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In this paper, a new scheme is proposed to connect wind turbines to the power grid. This scheme helps in limiting the fault currents as well as in voltage unbalance compensation. Voltage is injected in series with the transmission line to limit the fault currents as well as to balance the voltages. This scheme allows wind turbines to remain synchronized to the grid during faults or during low voltages and is useful for doubly fed as well as squirrel cage induction generators. Index Terms--Doubly fed induction generators (DFIGs), fault current limitation, voltage unbalance compensation.

Wind energy is often installed in rural, remote areas characterized by weak, unbalanced power transmission grids. In induction wind generators, unbalanced three-phase stator voltages cause a number of problems, such as overcurrent, unbalanced currents, reactive power pulsations, and stress on the mechanical components from torque pulsations. Therefore, beyond a certain amount of unbalance, induction wind generators are switched out of the network. This can further weaken the grid. In doubly fed induction generators (DFIGs), control of the rotor currents allows for adjustable speed operation and reactive power control. This paper presents a DFIG control strategy that enhances the standard speed and reactive power control with controllers that can compensate for the problems caused by an unbalanced grid by balancing the stator currents and eliminating torque and reactive power pulsations. Index Terms--AC generators, induction motors, power system dynamic stability, reactive power control, torque control, variable speed drives, wind energy.

A hybrid phase-modulated converter (HPMC) is a recent innovation in the family of soft-switching converters. It is a promising solution to most soft-switching issues. The principal bottleneck in achieving higher efficiency with this topology is the secondary side loss--mainly the losses in the transformer and the rectifier. For low-voltage high-current power supplies, the current-doubler rectification of HPMC addresses both the transformer conduction losses and the rectifier losses. The presence of an additional path for quiescent current in this scheme gives rise to a third mode of operation. There is also the possibility of magnetic integration of all the magnetic components into one, which can cause substantial reduction in magnetic requirements. These facts make the analysis of current doubler important. In this paper, all the operating modes are identified and corresponding equations and equivalent circuits that aid in filter and control design are derived. The zero-voltage-switching (ZVS) characteristics, filter requirement, small-signal transfer characteristics, device ratings, and magnetics size requirement are considered to compare this configuration with its center-tapped counterpart. The current-doubler scheme is found to have superior soft-switching characteristics in that it can achieve ZVS at lighter loads with a much lower peak magnetizing current in the transformer and leakage inductance. Also, a judicious choice of output current ripple can give an overall reduced magnetics requirement. The analyses are verified by simulation and hardware implementation. HPMC is found to be most advantageous for applications with input voltages essentially constant, but the output voltage widely varying, for example in battery chargers and converters with power factor correction front end. Index Terms--Current doubler, phase-modulated converter (PMC), zero-voltage switching (ZVS).

Dynamic modeling of switched reluctance motors (SRMs) is usually done on a per-phase basis. However, in most applications, SRMs are used with simultaneous excitation of more than one phase. Thus, a model accounting for mutual coupling in the presence of magnetic saturation is needed to predict and optimize their performance in terms of efficiency and torque ripple. This paper presents a dynamic two-phase excitation model of the SRM. Motor symmetry is used to reduce the amount of flux linkage data needed for the model. An experimental procedure to obtain the flux linkage data is described. Measured flux linkage data for the 8/6 SRM are also included. Details of the simulation model and comparison with experimental waveforms along with their implications for performance prediction are presented. Index Terms--Dynamic modeling, experimental characterization, switched reluctance motor (SRM), two-phase excitation.

This paper presents system modeling and control design for fast load voltage regulation using static compensators (STATCOMs). The modeling strategy gives a clear representation of load voltage magnitude and STATCOM reactive current on an instantaneous basis. The particular coordinate transformation employed here also facilitates extraction of linearized system dynamics in conjunction with circuit simulators. It is rigorously shown that the control problem of load voltage regulation using reactive current is nonminimum phase. Linear and nonlinear controllers for the regulation problem are designed and compared via simulation results. Internal dynamics of the STATCOM are modeled using the same strategy. Lyapunov based adaptive controllers are designed for controlling the STATCOM reactive current while maintaining its dc bus voltage. Simulation results of the controlled STATCOM integrated with the load bus voltage controller are presented to show efficacy of the modeling and control design. Index Terms--Flicker mitigation, nonlinear control, power quality, static compensators (STATCOMs), system modeling, voltage regulation.

