3-phase with 3 standalone(no connections between) all in one inverters?

[luk88]

There are inverters designed to supply 3 phase power. They are connected together and they sync their waveforms by communicating with eachother. This topic is not about tyese.

 

No. I'm talking about the cheap inverters like  EASUN SMH-ii  (most likely an  Axpert MKS II clone with no parallel board). I've done some benchtop testing and I noticed they they sync to the grid exactly even when in SBU mode.

 

So let's say one has a stable 3 phase grid and 3 such inverters that sync to all 3 phases. Can anyone think of a reason why one couldn't use a resistive 3 phase load connected in delta configuration? That resistive load is a tankless water heater (geyser).

 

 

[macafrican]

I presume this for connecting solar to, while on grid?     Not sure it is my council or NRS097 rule, but you’re not allowed to connect three discrete inverters onto a three phase utility supply (and remember limit is 13.8kVAac).     The three inverters must operate unity - when solar is surplus, equal kVA export to grid, per phase.

on your side of the fence, each inverter would be connected to a utility supply phase, so as long as you wire up your phase rotation correctly, a three phase load will work fine.   I have a three phase borehole but what I did add is a relay that does not allow borehole to try run unless all three phases operating (I have one three phase grid tied inverter and another battery inverter for loadshed)

[Beat]

It might work the way you described it. Each inverter synchronizes to the phase it is connected to. However if the grid fails (load shedding) they will no longer have a 3 phase reference and output non synchronized phases. Your 3 phase geyser would work in star connection but not well in delta. 3 phase motors would probably have problems.

[Scorp007]

36 minutes ago, macafrican said:

 The three inverters must operate unity - when solar is surplus, equal kVA export to grid, per phase.

on your side of the fence, each inverter would be connected to a utility supply phase, so as long as you wire up your phase rotation..... 

Correct me if I'm wrong. If 3 inverters are connected via comms and 1 phase generates 3 times the power from PV than another phase would some power be moved to the low phase in order to feed in the same per phase? 

3 single inverters should output at the same angle than the input if high voltage SCC so the output should match the input from grid if running in single mode or am I missing something? 

Thus 3 ph loads should just work as well as the grid.. 

Interesting to get more inputs on this topic. 

@Beat Great input about when the grid fails. Then a monitor for phase fail is a must to isolate the output of all 3.

Edited August 4 by Scorp007

[Beat]

6 minutes ago, Scorp007 said:

Great input about when the grid fails. Then a monitor for phase fail is a must to isolate the output of all 3.

I don't see that problem. They would be 3 single phase systems operating independently and non synchronized. However connecting a delta wired user would probably synchronize them all to one phase. Thus the delta wired user would get zero voltage.

[Scorp007]

36 minutes ago, Beat said:

I don't see that problem. They would be 3 single phase systems operating independently and non synchronized. However connecting a delta wired user would probably synchronize them all to one phase. Thus the delta wired user would get zero voltage.

Would each inverter not start their own sine wave at any point in time and one would have no control over what the ph angle would be between them when running from PV or battery???? 

[Beat]

18 minutes ago, Scorp007 said:

Would each inverter not start their own sine wave at any point in time and one would have no control over what the ph angle would be between them when running from PV or battery???? 

Yes, that is exactly what I made allusion to.

[luk88]

18 hours ago, Beat said:

Yes, that is exactly what I made allusion to.

This is not the case. I checked with an oscilloscope. Each inverter synchronises to each own phase from the grid when the grid is on.

 

When grid fails it continues running with this synch, but quickly drifts. So this can be used only for "off grid with grid backup" situations.

 

If it works well one can imagine a tiny 3 phase inverter providing the sync signal for a 100% set up. But its just an idea.

 

20 hours ago, Scorp007 said:

Correct me if I'm wrong. If 3 inverters are connected via comms and 1 phase generates 3 times the power from PV than another phase would some power be moved to the low phase in order to feed in the same per phase? 

There is no comms between inverters (other than my monitoring). They balance loads/PV/battery without any concern what other inverters are doing (to satisfy their each inputs/outputs).

20 hours ago, Scorp007 said:

3 single inverters should output at the same angle than the input if high voltage SCC so the output should match the input from grid if running in single mode or am I missing something? 

Thus 3 ph loads should just work as well as the grid.. 

Interesting to get more inputs on this topic. 

