Three-level Neutral Point Clamped Research Articles

Nonlinear control of three level NPC inverter used in PV/grid system: comparison of topologies and control methods

With the passage of time, the importance of using renewable energy systems to overcome energy consumption and improve the quality of the grid has emerged through the use of nonlinear control techniques and reliance on advanced types of inverters such as multi-level inverters. This research is focused on comparing two grid-connected converter topologies in a photovoltaic (PV) generation system connected to a three-phase grid that serves a non-linear load. Additionally, the study explores two different control techniques applied to this converter, evaluating their effects on the total harmonic distortion coefficient. A comparison has been made between the traditional inverter and the three-level inverter type neutral point clamped (NPC) inverter, with the use of integral backstepping (IBS) technique which was also compared with the proportional integral (PI) controller. The simulation results in MATLAB/Simulink are presented illustrating the performances and the strong effectiveness of the three-level NPC inverter controlled by the proposed technique (IBS).

A Three-Level Neutral-Point-Clamped Converter Based Standalone Wind Energy Conversion System Controlled with a New Simplified Line-to-Line Space Vector Modulation

Wind power systems, which are currently being constructed for the electricity worldwide market, are mostly based on Doubly Fed Induction Generators (DFIGs). To control such systems, multilevel converters are increasingly preferred due to the well-known benefits they provide. This paper deals with the control of a standalone DFIG-based Wind Energy Conversion System (WECS) by using a three-level Neutral-Point-Clamped (NPC) converter. The frequency and magnitude of the stator output voltage of the DFIG are controlled and fixed at nominal values despite the variable rotor speed, ensuring a continuous AC supply for three-phase loads. This task is achieved by controlling the DFIG rotor currents via a PI controller combined with a new Simplified Direct Space Vector Modulation strategy (SDSVM), which is applied to the three-level NPC converter. This strategy is based on the use of a line-to-line three-level converter space vector diagram without using Park transformation and then simplifying it to that of a two-level converter. The performance of the proposed SDSVM technique in terms of controlling the three-level NPC-converter-based standalone WECS is demonstrated through simulation results. The whole WECS control and the SDSVM strategy are implemented on a dSPACE DS 1104 board that drives a DFIG-based wind system test bench. The obtained experimental results confirm the validity and performance in terms of control.

Design and control of utility grid-tied bipolar DC microgrid

Abstract This paper explains in detail the design and control of a utility grid-connected bipolar DC microgrid, which consists of a solar photovoltaic system (SPV), a wind energy conversion system (WECS), a battery energy storage system (BESS) at the DC bus, and a three-level neutral point clamped (NPC) converter to connect the DC microgrid to the utility grid. A three-level bidirectional DC–DC converter at the battery energy storage system (BESS) is controlled with model predictive current control (MPCC), and a three-level NPC converter is controlled with a voltage-oriented control based proportional resonant (VOC-PR) controller. The proposed MPCC scheme for the three-level bidirectional DC–DC converter and the VOC-PR control scheme for the three-level NPC converter coordinate with each other to balance and regulate the voltage at the bipolar DC microgrid, control the power flow to the loads at the DC microgrid and the load at the point of common connection (PCC), and also inject or draw power from the utility grid and BESS based on the availability of Renewable Energy Sources (RES). Simulation and experimental results are presented to demonstrate the effectiveness of the proposed MPCC control scheme.

Design of 3-Level T-type NPC Inverter for EV Applications

In recent years, there has been a significant demand for lightweight converters that provide exceptional control performance while generating minimal acoustic noise. This has resulted in higher switching signals for hard- switched 2-level low voltage 3-phase inverters. However, the increased switching frequencies have a negative impact on the inverters’ efficiency, requiring the use of expensive switch technology to maintain high efficiency. This paper aims to tackle these challenges by introducing the design and implementation of a extremely effective 3-Level T-Type Neutral Point Clamped (NPC) inverter designed for Electric Vehicle (EV) applications. 3-Level T-Type NPC inverter associates the strengths of both NPC and T-Type inverters. Similar towards the NPC, it offers a lower breakdown voltage for the switching devices, thereby reducing switching losses. Additionally, it shares the characteristics of the T-Type inverter, resulting in fewer conduction losses. To validate the feasibility of the converter, simulation is performed using MATLAB software, followed by the development and testing of a lab-scale prototype. The proposed three-level T-type NPC inverter have several merits over traditional 2-level inverters, including reduced distortion, switching losses, and improved efficiency. These benefits contribute to enhanced motor drive performance, reduced electromagnetic interference (EMI), and extended battery life in EVs.

