Something I don't understand about inverters

moyboy06moyboy06 Registered Users Posts: 6
Ok so I'm new at this thing, and this might be a stupid question...but why are most inverters so low voltage on the DC side? I'm designing a solar array that will produce approx 1kW, and the voltage output of it is around 330Vdc. Most inverters I see are rated for a dc voltage of like 24 or at most 48 volts. Am I missing something? I assume these won't work for my system.

Comments

  • blackswan555blackswan555 Solar Expert Posts: 246 ✭✭
    Re: Something I don't understand about inverters

    It is common (well this side of the pond) To build battery based systems from 12 v > 60 v as most charge controllers only go to a max 150v, If it is a batery-less / sell to grid system, It is then up around the 300v area,

    Have a good one
    Tim
  • icarusicarus Solar Expert Posts: 5,355 ✭✭✭✭
    Re: Something I don't understand about inverters

    It has always been my understanding that battery voltages above ~48vdc get into a fairly dangerous DC voltage range. I believe that there are some significant code/installation issues above 48vdc. Also, to get to the 300vdc range would require a huge number of cells, each adding connections and failure points in a system.

    Tony
  • BB.BB. Super Moderators, Administrators Posts: 29,717 admin
    Re: Something I don't understand about inverters

    There are larger commercial/industrial UPS's out there that have 200 VDC+ battery packs. The Prius UPS project talks about them a little bit.

    Is there some reason you have to have such high voltage from your 1kW array? It is not a very large array and would normally be used with a fairly small inverter (~1kW maximum or so) for a normal solar off-grid system. Which would work fine with anything in the normal 12-48 VDC input range of inverters.

    If you absolutely need to have a >200 VDC array for some reason--you can use a Grid Tie Inverter (~200-600 VDC input) to back drive a 240 (or 120 if you can find a 120 VAC GT inverter) VAC True Sine Wave inverter which will charge a low(er) voltage battery bank (turns out that off-grid TSW inverters can be be connected to a GT Inverter and very happily supply stable AC power).

    However, there are issues--the main one is that you have to setup your own battery charger setup to prevent overcharging of your battery bank. Not impossible--but you have to be pretty knowledgeable to make a safe setup.

    Otherwise, if you want MPPT based charging--you are probably limited to Vmp of ~100 VDC maximum from your solar panel to drive a standard MPPT solar charge controller (Vbatt<Vmp<140vdc). There may be higher voltage commercially built solar RE charge controllers out there--but I have not seen any yet.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: Something I don't understand about inverters

    It is legacy and there is more code reg's above 60vdc.

    Just FYI, Honda Prius hybrid batteries are 550 vdc. Just waiting for news report of non-Toyota auto mechanic getting fried.
  • moyboy06moyboy06 Registered Users Posts: 6
    Re: Something I don't understand about inverters

    well i might just be missing something in my calculations them. I assumed I just treated the panels like a system.

    at max power of 70W, a single panel is rated at 65.5V and 1.07A....giving 61.21Ohms. 5 are in series, and there are 3 rows of those all connected in parallel. that would give 327.5V and 102.01Ohms approx....and a power of 1.05kW.

    I did a google search on 1kW off grid solar systems, and I just can't wrap my head around how some of these sets can use a normal 24V inverter with the systems. Like I said, I must just be misunderstanding how this all works.
  • WindsunWindsun Solar Expert Posts: 1,164 ✭✭
    Re: Something I don't understand about inverters

    Panels are considered a constant current device or source.

    Not sure how you got wrapped up in ohms, as that has no bearing on inverters or solar panels. While 61 ohms could be considered as the internal resistance of the panel, it has no relevance in your setup.
  • moyboy06moyboy06 Registered Users Posts: 6
    Re: Something I don't understand about inverters

    Well if thats the case, then how would I go about determining what size of an inverter I would need?
  • BB.BB. Super Moderators, Administrators Posts: 29,717 admin
    Re: Something I don't understand about inverters

    As WindSun says, Ohms does not have any meaning in the V=I*R equations for power sources...

