Improved Modulation Method Research Articles

An improved modulation method for modular multilevel converters based on particle swarm optimization

Modular multilevel converter (MMC) is the first topology choice for flexible DC transmission systems. This paper presents a particle swarm optimization (PSO) algorithm aided modulation method to calculate the improved selective harmonic elimination pulse width modulation (iSHE-PWM) switching angles with the aim of reducing the total harmonic distortion (THD) and output voltage error of MMC. The SHE-PWM has more controllable variables compared with the staircase modulation so that it can be used to improve the output current waveform quality. This method retains the advantages of easy to control and low frequency from the staircase modulation. But amplitude of its high-order harmonic is high and this paper improves it. Firstly the mathematical model of MMC is set up and corresponding expressions of THD and voltage error are given. The joint function of THD and voltage error is considered as the fitness function. The PSO algorithm was used to minimize the joint function for solving the optimal switching angles. Finally the proposed method is verified through simulation results showing its potential to successfully solve the problem of difficulty in obtaining the iSHE-PWM switching angles while minimizing the THD and output voltage error of the MMC.

Variable switching frequency PWM for quasi‐Z‐Source inverter based on DC current ripple prediction

AbstractHerein, a variable switching frequency pulse width (VSFPWM) modulation strategy for quasi‐Z‐source inverter (qZSI) based on a recently proposed modulation method, named as ZSVM8, is proposed. The formulae of quasi‐Z‐source inverter inductor current ripple of ZSVM8 are deduced in detail, and a prediction model for the current ripple is obtained. Combining with the VSFPWM technique, an improved modulation method named ZSVM8 VSFPWM is proposed to further reduce the switching times without increasing the maximum inductor current ripple. As a result, the switching losses can be reduced compared with the conventional ZSVM8. Besides, ZSVM8 VSFPWM also suppresses electromagnetic interference noise, such as current EMI and common‐mode voltage. Simulations and hardware experimental results are implemented to verify the efficacies of the proposed modulation method.

An Improved Modulation Method for Parallel Three-Level Rectifiers With Circulating Current Mitigation

An improved modulation method is proposed for two parallel three-level interleaved rectifiers to reduce current harmonics and high-frequency zero-sequence circulating current, which uses sinusoidal pulse width modulation. First, a model of the two parallel rectifiers is constructed, and the main factor affecting the line current performance and zero-sequence circulating current is discussed. Second, the current harmonics are the priority optimization target to guarantee better line current performance. Finally, an exchange of the large common-mode voltage between two converters is made to reduce the high frequency zero-sequence circulating current based on the low current harmonics. Performance comparisons between the proposed method and existing methods in terms of total current harmonics, zero-sequence circulating current value and switching loss are made in the experiment, which confirm the effectiveness of the proposed method.

Обзор методов модуляции ближайшего уровня на основе MMC

In view of the relevant situation of offshore wind power research, development and construction at home and abroad, as well as the energy crisis facing the world, we have ushered in a new opportunity and outbreak period for the development of new energy. Therefore, higher requirements are placed on the power supply, and the key technology for the design of offshore wind power flexible DC transmission high-power converters is proposed. The Modular Multi-level Converter (MMC) topology can only use a lower switching frequency to obtain higher power Waveform quality. The output voltage waveform can effectively reduce the switching loss and filter capacity and improve the efficiency and economy of the converter. Among them, the method based on Level Modulation (NLM) is usually the preferred method. This paper introduces the topology and working principle of the modular multilevel converter (MMC) in detail and describes the improved modulation method for NLM in detail. The advantages, disadvantages and application occasions of different improved modulation techniques are summarized, which provides a reference for the engineering application of MMC.

A New Configuration of Paralleled Modular ANPC Multilevel Converter Controlled by an Improved Modulation Method for 1 MHz, 1 MW EV Charger

In this article, a new configuration of the modular multilevel converter (MLC) based on the parallel connection of three-level active-neutral-point-clamped (3L-ANPC) cells as well as its improved modulation method is proposed for 1 MHz, 1 MW electric vehicle (EV) megacharger. In the proposed paralleled modular ANPC-MLC, only six high-frequency silicon carbide (SiC) power switches operating at 333 kHz are required to generate 1 MHz switching frequency spectrum. Moreover, the operating voltage of all power devices is halved, the magnitude of the first switching frequency harmonic cluster is decreased by the factor of five, and the load current is equally distributed between the 3L-ANPC legs by employing the proposed improved modulation method. Hence, the modularity, efficiency, and power density of the proposed converter are notably increased, whereas the value of passive components and the overall switching loss are remarkably decreased. In addition, an optimized design of the one 3L-ANPC cell of the proposed paralleled modular ANPC-MLC for 1 MHz, 1 MW EV megacharger using Ansys SIwave, Icepak, and Q3D finite element method platforms is presented and analyzed in detail. The provided experimental results of the down-scaled setup verify the feasibility and viability of the proposed configuration as well as its improved switching pattern.

