Madhav D. Manjrekar

A hybrid multilevel inverter topology for drive applications

The use of multilevel inverters has become popular in recent years for high power applications. Various topologies and modulation strategies have been reported for utility and drive applications in the recent literature. This paper is devoted to the investigation of a 500 hp induction motor drive based on a seven-level 4.5 kV hybrid inverter. The topological structure and operating principles of the proposed approach are presented. Various design criteria, spectral structures and other practical issues such as capacitor voltage balancing are discussed. The feasibility of the proposed approach is verified by computer simulations.

A generalized structure of multilevel power converter

Multilevel inverters are becoming popular in the past few years for high voltage and high power applications. Various topologies and modulation strategies have been investigated for utility and drive applications in the recent literature. However, most of the reported topologies for multilevel structures employ DC voltage sources of equal magnitudes. This paper is devoted to the investigation of a generalized structure of multilevel power converter where these discrete voltage sources are not necessarily equal. The topological structures, operating principles and performance characteristics resulting from this general approach are presented. Various design criteria and issues like spectral structure are discussed. A comparative evaluation of various alternatives obtained from the generalized structure is presented.

A reconfigurable uninterruptible power supply system for multiple power quality applications

A novel topology of an uninterruptible power supply (UPS) system is presented in this paper. The proposed UPS system presents the advantages of high power conversion efficiency as well as active front-end filtering which ensures unity input power factor (reactive power compensation) and low input total harmonic distortion (harmonic power compensation) in the voltages and currents. In addition, this UPS possess dynamic voltage sag compensation capabilities, hence eliminates any series transformer or boost converter which is normally used in conjunction with traditional stand-alone UPS systems. Meanwhile, during the backup (battery) mode of operation, this UPS has the capability of providing grid isolation without any backward power feed into the faulted utility gird, thus eliminates the needs of circuit breaker or static switch. In addition, it has the desirable characteristics of making seamless transition from normal to backup mode during power failures and vice versa, as well as providing controlled voltage charging at the dc bus link. The proposed UPS is also impervious to load variations, which enables it to operate under nonlinear load condition. Lastly, the circuit structure is simple, light weight, and can be implemented at a low cost using the standard commercial available components. A complete set of simulation as well as experimental results based on a 1 kW test prototype of the proposed UPS are presented here to demonstrate its viability and efficacy.

Performance improvement of half controlled three phase PWM boost rectifier

The potential of the half-controlled three-phase pulse-width modulated (PWM) boost rectifier is investigated based on theoretical analysis, simulations and experiments. The main advantages of this rectifier are: (1) a simpler and economical system compared to a full controlled PWM rectifier (reduced controlled switch count, single power supply for gate drives, and shoot-through free leg structure); and (2) better performance compared to a diode rectifier (actively controllable DC link voltage and lower input current total harmonic distortion (THD)). In particular, it is shown in this paper that the input current THD of this rectifier can be reduced by intentionally introducing a lagging power factor current command. Several issues for further performance improvement are pointed out for future work.

Investigations on a unified controller for a practical hybrid multilevel power converter

This paper is devoted to the investigation of a practically feasible and commercially viable hybrid multilevel power converter topology typically suitable for active rectifier or static reactive power compensation applications at distribution voltage levels. This system comprises a three-phase neutral point clamped (NPC) main inverter supported by three auxiliary single-phase H-bridge inverters fed from independent DC links. The main and auxiliary DC link voltages are configured in the ratio 3:1. A multilevel waveform is realized by adding the low-voltage, fast-switching auxiliary inverter outputs to the high-voltage, slow-switching main inverter output. A novel hybrid modulation scheme that incorporates stepped synthesis in conjunction with variable pulse width of the consecutive steps is described. In addition, the performance of a unified controller that enables the regulation of all the DC bus voltages is discussed.

Complementary half controlled three phase PWM boost rectifier for multi-DC-link applications

The half controlled three phase pulse width modulated (HC3/spl Phi/PWM) boost rectifier has simpler and more robust structure compared to the full controlled three phase (FC3/spl Phi/) PWM boost rectifier, and better performance compared to the diode rectifier. One of the main drawbacks is its even harmonic input current distortion which may cause undesirable resonance problems. Other issues of concern are fairly large AC side inductance and the necessity of intentional lagging power factor current command to obtain reasonably low input current THD. This paper describes methods with which these problems can be handled for a certain type of application by introducing complementary topologies of HC3/spl Phi/PWM boost rectifiers. Two types of complementary configuration, rectifier leg complementary and input transformer complementary, are introduced. The former comprises an emitter common and a collector common HC3/spl Phi/PWM boost rectifiers. The latter utilizes complementary polarity of the input transformer secondary windings. Two control schemes for this rectifier are presented: (a) independent local control (ILC), and (b) coordinated central control (CCC). ILC can lead to even harmonics cancellation with equally supplied loads by two DC links. CCC can reduce not only even harmonics but also nontriplen odd harmonics (5th, 7th, ...). In CCC, AC side inductor size can be reduced and no lagging power factor current command is necessary. In addition, CCC can handle load imbalance between two DC links to a certain extent. Power stage description, control principles, simulation results and experimental results are presented.

Flux Tracking Methods for Direct Field Orientation

Numerous techniques have been reported in the literature to accomplish the task of locating the rotor flux axis for the purpose of direct field orientation. This paper first summarizes the existing sensorless techniques using high-frequency test signal injection to determine the flux spatial location in an induction machine. A novel approach employing balanced and unbalanced excitation is then investigated to track the flux. It is first demonstrated that the rotating flux vector in the machine causes saturation-induced saliency in the phases under normal operating conditions. By employing high-frequency test signals, this saliency is monitored and then used to track the flux in the machine under test. The feasibility of the proposed approach is verified with experimental results.

A power electronic transformer (PET) fed nine-level H-bridge inverter for large induction motor drives

This paper is devoted to the investigation of a 500 HP induction machine drive based on a nine-level 4.16 kV H-bridge inverter. Previous work in the area of controlling H-bridge inverters at such power levels report enhancement in the performance of a conventional staircase modulation technique by employing optimization of switching angles to minimize voltage distortion. However, the DC link voltages of each level are typically held constant. Such a control scheme enables elimination of a maximum of three dominant harmonics (5/sup th/, 7/sup th/ and 11/sup th/) to synthesize a given fundamental voltage with a nine-level waveform. Moreover, at lower modulation depths, the nine-level operation degenerates into seven or even fewer levels. This restricts the number of harmonics that can be eliminated to two (5/sup th/ and 7/sup th/) or less. An alternative strategy to produce a required fundamental voltage with a nine-level waveform by controlling the DC bus voltages is presented in this paper. This control scheme enables elimination of four dominant harmonics (5/sup th/, 7/sup th/, 11/sup th/ and 13/sup th/) over the entire range of operation. The required DC link voltage control is achieved by employing active power electronic transformers for isolation, thereby supplying varying AC voltage to the front-end rectifiers. Operating principles, spectral structure and design consideration are discussed. Computer simulations backed up by experimental results are presented in the paper.