Interfacing renewable energy sources to the ac grid by a 43-level CMLI with many advantages

Abstract

This paper proposes a 43-level asymmetric cascaded multilevel inverter (CMLI) as an interface between renewable energy sources and the grid, that has the following advantages:1.The CMLI consists of only four H-bridges per phase, with separate dc sources. Renewable energy sources of different types are treated as the dc sources of the CMLI (e.g., dc voltage from PV panels, rectified ac voltage from wind turbines, etc.).2.It can interface different types of renewable energy sources instantaneously to feed the ac grid with a nearly sinusoidal voltage with very low total harmonic distortion (THD).3.It can accommodate natural fluctuations in the renewable energy sources and provide output voltage regulation against many fluctuations in these sources while keeping very low value of the THD for its output voltage.An approach using a mixed integer linear programming (MILP) optimization model is applied to determine the switching angles of the power switches of this CMLI to minimize the values of the undesired low order harmonics. A mathematical model is proposed, that can be applied for uniform step CMLIs, and this is applied for normal operation cases. Then a modification to this model is given that can be applied for some disturbance cases in the dc sources These models are applied for the single phase and three phase cases. For each case, an output voltage amplitude is selected first for normal operation at the nominal values of the dc sources, whose harmonics absolute values agree with the IEEE Standards 519-1992 for voltage distortion limits. Then the following disturbance cases are analyzed:1.Fluctuations of all the dc sources within ±20% of their nominal values.2.Dropping some dc sources below 80% of their nominal values.The models are applied under the required value of the output voltage with an allowed deviation within ±5%. Solutions are obtained that give the switching angles of the inverter at these disturbances in the dc sources while keeping low values of the output voltage THD, and still satisfying the IEEE standards for voltage distortion limits up to 59kV for single phase case and up to 161kV for three phase case.