Single Phase Bidirectional H6 Rectifier/Inverter

Transformerless photovoltaic (PV) inverters are more widely adopted due to high efficiency, low cost, light weight, etc. However, H5, HERIC, etc., transformerless PV inverters do not have the bidirectional capability for a solar energy storage system in the future. With topology derivation history reviewed from rectifier to inverter, the essence of bidirectional rectifier/inverter is revealed to find a reverse power flow approach. Therefore, this paper proposes an advanced bidirectional technique for a selected H6 inverter topology with only a modulation strategy modified, while the others remain the same. For the H6 circuitry in both rectifier and inverter modes, an excellent three level DM voltage feature is achieved, while leakage current issue is eliminated at the same time with improved modulation method. Simulations and experimental results verify the proposed single phase bidirectional H6 rectifier/inverter technique.

Analysis and optimization of hybrid modular multilevel converters under over-modulation conditions

This paper firstly analyzes the operating principles of a hybrid modular multilevel converter (MMC) which utilizes a combination of half-bridge submodules and full-bridge submodules, and proposes an improved modulation method. After that, the capacitor voltage fluctuation of sub-module is comprehensively analyzed in over-modulation condition. Furthermore, according to the above analysis, the optimal modulation index resulting in the smallest capacitor voltage ripple is derived based on the proposed mathematical relationship. In order to verify the proposed modulation method, capacitor voltage fluctuation analysis and modulation index optimization, a PSCAD/EMTDC based simulation model is constructed and some experiments are carried out on a scaled-down hybrid MMC prototype. Both simulation and experimental results confirm the correctness of the analysis and optimization.

Improved modulation method with reduced switching frequency for OW‐PMSM system with common DC bus

To reduce the switching frequency of open-winding permanent magnet synchronous motor (OW-PMSM) system with a common DC bus, this Letter proposes a novel modulation method. In this method, the independent modulation is implemented in each phase to generate the reference voltages in the abc reference frame, where only one bridge arm has switching actions. Compared with the existing modulation methods for OW-PMSM system, the total switching actions within one PWM period can be remarkably reduced to 6 with the control target of the zero-sequence current suppression achieved, and the modulation range can be kept unchanged. The experimental results validate the availability of the proposed modulation method for OW-PMSM system.

Improved modulation method of modular multilevel converter

Improved modulation method of modular multilevel converter

A Novel Modulation Method for Modular Multilevel Converters

Modular Multilevel has been widely applied in Flexible HVDC, electric locomotives and PET(Power Electronics Transformers). The modulation method is important for the converter performance. This paper presents an improved modulation method for Modular Multilevel Converter.

Design of a reduced component count single‐phase to three‐phase quasi‐Z‐source converter

This study proposes a cost-effective four-switch single-phase to three-phase converter with an improved space vector pulse width modulation method. This structure employs the quasi-Z-source (QZs) topology that can be utilised in induction motor drive applications. Compared with conventional four-switch structures, buck-boost operation is a unique property of this configuration. This four-switch QZs structure decreases the switching losses, load current total harmonic distortion and cost in comparison with traditional six-switch structures. Without using any extra filtering circuits, the applied modulation method can work well to reject the influence of DC-link voltage ripples on the converter voltage, which can be accomplished by either power supply or voltage variations in the four-switch structure. The closed-loop modulation strategy recognises via sensing any unbalanced voltage that occurs across two split capacitors, and by applying the changes in the vector's activation interval time, the three-phase output voltage can be balanced. The simulation and experimental results of the suggested converter with the improved modulation method have demonstrated the converter's performance and the potential of the structure to be further developed.

Breaking a chaos-based secure communication scheme designed by an improved modulation method

Recently Bu and Wang [Chaos, Solitons & Fractals 19 (2004) 919] proposed a simple modulation method aiming to improve the security of chaos-based secure communications against return-map-based attacks. Soon this modulation method was independently cryptanalyzed by Chee et al. [Chaos, Solitons & Fractals 21 (2004) 1129], Wu et al. [Chaos, Solitons & Fractals 22 (2004) 367], and \'{A}lvarez et al. [Chaos, Solitons & Fractals, accepted (2004), arXiv:nlin.CD/0406065] via different attacks. As an enhancement to the Bu-Wang method, an improving scheme was suggested by Wu et al. by removing the relationship between the modulating function and the zero-points. The present paper points out that the improved scheme proposed by Wu et al. is still insecure against a new attack. Compared with the existing attacks, the proposed attack is more powerful and can also break the original Bu-Wang scheme. Furthermore, it is pointed out that the security of the modulation-based schemes is not so satisfactory from a pure cryptographical point of view. The synchronization performance of this class of modulation-based schemes is also discussed.

New improved modulation method for a cycloconvertor driving an induction motor

This paper describes new modulation techniques that remove all the major limitations of the noncirculating current cycloconvertor, including low maximum frequency, subharmonics, poor bank crossover timing, and distortion due to discontinuous current, when used to drive an induction motor. The new modulation techniques work by keeping the flux in the induction motor (sensed by voltage feedback) as close to sinusoidal as possible and give superior performance to the present current feedback techniques. An experimental 18-thyristor 3-pulse cycloconvertor showed that the new modulation techniques allow smooth speed control of an induction motor from 0 to 25 Hz (limited by software) with a peak output voltage of 95% of the input voltage. It is expected that the same techniques applied to a 36-thyristor 6-pulse cycloconvertor will allow smooth speed control up to 50 Hz. To obtain fast speed response, vector control can be easily added to the new modulation techniques. The new modulation techniques can also be used to advantage in other cycloconvertor and convertor applications. Patents have been applied for.