CN118971100A - A grounding device for a flexible low-frequency power transmission system and a method for selecting the same - Google Patents
A grounding device for a flexible low-frequency power transmission system and a method for selecting the same Download PDFInfo
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- CN118971100A CN118971100A CN202411433728.3A CN202411433728A CN118971100A CN 118971100 A CN118971100 A CN 118971100A CN 202411433728 A CN202411433728 A CN 202411433728A CN 118971100 A CN118971100 A CN 118971100A
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01R—ELECTRICALLY-CONDUCTIVE CONNECTIONS; STRUCTURAL ASSOCIATIONS OF A PLURALITY OF MUTUALLY-INSULATED ELECTRICAL CONNECTING ELEMENTS; COUPLING DEVICES; CURRENT COLLECTORS
- H01R4/00—Electrically-conductive connections between two or more conductive members in direct contact, i.e. touching one another; Means for effecting or maintaining such contact; Electrically-conductive connections having two or more spaced connecting locations for conductors and using contact members penetrating insulation
- H01R4/58—Electrically-conductive connections between two or more conductive members in direct contact, i.e. touching one another; Means for effecting or maintaining such contact; Electrically-conductive connections having two or more spaced connecting locations for conductors and using contact members penetrating insulation characterised by the form or material of the contacting members
- H01R4/66—Connections with the terrestrial mass, e.g. earth plate, earth pin
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for AC mains or AC distribution networks
- H02J3/34—Arrangements for transfer of electric power between networks of substantially different frequency
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/32—Means for protecting converters other than automatic disconnection
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M5/00—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases
- H02M5/02—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases without intermediate conversion into DC
- H02M5/04—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases without intermediate conversion into DC by static converters
- H02M5/10—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases without intermediate conversion into DC by static converters using transformers
- H02M5/16—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases without intermediate conversion into DC by static converters using transformers for conversion of frequency
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Abstract
The invention discloses a grounding device of a flexible low-frequency power transmission system and a model selection method thereof. The grounding device of the present invention includes: an ac-to-ac converter for effecting conversion of the first frequency and the second frequency; the two sides of the first frequency transformer are respectively connected with the AC-DC converter and the first frequency AC power grid; a second frequency transformer comprising a second frequency grounding means for connecting the ac inverter to a second frequency ac grid; a second frequency grounding apparatus comprising: one end of the grounding resistor is connected with the neutral point of the second frequency transformer, and the other end of the grounding resistor is grounded to provide a zero potential reference point for the AC-DC converter; the zero sequence current collection winding is used as a third winding of the second frequency transformer to provide a main zero sequence current path. The invention ensures that the zero-sequence currents on the two sides of the second frequency transformer do not cross each other or are maintained at an allowable zero-sequence current low-crossing level through the matching of the grounding resistance and the zero-sequence current collecting winding impedance.
Description
Technical Field
The invention belongs to the technical field of power transmission and distribution of power systems, and particularly relates to a grounding device of a flexible low-frequency power transmission system and a model selection method thereof.
Background
Flexible low-frequency power transmission is a novel alternating current power transmission technology, and the power transmission frequency is between the power frequency and direct current. Due to the reduction of frequency, the flexible low-frequency power transmission combines the technical characteristics of power frequency and direct current power transmission, and has wide application prospect in the fields of urban power grid interconnection, new energy grid connection, long-distance power supply and the like. In particular to the application field of the mid-open sea wind power transmission, the flexible low-frequency power transmission technology provides a new means for the economical and efficient transmission of the mid-open sea wind power.
The ac converters are core devices for flexible low frequency power transmission and in order to prevent the ac converters from being too high to ground potential during operation, ground power needs to be provided on either side of the ac converters. However, the grounding device such as grounding transformer and reactor disclosed at present has the characteristics of large occupied area, high cost, heavy weight and the like, which is unfavorable for miniaturization and low cost of the flexible low-frequency converter station, so that the grounding device of the low-frequency power transmission system needs to be studied.
Disclosure of Invention
The invention aims to overcome the defects of large volume and high cost of an AC/AC converter grounding device in the prior art and provide a grounding device of a flexible low-frequency power transmission system and a model selection method thereof; the method of the invention ensures that the zero-sequence currents at the two sides of the second frequency transformer do not cross each other or are maintained at an allowable zero-sequence current low-crossing level, reduces the current stress of the AC-DC converter and ensures the sensitivity of relay protection.
Therefore, the invention adopts the following technical scheme.
