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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).

Utility capacitor-switching transients (CSTs) are responsible for numerous nuisance tripping of PWM adjustable speed drives (ASDs) and result in costly down times. During a CST event, the dc link voltage of the ASD can momentarily rise to greater than 1.3 p.u. resulting in nuisance tripping. In this paper, a new approach to mitigate nuisance tripping of PWM ASDs is discussed. In this approach, the soft-charge resistor available in most ASDs, is momentarily introduced in the series path of the power flow to effectively damping the CST. Further, damping effect is electronically adjusted. The proposed approach demonstrates a method that can be incorporated within an ASD to electronically damp the oscillatory transient generated during a CST event. The advantages of the proposed approach are i) electronic damping for CST event is achieved by low cost modifications to ASD hardware; ii) it adapts to several utility resonance conditions. The required additional hardware for ASDs to be immune to CSTs, can be viewed as an add-on option. This paper discusses the analysis, simulation and experimental performance on a 480 V, 16 kVA commercial ASD equipment. Index Terms--Adjustable speed drives, capacitor switching transients, electronic damping, power quality.

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.

Adjustable-speed drives under short-term power interruptions can interrupt a critical process. In this paper, an approach to provide ride-through with flyback converter modules powered by super capacitors is explored. The proposed approach is modular and facilitates additional modules to be added to suit higher voltage/power ratings. Both unidirectional and bidirectional flyback dc/dc converter topologies are examined. Simulation and experimental results are presented. Index Terms--Adjustable-speed drives, power quality, ride-through capability, short-term power interruption, super capacitors.

In this paper, a new hybrid active power filter topology is presented. A higher-voltage, low-switching frequency insulated gate bipolar transistor (IGBT) inverter and a lower-voltage high-switching frequency metal oxide semiconductor field effect transistor (MOSFET) inverter are used in combination to achieve harmonic current compensation. The function of the IGBT inverter is to support utility fundamental voltage and to compensate for the fundamental reactive power. The MOSFET inverter fulfills the function of harmonic current compensation. To further reduce cost and to simplify control, the IGBT and MOSFET inverters share the same dc-link via a split capacitor bank. With this approach harmonics can be cancelled over a wide frequency range. Compared to the conventional APF topology, the proposed approach employs lower dc-link voltage and generates less noise. Simulation and experimental results show that the proposed active power filter topology is capable of compensating for the load harmonics. Index Terms--Hybrid active power filter, IGBT inverter, MOSFET inverter, DSP.

In this paper, a new three-phase power-factor correction (PFC) scheme is proposed using two single-phase PFC modules. In this approach, the 'three' phase input is first transformed to 'two' phase by means of a 0.14-pu-rated autotransformer. Two standard single-phase PFC modules are then employed to process the 'two' phase power to dc output. Split inductors and diodes are employed to limit interaction between the two PFC stages. Due to cascade operation of two PFC stages, low-frequency (120 Hz) ripple in the dc-link capacitor is cancelled. Detailed analysis and simulation results are presented. A 220-V 1.5-kVA design example along with experimental results is shown. Index Terms--Harmonics, power-factor correction, power quality.

In this paper, a new real-time space-vector-based control strategy is presented for three-phase uninterruptible power supply systems powering nonlinear and unbalanced loads. The proposed control strategy generates the inverter reference and gating signals in closed loop and guarantees high-quality output voltages at the load terminals. The approach, which is implemented on a digital signal processor, adapts to a wide variation of nonlinear and unbalanced load conditions without specific knowledge of the output filter (L-C) component values. Analysis and experimental results on a 10-kVA prototype are presented. The results show that the output voltage is restored at heavy nonlinear and unbalanced load. Index Terms--Control, ground power units, modulation, power converters, unbalanced load, uninterruptible power supply.

