Grounding device for a flexible low-frequency power transmission system and its selection method

Abstract

The present invention discloses a grounding device for a flexible low-frequency power transmission system and a selection method therefor. The grounding device of the present invention includes: an AC converter for realizing the conversion between a first frequency and a second frequency; a first-frequency transformer, with both sides thereof respectively connected to the AC converter and a first-frequency AC power grid; a second-frequency transformer, including a second-frequency grounding device, for connecting the AC converter and a second-frequency AC power grid; the second-frequency grounding device includes: a grounding resistor, one end of which is connected to the neutral point of the second-frequency transformer and the other end of which is grounded to provide a zero-potential reference point for the AC converter; a zero-sequence current collection winding, serving as the third winding of the second-frequency transformer to provide a main path for zero-sequence current. The present invention, through the cooperation of the grounding resistor and the impedance of the zero-sequence current collection winding, enables the zero-sequence currents on both sides of the second-frequency transformer not to cross each other or to maintain at an allowable low zero-sequence current cross-through level.

Description

Technical Field

[0001] The present invention belongs to the technical field of power system transmission and distribution, and specifically relates to a grounding device for a flexible low-frequency power transmission system and a selection method thereof. Background Art

[0002] Flexible low-frequency power transmission is a new type of AC power transmission technology, and its transmission frequency is between the power frequency and DC. Due to the reduction of frequency, flexible low-frequency power transmission combines the technical characteristics of power frequency and DC power transmission, and has broad application prospects in fields such as urban power grid interconnection, new energy grid connection, and long-distance power supply. Especially in the application scenarios of medium and far sea wind power transmission, flexible low-frequency power transmission technology provides a new means for the economic and efficient transmission of medium and far sea wind power.

[0003] The AC / AC converter is the core equipment of flexible low-frequency power transmission. In order to prevent the ground potential of the AC / AC converter from being too high during operation, it is necessary to provide a grounding electrode on either side of the AC / AC converter. However, the currently disclosed grounding methods such as grounding transformers and reactor grounding have characteristics such as large floor area, high cost, and heavy weight of the grounding device, which are not conducive to the miniaturization and low cost of flexible low-frequency converter stations. Therefore, it is necessary to study the grounding device of the low-frequency power transmission system. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the defects of the large volume and high cost of the grounding device of the AC / AC converter in the above-mentioned prior art, and provide a grounding device for a flexible low-frequency power transmission system and a selection method thereof. The grounding resistance of the present invention provides a zero-potential reference point for the AC / AC converter to save the cost of the grounding device of the AC / AC converter in the low-frequency system; the selection method of the present invention enables the zero-sequence currents on both sides of the second-frequency transformer not to cross each other or maintain at an allowable low zero-sequence current crossing level, reduces the current stress of the AC / AC converter, and ensures the sensitivity of relay protection.

[0005] To this end, the present invention adopts the following technical solutions.

[0006] In a first aspect, the present invention provides a grounding device for a flexible low-frequency power transmission system, which includes:

[0007] An AC / AC converter for realizing the conversion between a first frequency and a second frequency, wherein the second frequency is lower than the first frequency;

[0008] A first-frequency transformer, with both sides thereof connected to the AC / AC converter and a first-frequency AC power grid respectively;

[0009] A second-frequency transformer, including a second-frequency grounding device; a first winding of the second-frequency transformer is connected to the AC / AC converter, and a second winding of the second-frequency transformer is connected to a second-frequency AC power grid;

[0010] The second-frequency grounding device includes: a grounding resistor, one end of which is connected to the neutral point of the second-frequency transformer and the other end is grounded to provide a zero-potential reference point for the AC converter; a zero-sequence current collecting winding, which serves as the third winding of the second-frequency transformer; through the cooperation of the grounding resistor and the impedance of the zero-sequence current collecting winding, the zero-sequence currents on both sides of the second-frequency transformer do not cross each other or are maintained at an allowable low zero-sequence current crossing level.

