Hi Marco; more below. Dan Fink Professor of Solar Energy Technology, Ecotech Institute IREC Certified Instructor™ for: ~ PV Installation Professional ~ Small Wind Installer Executive Director, Buckville Energy NABCEP Registered Continuing Education Providers™ 970.672.4342
On Sun, Dec 17, 2017 at 10:15 AM, Marco Mangelsdorf <[email protected]> wrote: > Aloha all, > > Here in the State of Aloha we’re leading the way as far as being the first > PV peeps to add AC-coupled storage to existing grid-tie NEM systems with > the benefit to the homeowner being having back-up power if/when the grid > goes down. > > During a grid outage the AC-coupled battery storage system effectively > provides a micro grid for the existing PV inverter(s) and allows the PV > array to power house loads and charge the battery. > > So what happens when the sun is out, the loads are all being met and the > battery is fully charged? Some AC-coupled back-up systems shift the AC > frequency higher to push the power quality out of the inverters’ specs > range thereby causing the PV to curtail its output. If the PV potential > goes down or the loads go up or the battery drops below 100 percent state > of charge, the frequency drops and the PV output goes up again. > > Makes sense so far. > > What I am concerned about is the fact that there are hundreds of thousands > of grid-tie inverters across our state dating back to 2001 when NEM first > came into being here. Over the past 16 years there have been many > different models and vintages with different operating parameters put into > service. > > Adding AC-coupled back-up power to existing systems with all these > varieties of inverters has me wondering how reliably and effectively all > these systems are going to work in the real world during a grid outage > period? > > Below are some of the questions I've come up with. > > * Will the existing PV inverters be able to effectively charge the battery > storage during grid outages? > *Yes, but the actual charging will be up to the new Multimode inverter you installed for the customer. * > > * Will the battery storage be able to reliably and effectively curtail the > output of the PV inverters during a grid outage by frequency shifting alone? > *That depends, there are 3 methods -- frequency shifting, blackout relay from the multimode inverter's aux battery voltage sensor or SOC meter, and a plain old fashioned diversion load controller and dump load.* > > * Just how high can the frequency be pushed up during a grid outage? Is > it dependent on time of day? Solar irradiation? Loads on or off in the > home? > *That depends on the manufacturer, see below. * > > * Will that curtailment of the PV inverters be gradual or all on or off? > *Frequency shift or blackout relay, all on or all off, often every 5 minutes as per UL1741. Don't tell your customer if they know anything about batteries, but in most places blackouts are rare and the 1-stage charge won't hurt anything for a couple hours or days.* > > * Will that high frequency shift during grid outages have the potential to > damage the PV inverters? > *Ask the inverter maufacturer, but I have never heard of that happening. * > > * Will that high frequency shift jeopardize the warranty coverage of the > PV inverters? > *Ask the inverter maufacturer, but I have never heard of that happening. * > > * Beyond potentially causing clocks and electronic equipment in the home > to behave abnormally during periods of high frequency shifting, will that > high frequency have the potential to actually damage loads in the home? > *Ask the inverter maufacturer, but remember that the frequency shift only lasts a couple seconds before it trigger the inverter into UL1741 mode. The multimode inverter takes over with the 60hz system "heartbeat" from there, very stable.* > > For example, the newer Enphase IQ micros now have a high frequency > tolerance as high as 68 hz. Whoa. Which means that the battery storage > system would need to push the frequency to above that to get them to > curtail. > *Enphase actually requires you to add a blackout relay or diversion load system because their frequency shifting range can be altered remotely from the web interface. And, yes, I'd be worried also about just using frequency shifting.....I go diversion controller and dump load, despite the expense, as it is the only AC coupled solution for 3-stage charging (we can get days-long blackouts out here in the mountains). The exception is the SMA cluster of 2 Sunnyboys and a Sunny Island; they all speak "SMA" and the sunny island can "warn" the sunnyboys that their freq is about to go weird and can provide 3-stage charging, that is the only way I can describe it without being an electronics nerd.* > > Any insight into this stuff would be appreciated. > > Mahalo, > > marco > > _______________________________________________ > List sponsored by Redwood Alliance > > List Address: [email protected] > > Change listserver email address & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: http://www.mail-archive.com/[email protected]. > org/maillist.html > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out or update participant bios: > www.members.re-wrenches.org > > _______________________________________________ List sponsored by Redwood Alliance List Address: [email protected] Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://www.mail-archive.com/[email protected]/maillist.html List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out or update participant bios: www.members.re-wrenches.org
