Solar protection system

In solar system, for lower kva applications, in India, generally people are using AC MCBs for protection instead of DC Disconnectors. I am a Distributor of DC Disconnectors. I need to educate my customers on the distinct advantage of using DC Disconnectors for protection. Can someone help me ? Thanks.

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Solar protection system

    What sort of system are we talking about?

    A standard grid-tie system with no batteries doesn't really need a DC disconnect, although its a good idea to have one. Typically this takes the form of circuit breaker(s) on the PV string(s). The AC side would have a circuit breaker at least, and some jurisdictions require an "accessible" AC disconnect in case of fire for all systems (somewhat redundant on a non battery system as the anti-islanding feature will kill output once the utility is disconnected).

    A battery-based system should have some form of disconnecting DC from the batteries to the inverter in addition to the inverter's "off" switch. Again this is usually a DC rated circuit breaker. There would be another such breaker or fuse & disconnect on the charge controller's output and similar circuit protection on the PV output.

    How you can convince people that such protection is necessary for safety is beyond me. I've been trying to educate people for decades, often to no avail.
  • BB.BB. Super Moderators, Administrators Posts: 30,642 admin
    Re: Solar protection system

    Here is a quick report written by Midnite Solar (boB, Robin, and Ryan/"Halfcrazy" post here and have their own forum too at Midnite Solar) about their testing of DC Breakers for ETL (similar to UL):

    http://www.midnitesolar.com/pdfs/MidNite_165vdc_ETLtest.pdf
    The first test consisted of applying 165 volts of DC to the 63amp breaker while running 99 amps of current through it. The test was done turning the breaker on for one second and then off for 9 seconds. This was done successfully 35 times. That was 16,335 watts of high voltage DC the little breaker had to switch on and off. The first time we ran this prior to ETL being there, all we got was a melted down breaker fused solid in the closed position! It’s a good thing we keep a hand on the knife switch for cases just like this. The breakers are marked with words stating “+ for DC”. Now we better understand why this marking exists. We had it backwards and that doesn’t allow the breakers internal arc chute to work effectively. Engineers rarely read directions before applying power to equipment. The + wording on the breaker was added by CBI on my recommendation years ago.

    More or less, DC Current is much harder to "break" than AC (alternating current has a zero crossing point every 1/2 cycle at 50 or 60 types a second).

    From a welding company--DC vs AC current:
    Q: What type of Stick welder works best for all-around use?

    A: A welder with an AC/DC output, whether its an electric arc machine like Miller's Thunderbolt or a gas engine drive like Miller's Bobcat.
    DC welding offers advantages over AC for most Stick applications, including: easier starts; fewer arc outages and sticking; less spatter/better looking welds; easier vertical up and overhead welding; easier to learn "how to weld" and a smoother arc. DC reverse polarity (electrode positive) provides about 10 percent more penetration at a given amperage than AC, while DC straight polarity (electrode negative) welds thinner metals better.
    Q: Does an AC output have any advantages?
    A: Yes, if you need to weld on material that's become magnetized from friction, such as when hay, feed or water constantly rub against a steel part. A DC output won't work because of "arc blow," where the magnetic field blows the molten filler metal out of the weld puddle. Because an AC output alternates between polarities, it enables you to weld magnetized parts.

    So, DC arcs start easier and sustain better than AC.

    Also, note a couple of effects here... First is that the effects of the DC arc are polarity sensitive. For a circuit breaker, as Bob (Robin?) notice saw, if a DC Breaker is connected "backwards" (positive vs negative), it can actually weld the breaker contacts closed.

    And a second effect... The use of magnetic fields/permanent magnets affect the arc (plasma and flow of metallic ions). DC Breakers and Relays can use magnetic fields to "blow out" the arcs--Which can also be polarity sensitive (instead of blowing an arc away into the arc chute--an assemble to stretch and cool the arc), the wrong polarity can pull the arc into the switch gear itself.

    In the end, many switches (and some breakers) are rated for both AC and DC use, and if you look at the specifications, you will see that DC ratings for such devices are much smaller than the AC ratings. (and the DC version of equivalent rated devices are much larger/heavier duty).

