Inductor sizing or other option?

Discussion in 'General Electronics Chat' started by izy, May 14, 2016.

  1. izy

    Thread Starter New Member

    Apr 19, 2011
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    Hi all,

    I've got a situation where I need to limit the rate of change in current that is presented from a bank of solar panels to the inverter's MPPT front end, basically the cloud effect is now causing over current issues as the MPPT doesn't act quick enough to clamp the over current when the panels cop a full burst of sun after a prolonged cloudy period.

    Based on some rough calcs, I am dealing with a change of approx 5A to 26A @ 300V, the MPPT front end only likes to deal with a max of 13A so you can see why it's burnt out when it can't respond quick enough (by changing the impedance presented to the panels to drive the MPPT voltage up higher, reducing the current and eventually limiting the power extracted from the panels).

    The most simple and elegant solution I can think of at this stage is to install a large inductor in series with the panels, effectively acting as a damper during these "cloud bursts", protecting the MPPT until it can respond to the sudden change.

    My question is now, how would one size an inductor to perform this task? I'd guess I need to go from 10-20ms 21A change and stretch it out to 100-200ms as a very rough starting point.

    Any help or alternative suggestions would be greatly appreciated.

    Cheers izy
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    If your solar panels can put out 26 amps but your MPPT charger can only work up to 13 amps you have a system design problem.

    Either buy a correctly rated charger or paint over some panels to reduce the max current.

    A charger that can only take 13 of 26 amps is hardly a MAX power point tracker anyway.

    And welcome to the forums!
     
  3. izy

    Thread Starter New Member

    Apr 19, 2011
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    Thanks for the reply, however it is not useful in the slightest.

    If you know anything about PV panels, you would understand they have an open circuit voltage (no current) and a short circuit current (no voltage) rating, the MPPT's job is to find the sweet spot where power is the greatest whilst not exceeding the inverter's rated capacity. It is common practice in the solar industry to install more panels then inverter capacity in light of the fact panels will only achieve their rated capacity for a short duration throughout the day and furthermore to allow for panels degrading over their useful life cycle.

    Again, the situation I am facing is that during cloudy periods the MPPT pulls the string voltage down to around the 300V mark @ approx 5 amps, the sweet spot for cloudy periods, when the sun comes out from behind the clouds the panels can peak to 120% of rated capacity for a short duration and as the MPPT doesn't respond quick enough (driving the voltage up and reducing the current) we have a situation where the front end is exposed to excess current and this is what I am trying to address.

    I have a feeling I am going to need a rather large inductor to the point it's installation will be impractical, either way I am trying to work out what size would be needed and how one could produce such an inductor.

    I've used partsim with a 300V source and 54 ohm load which roughly equals the cloudy operating conditions, doing an AC sweep and using 12.5Hz as roughly equal to the frequency I am trying to clamp, it suggests a 1H inductor to get a 50% reduction.

    Cheers izy
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
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    An inductor will be impractical. Why is you controller so slow?
    You might be able to turn on a "dump load" when the voltage gets to high.
     
  5. #12

    Expert

    Nov 30, 2010
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    Don't try to carry it up the ladder. Tie a rope to it and haul it up after you're on the roof.
     
  6. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Yes. It's common practice because it initially saves the customer money up front making the sale look good plus makes it possible to burn key components out in normal operation thusly leading to more service sales whereas putting components in that can handle the actual maximums that the panels can possibly produce would cost more up front but almost eliminate the potential for repeat service work cash flow later.

    Just because the industry does it all the time doesn't make it good or proper engineering practice. :(

    The closest thing I can come up with that would match his inductor requirements would be a 24" - 30" junkyard crane electromagnet and one of those still weighs several hundred pounds. :p
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Again. if your MPPT can be "exposed to excess current" it is not properly rated for your system.

    Have you placed the unit well away from your structure so it cannot light it on fire when it fails?
     
  8. izy

    Thread Starter New Member

    Apr 19, 2011
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    cheers Ernie, you really don't get what the cloud burst effect is do you?
     
  9. Papabravo

    Expert

    Feb 24, 2006
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    It is useful because it tells you you are headed in the wrong direction. People don't provide this kind of advice for fun, they do it to prevent serious harm or injury. Take the advice or ignore the advice, but don't expect any more when you treat it with such disdain. If you're so smart, why did you ask for advice in the first place?
     
    Last edited: May 15, 2016
  10. izy

    Thread Starter New Member

    Apr 19, 2011
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    the inverter spec sheet details a maximum short circuit current amongst other values, i am within this value and i can actually add an extra panel to each string and still be within the allowable range of all sizing criteria

    of course 1 answer is to simply remove panels however i need not post on this forum to come to that outcome, i came to this forum to ask about that use of an inductor as a possible solution and thoughts on how one might size such an item or an alternative suggestion should there be something more elegant, effective or both, this remains the focus of this discussion

    thanks

    cheers izy
     
  11. Papabravo

    Expert

    Feb 24, 2006
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    And the gist of the advice on the inductor is to fugeddaboudit. It's unlikely that you could buy one that size and assuming you could fabricate one you probably can't lift it without a crane. Nevermind what it will cost you. The use of other options is kind of an open ended requirement. The one that has been suggested is to match the number of panels with a charger that can handle the current. Engineering 101, it doesn't get much simpler.

