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Fuel cell stacks produce a dc output with a 2:1 variation in output voltage from no-load to full-load. The output voltage of each fuel cell is about 0.4 V at full-load, and several of them are connected in series to construct a stack. An example 100 V fuel cell stack consists of 250 cells in series and to produce 300 V at full-load requires 750 cells stacked in series. Since fuel cells actively convert the supplied fuel to electricity, each cell requires proper distribution of fuel, humidification, coupled with water/thermal management needs. With this added complexity, stacking more cells in series decreases the reliability of the system. For example, in the presence of bad or malperforming cell/cells in a stack, uneven heating coupled with variations in cell voltages may occur. Continuous operation under these conditions may not be possible or the overall stack output power is severely limited. In this paper, a modular fuel cell powered by a modular dc--dc converter is proposed. The proposed concept electrically divides the fuel cell stack into various sections, each powered by a dc--dc converter. The proposed modular fuel cell powered by modular dc--dc converter eliminates many of these disadvantages, resulting in a fault tolerant system. A design example is presented for a 150-W, three-section fuel cell stack and dc-dc converter topology. Experimental results obtained on a 150-W, three-section proton exchange membrane (PEM) fuel cell stack powered by a modular dc-dc converter are discussed. Index Terms--DC--DC converters, fuel cells, power conditioning, renewable power.

Fuel cell stacks produce a dc output with a 2:1 voltage variation from no load to full load. It is customary for a utility-scale fuel cell stack to consist of several hundreds of series-connected cells, each producing 0.6 V at full load. A power conditioner consisting of dc-dc and dc-ac converters is required for utility interface, which are operated in high frequency, employing pulsewidth-modulation control for voltage and current regulation. Due to their switch-mode nature, a common-mode voltage with respect to ground is generated. The common-mode voltage, in turn, contributes to the circulating ground current, which can interfere with the ground fault protection system. In addition, it also contributes to the neutral shift and electromagnetic interference. Moreover, the electrostatic potential to ground within the fuel cell stack needs to be limited for safe operation. This paper presents an analysis of common-mode voltage in several fuel-cell-powered converter topologies connected to the electric utility and discusses several mitigation methods suitable for utility-scale generation. Index Terms--Common-mode current, common-mode voltage, fuel cell, utility interactive.

This paper focuses on a new three-phase high power current-fed dc/dc converter with an active clamp. A three-phase dc/dc converter with high efficiency and voltage boosting capability is designed for use in the interface between a low-voltage fuel-cell source and a high-voltage dc bus for inverters. Zero-voltage switching in all active switches is achieved through using a common active clamp branch, and zero current switching in the rectifier diodes is achieved through discontinuous current Conduction in the secondary side. Further, the converter is capable of increased power transfer due to its three-phase power configuration, and it reduces the rms current per phase, thus reducing conduction losses. Moreover, a delta-delta connection on the three-phase transformer provides parallel current paths and reduces conduction losses in the transformer windings. An efficiency of above 93% is achieved through both improvements in the switching and through reducing conduction losses. A high voltage ratio is achieved by combining inherent voltage boost characteristics of the current-fed converter and the transformer turns ratio. The proposed converter and three-phase PWM strategy is analyzed, simulated, and implemented in hardware. Experimental results are obtained on a 500-W prototype unit, with all of the design verified and analyzed. Index Terms--Active clamp, leakage inductance, three-phase dc-dc converter, three-phase pulsewidth modulation (PWM) strategy, voltage transfer ratio, zero-voltage switching (ZVS).

In a three-phase four-wire utility system, three-phase--unbalanced loads and many single-phase nonlinear loads result in a neutral line current that is zero-sequence current. To deal with unbalanced and nonlinear loads in a high power system, this paper suggests a multilevel four-leg pulsewidth modulation (PWM) voltage-source inverter (VSI) as a topology for the high power applications where a function is required to control the zero-sequence component as well as dq components. This paper proposes a carrier-based PWM method for a multilevel four-leg PWM VSI along with introducing a novel offset voltage. The proposed offset voltage makes it possible for the switching sequence of all the legs to be optimized for minimizing the harmonic distortion of the output voltage irrespective of the number of inverter levels. The proposed PWM method based on digital signal processors is implemented and tested by using a prototype three-level four-leg VSI. The feasibility of the proposed PWM method is verified by the spectral analysis, simulation, and experimental results. Index Terms--Multilevel four-leg, offset voltage, optimal switching sequence, pulsewidth modulation, three-phase four-wire, voltage-source inverter, zero-sequence current.

