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A generalized formulation of quarter-wave symmetrical selective harmonic elimination (SHE) problems according to the rising and falling edges of the pulsewidth modulation (PWM) waveforms for multilevel inverters is proposed. The SHE modulation scheme features a broad solution space and can eliminate a large number of low-order harmonics. An effective method for obtaining initial values of switching angles is proposed. This method is based on rules of equal area and superposition of center of gravity (cg) of the PWM section with the sine reference signal. To illustrate the effectiveness of the proposed SHE method, a five-level voltage source inverter with elimination of 11 low-order harmonics is investigated. The SHE-PWM equations that can eliminate harmonics from 5th to 35th with modulation index M varying from 0 to 1.15 are formulated, and solutions are presented. Simulations and experiments based on two sets of solutions are carried out. It is demonstrated that the experimental results agree well with simulated ones, which proves the validity and practicability of the new method proposed. Index Terms--Multilevel inverter, pulsewidth modulation, selective harmonics elimination.

This paper proposes a novel three-phase nine-switch ac/ac converter topology. This converter features sinusoidal inputs and outputs, unity input power factor, and more importantly, low manufacturing cost due to its reduced number of active switches. The operating principle of the converter is elaborated; its modulation schemes are discussed. Simulated semiconductor loss analysis and comparison with the back-to-back two-level voltage source converter are presented. Finally, experimental results from a 5-kVA prototype system are provided to verify the validity of the proposed topology. Index Terms--AC/AC converter, pulsewidth modulation (PWM), reduced switch count topology.

The commercial power supply for the gate driver of integrated gate-commutated thyristors (GCTs) is an expensive proposition since the supply's output must withstand the high voltage of the GCT switch against all other relevant potentials in the converter. This voltage varies from a few thousand volts to several ten thousands of volts over the GCT application range. In this paper, a novel self-powered supply is proposed for the GCT gate driver, where the supply obtains energy from the snubber circuit of the GCT switch and then provides a regulated dc voltage for the gate driver. Since the snubber circuit is at the same potential as the GCT, the insulation level of the supply is reduced from thousands of volts to a couple of hundred volts, leading to a significant reduction in cost and size. The supply configuration is introduced and its operating principle is discussed. The design guidelines are discussed and experimental results from a 4160 V GCT current source rectifier are also provided. Index Terms--AC motor drives, power conversion, power electronics, power semiconductor devices, switched power supplies.

A novel control scheme for permanent-magnet synchronous generator is proposed in this paper, where a current-source converter is employed as the bridge between the generator and the grid for high-power wind energy conversion systems. In these medium voltage (2.3-13.8 kV) level applications, current-source converters not only have inherent advantages, but also present some challenges for controller design due to the dc link choke and filter capacitors. The control strategy is developed to achieve better performances with improved dynamic response. By maintaining the grid-side converter modulation index at the highest possible level, the proposed control scheme reduces the dc link current to a minimum value to reduce converter conduction loss. The system's dynamic performance is further improved by adopting generator-side power feedforward. Simulation and experimental results are provided to verify the proposed control scheme. Index Terms--Current source converter (CSC), permanent-magnet synchronous generator (PMSG), power feedforward control, wind energy conversion.

In this paper, a loss reduction and dc-link current minimization strategy for a high-power current-source inverter (CSI) fed drive is proposed. The proposed strategy consists of an inverter maximum modulation index control scheme and a flux optimization algorithm. Specifically, in the inverter modulation index control, the CSI modulation index is kept at the maximum value while the current-source rectifier (CSR) is used to regulate a reduced variable dc-link current, and therefore, to control the motor current magnitude. This control scheme can effectively reduce the dc-rink current, and at the same time, improve the line-side and motor-side waveforms. On the other hand, for the optimized flux control, the relationship between the rotor flux and the dc-link current is first investigated thoroughly. Based on this analysis, the dc-link current from the maximum inverter modulation index control can be minimized further by optimizing the rotor flux according to system variables such as the motor speed, the applied torque, and the motor-side capacitor size. With the proposed dc current minimization strategy, the losses in the semiconductor devices and the dc link can be reduced, and the drive current rating could be lowered. Both simulation and experimental results on a 4.16-kV 600-hp CSI-fed drive system are obtained to verify the effectiveness of the proposed strategy. Index Terms--Current-source converters, dc-link current minimization, field-oriented control, flux optimization, high-power drives, system efficiency.

This paper presents a novel switching sequence design for the space-vector modulation of high-power multilevel converters. The switching sequences are optimized for the improvement of harmonic spectrum and the minimization of device switching frequency. Compared to other commonly used switching sequences, the output spectrum of the proposed design shows higher inverter equivalent switching frequency. Meanwhile, the device switching frequency is reduced by using a flexible switching pattern. The proposed switching sequence has been simulated and experimentally tested on a 5-level neutral point clamped H-bridge based inverter. The results from both simulations and experiments consistently verify the above-mentioned features. Index Terms--Multilevel power converter, power conversion harmonics, space vector modulation (SVM), switching loss.

