Diode-clamped Multilevel Converters Research Articles

A Generalized Simplified Virtual Vector PWM to Balance the Capacitor Voltages of Multilevel Diode-Clamped Converters

This article presents a generalized simplified virtual vector pulsewidth modulation (SVVPWM) to attain the capacitor voltage balancing of multilevel diode-clamped converter (DCC) for all operational conditions, which is not possible by using conventional space vector pulsewidth modulation (PWM) and carrier-based PWM methods. Virtual vector PWM is a solution presented in the literature to sort out this issue but it becomes more complex for higher level converters. The proposed SVVPWM method is simple and easy to implement for any level converter because it converts the space vector (SV) diagram of any level converter to three-level SV diagram. The SVVPWM assures the natural voltage balancing under steady states and ideal operations. Under dynamic operations, a generalized equation that describes the influence of neutral point currents on dc-link capacitor's voltage has been derived and a general decoupled voltage balancing control for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> -level DCC is presented. The proposed modulation method is implemented on five-level and seven-level DCCs to authenticate the effectiveness of decoupled voltage balancing control. Simulation and experimental results demonstrate that the proposed modulation strategy is easy to implement and effectively controls the balance of the capacitor voltages for the whole range of modulation index and load power factor.

Pulse-Amplitude-Modulation Full-Bridge Diode-Clamped Multilevel LLC Resonant Converter Using Multi-Neighboring Reference Vector Discontinuous PWM

A full-bridge diode-clamped multilevel LLC resonant converter suitable for power conversion systems that use high input voltage, such as railway vehicles, is proposed in this paper. In order to eliminate the voltage deviations of the capacitors connected in series to the high voltage input DC link, a novel modulation strategy referred to as multi-neighboring reference vector discontinuous pulse-width modulation (MNRV DPWM) is proposed. Unlike the existing two-level resonant converter that varies the operating frequency to hold the output voltage constant, the proposed multilevel resonant converter modulates the amplitude of the fundamental wave input to a resonance tank while fixing the operating frequency at the resonance point. Therefore, the design of passive elements becomes easier, and stable operation is possible over a wide operating range with only one power conversion stage. In this paper, the control algorithm and operation characteristics of the newly proposed full-bridge diode-clamped four-level LLC resonant converter are analyzed in detail and design guidelines are presented. The feasibility of the proposed converter is verified through a simulation and an experiment with a prototype converter.

A Contactless Receptacle Applied to DC Power Distribution

The modern household power distribution with dc-to-dc power transfer is presented in this paper in order to demonstrate inductive power transfer technology over air-core and ferromagnetic-core. A high-frequency switching with a five-level diode clamped multilevel converter plays an important role in generating a low-%THD voltage waveform in order to increase the power density or reduce the size of the plug and receptacle. The major setbacks of using the dc-to-dc power transfer with electric shock, galvanic isolation issue and arc discharge on the contactor have been solved by using the inductive power transfer. In addition, an LLC series resonant tank is integrated with the power converter to satisfy the Zero-Voltage Switching (ZVS) condition of all active switching devices. The proposed contactless receptacle technique is validated by using MATLAB/Simulink and a developed hardware prototype. The efficiency of the power transfer while using the magnetic material core versus the air-core type is approximately 83.8% and 92.6%, respectively.

DC-Link Capacitor Voltage Balancing Control of a Five-Level Regenerative AC Electronic Load Using One-Cycle Control

High voltage electric power equipment requires rigorous regulation testing to specific standards which ensure proper and safe operation in the grid. Manufacturers conduct these tests in order to prove standard compliance and product liability. Variable linear or nonlinear loads are necessary for testing medium voltage (MV) high power AC power converters. Generally, those AC power converters or power supplies require performance validation, burn-in and/or lifetime testing under different load conditions, defined by the end-user or standards for the given applications. For flexible and efficient MV verification testing, this paper presents a five-level multilevel converter-based MV regenerative AC electronic load with one-cycle control (OCC), which is based on five-level diode-clamped multilevel converters with back-to-back structure and can emulate any impedance load. In this paper, especially the dc-link capacitor voltage balance of the proposed multilevel MV regenerative AC load is deeply analyzed. Simulation and experimental results are presented to verify the dc-link voltage balance performance of the proposed multilevel MV regenerative AC electronic load.

