Five-level Diode-clamped Converter Research Articles

Modulation Methods for Balancing the Capacitor Voltages of N-Level Diode-Clamped Converters

Diode-clamped converters (DCCs) suffer from the inherent capacitor voltage imbalance problem. Most software-based voltage-balancing methods fall into the space-vector-modulation (SVM) category and the carrier-based pulse-width-modulation (CB-PWM) category. Despite their effectiveness for the three-level DCCs, it is not easy to extend them to a general <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> -level topology. In this paper, a new modulation method of a category called good-switching (GS) PWM is proposed for balancing the capacitor voltages. The proposed method requires no parameter-tuning effort and is extremely simple to design even for the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> -level topology regardless of the number of phases. It is also potentially applicable to other multilevel converters. The principle, the implementation, and the stability analysis of GS-PWM are elaborated for a generic <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> -level DCC. A modified GS-PWM scheme and an on-off control scheme are proposed to reduce the switching transitions for further realizing the advantages of the GS-PWM. Particularly, the on-off control can be extended to many other voltage-balancing methods. Simulation and experiments with a five-level DCC validate the proposed methods.

Multi-phase inverter-controlled induction machine at varied rotor parameters

&lt;p&gt;This paper presents a step-wise modelling of a symmetrical six-phase induction machine driven by a six-phase diode clamped multi-level inverter at a varying rotor resistance and motor inertia. The machine drive process was considered in two stages. The first stage presents the dynamic behavior of the machine when a load torque of 0 Nm and 100 Nm is applied at a varied rotor external resistance value of (0.8 and 3.2) Ω with constant motor inertia. The second stage showcased the variations in the speed, electromagnetic torque and rotor current when motor inertia is varied at (0.5 and 1.5) Kg-m2 with rotor resistance held constant. A six-phase five-level diode clamped converter phase displaced by sixty degrees with a modulation index of 0.8 was modeled to drive the poly-phase machine at a reduced %THD. All machine models were simulated in MATLAB 7.11. The simulation results showed that reduced oscillations in rotor current, motor speed and torque pulsations were achieved at a varied external rotor resistance and motor inertia.&lt;/p&gt;

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.

Voltage balance for five-level DCC based on mixed-integer linear programming

In power converters, the modulation stage commands the switching devices to drive the converter outputs to the voltage required by the inner controllers in order to achieve their objectives. However, in multilevel converters, the modulation may be in charge of an additional goal since it also has to tackle with the capacitor voltage balancing issue. This paper formulates the modulation of multilevel three-phase power converters, specifically five-level diode-clamped converters (DCC), as a mixed-integer linear optimization problem. In this paper, it is shown that the presented optimization problem can deal with the capacitor voltage balance at the same time that the number of commutations is minimized. The problem is solved offline and its solution is stored in a lookup table to be used during normal operation. Then, an online procedure to obtain the levels that modulate each phase from the lookup table is given. Comparisons with model predictive control, space-vector-based algorithms and other modulation approaches are presented in simulations. Several experimental results are presented showing the feasibility of this approach with changes in the operating conditions.

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.

Suboptimal multirate MPC for five-level inverters

The application of multilevel converters to renewable energy systems is a growing topic due to their advantages in energy efficiency Regarding its control, model predictive control (MPC) has become very appealing due to its natural consideration of discrete inputs, its optimization capability, and the present-day availability of powerful processing hardware. The main drawback of MPC compared to other control techniques in this field is that the control input is held constant during the sampling period, and it is usually difficult or even impossible to reduce this sampling period because of hardware limitations. For this reason, a multirate MPC algorithm is proposed, which allows to change the control input several times within the sampling period. The optimization problem is simplified and made suboptimal to substantially decrease computational burden. This approach is tested in simulation on a three-phase, five-level diode-clamped converter (DCC) operating in inverted mode with a three-phase resistive load. Results show significant reduction in harmonic distortion at the cost of an increase in the number of commutations with respect to a standard MPC operating at the same sampling period.

