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Problem in parallel operation of two sinewave inverters

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abhichaotic

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Hi,
I have two sine wave inverters each of rating: Input 48V battery Output 230V/50HzAC 3KW
Now i want to connect both inverters output to a single load so they operate in parallel. Now i don't know how to make these inverters synchronize with each other. I can make both inverters to communicate with each other through CAN bus, but I don't know how to achieve this thing. I will be grateful if anybody helps me with the logic.
 

Ok. Are they the same model? If yes, what ic is used for the dc-dc conversion stage? You will need to make them work at the same duty cycle to ensure equal contribution to total power delivered to high voltage dc bus. Then use only one of the two inverter's h-brige signals to control the two h-brige mosfets. I hope you know how to parallel two different h-briges?
 

ok , thanks for this idea. But is there any other method using digital communication like CAN bus between two inverters??
 

But is there any other method using digital communication like CAN bus between two inverters??
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Yes, if the inverters are designed with a synchronization feature. No chance otherwise.
 

Hi, FvM thanks for your review. I have developed CAN bus communication for my inverter. Now for synchronization i need help for a scheme to synchronize the two inverters, means how to synchronize them so they can also share the currents equally among themselves.
 

Hi,
I have two sine wave inverters each of rating: Input 48V battery Output 230V/50HzAC 3KW
Now i want to connect both inverters output to a single load so they operate in parallel. Now i don't know how to make these inverters synchronize with each other. I can make both inverters to communicate with each other through CAN bus, but I don't know how to achieve this thing. I will be grateful if anybody helps me with the logic.
Click to expand...
Hi abhichaotic
If they don't have master / slave feed back you can't use them as two parallel sources , because they will fly ! and if they don't have that feed back and you want add it , it will be a bit hard to do ! you should find their feed back path and add a comparator and ... etc . ( if you design one of them it will be easier !!!! instead of adding that stage ) however there is another way available too , a combiner with HF component , ( it will consequence more price and i think it is not good too ) .
Best Wishes
Goldsmith
 

You need to know detailed schematics; theoreticaly need to use a single PWM clock for both inverters and be sure that phase difference between same power switchs is minimal. Even that is made correctly, will be problems with current sharing between inverters. Equal current mean equal internal impedance (dynamical) which are hard to obtain/maintain even for similar products.
 

Depending on where you connect them together, they can be synchronized in a few ways. Connecting the H bridges directly together is likely not an option because even if you synchronized the PWM controller clock and digital sine waves, you'd still be at risk for cross conduction. Connecting them in parallel after their output filters is a much safer method; then you don't have to perfectly synchronize the PWM frequency or duty cycle. You do, however, need to synchronize the digital sine wave, and use a shared error signal (assuming the inverters use some feedback).

But the short story is that doing any of this requires some heavy circuit modification. It's not something you can you with CAN interface alone.
 

Thanks mtwieg for showing your concern, as you are saying that some heavy circuit modification is required, will please tell what modifications are required actually, means some switchgears or some compensator type of thing?? Both the inverters are now working in parallel and as you have said I have connected them after output filter. But having problem in current sharing among them. Need help in that.

Hi, iop95, you are correct that for equal current internal impedance should be same and that is not possible even in two similar products, but how to compensate for this difference in impedance.
 

An exact load share can be only achieved by implementing balance controllers based on output current or power measurement. It will be difficult for inverters that have no current measurement at all. In practice, equal output voltage setpoints and a positive output impedance can be sufficient to achieve an acceptable load share.

Synchronization of sine carrier instantaneous phase is presumed of course.

The discussion suffers from the fact that we are talking about synchronization and load balancing details without knowing the exact inverter topology and control algorithm. Thus there's much guessing involved.
 

In DC is much easy to make a current sharing between more sources on same load. I think one method may be to add a current sensing circuit on each DC bus of inverters and a circuit to monitor/control these currents with outputs to each control circuit of inverters. This circuit will try to keep same DC currents by injecting an error signal to voltage/PWM loop of inverters; if in the front of inverters there are a DC converter (chopper) you may use current error signal in this stage. Maybe dynamical response of this circuit will be not so fast but will be ok for most of loads type them and a short overcurrent may be accepted by one or other inverter.
 
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    FvM

    FvM

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Yes, the mentioned output power balancing should be understood in the same direction. Implementation details are however a matter of inverter topology.
 

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