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? about parallel operation of inverters

? about parallel operation of inverters

? about parallel operation of inverters

(OP)
what i want to try to do, is to take two ups
inverters, tap the first at the logic drive to the FET's
driver, and jumper this logic over to a second inverter
which has had its logic disconnected.

the goal is to be able to then parallel connect the outputs of both inverters for more power

is this possible?

bob g

RE: ? about parallel operation of inverters

(OP)
ok, why?

is the logic not able to provide enough or sink enough current to provide for additional fet drivers?

if this is the case, what is keeping me from inserting a
intermediate driver, that takes the logic input and has enough capacity to provide for the fet driver darlington pairs?

or alternatively, maybe just change out the darlington pairs with higher power units and let them drive the additional fet's on the other inverter board?

i guess i had hoped for a bit more of an explanation
than a simple "no"

so if this method will not work, what would it take to
get both inverter boards in sync with one another, so that their output transformer secondaries could be connected in parallel?

thanks
bob g

bob g

RE: ? about parallel operation of inverters

These are not just open loop signals. They are closed loop. That means the driver needs to see what's happening as it creates the control signal.  Often they control the actual cycle profiles to specific levels.  This means the control signals are looking for specific signal shapes and amplitudes. These expected values are going to be badly distorted, confused, and wrong if you attempt to kludge two output stages together.

You also can't just hook two independent output stages together without investigating how they will share the load, and if they will share it equitably.

Further these things are engineered to great detail to just barely run correctly without the drive circuitry burning up from the loading it's subjected to.  Now you hook two full drive loads to one driver and you're going to be overloading the driver -badly-.  This will cause it to drive the output FETs badly -read slowly- which will cause their losses to skyrocket.  High losses lead rapidly to letting all the smoke out.

Just buy a bigger unit.  It will save you a fortune if you value your time at more than 10 cents and hour.

 

Keith Cress
kcress - http://www.flaminsystems.com

RE: ? about parallel operation of inverters

There are inverters that can be operated in parallel. Siemens 6SExx types can be parallelled if an optical link is connected between master and slave(s). The link synchronizes gating pulses.

It works in a mode similar to what brutus proposes. But doing it "just like that" would not work. The reason is that the EMI level in an inverter is quite high ("very" is a better description) and that parallel operation needs exact switching to be succesful. The need for exact switching precludes the filtering that is necessary to make the system operable.

The closed loop that Smoked mentions is there to ensure that every IGBT reacts properly to the gating pulses and that they do not have too high a residual Vsd (which could happen if there is a short on the load side). If the master gating is applied before that logic, the protection might work. The problem is that, even if it does work, the signals exist in the slave inverter's control processor and thus doesn't have any influence on the master pulses that actually control the inverter.

Parallelling of inverters surely can be done. See http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19640007846_1964007846.pdf for an early test. But it is not, as Smoked says, anything you should try at home.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: ? about parallel operation of inverters

What are you trying to achieve: redundancy or just to wring more power out of the drive?

If the former then it's realistically a non-starter, and for the latter wouldn't it be easier to install a single larger power module? The chances are you can drive a slightly larger power switch using the same driver circuitry within reasonable limits, although don't expect to take the control stage from a 1kW drive and apply it to a 100kW power stage because gate capacitance will overwhelm the driver and the maximum frequency big switches can achieve is much lower than those small switches are capable of.

With the cost of drives through regular suppliers and even on Ebay is there any likely cost benefit of what you're doing?
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: ? about parallel operation of inverters

As mentioned before, most modern UPS systems can be paralleled up but only with systems of the same design and manufacturer. You can also get systems that can have additional inverter modules inserted inside the unit to add power rather than redundancy.
I agree with all the other posts as to why what you are suggesting is not feesible.

UPS engineer http://www.powerups.co.uk

RE: ? about parallel operation of inverters

I could do it.  It would be more trouble than it is worth. If you have to ask this question and don't understand the potential problems, it means you can't.  A UPS is a nice source of parts for a project.  Other than that they are useless.  You can by a late model dead 2KW inverter on ebay for $20 and fix it in an hour.

