Author Topic: News from the GTI design workbench, Blog  (Read 3717 times)

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Offline fourtytwo42

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News from the GTI design workbench, Blog
« on: November 02, 2017, 02:51:13 am »
Hi all just so you know I am busy rather than asleep here are some updates to my ongoing GTI designs, basically I have one incarnation in service whilst designing its improved replacement, an ongoing process.

Those familiar with the typical LCL filter used for grid interconnect will understand that whilst it has many advantages it has severe penalties caused by its resonance and phase shift making the feedback around the current loop very difficult to stabilize.   This does not really have a parallel amongst simple feedback discussions such as type I, II & III. 99% of related treatise assume your a mathematician and have a copy of mathcad neither of which is true in my case. However reading material endlessly can procure results and I found some references mentioning the use of LCL capacitor current as a method to assist loop stability and the lights switched on for me!!

Capacitor current has a leading phase wrt to line current and solves the basic problem of attaining a feed-forward effect that I have long sought after, as in predicting line current before it happens :) I hope the enclosed plot illustrates the effect of mixing a proportion of capacitor current into the feedback loop ( the really nasty waveform is just line current, the better is with a proportion of capacitor current).

The next issue is that of sine reference, I must admit to being guilty of simply using the grid voltage!! Fool I am sure many say well OK and now I understand if your grid has a lot of harmonic distortion or your grid coupling impedance is highish your GTI acts as a harmonic amplifier!! So I shall finally resort to synthesized sine reference by micro phase locked to zeroX but why the hell does reference material scare people off with crazy wording such as "rotating reference frame" etc, for gods sake its a simple pll right!! maybe I grew up to long ago :)

Finally I would like a word about the venerable ACS712 hall current sensor, I used this in innocence in the presence of high powered ferrite inductors and transformers OMG I know I should have known but as warning guys it doesn't like external magnetic fields! Anybody know of a good analogue isolation barrier as I am thinking of dumping them in favor of resistive shunts, I have loads of high voltage auxiliary power available ?

Edit so there is the HCPL-7510 from Broadcom (Eww) but it's optical and there must be similar from real designers somewhere :)

   
« Last Edit: March 06, 2018, 07:13:11 am by fourtytwo42 »
 

Offline dmills

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Re: News from the GTI design workbench, LCL issues
« Reply #1 on: November 07, 2017, 05:56:20 am »
The HCPL-7510 was an Avago part that I think originated with HP/Agilent, it is probably fine (Those guys were good at isolation amps).

TI have some of the old Burr Brown iso amps still in the catalogue, may be worth a look?

Or you could maybe go old school with an IL300 or suchlike?

A 'Rotating reference frame" is just saying 'phasor diagram' to me, nothing too scary there.

Regards, Dan.
 
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Offline capt bullshot

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Re: News from the GTI design workbench, LCL issues
« Reply #2 on: November 07, 2017, 08:09:28 am »
Use a closed loop magnetic current sensor like the ones from LEM - these are quite insensitive to external fields.
The HCPL-7510 is also a good and reliable device, though there are newer parts with better linearity and bandwidth available from HP/Avage/Broadcom. Better not use the magnetic or capacitive coupled ones from TI  / ADI (to name the most known brands), they might be nasty sources of high frequency EMC. Stay away from coreless or open loop sensors in your environment. With some exeptions, e.g. when properly used, the Sensitec ones may perform well in your environment though beeing coreless.

Anyway, I'd prefer a LEM current transducer of appropriate size. Easy to use, +/- 15V or single supply 5V versions available, and isolation is for free.

Edit: If it is just the capacitor current to measure, there's no need for a DC path, you could use a simple small CT here.

Another one: If you have paralleled capacitors in your LCL, you can use these as a current divider (smaller cap in parallel to a larger has proportionally less current) together with a larger value (easier to get a larger signal amplitude) sense resisor.
« Last Edit: November 07, 2017, 08:13:49 am by capt bullshot »
Safety devices hinder evolution
 
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Offline fourtytwo42

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Re: News from the GTI design workbench, LCL issues
« Reply #3 on: November 07, 2017, 06:28:11 pm »
Many thanks for your ideas and help :) I must admit to being slightly biased against certain companies that have acquired technology through aggressive acquisition followed by downsizing of employees but sometimes there is no choice :( As for the name changing game, just Grrrrr

I like the idea of a small CT for capacitor current and I have never had a problem with small CT's in SMPS's so that should work :) As for the output current it is good to have the LEM recommendation although I recall these can be expensive OTH in a noisy environment a shunt can loose considerable power to provide enough signal.

