NPC Inverter Research Articles

Parallel Open-Circuit Fault Diagnosis Method of a Cascaded Full-Bridge NPC Inverter With Model Predictive Control

In this article, a parallel open-circuit fault diagnosis method (POFDM) is proposed for cascaded full-bridge neutral-point clamped inverters (CFNPCIs) employing the model predictive control. It can be inferred that the open-circuit fault will cause abnormal capacitor voltage changes in the submodule. Therefore, eight count variables are configured to correspond to the eight Insulated gate bipolar transistors (IGBTs) for each submodule. In the POFDM, the eight variables count all the abnormal changes, and the variables corresponding to the healthy switches are cleared to 0. Then, the switch whose corresponding variable is first added to the maximum is diagnosed as the fault device. Because the independent and parallel structure is adopted, the proposed POFDM can diagnose not only a single fault but also multiple faults in different submodules. And it can be easily extended to CFNPCIs with arbitrary submodules due to its parallel structure and modular software. Finally, the feasibility and validity of the POFDM are verified on the experimental prototype.

Mitigation of common mode voltage in five phase multilevel inverter

This paper exhibits the new modified sine wave pulse width modulation technique in a multilevel inverter to a decrement in the common mode voltage. It focuses on different schemes for decrease of CMV to reduce the circulating bearing current in poly phase induction motor drive. There are two different topologies considered for analysing the value of circulating bearing current in motor drives such as Neutral Point Clamped Inverter (NPC) topology and Cascaded inverter topology. The identified better topology to reduce circulating bearing current is cascaded topology. The simulation model of the proposed five phase five level inverter circuits are developed under the MATLAB-SIMULINK. The use of five phases in this technique is to enhance the productivity of machine and to reduce the losses in the machine unlike the three- phase machine which accounts for more losses compared to five phases. Simulation results are contrasted to those of theoretical calculations to examine the common mode voltage. Total Harmonic Distortion (THD) comparison is also done under Fast Fourier Transform (FFT) analysis between NPC inverter topology and cascaded inverter topology.

Fault Analysis and Reduction of Harmonics of Three-Level Three Wire and Four-Wire Neutral Point Clamped Inverter using PWM Switching Technique

Energy generation through wind and solar is increasing with the rising interest in the renewable and clean form of energy. Due to energy crises, the world is shifting from nonrenewable energy resources to renewable energy resources. Renewable energy resources are the green and eco-friendly sources and do not affect the climate like the other fossil fuels. Renewable fuels gain popularity due to their sustainability and low carbon emission. To get energy from renewable energy sources, well-defined mechanisms and control devices must be needed. Therefore, the reliability and stability of the system are required. The inverter is one of the tools which have a significant contribution to controlling the operation of the energy conversion system. The inverter is an accessory that changes a Direct Current (DC) source into an Alternative (AC) source. By using that AC source, we can easily derive AC appliances and drive the loads. To gain more power or to derive large power motors whose power ratings are up to several kilo/Megawatts, we need multi-level inverters. One of the types of multi-level inverters is the Neutral Point Clamped inverter (NPC). NPC inverters are used where considerable power is required, such as of several megawatts. The main contribution of this research paper is to design the three levels of neutral point clamped inverters by using the Pulse width Modulation (PWM) and Space Vector Modulation (SVM) Techniques.

Research of a Flexible Space-Vector-Based Hybrid PWM Transition Algorithm between SHEPWM and SHMPWM for Three-Level NPC Inverters

In this paper, one model of flexible space-vector-based hybrid pulse width modulation (HPWM) transition algorithm consisting of selective harmonic elimination pulse width modulation (SHEPWM) and selective harmonic mitigation pulse width modulation (SHMPWM) is applied and examined in a 10kV grid with a three-level neutral point clamped (3L-NPC) grid-connected inverter. These two modulation techniques are used to produce the appropriate firing pulses for 3L-NPC grid-connected inverters in different cases. SHMPWM is adopted to the grid-connected inverters to mitigate the required odd non-triplen harmonics according to the requirements of grid codes EN 50160 and CIGRE WG 36-05, while the firing pulses generated using SHEPWM is used to eliminate the primary low-order odd non-triplen harmonics completely. Meanwhile, one smooth and fast transition scheme is proposed by providing a suitable switching angles set at the transition point. Finally, it is demonstrated and validated by the MATLAB/SIMULINK model that smooth and quick transition is realized and there is no sudden change of current during the transition, as expected. Furthermore, this hybrid PWM technique is universal for different PWM methods based on the specific operating conditions.

