Flexible DC power grid adaptive reclosing method and system based on S transformation

Abstract

The invention discloses a flexible DC power grid adaptive reclosing method and system based on S transformation, and belongs to the field of power system relay protection. The method comprises the following steps: continuously recording fault voltage data by using a measurement end data acquisition device; performing S transformation on the fault voltage signal according to a set time window length; calculating an amplitude quadratic sum mean value of each column of the modular matrix after S transformation; and judging whether the mean value of the sum of squares of the amplitudes in the detection window suddenly changes or not, and in the maximum detection duration, if the maximum sudden change point of the mean value of the sum of squares of the amplitudes in the time window when the fault disappears does not exceed a threshold value, judging the fault as a permanent fault, and immediately locking the direct current circuit breaker. According to the method, self-adaptive reclosing is carried out after the direct-current circuit breaker of the flexible direct-current power grid trips, additional control does not need to be added, the fault property can be accurately and reliably recognized, the fault arc extinguishing moment can be determined under the instantaneous fault condition, the reclosing time can be effectively shortened, and the power supply recovery speed of a system can be increased.

Description

Technical Field

[0001] The present invention discloses a flexible DC grid adaptive reclosing method and system based on S-transform, belonging to the field of relay protection of power systems. Background Art

[0002] In a flexible DC grid, line protection and restoration schemes are mainly divided into two categories: the first category is based on a sub-module with fault self-clearing ability and a DC fast switch; the second category is based on a half-bridge sub-module without fault self-clearing ability and a DC circuit breaker. For the first type of restoration scheme, after a line fault, the converter station needs to be blocked to complete the restart of the system, which will cause a long-term power interruption, have a greater impact on the power transmission system, and it is difficult to be extended to the existing flexible DC system based on half-bridge sub-modules. For the second type of restoration scheme, the DC circuit breaker has a reclosing function, with a faster post-fault recovery speed and no global blocking. DC transmission lines are mostly overhead lines, with a high fault probability and mostly transient faults. Reclosing the DC circuit breaker in time after the fault arc is extinguished can shorten the outage time of the DC line and improve the power transmission reliability of the DC grid. In actual engineering, the DC circuit breaker adopts the method of automatic reclosing, that is, after the DC circuit breaker is disconnected, after a deionization time of 200-300 ms, the DC circuit breaker on one side of the DC line automatically recloses. If the DC voltage is established on the fault line within a certain time period, then the DC circuit breaker on the opposite side is reclosed, otherwise, the DC circuit breaker is tripped again. Whether the line has a transient fault or a permanent fault, the circuit breaker will reclose. If the circuit breaker recloses on a permanent fault, it will cause a secondary impact on the entire power transmission system. Therefore, an adaptive reclosing method is needed to discriminate the persistence of the line fault, avoid misreclosing and causing a secondary impact on the system, and accelerate the time for the system to resume power supply.

[0003] Currently, some experts have proposed that after the fault current drops to zero, a current signal is directly injected into the fault pole line by using a full-bridge modular multilevel converter (MMC), and the fault disappearance moment is identified by analyzing the port voltage response. However, this method is not applicable to half-bridge MMC converter stations. In addition, injecting energy into the line before the fault disappears may prolong the fault arc extinction time. Some other experts use the full-bridge MMC to generate a pulse signal and directly inject it into the healthy pole line, and identify the fault nature by detecting the capacitive coupling signal of the fault pole line. This method requires injecting a signal using the healthy pole and is only applicable to the case of a single-pole ground short circuit fault in a true bipolar system. Summary of the Invention

[0004] The object of the present invention is to provide a flexible DC grid adaptive reclosing method and system based on the S transform. This method does not require additional control, can accurately and reliably identify the nature of the fault, and can determine the fault arc extinction moment in the case of transient faults. Compared with the automatic reclosing scheme, it can effectively shorten the reclosing time and accelerate the system's power supply restoration speed.

[0005] The technical solution of the present invention is as follows: A flexible DC grid adaptive reclosing method and system based on the S transform. According to the principle of electrostatic induction, it is analyzed that before the fault arc extinction, since the fault point is always grounded, there is no induced voltage on the faulty pole. Therefore, the residual voltage of the faulty pole oscillates and decays around the zero axis until it becomes 0. After the fault arc extinction, there is an induced voltage on the faulty pole, resulting in the phenomenon that the residual voltage oscillates and decays around the zero axis before the fault disappears and offsets the zero axis after the fault disappears. Therefore, the fault is identified by detecting whether the residual voltage of the faulty pole offsets the zero axis during the decay oscillation process after the DC breaker trips.

