Unequal Dc-link Voltages Research Articles

HERIC-Based Cascaded H-Bridge Inverter for Leakage Current Suppression in PV Systems

Leakage current can flow through the transformerless photovoltaic (PV) systems because of no galvanic isolation. This undesirable current results in more losses, electromagnetic interferences, and total harmonic distortion. This article presents a hardware-based method to suppress the leakage current in transformerless cascaded H-bridge (CHB) based PV systems. The proposed scheme uses high-efficient and reliable inverter concept in CHB converters for the first time. Using this idea and proper utilization of ac-side filters leads to leakage current suppression in individual cells and, thus, the whole system. In contrast to the most existing solutions, the proposed technique can be easily extended to CHB inverters with an arbitrary number of voltage levels. Moreover, this solution is applicable in the system's conditions with unequal parasitic capacitances and unequal dc-link voltages of H-bridge cells. The validity of the proposed solution is verified by several simulations in the PSCAD/EMTDC environment. Then, the experimental results on a five-level CHB inverter are presented to confirm the theoretical and simulation results.

An Optimized Hybrid PWM Strategy for Five Level NPC VSI With Unequal DC-Links in a PV System

This article presents an optimized hybrid pulsewidth modulation (PWM) strategy for five-level neutral point clamped (5L-NPC) inverter with unequal dc-link capacitor voltages fed by differential power processing (DPP)-based photovoltaic (PV) system. The generation of balanced output from the 5L-NPC inverter with the unequal dc-link capacitor voltages becomes necessary in a PV system. The unequal dc-link capacitor voltages cause low order harmonics in inverter output voltage. A hybrid PWM strategy combining the advantages of space-vector PWM (SVPWM) and the nearest level modulation (NLM) methods is proposed to address the issues associated with unequal dc-link capacitor voltages in a five-level NPC inverter. The proposed hybrid PWM is optimized to achieve balanced output voltage with less total harmonic distortion at fundamental frequency. In-depth theoretical analysis of sector identification algorithm and dwell time computation for the proposed optimized hybrid PWM method are described. Simulation and experimental results are illustrated for several combinations of unequal dc-link capacitor voltages to validate the functionality and performance of the proposed optimized hybrid PWM method.

Enhanced Voltage Switching States for Direct Torque Controlled Open End Winding Permanent Magnet Synchronous Motor Drive

Direct torque control (DTC) is most popular technique used in industries for adjustable speed of drives. It has simple control algorithm without any pulse width modulation technique and vector transformations involvement. Two level voltage source inverter (VSI)-fed permanent magnet synchronous motor (PMSM) operated with DTC has higher torque and flux ripple problems due to the limited voltage vectors availability, which affects the performance of drive. To alleviate these problems, more number of voltage space vectors should be available for DTC of PMSM drive. In this article, DTC for open-end winding PMSM drive (OEW-PMSM) is proposed. A two level dual VSI-fed OEW-PMSM drive configuration attains three level inversion with an equal dc link voltage to VSIs. Similarly four level inversion can also be attained with an unequal dc link voltage of 2:1 ratio given to dual VSIs. Therefore availability voltage space vectors are increased for PMSM drive operation. The effective voltage switching tables for DTC operated OEW-PMSM drive at different speed conditions are proposed to make its performance more satisfactory, thus achieving less torque and flux ripples and also reduced common mode voltage (CMV). To validate this, Simulink/MATLAB and real time experimentation are performed on 3.7 kW PMSM drive.

A Novel Multilevel DC-Link Three-Phase T-Type Inverter

This research proposes a four-level T-type inverter that is suitable for low-power applications. The presented topology outranks other types of inverters in terms of a smaller number of semiconductor devices, absence of passive components such as clamping diodes and flying capacitors, low switching and conduction losses, and high efficiency. The proposed topology is free from voltage deviation and unbalanced voltage occurrences that are present in other multilevel converters having clamping diodes or flying capacitors. The proposed inverter can extend to N levels using unequal dc-link voltage sources for medium-voltage application. The inverter employs the simple fundamental frequency staircase modulation technique. Moreover, this paper presents a current commutation strategy to prevent the occurrences of short circuit and minimizing the number of required switching devices and switching transitions, resulting in improving the efficiency of the inverter. This paper also analyses the theoretical converter losses showing lower switching and conduction losses when compared to existing four-level inverters. The experimental validation of the proposed inverter shows its operating feasibility and a low output voltage THD.