Mitigation of Harmonics and Unbalanced Voltage Disturbance Compensation by MSVPWM Based- DVR in the Distribution Net

Recent power distribution networks comprise abundant sensitive loads, which extremely impact the power quality of source in electrical power networks. Voltage dip, voltage rise, imbalanced voltage, line notching and distortion of harmonics are problems of power quality frequently take place. Pre-disturbance voltage compensation strategy and phase-locked-loop (PLL) based dq- space vector control are presented to improve a Dynamic Voltage Restorer (DVR), which restore the magnitude of voltage disturbance and displacement of phase angle to prior of voltage disturbance. 3-phase Multilevel strategy of Space Vector Pulse Width Modulation (MSVPWM) based- Multilevel Diode Clamped Converter (MDCC) is proposed as switching pulse signals employed low frequency, which creates high levels of voltage and fewer harmonics in the output waveform in comparison to 2-level SVPWM based- DVR. 3-level SVPWM based- DVR under balanced and imbalanced distortion voltage disturbances included sags and swells injected appreciated quantities of voltage, thereby attained ideal sinusoidal waveform with lower Total Harmonic Distortion THD% compared to 2-level SVPWM based- DVR. Furthermore, real and imaginary powers balanced effectively at sensitive load during various distortion voltage disturbance conditions via presented work. The proposed simulation model of multi-level SVPWM based- DVR is implemented by dedicating the software system of MATLAB/SIMULINK. The results of simulation exhibit the effectiveness and efficiency of the presented work under different distortion voltage disturbance conditions.

Design and Analysis of the STATCOM Based on Diode Clamped Multilevel Converter Using Model Predictive Current Control Strategy

In recent decades, multi-level converters have become popular and used in many power systems applications. Compared with conventional converters, multi-level converters contribute to reducing the voltage stress on the switching devices and enhancing the power quality delivered to the load. In this paper, the study of the five-level diode clamped multilevel converter based static synchronous compensator has been accomplished. Model Predictive current control strategy which a type of modern control algorithms was employed for driving the proposed compensator. The suggested five level converter controlled by model predictive current control is firstly examined to verify that this control algorithm is appropriate for achieving the desired performance. Then the proposed converter and control combination is employed and simulated as a static synchronous compensator in distributed power system. Moreover, in order to examine the robustness of this compensator, the load status is suggested to be heavy inductive. Simulation process has been performed using MATLAB – SIMULINK software package. The results show that the implemented configuration (converter and control algorithm) provides high power quality improvement with adequate reactive power compensation.

Simulink Implementation of Voltage Stability Improvements Using STATCOM based 5-level Diode Clamped Converter

In this paper, a STATCOM based on Five-level Diode Clamped Multilevel Converter is used to maintain the bus voltage at specific bus bar in a suggested grid under various abnormal conditions like 2-phase to ground fault, load disturbance and source disturbance. The suggested grid consists of a single machine, transmission line and loads. The compensation procedure is carried out and compared between the two suggested control methods, the first method was the traditional control method using Phase Shifted-PWM controller which is used to improve the voltage profile. The Second one was the Finite Set-Model Predictive Controller for improving the voltage profile. The simulation results show that the two methods provided significant improvements of the bus voltage profile. Moreover, the improvement of voltage profile using FS-MPC method is less over shoot and attains a steady state better than the conventional PS-PWM when the system is subjected under many disturbances. The results of the harmonic effect show low values of total harmonic distortion (THD) of supply voltage and current.

