Load impedance estimation and iterative-learning control for a three-phase four-wire inverter

This paper presents a load impedance estimation and iterative-learning control for a three-phase four-wire inverter. The topology of a three-phase four wire inverter is adopted to uninterruptible power supply (UPS) applications, which can supply unbalanced load, linear load and rectified load. The proposed load impedance estimation scheme employs an RLC equivalent circuit concept to determine the load parameters. The inverter can track the sinusoidal reference voltage precisely and it is allowed to have wide inductance variation, reducing core size significantly. With the proposed iterative learning control, the steady-state error can be reduced significantly cycle by cycle. In the design and implementation, the inverter inductances corresponding to various inductor currents are measured off-line and tabulated into a single-chip microcontroller for tuning loop gain every switching cycle, while the RLC load parameters are estimated on-line to determine control (duty ratio), ensuring system stability. Experimental results measured from A 10 kVA system have verified the analysis and discussion.