Adaptive Neural Network-Based Virtual Flux Estimation for Sensorless Deadbeat Control of Grid-Connected Modular Multilevel Converters

This paper presents a grid-voltage-sensorless control scheme for a grid-connected modular multilevel converter based on an adaptive-linear-neuron quadrature signal generator for virtual-flux estimation (VF-ADALINE-QSG). The proposed estimator identifies the DC offset component and extracts the in-phase and quadrature fundamental components online without requiring offline training or specific initialization of the adaptive weights. By reconstructing VF exclusively from the identified fundamental components, the proposed method limits drift induced by DC offset and attenuates higher-order harmonics while avoiding the additional dynamic delays associated with conventional filtering-based estimators. The VF-ADALINE-QSG is integrated into a deadbeat predictive control framework incorporating grid-current regulation, circulating current suppression, and submodule-capacitor-voltage balancing. Its performance is compared to that of low-pass-filter (LPF) and second-order generalized-integrator (SOGI)-based VF estimators through real-time simulations on an OPAL-RT OP5700 platform. The evaluation considers startup, active-power-reference variations, a 20% DC offset, voltage distortion comprising 12.5% fifth- and 7.5% seventh-order harmonics, and the simultaneous occurrence of these disturbances. The proposed estimator achieves a settling time approximately one-sixteenth of that obtained with the LPF-based estimator. Under DC offset conditions, the estimated VF total harmonic distortion is limited to 0.26%, compared with 7.38% and 0.84% for the LPF- and SOGI-based estimators, respectively. The results demonstrate a faster transient response, improved estimation accuracy, and enhanced robustness against both DC offsets and harmonic disturbances.