A multi-direct-current commutation failure prevention device in an alternating-direct-current hybrid power grid

Abstract

The application provides a multi-direct-current commutation failure prevention device in an AC-DC hybrid power grid. The device is arranged between two adjacent LCC-HVDC converter stations, and the device comprises two groups of transformer units. Each group of transformer units comprises two multi-winding transformers connected back-to-back. Each multi-winding transformer comprises at least one angle-winding and one star-winding. The two angle-windings in each group of transformer units are connected to block a zero sequence passage. The two star-windings in each group of transformer units are connected to the corresponding LCC-HVDC converter station, or the two star-windings are not grounded to block a zero sequence loop. The voltage levels of the two star-windings are consistent with the voltage levels of the two adjacent LCC-HVDC converter stations. Thus, the application can improve the ability of the multi-direct-current feeding system to resist multi-direct-current commutation failure when the AC system is subjected to single-phase grounding fault and asymmetric fault.

Description

Technical Field

[0001] This application relates to the field of high voltage direct current transmission technology, and in particular to a multi-DC commutation failure defense device in an AC / DC hybrid power grid. Background Technology

[0002] High-voltage direct current (HVDC) transmission is a form of power transmission that uses high-voltage direct current (VDC) for energy transmission. Because AC transmission is limited in both capacity and distance, HVDC is widely used when the power source is far from the load center. LCC-HVDC is a type of DC transmission that extracts a portion of each phase of a three-phase AC power supply and combines them to form DC. The process of extracting a portion of each phase of the three-phase AC power supply requires the converter to be turned on alternately; this process is called commutation. LCC-HVDC converter stations use thyristors as commutation devices. Since thyristors are semi-controlled devices, they require a suitable external voltage as a commutation condition. When the system encounters an AC fault, it can easily cause commutation disorder or failure of the converter, a phenomenon known as commutation failure. When a thyristor converter experiences commutation failure, a short circuit often occurs between the upper and lower bridge arms, which will prevent the entire converter from outputting power to the AC system. Furthermore, when multiple LCC-HVDC converter stations are connected to an AC system, such as... Figure 1 As shown, Figure 1 A schematic diagram of the main wiring of an AC system provided in the prior art of this application; Figure 1 The AC system in the middle has multiple LCC-HVDC converter stations connected. Each two adjacent LCC-HVDC converter stations are connected by two AC lines. If the AC system fails, it will often cause multiple DC commutation failures. This phenomenon is called multiple DC commutation failure.

[0003] Currently, existing technologies mainly provide a longer time margin for the commutation process by increasing the arc-extinguishing angle. However, if the AC fault is severe, and the AC system voltage experiences a significant drop or asymmetry, commutation will still be impossible. For typical AC systems, single-phase grounding faults are the most common. If the fault causes a deep voltage drop, increasing the arc-extinguishing angle still cannot prevent commutation failure. Furthermore, by the time the LCC-HVDC control and protection system detects an AC fault, commutation failure has often already occurred, meaning that the control method of increasing the arc-extinguishing angle is often lagging behind the actual fault process. In addition, increasing the arc-extinguishing angle is a defense method for single LCC-HVDC commutation failure and has no optimization effect on multiple DC commutation failures.

[0004] Furthermore, existing technologies also use flexible DC to eliminate commutation failures; however, the converters used are expensive, and converters are the most costly equipment in high-voltage DC transmission projects. In addition, modifications would require alterations to valve halls, lines, and other related equipment. Therefore, this method is often not technically and economically viable for addressing multiple DC commutation failures. Summary of the Invention

[0005] The purpose of this application is to at least address one of the aforementioned technical deficiencies, particularly the lack of a defense device against multiple DC commutation failures in the prior art.

[0006] This application provides a multi-DC commutation failure defense device in an AC / DC hybrid power grid, wherein the device is installed between two adjacent LCC-HVDC converter stations;

[0007] The device includes two sets of transformer units, each set of transformer units includes two multi-winding transformers connected back to back, and each multi-winding transformer includes at least one delta winding and one star winding.

[0008] In each transformer unit, the two delta windings are connected, and the two star windings in each transformer unit are respectively connected to the corresponding LCC-HVDC converter station, or the two star windings are not grounded, and the voltage level of the two star windings is consistent with the voltage level of the two adjacent LCC-HVDC converter stations.

[0009] Optionally, if the insulation level of the multi-winding transformers in both transformer units is not lower than the preset threshold, then the voltage level of the two delta windings in each transformer unit is set to be not lower than the voltage level of the two star windings.

