Apparatus for detecting short-circuit for use in bi-polar d.c. transmission system

Abstract: There is disclosed an apparatus for detecting a short-circuit of a bi-polar d.c. transmission system in which one end (B) of a neutral line (6) is ground and the other end (A) thereof is not ground. This short-circuit detecting apparatus is adapted to detect a short-circuit between d.c. transmission lines on conditions that a current (I₆) flowing through the neutral line (6) is below a predetermined value, that a ground current (I g ) at she ground point (B) of the neutral line (6) is below a predetermined value, and that a difference between bi-polar current command values (I dp1 , I dp2 ) is above a predetermined value.

Description: TECHNICAL FIELD OF THE INVENTION This invention relates to an apparatus for detecting a short-circuit in a bi-polar d.c. transmission system. BACKGROUND OF THE INVENTION As one means for linking two a.c. systems, a d.c. transmission system is used because of the merits of cost and characteristic. As such a d.c. transmission system, a bi-polar transmission system with a neutral line (conductor) as shown in FIG. 1 is known. Roughly speaking, the system of Fig. 1 is such that two a.c. systems 20 and 25 are linked through d.c. transmission lines 5 and 7 and a neutral line 6. Two sets of power converters 1 and 3 are provided on the a.c. system 20 side, and two sets of power converters 2 and 4 are provided on the a.c. system 25 side. The a.c. terminals of the power converters 1 and 3 are connected to the first a.c. system 20 respectively through transformers 21 and 23, and the a.c. terminals of the power converters 2 and 4 are connected to the second a.c. system 25 respectively through transformers 22 and 24. The d.c. transmission line 5 constitutes a transmission line of a first pole, the d.c. transmission line 7 constitutes a transmission line of a second pole, and the neutral line 6 constitutes a line common to the both poles. Between the power converters 1 and 2, the first pole comprised of the power converter 1, a d.c. reactor 8, the d.c. transmission line 5, a d.c. reactor 9, the power converter 2, and the neutral line 6 is formed. Further, between the power converters 3 and 4, the second pole comprised of the power converter 3, the neutral line 6, the power converter 4, a d.c. reactor 11, the d.c. transmission line 7, and a d.c. reactor 10 is formed. Respective power converters 1 to 4 can be operated both as a rectifier and as an inverter. Ordinarily, one end B of the neutral line 6 is grounded on the a.c. system 20 side serving as a sending end. The other end A of the neutral line 6 is not grounded. Power converters 1, 2, 3 and 4 are operated and controlled as a rectifier or an inverter by converter control units 12, 13, 14 and 15, respectively. A current command value I dp1 is delivered from a current command output circuit 16 to the converter control units 12 and 13, and a current command value I dp2 is delivered from the current command output circuit 16 to the converter control units 14 and 15. Such a control circuit is described in U.S. Patent No. 4,578,743 as background of the invention. Explanation will be given in connection with a typical operating mode in the d.c. transmission system of the bi-polar configuration shown in FIG. 1 such that the power converters 1 and 3 are caused to be operated as rectifiers and the power converters 2 and 4 are caused to be operated as inverters in order to send a power from the first a.c. system 20 to the second a.c. system 25. In this case, since the voltage on the d.c. transmission line 5 of the first pole is positive, the first pole is called a positive pole depending upon circumstances. Further, since the voltage on the d.c. transmission line 7 of the second pole is negative, the second pole is called a negative pole also depending upon circumstances. In Japan, in order to avoid the problem of an electrical contact, etc. of underground installed equipment (service water pipe, etc.) by a ground current in the case of communication trouble or an earth return path system, the system of grounding one end of the neutral line 6 as described above is used. In other countries, however, there are many instances where the receiving end A is also grounded to constitute an earth return path to thereby omit the neutral line. Assuming now that the power converters 1 and 3 carry out a rectifier operation, and the power converters 2 and 4 carry out an inverter operation, voltage/current characteristics of respective poles are set as shown in FIGS. 2A and 2B. In FIGS. 2A and 2B, the X-axis and the Y-axis represent a d.c. current I d and a d.c. voltage V d , respectively. When it is assumed that the power converters 1 and 3 are respectively operated with constant current characteristics C₁ and C₃ and the power converters 2 and 4 are respectively operated with constant voltage characteristics C₂ and C₄, the operating points of the both poles are intersecting points X₁ and X₂ of the characteristic lines, respectively. As a result, a d.c. current I d1 flows in the first pole, and a d.c. current I d2 flows in the second pole. At this time, a current ΔI d = I d1 - I d2 indicative of a difference therebetween flows in the neutral line 6. It is to be noted that while constant current control values smaller by a current margin ΔI dp than constant current control values of the power converters 1 and 3 in the rectifier operation mode are set at the respective power converters 2 and 4 in the inverter