Diodes in series to limit voltage.

pechanpechan Solar Expert Posts: 92 ✭✭
My kestrel turbine came with a voltage limiter which consists of approximately 100 bridge rectifiers in series (just 2 poles connected).

While the nameplate says 245 vdc, my classic 250 still was going into hyper voc in high winds. After contacting kestrel they told me that my limiter limits to 280 vdc.

I am thinking about adding 30 or so more in series in order to further limit the voltage.
( I am not near my limiter so I cannot provide the part number of the bridge rectifier till tomorrow evening but I seem to recall they drop 1.1 volts each)

Any comments on this plan?
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Comments

  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Diodes in series to limit voltage.
    pechan wrote: »
    My kestrel turbine came with a voltage limiter which consists of approximately 100 bridge rectifiers in series (just 2 poles connected).

    While the nameplate says 245 vdc, my classic 250 still was going into hyper voc in high winds. After contacting kestrel they told me that my limiter limits to 280 vdc.

    I am thinking about adding 30 or so more in series in order to further limit the voltage.
    ( I am not near my limiter so I cannot provide the part number of the bridge rectifier till tomorrow evening but I seem to recall they drop 1.1 volts each)

    Any comments on this plan?

    I think you are looking at this backwards. Putting 100 rectifier diodes in series, with a roughly .7 to 1.1 volt forward drop, would give you effective limiting to around 70-100 volts. Using the correct two terminals of a bridge rectifier would put two diodes in series, for a drop of maybe 140-200 volts total. If you could somehow get four diodes in series in each rectifier pack, you would end up with 280-440 volts.
    Instead of adding more diodes to limit the voltage "more", you should be looking at removing some diodes so that they start conducting at a lower voltage. The current they have to drain would be about the same, so you might not have to use larger (higher current) diodes when doing this. By limiting at a lower voltage you are putting more of a load on the generator, and at a lower speed, so you should check with the manufacturer to confirm that this will be OK.
    Removing (or just bypassing) about 10 of the diode packages should reduce the clipping voltage by about 28 volts if my analysis is correct.
    SMA SB 3000, old BP panels.
  • pechanpechan Solar Expert Posts: 92 ✭✭
    Re: Diodes in series to limit voltage.

    Interesting, I will update you tomorrow with the schematic and info on the actual bridge rectifiers used.

    By the way kestrel turbines are rectified to dc in the turbine itself and dc is run down the tower. This is the 200 vdc 1000 watt turbine.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Diodes in series to limit voltage.

    "100 bridge rectifiers in series" sounds like a need for serious redesign.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Diodes in series to limit voltage.
    "100 bridge rectifiers in series" sounds like a need for serious redesign.

    i agree as there is quite a waste of power if they are using that many diodes in the first place. maybe this is a good candidate for the xantrex 600 volt mppt cc. the diodes should be of the right current capacity too.
  • NorthGuyNorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Diodes in series to limit voltage.

    My guess is that diodes are connected not in series, but in parallel to the load. While voltage is low, diodes do not conduct because their combined voltage drop is less than the total voltage at the load. As soon as the voltage goes above the combined voltage drop of all diodes, they open and start conducting (and dissipating a lot of heat, of course). This by-passes the load and drops voltage back to the limit.

    100 rectifiers will five you 200 diodes. If it does 280V then each diode should have 280/200 = 1.4V drop, which is a reasonable number for poor diodes.

    If that's the case, and OP wants less voltage, then he needs less diodes, not more diodes.
  • boBboB Solar Expert Posts: 1,014 ✭✭✭✭
    Re: Diodes in series to limit voltage.

    I have seen the Kestrel clipper. The one that I saw clipped at around 130V DC. That one was built for using the Outback MX60 CC
    with one of their turbines. It actually worked pretty well.

    They are all in series. Only down-side is that you cannot reduce the clipping voltage on the fly because
    there was not programmable bypass of any part of the string.

    The bridge rectifiers were mounted to a plate so that they could dissipate some watts a small voltage drop at a time.

    boB
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Diodes in series to limit voltage.
    niel wrote: »
    i agree as there is quite a waste of power if they are using that many diodes in the first place. maybe this is a good candidate for the xantrex 600 volt mppt cc. the diodes should be of the right current capacity too.
    This is only a waste of power at a time when you need to waste power. The idea is that instead of switching a fixed resistive load in and out you put the series string of diodes with a fixed voltage drop in parallel with the generator output.
    That way they will draw a variable current as needed to clamp the output voltage.
    Now a string of diodes is more expensive than a resistor, but the improved performance and the totally passive action gives a lot to recommend it.
    SMA SB 3000, old BP panels.
  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: Diodes in series to limit voltage.
    niel wrote: »
    i agree as there is quite a waste of power if they are using that many diodes in the first place.