RE: ? about parallel operation of inverters

(OP)
maybe i should have put a bit more info in to start with

the ups unit i am working with are common as dirt, scrapped when the batteries die usually, and can be had for less than 10 bucks each.

now then, with cheap inverters available, that are sine wave
it just seems a crime not to give it a go to try to stack them or use them in ways not originally planned by the oem.

what am i out if i fail? some time and a few bucks, and even that is not a big thing because i can use the transformers for another project.

as for taking the logic off board to control two inverter boards, i figured it possible (now i realize there are all sorts of reasons why i probably can't work) but,

i have seen it done in some inverters such as the 3600 watt vanner inverters,

if you open one of their cases, you will find one logic board that houses the microprocessor and all its related crap to generate the logic needed to form a sine wave

from that logic board the pulse are moved via a pair of equal lenth ribbon cables (same stuff as what is used to connect harddrives, floppy drives, etc in computers) to
remotely mounted FET driver/FET switcher boards. each of which is connected to its own transformer on the primary side, and then parallel connect on their secondary side.

it would seem that it is possible to do this only if the logic side was used, and not jumpering from the FET driver
to two seperate FET swithes where the power levels are much higher and emi would therefore be an issue.

at least the emi would be much more of an issue anywhere around those switches and drivers than it would be at the relatively low power levels of the logic drive.

also it is fair to note this is a project to see if it can be done, and what can be learned in the process.

so maybe there is less than a 1 in 10 chance of success?

or even 1 in a 100?  if successful there is practical use for such a system, by integrateing some microcontrol to bring in and out stages depending on loading the overall system efficiency can be much higher than if one were to simply buy a larger unit and run it at low power levels most of the time.

anyway thanks for the input guys, even if you are not optimistic about the project  :)

bob g

RE: ? about parallel operation of inverters

(OP)
i might also add

i won't be running in open loop
the idea being maintain the closed loop of the first inverter board to the logic as oem designed

then as the second board is brought in parallel on its output the feed back is maintained

otherwise there would be no feedback and certainly there would be problems.

the plan is to learn if this is possible, then take those
lessons learned to stack two or maybe three, 3kva inverters,

the 3kva ups inverters are much more expensive, so i would rather torch a pile of much cheaper and available 1.4kva units working the kinks out.

RE: ? about parallel operation of inverters

Good luck! It may even work.

Sometimes, ignorance helps. Guys that don't know about the perils of the journey can complete what others wouldn't even think of (remotely).

But don't come here for help if it doesn't work - we already told you.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: ? about parallel operation of inverters

(OP)
so the odds are something greater than 10 to 1 against?

not to worry, if it don't work, i won't be coming back to find out why.

however if it does work, does anyone want to hear about it?

if not, thanks for the input anyway

its appreciated

:)

bob g

RE: ? about parallel operation of inverters

Bob,

Let us know either way.

When you first energise it have a video camera rolling: that way if it's a really big bang you can put it on YouTube! tongue
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: ? about parallel operation of inverters

bob

I admire your adventurous spirit and would be very happy to hear if your project is a success. I am mainly worried about the current sensing side of the feedback loop. This will only be monitored on the output of the inverter that you will be taking the pulses from. This means that in theory, the other inverter could supply more curent that it can handle without any limitation. therefore if there is a parallel problem and this unlimited inverter feeds current into the other inverter (if out of phase for example)then all I can see is big bang and lots of smoke. Once again, have video camera rolling for you tube!!!!
Please let us know how things go.

UPS engineer http://www.powerups.co.uk

RE: ? about parallel operation of inverters

On that note, each individual fet in the H bridge feeds back to each individual power supply through an internal diode.  If one pair of fets shorts, the other inverter power supplies will supply current through a common ground without it being sensed.    

RE: ? about parallel operation of inverters

Bob!

Don't let them stop you! I am getting seriously interested in what you will arrive at. Carry on and KUTGW!

