A symmetrical monopole flexible DC grounding system and grounding parameter selection method thereof

Abstract

The present invention relates to a symmetrical monopole flexible DC grounding system and a grounding parameter selection method thereof, comprising a connection transformer arranged in a valve side AC field area; the grid side star winding of the connection transformer is grounded through one end of a connection transformer grid side lightning arrester arranged in the grid side AC field area, and the grid side star winding of the connection transformer is also connected in parallel to a connection transformer grid side neutral point lightning arrester and one end of a first current transformer; the valve side delta winding of the connection transformer is connected in parallel to a grounding reactor loop and one end of a connection transformer valve side current transformer through a starting loop, and the other end of the connection transformer valve side current transformer is connected to a converter arranged in a valve hall area through a strong bushing; the other ends of the connection transformer grid side lightning arrester, the connection transformer grid side neutral point lightning arrester, the first current transformer, the starting loop and the grounding reactor loop are all directly grounded, and the present invention can be widely used in the field of flexible DC power transmission.

Description

Technical Field

[0001] The invention relates to the field of flexible direct current power transmission, and in particular to a symmetrical monopole flexible direct current grounding system and a grounding parameter selection method thereof. Background Art

[0002] Flexible DC transmission has the advantages of being able to quickly and independently control active and reactive power, no need for filtering and reactive compensation devices, no commutation failure problems, being able to power passive systems, and being suitable for forming multi-terminal DC systems and DC power grids. Therefore, it has broad application prospects in renewable energy grid connection, asynchronous power grid interconnection, urban power grid expansion and transformation, power supply to remote areas, and multi-terminal network construction.

[0003] Flexible DC generally includes symmetrical bipolar flexible DC and symmetrical unipolar flexible DC structures. The symmetrical bipolar flexible DC structure is generally grounded through a neutral line system; the commonly used symmetrical unipolar DC structure is generally connected to the transformer neutral point high-resistance grounding. On the one hand, this grounding system has the transformer must adopt a triangular winding structure. In order to provide a third harmonic flow loop, an additional triangular third winding is required, which increases the complexity of transformer manufacturing. On the other hand, under the high-resistance grounding method, the transformer neutral point insulation design is difficult, and it is necessary to propose a new symmetrical unipolar flexible DC grounding system and parameter selection method to improve the flexibility and economy of the grounding system. Summary of the invention

[0004] In view of the above problems, the purpose of the present invention is to provide a symmetrical monopole flexible DC grounding system and a grounding parameter selection method thereof, which can effectively reduce the manufacturing difficulty of the connection transformer and improve the flexibility and economy of the grounding system.

[0005] To achieve the above object, the present invention adopts the following technical solutions: On the one hand, a symmetrical monopole flexible DC grounding system is provided, including a connection transformer arranged in the valve side AC field area;

[0006] The grid-side star winding of the connection transformer is grounded through one end of the connection transformer grid-side arrester arranged in the grid-side AC field area, and the grid-side star winding of the connection transformer is also connected in parallel to the connection transformer grid-side neutral point arrester and one end of the first current transformer;

[0007] The valve-side delta winding of the connection transformer is connected in parallel to the grounding reactor circuit and one end of the connection transformer valve-side current transformer through the starting circuit, and the other end of the connection transformer valve-side current transformer is connected to the converter arranged in the valve hall area through a strong bushing;

[0008] The other ends of the transformer grid-connected lightning arrester, the transformer grid-connected neutral point lightning arrester, the first current transformer, the starting circuit and the grounding reactor circuit are all directly grounded.

[0009] Further, the starting circuit includes a connecting transformer side lightning arrester, a second current transformer, a third current transformer, a connecting transformer side voltage transformer, a first isolation switch, a second isolation switch, a first grounding switch, a second grounding switch, a third grounding switch and a connecting transformer side starting circuit resistor;

[0010] One end of the connecting transformer valve side lightning arrester is connected in parallel to one end of the connecting transformer valve side voltage transformer and the valve side delta winding of the connecting transformer, the other end of the connecting transformer valve side lightning arrester is connected to one end of the second current transformer, the end of the connecting transformer valve side voltage transformer is also connected in parallel to the first isolation switch and one end of the first grounding switch, the other end of the first isolation switch is connected to one end of the third grounding switch in sequence through one end of the second grounding switch, the third current transformer and the connecting transformer valve side starting loop resistor, the end of the first grounding switch is also connected to one end of the third grounding switch through the second isolation switch, and the end of the third grounding switch is also connected in parallel to the grounding reactor loop and one end of the connecting transformer valve side current transformer;

[0011] The other ends of the second current transformer, the voltage transformer on the connecting transformer side, the first grounding switch, the second grounding switch and the third grounding switch are all directly grounded.

