Multilevel Power Converter Research Articles

Mixed-integer bilinear and piecewise-linear models for designing switching strategies of multilevel power converters

With the adoption of multilevel converters in the industry, the importance of their optimal operation has increased. Able to operate with more than three voltage levels, multilevel converters are often modulated by vectors defined in time, which can be optimized to reduce the number of commutations or total harmonic distortion. This papers presents two formulations that do not use trigonometric functions for selecting optimal switching angles, focusing on the minimization of the current weighted total harmonic distortion. The first relies on bilinear terms to yield a mixed-integer nonlinear programming (MINLP) formulation that can be tackled with MINLP algorithms. The second uses piecewise-linear approximation to enable the application of mixed-integer quadratic programming. Computational results from the application of these formulations are reported, in conjunction with a simulation of the optimal angles that were computed for a 5-level Neutral-Point Clamped converter.

Investigation of Fault-Tolerant Capabilities in an Advanced Three-Level Active T-Type Converter

A novel fault-tolerant three-level power converter topology, named advanced three-level active T-Type (A3L-ATT) converter, is introduced to increase the reliability of multilevel power converters used in safety-critical applications. This new fault-tolerant multilevel power converter is derived from the conventional T-Type converter topology. The topology has significantly improved the fault-tolerant capability under any open circuit or certain short-circuit faults in the semiconductor devices. In addition, under healthy condition, the redundant phase leg can be utilized to share overload current with other main legs, which enhances the overload capability of the converter. The conduction losses in the original outer devices can be reduced by sharing the load current with the redundant leg. Moreover, unlike other existing fault-tolerant power converters in the literature, full output voltages can be always obtained in this proposed A3L-ATT converter during fault-tolerant operation. A 13.5-kW ATT-A3L converter prototype was developed and constructed using silicon carbide MOSFETs. Simulation and experimental results were obtained to substantiate the theoretical claims of this new fault-tolerant power converter.

Interleaved multi‐level power converter for a battery energy storage system

This study proposes an interleaved multi-level power converter (IMPC) for a battery energy storage system. The proposed IMPC is composed of two power-electronic legs. The first power-electronic leg is an interleaved multi-level leg that contains eight power-electronic switches. Four power-electronic switches constitute a two-phase interleaved switch set and these are switched in a high-frequency interleaved pulse-width modulation. The other four power-electronic switches constitute a level-selection switch set that determines the dc bus voltage for the two-phase interleaved switch set according to the magnitude of the utility voltage. The second power-electronic leg is the low-frequency switching leg, which is composed of two power-electronic switches. Since the proposed IMPC integrates the performances of the multi-level power converter and interleaved power converter, the effective switching frequency is twice the practical switching frequency and the output voltage has five voltage levels. The ripple of the output current is significantly attenuated and the power capacity of the output filter is reduced. The proposed IMPC can also be used to adjust the real and the reactive power flows, in order to control the charging/discharging of battery sets and to compensate for the reactive power. A hardware prototype is developed to verify the performance of the proposed IMPC.

Single-Phase AC–DC–AC Multilevel Converter Based on H-Bridges and Three-Leg Converters Connected in Series

This paper investigates an ac–dc–ac multilevel power converter. The studied configuration is composed of two single-phase ac–dc–ac three-leg modules and series-connected H-bridges in the shared part of system. Because the proposed converter has shared legs between the input and output, it is employed in applications with same input and output frequency. Uninterrupted power supply and unified power quality conditioner are application examples for this converter. Such multilevel topology has lower dc-link voltage rating, which, consequently, presents low switch blocking voltages when compared to conventional topologies. System model, a space-vector pulsewidth modulation (PWM) strategy to symmetrical and asymmetrical dc-link voltages, and an overall control strategy to adjust the system variables are presented. A power flux analysis shows the operation zone in which the individual dc-link voltage balancing is possible. PWM and control strategies are developed to reduce the semiconductor total losses, harmonic distortion, and switching stress. Two ac–dc–ac multilevel conventional structures are used for comparison. Simulation and experimental results demonstrate the feasibility of the studied converter.

Single-Phase AC–DC–AC Multilevel Converter for Grid Overvoltage Based on an H-Bridge Connected in Series to the Five-Leg Converter

In this work, an ac–dc–ac multilevel power converter for grid overvoltage is investigated. The developed configuration is composed of one single-phase ac–dc–ac three-leg module and two H-bridges. One of them is connected to the shared part of the system to generate multilevel voltages at the input and output of the structure, and the other one is connected to the grid side to compensate the grid overvoltage. The converter can be employed in applications with same input and output frequency, such as uninterrupted power supply and unified power quality conditioner without isolation transformer. System model, a space vector pulsewidth modulation strategy to balance the individual dc-link voltages, and an overall control strategy to adjust the grid power factor, the amplitude, and the frequency of the load voltage are presented. The proposed converter is compared in terms of harmonic distortion and semiconductor losses with conventional structures. Simulation and experimental results demonstrate the operation of the proposed topology.