    A lead acid battery it pretty closely an ideal voltage source. V=12 volts from 0-100% rated current load (roughly). The ideal battery / voltage source has near zero Ohms.

    Solar panels have current output of I=4 amps from 0 volts to ~Vmp (Voltage maximum power). For voltages above Vmp (to Voc open circuit) drops form ~Imp to zero amps. The ideal current source has resistance near infinite Ohms.

    The above is a "weird concept"--how can a power source have infinite resistance and still output electricity... Basically, it is because you can use this equation to calculate resistance:

    V=I*R => R=V/I=Change in Voltage / Change in Current

    A perfect battery has zero change in resistance no matter the current.

    A perfect current source has zero change in current, no matter the voltage.

    With modern electronics--it is not hard to create these sorts of devices (voltage and current sources) which have huge numbers of real life application in the real world.

    You can look at a nice set of I*V curves for a generic solar panel here:

    data sheet for a crystalline silicon panel

    To size a panel array for a GT inverter (or MPPT type solar charge controller) is a non-trivial task.

    First, you have to look at the operating voltage and current range for the device (GT inverter may be ~200-600 VDC; an MPPT Solar Charge Controller may be roughly Vbatt<Vmp<140VDC).

    Then you look at the power rating of the inverter. If the inverter is 3 kWatts, then using P=I*V--that if V=300 volts, then current would be I=10 amps. If V=200 volts, then I=15 amps, etc...

    Also, solar panels are affected by temperature... Cold panels tend to output higher voltage; and hot panels tend to output lower voltage. So--most major vendors have created websites that allow people to plug in their requirements (size of inverter, Min/Max temperature, proposed brand/model of solar panels) and get a list of recommended strings that will match the inverter's operating requirements (usually a series/parallel connection of panel strings).

    For example, here is the Xantrex Sizing Calculator for the GT family of Inverters.

    Play with the settings and see what happens. More than happy to answer any questions you may still have. This is not an easy concept to grasp the first time it is seen.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Something I don't understand about inverters

    If we could re-focus on the OP's situation here ...

    Some trivia: 15VDC is "arc point" - above that you can start to weld metal. Put three standard car batteries in series and you have enough voltage and current potential to weld 1/4" steel. This is in fact done in 'emergency' situations by certain 4x4 enthusiasts I know.

    A "rule of thumb" for array voltage vs. system voltage when using an MPPT controller is "one step up":
    12V system = 24V array, 24V system = 48V array ... But it doesn't always have to be that way. You will always need higher array voltage than system voltage or else the batteries don't charge. That is why a "nominal" 12V panel will have an output of approximately 17-23 Volts.

    Some other "get you in the ballpark" info:

    Inverter size = total cumulative load.
    Battery bank size = amperage used over time * Depth Of Discharge factor (usually 2 or more).
    Array size = sufficient to provide charging for the battery bank at a minimum of 5% (up to 15% usually) of the bank's total amp/hour capacity.
    Charge controller = large enough to handle above current.
    Wiring = sized to meet current requirements over distance traveled.
    Circuit protection (fuses/breakers) = sized to handle current on given circuit under normal maximum loads.

    Please note: this data is not precise nor is it the last word in system design. Right? Just gives a bit of understanding to how things get sized up.
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Something I don't understand about inverters
    moyboy06 wrote: »
    I did a google search on 1kW off grid solar systems, and I just can't wrap my head around how some of these sets can use a normal 24V inverter with the systems. Like I said, I must just be misunderstanding how this all works.

    To re-iterate what Blackswan said, I think you're confusing how these systems are connected. Firstly there are 2 types of inverters (for the purposes of this discussion): battery based, and grid tied. Battery based inverters are the low voltage ones: 12V, 24V, 48V and they're connected to the batteries NOT the panels.
    Grid tied inverters are connected to the panels and they operate at higher voltages, upto 500VDC as input which they then supply to the grid as 240vac (or 120Vac in some countries).

