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Forum Index : Electronics : MAD/OZ AC/DC Charge Controller
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| Warpspeed Guru  Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
I had a home built inverter that ran for a couple of years without any problem, then suddenly spectacularly blew up, and kept blowing up randomly after replacing several sets of mosfets and gate driver chips. It took me quite a while to discover why. The quartz crystal metal can was intermittently shorting to the pads on the circuit board directly underneath. Fitting an insulating spacer between crystal and PCB fixed that problem. Even the most minor problem like a wiggly plug or a loose wire can produce something lethal to the mosfet switching sequence. If it ran for a couple of weeks, the basic concept obviously works. But something somewhere obviously failed. My faith in cheap Chinese components is not high, and a tantalum capacitor, mosfet, or electrolytic may have died. No real way of knowing for sure. Cheers, Tony. |
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oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
A failed IBGT in the GTI will introduce full DC bus voltage into the transformer and result in total saturation..... (just another reason I like galvanically isolated GTI's). It does not take a lot of dc to mess up a tranny. This will result in massive currents in the primary of the torroid, and kill the mosfets without hesitation..... nothing will save it in it's current configuration. I have pondered using a low side shunt instead of the CT in the ac circuit, as this would be a catch all... may have to give this more thought. If we catch the primary current, we should be able to protect from everything. Unfortunately, my over current protection is only on the output side, so will not catch this sort of over current. When you go to the aerosharp, this cannot happen, as the GTI transformer stops any chance of DC leakage into the AC circuits. The last GTI I fixed, had blown IBGT's, and after fixing them, the dreaded DC injection fault arrived. I think less than 1% is allowed into the grid. They seemed to do it by clamping another set of fets across the DC bus at cross over to stop any DC getting through to the output, so it is a serious problem with HF GTI's. They seem to have it well in hand though.... but when it gets away with the grid, just the GTI fails.... on a home system, the inverter will go with it, unless maybe we monitor the low side situation..... simple, I just did not bother with it. Warp, it is not just the chinamen with iffy stuff. I have three SMA units to look at when I get time.... they have been dropping like flies lately too.... oh and three Aurora 2kw units too. ...........oztules Village idiot...or... just another hack out of his depth |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Yes, if the 50 Hz switching halts on one half cycle, the inverter transformer saturates and the fun stops very quickly. That is exactly what happened to me. The crystal oscillator would hesitate or pause very briefly, then start up again but that killed my mosfets, leaving no obvious cause. Dc output protection for non galvanically isolated inverters can be provided with a crowbar circuit and series fuse. Something like a big triac straight down to neutral fed from a retriggerable monostable that only waits for about 15mS. If the zero crossings stop, then even a large fuse can be blown very fast indeed. Primary current limit is really only effective if a large non saturating choke in the primary has enough inductance to limit fault current rise to something manageable. As an example, a 48uH choke in a 48v inverter will limit the rate of fault current rise to 1 amp per microsecond. But only up to choke saturation. If the choke has plenty of excess current capacity, then even a fairly simple hardware current sensor and shutdown system can be relatively slow acting and still be effective. That is one reason why I don't like ferrite chokes. They work fine for reducing inverter idling current, but are totally useless under high power. The only thing limiting rate of primary fault current rise will be transformer leakage inductance, which will be almost zero. It makes an effective primary current limit circuit extremely difficult to design as the rate of fault current rise will be explosively fast. Cheers, Tony. |
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| oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Wish I had your depth of knowledge... I know only enough to be dangerous  ........oztules Village idiot...or... just another hack out of his depth |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Now long retired, but in a previous incarnation I used to do power electronics design for a day job. I have learned mainly by my mistakes and disasters, and there have been plenty of those over the years :-) Cheers, Tony. |
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Clockmanfr Guru Joined: 23/10/2015 Location: FrancePosts: 429 |
Ah yes, Madness has been trying out a HF GTI. Well I was going to try some stupidly cheap used/second hand HF, High Frequency No toroid GTI's. But as my knowledge is very limited, its back to the toroid GTIs for me. Not risking my beautiful OzInverters. Until that is, someone designs a OzInverter protection circuit for dodgy HF GTIs. However, reading the last few posts that looks unlikely. Everything is possible, just give me time. 3 HughP's 3.7m Wind T's (14 years). 5kW PV on 3 Trackers, (10 yrs). 21kW PV AC coupled SH GTI's. OzInverter created Grid. 1300ah 48v. |