[0011] In engineering, in order to reduce the current stress of the AC converter and ensure the sensitivity of relay protection, it is usually required that the zero-sequence currents on both sides of the second-frequency transformer do not cross each other or are maintained at an allowable low zero-sequence current crossing level. The allowable low zero-sequence current crossing level is determined by the zero-sequence circuit of the AC converter station, and the main determining parameters are the grounding resistor and the impedance of the zero-sequence current collecting winding.

[0012] Furthermore, for the first-frequency transformer, the winding connecting the AC converter adopts a delta type, and its main function is to isolate the zero-sequence current between the AC converter station and the first-frequency power grid; the winding connecting the first-frequency AC power grid adopts a star neutral point directly grounded method, which is suitable for the access of power grids with voltage levels of 220 kV and above.

[0013] Furthermore, for the second-frequency transformer, the first winding connecting the AC converter adopts a star type, and the second winding connecting the second-frequency AC power grid adopts a star neutral point directly grounded method.

[0014] Furthermore, due to other functional requirements such as station service power, the first-frequency transformer can adopt a three-winding transformer. The first winding connecting the AC converter adopts a delta type, the second winding connecting the first-frequency AC power grid adopts a star neutral point directly grounded method, and the third winding serves as the station service power supply winding, which can adopt a star or delta type.

[0015] Furthermore, the windings of the first-frequency transformer and the second-frequency transformer are directly connected to the AC converter, and there is no series-parallel grounding device (such as a grounding reactor, a grounding transformer, etc.) between them, thereby reducing the floor area of the AC converter station and lowering the investment cost of the AC converter station.

[0016] Furthermore, for the zero-sequence current collecting winding, it adopts a delta connection method and is dedicated to zero-sequence current collection without power supply extraction. In order to collect the main zero-sequence current, the zero-sequence current collecting winding adopts a low-impedance design, so this winding is not led out to prevent the short-circuit current of this branch from being too large in case of a line or load fault.

[0017] Further, when an asymmetric fault occurs on the first winding side of the second-frequency transformer or the first-frequency AC power grid, the zero-sequence current on the first winding side of the second-frequency transformer mainly passes through the zero-sequence current collecting winding of the second-frequency transformer, and a small part penetrates to the second winding side of the second-frequency transformer.

[0018] Further, when an asymmetric fault occurs on the second winding side of the second-frequency transformer or the second-frequency AC power grid, the zero-sequence current on the second winding side of the second-frequency transformer mainly passes through the zero-sequence current collecting winding of the second-frequency transformer, and a small part penetrates to the first winding side of the second-frequency transformer.

[0019] In a second aspect, the present invention provides a method for selecting the grounding device of the above flexible low-frequency power transmission system, that is, the grounding resistance R g and the impedance Z T2_3 of the zero-sequence current collecting winding for combined selection, which includes:

[0020] According to the requirements of the flexible low-frequency power transmission system or the AC / DC converter equipment, determine the allowable zero-sequence current low-penetration level of the second-frequency transformer, where the percentage of the zero-sequence current allowed to penetrate from the first winding of the second-frequency transformer to the second winding of the second-frequency transformer is K 12 , and the percentage of the zero-sequence current allowed to penetrate from the second winding of the second-frequency transformer to the first winding of the second-frequency transformer is K 21 ;

[0021] Determine the impedance of the first winding of the second-frequency transformer as Z T2_1 , and determine the impedance of the second winding of the second-frequency transformer as Z T2_2 ; Z T2_1 and Z T2_2 are mainly determined by the rated capacity, rated voltage of the second-frequency transformer and the short-circuit impedance percentage between windings;

[0022] Determine the impedance of the AC / DC converter as Z V , the AC / DC converter can conduct zero-sequence current, and its zero-sequence impedance is mainly composed of arm reactor;

[0023] Determine the impedance of the first-frequency transformer as Z T1 , which is mainly determined by the rated capacity, rated voltage of the first-frequency transformer and the short-circuit impedance percentage between windings;

[0024] Determine the equivalent zero-sequence impedance of the second-frequency AC power grid as Z f2 ;

[0025] The above parameters are all determined in the design stage of the flexible low-frequency power transmission system and are also the input conditions for the selection of the grounding device;