[0006] The specific steps are as follows:

[0007] Step1: When a fault occurs in the flexible DC grid transmission line, use the DC breakers at both ends of the line to cut off the faulty line;

[0008] Step2: Use the data acquisition device to collect the voltage data at the measurement points at both ends of the faulty line;

[0009] Step3: Perform the S transform on the fault voltage signal within the preset time window length at the measurement point;

[0010] Step4: Calculate the mean value of the sum of the squares of the amplitudes of each column of the modulus matrix after the S transform;

[0011] Step5: Determine the nature of the fault according to the obtained mean value of the sum of the squares of the amplitudes. If it is identified as a permanent fault, lock the DC breaker. If it is identified as a transient fault, execute Step6;

[0012] Step6: Calculate the fault arc extinction moment and reclose the DC breaker.

[0013] The specific content of Step1 is as follows:

[0014] After an in-zone fault occurs in the flexible DC grid, the DC breakers installed at both ends of the faulty line trip to isolate the fault.

[0015] The specific content of Step2 is as follows:

[0016] Step2.1: Install high-speed voltage signal acquisition devices at both ends of the transmission line;

[0017] Step 2.2: After a fault occurs in the line, use the voltage signal high-speed acquisition device to continuously sample the voltage of the faulty line.

[0018] The specific content of Step 3 is as follows:

[0019] Step 3.1: Define the length of each sliding time window as T and the sliding factor as a;

[0020] Step 3.2: Perform the S transform on the sampled voltage of the faulty line within each time window. The S transform S(τ, f) of the signal x(t) is defined as:

[0021]

[0022]

[0023] Where, is the Gaussian window, is the position parameter of the Gaussian window on the time axis is the position parameter, is the frequency, and j is the imaginary unit;

[0024] For the N discrete signal points x[i], i = 0, 1, …, N - 1, collected, perform the S transform using the above formula. The transformation result is a complex time-frequency matrix with n + 1 rows and m columns, denoted as the S matrix. Its columns correspond to the sampling time points, and its rows correspond to the frequencies.

[0025] The specific content of Step 4 is as follows:

[0026] According to Parseval's theorem, calculate the mean value X of the sum of the squared amplitudes of each column of the S matrix i :

[0027]

[0028] In the formula, f k is the frequency corresponding to the nth row of the S matrix; τ is the position parameter.

[0029] The specific content of Step 5 is as follows:

[0030] Step 5.1: Set the maximum detection time limit ΔT:

[0031] ΔT = T1 - T0

[0032] In the formula, T1 is the fixed time of reclosing, and T0 is the insulation recovery time of the arc path;

[0033] Step 5.2: Within the maximum detection time limit ΔT, construct an identification criterion using the mean value of the sum of the squared amplitudes of each column of the S matrix, and combine the mean value of the sum of the squared amplitudes of each column of the S matrix of each time window to identify the fault type: If it satisfies X within the (x + 1)th time window i>Y max , Y max is used as the setting criterion, if it is satisfied, it is judged as an instantaneous fault; otherwise, it is judged as a permanent fault.

[0034] Specifically, Step6 is as follows:

[0035] Step6.1: Calculate the arc extinction time t of the fault point end :

[0036] t end = t1 + x · a + q / f s

[0037] In the formula, t1 is the circuit breaker tripping time, x is the number of times the time window slides, q is the mutation point in the (x + 1)-th time window, and f s is the signal sampling frequency;

[0038] Step6.2: According to the calculated arc extinction time t end , wait for 100 ms for deionization and output a closing signal, then the closing time is t out = t end + 100, and the DC circuit breakers at both ends of the protection are closed.