    It sounds like you're talking about a battery based system, if this is the case, then your panels will be connected to a charge controller and the charge controller connected to the batteries. The charge controller will then determine the DC input voltage that it can accept from your panels (upto 150VDC for an MPPT controller).
    So in a battery based system, you choose your inverter based on:
    - Your maximum instantaneous loads (including the high startup current required for certain motor loads)
    - Your battery bank voltage
    - Other "nice to have" features that the inverter offers

    Hope this clarifies :D
  • blackswan555blackswan555 Solar Expert Posts: 246 ✭✭
    Re: Something I don't understand about inverters
    well i might just be missing something in my calculations them. I assumed I just treated the panels like a system.

    at max power of 70W, a single panel is rated at 65.5V and 1.07A....giving 61.21Ohms. 5 are in series, and there are 3 rows of those all connected in parallel. that would give 327.5V and 102.01Ohms approx....and a power of 1.05kW.

    I think you are missing something, Firstly forget about the Ohm`s, Secondly, yes 5 in series would give 327.5 v but only 70w per string, giving 210 watts (1.7 amps) if you parallel the 3 (Series add the voltage, the current stay`s the same, Parallel, add the current, the voltage stays the same)



    Have a good one
    Tim
  • blackswan555blackswan555 Solar Expert Posts: 246 ✭✭
    Re: Something I don't understand about inverters
    moyboy06 wrote: »
    Ok so I'm new at this thing, and this might be a stupid question...but why are most inverters so low voltage on the DC side? I'm designing a solar array that will produce approx 1kW, and the voltage output of it is around 330Vdc. Most inverters I see are rated for a dc voltage of like 24 or at most 48 volts. Am I missing something? I assume these won't work for my system.

    May be easier than explaining an lawful lot of what is going to be useless information, If you tell us what the system`s intended purpose is in what part of the world ?
    Have a good one
    Tim
  • icarusicarus Solar Expert Posts: 5,355 ✭✭✭✭
    Re: Something I don't understand about inverters
    RCinFLA wrote: »
    It is legacy and there is more code reg's above 60vdc.

    Just FYI, Honda Prius hybrid batteries are 550 vdc. Just waiting for news report of non-Toyota auto mechanic getting fried.

    [just the facts please. -Bill B. mod] The nominal voltage of a Prius battery s between 206 and 273vdc depending on the generation of the car. The genius of the Prius power management system is that maintains the battery SOC between 40-60% SOC, never fully charging, never deeply discharging. Contrary to urban myth, battery life is exceeding 500k km (~350k miles) in taxi service, the hardest service out there!

    There has never been a reported event of some one getting "fried" with a Prius system, tech, nor emergency worker.

    Tony

    [Correcting information is fine. Please leave it at that. -Bill B.]
  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: Something I don't understand about inverters

    Sorry, it was the Highlander that has over 500 vdc of boosted source to rear drive. I was thinking Toyota but listed Prius.
  • moyboy06moyboy06 Registered Users Posts: 6
    Re: Something I don't understand about inverters

    alright so im just all kinds of confused, so let me just rephrase my question.

    i have a 3X5 array of panels, each one rated at 65.5V and 1.07A at 70W. When connected, i have 1kW at 327.5V and 3.21A.

    Is it even practical to make this off-grid? How I'm understanding everything, i would need like atleast 14 24V batteries that would lead into the inverter. Is this correct? If not, why is it incorrect? also, can I used a grid-tie inverter for an off-grid use?

    I'm not looking to make something, like a house, completely off-grid. It would just be used to run a few items.
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Something I don't understand about inverters

    If you put all the panels in parallel, so the total array voltage would be 65.5V and total current 16A. Then you'd need an MPPT charge controller to convert from 65.5V panel voltage to 24V battery.

    You can use a grid tie inverter for off-grid, the only system that was designed to support this configuration is the SMA sunny island + sunny boy setup. You might be able to do this with other manufacturers inverters as well, but it will be more fiddly and complicated. For a 1kW array, probably not worth the trouble going this route - unless you already happen to own a Sunny Boy.

