If the PV panels in a distribution area generate more power than the area consumes the inverters will keep pumping the voltage up until it flows back out of the distribution area and into the main grid, if possible. Then when a cloud comes over, the opposite happens. In fringe areas with lots of PV panels and old lines this can cause voltage fluctuations of 30-50V p-p.
When the voltage gets too high the inverter is designed to cut out. But it will turn back on again when the voltage drops below a certain level. Since inverters don't talk to each other they can do this in unison which can fluctuate the voltage wildly.
All this can result in a lot of angry power customers. The PV owners complain that their PV systems are not exporting their expected output due to over voltage cutout. And many customers will complain that their appliances keep cutting in and out at various times of the day.
The power provider can try to fix this by re-tapping the distribution transformers to change the voltage but this wont work if the lines are old and have too much resistance and are now being expected to deliver the power in both directions due to the PV systems on the roofs.
Also, the power stations will try to throttle their generators to balance the load on the grid, maintain constant voltage and achieve their quota. This has to be done with great care and cooperatively to prevent the grid from becoming unstable.
But if there are a lot of PV & wind systems and no direct control of them, the grid can become like a wild beast that is difficult to control. Like a car that sometimes runs on all cylinders and sometimes not. Inverter manufacturers are attempting to help solve this problem by developing smart inverters that can accept communication from command central. But then the power companies might turn off the PV panels so that they can sell more power themselves and make more money! Who knows?
