Inverters in Parrallel ?? Volts and Kw's ??

[Chris-R]

I am playing dummie tonight and would like the experts on the forum ( Ha-ha Chris and I am not talking about you  ) to help me trying to understand this one.

1. If I connect 3 x 10kw Infini Solar 3 phase inverters in parallel - What is the total output ?

    3 x 10kw = 30kw ? @ 230v per phase  ( 4 x 12 v  100ah batteries in parallel = 12v 400ah bank )

2. If the inverter specs are only allowing a maximum load of approximately 3.3kw per phase, how can a thin parallel cable up the maximum load to double or even triple the normal. I have not       changed anything inside the inverter?

 

I know I am just being stupid, but this question has bothered me al week-end and the more I tried to sort it out, the more I confuse myself. (The same argument would be for 3 or 5 axperts in parallel on single phase.)

 

I am sure the guys out there can handle this one, in no time !

 

Thanks for the help GUYS !

[PaulF007]

@Chris-R I dont have the answer  but it is almost the same as the age old question if you have 3 phase and you measure 1 phase to neutral you get 220v but when you measure between phases you get 360v where did the other 300v go ?
3 x 220 = 660 and not 360 ....

If I have to guess on you question - you will still have only 10 kw but in 3 phase , which will only be useful if you have a 3 phase motor else you will still only have 10kw per phase available to use. 
You do have access to 30kw of power but can not exceed the max of 10kw per phase , I would also guess that on inverters phase balancing would also be quite important. The parallel cable would be to keep the volts and Hz in sync ... maybe ...

BUT I am only guessing as I have never really looked at parallel inverters   

[___]

37 minutes ago, PaulF007 said:

age old question

Indeed. The power is the voltage times the current, but because it's AC there is a difference between instantaneous power and the average (aka RMS) value. The instantaneous power on any particular phase would be the voltage multiplied by the current at that instant, so if you're about 30 degrees into that cycle, or π/6 radians, then your voltage on that phase would be around 115V and your power will be more or less proportional to the square of that voltage, assuming a resistive load.

But your phases are themselves 120 degrees apart, so each phase is at a different voltage at any instant in time, and so the instantaneous power on each phase is different. For example, the first phase might be at 0 degrees (so V=0), the second phase will be at 120 degrees (200V) and the third phase at 240 degrees (-200V). As you can see the voltages add up to zero. The current on the third phase (-200V) is however in phase with the voltage so you have a positive amount of power on each phase. Now, the 10KVA rating of your inverter is the instantaneous power values integrated over time (aka summed using really closely spaced samples, tending towards a spacing of zero... Calculus 178 was a decade ago...), basically, at any instant in time the three phases sum to 10kva.

In other words, if the switching electronics allow for it, you could have 10kva on any single phase, but then the other two cannot be used, and of course that would be a severely imbalanced setup which is usually a bad thing. That's the theory. In practice there will be a limit per phase and an overall limit across all three. So you may, for example, have a 4kva limit per phase but subject to an overall limit of 10kva.

I hope that made sense.

[___]

So to continue that, if your phases are properly balanced, then you can have 3.3kva on each phase, and if you had three inverters and paralleled them, then you'd have 10kva per phase for a total of 30kva. No way you're doing that with the small battery bank mentioned! :-)

I'm not sure how the thin parallel cable comes into it. I know some inverters have a "current sharing" cable, and my guess (without having the details) is that this cable is really a low-current signal indicating how hard that inverter works, so that the cooperating inverters on a phase all work equally hard. In other words, that wire doesn't carry the actual power, it does signalling. I'm guessing that that is what you're asking?

@Coulomb probably knows more about the current sharing wiring.

[Coulomb]

The cables that look like old EGA graphics card cables carry CAN bus messages; that's how the inverters agree on settings and I suspect they do a lot of synchronisation as well. 

The current sharing cables are a bit of a mystery to me, but certainly they carry signals not 230 V power. My guess is that they are connections to other inverters' current shunts. They probably need isolation. The signals are probably only tens of millivolts at full current. 

With respect to the question of where the voltage goes, consider two 12 V batteries in series. The total voltage will be 24 V or near 0 V, depending on which way they are connected. Where did the 24 V go in the second case? If you imagine switching one battery's polarity 100 times per second, you could see that a multimeter would read an average of 12 V.