[0012] Further, the grounding reactor loop includes a grounding loop inductor, a fourth current transformer, a fifth current transformer, a grounding resistor arrester and a grounding resistor;

[0013] One end of the grounding loop inductor is connected in parallel to the third grounding switch and the current transformer on the connecting valve side, the other end of the grounding loop inductor is connected in parallel to the grounding resistor and one end of the grounding resistor arrester through the fourth current transformer, and the other end of the grounding resistor is connected to one end of the fifth current transformer;

[0014] The other ends of the grounding resistance arrester and the fifth current transformer are directly grounded.

[0015] On the other hand, a method for selecting grounding parameters based on a symmetrical monopole flexible DC grounding system is provided, characterized by comprising:

[0016] Determine the influencing factors and design boundary conditions for grounding parameter selection of symmetrical monopole flexible DC grounding system;

[0017] Based on the determined influencing factors and design boundary conditions, the parameters of the grounding reactor circuit are determined.

[0018] Furthermore, the influencing factors include the effective value of the AC phase voltage on the connecting transformer side, the DC voltage value, the DC voltage control deviation, the modulation ratio of the flexible DC system, the fundamental frequency value, the maximum AC current value allowed to flow through the grounding inductance, and the maximum DC current value allowed to flow through the grounding inductance.

[0019] Further, the determining of the parameters of the grounding reactor circuit based on the determined influencing factors and design boundary conditions includes:

[0020] Based on the determined influencing factors and design boundary conditions, determine the parameters of the ground loop inductance and ground resistance;

[0021] Comprehensively considering steady-state measurement and transient protection requirements of the ground loop inductance and ground resistance, determining parameters of the fourth current transformer and the fifth current transformer;

[0022] Different reference voltages are selected for grounding resistance arresters. Considering the maximum transient stress on the grounding resistor when different types of faults occur at various locations in the symmetrical single-pole flexible DC grounding system, the reference voltage, coordination current, residual voltage and energy parameters of the grounding resistance arrester are determined.

[0023] Furthermore, the ground loop inductance is:

[0024]

[0025] Among them, U d is the DC voltage value, δ is the DC voltage control deviation, M is the flexible DC system modulation ratio, f is the base frequency value, and the maximum AC current value I allowed to flow through the grounding inductance 1 .

[0026] Furthermore, the grounding resistance is:

[0027]

[0028] Among them, I 2 is the maximum DC current allowed to flow through the ground inductance.

[0029] Further, the method of comprehensively considering the steady-state measurement and transient protection requirements of the ground loop inductance and the ground resistance to determine the parameters of the fourth current transformer and the fifth current transformer includes:

[0030] The rated currents of the fourth current transformer and the fifth current transformer must be able to cover the steady-state current flowing through the ground loop inductance and the ground resistance respectively;

[0031] At the same time, the transient measurement range must be able to cover the maximum fault current flowing through the ground loop inductance and ground resistance respectively.

[0032] The present invention adopts the above technical solution, which has the following advantages:

[0033] 1. The present invention eliminates the original star-shaped neutral point grounding large resistance on the valve side of the connection transformer and changes it to a triangle connection mode, providing a third harmonic flow loop and further eliminating the triangle third winding, which can greatly reduce the design and manufacturing difficulty of the connection transformer and improve the design reliability and economy of the connection transformer.

[0034] 2. The grounding reactor of the present invention is simple in design, occupies a small area, and has negligible reactive power consumption. It can provide a good grounding point without causing adverse effects on the design of the grounding system.

[0035] 3. The fourth current transformer in the grounding reactor circuit of the present invention can effectively monitor the operating status of the reactor, and judge whether the reactor is operating normally, short-circuited or open-circuited by current characteristics, providing an effective reference for operators.