I must confess to currently (pun) working in other area's of the project, namely the MPU that requires considerable overhaul and for me a completely new assembler to digest :)
 

Offline fourtytwo42

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Re: News from the GTI design workbench, PCB Costs
« Reply #4 on: February 05, 2018, 07:35:28 pm »
Things are once more moving to the prototype construction phase having created yet another new topology and control scheme. I have been looking at PCB costs and they are ever rising, the pain in part is caused by the size of the components mounted on the PCB so I have decided to revert to my valve ancestry and chassis mount the major components, in this case being the transformer, inductors, large capacitors and also avoid using a PCB for mounting heatsinks.  The only reason for a PCB at all is the voltages are to high (~500V) for stripboard construction and a compact layout is required around the switching devices/drivers.

Another lesson learnt is to divide the power circuits into two PCB's as this will make re-spin half the cost. I made the mistake before of trying to fit a prototype into a fairly tightly dimensioned enclosure and although this worked the PCB costs were much higher than had I given myself more space for a more distrubuted mechanical design.

The control function has not been an issue as it is constructed on stripboard and plugged into the power PCB, very simple to re-work at almost zero cost.

Hope this gives food for thought for others trying to reduce PCB costs in high voltage power circuits :)
« Last Edit: February 05, 2018, 07:43:49 pm by fourtytwo42 »
 

Offline fourtytwo42

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Re: News from the GTI design workbench, 2018 construction
« Reply #5 on: February 13, 2018, 08:00:55 pm »
I have completed the preliminary layout of the DC (input side) board and it shows a substantial size reduction is possible. Size is now 125*105mm and an estimated cost of $27 :) Both the main transformer and first stage inductor are now chassis mounted to save pcb real estate.

Outdoor metal cases for high voltage circuits are also very expensive in larger sizes however I discovered a neat steel lockable letterbox on an auction site for just £8 :) This is more than big enough being A4 sized and 85mm deep.  I have no sheet metal working facilities nor anywhere to buy stock so cannot fold up my own.

Onwards and upwards the AC board (output side) is waiting to be placement tuned and routed!
 
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Offline fourtytwo42

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Re: News from the GTI design workbench, 2018 construction
« Reply #6 on: February 14, 2018, 06:59:01 am »
Taking the plunge just to show what sort of issues arise here is a very early part routed AC board, the fight is on between track widths to accomodate current and spacing to accomodate voltages the TO220 mosfet packages are the worst area and this about as much power as you can squeeze out of them at nominal 240Vrms, any higher and the spacing vs track size just doesnt work even with conformal coating.

A problem with KiCad is you cannot specify spacing between particular nets, just a global spacing for a given net, causes headaches in multi-level high-voltage designs like this so a lot of manual gap checking :)
« Last Edit: February 14, 2018, 08:26:00 am by fourtytwo42 »
 

Offline fourtytwo42

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Re: News from the GTI design workbench, BLOG
« Reply #7 on: February 18, 2018, 03:31:40 am »
Work suspended for Chinese new year! That was bad timing on my part but hey everybody needs a holiday  :)
« Last Edit: March 06, 2018, 07:13:52 am by fourtytwo42 »
 

Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #8 on: March 06, 2018, 07:22:40 am »
Chinese new year over! DC boards (first sent out) arrived today, turns out held up longer in UK by snow than air-freight from China :)

Whilst waiting built and tested as much as possible of the control board, built on the ubiquitious stripboard as all low voltage. Socket at top is for processor.

DC-board just loaded with 5V regulator (just a linear from ~12V as consumption so low). 12V is derived from the panel voltage by a low-power switcher in the immersion (water heater) boost converter, better to only have one low power auxilery high voltage supply in the system, reduces complexity and costs whilst improving efficiency :)
 
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Offline ahbushnell

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Re: News from the GTI design workbench, Blog
« Reply #9 on: March 06, 2018, 09:39:02 am »
Looks nice. 
 
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Re: News from the GTI design workbench, Blog
« Reply #10 on: March 09, 2018, 03:00:43 am »
DC board fully loaded (except high energy storage caps) and tested along with control board.  Lots of thick heatsink cutting today with an angle-grinder to make heatsinks for both boards, frozen fingers out in the garage! AC board went out for fab yesterday.

