METHOD AND DEVICE FOR PREVENTING OVERSTABILIZATION OF LONGITUDINAL DIFFERENTIAL PROTECTION IN CASE OF INTERNAL FAULT

Abstract: A method and a device for preventing overstabilization of longitudinal differential protections in case of internal faults on power lines, which may take place when a fault situation, which is indicated as an external fault, is in reality an internal fault. The invention comprises criteria which finally provide information as to whether the longitudinal differential protection is to enter into operation. The criteria comprise level and directional determination of the currents which are measured in the terminals of the power lines, checking whether these have the same direction, and so on.

Description: Method and device for preventing overstabilization of longitudinal differential protection in case of internal fault TECHNICAL FIELD A differential protection is a type of protection used for preventing damage which may arise in connection with inter- nal faults on busbars, generators, motors, transformers, power lines, etc. The requirements on the protection are somewhat different depending on the object to be protected. An unjustified protective function must be prevented and this can be done by stabilizing the protection. This means that attempts are made to make the protection unsusceptible to certain internal and external electrical transients and to external electrical faults. It is necessary, for example, to prevent that inrush current surges, overmagnetization currents, continuous currents, saturation of current transformers, etc., lead to an unjustified protective function. The present invention relates to a special stabilizing problem which may arise when the differential protection is to protect against internal faults on power lines. Within the technical field in question, such a differential pro¬ tection is referred to as a longitudinal differential pro¬ tection. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a summary of the principle disclosed in the prior art concerning stabilization of a longitudinal differential protection. Figures 2, 3, 4 and 5 show different fault situations which may arise in a T-network. Figure 6 shows a combined flow chart and block diagram according to the invention for supplementation of the stabilization measures for a longitudinal differential protection. Figure 7 shows how the supplementation constitutes part of the stabilization measures of the longitudinal differential protection. BACKGROUND ART, THE PROBLEMS A differential protection is a type of protection which by means of current transformers measures and via a differen¬ tial connection compares incoming and outgoing current in the protective object. The intention is that, when a differential current arises which exceeds a set value, which shall normally indicate that a fault has arisen in the protective object, then the protection is to become activated. As mentioned, however, certain electrical transients both in the protective object and outside the protective object, as well as external electrical faults, may influence the protective function in such a way that the protection interprets the transient or the fault as a fault in the protective object and activates the protection. To prevent this, the protection is stabilized. Differential protections are often designed such that two different circuits can be distinguished, namely, a differen¬ tial circuit and a stabilizing circuit. The protection functions such that the difference current, I D - which in case of an internal fault arises between incoming and out¬ going currents, can be separated with the aid of the diffe¬ rential protection. The current which is used to stabilize the protection, Is, emanates in some way from the stabilizing circuit and it may be different for different protections, depending on what is to be stabilized against and on the technique used as well as the design of the protection in other respects. For example, the stabilizing current may be a reflection of the incoming current, obtained by means of a current trans¬ former, for busbars, for example, the sum of reflected incoming currents, any of its components, or the like. The function characteristic of a differential protection constitutes a graphic representation of the differential current as a function of stabilizing current, degree of stabilization S and function margin K, which is often expressed as ι D = S-is + K An example of the prior art regarding differential circuit and stabilizing circuit is described in SE 338 093, which relates to a differential protection for busbars. Here, incoming and outgoing currents are compared in accordance with the differential protection principle. Via a resistance • combination in a stabilizing circuit, the reflected currents give rise to a stabilizing voltage which, in case of current identity between incoming and outgoing currents, stabilizes the protection and prevents unjustified operation. In case of current difference, a differential current is formed which, via a resistance combination in the differential circuit, provides a differential voltage. When this voltage, with a certain margin, becomes greater than the stabilizing voltage, the protection enters into operation. SE 379 260 describes a longitudinal differential protection for a power line with two or more connected terminals. The protection in each one of the terminals comprises an output relay, a circuit which provides a stabilizing voltage and a differential circuit which provides a voltage in a tripping direction. Both the stabilizing circuit and the differential circuit consist of linear resistors. Otherwise, the sum of the currents arising in the differential circuits of all the terminals is equal to the sum of the product of the stabili- zing degree of the protection and the sum of the magnitudes of all the incoming currents, as well as a constant which depends substantially on the functional value of the above- mentioned output relay, that is, a direct parallel to the function characteristic of a differential protection accor¬ ding to the above. US 4,855,861 