Fault control method, device and isolated high-voltage DC transformer

Abstract

The present disclosure relates to a fault control method, device, and isolated high-voltage direct current transformer, which relates to the field of power system technology and solves the problem of low calculation speed and convergence speed caused by the influence of nonlinear power supply during the iterative calculation of short-circuit current, requiring a large number of series-parallel power electronic devices, and high fault handling costs. The method includes: when a DC fault is detected on the high-voltage side of the isolated high-voltage direct current transformer, controlling the isolated high-voltage direct current transformer to enter a DC power supply control mode; and controlling the high-voltage direct current circuit breaker to continue operating for a first preset time, and then controlling the current transfer branch and the main circuit breaker branch to disconnect, and controlling the lightning protection branch to conduct; and when the isolated high-voltage direct current transformer enters the DC power supply control mode for a second preset time, controlling the low-voltage side DC energy consumption device to conduct.

Description

Technical Field

[0001] The present disclosure relates to the technical field of power systems, and in particular to a fault control method, device, and an isolated high-voltage direct current transformer. Background Art

[0002] Due to the vast coverage of large-scale wind and solar power bases in western China, overhead lines are often used as DC transmission channels. The use of DC overhead lines introduces the potential for transient DC faults, and multi-voltage DC grids require extremely fast fault clearing. To avoid serious threats to equipment safety, multi-voltage DC grids must typically maintain fault clearing times of less than 5 milliseconds, significantly faster than the 50 milliseconds or longer fault clearing times of AC systems. This means that fault detection and protection in DC grids must be at least an order of magnitude faster than typical AC systems. Currently, high-voltage DC circuit breakers are used to handle DC faults. However, this approach requires the addition of numerous series-parallel power electronic devices to control the on / off switching of the high-voltage DC circuit breakers, resulting in high fault handling costs. Summary of the Invention

[0003] The present invention provides a fault control method, device, and isolated high-voltage DC transformer to at least address the problem of related prediction methods requiring a large number of series-parallel power electronic devices and high fault handling costs. The technical solution of the present invention is as follows.

[0004] According to a first aspect of an embodiment of the present invention, a fault control method is provided, which is applied to an isolated high-voltage direct current transformer. The method comprises: upon detecting a DC fault on the high-voltage side of the isolated high-voltage direct current transformer, controlling the isolated high-voltage direct current transformer to enter a DC power supply control mode; and, after controlling the high-voltage direct current circuit breaker to continue operating for a first preset time, controlling the current transfer branch and the main circuit breaker branch to be disconnected, and controlling the lightning protection branch to be turned on; and, when the isolated high-voltage direct current transformer enters the DC power supply control mode for a second preset time, controlling the low-voltage side DC energy consumption device to be turned on.

[0005] In one implementation, when a DC fault is detected on the high-voltage side of the isolated high-voltage DC transformer, before controlling the isolated high-voltage DC transformer to enter a DC power supply control mode, the method further includes: determining the DC current rise rate on the high-voltage side; determining that a DC fault has occurred when the absolute value of the DC current rise rate is greater than a preset threshold; and determining that no DC fault has occurred when the absolute value of the DC current rise rate is less than or equal to the preset threshold.

[0006] In another implementation, before determining the DC current rise rate on the high-voltage side, the method further includes: determining a preset threshold value based on the rated DC voltage on the high-voltage side and the equivalent inductance of the DC circuit on the high-voltage side.

[0007] In another implementation, a preset threshold is determined based on the rated DC voltage on the high-voltage side and the equivalent inductance of the DC loop, including: determining the equivalent inductance of the DC loop on the high-voltage side based on the sum of the positive and negative smoothing reactances on the high-voltage side and the high-voltage side bridge arm reactance of the isolated high-voltage DC transformer; determining the preset threshold based on the rated DC voltage on the high-voltage side of the isolated high-voltage DC transformer and the equivalent inductance of the DC loop on the high-voltage side.

[0008] In another implementation, the equivalent inductance of the DC circuit on the high-voltage side is determined based on the sum of the positive and negative smoothing reactances on the high-voltage side and the reactance of the high-voltage side bridge arm of the isolated high-voltage DC transformer. This includes determining the equivalent inductance of the DC circuit on the high-voltage side according to the following formula when the high-voltage DC transformer is a single-phase structure:

[0009] L dceq =L dc +L arm

[0010] Among them, L dceq is the equivalent inductance of the high-voltage side DC circuit of the isolated high-voltage DC transformer, L dc L is the sum of the positive and negative smoothing reactances on the high-voltage side of the isolated high-voltage DC transformer. arm It is the high-voltage side bridge arm reactance of the isolated high-voltage DC transformer.

