Active Neutral Point Clamped Research Articles

Model Predictive Controllers With Capacitor Voltage Balancing for a Single-Phase Five-Level SiC/Si Based ANPC Inverter

Employing both high bandwidth (HBW) controller and wide bandgap (WBG) devices in the structure of converters improve the system size, performance, and efficiency. In this paper, HBW model predictive controllers (MPCs) are proposed, with both fixed and unfixed switching frequencies, to control a single-phase five-level hybrid active neutral-point-clamped (ANPC) inverter. A hybrid modulation technique is considered in this paper, in which some of the switches are modulating with high frequency. Therefore, Silicon-Carbide (SiC) MOSFETs are employed in the converter structure to increase the switching frequency and consequently reduce the filter size and increase converter power density. To have the functionality of multilevel output voltage, some restrictions are defined in the adopted MPC with unfixed switching frequency. In the MPC with the constant switching frequency, predefined switching sequences are employed for all sectors. Moreover, to control the neutral point (NP) voltage, the applied times of both small voltage vectors are sets through a cost function. Finally, the simulation and experimental results prove the ability of both proposed methods to control the voltages of the load and NP.

An Active Hybrid Modulation Strategy for a Si/SiC Hybrid Multilevel Converter

This paper proposes a hybrid modulation strategy for a medium-voltage hybrid seven-level (7-L) converter, which consists of a three-phase silicon (Si) IGBT-based active-neutral-point-clamped (ANPC) stage and a cascaded silicon carbide (SiC) MOSFET-based H-bridge (HB) stage in each phase. Within the framework of the proposed modulation strategy, three floating HB converters are controlled using a high-frequency three-level (3-L) space-vector-modulation (SVM), while a low-frequency modulation is applied to the ANPC converter. To regulate the DC voltages across floating capacitors, a zero-sequence voltage (ZSV) injection method is adopted. The proposed method possesses three major advantages. First, it enables the simplification from a 7-L to a 3-L SVM, which significantly reduces the computational burden. Second, with all the high-frequency switching actions assigned to SiC devices in the HB stage, the power loss of the hybrid 7-L converter is reduced. Third, the hybrid modulation together with the ZSV injection enables a wider stable operating range compared to the existing 7-L SVM method. Finally, both simulation and experimental results performed on the scale-down and the full-scale 1 MVA prototype validate the effectiveness of the proposed hybrid modulation strategy.

Co-Reduction of Common Mode Noise and Loop Current of Three-Level Active Neutral Point Clamped Inverters

The increased switching frequency and speed of silicon carbide (SiC) MOSFETs lead to higher power density of inverters, but meanwhile resulting in weak electromagnetic interference (EMI). The impedance balance technique is a good way to reduce the common mode (CM) noise by making the voltage across the line impedance stabilization network (LISN) as small as possible. However, a side effect of this technique is the generation of relatively large loop current that circulates in the inverter. It can cause additional losses and cost, which can be a factor that stops the increase of the switching frequency by SiC MOSFET. This article, for the first time, analyzes the relationship between the CM noise and loop current of the three-level active neutral point clamped (ANPC) inverter, and proposes a co-reduction method for both. First, the CM noise and loop current are clarified for the ANPC inverter, and the analytical models for both are established. The conflict between the CM noise and loop current is introduced with a specific case by the existing design method. Then a co-reduction method is proposed and elaborated, which can both suppress the CM noise and the loop current. The extra cost and volume by the proposed method are also analyzed and are negligible. The design guideline is further shown for clarity. Finally, the analysis and proposed method is validated by the experiment.

