Three-phase Five-level Inverter Research Articles

A Three-Phase Five-Level Inverter With High DC Voltage Utilization and Self-Balancing Capacity of Floating Capacitor

Multilevel inverter are popular solutions in photovoltaic power station, wind farm, and other renewable energy generation. This article presents a three-phase five-level inverter with high dc voltage utilization. A significant feature of the inverter is that its maximum dc voltage utilization is twice that of the traditional three-phase inverters such as neutral point clamped, flying capacitor (FC), cascaded H-bridge, and active NPC. What is more, the inverter can realize the self-starting and self-balancing of the FC, and the self-balancing of the dc-link capacitors, so the complex FC voltage balancing strategy and bus midpoint voltage control method are not needed anymore. The voltage and current stresses analysis and the design guidelines of components are discussed in detail. A comparison with some existing topologies has also been made to highlight its performance. Taking one phase as an example, both the simulations and experiments are carried out to verify the validity of the proposed inverter and modulation strategy.

Torque ripple minimization with modified comparator in DTC based three‐level five‐phase inverter fed five‐phase induction motor

The proposed technology deals with the reduction in torque ripple of five-phase induction motor. The machine is controlled by direct torque control technique and fed by three-level five-phase inverter (TL-FPI). In the fraternity of multiphase induction motor, five phase is the most researched drive, considering complexity of control mechanism and advantages which multiphase machine has offered. The selection of TL-FPI is based on availability of more voltage vectors (243) than two-level five-phase inverter (32). The 41 voltage vectors are employed out of 243 based on simplicity and advantages of multiphase machine. The selected vectors are operated through modified seven-level torque comparator. An increase in levels of the comparator restrains the ripple in a predefined boundary. Here, modification in the predefined boundary of the comparator is suggested which allows the utilization of optimal voltage vectors. According to modification, zero voltage vectors are replaced with small active vectors when actual torque is lower than its reference value. To substantiate the claim various results of simulation and experimentation are given.

Combined Harmonic Reduction and DC Voltage Regulation of A Single DC Source Five-Level Multilevel Inverter for Wind Electric System

Wind power generation has increased in the past twenty years due to the development of power electronic converters. Power generation through wind has advantages over other renewable sources, such as having more efficiency, being pollution-free, and its abundant availability. Power electronic converters play a vital role in the wind energy conversion system. This paper presents a wind-electric system with a permanent magnet synchronous generator, diode rectifier, DC-DC converter (buck-Boost or Cuk converter), and a three-phase five-level inverter. The five-level inverter is a modified form of a cascaded H-bridge inverter that uses a single DC source as an input irrespective of several levels and phases. As the wind speed changes, the Permanent Magnet Synchronous Generator (PMSG) voltage and frequency changes, but for practical applications, these changes should not be allowed; hence, a voltage controller is used that maintains the output voltage of a DC converter, andthus a constant AC output is obtained. The DClink voltage is maintained at the desired voltage by a Proportional plus Integral (PI)-based voltage controller. The DC link voltage fed to the multilevel inverter (MLI) is converted to AC to feed the load. The MLI is controlled with a new Selected Harmonic Elimination (SHE), which decreases the total harmonic distortion (THD). The system is simulated with an Resistive plus Inductive (RL) load and is tested experimentally with the same load;the results prove that the Cuk converter has a better efficiency compared to the Buck-Boost converter, and the system has less THD when compared with the conventional SHE Pulse Width Modulation (PWM) technique.

Performance improvement of direct torque controlled five‐phase induction motor in context of common‐mode voltage, torque ripple and demagnetisation under low speed region

This study focuses on the performance improvement of direct torque controlled (DTC) five-phase induction motor (FPIM). The performance indices are common-mode voltage (CMV), torque ripple and demagnetisation during low-speed operation. The three-level five-phase inverter (TL-FPI) is used for feeding the FPIM, since more voltage vectors are available in TL-FPI than its two-level counterpart. Among available 243 voltage vectors, 41 non-vertex voltage vectors are employed. These 41 voltage vectors through seven-level torque comparator (SLTC) fires an inverter. The SLTC reduces the torque ripple. All the employed 41 voltage vectors have zero CMV output. The voltage vectors are selected so that the magnitude of radial component of the resultant stator flux under low-speed operation is more than the same under normal speed operation. This modification resolves the problem of demagnetisation during low-speed operation. The proposed method achieves the intended targets without losing simplicity and dynamics of the classical DTC method. The performance improvement of the five-phase machine is validated through several results.

A Multicarrier PWM Technique for Five Level Inverter Connected to the Grid

In this paper, a three-phase multilevel cascaded H-bridge inverter is developed for injecting renewable power energy into the grid through a filter. The main contribution of this work is to reduce the total harmonic distortion (THD) by using a passive filter and to enhance the output voltages by adopting a multicarrier pulse width modulation (MCPWM). The simulation results affirm a good performance of the proposed multicarrier PWM control using a three-phase five-level inverter. The proposed inverter is tested as well as the THD and the spectral analysis of the output voltage are calculated using Simulink/Matlab software.

