Improvement of control-law derivation for D-Σ digital controlled three-phase four-wire inverter

A division-summation (D-Σ) digital control can track sinusoidal reference current in four operation modes: grid-connection, rectification, PF leading and PF lagging mode. It can overcome the limitation of abc to α-β-γ frame transformation and cover wide filter inductance variation. However, the switching sequences for the four modes are different from each other, increasing complexity of firmware programming. In this paper, improvement of control-law derivation for D-Σ digital controlled three-phase four-wire inverter is presented. By selecting the zero crossing points of phase voltages as region transitions, the control laws and the related parameter tables for the four modes can be unified to a general form. The three-phase four-wire inverter topology can be also adopted as an uninterruptible power supply (UPS) application and it can supply unbalanced, linear and rectified loads. With the proposed load impendence estimation, the D-Σ digital control laws can be applied for voltage tracking. The switching sequences of the four modes and UPS mode are all unified to the switching pattern of the grid-connection mode. Additionally, a D-Σ transformation matrix is identified to simplify the derivation procedure of the division (D) and summation (Σ), which can obtain the control law directly. Experimental results measured from a 10 kVA inverter have verified the analysis and discussion.