[0036] 4. In the past, when the neutral point of the transformer was connected to a high-resistance grounding method, after a resistor failure occurred, the operating personnel could not know the equipment status. Once a grounding failure occurred in the flexible DC system, it would seriously affect the system safety. The present invention can well monitor the operating performance of the grounding device and equipment. When a defect occurs in the grounding device, the operating personnel can be notified immediately to handle it, thereby ensuring the safe operation of the system.

[0037] In summary, the present invention can be widely used in the field of flexible direct current transmission. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present invention. Throughout the accompanying drawings, the same reference numerals are used to represent the same components. In the accompanying drawings:

[0039] Figure 1 It is a schematic diagram of the configuration structure of the main electrical connection for connecting the star-shaped neutral point high-resistance grounding of the transformer valve side in the prior art;

[0040] Figure 2 It is a structural schematic diagram of a symmetrical monopole flexible DC grounding system provided in one embodiment of the present invention. DETAILED DESCRIPTION

[0041] The exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments described herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present invention and to fully convey the scope of the present invention to those skilled in the art.

[0042] It should be understood that the terms used herein are only for the purpose of describing specific example embodiments and are not intended to be limiting. Unless the context clearly indicates otherwise, the singular forms "one", "an" and "said" as used herein may also be meant to include plural forms. The terms "include", "comprise", "contain", and "have" are inclusive, and therefore specify the existence of stated features, steps, operations, elements and / or parts, but do not exclude the existence or addition of one or more other features, steps, operations, elements, parts, and / or combinations thereof. The method steps, processes, and operations described herein are not interpreted as necessarily requiring them to be performed in the specific order described or illustrated, unless the execution order is clearly indicated. It should also be understood that additional or alternative steps may be used.

[0043] Although the terms first, second, third, etc. can be used in the text to describe multiple elements, components, regions, layers and / or sections, these elements, components, regions, layers and / or sections should not be limited by these terms. These terms can only be used to distinguish an element, component, region, layer or section from another region, layer or section. Unless the context clearly indicates, terms such as "first", "second" and other numerical terms do not imply order or sequence when used in the text. Therefore, the first element, component, region, layer or section discussed below can be referred to as the second element, component, region, layer or section without departing from the teaching of the example embodiments.

[0044] For ease of description, spatially relative terms may be used herein to describe the relationship of one element or feature relative to another element or feature as shown in the figures, such as "inside", "outside", "inner side", "outer side", "below", "above", etc. Such spatially relative terms are intended to include different orientations of the device in use or operation in addition to the orientation depicted in the figures.

[0045] like Figure 1 As shown in the figure, the star-shaped neutral point of the transformer valve side is grounded through a large resistor in the prior art. This grounding method has the following technical defects: 1) The transformer valve side needs to adopt a star connection method, and the neutral point insulation level is high. In order to provide a third harmonic flow circuit, an additional triangle third winding needs to be configured, which significantly increases the difficulty of transformer design and manufacturing and reduces the design reliability of the transformer; 2) The neutral point of the transformer valve side is at zero point under normal operation. Therefore, when the grounding resistance is short-circuited or open-circuited, the fault cannot be detected by the current monitoring device. When a grounding fault occurs in the flexible DC system, it will cause a very large operation risk and protection measures will be difficult to deal with.

[0046] Based on the above defects, the symmetrical monopole flexible DC grounding system provided by the embodiment of the present invention is compared with Figure 1 The main differences of the technical solutions are as follows: 1) The large grounding resistor on the valve side is cancelled, the third winding is cancelled, and the star winding on the valve side of the transformer is changed to a triangle winding, which not only plays the role of connecting the transformer, but also provides the passage of the third harmonic, which can greatly reduce the manufacturing difficulty of the connecting transformer and improve the reliability and economy of the design. 2) The grounding is carried out separately on the valve side of the connecting transformer through a large reactance star resistor. The reactor is selected to be in the kilohenry level, and the resistor is selected to be in the kilo-ohm level. Both devices have the characteristics of low manufacturing difficulty, low loss, and small footprint. In addition, in the case of a short circuit or open circuit of the inductor, the current monitoring device can give an early warning; in the case of a short circuit of the resistor, it will not affect the operation of the system; in the case of an open circuit of the resistor, the parallel lightning arrester can play a corresponding alternative role.