Power supply voltages in digital systems have reduced considerably in recent years, and often digital components requiring different voltages are present on the same board. This has increased the demand for multiple output power distribution systems with tight individual load voltage regulation. This paper presents a power distribution system having a central power supply that acts as a controlled current source whose output is connected to individual loads on a time shared basis. Ali the magnetics are concentrated in the converter acting as the current source. The current source can be realized by a current controlled buck, buck-boost, or single-ended primary inductance converter (SEPIC). First, a comparative analysis of buck, buck-boost, and SEPIC based implementation of the current source is carried out. Next, a buck-based current source implementation with constant frequency pulse-width modulation control for the output voltages is described. Detailed component and control design, simulations, and experimental results for a 100-W prototype are presented. Index Terms--Buck based multiple output, controlled current source, individual load regulation, multiple output power supply, power distrbution system (PDS), single-ended primary inductance converter (SEPIC).

Torque output of switched reluctance machines (SRMs) can be increased by operating with high demagnetization voltage, especially for high speeds. This is first illustrated using computed results obtained from dynamic simulation. A power converter with the capability of providing increased demagnetization voltage is then presented. The converter operation is analyzed, ratings of various components are derived, and a controller for regulating the additional demagnetization voltage is designed. Experimental results illustrate expected operation of the converter, and improved performance of the overall SRM drive in terms of increased torque and power output. Index Terms--Fast demagnetization, power converter, switched reluctance motor drives.

Ned This paper explores a new configuration for modular dc-dc converters, namely, series connection at the input, and parallel connection at the output, such that the converters share the input voltage and load current equally. This is an important step toward realizing a truly modular power system architecture, where low-power, low-voltage, building block modules can be connected in any series/parallel combination at input or at output, to realize any given system specifications. A three-loop control scheme, consisting of a common output voltage loop, individual inner current loops, and individual input voltage loops, is proposed to achieve input voltage and load current sharing. The output voltage loop provides the basic reference for inner current loops, which is modified by the respective input voltage loops. The average of converter input voltages, which is dynamically varying, is chosen as the reference for input voltage loops. This choice of reference eliminates interaction among different control loops. The input-series and output-parallel (ISOP) configuration is analyzed using the incremental negative resistance model of dc-dc converters. Based on the analysis, design methods for input voltage controller are developed. Analysis and proposed design methods are verified through simulation, and experimentally, on an ISOP system consisting of two forward converters. Index Terms--Input-series connection, input-series and output-parallel (ISOP), load sharing, modular converter, voltage sharing.

Power electronics applications in utility systems are growing very rapidly and promise to change the landscape of future power systems in terms of generation, operation, and control. This paper attempts to justify the importance of introducing undergraduates to these applications in the first course in power systems, so they become familiar with current and future practices and technologies. It presents a roadmap of doing so, taking a top-down approach where first various utility applications of power electronics are briefly described along with the role of power electronics as an interface, and then the power electronics systems are discussed in appropriate detail to fulfill these roles. Index Terms--Education, power electronics, utility applications.

Since 1994, the University of Minnesota has been undertaking a long overdue restructuring of power electronics and electric machines/drives courses. This restructuring allows digital control to be integrated into first courses, thereby teaching students what they need to learn, making these courses appealing, and providing a seamless continuity to advanced courses. By a concise presentation in just two undergraduate courses, this restructuring motivates students to take related courses in programmable logic controllers, microcontrollers and digital signal processor applications. This ensures a first-rate education that is meaningful in the workplace as well as in graduate education leading to a research and development oriented career. This restructuring has several components to it. Outdated topics that waste time and mislead students are deleted. To integrate control in the first courses, unique approaches are developed to convey information more effectively. In the first course in power electronics, a building block is identified in commonly used power converter topologies in order to unify their analysis. In the field of electric drives, the use of space vectors is introduced on a physical basis to describe operation of ac machines in steady state in the first course, and to discuss their optimum control under dynamic conditions in the advanced course. Appropriate simulation software and software-reconfigurable hardware laboratories using a DSP-based rapid prototyping tool are used to support the analytical discussion. Index Terms--Digital control, education, electric drives, laboratories, power electronics.

Following the precepts of an ideal dc-dc power converter, the asymmetrical duty-cycle power converter, a zero-switching loss converter, is, in a demonstrable way, a new direction in dc power conversion. Its benefits include low conduction loss, low EMI generation, and lossless switching. This paper includes conceptual discourse on the nature of the asymmetrical converter, as well as experimental results.

This paper presents the application of zero-current switching in a three-phase rectification scheme with nearly sinusoidal line currents. The principle of third harmonic current circulation is used to reduce the distortion in the line currents. Zero-current switching is accomplished with the use of only two switches. The operation of the circuit is verified experimentally. Comparisons with a PWM based rectifier and a six-switch rectifier in terms of component stresses and EMI filter considerations are presented.

