Grid Tie Inverter Schematic

[Val Gretchev]

Re: Grid-Tie Inverter Schemetic

Here is what I have so far on the Signal Conditioning circuits.

 

[tcmtech]

I see on the switching device drivers your using the PowerEx M57161L-01 devices.
I am just curious as to if thats due to a particular system requirement or not.

I have used the International Rectifier IRF2110 half bridge gate drive IC's for some time now with no problems. They are logic level input compatible and are very cheap yet very durable. Plus with 500 volt isolation on the high side they work well with all most every common voltage.

I'm Just thinking about production cost savings!

So far it looks like a good solid concept! It will likely beat my best units for overall efficiency too!

 

[Val Gretchev]

Re: Grid-Tie Inverter Schematic

There are no particular system requirements. It's just another way of doing things. I got the idea from the PowerEx web site. They make pre-assembled driver boards with isolated +15V and -5V supplies. Then they mount them right onto the IGBT module. I am wondering if that is to minimize any wire inductance from driver to IGBT gate.

What I like about the PowerEx driver is that the components are mounted on a hybrid module. I don't have to mess around with a bunch of components. There are 6 drivers involved. That would be a lot of components to assemble individually onto the board.

Like I said, there are many different ways to achieve the same thing. I started off with probably the most expensive way and will, over time, try and cost reduce the design. I'll check out the circuit you suggested.

 

[Val Gretchev]

Re: Grid-Tie Inverter Schematic

Here is Rev. B of the Signal Conditioning circuits. Keep in mind that I haven't checked the design and probably will need to tweak a few components.

One interesting thought came to me when working on the Current Transformer Rectifier circuit. Taking the peak value of the AC current won't tell me the direction that the power is flowing. What if my PWM is producing less voltage than the mains. Won't the current flow from the grid to the inverter? My current measurement circuit will still read positive. Is there some directional circuit that can be used to ensure power is flowing from the inverter to the grid?

Every time I post something on this thread, the chatter seems to stop. Please give me some feedback. Is this stuff usefull?

 

[tcmtech]

Every time I post something on this thread, the chatter seems to stop. Please give me some feedback. Is this stuff usefull?

Hehehe...
I think the chatter stops because your at the front of the knowledge train. When you look for guidance to follow and everyone else is silent it just may be because your at the font of the line!
The lead engine on the train gets the best view but also has to hope like hell that the track he is on is clear. If not, everyone behind him that is blindly following will do their best to run his ass over when he does make a mistake! The leaders don't have any one to follow!
Kinda sucks when it becomes clear that you might just be looked at as the new leader. Wait until the nutters go after you for trivial and non relevant stuff! You will appreciate the silence then!

I will take a look when I can but now that the weather is better I have to work much much more. When I jumped in on this thread it was during my slow season and I had days of time to waste and just needed a way to keep myself occupied and still fell some what useful.

Now I am limited on the time I have for doing on line stuff.
I will think about what you posted and try to get back when I can.

But yea it would be nice if more were here to share ideas and what not!
But so far you have my full support and I do have a question or two as well!

 

[Mr RB]

Val, you do nice work, neat and professional, but... (maybe it's just me?) it seems really overcomplicated.

What about simplifiying to the very minimal hardware requirements?

1. Low voltage DC to HV 1/2 sine SMPS (generate the HV half sine in real time by PWM at the low volts DC side)

2. Four HV IGBT to commutate the half sine to a full sine and connect it to mains

3. one optocoupler to measure mains voltage (and phase), maybe you could use 2

4. one optocoupler to measure dc current drawn from the 1/2 sine circuit.

That reduces the whole circuit to 1 microcontroller or 494, couple of LV FETs, 4 IGBT, few diodes etc.

Re your question on measuring current, the mains can't supply current to the HV section of the circuit because its a high voltage dc supply which is a cap fed by diodes. It is only possible for the rmains to draw current, which can be measured on a simple shunt resistor on the HV dc ground rail.

My personal preference (I posted in another thread) would be to make the circuit "output current regulated", so the PWM controller feeds out current to the mains based on the instantaneous mains voltage, so if the mains waveform is 50v it feeds in 0.5A to the mains, if mains is 120v it feeds in 1.2A and so on.

This mimics a resistive load (but in reverse) so it naturally follows the mains waveform itself, so the PWM controller does not need to measure it's own output voltage or generate a sine voltage output etc (this will be automatic) which really simplifies things.

It also gives some easy protection for not feeding any current to the mains if the mains goes down, and can control phasing, so it kills a few birds with one stone.

 

[jsalsburg]

Grid Connected Inverter Tutorial

Go to my Tutorial on Grid Tied or Grid Connected Inverters. After reading some of the messages on this subject in this Forum, I feel compelled to share this information.

Grid Connected Inverter

One must understand the powerplay at work with this technology. Many forces are at work to resist or halt the development and commercial availability of this technology, both by Commercial concerns and by Governments who are under the influence of Lobbyists answering to special interests like Energy Fuels Companies and Power Utilities especially to resist inverter Net-Grid-Connection. Also, the manufacturing companies of low power Grid Tied Inverters, those below 500 Watts, have been swallowed up by International Corporations and subsequently closed down. The low power Grid Tied Inverter is available no more.