New three-port DC-AC inverter outputting a single-phase and a set of three-phase AC voltages

ABSTRACTA conventional DC-AC inverter can only output either a single-phase AC voltage or a set of three-phase AC voltages. A new three-port DC-AC inverter which can simultaneously output a single-phase AC voltage and a set of three-phase AC voltages is proposed in this paper. This three-port DC-AC inverter is based on the three-port T-type multi-level power converter which is composed of three T-type power electronic legs, a decoupling transformer set, a filter inductor set, a single-phase filter capacitor, and a three-phase filter capacitor set. The DC port of the proposed power converter is connected to a DC power source to act as the input port, and the single-phase AC port and the three-phase AC port serve as two output ports to supply power to the single-phase load and the three-phase load, respectively. The zero-sequence transformer is used to decouple the single-phase and three-phase AC components, which are generated by the three T-type power electronic legs. The operation principle of this three-port DC-AC inverter is analyzed, and a hardware prototype is established to verify the performance of the proposed three-port DC-AC inverter. The experimental results are as expected.

A Building Block IC for Designing Emerging Hybrid and Multilevel Converters

This paper introduces a practical power management integrated circuit (IC) building block, i.e., IC module, which enables the implementation of various emerging hybrid and multilevel power converter topologies in low-power high-frequency applications, operating at switching frequencies up to 10 MHz. Compared to conventional buck- and boost-based topologies, the emerging multilevel and hybrid converters offer significant volume reduction and power processing efficiency improvements. These key factors are significant contributions for modern portable and/or small footprint electronics devices, where size is of great concern. The IC building block (ICBB) consists of two segmented half bridges as well as all the elements needed for implementing mixed-signal current programmed mode (CPM) control. Mixed-signal CPM compatibility coupled with a segmented power stage not only provide inherent current protection and simple controller implementation but also allows for dynamic efficiency optimization. Operation at high-switching frequencies and simple modular design with ICBB are enabled through the development of novel architectures for key functional blocks of the ICBB and a cost-effective solution for level shifting, providing communication between external control signals and module inputs. For example, solutions for high-frequency low-power current sensing and fast digital-to-analog conversion are introduced as well as a capacitive coupling and coding schemes for data transfer. Several identical custom-designed ICBB modules are fabricated in the 0.13- $\mu \text{m}$ CMOS process, each providing up to 2 W of output power, while operating at a switching frequency of up to 10 MHz. The versatility and proper functionality of these modules are verified through simulation and experimental results.

Eight Levels Multilevel Voltage Source Inverter Modulation Technique

In recent years, there has been a substantial increase in interest in multilevel power conversion. This has led to the recent research of new converter topologies and modulation strategies. This article describes the operating principle of a novel 8-level inverter topology based on flying capacitors. It has 12 MOSFETs switching devices are arranged such that 6 are complementary and CD sources are used to generate the different voltage levels of the output waveform. The control technique used is the sinusoidal pulse-width modulation (SPWM).

DSPWM multilevel technique of 27-levels based on FPGA for the cascaded DC/AC power converter operation

Summary In this paper, a digital sinusoidal pulse width modulation (DSPWM) multilevel technique of 27-levels based on field programmable gate array (FPGA) is introduced, as an alternative to control of the direct current/alternating current multilevel power converters. The implementation of this technique with an FPGA XC3S500E model is achieved in the Xilinx Spartan-3E FPGA platforms. An experimental prototype is implemented by 3-cascaded H-bridges controlled by the DSPWM multilevel technique, generating high efficiency, low cost, and lower harmonic content. The efficiency of the DSPWM multilevel technique using R, RL, RC, and RLC loads connected to the power network is verified.

Selected converter topologies for interfacing energy storages with power grid

Abstract The paper presents different solutions applicable in power converter systems for connecting power grids with energy storage systems such as superconducting magnetic energy storage (SMES), supercapacitor energy storage (SES) or chemical batteries. Those systems are characterized by bidirectional current flow between energy storage and power grid. Two-level converters (AC-DC and DC-AC converters) dedicated for low power energy storage compatible with 3×400 V-type power grids are proposed. High power systems are connected with 3×6 kV-type power grids via transformers that adjust voltage to the particular energy storage or directly, based on multilevel power converters (AC-DC and DC-AC) or dual active bridge (DAB) systems. Solutions ensuring power grid compatibility with several energy storage systems of the same electrical parameters as well as of different voltage-current characteristics are also proposed. Selected simulation results illustrating operation of two system topologies of 200 kW power for two-level converter and neutral point clamped (NPC) three-level converter are presented.

