FLASHOVER BETWEEN POWER LINES SUSPENDED IN PARALLEL

Abstract: The invention relates to a method and a device for detecting a flashover between conductors in power transmission lines of different voltage levels suspended from the same towers and wherein the power lines are included in a power network where one of the power lines is stated to be a high-voltage power line (1) and where the other power lines are connected to the high-voltage line via transformers of a known transformer ratio (am) and internal impedance (ZXm) and wherein the line impendances (ZL) are known. Specific to the invention is that the fault current which is caused by a flashover is set to be equal to a detected current change in any of the phases in the high-voltage line. With this assumption an equation system can be set up with the aid of which the relative distance to fault n (0<n<1) can be determined. If the produced value of n lies within the stated region, this is interpreted as if an internal flashover has occurred.

Description: Flashover between power lines suspended in parallel TECHNICAL FIELD For environmental and cost reasons, particularly in densely populated areas, several power transmission lines with different voltage levels are allowed to be suspended from the same towers. This arrangement will hereinafter be referred to as a multi-circuit transmission line. Each one of the lines included is in the usual manner provided with protective devices in the form of impedance protection, ground-fault protection, differential protection etc. and with devices for fault location. All of these protective devices are based on current and voltage measurement either at one or at the other end or on simultaneous measurement at both ends of the respective line. Since the lines in a multi-circuit system are normally sus¬ pended in such a way that there may be only a few metres between conductors belonging to different voltage levels, factors such as heavy wind, ice formation on the individual conductors, fallen trees and other things may cause these conductors to come into contact with each other. In certain unfortunate cases, the contact between the conductors may last for quite a long time, among other things because of the conductors being welded together or the like. In the following such a fault will be referred to as a flashover fault. If the flashover fault occurs within the protective zone of the protective devices, the fault will be called an intersystem fault. Even if it is a question of a fault of very short duration, an intersystem fault gives rise to changed current and voltage conditions at the fault location and may in certain cases also entail a risk of personal injury. Such faults should, of course, be identified and be acted upon in some way as quickly as possible. However, the above- mentioned protective devices provide no, or insignificant, information as to the occurrence of an intersystem fault. The present invention relates to a method and a device for detection of an intersystem fault in a multi-circuit transmission line. BACKGROUND ART DISCUSSION OF THE PROBLEMS A method for detecting an intersystem fault in the multi¬ circuit transmission line which comprises two lines of different voltages is described in Cigre Conference paper 34-206, Paris 1990, by H Becker, J Nilges and H Dittrich entitled "Distance protection of double-circuit lines with different rated voltages during intersystem faults" . The method is based on the distance protection principle. To be able to selectively detect a short circuit according to the above-mentioned principle, it is necessary, when a short circuit occurs, to have access to correct values of currents and voltages at the measuring station. With the aid of the measured values at the instance of the fault, the circuit impedance, which corresponds to the positive- sequence impedance from the measuring station to the fault location, can be calculated. The method uses two current compensations which comprise the zero-sequence currents in the two lines and the corresponding voltages, which must be available at the same time. To detect an intersystem fault according to this method, it is thus assumed that the two lines can communicate with each other to be able to transmit measured fault quantities between the lines. This is, per se, a source of problems and it would therefore be a great advantage to have access to a method which is able to detect intersystem faults without having to communicate between the different lines. The present invention permits such a solution. BRIEF DESCRIPTION OF THE DRAWING Figure 1 shows a multi-circuit transmission line comprising two lines between which an intersystem fault has occurred. Figure 2 shows a multi-circuit transmission line comprising a plurality of lines between which an intersystem fault has occurred and how a flashover indicating device CALC can be connected to one of the lines. SUMMARY OF THE INVENTION The invention comprises a method for detecting a flashover between the conductors in power transmission lines of different voltage levels suspended from the same towers and wherein the power lines are included in a power network where one of the power lines is stated to be a high-voltage power line (1) and where the other power lines are connected to the high-voltage line via transformers of a known ratio (am) and internal impedance (ZXm) and wherein the line impedances (ZL) are known. The principle of the detection of an intersystem fault in a multi-circuit transmission line will be described starting from a system comprising two lines'according to Figure 1. This figure shows the voltage sources of the lines with the respective source impedances. The line impedances are designated ZL. The phase voltages in line 1 are designated U1R, UlS and UlT and the respective phase currents IlR, IIS and I1T. Further, it