A flexible low frequency power transmission system and low frequency fault ride-through method

Abstract

The application discloses a flexible low-frequency power transmission system and a low-frequency fault ride-through method, and belongs to the technical field of low-frequency power transmission systems. The low-frequency fault ride-through method comprises the following steps: detecting a first voltage between a second transformer and a low-frequency system, determining whether a low-frequency fault exists in the low-frequency system through the first voltage, and when the low-frequency fault exists, a voltage frequency controller and a power controller control the current and the voltage to a fault limiting state, so as to avoid overvoltage or overcurrent conditions and guarantee the safety of low-frequency equipment. Meanwhile, the voltage frequency controller and the power controller provide sufficient fault current for a low-frequency alternating current relay protection device, so that the low-frequency alternating current relay protection device acts and fault isolation is realized.

Description

Technical Field

[0001] This application relates to the field of low-frequency power transmission system technology, specifically to a flexible low-frequency power transmission system and a low-frequency fault ride-through method. Background Technology

[0002] Flexible low-frequency (LHF) transmission is a novel AC power transmission technology with frequencies lower than the power frequency. For LHF transmission systems used in dual- or multi-terminal network applications, overcurrent and overvoltage phenomena are highly likely to occur at each LHF converter station during low-frequency system faults, endangering their safety. Furthermore, since the LHF system is established by power electronic equipment, and both the low-frequency voltage and current are controlled by this equipment, without proper control during a LHF system fault, the system cannot provide sufficient fault current to the low-frequency AC relay protection devices, leading to device malfunction and failure to isolate the fault. Summary of the Invention

[0003] Purpose of the invention: The purpose of this application is to provide a flexible low-frequency power transmission system and a low-frequency fault ride-through method, which aims to solve the technical problem that when a low-frequency system fault occurs, each low-frequency current station is prone to overcurrent and overvoltage phenomena, endangering the safety of the current station.

[0004] Technical solution: In a first aspect, embodiments of this application provide a flexible low-frequency power transmission system, comprising:

[0005] Multiple converter stations, each including a converter for converting power frequency energy to low frequency energy, the multiple converter stations including a first converter station and at least one second converter station, wherein the converter in the first converter station is a first converter, the first converter including a voltage-frequency controller for controlling the voltage of the flexible low-frequency transmission system; the converter in the second converter station is a second converter, the second converter including a power controller for controlling the power of the second converter; and,

[0006] The low-frequency system includes a first converter station connected to the low-frequency system, used to control the low-frequency voltage to a fault-limited state when the low-frequency system fails; and a second converter station connected to the low-frequency system, used to control the low-frequency power to a fault-limited state when the low-frequency system fails.

[0007] In some embodiments, the converter station further includes a first switch, a second switch, a first transformer, and a second transformer; the first switch, the first transformer, the converter, the second transformer, and the second switch are connected in series in sequence; the second switch is connected to the low-frequency system, and the first switch is used to connect to the external power frequency grid.

[0008] In some embodiments, the output of the voltage frequency controller is connected to a first limiter;

[0009] The output of the power controller is connected to a second limiter.

[0010] Secondly, embodiments of this application provide a low-frequency fault ride-through method for a flexible low-frequency transmission system according to any one of the first aspects, comprising the following steps:

[0011] Obtain a first voltage, and determine whether the low-frequency system has a fault based on the first voltage;

[0012] In response to the low-frequency system fault, the voltage-frequency controller adjusts the voltage of the low-frequency system to a fault-limited state and controls the low-frequency current output by the first converter to a fault-limited state, providing fault current for the low-frequency AC relay protection device; the power controller adjusts the power of the second converter to a fault-limited state and controls the low-frequency current output by the second converter to a fault-limited state, providing fault current for the low-frequency AC relay protection device.

[0013] In response to the elimination of the low-frequency system fault, the voltage-frequency controller adjusts the voltage of the low-frequency system to the pre-fault state;

[0014] In response to the elimination of the low-frequency system fault, the power controller adjusts the power of the second converter to the pre-fault state.

[0015] In some embodiments, the first voltage includes the voltage between the second transformer and the low-frequency system.

[0016] In some embodiments, determining whether a fault exists in the low-frequency system based on the first voltage includes:

[0017] Obtain the effective value of the positive sequence component and the effective value of the negative sequence component of the low-frequency voltage of the first voltage;

[0018] In the operating state of the voltage frequency controller, the fault setting values ​​for the positive-sequence component and negative-sequence component of the low-frequency voltage are acquired. Combined with the effective values ​​of the positive-sequence and negative-sequence components of the first voltage, if the first criterion is met, it is determined that a fault exists in the low-frequency system.