Si/SiC hybrid 5-level active NPC inverter for electric aircraft propulsion drive applications

Medium voltage DC(MVDC) system is considered as a promising technology to improve the efficiency and power density of electric aircraft propulsion(EAP) drives. To adapt to the MVDC voltage level and achieve high drive performance, a five-level active neutral point clamped(5L-ANPC) inverter consisting of three-level ANPC and flying capacitor circuits is investigated, which possesses higher voltage capability, lower output harmonics, as well as mitigated dv/dt and common-mode voltage. To fulfill the requirements of high-speed operation and pursue further enhanced efficiency and power density of the inverter for the next-generation EAP drives, Silicon Carbide(SiC) semiconductor devices are considered for implementing the 5L-ANPC inverter. However, the large commutation loops associated with certain switching states of the inverter lessen the benefits of configuring all the switches as SiC devices. As a result, a hybrid Si/SiC 5L-ANPC inverter is developed with a synchronous optimal pulse(SOP) width modulation strategy for controlling the switches in cell 2 and finite-control-set model predictive controller(FCS-MPC) for those in cell 3 of the inverter. Consequently, in the proposed topology, the SiC devices are merely used for the high-frequency switches in cell 3 and the rest of the low-frequency switches are configured with Si IGBTs. This Si/SiC hybrid ANPC inverter concurrently provides high efficiency and low implementation cost at high-speed operation mode. Simulation and experimental results are provided to verify the effectiveness of the proposed hybrid inverter.

A Novel Single-Stage Five-Level Common-Ground-Boost-Type Active Neutral-Point-Clamped (5L-CGBT-ANPC) Inverter

The conventional active neutral-point-clamped (ANPC) inverter inherits excellent high-frequency common mode voltage (CMV) mitigation ability but with a restricted voltage gain. This letter proposes a novel five-level ANPC inverter that is able to accomplish voltage-boosting within a single-stage dc–ac power conversion. A common ground in the proposed topology not only mitigates the high-frequency CMV but also enhances the dc-link voltage utilization. The proposed topology exhibits lower voltage stresses and higher compactness when compared with a conventional two-stage five-level ANPC inverter. The proposed inverter saves three power switches and one capacitor while enhancing the overall efficiency. Simulations and experimental tests have validated the overall operation, efficacy, and practicality of the proposed ANPC inverter.

Active Voltage Balancing and Thermal Performance Analysis of Dual Flying-Capacitor Active Neutral-Point-Clamped (DFC-ANPC) Inverters

Multilevel inverters provide appealing advantages such as improved efficiency, superior thermal performance, better distribution of the switching and conduction power losses, smaller size of passive components including output ac filters and EMI filters, as compared to their two-level counterparts. The flying-capacitor (FC)-based topologies are attaining more attentions, and are finding lots of medium-voltage and low-voltage industrial and residential applications. The FC-based active neutral-point-clamped (ANPC) inverter, as a hybrid topology based on the three-level ANPC and the FC-based configurations, offers outstanding advantages including the reduced number of FCs as well as switching power cells, improved efficiency, and enhanced power quality. However, the utilization of high-voltage line-frequency switches, as a major drawback, hinders the ANPC converter from expansion and penetration into the high-voltage ranges. The dual flying-capacitor (DFC)-ANPC topology resolves this issue through introducing the zero-voltage-switching (ZVS) feature into the line-frequency devices. This soft-switching makes it possible to use the series connection of several low-voltage devices for the realization of the high-voltage switches without having the transient voltage imbalance issues. This article studies the thermal benefits that are achieved in the DFC-ANPC topology alongside the ZVS operation of its line-frequency high-voltage switches. Furthermore, an active voltage balancing technique, based on logic-equations, is proposed in this article for control of the FCs' voltages. The experimental results are provided to validate the improved thermal performance of the DFC-ANPC topology as well as the active balancing technique for the regulation of the FC voltages.

