In the P-f Droop control strategy for a three-phase Voltage Source Inverter using ABC-dq0, why is the current measured before as well as after the filter circuit or in other words why is the current before filtration taken as a parameter?
Is there any alternative to avoid measuring the current before the filter if one used a different inverter topology (like Z source converters)?
Figure source: Bouzid, A.E., Sicard, P., Yamane, A., & Paquin, J. (2016). Simulation of droop control strategy for parallel inverters in autonomous AC microgrids. 2016 8th International Conference on Modelling, Identification, and Control (ICMIC), 701-706.
This is because your filter increases the order of your controlled system and the filter output current no longer obeys the VSI's output voltage according to a first order delay. This is only the case for the filter input current.
So for a robust state control of your system you need the output and the input current of the filter (you might replace one of these currents by the capacitor current of the filter) and the capacitor voltage of the filter.
So you have three state variables as feedback from your third order system - which is appropriate for controlling it and to command the VSI's switching states in a robust manner.
If you use only one state variable (e.g. filter output current) fas feedback for your control system you risk to excite resonance oscillations in your filter.
As I think, Because of the current regulation scheme, the VSI is protected against overload conditions. Therefore, first the VSI AC-side current is controlled by a control scheme, through the VSI terminal voltage. Then, both real and reactive power are controlled by the phase angle and the amplitude of the VSI line current with respect to the common point voltage.
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