operation mode, the constant current control of the power converters 1 and 3 effectively functions in an ordinary state, but the constant current control function of the power converters 2 and 4 is ineffective. In this instance, consideration is taken such that it is not until the d.c. current value lowers by the current margin ΔI dp that the constant current control of the power converters 2 and 4 becomes effective. Here, at the first pole, for example, in the case where a current varies in a direction of increasing a current command value, the voltage/current characteristic of the power converter 1 of the first pole changes or shifts to the characteristic indicated by broken lines C₁₁ of FIG. 2A. Followed by this, the constant current characteristic of the power converter 2 shifts in a direction of increasing the current command value by the same value as above. A new operating point of the first pole is the intersecting point X₁₁ of the constant current characteristic line C₁₁ and the characteristic line C₂. Let now suppose the case where, at the first pole, the power converter 1 carries out a rectifier operation and the power converter 2 carries out an inverter operation in the same manner as stated above, whereas, at the second pole, the power converter 4 on the a.c. system 25 side carries out a rectifier operation and the power converter 3 on the a.c. system 20 side carries out an inverter operation unlike the above. In this case, at the first pole, a current from the a.c. system 20 side toward the second a.c. system 25 side takes places, and at the second pole, a current from the second a.c. system 25 side toward the first a.c. system 20 side takes place. Namely, the system will be operated by currents in directions different from each other with respect to the two poles. Such an operating state is generally called a power sending back operation. In the power sending back operation, current in different directions take place with respect to the poles. A difference between the transmission powers of the poles serves as an actual transmission power between a.c. systems 20 and 25. This power sending back operation is used for transmitting, between both a.c. systems, a power of a value smaller than the minimum operating powers of the individual power converters for a frequency correction of the a.c. system, or for other reasons. The voltage/current characteristics of respective poles at this time are as shown in FIGS. 3A and 3B, respectively. In FIGS. 3A and 3B, power converters 1 and 4 are operated in conformity with the constant current characteristics C₁ and C₅, and the power converters 2 and 3 are operated in conformity with the constant voltage characteristics C₂ and C₆. Accordingly, the operating points of the poles are the intersecting points X₁ and X₃ of the characteristic curves, respectively. Let now consider the behavior in the case where a short-circuit trouble takes place between d.c. transmission lines 5 and 7 under the state where a power sending back operation is carried out in the circuit of FIG. 1. In FIG. 4, a flow of currents in the case where a short-circuit takes place between the d.c. transmission lines 5 and 7 under the state where the power converters 1 and 4 carry out a rectifier operation and the power converters 2 and 3 carry out an inverter operation is indicated by broken lines. By occurrence of this short-circuit, between the power converters 1 and 3, there is formed a first current loop 18 in which a d.c. current I s1 flows through the power converter 1 (in the rectifier operation mode), the d.c. reactor 8, (a portion of) the d.c. transmission line 5, a short-circuited portion 17, (a portion of) the d.c. transmission line 7, the d.c. reactor 10, and the power converter 3 (in the inverter operation mode); and, between the power converters 4 and 2, there is formed a second current loop 19 in which a d.c. current I s2 flows through the power converter 4 (in the rectifier operation mode), the d.c. reactor 11, (a portion of) the d.c. transmission line 7, the short-circuited portion 17, (a portion of) the d.c. transmission line 5, the d.c. reactor 9, and the power converter 2 (in the inverter operation mode). By formation of these current loops, the operating points of the power converters 1 to 4 are caused to be newly in conformity with the voltage/current characteristics shown in FIGS. 5A and 5B unlike the case of FIGS. 3A and 3B. Namely, in the first current loop 18, since the power converter 1 is operated by the constant current characteristic C₁, and the power converter 3 is operated by the constant voltage characteristic C₆, the operating point X₄ is determined by both characteristics C₁ and C₆. On the other hand, in the second current loop 19, since the power converter 4 is operated in accordance with the constant current characteristic C₅, and the power converter 2 is operated in accordance with the constant voltage characteristic C₂, the operating point X₅ is determined by both characteristics C₂ and C₅. In the case where the transmission powers are equal to each other, i.e., d.c. currents are equal to each other with respect to the both poles before such short-circuit failure takes place, even if a short-circuit as described above takes place on the d.c. transmission lines, there is no change in the voltage/current of the respective operating points of FIGS. 3 and 5, so substantially no change takes place in the currents flowing in the respective d.c. transmission lines. For this reason, a current