Rafael Pindado

New feedforward space-vector PWM method to obtain balanced AC output voltages in a three-level neutral-point-clamped converter

In three-level neutral-point-clamped voltage-source inverters, proper modulation allows the average voltages of the DC-link capacitors to be kept at one-half the level of the DC-link voltage. However, in some operating conditions, a low-frequency ripple appears in the neutral-point potential and its frequency is three times that of the output voltages. As a result, the output voltages also have low-frequency distortion, which includes even and odd multiples of the fundamental. In this paper, a new space-vector pulsewidth-modulation strategy is presented to avoid that low-frequency output voltage distortion. This modulation method tries to balance the voltages of the capacitors by using only three vectors per modulation cycle. The duty ratios are directly calculated from the space-vector diagram, even when balance is not achieved. For all cases, the result is a balanced set of three AC output voltages. Some simulated and experimental results are given to validate the method.

Limits of the neutral-point balance in back-to-back-connected three-level converters

This paper explores the limits of neutral-point current control in back-to-back connected three-level converters. The theoretical analysis used is based on a mathematical neutral-point current model, which can be extended to apply to converters with higher numbers of levels. The low-frequency ripple, which appears in the neutral-point voltage for some operation conditions, can be removed for an extended operating area when two converters are connected back to back to the same dc bus. As a consequence, a lower voltage is applied to the devices, and the value of the capacitors can also be significantly reduced. Some practical graphics are given to represent the design according to the specifications of the application.

Effects of imbalances and nonlinear loads on the voltage balance of a neutral-point-clamped inverter

The effects of linear imbalances and nonlinear loads on the voltage balance of the neutral-point-clamped converter are described in this paper. The study reveals that a negative sequence of output currents (linear imbalance) may produce additional low-frequency oscillations to the neutral-point voltage. Similar consequences are produced by odd-order current harmonics from a nonlinear load, while even-order harmonics can cause the neutral-point voltage to shift. Furthermore, the second, fourth, and eighth output current harmonics might produce instability to the neutral-point voltage. The second and fourth harmonics are the worst components. The maximum amplitudes of these harmonics superposed to the current fundamentals that the system can tolerate are described. Simulated and experimental examples are presented.

Voltage-balance limits in four-level diode-clamped converters with passive front ends

Multilevel diode-clamped converters with more than three levels cannot maintain voltage balance in the DC-link capacitors for some operating conditions due to the existence of DC currents in the middle points. Since capacitors are either completely charged or discharged for those conditions, this circumstance severely limits practical application of these converters. The limit explored in this work is that the four-level converter cannot achieve voltage balance. Proper redundant vectors are selected in the space-vector diagram so that a quadratic parameter related to the currents in the middle points is minimized.

Evaluation of the low-frequency neutral-point voltage oscillations in the three-level inverter

The nearest vectors to the reference vector are commonly used in space-vector modulation strategies. The main advantages of these modulation strategies are the low switching frequencies of the devices, the good output voltage spectra and the low EMI. However, when these techniques are applied to the three-level neutral-point-clamped inverter, low-frequency oscillations appear in the neutral-point voltage for some operating conditions. As a result, the value of the DC-link capacitors must be increased in order to attenuate such oscillations. In this paper, these amplitudes are quantified for two modulation strategies that use nearest vectors to the reference vector. Owing to the nondimensional variables used in the analysis, the information provided will help for the calculation of the DC-link capacitors in a given specific application. Simulated and experimental examples are presented.

A Linear-Quadratic Regulator with Integral Action Applied to PWM DC-DC Converters

Averaging techniques have been widely applied to model PWM power electronic converters. The averaged models obtained by using this technique are very useful for the simulation of the systems as well as for the design of controllers. In this work, some general considerations about linear-quadratic regulators (LQR) are made. Then, an LQR is designed and applied to the particular case of a DC-DC buck converter. Some simulation and experimental results show very good performance of the system under start-up and transient events

Efficient Space-Vector Modulation Algorithm for Multilevel Converters with Low Switching Frequencies in the Devices

This paper presents a novel space-vector modulation scheme that can be processed quickly by a digital-signal processor, in addition to achieving minimum switching frequencies in the devices of the converter. The modulation algorithm takes advantage of symmetry in the space-vector diagram to process all the calculations within the first sextant. An equivalent reference vector in this sextant is used for the calculation of duty cycles and the definition of optimal sequences of vectors. By simply interchanging the final states of the legs of the inverter of this equivalent vector generates the original reference vector in the corresponding sextant. The total number of switching steps in the final sequences is preserved; thus, optimal sequences of vectors need only be defined in the first sextant. This strategy is especially attractive for multilevel converters in which the total number of vectors and sequences is very high. Furthermore, since multilevel converters are usually applied to high-power systems, reducing the switching frequencies of the devices is an important goal. Efficiency of the modulation algorithm is verified in the three-level converter by simulation and experiment

Design guidelines using selective harmonic elimination advanced method for DC-AC PWM with the Walsh transform

The use of the Walsh transform in DC-AC PWM waveform generation allows the calculation of the switching angles by means of linear equations which depend on the fundamental amplitude. However, when it is needed a wide regulation of the fundamental amplitude, conventional application of Walsh transform forces to switch among different equation sets, due to the range limitation of each one of them. In this paper it is described an advanced method to obtain the switching angles that permits full regulation of the fundamental amplitude, with only a switching interval vector, in single phase systems. The paper shows Matlab simulation results that proof the efficiency of the advanced method algorithm.

An Alternative Approach on Three-Dimensional Space-Vector Modulation of Three-Phase Inverters

This paper presents a new standpoint on the analysis of the space-vector modulation (SVM) in three-phase inverters. By means of a three-dimensional (3D) approach, a mathematically and physically sound explanation of the modulation process is obtained. As it is well known, a three-phase two-level voltage source inverter has eight switching states, which give rise to eight generating vectors (two of them are the zero-vectors). Taking into account that the reference voltage vector is synthesized by these generating vectors, it seems interesting to translate the reference vector from the natural a-b-c reference base to a new one, which is directly based on the generating vectors (generating base). In this new base, one of the three unitary vectors always follows the direction of the zero-vectors (zero-sequence axis). Moreover, in a three-leg full-bridge (TLFB) inverter, the voltage synthesized over the zero-sequence axis does not appear in the load side. Therefore, it is logical to cancel the contribution of the zero-vectors on the inverter output in order to obtain the maximum use of the dc-link voltage. This cancellation is really easy when the reference vector is expressed on the new generating base, since it is only necessary to eliminate the zero-vectors direction component. As a result of this analysis, a very efficient three-dimensional space-vector modulation (3D-SVM) algorithm is introduced. In this algorithm, the modulator input variables are given in the natural a-b-c reference frame. The proposed modulation method rests on the well-known carried-based modulation technique, and avoids trigonometric calculations, definition of optimal sequences, and control of the switching intervals. The proposed 3D-SVM technique is verified by simulation and experiment

Modulation Strategies for a Low-Cost Motor Drive

This paper is focused on the analysis of an economic single-phase to three-phase converter that has a reduced number of power switches and avoids the use of an input filtering inductance. This structure consists fundamentally on a four-leg converter, including the inverter and the rectifier. The topology is especially interesting for low-cost low-power motor drive applications. Continuous and discontinuous carrier-based PWM are applied and compared in this four-leg system to define which of them achieves better performance of the converter. Switching losses on the devices are also evaluated for the proposed modulation techniques. The work concludes showing some experimental results