Techno-economic evaluation of five-level nested neutral point clamped converter topology for transformer-less connection of high-power wind energy conversion systems

Developers and operators are interested in improving the reliability and reducing the associated costs of wind power plants (WPPs) because of the continuous increase in the power capacity of wind energy conversion systems (WECSs) and the increasing development of WPPs. The electrical subsystem of the WPP experiences the highest failure rate and constitutes a significant proportion of its total cost. Reliability of the WECS can be increased and its cost reduced by eliminating the wind turbine transformer from the electrical subsystem. This study gives a techno-economic evaluation of a five-level nested neutral point clamped (NNPC) converter topology for transformer-less connection of high- power WECSs. The approach entailed the calculation of reliability of five-level NNPC converter topology deployed in the grid-side of a WECSs. This method presents a mathematical formula for deriving the reliability of a five-level NNPC converter topology by using the reliability block diagram and reliability estimation-based models in the military handbook (MIL-HDBK-217F). The cost analysis model shows that the total cost of the five-level diode clamped converter topology was higher than the five-level NNPC converter topology. The study could be extended by carrying out accurate modelling of the mission profile of the presented converter by using multi-domain simulation technique.

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.

Modeling and Full Decoupling Control of a Grid-Connected Five-Level Diode-Clamped Converter

This paper presents a novel approach to deal with the regulation of the dc-link capacitor voltages and ac-side currents in a grid-connected five-level diode-clamped converter. Due to the controllability problems of this topology, guaranteeing a solid current control and, mainly, a correct dc-link voltage sharing, represents a complex technical challenge. With the purpose of coping with it, an averaged model that describes the system dynamics at both sides of the converter is presented, assuming that a modulation strategy is integrated in the system to generate the switching sequence. In order to derive the proposed model, no restriction concerning the use of only the three nearest vectors to the desired voltage reference is taken into account. Then, several changes of variables are carried out in the model equations to obtain control input decoupling for control purposes, while reducing the complexity of the model as well. Finally, the voltage and current controllers are designed separately using different control inputs in a straightforward way. Neither auxiliary hardware nor complicated mathematical calculations are required to achieve the control objectives. The effectiveness and good performance of the system under the proposed control approach are validated by simulation results, suggesting that the five-level diode-clamped converter can be a solid solution as an interfacing system connected to the utility grid for, e.g., industrial drives or renewable energy applications.

Implementation of artificial neural network based controller for a five-level converter based UPQC

To reduce the cost and complexity of the system, ANN controller is proposed for a five-level converter based UPQC which eliminates mathematical operations such as a-b-c to d-q-0 transformation and complex controllers such as DSP’s and FPGA’s,. This paper proposes a five-level DCC based UPQC with ANN based controller and its performance is tested with nonlinear unbalanced loads and harmonic supply voltage. In addition, voltage related issues such as sag and swell are also considered. ANN control scheme trained with Levenberg-Marquardt backpropagation algorithm is used in this paper for effective generation of reference signals and also for maintaining desired dc link capacitor voltage. Simulations are carried out in Matlab/Simulink software with two-level UPQC, and three-level and five-level diode clamped converter based UPQC using SRF based control and ANN control schemes. The results showed better performance with the proposed concept and are discussed in this paper. The response of dc link capacitor voltage with two-level, three-level and five-level converter based UPQC and its effect on supply currents are discussed. Comparison of %THD in load voltage and supply current with two-level, three-level and five-level converter based UPQC using SRF based control and ANN control schemes is presented to show the superiority of the proposed controller.

An Improved Capacitor Voltage-Balancing Method for Five-Level Diode-Clamped Converters With High Modulation Index and High Power Factor

The capacitor voltage imbalance is a critical issue of five-level diode-clamped converters (5L-DCC). To address this issue, an inner-hexagon-vector-decomposition-based space-vector modulation (VDSVM-H1) approach is provided in the literature, which obtains the capacitor voltage balancing with high modulation index and high power factor, but renders some drawbacks. To overcome these shortcomings, a novel capacitor voltage balancing method is proposed here. First, the previous VDSVM-H1 approach is modified by introducing six new vector sequences to each triangle and applying a new vector selection rule such that the converter will not violate the 5L-DCC switching mechanism in all operating conditions. Second, the variation of the line-to-line voltage output in one sampling period is restricted to one- or two-level in the optimized region, instead of the three-level in the previous VDSVM-H1 approach, which means less harmonics generating in the ac-side outputs. Finally, the simulation and experimental results show that the proposed method can improve the VDSVM-H1 with convincing results.