differential relay and/or a current directional relay for detecting changes in a current flowing direction, etc., generally provided at d.c. transmission lines cannot detect a short-circuit failure as described above. Here, in the case where only a current command value of the first pole is increased, constant current control values of the voltage/current characteristics of the power converters 1 and 2 shift in directions indicated by broken lines C₁₂ and C₂₁ of FIGS. 5A and 5B, respectively. In this case, a new operating point of the first current loop 18 in FIG. 4 changes to X₆ (FIG. 5A). On the contrary, in the second current loop 19, the current command value shifts in an increasing direction as indicated by the broken line C₂₁ in FIG. 5B by the power converter 2. However, the operating point X₅ determined by both characteristics C₂ and C₅ is not changed, so the operating point still remains at the point X₅. In the state of the voltage/current characteristic of FIG. 5B, since it becomes impossible to ensure a current margin ΔI dp of a predetermined value between power converters 4 and 2, which is determined by the characteristic curves C₅ and C₂₁, it becomes easy that constant current control by the power converter 2 in the inverter operation mode is carried out. As a result, interference in control may take place between the power converters 4 and 2. Here, when it is assumed that the current command value of the first pole is increased, the constant current control value indicated by the broken line in FIG. 5B further shifts in a direction indicated by an arrow in FIG. 5B, so the operating point cannot be ensured. Eventually, there is the possibility that the system may be down with respect to both poles, and/or any power converter may be damaged, so its bad influence will be exerted on the a.c. system. Accordingly, it is required to take measures to promptly and securely detect a short-circuit failure between d.c. transmission lines in the case where a power sending back operation is carried out in a bi-polar d.c. transmission system thus to prevent in advance inconveniences such as system down. SUMMARY OF THE INVENTION An object of this invention is to provide an apparatus for detecting a short-circuit in a bi-polar d.c. transmission system, which is capable of securely detecting a short-circuit between transmission lines, occurring in carrying out a power sending back operation in a bi-polar d.c. power transmission system. To achieve the above-mentioned object, in accordance with this invention, there is provided an apparatus for detecting a short-circuit in a bi-polar d.c. transmission system having a first transmission line, a second transmission line and a neutral line, in which one end of a neutral line is grounded and the other end thereof is not grounded and the current in the first transmission line is controlled by a first current command value while the current in the second transmission line is controlled by a second current command value; the short-circuit detecting apparatus comprising: first monitoring means for providing a detection output when the current flowing through the neutral line is below a predetermined value; second monitoring means for providing a detection output when the ground current at the ground point of the neutral line is below a predetermined value; third monitoring means for monitoring the current command values to provide a detection output when the difference between the current command values is above a predetermined value; and a logic circuit responsive to the detection outputs from the first, second and third monitoring means. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: FIG. 1 shows a typical d.c. transmission system of the bi-polar structure; FIGS. 2A and 2B are characteristic diagrams showing an operating point in an ordinary operating state of power converters used in the d.c. transmission system; FIGS. 3A and 3B are characteristic diagrams showing an operating point of power converters in the case where a power sending back operation is carried out; FIG. 4 is a diagram for explaining the flow of d.c. currents in the case where a short-circuit takes place in a d.c. transmission system at the time of a power sending back operation; FIGS. 5A and 5B are characteristic diagrams showing operating points of respective power converters in the case where a short-circuit takes place in a d.c. transmission system at the time of a power sending back operation; FIG. 6 is a block diagram showing an embodiment of an apparatus for detecting a short-circuit according to this invention; FIG. 7 is a diagram showing the flow of currents when a ground fault occurs at the time of a power sending back operation; and FIG. 8 is a block diagram showing a modification of an apparatus for detecting a short-circuit according to this invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 6 is a block diagram showing an embodiment of this invention. Respective elements or components of a power system in this figure are represented by using the same reference numerals as those of FIG. 1. In FIG. 6, power converters 1, 2, 3 and 4 are operated and controlled as rectifiers or inverters by converter control units 12, 13, 14 and 15, respectively. A current command value I dp1 corresponding to a transmission power of the first pole is delivered from a current command output circuit 16 to the converter control units 12 and 13, and a current command value I dp2 corresponding to a transmission power of the second pole