grid tie questions


I've a bunch (of questions) but perhaps I'll break them up into a solid one question with a second question on concept.

I've done a number of installations in Oaxaca and Chiapas but they've all been off the grid. I'm not a super expert but they've all worked fine and everyone is happy.

I figured I'd never have to worry about doing a grid-tie system because electricity is so cheap for the common person it doesn't work out. Also I didn't think CFE would cooperate. However a new client that I worked with on his ranch also wants a system for his in town freezer facility. He said that CFE will do net metering (he based his knowledge on WALMART which is a whole separate ball game with a nationwide agreement) so I went to check it out. First they said no and then sí after I read their regulations and pointed out there were regs for small producers. So it looks like we'll try the first in the 3 state area down here. You Arizona guys working in Northern Mexico probably already know this.

So I've been researching.....

This guy needs 3 phase and can't afford the whole thing at once. I figured we'd put in 20 of those Kyocera 205's and I've not yet decided on the inverter(s). (The changes in available technology since I started this a few years ago is just incredible!). What I really don't understand is how the inverters work with the grid. His normal load will be more than what he can produce from the 20 panels. CFE apparently uses bi-directional meters. So if he produces some of his power and they provide the rest of his power I don't get how the power flow works.

Using the simplistic Xantrex diagram as an example they both go to the same wires and distribution panel - CFE looks like a backfeed to the home system. You really couldn't have the home system inverters go only to CFE since when they're down you'd be producing power you can't use - unless you put in a switch. Also I've never seen a diagram that DIDN'T have the inverters going to the panel.

This client is relatively easy. He doesn't care that much if his system goes down when CFE fluctuates. He just wants to cut his costs. My figuring shows an easy 15 year payout and he's happy with that. So this long winded narrative is simply asking how grid tie systems work at the most basic level.

I'll add client 2 here.

She's got a large pastry making facility and consumes plenty of power - also 3 phase. She, however, has a different problem. CFE gives her ugly power. I've not analyzed yet all the problems to decide if they're mostly high surge, low voltage, bad harmonics - whatever. The upshot is they cost her a new piece of expensive digital Italian pastry making equipment a year. She wants stable, clean power above all else.

Off the grid systems are effectively UPS systems and I suppose I could suggest something like that but it'd be huge and unworkable financially for her. It'd be nicer to do grid-tie but have all her power come from her inverters - using them as a kind of stabilizer. Perhaps do a battery backup as well and size it for only 12 hours or so. I've seen some hybrid stuff but haven't researched it too much yet since I'm still confused about the basics. So my question here is more conceptional design. My son, an electrician also who works on very large systems, but wants to know nothing about alternative energy or anything DC, claims a stand alone transformer should work. She's so pissed off, however, at CFE she's willing to spend money just to give them less.

Sorry for the long winded explanation but I figured it'd help so you would all know where I'm coming from with the questions. I don't need to talk about sizing and nor installation. That I can figure out without too much difficulty. I might have a couple of add on questions since these will be the first 3 phase systems I've done also but so far from the research I've done they make sense. Thanks so much in advance for any suggestions, comments, and help you might offer. I'll be coming up to Arizona to get these and haul down. That'll be another first since previously I bought all my systems out of Texas but I'm changing my supplier.

Incidentally I'm bilingual and dual citizen and have brought plenty of stuff down. If anyone has any questions about Mexican regs and reality I'd be happy to give you my 2 cents worth.

Thanks again,



  • mike95490mike95490 Posts: 7,613Solar Expert ✭✭✭✭
    Re: grid tie questions

    1) If only the pastry maker is un-happy, a large UPS should clean it up, and keep it going for 5 minutes till the grid recovers.

    2) If the power is that bad, it will be knocking the GT inverters off-line all the time, they need 5 minutes to syc up, after each fault event.

    3) installing your own "Stand-Alone" transformer might help clean up the power, but really, the power company is supposed to have some standards to meet. Maybe a week of "line quality monitoring" would pay off, with some hard evidence to show the elec co.

