Nested Neutral Point Clamped Converter Research Articles

A low-frequency fluctuation suppressing method for a seven-level hybrid T-type nested neutral point clamped converter

This paper proposed a low-frequency fluctuation suppressing method for a seven-level hybrid T-type nested neutral point clamped (7L-HTNNPC) converter that can be applied in medium-voltage and high-power powertrain systems. The 7L-HTNNPC converter is a combination of a 3L-ANPC unit and a four-level T-type neutral point clamped (4L-TNNPC) unit. The 3L-ANPC unit is modulated to switch at a low fundamental frequency while the remained 4L-TNNPC unit is operated at a high switching frequency. Compared to the existing modulation method, the proposed method has the advantage of resuppressing flying capacitor voltage fluctuations and reducing current harmonics with a low flying capacitance. Simulation results have been presented to verify the proposed low-frequency fluctuation suppressing method for the 7L-HTNNPC converter.

A low-frequency fluctuation suppressing method for a seven-level hybrid T-type nested neutral point clamped converter

A low-frequency fluctuation suppressing method for a seven-level hybrid T-type nested neutral point clamped converter

An Optimized Capacitor Voltage Balancing Control for a Five-Level Nested Neutral Point Clamped Converter

This article presents a simple yet effective capacitor voltage balancing method for a five-level nested neutral point clamped (NNPC) converter. The NNPC has been recently developed for medium-voltage high-power applications and has interesting features. This hybrid topology has three flying capacitors in each leg. Like other flying capacitor-based topologies, it is necessary to regulate and balance the voltage of the flying capacitors in an NNPC converter to certain values. In this article, a new approach is proposed to accomplish this task. The proposed regulation technique employs line current direction and voltage deviation of flying capacitors to achieve regulation. The charging/discharging status of flying capacitors with respect to different switching states is used to define a priority index. This index is eventually used to select the proper switching state. A detailed investigation is presented to determine the best switching state for each level of the output voltage. The proposed method does not need any cost function and is very intuitive and simple to implement. Using simulations, the amplitude of the flying capacitor voltage ripple is measured, which shows reduction under all operating conditions. The feasibility and performance of the proposed method are confirmed by using a laboratory-type experimental setup.

Techno-economic evaluation of five-level nested neutral point clamped converter topology for transformer-less connection of high-power wind energy conversion systems

Developers and operators are interested in improving the reliability and reducing the associated costs of wind power plants (WPPs) because of the continuous increase in the power capacity of wind energy conversion systems (WECSs) and the increasing development of WPPs. The electrical subsystem of the WPP experiences the highest failure rate and constitutes a significant proportion of its total cost. Reliability of the WECS can be increased and its cost reduced by eliminating the wind turbine transformer from the electrical subsystem. This study gives a techno-economic evaluation of a five-level nested neutral point clamped (NNPC) converter topology for transformer-less connection of high- power WECSs. The approach entailed the calculation of reliability of five-level NNPC converter topology deployed in the grid-side of a WECSs. This method presents a mathematical formula for deriving the reliability of a five-level NNPC converter topology by using the reliability block diagram and reliability estimation-based models in the military handbook (MIL-HDBK-217F). The cost analysis model shows that the total cost of the five-level diode clamped converter topology was higher than the five-level NNPC converter topology. The study could be extended by carrying out accurate modelling of the mission profile of the presented converter by using multi-domain simulation technique.

A Multi-Layer Sequential Model Predictive Control of Three-Phase Two-Leg Seven-Level T-Type Nested Neutral Point Clamped Converter Without Weighting Factors

In this paper, a triple-layer sequential model predictive control (SMPC) approach without weighting factors is proposed for a three-phase two-leg 7-level T-type nested neutral point clamped (T-NNPC) converter. The main objectives of this paper are to eliminate the selection of weighting factors and enhance the reliability of system in the control process. In the proposed design, three cost functions are formulated in a cascaded way to avoid the weighting factors tuning while not compromising the computational complexity. Furthermore, the three-phase two-leg 7-level T-NNPC converter is investigated to improve the fault-tolerance ability in case of a faulty leg condition. Finally, the effectiveness of the control methodology for the three-phase two-leg 7-level T-NNPC converter is validated by comparative simulation studies.

An Optimal Voltage-Level Based Model Predictive Control Approach for Four-Level T-Type Nested Neutral Point Clamped Converter With Reduced Calculation Burden

Four-level T-type nested neutral point clamped (NNPC) converter is a newly developed multilevel voltage source converter for higher power density systems in low and medium voltage applications. The T-type NNPC converter can operate over a wide range of voltages and own less proportion of components compared with other four-level topologies, which makes it attractive among multilevel converters. In this paper, an optimal voltage-level based model predictive control (OVL-MPC) with reduced calculation burden is proposed for the T-type NNPC converter. The main objectives of this paper are to achieve the T-type NNPC converter load current control and the flying capacitors voltages balancing while remaining computationally feasible. In our proposed method, the output voltage levels are considered as the control options rather than the switching states in the conventional MPC scheme. Meanwhile, a simplified capacitor voltage balancing approach is employed to regulate the voltages of the capacitors at their desired values without punishing the computational complexity. The contribution of this paper lies in the following two aspects: first, the finite control set is decreased in the optimization processes. Second, a simplified capacitor voltage balancing strategy is introduced for the T-type NNPC converter. The performance of the proposed OVL-MPC approach for the T-type NNPC converter under steady-state, transient-operation, and unbalance mode are verified by the simulation results.