In this paper, a new control method is presented which enables equal sharing of linear and nonlinear loads in three-phase power converters connected in parallel, without communication between the converters. The paper focuses on solving the problem that arises when two converters with harmonic compensation are connected in parallel. Without the new solution, they are normally not able to distinguish the harmonic currents that flow to the load and harmonic currents that circulate between the converters. Analysis and experimental results on two 90-kVA 400-Hz converters in parallel are presented. The results show that both linear and nonlinear loads can be shared equally by the proposed concept. Index Terms--Control, ground power units, nonlinear loads, paralleling, power converters.

Adjustable - speed drive (ASD) ride-through issues have caused increased concerns due to the susceptibility of ASD's to power disturbances, and the costly results of process disruptions. These losses can be avoided for critical production processes by using ASD's with ride-through capabilities. This paper assesses industrial ride-through requirements through power quality surveys and the results of an ASD ride-through questionnaire conducted by the authors. ASD ride-through alternatives are evaluated based on design, implementation, and cost considerations, in order to determine the most suitable solutions for various kilovoltampere ratings and time duration requirements. Index Terms - Adjustable-speed drive, drives, energy storage, process disruptions, ride-through, tripping.

In this paper, the effect of pulsewidth modulation (PWM) voltage waveform on the voltage distribution among the stator windings of random wound cage induction motors is studied. First, a method of estimating the high-frequency distributed-circuit parameters of the motor using finite-element analysis is described. From these parameters, an equivalent circuit is formed with the windings represented by partially distributed and partially lumped parameters. Using this equivalent circuit, the voltage distribution among the turns and coils of the motor are simulated using the SABER simulation package. Through simulation, the effect of rise time of the PWM wavefront on the voltage distribution is studied, and it is shown that the rise time of the wavefront has influence on the additional voltage stress on the line-end coil. In order to validate the simulation procedure adapted, the simulation results are compared with experimental results. Index Terms - dv/dt, induction motor, PWM drives, transient effects, voltage stress.

In this paper, an improved inverter output filter is proposed for pulsewidth-modulated (PWM) drive systems. The proposed filter is shown to effectively reduce both the differential and common modes dr/dt at the motor terminals, even in the presence of long motor leads. Reducing differential mode dr/dt reduces overvoltages at the motor terminals and lowers the stress on the motor insulation. Lowering common mode dr/dt is shown to significantly reduce high-frequency leakage currents to ground and induced shaft voltage in the motor. An important advantage of the approach is that the filter can be installed within the inverter enclosure to achieve both the differential and common modes dr/dt reductions at the motor terminals. Thus, the use of the filter can contribute to enhanced bearing life and improve reliability of PWM drive systems. Analysis, design equations, and experimental results on a 480-V 20-hp PWM drive system are presented. The filter configuration is an excellent candidate for many new and retrofit PWM 480-V/575-V drive systems. Index Terms - Common mode dv/dt, common mode impedance, differential mode dv/dt, induction motor, inverter output filter, leakage current, long motor leads, power converter, pulsewidth modulation inverter, shaft voltage.

In this paper, a new method is proposed to avoid acoustic-resonance-related instabilities in metal halide lamps when operated with a high-frequency electronic ballast. Angle modulation of the inverter switching pattern has been used as a vehicle to randomize lamp driving frequency and to limit lamp power spectrum below the instability threshold. The optimal modulating pattern is obtained by studying the angle-modulated spectra by periodic and random signals. Analysis is supported by simulations and verified experimentally with the ballasting of 250-W lamps. Index Terms - Acoustic resonance, ballast, high-intensity discharge lamp, metal halide lamp, white-noise modulation.

In this paper, a zero-voltage-switching (ZVS) scheme for a boost converter is proposed and discussed. This lossless zero-voltage scheme is shown to improve the performance of active power-factor-correction circuits. The results are validated by both simulation and experimental results.

