My comment was based on the startup process that I understand is required. There is a certain wait period and also a test of the line. If you have a 1000w micro inverter (for example) and it can cause a voltage drop or a frequency shift on the AC power that it sees, it is not seeing a power company type (very stiff) line. I think one manufacturer said something like, "a test to see if the unit could perturb the line". I would guess that it varies from mfg to mfg and from the size of the inverter how much it could try to perturb the line in this testing phase of startup. No real experience but I would suspect you would need several KW of solid output to trick a well designed unit. If I were going to do any testing, I would avoid UL1741 units and use something dumber.
Another comment. I have 9.6kw micro inverter grid tied that is about 250 feet from the house with the main wire run not oversized....since you only see max power for like one or two hours. At my meter the voltage is 245v. At the panel array the voltage when the array is at max output is about 250v, with that 5v being dropped across the 250 foot wire run. What those facts say is that the micro inverter will do whatever, including raising the line voltage, in order to deliver its power to the grid. Your portable power station is trying to old a 120v AC output and has controls to regulate that based on the load. The grid tied feed would look like a negative load and taking the real load toward or to zero. The portable unit would try to back off the output to hold its normal output and would soon be swamped by the grid tie output. Now would it shut down nicely or with damage, who know. Maybe, "it depends", which is always a great answer.
