Sliding Mode Derivative Estimator for Optimal Zero-Sequence Voltage Balancing of Grid-Tied NPC Converters

This article presents a neutral-point (NP) voltage regulation for neutral-point clamped (NPC) converters that is aimed at grid-connected applications. The proposed control strategy combines an optimal zero-sequence voltage $(v_{0})$ with a sliding-mode (SM) super-twisting estimator aimed to evaluate the neutral current $(i_{0})$ causing the unbalancing on the two capacitors connected to the NPC DC link. The estimator derives an accurate real-time $i_{0}$ from the measurable capacitor voltages and is then used to synthesize a bounded $v_{0}$ that keeps the midpoint balanced. The study considers a grid-tied three-level NPC with an LCL filter and sinusoidal PWM. However, the proposed method is also compatible with carrier-based and space-vector modulation. The complete controller preserves the classical current loop and requires no retuning of grid-tie damping. Simulation and hardware tests are executed under the same set-points and additional trials impose deliberate DC-link de-balancing to assess robustness. Experimental results show fast and well-damped recovery of the midpoint, tunable and symmetric ripple on DC NP, and no deterioration of phase-current tracking. The synthesized $v_{0}$ remains within explicit bounds with low chattering phenomena in the presence of measurement noise and switching ripple. Therefore, the proposed approach offers an accurate solution for reliable NP control in grid-connected NPC converters without compromising power-quality performance.