[0047] Example 1

[0048] like Figure 2 As shown, this embodiment provides a symmetrical single-pole flexible DC grounding system, including a connection transformer grid-side lightning arrester F1 (a, b, c three-phase) arranged in the grid-side AC field area, a connection transformer TR (a, b, c three-phase) arranged in the valve-side AC field area, a connection transformer grid-side neutral point lightning arrester F2, a first current transformer T1, a starting circuit, a connection transformer valve-side current transformer T6 (a, b, c three-phase), a reinforcing bushing X1 (a, b, c three-phase) and a grounding reactor circuit, and a converter arranged in the valve hall area, wherein the starting circuit includes a connection transformer valve-side lightning arrester F3 (a, b, c three-phase), a second current transformer T2 (a, b, c three-phase), a third current transformer The invention relates to a circuit breaker comprising a first current transformer T3 (a, b, c three-phase), a voltage transformer PT1 (a, b, c three-phase), a first isolating switch Q11 (a, b, c three-phase), a second isolating switch Q12 (a, b, c three-phase), a first grounding switch Q21 (a, b, c three-phase), a second grounding switch Q22 (a, b, c three-phase), a third grounding switch Q23 (a, b, c three-phase) and a starting circuit resistor R1 (a, b, c three-phase) connected to the transformer side. The grounding reactor circuit includes a grounding circuit inductor L1 (a, b, c three-phase), a fourth current transformer T4 (a, b, c three-phase), a fifth current transformer T5, a grounding resistor arrester F4 and a grounding resistor R2.

[0049] The grid-side star winding of the connection transformer TR is grounded through one end of the connection transformer grid-side arrester F1, and the grid-side star winding of the connection transformer TR is also connected in parallel to the connection transformer grid-side neutral point arrester F2 and one end of the first current transformer T1.

[0050] The valve-side delta winding of the connection transformer TR is connected in parallel to the first isolation switch Q11 and one end of the first grounding switch Q21 through the connection transformer valve-side lightning arrester F3 and one end of the connection transformer valve-side voltage transformer PT1. The other end of the connection transformer valve-side lightning arrester F3 is connected to one end of the second current transformer T2. The other end of the first isolation switch Q11 is connected to one end of the third grounding switch Q23 through one end of the second grounding switch Q22, the third current transformer T3 and the connection transformer valve-side starting loop resistor R1. The first grounding switch Q The other end of 21 is connected to one end of the third grounding switch Q23 through the second isolation switch Q12, and the end of the third grounding switch Q23 is also connected in parallel to the grounding loop inductor L1 and one end of the current transformer T6 on the variable valve side. The other end of the grounding loop inductor L1 is connected in parallel to the grounding resistor R2 and one end of the grounding resistor lightning arrester F4 through the fourth current transformer T4. The other end of the grounding resistor R2 is connected to one end of the fifth current transformer T5, and the other end of the current transformer T6 on the variable valve side is connected to the converter through the strengthening bushing X1.

[0051] The other ends of the transformer substation side lightning arrester F1, the transformer substation side neutral point lightning arrester F2, the grounding resistance lightning arrester F4, the first current transformer T1, the second current transformer T2, the fifth current transformer T5, the transformer valve side voltage transformer PT1, the first grounding switch Q21, the second grounding switch Q22 and the third grounding switch Q23 are all directly grounded.

[0052] In a preferred embodiment, the connection transformer adopts a Y / d structure, and there is no need to configure a third winding. The d winding can provide the third harmonic flow loop required by the grounding system.

[0053] In a preferred embodiment, the fourth current transformer T4 of the grounding reactor loop can effectively monitor the operating status of the reactor, determine whether the reactor is operating normally, short-circuited or open-circuited based on current characteristics, and provide effective reference for operators.

