Capacitive voltage transformer for converter station of extra-high voltage direct current transmission project

Abstract

The invention relates to a capacitive voltage transformer for a converter station of an extra-high voltage direct current transmission project, which is characterized in that a secondary coil A, a secondary coil B and a secondary coil C in a secondary winding of an intermediate transformer have the same wire diameter, and when the secondary winding is wound, a secondary residual coil is wound on an insulating paper tube; the da end of the secondary residual coil is led out from the head end of the insulating paper tube, the dn end of the secondary residual coil is led out from the tail end of the insulating paper tube, then a secondary coil D is wound on the secondary residual coil, the 4a and 4n ends of the secondary coil D are led out from the tail end of the insulating paper tube, and then a secondary coil A, a secondary coil B and a secondary coil C are simultaneously wound on the secondary coil D in parallel, and 1a and 1n ends of the secondary coil A, 2a and 2n ends of the secondary coil B and 3a and 3n ends of the secondary coil C are led out from the head end of the insulating paper tube. The method has better accuracy performance and higher overvoltage bearing capacity, and can meet the use requirements of the converter station of the extra-high voltage direct current transmission project.

Description

Technical Field

[0001] This invention relates to the field of power equipment, specifically a capacitive voltage transformer for converter stations in ultra-high voltage direct current transmission projects. Background Technology

[0002] Capacitive voltage transformers (CVTs) are widely used in power systems. They are mainly used in voltage measurement, carrier communication, relay protection, telemetry and monitoring systems, etc. They can also isolate the high-voltage primary side and the low-voltage secondary side, protect the safe and stable operation of equipment, and are an important part of the power grid. With the development of technology, some capacitive voltage transformers with different functions have emerged.

[0003] For example, Chinese invention patent CN110907701B discloses a capacitive voltage transformer with harmonic measurement function, which includes a capacitive voltage divider, an electromagnetic unit, and a harmonic measurement module. The capacitive unit in the capacitive voltage divider includes a high-voltage capacitor, a medium-voltage capacitor, and a harmonic measurement capacitor connected in series. The medium-voltage terminal between the high-voltage capacitor and the medium-voltage capacitor is connected to the high-voltage side of the electromagnetic unit, and the harmonic measurement terminal between the medium-voltage capacitor and the harmonic measurement capacitor is connected to the low-voltage side of the electromagnetic unit. The harmonic measurement module is used to measure the output voltage across the harmonic measurement capacitor.

[0004] For example, Chinese invention patent CN106253777B discloses a method for improving the secondary output capacity of a capacitive voltage transformer. This method involves setting a secondary load detection device on the secondary winding of the capacitive voltage transformer and adjusting the number of turns in the primary winding and the number of turns in the compensating reactor based on the obtained secondary load, thereby improving the output capacity.

[0005] For example, Chinese invention patent CN104681261B discloses an equipotential shielded capacitive voltage transformer, wherein the secondary winding of the intermediate transformer includes a primary secondary winding and a remaining winding, and the remaining winding is connected in parallel with a ferroresonant damper to effectively suppress ferroresonant overvoltage.

[0006] In ultra-high voltage direct current (UHVDC) transmission projects, converter stations are equipped with CVTs on the converter station's incoming and outgoing lines, the converter transformer's incoming line, and the filter side. However, due to the special operating conditions of UHVDC converter stations, compared to CVTs used in conventional AC substations, UHVDC converter stations have special requirements for CVTs. These requirements include higher rated primary voltage levels, larger overvoltage withstand multiples, accuracy requirements to ensure accuracy under higher overvoltages, and excellent anti-ferromagnetic resonance performance and transient response characteristics. Existing conventional CVTs are difficult to meet the special operating conditions of UHVDC converter stations. Summary of the Invention

[0007] The purpose of this invention is to provide a capacitive voltage transformer for converter stations in ultra-high voltage direct current (UHVDC) transmission projects, which has better accuracy performance and higher overvoltage withstand capability, and can meet the usage requirements of UHVDC converter stations.

[0008] The objective of this invention is achieved through the following technical solution:

[0009] A capacitive voltage transformer for a converter station in an ultra-high voltage direct current (UHVDC) transmission project includes an electromagnetic unit and a capacitive voltage divider. The capacitive voltage divider includes a high-voltage capacitor C1 and a medium-voltage capacitor C2. The electromagnetic unit includes an intermediate transformer and a compensating reactor. The intermediate transformer includes a primary winding and a secondary winding. One end of the primary winding is connected to point M located between the high-voltage capacitor C1 and the medium-voltage capacitor C2, and the other end is connected to the compensating reactor. The secondary winding includes secondary coil A, secondary coil B, secondary coil C, secondary coil D, a secondary residual coil, and an insulating paper tube. C has the same wire diameter. When winding the secondary winding, the secondary residual coil is first wound on the insulating paper tube, and the da end of the secondary residual coil is led out from the head end of the insulating paper tube and the dn end is led out from the tail end of the insulating paper tube. Then, the secondary coil D is wound on the secondary residual coil, and the 4a and 4n ends of the secondary coil D are both led out from the tail end of the insulating paper tube. Then, the secondary coil A, secondary coil B and secondary coil C are wound in parallel on the secondary coil D, and the 1a and 1n ends of the secondary coil A, the 2a and 2n ends of the secondary coil B and the 3a and 3n ends of the secondary coil C are all led out from the head end of the insulating paper tube.