[0039] To achieve the above object, the present application also proposes a flexible DC grid adaptive reclosing system based on S-transform, including:

[0040] A fault processing module, which is used to isolate the faulty line when a fault occurs on the transmission line;

[0041] A numerical acquisition and calculation module, which is used to obtain the voltage data at the protection installation location of the faulty line, perform S-transform on the voltage data in each time window according to the set time window, and calculate the mean value X of the sum of the squares of the amplitudes of each column of the transformed modulus matrix i ;

[0042] A fault type identification module, which is used to construct an identification criterion using the mean value of the sum of the squares of the amplitudes of each column of the modulus matrix to identify the fault type, output a closing signal for an instantaneous fault, and output a blocking signal for a permanent fault;

[0043] A reclosing control module, which is used to receive the closing or blocking signal sent by the fault type identification module and control the DC circuit breaker to act.

[0044] Specifically, the fault processing module includes:

[0045] A signal receiving unit, which is used to receive the tripping signal;

[0046] An action starting unit, which is used to control the DC circuit breaker to trip.

[0047] Specifically, the numerical acquisition and calculation module includes:

[0048] A data acquisition unit, configured to extract and store voltage data at both ends of a faulty line;

[0049] An analog-to-digital conversion unit, configured to perform an S transform on the obtained faulty pole voltage to obtain an S transform modulus matrix;

[0050] A numerical calculation unit, configured to calculate the mean value of the sum of the squared amplitudes of each column of the S matrix according to Parseval's theorem.

[0051] The faulty type identification module specifically includes:

[0052] A setting determination time limit setting unit, configured to analyze the obtained faulty pole voltage data and set a maximum detection time limit ΔT;

[0053] A faulty type identification unit, configured to construct an identification criterion by using the mean value of the sum of the squared amplitudes of each column of the modulus matrix and identify the faulty type within the maximum detection time limit;

[0054] An action signal sending unit, configured to calculate the faulty arc extinction moment when it is identified as an instantaneous fault and output a closing signal after a fixed deionization; output a locking signal when it is identified as a permanent fault.

[0055] The reclosing control module specifically includes:

[0056] A closing unit, configured to receive a closing signal and control the reclosing of a DC circuit breaker;

[0057] A locking unit, configured to receive a locking signal and control the locking of a DC circuit breaker.

[0058] The present invention has the following beneficial technical effects:

[0059] (1) The present invention is directed to identifying the faulty type after a DC circuit breaker trips in a flexible DC power grid. Without actively injecting signals, it only performs calculation and analysis by using the residual voltage of the faulty line, which is easy to implement, can accurately and reliably identify the nature of the fault, and can determine the faulty arc extinction moment in the case of an instantaneous fault;

[0060] (2) The present invention uses the residual voltage of the line and the induced voltage of the non-faulty pole on the faulty pole to judge the persistence of the line fault, without the need for additional control;

[0061] (3) Through a large number of simulation analyses, compared with the automatic reclosing scheme, the present invention can effectively shorten the reclosing time and accelerate the power supply restoration speed of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 is the topology diagram of the DC circuit breaker of the present invention;

[0063] Figure 2Flow chart of the adaptive reclosing of the present invention;

[0064] Figure 3 Functional block diagram of the adaptive reclosing system of the present invention;

[0065] Figure 4 Schematic diagram of the fault identification result of Embodiment 1 of the present invention;

[0066] Figure 5 Schematic diagram of the fault identification result of Embodiment 2 of the present invention. Detailed implementation manners

[0067] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art.

[0068] Embodiment 1: Taking a flexible DC power grid as the background, the full length of the line is 200 km, and the voltage level is ±500 kV. It is set that a fault occurs at 20 km from the starting end of the line, the fault type is set as a positive pole grounding instantaneous fault, the fault duration is 10 ms, the transition resistance is set as 0 Ω, and the sampling rate is 10 kHz.

[0069] Based on the fault type identification flow chart of a flexible DC power grid adaptive reclosing method provided by the present invention, as shown in the appendix Figure 2 The specific implementation steps are as follows:

[0070] Step1: When a fault occurs in the transmission line of the flexible DC power grid, use the DC circuit breakers at both ends of the line to cut off the faulty line.

[0071] Specifically, after a fault occurs in the flexible DC power grid at time t0, the DC circuit breakers installed at both ends of the faulty line trip at time t1 = 6 ms to isolate the fault. The topological structure of the DC circuit breaker is as shown in the appendix Figure 1 shown.

[0072] Step2: Use the data acquisition device to collect the voltage data at the measurement points at both ends of the faulty line.

[0073] Specifically:

[0074] Step2.1: Install high-speed voltage signal acquisition devices at both ends of the transmission line;

[0075] Step2.2: After a fault occurs in the line, use the high-speed voltage signal acquisition device to continuously sample the voltage of the faulty line.