There is the additional issue of the Intelligent or Smart-Grid which will eleiminate the "Islanding" problem, and the safety hazard of live grids after disconnect.

 

[Val Gretchev]

To: Mr RB

Thanks for commenting.

Yes, my design looks complicated. I'm all for simplicity. However, this is as simple as I know how to make it. My goal was to maintain complete isolation both from the mains and also from the PV array. I've been involved with many designs that ultimately had to obtain VDE, UL, CSA approval. It's not easy. The schematic design is not the only thing they look at. The physical packaging is also very important. They will look at such things as... if a wire connected to the high voltage circuits got loose, could it come in contact with the low voltage circuits that humans could access.

I can't quite make out the circuit topology you are advocating. Perhaps it would help if you would illustrate with a sketch.

The design I am proposing is not new. I think you will find that most commercial units use this circuit topology. Xantrex publishes a partial schematic of their circuit topology in their manuals of their high power 3-phase inverters.

On the issue of back feeding from the mains to the inverter... the AC from the mains sees a resistor in series with a capacitor. The capacitor gets charged and discharged each cycle. Since there is resistance in the circuit from the wiring, some power is dissipated due to this current flowing in the resistance and not returned to the mains.

The PWM signal on the H-bridge induces a voltage on the filter components so that the voltage of the AC waveform is higher than the voltage from the mains. Power will then flow from the inverter to the grid. I still haven't been able to determine what components to use for the filter circuit.

I have thought about the question of islanding (not feeding power to the grid when there is a power outage). I hope to achieve shutdown in two ways. My power supply is getting power from the mains. If it goes down, so will my power supply no matter how much my inverter tries to keep the power alive. Secondly, the micro will keep monitoring the zero-crossing signal and shut everything down if it finds one pulse missing. I don't know if this will do the trick or whether some more sophisticated method needs to be employed.

 

[Mr RB]

Jsalsburg -nice web page! Typical small grid tie inverter. Are you saying that nobody is selling commercial small grid tie units any more??

Val Gretchev -on back feeding mains current into the inverter, this should not happen for reasons we both said. Surely you don't need to measure the few mW of the charge/discharge of your filter cap??

In the interest of absolute minimalisation (which always amuses me) I have just put up a minimalist micro grid tie topology design up on my web page you can see it here;
Micro Grid-Tie; solar panel tied to the mains grid

I look forward to comments.

 

[rx5]

Hi all... I have been reading all 10 pages BUT Im still missing the point..maybe?!

I have some set of questions hoping to get answered:

but this one first...

#1 what is the difference between an switching inverter (DC-DC then fed to H-bridge,chopped square wave to approximate sine) and a Grid Tie inverter?

I have here in the works, an inverter(12Vdc battery as input), using Hbridge using IRF840 mosfets..im using a PIC to do complementary "pulsed" modified sine SQUARE WAVE..... but still building on my DC-DC(maybe ill use a 494 or 3525 for this/push-pull)

goal: 220Vac @60Hz

 

[Val Gretchev]

To: rx5

Here is a brief explanation on Wikipedia.....Grid tie inverter - Wikipedia, the free encyclopedia

Just Google grid-tie for many more.

 

[Val Gretchev]

To: Mr RB

Way to go Roman! Nice theoretical design! Question is... why would you want to build such a small inverter, grid-tied or otherwise? The design I am proposing is bare bones minimum at 1500 watts. It can be scaled up with changes only to the inductors, filter components, and the output transformer. Obviously, the motivating factor here is to sell some power to the utility company during sunshine hours and buy it back at other times. Here in Ontario there is a proposal to pay home produced PV power almost twice per kWh as it costs to buy a kWh. So you would get 2 kWh for each one you sold. By this time next year, all homes will be outfitted with net power metering anyway when we will be billed different amounts depending on what time of day it is. The most expensive time will be during the day when the most sunshine is available. So, by offsetting some of that expense with a solar array will achieve a faster pay back for the installation.

I enjoyed reading your mini-autobiography on your web site.

 

[Mr RB]

Hi Val, thanks re the design. As for the why? Well these days I make my living doing commercial designs so any chance to take a diversion into a hobby "minimalist" design is the fun part of electronics for me.

I suppose the main "why" would be that the solar movement really needs more people entering at the bottom end. The promotion of solar education and even solar enthusiasm. Why do so many people buy 20w solar panels and use it to power something at home, even if just their lawn lights etc? It's like they are dipping their toe in the water.

Obviously the MICRO grid tie is very much at the amusement end of the solar movement, nobody would think their 20w solar panel is making any significant difference to their power bill... But its as FUN as any other micro solar home project and probably as credible, safety issues not withstanding.

There is just something cool about being able to connect your little 10w, 20w (up to 80w?) panel using a minimum amount of parts and have it take power directly off your home energy bill with a fairly high amount of efficiency. If you look out and see sun, it's like "Hehe that's putting power into my mains!" It sure beats filling up a battery and then thinking "Now what can I do with 12 volts?"