describes a longitudinal differential protec¬ tion based on the use of numerical technique and computer- controlled systems. To be able to determine if there is a fault within any line zone, also in this case it is a question of studying two situations, namely, whether all the currents are directed out from or inwards towards the rele¬ vant zone. In these two cases a fault is located within the relevant zone. Known problems in connection with differential protections is that they exhibit deficiencies as regards the stability in case of external faults and in case of networks with a large short-circuit power. In addition, false difference currents may arise in case of external faults owing to the fact that some of the current transformers becomes satura¬ ted. However, current transformer saturation may be detected in accordance with several known methods (see, inter alia, SE 9100917-5) . Information that saturation has occurred may be used for stabilization of a differential protection, which, however, entails a certain risk of overstabilization in case of an internal fault. A power line network may be fed from several voltage sources and, therefore, a longitudinal differential protection must receive current information from a corresponding number of terminals. A method for determining the direction to a fault which is based on a comparison of the polarity of the currents in the terminals is described in an article "Elektronischer Sammelschienenschutz" , published in Brown Boveri Mitt., 1966, No. 4/5, pages 3-16. This method can be used to avoid overstabilization if the polarity of the currents is such that all the currents flow towards an internal fault in the line network. A summary of the principle disclosed in the prior art regar- ding stabilization of longitudinal differential protections, as it has been described, is given in Figure 1. The terminal currents I and I n are conducted to a differential circuit 1, to a stabilizing circuit 2, to a current transformer saturation detector 3 as well as to a current directional detector 4. From the differential circuit there is obtained a differential current I D =∑I OC with α= l...n, from the sta¬ bilizing circuit there is obtained a stabilizing current Is=∑|lβl with β = l...n, from the saturation detector there is obtained a signal "s" when any of the current transformers becomes saturated, and from the directional detector there is obtained a signal "d" when the terminal currents have the same direction, that is, when it is a question of an inter¬ nal fault. To the function unit 5 of the longitudinal diffe¬ rential protection, which unit is to carry out the function characteristic I D = S-Is + K, there are conducted the diffe¬ rential current ID, the stabilizing current Is. and, via a contact pair ki and k 2 adapted such that either the first or the second contact is closed, a value Si or S 2 of the stabi¬ lizing degree S which is dependent on the output signals of the saturation detector and the directional detector, as well as a value of the relay constant K. Which of the con¬ tacts ki and k 2 of the contact pair is connected is deter¬ mined by a third contact k 3 , the closed/open state of which is dependent on the output signal of the directional detec- tor in such a way that it is opened only when the directio¬ nal detector signals an internal fault. The stabilization operates as follows: The contacts ki and k 3 are always closed in a normal and faultless state. The stabilizing degree Si is then part of the function charac¬ teristic. If the saturation detector signals saturation of any of the current transformers and no internal fault has been signalled, the stabilizing degree Si is disconnected and the stabilizing degree S 2 is connected in order to be part of the function characteristic. Irrespective of whether saturation has been signalled or not, however, the stabili¬ zing degree of the function characteristic will return to Si when an internal fault has been signalled. However, the described method according to the state of the art has the disadvantage that the directional information may be ambiguous, which can specially occur if the impedance ■ between two terminals is lower via an external path than internally via an internal nodal point. To illustrate this, a number of fault situations will be analyzed. A power transmission network connected according to Figure 2, that is, in the form of a so-called T-network, with a nodal point at A, is supplied from three different voltage sources Gl, G2 and G3 and is provided with one terminal at each line end at Tl, T2 and T3. If a fault occurs at F on the line distance between T2 and the nodal point A, the currents which are recorded in the respective terminals, that is, II, 12 and 13 will all be directed towards the location of the fault. The same current variation will occur independently of in which of the T-branches a fault occurs. Also if any of the current transformers of the terminals would become saturated, the currents in case of a fault in any of the T-branches between the terminals will exhibit the same current direction. Such a fault is clearly and unam¬ biguously an internal fault and the longitudinal differen¬ tial protection is to enter into operation. If a fault B occurs in a power network according to Figure 3 between the terminal T2 and the supply source G2, the current 12 will be oppositely directed relative to the currents II and 13. This is therefore a criterion of an external fault according to the prior art and none of the longitudinal differential protections in the three terminals is to enter into operation. The direction of the current is, of course, independent of whether any of the current trans¬ formers becomes saturated. If the power transmission network has a line connected in parallel between terminals T2 and T3 and the supply from G3 is small or insignificant while at the same time a fault F occurs relatively near the terminal T2, however between T2 and the nodal point A, that is, a situation according to Figure 4, the current 13 flows in the opposite direction relative to the currents 11 and 