Do modern Grid-tied inverters have all protection needed?

lazzalazza Solar Expert Posts: 336 ✭✭✭
Dear Forum

I am new to this forum, this is my first post, so hope it works :D

I have come across a 30 panel PV grid-tied system (2 series of 15 panels 170W) where the Inverter has failed (it is a Kaco 5001xi). We are looking at replacing the inverter. However there are some things that puzzle me.

My experience is mainly theoretical and in off-grid systems.

As I understand any system should have the following protection:
  • 3 varistors to protect against lighting and over-voltage,
  • Ground connection from the structure of the solar panels to Earth (separate from the inverter Earth)
  • Blocking diodes, to prevent reverse current at night

however this system seems to have none of these. My question is therefore as follows:

Do these types of Grid-tied Inverters have all these protections included in one box??

Thanks for any assistance you can give me

Larry

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Do modern Grid-tied inverters have all protection needed?

    Welcome to the forum, Larry.

    In short, no; the inverters are not where this sort of protection is found.

    You're quite right about the need for lightning protection if you're in a lightning-prone area. You can't stop it absolutely, but you can reduce the risk. Lightning arrestors: http://www.solar-electric.com/deliar.html

    Accessory blocking diodes probably aren't necessary, as there usually aren't any batteries in a grid-tied system so there's no Voltage source to leak back. When there are batteries (hybrid system) there's also a charge controller which should take care of the problem. Rarely are additional diodes needed.

    There are some differences of opinion (and occasional disagreements with code) as to how best to ground a PV array frame/rails/mounts. Most of us like a straight bit of 6 AWG wire right down the side of the building to a good Earth ground point.

    You've got 5 kW of panel there, so it is likely worthwhile replacing the inverter and fixing up the less-than-perfect install. I'll tell you right now everyone will want as many details about the existing system as you can provide before handing out any advice. :D
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Do modern Grid-tied inverters have all protection needed?

    Welcome to the forum Larry,

    Glad to see we got the IP problem fixed.

    There have been some changes over the years in GT inverter design--so there can be some minor wiring changes required.

    But, I believe this system is in Spain (not North America with the 120/240 VAC split phase wiring but probably 230 VAC @ 50 Hz).

    In the US, we typically have a ground rod at the main panel/power entrance to the building. And this would be tied to your building's safety ground.

    In the US (and probably much of the world), you just connect the AC output of the inverter to the appropriate sized circuit breaker in your main panel (many utilities/countries require a switch on an outside wall that can be locked off for lineman safety). And tie the AC safety ground back to the inverter.

    On the Solar panel side, there should be a + and - lead coming from the array (possibly several in parallel, perhaps with a combiner box with fuses/breakers--typically used if > 2 parallel solar panel circuits). These go directly to the GT inverter and, internal to the inverter, is a fused connection from AC safety ground to the solar panel negative ground (typical).

    If there is a Solar panel to ground short, it will pop the fuse and the GT inverter will shut down (and should indicate a ground fault).

    The metal frames of the solar panels should be tied to their own ground wire, which runs down the outside of the building (if roof mounted) to a ground rod, or to the main building AC ground rod (if close by).

    Many installation may connect the Solar Panel Frame ground to the AC safety ground... There is a lot of discussion about which is better/safer--And much of it depends on the exact installation (roof/ground mount/how far/etc.).

    If you choose to replace the GT inverter--Make sure you download and read the installation manual before you buy and make sure it is compatible with your current wiring and Array Voltage/Current setup (series/parallel connections of your solar panels).

    Different vendors have different solar panel input voltage/current requirements--Which could force you to reconfigure your existing solar array... A pain in itself, and sometimes costly too (for example, you may have one string of 15 panels with inverter A and require 2x8 parallel/series array for inverter B).

    We have a few posters here from Spain that can give you better information on local requirements and issues than us from North America.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RCinFLARCinFLA Solar Expert Posts: 1,397 ✭✭✭✭
    Re: Do modern Grid-tied inverters have all protection needed?

    There are two most likely ways to take influence from lightning. First, and probably highest probability is a strike to the power grid in the neighborhood. Second is a ground induced large voltage gradient due to a strike on a nearby object like a tree or other tall structure. These two examples do not include a direct strike to a PV panel or PV wiring which is usually rare and so destructive there is likely nothing going to save the system.