    Just rewire the array to a parallel configuration and invest in an MPPT charge controller.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Something I don't understand about inverters

    The short version of off-grid system design:

    1). Size the inverter to handle the maximum cumulative load (everything that will be on at once).
    2). Size the battery bank to supply enough Amp/hours to handle daily load (between charges).
    3). Size the array to provide sufficient charge current for the batteries (5%-13% of Amp/hr capacity). edited by niel to be 13% and not 15%.
    4). Size the charge controller to handle that current.
    5). Size the wiring and fuses/disconnects appropriate to the current at the various parts of the system.

    Panels should all be identical, and at system voltage for PWM controller or "one step up" for MPPT (12V system uses 24V array).
    Batteries in bank(s) should be identical.

    Sometimes we can get a bit carried away with all the little details about system design and device specs.
  • rrbvrrbv Registered Users Posts: 15 ✭✭
    Re: Something I don't understand about inverters

    Why do none of the charge controller manufacturers make a charge controller that will accept the 200 to 300 volt strings like a grid tied inverter and output to a 24 or better a 48 volt battery system? The newer panels are built for series connection strings. Does away with high current parallel strings and lowers overall cost.

    Or does someone make this device? Buck regulators are inherently efficient.

    rrbv
  • BB.BB. Super Moderators, Administrators Posts: 29,717 admin
    Re: Something I don't understand about inverters

    As far as I know--Buck Regulators (down converters) are actually pretty efficient. Up conversion is generally more difficult.

    Here is another thread that discussed the issues with High Voltage DC (and high(er) voltage DC battery banks).

    I have asked in times past in other threads (and even a PM) of solar charge controller designers here why not design and sell a 200-600 VDC to 48 volt solar charger. Even if it was fairly simple--it would be a great backup for those of us with GT systems that would like to use some portion of our large arrays for emergency backup.

    No bites yet...

    One alternative suggested by Solar Guppy was the use of GT inverter to back-drive an off grid True Sine Wave Inverter... Basically the Off-Grid inverter sets the voltage and frequency--and the GT Inverter tracks the Off-Grid inverter just like it was utility power.

    The Off-Grid inverter would supply energy when the load was greater than the GT inverter could supply--And in the case where the GT Inverter output was greater than the load, the GT Inverter would "back drive" the Off-Grid Inverter and charge the battery bank...

    This has been tested with several off grid inverters (Outback for one mfg., probably Xantrex too) and can work well...

    The "downside"--there is no battery charger/controller when the Off Grid Inverter is being back driven. You would have to design/configure your own charge controller(s) to make it work.

    There are several possible solutions that I can think of--One is to use a Dump Controller on the battery bank. Another would be to use a "dump like" controller to turn off a silicon relay in the output of the GT inverter when the batteries are full (Skystream Wind Turbines does/did a version to charge the first method of radio feedback to the turbine was not reliable, and used to have a charge controller listed, but no more--my guess).

    For safety, you probably need two charge controller systems in parallel to prevent the batteries from over charging if one of the controllers failed (NEC would require redundant systems if there is a case when when can fail and cause dangerous conditions).

    In the end, this is an "off specification" use of an Off-Grid TSW Inverter--And I am not sure that I have ever heard that any vendor would help you set up such a system... My guess is you would be on your own to set this up.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: Something I don't understand about inverters

    The buck switcher, in a simplified explaination is voltage transformed by the duty cycle of the switches. In the simplied first order explaination, which assumes no losses, a 50% duty cycle would produce half the output voltage.

    Going from 300 vdc to 25 vdc would require about 9 % duty cycle. This is very do-able but has some downsides.

    You like to keep the duty cycle greater then 5% to ensure easy stability in the switchers feedback control circuitry.