[0010] A shielding ring is wrapped around the outside of the secondary winding, and an insulating film is wrapped around the outside of the shielding ring.

[0011] A damping unit is connected in parallel between terminals 1a and 1n of secondary coil A. A first parallel branch and a second parallel branch are provided between terminals 1a and 1n. The damping unit includes a damping coil and a damping resistor provided on the first parallel branch, and a first parallel resistor provided on the second parallel branch. A third parallel branch is provided between terminals 2a and 2n of secondary coil B and between terminals 3a and 3n of secondary coil C. A second parallel resistor is provided on the third parallel branch. The first parallel resistor and the second parallel resistor have the same resistance.

[0012] The number of turns in the secondary winding is determined according to the following formula (1):

[0013]

[0014] In the above formula (1), Bm is the magnetic flux density; Usr is the rated secondary voltage; AFe is the cross-sectional area of ​​the intermediate transformer core; fr is the rated frequency; W2 is the number of turns of the secondary winding (102);

[0015] The number of turns in the primary winding is obtained according to the following formula (2):

[0016]

[0017] In equation (2) above, W1 is the number of turns in the primary winding; Ucr is the rated primary voltage of the intermediate transformer.

[0018] The compensating reactor includes a reactor core, a reactor main coil, and a reactor adjusting coil, wherein the reactor main coil is mounted on one side of the reactor core, and the reactor adjusting coil is mounted on the other side of the reactor core;

[0019] The reactor main coil includes a first main line segment and a second main line segment connected in series, and the first main line segment and the second main line segment are separated by a first insulating paper ring. The first main line segment, the second main line segment, and the first insulating paper ring are all located in a first insulating cavity formed by wrapping the first insulating paper.

[0020] The reactor adjustment coil includes a first adjustment line segment and a second adjustment line segment connected in series, and the first adjustment line segment and the second adjustment line segment are separated by a second insulating paper ring. The first adjustment line segment, the second adjustment line segment, and the second insulating paper ring are all located in a second insulating cavity formed by wrapping the second insulating paper.

[0021] The first main line segment has a first upward outlet at its upper end, and the second main line segment has a first downward outlet on one side of its lower end; the first adjustment line segment has a second upward outlet at its upper end, and the second adjustment line segment has a second downward outlet on one side of its lower end.

[0022] The reactor core has a rectangular cross-section and stepped protrusions on each side, with stepped stops formed at both ends of the stepped protrusions.

[0023] The reactor core has a first core section and a second core section on both sides, and an air gap δ is provided between the first core section and the second core section.

[0024] The compensating reactor includes an upper pressure plate, a lower pressure plate, and fastening screws. The upper pressure plate and the lower pressure plate both have a central protrusion and connecting plates on both sides. The upper end of the reactor core is placed in the central protrusion of the upper pressure plate, and the lower end is placed in the central protrusion of the lower pressure plate. The connecting plates on both sides of the upper pressure plate and the connecting plates on both sides of the lower pressure plate are respectively connected by fastening screws. Both ends of the fastening screws are provided with locking nuts.

[0025] The electromagnetic unit includes an oil tank, and the intermediate transformer, compensating reactor, and damping unit are all located in the oil tank. The upper end of the oil tank is provided with a first sleeve, and the electromagnetic unit cable is led out through the first sleeve and connected to point M on the capacitor voltage divider. The side of the oil tank is provided with a second sleeve, and the cable led out from the end of the medium-voltage capacitor C2 is led out through the second sleeve and connected to the oil tank. At the same time, the cable led out from the end of the compensating reactor is also connected to the oil tank. A grounding wire is provided on one side of the oil tank. The oil tank is provided with a secondary terminal box, and the secondary terminal box is provided with a rainproof cover.

[0026] The advantages and positive effects of this invention are as follows:

[0027] 1. The present invention redesigns the secondary winding of the intermediate transformer. The secondary winding, namely secondary coil A, secondary coil B and secondary coil C, are three secondary coils with an accuracy class of 0.2. They are made of enameled flat copper wire of the same diameter and wrapped with an insulating film, and the three are wound in parallel.

[0028] 2. In order to ensure that the three 0.2-level secondary coils have consistent accuracy performance, in addition to using the same wire and winding the secondary coils A, B and C in parallel, the three coils are also connected in parallel with resistors of the same resistance value. Furthermore, through the design of damping units, the present invention can also meet the antiferromagnetic performance and transient response requirements of CVT used in converter stations of UHVDC transmission projects.