[0026] Determine the grounding resistance R gand the impedance Z of the zero-sequence current collecting winding T2_3 coordination relationship

[0027] After selecting the value of the grounding resistance, it is necessary to determine the impedance value of the zero-sequence current collecting winding. Since the impedance of the excitation branch of the transformer is very large, the zero-sequence current of this excitation branch can be approximately equivalent to 0

[0028] When a fault occurs on the side of the second-frequency transformer connected to the AC converter, the total zero-sequence current on the side of the second-frequency transformer connected to the AC converter will be shunted between the zero-sequence current collecting winding and the second-frequency AC grid side of the second-frequency transformer. The zero-sequence current of the zero-sequence current collecting winding is denoted as I T2_3 , and the zero-sequence current crossing to the second-frequency AC grid side is denoted as I T2_2 , then

[0029] (1)

[0030] Furthermore, we get

[0031] (2)

[0032] When a fault occurs on the side of the second-frequency transformer connected to the second-frequency AC grid, the total zero-sequence current on the side of the second-frequency transformer connected to the second-frequency AC grid will be shunted between the zero-sequence current collecting winding and the side of the second-frequency transformer connected to the AC converter. The zero-sequence current of the zero-sequence current collecting winding is denoted as I T2_3 , and the zero-sequence current crossing to the side of the AC converter is denoted as I T2_1 , then

[0033] (3)

[0034] Furthermore, we get

[0035] (4)

[0036] Therefore, the grounding resistance R g and the impedance Z of the zero-sequence current collecting winding T2_3 need to simultaneously meet the requirements of formula (2) and formula (4).

[0037] According to the satisfied Z T2_3 , combined with the values of Z T2_1 and Z T2_2 , determine the short-circuit impedance percentages , and among the windings of the second-frequency transformer, where is the short-circuit impedance percentage between the first winding and the second winding, is the short-circuit impedance percentage between the first winding and the third winding, is the percentage of the short-circuit impedance between the second winding and the third winding:

[0038] (5).

[0039] Furthermore, the value selection of the grounding resistance R g is a process of comprehensive optimization. If the value of the grounding resistance is too large, the zero-sequence current during a fault is too small, the sensitivity of the relay protection device is insufficient, and it cannot effectively identify and protect against faults. At the same time, the requirement for the insulation level of the AC converter to the ground increases; if the value of the grounding resistance is too small, the zero-sequence current is too large, resulting in a large amount of energy in the grounding resistance and difficulty in equipment manufacturing. At the same time, the transient current stress of the AC converter increases; to meet the low-crossing characteristic of the zero-sequence current, the impedance value of the zero-sequence current collection winding is required to be small, but if it is too small, the manufacturing conditions cannot be met.

[0040] Therefore, the value selection of the grounding resistance should simultaneously meet the requirements of the sensitivity of zero-sequence current protection, the manufacturing level of the grounding resistance equipment, the transient current stress of the AC converter, and the manufacturing level of the zero-sequence current collection winding.

[0041] The beneficial effects of the present invention are as follows: The grounding resistance of the present invention provides a zero-potential reference point for the AC converter, saving the cost of the grounding device for the AC converter in the low-frequency system; the zero-sequence current collection winding of the present invention provides the main path for the zero-sequence current. Through the cooperation of the grounding resistance R g and the impedance Z T2_3 of the zero-sequence current collection winding, it is possible to prevent the zero-sequence currents on both sides of the second-frequency transformer from crossing each other or maintain them at an allowable low-crossing level of the zero-sequence current, which can reduce the transient current stress of the AC converter and ensure the sensitivity of the relay protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 is a schematic structural diagram of the grounding device of the flexible low-frequency power transmission system of the present invention;

[0043] Figure 2 is the zero-sequence equivalent circuit diagram of the flexible low-frequency power transmission low-frequency converter station in the specific embodiment of the present invention;

[0044] Figure 3 is the zero-sequence current flow diagram when an asymmetric grounding fault occurs on the side of the second-frequency transformer connecting the AC converter in the flexible low-frequency power transmission low-frequency converter station in the specific embodiment of the present invention;