Independent switching technique to remove abnormal output voltage in hybrid active NPC inverters

This paper proposes the independent switching technique to remove abnormal output voltage in hybrid active neutral-point clamped (ANPC) inverters. The hybrid ANPC inverter is composed by mixing silicon (Si) devices and silicon carbide (SiC) devices, unlike the conventional ANPC inverter which is generally composed of Si devices only. SiC devices provide low switching losses under a high switching frequency and a high withstand ability under high temperature compared with Si devices. Therefore, the hybrid ANPC inverter using SiC devices provides low power losses, high efficiency, and high reliability compared with the conventional ANPC inverters. In this hybrid ANPC inverter, abnormal output voltages are generated when the sinusoidal output current crosses the zero point. The abnormal output voltages are momentary pulses that must not be generated, and these pulses distort the output current and degrade the power quality. These pulses have two typical causes: one is from the switching operation time difference between switches, and the other is from deadtime. This paper proposes the independent switching technique to remove the abnormal voltages caused by the two reasons and verifies the proposed technique by simulation and experiment.

Soft switching circuit of high-frequency active neutral point clamped inverter based on SiC/Si hybrid device

Although the silicon carbide (SiC) metal–oxide–semiconductor field-effect transistor (MOSFET) is superior to the conventional silicon (Si) insulated gate bipolar transistor in terms of switching performance, the switching losses of SiC devices increase rapidly by hard switching when the switching frequency (fsw) increase to hundreds of kilohertz (kHz). This paper proposes an auxiliary zero-voltage-transition circuit to realize zero-voltage-switching for all of the high-frequency main switches of the active neutral point clamped (ANPC) inverter based on a SiC/Si hybrid device, and zero-current-switching for all of the auxiliary switches. Through soft switching, the switching losses and anti-parallel diode reverse recovery losses of high-frequency SiC MOSFET switches can be further reduced. First, the circuit topology and the operation principle of the soft switching are detailed followed by the design procedure of the parameters. Then, the efficiencies of hard switching and soft switching ANPC inverters are compared with fsw changing from 10 to 200 kHz. To further improve the efficiency of the soft-switching inverter, two improved methods are proposed. The two proposed methods are auxiliary switches paralleling the external diode and utilizing synchronous rectification technology. Finally, a 1 kW, up to 200 kHz frequency ANPC inverter has been built to validate the above analysis.

Model Predictive Control for a New Magnetic Linked Multilevel Inverter to Integrate Solar Photovoltaic Systems With the Power Grids

The multilevel inverters are becoming increasingly popular for use in the grid integration of wind and photovoltaic (PV) power plants due to their higher voltage handling capability and the better output power quality. There are several types of multilevel inverters that have been proposed in the literature; among them, the active neutral point clamp (ANPC) multilevel inverters have been drawing significant attention especially for solving the problems with other multilevel inverters. However, with the increase in the number of levels, the ANPC requires more electronic switches and flying capacitors, by which the complexity and the cost increase. In this article, an ANPC inverter with a reduced number of switches and flying capacitors is presented for the grid integration of the solar PV systems controlled using the model predictive control technique. This model predictive control technique uses a discrete-time model of the system to predict the future value of the active power and the reactive power for seven identified voltage vectors and selects the vector for the operation, which causes minimum cost function. The proposed power converter topology also effectively utilizes the dc bus voltage more when compared with the traditional ANPC converter. In the proposed inverter, a high-frequency transformer is used to eliminate the voltage balancing problems faced by the traditional ANPC inverters. The proposed magnetic linked power converter provides galvanic isolation, which is one of the most critical issues for traditional transformerless grid-connected PV systems. The proposed topology makes the control strategy simple and makes the power conversion system reliable for the PV power plants. The model predictive control-based power converter topology is simulated in MATLAB/Simulink environment and also validated in the laboratory test platform.

