A division-summation (D-Sigma) digital control for three-phase inverters to achieve active and reactive power injection has been presented. It can overcome the limitation of abc to dq frame transformation. However, the D-Sigma control requires four sets of control laws to cover four quadrant operations, namely, grid connection (power factor (PF) 1 similar to +/- 0.866), rectification with PF correction, PF leading (0 similar to 0.866), and PF lagging (0 similar to-0.866). Moreover, the switching sequences for the four modes are also different from each other, increasing complexity of firmware programming. In this paper, improvement of the control law derivation and region selection for the D-Sigma digital control 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 switching sequences of the four modes are also unified to the switching pattern of the grid-connectionmode. Additionally, a D-Sigma transformation matrix is identified to simplify the derivation procedure of the division (D) and summation (Sigma), which can obtain the control law directly. Measured results from a 10-kVA 3 phi bidirectional inverter have been presented to confirm the improvement.