Thyristor valve arrester

Abstract: To provide a thyristor valve arrester capable of measuring leak currents of arrester elements in a simple construction and a DC power transmission thyristor valve using the same, a plurality of laminated arrester sets (4A...4D) are arranged in parallel with a plurality of thyristor valves (2A...2D). Each arrester set is made of a plurality of arrester units arranged in parallel. Each pair of adjoining arrester sets have, through an insulator (10...13), an inter-stage connecting conductor (5...9) disposed therebetween at a position corresponding to the respective thyristor valve terminals. One end of each arrester set (4A...4D) and the respective inter-stage connecting conductors (5...9) have a leak current ammeter (15) and a surge counter (14) connected therebetween in electrically parallel fashion. Each inter-stage connecting conductor (5...9) and the respective thyristor valve stage (2A...2D) are interconnected by a thyristor valve connecting conductor (20).

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to an arrester for a thyristor valve used for protecting a direct current power transmission thyristor valve from an overcurrent, and a direct current power transmission valve using such an arrester. DESCRIPTION OF THE PRIOR ART Practical application of a direct current power transmission thyristor valve has been promoted in recent years with the increase in demand for electric power, and its operation voltage has increased from DC 125 KV to 250 KV and to 500 KV to accomplish a high voltage and a large current. To make economical insulation design of such a direct current power transmission thyristor valve, an excellent arrester for a thyristor valve is necessary. However, such direct current power transmission thyristor valves are stacked into four stages and are electrically connected in series with one another. Therefore, the overall height becomes about 11 m to about 15 m. If the arrester for a thyristor valve used for protecting such a direct current power transmission thyristor valve from overvoltage is accommodated in a single insulator and is so stacked into four stages as to correspond to each stage in the same way as in the case of ordinary alternating current arresters formed by accommodating an arrester element in a single insulator, sufficient vibration resistance cannot be obtained. Therefore, as described in the papers of '92 National Meeting of the Society of Electrical Engineering, S13-10, entitled "Arrester for AC/DC Conversion Station", a construction wherein the arrester for a thyristor valve is directly mounted to the main body of the thyristor valve has been proposed. However, with the DC power transmission thyristor valve constructed in such a way as described above, in testing the withstand voltage of the thyristor valve, the thyristor valve arrester must be separated from the DC power transmission thyristor valve to prevent affecting insulation of the thyristor valve arrester. After this, they must be assembled together. These are rather troublesome works. To solve the foregoing problems of the prior art, the same applicant proposed a thyristor valve arrester shown in Fig. 8 in the Japanese Patent Application No. 4-2314. In the proposed invention, there are two stacked stages of arrester units 30a and 30b each of which has an arrester element put in a supporting bushing. The two stacked stages are mounted through a plurality of supporting bushings 31 on a base 32. There is provided another same assembly set. These assembly stages are electrically connected in series to form the thyristor valve arrester of four arrester element stages as a whole. Each stage is connected with the respective stage of the thyristor valve through a conductor. However, the proposed thyristor valve arrester must have for each assembly set additional structures 33a and 33b for supporting the two stacked stages of arrester units 30a and 30b. The proposed thyristor valve arrester also must be isolated around 5 m from the thyristor valves 2A to 2D to insulate. As a result, the proposed thyristor valve arrester needs a large installation area and is complicated in the structures. The thyristor valve arrester of the kind described above uses zinc oxide elements as arrester elements. Their insulation deterioration must be monitored by measurement of leak current. The usual AC arrester can have a current transformer put in its grounding end, thereby being easily able to detect the leak current of around a few milliamperes. On the other hand, as for the thyristor valve arrester units 30a and 30b shown in Fig. 8, they are insulated from ground by the plurality of supporting bushings 31, and the leak current cannot be easily measured. They also would be affected by a transient translocation surge current of around hundreds of milliamperes due to the translocation vibrating voltage of the thyristor valve. The ammeter therefore must be capable of measuring the leak current of the thyristor valve arrester as wide as a few milliamperes to hundreds of milliamperes. A thyristor valve arrester comprising a plurality of interconnected arrester sets stacked upon each other with inter-stage connecting conductors arranged therebetween, as set forth in the first part of