John E. Quaicoe

Analysis and design of a multiple feedback loop control strategy for single-phase voltage-source UPS inverters

This paper presents the analysis and design of a multiple feedback loop control scheme for single-phase voltage-source uninterruptible power supply (UPS) inverters with an L-C filter. The control scheme is based on sensing the current in the capacitor of the load filter and using it in an inner feedback loop. An outer voltage feedback loop is also incorporated to ensure that the load voltage is sinusoidal and well regulated. A general state-space averaged model of the UPS system is first derived and used to establish the steady-steady quiescent point. A linearized small signal dynamic model is then developed from the system general model using perturbation and small-signal approximation. The linearized system model is employed to examine the incremental dynamics of the power circuit and select appropriate feedback variables for stable operation of the closed-loop UPS system. Experimental verification of a laboratory model of the UPS system under the proposed closed-loop operation is provided for both linear and nonlinear loads. It is shown that the control scheme offers improved performance measures over existing schemes, It is simple to implement and capable of producing nearly perfect sinusoidal load voltage waveform at moderate switching frequency and reasonable size of filter parameters. Furthermore, the scheme has excellent dynamic response and high voltage utilization of the DC source.

Selection of a curved switching surface for buck converters

A general procedure for the selection of a curved switching surface (SS) to control buck-type converters is presented in this letter. The analysis is based on the normalized representation of ideal SSs for different loading conditions. The normalization process leads to a unique representation of the SSs for any possible buck converter. A set of graphics in three dimensions is introduced to give a spatial sense of the behavior of the converter and its control requirements during transients. As a result of the investigation, a switching surface referred to in this letter as the natural unloaded SS is selected, providing excellent transient behavior and no overshoot during startup. For any buck converter with typical parameters, this control scheme produces, in one switching action, a minimum of 99% of the desired output voltage. The general concept of using second-order SS is also geometrically analyzed in this letter to clarify its characteristic features and disadvantages. Experimental results for a typical buck converter are presented to illustrate the transient behavior of the converter during startup and sudden load changes. The results confirm the virtues of the control scheme.

Performance Analysis of Delta Modulated PWM Inverters

An analysis is presented of a voltage source delta modulation (DM) inverter. The delta modulation method provides several advantageous characteristics over commonly used sine modulation process. It is relatively easy to implement and provides low harmonics at the inverter output. The inherent features of the delta modulation also provide control of ratio of voltage to frequency and commutation numbers of an inverter without much complexity in control circuit. These and other features of DM inverters are investigated theoretically and verified experimentally. Successful implementation of DM techniques in three-phase inverters are reported.

Advanced Boundary Control of Inverters Using the Natural Switching Surface: Normalized Geometrical Derivation

A curved switching surface (SS) for inverters control with superior characteristics is geometrically derived in this work. In order to avoid inaccuracies introduced by simplification or assumptions, the analysis is entirely performed using a versatile geometrical method in the normalized domain. Both the output voltage and the capacitor current are considered as varying references to establish a more accurate control law with enhanced performance. The proposed normalization technique provides remarkable insight into the behavior of system leading to a pure geometrical treatment that is general and applicable to any possible inverter. As a result, a control law for inverters defined as natural SS is proposed and thoroughly characterized. In addition to the enhanced dynamic response, fixed frequency operation is one of the key features of the proposed control scheme. In order to formally demonstrate fixed frequency operation, a transformation from the natural SS to its PWM equivalent is performed, revealing duality between boundary control using curved SS and traditional PWM. This is a significant advancement towards the unification and understanding of traditional modulation against modulation produced by curved SSs. Finally, an additional novel concept is explored: operation in mixed monopolar and bipolar mode using the natural SS. This new mixed operating mode overcomes physical limitations of the inverter structure in monopolar mode around the region of output voltage zero crossing (both the problem identification and solution are investigated). Experimental results of a 1.5 kVA inverter operating at fixed moderate frequency are presented to validate the natural SS performance, illustrate the benefits of the normalization technique, and demonstrate the monopolar and mixed operating mode.

Improvements in Boundary Control of Boost Converters Using the Natural Switching Surface

A boundary control scheme for boost converters with enhanced performance using the natural switching surface (NSS) is presented in this paper. The derivation of the switching surface (SS) is performed in the normalized domain to obtain a general geometrical representation that is applicable to any combination of boost converter parameters. The resulting SS provides an excellent dynamic response during start-up and sudden load changes, achieving steady state in only one switching action. In order to illustrate the characteristic features and the superior performance of the natural SS, a geometrical analysis is fully developed and compared to other recent SSs. A design example is presented to illustrate the benefits of the normalization technique, and the qualities of the natural SS are confirmed with simulation and experimental results of a 30 W, 10 V/22 V converter.