Run out of work so will have to start the case :)
 

Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #11 on: March 17, 2018, 08:39:10 pm »
AC boards arrived a while ago, stepwise loaded as each section tested, holdups due to unforseen MPU instability but now reached the stage where the main transformer and inductor are ready to be added (external to the pcb's) so the serious testing of the complete GTI can begin!
 
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Re: News from the GTI design workbench, Blog
« Reply #12 on: March 18, 2018, 03:18:18 am »
And here is the trial fitting in the case (letterbox) layout of everything designed to ensure the essential PicKit3 was going to fit in situ :) Run out of fiberglass sleeving so won't be wiring the magnetics till late next week! On the hunt now for something to keep the spiders out of the fan to avoid spidercide  :o
 

Offline ahbushnell

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Re: News from the GTI design workbench, Blog
« Reply #13 on: March 18, 2018, 09:34:35 am »
Looking good.  Is this going inside? 

 

Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #14 on: March 18, 2018, 06:04:16 pm »
Looking good.  Is this going inside?
Hello and thankyou :) It's going outside high up on a brick wall under an open sided shelter, the box has a steel door not shown here, the top vent (letterbox) will be filled with wire mesh to keep insects out and propped open 30deg to allow air out but keep water drops out (there shouldnt be any). Boards will be conformal coated after debug.
« Last Edit: March 18, 2018, 06:07:30 pm by fourtytwo42 »
 

Offline ahbushnell

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Re: News from the GTI design workbench, Blog
« Reply #15 on: March 19, 2018, 02:18:40 am »
Conformal coating is a good idea.  Condensation. 
 

Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #16 on: March 23, 2018, 05:28:56 am »
Fiberglass tube arrived so magnetics wired, here we are on the workbench starting debugging at low voltages/energies so far functionally ok but can be difficult to create realistic operation in a test harness. Something else I hate about windows is mouse freezing with heavy USB activity (both PicKit3 & Owon scope) if anybody else gets this try unplugging the (USB) mouse for a few seconds, usually works and saves a reboot!
 

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Re: News from the GTI design workbench, Blog
« Reply #17 on: March 26, 2018, 08:50:19 pm »
Busy testing, enclosed pic shows anti-islanding in action red being grid and yellow GTI DC input current. As an interesting aside the grid has a non-symetrical voltage variation that the GTI is trying to correct, this variation varies in frequency and comes and goes at different times of day acompanied by a 2-3dB drop in broadband SNR. Looks to me a bit like a cycloconverter!

Nasty smell from the bench when I last fired it up, have yet to have time to trace source but did have shower just in case :)
 

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Re: News from the GTI design workbench, Blog
« Reply #18 on: March 29, 2018, 06:50:14 am »
After some trials and tribulations I have to say I am tickled pink by the sine linearity performance compared to the previous design. Still early days in testing but the enclosed scope shot shows a nice clean grid voltage waveform, note at this stage transformers are used to reduce voltages for safety.
 

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Re: News from the GTI design workbench, Blog
« Reply #19 on: April 02, 2018, 03:08:51 am »
Ongoing debugging at moderate powers and voltages, continuing to find what poor software I write like the enclosed scope shot that shows the main inverter getting unloaded intermittently near zero-cross, turned out to be entirely due to lack of thought and the potential consequences of writing some code in a particular way! Meanwhile also discovered (and reported to Mchip) another silicon problem in the HSPWM laughebly just before the easter holiday but fortunatly was able to sucsesfully devise a workaround. The more you use powerful features of chips the more you expose yourself to nasties but if it was not for the high level of integration considerable external logic and linear discretes would be required.
 

