Multiple resonances mitigation of paralleled inverters in a solid-state transformer (SST) Enabled AC Microgrid

Abstract

Solid-state transformer (SST) becomes a promising technology in smart grid applications due to its multiple functionalities and control flexibility compared to line-frequency transformers. In a SST enabled ac microgrid, the grid impedance represented by the SST output impedance is frequency-dependent, which is different from the inductive grid impedance in a conventional ac microgrid. The frequency-dependent SST output impedance can interact with those of paralleled inverters and result in multiple resonances locating in a wide frequency range. In this paper, a combination of a lead-lag compensator and a negative impedance feedback control has been proposed to enhance the system stability. With proposed methods, the multiple resonances are decoupled and thus more feasible to be mitigated. The negative effect of digital control delay on the high frequency resonance damping is attenuated with proposed methods. In addition, the tradeoff design of virtual impedance is also analyzed with consideration of both system-level resonance mitigation function and single inverter stability. Consequently, the stability of paralleled inverters in the SST-enabled ac microgrid has been improved. Finally, an OPAL-RT-based hardware-in-the-loop verification has been provided to prove the validity of proposed methods.