Dual Input Three Stage NPC Inverter Fed PV – Grid System for Unbalanced AC Micro Grid

Dual Input Three Stage NPC Inverter Fed PV – Grid System for Unbalanced AC Micro Grid

Neutral-Point Voltage Balancing Method for Five-Level NPC Inverters Based on Carrier-Overlapped PWM

Neutral-point voltage imbalance is one of the main problems of five-level neutral-point clamped inverters that impedes their industrial application. In order to solve this problem, a closed-loop neutral-point voltage balancing method based on carrier-overlapped pulsewidth modulation (COPWM) is proposed in this article. The four dc-link capacitor voltages are balanced by three steps: voltage balancing between the two outer capacitors, voltage balancing between the two inner capacitors, and voltage balancing between the two inner and the two outer capacitors. The relationship between the neutral-point currents and the reference voltage is studied, and the top and the bottom dc-link capacitor voltages are balanced by zero-sequence voltage injection. The two inner capacitor voltages are balanced by adjusting the width of switching signals. Simulation and experimental results are presented to confirm the validity of this method.

Comparative Evaluation of Lifetime of Three-Level Inverters in Grid-Connected Photovoltaic Systems

The cost of the PV energy reduction is still required to increase the penetration level of PV systems in the energy market. The reliability of PV inverters is one of the important aspects to be enhanced in order to reduce the cost of PV energy, since it is closely related to the maintenance cost and the annual energy production. In this paper, the lifetime of NPC and T-type inverters, which are three-level inverter topologies that are widely used for PV systems, are comparatively evaluated with a 30 kW grid-connected PV system. It is performed by focusing on power devices since the power electronic components of both converters are the same except for the power devices. Therefore, this result can represent the comparison of the reliability performance of the NPC and T-type inverters. The power loss and temperature distributions of power devices are analyzed and their efficiencies are compared at different power levels with different switching frequencies. The lifetimes of the reliability-critical power devices in the NPC and T-type inverters are estimated, respectively with a one-year mission profile of the PV system, and the results are compared.

Robust Multilayer Model Predictive Control for a Cascaded Full-Bridge NPC Class-D Amplifier With Low Complexity

Model predictive control (MPC) has the advantages of good dynamic performance and the ability to handle multiple control objectives and constraints. However, it suffers from large amount of calculation and high dependence on system model when it is applied to cascaded inverters. In this article, to solve this problem, a robust multilayer MPC (RM-MPC) with strong robustness and less computation is proposed for Class-D amplifier using cascaded full-bridge NPC inverter. A Kalman filter-based disturbance observer is designed to estimate the lumped disturbance caused by the mismatches of the load parameters, which allows to implement robust control by compensating the disturbance to the predictive model during each control period. In the upper layer of the multilayer MPC, the control objectives that are linear with the output level are handled centrally, which allows the optimal level to be obtained directly without repetitive predictions and evaluations. The middle layer is used to allocate the optimal level to each submodule. And the lower layer is used to determine the switching state of each submodule. Finally, the feasibility and validity of the RM-MPC are verified on the designed experimental prototype.

New model of electric traction drive based sliding mode controller in field-oriented control of induction motor fed by multilevel inverter

This paper presents a new model of electric traction drive for electric vehicle. The sliding mode speed controller applied in the Indirect Rotor Field Oriented Control is used to control the induction motor fed by the five levels Neutral Point Clamped inverter (NPC). The simulation results showed the high performances dynamics and high robustness of the proposed model, that are reflected by faster startup and good speed tracking performances in terms of response time, oscillations and disturbance rejection. Also, a comparative study between different inverter topologies: two levels inverter, three levels NPC inverter and five levels NPC inverter has been carried out. From the spectral analysis, the five-levels NPC inverter fed drive provides better dynamics of voltage and current with reduced total harmonic distortion (THD).

PWM control techniques for three phase three level inverter drives

In this paper two very efficient pulse width modulation techniques were discussed named Sin pulse width modulation and space vector pulse width modulation. The basic structure of the three-level inverter neutral-point clamped is introduced and the basic idea about space vector pulse width modulation for three-level voltage source inverter has been discussed in detail. Nearest three vectors space vector pulse width modulation control algorithm is adopted as the control strategy for the three phase three level NPC inverter in order to compensate the neutral-point shifting. Mathematical formulation for calculating switching sequence has determined. Comparative analysis proving superiority of the space vector pulse width modulation technique over the conventional pulse width modulation, and the results of the simulations of inverter confirm the feasibility and advantage of the space vector pulse width modulation strategy over sin pulse width modulation in terms of good utilization of dc-bus voltage, low current ripple and reduced switching frequency. Space vector pulse width modulation provides advantages better fundamental output voltage and useful in improving harmonic performance and reducing total harmonic distortion.

