Flexible low-frequency power transmission system and low-frequency fault ride-through method

Abstract

The invention 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, and the low-frequency fault ride-through method comprises the steps: detecting a first voltage between a second transformer and a low-frequency system, judging whether the low-frequency system has a low-frequency fault or not through the first voltage, and when the low-frequency fault occurs, outputting the low-frequency fault; the voltage frequency controller and the power controller control current and voltage to be in a fault limiting state, so that the overvoltage or overcurrent condition is avoided, and the safety of low-frequency equipment is guaranteed; meanwhile, the voltage frequency controller and the power controller provide enough fault current for the low-frequency alternating-current relay protection device, so that the low-frequency alternating-current relay protection device acts, and fault isolation is achieved.

Description

Technical Field

[0001] The present application relates to the technical field of low-frequency power transmission systems, and in particular to a flexible low-frequency power transmission system and a low-frequency fault ride-through method. Background Art

[0002] Flexible low-frequency transmission is a new type of AC transmission technology with a frequency lower than the industrial frequency. For the networking application of the flexible low-frequency transmission system to realize the double-end or multi-end system, when a low-frequency system fault occurs, each low-frequency converter station is very likely to have overcurrent and overvoltage phenomena, endangering the safety of its converter station; at the same time, because the low-frequency system is established by power electronic equipment, the low-frequency voltage and low-frequency current are controlled by power electronic equipment. When a low-frequency system fault occurs, if it is not properly controlled, the system cannot provide enough fault current for the low-frequency AC relay protection device, resulting in the protection device being unable to operate and the fault being unable to be isolated. Summary of the invention

[0003] Purpose of the invention: The purpose of the embodiments of the present application is to provide a flexible low-frequency power transmission system and a low-frequency fault ride-through method, aiming to solve the technical problem that when a low-frequency system fails, each low-frequency current station is very likely to experience overcurrent and overvoltage, endangering the safety of the current station.

[0004] Technical solution: In a first aspect, the embodiment of the present application provides a flexible low-frequency power transmission system, including:

[0005] A plurality of converter stations, each of which includes a converter for realizing the conversion between industrial frequency electric energy and low frequency electric energy, the plurality of 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, the first converter includes a voltage-frequency controller, and the voltage-frequency controller is used to control the voltage of the flexible low-frequency power transmission system; the converter in the second converter station is a second converter, the second converter includes a power controller, and the power controller is used to control the power of the second converter; and,

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

[0007] In some embodiments, the converter station also 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 an external industrial frequency power grid.

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

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

[0010] In a second aspect, an embodiment of the present application provides a low-frequency fault ride-through method for a flexible low-frequency power transmission system according to any one of the first aspects, comprising the following steps:

[0011] Acquire 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, so as to provide a 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, so as to provide a fault current for the low-frequency AC relay protection device;

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

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

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

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

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

[0018] In the working state of the voltage-frequency controller, a low-frequency voltage positive-sequence component fault setting value and a low-frequency voltage negative-sequence component fault setting value are obtained, and combined with the effective value of the low-frequency voltage positive-sequence component and the effective value of the low-frequency voltage negative-sequence component of the first voltage, the first criterion is met, and it is determined that the low-frequency system has a fault:

[0019] First criterion:

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

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

[0022] Or, in the working state of the power controller, a low-frequency voltage positive sequence component fault setting value and a low-frequency voltage negative sequence component fault setting value are obtained, and combined with the effective value of the low-frequency voltage positive sequence component and the effective value of the low-frequency voltage negative sequence component of the first voltage, the second criterion is met, and it is determined that the low-frequency system has a fault:

[0023] Second criterion:

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

[0025] Among them, Ulf_pos_pqminset is the low-frequency voltage positive sequence component fault setting value under the working state of the power controller; Ulf_neg_pqmaxset is the low-frequency voltage negative sequence component fault setting value under the working 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] Obtaining a low-frequency voltage command value and a low-frequency voltage actual value, inputting the values ​​into a voltage-frequency controller, obtaining a first output result, and inputting the first output result into the first limiter;

[0028] Obtaining low-frequency current amplitude upper limit data and low-frequency current amplitude lower limit data, inputting the data into the first limiter, and obtaining first low-frequency current amplitude adjustment data in combination with the first output result;

[0029] The low-frequency current output by the first converter is adjusted to a fault limiting state based on the first low-frequency current amplitude adjustment data.