In response to electric energy crisis and power quality concerns, a simple and low-cost variable speed integrated doubly fed electric alternator/active filter (IDEA) for wind energy conversion systems is proposed. The proposed IDEA is capable of simultaneously capturing maximum power of wind energy with fluctuating wind speed and improving power quality, which are achieved by canceling the most significant and troublesome harmonics of the utility grid. Power factor correction and reactive power control are the other two significant features of the proposed technology. The back-to-back current-regulated power converters are employed to excite the rotor of IDEA. The control strategy of rotor side power converter is based on position sensorless field oriented control method with higher power density. A laboratory prototype has been fabricated and tested, and a TMS3202407 DSP-based controller has been used to control the back-to-back power converters. Analysis and experimental results are presented to demonstrate the effectiveness of the proposed integrated IDEA. Index Terms--Active filter, doubly-fed induction generator, electromechanical active filter, harmonics compensation, wind energy.

In this paper, an analysis and design of a wide input range dc-dc converter is proposed along with a robust power control scheme. The proposed converter and its control are designed to be compatible with a fuel cell power source, which exhibits 2 : 1 voltage variation as well as a slow transient response. The proposed approach consists of two stages: a three-level boost converter stage cascaded with a current-fed two-inductor boost converter topology, which has a higher voltage gain and provides galvanic isolation from the input source. The function of the front-end boost converter stage is to maintain a constant voltage at the input of the cascaded dc-dc converter to ensure optimal performance characteristics and high efficiency. At the output of the first boost converter, a battery or ultracapacitor energy storage is connected to handle slow transient response of the fuel cell (200 W/min). The robust features of the proposed control system ensure a constant output dc voltage for a variety of load fluctuations, thus limiting the power being delivered by the fuel cell during a load transient. Moreover, the proposed configuration simplifies power management and can interact with the fuel cell controller. Simulation and the experimental results confirm the feasibility of the proposed system. Index TermslBattery, current control, DC-DC power conversion, energy storage, fuel cells, hybrid power source, supercapacitors.

In this paper, an interleaved active-power-filter concept with reduced size of passive components is discussed. The topology is composed of two pulsewidth-modulation interleaved voltage-source inverters connected together on the ac line and sharing the same dc-link capacitor. The advantages of the proposed approach are as follows: 1) significant reduction in the linkage inductors' size by decreasing the line-current ripple due to the interleaving; 2) reduction of the switching stress in the dc-link capacitor, due to the shared connection; and 3) more accurate compensation for high-power applications, because the power sharing allows one to use a higher switching frequency in each inverter. This paper analyzes the design of the passive components and gives a practical and low-cost solution for the minimization of the circulation currents between the inverters, by using common-mode coils. Several simulation results are discussed, and experimental results with a three-phase 10-kVA 400-V unit are obtained to validate the theoretical analysis. Index Terms--Active filters, interconnected power systems, power-system harmonic, pulsewidth-modulated (PWM) inverters, reactive power.

High power telecommunication power supply systems consist of a three-phase switch mode rectifier followed by a dc/dc converter to supply loads at -48 V dc. These rectifiers draw significant harmonic currents from the utility, resulting in poor input power factor with high total harmonic distortion (THD). In this paper, a digitally controlled three-phase switch mode power supply based on a matrix converter is proposed for telecommunication applications. In the proposed approach, the matrix converter directly converts the low frequency (50/60 Hz, three-phase) input to a high frequency (10/20 kHz, one-phase) ac output without a dc-link. The output of the matrix converter is then processed via a high frequency isolation transformer to produce -48 V dc. Digital control of the system ensures that the output voltage is regulated and the input currents are of high quality under varying load conditions. Due to the absence of dc-link electrolytic capacitors, power density of the proposed rectifier is expected to be higher. Analysis, design example and experimental results are presented from a three-phase 208-V, 1.5-kW laboratory prototype converter. Index Terms--Three-phase switch mode rectifier, total harmonic distortion (THD).

In this paper, a fast detection method for voltage disturbances is explored. The proposed method provides reliable and fast detection for either single-phase or polyphase voltage disturbances such as voltage sags, voltage swells, flicker, frequency change in the utility voltage, and harmonic distortion. The algorithm is based on the theory that allows a set of three-phase voltages be represented as dc voltages in a d-q synchronous rotating frame. In this case, the utility input voltages are sensed and then converted to dc quantities in the d-q reference frame. Thus, any disturbance at the utility input voltages will be promptly reflected as disturbances in the d-q values. Further processing of the signals minimizes the point-on-wave effect and yields a trip signal indicating if the disturbance is a voltage sag or a voltage swell. Analysis, simulation, and experimental results are presented for a three-phase system. The proposed algorithm is implemented on a digital signal processor (DSP)-based system to provide ride-through for critical loads. Index Terms--Disturbances detection, disturbances mitigation and voltage disturbances, power quality.

