Variable Switching Frequency Control of AC/AC MMC for Low-Frequency Induction Heating Applications

In the production of special steel, the tundish induction heating power supply operates in a high-temperature, confined space with a high-dust environment. It is difficult to dissipate heat from the devices, resulting in increased losses in the internal components of the converter. This issue is particularly serious in low-frequency heating conditions, greatly increasing the risk of failure of special power supply semiconductor devices. In this article, an active thermal control strategy with variable switching frequency is proposed for the AC/AC MMC converter applied in the field of low-frequency induction heating. Firstly, the loss characteristics and thermal swing characteristics of AC/AC MMC induction heating power supply are analyzed in detail by establishing the thermal impedance model. Then, in order to analyze the harmonic characteristics of complex systems with multi-frequency coupling, the triple Fourier integral transform is proposed innovatively for the dual fundamental frequency structure. On this basis, the accurate mathematical expression of the grid current is derived. Furthermore, considering the THD of grid-connected current, efficiency and junction temperature, an optimized mathematical model for switching frequency is established and the Lagrange method of Karush-Kuhn-Tucker (KKT) condition is adopted to determine the optimal value. Finally, simulation and experiment results demonstrate the effectiveness of the proposed control strategy for reducing the junction temperature swing and system loss.