Electricity: power supply or regulation systems – For reactive power control – Using converter
Reexamination Certificate
1999-08-31
2001-06-19
Nguyen, Matthew (Department: 2838)
Electricity: power supply or regulation systems
For reactive power control
Using converter
Reexamination Certificate
active
06249108
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of active power filters and in particular to a control method based on one-cycle control.
2. Description of the Prior Art
In recent years, the usage of modem electronics equipment has been widely proliferating. This electronics equipment imposes nonlinear loads on the ac main or source that draw reactive and harmonic current in addition to active current. The reactive and harmonic current results in a low power factor, low efficiency, harmful electromagnetic interference to neighborhood appliances, as well as overheating of transformers. In order to solve these problems, many international agencies have proposed firm harmonic restrictions to electronic equipment. As a result, a vast number of power factor correction (PFC) techniques have been proposed to comply with these regulations.
Most techniques use a current shaper, whether in a two-stage multiple-switch configuration or a two-stage one-switch configuration, to shape the input current to a sinusoidal waveform. Since the current shaper is in the series path of the power, it requires high current and high voltage semiconductor devices and entails significant power losses. Therefore, PFC techniques are generally suitable for low to medium power applications. Furthermore, it is not convenient to insert a current shaper in existing electronic equipment, since significant redesign would be required. In high power applications, a parallel harmonic correction technique using an active power filter (APF) has been proposed and explored by many researchers. See, Fabiana Pottker and lov Barbi, “Power Factor Correction of Nonlinear Load Employing a Single Phase Active Power Filter: Control Strategy, Design Methodology and Experimentation” PESC 1997 Record 28
th
annual IEEE Power Electronics Specialists conference; D. A. Torrey, A Al-Zamel, “Single-phase active power filter for multiple nonlinear loads” IEEE Transactions on Power Electronics, Vol. 10, pp.263-271, May 1995; Simone Buso, Luigi Malesani, “Comparison of Current Control Techniques for Active Filter Applications” IEEE Trans on Industrial Electronics. Vol. 45. No. 5 October 1998; J.-C. Wu and H.-L. Jou “Simplified control method for the single-phase active power filter” IEE. Proc. Electr. power., Vol. 143, No. 3, May 1996; Hirofumi Akagi, “New trends in active filter for improving power quality” Proceeding of the 1996 International Conference on Power Electronics, Drives and Energy System for Industrial Growth; and J. Sebastian Tepper, Juan W. Dixon “A simple-frequency-independent method for calculating the reactive and harmonic current in a nonlinear load” IEEE Transaction on Industrial Electronics, Vol. 43, No. 6, December 1996.
An APF is a device that is connected in parallel to and cancels the reactive and harmonic currents from a group of nonlinear loads so that the resulting total current drawn from the ac source is sinusoidal. Ideally, the APF needs to generate just enough reactive and harmonic current to compensate the nonlinear loads in the line, thus it handles only a fraction of the total power to the load. Most APF control methods previously proposed need to sense the line voltage and the nonlinear load current, and then manipulate the information from these sensors to generate a current reference for the APF. Since the reference current has to reflect the load power of the nonlinear load, a multiplier is needed to scale the magnitude of the current reference. A control loop is necessary to control the converter to generate the reactive and harmonic current required by the nonlinear load. These functions are generally realized by a digital signal processing (DSP) chip with fast analog-to-digital (A/D) converters and high-speed calculations. The performance of these active power filters is based on three basic design criteria: the converter topologies, the control method used, and the method used to obtain the current reference. The complex circuitry results in high cost and unreliable systems, preventing this technique from practical applications.
What is needed is an API not subject to the inherent disadvantages of prior designs.
BRIEF SUMMARY OF THE INVENTION
The method of the invention is an unified constant-frequency integration (UCI) APF control method based on one-cycle control. It employs an integrator with reset as its core component to control the pulse width of an ac-dc converter so that its current draw is precisely opposite to the reactive and harmonic current draw of the nonlinear loads. The term, “reactive current”, shall be defined in this specificaiton and claims to include all current which is different in phase or at a different frequency from the AC source current, including the fundamental frequency, all harmonics and other nonlinear effects of the load. In contrast to all previously proposed methods, there is no need to generate a current reference for the control of the converter current, thus no need to sense the ac line voltage, the APF current, and the nonlinear load current. Only one current sensor and one voltage sensor (resister divider) are used to sense the ac source current and the voltage across the dc capacitor. The control method features constant switching frequency operation, minimum reactive and harmonic current generation, and simple analog circuitry. It provides a low cost and high performance solution for power quality control.
More specifically the invention is defined as a circuit comprising an AC source, a nonlinear load coupled to the AC source in which the nonlinear load has a reactive current drawn therefrom, and an active power filter coupled to the AC source in parallel with the nonlinear load. The active power filter is configured so that it has a current draw opposite to the reactive current draw of the nonlinear load and so that the reactive current draw of the nonlinear load is substantially cancelled out by the current draw of the active power filter.
The compensation of multiple nonlinear loads coupled to the AC source is included within the scope of the invention. In such a case, the active power filter is coupled to the AC source in parallel with each of the nonlinear loads.
The active power filter comprises a switched bridge circuit and a storage device coupled to the switched bridge circuit. In the illustrated embodiment the storage device is a capacitor, and the switched bridge circuit is a switched full wave-bridge. In another embodiment, the switched bridge circuit is a switched half wave-bridge, or a switched DC side directional boost circuit.
In all of the embodiments, the switched bridge circuit switches at a higher frequency than is characteristic of operation of the AC source and than the nonlinear load. The capacitor is configured to have a nearly constant voltage waveform across switching cycles.
The circuit further comprises an integrator with a reset circuit. The integrator has an input coupled to the active power filter. The integrator with the reset circuit has an output coupled to and controlling the switching of the active power filter. The integrator with the reset circuit is configured to control the switched bridge so that net current drawn from the AC source by the active power filter is equal to the fundamental active current drawn by the nonlinear load, and has substantially the same waveform and is in phase with the AC source.
In the illustrated embodiment, the active power filter comprises an AC to DC converter and a storage device coupled thereto having a control voltage, v
c
. The AC to DC converter is switched at a frequency characterized by a duty cycle, D. The nonlinear load and the active power filter are characterizable as an equivalent resistance, R
e
, which is coupled to the AC source. The AC source has a voltage, v
s
, and current, i
s
, and further comprises a sensing resistor, R
s
, which is coupled in series with the equivalent resistance, R
e
, and the AC source. The duty cycle, D, is controlled according to a control equation so that reactive and harmonic current of the nonline
Smedley Keyue M.
Zhou Luowei
Dawes, Esq. Daniel L.
Myers Dawes & Andras LLP
Nguyen Matthew
The Regents of the University of California
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