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

[0031] Obtaining a low-frequency active power command value, a low-frequency reactive power command value, a low-frequency active power actual value, and a low-frequency reactive power actual value, inputting the values ​​into the power controller, obtaining a second output result, and inputting the second output result into the second limiter;

[0032] Obtaining low-frequency current amplitude upper limit data and low-frequency current amplitude lower limit data, inputting the data into the second limiter, and obtaining second low-frequency current amplitude adjustment data in combination with the second output result;

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

[0034] In some embodiments, determining that the low-frequency system fault is eliminated includes:

[0035] Under the working state of the voltage-frequency controller of the first converter of the first converter station, a low-frequency voltage positive-sequence component fault recovery determination value, a low-frequency voltage negative-sequence component fault recovery determination value, a low-frequency voltage positive-sequence component effective value, and a low-frequency voltage negative-sequence component effective value are obtained, and the low-frequency system fault is determined to be eliminated if the third criterion is met:

[0036] The third criterion:

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

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

[0039] Under the working state of the power controller of the second converter of the second converter station, a low-frequency voltage positive sequence component fault recovery determination value, a low-frequency voltage negative sequence component fault recovery determination value, a low-frequency voltage positive sequence component effective value, and a low-frequency voltage negative sequence component effective value are obtained, and when the fourth criterion is met and the closing condition is met, it is determined that the low-frequency system fault is eliminated:

[0040] The fourth criterion:

[0041] Ulf_pos_rms≥Ulf_pos_pqminreset or Ulf_neg_rms≤Ulf_neg_pqmaxreset;

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

[0043] In some embodiments, the closing condition includes:

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

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

[0046] Beneficial effect: Compared with the prior art, the embodiment of the present application provides a flexible low-frequency power transmission system, including multiple converter stations and a low-frequency system, the converter station is used to realize the conversion of industrial frequency electric energy and low-frequency electric energy, and is connected 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, 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 the present application, there is a A first converter with a voltage and frequency controller is provided, and at least one second converter with a power controller is 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 be kept in a fault limiting state, and the power controller controls the power of the second converter to be kept in a fault limiting state to avoid overcurrent or overvoltage to ensure the safety of the equipment; at the same time, the voltage and frequency controller and the power controller control the current to be kept in a fault limiting state to provide sufficient fault current for the low-frequency AC relay protection device, so that the low-frequency AC relay protection device is activated to achieve fault isolation.

[0047] An embodiment of the present application provides a low-frequency fault ride-through method, which detects a first voltage between a second transformer and a low-frequency system, and determines whether a low-frequency fault exists in the low-frequency system through the first voltage. In the event of a low-frequency fault, a voltage-frequency controller controls the flexible low-frequency power transmission system to remain in a fault-limiting state, and a power controller controls the power of the second converter to remain in a fault-limiting state to avoid overcurrent or overvoltage to ensure equipment safety. At the same time, the voltage-frequency controller and the power controller control the current to remain in a fault-limiting state to provide sufficient fault current for the low-frequency AC relay protection device, so that the low-frequency AC relay protection device is activated to achieve fault isolation. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For those skilled in the art, other drawings can be obtained based on these drawings without creative work.

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

[0050] Figure 2 A schematic diagram of the connection between a voltage frequency controller and a first limiter in a flexible low-frequency power transmission system provided in an embodiment of the present 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 an embodiment of the present application;

[0052] Figure 4 A fault ride-through control flow chart of a voltage frequency controller in a low-frequency fault ride-through method provided in an embodiment of the present application;

[0053] Figure 5 A fault ride-through control flow chart of a power controller in a low-frequency fault ride-through method provided in an embodiment of the present application;

[0054] Figure 6 A flow chart of fault determination of a voltage frequency controller in a low-frequency fault ride-through method provided in an embodiment of the present application;

[0055] Figure 7 A flow chart of fault determination of a power controller in a low-frequency fault ride-through method provided in an embodiment of the present application;

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

[0057] Fig. 9 A schematic diagram of signal transmission of a power controller and a second limiter in a low-frequency fault ride-through method provided in an embodiment of the present application;

[0058] Fig.10 A flowchart of a low-frequency fault ride-through method provided in an embodiment of the present application;

[0059] Figure 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 DESCRIPTION

[0060] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present application.