[0011] In another implementation, the equivalent inductance of the DC circuit on the high-voltage side is determined based on the sum of the positive and negative smoothing reactances on the high-voltage side and the reactance of the high-voltage side bridge arm of the isolated high-voltage DC transformer. This includes determining the equivalent inductance of the DC circuit on the high-voltage side according to the following formula when the high-voltage DC transformer has a three-phase structure:

[0012] L dceq =L dc +2 / 3L arm

[0013] Among them, L dceq is the equivalent inductance of the high-voltage side DC circuit of the isolated high-voltage DC transformer, L dc L is the sum of the positive and negative smoothing reactances on the high-voltage side of the isolated high-voltage DC transformer. arm It is the high-voltage side bridge arm reactance of the isolated high-voltage DC transformer.

[0014] In another implementation, the DC power supply control mode includes a DC voltage drop control stage and a DC voltage recovery control stage; controlling the isolated high-voltage DC transformer to enter the DC power supply control mode includes: in the DC voltage drop control stage, determining the DC voltage command value on the high-voltage side and the AC side voltage command value; and reducing the AC side voltage command value and the high-voltage side DC voltage command value to a preset voltage value; the preset voltage value is within a preset range; in the DC voltage recovery control stage, adjusting the AC side voltage command value and the high-voltage side DC voltage command value to the steady-state voltage value before the fault.

[0015] In another implementation, reducing both the AC side voltage command value and the high-voltage side DC voltage command value to a preset voltage value includes: multiplying the AC side voltage command value and the high-voltage side DC voltage command value by corresponding first proportional coefficients, respectively, to reduce both the AC side voltage command value and the high-voltage side DC voltage command value to the preset voltage value;

[0016] Adjusting the AC side voltage command value and the high-voltage side DC voltage command value to the steady-state voltage value before the fault includes: multiplying the AC side voltage command value and the high-voltage side DC voltage command value by corresponding second proportional coefficients respectively to adjust the AC side voltage command value and the high-voltage side DC voltage command value to the steady-state voltage value before the fault.

[0017] According to a second aspect of an embodiment of the present invention, a fault control device is provided, which is applied to an isolated high-voltage DC transformer. The device includes: a first control unit, which is used to control the isolated high-voltage DC transformer to enter a DC power supply control mode when a DC fault is detected on the high-voltage side of the isolated high-voltage DC transformer; a second control unit, which is used to control the high-voltage DC circuit breaker to continue operating for a first preset time, and then control the current transfer branch and the main circuit breaker branch to be disconnected, and control the lightning protection branch to be turned on; and a third control unit, which is used to control the low-voltage side DC energy consumption device to be turned on when the isolated high-voltage DC transformer enters the DC power supply control mode for a second preset time.

[0018] According to a third aspect of an embodiment of the present invention, there is provided a fault control method for an isolated high-voltage direct current transformer configured to execute any one of the above implementations.

[0019] The technical solution provided by the embodiments of the present invention brings at least the following beneficial effects: by directly controlling the high-voltage DC transformer to disconnect the current transfer branch and the main circuit breaker branch and to conduct the lightning protection branch during the modulation and triggering links, active control of the DC voltage after a DC fault is achieved. In this way, by reducing the high-voltage side DC voltage during the fault, DC fault current suppression is achieved, the demand for the shutdown capacity of the high-voltage DC circuit breaker is reduced, and no additional power electronic devices are required for on-off control, thereby reducing the cost of fault handling.

[0020] It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings herein are incorporated into and constitute a part of the specification, illustrate embodiments consistent with the present disclosure, and together with the description are used to explain the principles of the present disclosure, and do not constitute an improper limitation of the present disclosure.

[0022] Figure 1 This is a flow chart of a fault control method according to an embodiment of the present invention;

[0023] Figure 2 A schematic diagram of a topological structure of a single-phase isolated high-voltage DC transformer according to an embodiment of the present invention;

[0024] Figure 3 A schematic diagram of a topological structure of a three-phase isolated high-voltage DC transformer according to an embodiment of the present invention;

[0025] Figure 4 A schematic structural diagram of a high-voltage DC circuit breaker according to an embodiment of the present invention;

[0026] Figure 5 A schematic diagram of a high-voltage DC transformer power supply circuit structure according to an embodiment of the present invention;

[0027] Figure 6 This is a schematic diagram of technical parameters of a DC transformer according to an embodiment of the present invention;

[0028] Figure 7 A waveform diagram of a high-voltage side DC voltage command value in a DC voltage drop control stage 1 under an ideal state shown in an embodiment of the present invention;

[0029] Figure 8 A schematic diagram of a simulated waveform of a DC voltage on the high-voltage side of a high-voltage DC transformer according to an embodiment of the present invention;

[0030] Figure 9 A schematic diagram of a simulated waveform of a DC voltage on the low-voltage side of a high-voltage DC transformer according to an embodiment of the present invention;

[0031] Figure 10 A schematic diagram of a simulated waveform of a DC current on the high-voltage side of a high-voltage DC transformer according to an embodiment of the present invention;

[0032] Figure 11 A schematic diagram of a simulated waveform of a DC current on the low-voltage side of a high-voltage DC transformer according to an embodiment of the present invention;

[0033] Figure 12The figure is a block diagram of a fault control device according to an exemplary embodiment. DETAILED DESCRIPTION

[0034] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned figure descriptions are intended to cover non-exclusive inclusions.