I didn't realise you were doing a commercial design (or is it a highly sophisticated home/hobby project?) but given 1500+W and the possibility of going commercial then yes it's critical to cover all possible safety issues.

Re my little autobiography, thanks! I really need to add the last 18 years in there but it would take so long to type in... Maybe i'll do a 20 year update.

 

[rx5]

ok Val got it...

my next question:

do all(or most) GTI use metal core for the step-up part?

 

[Val Gretchev]

To: rx5

If you are referring to the inductor of the booster circuit, then the core would be ferrite or powdered iron since the frequency is around 30 kHz. If you are talking about the isolation transformer, then that would be laminated iron core since it has to pass only the 60Hz (or 50Hz). The high frequency from the PWM is suppressed by the pi filter.

 

[tcmtech]

I Get a giggle every time someone says something has to be complex just to make it simple. I Laugh every time!
I make simple to be simple. Because I am simple. How simple is that?

As I have pointed out in several past posts there are countless ways to make one of these and as a simple man I came to the conclusion for my designs that the simple iron core low frequency method was the simplest, easiest, and cheapest way to feed the power back to the grid. And with the right Pi circuit it does give a clean sine wave that follows the grid wave form very cleanly and effectively.

I have never needed to monitor anything more than the instantaneous voltage of the line side wave form in order to get good clean returning power. All other line monitoring is just for safety, to make sure the line power is at the right voltage and frequency.

If the output amps get too high thats what fuses and circuit breakers are for. Same with the input side. If the input power exceeds the GTI capacity for too long the thermal protection limits shut it down and redirect the incoming power to a load dump circuit.
If the line voltage or frequency get to far off it again diverts the incoming power to the load dump device and then waits until the line side power returns to normal.

As far as using the HF switching to isolate the input from the output I see some gains and some losses. It does give a better and slightly more efficient use of the low end input power but is more complex and honestly does not have any overall great operating efficiency gains over a simple transformer. HF switching is much lighter physically but when you factor in the actual physical volume its actually bigger per watt than a standard transformer of the same capacity so nothing was gained size wise.

The LF iron core transformers are going to make the GTI heavier. But I figure, so what! The wind generators, solar panels or micro hydroelectric power source and the line connection are all stationary so what purpose does it serve or what gain do you get if the GTI is more portable? It cant actually go any place while its doing its job!
A lighter unit will save a customer a few dollars on shipping cost but a simpler and less complex unit is cheaper to build. And that savings will off set any added shipping costs.

From my personal experiences the standard voltage controlled switching device H-Bridge cant be beat! Its simple, easy to control, and protect.
It eliminates a need for multiple H-bridges or switching circuits to be used.
With the right control circuit design its capable of both current limiting and voltage limiting and PWM wave form shaping at 10's of KHz drive frequencies.

All other functions are done in the control side of the system. And once the control circuit is built and can do all of the input and output monitoring then its just a matter of changing voltage and current sensing
components in order to adapt it from a multi watt to a multi kilowatt control and monitoring capacity.

Just my opinion, but I do like the conceptual ideas and I am open to the possibilities that there could be other ways too.

I just think simple is simple and complex never makes anything actually simpler.

 

[Alan Dedier]

Grid Tie Interface

New to this my e-mail habits are obsidian age. I saw Val Gretchev work on inverters don't know if I can open .mcp. Chinese make cheap good inverters but the grid-tie Interface is the thing. A 20 year+ reliable seperate unit that can pass UL1741 would make easier getting grid tied solar systems signed off for the rebates without the really excessive grid-tied inverter costs. If the inverter only go 7 years still well worth it. I'm waiting for my NABCEP certificate for Solar Installer. I have access Protel layout, parts purchasing, through-hole dip solder and lead cutting machine.

 

[rx5]

@tcm,

good point! and nice explaination

dont worry I dont need ANY arguement right now.. LoL ;D

just wanted to make a portable and effecient inverter..to power those low wattage irons or something less than 200Watts.. youl never know WHEN youll need em..

in MY area, there is no need for GTI, dont have wind gen or PV panels to get alternate energy, all rely on grid power... sucks eh?

 

[Mr RB]

Rx5, those new soda-can sized inverters seem to be everywhere now, they are about $30 USD, have output 150W and fit in your soft drink holder in your car console. It seems everyone wants to plug their laptop into a car these days.

TCMTECH, I do like your simple micro grid tie inverter, I said so in your thread. But efficiency bothers me. If it uses a iron 50Hz transformer then you need to disconnect it for the other 18-20 hours of the day when the solar panel is not working. That adds a lot more complexity, like a relay to connect it, ie more parts and more power wastage. And when operating, the efficiency of iron transofrmer square wave inverter is not gonna be much over 60-65%.

 

[rx5]

Mr RB,

yes those small switching inverters are becoming VERY common nowadays... maybe I could buy one for myself

but I still need to finish my DIY..