Three-level Converter Research Articles

A Nine-Level T-Type Converter for Grid-Connected Distributed Generation

This article presents a new high-efficiency nine-level T-type converter (9L-T2C) for grid-connected applications based on the three-level T-type converter (3L-T2C). The proposed 9L-T2C outperforms other common dc-link nine-level converters in terms of the required number of active switches and capacitors, flying capacitors (FCs) voltage ratings, and efficiency. Only ten power switches, eight gate drivers, and two FCs are required for each phase. Exploiting the available pole-redundant states, an FCs balancing algorithm is developed to stabilize the two FCs with one voltage sensor in steady-state and dynamic operation. Moreover, an effective balancing method is proposed for dc-link capacitors without the need for further redundant states and integrated into FCs balancing. The FCs and dc-link balance are integrated into the phase-disposition pulsewidth modulation (PD-PWM) method, eliminating the need for an additional controller. Considering the designed PD-PWM method, a mathematical analysis is performed to establish the relationship between the FCs size and the desired ripple. A comprehensive comparison with other converters is provided to demonstrate the merits and application areas of 9L-T2C. The operation of the proposed 9L-T2C with the capacitors’ balancing scheme is validated for stand-alone and grid-connected operation via simulation investigations and experimental setup.

Switching Loss Optimization for a Pulsewidth Modulation Strategy With Reduced CMV and Improved Output Harmonic Distortion for a Three-Level NPC Converter

Switching Loss Optimization for a Pulsewidth Modulation Strategy With Reduced CMV and Improved Output Harmonic Distortion for a Three-Level NPC Converter

Fault-Tolerant Sequential MPC for Vertical Switch Open-Circuit Fault and ZSCC Suppression for Parallel T-Type Converters

A fault-tolerant control (FTC) is proposed for enhancing the reliability of power electronic systems. The software-based FTC of parallel-connected three-level T-type converters (3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> Cs) for suppressing zero-sequence circulating current (ZSCC) induced by a vertical switch open-circuit fault is investigated. A simplified sequential model predictive control (SSMPC)-based FTC technique is developed. Parallel-3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> Cs and their prediction models are introduced, followed by an SSMPC for the no-fault condition. The model inaccuracies under vertical switch faults are elaborated. For the faulty 3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> C, two fault-tolerant SSMPC (FT-SSMPC) methods are provided by creating a sequential predictive controller that considers ZSCC suppression and grid current tracking. The control policies for the ZSCC are changed from the standard feedback-free to feedback-based cost function (CF) optimization using the unimproved FT-SSMPC. Furthermore, an improved FT-SSMPC is proposed using a phase-deficient CF, which increases the accuracy of the mathematical relation of the fault condition. After fault diagnosis, the proposed method achieves neutral-point voltage balance and excellent point-of-common-coupling current, effectively suppresses the ZSCC, and increases the control reliability. Finally, experiments demonstrate the effectiveness of the proposed FT-SSMPC under various conditions.

Analysis of Capacitor Voltage Imbalance in Hybrid Converters and Inherently Balanced Operation Using Symmetric Architecture

This letter investigates the origin of the flying capacitor voltage imbalance in hybrid converters. An intuitive voltage–charge relationship is established to give a general explanation of the flying capacitor voltage balance in hybrid converters. The relationship is applied to devise a relatively simple and intuitive method to identify the behavior and performance difference of capacitor voltage and inductor current balancing performance in hybrid converters for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$V_{\text{out}}&lt; \frac{V_{\text{in}}}{N}$</tex-math></inline-formula> cases. Conventional hybrid converters with an even number of inductor charging intervals are shown to be susceptible to flying capacitor voltage imbalance, while flying capacitors in hybrid converters with inductors having odd charging intervals have inherently balanced operations. As a direct result of the analysis, a new symmetric architecture and the operation of three-level buck converters are introduced to achieve an inherent balance of flying capacitor voltages, which can be applied to other flying capacitor multilevel converters. Hardware prototypes are implemented and measured for the verification of the analytical analysis and the new symmetric operation

오프셋 전압 인가 방식의 펄스 크기 변조 풀브릿지 다이오드 클램프 3레벨 LLC 공진형 컨버터

In this paper, a full-bridge diode-clamped three-level LLC resonant converter with pulse-amplitude-modulation (PAM) suitable for a high-input voltage system has been studied. Unlike the existing frequency sweep converter, the proposed PAM method controls the output voltage by modulating the magnitude of fundamental component of leg voltage input to the resonance tank while fixing the operating frequency at the resonance point. In addition, the PWM switching pattern was easily implemented by applying an offset voltage while reducing the switching loss by using the ±180° discontinuous PWM (DPWM) technique. In this study, the control algorithm and operation characteristics were analyzed in detail, and the feasibility was confirmed through simulation.

