Electric power conversion systems – Current conversion – Having plural converters for single conversion
Reexamination Certificate
2000-05-10
2001-01-23
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
Having plural converters for single conversion
Reexamination Certificate
active
06178103
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to voltage source inverters, and more specifically, to a synchronization circuit for synchronizing the output of parallel voltage source inverters.
BACKGROUND
To generate high power outputs for an AC source, it is common to place a number of electronic voltage source inverters in parallel. Each of the voltage source inverters has an associated phase angle. One known problem with placing voltage source inverters in parallel is that if the phase angles are not the same in each of the parallel voltage source inverters, circulating real power between the voltage sources may be developed. Circulating real power reduces the capability and efficiency of the voltage sources. Further, circulating real power may also cause the malfunctioning of the voltage source inverter.
It is therefore one object of the invention to provide a voltage inverter circuit having the phase angles synchronized to minimize real power circulation.
SUMMARY OF THE INVENTION
It is therefore one object of the invention to provide a circuit having parallel voltage source inverters that are configured to minimize circulating real power among the inverters. The parallel voltage source inverters essentially become electrically equivalent by the present invention to eliminate the circulating real power.
In one aspect of the invention an inverter circuit comprises a synchronization signal source having a first frequency and a first phase. A first voltage source inverter circuit is coupled to a load and the synch source. The first voltage source inverter circuit comprises a cycle time counter generating a cyclic output signal and a first sampling circuit receiving the cyclic output signal and sampling said cyclic output signal. The first sampling circuit generates a magnitude signal and a phase signal in response to said cyclic output signal. The first inverter circuit generates an output signal having a first phase in response to the magnitude signal and the phase signal. A second voltage source inverter circuit is coupled to the load in parallel with the first voltage source inverter and to the synch signal source. The first voltage source inverter circuit has a second cycle time counter generating a cyclic output signal. The first sampling circuit receives the cyclic output signal and samples the cyclic output signal. The first sampling circuit generates a magnitude signal and a second phase signal in response to the cyclic output signal. The second phase signal and the first phase signal being substantially equal.
In a further aspect of the invention, a method for generating a voltage signal using a first inverter circuit and parallel with the second inverter circuit comprising the steps of:
generating a synch signal having a first frequency and a first phase;
continuously counting cycle times of the synch signal in a first inverter circuit;
sampling a first cycle time from the cycle times at a first sample moment;
calculating a first phase angle in response to the first cycle time;
generating an output signal from a first inverter circuit having the first phase angle;
counting cycle times of the synch signal in a second inverter circuit;
sampling a second cycle time at a second sampling moment;
calculating a second phase angle in response to said second cycle time;
generating a second output signal from said second inverter circuit having a second phase wherein said second phase angle is substantially equal to said first phase angle.
One advantage of the invention is that a sawtooth generator may be used to generate the cyclic output signal that represents the phase angle. This eliminates the usually strict requirement that the updating rate of each of the voltage source inverters must be the same and that the updating cycle must be synchronized for the phase angle synchronization among the voltage source inverters. Another advantage of the invention is that the fundamental operating frequency and the sampling frequency of the microprocessor do not need strict synchronization. Another advantage of the invention is that no analog-to-digital conversion is required between the microcontroller and the sawtooth wave generator. Yet another advantage of the invention is that the synchronous signal source is digital which has much better noise immunity than analog signals. Also, the digital signal is based on a relatively low frequency and thus the bandwidth requirements for transmission among the parallel voltage source inverters is low.
In combination, the advantages result in increased reliability, robustness and better performance of the voltage source.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.
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Bhavaraju Vijay
Deng Doug D.
Wang Kon-King Mike
Ford Global Technologies Inc.
Nguyen Matthew
Sparschu Mark S.
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