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Modular multilevel converters are increasingly gaining popularity in various high-voltage and high-power applications. However, their operation poses some technical challenges in implementing the control system, including issues, such as balancing submodule capacitor voltages and mitigating circulating currents. The presence of circulating currents leads to additional power losses, increases thermal stress on the power devices, and shortens their lifetime. This article introduces an innovative control technique to eliminate circulating currents by injecting a low-frequency alternating signal into a single submodule of each converter arm. The proposed method not only addresses circulating current mitigation but is also able to reduce capacitor voltage ripple and peak arm current, thereby enhancing the overall converter's lifespan and efficiency. Results of experimental tests on a seven-level converter are presented to prove the effectiveness of the proposed technique and compare its performance with existing state-of-the-art approaches.

Cascaded H-bridge (CHB) converters are good candidates for many industrial applications. Among the modulation techniques that can be applied to CHB converters, the phase-shifted pulse width modulation (PWM) method (PS-PWM) provides very good harmonic quality being the most popular modulation technique at the expense of presenting high switching losses. On the contrary block switching technique as well as selective harmonic elimination/mitigation techniques provides lower switching frequency and losses with a lower harmonic quality. Additionally, with these techniques the converter operation becomes stiff. In this work a modification of PS-PWM based on an extremely low frequency carrier is proposed with the objective of achieving both low switching losses and an acceptable harmonic quality, simultaneously. The main advantage of the proposed solution is the capability of adjusting the phase-shift angle among the carriers to compensate for harmonic grid voltage disturbance. Several experimental results have been reported to demonstrate the good performance of the proposed modulation technique.