Investigating the Effect of Transport Delay on the Grid Current THD of Single-Phase Converters With Proportional-Resonant Controllers

Minimizing grid current total harmonic distortion (THD) is a fundamental challenge in the control of grid-connected inverters. Typically, a proportional-resonant (PR) controller is used to accurately follow the sinusoidal reference current and mitigate low-order current harmonics in single-phase grid-connected inverters. The parameters of the PR controller are designed to achieve the desired dynamic characteristics, taking into account the system transfer functions, specifically the inverter transport delay. This delay is the time between the sampling instant of the grid current and the update instant of the modulating (reference) waveform. In this paper, the effect of this transport delay is experimentally investigated on the THD of the grid current when double-update sampling mode is utilized at a typical switching frequency of 10 kHz, resulting in a sampling frequency of 20 kHz. An innovative structure is introduced to minimize the transport delay as much as possible, and the results are compared with the standard control structure, where sampling is performed in the middle and the beginning of the control loop in double-update mode.