claim 1, is known from the article "Metal-Oxide Surge Arresters for the Thyristor Valves of the Dürnrohr HVDC Back-to-Back Converter Station" by Michael Crucius and Walter Flaßbeck, Siemens Power Engineering & Automation VII (1985) No. 5, pages 352 - 355. EP-A-0 190 547 discloses a thyristor valve arrester with a parallel circuit of an ammeter and a surge counter connected thereto to detect the leak current through the arrester. SUMMARY OF THE INVENTION It is an object of the present invention to provide a thyristor valve arrester of high mechanical resistance which is capable of measuring the leak current of an arrester element in a simple construction. This object is accomplished by the thyristor valve arrester defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Fig. 1 is a schematic diagram illustrating a front view of a DC power transmission thyristor valve that is an embodiment of the present invention. Fig. 2 is a detailed construction of an arrester unit of the thyristor valve arrestor shown in Fig. 1. Fig. 3 is a cross-sectional view taken across III-III of the arrester unit shown in Fig. 2. Fig. 4 is a cross-sectional view taken across IV-IV of the thyristor valve arrester unit shown in Fig. 2. Fig. 5 is a circuit diagram illustrating a surge counter and a leak current ammeter. Fig. 6 is a front view illustrating a detailed construction of an example of the DC power transmission thyristor valve of the present invention. Fig. 7 is a cross-sectional plan view taken across VII-VII of the DC power transmission thyristor valve shown in Fig. 6. Fig. 8 is a front view illustrating a usual thyristor valve arrester. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The following describes in detail embodiments according to the present invention by reference to the accompanying drawings. Fig. 1 depicts a schematic diagram illustrating a front view of a DC power transmission thyristor valve that is an embodiment of the present invention. In the figure, inputs of DC power transmission thyristor valves 2A to 2D are connected to an alternating current power source AC through converting transformers 1 and 3. The DC power transmission thyristor valves 2A to 2D feed out direct current powers DC from their outputs. Each of the four-stage thyristor valves 2A to 2D is connected in parallel with their respective arrester sets 4A, 4B, 4C, and 4D having a zinc oxide element each. The arrester sets 4A, 4B, 4C, and 4D are vertically stacked to four stages to form a thyristor valve arrestor LA. A mounting surface of an arrester set has an inter-stage connecting conductor 5 attached thereto which is insulated from ground by an insulating supporter 18. The inter-stage connecting conductor 5 has a flange formed on its upper portion. The upper portion of the inter-stage connecting conductor 5 is connected with a bottom of the above-mentioned arrester unit 4A through an insulator 10. The inter-stage connecting conductor 5 and the bottom of the above-mentioned arrester unit 4A, therefore, are insulated by the insulator 10. The both, however, are electrically connected by a leak current ammeter 15 and a surge counter 14 connected in electrical parallel between a conductor 17 connected with the bottom of the arrester set 4A and a conductor 16 connected with the inter-stage connecting conductor 5. The arrester set 4A has an inter-stage connecting conductor 6 having a flange on its end arranged on its upper end. The top of the arrester set 4A and the lower flange of the inter-stage connecting conductor 6 are electrically connected directly. Similarly, the bottom end of the arrester set 4B is connected with the upper flange of the inter-stage connecting conductor 6 through an insulator 11, and the top of the arrester set 4B and the lower flange of the inter-stage connecting conductor 7 having the flange on its end are electrically connected directly. The other arrester sets 4C and 4D are structured like the arrester sets 4A and 4B. As a result, four arrester sets 4A to 4D are vertically stacked. The inter-stage connecting conductors 6 to 9 are connected with the four stacked thyristor valves 2A to 2D through thyristor valve connecting conductors 20, respectively. The following describes the thyristor valve arrester LA in detail by reference to Figs. 2 to 4. As the arrester sets 4A to 4D are structured in virtually the same construction, only the arrester set 4A is described below. The arrester set 4A, as shown in Fig. 3, is formed of four arrester units 4a1, 4a2, 4a3, and 4a4 which are arranged in parallel at the respective vertexes of a virtual square. As shown in Fig. 2, the inter-stage connecting conductors 5 and 6 arranged on and under the arrester set 4A formed of the four arrester units 4a1, 4a2, 4a3 and 4a4 arranged in parallel have electric field relaxing shields 24A and 24B fixed thereon to enclose the upper and lower ends of the arrester units 4a1, 4a2, 4a3 and 4a4 to relax the electric field, respectively. The leak current ammeter 15 and the surge counter 14 are placed in parallel, and lower terminals thereof are connected with the inter-stage connecting conductor 5 through the conductor 16. That is, the conductor 16 and the inter-stage connecting conductor 5 are electrically connected therebetween