[0010] Optionally, if the insulation level of the multi-winding transformers in both transformer units is lower than the preset threshold, then the voltage level of the two delta windings in each transformer unit is set to be lower than the voltage level of the two star windings.

[0011] Optionally, when the multi-winding transformers in both sets of transformer units are both two-winding transformers, the device further includes a first reactive power compensation device.

[0012] The first reactive power compensation device is installed between the two angle-connected windings of one of the transformer units.

[0013] Optionally, the first reactive power compensation device is a switchable parallel capacitor.

[0014] Optionally, when two of the multi-winding transformers in at least one set of transformer units are three-winding transformers, the device further includes a second reactive power compensation device.

[0015] The second reactive power compensation device is mounted on the third winding of each of the two three-winding transformers.

[0016] Optionally, the second reactive power compensation device is a low-capacity reactive power compensation device or a low-voltage reactive power compensation device.

[0017] Optionally, the multi-winding transformers in both sets of transformer units are low-impedance transformers.

[0018] Optionally, the two sets of transformer units are connected in series on different AC lines between two adjacent LCC-HVDC converter stations.

[0019] Optionally, one set of transformer units is installed in one of the LCC-HVDC converter stations;

[0020] Alternatively, both sets of transformer units can be installed in the same LCC-HVDC converter station;

[0021] Alternatively, the two sets of transformer units can be installed in different LCC-HVDC converter stations.

[0022] As can be seen from the above technical solutions, the embodiments of this application have the following advantages:

[0023] This application provides a multi-DC commutation failure defense device in an AC / DC hybrid power grid. The device is installed between two adjacent LCC-HVDC converter stations and includes two sets of transformer units. Each set of transformer units includes two back-to-back multi-winding transformers, each multi-winding transformer including at least one delta winding and one star winding. The two delta windings in each transformer unit are connected to block the zero-sequence path, and the two star windings in each transformer unit are respectively connected to the corresponding LCC-HVDC converter station, or the two star windings are ungrounded to block the zero-sequence loop. The voltage level of the two star windings is consistent with the voltage level of the two adjacent LCC-HVDC converter stations. This allows the application to improve the multi-DC feed-in system's ability to resist multi-DC commutation failures during single-phase grounding faults and asymmetrical faults in the AC system. Furthermore, this application does not require the LCC-HVDC converter station to be converted into a flexible DC converter valve, exhibiting good technical and economic efficiency, and does not divide the AC system into different subgrids, thus maintaining system strength. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 A schematic diagram of the main wiring of an AC system provided in the prior art of this application;

[0026] Figure 2 A schematic diagram of one structure of a multi-DC commutation failure defense device obtained using the YNd connection method provided in an embodiment of this application;

[0027] Figure 3 This is a schematic diagram of another structure of the multi-DC commutation failure defense device obtained by using the YNd connection method provided in the embodiments of this application;

[0028] Figure 4 This is another structural schematic diagram of the multi-DC commutation failure defense device obtained using the YNd connection method provided in this application. Detailed Implementation

[0029] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0030] Currently, existing technologies mainly provide a longer time margin for the commutation process by increasing the arc-extinguishing angle. However, if the AC fault is severe, and the AC system voltage experiences a significant drop or asymmetry, commutation will still be impossible. For typical AC systems, single-phase grounding faults are the most common. If the fault causes a deep voltage drop, increasing the arc-extinguishing angle still cannot prevent commutation failure. Furthermore, by the time the LCC-HVDC control and protection system detects an AC fault, commutation failure has often already occurred, meaning that the control method of increasing the arc-extinguishing angle is often lagging behind the actual fault process. In addition, increasing the arc-extinguishing angle is a defense method for single LCC-HVDC commutation failure and has no optimization effect on multiple DC commutation failures.

[0031] Furthermore, existing technologies also use flexible DC to eliminate commutation failures; however, the converters used are expensive, and converters are the most costly equipment in high-voltage DC transmission projects. In addition, modifications would require alterations to valve halls, lines, and other related equipment. Therefore, this method is often not technically and economically viable for addressing multiple DC commutation failures. Therefore, this application proposes the following technical solution, as detailed below:

[0032] In one embodiment, this application provides a multi-DC commutation failure defense device in an AC / DC hybrid power grid, wherein the device is installed between two adjacent LCC-HVDC converter stations.