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Re: News from the GTI design workbench, Blog
« Reply #20 on: April 03, 2018, 04:11:09 am »
My BIG BANG theory is that it's related to overconfidence and always happens when things seem to be going swimmingly well  :rant: In my case I was retapping the isolation transformer to go up from 60Vrms to 120Vrms and I thought in the process I would ground the available centre tap for enhanced safety! All sounds fine, isolated scope, isolated laptop, what could possible go wrong  8) Well I like my music but most times when I am testing it's switched off and due to the audio jack of the laptop being right next to the heavely used USB's unplugged from the sound system, but today I was happy, I knew exactly what tests I had to do so managed to get the audio jack in with the USB's. Everything ready, cannot remember the tune but when I plugged in the isolation transformer BANG!!! :wtf: :scared: loud buzzing noise from the sound system, took a few seconds before I unplugged, EVERYTHING...nasty burning smell OMG  :-BROKE.......HOW  :-//

Well it turned out the problem was twofold, grounding the isolation transformer centre tap and having the sound system plugged into the laptop, those were both new things I had not done during previous testing  |O

The sound system provided a path from the laptop to mains earth via the cable screen.  The scope was monitoring something on both the grid side and solar side of the inverter connecting one leg of the grid to solar negative, thats ok normally as the scope is isolated anyway but now that referance was 60Vrms above ground and the final connection in the short circuit ? The PIC debugger referanced to solar negative and connected to the laptop USB  |O

The moral of the story, check and check again everytime a test harness is changed or something like a sound system added to it!! The sound system survived, the laptop had a track blown off by the headphone socket, the pickit3 is a bit of a mess with at least 3 burnt chips and some burnt tracks, most of the GTI survived but lots of control board chips were destroyed and sadly as I live in the sticks it will take over a week just to get the parts :(

I dont want to post any pics today, too embaressed!
 

Offline ahbushnell

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Re: News from the GTI design workbench, Blog
« Reply #21 on: April 03, 2018, 09:24:05 am »
Bummer.  :palm:
 
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Offline dmills

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Re: News from the GTI design workbench, Blog
« Reply #22 on: April 04, 2018, 01:41:20 am »
Such is the nature of power engineering, if you don't smoke something you are not trying.
It is **ALWAYS** something stupid that you wind up doing.....

I have lost count of the number of really expensive power devices I have killed over the years in screwups not completely unrelated.
It is when you do it to a 250A, 1,000V IGBT module that it really hurts, even when the resulting cloud of copper vapour stays safely inside the box.

Regards, Dan.
 
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Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #23 on: April 04, 2018, 04:07:07 am »
Ahh so true, I wonder if amplifiers also count as power engineering I had some expensive blowups there too over the years.

I think if I lost an IGBT module that big it wouldnt be just the noxious fumes making me cry......

 :blah: rambling alert! When I think I could sit back and write software to log temperatures or something that has no risk of destruction but then there wouldn't be the same challange and sense of acheivment for me at least, guess I like tiptoing around the edge of failure like someone walking around the top of a skyscraper or perhaps more meaningfully pushing the limits of innovation. :blah: I have always enjoyed electrically or mechanically oriantated engineering, something that moves or makes noise! Even this GTI crackles due to my less than perfectly wound magnetics as it reacts to itself or the grid changeing, quite a usefull aural feedback tool as long as you dont get overconfident and start listening to music during debugging sessions  :-DD
« Last Edit: April 04, 2018, 03:24:11 pm by fourtytwo42 »
 

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Re: News from the GTI design workbench, Blog
« Reply #24 on: April 10, 2018, 03:46:47 am »
Slowly crawling out of the pit I dug myself, repaired the PicKit3 by replacing not just the tranceivers but a lot of discreetes as well including MMBT3904/3906's several diodes and even an mlcc capacitor SMT not being my strongpoint this took a while as I had to order all the parts in sequentially as it's not possible to find all the dead bits in one go! Again the control board dead bits have been replaced but only a test with the processor running will determine if it's fully functional again. What a fag, I will definetly try NOT to repeat such a stupid mistake again  :palm: And so the trail of destruction continues more dead parts found on the control board that I do not have and have had to order, hopefully maybe these will be the last!
« Last Edit: April 11, 2018, 05:15:18 pm by fourtytwo42 »
 

Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #25 on: April 14, 2018, 04:51:07 am »
Having fixed all the blown up bit's including the control board testing has resumed at 120Vrms grid. I know how much you guys like pictures so todays shot is the error amplifier output when the DC-bus is to low a voltage to support the peak sinewave voltage causing the ea to saturate whenever the grid nears it's peak. This problem will simply be solved by stepping up the DCbus to the next test level (re-arrangeing transformer taps). What I like about the waveform is it's so nice and clean with no nasty resonances!
 