through an operation recorder 19 for recording operation of the arrester set 4A. Upper terminals of the leak current ammeter 15 and the surge counter 14 are connected to lower ends of the arrester units 4a1, 4a2, 4a3, and 4a4 through the conductor 17. In more detail, there is provided a common conductor 28 fixed on the upper flange of the inter-stage connecting conductor 5 through an insulator 36, and the common conductor 28 has the conductor 17 connected therewith. The common conductor 28, as shown in Fig. 3, electrically connects four connecting plates 37 which are electrically connected with lower ends of arrester elements 35 placed at the lowest ends of the arrester units 4a1, 4a2, 4a3, and 4a4. The lower ends of arrester elements 35 placed at the lowest ends of the arrester units 4a1, 4a2, 4a3 and 4a4 are electrically insulated from the inter-stage connecting conductor 5 by the insulator 36. The upper end of the arrester set 4A is constructed likely as described above. As a result, the arrester elements 35 of the arrester units 4a1, 4a2, 4a3 and 4a4 are electrically connected in parallel between the common conductors 28 placed at the highest ends and the lowest ends of the arrester units 4a1, 4a2, 4a3 and 4a4. As the other arrester sets 4B, 4C and 4D are constructed likely as described above, description of them is omitted here. Each of the arrester units 4a1, 4a2, 4a3, and 4a4, as shown in a longitudinal cross-section view of Fig. 4, is constructed in a way that a plurality of arrester elements 35 of zinc oxide are laminated in an insulating pipe 33 made of fiber reinforced plastics. The insulating pipe 33 is covered with a wrinkled insulating coat 34 made of, for example, plastics such as a silicon polymer. As the insulating pipe 33 of fiber reinforced plastics is lighter than a bushing, the thyristor valve arrestor LA has increased earthquake resistance. The wrinkled insulating coat 34 keeps a withstand voltage of the insulating pipe 33 from being dropped by contamination. In turn, the following describes the electrically parallel construction of the leak current ammeter 15 and the surge counter 14 in the thyristor valve arrester LA by reference to Fig. 5. Each of the arrester sets 4A, 4B, 4C, and 4D has a leak current ammeter 15 having an internal resistor R1 connected in series therewith. Each of the arrester sets 4A, 4B, 4C and 4D also has a surge counter 14 having an internal resistor R2 connected in series therewith. A protecting gap 27 is also provided. A current shunted to the leak current ammeter 15 by the internal resistor R1 is made to pass a rectifier 36 before being charged into a capacitor 37. The current then is made to flow to an indicator 38 through a resistor 39. A shunting ratio of the internal resistors R1 and R2 can be appropriately selected so easily that the indicator 38 of the leak current ammeter 15 can indicate a translocation vibrating current and the leak current in a wide range of 2 to 200 mA. As the surge counter 14 and the leak current ammeter 15 are connected in parallel, the arrester leak current is shunted. If the arrester leak current is 100 mA, for example, 80 mA flows to the leak current ammeter 15 and the remaining 20 mA to the surge counter 14. Instruments for the surge counter 14 and the leak current ammeter 15 are sufficient to endure the respective shunt currents. This means that the instruments can be made smaller and cheaper. The leak current of the thyristor valve arrester LA is made transient by the translocation vibrating current of the thyristor valves 2A to 2D. The transition current, however, can be suppressed by the capacitor 37 before coming to the indicator 38 so that a needle of the indicator 38 of the leak current ammeter 15 can be statically read. The surge counter 14 is formed of a rectifier 40, a capacitor 41, and a counter drive coil 42 to count a surge current. As the leak current ammeter 15 and the surge counter 14 are electrically connected in parallel, even if the leak current ammeter 15 is broken by some cause, a discharge current is made to flow through the surge counter 14 connected in parallel, thereby not preventing the normal operation. An operator, thus, can easily check the operation of the thyristor valve arrester LA by externally seeing deflection of the leak current ammeter 15 and a value counted on the surge counter 14. This means that the component reliability can be increased. Fig. 6 depicts the dc power transmission thyristor valve formed by combination of the above-described thyristor valve arrestor LA and the four thyristor valve stages 2A to 2D. Arrangements and parts in the figure identical with those in Fig. 1 are indicated by the same symbols as in Fig. 1. As shown in Fig. 6, the four-stage thyristor valve arrestor LA and the four thyristor valve stages 2A to 2D are arranged in parallel and electrically connected therebetween by thyristor valve connecting conductors 20 so as to correspond to the one in Fig. 1. In more detail, as shown in Fig. 7, the inter-stage connecting conductor 5 has a bracket 5a fixed thereto. The bracket 5a has ends of a plurality of thyristor valve connecting conductors 20a, 20b, 20c, and 20d connected thereto electrically and mechanically. The other ends of the thyristor valve