An improved model predictive control of back-to-back three-level NPC converters with virtual space vectors for high power PMSG-based wind energy conversion systems

In this study, an improved virtual space vector (VSV)-based two-stage model predictive control (MPC) scheme is presented for neutral point clamped (NPC) converters in high power-rated permanent magnet synchronous generator (PMSG)-based wind energy conversion system applications. The presented MPC scheme utilizes the VSVs and involves two-stage prediction for effectively reducing computation complexity. In stage-I prediction, a virtual space vector is identified based on the optimum cost function. Then, a set of sub-sector vectors are placed in stage II prediction based on the capacitor voltage levels. From this, the cost function predicts the optimum switching state. At the same time, the conditional selection of small voltage vectors in stage II prediction eliminates the neutral point voltage balancing constraint. Finally, the presented method is verified by the simulation of the back-to-back NPC converter of a high-power-rated PMSG-based wind energy conversion system. Further, the adaptability of the presented scheme for maximum power point tracking, reactive power control, and dc-link voltage control is also verified by simulation.

ZVS Design in Full-SiC Three-Level Neutral-Point-Clamped DC–DC Converter Considering Quasi-Linear Output Capacitance $C_{oss}$

The Three-level Neutral-point-clamped (TLNPC) dc-dc converter is a popular topology for applications with high input voltage due to its simplicity of modulation, capability of withstanding high input voltage, and Zero-voltage-switching (ZVS) operation. Regarding mitigating the voltage ringing on secondary side, energy recovery approach is preferred to RCD approach for the sake of high efficiency and small component counts. This paper demonstrates that, using recovery approach in a Silicon Carbide (SiC) based TLNPC dc-dc converter causes hard-switching of the outer switches due to the quasi-linear output capacitance <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_oss</i> of SiC MOSFETs. The ZVS design of a full-SiC TLNPC converter must not follow conventional methods for Silicon (Si) based converters. To address this issue, a modified design approach, which can guarantee ZVS of all switches in a full-SiC TLNPC dc-dc converter, is introduced and discussed. All of the analysis is verified on a 3 kW full-SiC TLNPC prototype, which has rated input voltage of 1.2 kV, output voltage of 600 V, and achieves ZVS in the output power range of 1.2 kW <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 3 kW.

Modulation Technique Design of An Improved Zero Common Mode Voltage (CMV)

Voltage imbalance of neutral-point and CMV are hot issues in the research of three-level inverters. This paper first introduces the three-level topology of the neutral-point clamped (NPC) inverter. Aiming to eliminate CMV of three-level NPC and balance the voltage of the neutral point simultaneously, a virtual space vector pulse modulation (VSVPWM) strategy is proposed. Virtual voltage vectors of this strategy are composed of 3 medium vectors. The voltage of the neutral point is balanced since the average current of the neutral point is 0. Therefore, the balance index is introduced to redefine virtual voltage vectors, so the voltage of the neutral point can recover quickly when the capacitors’ voltage of the dc-link is imbalanced.

Grid-Connected Solar PV System with Maximum Power Point Tracking and Battery Energy Storage Integrated with Sophisticated Three-Level NPC Inverter

In this research, a solar photovoltaic system with maximum power point tracking (MPPT) and battery storage is integrated into a grid-connected system using an improved three-level neutral-point-clamped (NPC) inverter. An NPC inverter with adjustable neutral-point clamping may achieve this result. To achieve this result, a modernized NPC inverter is used. Using the three-level vector modulation approach, the correct AC voltage may be generated when DC voltage conditions are present in an unbalanced situation involving an NPC inverter. A higher degree of precision is made possible by this. In-depth information on the many possible NPC inverter designs and modulation strategies was provided. By incorporating the necessary sophisticated algorithms into the NPC inverter, the necessary solar PV-MPPT functionality is made available, allowing for the regulation of power transfer among the solar PV system, the battery, and the grid. Modified INC method is proposed which has a tracking efficiency of 99.5% for varying irradiance. The use of sufficiently sophisticated algorithms makes this a reality. Using simulation with MATLAB/Simulink, we were able to examine an NPC inverter’s performance in several setups. Several amounts of solar irradiation were used to test the battery’s charging and discharging capabilities. The probe turned out to be fruitful. Laboratory testing ensures that the proposed method works by simulating real-world conditions.

