3KW Windmill Converter/Inverter

[Sten Martin]

Hi,

I am working with power conversion and want to use PICAEXE and the HPWM to convert restified Windmill-power to usable house-hold power.

My input is variable, 200 to 500V restified DC. I want to have an output of 1 Phase 230V AC @ 50HZ. The output power is 3 kW.

I need help from anyone that have done something similar or know how to do it with PICAEXE HPWM command.

 

[SAborn]

I have been around wind turbines for about 20 years and what you want to do is not very easy and almost not recommended in trying with a picaxe.

3kw is a big mill and i doubt you will see that sort of output on an average day, perhaps a few times a year.

You would be far better to use an inverter suited to wind turbines with MPPT, even then there is problems, and if you intend to back feed to the mains grid (GTI) you will need to comply with regulations like anti island controls etc.

It is not a project for the unskilled or beginner in electronics, there is few people here that would be skilled enough to complete a project like this and i very doubt it can be done over a forum.

How will you keep a loading on the mill in the event of a circuit failure or grid failure, lack of demand, etc.

 

[Sten Martin]

I know this is a major task, I did a complete compact military 10 kW 3-phase system for aircrafts over 15 years ago. I still have the complete spec with all the components. What I am trying todo is as scalled down 3 kW cheap version. And it is not just Wind-mill inputs, I have several other inputs in mind. What I was trying todo in the forum was only the PWM switching and the controlling part since the PICAEXE is relative cheap and simple to use.

I was first thinking of using the virtual labors you can find on Internet but wanted to try the Forum first with regards to the PWM switching programming part.

If you have any suggestions how this can be don I will be happy to here it. I can send you the specification I have of the hardware .
Of cource I am prepared to pay a fee for extended work.

 

[SAborn]

Have you worked with wind energy before, as it a much different animal than mechanical energy.

There is a lot more involved with getting a system to function correctly, and not overload or underload the mill.
Then you need to consider all the fail safes required to prevent a runaway mill situation you can not stop. (path to self destruction)

I know you are just asking for a PWM solution, but its a bit fruitless if all other aspects are not also considered.

200 - 500 VDC at 3kw is some serious voltage and current to deal with, one wrong mistake and its toast to you or the circuit.

What is the wind turbine you have in mind for use, and how will the output power be used, Off grid supply or Grid tied to backfeed to the grid.

Im happy to offer information and dont seek any payment, but do feel you might have overlooked many requirements, so a bigger pitcure of your intentions would fill in some gaps.

I really dont think the picaxe is anywhere fast enough to do the data processing to a high enough level, and also be reliable enough to trust a 3kw mill to in all weather conditions.

I can see a failure situation on the horozion, as so many have also found in the past.

 

[manuka]

Forget PICAXEs at this stage & ask the boring questions first -exactly where are you in Sweden & when is your demand likely to be greatest? What's your probable solar & wind resource? Why wind anyway?

3kW for a wind turbine is quite small beer, but serious consideration still has to be made to prevent storm destruction & maintenance has to regular and be VERY thorough. This may be costly if the gene is up a pole or in an isolated region...

I once witnessed here in NZ a near new 5kW (?-maybe 12kW?) mill frightenly spin out of control due to a bolt that had worked loose. A storm arose & the evil noise became akin to rotating plane propellors! Repair was impossible in the storm, & the gene became too dangerous to even be near. It self destructed after a few hours- much to the owner's anguish as he was out some US$50,000 as a result.

Further wind gene factors are noise if near buildings,visual pollution,need for elevation into laminar wind flows & likely US$10-$20 a metre cable run costs if sited in a windy spot away from dwellings. Ice & snow loadings may be issue too in Sweden.

Solar PV prices have recently collapsed to the extent that a 3kW PV array would now probably be cheaper to purchase, install & maintain than a similar output wind gene. Naturally it'd only work by day, & seasonal variations may be a significant issue. However it'd be easy to handle running repairs & tweaks, & also low profile too - the sun is just as strong at ground level as up a tall pole! Furthermore "old sol" is not subject to the inevitable once in a blue moon extremes that wind genes have to be over engineered to handle.

