SMA Inverters - Ratio between the Nominal AC power of the PV inverter and the battery inverter
I am modeling a hybrid AC-coupled PV-diesel system using the SMA Sunny Design Web and saw that one of the parameters used in the portal to check the system compatibility is the ratio between the Nominal AC power of the PV inverter (Sunny Tripower) and the battery inverter (Sunny Island), which should be lower than 2. What is the reasoning behind of this? As far I know, the power capacity of the PV inverter depends strictly on the rated power of the PV array, while the power capacity of the Sunny Island depends on the peak power of the expected load.
Thanks a lot for sharing your thoughts on this topic.
The concern is that your PV inverter typically doesn't have great controls for curtailing it's power output, so if you have a 10kw AC PV output, it will try to pass that to the battery inverter to charge the batteries, and if the battery inverter's max AC input power is exceeded it could shut off or be damaged (which would cause an outage at the facility). That's why it's important to design the PV inverter power output to stay within the guidelines set by SMA's battery based inverters. Also, for AC coupling, Schneider recommends 1:1 ratio, and Outback recommends 0.8:1 ratio PV to Battery AC power ratios. Hope this helps!
Also, for the Sunny Islands to form a grid for the Sunny Boys to follow, they must hold a voltage (fixed) and frequency (slightly variable) - that means that the PV inverter(s) will be able to export their full rated power into that artifiacial grid - which includes the battery as a load.
With Schneider and Outback, they can only process about what they are designed to produce - and be able to cut off the PV inverter by controlling it with frequency shift to protect the battery from overcharging.
SMA has another feature which causes the PV inverters to reduce their power output instead of just shutting off, so when you use an SMA PV inverter and and SMA battery inverter, the ratio is 2:1. See the attached graph depicting SMA power curtailment in a Sunny Boy as Hz shifts from 60 to 62.