Experimental Validation of a Hybrid PI–SO-SMC Approach for Zero-Sequence Circulating Current Suppression in Parallel Inverters

This paper presents an experimental investigation of a dual-inverter induction motor (IM) drive system for zero-sequence circulating current (ZSCC) suppression using a novel hybrid PI–SO-SMC strategy. The proposed method combines a proportional–integral (PI) regulator and a second-order sliding mode controller (SO-SMC) through a frequency-separation mechanism, exploiting their complementary steady-state and dynamic characteristics. By regulating the zero-vector dwell times and compensating for voltage mismatches between the parallel inverters, the proposed strategy directly addresses major sources of ZSCC. The complete drive system was implemented using real power converters, digital control hardware, and measurement instruments and experimentally evaluated under balanced operation and practical mismatch conditions, including a 0.4 μs dead-time difference and switching-frequency mismatch between 7 kHz and 10 kHz. The proposed controller was integrated with indirect field-oriented control (IFOC) and space vector modulation (SVM) to maintain high-performance motor operation. Simulation and experimental results demonstrate that the proposed strategy provides superior current quality and ZSCC suppression compared with conventional PI and standalone SO-SMC controllers. The total harmonic distortion (THD) of the machine current remained below 2.6% in the simulation and below 2.8% under experimental conditions throughout the investigated operating regimes, while the THD of the converter current was reduced by over 53% and 55% in the simulation and experimentally, respectively, under dead-time imbalance conditions, compared to the PI regulator. The proposed strategy also effectively removed the DC component of the circulating current and attenuated its oscillatory components under the considered mismatch conditions. These findings confirm the robustness and practical feasibility of the proposed hybrid control approach for improving current quality and mitigating circulating-current-related stresses in high-power parallel-inverter induction motor drives.