 

[Sten Martin]

Ok, I completely agree with you about Windmills. However my situation is this:
1. I am not going to connect to the grid.
2. All my experiment is done on my Yacht in Thailand where I live 10 month of the year.
3. On my Yacht I have wind-turbines, solar panels and hybrid electrical generators.
4. All those systems need an Inverter.
5. Today I have an expensive 3kW Victron converter.
5. All system but the hybrid Honda Electrical generator is charging a big battery bank that also need an Inverter.

I am convinst that the PICAXE X28 can drive the IGBT I need for the Inverter.
And I am going to develop the Inverter I need with or without PICAXE but I first want to know from you if it will be possible or not.
You know, difficult things take a few days to solve, impossible tasks take a little bit longer.


So aga

 

[Dippy]

Do you have any example inverter circuits already downloaded that you would like to consider?
There are many on the internet. I'm sure some are excellent and equally sure some are unsuitable.

How well does the o/p have to be regulated?

What sort of quality of sine wave do you need?
There are many simple switched push-pull transformer designs around but do you need a synthesised sine?

How 'bullet-proof' do you want it?


These things determine the complexity of the circuit.

How good are you at coding and electronics?
This will allow people to provide suggestions/solutions appropriate to your skill level.
And I'm sure people are Googling like mad to give you a circuit

 

[manuka]

Tropical Thailand -which part! Is this where the windmill is to be eventually sited? Aside from cyclones, winds in many parts of Asia are pretty tame (5-20 knot range). Most wind genes are rated for wind speeds of 10 m/sec (~20 knots), so a 3kW wind gene would only deliver a fraction of it's rated output.(The available power in wind goes up enormously -cubed- with wind speed increases of course)

Sorry to ask the boring questions, but -sigh- I've been down this track many times (especially pre cheaper solar PV) here in NZ. Unlike "on the roof" solar, wind gene cabling & ducting costs alone may be $$$ In 2009 an off grid mate spent $$$$ installing a ~1kW level wind turbine some distance from his sheltered house, only to have it end up "chasing it's tail" due to unforeseen non laminar wind flows. Delivered performance was pathetic & any tweaking at all needed a skyhook,calm day, work crew & well stocked beer fridge! With the recent plummet in PV prices he's instead now using door sized 250W PVs (available locally at ~US$1 a Watt), which he connected himself to great effect.

But I perhaps digress -why is your swish Victron inverter not suitable? What model wind gene are you considering & what blade diameter (important for yacht safety!) Stan.

 

[Sten Martin]

The Wind-turbine I am using have both under and over voltage/speed protection a several other security feature and will not be a problem for the inverter.

However, the most interesting part of my system for the Inverter is two Honda Civic Hybrid Generators (IMA) I have bought on eBay. They deliver variable voltages depending on load (speed) and the output is then similar to a Wind-mill.

If I can use IGBT transistors together with the new AVAGO (ACNV4506) optocoupler I will possible reduce noise and switching time.

What I want to do, if possible, with help of the Forum, is to investigate the possibility to use a PICAXE 28X2 for the PWM control of the circuit that will use a 2-pulse IGBT (IRG4PSH71UD) transistor/diode bridge with filters for 1-Phase system.

As I mentioned above I already have a detailed circuit I built as a prototype for the JAS Swedish aircraft. It is a very demanding sircuit and was expensive. Today we have much more inexpensive and better components. I belive most of us want to have an inexpensive 3kW Inverter? I am also working on an EM Generator with no mowing parts but that is another story.

The part of Thailand where I live is Phuket. My experience is over 30 years with building complex systems both hardware and software.