An Improved Three-Phase Five-Level Inverter Topology With Reduced Number of Switching Power Devices

This paper presents an optimized topology of 3- $\phi$ , multilevel inverter (MLI) configuration for five or higher level operation with reduced switch count. The proposed solution consists of two basic units (BUs) of multilevel converter and a T-structured 3-level inverter (3-LI). Both BUs are equally shared by the three phases maintaining symmetry among the phases. Moreover, for a higher level operation of the proposed MLI, the modifications required are at two BUs only and not at three legs of the inverter. It further helps in reducing the requirement of extra components for higher level operation compared to the conventional solutions. Furthermore, the topology also has the benefits of reduced switching and conduction loss. An algorithm is also devised to generate the switching pulses for the BUs and the 3-LI, using a carrier-based space vector modulation technique. Presented topology is compared with other existing topologies to prove its advantage. All the details regarding the operating modes and pulse width modulation techniques employed for the proposed system are given and supported by the simulation and experimental results.

Direct Torque Control of Five-Phase Induction Motor With Common-Mode Voltage and Current Harmonics Reduction

This paper proposes two direct torque control (DTC) strategies to reduce the common-mode voltage (CMV) in the five-phase induction motor driven by three-level five-phase inverter. In each technique, 31 voltage vectors consisting of 30 nonzero voltage vectors, and a zero voltage vector are selected out of available 243 voltage vectors. The selection of these voltage vectors are based on their capabilities of reducing the CMV, eliminating the x–y stator flux and maintaining the torque ripple under control. The ten large, ten small, and a zero voltage vector are same in both schemes, however only difference in designing the control strategy for the two schemes is the utilization of different ten medium voltage vectors. First proposal (DTC-I) reduces the CMV to $3V_{{\rm{dc}}}\,/ 10$ and second proposal (DTC-II) reduces to $V_{{\rm{dc}}}\,/ 10$ . These schemes are compared with DTC technique of five-phase induction motor fed by two-level five-phase inverter in order to judge the capabilities of proposed schemes in context of reducing the CMV. The simulation and experimental results validate the proposed DTC techniques.

Torque Ripple and Harmonic Current Reduction in a Three-Level Inverter-Fed Direct-Torque-Controlled Five-Phase Induction Motor

In a switching-table-based direct torque control method (ST-DTC), the torque ripple reduces with increasing number of levels of the torque comparator. The number of levels is decided by the availability of the number of voltage vectors according to the topology of the inverter. This paper proposes the seven-level torque comparator to reduce the torque ripple in the DTC-controlled five-phase induction motor fed by the three-level five-phase inverter. In the five-phase induction motor, an elimination of x − y stator flux is inevitable otherwise the stator phase current will be distorted. In order to eliminate the x − y stator flux, the virtual voltage vectors are formed. Experimental results are presented to validate the proposed DTC method.

Analytical and Experimental Comparison of Carrier-Based PWM Methods for the Five-Phase Coupled-Inductor Inverter

The five-phase coupled-inductor inverter is a newly proposed option for three-level five-phase inverter drives. This paper comprehensively analyzes the inverter including the coupled-inductor current, current stress on the dc-link capacitors, and a comparison of these performance aspects using three pulse width modulation (PWM) strategies. Mathematical derivations of the inverter currents and the associated current stress reveal that the current stress is not only influenced by the choice of modulation strategy, as in the neutral-point-clamped topology, but also by the value of coupled inductance; furthermore, the choice of the modulation strategy impacts the core loss of the coupled inductors. Three carrier-based PWM methods are considered: sinusoidal PWM (SPWM), and two methods with the common-mode voltage reduction (referred to as CMVR1 and CMVR2) for the five-phase coupled-inductor inverter. CMVR2 is shown to be the best solution if common-mode voltage reduction is required. The analysis demonstrates that SPWM performs better with respect to current stress and inverter losses, therefore is the preferable modulating strategy for the five-phase coupled-inductor inverter if common-mode voltage reduction is not required. The analysis is verified by simulations and experiments.

Selection of Voltage Vectors in Three-Level Five-Phase Direct Torque Control for Performance Improvement

This paper presents a Direct Torque Control (DTC) strategy for the five-phase induction motor driven by a three-level five-phase inverter in order to improve the performance of the five-phase induction motor. In the proposed DTC technique, only 22 voltage vectors out of 243 available voltage vectors in a three-level five-phase inverter are selected and are divided in 10 sectors each with a width of <TEX>$36^{\circ}$</TEX>. The four different DTC combinations (DTC-I, II, III and IV) for a three-level five-phase induction motor drive are investigated for improving the performance of five-phase induction motor. All four of the DTC strategies utilize a combination of the same large and zero voltage vectors, but with different medium voltage vectors. Out of these four techniques, DTC-II gives the best performance when compared to the others. This DTC-II technique is analyzed in detail for improvements in the performance of five-phase induction motor in terms of torque ripple, x-y stator flux and Total Harmonics Distortion (THD) of the stator phase current when compared to its two-level counterparts. To verify the effectiveness of the proposed three-level five-phase DTC control strategy, a DSP based experimental system is build. Simulation and experimental results are provided in order to validate the proposed DTC technique.

