Online Optimal Switching Frequency Selection for Grid-Connected Voltage Source Inverters

Abstract

Enhancing the performance of the voltage source inverters (VSIs) without changing the hardware structure has recently acquired an increased amount of interest. In this study, an optimization algorithm, enhancing the quality of the output power and the efficiency of three-phase grid connected VSIs is proposed. Towards that end, the proposed algorithm varies the switching frequency (f sw ) to maintain the best balance between switching losses of the insulated-gate-bipolar-transistor (IGBT) power module as well as the output power quality under all loading conditions, including the ambient temperature effect. Since there is a contradiction with these two measures in relation to the switching frequency, the theory of multi-objective optimization is employed. The proposed algorithm is executed on the platform of Altera ® DE2-115 field-programmable-gate-array (FPGA) in which the optimal value of the switching frequency is determined online without the need for heavy offline calculations and/or lookup tables. With adopting the proposed algorithm, there is an improvement in the VSI efficiency without degrading the output power quality. Therefore, the proposed algorithm enhances the lifetime of the IGBT power module because of reduced variations in the module's junction temperature. An experimental prototype is built, and experimental tests are conducted for the verification of the viability of the proposed algorithm.

On the Performance Optimization of Two-Level Three-Phase Grid-Feeding Voltage-Source Inverters

“…During any switching period, the inductor current is assumed linear, either in charging or discharging mode, with a constant fundamental component. For star-connected load, the current ripple per fundamental period can be expressed as in Equation (12). The TDD is expressed as in Equation (13).…”

“…Efficient optimization of these two objectives is necessary and must rely on an accurate mathematical representation of the powerelectronic system under study. Both objectives are mathematically linked together in this section, and the detailed derivations can be found in [12,[14][15][16][17][18][19][20]. If a sinusoidal current is flowing through a power-electronic module, PCQ and PCD can be formulated as in Equations (1) If a sinusoidal current is flowing through a power-electronic module, P CQ and P CD can be formulated as in Equations (1) and (2).…”

“…An online adaptive switching frequency (OASF) algorithm that considers the power quality of the output current and the efficiency of the VSI as a combined objective function is presented recently in [12]. Due to its simplicity and online capability, the variable f sw algorithm in [12] is adopted in this study.…”

“…The employed OASF depends on the weighting factor to achieve a trade-off between the power quality of the output current and the efficiency of the VSI. The selection of the weighting factor and the overall system optimization design have not been covered or discussed in [12]. A new degree of freedom is added to the employed OASF algorithm for the optimal selection of the weighting factor and the overall system optimization design.…”

“…Due to its simplicity and online capability, the variable f sw algorithm in [12] is adopted in this study. The employed OASF depends on the weighting factor to achieve a trade-off between the power quality of the output current and the efficiency of the VSI.…”