 

[rossko57]

What I want to do, if possible, with help of the Forum, is to investigate the possibility to use a PICAXE 28X2 for the PWM control of the circuit that will use a 2-pulse IGBT (IRG4PSH71UD) transistor/diode bridge with filters for 1-Phase system.

You're gonna have to spell out what you want the Picaxe to do. Produce PWM at XX KHz, changing the PWM level every nn mS, to approximate a 50Hz sine wave? To drive an H-bridge? Does the PWM need to track this variable DC source so as to regulate the AC output? Or are you wanting it to monitor the output in a closed-loop system? Will the Picaxe be required to also monitor temperatures, overload, overvolts etc and do anything about it? Or is that a function for another Picaxe perhaps? Fancy displays, datalogging, battery low warning SMS messages?

 

[Sten Martin]

In the first project I want to do the Inverter as simple as possible. I will use a fixed restified input voltage.

I already have a Maretron network sensing temperature, voltage and current with all kins of controls and alarms.

In this simpler case the input is a 1-phase 235V AC @ 7kHz that has to be rectified to 310V DC.

I want the Picaxe to do:
Produce PWM at 3.10 kHz, changing the PWM level every 20 ms, to approximate a 50Hz and to output a high quality 230V sine wave.
Drive a H-bridge

I don't need the Picaexe to do:
Monitor temperatures, overload, over volt etc.
Fancy displays, data logging, battery low warning SMS etc.
Track a variable DC source

The rectified DC-bus has a high frequency oscillation noise that has to be filtered.

This is done with a LC-filter which is low pass filtering the voltage but also providing an extra current buffer.

After filtering the DC-bus the transistor bridge will start chopping the voltage and forming the pulse width modulated sinus wave.

The way of chopping and switching is decided by the PICAXE program which gives signals to specified drives for switching the IGBT transistors forming a rough pulse width modulated sinus wave.

Assuming all the energy is stored in the capacitor bank a calculation gives a 467uF capacitor is needed.

As very few capacitors exist capable of handling this high voltage and still maintain a large capacitance multiple capacitors has to be connected in series.

Filtering and output stage
This rough sinus wave formed by the transistor bridge would inflict new noises and EMI both in wires and in the load due to its step nature and has to be low pass filtered directly after the transistor bridge.

This filtration can be done using LC-filters. The new filtered sinus wave have a higher frequent noise. With the new opto-drivers we can hopefully reduse the noise significantly, but I don't know that for now.

The charge and discharge time of the capacitor bank
I have calculated switching frequency of the PWM to be 3.107 kHz (321.8 us) based on 94.8 us ripple from the AC line.

As you can see from the input, what I realy need is a frequency converter from 7kHz to 50Hz but that is very much the same thing since I have to go through the hole conversion process anyway!

 

[rossko57]

Produce PWM at 3.10 kHz, changing the PWM level every 20 ms, to approximate a 50Hz and to output a high quality 230V sine wave.

You might be able to do this, speed is the limiting factor. The chip is perfectly capable of producing PWM, and you can set up a timer to interrupt every 20mS. You'd probably want an X2 part at 64MHz, because the restriction will be execution time of interpreted basic commands. Whatever code is in your interrupt handler must complete within 20mS before the next tick.
You'd need to get the PWM value for the next step - a look-up table?
And assuming you want to actually regulate the output, you will need a means to read and apply a correction value before setting the new PWM.
The correction value needn't be worked out every time, this might be spread over a cycle or twenty, depending how you implement it.

 

[AllyCat]

Hi Sten,

I want the Picaxe to do:
Produce PWM at 3.10 kHz, changing the PWM level every 20 ms, to approximate a 50Hz and to output a high quality 230V sine wave.

Personally I can't see how that works. IMHO if you only change the duty cycle of a PWM signal every 20 ms, then the most that I would expect to achieve is a "square wave" output at 25 Hz. Unless you have a 50Hz resonant load (in which case what does the 3.10 kHz do)?

Can you explain how it's intended to work, showing a waveform or schematic diagram?

