Small-Signal Modeling and Controller Design of a Grid-Connected Inverter for Solid State Transformer

Abstract

A single-phase grid-connected inverter for solid state transformer (SST) is presented. This work is aimed at the modeling and the design of control schemes for the inverter. In this thesis, a small-signal model for the grid-connected inverter with modified bipolar pulse width modulation method is developed. Small-signal analysis allows to predict the stability and dynamics of the inverter. Based on theoretical analysis, controllers are designed to improve dynamics and to guarantee stability of the system. A high power factor and low harmonics are desirable in transmission system to improve the efficiency of the utility line. The current controller of the single-phase grid-connected inverter has sinusoidal reference to achieve power factor correction (PFC) and harmonics suppression. Proportional-integral (PI) scheme is widely used to control the power converter. The PI controller has the infinite gain at dc so it is suitable to track the dc reference. However, it is not good for tracking sinusoidal reference due to its finite gain at ac. The proportional-resonant (PR) controller which introduces large gain at specific frequency can track the sinusoidal reference without steady-state error but the error remains in step input response. The proportional-integral-resonant (PIR) controller has the advantage of both tracking the sinusoidal reference and removing the steady-state error for the step input. In this thesis, PIR controller is proposed for current control and design procedure for the PIR controller is presented. In addition, the voltage controller is needed to regulate the dc link voltage. The voltage controller is designed to improve the audio-susceptibility and the output impedance characteristics. The dynamics and stability of the inverter system is analyzed. The proposed small-signal model and controllers are verified by PSIM simulation and experiments.