In a first aspect, the present invention provides a grounding device for a flexible low frequency power transmission system, comprising:
An ac-to-dc converter for effecting a conversion of a first frequency and a second frequency, wherein the second frequency is lower than the first frequency;
The two sides of the first frequency transformer are respectively connected with the AC-DC converter and the first frequency AC power grid;
The second frequency transformer comprises a second frequency grounding device; the first winding of the second frequency transformer is connected with the AC-DC converter, and the second winding of the second frequency transformer is connected with a second frequency AC power grid;
A second frequency grounding apparatus comprising: one end of the grounding resistor is connected with the neutral point of the second frequency transformer, and the other end of the grounding resistor is grounded to provide a zero potential reference point for the AC-DC converter; a zero sequence current collection winding is used as a third winding of the second frequency transformer; the zero-sequence currents on the two sides of the second frequency transformer do not cross each other or are maintained at an allowable zero-sequence current low-crossing level through the matching of the grounding resistance and the zero-sequence current collecting winding impedance.
In engineering, in order to reduce the current stress of the ac-dc converter and to ensure the sensitivity of the relay protection, it is generally 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 zero-sequence current low crossing level. The allowable zero-sequence current low-crossing level is determined by a zero-sequence loop of the exchange converter station, and the parameters playing a main role are the grounding resistance and the impedance of the zero-sequence current collecting winding.
Further, the first frequency transformer is connected with the AC/DC converter winding in a triangular mode and mainly used for isolating zero sequence current between the AC/DC converter station and the first frequency power grid; the windings connected to the first frequency ac power grid are directly grounded at the star neutral point, which is suitable for grid access at voltage levels of 220 kv and above.
Further, the first winding of the second frequency transformer connected with the AC-DC converter adopts a star-shaped mode, and the second winding connected with the second frequency AC power grid adopts a star-shaped neutral point direct grounding mode.
Further, due to other functional requirements such as station power consumption, the first frequency transformer can adopt a three-winding transformer, a first winding connected with the ac-dc converter adopts a triangle type, a second winding connected with the first frequency ac-dc power grid adopts a star neutral point direct grounding mode, and a third winding is used as a station power consumption power supply winding and adopts a star or triangle type.
Further, the windings of the first frequency transformer and the windings of the second frequency transformer are directly connected with the AC/DC converter, and no series-parallel connection grounding device (such as a grounding resistor, a grounding transformer and the like) is arranged between the windings, so that the occupied area of the AC/DC converter station is reduced, and the investment cost of the AC/DC converter station is reduced.
Furthermore, the zero-sequence current collection winding adopts a triangle wiring mode, is special for zero-sequence current collection, and does not lead out power supply. In order to collect main zero-sequence current, the zero-sequence current collection winding adopts a low-impedance design, so that the winding is not led out, and the short-circuit current of the branch is prevented from being overlarge when a lead-out line or a load fails.
Further, when the first winding side of the second frequency transformer or the first frequency alternating current power grid has an asymmetric fault, 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 of zero sequence current passes through the second winding side of the second frequency transformer.
Further, when the second winding side of the second frequency transformer or the second frequency alternating current power grid has an asymmetric fault, 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 of zero sequence current passes through the first winding side of the second frequency transformer.
In a second aspect, the present invention provides a method for selecting a grounding device of the above-mentioned flexible low-frequency power transmission system, that is, a matching type selection of a grounding resistor R g and an impedance Z T2_3 of a zero-sequence current collecting winding, which includes:
Determining a zero sequence current low crossing level allowed by the second frequency transformer according to the requirements of the flexible low frequency transmission system or the AC-AC converter equipment, wherein 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 is K 12, and 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 is K 21;
The impedance of the first winding of the second frequency transformer is determined to be Z T2_1, the impedance of the second winding of the second frequency transformer is determined to be Z T2_2;ZT2_1 and Z T2_2, and the impedance is mainly determined by the rated capacity, the rated voltage and the short-circuit impedance percentage among the windings of the second frequency transformer;
The impedance of the AC-DC converter is determined as Z V, the AC-DC converter can circulate zero sequence current, and the zero sequence impedance of the AC-DC converter mainly comprises bridge arm reactors;
The impedance of the first frequency transformer is determined as Z T1, and is mainly determined by the rated capacity and rated voltage of the first frequency transformer and the short-circuit impedance percentage between windings;
Determining the equivalent zero sequence impedance of the second frequency alternating current power grid as Z f2;
The parameters are determined in the design stage of the flexible low-frequency power transmission system and are input conditions for selecting the grounding device;
The matching relationship of the grounding resistance R g and the impedance Z T2_3 of the zero-sequence current collection winding is determined.