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

[0062] In the description of this application, it should be noted that, unless otherwise clearly specified and limited, the terms "connected" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, a direct connection, or an indirect connection through an intermediate medium, or it can be the internal communication of two elements. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0063] like Figures 1 to 4 As shown, the present application provides a flexible low-frequency power transmission system, including multiple converter stations 200 and a low-frequency system 300, the converter station 200 is used to realize the conversion of industrial frequency electric energy and low-frequency electric 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, 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 converter station and the second converter station are connected to the low-frequency system 300, which can realize the conversion of industrial frequency electric energy and low-frequency electric energy. conversion and realize low-frequency electric energy transmission; in the present application, there is a first converter with a voltage-frequency controller 411 and at least one second converter with a power controller 421. When a fault occurs in the low-frequency system 300, the voltage-frequency controller 411 controls the voltage of the flexible low-frequency power 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 to avoid overcurrent or overvoltage to ensure equipment safety; at the same time, 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, so that the low-frequency AC relay protection device is activated to 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 , and each converter station 200 is connected to an external industrial frequency power grid and is 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 embodiment of the present application includes three converter stations 200, that is, there are one first converter station and two second converter stations;

[0066] like Figure 2 and Figure 3 As shown, the converter 230 in the first converter station is the first converter, and the first converter includes a voltage-frequency controller 411. The voltage-frequency controller 411 is used to control the voltage of the flexible low-frequency power transmission system. When a fault occurs in the low-frequency system 300, the voltage and current of the low-frequency system 300 are controlled to be in a fault-limited state by controlling the voltage and current of the low-frequency system 300. At the same time, the current in the fault-limited state meets the fault current requirement of the low-frequency AC relay protection device, so as to provide sufficient fault current when the low-frequency system 300 fails, and ensure the action of the low-frequency AC relay protection device. To achieve the purpose of fault isolation; the converter 230 in the second converter station is the second converter, and the second converter includes a power controller 421. The power controller 421 is used to control the power of the second converter. When a fault occurs in the low-frequency system 300, the power and current are controlled in a fault-limited state by controlling the power and current of the second converter. At the same time, the current in the fault-limited state meets the fault current requirement 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, the 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; the 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 in sequence; 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 in sequence, so as to control the on and off of the power frequency current and 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 in sequence, so as to control the on and off of the low frequency current and transform the low frequency current.

[0069] like Figure 2 and Figure 3 As shown, in some embodiments, the output end 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 low-frequency voltage actual value, the low-frequency current amplitude upper limit data and the low-frequency current amplitude lower limit data, so as to adjust the voltage control of the low-frequency system 300 to a fault-limited state by adjusting the current, and at the same time, the adjusted current can ensure the protection action of the low-frequency AC relay protection device to achieve fault isolation; the output end 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 low-frequency active power actual value, the low-frequency reactive power actual value, the low-frequency current amplitude upper limit data and the low-frequency current amplitude lower limit data, so as to adjust the power control of the second converter to a fault-limited state by adjusting the current, and at the same time, the adjusted current can ensure the protection action of the low-frequency AC relay protection device to achieve fault isolation.

[0070] like Figures 4 to 10As shown, the embodiment of the present application also provides a low-frequency fault ride-through method of the flexible low-frequency power transmission system based on the present application, detecting the first voltage between the second transformer 240 and the low-frequency system 300, and determining whether the low-frequency system 300 has a low-frequency fault by the first voltage. In the event of a low-frequency fault, the voltage-frequency controller 411 controls the voltage of the flexible low-frequency power 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 to avoid overcurrent or overvoltage to ensure equipment safety; at the same time, 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, so that the low-frequency AC relay protection device is actuated to achieve fault isolation; the method specifically includes:

[0071] Step 1: Obtain a first voltage, and determine whether the low-frequency system 300 has a fault 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 value of the low-frequency voltage positive sequence component and the effective value of the low-frequency voltage negative sequence component of the first voltage.