connecting conductors 20a, 20b, 20c, and 20d connected to a terminal of the thyristor valve 2A electrically and mechanically. With the dc power transmission thyristor valve, the four-stage thyristor valve arrestor LA is placed in parallel adjacent to the four thyristor valve stages 2A to 2D. The thyristor valve arrestor LA also has the inter-stage connecting conductors 5 to 9 provided among the four arrester sets 4A to 4D. For this reason, axial lengths of the inter-stage connecting conductors 5 to 9 can be adjusted to make the inter-stage connecting conductors 5 to 9 correspond to the terminals of the thyristor valves 2A to 2D. The four arrester sets 4A to 4D having the plurality of arrester units arranged in parallel each have the earthquake resistance increased to a great extent as compared with the conventional one having four stacked stages each of which has a single arrester unit. For the lowest stage, as an example, the plurality of arrester units 4a1 to 4a4, can be arranged at the respective vertexes of the virtual square at appropriate intervals to make the structure highly resistive to earthquakes. Note that an insulation distance G between the thyristor valves and the inter-stage connecting conductors should be 1 to 1.5 m. The thyristor valves 2A to 2D and the thyristor valve arrestor LA can be easily connected therebetween by simple short conductors, such as the rigid thyristor valve connecting conductors 20a to 20d of circular cross section. A whole installation area of the thyristor valve arrestor LA and the thyristor valve stages 2A to 2D of the present invention can be made narrower than the conventional one. In thyristor valve arrestor LA, the bottoms of the arrester sets 4A to 4D are electrically insulated from the inter-stage connecting conductors 5 to 9 by the insulators 10 to 13, respectively. The leak current ammeter 15 and the surge counter 14 are connected at least in parallel between the conductor 17 connected with each of the bottoms of the arrester sets 4A to 4D and the conductor 16 connected with each of the connecting conductors 5 to 9. Thus, each leak current ammeter 15 can monitor the leak current flowing through the respective arrester element 33 of the arrester sets 4A to 4D. In addition, as each of the arrester units of the arrester sets 4A to 4D is formed of the plurality of laminated arrester elements 35 of zinc oxide in the insulating pipes 33 of fiber reinforced plastics, the structure can be made lighter and stiffer than that of the bushings. While both the thyristor valve arrester and the thyristor valve in the embodiment described above are made of four stages, the present invention is not limited thereto or thereby. Also, while the numbers of the arrester units of the arrester sets 4A to 4D of the thyristor valve arrester in the embodiment described above are four each, the present invention is not limited to thereto or thereby. As the thyristor valve arrester has the insulator put between each arrester set and the respective connecting conductor and has the leak current ammeter and the surge counter connected in parallel between each arrester set and the respective connecting conductors, the leak current can be measured easily and accurately. Should the leak current ammeter be broken, the surge counter can keep connection between the arrester set and the connecting conductor, thereby increasing the reliability of the apparatus. As the thyristor valve of the present invention has the multi-stage thyristor valve arrester arranged in the vicinity of the multi-stage thyristor valve and has the inter-stage connecting conductors provided among the multiple arrester sets, the axial lengths of the inter-stage connecting conductors can be adjusted. This allows the inter-stage connecting conductors to correspond to the terminals of the thyristor valves, thereby making easy connection of the thyristor valve arrester and the thyristor valve through the thyristor valve conductors of simple form. Also, the whole installation area can be made narrower than that of the prior art.

Claims: A thyristor valve arrester comprising a plurality of interconnected arrester sets (4A...4D) stacked upon each other with inter-stage connecting conductors (5...9) arranged therebetween, characterised in that an insulator (10...13) is arranged between each pair of arrester sets (4A...4D), a leak current ammeter (15) is connected between one end of each arrester set (4A...4D) and the corresponding inter-stage connecting conductor (5...8), and each arrester set (4A...4D) comprises a plurality of arrester units (4a1...4a4) arranged in parallel, each unit (4a1...4a4) having an arrester element (35) arranged in an insulating pipe (33) made of fibre reinforced plastics. The thyristor valve arrester of claim 1, wherein each inter-stage connecting conductor (5...9) carries a rigid conductor member (20a...20d) for connection to one end of a thyristor valve (2A...2D). The thyristor valve arrester of claim 1 or 2, wherein a surge counter (14) is connected in parallel with each leak current ammeter (15).

Inventors: SHIRAKAWA SHINGO, TERAKADO SHUICHI, SHIRAKAWA, SHINGO, TERAKADO, SHUICHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1993
Main classification: H02H9
IPC codes: H01C7, H02H3, H02H7