is assumed that line 2 receives its voltage via a transformer TR, connected to line 1, with the ratio a2 and an internal impedance ZX2. The phase voltages of line 2 are correspondingly designated U2R, U2S and U2T and the respective phase currents I2R, I2S, I2T. If we start from the assumption that line 1 is that of the two lines which has the highest voltage, then when an inter¬ system fault occurs, for example between phases Tl and R2 as shown in Figure 1, the current IlT before the fault will have a considerable fault current addition ΔllT. This fault current addition will then also be superimposed on the current I2R passing in phase R2 before the fault. This circuit is then closed via the transformer. To avoid commu¬ nication between the two lines to include I2R in the cal¬ culations, according to the invention it is assumed that I2R is equal in magnitude to ΔllT. Since the current addition ΔllT is normally also considerably higher than the phase current I2R before the fault, this assumption entails no major fault in the detection method according to the inven¬ tion. In the case of an intersystem fault between phases Tl and R2, the following equations may be set: Δ lT + I2R = 0 (1) U2R = UlR/a2 - I2R-ZX2 (2) UlT - n-ZL-HT + l2R-n-ZL - U2R = 0 (3) where n-ZL is the line impedance from the measuring station to the fault location at F at the relative fault distance n, that is, 0<n<l. The assumption according to equation (1) constitutes the main part of the inventive concept, namely, that the current in line 2 can be estimated without measuring it. Equation (2) shows how the phase voltage of line 2 can be calculated and equation (3) indicates a relationship between the voltage drops between the measuring point for voltage on line 1 and the imaginary measuring point for voltage on line 2. When current and voltage in line 2 have been obtained from equations (1) and (2), these values can be inserted in equation (3), that is, the only remaining quantity "n" can be solved. Now, if the value of "n" obtained in this way lies within the region of operation, that is, 0<n<l, this indicates that the flashover is an intersystem fault which is to lead to tripping of the relevant phases. In general terms, a flashover fault may occur between any of the phases of line 1 and any of the phases of line 2. To- obtain a fully covering protection for the two lines, "n" must therefore be continuously determined for all conceiv¬ able permutations of flashover faults. In transformers connected to a neutral point, ΔllT is allowed to replace both I2R and a possible neutral point current IN. The method for detection of intersystem faults described above can also be directly applied to a multi-circuit transmission line comprising an optional number of parallel lines. This is illustrated in Figure 2 by the lines 1 and 2 according to Figure 1 being supplemented by up to m parallel lines connected to line 1 via separate transformers. It is indicated here that the m'th transformer has a ratio equal to am and an internal impedance equal to ZXm. In a corre- sponding way, as described above, the phase voltages are designated UmR, UmS and UmT and the phase currents ImR, ImS and ImT. As an example of an intersystem fault between several lines, it is shown in Figure 2 that an inter¬ connection has been performed between phase T of line 1 and phase R of line 2 and with phase S of line . The equation system which applies to an optional number of parallel lines will then be as follows: Δllα + I2β + I3γ + ... + Imφ = 0 (4) U2β = Ulβ/a2 - !2β-ZX2 (5) U3γ = Ulγ/a3 - !3γ-ZX3 (5a) Umφ = Ulφ/am - imφ-ZXm (6) ulα - n-ZL-llα + n-ZL-!2β - U2β = 0 (7) Ulα - n-ZL-ilα + n-ZL-i3γ - U3γ = 0 (7a) Ulα - n-ZL-llα + n-ZL-lmφ - Umφ = 0 (8) where α, β, γ andφmay be R-, S- or T-phase. n these applications it is practical to assume line 1 to be the one with the highest voltage, hereinafter referred to as the hig -voltage line. When a flashover occurs, the high- voltage line is practically always involved. To sum up, equations (4) to (8) may be interpreted as follows: Equation (4) gives the relationship between currents on the high-voltage line and other lines of lower voltage. Equations (5), (5a) ... (6) indicate the phase voltage on the lines to which lower voltage is applied. Equations (7), (7a) ... (8) indicate the relationship between the voltage drops between the measuring point for the voltage on the high-voltage line and the imaginary measuring point for the voltage on the other lines. Now, if the number of lines suspended in parallel is "m", the equation system will comprise (2m+l) complex equations which comprise 2m complex unknown variables and the sought variable "n" which can thus be determined. 5 For purely practical reasons, there is a limit to how many et- parallel lines may be suspended from the same towers. In practice, this also limits the number of relevant equations and the permutations which continuously need to be completed 10 to be able to detect a flashover. Since, as mentioned above, the high-voltage line is prac¬ tically always involved when a flashover occurs, it is sufficient to connect to the measuring point for the high- 15 voltage line a flashover indicating device CALC, according to Figure 2, which is prepared with relevant equations as above. The flashover indicating device is fed with the voltages and currents of its own line and includes a possi¬ bility of determining a Δllα-value. In addition, the flash- 20 over indicating device shall be provided with knowledge of the line impedances as well as transformer ratio and impedance of all the lines involved. To reduce time and location for the continuous determination as to whether an "n" value lies within the protective zone, the determination 25 may start when the Δllα-value exceeds a limit value Δiαg, which normally originates from a flashover.