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Re: News from the GTI design workbench, Blog
« Reply #26 on: April 23, 2018, 04:51:58 am »
Been a bit quiet lately, apart from being the go crazy seed planting season I have had some very hard to trace mosfet popping episodes in the grid H-bridge  |O. This was discovered to be caused by the PIC driving both halves of the H-bridge on (active low) simeltaniosly some 20mS after power up even though at this time and condition no code exists to do so  :--. This is extremely disheartening! Further work discovered it was around the time another channel of the HSPWM was having its IOCTL register changed (the unfolder/H-bridge is driven by an HSPWM channel). After some additional attempts at localising the problem by using scope triggers the fault mysteriosly disapeared  :-//. This coincided with a long programming period (usually they are very short). So essentially the mosfets were lost due to an aberation in the PIC HSPWM, there are indeed some failings of the IO-override bits I use in the Errata though not this specific case however the Errata dates from August last year and obviosly will only contain those test cases so far reported.  Given I have reported several and had no responce it is quite likely other people have encountered and reported this bug already  :--.

As for a fix/work-around I am still considering this before sacrificing any more mosfets  :-BROKE
 

Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #27 on: May 10, 2018, 04:36:24 am »
So what can be the worst of all evils  >:D Juggling with changing the MPU to avoid the bugs has gotten me back pretty well where I started but here are some of the alternatives and why I dismissed them

1/ STM32F303 I was so impressed by the data sheet I went ahead and bought one, it was only in the latter stages of evaluation I realised a showstopper, the PWM although it did everything else did not support half-bridge or push-pull SMPS's and neither did any of there chips! Turns out there PWM is a very common piece of ARM related IP used by many other ARM MPU vendors so wiped all of them out too including the often mentioned NXP.

2/ Cypress PSOC5 Again very good spec and no worries regarding PWM weirdness as incorperates programmable logic where a custom design can be implimented. First indicator of problems was no errata on the website and when tech support contacted said no such document existed!! THEN I downloaded the IDE, OMG like a childs toy plaything, you are supposed to draw schematics of the design then it goes off and adds whatever boilerplate code it thinks is appropriate, the final straw came when I finally found the RTL description in a seperate document to the functional description with a flat index 45 pages long not even alphabetical, upshot, unuseable IDE and no alternatives (I asked).

3/ PLD was possible except all analogue peripherals would have to be external, but the killer was development system cost.

4/ A simpler PIC that I have used in the past with well known PWM's. Sadly re-discovered why I had chosen the PIC24EPxxxMCxxx in the first place as the PWM is an advance on all those preceding it.

It is very difficult to fix power mosfet drive problems with external logic as it must be secure against mis-operation in all power up/down circumstances. In the end I have decided to persevere with the PIC24EP as the least of all evils  >:D and I am at least now able to quantify some of those even though Microchip show no interest at all. For the time being I think I have devised a discreet circuit with known power cycle parameters that will work-around the PIC problem that was causing MOSFET loss, so the parts are on order and I hope I can replace the lost power mosfet 2nd time around without damage to the pcb, this being a hard ask on a high voltage part. In the meantime there is software to re-write to accomodate the changes and steer clear of the bugs :)

I hope my brief assesment of some alternatives might help others :)
« Last Edit: May 10, 2018, 04:45:46 am by fourtytwo42 »
 

Offline capt bullshot

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Re: News from the GTI design workbench, Blog
« Reply #28 on: May 10, 2018, 05:19:47 am »
I remember some Renesas controller having a PWM unit supporting half bridge, push-pull and phase-shift PWM modes. Can't tell which one it was, AFAIR a rather small one. We (at work) considered this one for a resonant converter, but did it for whatever reason in another way (using a CPLD). At least you're guaranteed their PWM has quite nothing in common with the ones you mentioned.
Safety devices hinder evolution
 
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Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #29 on: May 10, 2018, 05:57:06 pm »
Thankyou for the tip :) I think thats the old NEC, I seem to remember 78K from them many years ago!
I have had a look and there a bit bewildering at first with 20+ product lines and a selector thats too slow to work but early days, I just have to get a feel for them and learn there quirks, certainly plenty of choice :)
It seems there development boards are quite high-cost that may explain why I missed them in the first place.

EDIT Parts arrived so control board modified (getting busier) and ready for test BUT have to replace mosfets first  :-/O
« Last Edit: May 11, 2018, 12:57:22 am by fourtytwo42 »
 

Offline ahbushnell

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Re: News from the GTI design workbench, Blog
« Reply #30 on: May 11, 2018, 09:27:25 am »
TI has a Piccolo DSP which is designed for power supply and motor control.  And they have the big brother the Delfino.  Even one with 2 cores and it will run at 200 MHz. 
 