Continuous-Control-Set Model Predictive Control for Three-Level DC–DC Converter with Unbalanced Loads in Bipolar Electric Vehicle Charging Stations

Zero-emission transportation is currently a public priority, especially in big cities. For this reason, the use of electric vehicles (EVs) is receiving much attention. To facilitate the adoption of EVs, a proper charging infrastructure together with energy management is essential. This article proposes a design guideline for a direct current (DC) charging station with bipolar properties. A bipolar system can convert a two-wire system into three wires in a microgrid system with a neutral line. The configuration of the bipolar system supports different loads; therefore, the unbalanced operation is inherent to the system. The proposed bipolar DC charging station (CS) has a three-level balancing converter that reduces the step-down effort chargers. Moreover, this paper proposes the continuous-control-set model predictive control (CCS-MPC)-based balancing strategy that allows the handling of different output loads while keeping the neutral-line voltage efficiently regulated with improved dynamic performance compared to a traditional controller. Stability and parameter robustness analyses are also performed for the control parameter selection. To ensure the performance of the proposed method, both simulation and experimental results are presented and compared with those obtained from the traditional methods.

A novel neutral-point potential balance control method based on voltage feedback for neutral-point clamped three-level inverter

Neutral-point potential imbalance is not only an inherent problem of neutral-point clamped three-level converters, but also the key factor affecting their performance. The traditional capacitor neutral-point potential balance control methods based on space vector modulation, such as VSVPWM, have several shortcomings including the uncompensated region of neutral-point voltage and the slow dynamic response. A novel voltage feedback control method on neutral-point potential balance is proposed in this paper. A coefficient is introduced to define the degree of neutral-point potential imbalance. The composite signal composed of this coefficient and neutral point current is designed. Based on the improved reasonable sectors’ division, the suitable small vectors are selected by this composite signal to obtain the switching sequence of power devices. The voltage feedback control method is employed to realize accurate neutral-point potential balance of NPC three-level inverter. The experimental results show that this method can quickly balance the neutral-point potential fluctuation caused by disturbance. It has the advantages of low switching loss, stable output voltage and current waveform, and good dynamic performance.

Enhanced Performance of On-board EV Battery Charger with Universal Power Supply

This paper is intended to provide enhanced features of onboard EV battery chargers in terms of improved power quality feature, reduced voltage stress, reduced filter size, high efficiency and fast dynamic response for universal input voltage range (85–265 V). The proposed configuration comprises of three-level quadratic converter suitable to provide high voltage gain. The detailed operation, analysis and control scheme of the proposed converter rated for 3.2 kW, 400 V are studied under simulation and hardware in a loop environment. The on-board charger performances are assessed by charging a 40 Ah lithium ion EV battery under different charging modes. The state of charge of battery decides converter operating mode i.e. constant current and constant voltage mode to charge the battery. Enhancement in the onboard charger is observed in both operating modes tested for wide variation in supply voltages. The results reveal that the operation of converter depicts unity power factor operation, improvement in supply current harmonics and controlling output voltage/current. Under both operating modes, the proposed charger draws sinusoidal supply current from ac mains and its THD is reduced to specified limit prescribed by IEC-61000-3-2. Furthermore, the charger performance is also investigated for EV battery charging operation at wide input voltage variations and found satisfactory.