Design considerations for an inverter output filter to mitigate the effects of long motor leads in adjustable-speed drive (ASD) applications are presented. It is shown by analysis that, for a given length of cable, reducing the dv/dt of the pulsewidth modulated (PWM) inverter output voltage applied to the cable below a critical value will eliminate overvoltages due to voltage reflections. Design issues for a low-pass filter at the inverter output terminals to reduce the dv/dt of the inverter output pulse are examined in detail. The filter operation is verified for the entire variable frequency range of the inverter. The performance of the filter is evaluated through simulations and experimentally on a 460-V commercially available ac motor drive [PWM insulated gate bipolar transistor (IGBT)]. The proposed inverter output filter is then compared with a motor terminal shunt filter also designed to reduce overvoltages and ringing at the motor terminals. Index Terms - ASD, drives, overvoltage.

This paper proposes two new passive 24-pulse diode rectifier systems for utility interface of pulsewidth modulated (PWM) ac motor drives. The first approach employs an extended delta transformer arrangement, which results in near equal leakage inductance in series with each diode rectifier bridge. This promotes equal current sharing and improved performance. A specially tapped interphase transformer is then introduced with two additional diodes to extend the conventional 12-pulse operation to 24-pulse operation from the input current point of view. The proposed system exhibits clean power characteristics with fifth, seventh, eleventh, thirteenth, seventeenth, and nineteenth harmonics eliminated from the utility line currents. The second scheme is a reduced voltampere approach employing autotransformers to obtain 24-pulse operation. The voltampere rating of the polyphase transformer in the second scheme is 0.23 [P.sub.o] (PU). Detailed analysis and simulations verify the proposed concept, and experimental results from a 208-V 10-kVA rectifier system are provided. Index Terms - AC motor drives, clean power, 24-pulse, utility interface.

In this paper, a new active interphase reactor for twelve-pulse diode rectifiers is proposed. The proposed system draws near sinusoidal currents from the utility. In this scheme, a low kVA [0.02 [P.sub.o] (PU)] active current source injects a triangular current into an interphase reactor of a twelve-pulse diode rectifier. This modification results in near sinusoidal utility line currents with less than 1% THD. It is further shown that a low kVA, 12-pulse system with an autotransformer arrangement [kVA rating of 0.18 [P.sub.o] (PU)] can be implemented with the proposed active interphase reactor. The resulting system draws clean power from the utility and is suitable for powering larger kVA ac motor drive systems. Detailed analysis of the proposed scheme along with design equations is illustrated. Simulation results verify the concept. Experimental results are provided from a 208 V, 10 kVA rectifier system.

In this paper design considerations for 12-pulse diode rectifier systems operating under utility voltage unbalance and pre-existing harmonic voltage distortion are discussed. For a 12-pulse diode rectifier system connected in parallel to feed a common dc link via an interphase transformer it is shown that a small amount of impedance mismatch, utility voltage unbalance or pre-existing voltage distortion drastically affects the current-sharing capability of the rectifier bridges. This in turn generates additional uncharacteristic and characteristic harmonics thereby increasing the THD. In order to mitigate these effects and ensure proper operation of diode rectifiers, specially designed line reactors termed Harmonic Blocking Reactors (HBR's) are introduced. The analysis and design procedure for HBR's are discussed. Simulation results illustrate improved performance. Experimental results from a laboratory prototype system show close agreement with theory. Index Terms - Harmonics, 12-pulse, voltage distortion, harmonic blocking reactor

In this paper polyphase transformer arrangements with reduced kVA capacities are presented for harmonic current reduction in high power diode rectifier-type utility interface systems. Based on the concept of an autotransformer, a 12-pulse rectifier system is realized with a resultant transformer kVA rating of 0.18[P.sub.o] (pu). In this arrangement the 5, 7, 17, 19, etc. harmonics are absent from the utility input line current. In the second scheme an 18-pulse rectifier is realized with the kVA rating of 0.16[P.sub.o] (pu) and the 5, 7, 11, 13, etc. harmonics are canceled in the utility line currents. Analytical design equations are presented to facilitate the design of system components. Simulation results verify the proposed concept, and experimental results are provided from a 208 V, 10 kVA 12-pulse rectifier system. The advantage of employing the proposed system for utility interface of rectifier/PWM-inverter motor drive systems is also explained.