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Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #31 on: May 12, 2018, 02:35:16 am »
Thanks for the Piccolo tip :) Slight difficulty with protoboards here, seems to incorperate programmer/debugger on the same board but NOT as a breakoff module...... Anyway adds another string to my bow thanks :)
 

Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #32 on: May 15, 2018, 08:03:08 pm »
And there we have it, handling and fitting the mosfet with an anti-static foam tutu works  :-+
Fairly easy to remove afterwards with tweezers.
Of course having an EARTHED soldering iron helps too  :-[

EDIT :_ Just realised posting this here doesnt entirely make sense as I originally posted the weird friday mosfet problem in the tech forum, sorry!
« Last Edit: May 15, 2018, 11:45:12 pm by fourtytwo42 »
 

Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #33 on: May 17, 2018, 05:07:13 am »
Progress today, blew another complete set of unfolder mosfets  :palm: but that was after running a while and a VERY nasty test, removing and reconnecting the grid!

This time I cannot blame the PIC as my workaround circuit discretely avoids it's pwm problems so I just have to admit the mosfets are not man enough for real world spikes and surges. Of course it's pure chance what the phase of the grid is on reconnection and there's loads of spiky stuff as the contacts slide together so who knows what might be happening and the chances of capturing anything usefull on the scope are about nill  :-\

So I decided one instant improvement in beefiness is to replace the mosfets with igbt's, just a bit more surge capacity in the same package (TO220). I am beginning to wish I had used thyristors here in the first place ghaaaa nothing like belt n braces  :-+ anyway the igbt's are compatible with the same pcb assuming I can get the mosfets out without irreperable damage (one of them has been changed twice aready). Also changeing up from 800V mosfets to 1200V igbt's and there are 470V tvs's in there so whoaaaa must be current surge!!
Time will perhaps tell  :-\

Sorry no pics, mosfets look just the same as working ones and only a mild pop from a sand filled ceramic fuse announced the failure....
« Last Edit: May 17, 2018, 05:13:53 am by fourtytwo42 »
 

Offline ahbushnell

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Re: News from the GTI design workbench, Blog
« Reply #34 on: May 17, 2018, 12:33:08 pm »
Schematic? 

Andy
 

Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #35 on: May 17, 2018, 05:23:36 pm »
Here you are :) V1 is the grid and V+ is the DC source. There are TVS arresters across both the grid and DC output of the rectifier. However there is not one across any quadrent of the bridge and I am thinking that might be a mistake. In reality the grid connection is via an LCL filter not shown here and the TVS is on the H-bridge side of the LCL.
 

Offline f4eru

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Re: News from the GTI design workbench, Blog
« Reply #36 on: May 17, 2018, 08:57:46 pm »
Hey, fourtytwo42, your project is quite interesting ! I'm doing something similar, but island mode in the first version, with grid connection possibility for later dev.

Do you have the complete schematic with drivers, output filters ?
Your trouble probably comes from from the output filter vs protection etc etc etc...
What's your strategy to regulate the output current ?
how are your PWMs cycled in the positive and negative cycles ?
Do you have enough Tlow when the voltage is high to recharge the drv charge pump caps ? (what's your max pwm ?)
how do you sync to the grid ? PLL ?
do you have a sync waiting delay after reconnection ?
etc etc etc...

What's your DC filtering ?
 

Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #37 on: May 18, 2018, 12:21:55 am »
There are several reasons I would not publish actual schematics here nor discuss anything in more than general terms.  Those that come to mind are safety, commercial confidentiality and the intolerance some members exhibit to others when discussing high voltage circuits but thank you for your interest and good luck with your own project, perhaps you would like to start a blog on it too :) I should add there are silicon vendor examples of GTI's published on the web.
« Last Edit: May 18, 2018, 01:57:15 am by fourtytwo42 »
 

Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #38 on: May 18, 2018, 02:01:22 am »
IGBT's fitted but ohh what a mess! Unfortunatly I could not get hold of the TO220FP packages I wanted so these are standard TO220's with bushes, mica washers and copious amounts of heatsink gunge! Good enough for test but the insulation is definetly not so good  ::)
For comparison thats a TO220FP diode to the left.