Evaluation of Different Si/SiC Hybrid Three-Level Active NPC Inverters for High Power Density

Compared with full-SiC mosfets converters, the hybrid utilization of Si/SiC converters is an alternative approach to achieve the tradeoff between performance and cost. A systematic method is proposed to generate 2-SiC and 4-SiC hybrid three-level active neutral-point-clamped (3L-ANPC) inverter topologies from two types of switching cells based on the arrangement of freewheeling paths. The 2-SiC hybrid 3L-APNC inverter and the 4-SiC hybrid 3L-ANPC inverter are analyzed in detail with switching states and modulation strategies given. The power losses are quantitatively compared between the 2-SiC hybrid 3L-ANPC inverter and the 4-SiC hybrid 3L-ANPC inverter. The junction operating temperatures of active switches in different hybrid 3L-ANPC inverters are also estimated. With the same specifications and switching device parameters, the maximum output power of the 4-SiC hybrid 3L-ANPC inverter with modulation strategy I is almost 1.47 times than that of the 2-SiC hybrid 3L-ANPC inverter. A universal prototype for the full-SiC scheme, the full-IGBT scheme, and different Si/SiC hybrid schemes is built to evaluate these topologies at conversion efficiency and thermal characteristics. Experimental results and analysis show that the full-SiC 3L-ANPC inverter has the highest efficiency, whereas the 2-SiC hybrid 3L-ANPC inverter has the best cost performance. Further, the 4-SiC hybrid 3L-ANPC inverter with modulation strategy I has better thermal balance characteristic than that of the 2-SiC hybrid 3L-ANPC inverter, which shows significant superiority in high power density applications.

Direct power control with neutral-point voltage balance of grid-connected three-level NPC inverter

Multi-level power converters are nowadays the most appropriate solution for medium-voltage grid-connected renewable energy sources (RES) such as variable speed wind-turbines and photovoltaic (PV) generators. This paper presents a three-level direct power control (DPC) scheme for a grid-connected NPC inverter with neutral point voltage (NPV) balance. The developed DPC scheme provides the active (P) and reactive (Q) powers control and commonly it preserves the voltages of the two DC-link capacitors at one half of the full DC bus voltage. At each sampling period, the adequate space vector is selected from a predefined switching table designed regarding the effect of the inverter's space vectors on both, P and Q powers, and their ability to achieve the NPV balance. Numerical simulations are carried out, and confirm the effectiveness of the proposed DPC approach in controlling the P and Q powers and minimising the fluctuations of the DC link capacitors voltages.

Direct power control with neutral-point voltage balance of grid-connected three-level NPC inverter

Multi-level power converters are nowadays the most appropriate solution for medium-voltage grid-connected renewable energy sources (RES) such as variable speed wind-turbines and photovoltaic (PV) generators. This paper presents a three-level direct power control (DPC) scheme for a grid-connected NPC inverter with neutral point voltage (NPV) balance. The developed DPC scheme provides the active (P) and reactive (Q) powers control and commonly it preserves the voltages of the two DC-link capacitors at one half of the full DC bus voltage. At each sampling period, the adequate space vector is selected from a predefined switching table designed regarding the effect of the inverter's space vectors on both, P and Q powers, and their ability to achieve the NPV balance. Numerical simulations are carried out, and confirm the effectiveness of the proposed DPC approach in controlling the P and Q powers and minimising the fluctuations of the DC link capacitors voltages.

An Optimal Model Predictive Control Method for Five-Level Active NPC Inverter

Finite Control Set Model Predictive Control (FCS-MPC) can improve the control performance of Five-level Active NPC (5L-ANPC) inverter effectively. However, with the increasing of the levels of inverter, the traditional FCS-MPC method has many problems, such as large amount of rolling optimization calculation and difficulty in weight coefficient design. In order to solve those problems, an Optimal Model Predictive Control (O-MPC) method is proposed. First, an optimal control method based on level jump limitation is proposed to reduce the number of switch states. Second, in order to solve the conflict problem of multiple control objectives and reduce the design difficulty of weight factors, a satisfactory optimal control method based on hierarchy is proposed. After the optimization, the calculation time of the controller is greatly reduced and the control performance of the multi-objective control system will be significantly improved. At last, the effectiveness and feasibility of the proposed O-MPC are validated by the experimental results.