Indeed, that's how it looks like. My only reservation is that normally currently flows between live and neutral output. But in a balanced 3 phase set up current flows between phases only. So lets say there is a resistive load (my geyser) and it is delta connected (or star, but fully balanced). Then there will be current on phases, but no current on neutral... But the current has to get back somehow! So what would happen?

 

So this is my only reservation and the reason I haven't tried just connecting it yet.

20 hours ago, Scorp007 said:

@Beat Great input about when the grid fails. Then a monitor for phase fail is a must to isolate the output of all 3.

This could be resolved by a relay (to switch off if grid fails) as another user mentioned.

 

Let me add, this is for purely a resistive load. I'd be reluctant to run inductive loads like big motors with this.

 

I have no space in my home for a hot water tank (so I have to run tankless) the geyser is the only load I has to leave on the grid. On the other hand buying a 3 phase system for one load and spending 3x the money is something I decided against.

[Scorp007]

1 hour ago, luk88 said:

This is not the case. I checked with an oscilloscope. Each inverter synchronises to each own phase from the grid when the grid is on.

 

When grid fails it continues running with this synch, but quickly drifts. So this can be used only for "off grid with grid backup" situations.

 

If it works well one can imagine a tiny 3 phase inverter providing the sync signal for a 100% set up. But its just an idea.

 

There is no comms between inverters (other than my monitoring). They balance loads/PV/battery without any concern what other inverters are doing (to satisfy their each inputs/outputs).

Indeed, that's how it looks like. My only reservation is that normally currently flows between live and neutral output. But in a balanced 3 phase set up current flows between phases only. So lets say there is a resistive load (my geyser) and it is delta connected (or star, but fully balanced). Then there will be current on phases, but no current on neutral... But the current has to get back somehow! So what would happen?

 

So this is my only reservation and the reason I haven't tried just connecting it yet.

This could be resolved by a relay (to switch off if grid fails) as another user mentioned.

 

Let me add, this is for purely a resistive load. I'd be reluctant to run inductive loads like big motors with this.

 

I have no space in my home for a hot water tank (so I have to run tankless) the geyser is the only load I has to leave on the grid. On the other hand buying a 3 phase system for one load and spending 3x the money is something I decided against.

Thanks for adding detail. In the opening post you mention grid is stable but no mention of panel size or battery size or load of tank less geyser.

So we assume the inverters are used from PV for saving grid? 

 

[luk88]

1 minute ago, Scorp007 said:

Thanks for adding detail. In the opening post you mention grid is stable but no mention of panel size or battery size or load of tank less geyser.

So we assume the inverters are used from PV for saving grid? 

 

Yes, they are.

I didn't think these details were relevant, but for the sake of completness I'll add more  

I have 14kW peak PV array, but due to max current limitations and the way it is connected it operates at max 9kW peak power (I'm mostly concerned with low light production during rain/winter - I have too much electricity during summer anyway. I currently have 45kWh of battery storage (considering adding 10kWh more as I already have the cells, but they are very bad quality cells so I'm reluctant). This is a single system with 3 hybrid inverters, but not parallell. All inverters are connected to the same battery, but they power different loads. 2 inverters (Y&H 10.2kW) have each half of the array connected to them. The third inverter (EASUN SMH-ii-7k) has no PV currently. All have grid connected (each one has a different phase) and all work in SBU mode. All are configured to charge from the grid only if the battery voltage drops pretty low (which I hope will not happen during the most of the year). There is a transfer switch and a small gas generator (3kW) that can be manually connected to the EASUN inverter to charge the battery if the grid goes down for a long time in winter.

I also have a separate purely on-grid single phase system for selling power to the grid. This has a Deye 3kW inverter and 2400W of panels. This system paid for itself in the first 3 years of operation (as I got the panels from a bankrupcy sale and I made the ground mount myself), but the cost of electricity here in Europe is insane now (3x what it was 3 years ago).

So the purpose of the current system is to use the grid power as little as possible and where it is inevietable use power "saved" by the on-grid system (although due to recent changes it makes less and less sense to sell power to the grid).

The house consumes 10-20kWh per day depending on season and how much we want to save. Everything is electric. 

The geyser has no tank at all (it is a wall mounted unit that regulates water flow based on set temperature, we set it at 50C) and one can have a normal 10 min shower using only about 1.5kWh of electricity. However when one is not paying attention it is easy to use 5kWh in one use. It is not a lot in the grand scheme of things. But its "the last thing" on the grid in my house so I'm looking into how to switch it too. 

