Journals

PWM Common Mode Reference Generation for Maximizing the Linear Modulation Region of CHB Converters in Islanded Microgrids

Jacob Lamb, Behrooz Mirafzal, and Frede Blaabjerg, IEEE Transactions on Industrial Electronics, vol. , no. , pp. –, 2018.

Abstract— Abstract: Microgrid-interactive converters are increasingly expected to provide auxiliary services, particularly during islanded microgrid operation. Unbalanced, non-sinusoidal pulse width modulation (PWM) references may be required when converters provide auxiliary services, such as negative-sequence compensation. Since PWM references cannot exceed dc-bus voltage defined limits without causing overmodulation, the maximum positive-sequence voltage producible by microgrid-interactive converters may be diminished unless reference waveforms are adjusted. In this paper, a technique for maximizing the linear modulation region by injecting a common mode component into PWM references is proposed for microgrid-interactive cascaded H-bridge (CHB) converters. The technique presented: (i) can be implemented in real-time without lookup tables, (ii) is controller independent, (iii) is compatible with any carrier based PWM method, enabling its use for symmetric and asymmetric CHB, and (iv) enables continued operation after internal fault events when utilized with fault tolerant PWM schemes. Experimental data verify the common mode injection technique.

A Generator-Converter Topology With Zero DC-Link Impedance for Direct Drive Wind Turbines

Akanksha Singh, and Behrooz Mirafzal, IEEE Transactions on Energy Conversion (Letters), vol. 32, no. 4, pp. 1620 - 1623, December 2017.

Abstract- A new topology is proposed for direct drive wind turbines that eliminates the use of the failure prone dc-bus electrolytic capacitor, avoids the need for a dc-link inductor by utilizing the synchronous inductance of the wind turbine permanent-magnet generator (PMG), and provides a low-voltage generator design, which decreases the amount of permanent-magnet material used in PMG, and reduces the volume and weight of the generator.

Open-Circuit IGBT Fault Detection and Location Isolation for Cascaded Multilevel Converters

Jacob Lamb, and Behrooz Mirafzal, IEEE Transactions on Industrial Electronics, vol. 64, no. 6, pp. 4846 - 4856, June 2017.

Abstract— An open-circuit IGBT fault detection technique for cascaded h-bridge (CHB) multilevel converters is presented in this paper. This technique, designed to be implemented independently for each CHB leg, utilizes one current sensor and one voltage sensor to monitor a leg's current and output voltage. Measured voltages are compared to expected voltages, and deviations are used to determine open-circuit fault locations based on the deviation's magnitude and current flow direction. Once potential fault locations have been identified, the fault location is systematically isolated and then verified, reducing the possibility of unnecessary corrective actions due to fault misidentification, e.g. an intermittent gate misfiring fault being classified as an open-circuit fault. The proposed technique can be implemented for any number of cells, is independent of the PWM strategy used, and can be applied to symmetric and asymmetric CHB converters regardless of the cell input dc-source magnitudes utilized, i.e. cell input voltages are not required to be equal or to exist in specific ratios. For a CHB leg with M cells, the proposed technique identifies and isolates open-circuit switch faults in less than 2M measurement (sampling) cycles, and verification is completed in less than one full fundamental cycle. Experimentally obtained data demonstrate the efficacy of the proposed fault detection and isolation technique.

Grid-Interactive Cascaded H-Bridge Multilevel Converter PQ Plane Operating Region Analysis

Jacob Lamb, and Behrooz Mirafzal, IEEE Transactions on Industrial Electronics, vol. 64, no. 6, pp. 4846 - 4856, June 2017.

Abstract—Cascaded H-bridge (CHB) multilevel converters are being considered as a promising option for interfacing renewable energy resources with the grid due to their modularity, scalability, and increased efficiency when compared with traditionally used two-level inverters. When used as grid interfaces, CHB converters should be capable of controlling the injected active and reactive power while also satisfying operating criteria such as grid standards, e.g., IEEE 519. This paper aims to identify P Q plane operating points available when using symmetric and asymmetric grid interactive CHB converter topologies, and to determine the factors that prevent utilization of other converter output points. Notably, for asymmetric CHB converters, a technique is proposed to identify discordant output points where power is transferred between cells within a leg, e.g., one cell has a net regenerative power flow while the overall CHB leg does not. This work (i) provides distribution engineers with information regarding the active and reactive power that can feasibly be generated by grid-interactive CHB converter topologies, (ii) creates a framework for assessing operating point trajectories when altering the steady-state operation of grid-tied CHB converters, (iii) provides a basis for selecting and modifying steady-state pulse-width modulation generation techniques in order to meet desired performance criteria, and (iv) presents analytical techniques that can be built upon to facilitate analysis of grid-interactive CHB under conditions such as grid imbalance or during continued operation after CHB cell faults.