Control method of DC component for NNBI accelerator power supply

In the high voltage power supply of the negative ion source neutral beam injection system (NNBI) accelerator, the three-phase three-level neutral point clamping (NPC) inverter, as an inverter module, has a detectable DC component in its output. When it flows into the step-up transformer, it will lead to increased transformer loss and excitation saturation, thus reducing the service life of the transformer and increasing potential safety hazards. This paper analyzes the causes of the DC component of the NPC inverter, and a reliable DC component suppression method based on double-loop control is proposed. A suitable PI controller can effectively suppress the output DC component during the power supply operation by establishing the transfer function between the output DC component and the regulation rate of the switch’s duty cycle. Finally, the prototype experiments verify the effectiveness of the control method successfully.

Power quality enhancement of distribution grid using a photovoltaic based hybrid active power filter with three level converter

This paper employs a second-order sequence filter (SOSF) and proportional–integral (PI) controller to control a hybrid active power filter with photovoltaic (HAPF-PV) through three-level neutral point clamped (NPC) converters. The PV system uses perturbation and observation (P&O) method to track the maximum power point and drive a boost DC/DC converter. A reference signal of a shunt active power filter (SAPF) is generated by estimating the real (not imaginary) term of the polluted load current. The estimation of this term relies on adopting a SOSF and sampling that its input at points the load voltage crosses the zero. SAPF is responsible for compensating grid current harmonics. The suggested approach helps obtain fundamental real term of the polluted unbalanced load current with good accuracy and minimal mathematical computations. In addition to improve power quality, this system can produce green output using the PV arrays connected to the HAPF via a DC-link. HAPF-PV has advantages of power quality enhancement and distributed generation simultaneously. Also, in this system, three-level NPC converters are used in order to reduce harmonic currents injected into the grid by HAPF-PV. Escalating the converter output voltage had led to obtaining output sinusoidal waveform of the converter, and so in this paper, a three-level NPC converter with a neutral point is used to reduce harmonics due to PWM switching. Besides, a series active power filter (SeAPF) is presented for series compensation such as voltage sag/swell. The scheme was implemented in MATLAB considering various types of disturbances including the PCC voltage sag/swell, unbalanced load and changes in irradiance.

Open-Phase Fault-Tolerant DTC Technique for Three-Level NPC VSI-Fed Five-Phase Induction Motor Drives

The demand for motor drives based on multiphase machines (MMs) is increasing due to their inherent fault-tolerant capabilities. To achieve satisfactory post-fault operation, a proper fault-tolerant controller is needed. Some direct-torque-control (DTC) techniques have been reported for five-phase induction motor (FPIM) drives under an open-phase fault (OPF). DTC offers simplicity, fast response, and less parameter sensitivity. However, the existing fault-tolerant DTC methods were designed for two-level voltage-source inverters (VSIs). A three-level neutral-point-clamped (NPC) VSI allows greater flexibility in the switching states, lower current and torque ripple, and reduced common-mode voltage (CMV). However, the increased number of switching states requires the design of new DTC methods, able to tolerate OPFs while providing satisfactory performance in various aspects. In this article, two fault-tolerant DTC techniques are developed for three-level NPC VSI-fed FPIM drives under an OPF. The possible switching states are synthesized with suitable virtual vectors (VVs), addressing the dc-link voltage balance, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\text {d}v/\text {d}t}$ </tex-math></inline-formula> , CMV, and dc-link utilization. The two methods are compared under an OPF in these terms, as well as regarding the average switching frequency. Comparative experimental results are provided to assess the viability of the developed DTC schemes at steady-state and dynamic conditions.

Enhanced Predictive Torque Control for Three-Level NPC Inverter-Fed PMSM Drives Based on Optimal Voltage Magnitude Control Method

The major drawback of conventional predictive torque control (PTC) is that it fixes the magnitude of the reference voltage vector (VV) to 70%. This limitation results in large torque and flux ripples and slow dynamic response, especially when the permanent magnet synchronous motor operates in low-speed regions. A promising approach for reducing torque and flux ripples is by replacing the two-level inverter with a three-level neutral-point-clamped (NPC) inverter. Nevertheless, because of the elimination of zero VV during torque decrement in a conventional PTC, a large torque ripple appears in the low-speed region. In addition, high-speed operation is not applicable unless more dc-link is injected. To solve these problems, an effective, creative, and low-complex PTC based on a reference voltage vector magnitude control method (RVVCM) is proposed in this article. The proposed PTC effectively minimizes torque and flux ripples by utilizing only the required magnitude of reference VV of the three-level NPC inverter based on the variable voltage control concept. Furthermore, the dynamic response of the torque is improved by applying the full magnitude of applied VV. Simulation and experimental results are presented to validate the effectiveness of the proposed RVVCM-based PTC algorithm.

