High voltage direct current (HVDC) modular multilevel converter

Abstract

The present disclosure relates to a high voltage direct current (HVDC) modular multilevel converter. A HVDC modular multilevel converter for converting waveforms of current and voltage from alternating current (AC) to direct current (DC) or from direct current to alternating current includes power electronic units connected in series and positioned to form a stack and a conductive shielding structure arranged around the stack. When the converter operates, a magnetic field of irregular intensity is generated around the stack. The shielding structure includes at least one first portion made of a first conductive material and a second portion made of a second conductive material. The first conductive material is less susceptible to heating than the second conductive material when subjected to an electromagnetic field. The first portion is located in a first position in which there is a locally stronger magnetic field during operation of the converter, and the second portion is located in a second position in which there is a weaker magnetic field during operation of the converter than in the first position.

Description

Technical Field

[0001] This disclosure generally relates to the field of high-voltage direct current (HVDC) converter systems. More particularly, this disclosure relates to an HVDC modular multilevel converter for converting current and voltage waveforms from alternating current (AC) to direct current (DC) or vice versa. Background Technology

[0002] High-voltage direct current (HVDC) modular multilevel converters (hereinafter referred to as converters) are a vital component of the power grid. The core function of a converter is to transform power, that is, to seamlessly switch between alternating current (AC) and direct current (DC). An HVDC converter consists of many small parts called “power electronic units,” which are stacked together to form larger units called “valve.” Due to their complex design and high-voltage operation, valves require comprehensive protection to maintain their proper function. Shielding is deployed around each valve and across the top and bottom to prevent electrical breakdown between the valve, other nearby valves, other adjacent electrical equipment, and the grounding substation walls.

[0003] The shielding system, including side shields, top shields, and bottom shields, is typically made of thick aluminum, with thousands of side shields produced annually. In the example, the pair of side shields currently installed on the HVDC valve is bulky, weighing in the range of 60 kg. This means:

[0004] a) If the HVDC valve comprises two layers, there will be 8 pairs of side shields, and therefore the total mass of all side shields will be 480 kg.

[0005] b) If the HVDC valve comprises five layers, there will be 20 pairs of side shields, and the total mass of the side shields will be 1200 kg.

[0006] The total mass of a five-layer valve is approximately 25,000 kg, therefore the mass of the shielding is a significant component (almost 5%) of the total weight of the entire HVDC valve.

[0007] The current shielding design used in HVDC valves is a bottleneck in the development of efficient and cost-effective power grids. While the shielding provides adequate protection, its weight can complicate mechanical design and installation. Worse still, aluminum is a high-cost material, both in terms of raw materials and the cost of manufacturing components from aluminum. Therefore, the current shielding design for HVDC valves is not only bulky but also expensive. Furthermore, the existing shielding design may require robust supports for the installation of the side shielding.

[0008] Furthermore, in existing shielding, problems may arise when the temperature is high during converter operation, and hot spots may appear in the shielding due to localized strong magnetic fields. Utility Model Content

[0009] Therefore, there is a need for an improved high-voltage direct current (HVDC) modular multilevel converter with an improved shielding structure.

[0010] Therefore, the object of this disclosure is to provide an HVDC modular multilevel converter with an improved shielding structure for converting current and voltage waveforms from alternating current (AC) to direct current (DC) or vice versa. The improved shielding structure can mitigate, alleviate or eliminate all or at least some of the aforementioned disadvantages of currently known solutions.

[0011] This and other objectives are achieved by an HVDC modular multilevel converter as defined in the appended claims. The term "exemplary" should be understood in this context as an instance, example, or illustration.

[0012] According to a first aspect of this disclosure, a high-voltage direct current (HVDC) modular multilevel converter (hereinafter referred to as the converter) is provided for converting current and voltage waveforms from alternating current (AC) to direct current (DC) or from DC to AC. The converter includes power electronic units and a conductive shielding structure. The power electronic units are connected in series and positioned to form a stack. The conductive shielding structure is arranged around the stack. In operation of the converter, an irregular magnetic field of varying intensity is generated around the stack. The converter is characterized in that the shielding structure includes at least one first portion made of a first conductive material and a second portion made of a second conductive material, wherein the first conductive material is less susceptible to heating than the second conductive material under electromagnetic field conditions. The first portion is positioned in a first location, and the second portion is located in a second location. The irregular magnetic field of varying intensity is characterized by a locally stronger magnetic field in the first location and a weaker magnetic field in the second location during converter operation.