[0019] First criterion:

[0020] Ulf_pos_rms≤Ulf_pos_vfminset; or Ulf_neg_rms≥Ulf_neg_vfmaxset;

[0021] Wherein, Ulf_pos_rms is the effective value of the positive sequence component of the low-frequency voltage; Ulf_pos_vfminset is the fault setting value of the positive sequence component of the low-frequency voltage under the working state of the voltage frequency controller; Ulf_neg_rms is the effective value of the negative sequence component of the low-frequency voltage; and Ulf_neg_vfmaxset is the fault setting value of the negative sequence component of the low-frequency voltage under the working state of the voltage frequency controller.

[0022] Alternatively, in the operating state of the power controller, the fault setting values ​​for the positive-sequence component and negative-sequence component of the low-frequency voltage are acquired. Combined with the effective values ​​of the positive-sequence and negative-sequence components of the first voltage, if the second criterion is met, it is determined that a fault exists in the low-frequency system.

[0023] Second criterion:

[0024] Ulf_pos_rms≤Ulf_pos_pqminset; or Ulf_neg_rms≥Ulf_neg_pqmaxset;

[0025] Wherein, Ulf_pos_pqminset is the low-frequency voltage positive sequence component fault setting value under the operating state of the power controller; Ulf_neg_pqmaxset is the low-frequency voltage negative sequence component fault setting value under the operating state of the power controller.

[0026] In some embodiments, controlling the low-frequency current output by the first converter to a fault-limited state includes:

[0027] Obtain the low-frequency voltage command value and the actual low-frequency voltage value, input them to the voltage frequency controller, obtain the first output result, and input the first output result to the first limiter;

[0028] The upper limit data and lower limit data of low-frequency current amplitude are obtained and input into the first limiter. The first low-frequency current amplitude adjustment data are obtained by combining the first output result.

[0029] Based on the first low-frequency current amplitude adjustment data, adjust the low-frequency current output by the first converter to the fault-limited state.

[0030] In some embodiments, controlling the low-frequency current output by the second converter to a fault-limited state includes:

[0031] The low-frequency active power command value, low-frequency reactive power command value, low-frequency active power actual value and low-frequency reactive power actual value are obtained, input into the power controller, a second output result is obtained, and the second output result is input into the second limiter.

[0032] Obtain the upper limit data and lower limit data of low-frequency current amplitude, input them into the second limiter, and combine them with the second output result to obtain the second low-frequency current amplitude adjustment data;

[0033] The low-frequency current output of the second converter is adjusted to a fault-limited state based on the second low-frequency current amplitude adjustment data.

[0034] In some embodiments, determining that the low-frequency system fault has been cleared includes:

[0035] Under the operating state of the voltage frequency controller of the first converter in the first converter station, the fault recovery judgment value of the positive sequence component of low-frequency voltage, the fault recovery judgment value of the negative sequence component of low-frequency voltage, the effective value of the positive sequence component of low-frequency voltage, and the effective value of the negative sequence component of low-frequency voltage are acquired. If the third criterion is met, it is determined that the low-frequency system fault has been eliminated.

[0036] Third criterion:

[0037] Ulf_pos_rms≥Ulf_pos_vfminreset; or Ulf_neg_rms≤Ulf_neg_vfmaxreset;

[0038] Wherein, Ulf_pos_rms is the effective value of the positive sequence component of the low-frequency voltage; Ulf_neg_rms is the effective value of the negative sequence component of the low-frequency voltage; Ulf_pos_vfminreset is the fault recovery judgment value of the positive sequence component of the low-frequency voltage under the working state of the voltage frequency controller; and Ulf_neg_vfmaxreset is the fault recovery judgment value of the negative sequence component of the low-frequency voltage under the working state of the voltage frequency controller.

[0039] In the operating state of the power controller of the second converter in the second converter station, the fault recovery judgment value of the positive sequence component of low-frequency voltage, the fault recovery judgment value of the negative sequence component of low-frequency voltage, the effective value of the positive sequence component of low-frequency voltage, and the effective value of the negative sequence component of low-frequency voltage are acquired. If the fourth criterion is satisfied and the closing condition is also satisfied, it is determined that the low-frequency system fault has been eliminated.

[0040] Fourth criterion:

[0041] Ulf_pos_rms≥Ulf_pos_pqminreset or Ulf_neg_rms≤Ulf_neg_pqmaxreset;

[0042] Wherein, Ulf_pos_pqminreset is the fault recovery judgment value of the positive sequence component of low-frequency voltage under the working state of the power controller; Ulf_neg_pqmaxreset is the fault recovery judgment value of the negative sequence component of low-frequency voltage under the working state of the power controller.

[0043] In some embodiments, the closing conditions include:

[0044] The second switch of the first converter station is closed in three phases; and

[0045] The second switch of the second converter station is closed in three phases.

