A Random Sampling-Rate MPPT Method to Mitigate Interharmonics from Cascaded H-Bridge Photovoltaic Inverters

Interharmonics have become one challenging issue in power systems. One potential source of interharmonic emissions from photovoltaic (PV) systems is the perturbation of the maximum power point tracking (MPPT). In cascaded H-bridge (CHB) PV inverters, if the MPPT perturbations of CHB cells are in-phase, the oscillation on the equivalent total DC voltage will be multiplied, leading to large interharmonics in the grid. To address this issue, a phase-shifting MPPT (PS-MPPT) can be adopted. However, the interharmonic suppression performance of the PS- MPPT method is limited for CHB PV inverters with an odd number of cells. Moreover, when some CHB cells are operating under varying environmental conditions, the PS-MPPT may fail to minimize the oscillation on the total DC voltage, and thus, interharmonics appear again. To overcome these limitations, a random sampling-rate MPPT method for the CHB PV inverter is proposed in this paper. By randomly selecting the MPPT sampling-rate of each CHB cell, the dominant interharmonics can be effectively suppressed. Compared with the original PS-MPPT, the proposed method performs better in dynamic conditions in terms of interharmonic suppression, especially for CHB PV inverters with an odd number of cells. Simulations and experimental results have validated the effectiveness of the proposed method.