Electric power conversion systems – Current conversion – Including automatic or integral protection means
Reexamination Certificate
2000-12-27
2001-10-16
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
Including automatic or integral protection means
C363S095000, C307S087000
Reexamination Certificate
active
06304468
ABSTRACT:
BACKGROUND OF THE INVENTION
A power converter which absorbs or releases electric power in interconnected relation to a power system is described in JP-A-8-66048. In a controller for the converter described in JP-A-8-66048, self commutated operation is performed with respect to a load by switching the control of the converter at the time of the self commutated operation of the converter from current control using a first phase-locked loop (PLL) to voltage control using a second PLL. In a converter described in Japanese Patent Application Laid-open Hei 8-9648 (JP-A-8-9648), an arrangement is provided such that when returning from self commutated operation to grid connected operation, voltage phases and amplitudes of a system voltage and a converter output are determined, and the moment when they coincide with each other, a coincidence signal is imparted to close a circuit breaker, whereupon the converter is temporarily stopped and the operation is resumed, thereby switching over control to grid connected operation.
With the conventional technique described in the aforementioned Japanese Patent Application Laid-open Hei 8-66048 (JP-A-8-66048), at the time of the switching between the self commutated operation and the grid connected operation, cases occur in which controlled variables do not utterly converge, and the controlled variables fluctuate in steps. In addition, there are cases where a converter system stops due to an overcurrent at the time of returning from the self commutated operation to the grid connected operation. For this reason, the converter must be stopped to restore the control when a changeover to the grid connected operation is effected. In addition, with the conventional technique described in the aforementioned JP-A-8-66048, there are cases where an overcurrent occurs at the time of operation of the circuit breaker.
SUMMARY OF THE INVENTION
It has been found out from a study conducted by the present inventors that causes of the overcurrent and the sudden fluctuations of the controlled variables in the above-described conventional art lie in a phase shift occurring in the operation of a switching on/off means such as the circuit breaker upon ascertaining the moment of phase matching, the changeover between a phase detector for grid connected operation and a phase detector for self commutated operation, and the changeover between current control and voltage control.
Accordingly, in the power converter system in accordance with the present invention, the converter is controlled such that the phase of an output voltage of the converter matches the phase of the system voltage, and the circuit breaker is closed so as to avoid the occurrence of an overcurrent. In addition, a common control system is adopted for grid connected operation and self commutated operation so as to avoid sudden fluctuations of the controlled variables.
The power converter system in accordance with the present invention has a controller for adjusting an output phase of the converter so as to eliminate a phase difference between an ac voltage on a power system side of the switching on/off means and an ac voltage on a load side thereof. In the converter in accordance with the present invention has a controller for outputting a signal for opening or closing the switching on/off means in response to the phase difference between the aforementioned ac voltages. Preferably, the controller has a current control system for switching over a current command between grid connected operation and self commutated operation.
Further the power converter system in accordance with the invention has the following features:
(1) A common current control system is used for grid connected operation and self commutated operation.
(2) A feedforward component of the output voltage of the converter is set to a fixed value during self commutated operation.
(3) One (d-axis or q-axis) command value for current control is used as an output of voltage control, the other input deviation for current control is set to zero, and a phase detection signal is used in coordinate transformation in current detection.
(4) The two-phase/three-phase coordinate transformation is self-oscillated.
(5) Integral quantities of a power regulator and an automatic voltage regulator are made to conform to the integral quantity of the regulator being used for control.
(6) The amount of correction of a phase difference &Dgr;&thgr; during phase adjustment is provided with a limit, and the phase is adjusted with an amount of phase correction within an amount corresponding to the portion of frequency variation allowed for the system.
(7) The system-side voltage of the circuit breaker is used as a voltage command value of the voltage regulator, a limiter within the allowable range of the voltage variation which is determined by the system is provided for that value, and an output of the limiter is used as the command value.
(8) The feedforward detection value of the voltage of a converter controller is not fixed during self commutated operation.
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patent: 8009648 (1996-01-01), None
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Futami Motoo
Ichinose Masaya
Kiyofuji Yasuhiro
Maekawa Akira
Suzuki Kazuo
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Sterrett Jeffrey
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