An inductor-capacitor (LC) filter is normally required at the input of a pulse-width modulation (PWM) current source rectifier (CSR), to assist in the commutation of switching devices and to mitigate line current harmonics. To dampen the LC resonance introduced by the input filter, an effective damping method is proposed in this paper by using a hybrid combination of a virtual harmonic resistive damper and a three-step control signal compensator. The virtual harmonic resistor, which is immune to system parameter variations, can dampen the LC resonance caused by an external disturbance or control command changes without affecting the fundamental power flow. But the high-pass filter used for fundamental component elimination leads to poor dynamics. Therefore, a proposed three-step compensator is inserted into the DC current closed-loop control path which can effectively eliminate the resonance within the control loop by shaping the modulation signal. Compared to the two-step Posicast controller, the proposed three-step compensator has significant implementation advantages in medium voltage CSRs, where a low switching frequency is needed. By using a hybrid combination approach, dynamics of the virtual harmonic resistor can be improved with the transient overshoot mitigated by the three-step compensator. At the same time, the parameter sensitivity of the three-step compensator can be reduced, as any residual resonant oscillations can be dampened by the virtual resistor. Simulation and experimental results are provided to verify the effectiveness of the proposed approach. Index Terms--Active damping, current source rectifier, Posicast controller, three-step compensator, virtual harmonic damper.

The bottleneck of digital control for power factor correction (PFC) implementations is mainly due to three aspects: high calculation requirements, high cost, and limited switching frequency compared with analog implementations. A new duty cycle control strategy for boost PFC implementations is proposed in this paper. The duty cycle is determined based on the input voltage, reference output voltage, inductor current, and reference current. The duty cycle determination algorithm includes two terms, the current term and the voltage term, which can be calculated in parallel and requires only one multiplication and three additions (subtractions) operations in digital implementation. A 400-kHz switching frequency boost PFC based on field programmable gate array implementation and its test results show that the proposed new duty cycle control strategy has great potential in the next generation of high switching frequency PFC implementations, due to its lower calculation requirement, lower cost, and better performance than the conventional PFC control methods. Index Terms--Field programmable gate array (FPGA), power factor correction (PFC), switched mode power supplies (SMPS).

A predictive algorithm for digital control power factor correction (PFC) is presented in this paper. Based on this algorithm, all of the duty cycles required to achieve unity power factor in one half line period are calculated in advance by digital signal processors (DSPs). A Boost converter controlled by these precalculated duty cycles can achieve sinusoidal current waveform. One main advantage is that the digital control PFC implementation based on this control strategy can operate at a high switching frequency which is not directly dependent on the processing speed of DSP. Input voltage feed-forward compensation makes the output voltage insensitive to the input voltage variation and guarantees sinusoidal input current even if the input voltage is distorted. A prototype of Boost PFC controlled by a DSP evaluation board was set up to implement the proposed predictive control strategy. Both the simulation and experimental results show that the proposed predictive strategy for PFC achieves near unity power factor. Index Terms--Digital control, power factor correction (PFC), predictive algorithm.

A novel rectifier topology for high power (0.5 to 10 MVA) current source based ac motor drives is proposed. This rectifier is composed of a multi-winding transformer, a multi-level diode rectifier and a modified multi-level buck converter. The rectifier produces near unity input power factor and sinusoidal input current under any operating conditions. In addition, the proposed rectifier features reliable operation and low manufacturing cost. In this paper, the operating principle of the proposed rectifier is introduced. A number of design issues are investigated, which include PWM switching patterns, input power factor and line current harmonic distortion. Some design considerations such as the effect of the line inductance discrepancy on system performance are addressed. Experiments on a 5 kVA/208V four-level prototype are carried out for verification. Index Terms--Buck converter, medium voltage applications, multilevel converters, power factor control.

Three new integration algorithms for motor flux estimation are proposed in this paper. These algorithms are developed for use in high-performance sensorless ac motor drives. The first algorithm is used to elaborate the basic operating principle. The second one is designed for the drives that require a constant air-gap flux during operation. The third algorithm, in which an adaptive controller is used, can have wide industrial applications. The proposed algorithms can effectively solve the problems associated with pure integrators. These algorithms can be used to accurately measure the motor flux including its magnitude and phase angle over a wide speed range (1:100). The performance of the algorithms is investigated, compared, and verified experimentally. Index Terms - Adaptive control, flux estimation, integrator, sensorless drive.

The pulsewidth-modulated (PWM) current-source inverter (CSI) used in ac-drive applications can be implemented with symmetric gate turn-off thyristors (GTO's). One of the major difficulties in the optimization of the GTO switch and the snubber components of the inverter is the variation in different switching conditions encountered during normal operation. Past work has concentrated on the GTO and snubber components in voltage-source applications, where commutation of the GTO device is an independent process and does not affect the operation of the other inverter devices. This paper proposes the characterization of the GTO and the snubber components by formulation of the CSI equivalent circuit during the device commutation period. From the equivalent circuit, the state equations are derived, thereby obtaining accurate voltage and current waveforms of the GTO and associated snubbers. From the analysis, the component power loss can be calculated and optimization performed. Simulation results are verified by using both a laboratory prototype and medium-voltage drive system. Index Terms - Current-source inverter, snubber, symmetric GTO.

In order to reduce the cost and improve the efficiency of medium-voltage induction motor drives, it is desirable that no power transformers be used. The GTO current source inverter (CSI) drive can achieve this requirement. However, the transformerless design of the drive may introduce a high-voltage stress on motor windings, which may deteriorate the motor insulation life. In this paper, the line-to-ground and neutral-to-ground voltages of the motor fed by the GTO CSI are investigated. It is demonstrated that the maximum line-to-ground voltage applied to the motor could be twice as high as the motor-rated phase voltage. Computer simulation and experimental results from a 4000-V, 1250-hp drive are given to verify the theoretical analysis. The effects of dc link choke arrangement on the motor voltage stress are also discussed. This paper provides a valuable design guidance for the insulation of medium-voltage motors used in the GTO CSI drives.