[0036] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0037] It should be noted that the terms "first," "second," and the like in the specification and claims of the present disclosure and the above-mentioned drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or precedence. It should be understood that the numbers used in this manner are interchangeable where appropriate so that the embodiments of the present disclosure described herein can be implemented in an order other than those illustrated or described herein. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present disclosure. Instead, they are merely examples of apparatus and methods consistent with certain aspects of the present disclosure as detailed in the appended claims.

[0038] While this application provides method steps as described in the embodiments or flowcharts, more or fewer steps may be included based on routine or uninventive work. The order of steps listed in the embodiments is merely a fault control method among a variety of step execution sequences and does not represent the only execution sequence. The method should be implementable by software and / or hardware.

[0039] Research has found that my country's renewable energy generation will account for an increasing proportion of electricity, with wind power, photovoltaics, and other renewable energy sources dominating the new power system's power structure. Under current technical conditions, renewable energy power plants primarily connect to the grid using AC aggregation and ultra-high voltage DC transmission. Renewable energy generation is characterized by intermittent operation, low moment of inertia, and electronic power generation. Years of engineering experience have shown that this technical approach is prone to risks such as broadband harmonics and oscillations, which can seriously impact the safe and stable operation of the power system. As various large-scale renewable energy sources are connected to the grid, traditional power equipment, grid structures, and operating technologies are increasingly unable to accommodate ultra-large-scale renewable energy. Therefore, new technologies, equipment, and grid structures are essential to meet the profound changes in the future energy landscape.

[0040] The development of large-scale wind and solar power bases in the western Shagohuang region is a major growth driver for my country's future renewable energy development. These large-scale wind and solar power bases primarily fall into two scenarios: 1) DC transmission from large-scale photovoltaic power generation bases; and 2) renewable energy transmission from high-altitude areas. In the first scenario, due to the low voltage of individual photovoltaic power generation units, multi-stage DC-DC boosting to transmission levels is required, thus forming a multi-voltage photovoltaic DC transmission system. In the second scenario, the thin air and high cosmic ray intensity at high altitudes result in low inrush voltages of converter station main equipment and reduced external insulation strength. This necessitates the construction of high- and low-valve converter stations at separate locations, creating a multi-voltage DC transmission system. Therefore, adopting a multi-voltage DC grid solution for large-scale renewable energy transmission can effectively enhance system power supply reliability and operational flexibility, reduce system backup capacity, and mitigate fluctuations in renewable energy generation, thus meeting my country's demand for large-scale renewable energy transmission from west to east.

[0041] In AC systems, voltage level conversion can be achieved using ferromagnetic AC transformers based on the principle of electromagnetic induction. In multi-voltage DC grids, voltage level conversion cannot be achieved directly using AC transformers, and DC transformers are considered a key technology for multi-voltage DC grids. For low-voltage DC transformers, the conventional input parallel output series (IPOS) topology has achieved considerable technical maturity. At high-voltage and high-capacity levels, isolated high-voltage DC transformers are typically used to isolate the electrical coupling between different voltage levels. These isolated high-voltage DC transformers consist of modular multilevel converters (MMCs) located on the high-voltage DC side and the low-voltage DC side, with the high- and low-voltage MMCs achieving electrical isolation between the high- and low-voltage DC sides via converter transformers. Therefore, it is necessary to conduct a detailed study of a high-voltage DC fault control strategy for an isolated high-voltage DC transformer.

[0042] Figure 1 This is a flow chart of a fault control method provided in an embodiment of the present application. Figure 1 As shown in FIG, the fault control method includes the following steps. The method can be applied to an isolated high-voltage DC transformer.

[0043] S11 , when a DC fault is detected on the high-voltage side of the isolated high-voltage DC transformer, controlling the isolated high-voltage DC transformer to enter a DC power supply control mode.

[0044] S12, controlling the high-voltage DC circuit breaker to continue operating for a first preset time, then controlling the current transfer branch and the main circuit breaker branch to be disconnected, and controlling the lightning protection branch to be turned on.

[0045] S13, when the isolated high-voltage DC transformer enters the DC power supply control mode for a second preset time, controlling the low-voltage side DC energy consumption device to be turned on.

[0046] It is understandable that S11 is entered immediately after the fault occurs, while S12 and S13 are executed after a certain delay after the fault occurs. Moreover, the above S11, S12 and S13 are performed concurrently, but in a time-ordered manner.

[0047] Exemplarily, first, after a DC fault occurs and is detected, the isolated high-voltage DC transformer immediately enters a DC voltage control mode.

[0048] Secondly, after a DC fault occurs, the high-voltage DC circuit breaker disconnects the current transfer branch and the main circuit breaker branch after a certain time delay, and the lightning arrester branch is put into the fault current path, where the time delay is selected within the range of 2ms to 8ms.

[0049] Third, after a DC fault occurs, the isolated high-voltage DC transformer is put into operation after a certain time delay to switch to the low-voltage side DC energy consumption device, wherein the time delay is selected within the range of 2ms to 6ms.