[0045] Figure 4 is the zero-sequence current flow diagram when an asymmetric grounding fault occurs on the side of the second-frequency transformer connecting the second-frequency AC power grid in the flexible low-frequency power transmission low-frequency converter station in the specific embodiment of the present invention;

[0046] Figure 5Schematic diagram of a structure of a second - frequency AC power grid in a specific embodiment of the present invention;

[0047] Figure 6 is Figure 5 zero - sequence equivalent circuit diagram of. Specific embodiments

[0048] To describe the present invention more specifically, the technical solutions of the present invention will be described in detail below in conjunction with the specification drawings and specific embodiments.

[0049] This embodiment provides a grounding device for a flexible low - frequency power transmission system. As Figure 1 shown, it is composed of a first - frequency AC power grid, a first - frequency transformer, an AC - AC converter, a second - frequency transformer, and a second - frequency AC power grid.

[0050] The AC - AC converter is used to realize the conversion between the first frequency and the second frequency, where the second frequency is lower than the first frequency. In this embodiment, the AC - AC converter adopts a modular multilevel converter structure, which is composed of 9 bridge arms. Each bridge arm is composed of a plurality of full - bridge sub - modules and a bridge - arm reactor connected in series. The inductance value of the bridge - arm reactor is L b . The AC - AC converter can also adopt other structures similar to the modular multilevel converter.

[0051] In this embodiment, the first - frequency AC power grid is a power - frequency AC power grid with a frequency of 50 Hz. The second - frequency AC power grid is a wind power generation system with a frequency of 20 Hz.

[0052] The first - frequency transformer is used to connect the AC - AC converter and the first - frequency AC power grid. The winding connecting the AC - AC converter adopts a delta type, and the winding connecting the first - frequency AC power grid adopts a star - type neutral - point direct - grounding method. Taking the access to a 220 kV power - frequency AC power grid as an example, the winding of the first - frequency transformer connecting the first - frequency AC power grid adopts a star - type neutral - point direct - grounding method, and the winding connecting the modular multilevel converter side adopts a delta connection to block zero - sequence current. When the first - frequency transformer has a demand for station power supply, the first - frequency transformer can adopt a three - winding transformer. The first winding connecting the AC - AC converter adopts a delta type, the second winding connecting the first - frequency AC power grid adopts a star - type neutral - point direct - grounding method, and the third winding as the station - power - supply winding can adopt a star or delta method.

[0053] The second - frequency transformer is used to connect the AC - AC converter and the second - frequency AC power grid. The first winding connecting the AC - AC converter adopts a star type, and the second winding connecting the second - frequency AC power grid adopts a star - type neutral - point direct - grounding type.

[0054] The windings of the first frequency transformer and the second frequency transformer are directly connected to the AC-DC converter, and there is no series-parallel grounding device (such as a grounding reactor, a grounding transformer, etc.) between them, thereby reducing the floor area of the AC-DC converter station and lowering the investment cost of the AC-DC converter station.

[0055] The second frequency transformer described above includes a second frequency grounding device, and the second frequency grounding device includes: (1) a grounding resistor, one end of which is connected to the neutral point of the second frequency transformer and the other end is grounded to provide a zero-potential reference point for the AC-DC converter; (2) a zero-sequence current collection winding, which is used as the third winding of the second frequency transformer and is designed with a low impedance to prevent excessive short-circuit current after being led out. The third winding is dedicated to zero-sequence current collection and is not led out for power supply.

[0056] The zero-sequence equivalent circuit of the flexible low-frequency power transmission low-frequency converter station is as Figure 2 shown. When an asymmetric fault occurs on the first winding side of the second frequency transformer or the first frequency AC power grid, the zero-sequence current on the first winding side of the second frequency transformer mainly passes through the zero-sequence current collection winding of the second frequency transformer, and a small part of it crosses to the second winding side of the second frequency transformer, as Figure 3 shown. When an asymmetric fault occurs on the second winding side of the second frequency transformer or the second frequency AC power grid, the zero-sequence current on the second winding side of the second frequency transformer mainly passes through the zero-sequence current collection winding of the second frequency transformer, and a small part of it crosses to the first winding side of the second frequency transformer, as Figure 4 shown.