is delivered from the current command output circuit 16 to the converter control units 14 and 15. A current I₆ flowing in the neutral line 6 is detected by a current detector 60, and a ground current I g at the ground point B of the neutral line 6 is detected by a current detector 70. Detected outputs I₆ and I g from current detectors 60 and 70 and current command values I dp1 and I dp2 from the current command output circuit 16 are introduced to a short-circuit detecting unit 100. In the short-detecting unit 100, monitor circuits 51, 61 and 71 and an AND gate 80 are provided. The monitor circuit 51 monitors the current command values I dp1 and I dp2 of both poles to output a logic signal "1" when the difference between both command values is above a first predetermined value ΔS dp , i.e., |I dp1 - I dp2 | > ΔS dp . The first predetermined value ΔS dp is a little smaller than a current margin ΔI dp in the d.c. power transmission system, for example, equal to a 70 - 80% of the current margin ΔI dp . The monitor circuit 61 monitors neutral line current I₆ to output a logic signal "1" when that current is below a second predetermined value S₆ (i.e., I₆ < S₆). In addition, the monitor circuit 71 monitors ground current I g to output a logic signal "1" when that current is below a third predetermined value S g (i.e., I g < S g ). The second and third predetermined values S₆ and S g are nearly equal to zero, respectively. Output signals from these monitor circuits 51, 61 and 71 are introduced to the AND gate 80. In the state where current command values I dp1 and I dp2 of the both poles are not equal to each other, the difference therebetween is above the predetermined value (i.e., |I dp1 - I dp2 | > ΔS dp ), and a power sending back operation is carried out, currents I d1 and I d2 flow through routes or paths as shown in FIG. 1. For this reason, a difference current ΔI d = I d1 - I d2 flows in the neutral line 6. Assuming now that a short-circuit takes place between d.c. transmission lines 5 and 7, currents I s1 and I s2 flow through routes or paths as shown in FIG. 4. For this reason, no current flows in the neutral line 6, resulting in the state where relationship expressed as I₆ < S₆ holds. On the other hand, when a ground fault is assumed to take place at the point A at the time of a power sending back operation, as shown in FIG. 7, there are formed a first current loop 41 including the power converter 1, the d.c. reactor 8, the d.c. transmission line 5, the d.c. reactor 9, the power converter 2, the grounding point A, a ground return path 40, the current detector 70, and the ground point B, and a second current loop 42 including the power converter 4, the d.c. reactor 11, the d.c. transmission line 7, the d.c. reactor 10, the power converter 3, the ground point B, the current detector 70, the ground return path 40, and the grounding point A. In the case where a ground fault takes place at the point A as in the above case, while there only results the state where no current flows in the neutral line 6, there is no possibility that the operating points of respective power converters may change. In this case, a difference between currents of the both current loops 41 and 42 flows in the current detector 70, and the relationship expressed as I g > S g holds at this time. For this reason, there is no possibility that the monitor circuit 71 provides a logic "1" output. This ground fault is not a fault to be detected by the detecting apparatus of this invention. Accordingly, in this invention, the monitor circuit 71 is provided in order to detect a short-circuit failure of the d.c. transmission system in a manner to discriminate it from the ground fault. When the current command values are not equal to each other, and the difference between both command values has the relationship expressed as |I dp1 - I dp2 | > ΔS dp , the monitor circuit 51 outputs a logic signal "1". In addition, when a short-circuit fault takes place between d.c. transmission lines 5 and 7 in the state where a power sending back operation is carried out, the neutral line current I₆ ceases to flow, so there results the state where the relationship expressed as I₆ < S₆ (see FIG. 4) holds. Thus, the logical output of the monitor circuit 61 shifts to "1". Since the logical outputs of the monitor circuits 51, 61 and 71 are all "1" as stated above, the logical output of the AND circuit 80 becomes "1". Thus, it is possible to detect a short-circuit failure of the d.c. transmission system. FIG. 8 shows a modification of this invention. In this modification, currents I₁₁, I₁₂, I₂₃ and I₂₄ flowing in respective converters 1, 2, 3 and 4 are detected by current detectors 31, 32, 33 and 34, respectively. This apparatus of the modification further includes a current difference monitor circuit 91 adapted for outputting a logic signal "1" when a current difference ΔI₁ = |I₁₁ - I₁₂| of power converters 1 and 2 of the first pole is above a fourth predetermined value ΔS₁ (i.e., if there results the state where ΔI 1 = |I 11 - I 12 | > ΔS 1 holds); and a current difference monitor circuit 92 adapted for outputting a logic signal "1" when a current difference ΔI 2 = |I 23 - I 24 | of power converters 4 and 3 of the second pole is above a fifth predetermined value ΔS₂ (i.e., if there results the state where ΔI 2 = |I 23 - I 24 | > ΔS 2 holds). The fourth and fifth predetermined values ΔS₁ and ΔS₂ are a little smaller than a current margin ΔI dp in the d.c. power transmission system, respectively, for