[0033] The device includes two sets of transformer units, each set of transformer units includes two multi-winding transformers connected back to back, and each multi-winding transformer includes at least one delta winding and one star winding.

[0034] In each transformer unit, the two delta windings are connected, and the two star windings in each transformer unit are respectively connected to the corresponding LCC-HVDC converter station, or the two star windings are not grounded, and the voltage level of the two star windings is consistent with the voltage level of the two adjacent LCC-HVDC converter stations.

[0035] In this embodiment, in order to prevent multiple DC commutation failures, a multiple DC commutation failure defense device is installed between two adjacent LCC-HVDC converter stations in the AC system. This will not change the original internal structure of the LCC-HVDC converter station, but will also achieve the effect of preventing commutation failures.

[0036] Specifically, by appendix Figure 1 It is understood that two adjacent LCC-HVDC converter stations in the AC system are connected via AC line 1 and AC line 2. AC line 1 primarily connects the converter transformers of converter station A and converter station B, while AC line 2 connects converter station A and converter station B. Therefore, the multi-DC commutation failure prevention device in this application can include two sets of transformer units, which are used to connect to converter station A and converter station B. This connection method includes, but is not limited to, connecting the two sets of transformer units in series on AC line 1 and AC line 2 respectively, or placing the two sets of transformer units separately in converter station A and converter station B, or placing one set of transformer units in either converter station A or converter station B, etc., without limitation.

[0037] Furthermore, each transformer unit in this application may include two back-to-back multi-winding transformers. Each multi-winding transformer includes at least one delta winding and one star winding. The two delta windings in each transformer unit are connected to block the zero-sequence path. The two star windings in each transformer unit are respectively connected to the corresponding LCC-HVDC converter station, or the two star windings are not grounded to block the zero-sequence loop. The voltage level of the two star windings is consistent with the voltage level of the two adjacent LCC-HVDC converter stations.

[0038] It is understood that the connection method between the two back-to-back multi-winding transformers in this application mainly uses YNd and YNy connections to block zero-sequence paths. Furthermore, considering maintaining the original state before the line is disconnected, the two transformer units generally use the same connection group (e.g., both YNd11). Here, the uppercase letters indicate the connection method of the primary side (or primary winding) of the transformer, and the lowercase letters indicate the connection method of the secondary side (or secondary winding). Y (or y) indicates a star connection, "Yn" indicates a star connection with a neutral line, Y represents a star connection, n represents a connection with a neutral line, and D (or d) indicates a delta connection. The numbers use clock notation to represent the phase relationship between the primary and secondary line voltages. The primary line voltage phasor is used as the minute hand, fixed at 12 o'clock, and the secondary line voltage phasor is used as the hour hand. For example, "11" indicates that the secondary line voltage lags the primary line voltage by 330 degrees (or leads by 30 degrees).

[0039] It should be noted that the multi-winding transformer in this application can be a two-winding transformer or a three-winding transformer. The low-voltage side of the multi-winding transformer adopts a delta connection, and the two delta-connected windings of two multi-winding transformers are connected to block the zero-sequence path. The high-voltage side of the multi-winding transformer adopts a Y-connection because, under a certain line voltage, the phase voltage on the high-voltage side is only a small part of the line voltage, and the insulation is usually designed according to the phase voltage, so less material is needed. Furthermore, the voltage level of the star-connected winding on the high-voltage side can also be set to be consistent with the voltage level of the adjacent LCC-HVDC converter station. For example, if the LCC-HVDC converter station adopts a 500kV voltage level, the star-connected winding can also adopt a 500kV voltage level.

[0040] Indicatively, such as Figure 2 As shown, Figure 2 A schematic diagram of one structure of a multi-DC commutation failure defense device obtained using the YNd connection method provided in an embodiment of this application; Figure 2 In the diagram, the dashed box between converter station A and converter station B represents the multi-DC commutation failure defense device of this application. The device includes two sets of transformer units connected in series on different AC lines. Each set of transformer units includes two multi-winding transformers connected back-to-back. The two multi-winding transformers are connected by a YNd connection (Y-Δ connection). The star windings of converter station A and the two-winding transformers, as well as the star windings of converter station B and the two-winding transformers, are connected by a 500kV bus. The delta windings of the two two-winding transformers are connected by a 220kV bus. This effectively compensates for voltage drops and defends against multi-DC commutation failures.