One-Cycle Control of Three-Phase Five-Level Diode-Clamped STATCOM

This paper presents a new multilevel one-cycle control (OCC) technique for a five-level diode-clamped multilevel converter (DCMC)-based static VAR compensator (STATCOM). The OCC algorithm in combination with multi-carrier level-shifted PWM control is used in the proposed five-level DCMC-based STATCOM. The automatic dc link voltage balance is simply realized by four voltage compensators presented as the part of the OCC controller. In this paper the OCC principle of a two-level converter-based STATCOM is reviewed and extended to the five-level DCMC-based STATCOM. The proposed multilevel OCC control method of the five-level DCMC-based STATCOM in steady-state and dynamic operation modes is verified by simulation and experimental results.

Simplified hybrid space vector modulation for multilevel diode clamped converter

This study proposes a new simplified hybrid space vector modulation (HSVM) algorithm for multilevel diode clamped converter (DCC). The main idea consists in combining the advantages of both space vector modulation (SVM) algorithms synthesised in abc and αβ coordinates to establish a new hybrid, simplified and generalised SVM algorithm. The HSVM algorithm offers a simple method to compute the duration time, automatic generation of all redundant states of the adjacent switching vectors without triangles identification stage and switching sequence definition for any DCC level. Moreover, all the computational processes are performed in one unique sector thanks to the proposed new reference voltage vector defined in abc-coordinates. The proposed algorithm does not require any coordinates transformation or lookup tables. Also, it can be implemented with minimum computational burden using a low resources digital platform. The proposed HSVM is verified through MATLAB simulation and validated experimentally using three-level DCC laboratory prototype. For high-level DDC, the HSVM is tested in real time using hardware-in-the loop testing technology. Different case studies are performed to demonstrate the high performance of the proposed HSVM and its ability in lowering the computational resources requirement.

New Simplified and Generalized Three-Dimensional Space Vector Modulation Algorithm for Multilevel Four-Leg Diode Clamped Converter

This article aims to propose a new simplified and generalized three-dimensional space vector modulation (3DSVM) for four-leg multilevel diode clamped converter (FL-DCC). The idea is to use the three-dimensional space vector diagram of the multilevel FL-DCC in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">αβγ</i> space, while all calculation stages of the 3DSVM algorithm will be carried out in a new frame called <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ρστ</i> . The proposed 3DSVM algorithm provides, for any inverter level, 1) simple way to calculate the duration time; 2) automatic generation of the adjacent switching states; 3) balancing the dc capacitor voltages, and 4) switching sequence definition without prism and tetrahedron identification stage. All 3DSVM steps are implemented in only one sector, thanks to the proposed new reference voltage vector. Such feature makes possible to build a generalized 3DSVM that can be implemented with minimum computational burden in simple embedded digital platforms. Simulation and experimental results are provided to demonstrate the proposed concept.

Research on the Equivalent Virtual Space Vector Modulation Output of Diode Clamped N-level Converter under Multi-Modulation Carrier Modulation

Diode-clamped multi-level converters have DC-side capacitors in series, which will lead to the unbalance of DC-side capacitor voltage, the distortion of the output waveform, the increase of total harmonic distortion (THD), and even the damage of switching devices, which will make the system inoperable. The proposal of virtual space vector pulse-width modulation (VSVPWM) realizes the balanced control of the capacitor voltage, but when the output level of converter increases, the implementation of VSVPWM becomes very complicated, and the amount of calculation also increases greatly, thus hindering its application in the multi-level circuit. Compared with VSVPWM, the carrier-based pulse-width modulation (CBPWM) is simple to operate and easy to implement. If the equivalent relationship between CBPWM and VSVPWM can be found, the application of VSVPWM can be generalized to any level, and the advantages of VSVPWM can be fully utilized. This paper aims to study the inner relationship of VSVPWM and the multi-modulation carrier CBPWM (MCBPWM). After strict theoretical analysis, the equivalent relationship of VSVPWM and MCBPWM in the three-level and four-level and converter is realized by injecting the zero-sequence component into the modulation waves. Furthermore, the equivalent relationship between VSVPWM and MCBPWM is deduced to the N-level converter. Finally, the correctness of the relevant theoretical analysis is verified by the experiment.

