Simplified Small-Signal Model for Output Voltage Control of Asymmetric Cascaded HBridge Multilevel I

Abstract

This paper proposes a simplified small-signal model for output voltage control of a single-phase Asymmetrical Cascaded H-Bridge Multilevel Inverter (ACHMI). The ACHMI is an n-series connected H-bridge converter, each one with a unique value at the DC link and usually scaled at {1:2:6:..} or {1:3:9:..}. By assuming that the small-signal variation component is equal in all n converter terminal ports, a simplified small signal model is obtained. This assumption is carefully described and justified. To verify the veracity of the proposed model, two distinct control strategies are applied. One is a single-loop control scheme based on a modified Proportional-Integral (PI) controller. The other one is a double-loop control scheme based on a PI controller with feedforward action of the load current. Both controllers are tuned based on the dynamic behavior of the proposed model. Since the designed controllers based on the simplified model make the ACHMI output voltage to follow the reference a without steady-state error, the proposed simplified model truly represents the inverter.

Existing System

Staircase Modulation.

Proposed System

This paper proposed a simplified small-signal model for the output voltage control of an ACHMI. To obtain the model, the assumption that the small-signal variation components in all modules of the ACHMI are equals was made. Later, the proposed model was verified through two distinct control strategies. One was a single-loop control scheme based on a modified PI controller while the other one was a double-loop control scheme based on a PI controller with feedforward action of the load current. Both controllers were tuned based on the dynamic behavior of the proposed model. Since the designed controllers based on the simplified model made the ACHMI output voltage follow the reference without steady state error, it can be concluded that the proposed simplified model truly represents the inverter.

Architecture

The ACHMI connected to a PCC through an LC filter