example, equal to 70 - 80% of the current margin ΔI dp . Logical outputs from these current difference monitor circuits 91 and 92 are introduced also to the AND circuit 80. By employing such a configuration, the AND circuit 80 is caused to provide an operating output on condition that a current difference of the ends of the d.c. power transmission line 5 of the first pole and a current' difference of the ends of the d.c. power transmission line 7 of the second pole are beyond respective predetermined values. Accordingly, a more reliable short-circuit detection can be realized. As described above, in accordance with this invention, when a short-circuit is taking place in a d.c. transmission system in the state of the power sending back operation, a short-circuit of the d.c. transmission line can be detected before an attempt is made to alter a current command value of one pole. Accordingly, it is possible to take a procedure to allow bi-polar current command values to be in correspondence with respective original values, or an similar procedure before there results the state where the operating points of the voltage/current characteristics of the power converters cannot be ensured, thus to stop respective power converters in a stabilized manner. Thus, it is possible to prevent in advance inconveniences such that the power converter may be damaged and/or bad influence may be exerted on the a.c. system. Reference signs in the claims are intended for better understanding and shall not limit the scope.

Claims: An apparatus for detecting a short-circuit in a bi-polar d.c. transmission system, having a first transmission line (5), a second transmission line (7) and a neutral line (6), in which one end of a neutral line is grounded and the other end thereof is not grounded and the current in the first transmission line is controlled by a first current command value while the current in the second transmission line is controlled by a second current command value; characterized by: first monitoring means (60, 61) for providing a detection output when the current flowing through the neutral line (6) is below a predetermined value; second monitoring means (70, 71) for providing a detection output when the ground current at the ground point (B) of the neutral line (6) is below a predetermined value; third monitoring means (51) for monitoring the current command values (I dp1 ,I dp2 ) to provide a detection output when the difference between the current command values is above a predetermined value; and a logic circuit (80) responsive to the detection outputs from the first, second and third monitoring means (60, 61; 70, 71; 51). An apparatus for detecting a short-circuit in a bi-polar d.c. transmission system as set forth in claim 1, characterized in that the first monitoring means includes: a first current detector (60) for detecting the current flowing in the neutral line (6); and a first monitor circuit (61) for monitoring an output value of the first current detector (60). An apparatus for detecting a short-circuit in a bi-polar d.c. transmission system as set forth in claim 1, characterized in that the second monitoring means includes: a second current detector (70) for detecting the ground current at the ground point (B) of the neutral line (6); and a second monitor circuit (71) for monitoring an output value of the second current detector (70). An apparatus for detecting a short-circuit in a bi-polar d.c. transmission system as set forth in claim 1, characterized in that the logic circuit is an AND gate (80). An apparatus for detecting a short-circuit in a bi-polar d.c. transmission system as set forth in claim 1, further characterized by: fourth monitoring means (31, 32, 91) for providing a detection output when the difference between the currents at the ends of the first transmission line (5) of the bi-polar d.c. transmission system is above a predetermined value; and fifth monitoring means (33, 34, 92) for providing a detection output when the difference between the currents at the ends of the second transmission line (7) of the bi-polar d.c. transmission system is above a predetermined value; the logic circuit (80) additionally responding to the detection outputs from the fourth and fifth monitoring means (31, 32, 91; 33, 34, 92). An apparatus for detecting a short-circuit in a bi-polar d.c. transmission system as set forth in claim 5, characterized in that the fourth monitoring means includes: third and fourth current detectors (31, 32) for detecting the currents at the ends of the first transmission line (5) of the bi-polar d.c. transmission system; and a first current difference monitor circuit (91) for monitoring the difference between an output from the third current detector (31) and an output from the fourth current detector (32) to provide a detection output when that difference is above a predetermined value; and in that the fifth monitoring means includes: fifth and sixth current detectors (33, 34) for detecting the currents at the ends of the second transmission line (7) of the bi-polar d.c. transmission system; and a second current difference monitor circuit (92) for monitoring the difference between an output from the fifth current detector (33) and an output from the sixth current detector to provide a detection output when that difference is above a predetermined value.

Inventors: NAKAMURA NAOMI, NAKAMURA, NAOMI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1992
Main classification: H02H7
IPC codes: H02J3, H02J1, H02H7