AC Coupling in Utility-Interactive and Stand-Alone Applications: Page 15 of 16

Here in New York, I am seeing an increase in existing customers wanting to add battery backup to their grid-direct systems, as many of them experienced power outages for a week or more after Hurricane Irene last season. When the grid goes down, clients suddenly become aware that their PV system is not functional. During and after an extended power outage, that realization really starts to settle in. Because it doesn’t require any rewiring on the dc side, ac coupling is a good approach for retrofitting these systems. However, it has the disadvantage of adding system complexity and therefore cost. In New York, the financial incentives are based on kilowatts of installed solar modules, so there is no additional incentive money for ac-coupled systems.

What currently makes the SMA ac-coupled approach work well is that installers can set up all of the inverters to communicate with each other. However, in a retrofit, if the Sunny Boys and the Sunny Islands are not relatively close to each other (say the Sunny Boys are out in a field with the array and the Sunny Islands are down in the basement), then the communication part of the system can become problematic. The standard approach is running a hardwired communication cable between the inverters. However, it may not be practical or possible to connect them in retrofitted systems. In that case, the Sunny Island inverters still use frequency shift to regulate battery charging, but do not modulate the string inverter’s power output as they would with the communication cable in place. Using only frequency shift, the Sunny Boy inverters drop completely off-line and have to go through the 5-minute waiting cycle, which repeats until the combined load in the critical loads subpanel and the battery-charging power requirement is equal to or greater than the output of the Sunny Boy inverters.

We have installed ac-coupled systems only with SMA equipment and have some suggestions for how SMA could improve integration of these systems. The first is to make the Sunny Islands capable of providing 120/240 Vac output so that only one Sunny Island battery-based inverter is necessary. At present, either two Sunny Islands or a step-up/ step-down transformer is required. Two ac inputs, one for the grid and one for a generator, would be handy as well. Currently, you need to add a separate automatic or manual transfer switch if you want to include grid and backup-generator charging.

KENT OSTERBERG, SENIOR ENGINEER, BLUE MOUNTAIN SOLAR

One of the more common applications for ac-coupled systems is for homeowners who decide to add battery backup to a gridtied PV system. In most cases, I suggest to these customers that a generator is a better choice for occasional backup power. Folks in hurricane areas may be able to make a better argument for using batteries to back up grid power. If the PV array is a long distance from the batteries, they should consider ac coupling. However, with higher-voltage charge controllers such as the MidNite Solar Classic 250 and Xantrex XW-MPPT80-600 available, I would consider those alternatives first.

Keep in mind that in utility-interactive systems, the griddirect string inverter that is set up for ac coupling with a battery-based inverter is not operating in AC-Coupled mode except when grid power is down. So 99% of the time, you will not experience any of the annoyances that come with a specific ac-coupled system. If the system is off-grid, ac-coupled operation is the normal routine and any problems become quite apparent. For an off-grid system where the ac-coupled inverter is normally the primary charging source, an external diversion controller and load should be used in mixed inverter systems. Additionally, making the system fail-safe is important. For example, if the diversion controller is inadvertently turned off or fails, a relay-based approach should be in place to disconnect the ac-coupled inverter.

When considering ac coupling, one important question is how much support the inverter manufacturer is willing or able to provide. At one end of the spectrum, SMA has a fully integrated product line and a training program for its Sunny Island system. Other battery-based inverter manufacturers may offer only a short white paper on using their products in ac-coupled applications, and their technical support staff may have limited experience with these systems.

Benefits and Drawbacks of AC Coupling

Depending on the application and equipment used, ac-coupled systems may offer several advantages over their dc-coupled counterparts. In large stand-alone microgrid systems, such as those found in rural electrification projects in the developing world, ac coupling provides an extremely scalable and modular platform that can integrate multiple charging sources located throughout the system’s distribution network. Large standalone projects here in the US may also benefit from an ac-coupled rather than a dc-coupled design approach. The inverter/ battery cluster platform developed by SMA that is common in these large systems allows segmentation of battery storage into manageable bank sizes that limit the number of series battery strings required to achieve sufficient storage capacity.

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