Recommendation for Tough MPPT CC ?

VicVic Solar Expert Posts: 3,145 ✭✭✭✭
Have been using MX-60's in two roughly identical off-grid systems for the past 20 months or so.

Each runs Vmpp of about 106 VDC. In our mild winter climate, neither system has exceeded the Max Voc of the MX-60. However, these sysems are lightly loaded at this point, resulting in an average of over 300 minutes per day in Float during the Summer.

In addition to the customary fan issues with the MX's, one of them has had hurt FETs, as evidenced by that MX-60 "making power from darkness". Guess that the FETs were leaking Vbatt back to the PV input side, or something.

Have wondered if spending so many hours per day in Float @ Vin of 120-or-so VDC had damaged the FETs. And if so, if either of the newest MPPT Charge Controllers -- the XW and the Apollo, would be any tougher in this respect. My only back-up CC is a TS-60, which is ill equipped to handle the Vin of the current system, as wired.. So have been wondering about a spare, and perhaps one that is even tougher than the MX-60. Or perhaps there are some FETs now available which have a higher rating (?) Realize that there may also be some diodes etc which might need to be uprgaded as well.

Am quite happy with the MX in every regard, with the above noted exceptions. Like the convection-cooling of the XW, but it's Vin specs seem identical to the MX.

Any thoughts from the Gurus here ? TIA, Vic
Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.

Comments

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Recommendation for Tough MPPT CC ?

    i don't think the 106vmp will harm it, but the voc may as it could easily soar past 150v on occasion during normal operations. if edge of cloud or some other enhanced conditions occur, i would suppose that if there's any weak components present they may give way under the higher than recommended voltages. only my opinion.
  • boBboB Solar Expert Posts: 1,007 ✭✭✭✭
    Re: Recommendation for Tough MPPT CC ?

    Does the MX60 not go to sleep at night ?? The auto-restart would catch that (every hour or hour and 1/2 or so) though . What makes you say that the FETs have been hurt ? Did it smoke ? Or, are you just noticing the PV input voltage being something other than zero when it's dark out ? The reason I ask is because you may see some V in at night because of the design of the MX60, and not necessarily because of leakage. That voltage will (or should) definately be lower than battery voltage though.

    boB
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Recommendation for Tough MPPT CC ?

    Vic,

    My initial thought is similar to niel's: a Voc problem. The STC Voc spec for the Shell SQ-175 is 44.6 V, which would put your three modules-per-string array's STC Voc at 133.8 V.

    With the MX60's absolute max input voltage spec of 150 V, the available temperature correction factor is 112%. The NEC 690-7 uses a 113% correction factor for ambient temperatures between 14 F and 31 F.

    Looking at weather data for Los Banos (anywhere near you?), the average low in December is 35 F, and the record low was 14 F -- right in the NEC 113% range.

    But, Shell's Voc temperature coefficient spec is -145 mV/C. This spec suggests the series Voc would hit 150 V at ~-12 C, or ~10 F. Brrrr!!!

    Note that the some of the input voltage you see while the CC's are in float mode is actually higher than 120 V. 120 V is just the integrated value of the array Voc and the Vmp with respect to PWM duty cycle while supplying enough current to keep the batteries at float plus whatever loads you have. For example, if the displayed input voltage is 120 V, the array duty cycle is 10%, and the array's operating Vmp is 96 V, then the Voc is ~122.7 V.

    I've obviously made several assumptions here, but these numbers nonetheless suggest your system may be flirting with the MX60's 150 V absolute input voltage limit. Apollo and Xantrex use the very same limit for their MPPT controllers, so it looks like a technology cost/benefit trade-off to me. Also, the numbers above don't include any allowances for performance tolerances.

    After 20 months, what do your MX60's show as their (all time) Hi Voc's?

    Regards,
    Jim / crewzer
  • VicVic Solar Expert Posts: 3,145 ✭✭✭✭
    Re: Recommendation for Tough MPPT CC ?

