Carrier-based Modulation Scheme Research Articles

Research of the equivalent relationship between the space vector and the triangular carrier-based PWM modulation strategies in the flying capacitor multilevel inverters

ABSTRACTThe equivalent relationship between the carrier-based modulation (CBPWM) and the space vector modulation (SVPWM) based on the flying capacitor multilevel inverters has not been analysed systematically. Due to the redundancies of the flying capacitor multilevel inverters, this paper firstly focuses on the switching sequence and the working ratio of each switching state and then proposes a novel method of decomposing modulation wave. This method can distribute the ratio of redundancies arbitrarily to satisfy the requirements of the output. Taking three- and five-level inverters as examples, by injecting a zero-sequence voltage into the CBPWM modulation wave and then decomposing the new modulation wave, we can get the same sequence of SVPWM modulation. This method takes the advantages of using simple CBPWM to realise the complex output of SVPWM, such as double- and quadruple-frequency output. At last, we deduce the equivalent relationship between the SVPWM and CBPWM in n-level with an arbitrary number of segments. Experimental results verify the correctness of the equivalent relationship.

Voltage Balancing of a Modular Neutral-Point-Clamped Converter With a Carrier-Based Modulation Scheme

The five-level modular neutral-point-clamped (5L-MNPC) converter is realized by connecting two three-level neutral-point-clamped modules in series. For five-level operation, each dc-bus capacitor voltage of the modular neutral-point-clamped topology should be regulated at one-fourth of the net dc-bus voltage. In this letter, a simple voltage-balancing approach based on the carrier-based modulation scheme is proposed to achieve the above objectives. The proposed approach uses the redundancy switching states along with the current direction and the instantaneous value of the dc-bus capacitor voltage to achieve the voltage balancing in the 5L-MNPC topology. The dynamic performance of the 5L-MNPC topology with the proposed voltage-balancing approach is verified experimentally.

A New Two-Phase Five-Level Converter for Three-Phase Isolated Grid-Tied Systems With Inherent Capacitor Balancing and Reduced Component Count

This paper presents a single dc source fed five-level converter topology for medium-voltage high-power isolated grid-tied systems with reduced power switch count. The proposed converter generates two-phase systems of voltages that are further converted to three phases by using Scott-T transformer connection. Compared to existing topologies, the proposed converter requires fewer power switches, gate drivers, and power diodes. An added advantage is that only a single dc source is required. An important feature of this topology is that the dc-link capacitor voltages have an inherent ability to self-balance by themselves without the need for extra balance circuitry and special control algorithms. The voltage stress across the power devices of the proposed converter is equal to half of dc-link voltage. A level-shifted carrier-based pulsewidth modulation scheme is used to inject current into the grid at unity power factor. Analysis and simulation and experimental results are presented to validate the performance of the proposed converter under grid-tied operation.

A Generalized-Switch-Matrix-Based Space Vector Modulation Technique Using the Nearest Level Modulation Concept for Neutral-Point-Clamped Multilevel Inverters

Multilevel inverters (MLIs) have replaced conventional two-level inverters for medium-voltage high-power industrial applications. One of the best modulation techniques for MLIs is the space vector modulation (SVM). Although SVM has its own advantages, it has been overlooked for much simpler carrier-based modulation schemes in the industry. Nearest level modulation (NLM) is a simple alternative, which is functionally similar to SVM. In this paper, a new generalized SVM algorithm is presented, in which the SVM is achieved using the concept of NLM. A generalized switch matrix is introduced to derive gate pulses for the inverter, which further simplifies the algorithm. The algorithm is tested and verified for a three-phase, three-level, and five-level neutral-point-clamped inverters in both simulation and hardware.