Applying Response Surface Methodology to Small Planar Transformer Winding Design

Planar transformers provide a light-weight and low-profile solution for power electronic converters with highly reproducible parameters and simple manufacturability. Parasitic inductances, capacitances, and resistances in planar magnetics are difficult to model due to the complex interactions between the physical winding arrangement of each layer and the core geometry (track width, air gap, clearance, etc.). These nonlinear and multivariate magnetic devices play a key role in defining the performance of traditional, soft-switching, and resonant converters by ruling behaviors such as ringing, self-resonant frequency, conduction losses, and the current rate of change in these converters. In this paper, a methodology for determining parametric models for leakage and magnetizing inductance, inter- and intrawinding capacitances, and the winding resistance of small planar transformers is presented using a variety of winding arrangements. The models are employed to shape the winding design to control parasitic elements in order to optimize soft-switching and resonant converters. A central composite design based on the design of experiment methodology is employed on finite element simulations to provide the comprehensive models. Results from physical verification on a planar Ferroxcube ER18/3.2/10-3F3 core set are provided and show excellent correlation between models and verification tests. The method is later employed to effectively design an LLC resonant converter, which is also experimentally verified to illustrate the benefits of the proposed method. The methodology can be employed to characterize and design planar transformers and to predict their performances as part of a variety of power electronic converters.

An Embedded Frequency Response Analyzer for Fuel Cell Monitoring and Characterization

This paper presents an embedded frequency response analyzer for fuel cells (FCs) based on a robust measurement technique with simple implementation. A frequency response analysis technique provides valuable information of different electrochemical processes that occur inside the FC. The measurement system is implemented on a low-cost digital signal processor to perform frequency response and impedance tracking. The small-size and low-power consumption allows this special device to be embedded into the FC controller or power conditioning stage. The system is capable of measuring automatically the frequency response of the FC at different operating points, even when the FC is operating with a load. These measurements can be used to characterize the FC at the design stage and to perform online monitoring of the FC state during a continuous operation. The proposed instrument uses the lock-in amplification technique, which allows very accurate and precise measurements even in the presence of high noise levels. The proposed hardware and signal processing technique are described in this paper, including the experimental result of a 1.2-kW proton exchange membrane FC system.

Soft-Switching Techniques for Efficiency Gains in Full-Bridge Fuel Cell Power Conversion

This paper presents a set of novel soft-switching techniques to increase the power conversion efficiency in fuel cell (FC) systems using a full-bridge topology. For this purpose, a special right-aligned modulation sequence is developed to minimize conduction losses while maintaining soft-switching characteristics in the MOSFETs. Traditional auxiliary elements in the primary, such as series inductors that are impractical for realizing due to the extreme input current, are avoided and reflected to the output of the rectifier to minimize circulating current and generate soft transitions in the output diodes. As a result, the proposed combined techniques successfully reduce conduction losses, minimize reverse-recovery losses in the output rectifiers, minimize transformer ringing, and ensure low stress in all the switches. The high efficiency is maintained in the entire range of loading conditions (0%-100%) while taking into consideration remarkable challenges associated with FC power conversion: high input current, low voltage and poor regulation, and wide range of loading conditions. A detailed analysis of the techniques for efficiency gains are presented and a phase-shift zero-voltage switching topology is employed as a reference topology to highlight the mechanisms for performance enhancement and the advantages in the use of the special modulation. Experimental results of a 1-kW power converter are presented to validate the efficiency gains, illustrate the benefits of the special modulation, and demonstrate the soft-switching transitions.

A single-phase delta-modulated inverter for UPS applications

The performance characteristics of the rectangular wave delta modulation (RWDM) scheme for uninterruptible power supply (UPS) applications is investigated. Normalized characteristic curves that show the effect of various modulator parameters on the frequency spectrum of the inverter output voltage are obtained using discrete Fourier transform (DFT) and harmonic analysis techniques. The performance of a single-phase half-bridge inverter with an LC filter is discussed, and experimental results are provided to validate the predicted and simulated results. It is shown that the harmonic content of the inverter output waveform can be controlled through the control of the modulator parameters. >

Maximum Power Extraction from a Small Wind Turbine Emulator using a DC - DC Converter Controlled by a Microcontroller

An isolated small wind turbine emulator based on a separately excited DC motor is developed to emulate and evaluate the performance of a small wind turbine using different control strategies. The test rig consists of a 3HP separately excited DC motor coupled to a synchronous generator. A dump load is connected to the generator through a buck-boost converter controlled by a microcontroller. Wind turbine rotor and furling dynamics are incorporated in the emulator with the use of a PC based wind turbine model. Emulation of the wind turbine is confirmed by running the DC motor to track the theoretical rotational speed of the wind turbine rotor. A dynamic maximum power controller is implemented and tested. The controller uses the wind speed and rotor speed information to control the duty cycle of the buck-boost converter in order to operate the wind turbine at the optimum tip-speed ratio. Test results indicate that the proposed system accurately emulates the behavior of a small wind turbine system.