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| Clockmanfr Guru Joined: 23/10/2015 Location: FrancePosts: 429 |
Oztules do let me know how you get on with those SMA repairs. As you know I have a few. PM me if you need any info on the toroid ones. Everything is possible, just give me time. 3 HughP's 3.7m Wind T's (14 years). 5kW PV on 3 Trackers, (10 yrs). 21kW PV AC coupled SH GTI's. OzInverter created Grid. 1300ah 48v. |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138 |
Hmm, I quickly drew this up as a possible solution to monitoring DC current through each 1/2 of an H bridge, by using the internal Mosfets Rds(on) as a current sense. A current mirror arrangement could be deployed on each of the lower H Bridge Mosfets. Basically Q5 is monitoring the current through Q3, Q2 and expressing that value as a voltage across R5 referenced to 0 volts, this voltage turns on Q6 during overload on any half cycle, Q7, C1, R7 act as a blanking section that prevents erronious readings in the initial switch on event. I found a circuit similar to this on the web a few years ago, but I havent tested it in this application.  Cheers Mike |
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Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Very timely Warpspeed. i just finished making a primary choke from recycled aerosharp ones. I used two of the smaller C cores and converted them to an E core. This gives a bigger diameter coil plus twice the iron area. Anyway, it measured 36uH. My calculation tell me it would saturate at 12KW, which should be plenty. There are 9 turns of 45mm sq wire. The wire is made from 18x1.8mm dia enamelled, in parallel. Effective wire length is two meters. I will test it tomorrow but do you think it might prevent the catastrophic destruction mentioned here? Klaus |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Effective inverter short circuit protection should (?) protect the inverter. But it would take nerves of steel to deliberately dead short the output of a running inverter to test it. A Hall sensor could be placed between the electrolytic and the bridge to measure dc fault current. Although it is usually an inconvenient place physically, unless its a single large electrolytic with screw terminals. A more practical solution would be to place a Hall sensor in series with the choke. Even easier still would be a current transformer in the transformer primary. The choke should limit the rate of fault current rise to something that a shutdown circuit can then more easily cope with. All this assumes that upper and lower mosfets don't cross conduct. If they do, its all over in an instant regardless of anything else. Cheers, Tony. |
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| Clockmanfr Guru Joined: 23/10/2015 Location: FrancePosts: 429 |
Okay madness, Thanks, but honestly no worries. Send me a board one day so I can play around with it. Everything is possible, just give me time. 3 HughP's 3.7m Wind T's (14 years). 5kW PV on 3 Trackers, (10 yrs). 21kW PV AC coupled SH GTI's. OzInverter created Grid. 1300ah 48v. |
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Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
2017-10-04_214641_W47130ASR7_PV_Combined_regulator.zip Sorry Clockman I have too much on my plate at the moment with work etc. I have attached the Gerber files, do what you want with them, as far as I know the circuit works but I take no responsibility. It is designed to use an Arduino UNO with extra rows of pins to allow plugging it on top of the PCB. Such as this one. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Mike, That is similar to what I did way back for traction motor control. It was very effective... may investigate it further. It used to save using shunts to measure the current, and was virtually instant. I used to use the comparators output ( thats how I synced it too) to throttle back the controller, but in this case that would simply lower the AC output, and OFF would be better than low.... hmmm.... Always something else to play with. ..........oztules Village idiot...or... just another hack out of his depth |
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| Clockmanfr Guru Joined: 23/10/2015 Location: FrancePosts: 429 |
Thanks Madness, Will have a look and play. Everything is possible, just give me time. 3 HughP's 3.7m Wind T's (14 years). 5kW PV on 3 Trackers, (10 yrs). 21kW PV AC coupled SH GTI's. OzInverter created Grid. 1300ah 48v. |