A Four-Level Open-End Winding Induction Motor Drive With a Nested Rectifier–Inverter Combination With Two DC Power Supplies

This paper proposes a new nested rectifier–inverter circuit topology for a four-level (4-L) open-end winding induction motor drive (OEWIMD). To realize the 4-L inversion, the nested rectifier–inverter topology operates with unequal dc-link voltages, which are in the ratio of 2:1. The proposed topology has the inherent ability of avoiding the overcharging of the lower dc-link voltage capacitor by its higher voltage counter-part unlike in the case of the power circuit configuration proposed in the earlier literature. This paper also proposes a new sample-averaged zero-sequence elimination pulsewidth modulation (PWM) scheme for the proposed circuit configuration to avoid the flow of zero-sequence current through the motor windings. It is shown that the proposed power circuit is also capable of displaying immunity to a particular type of fault, as it can produce the same output voltage despite the unbalancing of one of the dc-link capacitor voltages without changing the modulation strategy. The proposed 4-L OEWIMD along with the associated PWM schemes is simulated and is validated with experimental results.

Unified Selective Harmonic Elimination for Cascaded H-Bridge Asymmetric Multilevel Inverter

The unequal dc-link voltages in the cascaded H-bridge (CHB) asymmetric multilevel inverters (AMLIs) lead to asymmetry of the equations for selective harmonic elimination (SHE), which increases the difficulty in solving the switching angles. On the other hand, the unequal dc-link voltages provide a large amount of synthesized waveforms which can be used to optimize the performance of inverters. However, as most of the traditional SHE approaches are derived according to a given output waveform, it cannot handle multiple synthesized waveforms. In this paper, the unified SHE equations are derived for all the possible synthesized waveforms under a specific switching angles distribution among the cells. By using the polynomial homotopy continuation (PHC) algorithm, all the possible solutions of the unified SHE equations can be found without the selection of initial values. As the waveforms generated by H-bridge are limited to three levels, some of the solutions are not physically realizable and should be discarded, hence, the criterion is proposed to distinguish these specious solutions. The case of a CHB converter with two cells and six switching angles is studied; in total, there exist 86 groups of candidate solutions and 14 of them are physically realizable. Experiments are also shown to validate the correctness of this unified SHE approach for CHB AMLIs.

Adaptive Phase-Shifted PWM based on Particle Swarm Optimization for Cascaded H-Bridge Inverter with Unequal Dc-link Voltages

The cascaded H-Bridge Multilevel Inverter has been found a promising technology in industrial applications because of its higher voltage with less distortion production. Various PWMs techniques have been proposed to push the harmonics frequencies higher than the switching frequency and thus reduces the THD as compared to non-carrier control technique based upon grid frequency. The Phase-Shifted PWM technique has an advantage over others PWM techniques because its harmonics orders are multiples of switching frequency and also depend on the number of levels of the inverter. The phase shifting angle is uniform when the equal voltage sources are adopted. However, in applications where sets of different voltage source levels feed the H-Bridge cells, the Phase Shifted PWM suffers its high order harmonics elimination capability. As a solution to alleviate this problem, an adaptive variable angle approach is proposed in this paper using Particle Swarm Optimization (PSO) algorithm to eliminate desired higher order harmonics. The algorithm is used to minimize the cost function based on high order sideband harmonics elimination equations. The results through MATLAB/Simulink environment shown in this paper confirm the reduction of sideband harmonics of higher orders, and the overall THD.

