Two inverters, two phases - problem - why?

[Guy Dewdney]

Hi

I Have a happily running hydro project (water wheel) driving a generator (3 phase, wild high voltage) which in turn powers two bridge rectumfriers, one powers inverter #1 - which is running fine, on phase #1 (and neutral). The problem comes when I connect the second inverter (identical Aurora 3.6Kw jobbie) to a second phase - they both trip out with 'earth fault'

Obv they are both earthed properly.

I can see why I would need an isolation transformer if they are both on the same phse - but they arnt.... so I am lost.

any ideas?

[Edgar Paternina]

Hi

I Have a happily running hydro project (water wheel) driving a generator (3 phase, wild high voltage) which in turn powers two bridge rectumfriers, one powers inverter #1 - which is running fine, on phase #1 (and neutral). The problem comes when I connect the second inverter (identical Aurora 3.6Kw jobbie) to a second phase - they both trip out with 'earth fault'

Obv they are both earthed properly.

I can see why I would need an isolation transformer if they are both on the same phse - but they arnt.... so I am lost.

any ideas?

Hi Guy and Everyone,

I have a similar problem with my Aurora inverter, during very high winds. In one phase there is 107v and the other 120v, it takes from granted that the grid failed, in spite it is not. I just put off the switch between controller and inverter and the one at the output of the inverter and it enters normally in 2 secs, so my guess there is a problem with the Aurora inverter when the phases are not balanced, and one of the things I think can be done is to install an isolating transformer with regulation. But must they be one for each phase, 120v and neutral?

I would appreciate too any recommendation from the experts too

Best regards

Edgar

[Rob Beckers]

Hi Guy,

First, welcome at GPT!
Is the hydro generator where you're using the 5kW Ginlong alternator? If you have some time, please start a hydro thread with a few pictures. I'd be interested to hear more about it.

As to the Power-One Aurora inverters: My business is a distributor for Power-One, and I know them very well. The Auroras are transformerless. The essence of this is that the DC input is not floating, but actually has both DC inputs moving "up and down" with the grid voltage with respect to ground. So, for us in North America, there's 240V AC superimposed on the DC input. Any time you want to use more than one Aurora on the grid that is tied to the same source (in your case the alternator) you need an isolation transformer for all but one of the Auroras. So two inverters = 1 transformer, 3 inverters = 2 transformers etc. Also if both inverters feed different phases, because the voltages (AC plus DC) that they are floating at on their DC inputs are entirely different.

I've written a Web page about stacking Aurora inverters (http://www.solacity.com/AuroraStack.htm), that may be of interest. It talks about feeding back a single phase (or split-phase), but the same applies for feeding multiple phases. In your case, a single 6kW inverter would have been a better way to go, the small imbalance in feeding back just one phase is inconsequential. Thanks to the price of copper, isolation transformers are not cheap, likely around $600 for a 4kVA one. You can sometimes find them on EBay for less, so that's one place to check out. Possibly the second inverter doesn't need to produce the full 3.6kW (you may be able to let the inverter without transformer do the bulk of the work), if so a smaller transformer could do the job.

In case you're wondering why they don't just stick a transformer in there to begin with: The Aurora wind inverters come from the solar world, in fact, they are identical in hardware to the Aurora solar inverters, just the firmware/software that they run is different. Solar the inverters are never stacked with the same source feeding them, so the transformerless design is not an issue. Without a build-in transformer the inverter costs less to produce and can gain a few percent in efficiency, and the Auroras are definitely as good as it gets in efficiency. Power-One is working on a 5kW wind inverter with isolation transformer. Unlike most inverter manufacturers that use transformers running at grid voltage and frequency for isolation, they are building it into the high-frequency switching stage, which makes the transformer much smaller and more efficient. This project is still a bit too early in its design to say when it'll be available, likely late 2010, possibly 2011.

-RoB-

[Guy Dewdney]

Thanks for the reply RoB

Yes - its the 5kw Ginlong I mentioned elsewhere.

As the inputs are isolated via the bridge rectumfrier of the wind interface - I'm surprised the DC float levels are relevant.

Ebay here I come I guess... :(


I'll dig out some pics tonight for the hydro page.

[Edgar Paternina]

Hi Guy,

First, welcome at GPT!
Is the hydro generator where you're using the 5kW Ginlong alternator? If you have some time, please start a hydro thread with a few pictures. I'd be interested to hear more about it.