Design and Software Implementation of Multilevel Diode Clamped Converter

The work presented in this paper concerned with the design and software synthesis of multilevel diode clamped converter. Two topologies of this type of converters have been considered and tested. The first topology was three-level diode clamped converter, while the second one was the five-level diode clamped converter. The major task of the proposed work is to feed a three-phase load with semi-pure (low distorted) sinewave three-phase voltages. Simulation results show that an approximately unity power factor, low harmonic distortion and balance of capacitors voltages are achieved. The work presented for three level and five level to verify the positive effect of increasing the level of converter. The digital simulation is performed using the MATLAB / Simulink software.

Equivalent Space Vector Output of Diode Clamped Multilevel Inverters Through Modulation Wave Decomposition Under Carrier-Based PWM Strategy

The carrier-based PWM method (CBPWM) and the space vector PWM method (SVPWM) are two main modulation methods for diode clamped multilevel converters, and there is a certain relationship between these two modulation strategies. For general 8 segments SVPWM modulation sequence in the multilevel inverter, the inherent relationship between these two modulation methods have been researched, but the relationship between CBPWM and SVPWM in an arbitrary level sequence has not been obtained. To solve this problem, an in-depth study of the relationship between the multilevel SVPWM method and the CBPWM method has been made in this paper. By decomposing the modulation wave, and the SVPWM can be achieved by multi-modulation waves CBPWM. And the relationship between SVPWM and CBPWM in N level with an arbitrary number of segments sequence are obtained. Thus, through simple multi-modulation waves CBPWM strategy, less THD in the spectrum and higher DC voltage utilization switching sequences of multilevel inverters can be obtained. The simulation and experimental results illustrate that the relationship between CBPWM and SVPMM is correct.

A Simplified Virtual Vector PWM Algorithm to Balance the Capacitor Voltages of Four-Level Diode-Clamped Converter

The voltage balancing problem of dc-link capacitors in multilevel diode-clamped converters limits their applications in industry under some operational conditions. Virtual vector PWM (VVPWM) can attain the capacitor voltage balancing under all operational conditions but it increases the complexity and computation burden by increasing the number of triangular regions for higher levels as compared to conventional space vector PWM (CSVPWM). In order to solve the dc-link voltage balancing problem in four-level diode-clamped converter, a simplified virtual vector PWM (SVVPWM) algorithm is proposed in this paper. The pair of new virtual vectors is defined in such a way that the space vector diagram of four level converter is transformed into three level space vector diagram that lessens the complexity and computation burden by decreasing the number of triangular regions. The balancing of capacitor voltages can be achieved by using proposed SVVPWM algorithm for the whole range of modulation index load power factor. Steady state and dynamic performance of the SVVPWM algorithm is verified through the simulations and experiments.

A Neutral-Point Diode-Clamped Converter With Inherent Voltage-Boosting for a Four-Phase SRM Drive

This article proposes a new asymmetric neutral-point diode-clamped (NPC) multilevel converter for a four-phase switched reluctance motor drive. The inbuilt NPC clamping capacitors are used for both voltage level clamping and also as dc rail voltage-boosting capacitors to increase the output power of the motor, particularly for high-speed electric vehicle applications. The new converter allows regenerative energy to be recovered back to the dc supply for rapid machine braking, thus increasing overall drive efficiency. Analysis of the different modes of converter operation, along with design equations for sizing the voltage-boosting capacitors, is detailed. The effect of capacitance on boost voltage and increased motor base speed is presented. Simulation and experimental results confirm the effectiveness of the proposed converter.

