Electric power conversion systems – Current conversion – With means to introduce or eliminate frequency components
Reexamination Certificate
2000-12-07
2002-06-11
Vu, Bao Q. (Department: 2838)
Electric power conversion systems
Current conversion
With means to introduce or eliminate frequency components
C363S056020, C363S098000, C363S017000
Reexamination Certificate
active
06404655
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to power converters and distribution schemes for power distribution. More specifically, the present invention relates to a transformerless output 3 phase power inverter topology and control method to facilitate AC power with very low DC offset content, high quality source, and low output total harmonic distortion (THD).
2. Background Art
Electric power distribution is a necessary component of systems that operate with electronic power or in the distribution of electronic power. For example, most electronic equipment is connected to a utility grid wherein power arrives in one form and is transferred and transformed into a form more suitable for the equipment.
The distribution of electric power from utility companies to households and businesses utilizes a network of utility lines connected to each residence and business. The network or grid is interconnected with various generating stations and substations that supply power to the various loads and that monitor the lines for problems. Distributed electric power generation, for example, converting power from photovoltaic devices, micro-turbines, or fuel cells at customer sites, can function in conjunction with the grid. Loads that are connected to the grid take the generated power and convert it to a usable form or for supplementing the grid.
An electric utility grid generally can also consist of many independent energy sources energizing the grid and providing power to the loads on the grid. This distributed power generation is becoming more common throughout the world as alternative energy sources are being used for the generation of electric power. In the United States, the deregulation of electric companies has spurred the development of independent energy sources co-existing with the electric utility. Rather than have completely independent energy sources for a particular load, these alternative energy sources can tie into the grid and are used to supplement the capacity of the electric utility.
The number and types of independent energy sources is growing rapidly, and can include photovoltaic devices, wind, hydro, fuel cells, storage systems such as battery, super-conducting, flywheel and capacitor types, and mechanical means including conventional and variable speed diesel engines, Stirling engines, gas turbines, and micro-turbines. In many cases these energy sources can sell the utility company excess power from their source that is utilized on their grid.
Each of these independent energy sources needs some type of power converter that feeds energy to the grid or used to directly power the various loads. There must also be some means to provide protection when the grid becomes unstable. In most scenarios the utility company is still the main power source and in many cases controls the independent source to some extent.
A problem with the prior art system is that the distribution system is subject to non-linear, high harmonic content and unbalanced loading. This is especially true where the distributed generation system operates independent of the utility grid, and must therefore provide all of the load required harmonic currents. In distributed power applications, high harmonic content or unbalanced loads may lead to utility grid instability, resonances or other unanticipated distribution system behavior that may cause catastrophic failure of the distribution system components. Such a failure can result in damage to equipment and possibly personal injury.
Power converters such as inverters are necessary in modem power systems for the new energy generating devices such as photovoltaic devices, micro-turbines, fuel cells, superconducting storage, etc., that generate AC or DC electricity that needs to be converted to a conditioned AC for feeding into the power grid or for direct connection to loads.
Grid independent DC-AC inverters generally behave as sinusoidal voltage sources that provide power directly to the loads. This type of power distribution architecture is generally required to provide power to both 3 phase and single phase, or line to neutral connected loads. Typically, 3 phase power inverters meet this 3 phase+neutral requirement by isolating the power inverter from the loads with a delta-wye power transformer.
Grid connected DC-AC inverters generally behave as a current source that injects a controlled AC sinewave current into the utility line. The controlled AC current is generated in sync with the observed utility zero crossings, and may be exactly in phase, generating at unity power factor where upon real power only is exported. It is also possible to generate a variable amount out of phase—at other than unity power factor where upon real and reactive power is exported to the grid. An effective change in reactive power output can be made by either phase shifting the output current waveform with respect to voltage or by creating an assymetric distortion to the output current waveform.
Whether grid connected or grid independent, typical delta-wye transformer isolated 3 phase power inverters demonstrate poor output waveform THD when connected to non-linear loads. This is particularly true in the case of even order harmonic currents (2
nd
, 4
th
, 6
th
, 8
th
etc.). Specifically, typical power transformers common to most power distribution systems demonstrate a tendency to saturate when exposed to even order or DC content load generated non-linear currents. This causes the transformer's impedance to instantaneously decrease, thereby allowing excessive asymmetric currents to flow through the transformer's windings, while decreasing the power actually coupled from primary to secondary. A variety of factors define how steep this saturation transition will occur, including magnetic core material and construction, magnitude of even order harmonics, and transformer operating temperature. At the least, very poor output power quality, nuisance circuit breaker tripping, increased distribution system components losses and increased operating temperatures will be observed.
Although distribution system transformer saturation is not as likely to occur in utility grid connected systems (due primarily to the utility grid's typically lower impedance than the grid connected inverter system), distortion and instability may still occur. This problem is greatly aggravated where power inverters act as “stand alone” voltage sources, where the inverter comprises the only power source to the local distribution system.
These problems are currently solved in the distribution system by over sizing the distribution transformers. For power inverters, expensive gapped core type isolation transformers are commonly employed to decrease the power conditioning system susceptibility to even order harmonic currents, as well as isolate inverter generated DC voltage offsets from the distribution system. The increased cost and space requirements for the isolation transformers are problems that are well known in the industry.
Inverters that perform DC-AC conversion function, and are connected to the grid, are known as “Utility-Interactive Inverters” and are the subject of several US and international codes and standards, e.g., the National Electrical Code, Article 690—Photovoltaic Systems, UL 1741, Standard for Photovoltaic Inverters, IEEE 929—Recommended Practice for Utility Interface of Photovoltaic (PV) Systems.
Pulse width modulator (PWM) inverters are used in three phase bridges, H-bridges, and half-bridge configurations. The bus capacitors, typically electrolytic, consist of two or more capacitors connected in series that are fed from a rectifier or actively switched front end section.
In order to reduce the aforementioned problems, attempts have been made to produce an improved dispensing system. The prior art systems have general short-comings and do not adequately address the aforementioned problems.
What is needed is a means of efficiently operating power inverters, especially for non-linear, high harmonic content, and/or unbalanced load
Asmus Scott J.
Maine Vernon C.
Maine & Asmus
Semikron, Inc.
Vu Bao Q.
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