Cross-Regulation Assessment of DIDO Buck-Boost Converter for Renewable Energy Application

When medium- or high-voltage power conversion is preferred for renewable energy sources, multilevel power converters have received much of the interest in this area as methods for enhancing the conversion efficiency and cost effectiveness. In such cases, multilevel, multi-input multi-output (MIMO) configurations of DC-DC converters come to the scenario for integrating several sources together, especially considering the stringent regulatory needs and the requirement of multistage power conversion systems. Considering the above facts, a three-level dual input dual output (DIDO) buck-boost converter, as the simplest form of MIMO converter, is proposed in this paper for DC-link voltage regulation. The capability of this converter for cross regulating the DC-link voltage is analyzed in detail to support a three-level neutral point clamped inverter-based grid connection in the future. The cross-regulation capability is examined under a new type of pulse delay control (PDC) strategy and later compared with a three-level boost converter (TLBC). Compared to conventional boost converters, the high-voltage three-level buck boost converter (TLBBC) with PDC exhibits a wide controllability range and cross regulation capability. These enhanced features are extremely important for better regulating variable output renewable energy sources such as solar, wind, wave, marine current, etc. The simulation and experimental results are provided to validate the claim.

A Modular Multilevel Converter with Power Mismatch Control for Grid-Connected Photovoltaic Systems

A modular multilevel power converter configuration for grid connected photovoltaic (PV) systems is proposed. The converter configuration replaces the conventional bulky line frequency transformer with several high frequency transformers, potentially reducing the balance of systems cost of PV systems. The front-end converter for each port is a neutral-point diode clamped (NPC) multi-level dc-dc dual-active bridge (ML-DAB) which allows maximum power point tracking (MPPT). The integrated high frequency transformer provides the galvanic isolation between the PV and grid side and also steps up the low dc voltage from PV source. Following the ML-DAB stage, in each port, is a NPC inverter. N number of NPC inverters’ outputs are cascaded to attain the per-phase line-to-neutral voltage to connect directly to the distribution grid (i.e., 13.8 kV). The cascaded NPC (CNPC) inverters have the inherent advantage of using lower rated devices, smaller filters and low total harmonic distortion required for PV grid interconnection. The proposed converter system is modular, scalable, and serviceable with zero downtime with lower foot print and lower overall cost. A novel voltage balance control at each module based on power mismatch among N-ports, have been presented and verified in simulation. Analysis and simulation results are presented for the N-port converter. The converter performance has also been verified on a hardware prototype.

A Fast On-Line Diagnostic Method for Open-Circuit Switch Faults in SiC-MOSFET-Based T-Type Multilevel Inverters

On-line condition monitoring is of paramount importance for multilevel power converters used in safety-critical applications. A novel on-line nonintrusive diagnostic method for detecting open-circuit switch faults in silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFETs)-based T-type multilevel converters is introduced in this paper. The principle of this method is based on monitoring the abnormal variations of the dc-bus neutral-point current in combination with the existing information on instantaneous switching states and phase currents. Advantages of this method include faster detection speed and simpler implementation compared to other existing diagnostic methods in the literature. Moreover, this diagnostic method is immune to the disturbances of inverter's dc-bus voltage unbalance and load unbalance. In this method, only one additional current sensor is required for measuring the dc-bus neutral-point current; therefore, the implementation cost is low. Simulation and experimental results based on a lab-scale 20 kVA adjustable speed drive with a three-level SiC T-type inverter validate the effectiveness and robustness of this novel diagnostic method.

Research on Output Voltage Modulation of a Five-Level Matrix Converter

The matrix converter (MC) is a direct ac-ac power converter without an intermediate dc link. It has several attractive features, such as high power density, four-quadrant operation, and bidirectional power flow. However, the application of MCs in high-voltage and high-power systems is still limited by the device voltage levels. It is well known that the multilevel power converter technology can be a solution to enhance the voltage rating and improve harmonic performance in many applications. Therefore, an application of a multilevel topology based on a direct ac-ac power converter is proposed in this paper. This paper first introduce the topology structure, the switching pattern grouping method, and the proposed output voltage modulation method for a five-level capacitor clamped multilevel matrix converter (5LMC). Amplitude coefficient adjustment is also introduced as a part of modulation procedure to simplify the duty cycles calculation. Simulation from MATLAB/Simulink is given to demonstrate the theoretical work. Experimental results from a five-level matrix converter prototype are presented to validate the theoretical findings.