Power electronics have become increasingly reliable and cost effective at the transmission voltage level. New applications of FACTS devices are just emerging, Thyristor Controlled Phase-Angle Regulator (TCPAR) being one of them. This paper identifies and discusses several gate control and system issues for application of single core transformer Thyristor Controlled Phase-Angle Regulator. Alternative TCPAR designs are proposed.

Use of nonlinear loads and devices in power systems is expected to grow rapidly. Examples are thyristor-controlled inductors for FACTS, converters for HVDC transmission and large adjustable speed drives. All of these loads inject harmonic currents into the power system. A novel hybrid filter topology and its control, to prevent such harmonic currents from entering the power system, are presented in this paper. Analysis shows that in the proposed filter, the power electronic converter, required to generate harmonic currents for compensation, has a rating of only 9 % when compared to the converter rating in an active filter and approximately one-half of that in a series-hybrid filter discussed in the literature. The proposed topology enables practical implementation of active harmonic current filters. The control of this filter under transient conditions such as start-up and during steady state is demonstrated by means of simulations. Results from a scaled-down hardware prototype are presented to verify the simulations.

Utility-interactive photovoltaic, wind-electric and fuel-cell systems are being planned at sufficiently large power levels. The technical and economic feasibility of such systems partially depends on the reliability, cost, and the efficiency of their grid interface. Here, a novel interface, which takes advantage of the system conditions in a utility-scale application is presented. The paper describes its basic principle of operation and characteristics. It is shown that the Total Harmonic Distortion in the line currents meets the 5 percent limit recommended by the IEEE 519-1992, with one-third the high frequency switch kVA ratings as compared to other topologies. Various options are discussed for utility-scale applications. Simulation results for a 25 kW unit are presented. The new conceptual circuit is validated by a prototype hardware design at 2.2 kW power level. Keywords: Grid interface, utility-interactive, power electronics inverter with sinusoidal currents, photovoltaic interface, wind-electric interface, interface for fuel cells.

A novel approach to achieve nearly sinusoidal line current rectification of three-phase utility voltages is presented. The scheme incorporates two step-up dc-dc converters to modulate the de-link currents at the third-harmonic frequency. The modulated currents are then circulated through the ac-side of the diode-bridge rectifier, using a zigzag autotransformer. Simulation results for a three-phase, 208-V (line-to-line), 1-kW system are presented. Results from experimental investigations on a laboratory model are included, supporting the concept.

Power electronic loads inject harmonic currents into the utility system. This paper presents a comparative evaluation of harmonic reduction techniques which satisfy the current harmonic limits specified by the IEEE Standard 519, and at the same time provide a regulated dc output voltage. The techniques considered include active and hybrid filters, and various current waveshaping approaches for a three-phase utility interface. These techniques are compared in terms of their complexity (number of switches) and their component ratings. Based on the application requirements and the cost of active and passive components, this paper enables the estimation of the minimum cost topology.

A zero-voltage-switching (ZVS) three-phase PWM inverter which uses a newly proposed Parallel-Resonant DC-Link (PRDCL) circuit is further examined in this paper. The proposed PRDCL circuit is aimed at both providing zero-dc-link voltage periods for PWM inverter switchings and imposing minimum dc bus voltage stress to PWM inverters. A simple circuit control scheme and the design formulas are offered. To confirm the circuit theoretical analyses and results, a 50-watt ZVS single-phase PWM inverter experimental prototype using the proposed PRDCL circuit was fabricated and tested. The test results have proved the proposed circuit and concept. The paper concludes with the power device selections and device stress evaluations as well as a detailed comparison of the proposed PRDCL ZVS inverter system with conventional voltage source PWM inverters.

It has been estimated that by the end of this decade, as much as 50% of the electrical load in the U.S. will be supplied through power electronics interface. Very often in such systems, line-frequency ac voltages are converted into an uncontrolled dc voltage by means of diode-bridge rectifiers, which draw a highly distorted current from the utility source. This paper presents a novel approach to achieve nearly sinusoidal line current rectification of 3-phase utility voltages by dc-link current modulation. Significant other benefits of this approach are as follows: (i) only two transistor switches are required, (ii) a regulated dc voltage is provided at the output, thus simplifying the design of the rest of the power electronics equipment, (iii) the output dc voltage which can be quite large is automatically shared equally by two series-connected capacitors, and (iv) the usual disadvantages of passive filters are avoided. This novel approach can be applied to most of the power electronics equipment with 3-phase utility input, as well as to interface generation using renewable energy sources such as wind, solar, etc.

Operating a bridge-type PWM switch mode power converter with asymmetrical duty ratios can eliminate switching losses with no increase in conduction loss. This is a new circuit topology that combines the best features of resonant (zero switching loss) and switch mode (low conduction loss) circuits. Design equations for just such a circuit are presented.