Cheers, Alan.

 

[rossko57]

You're right about those timings Allycat; I hadn't thought about the given specs. Those are Picaxe-able but won't give 50 cycle AC.

I would say one would want five or six update steps per-half cycle to aprroximate a sine wave, which allows at most 2mS to set up the PWM for the next step. I doubt a Picaxe can do that usefully, purely because of the interpreted Basic execution speed.

 

[Sten Martin]

I have found on Internet a Ti solution that does a 3-5 kW Inverter. http://www.ti.com/lit/an/slaa602/slaa602.pdf. The heart of
the system is the H-Bridge. The Ti system does much more then I need but is a good reference. The trick is to program the PICAEXE so I get a pure Sinewave at 230V 50 Hz. I think rossko57 is on the right track and I have to test if speed is the limiting factor. Do you have any suggestion of a code sample to test?

 

[Sten Martin]

This is another good reference how to do a Sine Wave generator http://www.csulb.edu/~hill/ee470/Lab 2d - Sine_Wave_Generator.pdf. They are using the Arduono ATmega328P micro controller. The reference includes detailed specifications as well as C-code.

 

[Sten Martin]

Frequency change from 12 kHz to 50 Hz

From the references abow my thoughts is to change the frequency the simplest possible way. What do you think?
I have a fixed digital pulse-train at 7kHz (base frequency) as input and can possible use PWM with a PLL (frequency divider, ) locked to the system clock and a timer "T".

The output can be combined with an analog filter to generate a Pure Sinus analog output signals (digital to analog converter (DAC)).

I don't need to generate different analog levels, only changing the output frequency to 50 Hz?

To change the PWM base frequency, the timer clock frequency has to be changed.

To be able to generate an output from the PWM, I need a PLL (frequency divider, ) locked to the system clock and a timer "T".

Your comments please.

 

[rossko57]

That doesn't seem to have anything to do with microcontrollers. It's not at all clear how you're proposing to generate 50hz power from DC now.

 

[AllyCat]

Hi Sten,

I don't need to generate different analog levels, only changing the output frequency to 50 Hz?.

But the low frequency output sine wave consists of different analogue levels, that's how a PWM inverter works! I supsect that you are (now) somehow trying to "shape" the output with a (passive) "filter" (low pass or resonant). Do you have any idea what a 3 kW, 50 Hz filter looks like?

Sorry, but IMHO you are totally out of your depth with this "idea" (I refuse to call it a project); currently, I can't see any possibility of success. But if you want to (try to) prove me wrong, start with a few simple experiments "on the bench" (call them scale prototypes if you wish) with modulated PWM signals (or a PLL or frequency divider or D-A converter or whatever you think might work), running at a fraction of a watt from a nice safe low voltage battery.

Cheers, Alan.

PS: It's a bad idea to edit posts without explaining what has been changed, after others may have read them.

 

[hippy]

I would say one would want five or six update steps per-half cycle to aprroximate a sine wave, which allows at most 2mS to set up the PWM for the next step. I doubt a Picaxe can do that usefully, purely because of the interpreted Basic execution speed.

An 08M2 at 32MHz seems to be able to update a PWM duty every 100us so that would seem to fit the bill. An X2 at 64MH would presumably update in half that giving twice as many steps.

This is another good reference how to do a Sine Wave generator http://www.csulb.edu/~hill/ee470/Lab 2d - Sine_Wave_Generator.pdf.