Discontinuous decoupled SVPWM schemes for a four‐level open‐end winding induction motor drive with waveform symmetries

A four-level open-end winding induction motor drive (OEWIMD) is realised by feeding the open stator windings of a three-phase induction motor from either end with two-level voltage source inverters (VSIs). These VSIs are operated with unequal DC-link voltages, which are in the ratio of 2:1. This circuit configuration has the disadvantage of overcharging the DC-link capacitor of the inverter operated with the lower input voltage. Decoupled space vector pulse-width modulation (DSVPWM) schemes, which were suggested in the previous literature to circumvent this problem, result in higher switching power loss in the dual-inverter topology. It is known that the discontinuous pulse-width modulation (PWM) schemes reduce the switching power loss. However, lack of structural symmetry of the power circuit renders it unwieldy to devise these PWM schemes. This study explores the applicability of discontinuous decoupled SVPWM (DDPWM) techniques for the four-level OEWIMD, without compromising on the waveform symmetries. With the aid of an improvised loss model, it is shown that these PWM schemes achieve the reduction of the overall loss of the four-level OEWIMD compared to the DSVPWM schemes. It has also been observed that one of the proposed DDPWM schemes result in reduced dv⁄dt in the motor phase voltages.

Predictive torque control of open‐end winding induction motor drive fed with multilevel inversion using two two‐level inverters

This study proposes direct torque and flux control of dual-inverter-fed open-end winding induction motor (OEWIM) with the help of model predictive control. OEWIMs are extensively used in electric vehicles and for ship propulsion but they require a high dynamic performance. Predictive torque control (PTC) retains the features of direct torque control and offers a high dynamic performance by eliminating start-up problems. In this study, predictive torque control is implemented for multilevel inversion-fed OEWIMs. Multilevel inversion is obtained by operating two two-level inverters with equal and unequal DC link voltages. The proposed study gives a comparative analysis of PTC of OEWIM for various speeds and numerical analysis of torque ripple and flux ripple. The proposed methods are simulated using MATLAB/SIMULINK and experimental response shows the validity of the developed methods.

Open-End Nine-Phase Machine Conversion Systems

This paper proposes four open-end nine-phase conversion systems with a reduced number of controlled switches. For this to be achieved, a converter is shared among the three three-phase groups that compose the machine. In addition, in order to reduce the number of controlled switches even more, three of the studied topologies with a shared converter make use of diodes replacing some of the switches. Because of the shared converter, the configurations that make use of diodes are only applied for machines with no spatial shift between the machine three three-phase groups. Analysis of harmonic distortion for equal and unequal dc-link voltages is presented. It is shown that the harmonic distortion for configurations with diodes are higher when unequal dc-link voltages are used. Besides, study of machine torque ripple is performed and a pulse-width modulation strategy modification is made aiming torque ripple reduction for two of the proposed conversion systems. Simulation and experimental results are presented to validate the theoretical approach.

Decoupled PWM Control of a Dual-Inverter Four-Level Five-Phase Drive

This paper studies pulse-width modulation (PWM) techniques suitable for a four-level five-phase open-end winding (OeW) drive. The drive comprises a five-phase induction machine, supplied using two two-level voltage source inverters (VSIs) with isolated and unequal dc-link voltages, in the ratio 2:1. A decoupled carrier based (CB) PWM modulation strategy, based on unequal voltage reference sharing between the two converters, is introduced in this paper. The stability of dc-link voltages in OeW drives is investigated next, using a novel analysis technique. Several modulation methods are analysed and the results show that application of the coupled pulse width modulation technique, with carriers having in-phase disposition (PD), leads to overcharging of the capacitor in the dc-link of the inverter intended to operate with the lower dc-link voltage. On the other hand, the proposed decoupled CB PWM scheme naturally eliminates the dc-link capacitor overcharging problem. These findings are verified experimentally, using open-loop V/f control. Two different decoupled CB modulation methods are compared and the best performing modulation method is selected and incorporated further into an OeW drive with field-oriented control (FOC). The presented steady state and transient experimental results demonstrate that the decoupled CB PWM technique is suitable for high performance variable speed drive applications.