[0013] Therefore, the embodiments described herein provide a shielding structure based on the magnetothermal behavior of an HVDC modular multilevel converter. The first and second portions of the shielding structure are made of different conductive materials and are positioned based on the magnetic field generated during converter operation. Both the first and second conductive materials are lightweight.

[0014] In some embodiments, the first material has a higher electrical conductivity than the second material.

[0015] In some embodiments, the converter includes at least one terminal extending from the stack of electrical units, wherein a first position is a position close to the terminal and a second position is a position further away from the terminal.

[0016] In some embodiments, the first conductive material includes a non-magnetic material.

[0017] In some embodiments, the first conductive material comprises austenitic steel.

[0018] In some embodiments, the austenitic steel is a stainless steel grade containing at least 10.5 wt.% chromium.

[0019] In some embodiments, the first conductive material includes any one of aluminum, aluminum alloy, copper, or copper alloy.

[0020] In some embodiments, the first conductive material includes copper or a copper alloy.

[0021] In some embodiments, the second conductive material comprises ferritic steel.

[0022] Advantageously, in the proposed embodiments, different materials are used at different locations (e.g., a first location and a second location) of the shielding structure, depending on the stronger magnetic field typically caused by the presence of the terminals. Hot spots may appear in the shielding structure due to locally strong magnetic fields. The proposed embodiments address the hot spot problem and reduce losses and temperatures in the shielding structure by using suitable materials in the corresponding locations where hot spots appear. Therefore, a non-magnetic material is used in the first part of the shielding structure because the first part is located near the terminals at a first location, and the magnetic field is stronger at the first location during converter operation. Further, a second conductive material comprising ferritic steel is used in the second part of the shielding structure because the second part is located away from the terminals at a second location, and the magnetic field is weaker at the second location during converter operation.

[0023] In some embodiments, the ferritic steel is a carbon steel grade.

[0024] Compared to traditional aluminum, carbon steel offers several advantages when used as a shielding material for structural components. The carbon steel second section of the shielding structure is at least 57% lighter than its aluminum counterpart, thus reducing the overall weight of the shielding structure. Furthermore, because the second section of the shielding structure is made of carbon steel, it performs exceptionally well in blocking low-frequency magnetic fields generated by the active portion of the transducer, ensuring superior electromagnetic compatibility (EMC). In addition, using carbon steel to manufacture the second section of the shielding structure significantly reduces production costs (including material and manufacturing costs, with raw material costs reduced by 80%).

[0025] Advantageously, carbon steel grades have low levels of alloying elements, and are therefore generally cheaper, which significantly reduces the production cost of shielding structures.

[0026] In some embodiments, the conductivity of the first conductive material is at least twice that of the conductivity of the second conductive material.

[0027] In some embodiments, the first portion includes an insert disposed on and attached to the second portion.

[0028] In some embodiments, the insert is attached to the side of the second portion that is oriented toward the stack of electrical cells.

[0029] Advantageously, the insert is made of a conductive material and is disposed inside the second part of the shielding structure, which helps to reduce losses and temperature hotspots in the second part of the shielding structure.

[0030] In some embodiments, the shielding structure includes sheet-like parts arranged to cover opposite ends of the stack of electrical units.

[0031] The proposed shielding structure is lightweight because it uses different lightweight materials in different parts. Furthermore, the proposed shielding structure is cost-effective due to the low cost of the materials used.

[0032] Other advantages may be apparent to those skilled in the art. Some embodiments may have some or all of the advantages described. Attached Figure Description

[0033] The foregoing will become apparent from the following more detailed description of the exemplary embodiments illustrated in the accompanying drawings, in which the same reference numerals refer to the same parts in all different views. The drawings are not necessarily drawn to scale, but rather focus on illustrating the exemplary embodiments.

[0034] Figure 1A A prior art example of a converter system is disclosed, which illustrates hot spots on a shielding structure;

[0035] Figure 1B A schematic diagram of an example high-voltage direct current (HVDC) modular multilevel converter according to some embodiments is disclosed;

[0036] Figures 2A to 2C show external and internal views of a first or second portion of an example shielding structure according to some embodiments;

[0037] Figure 3A shows a cross-sectional view of a prior art side shielding structure;

[0038] Figure 3B shows a cross-sectional view of an example second part of a shielding structure according to some embodiments; and

[0039] Figure 3C shows a cross-sectional view of an example of a first portion according to some embodiments, wherein an insert is disposed on and attached to a second portion of the shielding structure. Detailed Implementation

[0040] The aspects of this disclosure will be described more fully below with reference to the accompanying drawings. However, the high-voltage direct current (HVDC) modular multilevel converter (hereinafter referred to as the converter) disclosed herein can be implemented in many different forms and should not be construed as being limited to the aspects set forth herein. Throughout the drawings, the same reference numerals refer to the same elements.

