An application of the well-developed frequency-domain approach to detect oscillations in nonlinear feedback systems with time delay is presented. The method depends on an early proof of the Hopf bifurcation theorem known as the Graphical Hopf Theorem (GHT). Several nondegeneracy conditions are included to apply the GHT in nonlinear systems with time delay. The singular conditions corresponding to degeneracies, which include static and dynamic bifurcations, as well as some special cases of degenerate Hopf bifurcations and multiple crossings, are also discussed. Two Single-Input Single-Output (SISO) feedback systems with odd nonlinearities are presented as examples to show that the proposed technique and a standard simulation method have very good agreement in the results, yet the GHT is much simpler in calculation. The first one shows an application of the GHT under classical Hopf conditions while the second emphasizes the presence of degenerate Hopf bifurcations and multiple crossings. For both examples, and others which have appeared recently in the literature, a considerable simplification of the formulas for recovering periodic solutions is also provided in this paper.

In this paper a new control strategy for voltage-source converters (VSC) is introduced. The proposed strategy consists of a nonlinear feedback controller based on feedback linearization plus a feedforward compensation of the estimated load current. In our proposal an energy function and the direct-axis current are considered as outputs, in order to avoid the internal dynamics. In this way, a full linearization is obtained via nonlinear transformation and feedback. An estimate of the load current is feedforwarded to improve the performance of the whole system and to diminish the capacitor size. This estimation allows to obtain a more rugged and cheaper implementation. The estimate is calculated by using a nonlinear reduced-order observer. The proposal is validated through different tests. These tests include performance in presence of switching frequency, measurement filters delays, parameters uncertainties and disturbances in the input voltage.

The electromechanical interface is a Synchronous Machine because its field winding permits direct management of the magnetization during speed variations. For systems with a common dc-link for the drive and excitation converters, the efficiency is increased if the excitation drive has boosting capability. It is shown that with the proposed control strategy the Z-source converter is suitable for this application, becoming a better alternative than the typically used buck converter. The Z-source converter in combination with the proposed multi-loop control law can achieve the desired voltage reference swing and high-performance tracking. An analytical comparison between the dominant losses of the buck topology, typically used in FESS, and the Z-source converter shows that the latter has higher efficiency for this application. The parameters of the converter prototype were experimentally identified and used to implement the proposed controller. The control strategy uses the two duty cycles as manipulated variables, one to allow tracking fast changes in the reference signal and the other to adapt the system to the slow changes. The combined action on both inputs contribute to the compensation of the non-minimum phase response of the Z-converter. Experimental results show the potential of the controller for tracking typical FESS application waveforms.

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This paper proposes a novel method to generate a sinusoidal waveform synchronized with any measurable periodic signal whose frequency is within a given neighborhood. The synthesized sinusoidal signal could be used as a reference current for certain applications of parallel active power filters or any other where such synchronization would be necessary (e.g., ac/dc converters for renewable energy resources, power factor correctors, power supplies, UPS, etc.) The method is based on the behavior of a dynamical system and avoids employing the usual combination of phase-locked loop (PLL) and lookup table found in most parallel active filters synthesizing a sinusoidal source current (a table also means using significant storage memory). The novel method produces two high-quality sinusoidal waveforms that are in quadrature and is applicable to those parallel active filters whose control methodology is in the reference frame, or alternatively, it produces three sinusoidal waveforms shifted 120 degrees for designs that work in the frame. Here, a (2 + 1)th order implementation is described, including a proof of convergence. For its most simple implementation ( = 1), simulated and experimental results are included.

Abstyact-The design of a swltching power supply (Buck DC-DC converter) with regulated output voltage for a wide range of current consumption is considered. T h e control objective is to regulate the output voltage despite the exlstence of current load variations with bounded time derivative. A variable-structure technique is used t o design a continuous time controller. A discrete and an analog version of the same controller are compared. Laboratory experiences are performed to obtain conclusions about the real performance of the proposed controller with satisfactory results.