[0074] Step 1.3: If Figures 4 to 7 As shown, based on the effective value of the low-frequency voltage positive sequence component and the effective value of the low-frequency voltage negative sequence component of the first voltage, fault detection is performed in the working state of the voltage-frequency controller 411 and the power controller 421 respectively. If the detection result in the working state of the voltage-frequency controller 411 or the power controller 421 has a fault, it is determined that the low-frequency system 300 has a fault:

[0075] Step 1.3.1: Figure 4 and Figure 6 As shown, the fault determination is performed in the working state of the voltage-frequency controller 411:

[0076] Obtain a low-frequency voltage positive-sequence component fault setting value and a low-frequency voltage negative-sequence component fault setting value, combine the low-frequency voltage positive-sequence component effective value and the low-frequency voltage negative-sequence component effective value of the first voltage, and satisfy the first criterion to determine 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] Among them, Ulf_pos_rms is the effective value of the low-frequency voltage positive sequence component; Ulf_pos_vfminset is the fault setting value of the low-frequency voltage positive sequence component under the working state of the voltage frequency controller 411; Ulf_neg_rms is the effective value of the low-frequency voltage negative sequence component; Ulf_neg_vfmaxset is the fault setting value of the low-frequency voltage negative sequence component under the working state of the voltage frequency controller 411.

[0080] Step 1.3.2: Figure 5 and Figure 7 As shown, the fault determination is performed under the working state of the power controller 421:

[0081] Obtain a low-frequency voltage positive-sequence component fault setting value and a low-frequency voltage negative-sequence component fault setting value, combine the low-frequency voltage positive-sequence component effective value and the low-frequency voltage negative-sequence component effective value of the first voltage, and satisfy the second criterion to determine 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 when the power controller 421 is in the working state; Ulf_neg_pqmaxset is the low-frequency voltage negative sequence component fault setting value when the power controller 421 is in the working state.

[0085] Step 2: If 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, so as to provide 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, so as to provide sufficient fault current for the low-frequency AC relay protection device:

[0086] Step 2.1: 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 that a fault exists in the low-frequency system 300, the low-frequency voltage command value Ulf_ref is switched from a rated value to a fault limiting state, the low-frequency voltage command value Ulf_ref and the low-frequency voltage actual value Ulf at this time are obtained, input into the voltage-frequency controller 411, the first output result is obtained, and the first output result is input into the first limiter 412;

[0088] Switch the low-frequency current amplitude upper limit data Ilf_vfref_max and the low-frequency current amplitude lower limit data Ilf_vfref_min from the steady-state limit value to the fault limit state, obtain the low-frequency current amplitude upper limit data Ilf_vfref_max and the low-frequency current amplitude lower limit data 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 in combination with the first output result;

[0089] Combined with the first low-frequency current amplitude adjustment data Ilf_vfref, the current output by the first converter is adjusted to a fault limiting state to ensure that no excessive fault current is generated. At the same time, sufficient fault current is provided 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: If Figure 5 and Fig. 9 As shown, the power controller 421 adjusts the power of the second converter to a fault limiting state, and controls the low-frequency current output by the second converter to a fault limiting state, providing a fault current for the low-frequency AC relay protection device:

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

[0092] Switch the low-frequency current amplitude upper limit data Ilf_pqref_max and the low-frequency current amplitude lower limit data Ilf_pqref_min from the steady-state limit value to the fault limit state, obtain the low-frequency current amplitude upper limit data Ilf_pqref_max and the low-frequency current amplitude lower limit data 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 in combination with the second output result;

[0093] Based on the second low-frequency current amplitude adjustment data Ilf_pqref, the low-frequency current is adjusted to a fault limiting state. At the same time, the current in the fault limiting state can 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.