    4) I don't think you want to get into a huge battery system for the bakery, it would be too unwieldy.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
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  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: grid tie questions

    if the pastry machinery is that expensive it might pay for your client to purchase her own 3 phase generator. any gt system will only track (or at least try to) what the electric company gives. as was stated she will need proof of the quality of electric coming to her, not so much to prove to the electric company as they already know they are putting out garbage, but to use it to go through political processes to regulate that said industry (or company) to bring them into some standards. note here that i am not familiar with the comings and goings with power in mexico be it political or electrical. her only recourse is to be independent of the dirty power if complaining doesn't work, imho.
  • BB.BB. Posts: 27,354Super Moderators admin
    Re: grid tie questions

    Also, review what is getting fried... If it is controllers and servos--then powering those "low power devices" from a continuous conversion UPS is probably very practical... If large heating elements/multi-horse power motors/etc. need "cleaner power"--may be much more expensive/impractical to achieve.

    GT inverters themselves will not clean up any power as Niel said.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dellsworthdellsworth Posts: 20Registered Users
    Re: grid tie questions

    I was in the jungle looking at some panels and just got back. Thanks for your patience and your responses. Everyone made great suggestions - each looking at slightly different approaches. It's obvious the GT isn't going to work for the pastry chef owner. I've read all of your stuff on other posts and it's always a pleasure.

    Neil. I had to laugh. CFE is a monopoly and it's nationalized. They do what they want and there is no means whatsoever to pressure them. They've survived privatization marches, strikes, and political shenanigans since the privatization threat raised it's head in Mexico about 15 years ago because of their service and commercial pricing.

    To try to be more specific on the first question. If my panels are putting a certain about of voltage (and amps) on the line to the inverter and then to the panel what then what keeps the CFE voltage from overwhelming the system. Say I'm contributing 3000 watts from my system on some apparatuses that require 7000. What allows my system to contribute all I can produce (the 3000) and CFE contribute the other 4000? Why wouldn't my load come entirely from CFE and even more important what allows the apparatuses load to pull entirely from my system and just take the residual it needs from CFE? For that matter how does my generation feed into their system if everything on my side is down for some reason?

    I have this vision of them being all powerful voltage wise (not to mention unstable as I mentioned before). I guess I want my voltage to overcome theirs and so strongly that there is no backup on the amount of amperage I can shove at them. I want an open spigot from my system to my own equipment or their grid if I'm not using it what I generate for myself. How does that work? Does it work the same whether I'm running single phase or 3 phase - at whatever voltage I choose on my 3 phase (or single phase) lines?

    NOW the extra question after considering the above. Will their instability of voltage (or whatever) cause me any problems or suggest ways I should design. For instance. Does it make sense for me to get an extra meter that has no load and I shove all my generation at them through that meter (even though they are bidirectional) and another meter for my load so there is no clash of varying voltages or harmonics?

    I apologize if I'm sounding hopelessly naive. Off the grid is so easy...

    Thanks again.
  • BB.BB. Posts: 27,354Super Moderators admin
    Re: grid tie questions

    Basically, the reason why a GT Inverter will not "stabilize" your local voltage:
    1. The utility is a very low impedance (resistance) network (technically for Electricity 101 a voltage source)... Basically the AC equivalent of a "giant" lead acid battery or Filter Cap. The utility network "sets the voltage and frequency" for the power at the customer's location. If the utility power is dirty--you are hosed unless you install something to "clean up the power" (not a GT inverter).
    2. The GT Inverter is the "opposite" of a lead acid battery. Basically, it looks like a "current source". The impedance of a current source is very high (in theory, approaches infinity). So, a GT inverter does not do anything to "smooth out" the local voltage/frequency fluctuations. In practice, the GT Inverter is a "slave" to the utility voltage/frequency. (of course, there are more details in that the GT inverter is a "constant power source" which could, in theory, somewhat reduce voltage surges--but in practice, I would guess the GT Inverter provide minimal smoothing of AC power--and would do nothing at all outside ~9am-3pm--when the sun is not available).
    There are certain conditions where having a GT Inverter could make things worse... For example, a utilty running "high voltage" with a long run from the pole to the point of use. The GT inverter would supply more current (out to the pole)--which creates a "voltage rise" (V=I*R) across the too small of gauge of wire/too long wire run.

    So what ends up happening is the GT Inverter detects an Over Voltage (high utility voltage + voltage rise caused by GT inverter) and shuts down for 5 minutes. And the cycle can repeat.

    GT Inverters should only be used on "clean lines" that are at nominal voltage. Lines that run too high (or too low) will cause the inverters to shutdown because the lines are out of spec.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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