    The diodes are effectively out of the circuit until the upper voltage limit is approached - but at that point you _want_ to "waste power" to clamp the voltage at a safe level. It is effectively a large zener. (Zeners are often used as clamps.)
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Diodes in series to limit voltage.
    The diodes are effectively out of the circuit until the upper voltage limit is approached - but at that point you _want_ to "waste power" to clamp the voltage at a safe level. It is effectively a large zener. (Zeners are often used as clamps.)

    i'm well aware of what it is as i have an electronics background, but it is still a very inefficient way of doing this. if we all thought that was the only way to go about things then we wouldn't have mppt controllers.
  • pechanpechan Solar Expert Posts: 92 ✭✭
    Re: Diodes in series to limit voltage.
    niel wrote: »
    i agree as there is quite a waste of power if they are using that many diodes in the first place. maybe this is a good candidate for the xantrex 600 volt mppt cc. the diodes should be of the right current capacity too.

    The xantrex controlers do not do wind.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Diodes in series to limit voltage.

    so why wouldn't the 600v xantrex mppt work as controllers don't know the source of the voltage being inputted?
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Diodes in series to limit voltage.
    niel wrote: »
    so why wouldn't the 600v xantrex mppt work as controllers don't know the source of the voltage being inputted?
    They do know if you tell them via configuration settings!
    One reason would be that there is inertia in wind systems and so a fast change in input to try to seek out the MPP can get to a point where the turbine will slow down because the power from the wind is not great enough to keep it turning at that speed against that load.
    Chris Olson will tell you that the best operation, based on a knowledge of the wind turbine characteristics, is to have the controller choose the current drain based on the Voc at the moment, using internal lookup tables which are different for different types and sizes of turbine. This is not the same algorithm used for MPPT from a PV array.
    The Midnite Solar controllers can do this.
    A second reason is that if possible you also want the controller to provide for or control a dump load to sink turbine energy when the batteries do not need charging, or will accept only limited current, as in Absorb mode.
    You can use an entirely separate dump load controller like the Midnite Clipper, but the Xantrex by itself may not be able to perform this function.

    To take your question to an extreme, "why can't you connect the input of the xantrex MPPT to a battery, as it cannot tell the source of the voltage and you do want it to pull maximum power, right?"
    SMA SB 3000, old BP panels.
  • pechanpechan Solar Expert Posts: 92 ✭✭
    Re: Diodes in series to limit voltage.

    Here are some pictures showing the voltage limiter.
    There is also a closeup showing the part number of the bridge rectifier they are using.
    There are 180 bridge rectifiers used.
    Here is a link to the data sheet:

    http://www.mouser.com/ds/2/115/ds21210-73668.pdf

    Attachment not found.
    Attachment not found.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Diodes in series to limit voltage.

    i count 120.:confused:
  • pechanpechan Solar Expert Posts: 92 ✭✭
    Re: Diodes in series to limit voltage.

    These suckers are 3 rows deep!
    niel wrote: »
    i count 120.:confused:
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Diodes in series to limit voltage.
    pechan wrote: »
    These suckers are 3 rows deep!

    so you are saying there are 3 panels that look like that in pic 2? if so and they are using the diodes in series/parallel per bridge then that would be 120x3=360 in series. yikes! there's gotta be a better way, but i understand why they took that approach.
  • pechanpechan Solar Expert Posts: 92 ✭✭
    Re: Diodes in series to limit voltage.
    niel wrote: »
    so you are saying there are 3 panels that look like that in pic 2? if so and they are using the diodes in series/parallel per bridge then that would be 120x3=360 in series. yikes!

    I count 60 per layer.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Diodes in series to limit voltage.

    each bridge has 4 diodes. most in series is 2 and other 2 are parallel to it. i count the 2 in series per bridge in 6 rows with 10 per row. 3 layers of that and that's 360. they'd be foolish to only use 1 diode per bridge or they would be able to use single diodes instead.
  • pechanpechan Solar Expert Posts: 92 ✭✭
    Re: Diodes in series to limit voltage.

    Here is a short video I made the day of the install.
  • pechanpechan Solar Expert Posts: 92 ✭✭
    Re: Diodes in series to limit voltage.

    Got ya I was talking bridge rectifiers and you were talking diodes.
    niel wrote: »
    each bridge has 4 diodes. most in series is 2 and other 2 are parallel to it. i count the 2 in series per bridge in 6 rows with 10 per row. 3 layers of that and that's 360. they'd be foolish to only use 1 diode per bridge or they would be able to use single diodes instead.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Diodes in series to limit voltage.

    no, i don't think you do as each bridge has 2 diodes that can be used in series, not 1. you have 180 bridges and 2 diodes inside the bridge useable in series for 360 diodes. a bridge is actually 4 diodes in a 2sx2p arrangement.
  • pechanpechan Solar Expert Posts: 92 ✭✭
    Re: Diodes in series to limit voltage.