[0041] The terminology used herein is for the purpose of describing specific aspects of this disclosure only and is not intended to limit the invention. It should be emphasized that the term "comprising / including" as used in this specification is used to specify the presence of the stated feature, integral, step, or component, but does not exclude the presence or addition of one or more other features, integrals, steps, components, or groups thereof. Unless the context clearly indicates otherwise, the singular forms "a," "an," and "described" are intended to include the plural forms as well, as used herein.

[0042] The embodiments of this disclosure will be described and illustrated more fully below with reference to the accompanying drawings. However, the solutions disclosed herein can be implemented in many different forms and should not be construed as limited to the examples set forth herein.

[0043] In the following description of exemplary embodiments, the same reference numerals denote the same or similar parts.

[0044] In existing technologies, shielding components are heavy, which can complicate mechanical design and installation. The shielding structure design of HVDC modular multilevel converters is not only bulky but also expensive.

[0045] Further, refer to Figure 1A This illustrates a prior art example of a converter system. For example... Figure 1A As shown, a hot spot (100b) exists on the shielding structure (100a). Problems may occur when the temperature rises during converter operation, and hot spots may appear in the shielding structure due to localized strong magnetic fields.

[0046] In contrast to existing technologies, the embodiments described herein provide a high-voltage direct current (HVDC) modular multilevel converter with an improved shielding structure.

[0047] In an embodiment, Figure 1BA schematic diagram of an example high-voltage direct current (HVDC) modular multilevel converter (100) (hereinafter referred to as the converter) is disclosed. The converter (100) converts the waveforms of current and voltage from alternating current (AC) to direct current (DC), or vice versa. The converter (100) includes power electronic units (102). The power electronic units (102) are connected in series and positioned to form a stack. Figure 1B As shown, the converter (100) further includes a conductive shielding structure (104) arranged around the stack. During operation of the converter (100), an irregular magnetic field of varying strength is generated around the stack. The shielding structure (104) is arranged around the stack and across the top and bottom of the stack to prevent electrical breakdown with other adjacent electrical devices.

[0048] The shielding structure (104) includes a second portion (108) and at least one first portion (106). At least one first portion (106) is made of a first conductive material. The second portion (108) is made of a second conductive material. The first conductive material is less susceptible to heating than the second conductive material when subjected to an electromagnetic field. In the example, at least one first portion (106) and the second portion (108) are arranged in at least one side position of a stack disposed in the converter (100). The first portion (106) is positioned in a first position where a locally strong magnetic field exists during operation of the converter (100). The second portion (108) is located in a second position where a weaker magnetic field exists during operation of the converter than when it is in the first position.

[0049] In existing converter systems, the entire shielding structure is primarily designed using heavy materials such as aluminum. Furthermore, aluminum is a high-cost material, both in terms of raw materials and the cost of manufacturing components with aluminum.

[0050] Therefore, the embodiments herein provide a shielding structure (104) based on the magnetocaloric behavior of the converter (100). A first portion (106) and a second portion (108) of the shielding structure (104) are made of a first conductive material and a second conductive material. The first portion (106) and the second portion (108) of the shielding structure (104) are positioned based on the magnetic field generated during operation of the converter (100). The first and second conductive materials are lightweight.

[0051] In some embodiments, the first conductive material has a higher conductivity than the second conductive material.

[0052] In some embodiments, such as Figure 1BAs shown, the converter (100) includes at least one terminal (110). At least one terminal (110) extends from the stack of electrical units (102). A first position is a position close to the terminal (110), and a second position is a position further away from the terminal (110).

[0053] In some embodiments, the first conductive material includes a non-magnetic material.

[0054] In some embodiments, the first conductive material comprises austenitic steel. Austenitic steel is a stainless steel grade containing at least 10.5 wt.% chromium.

[0055] In the example, the austenitic steel may contain a percentage of carbon (<1.2%), a percentage of chromium (>10.5%), and iron, as well as other components.

[0056] In some embodiments, the first conductive material includes any one of aluminum, aluminum alloy, copper, or copper alloy.