Improvised SVPWM Strategies for an Enhanced Performance for a Four-Level Open-End Winding Induction Motor Drive

A four-level open-end winding induction motor drive (OEWIMD) is obtained when an induction motor with open windings is fed from either side with two two-level 3-Ph voltage-source inverters (VSIs) with unequal dc-link voltages, which are in the ratio of 2:1. This drive principally suffers from two problems namely: the circulation of zero sequence current in motor phases and overcharging of the lower dc-link capacitor by its counterpart with higher dc-link voltage. It has been reported earlier, that the decoupled space vector pulse width modulation (SVPWM) control strategies avoid both of these problems. This paper proposes an improvised SVPWM scheme that achieves a better harmonic performance compared to the decoupled SVPWM scheme without altering either the power circuit configuration or the voltage ratings of the devices. The proposed PWM strategy clamps the inverter with lower dc-link voltage with an active state, while the other inverter is switched around it. It has been shown that the proposed SVPWM scheme results in an overall reduction of losses for the drive system while resulting in a considerably lower d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</sub> /d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> compared to the decoupled SVPWM schemes. The four-level OEWIMD employing the proposed SVPWM scheme is simulated and is validated with experimental results.

An Open-End Winding Four-Level Five-Phase Drive

A four-level five-phase open-end winding (OeW) drive topology is introduced in this paper. The drive comprises a five-phase induction machine with open-end stator windings, supplied using two two-level voltage-source inverters with isolated and unequal dc-link voltages, in the ratio 2 : 1. The topology offers the advantages of a modular structure with fewer semiconductor components and has a greater potential for fault tolerance, as compared with an equivalent single-sided four-level drive. Due to the large number of switching states, development of a suitable space vector pulsewidth-modulation (PWM) method can be challenging. Hence, this paper examines the implementation of two-level-shifted carrier-based PWM methods. The effect of dead time on the drive performance is discussed, and it is shown that simultaneous PWM switching of both inverters can lead to degraded output phase voltage waveforms. Detailed analysis of this phenomenon is presented, a solution is proposed, and the modified modulation techniques are incorporated in an experimental setup, at first in conjunction with $V/f$ control. Once the proof of concept has been provided, full field-oriented control is implemented in this OeW drive topology for the first time; detailed experimental testing is conducted, and results are reported.

Performance analysis of cascaded h-bridge multilevel inverter using mixed switching frequency with various dc-link voltages

Multilevel inverters have been widely used in many applications since the technology is advantageous to increase the converter capability as well as to improve the output voltage quality. According to the applied switching frequency, multilevel modulations can be subdivided into three classes, i.e: fundamental switching frequency, high switching frequency and mixed switching frequency. This paper investigates the performance of cascaded H-bridge (CHB) multilevel inverter that is modulated using mixed switching frequency (MSF) PWM with various dc-link voltage ratios. The simulation results show the nearly sinusoidal load output voltages are successfully achieved. It is revealed that there is improvement in output voltages quality in terms of THD and low-order harmonics content. The CHB inverter that is modulated using MSF PWM with equal dc-link voltage ratio (½ Vdc: ½ Vdc) produces output voltage with the lowest low-order harmonics (less than 1% of fundamental) while the CHB inverter that is modulated using MSF PWM with un-equal dc-link voltage ratio (2/3 Vdc: 1/3 Vdc) produces a 7-level output voltage with the lowest THD (16.31%) compared to the other PWM methods. Improvement of the output voltage quality here is also in line with improvement of the number of available levels provided in the output voltage. Here only 2 cells H-bridge inverter (contain 8 switches) are needed to produce a 7- level output voltage, while in the conventional CHB inverter at least 3 cells of H-bridge inverter (contain 12 switches) are needed to produce a 7-level output voltage. Hence it is valuable in term of saving number of component.