[0029] 3. After determining the secondary winding structure, this invention designs the intermediate transformer with a rated voltage factor of 1.9 times for 8 hours to give the secondary winding a larger number of turns. This allows the intermediate transformer to have a lower rated magnetic flux density. Furthermore, this invention optimizes the design based on load conditions to determine a smaller rated primary voltage for the intermediate transformer. On the other hand, it specially designs the compensating reactor to ensure that the CVT has better error performance and can withstand greater overvoltage. This solves the problem of increased primary winding resistance caused by an increase in the number of primary winding turns as the secondary winding increases. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the electrical principle of the present invention.

[0031] Figure 2 for Figure 1 Front view of the electromagnetic unit.

[0032] Figure 3 for Figure 2 Left view of the electromagnetic unit.

[0033] Figure 4 for Figure 2 Top view of the electromagnetic unit.

[0034] Figure 5 for Figure 1 A schematic diagram of the secondary winding structure.

[0035] Figure 6 for Figure 5 A schematic diagram of the winding principle of the secondary winding.

[0036] Figure 7 for Figure 5 Left view of the secondary winding.

[0037] Figure 8 for Figure 7 A schematic diagram of the unfolded shielding ring.

[0038] Figure 9 for Figure 1 The diagram shows the resistor connection at point D.

[0039] Figure 10 for Figure 1 The diagram shows the resistor connection at point E.

[0040] Figure 11 for Figure 1 The diagram shows the resistor connection at point F.

[0041] Figure 12 for Figure 4 Front view of the compensation reactor.

[0042] Figure 13 for Figure 12 Left view of the compensation reactor.

[0043] Figure 14 for Figure 12 Top view of the compensation reactor.

[0044] Figure 15 for Figure 12 Front view of the reactor core.

[0045] Figure 16 for Figure 15 AA view in

[0046] Figure 17 for Figure 12 Schematic diagram of the main coil of the medium-voltage reactor.

[0047] Figure 18 for Figure 12 Schematic diagram of the structure of the regulating coil of the reactor.

[0048] Figure 19 This is a structural schematic diagram of an application example of the present invention.

[0049] Wherein, 1 is the electromagnetic unit, 101 is the intermediate transformer, 102 is the secondary winding, 1021 is the secondary coil A, 1022 is the secondary coil B, 1023 is the secondary coil C, 1024 is the secondary coil D, 1025 is the secondary residual coil, 1026 is the insulating film, 1027 is the shielding ring, 10271 is the grounding lead, 10272 is the capacitor paper, 1028 is the insulating diaphragm, 1029 is the insulating paper tube, 103 is the primary winding, 104 is the compensating reactor, 1041 is the reactor core, 10411 is the first core segment, 10412 is the second core segment, 10413 is the stepped stop, 1042 is the reactor main coil, 10421 is the first main line segment, 10422 is the second main line segment, 10423 is the first insulating paper ring, 10424 is the first insulating paper, 104... 25 is the first upper exit, 10426 is the first lower exit, 1043 is the reactor adjusting coil, 10431 is the first adjusting segment, 10432 is the second adjusting segment, 10433 is the second insulating paper ring, 10434 is the second insulating paper, 10435 is the second upper exit, 10436 is the second lower exit, 1044 is the upper pressure plate, 1045 is the lower pressure plate, and 1046 is the lock nut. 1047 is a fastening screw, 1048 is a bracket, 105 is a damping unit, 1051 is a damping coil, 1052 is a damping resistor, 1053 is the first parallel resistor, 1054 is the second parallel resistor, 106 is the second bushing, 107 is a secondary terminal box, 108 is a rain cover, 109 is a grounding switch, 110 is the first bushing, 111 is an oil tank, 2 is a high-voltage terminal, and 3 is a capacitor voltage divider. Detailed Implementation

[0050] The invention will now be described in further detail with reference to the accompanying drawings.

[0051] like Figures 1-19 As shown, the present invention includes an electromagnetic unit 1 and a capacitive voltage divider 3, wherein, as Figure 1 As shown, the capacitor divider 3 includes a high-voltage capacitor C1 and a medium-voltage capacitor C2. The electromagnetic unit 1 includes an intermediate transformer 101 and a compensating reactor 104. The intermediate transformer 101 includes a primary winding 103 and a secondary winding 102. One end of the primary winding 103 is connected to point M located between the high-voltage capacitor C1 and the medium-voltage capacitor C2, and the other end is connected to the compensating reactor 104. The secondary winding 102 includes multiple secondary coils. The above structures are all known technologies in the art, for example, see patent CN201282017Y.

[0052] However, this invention is for converter stations in ultra-high voltage direct current transmission projects, where the accuracy requirements are relatively high.

[0053] The accuracy class combination for the secondary winding of a conventional 750kV CVT in an AC substation is as follows:

[0054]

[0055] The accuracy class combination of the CVT secondary winding used in the converter station of the UHVDC transmission project is as follows:

[0056]

[0057] As can be seen from the comparison of the two tables above, the CVT display of the converter station of the UHVDC transmission project shows that three secondary windings have an accuracy class of 0.2. Therefore, its secondary winding structure needs to be specially designed to meet the system's requirements for more accurate measurement of the primary voltage.