Flying capacitor voltage balance and neutral point voltage regulation of PV array fed active neutral point clamped converter in real‐time HIL

This study deals with a new control strategy for flying capacitor (FC) based multilevel converter, in order to attain the desired reference value for five-level output voltage in active neutral point clamped (ANPC) converter. The balancing of capacitor voltages is one of the crucial aspects of grid-tied photovoltaic (PV) fed ANPC converter. The accurate reference voltage for FC is required to experience minimum power pulsation and capacitor voltage imbalance. An offset injection-based FC voltage regulator is implemented to track the desired voltage reference for FC. A simple proportional–integral controller generates the offset signal, which is added to the sinusoidal modulating wave to eliminate the FC imbalance. Moreover, neutral point voltage (NPV) balancing is achieved with the help of DC component injection control. The DC–AC power transfer, FC balance, NPV balance and unity power factor operation are achieved simultaneously. The performance of the closed-loop controller for grid-tied PV-fed ANPC converter is analysed at constant and variable irradiances. The system is modelled in MATLAB/Simulink for offline simulation. Furthermore, offline simulation results are validated using real-time controller hardware in loop (HIL) test bench. Moreover, active power is injected into the AC grid seamlessly and results are found compliant with the IEEE 519 standard.

Open-Circuit Fault Tolerance Method for Three-Level Hybrid Active Neutral Point Clamped Converters

Three-level converters are the most important technologies used in high power applications. Among these technologies, active neutral point clamped (ANPC) converters are mainly used for industrial applications. Meanwhile, recent developments have reduced losses and increased efficiency by using a hybrid combination of Si-IGBT and SiC-MOSFET switches to achieve hybrid ANPC (HANPC) converters. Open-circuit failure is regarded as a common and serious problem that affects the operational performance. In this paper, an effective fault-tolerant method is proposed for HANPC converters to safely re-utilize normal operation and increase the reliability of the system under fault conditions. Sequentially, regarding different topologies with reference to earlier fault tolerance methods which could not be applied to the HANPC, the proposed strategy enables continuous operation under faulty conditions effectively without using any additional devices by creating new voltage references, voltage offset, and switching sequences under the faulty conditions. Consequently, no additional costs or changes are associated with the inverter. A detailed analysis of the proposed strategy is presented highlighting the effects on the voltage, currents, and the corresponding total harmonic distortion (THD). The simulation and experimental results demonstrate the capability and effectiveness of the proposed method to maintain normal operation and eliminate the output distortion.

Enhanced Hybrid Active-Neutral-Point-Clamped Converter With Optimized Loss Distribution-Based Modulation Scheme

The active-neutral-point-clamped (ANPC) converter is a prominent topology for medium voltage applications. To realize a high-power-density three-level ANPC converter, silicon carbide (SiC) MOSFETs may be used but they also lead to an extremely high cost. Thus, it is important to properly utilize the SiC switches in order to achieve not only a highly efficient but also a low-cost system. In this article, a three-level enhanced hybrid ANPC (E-HANPC) structure is proposed, which optimally utilizes both Si insulated gate bipolar transistors (IGBTs) and SiC MOSFETs to achieve these objectives. A dedicated modulation scheme is also proposed that enables a reduction in the conduction and switching losses and, hence, higher efficiency is achieved. The presented method optimally distributes the losses amongst the converter switches that leads to enhanced power handling capability of the E-HANPC converter. Moreover, the proposed converter also achieves short-length commutation loops for the high-frequency SiC MOSFETs, which imparts fast-switching capability with reduced overvoltage stress on these switches. To validate the efficacy of the proposed converter and modulation scheme, extensive simulation and analytical studies are performed. A prototype of the single-phase leg of the E-HANPC converter is also developed to validate its feasibility, proposed principles, and reconfirm the simulation and analytical findings.

Analysis and Design of a Novel Six-Switch Five-Level Active Boost Neutral Point Clamped Inverter

This article presents an analysis and design of a new boost type six-switch five-level (5L) active neutral point clamped (ANPC) inverter based on switched/flying capacitor technique with self-voltage balancing. Compared to major conventional 5L inverter topologies, such as neutral point clamped, flying capacitor, cascaded H-bridge, and ANPC topologies, the new topology reduces the dc-link voltage requirement by 50%. Whilst reducing the dc-link voltage requirement, the number and the size of the active and passive components are also reduced without compromising the reactive power capability. The analysis shows that the proposed topology is suitable for wide range of power conversion applications (for example, rolling mills, fans, pumps, marine appliances, mining, tractions, and most prominently grid-connected renewable energy systems). Experimental results from a 1.2 kVA prototype justifies the concept of the proposed inverter with a conversion efficiency of around 97.5% ± 1% for a wide load range.