Claims: CLAIMS 1. A method for detecting a flashover between conductors in power transmission lines of different voltage levels suspen¬ ded from the same towers and wherein the power lines are included in a power network where one of the power lines is stated to be a high-voltage power line (1) and where the other power lines are connected to the high-voltage line' via transformers of a known transformer ratio (am) and internal impedance ZXm and wherein the line impedances ZL are known, said method being characterized in that the fault current which is caused by the flashover is set to be equal to a detected current change Δllα of any of the phases in the high-voltage line, whereby the following equation system for an optional number of parallel lines can be set up: Δ lα + I2β + I3γ + + mφ = 0 (4) U2β = Ulβ/a2 - l2β-ZX2 (5 ) U3γ = Ulγ/a3 - I3γ-zx3 (5a) Umφ = Ulφ/am - mφ-ZXm (6 ) Ulα - n-ZL-llα + n-ZL-l2β - U2β = 0 (7 ) Ulα - n-ZL-llα + n-ZL-i3γ - U3γ = 0 (7a) Ulα - n-ZL-llα + n-ZL-imφ - Umφ = 0 (8 ) where α, β, φ and γ may be R-, S- or T-phase and where n-ZL is the line impedance from the measuring station to the fault location F at the relative distance to fault n, whereby n can be determined and, if the value of n obtained in this way lies within 0<n<l, a flashover has occurred between any of the conductors. 2. A method for detecting a flashover between conductors in power transmission lines of different voltage levels suspen¬ ded from the same towers according to claim 1, characterized in that the determination of n is started only when the detected current change Δllα exceeds a pre-set limit value Δiαg. 3. A device for carrying out the method according to claim 1 for detecting a flashover between conductors in power trans¬ mission lines of different voltage levels suspended from the same towers and wherein the power lines are included in a power network where one of the power lines is stated to be a high-voltage power line (1) and where the other power lines are connected to the high-voltage line via transformers of a known transformer ratio (am) and internal impedance ZXm and wherein the line impedances ZL are known, which device is formed as a flashover indicating device (CALC) , characterized in that the phase currents Ilα and phase voltages Ulαof the high-voltage line are arranged as inputs to the flashover indicating device together with information about the ratio (am) and internal impedance ZXm of the transformers as well as the line impedances ZL, that the flashover indicating device is adapted to detect a current change Δllα in any of the phases in the high-voltage line and that the flashover indicating device is provided with an equation system for an optional number of parallel lines according to Δllα + I2β + I3γ + ... + Imφ = 0 (4) U2β = Ulβ/a2 - l2β-ZX2 (5 ) U3γ = Ulγ/a3 - l3γ-ZX3 (5a) Umφ = Ulφ/am - Imφ-ZXm ( 6 ) Ulα - n-ZL-llα + n-ZL-l2β - U2β = 0 (7 ) Ulα - n-ZL-llα + n-ZL-l3γ - U3γ = 0 (7a) Ulα - n-ZL-llα + n-ZL- mφ - Umφ = 0 ( 8 ) where , β, φ and γ may be R-, S- or T-phase and where n-ZL is the line impedance from the measuring station to the fault location (F) at the relative distance to fault n, which can thus be determined,, and that the flashover indicating device is adapted to supply a signal IF which indicates that a flashover has occurred within the protective zone if 0<n<l.

Inventors: ERIKSSON LEIF, MOHAN SAHA MURARI, ERIKSSON, LEIF, MOHAN SAHA, MURARI
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1992
Main classification: G01R31
IPC codes: G01R31, H02H7