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Application Note

When the sun comes out the next day, the system reverts to the power flow as shown in Figure 3, with any extra

energy used for recharging the batteries. If no excess is available then some manual load-shedding (divesting of

less critical loads to prioritize the most critical, such as lighting and refrigeration) may be necessary through the

turning-off of the critical load panel connected devices until the batteries are charged. If the critical loads are

absolutely essential and load shedding is not an option, then adding a generator to the backup system can satisfy

the critical load demand while charging the batteries.

One might ask: “why not just add the generator and forget about the BB based inverter? “That might be a viable

option for some, but there are some important things to consider before making that decision.

1. The generator will need to run the entire time that electrical energy is demanded by the building’s loads,

perhaps 12-18 hours per day. Aside from the noise factor, many low-cost generator motors require

frequent maintenance, and are very inefficient at low power output.

2. As an example, higher efficiency can mean the difference between filling up a 5-gallon gas tank once a day

versus once a week if the generator is tied into a battery-based backup system that is running the

generator just a couple of hours a day. News photos and videos of the aftermath of extreme weather and

other emergency events show people standing in long gas lines for hours, and even then not always

successfully. Many of the gas stations that had gas couldn’t pump their storage tanks without available

grid power!

3. In a BB inverter system augmented with generator power, the batteries can greatly extend generator run-

time—the same fuel will go a lot further since the generator does not need to run 24/7.

Typical AC-Coupling solutions: Like any solution, the devil is in the details, and AC Coupling a GT and a BB

inverter together is no different, especially if one wants a “one size fits all” solution for all applications. That “one

size fits all” package usually includes one or more diversion loads, possibly a blackout relay or some other method

to take the GT inverter offline to prevent it from overcharging the batteries on the BB inverter. And it requires a

lot more upfront design to be sure all devices in the system can handle all possible conditions.

Some battery-based inverter manufacturers attempt to simplify their AC Coupling implementations by “dithering”

or altering the frequency of their power to the GT inverter outside its operational window of 59.5 to 60.5 hertz.

This will effectively shut it off when the batteries are full instead of using a blackout relay, thereby saving the cost

of the relay. However, in many instances frequency dithering prohibits the use of a generator as the generator

frequency is not always stable enough for the GT inverter to synchronize to. Even if it synchronizes, there is a risk

of back-feeding and damaging the generator in low or no load conditions.

It should also be noted that the “one size fits all” AC Coupling solution really needs diversion loads to divert any

excess energy in systems that either have too much PV power and/or too small of a battery bank which could put

dangerous charge levels into the batteries. While it can be argued this energy can be used to heat water or run

pumps, that “benefit” often comes when not necessarily demanded, and if the diversion load can no longer

accept the available energy, the GT inverter must still be shut down. In addition to the complexity, diversion load

implementations can also be expensive, requiring undesirable invasive installations which can make this design a

poor choice for those who want simplicity and cost control in their AC Coupling system. The extra expense of all

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PV Powered Grid-Tied Bedienungsanleitung Seite 6