As to the Power-One Aurora inverters: My business is a distributor for Power-One, and I know them very well. The Auroras are transformerless. The essence of this is that the DC input is not floating, but actually has both DC inputs moving "up and down" with the grid voltage with respect to ground. So, for us in North America, there's 240V AC superimposed on the DC input. Any time you want to use more than one Aurora on the grid that is tied to the same source (in your case the alternator) you need an isolation transformer for all but one of the Auroras. So two inverters = 1 transformer, 3 inverters = 2 transformers etc. Also if both inverters feed different phases, because the voltages (AC plus DC) that they are floating at on their DC inputs are entirely different.

I've written a Web page about stacking Aurora inverters (http://www.solacity.com/AuroraStack.htm), that may be of interest. It talks about feeding back a single phase (or split-phase), but the same applies for feeding multiple phases. In your case, a single 6kW inverter would have been a better way to go, the small imbalance in feeding back just one phase is inconsequential. Thanks to the price of copper, isolation transformers are not cheap, likely around $600 for a 4kVA one. You can sometimes find them on EBay for less, so that's one place to check out. Possibly the second inverter doesn't need to produce the full 3.6kW (you may be able to let the inverter without transformer do the bulk of the work), if so a smaller transformer could do the job.

In case you're wondering why they don't just stick a transformer in there to begin with: The Aurora wind inverters come from the solar world, in fact, they are identical in hardware to the Aurora solar inverters, just the firmware/software that they run is different. Solar the inverters are never stacked with the same source feeding them, so the transformerless design is not an issue. Without a build-in transformer the inverter costs less to produce and can gain a few percent in efficiency, and the Auroras are definitely as good as it gets in efficiency. Power-One is working on a 5kW wind inverter with isolation transformer. Unlike most inverter manufacturers that use transformers running at grid voltage and frequency for isolation, they are building it into the high-frequency switching stage, which makes the transformer much smaller and more efficient. This project is still a bit too early in its design to say when it'll be available, likely late 2010, possibly 2011.

-RoB-

Hi Rob,

Now by reading your reply I think I made the wrong decision by buying the Aurora transformeless inverter instead of the chineese Sungrow inverter in the attachment that for sure manages the grid in a more natural electrical way as it has low frequency transformer isolation and its price is much better.

My best regards

Edgar:wondering:

[Guy Dewdney]

Heres one bit of info that I dont know if it applies to the states:-

Neutral and Earth are the same. Exactly the same. If I tied DC -ve to earth - on both systems - wouldnt that remove the DC injection problem? Or am I being simplistic (likely...)

edit - quick poke with the multimeter shows 250 odd volts from -ve to earth AND from +ve to earth. Odd.

[Rob Beckers]

Edgar, we can't use Chinese inverters over here, as none are UL or CSA listed (we need CSA listing in Canada, but some inspectors will let one get away with 'just' UL listing). My second-hand experience from people that do use Chinese inverters confirm the much lower price, however, they tend to be designed to minimums, and won't take much to blow up semiconductors. I've sold several inverters to people that started out with Chinese stuff and this was their experience. The Auroras are very robust, and I'm not saying that because I sell them: Of all the pallets of inverters sold over the past few years I have not had a single one, not one, return due to failure! We've had one where the back-up battery failed prematurely, causing strange behavior. Switched out the battery (a 'coin' type small battery) and it's working fine again. Today may be the first one that needs a replacement: Someone called to say the USB port on one of his stacked inverters failed so he can't talk to that inverter any more. I suspect failure on the inverter electronics for that port but that'll need more investigating. If so, that'll be the first I replace in 4 years of selling Auroras!

For single-inverter setups I believe the Aurora can't be beat. It's MPPT range, programming flexibility, robustness, and very high efficiency just has no equal for wind/hydro inverters. If you're going to stack multiple inverters for wind then maybe the WindyBoy is an easier choice, because it has a build-in transformer. It's far less refined than the Aurora, has its own set of issues, but it stacks far easier.

Guy, neutral and earth/ground are not the same, even if both are (mostly) at the same potential. Neutral is meant to carry current, ground should never carry current unless there's a fault (and then there should be some mechanism to detect it, such as a GFI, and disconnect the source). Ground is there for safety, so chassis and other parts won't carry grid voltage that electrocute you. There should be only one place where ground and neutral are connected, and that is in your main load center (the breaker box).