[0050] Through the above-described implementation, active control of the DC voltage after a DC fault is achieved by directly controlling the HVDC transformer during the modulation and triggering stages to disconnect the current transfer branch and the main circuit breaker branch, and to conduct the lightning protection branch. This reduces the DC fault current by lowering the high-voltage DC voltage during the fault, reducing the demand for the HVDC circuit breaker's shutoff capacity. This eliminates the need for additional power electronic devices for on-off control, thereby reducing fault handling costs. Furthermore, this implementation is simple and applicable not only to single-phase isolated HVDC transformers but also to three-phase isolated HVDC transformers, making it highly applicable and highly valuable in engineering applications.

[0051] The fault control method of the present application can be applied to Figure 2 and Figure 3 The single-phase and three-phase isolated HVDC transformers shown in the figure include a DC energy dissipation device, multiple half-bridge submodules (SMs), and two HVDC circuit breakers. The DC energy dissipation device is connected to both ends of the low-voltage side of the isolated HVDC transformer, and the two HVDC circuit breakers are respectively installed on two lines on the high-voltage side of the HVDC transformer.

[0052] The above-mentioned high voltage DC circuit breaker is as follows Figure 4 Shown are the current transfer branch, main circuit breaker branch, and lightning arrester branch.

[0053] like Figure 5 As shown, the photovoltaic power generation unit provides a 20kV voltage in the positive and negative directions to the high-voltage DC transformer, which is converted by the high-voltage DC transformer to obtain a 200kV voltage in the positive and negative directions.

[0054] In one embodiment, immediately after a fault occurs and before entering S11 , the fault may be determined first. The fault determination process is as follows.

[0055] First, determine the DC current rise rate on the high voltage side.

[0056] Secondly, when the absolute value of the DC current rising rate is greater than a preset threshold, it is determined that a DC fault has occurred.

[0057] Third, when the absolute value of the DC current rising rate is less than or equal to a preset threshold, it is determined that no DC fault has occurred.

[0058] In this embodiment, the absolute value of the DC current rising rate is used as a criterion. When the absolute value of the DC current rising rate is greater than a preset threshold, it is considered that a DC fault has occurred; otherwise, it is determined that no fault has occurred.

[0059] In one embodiment, before determining the DC current rise rate on the high-voltage side, the method further includes: determining a preset threshold value according to the rated DC voltage on the high-voltage side and the equivalent inductance of the DC circuit on the high-voltage side.

[0060] Specifically, the preset threshold is k*U dc0 / L dceq , where the constant k can range from 0.1 to 0.5, U dc0 is the rated DC voltage on the high side of the isolated high-voltage DC transformer, L dceq It is the equivalent inductance of the DC circuit on the high-voltage side of the isolated high-voltage DC transformer.

[0061] For a single-phase high-voltage DC transformer, L dceq =L dc +L arm, for a three-phase high voltage DC transformer, L dceq =L dc +2 / 3L arm , where L dc Equal to the sum of the positive and negative smoothing reactances on the high-voltage side of the isolated high-voltage DC transformer, L arm Equal to the reactance of the high-voltage side bridge arm of the isolated high-voltage DC transformer.

[0062] The preset threshold is determined according to the rated DC voltage on the high-voltage side and the equivalent inductance of the DC circuit. The specific process includes:

[0063] First, the equivalent inductance of the DC circuit on the high-voltage side is determined based on the sum of the positive and negative smoothing reactances on the high-voltage side and the high-voltage side bridge arm reactance of the isolated high-voltage DC transformer.

[0064] Secondly, a preset threshold is determined according to the rated DC voltage on the high-voltage side of the isolated high-voltage DC transformer and the equivalent inductance of the DC loop on the high-voltage side.

[0065] In one embodiment, the equivalent inductance of the DC circuit on the high-voltage side is determined based on the sum of the positive and negative smoothing reactances on the high-voltage side and the high-voltage side bridge arm reactance of the isolated high-voltage DC transformer, including: when the high-voltage DC transformer is a single-phase structure, the equivalent inductance of the DC circuit on the high-voltage side is determined according to the following formula:

[0066] L dceq =L dc +L arm

[0067] Among them, L dceq is the equivalent inductance of the high-voltage side DC circuit of the isolated high-voltage DC transformer, L dc L is the sum of the positive and negative smoothing reactances on the high-voltage side of the isolated high-voltage DC transformer. arm It is the high-voltage side bridge arm reactance of the isolated high-voltage DC transformer.

[0068] In one embodiment, the equivalent inductance of the DC circuit on the high-voltage side is determined based on the sum of the positive and negative smoothing reactances on the high-voltage side and the high-voltage side bridge arm reactance of the isolated high-voltage DC transformer, including: when the high-voltage DC transformer has a three-phase structure, the equivalent inductance of the DC circuit on the high-voltage side is determined according to the following formula:

[0069] L dceq =L dc +2 / 3L arm

[0070] Among them, L dceq is the equivalent inductance of the high-voltage side DC circuit of the isolated high-voltage DC transformer, L dcL is the sum of the positive and negative smoothing reactances on the high-voltage side of the isolated high-voltage DC transformer. arm It is the high-voltage side bridge arm reactance of the isolated high-voltage DC transformer.

