Low Voltage Cutoff on Grid Tie Inveter

philg · July 2019

I've got a SolarEdge system with power optimizers installed as grid tie.

Unfortunately, it appears that the grid transformer serving my neighborhood is undersized, so during the summer when all the AC units kick on in the neighborhood, the voltage drops. Normally I receive 240AC to my single phase SE6000H HD-Wave Solaredge system. Over the last few days I've see the grid voltage drop as low as 203AC with all the additional draw in my neighborhood.

The SolarEdge is tripping into a low voltage fault mode for any voltage below 211.5 VAC. In fault mode, it stops producing power until the voltage again rises above 211.5.

Couple of questions:
(1) Currently I'm running the inverter in default install mode rather than Rule 21 mode. Would switching to Rule 21 mode help with this situation? (for example lower low voltage trip levels and/or longer low voltage ride through)

(2) There a setting in the SE6000H for adjusting the low voltage cut off (to be done in guidance with local utility, which in my case is LA DWP). Assuming that LADWP allows for a lower threshold for voltage cut off, is there any downside to running an inverter at a lower voltage cutoff. Lets say setting the cutoff to 205 rather than 211.5. Based on the size of my system, I still would not be hitting the 25A rating if I was collecting and inverting at 205 (205VAC * 25A = 5.2KW vs 240*25 = 6KW)

BB. · July 2019

I believe you are correct... Your inverter is going to output 25 Amps maximum @ xxx VAC... So your inverter should not "over current" at that low voltage. So no damage to your inverter/mains panel.

If LA DWP allows you to run the inverter to a lower voltage down to 182 VAC or so (low 208 VAC rating) without issue (regarding GT Hardware)... If LA DWP says no, then you cannot.

The other would be to light a fire under LA DWP and say they are running your AC mains below ~105 VAC (or whatever they say low line is). The utility can, in theory, be responsible for any damage to your appliances (and possibly your food in your fridge) because they are running in "brownout" conditions (they should turn off power when there is low line voltage).

Of course, they could play hardball, and simply turn off your AC power when there is a brownout present (when you may have chosen to keep running your home otherwise).

Another option for your critical loads at home (say refrigerator/freezers/etc.) is to get an AVR / Line Stabilizer... This are automatic variable transformers that will (in theory) bring ~102 VAC low line (some products spec. 96.5 volts as "normal minimum" voltage to ~147 VAC to ~110-120 VAC:

https://www.amazon.com/s?k=avr+voltage+stabilizer

Obviously, the correct solution is to have LA DWP replace/upgrade their distribution system... Which may be a long waite.

-Bill

philg · July 2019

I agree that it would be ideal to upgrade the transformer but I think that's not going to happen until they burn this one out... I'm in an are of Los Angeles where the power and transformers are underground. They blew one up a few years ago and the voltage improved for a while after it's replacement, but because the neighborhood is gentrifying and being redeveloped, more and more load is being added. I only put in PV this year, so didn't realize that the low voltage was going to be an issue until I started to see my inverter enter low volt fault these past few days as summer kicks into gear. Ironically, there's high PV penetration in my neighborhood, so I would imagine all those systems are also dropping out during the peak of the day (afternoon from about 2pm to 530pm), ironically making the situation even worse.

So it sounds like, what you're saying is that running at a lower cutoff voltage doesn't harm the inverter, assuming the PV system doesn't produce more than the rated 25A.

Any thoughts on Rule 21 settings? (I've read a bit about Rule 21, but frankly am not sure I fully understand it.)

BB. · July 2019

Yep, in one way your are correct. The GT Solar systems would help low line voltage.

On the other hand, from the utility point of view, "too many" GT solar systems output current/power can exceed the local loads and actually run into high line voltage issues. Your "typical loads" are over 24 hours, and the typical GT solar is over ~6 hour "full sun" cycle... So, the typical GT Solar can output (on average) ~4x more power in that 6 hours than the average a home uses over a 24 hour cycle.

For the utility, sizing for GT Solar is just as big (if not bigger) issue than sizing for neighborhood loads. In general, in California the peak loads are actually in the evening when people get home from work (and after sunset):

http://www.caiso.com/TodaysOutlook/Pages/default.aspx

CAISO is an interesting website to understand "state level" power usage/needs/utility operations.

In Puerto Rico, there is (was) a large military base that installed a large GT solar system, and took the whole local grid to over voltage because of GT in the sunny afternoons.

-Bill

solar_dave · July 2019

Not all homes here have solar but the penetration is getting greater and greater. APS complains about this as well but I find it hard to believe that my system when in pretty much full blown back-feed does much to anything but the local transformer. Not all my neighbors have solar but it is approaching 25% SWAG.

AZ is a pretty high consumption state

BB. · July 2019

In general, when actual solar generation exceeds about 10% of the Utility's local generation--That is when GT Solar went from "don't care", to having the network (power) engineers sharpening their pencils.

25% GT Solar in a neighborhood, and you will have zero or even power flow reversal in the local transformer(s).

And all of this is a major pain for a utility... The GT inverters "follow" the AC input voltage/frequency. It is not in control of the utility. And you can have major powerflow management issue (cool spring day, no major A/C loads, high GT generation (reversal of energy flows).

Other issues--Power glitch and it knocks GT inverters off line for 5 minutes (full neighborhood loads supplied by utility). 5 minutes later, neighborhood goes to near zero or even generating power. The utility has to be able to source/sink these highly variable loads.

Utility power management is very much a statistical process... And when you have "coordinated" "large scale events"--It becomes a different animal.

-Bill

solar_dave · July 2019

One thing about AZ is that all the new construction over the last 20-30 years has produced a robust electric grid. Sure APS has it challenges but they are probably more prepared than most states. It is funny, most of the glitchy voltage activity is during the low load times overnight, I suspect they are then bringing all the peakers offline.

Low Voltage Cutoff on Grid Tie Inveter

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