If n number of VSI are connected in parallel, what will be effect on overall efficiency of system?Which factors will be improved / reduced?
Hi,
I wrote my diploma thesis exactly about this.
If VSI are connected in parallel it may be observed a circulating current. This current depends on the inductance and resistance in the loops between the VSIs. A pair of not syncroniced VSI drives a circulating current with is given by I=U_ac / (2*pi*fs*L_loop).
If the VSI are syncroniced and there is no U_ac in the loop the current depends on e.g. the R_on (Semiconductor-Resitance) and the other resitances.
The problem could be solved with a control-loop. The circulating current must be measured and controlled to be zero. It may be used a d/q Transformation...
kind regards Tobi
yes , we will take care of circulating current but what will be effect on current drawn from VSI ? As both VSI are acting as source to load. does current will be shared by two VSI ?
Dear Ranvir:
Adding to Tobias, the parallel connections of electrical sources including the inverters is an important topic. Connecting electric sources in parallel must be subjected to preconditions and precautions for the sources not to be damaged.
There are types of sources: voltage sources and current sources.
At first assume that the sources are ideal and then assume to connect n sources of the constant voltage sources then Vt= total voltage = SUM of all source voltages= SUM VSi from i=1 to n. If all voltages are equal then Vt= n VSi. The series connection of the voltage sources even when not ideal will impart no problems.
If one require to connect constant voltage sources in parallel, then they have to satisfy the precondition:
-VSi must be equal through out all sources
- Since the sources are not ideal they normally have source resistance RSi even if it is small, purposely for symmetry also all RSi must be strictly equal
- If the sources are AC sources, they have to be coherent which means that they have to be of the same frequency and phase as with the inverter.
When connecting sources that deviate from the above ideal conditions, the sources will push current in each other and the operating efficiency will be smaller since more energy will be lost in the internal series resistances of practical voltage sources. More important is that the current driven by the stronger source may cause excessive current to path through the sources and cause its damage if exceeded the rated value.
The only gain by operating the inveters in parallel is to multiply the output current.
To operate the inverters in parallel it is a normal practice is let one fix the voltage of the load and operate the others in constant current mode.
In this way one can solve the challenges of the parallel connection of same rated inverters.
For more information please follow the link: http://www.keysight.com/main/editorial.jspx?cc=US&lc=eng&ckey=520808&nid=-11143.0.00&id=520808
Best wishes
If load is ac motor, current shared by both VSI having same frequency will be equal or varies?
Hello Ranvir, in addition to the valuable previous answers, you must achieve synchronization between the VSIs. Please have a look at the following papers:
https://www.researchgate.net/publication/281855509_Stability_Analysis_of_Parallel_Inverters_Microgrid
https://www.researchgate.net/publication/312937689_Modeling_and_Control_of_Master-Slave_Microgrid_with_Communication_Delay
Regarding your question about the inductive load the following paper "Control of parallel inverters based on CAN bus in large-capacity motor drives" addresses the control of parallel VSIs to derive a motor load:
https://www.researchgate.net/publication/224313726_Control_of_parallel_inverters_based_on_CAN_bus_in_large-capacity_motor_drives
Connecting converters in parallel offers many advantages such as increased power processing capabilities, improved reliability, easy maintenance, future expansion, and modularity.
I hope this will help you.
Best Regards
Conference Paper Stability Analysis of Parallel Inverters Microgrid
Conference Paper Modeling and Control of Master-Slave Microgrid with Communic...
Conference Paper Control of parallel inverters based on CAN bus in large-capa...
Ranvir,
I hinted above all sources will drive the same current in the load if they are assumed identical.
Best wishes
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