[0058] like Figure 1 and Figures 5-7 As shown, the secondary winding 102 of the intermediate transformer 101 of the present invention includes secondary coil A1021, secondary coil B1022, secondary coil C1023, secondary coil D1024, secondary residual coil 1025, and insulating paper tube 1029. Secondary coils A1021, B1022, and C1023 are three secondary coils with an accuracy class of 0.2, made of enameled flat copper wire of the same diameter and wrapped with an insulating film, and wound in parallel. The present invention arranges the number of turns and layers per layer according to the design number of turns and core size of the intermediate transformer 101. When the secondary winding 102 is wound, as follows... Figure 6 As shown, a secondary residual coil 1025 is first wound on an insulating paper tube 1029, with the da end of the secondary residual coil 1025 leading out from the head end of the insulating paper tube 1029 and the dn end leading out from the tail end of the insulating paper tube 1029. Then, a secondary coil D1024 is wound on the secondary residual coil 1025, with the 4a and 4n ends of the secondary coil D1024 both leading out from the tail end of the insulating paper tube 1029. Then, secondary coils A1021, B1022, and C1023 are simultaneously wound in parallel on the secondary coil D1024, with the 1a and 1n ends of secondary coil A1021, the 2a and 2n ends of secondary coil B1022, and the 3a and 3n ends of secondary coil C1023 all leading out from the head end of the insulating paper tube 1029. Additionally, as shown... Figure 7 As shown, insulating diaphragms 1028 are used to separate the coil layers and coils during winding. After each coil of the secondary winding 102 is wound, a shielding ring 1027 is wrapped around the outside of the secondary winding 102, and finally an insulating film 1026 is wrapped around the outside of the shielding ring 1027. The shielding ring 1027 is designed to prevent high voltage overvoltage from being transmitted to the secondary side, as shown in the figure. Figure 8As shown, in this embodiment, the shielding ring 1027 is made of copper foil with a length of L, and the four corners of the copper foil are rounded. In addition, the end of the copper foil is wrapped with capacitor paper 10272 and glued with tape. The copper foil is provided with a grounding lead 10271.

[0059] like Figure 1 and Figures 9-11 As shown, to ensure consistent accuracy performance of the three 0.2-level secondary coils, in addition to using the same wire and winding them in parallel, secondary coils A1021, B1022, and C1023 are also connected in parallel with resistors of the same value. For example... Figure 1 and Figure 9 As shown, a damping unit 105 is connected in parallel between terminals 1a and 1n of the secondary coil A1021. A first parallel branch and a second parallel branch are provided between terminals 1a and 1n. The damping unit 105 includes a damping coil 1051 and a damping resistor 1052 disposed on the first parallel branch, and a first parallel resistor 1053 disposed on the second parallel branch. Figure 1 and Figures 10-11 As shown, a third parallel branch is provided between the 2a and 2n terminals of the secondary coil B1022 and between the 3a and 3n terminals of the secondary coil C1023, and a second parallel resistor 1054 is provided on the third parallel branch. The first parallel resistor 1053 and the second parallel resistor 1054 have the same resistance.

[0060] Furthermore, compared to conventional CVTs, the antiferromagnetic performance requirements for CVTs used in UHVDC converter stations are increased by 3 times, and the transient response characteristic requirements are increased by 2 times, making the requirements more stringent. This invention, through the design of the aforementioned damping unit 105, can also meet the antiferromagnetic performance and transient response requirements of CVTs used in UHVDC converter stations. Specifically, based on a 750kV capacitive voltage transformer used in conventional AC substations with a rated voltage factor of 1.5 times for 30 seconds, this invention verifies the design of the damping unit 105 by conducting a design verification at a rated voltage factor of 1.9 times for 8 hours. This involves simulating the antiferromagnetic resonance characteristics at 1.5 times, and also calculating the antiferromagnetic resonance characteristics at 1.6 times, 1.7 times, 1.8 times, and 1.9 times respectively. By combining these verification points, the CVTs used in UHVDC converter stations can achieve excellent antiferromagnetic resonance performance and transient response characteristics.

[0061] After determining the structure of the secondary winding 102, this invention, while ensuring consistent accuracy performance of the secondary coils A1021, B1022, and C1023, designs the intermediate transformer 101 according to the 8-hour requirement of a rated voltage factor of 1.9, so that the secondary winding 102 has a larger number of turns (70 turns or more). This allows the intermediate transformer 101 to have a lower rated magnetic flux density (0.4T or less). However, increasing the number of turns in the secondary winding 102 also leads to an increase in the number of turns in the primary winding 103, which in turn increases the resistance of the primary winding 103.

[0062] To address the aforementioned issues, this invention optimizes the design based on load conditions to determine a smaller rated primary voltage for the intermediate transformer 101 (typically, the rated primary voltage Ucr ≤ 10kV). This reduces the resistance of the primary winding 103, thereby lowering the overvoltage value on the intermediate transformer 101 while maintaining error performance. This ensures that the CVT used in the converter station of the UHVDC transmission project has better accuracy and insulation performance. Specifically:

[0063] 1. Determine the magnetic flux density Bm of the intermediate transformer 101.