In this paper, filtering techniques are investigated to reduce the motor terminal overvoltage, ringing, and dv/dt in inverter-fed ac motor drive systems where long leads are required. Analysis is presented to show that the distributed LC of a cable and the pulse-width modulated (PWM) inverter switching action lead to motor terminal voltage reflections and ringing that stress the turn insulation and contribute to bearing currents. First- and second-order shunt filter designs are analyzed and compared. Detailed design equations to match the filter impedance to the cable surge impedance and to determine the filter component values are presented. It is demonstrated that a first-order shunt filter connected at the motor terminals significantly reduces the overvoltage stress and ringing, and also lowers the dv/dt of the PWM switching pulse. Simulation and experimental results are presented to verify the proposed filter designs for 50-ft and 100-ft cable lengths for 460-V PWM insulated gate bipolar tranistor (IGBT) ac motor drive systems.

In this paper a new configuration for an active line conditioner is proposed to correct dynamically voltage unbalances in a three-phase ac system. In the proposed system it is shown that the injection of a correction voltage [V.sub.inj] in one phase is sufficient to nullify the negative sequence voltage component in the incoming three phase supply. The resulting three phase voltages at the load terminals are essentially positive sequence voltages and hence are balanced. It is further shown that the kVA requirement of the proposed active line conditioner is small, typically 3% for a ten percentage unbalance in the input supply. The dynamic cancellation of the negative sequence voltage component by the proposed scheme drastically improves the performance of induction motor loads connected to a weak ac system. A thorough analysis of the scheme along with the suitable design guides are presented. Finally selected experimental results on a laboratory prototype active line conditioner are detailed.

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.

In this paper, a new series resonant ac-to-dc rectifier with high-frequency isolation is introduced. The proposed approach employs a PWM controlled ac controller, a series resonant tank and a high-frequency isolation transformer. With this approach, the single phase input ac is directly processed via the ac-to-ac converter eliminating the ac-to-dc rectification stage present in the conventional system. The output of the HF transformer is rectified and processed via a filtering stage to obtain a dc output. With the addition of an input filter, the input current is near sinusoidal at unity power factor. Simulation and experiment results are presented to verify the basic concept.

Recent surveys of 208/120 V three-phase four-wire electric systems, buildings, and industrial plants with computers and nonlinear loads show excessive currents in the neutral. These neutral currents are fundamentally third harmonic, and their presence is tied to wiring failures, elevating of neutral potentials, transformer overheating, etc. In response to these concerns, this paper proposes a new active power filter scheme to cancel neutral current harmonics. The proposed approach employs a star/delta transformer along with a two-switch PWM-controlled active filter. The closed loop control of the active power filter guarantees cancellation of neutral current harmonics under varying load conditions. The proposed system drastically improves the overall system performance and virtually eliminates transformer overheating due to harmonics. Experimental results from a prototype active power filter confirm the suitability of the proposed approach.

In this paper, a new approach to improve power factor and reduce harmonics generated by a three-phase diode rectifier type utility interface is proposed. The proposed approach is passive and consists of a novel interconnection of a star/delta transformer between the ac and dc sides of the diode rectifier topology. This novel interconnection, in combination with the 120 [degrees] conduction intervals of each diode, is shown to generate a circulating third harmonic current between the ac and dc side of the rectifier bridge. The circulating third harmonic current between the ac and dc side of the rectifier bridge. The circulating third harmonic current is then shown to drastically improve the performance of the diode rectifier type interface. The resulting input current is near sinusoidal in shape with significant reduction in line current harmonics. The proposed system can be viewed as a cost-effective retrofit to the existing diode rectifier utility interface applications. Analysis and design of the proposed approach are examined in per-unit quantities. A suitable design example illustrates the sizing of the necessary passive components. Selected results are verified experimentally on a laboratory prototype system.