    HI Neil & boB,

    Thanks for the replies.

    boB, Nothing smoked, or even smelled bad, but, would note that hours after sunset -- total darkness, the PV input would be about 100 VDC. Would switch the input (PV) breaker to the MX to the off position, and the voltage would rise a bit above that. The MX-60 got a bit warm, the fan ran, and power was totalized as if the MX was really making power. Think that it WAS making power from the batteries via leakage (in my uninformed opinion) -- but of course it was a net negetive. First noticed this 'cause, as a Ham (also) noted the little birdie @ 3759 Khz at night. This has always come from the MX. Went to look, and there it was. That night happened to be a full moon. Commented to someone that was making power from moonlight. But the second night switched off the PV breaker and noted reality.

    That MX went back to OB, and asked what was wrong with it. FETs was the answer. I do not know the real topology of the MX's buck converter, just thought that basically, the FETs were connected twix the PV input and the Battery output with an inductor/diode(s) and some filter caps etc.

    My max Voc was recorded as 136 VDC as I recall ... close to the limit. Our climate is mild, so very few days per year is it above 130 VDC, and then for fairly short periods.

    Our peak ambient temps are about 95 degrees F in the power room, this occurrs on many summer afternoons. And at the peak Ta, we are in Float (for hours) at about 122 VDC Vin.

    Had wondered if this long duration elevated Vin and Ta could take a toll on the FETs, more than the unusual morning or two at 130-ish Voc.

    Realize that your Midnite Classic may be offered (later) in an HV configuration, but think that I am on the way to hurting a second set of FETs. Now, when the PV in breaker is switched to off, see about 45 VDC Vin, as reported by the MX. Have no spare MPPT CC here, and am thinking about finding an even tuffer spare. Some/all of this may be illogical, perhaps too much guessing from too little data.

    Edit: AND Crewaer, thanks for the thoughts ... Well we had an unusually COLD winter this past season. The lows were in the 24 Degrees F ... locals said thet this is the coldest noted in the past 12 years, but colder will happen.

    Interesting thoughts re the summer Voc, but will add that will always have stacked inverters to run as absolute minimum loads, which prob takes a bit off the ultimate Voc value, short of a fault condition. Have noted that even on summer afternoons can supply hundreds of watts at 120 VDC Vin, however. So any clipper that I make needs to be good for about 500 Watts or so. Perhaps I need to make better use of the XS power. Had wondered about dumping the PV DC into a medium sized electirc water heater, but might need to add a heat exchanger to keep that load constant.

    thanks, all ... (end edit)

    Anyway 73 for now es TU, Vic K6IC
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Recommendation for Tough MPPT CC ?

    Adding to one of boB's comments: There's a resistor inside the MX60 that precharges the PV input capacitors. It's normal and expected for the PV input voltage display value to reach near the battery voltage when the array is disconnected, i.e., at night.

    HTH,
    Jim / crewzer
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Recommendation for Tough MPPT CC ?

    i don't think that's it jim as he indicated the mx got very warm and the fan kicked on. did you try placing an ammeter inline to see if current is actually backflowing?
    3759khz bummer. don't feel bad as i have some nice bcb harmonics and maybe a few intermod products as well showing up on 75/80mtrs, moreso than i get on 160mtrs. go figure? i'm hoping to put up another dipole before the snow flies, but i may need a filter for the bcb interference as this dipole will be cut 240ft long giving rise to more signal captured off of the bcb. yes, i tried making one, but i guess i'm not good with figuring the proper uh on toroids as it didn't work.:roll:
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Recommendation for Tough MPPT CC ?

    niel,

    You may be right about Viv's indicated PV voltages. Looking at the time stamps, I think that several posts got cross-mojo-nated and some of my comments are therefore OBE.

    Regards,
    Jim / crewzer
  • VicVic Solar Expert Posts: 3,145 ✭✭✭✭
    Re: Recommendation for Tough MPPT CC ?

    Thaks Neil and Jim,

    First, boB, as I recall, the MX did go to sleep as the sun faded, but awoke later and making the phantom power (perhaps that function needs a patent !)

    Yep, While I was writing and posting my War and Peace reply to the first two posts, Jim's post was there.

    Edited my orig post to try to respond to Jim ... perhaps not a good idea.

    The closest town to me is Tracy, CA ... abt 8-10 airline miles.

    Thanks Crewzer for the info on the cap precharge function ... guess that had not switched off the PV at night before and noted the result .. good info.

    Neil, no did not look at the current and its direction, as it was funny to see, but the MX was obviously in need of repair, so back it went.