[0046] Beneficial Effects: Compared with the prior art, the embodiments of this application provide a flexible low-frequency power transmission system, including multiple converter stations and a low-frequency system. The converter stations are used to convert power frequency energy to low-frequency energy and connect to the low-frequency system. The multiple converter stations include a first converter station and at least one second converter station. The converter in the first converter station is a first converter, and the converter in the second converter station is a second converter. The first converter includes a voltage-frequency controller for controlling the voltage of the low-frequency system, and the second converter includes a power controller for controlling the power of the second converter. In this application, a... A first converter with a voltage and frequency controller and at least one second converter with a power controller are provided. When a fault occurs in the low-frequency system, the voltage and frequency controller controls the voltage of the flexible low-frequency transmission system to remain in the fault-limited state, and the power controller controls the power of the second converter to remain in the fault-limited state to avoid overcurrent or overvoltage and ensure equipment safety. At the same time, the voltage and frequency controller and the power controller control the current to remain in the fault-limited state, providing sufficient fault current for the low-frequency AC relay protection device, so that the low-frequency AC relay protection device can be activated and fault isolation can be achieved.

[0047] The embodiments of this application provide a low-frequency fault ride-through method. A first voltage between a second transformer and a low-frequency system is detected. The presence of a low-frequency fault in the low-frequency system is determined by the first voltage. During a low-frequency fault, a voltage-frequency controller keeps the flexible low-frequency transmission system in a fault-limited state, and a power controller keeps the power of the second converter in a fault-limited state to prevent overcurrent or overvoltage and ensure equipment safety. Simultaneously, the voltage-frequency controller and power controller maintain the current in a fault-limited state, providing sufficient fault current for the low-frequency AC relay protection device, enabling the device to operate and achieve fault isolation. Attached Figure Description

[0048] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying 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.

[0049] Figure 1A system diagram of a flexible low-frequency power transmission system provided in an embodiment of this application;

[0050] Figure 2 This is a schematic diagram showing the connection between a voltage frequency controller and a first limiter in a flexible low-frequency power transmission system provided in an embodiment of this application.

[0051] Figure 3 A schematic diagram of the connection between a power controller and a second limiter in a flexible low-frequency power transmission system provided in this application embodiment;

[0052] Figure 4 This is a flowchart illustrating the fault ride-through control of the voltage frequency controller in a low-frequency fault ride-through method provided in this application embodiment.

[0053] Figure 5 This application provides a flowchart of the fault ride-through control of the power controller in a low-frequency fault ride-through method.

[0054] Figure 6 This is a flowchart illustrating the fault detection process of a voltage frequency controller in a low-frequency fault ride-through method provided in this application embodiment;

[0055] Figure 7 This application provides a flowchart of a fault detection process for a power controller in a low-frequency fault ride-through method.

[0056] Figure 8 This is a schematic diagram of the signal transmission between the voltage frequency controller and the first limiter in a low-frequency fault ride-through method provided in an embodiment of this application;

[0057] Figure 9 This is a schematic diagram of the signal transmission of the power controller and the second limiter in a low-frequency fault ride-through method provided in an embodiment of this application;

[0058] Figure 10 A flowchart illustrating the steps of a low-frequency fault ride-through method provided in this application embodiment;

[0059] Reference numerals: 200, converter station; 210, first switch; 220, first transformer; 230, converter; 240, second transformer; 250, second switch; 300, low-frequency system; 411, voltage-frequency controller; 412, first limiter; 421, power controller; 422, second limiter. Detailed Implementation

[0060] 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 skilled in the art without creative effort are within the scope of protection of this application.

[0061] The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, unless otherwise stated, "a plurality of" means two or more.

[0062] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "connected" and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection, a direct connection, or an indirect connection through an intermediate medium; they can refer to the internal communication between two components. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0063] like Figures 1 to 4 As shown, this application provides a flexible low-frequency power transmission system, including multiple converter stations 200 and a low-frequency system 300. The converter stations 200 are used to convert power frequency energy to low-frequency energy and input it into the low-frequency system 300. The multiple converter stations 200 include a first converter station and at least one second converter station. The converter 230 in the first converter station is a first converter, and the converter 230 in the second converter station is a second converter. The first converter includes a voltage-frequency controller 411 for controlling the voltage of the low-frequency system 300, and the second converter includes a power controller 421 for controlling the power of the second converter. The first and second converter stations are connected to the low-frequency system 300, enabling the conversion of power frequency energy to low-frequency energy. The system converts and transmits low-frequency power. In this application, a first converter with a voltage-frequency controller 411 and at least a second converter with a power controller 421 are included. When a fault occurs in the low-frequency system 300, the voltage-frequency controller 411 controls the voltage of the flexible low-frequency transmission system to remain in a fault-limited state, and the power controller 421 controls the power of the second converter to remain in a fault-limited state, preventing overcurrent or overvoltage to ensure equipment safety. Simultaneously, the voltage-frequency controller 411 and the power controller 421 control the current to remain in a fault-limited state, providing sufficient fault current for the low-frequency AC relay protection device, enabling the low-frequency AC relay protection device to operate and achieve fault isolation.

