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ABSTRACT: Multilevel voltage-source converters (VSC) are attractive for high-voltage and high-power applications. However, the increased part counts and control complexity also complicate the converter modeling and control. One particular control problem is the balancing of dc capacitor voltages. Conventional voltage balancing control design has been based on periodic steady-state or line-cycle averaged models of the converter that don't correctly predict the capacitor voltage dynamics within a line cycle and under dynamic conditions. This paper develops an averaged model for three-level neutral-point clamped (NPC) converters that is valid under all conditions and involves switching-cycle averaging only. Unlike previous works, no assumption is made about the balance of ac line voltages, making the resulting model suitable for analysis and simulation of large, possibly unbalanced electric power systems. Applications of the model include more efficient system simulation, controller analysis and design for ac current regulation, dc bus voltage regulation and dc capacitor voltage balancing. Simulation results are presented to validate the proposed models.
Control and Modeling for Power Electronics (COMPEL), 2010 IEEE 12th Workshop on; 07/2010
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ABSTRACT: This paper addresses stability problems of power systems with actively controlled loads that exhibit constant-power behavior. Instability occurs in such systems due to the negative incremental resistance of the constant-power loads (CPL). Existing approaches to stabilizing such systems require modification of the source and/or the load control characteristics, or isolating the CPL from the rest of the system by additional active devices, which are difficult to implement and often conflict with other system requirements such as control bandwidth, size, weight, and cost. In this work, we propose passive damping as a general method to stabilize power systems with CPL. Using a representative system model consisting of a voltage source, an LC filter, and an ideal CPL, we demonstrate that a CPL system can always be stabilized by a simple passive damping circuit added to one of the filter elements, no matter how the original system behaves. Three different damping methods are considered and for each analytical models are developed to define their parameters required for stabilizing the system. The different damping methods are also compared in terms of their stabilization capabilities and impact on other system performances such as filter attenuation. Time- and frequency-domain measurements from an experimental system are presented to validate the proposed methods.
Applied Power Electronics Conference and Exposition (APEC), 2010 Twenty-Fifth Annual IEEE; 03/2010
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ABSTRACT: This paper presents asymmetric interleaving as a new method to operate parallel voltage source converters (VSC) in which the carrier phase shifts don't add to exactly one carrier cycle and may also vary from one pair of modules to another. Double Fourier analysis is applied to develop analytical spectral models for individual module outputs, from which a scaling factor is defined as a measurement of harmonic cancellation effects and is found to be a function of only the carrier phase shifts and independent of the specific PWM method used. Using the scaling factor as a mathematical tool, we investigate practical asymmetric interleaving methods that can achieve different system optimization objectives, including the avoidance of resonance at certain frequencies by selectively eliminating certain carrier harmonics and their sideband components, the minimization of total harmonic voltage or current distortion, and the minimization of EMI filter size. Applications of these methods are illustrated by examples and verified by numerical simulation.
Energy Conversion Congress and Exposition, 2009. ECCE 2009. IEEE; 10/2009
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ABSTRACT: This paper presents a general analysis of interleaved operation of three-phase voltage-sourced converters (VSC). Both the conventional symmetric interleaving and the more general, asymmetric interleaving with unequal carrier phase shifts are studied. For each case, previously reported analytical phase voltage spectral models are generalized to include the effects of harmonic injection into the reference (modulation) signals. The spectral models are then used to determine and quantify harmonic cancellation effects in the phase current, common-mode voltage, as well as dc-link current. The analyses will prove that harmonic injection doesn't affect harmonic cancellation in any of the variables mentioned above under symmetric interleaving. For asymmetric interleaving, a scaling factor is defined to model the effects of variable interleaving angles on individual harmonics. Examples are provided to demonstrate the potential benefits of asymmetric interleaving, such as minimization of EMI filter size. The analyses are further extended to regular-sampling PWM where complete harmonic cancellation usually doesn't happen to all sideband harmonics simultaneously. Numerical results are included to validate the analytical models.
Power Electronics Specialists Conference, 2008. PESC 2008. IEEE; 07/2008
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ABSTRACT: This paper presents a systematic study of the effects of interleaving on three-phase voltage-sourced converters (VSC). Analytical results are developed for the spectra of converter phase currents, common-mode voltages, and dc-link currents for the general case of interleaving N parallel modules with a common dc link. It will be shown, both analytically and numerically, that ripple cancellation effects similar to what have been widely observed in buck dc-dc converters, that is, the elimination of harmonic sidebands other than those centered at TV multiples of the carrier frequency, can also be achieved by proper interleaved operation of N three-phase VSC modules. This will be shown to be true not only for the ac phase currents, as previously known, but also for the common-mode voltage as well as the dc-link current. Analytical results are also presented for the circulating current among parallel modules. Additionally, a relationship between interleaved VSCs and capacitor-clamped multilevel converters is established and used to extend the analytical results developed for interleaved VSCs to multilevel converters.
Power Electronics Specialists Conference, 2007. PESC 2007. IEEE; 07/2007