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

[0095] Step 3.1: Check whether the fault of the low-frequency system 300 is eliminated:

[0096] Step 3.1.1: First, in the working state of the voltage-frequency controller 411, determine whether the fault of the low-frequency system 300 is eliminated:

[0097] When the voltage-frequency controller 411 is in working state, the low-frequency voltage positive-sequence component fault recovery determination value, the low-frequency voltage negative-sequence component fault recovery determination value, the low-frequency voltage positive-sequence component effective value, and the low-frequency voltage negative-sequence component effective value are obtained, and the third criterion is met to determine that the low-frequency system 300 fault is eliminated:

[0098] The third criterion:

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

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

[0101] Step 3.1.2: Under the working state of the power controller 421, determine whether the fault of the low frequency system 300 is eliminated:

[0102] When the power controller 421 is in working state, the low-frequency voltage positive sequence component fault recovery determination value, the low-frequency voltage negative sequence component fault recovery determination value, the low-frequency voltage positive sequence component effective value and the low-frequency voltage negative sequence component effective value are obtained, and the closing condition is met while the fourth criterion is met, and it is determined that the low-frequency system 300 fault is eliminated:

[0103] The fourth criterion:

[0104] Ulf_pos_rms≥Ulf_pos_pqminreset or Ulf_neg_rms≤Ulf_neg_pqmaxreset;

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

[0106] The closing conditions include: the three-phase closing of the second switch 250 of the first converter station and the three-phase closing of the second switch 250 of the second converter station.

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

[0108] After determining that the fault of the low-frequency system 300 has been cleared, the voltage-frequency controller 411 restores the low-frequency voltage by restoring the low-frequency voltage command value Ulf_ref, and the recovery rate must meet the requirements of stable operation of the system; 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 recovery rate must meet the requirements of stable operation of the system.

[0109] The present application has introduced in detail a flexible low-frequency power transmission system and a low-frequency fault ride-through method provided in the embodiments of the present application. Specific examples are used in this article to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the method of the present application and its core idea. At the same time, for technical personnel in this field, according to the idea of ​​the present application, there will be changes in the specific implementation method and application scope. In summary, the content of this specification should not be understood as a limitation on the present application.

Claims

1. A flexible low-frequency power transmission system, characterized in that: include: A plurality of converter stations (200), each of the converter stations (200) comprising a converter (230) for realizing conversion between industrial frequency electric energy and low frequency electric energy, the plurality of converter stations (200) comprising 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 comprising a voltage frequency controller (411), the voltage frequency controller (411) being used to control the voltage of the flexible low frequency power transmission system; the converter (230) in the second converter station being a second converter, the second converter comprising a power controller (421), the power controller (421) being used to control the power of the second converter; and, A low-frequency system (300), wherein the first converter station is connected to the low-frequency system (300) and is used to control the low-frequency voltage to a fault-limited state when the low-frequency system (300) fails; and the second converter station is connected to the low-frequency system (300) and is used to control the low-frequency power to a fault-limited state when the low-frequency system (300) fails.

2. The flexible low-frequency power transmission system according to claim 1, characterized in that: The converter station (200) further comprises 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 sequentially 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 an external industrial frequency power grid.

3. The flexible low-frequency power transmission system according to claim 1, characterized in that: The output end of the voltage-frequency controller (411) is connected to a first limiter (412); The output end of the power controller (421) is connected to a second limiter (422).

4. The low-frequency fault ride-through method of the flexible low-frequency power transmission system according to any one of claims 1 to 3, characterized in that: The steps include: Acquiring a first voltage, and determining 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, thereby providing a fault current for a 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, thereby providing a fault current for a low-frequency AC relay protection device; In response to the low-frequency system (300) fault being eliminated, the voltage-frequency controller (411) adjusts the voltage of the low-frequency system (300) to a pre-fault state; In response to the low frequency system (300) fault being eliminated, the power controller (421) adjusts the power of the second converter to a pre-fault state.

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

6. The low-frequency fault ride-through method of the flexible low-frequency power transmission system according to claim 4, characterized in that: The determining whether the low-frequency system (300) has a fault based on the first voltage comprises: Acquire an effective value of a low-frequency voltage positive-sequence component and an effective value of a low-frequency voltage negative-sequence component of the first voltage; In the working state of the voltage-frequency controller (411), a low-frequency voltage positive-sequence component fault setting value and a low-frequency voltage negative-sequence component fault setting value are obtained, and combined with the low-frequency voltage positive-sequence component effective value and the low-frequency voltage negative-sequence component effective value of the first voltage, a first criterion is met, and 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 low-frequency voltage positive sequence component; Ulf_pos_vfminset is the low-frequency voltage positive sequence component fault setting value under the working state of the voltage-frequency controller (411); Ulf_neg_rms is the effective value of the low-frequency voltage negative sequence component; Ulf_neg_vfmaxset is the low-frequency voltage negative sequence component fault setting value under the working state of the voltage-frequency controller (411); Or, in the working state of the power controller (421), a low-frequency voltage positive sequence component fault setting value and a low-frequency voltage negative sequence component fault setting value are obtained, and combined with the effective value of the low-frequency voltage positive sequence component and the effective value of the low-frequency voltage negative sequence component of the first voltage, a second criterion is satisfied, and 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; Among them, Ulf_pos_pqminset is the low-frequency voltage positive sequence component fault setting value when the power controller (421) is in the working state; Ulf_neg_pqmaxset is the low-frequency voltage negative sequence component fault setting value when the power controller (421) is in the working state.