With AC, it turns out that you can get more than two polarities, you can get any phase angle between - 180° and +180°. It turns out that you can think of the AC voltages as vectors, i.e. things with magnitude (what an AC multimeter will read) and direction or angle. There are meters that can measure the angle between two AC quantities. The voltage and current drawn by an AC motor have a non-zero angle; that's why the power factor is less than one (typically about 0.8), and why volt-amps are more than watts. Where did the "missing" watts go? Same place as the 24 V from the anti-series batteries ; it's all about opposing voltages. 

Edit: Now imagine two mechanical switches changing the polarity of the batteries, and imagine that these are synchronised by a mechanical shaft, so they switch at exactly the same speed (same frequency). But you arrange the brushes or whatever so that while they open and close at the exact same 100 times per second, there can be a variable delay set by the position of the brushes. Now you can imagine seeing an average of -12 V to +12 V depending on the phase of the switches. AC is like that, except that instead of being +12 V or -12 V, the voltage changes smoothly over all values from about -18 V to +18 V. The average is zero, but another measure, the Root Mean Squared voltage, is 12 V, so that 12 V AC will light a bulb the same as a 12 V battery would. When you consider the instantaneous voltages of two 12 VAC sources in series, with the exact same frequency but different phases, it turns out you can measure anything from 0 VAC to 12 VAC depending on the phase difference. The 120° phase difference between phase to neutral voltages in a three phase system introduces a factor of √3 (2 x sin(120°) = 2.√3/2 = √3). √3 x 220 V ~= 380 V (1.7321 x 220 = 381.06). In a "split phase" system like they use in the USA, the voltages are 180° apart, and so their 120 V phases add to 240 V. 

Edit: 18 V -> 12 V.

[ghatikar]

The current cables are only used if inverters are in parallel on same phase -  e.g For 3 phase setup no current sharing cable is required only data cables - http://www.voltronicpower.com/oCart2/files/manual/Parallel-installation-AxpertKS+MKS-4K-5K-20141014.pdf

 

The current sharing is required to prevent what is known as circulating current where one inverter tries to feed other inverter instead of load causing either overheating / loss of power or worst case failure . The difference comes from small differences in electronics, temperatures , cabling length etc. 

In conventional setups this also happens and closely monitored e.g. paralleling of two transformers  or generating sets. It is just that transformers by their virtue are much more resistant to circulating currents compared to High frequency  transformerless inverters . 

 

In your case the per phase capcity will be 10 KW ( 3.33x 3 units per phase)

 

wbr

 

Anil

 

[Chris-R]

On ‎5‎/‎10‎/‎2017 at 6:59 PM, ghatikar said:

The current cables are only used if inverters are in parallel on same phase -  e.g For 3 phase setup no current sharing cable is required only data cables - http://www.voltronicpower.com/oCart2/files/manual/Parallel-installation-AxpertKS+MKS-4K-5K-20141014.pdf

 

The current sharing is required to prevent what is known as circulating current where one inverter tries to feed other inverter instead of load causing either overheating / loss of power or worst case failure . The difference comes from small differences in electronics, temperatures , cabling length etc. 

In conventional setups this also happens and closely monitored e.g. paralleling of two transformers  or generating sets. It is just that transformers by their virtue are much more resistant to circulating currents compared to High frequency  transformerless inverters . 

 

In your case the per phase capcity will be 10 KW ( 3.33x 3 units per phase)

 

wbr

 

Anil

Hi Anil

Thanks for the response and all the other guys for their input, highly appreciated.

I found the following document ( as per link ) and you are right, but looks like it is only when you use single phase inverters to be setup for a 3phase operation.

According to the 3phase 10kw installation you do require the current cables as well as the communication cables. Please check whether my assumption is correct ?

http://www.mppsolar.com/manual/MPI 10K HYBRID 3-PHASE/10k hybrid 3 phase parallel guide-20150626.pdf

.Thanks guys for all your input

 

 

[ghatikar]

Yes . Current sharing cable is required between two 3 phase inverters  

 

It is same as if you were paralleling 6 inverters ( 2 in each phase) 

[Coulomb]

Right, But the possibly confusing case is three phase with 3 inverters. You need to use the "parallel communication cables" but not the current sharing cables. Here there is a master and two slaves, and they definitely need the parallel communication cables because they sure as hell need to be talking to each other to make sure that they get their phases right. So that you don't get two B phases, one A phase, and no C phase, for example. Plus, they need something to get the timing right, so they don't drift out of synchronisation, and are always 120° apart. 

The Parallel Guide is clear on the current sharing cables:

Quote

 

WARNING: Do not connect the current sharing cable between the inverters which are in different phases.
Otherwise, it may damage the inverters.