12. With known criteria for an external and an internal fault, respectively, this fault is an external fault for which the longitudinal differential protections are to be stabilized, although in reality it is an internal fault. This means that the longitudinal diffe- rential protections are overstabilized for such a fault situation. If, using the same network configuration, a fault occurs between the terminal T2 and the nodal point A, however at a point further along the line counting from T2 (see Figure 5) , all the terminal currents will be directed in the same direction. The fault situation will therefore be identified as an internal fault and the longitudinal differential pro¬ tections enter into operation. The ambiguity regarding the function of a longitudinal differential protection depending on the location of the fault, which is clear from the above, is a problem as such and may lead to overstabilization in case of an internal fault and entail serious consequences. SUMMARY OF THE INVENTION A method and a device according to the invention constitute a supplementation of the method and the device for stabili¬ zation of a longitudinal differential protection according to the prior art. This supplementation eliminates the risk of overstabilization in case of an internal fault, due to the ambiguity which may arise in certain situations, as described above, regarding the determination as to whether a fault is an external fault or an internal fault. If it is a question of an external fault, a longitudinal differential protection is to be stabilized for this, that is, the pro¬ tection shall not enter into operation. As described above, certain situations may arise where, according to the current direction criterion, it appears to be a question of an external fault whereas in reality it is an internal fault, which is not to be stabilized. When, according to the criteria of the invention, it is determined that a fault which is interpreted as an external fault in reality is an internal fault, the stabilization actions are to be dis¬ connected, whereby the protection can enter into operation. A simple fault situation is always connected with a current increase. It is suitable to take into account only those currents of the terminals which exceed a certain minimum fault current level, k-I r , where I r is the rated current of the terminals. In this way the advantage is obtained that terminals which provide a small or no contribution to the fault current, and which hence reasonably have a somewhat more uncertain possibility of a correct current direction determination, are excluded from further processing in the stabilization process. On the other hand, the value of k must be selected lower than the greatest current that may be measured without current transformer saturation occurring. To sum up, the supplementation comprises the following steps: checking by means of a first criterion whether at least two of the terminal currents have a value greater than k-l r and then determining the current direction for these currents, and checking by means of a second criterion whether the current ' directions are different, and if the fault situation is interpreted as an external fault, finding out the level I B of that terminal current which has a direction opposite to that of the other terminal currents, and also finding out the level I A of the greatest of the other currents, and checking by means of a third criterion whether IB is smaller than a certain part of I A - If this is the case, the fault situation is interpreted as an internal fault which is to be acted upon. In the opposite case, the fault situation is interpreted as an external fault for which the longitudinal differential protection is not to be stabilized. What distinguishes the invention from the prior art is, thus, that instead of only directional detection according to 4 in Figure 1, a supplementation has been introduced which comprises a number of criteria which include level detection, directional detection, control of current direc¬ tion, maximum value determination, and a relative comparison between the terminal currents. When the supplementation device signals an internal fault, this signal shall replace the signal "d" in Figure 1 and disconnect a possible saturation signal "s", whereby the stabilizing degree Si will be the one that determines the function characteristic. DESCRIPTION OF THE PREFERRED EMBODIMENTS The supplementation of the existing stabilization measures, as described under the summary of the invention, means that that part which comprises the directional detection 4 accor- ding to Figure 1 is replaced by a number of criteria which, however, comprise, inter alia, a directional detection. A supplementation device for carrying out the described method according to the invention may have a plurality of different embodiments built up of analog and/or digital components which together may constitute a more or less integrated design. An embodiment KA of the supplementation device according to the invention will be described in greater detail with reference to a combined flow chart and block diagram according to Figure 6. All the terminal currents are conducted to a level detector unit and a first selector 6 for deciding upon the first criterion. If at least two of the terminal currents have a value which is greater than k-I r , according to this crite- rion a fault situation exists. If this is the case, a sepa¬ ration of positive and negative currents for each half-cycle takes place in a separation unit 7, that is, a determination of the current direction for the two or more currents which are produced from the first criterion. The information about the current directions is input information to a second selector 8 for determination of the direction, which determination constitutes the second criterion of the supplementation. Here it is asked whether the current directions are different. If the current directions are not different, the fault situation is an internal fault which is to be acted upon in the usual way according to the state of the art, that is, the