The design of power supply for HF MRI superconducting magnet

High-field (HF) magnet resonance imaging (MRI) is designed as a large aperture human body imaging system. The superconducting magnet provides magnetic field B0 for HF MRI. The power supply is a key component, provides excitation and demagnetization functions for superconducting magnet. According to the requirements, the superconducting magnet power supply needs to be designed and developed with small volume and high reliability and bidirectional energy flow. A combination topology is proposed, including a pulse-width-modulation (PWM) rectifier, a dual-active-bridge (DAB), and a H-bridge. In this paper, the principle of the combination topology is introduced briefly. Meanwhile, the analysis and test results of a PWM rectifier based on hybrid active-neutral-point-clamped (ANPC) three-level are given.

Maximum power exploitation for grid-connected PV system under fast-varying solar irradiation levels with modified salp swarm algorithm

With the rapid development of photovoltaic (PV) power generation technology, PV generation system has been applied into planes, cars, boats and trains. Solar irradiation levels will change rapidly in these PV systems. Fast-varying solar irradiation can invalidate the traditional configurations of PV system and maximum power point tracking (MPPT) control strategy, which will reduce the energy conversion efficiency. In order to extract maximum power exploitation, this paper proposes a novel maximum power exploitation configuration combined with the proposed control scheme for grid-connected PV system under fast-varying solar irradiation levels. In the proposed configuration, each PV panel cascades a step-up boost converter, hence allowing independent control corresponding to the irradiation levels, which generates multiple-levels of dc-voltage and may be converted to ac via an active neutral point clamped (ANPC) inverter. Meanwhile, a modified salp swarm algorithm with dynamic w factor (DWSSA) based MPPT method is proposed for each boost converter units, which can effectively accelerate convergence velocity and stability of iteration to make the searching process rapidly adapting to different fast-varying solar irradiation levels. In order to express the dynamic variation of the PV system, the mathematical model of the proposed configuration is established. Moreover, the performance of the proposed DWSSA algorithm is investigated by the stability analysis based Lyapunov theory and convergence analysis on 23 benchmark functions and CEC 2005 benchmark functions. Finally, the constructed simulation test platform is implemented and the results demonstrate that the configuration can exploit the variable converter ratios, and the proposed DWSSA method has faster tracking speed and higher energy extraction efficiency compared with the previous MPPT methods in most cases, especially, the power extraction percentage of 97.89% and the tracking time of 0.761s under the most severe uneven solar irradiation levels.

Investigation of Three-Level ANPC IGCT Converter for Industrial Medium-Voltage Drives

High-power three-level neutral-point-clamped (NPC) integrated gate commutated thyristor (IGCT) voltage-source converters have been widely used in industry applications. The novel three-level active neutral-point-clamped (ANPC) IGCT voltage-source converters can provide three additional switch states compared to the the NPC counterparts. Thus, the ANPC converter can increase the rated power compared to NPC. In this paper, the switching states of the three-level ANPC IGCT voltage-source converters are illustrated. The commutations and power loss distribution of the IGCT devices are investigated. The strategy is implemented on a high power three-level ANPC voltage-source converter equipped with 4.5kV/4kA IGCT devices. The experimental results prove that the three-level ANPC voltage-source converter has higher power density compared to the conventional NPC counterparts.