[0057] In order to ensure that the zero-sequence current crossing to the other side of the second frequency grounding device remains at a low value after an asymmetric grounding fault occurs on one side of the second frequency grounding device, it is required that the impedance of the grounding resistor and the zero-sequence current collection winding cooperate with each other.

[0058] The selection value of the grounding resistor R g is a comprehensive optimization process. If the resistance value of the grounding resistor is too large, the zero-sequence current during a fault is too small, and the sensitivity of the relay protection device is insufficient to effectively identify and protect the fault. At the same time, the requirement for the insulation level of the AC-DC converter to the ground is increased; if the resistance value of the grounding resistor is too small, the zero-sequence current is too large, resulting in a large amount of energy in the grounding resistor and difficulty in equipment manufacturing. At the same time, the transient current stress of the AC-DC converter increases; to meet the low-crossing characteristic of the zero-sequence current, the impedance value of the zero-sequence current collection winding is required to be small, but if it is too small, the manufacturing conditions cannot be met. Therefore, the selection value of the grounding resistor should simultaneously meet the requirements of the sensitivity of zero-sequence current protection, the manufacturing level of the grounding resistor equipment, the transient current stress of the AC-DC converter, and the manufacturing level of the zero-sequence current collection winding.

[0059] In this embodiment, the rated capacity of the first frequency transformer is S T1 , and the rated voltage of the high-voltage side is U 1_T1, the percentage of short-circuit impedance is U k_T1 .

[0060] The rated capacity of the second-frequency transformer is S T2 , the rated voltage of the high-voltage side is U 1_T2 , the percentage of short-circuit impedance between the first winding and the second winding is , the percentage of short-circuit impedance between the first winding and the third winding is , the percentage of short-circuit impedance between the second winding and the third winding is .

[0061] This embodiment also provides a method for selecting the grounding device of the above flexible low-frequency power transmission system, that is, the grounding resistance R g and the impedance Z of the zero-sequence current collecting winding T2_3 for coordinated selection, and the steps are as follows:

[0062] First, according to the requirements of the system relay protection device and the AC converter equipment, determine the allowable zero-sequence current low-penetration level (i.e., the percentage of zero-sequence current) of the second-frequency transformer. Among them, the percentage of zero-sequence current allowed to pass from the first winding to the second winding is K 12 , the percentage of zero-sequence current allowed to pass from the second winding to the first winding is K 21 , K 12 and K 21 are input conditions, and the selection of the grounding device is determined according to K 12 and K 21 . K 12 and K 21 should be as small as possible under allowable conditions, generally within 15%, preferably within 10%.

[0063] Then, determine the impedance of the first winding of the second-frequency transformer as Z T2_1 , determine the impedance of the second winding of the second-frequency transformer as Z T2_2 , Z T2_1 and Z T2_2 are mainly determined by the rated capacity, rated voltage of the second-frequency transformer and the percentage of short-circuit impedance between windings. In this embodiment:

[0064] (5)

[0065] Determine the impedance of the AC converter as Z V , the AC converter can conduct zero-sequence current, and its zero-sequence impedance is mainly composed of bridge arm reactors. In this embodiment:

[0066] (6)

[0067] In the formula, represents the angular frequency of zero-sequence current; ZB is the impedance reference value of the second - frequency transformer;

[0068] (7)

[0069] Determine the impedance of the first - frequency transformer as Z T1 , which is mainly determined by the rated capacity, rated voltage of the first - frequency transformer and the short - circuit impedance percentage between windings. In this embodiment:

[0070] (8)

[0071] Determine the equivalent zero - sequence impedance of the second - frequency AC power grid as Z f2 .

[0072] Substitute formulas (6) to (8) and Z f2 into formula (9), and the value range of Z T2_3 can be obtained. Then substitute it into formula (5) to get the short - circuit impedance percentage of the feasible second - frequency transformer and .