    Depending on the size of your business (or your potential sales of DC switch gear)--You can see how easy it was for Midnite to connect a dozen mid size batteries in series to give them a high voltage DC source with ~2,000 Amperes of dead short potential.

    You could, (if you have the desire, money, and are vary careful) do your own demonstration testing with heavy cable, a safety switch (large/high Amperage rated knife switch while wearing a welding glove/face shield) and make a simple video showing the use of AC vs DC breakers. You probably can get some pretty impressive photos--And you could keep a couple of test results (melted pile of AC breakers vs DC breakers with the same test) to show potential customers.

    Obviously, this is a vary dangerous test--So doing it away from combustible materials and in an area with good ventilation and a couple of fire extinguishers is important (remember the batteries are their own source of acid/hydrogen gas (during charging) too.

    I tried a quick look on the web for photos/videos that tested AC vs DC breakers and could not find any that would be a concise demonstration of the issues.

    One last comment, I believe that somebody here (boB or Robin?) said that the agencies are working on releasing requirements for DC Breakers that are not polarity sensitive--There are many applications in solar RE where the direction of current flow through the breakers is bidirectional (charging/discharging)--And polarized breakers are not a great idea... But, in general, we always assume that the Battery Bank is the source of near unlimited current and that is what the breaker is usually protecting as the source.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Solar protection system
    BB. wrote: »
    One last comment, I believe that somebody here (boB or Robin?) said that the agencies are working on releasing requirements for DC Breakers that are not polarity sensitive--There are many applications in solar RE where the direction of current flow through the breakers is bidirectional (charging/discharging)--And polarized breakers are not a great idea... But, in general, we always assume that the Battery Bank is the source of near unlimited current and that is what the breaker is usually protecting as the source.
    -Bill

    Wow! I really appreciate the pointer to this fascinating subject and to the PDF from the guys at Midnight. Setting up fault test conditions is a whole separate area of Engineering from designing the components to meet the specs in the first place!

    One very clear problem that this points out is that if you are putting a breaker like this in a PV combiner, you have a rock and hard place choice.

    If you want to be able to use the breaker as a DC disconnect under load, the + has to be pointed at the panel array. But if you want it to protect the array against a CC failure that backfeeds battery into a potentially shorted array, you had better have the + pointed towards the inverter.

    If the breaker would open successfully only once under back feed and destroy itself in the process, that would probably be OK from a safety perspective, but if you want to be able to use it multiple times as a DC load break, then you have no choice but to put the + toward the array (unless the breaker can survive load breaking in the reverse direction as long as the current is low enough.)
    Until we get non-polarized DC breakers, it looks like we would have to put in two back-to-back breakers and be careful which one we use for manual actuation or else orient the breaker for use solely as a disconnect switch and include a (larger?) DC fuse for reverse overcurrent protection.
    Wait till John Wiles sinks his teeth into this!
    SMA SB 3000, old BP panels.
  • boBboB Solar Expert Posts: 975 ✭✭✭✭
    Re: Solar protection system
    BB. wrote: »

    One last comment, I believe that somebody here (boB or Robin?) said that the agencies are working on releasing requirements for DC Breakers that are not polarity sensitive--There are many applications in solar RE where the direction of current flow through the breakers is bidirectional (charging/discharging)--And polarized breakers are not a great idea... But, in general, we always assume that the Battery Bank is the source of near unlimited current and that is what the breaker is usually protecting as the source.

    -Bill


    Yes, it is going the way of non-polaraized for the NEC 2014. This is quite a big deal for us at MidNite. Robin is in South Africa right now talking with
    one of our breaker manufacturers about this very thing.

    As far as PV combiner breakers, luckily, the need for string over current protection is very rare so the polarity issue doesn't
    come up very often in real life.

    boB
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Solar protection system
    boB wrote: »
    As far as PV combiner breakers, luckily, the need for string over current protection is very rare so the polarity issue doesn't
    come up very often in real life.
    boB
    So, which way do you actually recommend? The way that acts as a string over current protection (insane panels? Power line falling on panels?) and is also usable as a load-breaking disconnect? Or the way that protects against feedback from the battery in the event of a double failure (the more serious but far less frequent event?)
    SMA SB 3000, old BP panels.
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