Analysis and Comparison of Peak-to-Peak Current Ripple in Two-Level and Multilevel PWM Inverters

Three-phase multilevel inverters are used in many medium- and high-power applications such as motor drives and grid-connected systems. Despite numerous PWM techniques for multilevel inverters have been developed, the impact of these modulation schemes on the peak-to-peak output current ripple amplitude has not been addressed yet. In this paper the analysis and the comparison of current ripple for two- and three-level voltage source inverters are given. Reference is made to optimal and popular modulation, so-called centered PWM, easily obtained by both carrier-based modulation (phase disposition, with proper common-mode voltage injection) and space vector modulation (nearest three vectors). It is shown that the peak-to-peak current ripple amplitude in three-level inverters can be determined on the basis of the ripple in two-level inverters, obtaining the same re-sults as by directly analyzing the output voltage waveforms of the three-level inverters. This procedure can be readily extended to higher level numbers. The proposed analytical developments are verified by both numerical simulations and experimental tests

Impact of PWM Voltage Waveforms in High-Speed Drives: A Survey on High-Frequency Motor Models and Partial Discharge Phenomenon

The insulation system’s dielectric of the electric motor is very often subjected to severe electrical stress generated by the high dv/dt seen at the machine’s terminals. The electrical stress and high reflected wave transient overvoltage are even more evident in case of high-speed machines fed by high-frequency (HF) converters featuring very fast wide-bandgap devices. They are promoting the occurrence of partial discharges and consequently accelerate ageing. As this is serious issue and the main cause of the drive failure, it is important to analyse and characterise the surges at the motor terminals. Several HF models of motors have been proposed in the literature for this purpose. This article presents a survey on HF motor models, which is crucial in understanding and studying the most critical parameter identification and overvoltage mitigation techniques. Moreover, it offers a comparison of the models’ main features as well as a comparison with the experimental voltage waveform at motor terminals. A general overview of the partial discharge (PD) phenomenon is also provided, as it is favoured by HF operation and together with HF motor modelling provides key insights to the insulation ageing issue. In particular, an analysis of the effects of PWM waveform affecting insulation is given, as well as useful methods for developing strategies for the inspection and maintenance of winding insulation

Wealth effects of convertible-bond and warrant-bond offerings: a meta-analysis

The literature on wealth effects associated with the announcements of convertible-bond and warrant-bond offerings is reviewed. The findings of 35 event studies, which include 84 sub-samples and 6310 announcements, are analysed using meta-analysis. We find a mean cumulative abnormal return of −1.14% for convertibles compared with −0.02% for warrant bonds, the significant difference confirming a relative advantage for warrant bonds. Abnormal returns for hybrid securities issued in the USA are significantly more negative than those issued in other countries. In addition, issuing hybrid securities to refund debt does not seem to be favoured by investors. Finally, several factors identified as important by theory or in prior research are not significant within our cross-study models, suggesting that more evidence is needed to confirm whether they are robust

Analysis of Dead-Time Effects in Multiphase Voltage Source Inverters

Inverter dead-time effects have been investigated in past for three-phase voltage source inverters. Also, there have been some studies about multi-phase (more than three phase) inverters with reference to multi- phase drives. Recently, a great deal of research has concentrated on PWM methods suitable for multi-phase VSIs. All PWM methods theoretically produce sinusoidal output voltages without low-order harmonics. This paper analyzes the dead\u2013time effects on output variables of voltages source inverters if no any compensation technique is applied. In particular, an overview of three-phase inverters is considered first. Then, a generalized extension to multiphase inverters is proposed and investigated in details. Theoretical analysis has shown that particular low-order harmonics appear as result of dead-times, and simulation results confirm the proposed analytical approach

Space Vector Analysis of Dead-Time Voltage Distortion in Multiphase Inverters

Inverter dead-time effects have been widely investigated in past for three-phase voltage source inverters (VSIs). Recently, a great deal of research has concentrated on PWM methods suitable for multi-phase VSIs (with more than three phases). All PWM methods theoretically produce sinusoidal output voltages without low-order harmonics. This paper analyzes the dead-time effects on output variables of multi-phase VSIs in the case of carrier-based PWM techniques if no any compensation technique is applied. In particular, a generalized analysis for n-phase inverters is introduced first, and numerical verification for three-, five- and seven-phase inverters are performed to verify the analytical developments. The dead-time effect on load voltage is also represented in terms of multiple space vectors, and analysis of harmonics content for each alfa-beta plane is given

Interconnection strategies of point absorber type wave energy converters and rectifier units

none3siDirect-driven point absorber type tubular wave energy converters, reflect the random nature of wave energy input to its output voltage waveform. The better conditions in the generator output stage are the first step that allows the reduction in the need of complicated power conversion system. This work presents the preliminary analysis and comparison of two important interconnection strategies of WEC-rectifier units. Two different interconnection strategies are considered based on how they build up the DC-link, i.e. parallel (well- known and used by many researchers in wave area) and cascaded (less considered) configuration of WEC-rectifier units. Conventional two-level AC/DC/AC power conversion system is used in both cases. Matlab/Simulink based system simulation is used to compare the different interconnection strategies. Various results are presented and discussed, together with the voltage ripple calculations of the DC-link voltage and output phase voltage and current, being it important for the sizing and the cost of the system.mixedLoncarski J.; Soman D.E.; Frontoni E.Loncarski J.; Soman D.E.; Frontoni E

Effects of Current Ripple on Dead-Time Analysis of Three-Phase Inverters

Inverter dead-time distortion in output voltages have been widely investigated in the past for three-phase PWM voltage source inverters. Also, there have been some studies about multi-phase inverters with reference to multi-phase drives. Usually, almost sinusoidal output currents are considered, with a single zero crossing for every half fundamental period. High output current ripple introduces multiple zero crossing, leading to modified output voltage distortions. In this paper the effect of the output current ripple has been taken into account, in the case of carrier-based PWM techniques when no any dead-time compensation technique is applied. In particular, the harmonic spectrum of the output voltage distortion has been evaluated on the basis of the multiple zero crossing time interval of output currents. In case of relevant current ripple it is verified a relevant deviation of output voltage harmonics in comparison to the case of almost sinusoidal currents. Theoretical analysis has shown that particular low-order harmonics are affected by the current ripple amplitude, and simulation results confirm the developed analytical approach

DC-link stress analysis for the grid connection of point absorber type wave energy converters

Highly random nature of input power from wave energy converters (WEC), especially from direct-driven point absorbers, demands customized power electronic converters for grid connection. In this paper, analysis and comparison of the DC-link stresses in the converter systems for two cases - a single and three collective units, of wave energy converters is given. The AC/DC/AC converter system includes a conventional uncontrolled three phase rectifier, a DC/DC converter to boost the DC-link voltage and an inverter with RL load. The system has been studied under two different controller actions for the DC/DC converter: with constant boost factor and with constant DC-link voltage. A Proportional Integral controller has been used to regulate the voltage in the latter case. Matlab/Simulink based system simulation has been done to compare the DC-link stress. The analysis shows the comparison in DC-link stresses and the requirements of the system for different cases, proving the advantages and the importance of having customized active power conversion methods for minimizing the DC-link stresses