    So go ahead down your chosen path, you probably will anyway regardless of anything we might say.
     
  12. tcmtech

    Well-Known Member

    Nov 4, 2013
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    So what exactly do you want from us? You admit that the solar panels can and regularly do create an output substantially beyond your MPPT units capacity which to most of us with any electronics experience is to us an obvious design flaw in the system.

    If it's not, then explain and then explain why if a 13 amp unit works why can't a 6 amp or 2 amp unit be used if it's short circuit rating is over 26 amps?

    The thing is for all of us who work with electrical and electronic power systems and devices to us the term safe overhead capacity is a standard of design as in designing so that the working components have a reserve capacity above and beyond the limits of what is likely to ever be expected of them and in your case using a 13 amp unit on a 26 amp capable system is an obvious design error plain and obvious.

    As for an inductor now it won't help. Properly sizing your MPPT unit to handle your solar panels maximum output and a bit more is the solution.
     
    Papabravo likes this.
  13. Papabravo

    Expert

    Feb 24, 2006
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    Inductors are useful for many things, but limiting current is not one of them. If the DC resistance of the winding in a 1 H inductor was say 10Ω and it was passing 26 Amperes then it would be dissipating 6,760 Watts which is more than enough to grill steaks. Why would anybody waste that much power to charge a battery? Design flaw -- gee do ya think?
     
  14. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Alternate:
    http://forum.allaboutcircuits.com/threads/inductor-sizing-or-other-option.124098/#post-999569
     
  15. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    On the contrary, I completely understand that topic, along with some other really hard terms like "voltage" and "current."

    Since you are new here let me let you into something the old hands know very well:

    It is typically far more effective to offer suggestions to fix your problem, rather than to offer suggestions to fix your solution.

    The inductor idea is not workable. Getting a converter properly sized to your panel array is the best way to go.
     
  16. Papabravo

    Expert

    Feb 24, 2006
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    I should also point out that this is hardly the first time that a poster did not like the advice he was given because it did not conform to some preconceived notion of what it should look like. In slightly over a decade I cannot recall a single instance of a poster actually succeeding with their approach in the face of advice to the contrary. @izy might be the first, but I think the chances are "rapidly approaching a very low order of probability."
     
  17. BR-549

    Well-Known Member

    Sep 22, 2013
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    If you take two 1 H coils in series and mutually couple them, you will get 4 H.

    Another set will give you another 4 H. In theory, if you couple the first set to the second set.....you get 16 H.

    Flux Magic. One won't get all 4 H. Depending on one's craftsmanship and understanding, one can get close.

    I know nothing about controlling solar panels, but a coil is a great way to limit and save changing current temporarily.
     
    Last edited: May 15, 2016
  18. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi there,

    When you specify an inductor for an application you need to specify more than just the value in Henries. You need to also know the required max DC resistance, the maximum current, and if it will be used for DC you need to know the saturation current level.

    The value of the inductor and the max DC resistance tells you a lot about the cost. A high value inductance combined with a high value DC series resistance means the inductor will be relatively low cost, while a high value combined with a very low DC resistance means the inductor will be high cost. Power inductors are usually higher in cost because they have to have low DC resistance in order to prevent wasting energy and cause low efficiency. They also have to have high saturation current so that they dont turn into a big hunk of low resistance wire when there is DC current passing through them which also makes them cost more.

    Because you would need an inductor as high as 0.5 or 1 Henry and 26 amps DC current, the inductor would be expensive. There are still other considerations too though that are particular to inductors.

    Inductors have a tendency to produce very high voltages when disconnected from a circuit after passing current. This can happen unexpectedly in circuits that have poor connectors or even good connectors when the product is serviced. Special arrangements have to be made to prevent anything like this from happening.

    These problems, when taken together, strongly suggest that an inductor is not the right way to do this. The right way would be to modify the controller, but if that's not possible then add a regulator circuit to handle times when the existing circuit can not handle the load properly.

    A regulator for this kind of circuit would not be especially easy either because it would probably involve using a Pulse Width Modulated regulator that can pre-regulate the incoming power to make sure it never allowed either too high of a voltage or too high of a current, and do this without dissipating a ton of energy which would cause too much heating. I dont think you want to have a 10000 watt heater running just because power went too high :)

    If the problem is only for a short time then maybe a dummy load would work to absorb the extra power for that short time. That would just require a switching circuit and some resistive load bank to absorb the extra power for that short time. Someone else suggested this too and it is probably the cheapest way and is effective too as long as the overload time is short and does not occur too often.
     
  19. Papabravo

    Expert

    Feb 24, 2006
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    Just to be clear, an inductor does not limit steady state DC current. It can limit the rate of change of current by creating very high voltages like an ignition coil. IMHO an inductor is completely unsuitable for the TS/OP's situation. The whole point of doing solar power generation and storage is summarized in three words, Efficiency, efficiency, and efficiency. This is going in the opposite direction.
     
    BR-549 likes this.
  20. crutschow

    Expert

    Mar 14, 2008
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    Can you shut down the MPPT unit if an input overcurrent is detected and then have it restart?
     
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