Bridge rectifiers do not provide isolation; part of the AC cycle the bridge conducts and ties the input to the output for both AC and DC, so the input is not independent (in DC) from the output. The Aurora negative (DC-) input cannot be tied to ground, because it has both a DC value different from ground, as well as 240V AC superimposed on top of that. Connecting it to ground while the inverter is running will result in a big spark, probably a blown breaker, and the GFI error on the inverter will come on. I'm not sure if it damages the inverter, that would depend if they have current limiting build into the H-bridge in there (the few accidents where the input was accidentally grounded did not seem to do any damage to the inverter, but I cannot say for sure that will always be the case).

-RoB-

[Edgar Paternina]

Edgar, we can't use Chinese inverters over here, as none are UL or CSA listed (we need CSA listing in Canada, but some inspectors will let one get away with 'just' UL listing). My second-hand experience from people that do use Chinese inverters confirm the much lower price, however, they tend to be designed to minimums, and won't take much to blow up semiconductors. I've sold several inverters to people that started out with Chinese stuff and this was their experience. The Auroras are very robust, and I'm not saying that because I sell them: Of all the pallets of inverters sold over the past few years I have not had a single one, not one, return due to failure! We've had one where the back-up battery failed prematurely, causing strange behavior. Switched out the battery (a 'coin' type small battery) and it's working fine again. Today may be the first one that needs a replacement: Someone called to say the USB port on one of his stacked inverters failed so he can't talk to that inverter any more. I suspect failure on the inverter electronics for that port but that'll need more investigating. If so, that'll be the first I replace in 4 years of selling Auroras!

For single-inverter setups I believe the Aurora can't be beat. It's MPPT range, programming flexibility, robustness, and very high efficiency just has no equal for wind/hydro inverters. If you're going to stack multiple inverters for wind then maybe the WindyBoy is an easier choice, because it has a build-in transformer. It's far less refined than the Aurora, has its own set of issues, but it stacks far easier.

-RoB-

Hi Rob,

Thank for your reply, and I really agree with you regarding robutness of the Aurora, but I think it is too fussy regarding the state of the grid. I don't understand as a client why I have to install two line conditioners one for each phase to solve the problem that I have, regarding unbalance phases(107v and 120v), when there is no problem in my household circuits. It is just a wrong condition of the Aurora: as I said, if I reset the Aurora by putting off the corresponding switches in and out everything seems to be ok too.

Up to now my experience with the chineese wtg and controller is quite good, it has behave very well in very high winds, its MPPT is 100-400V, with a nominal of 240v and I have already seen 300v, when the Aurora failed. It was very disappointing for me to see that failure after a month of very low wind. So now I am much more confident about chineese products.

Best regards

Edgar:unsure:

[Rob Beckers]

Hi Edgar,

There's a simple work-around for unbalanced phase issues with the Aurora inverters (I'm assuming you have split-phase over there since balance is an issue?). Just switch the inverter to single-phase 208V. Then use the AuroraInstaller program to increase the grid voltage setting to 240V, or 245V. Now the inverter will ignore the neutral (you can leave it connected or disconnect it, makes no difference at this point).

By the way, that sensitivity for phase balance is not something Power-One invented. It's a UL1741 requirement. They are simply doing what the rules require them to do. If those Chinese inverters were UL listed they would also quit on the same phase imbalance.

-RoB-

[Edgar Paternina]

Hi Edgar,

There's a simple work-around for unbalanced phase issues with the Aurora inverters (I'm assuming you have split-phase over there since balance is an issue?). Just switch the inverter to single-phase 208V. Then use the AuroraInstaller program to increase the grid voltage setting to 240V, or 245V. Now the inverter will ignore the neutral (you can leave it connected or disconnect it, makes no difference at this point).

By the way, that sensitivity for phase balance is not something Power-One invented. It's a UL1741 requirement. They are simply doing what the rules require them to do. If those Chinese inverters were UL listed they would also quit on the same phase imbalance.

-RoB-

Hi Rob,

Thank you again! Yes, I have split phase, and my main idea to solve the imbalance problem is to install 2 line conditioners, but I will try your recomendation about single phase too.

Here in my country rules are not so strict, what can be both an advantage from the point of view of research, but a disavantage if I am going to buy an electrical product from USA or Canada; then I will need to solve the problem like in this case.

Best regards

Edgar:amuse:

[Guy Dewdney]

Right!

inverter now connected via an isolating transformer. No more earth faults :)

However - theres no power coming from the inverter - weirdly. It sees the outside mains voltage (240V 50Hz) and the generated voltage (500V DC) and the power graph is installed and loaded.

any ideas?


edit - reset the graph, then re-wrote it - and it works! :)