[0057] In some embodiments, the first conductive material includes copper or a copper alloy.

[0058] Advantageously, in the proposed embodiment, different materials are used at different locations (e.g., a first location and a second location) in the shielding structure (104) based on the stronger magnetic field typically caused by the presence of the terminal (110). Hot spots (not shown) may appear in the shielding structure (104), and these hot spots are caused by locally strong magnetic fields. The proposed embodiment addresses the hot spot problem and reduces the loss and temperature of hot spots in the shielding structure (104) by using suitable materials in the corresponding portions where hot spots appear. Therefore, a non-magnetic material is used in the first portion (106) of the shielding structure (104) because the first portion (106) is located close to the terminal (110) at the first location, and the magnetic field is stronger at the first location during converter (110) operation. Further, a second conductive material comprising ferritic steel is used in the second portion (108) of the shielding structure (104) because the second portion (108) is located away from the terminal (110), and the magnetic field is weaker during converter (100) operation.

[0059] In some embodiments, the second conductive material comprises ferritic steel.

[0060] In some embodiments, the ferritic steel is a carbon steel grade.

[0061] In some embodiments, carbon steel offers several advantages over conventional aluminum when used as a shielding material for structural elements. The second part (108) of the shielding structure (104), made of carbon steel, is at least 57% lighter than its aluminum counterpart, thereby reducing the overall weight of the shielding structure (104). Furthermore, since the second part (108) of the shielding structure (100) is made of carbon steel, it performs exceptionally well in blocking the low-frequency magnetic field generated by the active portion of the converter (100), thus ensuring superior electromagnetic compatibility (EMC). In addition, using carbon steel to manufacture the second part (108) of the shielding structure significantly reduces production costs (including material and manufacturing costs, with raw material costs reduced by 80%).

[0062] Advantageously, carbon steel grades have low levels of alloying elements and are therefore generally cheaper, thus significantly reducing the production cost of the shielding structure (100).

[0063] In the example, ferritic steel may contain carbon content (a percentage selected from 0.05 to 2.1%), iron content, and other components.

[0064] In the example, the material properties of stainless steel and carbon steel (at 20°C) are as follows:

[0065] Non-alloy structural steel (carbon steel): (ferro)magnetic, σ>2E+6 S / m, ρ = 7900 kg / m3 ± 5%;

[0066] Stainless steel: non-magnetic, σ>8E+5 S / m, ρ = 7900 kg / m3 ± 5%.

[0067] In some embodiments, the conductivity of the first conductive material is at least twice that of the conductivity of the second conductive material.

[0068] In some embodiments, Figures 2A and 2B show an external view and an internal view of the shielding structure (104) of the example shielding structure, respectively.

[0069] In some embodiments, as shown in FIG2C, the first portion (106) includes an insert (112) disposed in the second portion (108) of the shielding structure (100) (e.g. Figure 1B (as shown) and attached to the second part.

[0070] In some embodiments, the insert (112) is attached to the side of the second portion (108) oriented toward the stack of electrical units (102). The insert (112) may include a first conductive material having a conductivity at least twice that of a second conductive material. In an example, the insert (112) is selected from at least one material selected from the group consisting of copper, aluminum, silver, gold, etc. In an example, the insert (112) is selected to have a thickness in the range of 0.5 mm to 2 mm. In an example, the material of the insert (112) is selected as copper, and the thickness of the insert (112) is 2 mm, thereby better reducing temperature.

[0071] Advantageously, the insert (112) is made of a conductive material and is disposed inside the second part (108) of the shielding structure (104), which helps to reduce losses and temperature hotspots in the second part (108) of the shielding structure (104).

[0072] In some embodiments, the shielding structure (104) includes sheet-like parts (114, 116) arranged to cover opposite ends of the stack of electrical units (102). Figure 1B As shown, sheet (114) is arranged on top of the stack of electrical units (102), and sheet (116) is arranged on the bottom of the stack of electrical units (102). In the example, sheets (114, 116) may be made of one or more non-alloy structural materials selected from at least one of carbon steel, aluminum, or stainless steel.

[0073] In the example, the material selection for the converter (100) is given according to specific standards. For example, non-alloy structural steel is selected according to EN 10025-2; stainless steel is selected according to EN 10088-2; and copper is selected according to EN 1652.