A High-Efficiency Three-Level ANPC Inverter Based on Hybrid SiC and Si Devices

Silicon carbide (SiC) devices have excellent performance, such as higher switching frequency and lower switching loss compared with traditional silicon (Si) devices. The application of SiC devices in inverters can achieve higher efficiency and power density. In recent years, the production process of SiC devices has become more mature, but the cost is still several times that of traditional Si devices. In order to balance cost and efficiency, replacing only some of the Si devices with SiC devices in a topology is a better choice. This paper proposed a high-efficiency hybrid active neutral point clamped (ANPC) three-level inverter which has only two SiC devices and the other devices are Si devices. A specific modulation strategy was applied to concentrate switching losses on the SiC devices and reduce the on-state loss through parallel operation during freewheeling intervals. Theoretical efficiency curves and experimental verification of the proposed hybrid scheme with Si-only and SiC-only schemes were carried out.

A Magnetic-Linked Multilevel Active Neutral Point Clamped Converter With an Advanced Switching Technique for Grid Integration of Solar Photovoltaic Systems

Nowadays, the grid-connected solar photovoltaic (PV) system has been drawing significant attention due to the rapid development and the decreasing cost of solar panels. The efficiency and reliability of a grid-connected PV system mainly depend on the power conversion system and the control strategy. This article presents a new magnetic-linked 7-level active neutral point clamped (ANPC) converter, which requires less number of flying capacitors and also reduces the control complexity of the grid-connected PV system. The proposed converter topology utilizes the input dc bus voltage better than the traditional ANPC multilevel converter and also provides galvanic isolation to the grid-connected PV systems. The lack of galvanic isolation is one of the biggest problems faced by the power frequency transformerless grid-connected PV systems. The proposed magnetic-linked power converter is also validated through simulations in MATLAB/Simulink and through tests in a laboratory test platform.

Hybrid Model Predictive Control of ANPC Converters With Decoupled Low-Frequency and High-Frequency Cells

Active-neutral-point-clamped (ANPC) converters can be decomposed into a low-frequency cell (LFC) and a high-frequency cell (HFC), and fundamental-frequency operation of the LFC is becoming popular. However, balancing the dc-link voltage and three flying capacitor voltages while maintaining very fast control of the output current is always a challenging task, especially when the flying capacitor is small. This article proposes a hybrid model predictive control (MPC) to guarantee the fundamental-frequency operation of the LFC and achieve the fixed-switching frequency operation of the HFC. In the hybrid MPC, the classical MPC regulates the LFC, whereas the duty-cycle-optimized MPC regulates the HFC. The capacitor voltage balance is achieved by adjusting the duty cycles to avoid the weighting factor tuning process. The proposed MPC features a fixed switching frequency and improves the steady-state performance while guaranteeing the fundamental-frequency operation of the LFC. Simulation and experimental results on a five-level ANPC are presented to validate the advantages of the proposed hybrid MPC method over the classical MPC.

Control of MW-Scale High-Frequency “SiC+Si” Multilevel ANPC Inverter in Pump-Back Test for Aircraft Hybrid-Electric Propulsion Applications

High-power-density high-speed electric motors for aircraft hybrid-electric propulsion (HEP) applications require high fundamental output frequency from power inverters. Conventional silicon (Si)-based megawatt (MW)-scale power inverters typically have low switching frequency that is not sufficient to meet the dynamic and harmonic requirements for such applications. An MW-scale medium-voltage three-level active neutral-point-clamped (ANPC) inverter based on a hybrid utilization of silicon carbide (SiC) and Si power devices (i.e., “SiC+Si”) has been developed for high-speed HEP drive applications, which has a rated output frequency of 1.4 kHz at 1-MW active power. To evaluate the power capability and efficiency of this ANPC inverter in the laboratory, power pump-back tests need to be carried out. In this article, control methods for single- and three-phase inverter pump-back tests have been developed to evaluate the performance of such high-frequency propulsion drives. The implementation and experimental results are presented to verify the efficacy of the control methods and the performance of the MW-scale “SiC+Si” ANPC inverters.