An Adaptive SPWM Technique for Cascaded Multilevel Converters with Time-Variant DC Sources

Jacob Lamb, and Behrooz Mirafzal, IEEE Transactions on Industry Applications, vol. 52, no. 5, pp. 4146 – 4155, September/October 2016.

Abstract— An adaptive sinusoidal pulse width modulation (SPWM) technique for cascaded h-bridge (CHB) multilevel converters is presented in this paper. The proposed technique enables the CHB topology to provide a symmetrical ac voltage from a set of asymmetrical time-variant input dc source voltages. A sensor-per-source (SPS) algorithm and a sensor-per-leg (SPL) algorithm are described as alternatives for implementing the adaptive SPWM technique. Software simulations and experimental results are presented to demonstrate the efficacy of the presented SPS and SPL algorithms.

Rapid Implementation of Solid-State Based Converters in Power Engineering Laboratories

Jacob Lamb, Akanksha Singh, and Behrooz Mirafzal, IEEE Transactions on Power Systems, vol. 31, no. 4, pp. 2957 – 2964, July 2016.

Abstract— As the number of renewable energy generation units and, consequently, solid-state based converters increases in the power grid, many power electronics concepts should be integrated into power system analysis and design. Thus, a power laboratory course with emphasis on the nexus between power electronics and power system is desired. However, the flexibility of instructors in designing a wide range of educational experiments is limited by the use of specialized educational setups. This problem can be resolved by allowing students to assemble their setups using laboratory scaled-down devices, however, implementation times can limit students to focus on the main concepts of the experiments. In particular, lacking experience with hardware description languages to develop switching patterns and control schemes for solid-state based converters further increases the implementation times. In this paper, a technique for rapidly implementing switching patterns and control schemes for power converters is proposed. This technique provides an easy-to-use laboratory allowing students to focus on reinforcing the theory learned in energy conversion, motor-drive, power electronics, and power system classes while still providing instructors with the flexibility to design a wide range of educational experiments.

On Dynamic Models and Stability Analysis of Three-Phase Phasor PWM-Based CSI for Stand-Alone Applications

Akanksha Singh, Ali K. Kaviani, and Behrooz Mirafzal, IEEE Transactions on Industrial Electronics, vol. 62, no. 5, pp. 2698 – 2707, May 2015.

Abstract— The Phasor PWM (PPWM)-based Current Source Inverter (CSI) is a boost inverter. The difference between conventional CSIs and the boost inverter is that the boost ratio (V_LLrms/V_dc) in the boost inverter can easily be above 3.0, whereas this ratio is around one in conventional CSIs. In order to realize potential capabilities of the boost inverter and to assist its penetration into renewable energy systems, the boost inverter dynamic behaviors are studied in this paper. First, the large- and small-signal models as well as the dq-equivalent circuits of the boost inverter are presented. Then, the developed models are verified using circuit simulations and experiments on a laboratory-scale 2kW, 208/240VLLrms, V_dc= 65V boost inverter made of reverse-blocking IGBTs (RB-IGBTs). Finally, the developed small-signal model is used to study the stability of the boost inverter through root locus of small signal poles (eigenvalues) as control inputs and load parameters vary within the boost inverter's operating limits.

Survey of Fault-Tolerance Techniques for Three-Phase Voltage Source Inverters

Behrooz Mirafzal, IEEE Transactions on Industrial Electronics, vol. 61, no. 10, pp. 5192-5202, Oct. 2014.

Abstract— Inverters play key roles in motor-drives, flexible power transmissions, and recently grid-tied renewable energy generation units. Therefore, availability and reliability of inverters has become increasingly important. Following early-stage fault detections in inverters, remedial actions can extend normal operation of inverters and, in some cases, derate the system to prevent unexpected shutdowns. A remedial action typically contains a combination of hardware and software reconfigurations. The main purpose of this paper is to provide an instructive survey of existing fault-tolerance (remedial) techniques for three-phase two-level and multilevel inverters.