[0071] As an embodiment, the DC power supply control mode in step S11 includes a DC voltage drop control phase and a DC voltage recovery control phase. The DC voltage drop control phase includes the following two phases: Phase 1, determining the high-voltage side DC voltage command value and the AC side voltage command value; Phase 2, reducing the AC side voltage command value and the high-voltage side DC voltage command value to a preset voltage value. The preset voltage value is within a preset range.

[0072] The DC voltage recovery control stage is stage 3, that is, the AC side voltage command value and the high-voltage side DC voltage command value are adjusted to the steady-state voltage values ​​before the fault.

[0073] It is understandable that controlling the isolated high-voltage DC transformer to enter the DC power supply control mode includes: stage 1, stage 2 and stage 3.

[0074] Exemplarily, the DC voltage control of the isolated high-voltage DC transformer is divided into three stages in chronological order, including DC voltage drop control stage 1, DC voltage drop control stage 2 and DC voltage recovery control stage 3, and the DC voltage drop control stage 1 is immediately entered after the DC fault is detected, and the DC voltage drop control stage 2 is entered after a short time delay (the short time delay can be selected within the range of 0.1ms to 1ms), and the DC voltage recovery control stage 3 is entered after the DC fault current is completely cleared.

[0075] When the DC voltage recovery control stage 3 is executed, the steps S12 and S13 have been completed, because the DC circuit breakers and DC energy consumption devices involved in the steps S12 and S13 do not need to be recovered synchronously.

[0076] In one embodiment, the specific process of reducing both the AC side voltage command value and the high-voltage side DC voltage command value to preset voltage values ​​includes: multiplying the AC side voltage command value and the high-voltage side DC voltage command value by corresponding first proportional coefficients respectively, so as to reduce both the AC side voltage command value and the high-voltage side DC voltage command value to preset voltage values.

[0077] The above-mentioned high-voltage side DC voltage command value U dc_ref (Per unit value) changes as follows.

[0078] The instantaneous value of the high-voltage side MMC AC voltage (single-phase structure) or the a-phase AC voltage (three-phase structure) is in the increasing stage and the time of the most recent zero crossing is recorded as t0; for time t, first find n (n is an integer) that satisfies -π / 6≤ω0(t-t0)-nπ / 3≤π / 6, where ω0 is the rated operating frequency of the AC side of the high-voltage DC transformer; the expression of the high-voltage side DC voltage command value can be written as Among them U ac0 Indicates the effective value of the high-voltage side MMC AC voltage (single-phase structure) or the effective value of the AC line voltage (three-phase structure), U dc0 Indicates the rated DC voltage on the high-voltage side.

[0079] By implementing the steps, the required AC voltage command value is determined. The voltage command value is equal to the maximum instantaneous value of the three-phase AC voltage. Mathematically, the zero-crossing moment is the moment when the voltage command value is equal to the maximum instantaneous value of the three-phase AC voltage.

[0080] In the above implementation process, it is necessary to set the AC side voltage command value U of the isolated high-voltage DC transformer to ac_ref (per unit value) and the high-voltage side DC voltage command value U dc_ref (per-unit value) decreases at the same time, both are equal in size and their values ​​are within the range of 0.1pu to 0.3pu.

[0081] AC side voltage command value U ac_ref (per unit value) and the high-voltage side DC voltage command value U dc_ref (Per-unit value) Before they drop at the same time, the two are the same, and after they drop, they are also the same.

[0082] Furthermore, the AC side voltage command value and the high-voltage side DC voltage command value are adjusted to the steady-state voltage value before the fault, including: multiplying the AC side voltage command value and the high-voltage side DC voltage command value by the corresponding second proportional coefficient respectively to adjust the AC side voltage command value and the high-voltage side DC voltage command value to the steady-state voltage value before the fault.

[0083] The AC side voltage command value U of the above-mentioned isolated high-voltage DC transformer ac_ref (per unit value) and the high-voltage side DC voltage command value U dc_ref (per unit value) are restored to the steady-state value before the fault, preparing for the rapid recovery after the subsequent DC circuit breaker reclosing.

[0084] The above implementation process is directly modified in the MMC modulation link and trigger link (ie, the half-bridge submodule modulation link and trigger link), that is, the AC side voltage command value U ac_ref (per unit value) and the high-voltage side DC voltage command value U dc_ref(Per-unit values) are directly modified in the MMC modulation and triggering links by multiplying the original AC voltage and DC voltage signals by the corresponding proportional coefficients.

[0085] As a specific implementation method, Figure 6 The following describes the high-side DC fault control process for an isolated HVDC transformer, using the parameter settings of various medium- and high-voltage DC transformers as an example. These parameters include the DC voltages on the high- and low-voltage sides, the number of cascaded submodules (SMs), the rated AC frequency, the commutation transformer leakage reactance, the commutation transformer ratio, the submodule capacitance of the cascaded submodules, and the arm reactance and smoothing reactance.