Power Sharing in Three-Level NPC Inverter Based Three-Phase Four-Wire Islanding Microgrids With Unbalanced Loads

The droop-based control schemes are widely used for power-sharing and control of the isolated microgrids. The conventional droop control scheme with inner current and voltage controllers can work effectively under ideal voltage. However, the existence of unbalance tends its performance to worsen. This paper aims to propose an enhanced droop control method to improve the power-sharing, and maintain the distributed generators’ (DGs) terminal voltages and the system frequency under the balanced as well as unbalanced loads. The enhancements of the proposed control scheme include the droop controller, inner current and voltage controller, and virtual impedance loop. The proposed virtual impedance deals with the accurate sharing of powers in the case of unequal line impedance and the control of zero sequence current. Moreover, to enable the proposed control to compensate for the unbalance, three-level neutral point clamped (NPC) inverters are used to form a three-phase four-wire microgrid. With this control scheme, the voltage unbalance factors (VUFs) of the DGs’ terminal voltages are reduced from 4% to less than 1%, negative sequence reactive powers, which are also an indication of unbalance, have been reduced significantly, and the system frequency is maintained within the standard permissible limit of 2.5%. Furthermore, a local load controller is also proposed to control the reactive loading of the DGs when they have local loads supplied from their terminals. The investigations have been carried out under simulation as well as lab-based environments to validate the efficacy of the proposed control method.

A Simultaneous Diagnosis Method for Power Switch and Current Sensor Faults in Grid-Connected Three-Level NPC Inverters

A simultaneous diagnosis method for power switch open-circuit faults and current sensor faults of grid-connected three-level neutral point clamped (NPC) inverters is proposed herein. First, by designing the adaptive reaching law and reducing the steady-state resonance, a novel interval sliding model observer is developed to track and estimate the three-phase currents accurately and rapidly. Next, a faulty phase detection scheme using an adaptive threshold is proposed to achieve sensitive and robust detection results. Also, the measured three-phase currents sum and estimated three-phase currents sum are used to distinguish power switch open-circuit faults and current sensor faults. Subsequently, a faulty switch location method and the current sensor fault type identification method are proposed, using some of the same identification variables. Finally, combining the faulty phase detection variables with the identification methods ensures the feasibility of the detection and identification of 12 open-circuit faults and 9 current sensor faults. The hardware-in-the-loop test results are performed to validate the effectiveness and robustness of the proposed method.

Assessment of a High-Order Stationary Frame Controller for Two-Level and Three-Level NPC Grid-Connected Inverters

Most grid-connected DC/AC inverters use traditional proportional–integral (PI) controllers in a synchronous frame. In addition to poor disturbance rejection capabilities, these PI controllers also exhibit steady-state errors for sinusoidal reference signals. To address these drawbacks, this article investigates the use of a high-order controller in the stationary frame and then compares it with the standard PI controller. The effectiveness of the high-order controller in the stationary frame has been examined by providing an infinite gain at a resonance frequency. In this work, the design of high-order and PI controllers and tuning instructions are given. Furthermore, both high-order and PI current-controlled two-level and three-level neutral point clamped (NPC) inverters are compared. Various operational conditions are used for the comparison. The high-order controller reduced the total harmonic distortion (THD) of the injected current by 1.15% for the two-level inverter in normal conditions as compared to the PI controller and 0.9% for the three-level NPC inverters. Furthermore, it reduced the THD in balanced abnormal conditions by 0.5% for the two-level inverter and 0.18% for the three-level NPC inverters. However, the dq controller has a lower THD during unbalance and short circuit conditions.