An SVPWM-based switching pattern for stand-alone and grid-connected three-phase single-stage boost-inverters

Behrooz Mirafzal, Mahdi Saghaleini, and Ali K. Kaviani, IEEE Transactions on Power Electronics, vol.26, no. 4, pp. 1102-1111, April 2011.

Abstract- In many modern energy conversion systems, a DC voltage, which is provided from a sustainable energy source or energy storage device, must be boosted and converted to an AC voltage with a fixed amplitude and frequency. In this paper, a switching pattern based on the concept of the conventional space-vector pulse-width-modulated (SVPWM) technique is developed for single-stage, three-phase boost-inverters using the topology of current source inverters. The six main switching states, and two zeros, with three switches conducting at any given instant in conventional SVPWM techniques are modified herein into three-charging states and six discharging states with only two switches conducting at any given instant. The charging states are necessary in order to boost the DC input voltage. The developed switching pattern was experimentally verified through a laboratory scaled three-phase 500W boost-inverter and the results are presented in this paper.

A Method of Seamless Transitions Between Grid-Tied and Stand-Alone Modes of Operation for Utility-Interactive Three-Phase Inverters

David S. Ochs, and Behrooz Mirafzal, IEEE Transactions on Industry Applications, vol. 50, no. 3, pp. 1934-1941, May/June 2014.

Abstract- A method for the seamless transition of three-phase inverters switched between grid-tied and stand-alone modes of operation is presented in this paper. In this method, only the inverter current and voltage sensors are utilized and no control over the grid-side static transfer switch is needed. The presented method contains two strategies for grid-tied-to-stand-alone and stand-alone-to-grid-tied transitions. In the stand-alone-to-grid-tied transition strategy, a novel algorithm is presented for estimating the grid angle nearly instantaneously, which allows the three-phase inverter to respond very quickly if the grid and point of common coupling voltages are out of phase. This fast response allows the inverter to effectively eliminate the transient overcurrent that would normally occur if it was connected to the grid without first being synchronized. The fast response also allows the inverter to return to normal operation very quickly after such an event. The strategy for the seamless transition from grid-tied to stand-alone mode is also presented. These strategies have been verified through experiments, and the results are presented in this paper.

Fault-tolerant technique for ∆-connected AC motor-drives

Ahmed Sayed-Ahmed, Behrooz Mirafzal, and Nabeel Demerdash, IEEE Transactions on Energy Conversion, vol. 26, pp. 646 - 653, June 2011. (IEEE Society (PES) Prize Paper)

Abstract- A fault-tolerant technique for motor-drive systems is introduced in this paper. The technique is merely presented for AC motors with Δ-connected circuits in their stator windings. In this technique, the faulty phase is isolated by solid-state switches after the occurrence of a failure in one of the stator phases. Then, the fault-tolerant technique manages current-flow in the remaining healthy phases. This technique is to significantly mitigate torque pulsations, which are caused by an open-Δ configuration in the stator windings. The performance of the fault-tolerant technique was experimentally verified using a 5-hp 460V induction motor-drive system and the results are presented in this paper.

A Time-Coordination Approach for Regenerative Energy Saving in Multi-Axis Motor-Drive Systems

Ali K. Kaviani, Brian Hadley, and Behrooz Mirafzal, IEEE Transactions on Power Electronics, vol. 27, pp. 931 - 941, Feb. 2012.

Abstract- An analytical approach for the management of regenerative energies in multi-axis servo-motor-drives is presented in this paper. This energy management is achieved through a proper time-coordination between the speed commands of multi-axis drives. Moreover, the proposed approach significantly limits the peak value of the AC input current in these systems. It is mathematically proved that a time-delay can significantly increase the amount of utilized regenerative energy and consequently decreases the amount of dissipated energy. In this paper, a set of closed-form formulas is developed for different acceleration-deceleration time ratios, where motor losses are neglected. The findings of this investigation were experimentally verified using a two-axis permanent-magnet (PM) motor-drive system, and the results are presented in this paper. The experimental results are in remarkable agreement with the developed closed-form formulas.

A failure mode for PWM inverter-fed AC motors due to the antiresonance phenomenon

Behrooz Mirafzal, Gary Skibinski, and Ranga Tallam, IEEE Transactions on Industry Applications, vol.45, pp. 1697-1705, September/October 2009.