Same thing done with a PICAXE, poking the duty register SFR direct rather than using PWMOUT or PWMDUTY for speed. The 0 in the middle was just so I could measure how long between changes on the logic analyser -

#Picaxe 08M2

Symbol CCPR1L           = $B1   ' $291

SetFreq M32
PwmOut 2, 100, 0

Do
  PokeSfr CCPR1L,   0
  PokeSfr CCPR1L,  10
  PokeSfr CCPR1L,  20
  PokeSfr CCPR1L,  30
  PokeSfr CCPR1L,  40
  PokeSfr CCPR1L,  50
  PokeSfr CCPR1L,  60
  PokeSfr CCPR1L,  70
  PokeSfr CCPR1L,  80
  PokeSfr CCPR1L,  90
  PokeSfr CCPR1L, 100
  PokeSfr CCPR1L,   0
  PokeSfr CCPR1L, 100
  PokeSfr CCPR1L,  90
  PokeSfr CCPR1L,  80
  PokeSfr CCPR1L,  70
  PokeSfr CCPR1L,  60
  PokeSfr CCPR1L,  50
  PokeSfr CCPR1L,  40
  PokeSfr CCPR1L,  30
  PokeSfr CCPR1L,  20
  PokeSfr CCPR1L,  10
  PokeSfr CCPR1L,   0
Loop

 

[rossko57]

An 08M2 at 32MHz seems to be able to update a PWM duty every 100us so that would seem to fit the bill.

That's impressive!
Next challenge ... do it at defined intervals (for the target sinewave accuracy)?
One approach might be to set up a timer interrupt every 2mS ... but I understood the call and return overheads (stack handling etc.) would be prohibitive at that sort of time?

 

[hippy]

Next challenge ... do it at defined intervals (for the target sinewave accuracy)?

I'd simply do it as I have done using as many POKESFR as required and some PAUSEUS padding to get it close to 50Hz. Put at the peak of the sine wave that should be pretty transparent. It perhaps depends on just how accurate to 50Hz it needs to be.

It should work well enough for generating a DC waveform via an RC but I have no idea if it's suitable for a 3kW system. Many UPS use simple square waves, pulses, or half-height square waves with a pulse on top to approximate a sine wave so I would have thought it would equal or better those.

I would guess any pre-production solution needs the output analysing and then the code tweaking so it matches what's required. I imagine it's not as simple as 'this generates a perfect sine wave using an RC circuit so it still will when using an H-bridge chopper and the output passed through a coupling transformer'.

 

[Sten Martin]

Thank you all for your comments they are verry important.
I have been away for a few days working on Aurdino 328P micro controller because they have a solution to produce
a Sinewave. The important part is the Sinewave table that has to be red 256 times for each period to output at 50 Hz.
I have been doing a C-program that produce the Sinewave and are working on to convert it to PICAEXE.
They are using a simple RC filter to shape the Sinewave. Ofcource the 3kW power requirement has to be calculated.
I already have a 3 kW inverter (not working after a power failier in the marina) and that is a big and heavy object, hope I can do it more compact?
I will put together all your comments and suggestions together with what I learned from the other project and see what I can do.

 

[hippy]

The important part is the Sinewave table that has to be red 256 times for each period to output at 50 Hz.

256 updates per 20ms would be an update every 78us or so. From my testing done earlier it appears an 08M2 can match that, though poking the desired values directly rather than looking them up in a table.

The thing is though, with such frequent updates, you would need a PWM output in excess of 12kHz to see a resultant change at every update.

Most inverters I have seen use far fewer updates per full phase cycle - just 4 for a modified square wave and 8 for a modified sine wave with pulse on top - so having so many updates may be engineering overkill and unnecessarily complicating things.

 

[AllyCat]

Hi,

I want the Picaxe to ... Produce PWM at 3.10 kHz,.... and to output a high quality 230V [50 Hz] sine wave.

There's not much point in changing the PWM value 256 times if there are only 62 cycles of PWM ! However, I have always thought your value was much too low (didn't you orginally quote a 6.xx kHz frequency to three significant figures?). It's typical to run switched-mode converters above 20 kHz, if only to avoid acoustic noise, and frequencies an order of magnitude higher can provide enormous benefits in the physical size of the filter(s).

Those frequencies move outside the realm of a PICaxe, but why use one anyway? If the program is only POKEing values into the PWM register of a PIC (and there's no time to do anything else) then you might as well compile code directly in assember, or a high level language, and work with precise 1 us (or higher) instruction cycles and not with the "undefined" delays from a PICaxe.

