Electric power conversion systems – Current conversion – With voltage multiplication means
Patent
1998-04-01
1999-11-16
Riley, Shawn
Electric power conversion systems
Current conversion
With voltage multiplication means
363140, H02M 318
Patent
active
059869070
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to AC to DC power conversion, rectification, derectification, and power flow control.
Standard rectification using diode brides causes both harmonics and a reactive power on the AC line. This is because power is drawn from the line when its AC voltage is higher than the output voltage and no power is drawn from the line when its AC voltage is lower than the output voltage. This uneven loading of the line throughout the AC cycle introduces harmonics onto the line. Current is only drawn out of the system when the voltage is high relative to the output voltage. As a consequence, conventional bridge or half wave rectification techniques seriously distort the input waveform. This problem is solved by using harmonic filters and capacitors to eliminate the harmonics. In addition, filtering is added on the DC side to reduce ripple that the processes tends to cause.
In an earlier patent (i.e., U.S. Pat. No. 5,270,913 filed Apr. 6, 1992, and incorporated herein by reference), I described a transformerless power conversion system (referred to hereinafter as PCS). In very general terms, the PCS works by charging a set of capacitors from a power source, possibly transforming the voltage across the set of capacitors by inverting the voltages on selected capacitors, and then discharging the set of capacitors at the transformed voltage into a distribution node or load. In other words, a complete cycle of operation in the PCS includes a charging phase, possibly an inversion phase, and a discharging phase. By employing many cycles of operation per second (e.g. 1 to 2 kHz), the PCS can extract charge from the power source and inject it into the distribution node or load to reconstruct an output having a desired waveform. The PCS is extremely versatile in the transformations which it can be configured to perform. For example, it can be configured to convert AC to DC, DC to DC with step-up or step-down, DC to AC, or AC of one frequency to AC of another frequency, to name a few.
In the case of AC to DC conversion, charging the PCS from a low voltage source (e.g. when the instantaneous voltage of the input AC waveform is low) presents the same type of problem that is encountered with conventional rectification. If the transformed voltage in the PCS is less than at least two times the output voltage, it will not be possible to fully discharge the capacitors into the DC output terminal. Therefore, it follows that the PCS system can also impose a nonuniform load on the input line and thereby distort the input waveform by introducing harmonics back onto the input line.
As described in the earlier patent, however, this problem can be solved by using multiple charging cycles per discharge cycle. In this way, the output voltage of the PCS can be made sufficiently high to permit a complete discharge of the storage capacitors during the discharge cycle. Though that technique works, it may be more complex than necessary, it involves more computation, and it requires capacitors with higher voltage ratings.
SUMMARY OF THE INVENTION
In general, in one aspect, the invention is a system for controlling VAR of a multiphase grid. The system includes a plurality of charge storage elements; a plurality of charge transfer circuits each connected to a corresponding phase of the multiphase grid and to a corresponding one of the plurality of charge storage elements; and a charge redistribution circuit connected to the plurality of charge storage elements, wherein during operation the charge redistribution circuit redistributes charge among the plurality of charge storage devices.
In preferred embodiments, the system further includes a controller which operates the plurality of charge transfer circuits and the charge redistribution circuit, wherein during operation the controller causes the plurality of charge transfer circuits to transfer charge to the plurality of charge storage elements, causes the charge redistribution circuit to redistribute the charge that was transferred to the plurality char
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