[0064] like Figure 1 As shown, in some embodiments, the flexible low-frequency power transmission system includes multiple converter stations 200 and a low-frequency system 300. Each converter station 200 is connected to an external power frequency grid and is also connected to the low-frequency system 300.

[0065] like Figures 1 to 3 As shown, in some embodiments, the plurality of converter stations 200 include a first converter station and at least one second converter station, such as... Figure 1 As shown, the embodiments of this application include three converter stations 200, which have one first converter station and two second converter stations;

[0066] like Figure 2 and Figure 3 As shown, converter 230 in the first converter station is the first converter. The first converter includes a voltage-frequency controller 411, which controls the voltage of the flexible low-frequency transmission system. When a fault occurs in the low-frequency system 300, the controller controls the magnitude of the voltage and current of the low-frequency system 300 to keep them within the fault-limited state. Simultaneously, the magnitude of the current in the fault-limited state meets the fault current requirements of the low-frequency AC relay protection device, thereby providing sufficient fault current to ensure the operation of the low-frequency AC relay protection device when the low-frequency system 300 fails. To achieve fault isolation, the converter 230 in the second converter station is the second converter. The second converter includes a power controller 421, which controls the power of the second converter. When a fault occurs in the low-frequency system 300, the power and current of the second converter are controlled to keep them within the fault limit state. At the same time, the current in the fault limit state meets the fault current requirements of the low-frequency AC relay protection device, so as to provide sufficient fault current when the low-frequency system 300 fails, ensure the operation of the low-frequency AC relay protection device, and achieve the purpose of fault isolation.

[0067] like Figures 1 to 3 As shown, in some embodiments, a first converter is connected to the low-frequency system 300 and is used to control the low-frequency voltage in a fault-limited state when the low-frequency system 300 fails; a second converter is connected to the low-frequency system 300 and is used to control the low-frequency power in a fault-limited state when the low-frequency system 300 fails.

[0068] like Figure 1As shown, in some embodiments, the flexible power transmission system further includes a first switch 210, a second switch 250, a first transformer 220, and a second transformer 240; the first switch 210, the first transformer 220, the converter 230, the second transformer 240, and the second switch 250 are connected in series; the second switch 250 is connected to the low-frequency system 300, and the first switch 210 is used to connect to the external power frequency grid. In some embodiments, the first switch 210 is a power frequency switch, the first transformer 220 is a power frequency transformer, the second switch 250 is a low-frequency switch, and the second transformer 240 is a low-frequency transformer. Between the converter 230 and the power frequency grid, the power frequency switch and the power frequency transformer are connected in series from the power frequency grid to the converter 230 to control the on / off of the power frequency current and to transform the power frequency current; between the converter 230 and the low-frequency system 300, the low-frequency transformer and the low-frequency switch are connected in series from the converter 230 to the low-frequency system 300 to control the on / off of the low-frequency current and to transform the low-frequency current.

[0069] like Figure 2 and Figure 3 As shown, in some embodiments, the output terminal of the voltage frequency controller 411 is connected to a first limiter 412. By combining the low-frequency voltage command value, the actual low-frequency voltage value, the upper limit data of the low-frequency current amplitude, and the lower limit data of the low-frequency current amplitude, the first low-frequency current amplitude adjustment data is obtained. This allows the voltage of the low-frequency system 300 to be controlled in a fault-limited state by adjusting the current. At the same time, the adjusted current can ensure the protection action of the low-frequency AC relay protection device, achieving fault isolation. The output terminal of the power controller 421 is connected to a second limiter 422. By combining the active power command value, the low-frequency reactive power command value, the actual low-frequency active power value, the actual low-frequency reactive power value, the upper limit data of the low-frequency current amplitude, and the lower limit data of the low-frequency current amplitude, the second low-frequency current amplitude adjustment data is obtained. This allows the power of the second converter to be controlled in a fault-limited state by adjusting the current. At the same time, the adjusted current can ensure the protection action of the low-frequency AC relay protection device, achieving fault isolation.

[0070] like Figures 4 to 10As shown, embodiments of this application also provide a low-frequency fault ride-through method for a flexible low-frequency transmission system based on this application. The method detects a first voltage between the second transformer 240 and the low-frequency system 300, and determines whether a low-frequency fault exists in the low-frequency system 300 based on the first voltage. In the event of a low-frequency fault, the voltage-frequency controller 411 controls the voltage of the flexible low-frequency transmission system to remain in a fault-limited state, and the power controller 421 controls the power of the second converter to remain in a fault-limited state, preventing overcurrent or overvoltage to ensure equipment safety. Simultaneously, the voltage-frequency controller 411 and the power controller 421 control the current to remain in a fault-limited state, providing sufficient fault current to the low-frequency AC relay protection device, causing the low-frequency AC relay protection device to operate and achieving fault isolation. The method specifically includes:

[0071] Step 1: Obtain the first voltage and determine whether there is a fault in the low-frequency system 300 based on the first voltage:

[0072] Step 1.1: Collect the voltage between the second transformer 240 and the low-frequency system 300 as the first voltage.