Simplified Gate Driver Design Technique for Multi-MHz Switching GaN FETs in Three-Level Buck-Based 4G/5G Envelope Tracking

Multi-MHz switching frequency converter-based envelope tracking dc power supplies are widely adopted in modern 4G/5G communication base stations to improve the efficiency of radio frequency power transmitters. The switching loss of buck converter at MHz switching frequency increases with the input dc voltage. To obtain higher output voltage range without much compromise on the efficiency, multilevel converters with discrete low voltage gallium nitride (GaN) field-effect transistors (FETs), switching at multi-MHz frequency, are used. However, the gate driving circuit and PCB layout of a multilevel buck converter are complex and occupy major portion of the board when compared with the power GaN FETs. This article proposes a simplified gate driving technique in a cascaded bootstrapping architecture for a three-level buck converter with reduced number of components that can support up to 40 MHz switching frequency. The converter can reproduce any reference envelope signal with a maximum bandwidth of 8 MHz and output voltage variation from 6 to 42 V. Hardware prototype is developed, and experimental results are presented to validate the concept.

Sequential Model Predictive Fault-Tolerance Control for T-Type Three-Level Grid-Connected Converters With LCL Filters

Recently, fault-tolerance control (FTC) methods have been proposed to further improve the reliability of T-type three-level converters. However, the application of model predictive control in FTCs is rare, and the neutral point (NP) voltage imbalance before an open-switch fault occurs has not been solved. This paper focuses on these two problems and proposes two FTC control methods: sequential model predictive control (SMPC) and sequential model predictive tolerance control (SMPTC). Both methods are implemented by designing a sequential predictive control that first considers the NP voltage balance and then grid current tracking. Through the staggered operation of the two methods, a T-type grid-connected converter with an LCL filter can operate normally after an open-switch fault occurs. Moreover, both methods can achieve the NP voltage balance and excellent grid current outputs, eliminate the time-consuming selection of weighting factors, and reduce the number of calculation loops compared with the traditional model predictive tolerance control, thereby improving the control flexibility. The performance and comparison of the open-switch FTC with respect to the NP voltage oscillations, total harmonic distortion of the grid current, and pole voltage were demonstrated using numerous simulation and experimental results.

A New Neutral-point Voltage Balance Control Based on Virtual Space Vector Modulation for Three-Level NPC Converter

A New Neutral-point Voltage Balance Control Based on Virtual Space Vector Modulation for Three-Level NPC Converter

Baseline for Split DC Link Design in Three-Phase Three-Level Converters Operating with Unity Power Factor Based on Low-Frequency Partial Voltage Oscillations

The study sets a baseline for split DC link capacitance values and voltage set points in three-phase three-level AC/DC (or DC/AC) converters operating with unity power factor. In order to equalize the average values of partial DC link voltages, the controller generates a zero-sequence containing DC components only while employing neither dedicated DC link capacitance balancing hardware nor high-order zero-sequence component injection. Such a baseline is required in order to evaluate the effectiveness of different DC link capacitance reduction methods proposed in the literature. Unlike most previous works, utilizing neutral point current based on cumbersome analytical expressions to determine neutral point potential oscillations, the instantaneous power balance-based approach is employed in this paper, resulting in greatly simplified and more intuitive expressions. It is demonstrated that while the total DC link voltage is low-frequency ripple-free under unity power factor balanced AC-side operation, split DC link capacitors absorb triple-fundamental frequency power components with one-sixth load power magnitude. This yields significant opposite phase partial voltage ripples. In such a case, selection of DC link capacitances and voltage set points must take into account the expected values of AC-side phase voltage magnitude and split DC link capacitor voltage and current ratings. Simulation and experimental results validate the proposed methodology by application to a 10 kVA T-type converter prototype.

Bidirectional H8 AC–DC Topology Combining Advantages of Both Diode-Clamped and Flying-Capacitor Three-Level Converters

A three-level bidirectional ac–dc converter with H8 topology is proposed to combine the advantages of both neutral-point clamped and flying-capacitor (FC) three-level converters for medium-voltage (MV) ac–dc solid-state transformer (SST) applications. The innovative H8 converter features self-balanced capacitor voltage, common-mode voltage elimination, and small capacitance of the FCs. The proposed carrier-based modulation enables the H8 converter to operate under both active and reactive power conditions. A 10-kVA ac–dc converter prototype with 1.7-kV SiC MOSFETs and 3.3-kV SiC diodes verifies the feasibility and advantages of the proposed topology and the modulation method. The capacitance of the FCs in the H8 converter is reduced by 45 times compared to that of the FC converters. The H8 converter efficiency is tested up to 99% at 2-kV dc voltage. The proposed H8 converter is suitable for the next-generation PV inverter, grid-forming inverter, MV fast charger, and SST applications.

