Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of ground fault indication
Patent
1996-03-01
1998-06-09
Karlsen, Ernest F.
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of ground fault indication
324522, 324543, 361 88, G01R 3114
Patent
active
057640645
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 of application No. PCT/SE94/00892, filed Sep. 28, 1994.
TECHNICAL FIELD
For protection of cables and overhead lines in a power network, normally so-called distance protections are used. These protections are often based on the impedance principle, that is, they indicate a fault when the quotient of voltage and current measured at a measuring point is lower than a pre-set value. Besides being able to indicate faults and hence initiate disconnection of the phase or phases which is or are faulted, these protections also have other properties which extend the protective effect, such as directional and reach properties. Furthermore, they can be used for calculating the distance from a measuring station to a possible fault and can also calculate the magnitude of the fault resistance.
BACKGROUND ART, DISCUSSION OF THE PROBLEM
When a fault has been indicated, it is desirable to be able to take different measures depending on the type of fault, for example if it is a single-phase or a multi-phase fault. If it is known that it is a single-phase fault, normally the faulty phase is disconnected and a new connection is made after a short while, for example after half a second. The reason for doing so is that a single-phase fault is often a fault of a very short duration. Therefore, this method entails a minimum of disturbances on the power network.
Now, however, even if it really is a question of a single-phase fault, it is possible, because of zero-sequence currents and other current supply to the non-faulted phases, that the protections for the other phases also indicate a fault which leads to tripping of all the phases. This, in turn, leads to unnecessarily heavy disturbances on the power network.
When studying the occurrences of faults, it has turned out that 75-85% of all faults in a power network originate from single-phase faults. It would therefore be of great value to be able to determine in a very fast manner whether a fault which has occurred on a power network is a single-phase fault or not. The present invention permits a possibility of determining, as soon as a fault has occurred, whether the fault is single-phase fault. This, in turn, means that unnecessary disconnection of more phases than the faulty one can be avoided.
To be able to place the invention in its proper context and also to be able to demonstrate the value of the invention, a relatively detailed description of the state of the art as regards the use of a distance protection as a fault locator, and of the problems which are associated with the current technique within this field, will first be given.
The basic criterion for tripping of a power transmission line, in which a distance protection based on the impedance principle is used, is based on a check whether aid of phase voltage U.sub.A and phase current I.sub.A is smaller than a preset value Z.sub.in. This check can suitably be performed with a common underimpedance relay with a setting Z.sub.in lower than the normal load impedance.
When the distance protection is to be used as a fault locator, however, a considerable extension of the basic concept is required to achieve the desired accuracy and speed in the evaluation when a fault has been indicated on the power transmission line.
Most fault locators are based on measuring the reactance between a short-circuit and that end of the power transmission line where the fault locator is placed. However, the accuracy in the distance calculation is influenced by the fault resistance. The reason for this is that the current which flows through the fault resistance is somewhat offset in phase in relation to the phase position of the current measured at the end of the power transmission line. This means that the fault resistance is interpreted as an apparent impedance with one resistive and one reactive component. It is this reactive component which gives rise to the inaccuracy or the fault in the distance calculation since it influences the measured reactance.
The principles of fault location and calculation of faul
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Eriksson, Leif, "An Accurate Fault Locator with Compensation for Apparent Reactance in the Fault Resistance Resulting from Remote-End Infeed", IEEE Trans. on PAS, V.104, N.2, p. 427, Feb. 1985.
Eriksson Leif
Saha Murari Mohan
Asea Brown Boveri AB
Karlsen Ernest F.
Valone Thomas
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