Carrier-Based Modulation Strategies With Reduced Common-Mode Voltage for Five-Phase Voltage Source Inverters

Two modulation strategies based on carrier-based modulation (CBM) scheme (named RCMV-CBM1 and RCMV-CBM2) are proposed to reduce the common-mode voltage (CMV) of a five-phase voltage source inverter. The basic characteristic of them is that two kinds of carriers with opposite phase are adopted. By applying the opposite carrier to some specific phases, the switching states with higher CMV absolute value can be avoided. In RCMV-CBM1, the output phase with the third largest modulated signal uses the opposite carrier, and the peak-to-peak value of CMV is reduced by 40%. In RCMV-CBM2, the output phases with the second largest and fourth largest modulated signals use the opposite carrier, and the peak-to-peak value of CMV is decreased by 80%. However, the ripple analysis reveals that RCMV-CBM1 has an advantage over RCMV-CBM2 in current quality. In addition, the optimized RCMV-CBM1 and RCMV-CBM2 are presented for reducing the output current ripple. Finally, a scaled-down prototype is built to verify the correctness and effectiveness of the proposed modulation strategies.

A Carrier-Based Modulation Scheme to Reduce the Third Harmonic Component of Common-Mode Voltage in a Three-Phase Inverter Under High DC Voltage Utilization

High dc voltage utilization is one of the important requirements in a three-phase inverter. Under some carrier-based pulse width modulations (PWMs), the dc voltage utilization can reach the same maximum as that under the space vector modulation. But the third harmonic component of common-mode voltage (CMV) will be increased markedly while using these carrier-based PWMs in inverters. To improve the situation, this paper proposes that a midpoint-fluctuation carrier (MFC) scheme should be used in the three-phase inverter. The MFC is formed by adding a midpoint-fluctuation signal on the standard symmetrical triangular carrier. By analyzing the linear modulation region of the midpoint-fluctuation signals, a minimum fluctuant signal in the linear modulation region is designed in the paper. Compared with the use of other conventional schemes in the three-phase inverter, the use of minimum MFC scheme has higher fundamental amplitude in the output line voltage and smaller magnitude of the third harmonic in the CMV. The feasibility and validity of the MFC are proved by experiment results in a three-phase inverter.

A Space Vector PWM Technique for a Three-Level Symmetrical Six-Phase Drive

A space vector pulse-width modulation (SVPWM) algorithm for a three-level symmetrical six-phase drive, based on a vector space decomposition approach, is, for the first time, presented and experimentally proven in this paper. The process how to correctly select the optimal switching sequences, based on several starting requirements and conditions for the analyzed topology, such that the output phase voltage waveforms do not contain any low-order harmonics, is explained in detail. The developed SVPWM algorithm is verified experimentally using a three-level neutral-point-clamped converter and a symmetrical six-phase induction machine. Obtained results prove the validity of the developed SVPWM algorithm. The performance of the SVPWM algorithm is compared with the corresponding carrier-based modulation strategy, and it is shown that the two techniques yield identical performance. Finally, both simulation and experimental analysis of the voltage and current total harmonic distortion (THD) are reported.

Modified hybrid modulation scheme with even switch thermal distribution for H‐bridge hybrid cascaded inverters

Hybrid cascaded multilevel inverters have been recognised as an important alternative in the medium-voltage inverter market due to their high-quality output and outstanding availability. To optimise the performance of voltage-source converters, modulation technologies have attracted more and more attention. With conventional modulation technologies, the switches within a cell are not utilised at a uniform switching frequency, which is not benefit for reliability and modularity. To solve this problem, a modified hybrid modulation strategy based on unipolar frequency doubling is proposed for H-bridge hybrid cascaded inverters. The equivalence of different multilevel carrier-based modulation strategies is revealed and corresponding implement methods are provided. Based on the equivalent hybrid modulation strategy, a modified hybrid modulation strategy is presented by using unipolar frequency doubling. Compared with the conventional modulation strategies, the proposed strategy makes a more uniform use of the switches within a cell. The proposed strategy keeps the output quality and efficiency while the maximum switching frequency is decreased by half. The strategy has been successfully substantiated by the simulation and experimental results of a seven-level hybrid cascaded inverter.