[0086] First, a DC pole-to-pole fault on the high-voltage side occurs within 2 seconds. Thereafter, when the rate of increase in the absolute value of the DC current on the high-voltage side exceeds 2A / us (20us after the fault occurs), a DC fault is considered to have occurred, and the isolated high-voltage DC transformer immediately enters the DC voltage control mode. Six milliseconds after the fault occurs, the current transfer branch and the main circuit breaker branch of the high-voltage DC circuit breaker are disconnected, and the lightning arrester branch is put into the fault current path. Three milliseconds after the fault occurs, the DC energy consumption device on the low-voltage side is put into operation.

[0087] Secondly, after the isolated HVDC transformer enters the DC voltage control mode, it immediately goes through DC voltage drop control stage 1, DC voltage drop control stage 2, and DC voltage recovery control stage. It enters DC voltage drop control stage 2 0.2ms after entering DC voltage drop control stage 1. 7.15ms after the fault occurs, the DC fault current is completely cleared, and the isolated HVDC transformer enters the DC voltage recovery control stage.

[0088] Third, in the DC voltage drop control phase 1, the high-voltage side DC voltage command value U dc_ref (Per unit value) as follows Figure 7 The pulsation wave mode changes shown in FIG. 1 , without considering the subsequent DC voltage drop control stage 2 and DC voltage recovery control stage, U dc_ref (Per unit value) fluctuation range is 1.0~0.866.

[0089] Fourth, in the DC voltage drop control stage 2, the AC side voltage command value U of the isolated high-voltage DC transformer ac_ref (per unit value) and the high-voltage side DC voltage command value U dc_ref (per unit value) at the same time dropped to 0.1 pu.

[0090] Fifth, starting from 2.00715s, the isolated high-voltage DC transformer enters the DC voltage recovery control stage, and the AC side voltage command value U ac_ref (per unit value) and the high-voltage side DC voltage command value Udc_ref (Per-unit value) are restored to the steady-state value before the fault, 1.0pu.

[0091] Figure 8 and Figure 9 The simulation waveforms of the DC voltage on the high-voltage side and low-voltage side of the high-voltage DC transformer are given respectively. Figure 10 and Figure 11 The simulation waveforms of the DC current on the high-voltage side and low-voltage side of the high-voltage DC transformer are given. As can be seen from the figure, the simulation results prove the effectiveness of the present invention, which can effectively handle the DC fault on the high-voltage side of the high-voltage DC transformer. In addition, if the proposed fault control strategy is not adopted, according to U dc0 / L dceq Keeping the same principle, it can be estimated that the high-voltage side fault DC current will exceed 30kA. However, after adopting the present invention, the high-voltage side fault DC current is less than 0.8kA, which can greatly reduce the requirement for the breaking capacity of the high-voltage DC circuit breaker.

[0092] In order to achieve the above functions, the fault control device includes hardware structures and / or software modules corresponding to the execution of each function. It should be easily appreciated by those skilled in the art that, in combination with the algorithm steps of each example described in the embodiments disclosed herein, the present application can be implemented in the form of hardware or a combination of hardware and computer software. Whether a function is executed in the form of hardware or computer software driving hardware depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0093] The present disclosure also provides a Figure 12 A fault control device is shown, which includes a first control unit 121, a second control unit 122 and a third control unit 123.

[0094] The first control unit 121 is used to control the isolated high-voltage DC transformer to enter a DC power supply control mode when a DC fault is detected on the high-voltage side of the isolated high-voltage DC transformer; the second control unit 122 is used to control the high-voltage DC circuit breaker to continue operating for a first preset time, then control the current transfer branch and the main circuit breaker branch to be disconnected, and control the lightning protection branch to be turned on; the third control unit 123 is used to control the low-voltage side DC energy consumption device to be turned on when the isolated high-voltage DC transformer enters the DC power supply control mode for a second preset time.

[0095] In one implementation, when a DC fault is detected on the high-voltage side of the isolated high-voltage DC transformer, before controlling the isolated high-voltage DC transformer to enter a DC power supply control mode, the device is further used to: determine the DC current rise rate on the high-voltage side; when the absolute value of the DC current rise rate is greater than a preset threshold, determine that a DC fault has occurred; when the absolute value of the DC current rise rate is less than or equal to the preset threshold, determine that no DC fault has occurred.

[0096] In another implementation, before determining the DC current rise rate on the high-voltage side, the device is further configured to: determine a preset threshold value based on the rated DC voltage on the high-voltage side and the equivalent inductance of the DC circuit on the high-voltage side.

[0097] In another implementation, the device is also used to: determine the equivalent inductance of the DC circuit on the high-voltage side based on the sum of the positive and negative smoothing reactances on the high-voltage side and the reactance of the high-voltage side bridge arm of the isolated high-voltage DC transformer; and determine the preset threshold value based on the rated DC voltage on the high-voltage side of the isolated high-voltage DC transformer and the equivalent inductance of the DC circuit on the high-voltage side.