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.

Voltage Balancing in Five-Level Diode-Clamped Power Converters

Abstract This paper addresses the voltage imbalance problem of the dc-link capacitors in multilevel power converters. Considering the five-level diode-clamped converter, a mathematical analysis of the capacitor voltage difference dynamics is carried out. It leads to a new problem statement that relates the voltage balancing objective to a problem of ensuring the practical stability of a nonlinear system in the presence of disturbances. Then, exploiting the properties and knowledge of the disturbance patterns, a novel and simple controller is presented. Simulation results are included to validate the performance of the proposed controller.

Harmonic Reduction in Space Vector PWM Controlled Diode Clamped Multilevel Inverter for Motor drives

This paper describes a transformer less medium voltage adjustable-speed induction motor drive consisting of two five-level diode-clamped converters connected back-to-back. It deals mainly with the voltage balancing problem that occurs when split dc capacitors are used. The switching methods by which this problem can be reduced is being described in this work. Space vector PWM is used ahead of multi carrier pulse width modulation. The harmonic values of voltage and current waveforms of the induction motor output were reduced by using the space vector PWM switching method.

Compensation of Nonlinearities in Diode-Clamped Multilevel Converters

The application of multilevel converters for exciting permanent-magnet machines with low-phase inductance to dc-link voltage ratios facilitates a reduction in high-frequency switching harmonics. However, converter nonlinearities and, in-particular, on-state device voltage drops, create additional low-frequency harmonics. This paper therefore proposes a generic compensation scheme to accommodate the effects of such converter nonlinearities and, in doing so, improve the harmonic quality of the machine phase currents. Experimental results gathered from a prototype five-level diode-clamped converter validate the benefits of the proposed scheme by showing quantitative reductions in low-frequency harmonics.

Control and DC-capacitor voltage balancing of a space vector-modulated five-level STATCOM

A space vector modulation (SVM)-based switching strategy for a five-level diode-clamped converter that is adapted as a static compensator (STATCOM) is presented. The main feature of the proposed SVM switching strategy is that it enables voltage-balancing of the DC capacitors, under steady-state and transient conditions, with no requirements for additional control actions or auxiliary devices. A dynamic model of the five-level STATCOM for power system studies is also developed. On the basis of the developed model, a voltage controller to regulate the DC-side voltage and a decoupled current controller to control the AC-side real and reactive power are designed. Performance of the STATCOM and the corresponding controllers are evaluated on the basis of digital time-domain simulation studies in the PSCAD/EMTDC environment. The studies demonstrate capabilities of the proposed space vector modulation (SVM) strategy to maintain voltage balance of the DC capacitors and control the STATCOM reactive power exchange for various operating conditions.

Analysis and Control of DC-Capacitor-Voltage-Drift Phenomenon of a Passive Front-End Five-Level Converter

The phenomenon of dc-capacitor-voltage drift is the main technical drawback of a passive front-end multilevel diode-clamped converter (DCC). This paper formulates and analyzes the dc-capacitor-voltage-drift phenomenon of a passive front-end five-level DCC, which operates based on a sinusoidal pulsewidth-modulation (SPWM) switching strategy. The analysis shows dependence of the voltage drift on the modulation index and the ac-side power factor of the DCC. The analysis concludes that an SPWM strategy, without the use of auxiliary power circuitry, is not able to prevent the voltage-drift phenomenon of a five-level DCC. This paper also proposes a space-vector-modulation (SVM)-based switching strategy that takes advantage of redundant switching vectors of the SVM method to counteract the voltage-drift phenomenon. The limit to the range of operation of a five-level DCC, which is based on the proposed SVM strategy, is also presented. The salient feature of the proposed strategy is that it enables voltage balancing of the dc capacitors with no requirements for additional controls or auxiliary-power circuitry, within the specified range of operation. The performance of a DCC under various operating conditions, based on time-domain simulation studies in the MATLAB/SIMULINK environment, is evaluated. This paper demonstrates capability of the proposed SVM strategy to control and maintain voltage balance of dc capacitors.