Abstract- A failure mode for pulsewidth-modulation inverter-fed ac motors due to the antiresonance phenomenon is introduced and investigated in this paper. At high frequencies, an ac motor behaves as a series RLC circuit with a resonance point, so-called antiresonance, typically above 1.0 MHz. If the voltage oscillation frequency caused by the reflected-wave phenomenon matches the motor antiresonance frequency, an amplified voltage will appear internally between the turns of the stator windings. This externally unobservable state can cause winding insulation failure in the ac motor. This phenomenon is experimentally verified using a rewound permanent-magnet motor with accessible taps along the stator windings, and the results are presented in this paper.

Voltage stability improvement for wind farms using shunt FACTS devices based on dynamic modeling

Vahid Salehi, Saeed Afsharnia, and Behrooz Mirafzal, International Journal of Distributed Energy Resources, ISSN 1614-7138, vol.6, no. 2, pp. 109-130, April-June 2010.

Abstract-

Universal induction motor model with low-to-high frequency response characteristics

Behrooz Mirafzal, Gary Skibinski, Ranga Tallam, David Schlegel, and Richard Lukaszewski, IEEE Transactions on Industry Applications, vol.43, pp. 1233 - 1246, September/October 2007. (IEEE Society (IAS) Prize Paper)

Abstract- A three-phase induction motor model that depicts the motor behavior over a wide range of frequencies from 10 Hz to 10 MHz is presented in this paper. The model is universal in the sense that common-mode, differential-mode, and bearing circuit models are combined into one three-phase equivalent circuit model. The proposed model is basically an extension of the low-frequency IEEE Standard 112 circuit model. The proposed model was experimentally simulated and verified with results presented.

Determination of parameters in the universal induction motor model

Behrooz Mirafzal, Gary Skibinski, and Ranga Tallam, IEEE Transactions on Industry Applications, vol.45, pp. 142 - 151, January /February 2009.

Abstract- A systematic procedure to determine the parameters of a previously proposed low- to high-frequency induction motor model is presented. An analysis of the high-frequency behavior with regard to the impact of magnetic core selection, parasitic interturn and winding-to-frame capacitors, and skin effects of windings is investigated in greater detail for the proposed universal model of an induction motor. The model is universal in the sense that it is derived by extending the low-frequency standard T-equivalent circuit (IEEE Standard 112) to include high-frequency effects under both common- and differential-mode domains and can be used for transient reflected-wave studies and electromagnetic interference emissions in motor drive systems. A test-based method and an analytical approach, which are useful in application and design stages, respectively, are presented to determine the frequency characteristics of the induction motor. Findings of the investigation were verified experimentally with results presented in this paper.

Inter-turn fault diagnosis in induction motors using pendulous oscillation phenomenon

Behrooz Mirafzal, Richard Povinelli, and Nabeel A.O. Demerdash, IEEE Transactions on Energy Conversion, vol.21, pp.871-882, December 2006.

Abstract- A robust interturn fault diagnostic approach based on the concept of magnetic field pendulous oscillation, which occurs in induction motors under faulty conditions, is introduced in this paper. This approach enables one to distinguish and classify an unbalanced voltage power supply and machine manufacturing/construction imperfections from an interturn fault. The experimental results for the two case studies of a set of 5-hp and 2-hp induction motors verify the validity of the proposed approach. Moreover, it can be concluded from the experimental results that if the circulating current level in the shorted loop increases beyond the phase current level, an interturn fault can be easily detected using the proposed approach even in the presence of the existence of motor manufacturing imperfection effects

On innovative methods of induction motor inter-turn and broken-bar fault diagnostics

Behrooz Mirafzal, and Nabeel A.O. Demerdash, IEEE Transactions on Industry Applications, vol.42, pp.405-414, March/April 2006.

Abstract- A fault indicator, the so-called swing angle, for broken-bar and interturn faults is investigated in this paper. This fault indicator is based on the rotating magnetic-field pendulous-oscillation concept in faulty squirrel-cage induction motors. Using the "swing-angle indicator," it will be demonstrated here that an interturn fault can be detected even in the presence of machine manufacturing imperfections. Meanwhile, a broken-bar fault can be detected under both direct-line and PWM excitations, even under the more difficult condition of partial-load levels. These two conditions of partial load and motor manufacturing imperfections, which are considered as difficult situations for fault detection, are investigated through experimentally obtained test results for a set of 2- and 5-hp induction motors

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