Also: Be careful! Exploding power transistors and electrolytic capacitors are not nice to get into one's face. Wear protective goggles! At least.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: ? about parallel operation of inverters

(OP)
not knowing what KUTGW means i guess it is full steam ahead

i picked the inverter with the failed fan circuit to do the chopjob on, and lifted the main board out of the case

the solder side is the exposed side, and the component side is down in the case, so in case of explosion it is well contained.

the topology is as follows as near as i can tell

working from the output back

there is an output transformer that puts out 120ac on the secondaries and input 13.5 vac no load to ~15.8 vac at 3/4 load, so i am not dealing with terribly lethal voltages here at least on the drive side.

driving the primary side are two banks of FETs each having a parallel set

each set is driven by a pn2222a and a 2907 transistor
which as near as i can make out so far make up what in effect is a darlington pair

this pair is driven directly from the logic off the processor ic.

what i have in mind on the slave board is disconnecting the logic to the darlington pairs and jumpering the logic from the master boards logic

that way one logic controls two seperate sets of darlingtons which in turn drive two seperate sets of FET's
which in turn drive two seperate transformers

now there is no feedback from the second transformer output so i will have to parallel connect the two transformer secondaries in order for the master to be able to monitor the combined output and make correction if the voltage goes high or low.

at this point it looks fairly straight forward, but
i am thinking i should make up a tree type jumper set so that all the leads to both sets of darlingtons are equal length and therefore resistance.

it would appear to me that what i am contemplating doing is just doubling the same archetecture that is already on one board, they use a darlington pair to drive each bank of fet's, i am just adding more banks of both darlingtons and FET's.

i did find the current transformer and its output goes directly to the microprocessor via a couple of resistors, it might be that i can connect the slave CT back to the micro in tandem with the master CT? gotta think about that a bit more. i really don't think i will need to do that as long as the outputs are connected in parallel and not run seperately where one would have feedback and the other would be running open loop.

just got to take a bit more time to sketch up the connections from the board, double check and as the say in carpentry, "measure twice and cut once"

i was somewhat relieved to see that there is no DC/DC converter in the front section that boosted the 24vdc buss
to something over 150volts or somesuch.

working with a driver that is under 24vdc albeit at fairly high amperage is much safer poking around in :)

at least for me

trying to take my time, be patient and think it through well is the challenge at this point.

i just wanna do it and see what happens

thanks guys

bob g

RE: ? about parallel operation of inverters

Keep Up The Good Work (KUTGW)

The 2N2222 and 2N2907 are not a darlington pair. They are a complementary pair with 2N2907 being a PNP transistor and 2N2222 an NPN. Needed to drive the gate capacitance on/off.

Don't forget the camera! And remember the goggles! Insulated gloves is a good idea, too.

 

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: ? about parallel operation of inverters

(OP)
Gunnar:

boy you sure are something less than optimistic huh?

:)

thats ok, i frequently work around 440/440 and am aware
of getting bit, and have had caps explode on me as well,
thats always good for getting the heart rate up as well.

one question though
the two transistors 2222 and 2907
are they not situated to make up the equivalent of a darlington pair. in that one takes the lower level
and raises it while the second handles a bit more power.

its been about 30 years since i was in electronics tech school, so i am working to relearn as i go.

anyway looks like it might be exciting huh?

placing any odds on success?

:)

bob g

RE: ? about parallel operation of inverters

Darlingtons always come in NPN or PNP. Never both in one pair. What you see is probably a level shift plus a driver. Difficult to say when no diagram available.

I AM very optimistic. But also somewhat careful. 10:1 seems to be a fair bet. Or 100:1.  

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: ? about parallel operation of inverters

(OP)
having spent a huge chunk of time reverse engineering the koyo/ge/automation direct d305 series plc so that i could insert my own choosing of micro processors, in a master and multiple slave configuration this project seems like it should be a bit easier to plot out.

i caught alot of flack over that project as well, and finally got it to working very well, and in the end learned
a lot in the process.

the more i dig into this the better the odds of it working appear to me, but we shall see

know where i can get a good flak jacket?

:)

its been a while since i release the magic smoke or made an molten pile of scrap.

i don't have a movie cam, but have a digital camera
so if it all goes boom i can document the aftermath
and present it here for you guys entertainment.

thanks for the help

bob g

RE: ? about parallel operation of inverters

Now that I see you are using really old technology with line frequency transformers, I give you a better chance of success.

RE: ? about parallel operation of inverters

From what I can tell reading about the topology being proposed, it sounds like it should work.  In effect, the goal is to parallel the FETs/IGBTs, presumably across the same DC link, and use one controller to drive both simultaneously.  This is considerably less complicated than attempting to apply a closed loop control system to co-ordinate the operation of two separate inverters, coupled on the AC side.

