Three-level Neutral Point Clamped Rectifier Research Articles

Three-Phase Three-Level Neutral Point Clamped Rectifier with Predictive Control Method without Employing Weighting Factor

A predictive control method using injected offset voltage to achieve neutral point (NP) voltage balance of three-phase three-level neutral point clamped (NP) rectifiers, without employing a weighting factor, is proposed in this study. One of the biggest problems with the three-level NP rectifiers is the dc link capacitor voltage imbalance. Therefore, it is necessary to maintain the balance of the NP voltage in addition to synthesize the three-phase sinusoidal input current by control methods. Conventional predictive control methods for the NP rectifiers have used a weighting factor in a cost function that determines the control ratio of the input currents and the capacitor voltage balance. As a result, it is burdensome to empirically redesign the weighting factor when the rectifiers’ parameter values and control conditions change. Unlike the conventional methods, the proposed approach without the weighting factor can significantly eliminate differences between two DC capacitor voltages by utilizing an offset voltage, which is generated by using the difference between the upper and lower capacitor voltages. Consequently, the proposed approach using the offset voltage injection can control the input currents and retain the balance of NP voltage. Simulation and experiments are presented to verify the correctness of the NP voltage balancing of the proposed control method.

Vector Shifted Model Predictive Power Control of Three-Level Neutral-Point-Clamped Rectifiers

Model predictive power control (MPPC) has been emerging as one of the most promising control schemes for three-level neutral-point-clamped (NPC) rectifiers. However, conventional MPPC (C-MPPC), which only selects one switching state during the entire sampling period, leads to high active and reactive power ripples. Moreover, the heavy computational burden and variable switching frequency limit the applications of MPPC. In this article, vector shifted MPPC (VS-MPPC) methods are investigated. With the shifted vectors, the constant-switching-frequency MPPC of three-level NPC rectifiers can be simplified as that of a two-level rectifier, and the balanced neutral capacitor voltage can be easily achieved by adjusting the duty cycle of the redundant switches without any weighting factor employed. Only eight voltage vectors are calculated and shifted based on the small hexagon selection. Consequently, the computational burden is significantly reduced, even 35% less than that of C-MPPC. Furthermore, the proposed VS-MPPC presents a constant switching frequency and better steady-state control performance without evaluating all the switching states. Simulation and experimental evaluations of the proposed VS-MPPC methods with C-MPPC have been conducted to validate the superiority of the proposed VS-MPPC methods.

Integrated Control and Modulation for Three-Level NPC Rectifiers

This paper uses a novel approach for the control of three-level neutral-point-clamped (NPC) rectifiers in order to tackle the capacitor voltage balance problem. A distinctive feature of the new control approach is that it is based on a model which is written in terms of the duty ratios for each phase at each level. Hence, the system model presents nine duty cycle variables. Despite the fact that this formulation is different from the usual ones, it is shown that the control problem of currents and dc-link voltage can be formulated in a similar way to conventional methods. Furthermore, the control of the capacitor voltage balance can be expressed by means of equations that are decoupled from the currents and dc-link voltage dynamics, which results in a specific controller for the voltage balancing that does not affect the previous dynamics. A key point of the proposed approach is that part of the modulation stage is implicit in the formulation. Two particular controllers are compared in this paper. The first one fulfills the different control objectives at the expense of a large number of commutations. This problem is overcome in a new proposed controller, which presents similar performance and a satisfactory number of commutations. Experimental results are performed showing the effectiveness compared with a modified virtual space vector modulation with capacitor voltage balance capabilities.

Three-level NPC rectifier-based midpoint converter-fed SRM drive

This paper deals with a three-level Neutral Point Clamped (NPC) rectifier-based midpoint converter-fed Switched Reluctance Motor (SRM) drive. The three-level NPC rectifier for midpoint converter-fed SRM drive is used for improving power quality at AC mains and to balance the DC link capacitor voltages. The proposed three-level NPC rectifier is designed, modelled and its performance simulated using MATLAB/Simulink environment. The performance of proposed three-level NPC rectifier-based midpoint converter-fed SRM drive is compared with a six-pulse bridge converter-fed SRM drive to demonstrate its improved power quality.

A Comparative Study on Control Algorithm for Active Front-End Rectifier of Large Motor Drives Under Unbalanced Input

This paper investigates unbalance compensating control algorithms for the active front-end rectifier employed in high-power motor drive systems. The generic voltage-source-converter topology of the three-level neutral-point clamped rectifier is selected for the comparative study. Three different control algorithms have been formulated based on the symmetrical components and dq synchronous frames under generalized unbalanced operating conditions. Control algorithms have been investigated with respect to major design factors in large drive systems: fault ride-through capability, harmonic distortions, and efficiency. The control algorithm having zero amplitude of negative-sequence input current shows the most optimized and cost-effective performance regarding the fault ride-through capability and efficiency aspects. The control algorithm nullifying the oscillating components of the instantaneous active power generates the least harmonic distortions. A combination of these two control algorithms depending on system requirements and operating modes provides improved performance factors of fault ride-through capability, low harmonic distortions, and efficiency under generalized unbalanced operating conditions.