[0098] In another implementation, when the high-voltage DC transformer is a single-phase structure, the equivalent inductance of the DC circuit on the high-voltage side is determined according to the following formula:

[0099] L dceq =L dc +L arm

[0100] Among them, L dceq is the equivalent inductance of the high-voltage side DC circuit of the isolated high-voltage DC transformer, L dc L is the sum of the positive and negative smoothing reactances on the high-voltage side of the isolated high-voltage DC transformer. arm It is the high-voltage side bridge arm reactance of the isolated high-voltage DC transformer.

[0101] In another implementation, when the high-voltage DC transformer is a three-phase structure, the equivalent inductance of the DC circuit on the high-voltage side is determined according to the following formula:

[0102] L dceq =L dc +2 / 3L arm

[0103] Among them, L dceq is the equivalent inductance of the high-voltage side DC circuit of the isolated high-voltage DC transformer, L dc L is the sum of the positive and negative smoothing reactances on the high-voltage side of the isolated high-voltage DC transformer. arm It is the high-voltage side bridge arm reactance of the isolated high-voltage DC transformer.

[0104] In another implementation, the DC power supply control mode includes a DC voltage drop control stage and a DC voltage recovery control stage; the first control unit 121 is specifically used to: control the isolated high-voltage DC transformer to enter the DC power supply control mode, including: in the DC voltage drop control stage, determining the DC voltage command value on the high-voltage side and the AC side voltage command value; and reducing the AC side voltage command value and the high-voltage side DC voltage command value to a preset voltage value; the preset voltage value is within a preset range; in the DC voltage recovery control stage, adjusting the AC side voltage command value and the high-voltage side DC voltage command value to the steady-state voltage value before the fault.

[0105] In another implementation, the first control unit 121 is specifically configured to: multiply the AC side voltage command value and the high-voltage side DC voltage command value by corresponding first proportional coefficients, respectively, to reduce both the AC side voltage command value and the high-voltage side DC voltage command value to preset voltage values;

[0106] The first control unit 121 is specifically configured to multiply the AC side voltage command value and the high-voltage side DC voltage command value by corresponding second proportional coefficients, respectively, to adjust the AC side voltage command value and the high-voltage side DC voltage command value to steady-state voltage values ​​before the fault.

[0107] Regarding the device in the above embodiment, the specific manner in which each unit module performs operations has been described in detail in the embodiment of the method, and will not be elaborated here.

[0108] The present application also provides a computer-readable storage medium. When the instructions in the computer-readable storage medium are executed by a processor of a fault control device or electronic device, the fault control device or electronic device can perform the fault control method of any of the possible implementations described above. The method can achieve the same technical effects, and to avoid repetition, it will not be described here.

[0109] The present application also provides a computer program product including a computer program or instructions, wherein the computer program or instructions are executed by a processor to implement the fault control method according to any of the above possible implementations. The computer program or instructions can achieve the same technical effects, and to avoid repetition, they are not described here.

[0110] Those skilled in the art will readily appreciate other embodiments of the present application after considering the specification and practicing the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of the present application that follow the general principles of this application and include common knowledge or customary techniques in the art not disclosed herein. The description and examples are to be considered as exemplary only, and the true scope and spirit of the present application are indicated by the following claims.

[0111] It should be understood that the present application is not limited to the exact structures described above and shown in the drawings, and that various modifications and changes may be made without departing from the scope thereof. The scope of the present application is limited only by the appended claims.

Claims

1. A fault control method, characterized in that: Applied to an isolated high-voltage DC transformer, the method includes: When a DC fault is detected on the high-voltage side of the isolated high-voltage DC transformer, controlling the isolated high-voltage DC transformer to enter a DC power supply control mode; and, controlling the high-voltage direct current circuit breaker to continue operating for a first preset time, then controlling the current transfer branch and the main circuit breaker branch to be disconnected, and controlling the lightning protection branch to be turned on; and, when the isolated high-voltage DC transformer enters the DC power supply control mode for a second preset time, controlling the low-voltage side DC energy consumption device to be turned on; Before controlling the isolated high-voltage DC transformer to enter a DC power supply control mode when a DC fault is detected on the high-voltage side of the isolated high-voltage DC transformer, the method further includes: Determining the DC current rising rate on the high voltage side; When the absolute value of the DC current rising rate is greater than a preset threshold, determining that a DC fault occurs; When the absolute value of the DC current rising rate is less than or equal to the preset threshold, determining that no DC fault has occurred; The DC power supply control mode includes a DC voltage drop control stage and a DC voltage recovery control stage; and controlling the isolated high-voltage DC transformer to enter the DC power supply control mode includes: In the DC voltage reduction control stage, the DC voltage command value of the high-voltage side and the AC side voltage command value are determined; and the AC side voltage command value and the high-voltage side DC voltage command value are reduced to preset voltage values; the preset voltage values ​​are within a preset range; In the DC voltage recovery control stage, the AC side voltage command value and the high-voltage side DC voltage command value are adjusted to pre-fault steady-state voltage values.

