Fig. 1. Direct parallel three-phase VSIs.
Fig. 2. Path of the circulating current via lower switches ofinverter 1 and lower switches of inverter 2.
Fig. 3. Path of the circulating current via upper switches of inverter1 and lower switches of inverter 2.
Fig. 4. Single phase of directly connected parallel VSIs.
Fig. 5. Methods for removing the path of the circulating current.
Fig. 6. Simplified block diagrams of conventional closed-loopcurrent control in z-domain. (a) d-axis, (b) q-axis, and (c) z-axiscurrent control loops.
Fig. 7. Proposed current controller for parallel three-phase VSIs.
Fig. 8. Nyquist diagram of PI controller.
Fig. 9. Proposed ZSCC control loop.
Fig. 10. Trajectory of Rcl(z).
Fig. 11. Simulated waveforms with conventional ZSCC. (a)Currents of inverters 1. (b) Currents of inverters 2. (c) Inverter 1and 2 currents in phase a. (d) ZSCC.
Fig. 12. Simulated waveforms with the proposed ZSCC. (a)Currents of inverters 1. (b) Currents of inverters 2. (c) Inverter 1and 2 currents in phase a. (d) ZSCC.
Fig. 13. Diagram of parallel three-phase VSIs with L filters.
Fig. 14. Experimental result of the current waveforms. (a)Conventional controller. (b) Proposed controllers with thedifferent current references( * 1,inv di =30 A, 2,inv di =10 A) and filter *inductances(Lf1 = 2 mH and Lf1 = 3 mH).
Fig. 15. Experimented current waveforms. (a) Conventionalcontroller. (b) Proposed controllers with the same current reference( * 1,inv di =30A, 2,inv di =30A) and different filter inductances(Lf1 = 2 *mH and Lf1 = 3 mH).
TABLE I SI BASE UNITS
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