stabilizing degree Si according to Figure 1 is to be determining for the function characteristic. If any of the terminal currents has a current direction opposite to that of the other terminal currents, that is, that it would entail an external fault according to the state of the art, the supplementation according to the invention comprising first finding out, in a level deter¬ mination unit 9, the level I B of that terminal current which has the opposite direction and also finding out the level I A of the greatest of the other terminal currents. Further, the supplementation comprises utilizing this information to find out, as the third criterion in a third selector 10, if the level I B of the terminal current which has an opposite direction is lower than a certain part "f" of the level of the greatest of the other terminal currents, that is, if |I B I < f-|IAI• The value of "f" is selected to correspond to the current distribution at the nodal point nearest the terminal which has a deviating current direction. The current distribution is dependent on the geographical con¬ ditions and the impedances of the power network and will normally be between 0.25 and 0.5. If the answer is YES, that is, when the supplementation device 11 interprets the fault situation as an internal fault, a signal "i" is obtained which is to replace the signal "d" according to Figure 1. If the answer is NO, the fault situation is to be interpreted as an external fault, and the stabilizing degree S 2 accor¬ ding to the prior art and Figure 1 is to be connected into the function unit 5. Figure 7 shows how the supplementation device 11, KA, accor¬ ding to Figure 6 is integrated into the other stabilization measures by replacing the directional detection 4 according to Figure 1. When, according to the invention, an internal fault is signalled, a possible signal "s" indicating a satu¬ rated transformer is to be disconnected in the usual way according to the prior art, and the stabilizing degree Si according to Figure 1 is to be determining for the function characteristic. In a preferred embodiment, the following is set: k = 1.5 and 5 = 0.5. In an embodiment used in practice, the supplementation device comprises input interface with analog-to-digital conversion, filters, etc.

Claims: CLAIMS 1. A method for preventing overstabilization of longitudinal differential protections in case of internal faults on power lines which are each provided with a terminal with rated current I r where the terminal current in question is measured and the values II, ... In of all terminal currents are supplied to the longitudinal differential protection, ' which method constitutes a supplementation of the other stabilization measures for the longitudinal differential protection and which is characterized by three criteria which together determine whether a fault situation is to be interpreted as an internal fault, wherein the first criterion is fulfilled when at least two of the terminal currents, the value of which is continuously sensed by a level detector, have a value which exceeds a minimum fault current level k-I r and that the direction of these terminal currents is determined, the second criterion comprising checking if the fault situation is interpreted as an exter- nal fault because the directions of these terminal currents are different, and, if this is the case, determining the level I B of the terminal current which has a direction opposite to that of the other terminal currents as well as the level I A of the greatest of the other terminal currents, and the third criterion comprising investigating whether |I B I < f-|I A ! and, if this is the case, the fault situation is to be interpreted as an internal fault which requires action. 2. A method for preventing overstabilization of longitudinal differential protections in case of internal faults on power lines according to claim 1 and which is characterized in that the factor "f" in the smallest fault current level k-I r is set at 1.5 and that the factor "f" in the inequality |I B | < f-|l l s set at 0.5. 3. A device for carrying out the method according to claim 1 for preventing overstabilization of longitudinal differen¬ tial protections in case of internal faults on power lines which are each provided with a terminal with rated current I r g for measuring the terminal current in question and where the values II, ... In of all terminal currents are arranged as input quantities to the longitudinal differential protec- tion and wherein the device is intended to constitute a supplementation of the other stabilization measures for longitudinal differential protections, which device is characterized by determining with the aid of a level detector and a first selector (9) whether at least two of the terminal currents have a value which exceeds a minimum fault current level k-I r and, if this is the case, causing a separation and direction determining unit (7) to determine the direction of these currents by means of separation of the positive and negative half-cycles of these currents, whereafter a second selector (8) is adapted to determine whether the current directions are different and, if this is the case, a level determination unit (9) is adapted to determine the level IB of the terminal current which has a direction opposite to the directions of the other terminal currents as well as the level I of the greatest of the other terminal currents, whereupon a third selector (10) is adapted to determine whether |I B I < f-|I A I and, if this is the case, the fault situation is interpreted as an internal fault and the device delivers an action signal "i". 4. A device for preventing overstabilization of longitudinal differential protections in case of internal faults on power lines according to claim 3, characterized i that the smallest fault current level is set at 1.5 and that the factor "f" in the inequality |lβ| < f |I A I is set at 0.5.

Inventors: ERIKSSON LEIF, MOHAN SAHA MURARI, ERIKSSON, LEIF, MOHAN SAHA, MURARI
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1992
Main classification: H02H3
IPC codes: H02H1, H02H3