But at least you appear to be aware (?) that you can't use an RC filter at 3 kW.

Cheers, Alan.

 

[rossko57]

If the program is only POKEing values into the PWM register of a PIC (and there's no time to do anything else)

I think that's the crux of it; little point using a PIC of any kind as a PWM generator if it has no time to do any regulation. That requires some reading of output / assessing against target / manipulation of PWM value somewhere along the way.

I wouldn't think an RC filter would be very desireable at 3kW, bit hot. That's why the failed commercial unit is heavy, LC filter with chunky inductors.

 

[Sten Martin]

I am currently investigating how I can get the size and weight down on the 3kW Inverter. You are right about the filter they will not work for 3KW. Attached is the table used in the other micro controller.

const int sinewave_length=256;
const unsigned char sinewave_data[] PROGMEM = {

0x80,0x83,0x86,0x89,0x8c,0x8f,0x92,0x95,0x98,0x9c,0x9f,0xa2,0xa5,0xa8,0xab,0xae,
0xb0,0xb3,0xb6,0xb9,0xbc,0xbf,0xc1,0xc4,0xc7,0xc9,0xcc,0xce,0xd1,0xd3,0xd5,0xd8,
0xda,0xdc,0xde,0xe0,0xe2,0xe4,0xe6,0xe8,0xea,0xec,0xed,0xef,0xf0,0xf2,0xf3,0xf5,
0xf6,0xf7,0xf8,0xf9,0xfa,0xfb,0xfc,0xfc,0xfd,0xfe,0xfe,0xff,0xff,0xff,0xff,0xff,

0xff,0xff,0xff,0xff,0xff,0xff,0xfe,0xfe,0xfd,0xfc,0xfc,0xfb,0xfa,0xf9,0xf8,0xf7,
0xf6,0xf5,0xf3,0xf2,0xf0,0xef,0xed,0xec,0xea,0xe8,0xe6,0xe4,0xe2,0xe0,0xde,0xdc,
0xda,0xd8,0xd5,0xd3,0xd1,0xce,0xcc,0xc9,0xc7,0xc4,0xc1,0xbf,0xbc,0xb9,0xb6,0xb3,
0xb0,0xae,0xab,0xa8,0xa5,0xa2,0x9f,0x9c,0x98,0x95,0x92,0x8f,0x8c,0x89,0x86,0x83,

0x80,0x7c,0x79,0x76,0x73,0x70,0x6d,0x6a,0x67,0x63,0x60,0x5d,0x5a,0x57,0x54,0x51,
0x4f,0x4c,0x49,0x46,0x43,0x40,0x3e,0x3b,0x38,0x36,0x33,0x31,0x2e,0x2c,0x2a,0x27,
0x25,0x23,0x21,0x1f,0x1d,0x1b,0x19,0x17,0x15,0x13,0x12,0x10,0x0f,0x0d,0x0c,0x0a,
0x09,0x08,0x07,0x06,0x05,0x04,0x03,0x03,0x02,0x01,0x01,0x00,0x00,0x00,0x00,0x00,

0x00,0x00,0x00,0x00,0x00,0x00,0x01,0x01,0x02,0x03,0x03,0x04,0x05,0x06,0x07,0x08,
0x09,0x0a,0x0c,0x0d,0x0f,0x10,0x12,0x13,0x15,0x17,0x19,0x1b,0x1d,0x1f,0x21,0x23,
0x25,0x27,0x2a,0x2c,0x2e,0x31,0x33,0x36,0x38,0x3b,0x3e,0x40,0x43,0x46,0x49,0x4c,
0x4f,0x51,0x54,0x57,0x5a,0x5d,0x60,0x63,0x67,0x6a,0x6d,0x70,0x73,0x76,0x79,0x7c}

 

[hippy]

Attached is the table used in the other micro controller.

