Method of measurement for locating line faults on HVDC lines

TECHNICAL FIELD

When a fault occurs on a power transmission line between two stations, independently of whether it is a question of an ac line or of a high-voltage direct current line, that is, a so-called HVDC line, it is of great value to rapidly determine the distance from any of or both of the stations to the fault location. For this purpose, so-called line fault locators, known under the generally accepted abbreviation LFL, are provided.

Published methods for such fault-distance determination deal predominantly with fault-distance determination on ac lines. A summary of and a short description of methods and measuring principles for this purpose are given, inter alia, in an article "Fault Location Methods for Overhead Lines", published in Trans AIEE, 7G (1957), part III, pp. 518-530. This article describes a measuring principle which comprises detection of a fault and evaluation of the distance to the fault as a travelling wave time principle. This measuring principle is also used in connection with the present invention.

When a fault occurs on a line, a transient occurs between the states prior to and after the fault in the form of travelling waves which propagate from the fault in respective directions towards the two stations. There are different ways of measuring the propagation times of the travelling waves in order to locate the fault. Since this invention relates to a method of measurement for these travelling waves, the actual evaluation method for the fault-distance determination will be dealt with very briefly.

A very common method, which is described in an article "Automatic Fault Locator for Bonneville Power Administration" by D.J. Marihart and N.W. Haagenson, published as Conference Paper, IEEE, PES, July 1972, is to determine the difference in time between the arrival of the travelling waves at the two stations. With knowledge of this difference in time, it is, in principle, relatively simple to obtain a fault-distance determination. The direct measurement of the difference in time may be performed in different ways but nowadays normally takes place with the aid of synchronized clocks at the two stations. Variants of such determination are also described in US 3,609,533, "Fault Location System For Determining Distance of Fault on a Transmission Line From a Predetermined Position Thereon" . During fault- distance determination while determining the time at the two stations, it is sufficient to detect either the voltage or the current which is associated with the travelling waves.

To eliminate the communication between the two stations which is necessary according to the above-mentioned method, the fault-distance determination may also be performed by measuring the travelling wave, or waves reflected by the travelling wave, at one of the stations only. To be able to determine the fault distance, however, this method requires determination of both the current and the voltage which are associated with the travelling wave.

The references and methods mentioned above relate to fault- distance determination on ac lines, where, for measurement of the current and the voltage which are associated with these waves, current and voltage transformers are used.

Using travelling waves for fault determination on dc lines is also fully possible, and an article entitled "Development and Field-Data Evaluation of Single-End Fault Locator For Two Terminal HVDC Transmission Lines" by M. Ando , E.O. Schweitzer and R.A. Baker, published in IEEE, Vol.Pas-104, No. 12, Dec. 1985, pp. 3524-3537, describes a method for fault-distance determination based on measurement at one station of both the current and the voltage which are associated with both travelling waves and reflected waves. The article shows that measurement of the current is per- formed via shunt resistances and that the voltage is measured with the aid of "compensated" voltage dividers.

The above-mentioned invention deals with determination of travelling waves on HVDC lines and relates to a method wherein the difference in time is determined between the point where travelling waves from a fault reach the end stations of the line. It is thus assumed that the time determination takes place with synchronous clocks and communication between the two stations. According to the above, this measuring and determination principle means that only that voltage or that current which is associated with the travelling wave is needed for the fault-distance determination .

One way of indirectly measuring the voltage which is associated with the travelling way is clear from the accompanying Figure 1. A converter station 1 feeds an HVDC line 2 on which a fault has occurred at F. In the station, a capacitor 3 is connected between the HVDC line and ground. For measuring the current through the capacitor, a current pulse transformer 4 is connected between the capacitor and ground. The signal S which is generated in this current pulse transformer thus corresponds to the derivative of the voltage at the travelling wave. With the aid of a corresponding device in the receiver station and synchronized clocks, the difference in time between the arrival of the signals at the stations may be obtained and the fault distance be determined.

In connection with HVDC, it may occur that the power transmission takes place via two or more parallel-connected HVDC lines, exemplified as 2a and 2b in Figure 2. If the same measuring principle as shown in Figure 1 should be used also for parallel-connected two- or multi-line transmissions, the measurement would not function owing to the fact that the voltage and the voltage derivative, respectively, of the travelling waves would cancel each other across the capacitor since the voltage wave, via capacitive coupling between the lines, will be transmitted on the other line, although with reverse polarity. To obtain an adequate measure of the travelling wave, according to the invention the current which is associated with the travelling wave is instead determined. Also the current will be transmitted via inductive coupling to the other line, but these two will concur at the points of connection. The current measurement may be performed in different ways, for example according to the method described above via a shunt resistance, with the aid of dc instrument transformers 5 and 6, so-called DCCT, on each parallel-connected line according to Figure 2 or via a Rogowski coil. Both during current measurement via a shunt resistance and DCCT, the direct current which flows in each line will be continuously measured. To identify the current which is associated with a travelling wave, a filtering is performed such that only signals from the shunt resistance and each DCCT which are considered to originate from a travelling wave, and have a frequency above a certain preset value, are detected for further evaluation of the fault distance. In addition, it is assumed that the line fault protection devices of the lines have identified a line fault.