[0073] Step 1.2: Extract the effective values ​​of the positive sequence component and the negative sequence component of the low-frequency voltage of the first voltage.

[0074] Step 1.3: As Figures 4 to 7 As shown, based on the effective values ​​of the positive-sequence component and the negative-sequence component of the low-frequency voltage of the first voltage, fault detection is performed under the operating states of the voltage frequency controller 411 and the power controller 421, respectively. If a fault is detected in the detection results under the operating states of the voltage frequency controller 411 or the power controller 421, then the low-frequency system 300 is determined to have a fault.

[0075] Step 1.3.1: As Figure 4 and Figure 6 As shown, fault determination is performed under the operating conditions of the voltage and frequency controller 411:

[0076] Obtain the fault setting values ​​for the positive-sequence component and negative-sequence component of the low-frequency voltage. Combine these with the effective values ​​of the positive-sequence component and negative-sequence component of the first voltage. If the first criterion is met, it is determined that a fault exists in the low-frequency system 300.

[0077] First criterion:

[0078] Ulf_pos_rms≤Ulf_pos_vfminset; or Ulf_neg_rms≥Ulf_neg_vfmaxset;

[0079] Wherein, Ulf_pos_rms is the effective value of the positive sequence component of the low-frequency voltage; Ulf_pos_vfminset is the fault setting value of the positive sequence component of the low-frequency voltage under the working state of the voltage frequency controller 411; Ulf_neg_rms is the effective value of the negative sequence component of the low-frequency voltage; and Ulf_neg_vfmaxset is the fault setting value of the negative sequence component of the low-frequency voltage under the working state of the voltage frequency controller 411.

[0080] Step 1.3.2: As Figure 5 and Figure 7 As shown, fault diagnosis is performed under the operating conditions of the power controller 421:

[0081] Obtain the fault setting values ​​for the positive-sequence component and negative-sequence component of the low-frequency voltage. Combine these with the effective values ​​of the positive-sequence and negative-sequence components of the first voltage. If the second criterion is met, it is determined that a fault exists in the low-frequency system 300.

[0082] Second criterion:

[0083] Ulf_pos_rms≤Ulf_pos_pqminset; or Ulf_neg_rms≥Ulf_neg_pqmaxset;

[0084] Among them, Ulf_pos_pqminset is the low-frequency voltage positive sequence component fault setting value under the working state of power controller 421; Ulf_neg_pqmaxset is the low-frequency voltage negative sequence component fault setting value under the working state of power controller 421.

[0085] Step 2: As Figures 5 to 10 As shown, in response to a fault in the low-frequency system 300, the voltage-frequency controller 411 adjusts the voltage of the low-frequency system 300 to a fault-limited state and controls the low-frequency current output by the first converter to a fault-limited state, providing sufficient fault current for the low-frequency AC relay protection device; the power controller 421 adjusts the power of the second converter to a fault-limited state and controls the low-frequency current output by the second converter to a fault-limited state, providing sufficient fault current for the low-frequency AC relay protection device.

[0086] Step 2.1: As Figure 4 and Figure 8 As shown, the voltage-frequency controller 411 adjusts the voltage of the low-frequency system 300 to a fault-limited state and controls the low-frequency current output by the first converter to a fault-limited state, providing sufficient fault current for the low-frequency AC relay protection device.

[0087] After detecting a fault in the low-frequency system 300, the low-frequency voltage command value Ulf_ref is switched from the rated value to the fault limit state. The low-frequency voltage command value Ulf_ref and the actual low-frequency voltage value Ulf at this time are obtained and input into the voltage frequency controller 411. The first output result is obtained and input into the first limiter 412.

[0088] Switch the upper limit data of low-frequency current amplitude Ilf_vfref_max and the lower limit data of low-frequency current amplitude Ilf_vfref_min from steady-state limit values ​​to fault limit state, obtain the upper limit data of low-frequency current amplitude Ilf_vfref_max and the lower limit data of low-frequency current amplitude Ilf_vfref_min at this time, input them to the first limiter 412, and obtain the first low-frequency current amplitude adjustment data Ilf_vfref by combining the first output result;

[0089] The output current of the first converter is adjusted to the fault limit state by combining the first low-frequency current amplitude adjustment data Ilf_vfref, so as to ensure that the fault current is not too large and at the same time provide sufficient fault current for the low-frequency AC relay protection device to ensure the protection action of the low-frequency AC relay protection device and achieve fault isolation.

