If only a few people had grid tied solar, we would not need complex grid codes. But thanks to Germany and Hawaii, a problem did surface. In the middle of the day, when all of the solar panels were cranking out maximum power, the grid actually started to unload. The power plant turbines could not react fast enough. Even through they were throttling down, the turbine generators started to spin a little faster. This caused the grid frequency to rise. Under the old rules, once the frequency hit a certain level, ALL of the solar grid tied inverters just shut off. This was really bad. The throttled down generating plants went from basically unloaded, to instantly having to supply the entire load. They could not handle that load, and they slowed very quickly, and the system basically crashed.
Frequency watt control and specific delays in the system are there to stabilize the grid. IF the frequency starts to climb up, then the solar inverters can quickly respond and reduce how much current they are pushing to the grid. The utility (SHOULD) still throttle back when they see this happening, but it gives them a lot more time to get it right. As the frequency starts to drop back down, the solar inverters ramp back up and keep the grid stable. In nearly 3 years of logs on my Enphase gear, I only saw one time, before the XW was installed, where the grid frequency went up enough to cause a power dip. A few of my iQ7 inverters did go offline, but the system recovered and was back to full production in under 3 minutes. It didn't even take the normal 5 minutes. It was all back to normal on the next log entry. I think the short delay was because the grid never lost voltage. Another time the grid frequency rose, I am not sure how long it really lasted. My XW-Pro actually disconnected from the grid and went to 60Hz on my sub panel. That caused my iQ7's to go back to full production, and it was charging my battery. The XW-Pro re-qualified the grid and it was back at 60 Hz and full production in a total of about 6 minutes.
On a real power failure, my system was running off grid for several hours. When the clouds cleared and we got full sun on the panels, the solar was pushing over 2,000 watts back into the XW. Had I ben home, I would have raised my maximum charge rate, as I had it set to charge at just 18% with the grid up. This low current setting caused the XW-Pro to ramp the frequency up to try to ramp the solar down to maintain my set charge current. I can see in the logs where it caused 5 of my iQ7's to shut off. Then the current would fall, the XW would ramp the frequency back down, the inverters turned back on, and the cycle repeated several times.
I have not gotten a solid answer, but I do nave a hypothesis. I think Enphase's Rule-21 grid code is only changing the power limit. My iQ7's are rated for 240 watts. But if they are only making 120 watts, the frequency can shift quite a lot before it will try to ramp below 50% power to start reducing output. The XW ramped a bit too high, and due to tolerances, 5 of them shut off from grid frequency too high, the other 11 stayed working. On each cycle, it was always the same 5. Too bad, I was not home to truly monitor it in real time, I had to go from what was saved in the logs. No one in the house had any idea this was going on. They knew the power was out, and the stuff running was off the batteries and solar, but nothing ever showed a problem with the power. Had I set the charge current higher, it may have even been able to top up the batteries before sun down, but with it limiting to 18% charge rate, it only got them up to 75%. Not too bad.
The grid codes are not perfect, and so far the utilities have not tried to exploit it. I check my grid frequency from time to time, and check my logs. If one day the grid is sitting at 61.5 Hz all day, then we know they are trying to turn down our solar production for no reason. But if they do that, the utility also needs to produce more power, costing them money as well.
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