After the ground resistance is selected, the impedance value of the zero sequence current collection winding needs to be determined. Because the impedance of the excitation branch of the transformer is large, the zero sequence current of the excitation branch can be approximately equivalent to 0.
When the second frequency transformer is connected with the AC-DC converter side to generate faults, the total zero-sequence current of the second frequency transformer connected with the AC-DC converter side is split between the zero-sequence current collecting winding and the second frequency AC power grid side of the second frequency transformer, the zero-sequence current of the zero-sequence current collecting winding is recorded as I T2_3, the zero-sequence current passing through the second frequency AC power grid side is recorded as I T2_2, and then:
（1）
And then obtain:
（2）
When the second frequency transformer is connected with the second frequency alternating current power grid side to generate faults, the total zero-sequence current of the second frequency transformer connected with the second frequency alternating current power grid side is split between the zero-sequence current collecting winding and the second frequency transformer alternating current converter side, the zero-sequence current of the zero-sequence current collecting winding is recorded as I T2_3, the zero-sequence current passing through the alternating current converter side is recorded as I T2_1, and then:
（3）
And then obtain:
（4）
Therefore, the ground resistance R g and the impedance Z T2_3 of the zero sequence current collection winding need to satisfy the requirements of both equation (2) and equation (4).
Determining the short-circuit impedance percentage between windings of the second frequency transformer according to Z T2_3 meeting the requirements and combining the values of Z T2_1 and Z T2_2 、AndWherein, the method comprises the steps of, wherein,As a percentage of the short-circuit impedance between the first winding and the second winding,As a percentage of the short-circuit impedance between the first winding and the third winding,The short-circuit impedance percentage between the second winding and the third winding is as follows:
（5）。
Further, the selection of the ground resistance R g is a comprehensive optimization process. When the resistance value of the grounding resistor is overlarge, the zero-sequence current is small during faults, the sensitivity of the relay protection device is insufficient, faults cannot be effectively identified and protected, and meanwhile, the requirement on the ground insulation level of the AC-to-AC converter is improved; the grounding resistance is too small, zero sequence current is too large, so that the grounding resistance is high in energy and difficult in equipment manufacturing, and transient current stress of the AC-DC converter is increased; in order to meet the low-pass characteristic of the zero-sequence current, the impedance value of the zero-sequence current collecting winding is required to be small, however, too small a size does not satisfy the manufacturing conditions.
Therefore, the ground resistance value should meet the requirements of zero sequence current protection sensitivity, ground resistance equipment manufacturing level, AC-to-AC converter transient current stress and zero sequence current collecting winding manufacturing level at the same time.
The invention has the following beneficial effects: the grounding resistor provides a zero potential reference point for the AC-DC converter, so that the cost of the grounding device of the AC-DC converter of the low-frequency system is saved; the zero-sequence current collecting winding provides a main zero-sequence current path, and the zero-sequence currents on two sides of the second frequency transformer can not pass through each other or can be maintained at an allowable zero-sequence current low pass-through level through the matching of the grounding resistor R g and the impedance Z T2_3 of the zero-sequence current collecting winding, so that the transient current stress of the AC-AC converter can be reduced, and the sensitivity of relay protection is ensured.
Drawings
Fig. 1 is a schematic structural view of a grounding device of a flexible low-frequency power transmission system according to the present invention;
Fig. 2 is a zero sequence equivalent circuit diagram of a flexible low frequency transmission low frequency converter station in an embodiment of the invention;
Fig. 3 is a zero sequence current flow diagram when an asymmetric ground fault occurs on the second frequency transformer connection ac-dc side of the flexible low frequency power transmission low frequency converter station in the embodiment of the present invention;
Fig. 4 is a zero sequence current flow diagram when an asymmetric ground fault occurs on a side of a second frequency transformer of the flexible low frequency power transmission low frequency converter station connected to a second frequency ac power grid in an embodiment of the present invention;
FIG. 5 is a schematic diagram of a second frequency AC power grid according to an embodiment of the present invention;
Fig. 6 is a zero sequence equivalent circuit diagram of fig. 5.
Detailed Description
In order to more specifically describe the present invention, the following detailed description of the technical scheme of the present invention is given with reference to the accompanying drawings and the specific embodiments.