[0090] Step 2.2: As Figure 5 and Figure 9 As shown, the power controller 421 adjusts the power of the second converter to a fault-limited state and controls the low-frequency current output by the second converter to a fault-limited state, providing fault current for the low-frequency AC relay protection device.

[0091] After detecting a fault in the low-frequency system 300, the low-frequency active power command value P_ref, low-frequency reactive power command value Q_ref, low-frequency active power actual value P_lf, and low-frequency reactive power actual value Q_lf are obtained and input into the power controller 421 to obtain the second output result. The second output result is then input into the second limiter 422.

[0092] Switch the upper limit data of low-frequency current amplitude Ilf_pqref_max and the lower limit data of low-frequency current amplitude Ilf_pqref_min from steady-state limit values ​​to fault limit state, obtain the upper limit data of low-frequency current amplitude Ilf_pqref_max and the lower limit data of low-frequency current amplitude Ilf_pqref_min at this time, input them into the second limiter 422, and obtain the second low-frequency current amplitude adjustment data Ilf_pqref by combining the second output result;

[0093] Based on the second low-frequency current amplitude adjustment data Ilf_pqref, the low-frequency current is adjusted to the fault limit state. At the same time, the current in the fault limit state can provide sufficient fault current for the low-frequency AC relay protection device, ensuring the protection action of the low-frequency AC relay protection device and realizing fault isolation.

[0094] Step 3: As Figure 6 and Figure 7 As shown, in response to the clearing of a 300 fault in the low-frequency system, the current and voltage are adjusted to the pre-fault state:

[0095] Step 3.1: Check if the fault in the low-frequency system 300 has been eliminated:

[0096] Step 3.1.1: First, while the voltage and frequency controller 411 is operating, determine whether the fault in the low-frequency system 300 has been eliminated:

[0097] Under the operating state of the voltage frequency controller 411, the fault recovery judgment values ​​of the positive sequence component of the low-frequency voltage, the fault recovery judgment value of the negative sequence component of the low-frequency voltage, the effective value of the positive sequence component of the low-frequency voltage, and the effective value of the negative sequence component of the low-frequency voltage are acquired. If the third criterion is met, the fault of the low-frequency system 300 is determined to be eliminated.

[0098] Third criterion:

[0099] Ulf_pos_rms≥Ulf_pos_vfminreset; or Ulf_neg_rms≤Ulf_neg_vfmaxreset;

[0100] Wherein, Ulf_pos_rms is the effective value of the positive sequence component of the low-frequency voltage; Ulf_neg_rms is the effective value of the negative sequence component of the low-frequency voltage; Ulf_pos_vfminreset is the fault recovery judgment value of the positive sequence component of the low-frequency voltage under the working state of the voltage frequency controller 411; and Ulf_neg_vfmaxreset is the fault recovery judgment value of the negative sequence component of the low-frequency voltage under the working state of the voltage frequency controller 411.

[0101] Step 3.1.2: With the power controller 421 in operation, determine whether the fault in the low-frequency system 300 has been eliminated:

[0102] When the power controller 421 is in operation, it acquires the fault recovery judgment value of the positive sequence component of the low-frequency voltage, the fault recovery judgment value of the negative sequence component of the low-frequency voltage, the effective value of the positive sequence component of the low-frequency voltage, and the effective value of the negative sequence component of the low-frequency voltage. If the fourth criterion is met and the closing condition is also met, it is determined that the fault in the low-frequency system 300 has been eliminated.

[0103] Fourth criterion:

[0104] Ulf_pos_rms≥Ulf_pos_pqminreset or Ulf_neg_rms≤Ulf_neg_pqmaxreset;

[0105] Among them, Ulf_pos_pqminreset is the fault recovery judgment value of the positive sequence component of low frequency voltage under the working state of power controller 421; Ulf_neg_pqmaxreset is the fault recovery judgment value of the negative sequence component of low frequency voltage under the working state of power controller 421.

[0106] The closing conditions include: closing the second switch 250 of the first converter station in three phases, and closing the second switch 250 of the second converter station in three phases.

[0107] Step 3.2: Adjust the current and voltage to the pre-fault state:

[0108] After the fault in the low-frequency system 300 is cleared, the voltage-frequency controller 411 restores the low-frequency voltage by restoring the low-frequency voltage command value Ulf_ref, and the restoration rate must meet the requirements for stable system operation; the power controller 421 restores the low-frequency power by restoring the limit values ​​Ilf_pqref_max and Ilf_pqref_min of the low-frequency current command value, and the restoration rate must meet the requirements for stable system operation.

