The system below is a dc microgrid. During the islanded mode, the battery buck-boost converter regulates the 400V dc across the dc load. While the ac-dc converter(after the transformer) regulates the 400V dc bus during grid connected mode. Both the ac and dc loads are supplied by the dc microgrid in islanded mode. But in grid connected mode, I have given a threshold to the battery power so that when it's charged up power exceeds that limit, the battery will supply the surplus amount to the grid connected inverter i.e ac load. Otherwise if it is under that threshold the ac load will be supplied by the grid in the grid-connected mode.

So the controller of battery buck-boost converter runs in voltage tracking mode during islanded mode and in power tracking mode(senses the 'PV_power' & 'P_dc_load' to find the required amount of power need to be generated or consumed by the battery) when the system is grid connected. As mentioned earlier, dc bus voltage will be regulated by the ac-dc converter to 400v as battery is not regulating the voltage in the latter mode.

My dc microgrid is working properly in the islanded mode as the voltage is kept constant 400v dc across the load by battery buck-boost converter.

Furthermore, the main grid without the dc microgrid is also performing fine and keeping the dc bus at 400v and supplying the ac load with a decent THD(below 5%).

But when i connect my dc microgrid with the main grid dc bus(i.e grid connected mode) the whole system goes unstable.

I have used separate output capacitors for each converter. But i recently saw some papers where they used a common output capacitor for PV MPPT boost converter, battery buck-boost converter and main grid's ac-dc converter.

Problem is that if i use a common capacitor for all these converters, they individually do not perform well. And it also hampers the dc microgrid in islanded mode which is working right now with different capacitor values.

I am not sure what actually is causing this and how can i solve this interface between microgrid and main utility grid? Please help!

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