The biggest potential problem that I see is current hogging by one set of transistors.  It will be necessary to either use a type of transistor that naturally shares current when operated in parallel, such as a MOSFET, or some form of feedback will be required, such as a small resistor in the emitter path (assuming this is a problem with parallel IGBTs).


 

RE: ? about parallel operation of inverters

(OP)
the oem topology already uses current limiting resistors
between the pnp driver emitter and each of the FET's on each board, so they ought to current share as designed.

doing a bit more reverse engineering it appears the 2222a
and 2907a are setup in a pushpull arrangement, so that the fet's can be driven on and driven off, but i gotta look a bit deeper. my eyes are not as good as they once were.

the base of the 2222a is driven at approx 3.4 v at no load
and increases to 4.8 v at near full load, i am assuming that
what i am seeing is a change in the pwm average voltage?

soon as i am feeling a bit better, i will drag out the scope and take a look and see if this is the case.

the unknown then becomes is the pwm drive sufficient to drive another set of 2222a's

if so it looks like i am golden in that regard, and maybe i won't smoke the logic drive.

the unit seems fairly well protected from overload, if it gets anywhere close to an overload or a short it shuts down instantly, without damage and will restart when the overload or fault is removed.

so maybe it can protect itself from my frankenstein approach

interesting project so far anyway.

so are my odds of success improving?

:)

bob g

RE: ? about parallel operation of inverters

(OP)
after tracking down a bunch more traces i found one that is obviously a feedback loop trace from the output of the FET bank back up to an IC that is near the microcontroller.

so..

now i am thinking rather than connect the secondaries of the output stepup transformers (secondaries) i should connect the primary side so the feedback circuit can see both outputs in tandem rather than seeing one direct and the other from the second board via induction back from its secondary.

plotting along

bob g

RE: ? about parallel operation of inverters

This thread - and Dilbert- is my favourite morning reading nowadays. smile

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: ? about parallel operation of inverters

(OP)
i guess i can add to "Red Green's"

"if the women don't find you handsome they oughta find you handy"

"if the engineering forum don't find your project interesting, they oughta find it entertaining"

:)

bob g

RE: ? about parallel operation of inverters

(OP)
update:

well i learned a bit today :)

you don't want to get in a hurry and messup connecting wires

i managed to heat up one bank of the final drive, likely damaging it, but it seemed to work fine after i let it cool down and corrected my boneheaded mistake.

got it to drive both output transformers no problem
got the logic jumpered across to the other driver, and that seemed to work just fine, but i had no output voltage coming out of the recepticals because the 2nd board was not powered up to close the output relays and connect the secondaries to the recepticals,, but it was working

then i decided to do something nearly as boneheaded as the misconnection in the earlier step.
i decided to removed the connecting jumper from the primaries of the two transformers.

now either that is a no/no, or i had damaged the final drive in the earlier short/overheat, but the result was
not very dramatic or interesting.

several pops and that acrid smell of magic smoke!
one bank FET's of the primary inverter took a dump, it was the one i had overheated badly. so i am not sure if the earlier short had damaged it or my disconnection of the two board outputs broke the feedback or what?

so i think i am close, i just need to go find another 10 dollar inverter, and take what i have learned and try again
this time not making the earlier mistakes.

lessons learned?
1.one logic driver apparently runs two inverter sections without blowing up.
2. don't make bad connections that result in shorts of course.
3. don't disconnect the jumpers between the output transformers? (this might have caused the failure, but not sure, however all was well till i did that)
4. a video camera would have been a waste of time, all the
 fireworks are contained inside the case, so you can't see the carnage.
5. got another transformer i have been needing for another project, that has a much higher chance of success, along with a pile of other useful stuff.
6. this is too much trouble! :)

conclusion:
i got a better understanding of how inverters work, particularly the types used in small ups systems.
i have learned a lot about what more i need to learn
and i guess that is worth the price of admission.

thanks guys

bob g

RE: ? about parallel operation of inverters

Ever disconnected a car's alternator from the battery while the engine is running?  It does bad things to unprotected circuits...

Dan - Owner
http://www.Hi-TecDesigns.com

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