    GT inverters that use a transformerless design are much more common in Europe then in U.S. Most inverters sold in U.S. are transformer isolated between PV/MOSFET switching side and output side of transformer which is pretty much just L-C filtering. There was a lot of fear that transformerless GT inverters are prone to subjecting MOSFET's to grid lightning hits and grid voltage transients.

    On the ground induced hit, there can be thousands of volts of ground gradient over 10 to 30 meters between separate ground rods. An isolated PV frame should be okay to ground separately but PV wiring (negative line) should be ground referenced back to where inverter is grounded, which should be a common ground rod with the grid entrance point.
  • lazzalazza Solar Expert Posts: 336 ✭✭✭
    Re: Do modern Grid-tied inverters have all protection needed?

    HI Forum

    Thanks for all your replies- a great help.

    So if I understand correctly:

    1. Lightening Arrestors (varistors) on the DC side are advisable but not essential.
    2. The ground wire from the panel array structure is connected to the Inverter ground- and this is ok.
    3. Blocking diodes are not necessary for a GT system.

    My only dilemma therefore is if it is worth adding DC-side lightening protection.

    (the AC side has both Overvoltage conductors/varistors on the grid side and Grid-side varistors.)

    Cheers
    Larry
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Do modern Grid-tied inverters have all protection needed?
    lazza wrote: »
    1. Lightening Arrestors (varistors) on the DC side are advisable but not essential.
    Watch the voltage rating of the Arrestors... A GT inverter can be rated to 600 VDC (or even higher). For example, these Delta Arrestors are rated to 500 VDC for one model and 800 VDC maximum for a second model.
    2. The ground wire from the panel array structure is connected to the Inverter ground- and this is ok.

    We have held lots of discussions about lightning here... The US National Electric Code has recommendations for grounding that probably are not optimal (have frame ground of the array go with the DC power leads into the home where the inverter (may be) installed.

    In general, it has always been recommended that Lightning grounds (air electrodes, metal roof component bonding) should aways be taken to the outside envelope of the buildings... And that multiple lightning ground runs be distributed equally around the outside of the building. And that runs not be in the middle of the building (go through the interior) or bunched together on one side of the building (as an example).

    The reason is that high high frequency current (lightning is more radio frequency power vs DC current flow) and tends to force itself to the outside/farthest apart conductors (known as the skin effect for RF current in conductors).

    So, the long way around, is that the Frame Grounds (in my humble opinion) should be routed outside the building to an earth ground rod/system at the edge of the building envelope. This will be a much safer path for lightning conduction.

    However, to also settle the need for safety ground (if there is an array to earth ground short circuit--such as if a solar panel gets broken or a wire shorted to the frame work), a wire should be attached from the lightning earth ground rod to the ground rod used by the main panel/earth safety ground.
    3. Blocking diodes are not necessary for a GT system.
    Correct.
    My only dilemma therefore is if it is worth adding DC-side lightening protection.
    Don't know--It is probably one of those things that won't hurt. They are not too expensive (less than $40 USD for the 500 VDC version).
    (the AC side has both Overvoltage conductors/varistors on the grid side and Grid-side varistors.)

    An interesting point--Our administrator from Northern Arizona Wind & Sun has seen most lightning induced failures in off grid system at the AC output of the Off-Grid Inverters.

    So far, have not had many discussions of actual system failures from lightning strikes.

    A couple threads about Lightning:

    Off Grid Grounding Technique?
    Another Question, this time about Lightning

    Note, the above are discussions, not a do A, B, and C--and you will be "safe". There probably is no such thing with lightning. Several different techniques are discussed--and a few of those posters even have experience with lightning. :cool:

    And our host's consolidated FAQ page:

    www.windsun.com
    Lightning Protection for PV Systems

    From other past posts here, Windsun (admin/owner of NAWS), he said that most of lighting induced failures he saw were in the Inverters' AC output section.

    Towards the end of this thread is a very nice discussion of proper generator grounding.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lazzalazza Solar Expert Posts: 336 ✭✭✭
    Re: Do modern Grid-tied inverters have all protection needed?

    Hi, thanks for all your replies, a real help

    I just visited another fairly badly set up system yesterday (doing a tour of all the existing PV-Wind systems in the area)- so i have a few more questions I'll be posting if you guys dont mind

    really happy to be on the forum

    Larry
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