The vast majority of grid tie PV systems are useless when grid failures occur, shutting down at the very time that the electricity generated on site is most needed. There are a number of ways to ensure that a PV array does not become stranded during utility outages, the common thread between them all is that they use energy storage, additional inverters and a variety of different power electronics depending on the system design. SMA offers a “secure power” feature that puts out up to 15 amps AC during utility interruptions however the output varies with the available solar resource. Because the output of these inverters is not stable and they don’t provide power at night they are not addressed in this article.
Each of the systems outlined below will achieve the goals of providing secure AC power during utility outages. The systems all utilize existing onsite PV for recharging batteries and powering daytime loads.
- Battery Based Grid Tie Systems have a long and proven history. Some of the very first residential grid tie systems in North America were battery based and more than twenty years later many are still going strong. However, these systems are less efficient during grid-tied operation making them less cost effective. They are generally not a good choice unless the utility connected customer wants the “off grid” experience. Examples include the Schneider XW series, and Outback GTFX modes.
- AC Coupled Back Up Power Systems use a battery-based inverter/charger to “fool” the GT inverter to “thinking” that is still connected to the grid by supplying it with a AC signal. Examples include Outback Radian and some Magnum inverters. Once the GT inverter “qualifies” the AC signal (five minutes) it starts producing power from the array, which is sent to the AC charger input of the battery based inverter/charger. The power is used to power daytime loads and recharge the batteries. AC Micro inverters are also used in AC coupling applications. The principle is generally the same as with the larger string inverters although they offer somewhat more design flexibility due to their more modular nature.
More advanced AC Coupled PV systems are designed to operate in stand-alone mode or utility interactive mode with a more seamless operation between the GT inverter and battery-based inverter/charger. Examples include the SMA Sunny Island series. These systems integrate a battery-based inverter/charger that takes over controlling the grid-tie inverters when utility failures occur. They are very efficient, represent the current state-of-the-art technology and are the most costly choice for most emergency power applications.
Retrofitted AC Coupled systems can represent a series of design compromises.
Systems must be designed so that the backup inverter and battery bank can handle the entire output of the grid tie inverter plus a safety margin. This results in system designs that are often unnecessarily large for the intended purpose of providing a limited amount of backup power during utility failures.
Although AC coupled solutions can be more efficient for daytime loads, the battery charging is less efficient than traditional battery based grid tied systems that use a solar charge controller to charge the battery directly from the array.
Another casualty with the less advanced AC Coupled design approach is the quality of battery charge control during power outages. These systems often use a single stage approach that turns the GT inverter off when battery voltage reaches a predetermined level. Battery voltage is allowed to drop until a “reconnect” threshold is reached at which time the GT inverter is turned back on. After a 5-minute qualifying period the GT inverter begins supplying AC power for use by local loads and the battery charger again. This single-stage method of charging may reduce the state of charge at the end of the day by as much as 20%. Unfortunately, the AC Coupled system enters the second day of an extended power outage with a battery of diminished capacity.
There are a number of pre- engineered AC Coupled system designs that use Morningstar controllers to improve the effectiveness of battery charging management. In string inverter applications Morningstar TriStar diversion controllers are used to “burn off” energy to keep the battery voltage below the threshold that turns off the GT inverter. In micro inverter systems, the Morningstar Relay Driver is used to control groups of micro inverters. When demand for current decreases, the micro inverters are turned off in groups. By doing so, decreasing amounts of the available PV energy can be supplied to the batteries to support their requirements. When demand increases, the Morningstar Relay Driver reconnects the AC signal to the micro inverters that are turned off. After the five-minute qualifying period, they begin to contribute to the load and charging again.
DC-coupled back-up power systems divert the output of the PV array away from the GT inverter to a MPPT charge controller to convert the PV array string voltage to battery voltage (usually 48 Vdc). They operate independently of the GT inverter and as such the connected battery based inverter/charger or UPS system can be sized to the customers’ needs. This design approach often results in a simpler system that can be installed at a lower initial cost.
Advantages of DC-coupled systems
DC-coupled systems, by their very nature, put more power back into the batteries during a power failure. The MPPT controller uses 4-Stage voltage regulation to fully recharge the batteries during the solar day (providing sufficient solar resource is available). This is a very important advantage if the power outage extends beyond a few hours and into the evening hours because batteries in a DC-coupled system will end the first and subsequent days of a power outage at a higher state of charge.
An additional advantage of this system is that it can accept an “oversized” PV array without harm to the controller. During sunny conditions the controller simply “ignores” input power that is in excess to its capabilities or requirements. During cloudy weather the controller can harvest energy from the entire array and as such the likelihood of achieving a full charge is greatly increased.
The inherent design flexibility of a DC-coupled system allows the inclusion of backup power into one leg of a three-phase PV array. The design paradigm also allows DC installation on sub-arrays in large systems.
DC coupling can also be used in combination with an AC coupled system. This can be useful where there there are multiple string inverters. This increases the charging capacity of the system in a more cost effective way than having to add more AC coupled inverter/chargers for each additional GT inverter. The effective capacity of the installed battery is “increased” because the DC coupled part of the system completes a full charge cycle when sufficient solar resource is available. This also solves any problems with a low voltage disconnect with the AC coupled inverter/charger since the solar controller will remain on when the inverter/charger has shut off due to low voltage disconnect.
Written by Douglas Grubbs, Product Applications Sales Engineer, Morningstar Corporation