The embodiment provides a grounding device of a flexible low-frequency power transmission system, as shown in fig. 1, which consists of a first frequency alternating current power grid, a first frequency transformer, an alternating current converter, a second frequency transformer and a second frequency alternating current power grid.
The AC-DC converter is used for realizing the conversion of a first frequency and a second frequency, wherein the second frequency is lower than the first frequency. In the embodiment, the ac-dc converter adopts a modularized multi-level converter structure, and is composed of 9 bridge arms, each bridge arm is composed of a plurality of full-bridge sub-modules and bridge arm reactors in series, and the inductance value of the bridge arm reactors is L b. The ac converters may also take other configurations like modular multilevel converters.
In this embodiment, the first frequency ac power grid is a power frequency ac power grid, and the frequency is 50Hz. The second frequency alternating current power grid is a wind power generation system, and the frequency is 20Hz.
The first frequency transformer is used for connecting the AC-DC converter and the first frequency AC power grid, wherein the winding connected with the AC-DC converter adopts a triangle type, and the winding connected with the first frequency AC power grid adopts a star neutral point direct grounding mode. Taking 220kV power frequency alternating current power grid as an example, a winding of the first frequency transformer connected with the first frequency alternating current power grid adopts a star neutral point direct grounding mode, and a connection modularized multi-level converter side adopts triangle wiring to block zero sequence current. When the first frequency transformer has the power supply requirement of station power utilization, the first frequency transformer can adopt a three-winding transformer, a first winding connected with the AC-DC converter adopts a triangle type, a second winding connected with the first frequency AC power grid adopts a star neutral point direct grounding mode, and a third winding is used as the power supply winding of station power utilization, and can adopt a star or triangle mode.
The second frequency transformer is used for connecting the AC-DC converter and the second frequency AC power grid, wherein a first winding connected with the AC-DC converter adopts a star type, and a second winding connected with the second frequency AC power grid adopts a star neutral point direct grounding type.
The windings of the first frequency transformer and the windings of the second frequency transformer are directly connected with the AC/AC converter, and a grounding device (such as a grounding reactor, a grounding transformer and the like) is not connected between the windings of the first frequency transformer and the windings of the second frequency transformer in series and parallel, so that the occupied area of the AC/AC converter station is reduced, and the investment cost of the AC/AC converter station is reduced.
The second frequency transformer comprises a second frequency grounding device, and the second frequency grounding device comprises: (1) One end of the grounding resistor is connected with the neutral point of the second frequency transformer, and the other end of the grounding resistor is grounded to provide a zero potential reference point for the AC-AC converter; (2) The zero sequence current collecting winding is used as a third winding of the second frequency transformer, and is designed with low impedance so as to prevent overlarge short-circuit current after the third winding is led out, and the third winding is specially used for zero sequence current collection and does not lead out power supply.
When the first winding side of the second frequency transformer or the first frequency alternating current power grid has an asymmetric fault, 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 of the zero-sequence current passes through the second winding side of the second frequency transformer, as shown in fig. 3. When the second winding side of the second frequency transformer or the second frequency alternating current power grid has an asymmetric fault, 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 of zero-sequence current passes through the first winding side of the second frequency transformer, as shown in fig. 4.
In order to ensure that the zero sequence current passing through the other side of the second frequency grounding device after an asymmetrical ground fault occurs on one side of the second frequency grounding device is kept at a low value, the grounding resistance and the impedance of the zero sequence current collecting winding are required to be matched with each other.
The selection of the value of the ground resistor R g is a comprehensive optimization process. When the resistance value of the grounding resistor is overlarge, the zero-sequence current is small during faults, the sensitivity of the relay protection device is insufficient, faults cannot be effectively identified and protected, and meanwhile, the requirement on the ground insulation level of the AC-to-AC converter is improved; the grounding resistance is too small, zero sequence current is too large, so that the grounding resistance is high in energy and difficult in equipment manufacturing, and transient current stress of the AC-DC converter is increased; in order to meet the low-pass characteristic of the zero-sequence current, the impedance value of the zero-sequence current collecting winding is required to be small, however, too small a size does not satisfy the manufacturing conditions. Therefore, the ground resistance value should meet the requirements of zero sequence current protection sensitivity, ground resistance equipment manufacturing level, AC-to-AC converter transient current stress and zero sequence current collecting winding manufacturing level at the same time.
In this embodiment, the rated capacity of the first frequency transformer is S T1, the rated voltage of the high-voltage side is U 1_T1, and the short-circuit impedance percentage is U k_T1.