Beginning fall 2001, the Electrical Engineering Department at Texas A&M University, College Station, significantly altered the instructional philosophy of the ELEN 214 Electric Circuit Theory course by introducing more engineering design into the curriculum and adopting the WebCT-based interactive homework submission system. This paper will discuss the use of the Quiz tool within WebCT for the construction of question banks and their publication to a WebCT server. An example, deriving mathematical expressions, which describes electric circuit behavior and helps customize the homework problems to each individual student, are discussed. In other words, in a class of 200 students, each student is presented with an individual homework assignment with a unique set of problems not repeated to anyone else via WebCT. A help desk staffed by senior undergraduates assists the course students in completing the WebCT-based homework on time. WebCT is an essential ingredient in the delivery of the course. The approach presented in this paper can be adapted to any other course in engineering/science that involves mathematical calculations. So far, the course evaluations suggest that the students are more motivated and excited about electrical and computer engineering as a career. Index Terms--Automated homework, engineering education, Internet, Web-based materials, WebCT.

In this paper, the development of a low cost fuel cell inverter system is detailed. The approach consists of a three-terminal push-pull dc-dc converter to boost the fuel cell voltage (48 V) to [+ or -]200 VDC. A four switch [insulated gate bipolar transistor (IGBT)] inverter is employed to produce 120-V/240-V, 60-Hz ac outputs. High performance, easy manufacturability, lower component count, safety and cost are addressed. Protection and diagnostic features form an important part of the design. Another highlight of the proposed design is the control strategy, which allows the inverter to adapt to the requirements of the load as well as the power source (fuel cell). A unique aspect of the design is the use of the TMS320LF2407 DSP to control the inverter. Two sets of lead-acid batteries are provided on the high voltage dc bus to supply sudden load demands. Efficient and smooth control of the power drawn from the fuel cell and the high voltage battery is achieved by controlling the front end dc-dc converter in current mode. The paper details extensive experimental results of the proposed design on Department of Energy (DoE) National Energy Technology Laboratory (NETL) fuel cell. Index Terms--Current mode, dc-dc converter, fuel cell.

In this paper, a robust anti-islanding algorithm for distributed fuel cell powered generation (DFPG) is proposed. Three different islanding scenarios are explored and presented based on analysis of real and reactive power mismatch. It is shown via analysis that islanding voltage is a function of real power alone, whereas its frequency is a function of both real and reactive power. Following this analysis, a robust anti-islanding algorithm is developed. The proposed algorithm continuously perturbs ([+ or -]5%) the reactive power supplied by DFPG while simultaneously monitoring the utility voltage and frequency. If islanding were to occur, a measurable frequency deviation takes place upon which the real power of DFPG is further reduced to 80%; now a drop in voltage positively confirms islanding, and the DFPG is safely disconnected. This method of control is shown to be robust, able to detect islanding under resonant loads, and fast acting (operable in one cycle). Possible islanding conditions are simulated and verified with analysis. Experimental results on a 0.5-kW fuel cell inverter system connected to 120-V 60-Hz utility are discussed. Index Terms--Anti-islanding algorithm, distributed fuel cell powered generation (DFPG), islanding.

In this paper, a new parallel-connected single phase power factor correction (PFC) topology using two flyback converters is proposed to improve the output voltage regulation with simultaneous input power factor correction and control. This approach offers lower cost and higher efficiency by parallel processing of the total power. Flyback converter-I primarily regulates output voltage with fast dynamic response and processes 55% of the power. Flyback converter-II with ac/dc PFC stage regulates input current shaping and PFC, and processes the remaining 45% of the power. This paper presents a design example and circuit analysis for 200 W power supply. A parallel-connected interleaved structure offers smaller passive components, less losses even in continuous conduction inductor current mode, and reduced volt-ampere rating of dc/dc stage converter. TI-DSP, TMS320LF2407, is used for implementation. Simulation and experimental results show the performance improvement. Index Terms--Circuit analysis, PFC.