[0073] (9)

[0074] Application Example

[0075] Apply the embodiment to an offshore wind power low - frequency transmission system. Its on - shore part and off - shore part are respectively as Figure 1 and Figure 5 shown, and its zero - sequence equivalent circuit is as Figure 2 and Figure 6 shown. The main parameters are shown in Table 1

[0076] Table 1

[0077]

[0078] The equivalent capacity of the fan transformer TW is S TW , and the short - circuit impedance percentage is U k_TW .

[0079] The impedance Z TW of the fan transformer is:

[0080] (10)

[0081] The rated capacity of the offshore transformer T3 is S T3 , the rated voltage of the high - voltage side is U 1_T3 , the short - circuit impedance percentage between the first winding and the second winding is U k_T3_12 , and the short - circuit impedance percentage between the first winding and the third winding is U k_T3_13, the percentage of the short - circuit impedance between the second winding and the third winding is U k_T3_23 .

[0082] The impedance Z of the first winding of the offshore transformer T3_1 , the impedance Z of the second winding T3_2 and the impedance Z of the third winding T3_3 are as follows:

[0083] (11)

[0084] According to the actual parameters in Table 1, combined with formulas (6) to (8), formula (10) and formula (11), the zero - sequence impedances can be obtained, as shown in Table 2.

[0085] Table 2

[0086]

[0087] According to Figure 6 and Table 2, by calculating the equivalent zero - sequence impedance Z of the second - frequency AC power grid through circuit theory f2 , its per - unit value is 0.7224 + j*0.7218.

[0088] In this example, K 12 and K 21 both take 10%.

[0089] According to the general design of the transformer, considering that the percentage of the impedance of the high - low windings (i.e., the percentage of the short - circuit impedance between the first winding and the third winding) is slightly greater than the sum of the percentage of the impedance of the high - middle windings (the percentage of the short - circuit impedance between the first winding and the second winding) and the percentage of the impedance of the middle - low windings (the percentage of the short - circuit impedance between the second winding and the third winding), the difference is taken as 3% in this example.

[0090] According to Z T1 , Z T2_1 , Z T2_2 , Z V , Z f2 , R g , K 12 and K 21 and formula (9), the value range of Z T2_3 is 0 - 9.5%, thus obtaining a set of feasible percentage of the short - circuit impedance of the second - frequency transformer [U k_T2_12 , U k_T2_23 , U k_T2_13 as [15%, 9.5%, 27.7%], and the corresponding impedance of the second - frequency transformer [Z T2_1 , Z T2_2 , Z T2_3When it is [j*0.165, -j*0.015, j*0.11], substituting into formulas (1) and (3) at this time, K is obtained. 12 and K 21 are 9.98% and 5.72% respectively, meeting the requirement that the zero-sequence current penetration level is less than 10%.

[0091] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than limiting it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included in the protection scope of the present invention.

Claims

1. A grounding device for a flexible low-frequency power transmission system, characterized in that: include: an alternating current converter, used to realize conversion between a first frequency and a second frequency, wherein the second frequency is lower than the first frequency; A first frequency transformer, two sides of which are respectively connected to the AC converter and the first frequency AC power grid; a second frequency transformer including a second frequency grounding device; The first winding of the second frequency transformer is connected to the AC converter, and the second winding of the second frequency transformer is connected to the second frequency AC power grid; The second frequency grounding device includes: a grounding resistor, one end of which is connected to the neutral point of the second frequency transformer and the other end is grounded to provide a zero potential reference point for the AC converter; a zero-sequence current collection winding, which serves as the third winding of the second frequency transformer; Through the coordination of the grounding resistance and the zero-sequence current collection winding impedance, the zero-sequence currents on both sides of the second frequency transformer do not cross each other or are maintained at an allowable zero-sequence current low crossing level; The grounding resistance and the impedance of the zero-sequence current collection winding must simultaneously satisfy the following coordination relationship: ， ， In the formula, R g Represents the grounding resistance, Z T2_3 Represents the impedance of the zero-sequence current collection winding, Z T2_1 The impedance of the first winding of the second frequency transformer, Z T2_2 The impedance of the second winding of the second frequency transformer, K 12 Indicates the percentage of zero-sequence current allowed to pass from the first winding of the second frequency transformer to the second winding of the second frequency transformer, K 21 It indicates the percentage of zero-sequence current allowed to pass from the second winding of the second frequency transformer to the first winding of the second frequency transformer, Z V The impedance of the AC converter, Z T1 represents the impedance of the first frequency transformer, Z f2 Represents the equivalent zero-sequence impedance of the second frequency AC power grid.