Power Losses Reduction of T-Type Grid-Connected Converters Based on Tolerant Sequential Model Predictive Control

Three-level T-type converters (3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> C) with inductance–capacitance–inductance filters have high power quality of grid current, particularly in the low-voltage system. However, the switching loss and conduction losses degrade the overall efficiency of 3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> C. The generated heat due to the losses endangers the reliability of the device and shortens its service life. To relieve the power losses as well as maintain high-quality current output, a low-loss-tolerant sequential model predictive control (LL-TSMPC) is proposed. First, based on the energy loss analysis under switching transitions per commutation, the prediction models of both switching loss and conduction loss are established. Second, the total loss reduction is considered as one control objective in LL-TSMPC. Finally, because multiobjective model predictive control (MPC) requires selecting appropriate weighting factors, complicating the design to solve this problem, TSMPC-based lexicographic optimization is introduced to eliminate the tradeoff factors and simplify the MPC controller design. The proposed method is tested on the hardware platform with the rated power of a 10-kW prototype. The experimental results show that the LL-TSMPC method can effectively reduce the switching losses while maintaining the high power quality of the grid current.

A Hybrid Three-Level ZVZCS Converter for Photovoltaic Power Connecting to MVDC Collection System

Compared with the medium voltage ac (MVAC) collection system, the medium voltage dc (MVDC) one for renewable energy sources has many advantages. High-power dc/dc converters are one of the key stages of the MVDC collection system to boost the voltage generated by photovoltaic or wind turbine. A novel hybrid three-level dc/dc converter utilizing a blocking capacitor to realize zero-voltage zero-current-switching (ZVZCS) is proposed. A higher overall efficiency can be achieved by reducing conduction losses. Detailed experimental results on a scaled-down hardware prototype rated at 150 V/750 V/1 kW are demonstrated to verify the proposed converter performance.

Performance evaluation of model predictive control method for neutral point clamped inverter

The neutral-point clamped (NPC) inverter is a popular three-level converter topology used in motor drive applications and other dc/ac converter systems. In this paper, the performance evaluation of the model predictive control is performed to investigate its applicability in controlling NPC inverters. The model predictive control (MPC) is a promising closed-loop control strategy in applications where multiple control goals are considered. The ease of adding the objectives to the control law improves the reputation of the MPC. The numerous control goals can be regulated in single feedback. Thus, the adequate bandwidth is noticeably higher compared to the traditional linear controllers. However, controlling the multiple objectives require the use of weighting factors to tune the system performance. Regarding the system performance, multiple reference tracking performance is investigated in this study. Our case study considers three control goals: output load current, switching frequency control, and capacitor voltage balancing. The predictive control is designed to regulate these dynamics, and comprehensive performance analyses are performed. The designed controller is tested using a simulation tool. The simulation results prove that predictive control offers an excellent multi-objective control performance provided that the weighting factors and other design parameters are finely adjusted. The poor selection of the design parameters affects the closed-loop performance, and the conducted analyses show the effects of the controller parameters.

Detailed Power Loss Analysis of T-Type Neutral Point Clamped Converter for Reactive Power Compensation

The paper presents an analysis of power losses in a three-phase T-type neutral point clamped converter with insulated gate bipolar transistors. The paper’s main aim is to perform a detailed analysis of power losses in the converter operating as an active power filter. The study is based on the use of characteristics of semiconductor devices provided by the manufacturer in the module datasheet. Thanks to the analysis, it is possible to recognise the value of power losses, which facilitates the design of the converter cooling system. Identifying how power losses are distributed between the module switching devices is also possible. Power losses are shown as functions of the output current and module temperature. The analysis results were successfully verified by measuring power losses using a laboratory model of the converter with rated currents of 10 and 20 A. The obtained results indicate relatively low power losses and a relatively even distribution of power losses between the semiconductor devices. This is a superior feature of the three-level T-type neutral point clamped converter topology.