[0064] II. Determine the number of turns of the secondary winding 102 according to the following formula (1):

[0065]

[0066] In the above formula (1), Bm is the magnetic flux density of intermediate transformer 101; Usr is the rated secondary voltage; AFe is the cross-sectional area of ​​the core of intermediate transformer; fr is the rated frequency; and W2 is the number of turns of secondary winding 102.

[0067] III. The number of 103 turns of the primary winding is obtained according to the following formula (2):

[0068]

[0069] In the above formula (2), W1 is the number of 103 turns of the primary winding; Ucr is the rated primary voltage of the intermediate transformer, which is also the rated intermediate voltage of the capacitor divider.

[0070] On the other hand, this invention employs a special design for the compensating reactor 104 to ensure that the CVT has better error performance and can withstand greater overvoltage, such as... Figures 12-18As shown, the compensating reactor 104 includes a reactor main coil 1042 and a reactor adjusting coil 1043 respectively mounted on both sides of the reactor core 1041. The primary winding 103 of the intermediate transformer 101, the reactor adjusting coil 1043, and the reactor main coil 1042 are connected in series. This is a well-known technology in the field; for example, see patent CN201282017Y. However, this invention changes the conventional reactor main coil and reactor adjusting coil, which typically consist of only one coil segment, to include two coil segments to improve the reactor's ability to withstand overvoltage. Specifically:

[0071] like Figure 17 As shown, the reactor main coil 1042 includes a first main line segment 10421 and a second main line segment 10422 connected in series, and the first main line segment 10421 and the second main line segment 10422 are separated by a first insulating paper ring 10423. The first main line segment 10421, the second main line segment 10422 and the first insulating paper ring 10423 are all disposed in a first insulating cavity formed by wrapping the first insulating paper 10424. A first upper outlet 10425 is led out from the upper end of the first main line segment 10421, and a first lower outlet 10426 is led out from one side of the lower end of the second main line segment 10422.

[0072] like Figure 18 As shown, the reactor adjusting coil 1043 includes a first adjusting segment 10431 and a second adjusting segment 10432 connected in series, and the first adjusting segment 10431 and the second adjusting segment 10432 are separated by a second insulating paper ring 10433. The first adjusting segment 10431, the second adjusting segment 10432, and the second insulating paper ring 10433 are all disposed in a second insulating cavity formed by wrapping the second insulating paper 10434. The upper end of the first adjusting winding segment 10431 leads to the second upper outlet 10435, and the lower end of the second adjusting winding segment 10432 leads to the second lower outlet 10436. The second lower outlet 10436 is connected to the first lower outlet 10426 to realize the series connection between the reactor main coil 1042 and the reactor adjusting winding coil 1043. Sufficient insulation must be wrapped at the connection point. The reactor adjusting winding coil 1043 is used for fine adjustment when adjusting the error of the finished product.

[0073] In addition, the present invention also redesigns the reactor core 1041, such as... Figure 15 As shown, the reactor core 1041 has an overall U-shaped structure, and as... Figure 16 As shown, the reactor core 1041 has a rectangular cross-section with stepped protrusions on each side. The stepped protrusions form stepped stops 10413 at both ends. This cross-section has a 25% larger cross-sectional area than a conventional rectangular core. Meanwhile, as... Figure 15As shown, this invention provides air gap notches δ between the first core section 10411 and the second core section 10412 on both sides of the reactor core 1041 to adjust the air gap length, thereby adjusting the inductance value to make the inductive reactance equal to the capacitive reactance, thus reducing the internal impedance of the CVT to a minimum and ensuring a more precise accuracy level for the CVT. The adjustment process is as follows:

[0074] 1. Determine the inductance value of the compensating reactor 104 according to the following formula (3):

[0075]

[0076] In equation (3) above, L P The inductance of the reactor is used to compensate for the voltage; C1 is the high-voltage capacitor; C2 is the medium-voltage capacitor; fr is the rated frequency.

[0077] 2. The inductance of the compensating reactor 104 should be adjusted on the complete electromagnetic unit 1 when it is a semi-finished product. During the adjustment, the secondary winding 102 of the intermediate transformer 101 must be short-circuited. The adjustment method is as follows: adjust the air gap cut δ between the first iron core section 10411 and the second iron core section 10412, then apply a voltage of 200V to the compensating reactor 104 and use an instrument to detect the current value. The deviation between the measured current value and the calculated value should not exceed ±5%, which is considered qualified. The calculated current value Ic is obtained by the following formula (4):

[0078] I C =200×2πf r (C1+C2)×10 -9 (4).

[0079] Conventional capacitive voltage transformers typically have an overvoltage protector connected in parallel with the compensating reactor. However, this overvoltage protector needs to be replaced after multiple trips, which complicates operation and maintenance. The compensating reactor 104 of this invention, due to its special structure, can withstand a high level of overvoltage, thus eliminating the need for a parallel overvoltage protector and avoiding the problem of replacing it during operation and maintenance.