2. The grounding device of a flexible low-frequency power transmission system according to claim 1, characterized in that: The first frequency transformer has a triangle-shaped winding connected to the AC converter, and a star-shaped neutral point direct grounding winding connected to the first frequency AC power grid.

3. The grounding device of a flexible low-frequency power transmission system according to claim 1, characterized in that: The second frequency transformer has a first winding connected to the AC converter in a star shape, and a second winding connected to the second frequency AC power grid in a star-shaped neutral point direct grounding manner.

4. The grounding device of a flexible low-frequency power transmission system according to claim 1, characterized in that: The first frequency transformer adopts a three-winding transformer, the first winding connected to the AC converter adopts a triangle type, the second winding connected to the first frequency AC power grid adopts a star neutral point direct grounding method, and the third winding is used as the station power supply winding, adopting a star or triangle type.

5. The grounding device of a flexible low-frequency power transmission system according to claim 1, characterized in that: The winding of the first frequency transformer and the winding of the second frequency transformer are directly connected to the AC converter without any series-parallel connection device between them.

6. The grounding device of a flexible low-frequency power transmission system according to claim 1, characterized in that: The zero-sequence current collection winding adopts a triangle connection method, is specially used for zero-sequence current collection, and does not lead out power supply.

7. The grounding device of a flexible low-frequency power transmission system according to claim 1, characterized in that: When an asymmetric fault occurs on the first winding side of the second frequency transformer or the first frequency AC power grid, the zero-sequence current on the first winding side of the second frequency transformer mainly passes through the zero-sequence current collection winding of the second frequency transformer, and a small part passes through the second winding side of the second frequency transformer.

8. The grounding device of a flexible low-frequency power transmission system according to claim 1, characterized in that: When an asymmetric fault occurs on the second winding side of the second frequency transformer or the second frequency AC power grid, the zero-sequence current on the second winding side of the second frequency transformer mainly passes through the zero-sequence current collection winding of the second frequency transformer, and a small part passes through the first winding side of the second frequency transformer.

9. The method for selecting a grounding device of a flexible low-frequency power transmission system according to any one of claims 1 to 8, characterized in that: include: According to the requirements of the flexible low-frequency power transmission system or AC converter equipment, determine the allowable zero-sequence current low ride-through level of the second frequency transformer, including: the zero-sequence current percentage K allowed to pass from the first winding of the second frequency transformer to the second winding of the second frequency transformer 12 , the percentage of zero-sequence current K allowed to pass from the second winding of the second frequency transformer to the first winding of the second frequency transformer 21 ; Determine the impedance Z of the first winding of the second frequency transformer T2_1 , determine the impedance Z of the second winding of the second frequency transformer T2_2 , determine the impedance Z of the AC converter V , determine the impedance Z of the first frequency transformer T1 ; Determine the equivalent zero-sequence impedance Z of the second frequency AC power grid f2 ; Determine the ground resistance R g and zero-sequence current collection winding impedance Z T2_3 cooperation relationship.

10. The selection method according to claim 9, characterized in that: According to the Z that meets the requirements T2_3 , combined with Z T2_1 and Z T2_2 The value of is used to determine the short-circuit impedance percentage between the windings of the second frequency transformer. The formula is as follows: ， in, is the short-circuit impedance percentage between the first winding and the second winding, is the short-circuit impedance percentage between the first winding and the third winding, is the short-circuit impedance percentage between the second winding and the third winding.

Images