The rated capacity of the second frequency transformer is S T2, the rated voltage of the high-voltage side is U 1_T2, and the short-circuit impedance percentage between the first winding and the second winding isThe short-circuit impedance percentage between the first winding and the third winding isThe short-circuit impedance percentage between the second winding and the third winding is。
The embodiment also provides a method for selecting the grounding device of the flexible low-frequency power transmission system, namely matching selection of the grounding resistance R g and the impedance Z T2_3 of the zero-sequence current collecting winding, which comprises the following steps:
First, the allowable zero-sequence current low-crossing level (i.e., zero-sequence current percentage) of the second frequency transformer is determined according to the requirements of the system relay protection device and the ac converter equipment, wherein the zero-sequence current percentage allowed to pass from the first winding to the second winding is K 12, the zero-sequence current percentage allowed to pass from the second winding to the first winding is K 21,K12 and K 21, which are input conditions, and the type of the grounding device is determined according to K 12 and K 21. K 12 and K 21 are as small as the conditions allow, typically within 15%, preferably within 10%.
Then, the impedance of the first winding of the second frequency transformer is determined to be Z T2_1, and the impedance of the second winding of the second frequency transformer is determined to be Z T2_2,ZT2_1 and Z T2_2, which are mainly determined by the rated capacity, rated voltage and short-circuit impedance percentage between windings of the second frequency transformer. In this embodiment:
（5）
the impedance of the ac converter is determined to be Z V, the ac converter can flow zero sequence current, and the zero sequence impedance of the ac converter mainly comprises bridge arm reactors, in this embodiment:
（6）
In the formula, Representing the zero sequence current angular frequency; z B is the second frequency transformer impedance reference value;
（7）
the impedance of the first frequency transformer is determined as Z T1, which is mainly determined by the rated capacity, rated voltage and short-circuit impedance percentage between windings of the first frequency transformer, in this embodiment:
（8）
and determining the equivalent zero sequence impedance of the second frequency alternating current power grid as Z f2.
Substituting the formulas (6) to (8) and Z f2 into the formula (9) to obtain the value range of Z T2_3, and substituting the value range into the formula (5) to obtain the feasible short-circuit impedance percentage of the second frequency transformerAnd。
（9）。
Application example
The embodiment is applied to an offshore wind power low-frequency sending-out system, the land part and the offshore part of which are respectively shown in fig. 1 and 5, the zero sequence equivalent circuits of which are shown in fig. 2 and 6, and the main parameters of which are shown in table 1.
TABLE 1
The equivalent capacity of the fan transformer TW is S TW and the short-circuit impedance percentage is U k_TW.
The impedance Z TW of the fan transformer is:
（10）
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, the short-circuit impedance percentage between the first winding and the third winding is U k_T3_13, and the short-circuit impedance percentage between the second winding and the third winding is U k_T3_23.
The impedance Z T3_1 of the first winding, the impedance Z T3_2 of the second winding and the impedance Z T3_3 of the third winding of the offshore transformer are:
（11）
according to the actual parameters of table 1, the zero sequence impedances can be obtained by combining the formulas (6) to (8), the formulas (10) and the formulas (11), as shown in table 2.
TABLE 2
According to fig. 6 and table 2, the equivalent zero sequence impedance Z f2 of the second frequency ac grid is calculated by circuit theory, and its per unit value is 0.7224+j×0.7218.
In this case, K 12 and K 21 each represent 10%.
According to the general design of the transformer, consider that the high-low winding impedance percentage (i.e. the short-circuit impedance percentage between the first winding and the third winding) is slightly larger than the sum of the high-medium winding impedance percentage (the short-circuit impedance percentage between the first winding and the second winding) and the medium-low winding impedance percentage (the short-circuit impedance percentage between the second winding and the third winding), and the difference is 3% in this example.
According to Z T1、ZT2_1、ZT2_2、ZV、Zf2、Rg、K12, K 21 and formula (9), the value range of Z T2_3 is 0-9.5%, so that a group of feasible short-circuit impedance percentages [ U k_T2_12,Uk_T2_23,Uk_T2_13 ] of the second frequency transformer are [15%,9.5% and 27.7% ], and when the corresponding second frequency transformer impedance [ Z T2_1,ZT2_2,ZT2_3 ] is [ j 0.165, -j 0.015 and j 0.11], formulae (1) and (3) are substituted at the moment, and K 12 and K 21 are 9.98% and 5.72% respectively, so that the requirement that the zero sequence current crossing level is less than 10% is met.