[0109] This application has provided a detailed description of a flexible low-frequency power transmission system and a low-frequency fault ride-through method provided in the embodiments of this application. Specific examples have been used to illustrate the principles and implementation methods of this application. The description of the above embodiments is only for the purpose of helping to understand the method and its core ideas. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A flexible low-frequency power transmission system, characterized in that, include: Multiple converter stations (200), each converter station (200) including a converter (230) for converting power frequency energy to low frequency energy, the multiple converter stations (200) including a first converter station and at least one second converter station, the converter (230) in the first converter station being a first converter, the first converter including a voltage frequency controller (411) for controlling the voltage of the flexible low frequency transmission system; the converter (230) in the second converter station being a second converter, the second converter including a power controller (421) for controlling the power of the second converter; and A low-frequency system (300) is connected to the first converter station. It is used to control the low-frequency voltage in a fault-limited state when the low-frequency system (300) fails, and to provide fault current for the low-frequency AC relay protection device. Specifically, the output terminal of the voltage frequency controller (411) is connected to a first limiter (412). The first low-frequency current amplitude adjustment data is obtained by combining the low-frequency voltage command value, the actual low-frequency voltage value, the upper limit data of the low-frequency current amplitude, and the lower limit data of the low-frequency current amplitude, so as to adjust the voltage of the low-frequency system (300) in a fault-limited state by adjusting the current. The second converter station is connected to the low-frequency system (300) and is used to control the low-frequency power in a fault-limited state when the low-frequency system (300) fails, so as to provide fault current for the low-frequency AC relay protection device; specifically, the output terminal of the power controller (421) is connected to a second limiter (422), which obtains second low-frequency current amplitude adjustment data by combining the active power command value, the low-frequency reactive power command value, the actual value of low-frequency active power, the actual value of low-frequency reactive power, the upper limit data of low-frequency current amplitude, and the lower limit data of low-frequency current amplitude, so as to adjust the power control of the second converter in a fault-limited state by adjusting the current.

2. The flexible low-frequency power transmission system according to claim 1, characterized in that, The converter station (200) also includes a first switch (210), a second switch (250), a first transformer (220), and a second transformer (240); the first switch (210), the first transformer (220), the converter (230), the second transformer (240), and the second switch (250) are connected in series; the second switch (250) is connected to the low-frequency system (300), and the first switch (210) is used to connect to the external power frequency grid.

3. The low-frequency fault ride-through method for a flexible low-frequency transmission system according to any one of claims 1 to 2, characterized in that, Includes the following steps: Obtain a first voltage, and determine whether the low-frequency system (300) has a fault based on the first voltage; In response to a fault in the low-frequency system (300), the voltage-frequency controller (411) adjusts the voltage of the low-frequency system (300) to a fault-limited state and controls the low-frequency current output by the first converter to a fault-limited state, providing fault current for the low-frequency AC relay protection device; specifically, the output terminal of the voltage-frequency controller (411) is connected to a first limiter (412), and the first low-frequency current amplitude adjustment data is obtained by combining the low-frequency voltage command value, the actual low-frequency voltage value, the upper limit data of the low-frequency current amplitude, and the lower limit data of the low-frequency current amplitude, so as to adjust the voltage of the low-frequency system (300) to be controlled in a fault-limited state by adjusting the current; The power controller (421) adjusts the power of the second converter to a fault-limited state and controls the low-frequency current output by the second converter to a fault-limited state, providing fault current for the low-frequency AC relay protection device; specifically, the output terminal of the power controller (421) is connected to a second limiter (422), and the second low-frequency current amplitude adjustment data is obtained by combining the active power command value, the low-frequency reactive power command value, the actual value of low-frequency active power, the actual value of low-frequency reactive power, the upper limit data of low-frequency current amplitude, and the lower limit data of low-frequency current amplitude, so as to adjust the power of the second converter to be controlled in the fault-limited state by adjusting the current; In response to the elimination of the fault in the low-frequency system (300), the voltage frequency controller (411) adjusts the voltage of the low-frequency system (300) back to the pre-fault state; In response to the elimination of the fault in the low-frequency system (300), the power controller (421) adjusts the power of the second converter to the pre-fault state.

4. The low-frequency fault ride-through method for a flexible low-frequency transmission system according to claim 3, characterized in that, The first voltage includes the voltage between the second transformer (240) and the low-frequency system (300).

