Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Patent
1996-10-09
1997-12-23
Sterrett, Jeffrey L.
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
363 51, H02J 336
Patent
active
057012415
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method for recovery of transmitted power in an installation for transmission of high-voltage direct current after a transient short circuit-type fault on the dc connection of the installation or in ac networks connected to the installation, and to a device for carrying out the method.
The device comprises a signal-forming member for forming a state signal, indicating that a short circuit-type fault has occurred.
BACKGROUND OF THE INVENTION
An installation for transmission of high-voltage current between two ac networks usually comprises two converters, each one on its ac side being connected to a respective one of the ac networks, as well as a dc connection which connects the direct-current terminals of one of the converters to the corresponding direct-voltage terminals on the other converter. The dc connection may be in the form of an overhead line and/or a cable and may also, in part, consist of ground or water instead of a metallic conductor. In certain cases, the converters are erected in the immediate vicinity of each other, so-called back-to-back erection, in which case the dc connection may consist of short busbars. During normal operation, one of the converters, hereinafter referred to as the rectifier, operates in rectifier operation, and the other converter, hereinafter referred to as the inverter, operates in inverter operation. The inverter is usually controlled to a maximum direct voltage suitable for the operating conditions of the installation, whereas the rectifier is then controlled in such a way that the direct current and hence the transmitted active power remain at a desired value. For this purpose, a current regulator is activated in the control system of the rectifier, which current regulator, in dependence on a comparison between a current order and a measured value of the actual direct current, controls the voltage of the rectifier such that the difference between ordered and actual current approaches zero. The current order is generated in a calculating member as the quotient between a power order supplied to this member and a measured value of the direct voltage at the rectifier. The measured direct voltage contains a certain measure of harmonics and also other noise and therefore it is filtered in a filter of low-pass character before its value is supplied to the above-mentioned calculating member. For a first-order filter with a transfer function corresponding to a single time constant, the time constant may typically be of the order of magnitude of one second.
Both the ac networks and the dc connection are subjected to short circuit-type faults or to single- or multi-phase ground faults. These faults lead to the voltage of the installation breaking down or at least significantly dropping, whereby the dc voltage of the converters is reduced to a voltage which essentially is zero. This also results in a reduction of the transmitted power to essentially zero. The above-mentioned faults are often of a rapidly changing nature, typically of a duration of the order of magnitude of <0.5 seconds, and it is desired and also often specifically required by the installation operator to be able to rapidly return the installation to the operating condition it had before the occurrence of the fault. It is a known problem, however, that the time it takes to recover the power in this kind of installations depends at the level of the direct current on the occurrence of the fault in such a way that lower current levels entail longer times for recovery of the power. This problem is further accentuated when the dc connection comprises long cables which possess a high capacitance.
SUMMARY OF THE INVENTION
The invention aims to provide a method of the kind described in the introductory part of the description, which permits a considerably shorter time for recovery of power in the installation, and a device for carrying out the method.
What characterizes a method and a device according to the invention will become clear from the appended clai
REFERENCES:
patent: 3992659 (1976-11-01), Ekstrom
patent: 4639848 (1987-01-01), Sakai
patent: 4641042 (1987-02-01), Miyazawa
patent: 4727467 (1988-02-01), Bendl et al.
patent: 4739464 (1988-04-01), Nishihiro et al.
Kimbark, Direct Current Transmission, vol. I, 1971, Wiley-Interscience, New York, Clearing Line Faults and Reenergizing the Line, Chapter 7-6, pp. 272, 279.
Dofnas Lars
Hyttinen Mats
Asea Brown Boveri AB
Sterrett Jeffrey L.
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