Milan Srndovic

Simple Time Averaging Current Quality Evaluation of a Single-Phase Multilevel PWM Inverter

This paper addresses theoretical calculation of a single-phase multilevel pulsewidth modulation (PWM) inverter current total harmonic distortion (THD). Analytical approach introduced in late 1980s for a two-level inverter is generalized for an arbitrary level count for a single-phase inverter. Although the obtained closed-form piecewise analytical solutions assume (infinitely) large ratio of switching and fundamental frequencies and pure inductive load, they are practically very accurate for the ratios of (apparent) switching and fundamental frequencies larger than 25-30 and inductance-dominated loads. Along with strict mathematical solutions, simple accurate Bessel function approximation and hyperbolic average trend one are suggested. The formulas clearly reveal a single-phase PWM inverter current THD dependence on modulation index for an arbitrary voltage level count and are easily modified to cover grid-connected cases.

Evaluation of DC voltage ripple in single-phase H-bridge PWM inverters

In this paper the complete analysis of the dc-link voltage ripple is given for single-phase H-bridge PWM inverters. The dc voltage ripple amplitude is analytically determined as a function of the modulation index and the load phase angle, considering both the switching frequency component and the double fundamental frequency component (i.e. 100 Hz). Peak-to-peak distribution, maximum amplitude, and rms values of the voltage switching ripple over the fundamental period are analytically determined and numerically verified by numerical simulations carried out by Matlab/Simulink. Simple and effective guidelines to design the dc-link capacitor are given on the basis of the dc-voltage ripple requirements.

Asymptotic time domain evaluation of a single-phase multilevel PWM inverter current quality

This paper addresses theoretical calculation of a single-phase multilevel PWM inverter current Total Harmonic Distortion (THD). Analytical approach introduced in late 1980s for a 2-level inverter is generalized for an arbitrary level count for a single-phase inverter. Though the obtained closed-form piece-wise analytical solutions assume (infinitely) large ratio of switching and fundamental frequencies and pure inductive load, they are practically very accurate for the ratios of (apparent) switching and fundamental frequencies larger than 25-30 and inductance dominated loads. Along with strict mathematical solutions suggested are simple accurate Bessel function approximation and hyperbolic average trend one. The formulas clearly reveal a single-phase PWM inverter current THD dependence on modulation index for an arbitrary voltage level count and are easily modified to cover grid-connected cases.

Current ripple evaluation in dual three-phase inverters for open-end winding EV drives

Ac motor current ripple in electric vehicles is source of electromagnetic interferences and audio noise, from the inverter-motor power connection cables and from the motor itself. By increasing the inverter switching frequency the ripple amplitude is reduced, but the drive efficiency decreases due to the proportionally increased switching losses. A viable solution to reduce the current ripple amplitude is to introduce a multilevel inverter instead of a standard two-level (2L) three-phase inverter. A simple and effective way to achieve a three-level (3L) inverter in battery-supplied electric vehicles consists of using two standard three-phase 2L inverters with the open-end winding motor connection. In this paper the peak-to-peak ripple amplitude of the dual-2L inverter is evaluated and compared with the corresponding ripple of the single 2L inverter, considering the same voltage and power motor ratings. The ripple analysis is carried out as a function of the modulation index to cover the whole inverter modulation range.

Simultaneous Selective Harmonic Elimination and THD Minimization for a Single-Phase Multilevel Inverter With Staircase Modulation

Power electronic converters for medium- and high-power/voltage applications employ low-frequency fundamental switching frequencies and, therefore, minimize switching losses. There are two main control techniques of switching angle estimation; total harmonic distortion (THD) minimization and selective harmonic elimination (SHE). In case of the first control technique, the switching angles are found based on THD minimization without being specifically focused on eliminating some exact harmonics. For the SHE control technique, all degrees of freedom, due to available number of switching angles, is used for the elimination of certain low-order harmonics. The basic idea of combining those two control techniques consists of using some degrees of freedom for eliminating certain harmonics and using the others for minimizing the remaining THD content. This generalized problem formulation includes classic minimal THD and SHE problems as special cases. Theoretical developments are verified by the set of experimental cases for the voltage and current THDs selecting characteristic working points over the modulation index range.

Evaluation of DC voltage ripple in three-phase PWM voltage source inverters

Determination of dc-link voltage switching ripple in three-phase PWM voltage source inverters (VSI) is important for the selection and design of the dc-link capacitor. In this paper the complete analysis of the dc-link voltage ripple is given for three-phase PWM VSIs, in case of dc source impedance and balanced load. The dc voltage ripple amplitude is analytically determined as a function of amplitude and phase angle of output current, and modulation index. In particular, peak-to-peak distribution and maximum amplitude over the fundamental period of the dc voltage switching ripple are obtained. Guidelines for designing the dc-link capacitor are given on the basis of the dc-voltage ripple requirements. Simulations results carried out by Matlab/Simulink are provided to verify the analytical developments.

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.