• Energy Research

The multilevel inverters are becoming increasingly popular for use in the grid integration of wind and photovoltaic (PV) power plants due to their higher voltage handling capability and the better output power quality. There are several types of multilevel inverters that have been proposed in the literature; among them, the active neutral point clamp (ANPC) multilevel inverters have been drawing significant attention especially for solving the problems with other multilevel inverters. However, with the increase in the number of levels, the ANPC requires more electronic switches and flying capacitors, by which the complexity and the cost increase. In this article, an ANPC inverter with a reduced number of switches and flying capacitors is presented for the grid integration of the solar PV systems controlled using the model predictive control technique. This model predictive control technique uses a discrete-time model of the system to predict the future value of the active power and the reactive power for seven identified voltage vectors and selects the vector for the operation, which causes minimum cost function. The proposed power converter topology also effectively utilizes the dc bus voltage more when compared with the traditional ANPC converter. In the proposed inverter, a high-frequency transformer is used to eliminate the voltage balancing problems faced by the traditional ANPC inverters. The proposed magnetic linked power converter provides galvanic isolation, which is one of the most critical issues for traditional transformerless grid-connected PV systems. The proposed topology makes the control strategy simple and makes the power conversion system reliable for the PV power plants. The model predictive control-based power converter topology is simulated in MATLAB/Simulink environment and also validated in the laboratory test platform.

The high-frequency common magnetic-link made of amorphous material, as a replacement for common dc-link, has been gaining considerable interest for the development of solar photovoltaic medium-voltage converters. Even though the common magnetic-link can almost maintain identical voltages at the secondary terminals, the power conversion system loses its modularity. Moreover, the development of high-capacity high-frequency inverter and power limit of the common magnetic-link due to leakage inductance are the main challenging issues. In this regard, a new concept of identical modular magnetic-links is proposed for high-power transmission and isolation between the low and the high voltage sides. Third harmonic injected sixty degree bus clamping pulse width modulation and third harmonic injected thirty degree bus clamping pulse width modulation techniques are proposed which show better frequency spectra as well as reduced switching loss. In this paper, precise loss estimation method is used to calculate switching and conduction losses of a modular multilevel cascaded converter. To ensure the feasibility of the new concepts, a reduced size of 5 kVA rating, three-phase, five-level, 1.2 kV converter is designed with two 2.5 kVA identical high-frequency magnetic-links using Metglas magnetic alloy-based cores.