48-Pulse Voltage-Source Converter Based on Three-Level Neutral Point Clamp Converters for Solar Photovoltaic Plant

The demand for high-power converters for use in large-scale solar photovoltaic (PV) plants increases as the plant size continues to grow. The multipulse voltage-source converter (VSC) configurations are the viable option for high-power solar PV converter applications because they improve conversion efficiency due to low switching losses and allow direct integration with a medium-voltage (MV) grid. In this article, a 48-pulse VSC based on the fundamental frequency switching (FFS) three-level neutral point clamp (NPC) converter with a start–delta transformer configuration is used for large-scale solar PV plant application. It is also used as a PV-static synchronous compensator (STATCOM) to provide the required reactive power support to the grid to regulate the voltage. The solar PV plant configuration for a 100-MW capacity is modeled in MATLAB and implemented in the real-time OPAL-RT platform to verify its design and controls. The harmonics, static, and dynamic performances are demonstrated in detail.

A Novel Bidirectional Five-Level Multimode CLLC Resonant Converter

In order to widen voltage gain of bidirectional on-board charger to meet requirements of electric vehicle with different voltage levels, a novel bidirectional five-level multimode <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CLLC</i> resonant converter is proposed by combining the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CLLC</i> resonant tank module (RTM) with the five-level cascaded neutral-point clamp active bridge which is composed of two three-level neutral-point clamp (3L-NPC) bridge arms. Furthermore, the converter can, respectively, establish three voltage gain modes of high, medium, and low to effectively widen voltage gain adjustment capabilities through the fixed-frequency plus phase-shift hybrid modulation independently adopted in each 3L-NPC bridge arm, while soft-switching is realized by phase control between resonant current and input voltage of RTM. Meanwhile, the key parameters of RTM would be symmetrically designed to make bidirectional power transmission characteristics, which become consistent based on soft-switching and monotonic voltage gain control. Finally, according to the simulation platform and experimental prototype, it can be shown that the practical voltage gain is close to the theoretical design gain and converter has good dynamic performances after realizing soft-switching in each mode.

A Carrier-Based Common-Mode Voltage Elimination Method for Back-to-Back Three-Level NPC Converters

As the increase of power and voltage level of back-to-back three-level neutral-point clamped (NPC) converters, the high-amplitude and high-frequency common-mode voltage (CMV) has become an urgent problem. To address this, a CMV elimination method based on pulsewidth modulation pulse alignment for back-to-back three-level NPC converter is proposed. On this basis, a neutral-point potential balance method based on zero-sequence voltage (ZSV) is also proposed. In order to decouple the CMV elimination and the neutral-point potential balance, the same optimal ZSV is injected into both the rectifier and inverter sides, and the CMV elimination will not be affected. Simulation and experiments are presented to verify the effectiveness of the proposed method.

24 Sectors DTC Control with Fuzzy Hysteresis Comparators for DFIM Fed by the Three-level NPC Inverter

This article presents 24 sectors direct torque control (DTC) with fuzzy hysteresis comparators for the doubly-fed induction motor (DFIM) using a three-level neutral point clamped (NPC) inverter. The designed DTC technique of the DFIM combines the advantages of the DTC strategy and fuzzy logic controller. The reaching conditions, stability, and robustness of the DFIM with the designed DTC technique are guaranteed. The designed DTC technique is insensitive to uncertainties, including parameter variations and external disturbances in the whole control process. Finally, the designed DTC technique with fuzzy hysteresis comparators is used to regulate the electromagnetic torque and the flux of the DFIM fed by the three-level NPC inverter and confirms the validity of the designed DTC technique. Results of simulations containing tests of robustness and tracking tests are presented.

Improved Temperature Monitoring and Protection Method of Three-Level NPC Application Based on Half-Bridge IGBT Modules

The overheating stress of power devices is the main cause of converter system failures, so real-time temperature acquisition, as well as timely over-temperature protection, under various operating conditions are key to achieving a more reliable energy conversion. In this paper, an improved three-dimensional (3-D) coupled thermal model for insulated gate bipolar transistors (IGBTs) and related heatsink is constructed by considering the thermal coupling effects at both the device- and the module level. Furthermore, the coupled thermal impedance parameters are thereby extracted based on the finite-element method (FEM), so that the dynamic loss and transient temperature distribution under actual working conditions can be obtained based on this electro-thermal coupling model. Meanwhile, to avoid the overheating damage of power devices due to untimely action of the negative temperature coefficient (NTC) thermistor over-temperature protection, this article aims at the idle IGBT device in the three-level neutral-point clamped (NPC) application using IGBT half-bridge modules, combined with the emitter terminal temperature, and finally realizes the multiple over-temperature protection settings. The improved temperature monitoring methods proposed in this paper are realized by the drive and control circuits and verified through simulation and experimental results.