Microgrid Power Quality Improvement by using Dual Output Four-Leg Inverter

In this paper, a new system is proposed for connecting a microsource to a weak utility. The proposed system simultaneously conditions the quality of the power supplied to the microgrid loads. This system employs a recently introduced reduced switch count dual-output inverter in a so-called equal frequency mode which offers its best operation features such as maximum dc bus voltage utilization and minimum device ratings. Compared to the counterpart system, the proposed configuration enjoys an integrated structure and uses less number of semiconductor switches. Moreover, it retains the desirable features of the conventional system such as ability to compensate unbalanced/sagged utility and to supply three-phase unbalanced loads with balanced and ceaseless voltage. The proposed system is introduced and its carrier-based modulation scheme is elaborated. Subsequently, a detailed study on the maximum achievable modulation index under different working conditions is carried out. The report concludes with a discussion on control of the system and its controller design. The validity of operation of the proposed system is verified through simulation. Keywords —Distributed generation (DG), dual-output four leg inverter, microgrid.

Carrier-Based Modulation Strategies for Multimodular Matrix Converters

Based on the equivalency between the full-bridge indirect matrix converters (MCs) and the 3 × 1modular MCs, the carrier-based modulation strategy is first tailored for the 3 3×1-modular MC. It can be directly expanded to 3×N-modular MC, but in order to improve the power quality, the phase-shifted (PS) method and phase disposition (PD) method are proposed. The former is suitable for the 3×N-modular MC with any multiwinding transformers (including phase-shifting transformers). Moreover, the latter is only applicable to the 3×N-modular MC with ordinary multiwinding transformers. The PD method has advantages over the PS method in the output current quality and efficiency. However, the PS method is superior to the PD method in the input current quality. Experimental results verify the correctness and effectiveness of the proposed modulation schemes.

Modulation Strategy for Multiphase Neutral-Point-Clamped Converters

This paper presents a novel modulation strategy for n -phase neutral-point-clamped (NPC) converters. The proposed modulation strategy is able to control and completely remove the low-frequency neutral-point (NP) voltage oscillations for any operation point and load types. Even when unbalanced and/or nonlinear loads are considered, the NP voltage remains under total control. Consequently, the strategy is very attractive for n -phase active filters. In addition, it enables the use of low-capacity film capacitors in NPC converters. The proposed modulation takes the carrier-based modulation strategy as a basis. It is formulated following a generalized approach that makes it expandable to n -phase NPC converters. In addition, the NP voltage is controlled directly using a closed-loop algorithm that does not rely on the use of the linear control regulators or the additional compensators used in other modulation algorithms. Therefore, no tuning of parameters is required and it performs optimally for any operating conditions and kind of loads, including unbalanced and nonlinear loads. Although the high-frequency harmonic content of the output voltages may increase, the weighted total harmonic distortion generated by the proposed strategy is similar to that of a standard sinusoidal pulse width modulated strategy. The proposed modulation algorithm has been tested in a four-leg NPC converter prototype performing as a three- and four-phase system and operating with balanced and unbalanced loads.

An Effective Carrier-Based Modulation Strategy to Reduce the Switching Losses for Indirect Matrix Converters

In this paper, an effective carrier-based modulation (CBM) strategy to reduce the switching losses for indirect matrix converters (IMCs) is presented. The discontinuous pulse width modulation method is applied to decrease the switching numbers in one carrier cycle, and an optimum offset voltage is selected to avoid commutations of the high output phase currents. By decreasing the switching numbers along with avoiding commutation of the high currents, the proposed CBM strategy significantly reduces the switching losses in IMCs. In addition, the proposed CBM strategy is independent of load conditions, such as load power and power factor, and it has good performance in terms of the input/output waveforms. Simulation and experimental results are provided to verify the effectiveness of the proposed CBM strategy.