In this paper, a new control scheme is proposed for controlling multiple single-phase power-factor-correction (PFC) modules with a single low-cost digital signal processor (DSP). The proposed scheme allows for multiple PFC modules of different current ratings to be operated in parallel and controlled via a single DSP. DSP-based control handles simple current sharing and provides size reduction. The paper describes a current sense technique for each PFC module and a closed-loop control algorithm for output voltage and input current control for operating multiple modules. In the example design, switching frequency is set at 120 kHz and two continuous conduction mode PFC stages are operated in parallel and controlled via a single TMS320LF2407 DSP. Experimental results show that the proposed scheme is capable to be used for modern switching power supplies. Index Terms--Digital signal processor (DSP), multiple modules, power-factor correction (PFC).

Modern telecommunication power supply systems have several parallel-connected switch-mode rectifiers to provide -48 Vdc. A typical switch-mode rectifier configuration includes a three-phase diode rectifier followed by a dc-dc converter. Such a system draws significant harmonic currents for the utility, resulting in poor input power factor and high total harmonic distortion. In this paper, a throe-phase active harmonic rectifier (AHR) scheme is proposed. In the AHR scheme, a diode rectifier module is replaced by a six-insulated-gate-bipolar-transistor pulsewidth-modulation rectifier to supply load harmonics as well as its own active power. Each dc-dc converter module is connected to a shared 48-V dc link. The AHR module together with parallel-connected switch-mode rectifiers is controlled to achieve clean input power characteristics. The VA ratings of the AHR scheme is compared with an active power filter approach. The control design is based on the synchronous reference frame approach. Analysis, simulation, and experimental results show that the AHR offers several advantages such as lower VA rating, better current control response, efficient use of the AHR dc link, small size, and stable dc-link voltage control. Index Terms--Active harmonic rectifier (AHR), active power filter (APF), telecom rectifier, total harmonic distortion (THD).

In this paper, an approach to reduce common-mode voltage at the output of a matrix converter is discussed. A new modulation strategy which reduces the common-mode voltage at the output is presented. The proposed method maintains the active voltage vector and distributes zero vector equally within a sampling period and reduces square rms of ripple components of input current. Advantages of the scheme include: 34% reduction in the common-mode voltage (peak value); improved harmonic spectrum; reduced switching losses and no more additional switching instants over one sampling period; and simplified implementation via software. Further, the voltage transfer ratio is unaffected by the proposed scheme. Simulation results are shown to demonstrate the advantages of the new pulsewidth modulation approach. Experimental results on a 230-V 3-kVA matrix converter-fed adjustable-speed drive based on a digital signal processor controller are presented. Index Terms--Common-mode voltage, indirect space-vector pulsewidth modulation (ISV-PWM), leakage current, matrix converter.

In this paper, a new voltage sag compensator for powering critical loads in electric distribution systems is discussed. The proposed scheme employs a pulsewidth-modulation ac-ac converter (four insulated gate bipolar transistors per phase) along with an autotransformer. During a disturbance such as voltage sag, the proposed scheme supplies the missing voltage and helps in maintaining rated voltage at the terminals of the critical load. The approach does not employ any energy storage components such as bulk capacitors/inductors and provides fast response at low cost. Under normal conditions the approach works in bypass mode, delivering utility power directly to the load; this method of control allows the transformer to be rated only for transient conditions, thus reducing its required size. A four-step switching technique to drive the ac-ac converter is employed to realize snubberless operation. A design example is presented, and simulation results are shown for a three-phase 230-V 5-kVA system. Experimental results on a single-phase unit are discussed. The proposed approach can be easily integrated into a distribution transformer supplying critical loads. Index Terms--AC chopper, critical loads, duty cycle, four-step switching, insulated gate bipolar transistor (IGBT), pulsewidth-modulation (PWM) ac-ac converter, voltage sags.

Power supply systems in telecommunication applications employ several parallel-connected ac-to-dc and dc-to-dc power converters. Such a system offers modularity, redundancy, and is easily scalable to higher power levels. Such parallel-connected systems normally consist of several single-phase power-factor-correction (PFC) stages connected to the same input utility. In this paper, a modular single-phase PFC scheme with an integrated harmonic filtering function is presented. The proposed approach demonstrates that, with suitable modifications to the PFC control, harmonic filtering capability can be added. In other words, the PFC stage can compensate for harmonics generated by other rectifier loads connected to the same ac input terminals. The paper presents an example employing three ac-dc rectifier stages with only one ac-dc rectifier stage with PFC capability. It is shown that one PFC stage with the proposed control can compensate for harmonics generated by the other two uncompensated rectifier stages. Results from a laboratory prototype system demonstrate that the overall system meets the EN 61000-3-2 harmonic limits. Index Terms--Digital signal processor (DSP), harmonic filtering, modular, nonlinear load, power-factor correction (PFC).