I'm also considering running the heaters on 240V instead (3 separate phases), but I'm not sure if it will be easy to switch and if it'll work (its a "smart" heater so it may detect the voltage and refuse to run). I'm also thinking about adding an outdoor burried underground tank of hot water, but the cost to do that will be quite high as if I was doing such a big job I'd want the tank to also be used for storing house heating water (I already have a heat pump, but it is currently only used for cooling the house during the summer).

[Scorp007]

2 hours ago, luk88 said:

Yes, they are.

I didn't think these details were relevant, but for the sake of completness I'll add more  

I have 14kW peak PV array, but due to max current limitations and the way it is connected it operates at max 9kW peak power (I'm mostly concerned with low light production during rain/winter - I have too much electricity during summer anyway. I currently have 45kWh of battery storage (considering adding 10kWh more as I already have the cells, but they are very bad quality cells so I'm reluctant). This is a single system with 3 hybrid inverters, but not parallell. All inverters are connected to the same battery, but they power different loads. 2 inverters (Y&H 10.2kW) have each half of the array connected to them. The third inverter (EASUN SMH-ii-7k) has no PV currently. All have grid connected (each one has a different phase) and all work in SBU mode. All are configured to charge from the grid only if the battery voltage drops pretty low (which I hope will not happen during the most of the year). There is a transfer switch and a small gas generator (3kW) that can be manually connected to the EASUN inverter to charge the battery if the grid goes down for a long time in winter.

I also have a separate purely on-grid single phase system for selling power to the grid. This has a Deye 3kW inverter and 2400W of panels. This system paid for itself in the first 3 years of operation (as I got the panels from a bankrupcy sale and I made the ground mount myself), but the cost of electricity here in Europe is insane now (3x what it was 3 years ago).

So the purpose of the current system is to use the grid power as little as possible and where it is inevietable use power "saved" by the on-grid system (although due to recent changes it makes less and less sense to sell power to the grid).

The house consumes 10-20kWh per day depending on season and how much we want to save. Everything is electric. 

The geyser has no tank at all (it is a wall mounted unit that regulates water flow based on set temperature, we set it at 50C) and one can have a normal 10 min shower using only about 1.5kWh of electricity. However when one is not paying attention it is easy to use 5kWh in one use. It is not a lot in the grand scheme of things. But its "the last thing" on the grid in my house so I'm looking into how to switch it too. 

I'm also considering running the heaters on 240V instead (3 separate phases), but I'm not sure if it will be easy to switch and if it'll work (its a "smart" heater so it may detect the voltage and refuse to run). I'm also thinking about adding an outdoor burried underground tank of hot water, but the cost to do that will be quite high as if I was doing such a big job I'd want the tank to also be used for storing house heating water (I already have a heat pump, but it is currently only used for cooling the house during the summer).

Thanks for all the detail. No my question was only to get an idea of your system while you are exporting and also using 3 cheaper inverters running on a 3 oh system. 

At times one tries to give leads on possibilities but not knowing how and what size system one is using the leads might not be suitable at all and can be costly mistakes. 

This even more true when not knowing the knowledge of the person asking. Not applicable in this case

We see a number of questions from members that have to give information to their installers and electricians. 

[Beat]

3 hours ago, luk88 said:

So lets say there is a resistive load (my geyser) and it is delta connected (or star, but fully balanced). Then there will be current on phases, but no current on neutral... But the current has to get back somehow! So what would happen?

Understanding the 3 phase system. Yes, in a balanced 3 phase system there is no current in the neutral. In the star wired generation the phase currents cancel themself out at theyr neutral connection point. But only if the load is balanced and the phases are accurately 120° off each other. As you assume without grid the 3 inverters will drift - no more correct 3 phase system. As I mentioned they will operate as 3 individual single phase systems. If a 3 phase load (geyser) has star wired elements each element is a single phase load to the corresponding inverter. They couldn't care less of the phase angle as opposed to a 3 phase motor.  However if the load is delta wired the resistor of the heating elements connect and pull the output of the 3 inverters together. They most likely will synchronize to one phase thus no voltage between them. Or the phase to phase voltages will vary all over the place as they continue to drift. Could go up to 460V. You can find out by measuring the voltage between the phase of the inverters. It should be steady around 400V. If not the inverters are drifting.