A Two-Loop Feedback Control Strategy for Neutral Point Voltage Balancing of Three-Level Converter and Vienna Rectifier With a Carrier-Based Implementation

Neutral point voltage unbalancing is a critical issue in neutral-point-clamped three-level converters (TLCs) and Vienna rectifiers (VRs), where the voltages of two series-connected capacitors across the DC link become unequal. This imbalance leads to line current distortion, increased device stress, and potential failure. The primary cause is the non-zero average neutral point current, which depends on the modulation strategy. An effective voltage balancing strategy must dynamically maintain half of the total DC-bus voltage across each capacitor while minimizing low-frequency voltage pulsations. The distribution of dwell time among space-vector's redundant switching states influences the neutral point current. Existing literature leverages this distribution factor for voltage balancing, but the associated plant model is nonlinear. The small-signal linearized model varies significantly across operating conditions, making compensator design challenging. Additionally, the conventional practice of maintaining the balancing loop bandwidth equal to that of the outer DC-bus control in the rectifier limits the effectiveness of this approach in mitigating triplen-harmonic voltage pulsations. This paper proposes an alternative control strategy that transforms the plant into a linear, exact, and large-signal valid system, simplifying compensator design. The balancing action is structured into a two-loop architecture: the DC offset correction operates at the same speed as the outer DC-bus control loop, while a faster loop mitigates third-harmonic voltage pulsations. Furthermore, a carrier-comparison-based implementation incorporating the proposed control is introduced. The effectiveness of the control strategy and its implementation is validated through simulations and experimental results on two hardware prototypes—one for TLC and another for VR.