So here we have three inverters working together, but none have their outputs paralleled. In fact, there is 400 V between any pair of outputs.

But now add one more inverter for a total of four. You have to choose one phase to parallel your fourth inverter with. Now you have two inverters paralleled, and two not paralleled. The two that are paralleled (the two on the same phase) do need current sharing cables. The two that are not paralleled must not have current sharing cables. All four inverters are daisy chained together to talk to each other, and you'd better have the correct settings on the four inverters (e.g. 3P1, 3P2, 3P2, and 3P3 if paralleling on phase B ) so they know what to do.

As I was typing this, I was wondering about the potentially random way that masters and slaves is assigned (in practice, one seems to become master like 95% of the time, but 95% is not 100%), does this guarantee that pumps will operate in the same direction every time you reconnect the battery? But of course, even though the first or second inverter might be the master at any time, the 3P1 setting in one inverter and 3P2 in the other will guarantee that master or slave, the one with 3P1 will always have the same phase relationship to the one with 3P2. It might be that today the 3P1 inverter is master, and tomorrow the 3P2 or 3P3 inverter will be master, but regardless, the one with the dark blue active (using South African or Australian standards) will always be phase C. (In Australia, the three phases are red, white, and dark blue; it looks like South Africa uses red, yellow, and dark blue.)

Off topic: won't it be fun when they introduce the new international standards, and black will change from neutral to L2 active! What could possibly go wrong?

[Chris Hobson]

6 hours ago, Coulomb said:

Off topic: won't it be fun when they introduce the new international standards, and black will change from neutral to L2 active! What could possibly go wrong?

Oh my! Why can they not leave convention alone? Conventional current flow is from positive to negative. Franklin's convention predates the discovery of electrons and electron flow. Wisely Franklin's convention remains why change a colour coding convention mid-stream?

[___]

5 hours ago, Chris Hobson said:

Oh my! Why can they not leave convention alone? Conventional current flow is from positive to negative. Franklin's convention predates the discovery of electrons and electron flow. Wisely Franklin's convention remains why change a colour coding convention mid-stream?

It's AC. None of Franklin's conventions apply anyway, literally half the time the current flows up the wrong way via the black connector and back out the red one, and the other half of the time it does it properly as per Franklin. That's life.

What Coulomb is referring to is the colour coding on three-phase wiring. Presently that's red (L1), white or yellow (L2), and blue (L3) for the three phases, and black for neutral. Internationally, it's Red (L1), black (L2), and blue (L3), and white/gray (Neutral).

So they are literally swapping L2 with neutral. What could possibly go wrong.... indeed!

[Chris Hobson]

52 minutes ago, plonkster said:

It's AC. None of Franklin's conventions apply anyway.......

I was not trying to say anything about current flow but more about convention. If you have a convention don't change it. I stripped out red and black cloth covered wiring in a shed which dated from about the 1940s. So red and black have denoted live and neutral in both single phase and 3P for some time. 

The reference to Franklin was perhaps unfortunate. His convention dates from  the mid 18th century (1751 I think). The discovery of electrical flow by JJ Thomson 150 years later created  confusion. Yet Franklin's convention stands. It would appear that authorities are not content and want to add to the confusion.

[___]

5 minutes ago, Chris Hobson said:

If you have a convention don't change it.

I think the idea is to get it in line with international standards.

Take for example the electrical socket thing. South Africa actually wants to move to something that IS an international standard... but apart from Brazil we're the only people using it. :-)

The nice thing about standards is that there are so many to choose from.

And, in software, we often have this cycle: There are 9 competing standards for frobnicating a bar. I am going to unify them into one grand standard to remove all the confusion. Six months later someone else comes along... There are 10 competing standards, we're going to fix it...

:-)

[Coulomb]

12 hours ago, plonkster said:

Internationally, it's Red (L1), black (L2), and blue (L3), and white/gray (Neutral).

I thought we were all going to brown (L1), black (L2), grey (L3) and light blue for neutral. Most mains cords here in Australia have used the brown and light blue for decades. 

The biggest hassle has been the USA using black for active and white for neutral, when so many countries use black for neutral. Australia uses white for an active, so that's completely opposite to the USA convention. 

[Mark]

Brown, Black Grey plus Blue.  South Africa going to get a shock

[Gundog]

This change took place  in the UK about 5 years ago too without too many problems.  When I was still over there you couldn't use exposed earth either, it had to be insulated with the standard yellow/green.  I didn't mind it but I can see it being a problem with 3 phase systems, especially here where there are too many "self" trained electricians.