Design and Implementation of 20 kVA D-Σ Digital Controlled Three-Phase Four-Wire Multi-Function Inverter System

This paper presents design and implementation of a 20 kVA division-summation (D-Sigma) digital controlled three-phase four-wire multi-function inverter system. The inverter can be operated in grid-connected or stand-alone mode to act as a renewable-energy ac power-supply system. In the stand-alone mode, since the D-Sigma digital control law includes load impedance estimation, the inverter can track sinusoidal output voltage under linear or nonlinear load. For the grid-connected mode and in order to achieve a specific PF and stabilize grid voltage and frequency, the D-Sigma digital control law is supplemented by a droop control based linear P-Q compensation. Moreover, the D-Sigma digital control law in this research is based on Two-Phase Modulation (TPM) scheme and associated with the switching sequence derived from space vector pulse width modulation (SVPWM) scheme. The inductance variation has been taken into account in the control-law derivation, reducing core size significantly. The analysis and discussion have been verified with measured results from a 20 kVA three-phase four-wire inverter system.