The above embodiments are only for illustrating the technical solution of the present invention, and are not limiting; although the invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical scheme described in the foregoing embodiments can be modified or some technical features thereof can be replaced by equivalents; such modifications and substitutions do not depart from the spirit and scope of the technical solutions of the embodiments of the present invention, and are intended to be included in the scope of the present invention.
Claims (10)
1. A grounding device for a flexible low frequency power transmission system, comprising:
An ac-to-dc converter for effecting a conversion of a first frequency and a second frequency, wherein the second frequency is lower than the first frequency;
The two sides of the first frequency transformer are respectively connected with the AC-DC converter and the first frequency AC power grid;
The second frequency transformer comprises a second frequency grounding device; the first winding of the second frequency transformer is connected with the AC-DC converter, and the second winding of the second frequency transformer is connected with a second frequency AC power grid;
A second frequency grounding apparatus comprising: one end of the grounding resistor is connected with the neutral point of the second frequency transformer, and the other end of the grounding resistor is grounded to provide a zero potential reference point for the AC-DC converter; a zero sequence current collection winding is used as a third winding of the second frequency transformer;
The zero-sequence currents on the two sides of the second frequency transformer do not cross each other or are maintained at an allowable zero-sequence current low-crossing level through the matching of the grounding resistance and the zero-sequence current collecting winding impedance.
2. The grounding device of a flexible low-frequency power transmission system according to claim 1, wherein the first frequency transformer is connected with the ac converter winding in a delta-shaped form, and the winding connected with the first frequency ac power grid is directly grounded by a star-shaped neutral point.
3. The grounding device of a flexible low-frequency power transmission system according to claim 1, wherein the first winding of the second frequency transformer connected to the ac converter is in a star-type form, and the second winding of the second frequency ac power network is directly grounded by a star-type neutral point.
4. The grounding device of a flexible low-frequency power transmission system according to claim 1, wherein the first frequency transformer is a three-winding transformer, the first winding connected with the ac-to-ac converter is in a delta-type, the second winding connected with the first frequency ac power grid is in a star-type neutral point direct grounding mode, and the third winding is used as a station power supply winding in a star-type or delta-type.
5. The grounding device of a flexible low frequency power transmission system of claim 1, wherein the windings of the first frequency transformer and the windings of the second frequency transformer are directly connected to an ac-to-ac converter without a series-parallel connection of the grounding device therebetween.
6. The grounding device of a flexible low-frequency power transmission system according to claim 1, wherein the zero-sequence current collecting winding adopts a delta connection mode, is specially used for zero-sequence current collection, and does not lead out power supply.
7. A grounding device for a flexible low frequency power transmission system according to claim 1, wherein when an asymmetric fault occurs on the first winding side of the second frequency transformer or on the first frequency ac grid, the zero sequence current on the first winding side of the second frequency transformer passes mainly through the zero sequence current collecting winding of the second frequency transformer, and passes through to the second winding side of the second frequency transformer at least partially.
8. A 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 on the second frequency ac grid, the zero sequence current on the second winding side of the second frequency transformer passes mainly through the zero sequence current collecting winding of the second frequency transformer and passes through to the first winding side of the second frequency transformer at least partly.
9. A method of selecting a type of grounding device for a flexible low frequency power transmission system as claimed in any one of claims 1 to 8, comprising:
Determining a zero sequence current low crossing level allowed by the second frequency transformer according to the requirements of the flexible low frequency transmission system or the AC-AC converter equipment, wherein 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 is K 12, and 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 is K 21;
determining the impedance of a first winding of the second frequency transformer as Z T2_1, determining the impedance of a second winding of the second frequency transformer as Z T2_2, determining the impedance of an AC-to-AC converter as Z V, and determining the impedance of the first frequency transformer as Z T1;
Determining the equivalent zero sequence impedance of the second frequency alternating current power grid as Z f2;
The matching relationship between the grounding resistance R g and the zero-sequence current collection winding impedance Z T2_3 is determined.
10. The method according to claim 9, wherein the ground resistance R g and the impedance Z T2_3 of the zero sequence current sink winding simultaneously satisfy the following matching relationship:
，
，
Determining the short-circuit impedance percentage between windings of the second frequency transformer according to Z T2_3 meeting the requirements and combining the values of Z T2_1 and Z T2_2 、AndWherein, the method comprises the steps of, wherein,As a percentage of the short-circuit impedance between the first winding and the second winding,As a percentage of the short-circuit impedance between the first winding and the third winding,The short-circuit impedance percentage between the second winding and the third winding is as follows:
。
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