EDIT I am releived the igbt's seem to have solved the problem though I did also add a TVS from the AC to DC sides of the unfolder and tweek the software to try and avoid noise at grid restart at the same time! But I am not hurt by the extra TVS, the igbt's were cheaper than the original mosfets and software is free as they say hahaha
NOW I need some igbt's in TO220FP because the creepage on the present assembly gives me the creeps  :-DD
« Last Edit: May 18, 2018, 06:30:49 pm by fourtytwo42 »
 

Offline f4eru

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Re: News from the GTI design workbench, Blog
« Reply #39 on: May 19, 2018, 10:02:43 pm »
Quote
k you for your interest and good luck with your own project, perhaps you would like to start a blog on it too :) I should add there are silicon vendor examples of GTI's published on the web.
Yep, I understand.
I'm also not allowed to publish it as it'sa commercial thing, but I can share a few insights, I'll not write a blog.
 

Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #40 on: May 20, 2018, 06:44:21 pm »
Today's problem is classic transformer flux walking, the yellow trace in the picture is the primary current and you can clearly see the knee of magnetic saturation, this has been caused by several cycles of unequal conduction that in turn are caused by instability in the control system.

I am pretty sure this is what led to the tracks being blown off the board last year but now the primary overcurrent protection has been completely redesigned the problem can be examined without loosing the pcb!
 

Offline f4eru

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Re: News from the GTI design workbench, Blog
« Reply #41 on: May 20, 2018, 09:33:40 pm »
Interesting, I didn't get a problem yet with flux walking, perhaps my primary is balanced better.
For the overload on tests, to not blow up everything, and test with no proper cooling, I usually pulse the system 10ms on, 1s off until everything is stable
 

Offline ahbushnell

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Re: News from the GTI design workbench, Blog
« Reply #42 on: May 21, 2018, 01:24:10 am »
Today's problem is classic transformer flux walking, the yellow trace in the picture is the primary current and you can clearly see the knee of magnetic saturation, this has been caused by several cycles of unequal conduction that in turn are caused by instability in the control system.

I am pretty sure this is what led to the tracks being blown off the board last year but now the primary overcurrent protection has been completely redesigned the problem can be examined without loosing the pcb!
If the current is not too high you can put a capacitor in series with the transformer and if there is a imbalance a DC voltage will correct for the walking.
Or you can use a current probe and detect the flux walking by the current on one side.  The probe has to be able to measure a DC component.  We buy probes from LEM. 

Andy
 

Offline ahbushnell

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Re: News from the GTI design workbench, Blog
« Reply #43 on: May 21, 2018, 01:25:40 am »
Interesting, I didn't get a problem yet with flux walking, perhaps my primary is balanced better.
For the overload on tests, to not blow up everything, and test with no proper cooling, I usually pulse the system 10ms on, 1s off until everything is stable
In the early days of power electronics people didn't know about flux walking.  Products were shipped and most worked but some would blow up.  It took awhile before it was solved. 
 

Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #44 on: May 21, 2018, 04:13:50 am »
Thanks for the replies :) I dont worry to much about the walking as it's an effect rather than the cause, just pleased the current limiter catches it :) Yes the trafo is capacitor coupled, it's a half bridge but the capacitance has to be large enough to deal with the power concerned. 10mS is certainly long enough to destroy any semiconductor I know of!

The problem here is actually the control loop does not attinute Fo suficiently allowing the modulator to occasionally lock into this unfortunate situation. Anybody familier with GTI's will know the control loop is a delicate balancing act and of course having changed its responce some new issues arise.........

I should add I do use a LEM in one location but they take up a lot of space compared to a simple current transformer, I originally used the Allegro things but they are badly affected by external fields, the LEM is sooo much better :)

I remember the old days of doing 48V input push-pull with bipolars, now that had some serious unexpected saturation issues way back in the early 80's :) Worked in the end just made the trafo bigger so it could handle more abuse  :-DD
« Last Edit: May 21, 2018, 05:48:19 pm by fourtytwo42 »
 

Offline f4eru

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Re: News from the GTI design workbench, Blog
« Reply #45 on: May 22, 2018, 01:46:13 am »
why not use a simple shunt resistor instead ?
I usually put a shunt on the lower leg of the H bridge, that can be acessed and measured very very easily.
Which driver do you use ?
I use the NCP5104
 

Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #46 on: May 22, 2018, 03:28:48 am »
why not use a simple shunt resistor instead ?
I usually put a shunt on the lower leg of the H bridge, that can be acessed and measured very very easily.
Which driver do you use ?
I use the NCP5104
I am sorry I think we are talking different era's or technoligies, shunt resistor ? H-bridge, what H-bridge where ?
How many watts is this inverter of yours, from what input voltage and as I dont know what country your from what grid voltage ? and does it use a ferrite or iron transformer ?
« Last Edit: May 22, 2018, 03:30:32 am by fourtytwo42 »
 

Offline f4eru

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Re: News from the GTI design workbench, Blog
« Reply #47 on: May 22, 2018, 06:34:23 am »
Hello,

The inverter I design currently has a 48-> 310V     40 kHz DC/DC followed by a 310V H-bridge, in order to create a 50Hz 220V sinus.
output is approx 250 VA.

The current sensing is through a single lower leg resistor.

For the h-bridge, you have also shown a h-bridge in your sim schematic....
« Last Edit: May 22, 2018, 06:37:09 am by f4eru »
 

Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #48 on: May 22, 2018, 02:31:31 pm »
Ahh now I understand :) But if you sense current in the output using a resistor don't you then have to transfer that signal across the isolation bridge to your controller on the input side ? Thats why I use current transformers or the LEM they have built in isolation and with a resistor you have the signal level vs losses connundrum.

My grid voltage is a little higher than yours around 240Vrms here in the UK, guess your in mainland Europe then ? But I have seen it rise to 250V here and fall as low as 225V thats what you get for living in a small village sharing one large 11Kv distribution  transformer on a pole, I think it's either 50 or 75Kva.

My GTI runs from ~180Vdc input and is designed for 1600W that being 50% more than my presently installed PV.

I didnt get to look at it yesterday, to much sun outside and gardening to do, trying to clear the house of seedlings and get them outdoors hahaha
 

Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #49 on: May 23, 2018, 04:21:59 am »
This evening I went trafo hunting, this is similar to truffle hunting and for engineers can be just as rewarding  :-DD
In this case I am upgrading the test rig to increase voltages and powers towards rated and had exhausted the present capabilities. So I was trafo hunting to increase the available input voltage and found this beauty, once part of a linear computer power supply. I used to have a lot of these and bigger but you know how it is moving houses and wives, things get downsized even the junk pile  :-DD
 
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Offline f4eru

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Re: News from the GTI design workbench, Blog
« Reply #50 on: May 23, 2018, 05:53:40 am »
my controller is on the "grid" side of the isolation barrier (when grid tied, else the isolation barrier is shunted)
so no need for all these isolaion devices. KISS principle.
Are you using a 50 Hz transforer after your LV H-bridge ?
 

Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #51 on: May 23, 2018, 06:19:34 pm »
my controller is on the "grid" side of the isolation barrier (when grid tied, else the isolation barrier is shunted)
so no need for all these isolaion devices. KISS principle.
Far from KISS there's no way I would want all my test gear or myself connected to the grid thank you :) I prefer to do my debugging strictly at earth potential hahaha

Are you using a 50 Hz transforer after your LV H-bridge ?
No its ferrite, the large trafo in the photo is just going to be added to the part of the test rig emulating solar panels.
I dont have an LV H-bridge, the H-bridge is connected to the grid.
 

Offline fourtytwo42

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Re: News from the GTI design workbench, Blog
« Reply #52 on: May 24, 2018, 04:02:27 am »
I think in hindsite I have published way to much information here already for safety. Whilst I am an ardent DIY enthusiast I do it with knowlage having spent a lifetime as an electronics designer. I would not wish to attract the kind of copyists who I see on many forums requesting schematics to copy with little idea of what they are doing.

I would like to thank the contributors to this blog who have helped me with insightful sugestions to some of my problems and I hope I have provided some alternative ideas in responce.

High voltage high energy electronics are extremely dangerious and should only be undertaken by the significently experienced IMOP.
 

Offline f4eru

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Re: News from the GTI design workbench, Blog
« Reply #53 on: May 24, 2018, 04:07:04 am »
Yep sure, safety is priority :)
Quote
I prefer to do my debugging strictly at earth potential hahaha
Yep, for that I use an isolation transformer, as a test setup.
 


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