Modeling, control and analysis of cascaded inverter based grid-connected photovoltaic system

Abstract This paper proposes a vector controlled isolated source cascaded two-level inverter (CTLI), for grid connected photovoltaic (PV) system. The system is controlled to operate with variable solar irradiance, supplying different levels of active power. The PV systems are designed, modeled and tested with the proposed controller, to provide maximum power output. Additional operation as a reactive power supplier, in the absence of solar radiation, is also tested. A simple sinusoidal pulse width modulation (PWM) technique is used, instead of referred space vector PWM (SVPWM) technique, for the operation. Two different schemes have been considered to operate the inverter with equal and unequal DC-link voltages. The control scheme has been found working, for both active and reactive power supply in steady state and transient conditions. The power supplies of both the schemes are analyzed. The controller performance is found to be satisfactory for both the schemes to extract maximum power at the considered working conditions.

A Dual Inverter Fed Four-Level Open-End Winding Induction Motor Drive With a Nested Rectifier-Inverter

A four-level inverter drive could be obtained by feeding an open-end winding induction motor by two two-level inverters from either side with unequal dc-link voltages, which are in the ratio of 2:1. This inverter modulation scheme is capable of producing 64 space-vector combinations. Some of the space vector combinations could result in the overcharging of the dc-link capacitor corresponding to the inverter operating with the lower voltage. In this paper, a new power circuit topology is proposed, in which, a rectifier-inverter combination is nested within a conventional two-level inverter configuration. The output of the conventional two-level inverter feeds one end of the open-end winding induction motor, while the output of the nested rectifier-inverter combination feeds the other end. The total dc-link voltage needed in the proposed topology is only 77% of the one needed in the conventional multilevel inverters. Also, the problem of zero-sequence current, commonly encountered in open-end winding induction motor drives, is avoided by resorting to a decoupled space vector modulation PWM scheme, which eliminates the zero-sequence voltage of the dual-inverter system by forcing the sampled average of the zero sequence voltage of individual inverters to zero.

Harmonic elimination technique for a single-phase multilevel converter with unequal DC link voltage levels

Multilevel converters, because of the benefits they attract in generating high quality output voltage, are used in several applications. Various modulation and control techniques are introduced by several researchers to control the output voltage of the multilevel converters like space vector modulation and harmonic elimination (HE) methods. Multilevel converters may have a DC link with equal or unequal DC voltages. In this study a new HE technique based on the HE method is proposed for multilevel converters with unequal DC link voltage. The DC link voltage levels are considered as additional variables for the HE method and the voltage levels are defined based on the HE results. Increasing the number of voltage levels can reduce lower order harmonic content because of the fact that more variables are created. In comparison to previous methods, this new technique has a positive effect on the output voltage quality by reducing its total harmonic distortion, which must take into consideration for some applications such as uninterruptable power supply, motor drive systems and piezoelectric transducer excitation. In order to verify the proposed modulation technique, MATLAB simulations and experimental tests are carried out for a single-phase four-level diode-clamped converter.

Decoupled Space-Vector PWM Strategies for a Four-Level Asymmetrical Open-End Winding Induction Motor Drive With Waveform Symmetries

In this paper, two space-vector-based pulsewidth modulation (PWM) (SVPWM) strategies named equal- and proportional-duty SVPWMs are described, which are used to synthesize a four-level waveform from an open-end winding configuration of an induction motor. Two isolated dc-link voltages, which are in the ratio of 2 : 1, are employed to achieve this objective. Both of these PWM strategies achieve the avoidance of overcharging of the dc-link capacitor of an inverter operating with lower dc-link voltage by its counterpart with higher dc-link voltage. Implementation of these PWM strategies requires only the instantaneous three-phase reference voltages, eliminating the need of sector identification or lookup tables. The numbers of samples per cycle for individual inverters are selected in such a way that the quarter-wave, half-wave, and three-phase symmetries are achieved for the dual-inverter drive despite unequal dc-link voltages for the constituent inverters. It is also shown that one of the two PWM techniques, called the proportional-duty SVPWM, results in a better spectral performance and lower switching power loss in the overall dual-inverter system compared to the equal-duty SVPWM.