Photovoltaic grid-connected with transformer less renewable source system with multilevel NPC inverter

Photovoltaic grid-connected with transformer less renewable source system with multilevel NPC inverter

Photovoltaic grid-connected with transformer less renewable source system with multilevel NPC inverter

Photovoltaic grid-connected with transformer less renewable source system with multilevel NPC inverter

Study on Effect of Installation Location on Lifetime of PV Inverter and DC-to-AC Ratio

The reliability improvement of a PV inverter is one of the important aspects to decrease the cost of PV energy. Furthermore, oversizing the PV arrays is a commonly applied strategy to achieve the cost of PV energy reduction but it may also have a negative impact on the cost of PV energy since it accelerates the wear-out of the PV inverter. Therefore, the lifetime of the PV inverter has to be considered to find a safe limit for the oversizing of the PV arrays but there is a lack of study on this effect. This paper studies the effect of the installation location on the lifetime of the PV inverter together with DC/AC ratio. The lifetime evaluation of NPC inverter is carried out with three installation locations inclusive of the effect of the cooling capacity. Furthermore, the impact of DC/AC ratio on the lifetime of the PV inverter is investigated by taking into account the influence of the installation location to show the importance of the lifetime of the PV inverter to select the optimum DC/AC ratio to minimize the cost of PV energy.

Low-Speed Performance Improvement of Direct Torque Control for Induction Motor Drives Fed by Three-Level NPC Inverter

Classical direct torque control (DTC) is considered one of the simplest and fastest control algorithms in motor drives. However, it produces high torque and flux ripples due to the implementation of the three-level hysteresis torque regulator (HTR). Although, increasing the level of HTR and utilizing multilevel inverters has a great contribution in torque and flux ripples reduction, stator flux magnitude of induction motor (IM) droops at every switching sector transition, particularly at low-speed operation. This problem occurs due to the utilization of a zero voltage vector, where the domination of stator resistance is very high. A simple flux regulation strategy (SFRS) is applied for low-speed operation for DTC of IM. The proposed DTC-SFRS modifies the output status of the five-level HTR depending on the flux error, torque error, and stator flux position. This method regulates the stator flux for both forward and reverse rotational directions of IM with retaining the basic structure of classical DTC. By using the proposed algorithm, the stator flux is regulated, hence pure sinusoidal current waveform is achieved, which results in lower total harmonics distortion (THD). The effectiveness of the proposed DTC-SFRS is verified through simulation and experimental results.

A novel region selection approach of SVPWM for a three-level NPC inverter used in electric vehicle

This paper confers an investigation of a space vector modulation based control strategy of induction motor using a three-level inverter for electric vehicle application. The proposed controller uses a simple v/f control with a novel SVPWM technique. A new method for region selection of SVPWM for a multilevel inverter is implemented for the closed-loop system. Previously the region selection procedure was not considered by most of the researcher in the literature for developing the PWM algorithm. This approach is based on some algebraic equations. The remarkable point here is that it is identical to all the remaining sectors. Hence calculation complexity reduces by making it a simpler implementation. This method can be applied to any number of levels. The potency of the proposed controller is validated through the MATLAB/SIMULINK environment. The performance of the overall system is inspected through transient and steady-state analysis. The neutral point balance of the 3L NPC inverter is established by adopting proper switching sequences.

Topology and Modulation Scheme for Three-Phase Three-Level Modified Z-Source Neutral-Point-Clamped Inverter

This paper presents the topology and modulation technique of a three-phase three-level modified Z-source neutral-point-clamped (MZS-NPC) inverter, which combines a modified Z-source impedance network and a three-phase three-level NPC inverter. The boost factor of the proposed MZS-NPC inverter is twice as high as the three existing representative topologies combing an impedance network with a three-level NPC inverter. A modulation scheme for the proposed topology, based on a maximum boost control method, is designed to achieve the maximum voltage gain with simple implementation and to balance the dc-link neutral-point voltage. A closed-loop control of the ac load voltage in the fuel-cell or photovoltaic applications based on the proposed inverter is realized, in order to supply a desired voltage to the critical load in islanding mode of a microgrid. The boosting ability and operation validity of the proposed topology and modulation technique are demonstrated with simulation and experimental results.