[0080] like Figures 12-14As shown, in this embodiment, the compensating reactor 104 includes an upper pressure plate 1044, a lower pressure plate 1045, and fastening screws 1047. Both the upper pressure plate 1044 and the lower pressure plate 1045 have a central protrusion and connecting plates on both sides. The upper end of the reactor core 1041 is placed in the central protrusion of the upper pressure plate 1044, and the lower end is placed in the central protrusion of the lower pressure plate 1045. The connecting plates on both sides of the upper pressure plate 1044 and the connecting plates on both sides of the lower pressure plate 1045 are respectively connected by fastening screws 1047 to clamp and fix the compensating reactor 1044 as a whole. Each end of the fastening screw 1047 is provided with a locking nut 1046. Tightening the locking nuts 1046 at both ends of the fastening screw 1047 achieves the function of clamping and fixing the reactor core 1041 by the upper pressure plate 1044 and the lower pressure plate 1045. Additionally, as shown... Figure 14 As shown, a bracket 1048 is provided on one side of the upper pressure plate 1044, the electromagnetic unit 1 includes an oil tank 111, and the bracket 1048 is used to install the entire compensation reactor 104 in the oil tank 111.

[0081] like Figures 2-4 As shown, the intermediate transformer 101, compensating reactor 104, and damping unit 105 of the electromagnetic unit 1 are all housed in the oil tank 111. The upper end of the oil tank 111 is provided with a first sleeve 110 for the electromagnetic unit 1 cable to be led out and connected to point M on the capacitor voltage divider 3. One side of the oil tank 111 is provided with a second sleeve 106, and the other side is provided with a grounding switch 109. (The last sentence appears to be incomplete and possibly refers to a different topic.) Figure 1 As shown, the cable leading from the end of the medium-voltage capacitor C2 is connected to the oil tank 111 after being led out through the second bushing 106. Simultaneously, the cable leading from the end of the compensating reactor 104 is also connected to the oil tank 111. A grounding wire is provided on one side of the oil tank 111. Figure 3 As shown, the oil tank 111 is equipped with a secondary terminal box 107, and the secondary terminal box 107 is equipped with a rain cover 108, such as... Figure 1 As shown, terminals 1a, 1n, 2a, 2n, 3a, 3n, 4a, 4n, da, and dn of the secondary winding 102 are all located on the secondary terminal box 107. The secondary terminal box 107 has an IP67 protection rating, which effectively balances the pressure inside and outside the terminal box and reduces condensation. It also prevents solid and liquid contaminants from entering the terminal box, improving the safety and reliability of the equipment.

[0082] The working principle of this invention is as follows:

[0083] The rated primary voltage of a 750kV CVT in a conventional AC substation is The purpose of this invention is to provide a 750kV CVT product for converter stations in ultra-high voltage direct current transmission projects, with a rated primary voltage of [missing information]. The rated insulation levels of conventional 750kV CVTs for AC substations are 800 / 975 / 2100 / kV. The rated insulation level of this invention must not only meet the above requirements, but the product must also meet the requirements for transient overvoltage and short-time overvoltage in AC systems, as shown in the table below:

[0084]

[0085] Conventional AC substations using CVT at 750kV The radio interference voltage is 2500μV. The radio interference voltage requirement of this invention is... ≤500μV. The accuracy class of CVT protection used in a conventional 750kV AC substation is... (Note: Upr is the rated voltage of the device), this invention requires that... (Note: Um is the highest voltage of the equipment) It meets the protection accuracy requirements. In addition, compared with the conventional 750kV CVT used in AC substations, the accuracy class of the three secondary winding coils of this invention is 0.2, which is a higher accuracy requirement.

[0086] Therefore, in summary, this invention requires better accuracy performance and higher overvoltage withstand capability.

[0087] To solve the above problems, the present invention first redesigns the secondary winding 102 of the intermediate transformer 101, such as... Figure 1 and Figures 5-6 As shown, the secondary winding 102 consists of three secondary coils A1021, B1022, and C1023 with an accuracy class of 0.2. They are made of enameled flat copper wire of the same diameter and wrapped with an insulating film, and are wound in parallel.

[0088] Secondly, as Figure 1 and Figures 9-11 As shown, to ensure consistent accuracy performance of the three 0.2-level secondary coils, in addition to using the same wire and winding them in parallel, secondary coils A1021, B1022, and C1023 are also connected in parallel with resistors of the same resistance value. For example... Figure 1 and Figure 9 As shown, a damping unit 105 is connected in parallel between terminals 1a and 1n of the secondary coil A1021, and the damping unit 105 includes a damping coil 1051 and a damping resistor 1052 disposed on the first parallel branch, and a first parallel resistor 1053 disposed on the second parallel branch. Figure 1 and Figures 10-11As shown, a second parallel resistor 1054 is provided between terminals 2a and 2n of the secondary coil B1022 and between terminals 3a and 3n of the secondary coil C1023, and the first parallel resistor 1053 and the second parallel resistor 1054 have the same resistance. Furthermore, this invention is for a CVT used in converter stations of UHVDC transmission projects, where the requirements for antiferromagnetic performance are increased by 3 times and the requirements for transient response characteristics are increased by 2 times, making the requirements even more stringent. Through the design of the aforementioned damping unit 105, this invention can also meet the antiferromagnetic performance and transient response requirements of the CVT used in converter stations of UHVDC transmission projects.