That table would seem to be for controlling a DAC output or PWM through an RC, generating a positive phase output of 2.5V to 5V and back to 2.5V, then the negative phase output 2.5V to 0V then back to 2.5V. Poorly represented in ASCII art as -

5V    _
    _/ \_
         \_/
0V

I am not sure how that table helps with generating PWM. PWM requires the same PWM duty in the positive and the negative phases, but applied to different halves of the H-bridge chopper -

           _   __   _        _   __   _
PWM    _|_| |_|  |_| |_|__|_| |_|  |_| |_|__
       _________________                   _
Phase                   |_________________|
           _   __   _
+Ve H  _|_| |_|  |_| |_|____________________
                             _   __   _
-Ve H  ___________________|_| |_|  |_| |_|__
         
         ____------____
Out   _--              --_                 _ 
                          --____     ____--
                                -----

I am confused because we seem to be talking about one thing ( using PWM to control an H-Bridge chopper to create AC ) but keep referencing something else entirely ( using PWM to generate a DC sine wave voltage through an RC circuit ).

 

[rossko57]

hippy, your ascii-art is nifty - clear and concise - bravo

 

[SAborn]

I think i said back in post #2 a picaxe was not suited to the job.

Why reinvent the inverter when there is many already on the market.

3kw GTI (grid tied inverter) is not that costly, then all you need to do is trick it into thinking it is connected to a grid (another inverter)
Your DC voltage range you quoted is within the average GTI range, it will also give you a MPPT built in.

I think you mentioned this was for a yacht, i would feel much safer using commercial products, than a home built system not previous tested.
3kw is a good amount of energy, and a big heater is something goes wrong, i doubt i would want to be bobbing around the ocean somewhere, and rely on a Picaxe preventing a meltdown.
You are talking reasonably high voltage DC, (200+ VDC), and 240 VAC, either is high enough to kill you, and there is enough current involved to set fire to the system.

If you use a transformer less, GTI then the weight is very light.

Personally i like the SMA sunny boy inverters (GTI) as they can be reprogrammed for off grid use (supported software).
One drawback is they are a transformer based inverter, so the weight is greater.

They are a AC coupled system and designed to work with other SMA inverters (when set to off grid mode), all the controlling inverter do is increase the mains frequency by 2hz (52hz) and the Sunny Boy (SB) inverters back off to maintain demand, if the demand increases the frequency drops and the SB increases supply.

You could possibly build a smallish Picaxe inverter to supply a false grid (GTI) and leave all the heavy power requirements to a standard off the shelf GTI.
No false grid, no meltdown, as SB automatically shuts down. (AC coupled)

 

[hippy]

hippy, your ascii-art is nifty - clear and concise - bravo

Years of practice. Thanks.

I think i said back in post #2 a picaxe was not suited to the job.

I don't agree. The PICAXE looks perfectly capable of generating a 50Hz sine wave via an RC circuit or through H-Bridge chopper control.

The 08M2 can update PWM every 100us giving 200 steps per 50Hz mains cycle and it really is just a typical 'bit-banging' exercise, 'byte banging' in this case. That may not be as good as some other controllers but it seems perfectly adequate to achieve the task. The only issue I can see is, the higher the PWM frequency the fewer duty steps available, so one has to balance frequency against granuality of duty control.

The problem seems to be in clearly defining what the PICAXE controls which dictates what the PICAXE must do.

I think part of the problem is that many designs are purely analogue; a reference sine wave is generated which is 'anded' with a high frequency clock. That creates a PWM signal which is then used to drive the chopper.

In a digital system the chopper signal is generated directly by the micro. That can either be via an actual PWM controller or bit-banged as AllyCat alluded to earlier in post #25. The PICAXE is not fast enough to do bit-banging at the speed required but can adjust the PWM controller to achieve the same and do it fast enough to have quite good control over PWM duty throughout the phase of the generated signal.