[0054] In a preferred embodiment, by configuring the grounding resistor arrester F4, the voltage stress borne by the grounding resistor in the transient state can be effectively limited, and the reliable operation of the grounding resistor can be protected. In the case of a short circuit of the grounding resistor, the resistance of the grounding reactor itself can limit the unbalanced DC voltage borne by the AC busbar on the valve side; in the case of an open circuit of the grounding resistor, the grounding resistor arrester F4 can replace the grounding circuit to play a corresponding role.

[0055] Example 2

[0056] This embodiment provides a method for selecting grounding parameters of a symmetrical monopole flexible DC grounding system, which includes the following steps for a currently widely used ±100kV-±500kV symmetrical monopole flexible DC grounding system:

[0057] 1) Based on the design principle of flexible DC, the influencing factors and design boundary conditions of the grounding parameter selection of the symmetrical monopole flexible DC grounding system are determined.

[0058] Specifically, the influencing factors include the effective value of the AC phase voltage (Um) on the connecting transformer side, the DC voltage value U d , DC voltage control deviation δ, flexible DC system modulation ratio M, fundamental frequency value f, maximum AC current value I allowed to flow through the ground inductance 1 and the maximum DC current value I allowed to flow through the ground inductance 2 .

[0059] Specifically, the design boundary conditions are: the DC voltage control deviation δ is generally ±0.5% or ±1.0%, the flexible DC system modulation ratio M is generally 0.75-1.05, and the maximum AC current value I allowed to flow through the grounding inductance is 1 and the maximum DC current value I 2 The difficulty of R&D and manufacturing and the economy of the equipment are mainly considered, and the value is generally in the range of 0.1A to 1A.

[0060] 2) Based on the determined influencing factors and design boundary conditions, the parameters of the ground loop inductance L1 and the ground resistance R2 are determined.

[0061] Specifically, the ground loop inductance is:

[0062]

[0063] The grounding resistance R2 is:

[0064]

[0065] 3) Taking into account the steady-state measurement and transient protection requirements of the ground loop inductance L1 and the ground resistance R2, the parameters of the fourth current transformer T4 and the fifth current transformer T5 are determined.

[0066] Specifically, the rated currents of the fourth current transformer T4 and the fifth current transformer T5 need to be able to cover the steady-state currents flowing through the grounding loop inductance L1 and the grounding resistance R2, respectively, and meet the 0.2 level measurement requirements at the maximum and minimum steady-state currents; at the same time, the transient measurement range needs to be able to cover the maximum fault currents flowing through the grounding loop inductance L1 and the grounding resistance R2, respectively, and meet the 5% protection requirements.

[0067] 4) Different reference voltages are selected for the grounding resistor arrester F4, and the maximum transient stress borne by the grounding resistor arrester F4 when different types of faults occur at various locations in the symmetrical single-pole flexible DC grounding system is considered, that is, the arrester coordination current, residual voltage and energy under the reference voltage. The reliability and economy of the grounding loop inductance L1, the grounding resistance R2 and the grounding resistor arrester F4 under different reference voltages are comprehensively considered to determine the reference voltage, coordination current, residual voltage and energy parameters of the grounding resistor arrester F4.

[0068] The above embodiments are only used to illustrate the present invention, wherein the structure, connection mode and manufacturing process of each component may be changed. Any equivalent transformations and improvements based on the technical solution of the present invention should not be excluded from the protection scope of the present invention.