[0041] In the above embodiments, the multi-DC commutation failure defense device is installed between two adjacent LCC-HVDC converter stations. The device includes two sets of transformer units, each set of transformer units including two back-to-back multi-winding transformers. Each multi-winding transformer includes at least one delta winding and one star winding. The two delta windings in each set of transformer units are connected to block the zero-sequence path, and the two star windings in each set of transformer units are respectively connected to the corresponding LCC-HVDC converter station, or the two star windings are not grounded to block the zero-sequence loop. The voltage level of the two star windings is consistent with the voltage level of the two adjacent LCC-HVDC converter stations. This allows the present application to improve the ability of the multi-DC feed-in system to resist multi-DC commutation failure in the event of single-phase grounding faults and asymmetrical faults in the AC system. Furthermore, the present application does not require the LCC-HVDC converter station to be modified into a flexible DC converter valve, which has good technical and economic efficiency, and does not divide the AC system into different subnets, thus maintaining the system strength.

[0042] In one embodiment, if the insulation level of the multi-winding transformers in both transformer units is not lower than a preset threshold, then the voltage level of the two delta windings in each transformer unit is set to be not lower than the voltage level of the two star windings, such as 1000kV or 750kV.

[0043] In one embodiment, if the insulation level of the multi-winding transformers in both transformer units is lower than the preset threshold, then the voltage level of the two delta windings in each transformer unit is set to be lower than the voltage level of the two star windings, such as 220kV.

[0044] In one embodiment, such as Figure 2 As shown, when the multi-winding transformers in both sets of transformer units are both two-winding transformers, the device also includes a first reactive power compensation device. The first reactive power compensation device is installed between the two angle-connected windings of one of the transformer units. In this way, when the original line is long and the angle-connected winding side may lack reactive power under heavy line load, reactive power compensation is performed through the reactive power compensation device to reduce line losses and improve power grid quality.

[0045] In one embodiment, the first reactive power compensation device is a switchable parallel capacitor, which can be used to compensate for the reactive power of inductive loads in the power system, improve the power factor, improve voltage quality, and reduce line losses.

[0046] In one embodiment, when two of the multi-winding transformers in at least one set of transformer units are three-winding transformers, the device further includes a second reactive power compensation device; the second reactive power compensation device is respectively mounted on the third winding of the two three-winding transformers.

[0047] In this embodiment, as Figure 3 As shown, Figure 3 This is a schematic diagram of another structure of the multi-DC commutation failure defense device obtained by using the YNd connection method provided in the embodiments of this application; Figure 3 In the diagram, the dashed box between converter station A and converter station B represents the multi-DC commutation failure prevention device of this application. This device includes two sets of transformer units connected in series on different AC lines. Each transformer unit includes two back-to-back multi-winding transformers connected in a YNd (Y-Δ) connection. Converter station A is connected to the star windings of the two-winding and three-winding transformers, and converter station B is connected to the star windings of the two-winding and three-winding transformers via 500kV busbars. The delta windings of the two two-winding transformers are connected via a 220kV busbar, and the delta windings of the two three-winding transformers are also connected via a 220kV busbar. This effectively compensates for voltage dips and prevents multi-DC commutation failure. Figure 3 As can be seen, the multi-winding transformers connected back-to-back in this application use reactive power compensation equipment on the corner winding side to cope with possible reactive power demand. If the technical conditions of the multi-winding transformer allow, a three-winding transformer can also be used, and the reactive power compensation equipment can be mounted on the third winding of the two three-winding transformers respectively. The voltage level of the bus connecting the third winding and the reactive power compensation equipment can be 35kV.

[0048] In one embodiment, the second reactive power compensation device is a low-capacity reactive power compensation device or a low-voltage reactive power compensation device, which can be used to compensate for the reactive power of inductive loads in the power system, improve the power factor, improve voltage quality, and reduce line losses.

[0049] In one embodiment, the multi-winding transformers in both sets of transformer units are low-impedance transformers, meaning they have low short-circuit impedance. Short-circuit impedance refers to impedance voltage, which is usually expressed as a percentage. Impedance voltage short-circuits the secondary winding of the transformer, causing the primary winding voltage to gradually increase. Once the short-circuit current in the secondary winding reaches the rated current, the ratio of the short-circuit voltage applied to the primary winding to the rated voltage is the impedance voltage, i.e., Uk% = short-circuit voltage / rated voltage * 100%. When the transformer is running at full load, the level of short-circuit impedance has a certain impact on the output voltage on the secondary side. A lower short-circuit impedance results in a smaller voltage drop, and a higher short-circuit impedance results in a larger voltage drop. When a short circuit occurs in the transformer load, a lower short-circuit impedance results in a larger short-circuit current, and the transformer experiences a greater electrodynamic force. Conversely, a higher short-circuit impedance results in a smaller short-circuit current, and the transformer experiences a smaller electrodynamic force.

