AC coupling for battery hybrid grid interactive inverters usually do use the attributes of anti-islanding to control GT inverters,.
It can work differently for high freq hybrid inverters and low freq (heavy) hybrid inverters.
A hybrid grid interactive inverter synchronizes its inverter to grid. A relay on AC input closes, after inverter has matched grid, and the inverter and grid are operating in parallel, just like multiple utility power plants do. If a hybrid inverter pushes power to grid it must meet all the requirements of anti-islanding that a normal GT inverter does.
First thing, generated power has to go somewhere.
When a low freq inverter is working without grid power on batteries with AC coupled GT inverters on its output, power generated by PV GT inverters is normally consumed by house AC loads and/or back fed into hybrid inverter where it is used to charge battery. Without grid being present, the hybrid inverter is acting as surrogate grid for the GT inverters.
Problem comes in when PV GT power production exceeds demand. The LF hybrid inverter will change its output frequency slightly to be out of the narrow freq range of a valid grid specification. The anti-islanding function in GT inverter senses the out of spec frequency and shuts down which solved the over production problem. Some newer GT inverter respond to a slight AC freq shift with a reduction in their output power. This allows the hybrid inverter the ability to back down GT production to meet the AC load demand without totally shutting off GT inverter production.
HF hybrid inverters have a problem. They don't have the large heavy power transformer. HF inverters are two stage. First section is a DC to high voltage DC converter. Second stage PWM chops the HV DC and filters it to make sinewave output.
Problem for AC couping in the HF hybrid inverter is it has to make a mode change on the first stage DC to HV DC converter to reverse its direction. This takes more time then a LF hybrid inverter which is immediate.
The toughest thing for AC coupling is the situation where PV production is almost meeting AC load demands and a small about of power is being added by hybrid inverter to suppliment the total AC load. Then someone turns off a large AC load and all of a sudden there is more GT power generation than is being consumed.
A LF hybrid inverter will start the process of shifting AC output frequency, which it should not do too quickly because of possible induction motor AC loads, to shut down GT inverter production. This process can take a couple of seconds, mean time the batteries are absorbing the overproduction.
A HF hybrid inverter has to make a mode change to push power to batteries. It is not fast enough to prevent over PV production from causing problems with the HF hybrid inverter HV DC supply level. To prevent blowing out the HV DC filter capacitor, a HF hybrid inverter will usually employ a relay to immediately disconnect the PV GT inverter. SolArk feeds the GT inverter PV input through the Generator input port so it can use the generator connect relay to pull the plug on connected PV GT inverter. The GT inverter's anti-islanding sensing the 'grid' going down shuts off.
Another way for a HF inverter to handle the immediate over production is to have a 'dummy load', like perhaps a connected hot water heater, to absorb the overproduction until its DC to HV DC converter reverses direction.
It is theoretically possible to make a true bi-directional immediately responding high frequency switching DC to HV DC converter but no one has figured out how to do it cost effectively and reliably.