There is a 'sine wave table' involved but no sine wave is actually generated. It is really a table of what PWM duty should be used at any particular time through the generated cycle to produce that cycle. The external hardware converts that PWM to an actual AC waveform.

So all the PICAXE has to do is generate PWM with the right duty rate at the required times and I believe it is entirely capable of doing that. In the designs I have seen which use a micro to do no more than generate the PWM at the required time I believe you could drop a PICAXE into those and still achieve satisfactory results. I must admit I've never tried it though so there may be issues I am not aware of.

I would expect it to be possible to produce proof of concept code within an hour and working H-Bridge chopper control in under a day. The actual inverter hardware beyond that I have no experience of.

 

[fernando_g]

I have significant experience troubleshooting and designing power circuits, and thus will provide my two yen's worth of opinion how to prevent high power H-bridge failures:

1) Not giving enough thought to the gate drive circuit. This not only includes a fast, very stiff driver capable of sourcing and sinking lots of amps in a few microsecs, but also in its protection features...for instance I would not rely solely on a microcontroller to prevent a shoot-thru conduction and/or a fast disable. The driver design should incorporate that.

2) Not checking for transient load operation. i.e. What happens when you add/remove a load suddenly? How about inrush currents?

3) Not checking the operation with low-power-factor loads. The reactive current will flow thru a Mosfet's intrinsic body diode, and not all are doped properly to have good recovery characteristics. When in doubt, consider external ultra-fast diodes very close to the Mosfet itself..

4) Not taking the time to minimize parasitic elements, and/or applying snubbers to tame the resultant ringing.

5) Not verifying converter operation at the envelope edges. I.e. max load with min inut voltage, and min load with max input voltage.

6) PWBoard layout. This is an art and a science on itself.

7) Last but not least: thermal management. Not all thermal pads are equal. Not all heatsinks have the same thermal impedance. Not all mounting methods provide good simultaneously good thermal paths and high electrical isolation. Fans are great, but may also ingest debris and throw that into a high voltage path.

 

[Sten Martin]

hippy is right about the confusion with analog Inverters. I have gon through my earlier designes and many Inverters on Internet. All of them is analog. I have been confused myself. The digital Inverter is much easier todo, a few line of cod and much less components. I have used nanocristaline cores in filters making the system much smaller and lighter. My calculation of the weight of this 3kW system is about 2.5kg. All the other Inverters for 3KW is about 10Kg. I want to go further with hippy's suggestion and see if we can make the code with PICAXE and the H-Bridge chopper. Size is important in any mobile application. Thanks Sten

 

[rossko57]

The digital Inverter is much easier todo, a few line of cod and much less components.

I wouldn't think that. As fernando points out, the "exciting" part is the H-bridge and associated circuitry; any design error or oversight will fail spectacularly.

Have you given any thought to regulation? I'm sure you said the input DC was variable, that would be reflected in the output with fixed-table PWM. What happens at no-load conditions?

 

[Sten Martin]

I have on My floor, in parts, one 3-5 kW Inverter solar panel battery charger. They have used exactly the same main parts I am going to use except the filters and the micro controller. With a micro kontroller producing the PWM there is fewer things that can go wrong. The H-Bridge is of course the most important part when it comes to safety since all the power is switched there. And yes I have thought about the regulation and in this first simple test version there is no need for regulation the speed is fixed from an EM generator (with help from hippy in another project). I think I have a pretty good idea about the hardware and will focus on if it is possible or not to program the PICAXE to feed the H-Bridge with the PWM signal. I will let you all know about the test results from both hardware and software.

 

[Sten Martin]

rossko57 have stressed the safety features. All my engines and electrical generators have automatic shutdown at over temperature. Why not have a temperature sensor and a red LED (and a summer) to warn fore over temperature (the most common indication of a failier). If the temp in the Iverter box goes over say 60 degree Cellsius the system shut down itself. I will think about some more common problems I can use as indicators to shutdown the system.