Impact of Modulation Schemes on the Power Capability of High-Power Converters with Low Pulse Ratios

The design of modulation schemes at low pulse ratios is a known topic. However, most of previous work has focused on optimized pulse patterns, e.g., selective harmonic elimination. For industrial applications, standard carrier modulators are still highly attractive in many cases. Unfortunately, few papers presented insightful studies on carrier modulators at extremely low pulse ratios. This paper covers this gap. At extremely low pulse ratios, several special phenomena of carrier modulators are introduced for the first time. For example, a significant performance variation effect can be observed when carrier modulators are used. Moreover, the thermal unbalance of the hotspot devices in the converter may lead to significant power capability degradation of the converter unit. In this paper, the mechanisms behind this are explored and explained in detail for the first time. A quantitative comparison between different carrier-based modulation schemes is presented. Based on that, several design instruction for modulators at low pulse ratios are summarized at the end.

Improved Space Vector Modulation for Three-to-Seven Phase Matrix Converter with Reduced Number of Switching Vectors

This paper proposes a space vector modulation (SVM) scheme for a three-to-seven-phase matrix converter (MC), feeding a variable-voltage variable-frequency multiphase drive. The main feature of the proposed technique is that it utilizes only 129 out of 2187 possible active space vectors for a successful ac-ac power conversion. Since the number of vectors is significantly reduced, the switching patterns are simplified, and the execution time of the algorithm is shortened. Despite the drastic reduction in the number of active vectors, it is found that there is no significant degradation in the performance of the MC. Furthermore, the SVM also produces balanced sinusoidal input currents with a unity power factor over a wide operational frequency range (1-110 Hz). In this paper, the theoretical analysis is supported by simulation and validated using a hardware prototype. The output voltage can reach up to 76.93% of the input voltage, which is the maximum physical limit of a three-to-seven-phase MC. In addition, it exhibits a better harmonic profile than the carrier-based modulation scheme; the total harmonic distortion for the output voltage waveform is measured to be below 5% over the entire operating frequency range.

Investigation of Carrier-Based PWM Techniques for a Five-Phase Open-End Winding Drive Topology

This paper discusses the implementation of level-shifted and phase-shifted carrier-based modulation methods, in conjunction with a multiphase open-end winding drive topology. The considered drive is supplied using two five-phase two-level voltage source inverters (VSIs), with input provided from two isolated supplies of equal dc voltages. The topology is known to yield the same space-vector pattern as a corresponding three-level inverter in single-sided supply mode. It is shown in this paper that, with the application of a simple logic, the same phase voltage waveforms result as those obtainable with the appropriate carrier-based modulation scheme applied to the three-level VSI in single-sided supply mode. While the outcomes of the modulation techniques are the same, the open-end winding topology offers certain advantages, such as modularity and absence of capacitor voltage balancing requirements. The analysis is conducted for selected modulation methods using voltage and current waveforms, spectra Fast Fourier Transform (FFT), and total harmonic distortion as figures of merit. Theoretical considerations are verified by means of simulation and experimental results.

Design of Neutral-Point Voltage Controller of a Three-Level NPC Inverter With Small DC-Link Capacitors

A neutral-point-clamped three-level inverter with small dc-link capacitors is presented in this paper. The inverter requires zero average neutral-point current for stable neutral-point voltage. The small dc-link capacitors may not maintain capacitor voltage balance, even with zero neutral-point current. This may happen due to nonlinearities present in the circuit. This requires a fast control of the neutral-point voltage. A simple carrier-based modulation strategy which allows modeling of the neutral-point voltage dynamics as a continuous function of power drawn from the inverter is proposed. This continuous model shows that the neutral-point current is proportional to the power drawn from the inverter, and it enables the use of a well-established classical control theory for the neutral-point voltage controller design. A simple proportional integral controller is designed for the neutral-point voltage control on the basis of the continuous model. The design method for optimum performance is discussed. The implementation of the proposed modulation strategy and the controller is very simple. The controller is implemented in a 7.5-kW induction machine-based drive with only 14 μF dc-link capacitors. Also, the experimental results show that fast and stable performance of the neutral-point voltage controller are achieved and thus verify the validity of the proposed control approach.