A Novel Hybrid Voltage Balance Method for Five-Level Diode-Clamped Converters

Voltage balance control of dc capacitors is the key problem of multilevel diode-clamped converters (DCCs). In this paper, a novel hybrid voltage balance method is proposed for five-level DCCs. First, to balance the upper or lower two capacitors, a novel flying-capacitor-based voltage balance circuit (VBC) is proposed, and the time-domain average model of the VBC is established. Then a state-machine-based control strategy is developed to balance capacitor voltages as well as to limit overcurrent of inductors in a transition process. To keep the midpoint voltage stable, a zero-sequence injection method is proposed, which inherently ensures the improved modulation signals locate within the linear modulation ranges. With the flying capacitor transferring energy between dc capacitors, the device voltage stresses of the VBC are equalized, and the steady-state current ripples of the inductors are much smaller than those of the conventional chopper-based VBCs, which can reduce the volume and weight of the inductor. Simulation and experimental results for both inverter and rectifier operations prove that the proposed hybrid voltage balance method can balance dc capacitors of five-level DCCs in both steady states and transition states, and the inductor currents of VBCs can also be restricted within safe ranges.

A controller for practical stability of capacitor voltages in a five-level diode-clamped converter

The balancing of the dc-link capacitor voltages represents the main technical challenge in multilevel diode-clamped converters. In particular, when the number of capacitors is greater than two, these power conversion systems must be actively controlled to achieve a correct voltage sharing. Focusing on the five-level diode-clamped converter topology, this paper deals with the voltage imbalance phenomenon from a control point of view. For that purpose, the voltage balancing objective is addressed as a problem of ensuring the practical stability of a nonlinear system under the presence of external disturbances. Considering this approach, a novel control method is proposed to regulate the outputs of the system. It is based on several steps, which should be carried out during the controller design stage, that lead to define a discrete-time controller with sampling frequency corresponding to the triple of the ac-source frequency. Finally, the performance of the proposed controller is analyzed under varying operating conditions to demonstrate the capability and viability of the controller to attain the control goals.

Generalised approach for predictive control with common‐mode voltage mitigation in multilevel diode‐clamped converters

This study proposes a generalised approach based on model predictive strategy for the current control, dc-link capacitor voltages balancing, switching frequency reduction and common-mode voltage mitigation in multilevel diode-clamped converters. A generalised discrete-time model of the converters is presented, where all the control objectives are formulated in terms of the switching states. The control goals are expressed as a cost function, and with the help of suitable weighting factors these goals are met simultaneously. The cost function minimisation is used as criteria for choosing the best switching state which would be applied to the converter during next sampling interval. The real-time digital control issues such as computational burden and delay compensation are also discussed. The feasibility of the proposed method is verified by simulations in three- to six-level converters, and by experiments in three- and four-level converters.

Multilevel Converters for Industrial Applications [Book News

This book first emphasizes the generalized multilevel converter topology with a common dc bus with its basic cell and then details the genesis of the neutral point clamped (NPC) topology and diode-clamped multilevel converters. It also includes the operation of the flying capacitor multilevel converter, the cascaded cell multilevel converter, and the hybrid asymmetric topologies. The book also covers state-of-the art multilevel modulation methods, including space vector modulation, the derivation of the boundary of voltage balancing in NPC multilevel converters, and the optimization of the dc capacitor voltage balance. Two case studies are included: 1) The application of cascade asymmetric multilevel converters in distribution static compensator (DSTATCOM) and 2) a medium-voltage induction motor (IM) drive using two back-to-back connected, five level NPC converters.

Five‐level diode‐clamped active power filter using voltage space vector‐based indirect current and predictive harmonic control

This study presents an active power filter (APF) based on diode-clamped multilevel circuits with auxiliary dc-link voltage balancing circuits and harmonic compensation control. The control algorithm is composed of indirect current loop for active and reactive power control, predictive current loop for harmonic filtering control and independent control for auxiliary balancing circuits. Among these modules, the indirect current along with the predictive current control will generate a combined voltage reference to the module of space vector pulse-width modulation, which could output optimal modulation signals to the voltage control of a diode-clamped multilevel converter. Comparing with traditional dc control for two-level APF, this system takes advantages of indirect current control and multilevel circuits, including improvement in waveform quality, increase in system power rate and reduction in voltage stress and switching frequency on power semiconductors. Simulation and experimental results validate the effective control of the presented system.