2. The fault control method according to claim 1, characterized in that: Before determining the DC current rise rate on the high-voltage side, the method further includes: The preset threshold is determined according to the rated DC voltage of the high-voltage side and the equivalent inductance of the DC circuit on the high-voltage side.

3. The fault control method according to claim 2, characterized in that: The determining the preset threshold value according to the rated DC voltage of the high-voltage side and the DC loop equivalent inductance includes: Determine the equivalent inductance of the DC circuit on the high-voltage side according to the sum of the positive and negative smoothing reactances on the high-voltage side and the high-voltage side bridge arm reactance of the isolated high-voltage DC transformer; The preset threshold is determined according to the rated DC voltage on the high-voltage side of the isolated high-voltage DC transformer and the equivalent inductance of the DC loop on the high-voltage side.

4. The fault control method according to claim 3, characterized in that: The determining of the equivalent inductance of the DC circuit on the high-voltage side according to the sum of the positive and negative smoothing reactances on the high-voltage side and the high-voltage side bridge arm reactance of the isolated high-voltage DC transformer includes: When the high-voltage DC transformer is a single-phase structure, the equivalent inductance of the DC circuit on the high-voltage side is determined according to the following formula: L dceq =L dc +L arm Among them, L dceq is the equivalent inductance of the high-voltage side DC circuit of the isolated high-voltage DC transformer, L dc L is the sum of the positive and negative smoothing reactances on the high-voltage side of the isolated high-voltage DC transformer. arm is the high-voltage side bridge arm reactance of the isolated high-voltage DC transformer.

5. The fault control method according to claim 3, characterized in that: The determining of the equivalent inductance of the DC circuit on the high-voltage side according to the sum of the positive and negative smoothing reactances on the high-voltage side and the high-voltage side bridge arm reactance of the isolated high-voltage DC transformer includes: When the high-voltage DC transformer is a three-phase structure, the equivalent inductance of the DC circuit on the high-voltage side is determined according to the following formula: <h2 style=";text-align:left;direction:ltr">L<h2 style=";text-align:left;direction:ltr"> dceq <h2 style=";text-align:left;direction:ltr"> =L<h2 style=";text-align:left;direction:ltr"> dc <h2 style=";text-align:left;direction:ltr"> +2 / 3L<h2 style=";text-align:left;direction:ltr"> arm Among them, L dceq is the equivalent inductance of the high-voltage side DC circuit of the isolated high-voltage DC transformer, L dc L is the sum of the positive and negative smoothing reactances on the high-voltage side of the isolated high-voltage DC transformer. arm is the high-voltage side bridge arm reactance of the isolated high-voltage DC transformer.

6. The fault control method according to claim 1, characterized in that: The step of reducing both the AC side voltage command value and the high-voltage side DC voltage command value to preset voltage values ​​includes: multiplying the AC side voltage command value and the high-voltage side DC voltage command value by corresponding first proportional coefficients respectively, so as to reduce both the AC side voltage command value and the high-voltage side DC voltage command value to the preset voltage value; The step of adjusting the AC side voltage command value and the high-voltage side DC voltage command value to steady-state voltage values ​​before a fault comprises: The AC side voltage command value and the high-voltage side DC voltage command value are respectively multiplied by corresponding second proportional coefficients to adjust the AC side voltage command value and the high-voltage side DC voltage command value to the pre-fault steady-state voltage value.

7. A fault control device, characterized in that: Applicable to an isolated high-voltage DC transformer, the device comprises: a first control unit, configured to control the isolated high-voltage DC transformer to enter a DC power supply control mode when a DC fault is detected on the high-voltage side of the isolated high-voltage DC transformer; a second control unit, configured to control the high-voltage DC circuit breaker to continue operating for a first preset time, then control the current transfer branch and the main circuit breaker branch to be disconnected, and control the lightning protection branch to be turned on; a third control unit, configured to control the low-voltage side DC energy consumption device to be turned on when the isolated high-voltage DC transformer enters the DC power supply control mode for a second preset time; Before controlling the isolated high-voltage DC transformer to enter a DC power supply control mode when a DC fault is detected on the high-voltage side of the isolated high-voltage DC transformer, the device is further configured to: determine a DC current rise rate on the high-voltage side; determine that a DC fault has occurred when the absolute value of the DC current rise rate is greater than a preset threshold; and determine that no DC fault has occurred when the absolute value of the DC current rise rate is less than or equal to the preset threshold; The DC power supply control mode includes a DC voltage drop control stage and a DC voltage recovery control stage; the first control unit is specifically used to: determine the DC voltage command value and the AC side voltage command value on the high-voltage side in the DC voltage drop control stage; and reduce the AC side voltage command value and the high-voltage side DC voltage command value to a preset voltage value; the preset voltage value is within a preset range; and adjust the AC side voltage command value and the high-voltage side DC voltage command value to a steady-state voltage value before the fault in the DC voltage recovery control stage.

8. An isolated high-voltage DC transformer, characterized in that: The isolated high-voltage DC transformer is configured to execute the fault control method according to any one of claims 1 to 6.

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