output of the battery-based inverter is connected to the same circuit as the utility-interactive string inverter, the string inverter synchronizes with the battery-based inverter’s ac-output voltage and frequency. After a required 5-minute disconnect, the string inverter reconnects and starts processing the power from the PV array, charging the battery and supplying power to the critical loads.

With the string inverter synchronized to the output waveform of the Magnum inverter/charger, the string inverter processes all the energy the PV array generates. When the grid is present, household loads normally consume this energy, with any excess exported to the utility grid. During a power outage, however, noncritical loads terminated in the main service panel are not connected to the system, and the utility grid is no longer present. As a result, any excess power that the loads connected to the system’s subpanel do not consume is pushed back through the ac output of the battery-based inverter and into the battery bank.

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