    For me the birdies are small, and happne during the day when the MX is active doing PV work. Am never on the lower bands then. With each octive increase in frequency, the artifacts diminish quite a lot. And they were quite small to begin with -- have lotta shielding and ferrite in the system. Were it for my extension cord or two, probably these thingies would be another 10 Db or more lower. These big switching MPPT boxes sure work well. To me the MX-60 is amazingly well designed. More info than I need presented in an exceedingly intuitive fashion. Have never seen a piece of gear which had such an intuitive interface. And find the Aux functionality to be quite flexible. Great product ! I just should have gone with 71 Vmp, wanted to reduce the IR drops in the PV runs. 71 Vmp is high enuf to allow Solar EQ even on hot summer days.

    Neil, the BCB stuff is tuff, but a multi section toroidal filter will do the job, unless you have multiple 10 Kw or larger stations close to you. The main reason for this off-grid site is to get away from the awful Power Line interference and ALL of the unsuppressed switching gizmos in people's homes. Now, the Treck Lighting switching power supplies are by far the largest source of interference. The off-grid site has S-zero background noise save for Lightning static and noises that I make. Thanks again for all the info. Vic
    Thanks a
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • boBboB Solar Expert Posts: 1,007 ✭✭✭✭
    Re: Recommendation for Tough MPPT CC ?
    crewzer wrote: »
    Adding to one of boB's comments: There's a resistor inside the MX60 that precharges the PV input capacitors. It's normal and expected for the PV input voltage display value to reach near the battery voltage when the array is disconnected, i.e., at night.
    HTH,
    Jim / crewzer

    That's right Jim but not quite ~that~ high of voltage (100V?), especially with the PV still connected at night. The PV being connected tends to bleed that pre-charge voltage down pretty much.

    Now, if the MX was still switching away, like that 75 meter band birdie probably indicates, then it means the MX did not go to sleep, or re-woke up
    and is still generating PV voltage above the battery voltage. At night that's the only way I can think of that 100 or so volts can be at those terminals.

    The PV voltage being high by itself shouldn't hurt anything. 136 V is a VERY common max for a 72V nominal array. I wouldn't worry about that at all.

    If, when the unit went back to OB, they updated your software, that's even more reason to trust it now. When was that unit made ?? How old
    is it ?


    boB
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Recommendation for Tough MPPT CC ?
    That's right Jim but not quite ~that~ high of voltage (100V?),
    Right you are, granpA boB... part of the cross-mojo-nation problem...:roll:

    Regards,
    Jim / crewzer
  • VicVic Solar Expert Posts: 3,145 ✭✭✭✭
    Re: Recommendation for Tough MPPT CC ?

    Thanks boB,

    Both units came thru distribution about 23 months ago. Both got fan upgrades on the original units, and the current FW at the time. The one with FET issues was a Warranty swap in February, 07, as I recall. That one is 5.11 Firmware, the other is 5.10 as I recall.

    Thanks for the confidence-building info. Think that Crewzer Jim was referring to the current situation, where the PV goes up to about 45 VDC when the PV input breaker is OFF and no sun. That made me think that was going down the same road as the previous unit.

    AND, think that the unit with hurt FETs did go to sleep as the sun faded, and awoke later (as I reall).

    Just from my limited perspective, sure do like the concept of convection-cooling. With, perhaps, an emergency back-up fan. Or perhaps just shedding current to keep the electronics at/below max. temp spec. Just MHO.

    Think that you guys were talking about a clipper. Would like to set Vin max to about 130 VDC, and clamp it.

    But perhpas this thread is about complete, now. Thanks again for all the info. Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Recommendation for Tough MPPT CC ?

    nope, not complete with cross polli-nation. part of the weird, but true sequence of the forum.:cry::roll::p

    btw, there are 2 >10kw and 2 or 3 smaller interacting with each other within a 5 mile radius. the toroid should've shown some loss if i had done it right and i saw none. maybe i should go buy a filter and get it over with.:confused:
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Recommendation for Tough MPPT CC ?
    Think that Crewzer Jim was referring to the current situation, where the PV goes up to about 45 VDC when the PV input breaker is OFF and no sun.
    Vic,
    That is indeed what I was thinking about! 8)

    Regards,
    Jim / crewzer
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