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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.

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.

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).

In this paper, the modularity concept applied to medium-voltage adjustable speed drives is addressed. First, the single-phase cascaded voltage-sorce inverter that uses series connection of insulated gate bipolar transistor (IGBT) H-bridge modules with isolated dc buses is presented. Next, a novel three-phase cascaded voltage-source inverter that uses three IGBT triphase inverter modules along with an output transformer to obtain a 3-p.u. multilevel output voltage is introduced. The system yields in high-quality multistep voltage with up to four levels and low dv/dt, balanced operation of the inverter modules, each supplying a third of the motor rated kVA. The concept of using cascaded inverters is further extended to a new modular motor--modular inverter system where the motor winding connections are reconnected into several three-phase groups, either six-lead or 12-lead connection according to the voltage level, each powered by a standard triphase IGBT inverter module. Thus, a high fault tolerance is being achieved and the output transformer requirement is eliminated. A staggered space-vector modulation technique applicable to three-phase cascaded voltage-source inverter topologies is also demonstrated. Both computer simulations and experimental tests demonstrate the feasibility of the systems. Index Terms--Medium-voltage adjustable-speed drives, modulation strategies, multilevel converters.

Many facilities, such as patient health care centers, data processing systems, and critical telecommunication links, rely on uninterruptible power supplies (UPS) to maintain a continuous supply of power in case of a line outage. In addition to requiring continuous power, many critical nonlinear loads are sensitive to incoming line transients and input harmonic voltage distortion. Conventional UPS systems operate to protect against such disturbances using complex filtering schemes, often employing large passive components. This paper presents the advantages of using real time digital signal processing (DSP) control of UPS systems. A DSP controlled UPS inverter and harmonic conditioning system is described and the performance is verified on a 150 kVA system.

Back electromotive force (EMF)-based sensorless vector control of permanent magnet synchronous motor (PMSM) drives provides a simple, robust, and low-cost solution for applications requiring only medium- and high-speed control. However, during the low-speed operation, back EMF is nearly zero, and hence, for starting and speed ramp-up, open-loop I−ƒ control is used. Once the motor attains a sufficient speed, the control is transferred to sensorless vector control. However, an arbitrary choice of the slope of frequency ramp leads to pole slipping. Pole slipping is usually prevented by fixing the frequency ramp slope to a minimum value, which deteriorates the dynamic performance during starting. In this article, a torque controller is proposed to dynamically vary the frequency slope depending on the moment of inertia and load torque present in the system, achieving fast and reliable starting. A reference frequency modulation algorithm for overcoming the inherent mid-frequency instability of the open-loop control is also included. An additional slope attenuator is proposed to minimize the torque oscillations caused by the reference frequency modulation during the transition from a ramp-up interval to a constant-speed interval with I−ƒ control. The proposed methods are experimentally verified on a 25-kW PMSM drive, which is started using open-loop I−ƒ control. [ABSTRACT FROM AUTHOR]

This paper proposes a robust three-phase active power-factor-correction (PFC) and harmonic reduction scheme suitable for higher power applications. The proposed system is a unique combination of a low-kilovoltampere 12-pulse rectifier system with a single-phase boost PFC scheme to shape the input current to near sinusoidal waveshape. The voltampere rating of the active PFC converter is 0.05 pu and is not exposed to line transients under varying load conditions. The proposed system is suitable for utility interface of higher power rectifiers employed in power supplies and adjustable-speed drive systems which demand clean input power characteristics in the range of 1-500 kW. The proposed system is rugged and, in the event the active control were to fail, the system reverts to 12-pulse operation with fifth and seventh harmonic cancellation. Analysis and design of the system is examined in detail, and simulation and experimental results on a 10-kVA prototype are shown. Index Terms - Clean power, harmonic reduction, three-phase rectifier.

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. [ABSTRACT FROM AUTHOR]

Analyzes the modularity concept applied to medium-voltage adjustable speed drives. Features of the insulated gate bipolar transistor H-bridge modules; Information on the type of voltage produced by the system; Details on the operating principles of the modular motor-modular inverter system.