    The higher the breakdown voltage rating of the MOSFET's switches, the higher their series resistance (Rs) in comparision to a low voltage MOSFET of the same silicon area. In other way, to get the same Rs on a higher voltage capable MOSFET requires a larger (and more expensive) MOSFET. Because it is larger die, it has higher input capacitance to be driven by the switcher control circuit. Power taken to drive the MOSFET input capacitance is CFV^2, where C is driven input capacitance, V is the voltage swing on the gate drive (10-12v), and F is the switching frequency.

    All this means the higher the input voltage capability, the lower the efficiency of the switcher. This is particularly painful at low input power ranges.
  • Dave AngeliniDave Angelini Solar Expert Posts: 4,842 ✭✭✭✭✭
    Re: Something I don't understand about inverters
    BB. wrote: »

    In the end, this is an "off specification" use of an Off-Grid TSW Inverter--And I am not sure that I have ever heard that any vendor would help you set up such a system... My guess is you would be on your own to set this up.

    -Bill

    Bill,
    The Sunny Island inverter/charger is used with their grid-tie inverter in an "AC coupled" set-up in this months Home Power. Looks pretty straight forward to me and it does charge the batteries with a generator.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • BB.BB. Super Moderators, Administrators Posts: 29,717 admin
    Re: Something I don't understand about inverters

    Yes, the Sunny Island is an option... However, because they use slight variations to the AC power as an indication to cut back on GT power output--you would need to install a SMA compatible GT inverter only... Will probably not work with other vendor equipment.

    Has anyone here installed a Sunny Island system?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Something I don't understand about inverters
    BB. wrote: »
    Has anyone here installed a Sunny Island system?

    Not yet :D I attended their training course in Germany on the Sunny Island and Sunny Boy, but for my own setup, I'll be using the Sunny Island with someone else's DC charge controller. I blogged a bit about their AC system vs. traditional DC here: http://www.casanogaldelasbrujas.com/blog/2009/05/13/sma-sunny-island-ac-coupling/

    I think the AC coupling is good for a few specific cases, but in general for small-medium off-grid homes, personally, I'd go with DC coupling because of lower costs and better winter efficiency.

    Another thing about the SI AC system is that there's only a single charger in the system; so for example if the sun's out and you have the generator on, then you'll still be limited by the maximum charging current of the Sunny Island.
    Which is also something to consider from a failure point of view - if the sunny island fails, then you have no way of charging your batteries.

    Regarding using the SI with other GT inverters, it might work if you can configure the GT to disconnect from the grid within the frequency range that the SI uses to control power. I.e. if the SI varies the frequency between 60-62Hz, then configure the GT to disconnect at 61Hz.
    (I think we may have already discussed this here...?) Using it this way won't be as smooth as using the gradual control system of the sunny boys, but I don't see why it shouldn't work.
  • BB.BB. Super Moderators, Administrators Posts: 29,717 admin
    Re: Something I don't understand about inverters

    I believe that US compliant GT inverters will disconnect/turn off outside the 59.5 to 60.5 Hz range.

    May be an interesting solution... $5,200 for a 5kW SMA "Island" inverter/charger--vs $3,200 for a 6kW Xantrex XW inverter (plus more parts depending on solution).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Something I don't understand about inverters

    Hehe, it's the other way around over here, the Xantrex 6048 for 4100 Euro and the Sunny Island 5048 for 3300 Euro. That darn Atlantic Ocean.

    [Edit]
    The Sunny Island has an integrated display and integrated generator start - which I believe are optional add-ons for the Xantrex
  • nigtomdawnigtomdaw Solar Expert Posts: 705 ✭✭
    Re: Something I don't understand about inverters

    Yep the Atlantic Ocean is a pain, I like American RE products a lot, I bow to Steven on SMA stuff from the EU, but Im not considering SMA, at the mo, Im focusing on supplying smaller sized equipment to people as most off gridders in Spain and I guess other sunny locals need 2 person small house AC requirements. PV Prices have crashed over her since the credit crisis, the Spanish Goverment have slashed the subsidy for PV supplied solar farms by 70% and PV solar Suppliers are turning to the off grid market like fish out of water.

    Nigel
Sign In or Register to comment.