    Hehe I think we are still not talking apples to apples.
    I originally said that there are 180 bridge rectifiers not diodes.
    I realize that there are multiple diodes per bridge rectifier (usually 4).

    pechan wrote: »
    Here are some pictures showing the voltage limiter.
    There is also a closeup showing the part number of the bridge rectifier they are using.
    There are 180 bridge rectifiers used.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Diodes in series to limit voltage.
    pechan wrote: »
    Hehe I think we are still not talking apples to apples.
    I originally said that there are 180 bridge rectifiers not diodes.
    I realize that there are multiple diodes per bridge rectifier (usually 4).

    Yes, but although a rectifier bridge package contains 4 diodes, there is no way to use all four of them in a forward biased series connection. The internal wiring prevents that. The best you can do is get two in series, with the other two not contributing at all.

    And the initial calculations in this thread (mine at least) were based on your estimate in your first post that there were about 100. A far cry from 180.

    360 diodes in series, with a voltage drop of .7 volts per diode at the point where they just start to conduct would be 252 volts. Using a more accurate figure for the single diode voltage drop and allowing for some minimum current to be effective in limiting the turbine output, you can easily get the 280 volts that the manufacturer quoted to you.
    And still, removing about 18 diode packs from the string should drop the regulating voltage down to about 255 volts. So you need to go a little farther than that.
    SMA SB 3000, old BP panels.
  • pechanpechan Solar Expert Posts: 92 ✭✭
    Re: Diodes in series to limit voltage.

    Thanks for your help inetdog.
    Just want to make sure that this will indeed lower the voltage before doing so in order to not let the smoke out of my classic 250. I think I will start by removing 5 at a time and then gauge the results before proceeding further.

    On a side note that turbine made 9.2 kwh yesterday I am quite impressed with kestrel so far.
  • NorthGuyNorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Diodes in series to limit voltage.

    Why did they use bridges instead of single diodes? I guess they wanted it to work in opposite direction too. So, it doesn't matter where you connect plus and where you connect minus. But they wouldn't go into the expense of doubling the number of diodes just to prevent a polarity connection error. Then why? The answer is simple - this clipper is designed for AC.

    According to the spec sheet, the voltage drop of a diode is 1.1V. 360 (I didn't count them, but I trust people who did) x 1.1V = 396V. It will clip at 396V!!!

    If this is AC, then 396V peak gives you 281V rms. That's exactly the number that tech support referred to. They say it clips at 280V. Was it rms?

    Classic requires DC, so we can assumed you wired the clipper on DC line. If you want it to clip at 240V, you only need 240/1.1 = 218 diodes or 109 rectifiers. 180-109 = 71 rectifier to be bypassed. You can open a little rectifier store :D

    That's my theory, but, of course, this needs to be tested with voltmeter in real life. Should be as easy as measuring open circuit voltage of the turbine, then connecting the clipper without load and measuring voltage again. By measuring, you can find the exact point that gives you exactly 240V.

    However, I doubt the turbine can put out voltages that high.
  • pechanpechan Solar Expert Posts: 92 ✭✭
    Re: Diodes in series to limit voltage.

    Not sure about that as kestrel only makes turbines that output dc.
  • mike95490mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Diodes in series to limit voltage.

    Bridge Diode Packs are often easy to use, with large terminals and isolated heat sinks. BUT only 2 useable diodes if you are trying to hook all in series.
    Some better diodes come in TO-220 style packages, but could be harder to wire. Some come in stud mount, but you have to remember, the heat sinks are electrically connected to one terminal, and you have to isolate with mica or ceramic washers and heat sink grease.

    But even more important than Vf (forward Conduction voltage) is the curve plot of that voltage as amps change. At low amps, the voltage is low, as the amps go up, the Vf goes up, and with a huge chain of diodes, you can get into charge controller BBQ area quickly.

    Why not use a bank of a dozen or so, zener diodes, rated at the full amps of the generator, and let the zener & heatsinks do the work? Much fewer connections = more reliable.
    http://www.evilmadscientist.com/2012/basics-introduction-to-zener-diodes/
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  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Diodes in series to limit voltage.

    All turbines produce AC.
    Run it through a bridge rectifier you get DC.
    Without seeing how this is all wired together it's hard to say just what is going on.

    Quick sketch of a bridge rectifier so people will know what we're talking about here.
  • pechanpechan Solar Expert Posts: 92 ✭✭
    Re: Diodes in series to limit voltage.

    Yes I realize turbines produce ac. Kestrel rectfies it to dc in the turbine itself.


    I just measured voc on the turbine~720 vdc

    Hooked it through the limiter ~ 275-280 vdc
  • pechanpechan Solar Expert Posts: 92 ✭✭
    Re: Diodes in series to limit voltage.

    Ok now I bypassed 20 of the rectifiers and am now sitting at 244 vdc out of the limiter in these winds. I will bypass more as I experience higher winds and gauge the voltage untill I hit the winds where the kestrel blades are fully furled?
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