Integrated Solution for Microgrid Power Quality Assurance

In this paper, a new interface configuration is proposed for connecting a microsource to a weak utility. The proposed configuration simultaneously conditions the quality of the power supplied to the microgrid loads. This system employs a recently introduced reduced switch count dual-output inverter in a so-called equal frequency mode which offers its best operation features such as maximum dc bus voltage utilization and minimum device ratings. Compared to the counterpart system, the proposed configuration enjoys an integrated structure and uses less number of semiconductor switches. Moreover, it retains the desirable features of the conventional system such as ability to compensate unbalanced/sagged utility and to supply three-phase unbalanced loads with balanced and ceaseless voltage. The proposed system is introduced and its carrier-based modulation scheme is elaborated. Subsequently, a detailed study on the maximum achievable modulation index under different working conditions is carried out. The report concludes with a discussion on control of the system and its controller design. The validity of operation of the proposed system is verified through simulation.

Carrier-Based Modulation Strategy and its Dead-Time Compensation Method on Two-Stage Matrix Converter

To deal with the problem that dead time is indispensable and that its compensation is difficult to implement especially when taking Space Vector Modulation method to modulate the inverter stage of Two-Stage Matrix Converter (TSMC), this paper presents a Carrier-based Modulation (CBM) strategy suitable for the TSMC and amplifies concerned the uniqueness of the proposed CBM. Based on this, the dead-time effect to TSMC is analyzed, formula for describing dead-time effect is induced, and two compensation methods are given, namely modulation voltage modification method and time compensation method. At last, the uncompensated and compensated experiments are carried out on the prototype, and the results prove that this strategy meets the satisfaction.

Generalized Carrier-Based Modulation Strategy for Cascaded Multilevel Converters Operating Under Fault Conditions

This paper proposes a simple modulation approach for symmetrical and asymmetrical cascaded multilevel converters, suitable for operation under normal and fault conditions. Initially, the set of all possible converter common-mode voltages for linear operation is derived, considering the status of each converter cell. It is demonstrated that the methods previously described in the literature belong to the derived set. By properly selecting the common-mode voltage, the entire converter voltage synthesis capability can be achieved under any fault condition. The proposed modulation approach has been applied for both symmetrical and asymmetrical cascaded multilevel converters. Simulation and experimental results show the effectiveness of the proposed modulation approach and validate the theoretical analysis.

Analysis of Advanced Pulse-Width Modulation Schemes for Z-Source Inverters

Modified switching patterns for Z-source inverters (ZSIs) from the traditional inverter schemes have reached the point where further improvements in firing the switches and inserting the shoot-through (simultaneous conduction of same phase leg switches) periods bring crucial benefits. A large variety of control methods, different in concept and performance, have been developed and described. This paper analyses the implementation logics, and comparative merits and demerits of recently-developed carrier-based and space-vector-based modulation techniques of ZSIs. Carrier-based pulse-width modulation (PWM) schemes, such as maximum boost control with third harmonic injection and constant boost control with third harmonic injection, and space vector modulation-based PWM schemes for three-phase ZSIs are investigated. The suitability of these techniques for various applications is also outlined. The objective of this paper is to provide a comprehensive assessment of the above PWM schemes, thereby giving a framework and guidance for the selection of the best technique for each area of application. The analysis is performed with respect to switching stress, boosting capability, voltage gain, total harmonic distortion, ripples in impedance network elements and the range of controllability. All the above control methods are simulated by Matlab/Simulink software and verified by experiments.