Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2002-10-18
2004-06-29
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
Reexamination Certificate
active
06757185
ABSTRACT:
I—TECHNICAL BACKGROUND
1 Technical Field
The present invention is related to the electrical rectifier field, more specifically, to the field of control techniques for the three-phase three-level boost type rectifier.
This work introduces a new control method and control circuit for a three-phase three-level boost-type rectifier. Under the proposed control method and control circuit, the rectifier performs three-phase AC-DC conversion generating sinusoidal input currents with reduced ripple and near unity power factor while balancing the two DC output voltages. The control circuit uses two triangular carriers with the same amplitude but 180 degrees phase-shifted; Trg1 is used during the positive halve cycle and Trg2 for the negative one. The advance of the method is that the switching pulses are synchronized and appropriately distributed, allowing the reduction of the input current ripple. An effective balance of the output voltage is obtained by adding a sensor that acts over the input current waveform reference. In addition, the control system features simplicity and low cost.
2 Background
With the development of power electronics technology and the generalized utilization of equipment with non-linear characteristics such as electronic rectifiers or static converters AC-DC, there have been increasing utility pollution problems produced by large distorted harmonics currents drawn from the power distribution systems [See P. Rioual and T. Deflandre, “Impact on the Distribution and Transmission Systems of Harmonic Current Injection due to Capacitive Load Rectifiers in Commercial, Residential and Industrial Installations”, Conference Records of EPE'95, pp. 3.503-3.508, 1985, and Yu-Kang Lo and Neng-Chin Lia, “On Evaluating the Input Current Distortion with Current Slopes for Single-Phase Switch-Mode Rectifiers”, Conference records of PESC'98, pp. 77-80, 1998]. Harmonic currents cause additional harmonic losses in the utility system and may excite electrical resonances, leading to large overvoltages; thus disturbing other sensitive loads connected to the same supply [See Ned Mohan, Tore M. Undeland, William P. Robbins, “Power Electronics Converters, Applications, and Design”, John Wiley & Sons, 1989].
The uncontrolled and thyristor bridge rectifiers are widely used interface between utility and power electronics systems. Although they are very simple in structure and robust in operation, they have the disadvantages of drawing a large amount of harmonic current from utility [See Hirofumi Akagi, “New Trends in Active Filters for Power Conditioning”, IEEE Transactions on Industry Applications, Vol. 32, No. 6, pp. 1312-1322, 1996]. Recently the harmonics produced by such nonlinear loads have become a serious problem in many countries.
To prevent the problem to become intolerable, various standards and guidelines such as the IEEE 519 and the IEC-555, have been established which specifies limits on the magnitudes of harmonic currents and harmonic voltage distortion at various frequencies [Ned Mohan et al., op. cit.]. Simultaneously, much effort has been carried out to develop power converters with low harmonic current injection to utility and capable to control the input power factor.
Modern AC-DC converters are expected to draw sine-wave current from the mains, with power factor very close to unity. In other words it is required that the converter presents a resistive load to the mains [Javier Sebastian and Miguel Jaureguizar, “Tendencias Futuras en la Corrección del Factor de Potencia en Sistemas de Alimentación”, Conference records of CIEP'93, pp. 136-153, 1993]. In general, besides those characteristics, the target is low circuit complexity, low component stress, high power density, high efficiency, high reliability, and controllability of the output voltage.
In single-phase applications, one of the most popular power circuit schemes for power factor correction is the boost converter topology. A frond-end rectifier diode bridge followed by a boost DC-DC converter composes the topology. The technique has been extended to three-phase applications using a three-phase diode bridge and a single switch boost DC-DC converter operating in discontinuous conduction mode [A. R. Prasad, P. D. Ziogas, and S. Manias, “An Active Power Factor Correction Technique for Three-Phase Diode Rectifiers”, IEEE Transactions on Power Electronics, Vol. 6, No. 1, pp. 83-92, 1991] The single active switch three-phase preregulator presents low cost and simplicity but the topology has as drawbacks high conduction losses, high switching stresses, and radio interference emission regulations resulting in a large input filter.
Circuits with multiple configuration and advanced designs are actually considered to increase the power levels and improve the generated waveforms [See A. Nabae, I. Takahashi and H. Akagi, “A New Neutral-Point-Clamped PWM Inverter”, IEEE Transactions on Industry Applications, vol. IA-17, No. 5, pp. 518-523, 1981; and J. W. Kolar, H. Ertl and F. C. Zach, “Design and Experimental Investigation of a Three-Phase High Power Density High Efficiency Unity Power Factor PWM (VIENNA) Rectifier Employing a Novel Integrated Power Semiconductor Module”, Conference records of APEC'96, pp. 514-523, 1996].
In fact, multilevel converters are starting to be used in industry and traction applications and its future seems promissory [See T. Katta, Y. Kurokawa, “Advanced Inverter Control System Using High Voltage IGBT for EMU” Conference records of IPEC'95, pp. 1060-1065, 1995 and S. Bernet, “Recent Developments of High Power Converters for Industry and Traction Applications” Conference records of COBEP'99, pp. 3-16, 1999].
The multilevel converters present some advantages over conventional two level converters such as:
The voltage imposed across the switching devices is reduced proportionally with the number of levels. In this way, it is possible to use them in high voltage systems increasing the converter capability [R. Rojas, T. Ohnishi, T. Suzuki, “PWM Control Method for a Four-Level Inverter”, IEE Proceedings on Electric Power Applications, vol. 142, Issue 6, pp. 390-396, 1995].
Improve significantly the voltage and current waveforms, which results in a substantial reduction of the harmonic distortion [N. S. Choi, J. G. Cho and G. H. Cho, “A General Circuit Topology of Multilevel Inverter”, Conference records of PESC'91, pp. 96-103, 1991 and J. Lai and F. Peng, “Multilevel Converters—A New Breed of Power Converters”, IEEE Transactions on Industry Applications, vol. IA-32, No. 3, pp. 509-517, 1996]. This is probably the best characteristic of the multilevel converters because the actual tendency is to improve the waveforms of the power electronic circuits.
The electromagnetic interference is smaller than that of conventional converters because the voltage variation (dv/dt) at the commutation instant is divided by the number of levels [T. A. Meynard and H. Foch, “Multi-Level Conversion: High Voltage Choppers and Voltage-Source Inverters”, Conference records of PESC'92, pp. 397-407, 1992]. In the European countries there is already a strict norm to limit the electromagnetic interference and other counties will follow their example.
An interesting configuration for a three-phase three-level rectifier has been considered lately by several researchers [See Ewaldo L. M. Mehl and Ivo Barbi, “An Improved High Power Factor and Low Cost Three-Phase Rectifier”, Conference records of APEC'95, pp. 835-841, 1995; and J. W. Kolar, H. Ertl and F. C. Zach, “Design and Experimental Investigation of a Three-Phase High Power Density High Efficiency Unity Power Factor PWM (VIENNA) Rectifier Employing a Novel Integrated Power Semiconductor Module”, Conference records of APEC'96, pp. 514-523, 1996]. The converter is composed by a combination of diodes and switches adjusted to the requirements of generating an input current nearly sinusoidal with high power factor and
E.E.S. Sistemas de Energia Ltda.
Harness Dickey & Pierce PLC
Sterrett Jeffrey
LandOfFree
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