Claims

1. A symmetrical monopole flexible DC grounding system, characterized in that: It includes a connection transformer disposed in the valve-side AC field area; The grid-side star winding of the connection transformer is grounded through one end of the connection transformer grid-side arrester arranged in the grid-side AC field area, and the grid-side star winding of the connection transformer is also connected in parallel to the connection transformer grid-side neutral point arrester and one end of the first current transformer; The valve-side delta winding of the connection transformer is connected in parallel to the grounding reactor circuit and one end of the connection transformer valve-side current transformer through the starting circuit, and the other end of the connection transformer valve-side current transformer is connected to the converter arranged in the valve hall area through a strong bushing; The other ends of the connection transformer grid side lightning arrester, the connection transformer grid side neutral point lightning arrester, the first current transformer, the starting circuit and the grounding reactor circuit are all directly grounded; The starting circuit includes a connecting transformer side lightning arrester, a second current transformer, a third current transformer, a connecting transformer side voltage transformer, a first isolation switch, a second isolation switch, a first grounding switch, a second grounding switch, a third grounding switch and a connecting transformer side starting circuit resistor; One end of the connecting transformer valve side lightning arrester is connected in parallel to one end of the connecting transformer valve side voltage transformer and the valve side delta winding of the connecting transformer, the other end of the connecting transformer valve side lightning arrester is connected to one end of the second current transformer, the end of the connecting transformer valve side voltage transformer is also connected in parallel to the first isolation switch and one end of the first grounding switch, the other end of the first isolation switch is connected to one end of the third grounding switch in sequence through one end of the second grounding switch, the third current transformer and the connecting transformer valve side starting loop resistor, the end of the first grounding switch is also connected to one end of the third grounding switch through the second isolation switch, and the end of the third grounding switch is also connected in parallel to the grounding reactor loop and one end of the connecting transformer valve side current transformer; The other ends of the second current transformer, the voltage transformer at the connecting transformer valve side, the first grounding switch, the second grounding switch and the third grounding switch are directly grounded; The grounding reactor loop includes a grounding loop inductor, a fourth current transformer, a fifth current transformer, a grounding resistor arrester and a grounding resistor; One end of the grounding loop inductor is connected in parallel to the third grounding switch and the current transformer on the connecting valve side, the other end of the grounding loop inductor is connected in parallel to the grounding resistor and one end of the grounding resistor arrester through the fourth current transformer, and the other end of the grounding resistor is connected to one end of the fifth current transformer; The other ends of the grounding resistance arrester and the fifth current transformer are directly grounded.

2. A grounding parameter selection method for a symmetrical monopole flexible DC grounding system according to claim 1, characterized in that: include: Determine the influencing factors and design boundary conditions for grounding parameter selection of symmetrical monopole flexible DC grounding system; Based on the determined influencing factors and design boundary conditions, the parameters of the grounding reactor circuit are determined.

3. The method for selecting grounding parameters of a symmetrical monopole flexible DC grounding system according to claim 2, characterized in that: The influencing factors include the effective value of the AC phase voltage on the connecting transformer side, the DC voltage value, the DC voltage control deviation, the modulation ratio of the flexible DC system, the fundamental frequency value, the maximum AC current value allowed to flow through the grounding inductance, and the maximum DC current value allowed to flow through the grounding inductance.

4. The method for selecting grounding parameters of a symmetrical monopole flexible DC grounding system according to claim 3, characterized in that: The step of determining the parameters of the grounding reactor circuit based on the determined influencing factors and design boundary conditions includes: Based on the determined influencing factors and design boundary conditions, determine the parameters of the ground loop inductance and ground resistance; Comprehensively considering steady-state measurement and transient protection requirements of the ground loop inductance and ground resistance, determining parameters of the fourth current transformer and the fifth current transformer; Different reference voltages are selected for grounding resistance arresters. Considering the maximum transient stress on the grounding resistor when different types of faults occur at various locations in the symmetrical single-pole flexible DC grounding system, the reference voltage, coordination current, residual voltage and energy parameters of the grounding resistance arrester are determined.

5. The method for selecting grounding parameters of a symmetrical monopole flexible DC grounding system according to claim 4, characterized in that: The ground loop inductance is: in, is the DC voltage value, is the modulation ratio of the flexible DC system, is the fundamental frequency value, the maximum AC current value allowed to flow through the ground inductance .

6. A method for selecting grounding parameters of a symmetrical monopole flexible DC grounding system according to claim 5, characterized in that: The grounding resistance is: in, is the maximum DC current allowed to flow through the ground inductance, is the DC voltage control deviation.

7. The method for selecting grounding parameters of a symmetrical monopole flexible DC grounding system according to claim 4, characterized in that: The method of comprehensively considering the steady-state measurement and transient protection requirements of the ground loop inductance and the ground resistance to determine the parameters of the fourth current transformer and the fifth current transformer includes: The rated currents of the fourth current transformer and the fifth current transformer must be able to cover the steady-state current flowing through the ground loop inductance and the ground resistance respectively; At the same time, the transient measurement range must be able to cover the maximum fault current flowing through the ground loop inductance and ground resistance respectively.

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