In this paper an Active Power Filter is proposed for balancing unbalanced loads. A 3-phase, PWM ac to dc converter topology with inductive energy storage is controlled to continuously inject desired negative sequence currents into the system to achieve balancing of continuously varying unbalanced loads. The principal of operation is as follows: the negative sequence components of the load currents are measured in magnitude and in phase and the PWM-controlled active power filter is controlled to inject currents opposite to these quantities, thereby achieving the balancing function. The operation of the proposed system along with the selection of the active power filter components are discussed in detail. Finally, experimental filter operation is verified for an unbalanced load.

Abnormal harmonics of significant magnitude are generated at the output and input terminals of a PWM ac to dc converter under unbalanced operating conditions. A new control strategy is presented here to selectively cancel the generated lower order abnormal harmonics at the output and input terminals and thereby to preserve the high performance features of a PWM ac to dc converter. The proposed technique essentially involves computing the sequence components of the unbalanced input supply and suitably counter-unbalances the PWM gating signals of the converter switches to cancel the generated abnormal harmonics. The technique is essentially a feed forward approach and is suitable for higher power G.T.O. type PWM ac to dc converters. A procedure for implementing this technique in real time is discussed. Finally selected results are verified experimentally on a prototype PWM ac to dc converter.

In this paper, several new component minimized circuit topologies for single-phase to three-phase conversion are proposed. The proposed topologies employ fewer semiconductor devices and generate high-quality output voltages. Suitable modification to achieve active input current shaping feature is also illustrated in detail. Analysis and simulation of the proposed schemes is carried out to show the high-performance features. Suitable guides for the selection of filter components and for facilitating circuit design is presented. Finally, selected results are verified experimentally on laboratory prototype converters.

In this paper, a new current control scheme based on programmed PWM switching patterns is presented for ac motor drives. In this scheme, reference and feedback current correspondence is maintained with simultaneous optimization of a performance objective. The proposed scheme overcomes the disadvantages of the existing current regulators employing the direct control approach and exhibits several advantages in terms of reduced switching frequency, minimum current/torque ripple, higher efficiency, and improved performance in the ac motor drive. Analytical and experimental investigations on a current-regulated induction motor drive confirm the predicted advantages of the scheme.

In this paper analysis of common mode voltage or 'neutral shift' in the new emerging voltage source inverter type medium voltage adjustable speed drive (MV-ASD) systems is presented. Both cascaded multilevel (CML) inverter and 3-level NPC inverter topologies are analyzed. An equivalent circuit model to determine the common mode voltage stress is presented. Analysis and simulation results are discussed and worst case common mode voltage excursion is computed for an example 800 Hp, 4160 V MV-ASD. It is shown that certain system components are excessively stressed and in the MV-ASD system these data are particularly useful in specifying system components and for proper design of the system. Possible effects of common mode voltage and its dv/dt on medium voltage motor bearings are discussed. A new multilevel PWM strategy is introduced which results in zero common-mode voltage. Simulation results are shown to illustrate the effectiveness of these schemes. Finally, experimental results from a 800 hp, 4160 V, MV-ASD system are presented. Index Terms--Common mode, medium voltage drove.

In this paper, an integrated single-switch approach to improve harmonic performance of standard pulsewidth modulation adjustable-speed drives (ASD's) is presented. The approach is essentially an add-on solution to a standard ASD topology. The approach is based on circulating a third harmonic current to reduce the harmonics of the line current. The dynamic braking chopper available in standard ASD's is used to control the amplitude of the third harmonic circulating current. Analysis, design, and simulations are presented to determine the performance of the proposed scheme with different line impedance and with and without dc-link inductance in the ASD. Utility input current distortion well below 15% is obtained. This paper also presents how this new approach is capable of reducing the harmonic distortion in multiple drives. The paper includes experimental results using a 480-V 10-kW commercial ASD retrofitted with the proposed approach. Index Terms--Adjustable-speed drive, harmonic distortion, power quality, rectifiers, third harmonic injection.