[0050] In one embodiment, such as Figure 2 , 3As shown, the two sets of transformer units are connected in series on different AC lines between two adjacent LCC-HVDC converter stations.

[0051] In one embodiment, one set of transformer units is installed in one of the LCC-HVDC converter stations; or, both sets of transformer units are installed in the same LCC-HVDC converter station; or, the two sets of transformer units are installed in different LCC-HVDC converter stations.

[0052] In this embodiment, considering factors such as land occupation, back-to-back connected multi-winding transformers can also be installed in the LCC-HVDC converter station. (Illustratively, as shown...) Figure 4 As shown, Figure 4 This is another structural schematic diagram of the multi-DC commutation failure defense device obtained using the YNd connection method provided in this application; by Figure 4 It is understood that this application can install one set of transformer units in one of the LCC-HVDC converter stations; of course, this application can also install both sets of transformer units in the same LCC-HVDC converter station; or, the two sets of transformer units can be installed in different LCC-HVDC converter stations, thereby reducing the footprint.

[0053] Finally, it should be noted that in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0054] The various embodiments in this specification are described in a progressive manner. Each embodiment focuses on the differences from other embodiments. The various embodiments can be combined as needed, and the same or similar parts can be referred to each other.

[0055] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A multi-DC commutation failure defense device in an AC / DC hybrid power grid, characterized in that, The device is installed between two adjacent LCC-HVDC converter stations; The device includes two sets of transformer units, each set of transformer units includes two multi-winding transformers connected back to back, and each multi-winding transformer includes at least one delta winding and one star winding. In each transformer unit, two delta windings are connected, and two star windings in each transformer unit are connected to the corresponding LCC-HVDC converter station. The two star windings are not grounded, and the voltage level of the two star windings is consistent with the voltage level of the two adjacent LCC-HVDC converter stations.

2. The multi-DC commutation failure defense device in an AC / DC hybrid power grid according to claim 1, characterized in that, If the insulation level of the multi-winding transformers in both transformer units is not lower than the preset threshold, then the voltage level of the two delta windings in each transformer unit is set to be not lower than the voltage level of the two star windings.

3. The multi-DC commutation failure defense device in an AC / DC hybrid power grid according to claim 2, characterized in that, If the insulation level of the multi-winding transformers in both transformer units is lower than the preset threshold, then the voltage level of the two delta windings in each transformer unit is set to be lower than the voltage level of the two star windings.

4. The multi-DC commutation failure defense device in an AC / DC hybrid power grid according to claim 1, characterized in that, When the multi-winding transformers in both sets of transformer units are two-winding transformers, the device also includes a first reactive power compensation device. The first reactive power compensation device is installed between the two angle-connected windings of one of the transformer units.

5. The multi-DC commutation failure defense device in an AC / DC hybrid power grid according to claim 4, characterized in that, The first reactive power compensation device is a switchable parallel capacitor.

6. The multi-DC commutation failure defense device in an AC / DC hybrid power grid according to claim 1, characterized in that, When at least two of the multi-winding transformers in a transformer unit are three-winding transformers, the device further includes a second reactive power compensation device. The second reactive power compensation device is mounted on the third winding of each of the two three-winding transformers.

7. The multi-DC commutation failure defense device in an AC / DC hybrid power grid according to claim 6, characterized in that, The second reactive power compensation device is a low-capacity reactive power compensation device or a low-voltage reactive power compensation device.

8. The multi-DC commutation failure defense device in an AC / DC hybrid power grid according to any one of claims 1-7, characterized in that, Both sets of transformer units contain multi-winding transformers that are low-impedance transformers.

9. The multi-DC commutation failure defense device in an AC / DC hybrid power grid according to any one of claims 1-7, characterized in that, The two sets of transformer units are connected in series on different AC lines between two adjacent LCC-HVDC converter stations.

10. The multi-DC commutation failure defense device in an AC / DC hybrid power grid according to any one of claims 1-7, characterized in that, One set of transformer units is installed in one of the LCC-HVDC converter stations; Alternatively, both sets of transformer units can be installed in the same LCC-HVDC converter station; Alternatively, the two sets of transformer units can be installed in different LCC-HVDC converter stations.

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