A Hybrid Nine-level Inverter Topology for an Open-end Stator Winding Induction Motor

In this article, a new hybrid nine-level inverter topology for an induction motor with an open-end stator winding is proposed. In the proposed topology, one end of the open-end winding is fed by a three-phase five-level inverter, and the other end is connected to a conventional three-phase two-level inverter. The three-phase five-level structure is realized by cascading a three-level flying capacitor inverter with capacitor-fed H-bridge in each phase. In the proposed topology, a total of 17,576 space vector combinations are available; hence, the capacitor voltages can be maintained at desired levels using switching redundancies. As compared to conventional topologies that require 48 switches, the proposed topology requires only 30 switches and completely eliminates the requirement of clamping diodes, which are required in a neutral point clamped inverter. Also, it requires only two capacitors per phase, whereas the nine-level flying capacitor inverter requires 28 capacitors per phase. In case of fault in any H-bridge, the system can be operated as a five-level inverter at its full power rating and improves the reliability of the inverter. The performance of the proposed system has been experimentally investigated on a 1.5-kW open-end stator winding induction motor, and results are presented.

Simplified Three-Level Five-Phase SVPWM

A simplified space vector pulse width modulation (SVPWM) is proposed for a three-level five-phase inverter. The proposed method generates the duty cycle of the three-level inverter switches based on dwell times of the two-level inverter and carrier index. The proposed method automatically determines the eligible vectors, region, and switching sequence of optimized five vectors based on the modulation index. Out of 243 available vectors, 113 most eligible vectors are used for generation of desired voltage reference in main subspace, while zeroing the average voltage in the auxiliary subspace by using the proper switching sequence. This method also uses the redundant vectors in each subcycle thus balances the dc-link capacitor voltages and no additional algorithm or techniques are needed to balance the dc-link capacitor voltage. The identification of the reference location with the carrier index using the signum function simplifies the algorithm implementation. Thus, the proposed method eases the implementation of optimum five vectors to a greater extent. Based on only changing the carrier index, the proposed method can be easily extended for any multiphase multilevel (5, 7,…, n) inverter. The simulation and hardware results of the three-level five-phase inverter validate the proposed simplified method.

Selective harmonic elimination strategy for cascaded H‐bridge five‐level inverter with arbitrary power sharing among the cells

Selective harmonic elimination PWM (SHEPWM) is a well-known fundamental frequency switching method for cascaded multilevel inverters. The main goal in the conventional SHEPWM is to eliminate some lower order harmonics of the output voltage while the fundamental component is satisfied. Thus, this method only regards the inverter output voltage quality, which is not sufficient in many applications. Different cells of the cascaded H-bridge multilevel inverter are supplied by isolated dc sources which may have different amounts of power. To deliver maximum power of these sources to a load or network, power should be properly shared among the cells. This study presents a modified SHEPWM method to control quality of the output voltage of the cascaded H-bridge five-level inverter in wide range of the modulation index while desired power sharing among the cells is also considered as the main priority in the whole range of the modulation index. The method forces new constraint in obtaining optimal switching angles and it may require additional switching. To study the problem, a genetic algorithm optimisation program, based on the modified method, is employed for a three-phase five-level inverter. Simulation and experimental results show the effectiveness of the proposed switching method.

A three-phase five-level inverter for DTC drives application

This paper presents a new three-phase five-level inverter for direct torque control (DTC) drives application. The advantages of the inverter are reduced output harmonics, switching losses and number of power switches. The multilevel PWM modulation method as well as output voltage analysis based on symmetrical sampling are presented. The proposed inverter is compared with the conventional inverter in terms of its current's total harmonics distortion (THD). Experimental results of DTC drive of permanent magnet synchronous motor (PMSM) are also presented for verification of its proposed function.

A Space Vector Switching Strategy for Three-Level Five-Phase Inverter Drives

A novel space vector modulation (SVM) technique for a three-level five-phase inverter is described based on an optimized five vectors concept. The concept utilizes a novel vector minimization technique that reduces the number of vectors in the d1-q1 vector space by identifying candidate vectors in each of the ten sectors that comprise the decagon vector space. The candidate vectors are selected based on the inequality relationship between the five-phase voltages during each switching cycle. Using this technique, the original 243 inverter states are reduced to 113 candidate vectors, and from the remaining states ten possible switching sequences in each sector are utilized to develop the desired voltage reference in the d1-q1 vector space while forcing a null vector in the d3-q3 vector space. A novel region determination technique is also introduced to identify the subregion that the d1-q1 voltage vector occupies. This technique significantly reduces the computational overhead required when implementing SVM techniques with multilevel and multiphase inverters. The space vector technique can utilize redundant vectors to assist in balancing subcycle variation of the dc-link capacitor voltage under unbalanced load conditions. Experiments validate simulation results where the low-order voltage harmonics show that the d3-q3 voltage vector is null.