5. The low-frequency fault ride-through method for a flexible low-frequency transmission system according to claim 3, characterized in that, The step of determining whether the low-frequency system (300) has a fault based on the first voltage includes: Obtain the effective value of the positive sequence component and the effective value of the negative sequence component of the low-frequency voltage of the first voltage; In the operating state of the voltage frequency controller (411), the fault setting values ​​of the positive sequence component and the negative sequence component of the low-frequency voltage are acquired. Combined with the effective values ​​of the positive sequence component and the negative sequence component of the low-frequency voltage of the first voltage, if the first criterion is met, it is determined that the low-frequency system (300) has a fault: First criterion: Ulf_pos_rms≤Ulf_pos_vfminset; or Ulf_neg_rms≥Ulf_neg_vfmaxset; Wherein, Ulf_pos_rms is the effective value of the positive sequence component of the low-frequency voltage; Ulf_pos_vfminset is the fault setting value of the positive sequence component of the low-frequency voltage under the working state of the voltage frequency controller (411); Ulf_neg_rms is the effective value of the negative sequence component of the low-frequency voltage; and Ulf_neg_vfmaxset is the fault setting value of the negative sequence component of the low-frequency voltage under the working state of the voltage frequency controller (411). Alternatively, in the operating state of the power controller (421), the fault setting values ​​of the positive sequence component and negative sequence component of the low-frequency voltage are obtained, and combined with the effective values ​​of the positive sequence component and negative sequence component of the low-frequency voltage of the first voltage, if the second criterion is met, it is determined that the low-frequency system (300) has a fault: Second criterion: Ulf_pos_rms≤Ulf_pos_pqminset; or Ulf_neg_rms≥Ulf_neg_pqmaxset; Wherein, Ulf_pos_pqminset is the low-frequency voltage positive sequence component fault setting value under the working state of the power controller (421); Ulf_neg_pqmaxset is the low-frequency voltage negative sequence component fault setting value under the working state of the power controller (421).

6. The low-frequency fault ride-through method for a flexible low-frequency transmission system according to claim 3, characterized in that, Controlling the low-frequency current output by the first converter to a fault-limited state includes: Obtain the low-frequency voltage command value and the actual low-frequency voltage value, input them to the voltage frequency controller (411), obtain the first output result, and input the first output result to the first limiter (412); The upper limit data and lower limit data of low frequency current amplitude are obtained and input to the first limiter (412). The first low frequency current amplitude adjustment data are obtained by combining the first output result. Based on the first low-frequency current amplitude adjustment data, adjust the low-frequency current output by the first converter to the fault-limited state.

7. The low-frequency fault ride-through method for a flexible low-frequency transmission system according to claim 3, characterized in that, The step of controlling the low-frequency current output by the second converter to a fault-limited state includes: The low-frequency active power command value, low-frequency reactive power command value, low-frequency active power actual value and low-frequency reactive power actual value are obtained and input into the power controller (421), and the second output result is obtained and input into the second limiter (422). Obtain the upper limit data and lower limit data of low frequency current amplitude, input them into the second limiter (422), and obtain the second low frequency current amplitude adjustment data by combining the second output result; The low-frequency current output of the second converter is adjusted to a fault-limited state based on the second low-frequency current amplitude adjustment data.

8. The low-frequency fault ride-through method for a flexible low-frequency transmission system according to claim 3, characterized in that, The closing conditions include: The second switch (250) of the first converter station is closed in three phases; and The second switch (250) of the second converter station is closed in three phases.

9. The low-frequency fault ride-through method for a flexible low-frequency transmission system according to claim 8, characterized in that, Determining that the fault in the low-frequency system (300) has been cleared includes: In the operating state of the voltage frequency controller (411), the fault recovery judgment value of the positive sequence component of low-frequency voltage, the fault recovery judgment value of the negative sequence component of low-frequency voltage, the effective value of the positive sequence component of low-frequency voltage, and the effective value of the negative sequence component of low-frequency voltage are acquired. If the third criterion is met, it is determined that the fault of the low-frequency system (300) has been eliminated. Third criterion: Ulf_pos_rms≥Ulf_pos_vfminreset; or Ulf_neg_rms≤Ulf_neg_vfmaxreset; Wherein, Ulf_pos_rms is the effective value of the positive sequence component of the low-frequency voltage; Ulf_neg_rms is the effective value of the negative sequence component of the low-frequency voltage; Ulf_pos_vfminreset is the fault recovery judgment value of the positive sequence component of the low-frequency voltage under the working state of the voltage frequency controller (411); and Ulf_neg_vfmaxreset is the fault recovery judgment value of the negative sequence component of the low-frequency voltage under the working state of the voltage frequency controller (411). In the operating state of the power controller (421), the fault recovery judgment value of the positive sequence component of low-frequency voltage, the fault recovery judgment value of the negative sequence component of low-frequency voltage, the effective value of the positive sequence component of low-frequency voltage, and the effective value of the negative sequence component of low-frequency voltage are acquired. If the fourth criterion is satisfied and the closing condition is also satisfied, it is determined that the fault of the low-frequency system (300) has been eliminated. Fourth criterion: Ulf_pos_rms≥Ulf_pos_pqminreset or Ulf_neg_rms≤Ulf_neg_pqmaxreset; Wherein, Ulf_pos_pqminreset is the fault recovery judgment value of the positive sequence component of low frequency voltage under the working state of the power controller (421); Ulf_neg_pqmaxreset is the fault recovery judgment value of the negative sequence component of low frequency voltage under the working state of the power controller (421).

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