[0089] After the structure of the secondary winding 102 is determined, the present invention designs the intermediate transformer 101 according to the requirement of 1.9 times the rated voltage factor for 8 hours, so that the secondary winding 102 has more turns (70 turns or more). This allows the intermediate transformer 101 to have a lower rated magnetic flux density (0.4T or less), but this also leads to an increase in the resistance of the primary winding 103 due to the increased number of turns.

[0090] To address the aforementioned issues, this invention optimizes the design based on load conditions to determine a smaller rated primary voltage for the intermediate transformer 101. This reduces the resistance of the primary winding 103, thereby lowering the overvoltage value on the intermediate transformer while maintaining error performance. This ensures that the CVT used in the converter station of the UHVDC transmission project has better accuracy and insulation performance.

[0091] On the other hand, this invention employs a special design for the compensating reactor 104 to ensure that the CVT has better error performance and can withstand greater overvoltage, such as... Figures 12-18 As shown, this invention firstly changes the conventional reactor main coil and reactor regulating coil, which consist of only one coil segment, to include two coil segments to improve the reactor's ability to withstand overvoltage. Secondly, this invention designs the reactor core 1041 cross-section as rectangular with stepped protrusions on each side. The stepped protrusions form stepped stops 10413 at both ends. This cross-section has a 25% larger area than the conventional rectangular core cross-section. Furthermore, as... Figure 15As shown, an air gap slit δ is provided between the first core section 10411 and the second core section 10412 on both sides of the reactor core 1041. This air gap length can be adjusted to adjust the inductance value so that the inductive reactance and capacitive reactance are equal, thereby minimizing the internal impedance of the CVT and ensuring a more accurate CVT. Therefore, compared with conventional capacitive voltage transformers, this invention can withstand higher overvoltage levels. Moreover, conventional capacitive voltage transformers usually have an overvoltage protector connected in parallel with the compensating reactor. The overvoltage protector needs to be replaced after multiple trips, which brings difficulties to operation and maintenance. However, due to the above-mentioned special structure, the compensating reactor 104 of this invention does not need to be connected in parallel with an overvoltage protector, which also avoids the problem of replacing the overvoltage protector during operation and maintenance.

[0092] like Figure 19 As shown, in one application example of the present invention, the capacitor voltage divider 3 can be divided into four sections. The top three sections are coupling capacitors, and the bottom section is a voltage divider. The intermediate voltage is extracted through the bottom voltage divider. These four sections can be insulating sleeves of the same height. Alternatively, considering transportation height restrictions, the bottom insulating sleeve can be lower than the height of the top three insulating sleeves. In this application example, the CVT creepage distance reaches 27280mm, and the altitude can reach 2500m. Additionally, as... Figure 19 As shown, this application example has a high-voltage terminal 2 at the upper end, which is equivalent to Figure 1 Terminal A in the middle.

Claims

1. A capacitive voltage transformer for a converter station in an ultra-high voltage direct current (UHVDC) transmission project, comprising an electromagnetic unit and a capacitive voltage divider, wherein the capacitive voltage divider includes a high-voltage capacitor C1 and a medium-voltage capacitor C2, the electromagnetic unit includes an intermediate transformer and a compensating reactor, the intermediate transformer including a primary winding and a secondary winding, and one end of the primary winding being connected to point M located between the high-voltage capacitor C1 and the medium-voltage capacitor C2, and the other end being connected to the compensating reactor, characterized in that: The secondary winding (102) includes secondary coil A (1021), secondary coil B (1022), secondary coil C (1023), secondary coil D (1024), secondary residual coil (1025), and insulating paper tube (1029). Secondary coils A (1021), B (1022), and C (1023) have the same wire diameter. When winding the secondary winding (102), the secondary residual coil (1025) is first wound on the insulating paper tube (1029). The da end of the secondary residual coil (1025) is led out from the head end of the insulating paper tube (1029), and the dn end is led out from the head end of the insulating paper tube (1029). 29) The tail end is led out, and then a secondary coil D (1024) is wound on the secondary remaining coil (1025). The 4a and 4n ends of the secondary coil D (1024) are both led out from the tail end of the insulating paper tube (1029). Then, secondary coil A (1021), secondary coil B (1022) and secondary coil C (1023) are wound in parallel on the secondary coil D (1024). The 1a and 1n ends of the secondary coil A (1021), the 2a and 2n ends of the secondary coil B (1022) and the 3a and 3n ends of the secondary coil C (1023) are all led out from the head end of the insulating paper tube (1029).

2. The capacitive voltage transformer for converter stations in ultra-high voltage direct current transmission projects according to claim 1, characterized in that: A shielding ring (1027) is wrapped around the outside of the secondary winding (102), and an insulating film (1026) is wrapped around the outside of the shielding ring (1027).