Heat Spreader Thermal Switch for Power Converter Isothermalization

Power module heat dissipation with spatial inhomogeneity and induced nonuniform temperature distribution presents a challenging concern for system reliability due to thermo-mechanical stresses. These reliability challenges are especially important for nonplanar designs using 3-D packaging principles. Here, we develop a heat spreader thermal switch capable of actively reducing temperature gradients between silicon carbide (SiC) devices in a three-level T-type power converter, which can change depending on electronic operating conditions. The heat spreader thermal switch consists of a stainless-steel (SS) heat spreader and a copper sliding switch embedded within the spreader. Heat transfer from the SiC devices and within the spreader can be controlled by moving the slider between positions within the spreader. To understand heat transfer mechanisms and design the heat spreader thermal switch, we conducted 3-D finite element method simulations to calculate the minimum attainable temperature difference between SiC devices. We used the finite element simulation to quantify the reduction in junction temperature swing during dynamic operation and coupled the results to the Coffin–Manson reliability model to quantify lifetime to failure. We integrated the heat spreader thermal switch with one phase of a three-phase T-type converter and demonstrated isothermalization at different working conditions. At 2.4-kW converter power, each hot SiC device dissipated 4.7 W of heat, resulting in a device case temperature of 43 °C, with each cold SiC device dissipating 1.7 W at 38 °C. The device-to-device temperature difference was decreased from 5 °C to 0 °C by moving the switch a distance of 20 mm. Finite volume method (FVM) simulations of the conjugate heat transfer problem validate the experimental results and support analysis of key performance parameters. This work demonstrates successful isothermalization of a power converter with an active heat spreader thermal switch and develops simulation and design guidelines for successful electro-thermal codesign and implementation of thermal switches for other electronics applications for which isothermalization and enhanced device reliability is a key issue.

Rüzgâr Türbin Sisteminde İki ve Üç Seviyeli Dönüştürücü Tasarımı ve Güç Analizinin Yapılması

Günümüzde artarak devam enerji ihtiyacı ülkeleri yenilenebilir enerji kaynaklarından üretilen enerjilere yönlendirmiştir. Bu çalışmada, büyük güç gereksinimi duyulan rüzgâr türbinlerine uygun, çift beslemeli asenkron generatör (ÇBAG) tabanlı rüzgâr türbin sistemi incelenmiş ve benzetimi yapılmıştır. Bu türbin sistemi 6 adet 1,5 MW toplam 9MW gücünde 25 kV’luk dağıtım sistemine entegre, 30 km’lik bir hat uzunluğuna sahip, 25 kV’luk transformatör ile 120 kV’luk şebekeye güç üretmektedir. ÇBAG tabanlı rüzgâr türbin sistemi çift yönlü enerji akışına uygun olmasından dolayı arka arkaya bağlı iki adet dönüştürücüden oluşmaktadır. Matlab/ Simulink programında üç fazlı iki seviyeli dönüştürücü ve üç fazlı üç seviyeli dönüştürücünün tasarımı yapılmış ve sinüzoidal darbe genişlik modülasyon tekniği ile anahtarlama sinyalleri üretilmiştir. Bu dönüştürücüleri tasarladığımız ÇBAG tabanlı rüzgâr türbini modellemesinde kullanarak ve 15 m/sn rüzgar hızında toplam güç, reaktif güç ve sistemin çıkış gerilimleri değerleri kıyas edilmiştir. Benzetimi yapılan sistem için üç seviyeli dönüştürücü ile daha kararlı ve stabil sonuçlar alınmıştır.

Improved Model Predictive Current Control of NPC Three-Level Converter Fed PMSM System for Neutral Point Potential Imbalance Suppression

For the neutral point clamped (NPC) three-level converter fed permanent magnet synchronous motor (PMSM) system, the performance of the conventional model predictive current control (MPCC) algorithm will be deteriorated if the amplitude of the neutral point potential (NPP) is large. Additionally, the adjustment process of the weighted coefficients of the conventional MPCC algorithm is complex because of numerous control terms in the cost function. To solve the above issues, an improved MPCC algorithm is proposed in this paper. Firstly, Newtonian iteration is used to transfer the stator current into stator voltage in the cost function. Then, the NPP term in the conventional cost function can be eliminated by introducing the partition control of the NP potential, which also eliminates the whole adjustment process of weighting coefficients. Finally, based on the amplitude of the NPP, the amplitude and phase angle of medium and small vectors are modified to improve the control performance of the torque and flux. Experimental results show that the fluctuation of the neutral point potential can be suppressed rapidly. Meanwhile, the performance of the torque, flux and current are also improved compared with the conventional MPCC.