Carrier-Based Stair Edge PWM (SEPWM) for Capacitor Balancing in Multilevel Converters With Floating Capacitors

Multilevel converters with floating capacitors (FCs) are widely applied in recent years in a wide range of industrial applications from high-voltage direct current systems to high power drives. However, some FC topologies lack complete FC voltage balancing capability due to the inherent topology limitation or the insufficient switching state for capacitor balancing. This will result in large ripples on capacitors at low frequency (such as fundamental frequency), limiting their performance and application, particularly, when the fundamental frequency is very low in low-speed drive operations. Typical examples of those multilevel converters are modular multilevel converters (MMC), nested neutral-point-clamped (NNPC) converter, etc. In order to improve the FC balancing for this kind of multilevel converters, this paper proposes a carrier-based pulse-width modulation (PWM) method, named stair edge PWM method. This method can obtain sufficient redundant switching states for FC voltage balancing by producing multiple levels in one PWM period. The FC voltage balancing is thus achieved at switching frequency, which can greatly reduce the capacitor voltage ripple and therefore enable the use of much smaller FCs. Meanwhile, the voltage stress on each device and low $dv/dt$ feature are still the same as the normal multilevel operation, reserving two of the most salient features of multilevel converters. The proposed method also features much simpler and easier implementation compared with space-vector-based approaches, particularly when high-level converters are considered. An application example using a four-level NNPC converter is provided in this paper. The effectiveness and performance of the proposed method are verified by both simulation and experiment.

A Space Virtual-Vector Modulation With Voltage Balance Control for Nested Neutral-Point Clamped Converter Under Low Output Frequency Conditions

The recently-developed nested neutral-point clamped (NNPC) converter is a four-level converter for medium-voltage high-power applications. It has some merits such as operating over wide voltage ranges (2.4-7.2 kV) without switching devices in series and less diodes or capacitors when compared with classical four-level topologies. But it has severe voltage balancing problems under low output frequency conditions when conventional space vector modulation (SVM) or sinusoidal pulse width modulation (SPWM) is applied, which limits its wide applications. In order to solve this issue, a space virtual-vector modulation (SVVM) along with a voltage balance control (VBC) algorithm is proposed in this paper. It helps reduce the voltage ripples greatly and maintain them within an acceptable region under low output frequency conditions. Therefore, it guarantees even voltage stresses among switching devices and extends the NNPC converter's applications. Principles of the proposed SVVM and VBC algorithms are elaborated in detail, and the voltage balancing performance is investigated and compared with the cases where conventional SVM and SPWM with the same VBC are applied. The relationship curves between voltage ripples and flying capacitance, output power, and output frequency have been explored and compared for both conventional SVM and proposed SVVM. Simulation and experimental results are presented to verify the effectiveness of the proposed method.

Finite Control-Set Model Predictive Control (FCS-MPC) of Nested Neutral Point-Clamped (NNPC) Converter

This paper proposes a model predictive control (MPC) strategy for a nested neutral point-clamped (NNPC) converter to control output currents and voltages of flying capacitors. The NNPC converter is a four-level converter topology for medium-voltage applications with interesting properties such as operating over a wide range of voltages (2.4–7.2 KV) without the need for connecting power semiconductor in series, high quality output voltage, less number of components compared to other classical four-level topologies. A discrete-time model of the converter is presented and all the control objectives are formulated in terms of the switching states. During each sampling interval, the predicted variables are assessed by the cost function and the best switching state which gives minimum value for the cost function is selected and applied to the converter gating terminals. The performances of the NNPC converter and predictive control scheme are verified through MATLAB/Simulink simulations and their feasibility is evaluated experimentally.

A Novel and Simple Single-Phase Modulator for the Nested Neutral-Point Clamped (NNPC) Converter

The nested neutral-point clamped (NNPC) converter is a four-level converter topology for medium-voltage applications with interesting properties such as operating over a wide range of voltages (2.4–7.2 KV) without the need for connecting the power semiconductor in series, high quality output voltage, and less number of components in compare to other classical four-level topologies. The control and balance of the flying capacitors (FCs) of the NNPC converter can be done by different control techniques taking advantage of the large number of redundant switching states. This paper presents a simple single-phase modulator for the NNPC converter, which can be applied to each phase of a three-phase NNPC converter. The proposed simple technique can control and balance the FCs to their desired values. Performance of the proposed technique under different operating conditions is investigated in the MATLAB/Simulink environment. The feasibility of the proposed converter is evaluated experimentally.