Electric power conversion systems – Current conversion – With means to introduce or eliminate frequency components
Reexamination Certificate
2001-10-29
2003-11-25
Vu, Bao Q. (Department: 2838)
Electric power conversion systems
Current conversion
With means to introduce or eliminate frequency components
C363S160000, C363S131000, C363S135000
Reexamination Certificate
active
06654261
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to power distribution, more specifically, to power regulation and conditioning.
2. Background Art
In the current power environment, power plants are interconnected to loads via utility grids to deliver large amounts of power. Efficient distribution over long distances power is delivered as low frequency three phase AC current. However, low frequency AC current is not suitable for end use at some loads. Therefore, prior to the end use, the utility grid power has to be converted to a more useable form. A typical power “conditioning” configuration includes an AC-to-DC rectifier that converts the utility AC power to DC across positive and negative DC buses, across a DC link, and an inverter linked to the DC link that converts the DC power back to three phase AC power having an end useable form, namely three phase high frequency quality AC voltage. A controller controls the inverter to provide the voltage waveforms required by the load.
Besides power plants connected to a grid, local power systems can be interconnected to the grid. And, backup power supplies provide power to the local load when utility grid power is cut off. In the distributed grid, the local power system transfers power to the grid or takes power from the grid as needed. There are many forms of local power systems such as fuel cells and microturbines, each having particular requirements and operating parameters. Thus, there are many applications for an efficient power conditioning unit that can generate the proper power to the load with minimal distortion.
Typical DC-DC voltage regulators are well-known in the art, and they primarily provide a constant DC voltage to an inverter which then pulse width modulates (PWM's) the DC voltage to produce an AC output voltage. PWM inverters and various techniques that convert DC to AC are also well known in the art. A PWM pattern is generally a set of switching transients that is applied to the DC signal via an inverter and produces a sinusoidal AC signal.
The combination of an inverter stage connecting to the DC-DC converter is also commonplace, and the inverter converts the cleaned DC output of the converter into an AC output of the inverter.
As recognized in the industry, there are many limitations in the existing designs and considerable room for improvement. For example, the systems have difficulty addressing unbalanced load conditions, there are problems with ripple frequencies that require expensive and large inductors and capacitors. There have been various attempts in the prior art to regulate the AC output signal and reduce distortions and ripple on the AC output. Typically these attempts measured the voltage and current of the AC signal and changed the PWM switching patterns to minimize distortions.
In a typical application, the DC-DC converter conditions and regulates power from an regulated DC source and tries to produce a tightly regulated DC output voltage for use by a PWM output inverter. In a 240/120VAC 60 Hz application, the output power (or DC current) as seen by the dc link occurs at a 120 Hz frequency. Typically an output PWM filter is used to filter off the PWM inverter “switching ripple” thereby producing a “clean” sinusoidal output. Voltage and consequently power loss developed across the PWM filter inductor increases as a function of the difference between the DC link volts and the output AC voltage. The greater the difference between the DC links voltage and the output voltage, the more extreme the voltage levels across the output filter components.
U.S. Pat. Nos. 4,935,859 and 4,935,860 describe a VCSF system that has an inverter that applies a PWM pattern of switching transients to the DC signal to produce an AC signal. A feedback circuit reduces short duration switching transients in the PWM pattern by analytically determining DC link distortion. These patents relate to methods for reducing insulated gate bipolar transistor (IGBT) short pulses by modulating the DC link. A related patent, U.S. Pat. No. 4,937,720, is for a DC link harmonic elimination for AC inverters.
There are other inventions intended to reduce semiconductor switching losses with a variety of soft switch techniques, such as U.S. Pat. Nos. 5,559,685, 5,592,371 and 5,841,644. While these various schemes have certain advantages, they are not intended nor allow for reducing filter inductor losses.
Besides utility and local power conditioning, there are many other applications for power conditioning such as uninterruptible power supplies (UPS), that benefit from more efficient conditioning.
What is needed is a system that can reduce the voltage or switching ripple across the PWM output filter inductor. For safety purposes, the system should also provide fast circuit breaking in fault conditions. Such a system should also allow for lower cost, lighter weight and more efficient PCU systems.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the aforementioned background. One object of the invention is a modulated dc link scheme for dramatically reducing the switching frequency ripple developed across the PWM output filter inductor.
Another object of the invention is to allow greater overall power conditioning unit (PCU) inverter efficiency because of the reduced PWM filter inductor losses, without any loss of AC output waveform dynamic performance. Typically a voltage “feed forward” term from the dc volts feedback to the PWM controlling DSP can allow for stable inverter control.
A further benefit is that lower cost PWM filter inductor core materials may be used as the core losses are primarily driven by the high frequency ripple content of the PWM output. Inductor size and weight may also be reduced.
During excessive overloads such as occur during a circuit breaker protected branch short circuit where, the power conditioning inverter is required to feed energy into the fault to cause the protection circuit breaker to trip. Utilizing the dc modulation scheme of the present invention automatically reduces the DC link voltage to track the output AC volts. When the load impedance is very small, or shorted, the output AC volts is consequently reduced to limit output current. The output inverter provides more energy faster into the fault to clear the circuit breaker. This is primarily because the inverter switch losses are reduced (reduced VDC), and the filter inductor magnetic core normally absorbs a large percentage of the total output power during an overload fault.
Further passive component cost reductions, and lifetime enhancements can be realized with this dc link modulation scheme. For example DC link bulk capacitance can be reduced as the required energy storage is reduced, thereby allowing for smaller and less expensive capacitors. PWM filter costs (capacitors and inductors) are also reduced. And, the longevity and stability of the system is enhanced by avoiding excessive power handling primarily due to undesirable switching ripple.
Electromagnetic interference (EMI) is also reduced as the PWM output voltage ripple, and therefore higher frequency harmonics (up into the RF range), are dramatically reduced. This allows the output EMI/RF filter to be more efficiently designed, while achieving similar EMI performance. Conversely, a high quality RE/EMI filter may be retained thereby providing significant EMI/RF emissions reductions. In certain applications that require low noise emissions, such as military applications, the present scheme has particular applicability.
The dc modulation scheme allows very efficient power conditioning at a lower cost, particularly when using of a high frequency transformer (20-50 kHz) in the main DC/DC converter. High-frequency links have rapidly become the preferred technology in grid-connected, photovoltaic inverter applications, and have the advantage of providing dc isolation and inversion without the need of 60 Hz transformer. As a result, the designs are smaller and lighter, and advantages are further described in U.S. Pat. No. 4,641
Beckedahl Peter
Bond Karl
Griessel Richard E
Welches Richard Shaun
Maine & Asmus
Vu Bao Q.
Youtility, INC
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