3. The capacitive voltage transformer for converter stations in ultra-high voltage direct current transmission projects according to claim 1, characterized in that: A damping unit (105) is connected in parallel between terminals 1a and 1n of secondary coil A (1021). A first parallel branch and a second parallel branch are provided between terminals 1a and 1n. The damping unit (105) includes a damping coil (1051) and a damping resistor (1052) provided on the first parallel branch, and a first parallel resistor (1053) provided on the second parallel branch. A third parallel branch is provided between terminals 2a and 2n of secondary coil B (1022) and between terminals 3a and 3n of secondary coil C (1023). A second parallel resistor (1054) is provided on the third parallel branch. The resistances of the first parallel resistor (1053) and the second parallel resistor (1054) are the same.

4. The capacitive voltage transformer for converter stations in ultra-high voltage direct current transmission projects according to claim 1, characterized in that: The number of turns of the secondary winding (102) is determined according to the following formula (1): In the above formula (1), Bm is the magnetic flux density; Usr is the rated secondary voltage; AFe is the cross-sectional area of ​​the intermediate transformer core; fr is the rated frequency; W2 is the number of turns of the secondary winding (102); The number of turns of the primary winding (103) is obtained according to the following formula (2): In the above formula (2), W1 is the number of turns of the primary winding (103); Ucr is the rated primary voltage of the intermediate transformer.

5. The capacitive voltage transformer for converter stations in ultra-high voltage direct current transmission projects according to claim 1, characterized in that: The compensating reactor (104) includes a reactor core (1041), a reactor main coil (1042), and a reactor adjusting coil (1043), wherein the reactor main coil (1042) is fitted on one side of the reactor core (1041), and the reactor adjusting coil (1043) is fitted on the other side of the reactor core (1041); The reactor main coil (1042) includes a first main line segment (10421) and a second main line segment (10422) connected in series, and the first main line segment (10421) and the second main line segment (10422) are separated by a first insulating paper ring (10423). The first main line segment (10421), the second main line segment (10422), and the first insulating paper ring (10423) are all disposed in a first insulating cavity formed by wrapping the first insulating paper (10424). The reactor adjustment coil (1043) includes a first adjustment line segment (10431) and a second adjustment line segment (10432) connected in series, and the first adjustment line segment (10431) and the second adjustment line segment (10432) are separated by a second insulating paper ring (10433). The first adjustment line segment (10431), the second adjustment line segment (10432), and the second insulating paper ring (10433) are all located in a second insulating cavity formed by wrapping the second insulating paper (10434).

6. The capacitive voltage transformer for converter stations in ultra-high voltage direct current transmission projects according to claim 5, characterized in that: The first main line segment (10421) has a first upper outlet (10425) at its upper end, and the second main line segment (10422) has a first lower outlet (10426) at one side of its lower end; the first adjustment line segment (10431) has a second upper outlet (10435) at its upper end, and the second adjustment line segment (10432) has a second lower outlet (10436) at one side of its lower end.

7. The capacitive voltage transformer for converter stations in ultra-high voltage direct current transmission projects according to claim 5, characterized in that: The reactor core (1041) has a rectangular cross-section and stepped protrusions on each side, with stepped stops (10413) formed at both ends of the stepped protrusions.

8. The capacitive voltage transformer for converter stations in ultra-high voltage direct current transmission projects according to claim 5, characterized in that: The reactor core (1041) has a first core section (10411) and a second core section (10412) on both sides, and an air gap cutout δ is provided between the first core section (10411) and the second core section (10412).

9. The capacitive voltage transformer for converter stations in ultra-high voltage direct current transmission projects according to claim 5, characterized in that: The compensating reactor (104) includes an upper pressure plate (1044), a lower pressure plate (1045), and fastening screws (1047). The upper pressure plate (1044) and the lower pressure plate (1045) both have a central protrusion and connecting plates on both sides. The upper end of the reactor core (1041) is placed in the central protrusion of the upper pressure plate (1044), and the lower end is placed in the central protrusion of the lower pressure plate (1045). The connecting plates on both sides of the upper pressure plate (1044) and the connecting plates on both sides of the lower pressure plate (1045) are respectively connected by fastening screws (1047). Both ends of the fastening screws (1047) are provided with locking nuts (1046).

10. The capacitive voltage transformer for converter stations in ultra-high voltage direct current transmission projects according to claim 1, characterized in that: The electromagnetic unit (1) includes an oil tank (111), and the intermediate transformer (101), the compensating reactor (104), and the damping unit (105) are all located in the oil tank (111). The upper end of the oil tank (111) is provided with a first sleeve (110), and the cable of the electromagnetic unit (1) is led out through the first sleeve (110) and connected to point M on the capacitor voltage divider (3). A second sleeve (106) is provided on one side of the oil tank (111), and the cable led out from the end of the medium voltage capacitor C2 is led out through the second sleeve (106) and connected to the oil tank (111). At the same time, the cable led out from the end of the compensating reactor (104) is also connected to the oil tank (111). A grounding wire is provided on one side of the oil tank (111), and a secondary terminal box (107) is provided on the oil tank (111), and a rain cover (108) is provided on the secondary terminal box (107).

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