[0074] Figure 3a A cross-sectional view of a prior art example side shielding structure 300A is shown. The side shielding structure (300A) is made of aluminum (302) and has a thickness of 4 mm. Aluminum is a high-cost material, both in terms of raw materials and the cost of manufacturing components with aluminum. The prior art shielding structure 300A is not only bulky but also expensive.

[0075] like Figure 3b As shown, a cross-sectional view of an example second part (304) of the shielding structure (300B) is presented. The second part (304) of the shielding structure (300B) is made of carbon steel and has a thickness of 0.6 mm.

[0076] like Figure 3c As shown, an example of the first part is illustrated, wherein the insert (306) is disposed on and attached to the second part (304) of the shielding structure (300B). The second part (304) of the shielding structure (300B) is made of carbon steel and has a thickness of 0.6 mm. The insert (306) is made of copper and has a thickness of 2 mm.

[0077] In some embodiments, as shown in Figures 3B and 3C, the shielding structure may include a plurality of C-shaped or L-shaped profiles arranged to surround the sides of the stack of electrical units, wherein inserts are disposed inside the corners of the C-shaped or L-shaped profiles.

[0078] The foregoing description of specific embodiments will fully reveal the general nature of the embodiments herein, such that others can readily modify and / or adapt such specific embodiments for various applications by applying existing knowledge without departing from the overall concept, and therefore, such modifications and adaptations should and are intended to be understood as being within the meaning and scope of equivalents of the disclosed embodiments. It should be understood that the wording or terminology used herein is for descriptive purposes and not restrictive. Therefore, although embodiments herein have been described with respect to preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modifications within the scope of this disclosure.

Claims

1. A high-voltage direct current (HVDC) modular multilevel converter, the HVDC modular multilevel converter being used to convert current and voltage waveforms from alternating current (AC) to direct current (DC), or from DC to alternating current, the converter (100) comprising power electronic units (102) and a conductive shielding structure (104), the power electronic units being connected in series and positioned to form a stack, the conductive shielding structure being arranged around the stack, wherein, When the converter (100) is in operation, an irregular magnetic field of varying intensity is generated around the stack, wherein the converter (100) is characterized in that the shielding structure (104) includes at least one first portion (106) made of a first conductive material and a second portion (108) made of a second conductive material, wherein the first conductive material is less susceptible to heating than the second conductive material when subjected to an electromagnetic field, wherein the first portion (106) is located in a first position and the second portion (108) is located in a second position, wherein the irregular magnetic field of varying intensity is that during the operation of the converter (100), there is a locally stronger magnetic field in the first position and a weaker magnetic field in the second position than in the first position.

2. The high voltage direct current (HVDC) modular multilevel converter of claim 1, wherein, The first conductive material has a higher conductivity than the second conductive material.

3. The high voltage direct current (HVDC) modular multilevel converter of claim 1 or 2, wherein, The first conductive material includes a non-magnetic material.

4. The high voltage direct current (HVDC) modular multilevel converter of claim 1 or 2, wherein, The first conductive material includes austenitic steel.

5. The high voltage direct current (HVDC) modular multilevel converter of claim 4, wherein, The austenitic steel is a stainless steel grade containing at least 10.5 wt.% chromium.

6. The high voltage direct current (HVDC) modular multilevel converter of claim 1 or 2, wherein, The first conductive material includes any one of aluminum, aluminum alloy, copper, or copper alloy.

7. The high voltage direct current (HVDC) modular multilevel converter of claim 1 or 2, wherein, The first conductive material includes copper or a copper alloy.

8. The high voltage direct current (HVDC) modular multilevel converter of claim 1 or 2, wherein, The conductivity of the first conductive material is at least twice that of the conductivity of the second conductive material.

9. The high voltage direct current (HVDC) modular multilevel converter of claim 1 or 2, wherein, The first part includes an insert disposed on and attached to the second part (108).

10. The high-voltage direct current (HVDC) modular multilevel converter according to claim 9, wherein, The insert (112) is attached to the side of the second portion that is oriented toward the stack of the power electronics unit (102).

11. The high voltage direct current (HVDC) modular multilevel converter of claim 1 or 2, wherein, The shielding structure (104) includes sheet-like parts (114, 116) arranged to cover opposite ends of the stack of the power electronic units (102).

12. The high voltage direct current (HVDC) modular multilevel converter of claim 1 or 2, wherein, The high-voltage direct current (HVDC) modular multilevel converter includes at least one terminal (110) extending from the stack of the power electronics unit (102), wherein the first position is a position close to the terminal (110) and the second position is a position further away from the terminal (110).

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