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| oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Ideas anyone? To do the current sense of the fets, we will have to sample both the low side fets, ie one with the 50hz carrier, and one with the 20khz spwm. I am thinking this may best be done with a lm339 with 2 op amps per leg. One to do the sync, and one to do the comparison of each leg. ( use all four) By their nature the rds on measurement is going to change effective resistance by about 30% over the max temp range. This will be mostly relevant to the spwm, as it's switching losses are higher, so will display differently. The 339 response time is in the 2us range, so I am thinking this may be doable, and perhaps drive a 4069 hex inverter to drive various shut down parameters... eg the 8010 current sense, and the 2110 shut down and panic led perhaps.... These can be latched with high speed diodes, so that off is off forever, or maybe just drive the present current shut down as well.... we cannot have jitter with the torroid lurking about the place.... off must be off and then a soft start from a reset only. Propagation of the 4069 is in the <200ns range. Does this sound like a catchall worthy of me trying it out?.. or am I going about this all wrong. I think the comparator is fast enough, and the hex inverter gives me lots of avenues to process the results. I think the time base will catch things in less than 10% of the max pulse width... but is that enough to aim for. ..........oztules Village idiot...or... just another hack out of his depth |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
I for one am looking forward to what you come up with, its certainly a worthwhile feature. As is anything that safes the inverter from destruction  .If it can be easily retrofitted to existing inverters so much the better. Klaus |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138 |
Great idea and worthy of experiment, possibly a more modern faster comparator with push pull outputs like TLV3202 (50 ns) would work here, couple that with a precision voltage ref chip and something like an RS Latch or similar for lockout until manual reset. 4069 hex inverters would be fine. There will have to be some adjustment (multi-turn Pot)to allow for the expected increase in RDS(on) with temperature and various numbers of parallel mosfets employed. When the mosfets first start to turn on and in their linear region, the over current sensing will have to be turned off to prevent false shut downs. Cheers Mike |
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Revlac Guru Joined: 31/12/2016 Location: AustraliaPosts: 1026 |
I have a used Xantrex GT5 and eventually I would like to hook it up to the off grid inverter however it is a HF GTI and eventually it will stuff it self and the inverter connected to it. To test this GTI I would first connect the panels etc then fire it up using a small generator (2000W Taurus) and see if it behaves it self. I have tested a few small electric motors this way to see if they work OK, before I try and use them on the HF inverter that is running the house, and sure enough I found 2 motors with intermittent faults, (turns out to be the centrifugal start switch) stalled the little Genny completely never had a chance to pop the breaker. perhaps thats 1 way to check a suspect GTI before trying it on the off grid inverter. As for protecting the inverter from a failed HF GTI, Galvanic Isolation seems the most effective so far, I think That would require a 1:1 transformer between the GTI and the off grid L N out, along with a fast blow fuse or circuit breaker between the GTI and the off grid inverter. The way I see it, the GTI can sh*t it self on that side of the transformer and the inverter may not even smell it, the inverter should just see it as an overload and or the breakers between the isolation transformer and the inverter output should pop leaving the inverter safe and perhaps still running the house. Just my theory I could be seriously missing something here. I think it would be difficult to limit the current at the primaries as it has already saturated the inverter transformer. Have being Wrong before Maybe someone will find something useful out of this.  Cheers Aaron Cheers Aaron Off The Grid |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
Gidday Aaron, I have been thinking about this too. The MOSFETs in the OGI are isolated from what ever is coming out of the GTI by the Inverters transformer. So if 300V dc was on the house AC circuit it can't get directly to the MOSFETs. When the inverter tries to push against the dc it would appear as a short? Is having another transformer in the circuit going to make any difference????? Still trying to get my single brain around it, much easier for Tasmanians with 2 heads. One thing I should have done is replace the 63A breaker I had on the GTI AC out with a 20A. Maybe that would have helped, anyway I have a couple on the way. I have checked the GTI and of the 6 IGBT's and 2 diodes in it's output side 2 of the FGA40N65SMD IGBT, 80 A 650 V, are dead short. They are just over $4 from RS so I have ordered some and will see if I can repair it. I still have the grid to test it. I saw a video today about testing GTI's using a variac connected to a transformer for isolation then a rectifier with a cap across it. This gave a controlled isolated DC source to get the GTI operating. I still have the grid for testing, I won't be reconnecting it to any Inverter till I am sure it right. Been thinking too these 80 A 650 V IGBT's (sounds a bit like the LGBT plebiscite thing) would be very good for controlling a GTI and they are cheap. After all the dramas I had I have my smaller single core toroid inverter running the house again until I get MOSFETs for the big inverter. China has been on holidays again this last week, it is amazing how when they have a public holiday they really do it properly. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Revlac Guru Joined: 31/12/2016 Location: AustraliaPosts: 1026 |
On holidays is about right I ordered some IRGP4066DPbF some time ago still not here yet. Hopefully the gate drive on the GTI is still OK. Cheers Aaron Cheers Aaron Off The Grid |
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