7. The low-frequency fault ride-through method of the flexible low-frequency power transmission system according to claim 4, characterized in that: The controlling the low-frequency current output by the first converter to a fault limiting state includes: Obtaining a low-frequency voltage command value and a low-frequency voltage actual value, inputting them into a voltage-frequency controller (411), obtaining a first output result, and inputting the first output result into the first limiter (412); Obtaining low-frequency current amplitude upper limit data and low-frequency current amplitude lower limit data, inputting the data into the first amplitude limiter (412), and obtaining first low-frequency current amplitude adjustment data in combination with the first output result; The low-frequency current output by the first converter is adjusted to a fault limiting state based on the first low-frequency current amplitude adjustment data.

8. The low-frequency fault ride-through method of the flexible low-frequency power transmission system according to claim 4, characterized in that: The controlling the low-frequency current output by the second converter to a fault limiting state comprises: Obtaining a low-frequency active power command value, a low-frequency reactive power command value, a low-frequency active power actual value, and a low-frequency reactive power actual value, inputting the values ​​into the power controller (421), obtaining a second output result, and inputting the second output result into the second limiter (422); Obtaining low-frequency current amplitude upper limit data and low-frequency current amplitude lower limit data, inputting the data into the second amplitude limiter (422), and obtaining second low-frequency current amplitude adjustment data in combination with the second output result; The low-frequency current output by the second converter is adjusted to a fault limiting state based on the second low-frequency current amplitude adjustment data.

9. The low-frequency fault ride-through method of the flexible low-frequency power transmission system according to claim 4, characterized in that: Determining whether the fault of the low-frequency system (300) is eliminated comprises: In the working state of the voltage-frequency controller (411), a low-frequency voltage positive-sequence component fault recovery determination value, a low-frequency voltage negative-sequence component fault recovery determination value, a low-frequency voltage positive-sequence component effective value, and a low-frequency voltage negative-sequence component effective value are obtained, and a third criterion is met to determine that the low-frequency system (300) fault is eliminated: The 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 low-frequency voltage positive sequence component; Ulf_neg_rms is the effective value of the low-frequency voltage negative sequence component; Ulf_pos_vfminreset is the fault recovery judgment value of the low-frequency voltage positive sequence component under the working state of the voltage-frequency controller (411); Ulf_neg_vfmaxreset is the fault recovery judgment value of the low-frequency voltage negative sequence component under the working state of the voltage-frequency controller (411); In the working state of the power controller (421), a low-frequency voltage positive sequence component fault recovery determination value, a low-frequency voltage negative sequence component fault recovery determination value, a low-frequency voltage positive sequence component effective value, and a low-frequency voltage negative sequence component effective value are obtained, and when the fourth criterion is met and the closing condition is met, it is determined that the fault of the low-frequency system (300) is eliminated: The fourth criterion: Ulf_pos_rms≥Ulf_pos_pqminreset or Ulf_neg_rms≤Ulf_neg_pqmaxreset; Among them, Ulf_pos_pqminreset is a low-frequency voltage positive sequence component fault recovery judgment value when the power controller (421) is in the working state; Ulf_neg_pqmaxreset is a low-frequency voltage negative sequence component fault recovery judgment value when the power controller (421) is in the working state.

10. The low-frequency fault ride-through method of the flexible low-frequency power transmission system according to claim 9, characterized in that: The closing conditions include: The second switch (250) of the first converter station is switched on in three phases; and The second switch (250) of the second converter station is closed in three phases.

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