AC Coupling in Utility-Interactive and Stand-Alone Applications: Page 14 of 16

My first experience with ac coupling was in 2003 while I was doing graduate studies in renewable energy in Germany. For my thesis project, I had the opportunity to design and install a hybrid PV-and-generator minigrid system that provided power for a rural boarding school located in Bulyansungwe, Uganda. This project included two 1.7 kW Sunny Boy 1700E inverters and a 3.3 kW Sunny Island 3300, and was one of the first deployments of SMA’s ac-coupled Sunny Island system.

One of the key advantages of ac coupling is higher systemproduction efficiency. Here in Washington state, we have a production incentive that pays an elevated rate per ac kWh produced. With an ac-coupled system, we can take advantage of the high conversion efficiencies of grid-direct string inverters— instead of the lower efficiencies of battery-based inverters. This results in a substantial increase in production incentive payments. The increased efficiency is also a benefit for off-grid systems with a high percentage of daytime loads.

The frequency control used in SMA’s Sunny Island system allows the Sunny Island battery-based inverters to incrementally decrease the Sunny Boys’ production when the batteries are approaching a full state of charge and ac loads are low. This is a very elegant solution. There is a real advantage to keeping the array generating at partial capacity instead of turning the grid-direct inverters completely off and pulling energy from the batteries. This can, of course, also be accomplished in mix-andmatch inverter systems by using a diversion controller and load.

One of the things to watch for in ac-coupled system design is that the battery-based inverters typically cannot pass through to the grid more power than their rated inverting capacity. This means that the battery-based inverter capacity has to be at least as large as your grid-direct string inverter capacity, which can present an unfortunate bottleneck when you have the typical large grid-direct system and only a small requirement for backup power. To solve this scenario, you can use an automatic transfer switch or divide the grid-direct inverter and array capacity between the main panel and the critical loads panel.


In 2008, Oasis Montana installed an off-grid ac-coupled system that integrated two Sunny Boy 3000-US inverters with a quad stack of four OutBack VFX3648 battery-based inverters. Because of tall trees around the home, the 5.6 kW polemounted PV array was located 685 feet from the inverters and battery. By going with ac coupling, we were able to configure the array at high voltage, which reduced wire costs significantly. The ac-coupled approach did require more components and increased the complexity of the system, however.

In an ac-coupled configuration, ac power produced by the Sunny Boy inverters is back-fed through the ac-out terminals of OutBack VFX inverters. When you back-feed the Outback inverters, all the charging parameters are bypassed and the charge you are applying to the batteries is unregulated. In this system, we used a series of relays to switch dump loads on and off, simulating a three-stage charging process.

A Morningstar Relay Driver monitors the battery-bank voltage and controls four different relays. The first three relays control three 1,500 Wac electric heaters. Each relay is set at a slightly higher regulation voltage. A fourth relay is set at 62 Vdc and acts as our fail-safe in case one of the three diversion loads malfunctions. This relay is configured to disconnect the combined 240 Vac output of the two Sunny Boy inverters.

In the past few years, higher voltage dc-charge controllers such as the MidNite Solar Classic and Xantrex XW-MPPT80-600 have come to market. On midrange to long wire runs, consider using a higher voltage charge controller rather than ac coupling. If you decide to go with ac coupling, I would recommend the SMA Sunny Island system because it is fully integrated. Based on our crew’s experience, we would not recommend mixing equipment from different manufacturers in residentialscale off-grid ac-coupled systems. We learned this the hard way. Some grid-direct inverter manufacturers do not support or warranty their products in ac-coupled applications.


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