Selective Harmonic Elimination Model Predictive Control for Multilevel Power Converters

In this study, a model predictive control (MPC) strategy that combines finite-control-set MPC with selective harmonic elimination (SHE) modulation pattern in its formulation is proposed to govern multilevel power converters. Based on a desired operating point for the system state (converter current reference), an associated predefined SHE voltage pattern is obtained as a required steady-state control input reference. Then, the cost function is formulated with the inclusion of both system state and control input references. According with the proposed reference and cost function formulation, the predictive controller prefers to track the converter output current reference in transients, while preserving the SHE voltage pattern in steady state. Hence, as evidenced by experimental results, a fast dynamic response is obtained throughout transients while a predefined voltage and current spectrum with low switching frequency is achieved in steady state.

A Comparative Study Between the Nearest Three Vectors and Two-Level Hexagons Based Space Vector Modulation Algorithms for Three-Level NPC Inverters

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. Basically, there exist two space vector modulation (SVPWM) schemes which are commonly used to modulate the output voltages provided by three-level inverters . For the first method, namely the nearest three vectors (NTV), the position of the space vector is identified in one of the triangles forming the space vector diagram. As for the second method that is referred to as the hexagon technique, the opportune position of the space vector is determined within a two-level hexagon. This paper is therefore focused on a comprehensive comparative study between the performances of the two aforementioned techniques. The evaluation is performed in terms of Common Mode Voltages (CMV), Total Harmonic Distortion (THD), conducted emissions, and efficiency. The proposed comparison is based on numerical simulations and experimental tests. The obtained results confirm that the NTV technique provides the better performances in terms of THD and CMV for low modulation index values. It also provides less common mode emissions when the modulation index is near its maximum value. As for the efficiency, both methods provide the same performance for a modulation index superior or equal to 0.8.

New Flying-Capacitor-Based Multilevel Converter With Optimized Number of Switches and Capacitors for Renewable Energy Integration

The flying-capacitor-based multilevel converter is one of the well-known breeds of the multilevel power converters. This paper proposes a new flying-capacitor-based multilevel converter to minimize the number of flying capacitors (FCs) and power switches. The advantage of the proposed FC-based multilevel converter in comparison with the conventional flying-capacitor multicell converter is that it needs fewer FCs. Also, in comparison with the stacked multicell converter, the proposed multilevel converter requires fewer semiconductor switches. In order to balance the voltage of the FCs in proposed multilevel converter, a new active voltage balancing method which is fully implemented using logic-form equations is presented. The proposed voltage balancing method measures output current and FC voltages to generate switching states to produce the required output voltage level, as well as balance the FCs voltages at their reference values. The output voltage of the proposed multilevel converter controlled with suggested active voltage balancing method can be modulated with any pulse-width-modulation (PWM) method, such as phase-shifted-carrier PWM or level-shifted-carrier PWM. Simulation results and experimental measurements of proposed FC-based multilevel converter are presented to verify the performance of the proposed converter, and its novel switching and modulation strategy, which is based on the active voltage balancing method.

Binary Control Design for a Class of Bilinear Systems: Application to a Multilevel Power Converter

This brief studies the local stabilization of bilinear systems with binary inputs. A novel convex optimization procedure for the robust design of switching surfaces is proposed using convex optimization tools and linear matrix inequalities. The procedure takes into account the robustness to parametric uncertainties in the system matrices. An experimental illustration for the stabilization of a three-cell multilevel power converter is given.

Control Design Strategy for Flying Capacitor Multilevel Converters Based on Petri Nets

Multilevel power converters belong to a class of hybrid systems, where continuous and discrete variables coexist, representing a challenge for their control design. Flying capacitor multilevel converters require controlling the flying capacitor voltages and the output current. In this regard, several works have studied this problem. However, the discrete nature of the converter has not been directly included in the control design. In this paper, a hybrid representation of the converter is considered, and using Petri net (PN) methodology, a control scheme is proposed to regulate the flying capacitor voltages and the output current. An analysis of stability, using Lyapunov method, is presented, and sufficient stability conditions are obtained. From these conditions, the commutation rules are defined and used to define the transition rules for the PNs. Experimental results validate the performance of the proposed scheme.

Conception of a modular multilevel converter for high-scale photovoltaic generation based on efficiency criteria

This work presents the design of a modular multilevel converter for high-scale grid-connected photovoltaic generation. Converter modules are used in order to divide between them the power to deliver, giving more advantages in comparison with using only one total converter. Losses and efficiencies are derived for each stage of a modular component, in order to select the most adequate number of converter modules according to efficiency criteria. First, a modular multilevel converter topology is proposed, rising its advantages over other reported topologies. Second, the calculation of losses of each stage of the converter module is performed. Then, the design of the modular multilevel converter is developed for two practical examples, one for a 10kW–1.32kV generation system, and another one for a 1MW–13.2kV medium-voltage generation system. These designs are developed considering different number of converter modules, and an analysis to select the most adequate number of them is made for both cases. One of the intentions of this work is to draw the highest efficiency possible when implementing a multilevel power converter for interconnecting photovoltaic generation to the grid.