Electricity: electrical systems and devices – Safety and protection of systems and devices – Ground fault protection
Patent
1998-04-28
1999-08-31
Gaffin, Jeffrey
Electricity: electrical systems and devices
Safety and protection of systems and devices
Ground fault protection
361 71, 361 72, 361 86, H02H 300
Patent
active
059461721
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an installation for power transmission by means of high-voltage direct current (HVDC installation) comprising two converter stations inter-connected by means of a dc line, wherein at least one station has a line protection device adapted to fault is that the line voltage falls below a predetermined level during a predetermined first time interval, and the voltage and the current of the line and, thereafter, to make a restart attempt.
BACKGROUND OF THE INVENTION
An HVDC installation usually has two converter stations. The converters in one station normally operate as rectifiers and the converters in the other station as inverters. If a ground fault occurs on the dc line in such an installation, this is detected by a line protection device. Such protection devices are well known. A typical line protection device makes use of two criteria for detection of a ground fault. The derivative part of the line protection device indicates a ground fault if the direct voltage decreases to a low level but at a certain speed (possibly it is also required that the direct current increases). The level part of the protection device indicates a ground fault if the direct voltage falls below a low limit value for a certain minimum period of time. This time has to be chosen so low, for example 150-300 ms, that unjustified ground-fault indications are not obtained during normal switching processes or during such other disturbances, for example loss of the alternating voltage of one of the stations, which also give rise to a low direct voltage.
When the line protection device indicates a ground fault on the dc line, the rectifier is controlled in a known manner to inverter operation, and the current and voltage of the line are reduced to zero. After a certain time, a restart attempt is then made. This time is so chosen (e.g. 100-200 ms) that arcs which occur during temporary ground faults on an overhead line should have had time to become extinguished and deionized. During a restart attempt, the voltage and current of the line are increased. If the fault no longer remains, normal operation is resumed. If the fault remains, the protection device reacts again. Usually, a certain number of restart attempts are made, whereupon, in case of a remaining fault, the operation of the installation (or the pole concerned) is interrupted.
Installations of the above kind are previously known from, for example, 1975, e.g. p. 221 1965, e.g. pp. 131-132 173-175
A dc line in an HVDC installation may comprise an overhead line or a cable or, perhaps more often, partially an overhead line and partially a cable. Ground faults on an overhead line are often temporary. In the event of such faults, it is desirable to resume normal operation as quickly as possible, and this can be done with minimum disturbance of the operation in the manner described above. Ground faults on a cable, on the other hand, are normally permanent. Making one or more restart attempts during such a ground fault is ineffective. The restart attempt or attempts only delay(s) the definite interruption of the operation. Further, during the restart attempts repeated high currents are fed through the location of the fault, each time for a relatively long time (such a long time that the line protection device has time to trip). This entails, on the one hand, a risk of additional damage of the cable, and on the other hand, heavy disturbances in alternating-voltage networks connected to the installation, and especially in weak networks.
SUMMARY OF THE INVENTION
The object of the invention is to provide an installation of the kind described above, which makes possible a rapid and reliable detection of permanent low-impedance ground faults on the dc line, and, in particular, of cable faults, whereby unnecessary restart attempts with the drawbacks mentioned above can be avoided.
What characterizes an installation according to the invention will become clear from the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will
REFERENCES:
patent: 5303112 (1994-04-01), Zulaski et al.
Uhlmann, Power Transmission by Direct Current, Berlin Heidelberg New York, 1975, p. 221.
Cory et al., High Voltage Direct Current Convertors and Systems, London 1965, pp. 131-132.
Arrilaga, High Voltage Direct Current Transmission, Peter Peregrinus Ltd. (7.3-7.12), London 1983, pp. 173-175.
Kimbark, Direct Current Transmission, vol. 1 Wiley Interscience, 1971, pp. 272-276.
Hansson Erik
Hyttinen Mats
Miram Per
Area Brown Boveri AB
Gaffin Jeffrey
Sherry Michael J.
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