[0076] Step3: Perform S transform on the fault voltage signal within the preset time window length at the measurement point.

[0077] Specifically:

[0078] Step 3.1: Define the length of each sliding time window as T and the sliding factor as a. In this example, T = 10 ms and a = 1 ms.

[0079] Step 3.2: Perform the S-transform on the voltage of the faulty line collected within each time window. The S-transform S(τ, f) of the signal x(t) is defined as:

[0080]

[0081]

[0082] where, is the Gaussian window, is the position parameter of the Gaussian window on the time axis and is the frequency, and j is the imaginary unit;

[0083] For the N discrete signal points x[i], i = 0, 1, …, N - 1 collected, perform the S-transform using the above formula. The transformation result is a complex time-frequency matrix with n + 1 rows and m columns, denoted as the S matrix. Its columns correspond to the sampling time points, and its rows correspond to the frequencies.

[0084] Step 4: Calculate the mean value of the sum of the squared amplitudes of each column of the modulus matrix after the S-transform.

[0085] Specifically:

[0086] According to Parseval's theorem, calculate the mean value of the sum of the squared amplitudes of each column of the S matrix X i :

[0087]

[0088] where f k is the frequency corresponding to the nth row of the S matrix.

[0089] Step 5: Determine the nature of the fault based on the obtained mean value of the sum of the squared amplitudes. If it is identified as a permanent fault, lock out the DC circuit breaker. If it is identified as a transient fault, execute Step 6.

[0090] Specifically:

[0091] Step 5.1: Set the maximum detection time limit ΔT = 100 ms;

[0092] ΔT = T1 - T0 = 200 - 100 = 100 ms

[0093] where, the fixed time of reclosing T1 is 200 ms, and the insulation recovery time of the arc path T0 is 100 ms. According to the maximum discrimination time limit, the maximum number of sliding times x is calculated to be 91.

[0094] Step5.2: Construct an identification criterion using the mean of the sum of the squared amplitudes of each column of the S matrix within the maximum detection time limit ΔT. Combine the mean of the sum of the squared amplitudes of each column of the S matrix for each time window to identify the fault type: If it satisfies X i >Y max in the (x + 1)-th time window, and Y max is the setting criterion, then it is judged as an instantaneous fault; otherwise, it is judged as a permanent fault. To retain a certain margin, in the present invention, Y max is taken as 0.5.

[0095] As can be seen from Figure 4 , there is an obvious mutation at the 52nd sampling point in the first time window, that is, the mean of the sum of the squared amplitudes of each column of the S matrix exceeds the threshold Y max = 0.5, and it can be judged as an instantaneous fault.

[0096] Step6: Calculate the arc extinction time of the fault point and reclose the DC circuit breaker.

[0097] Specifically:

[0098] Step6.1: Calculate the arc extinction time t end of the fault point;

[0099] t end = t1 + x·a + q / f s = 6 + 0 + 52 / 10 = 11.2 ms

[0100] In the formula, t1 is the circuit breaker tripping time, which is 6 ms, the time window sliding times x = 0, the mutation point q = 52 in the first time window, and the signal sampling frequency f s = 10 kHz.

[0101] As can be seen from Figure 4 , the fault extinguishes at the 52nd sampling point, that is, 5.2 ms after the circuit breaker disconnects. Then, starting from the start of protection activation, the grounding fault extinguishes at t end = 11.2 ms, with an error of only 1.2 ms from the actual fault disappearance time of 10 ms.

[0102] Step6.2: According to the calculated arc extinction time t end , wait for 100 ms for deionization, output a closing signal, and the closing time is t out = t end + 100 = 111.2 ms, and the DC circuit breakers at both ends of the protection close.

[0103] Figure 3 The figure shows the functional block diagram of a flexible DC grid adaptive reclosing system based on the S transform provided by the present invention, which mainly includes:

[0104] A fault handling module, which is used to isolate a faulty line when a fault occurs in the transmission line;

[0105] A numerical acquisition and calculation module, which is used to obtain voltage data at the protection installation location of the faulty line, perform an S-transform on the voltage data within each time window according to a set time window, and calculate the mean value X of the sum of the squares of the amplitudes of each column of the transformed modulus matrix i ;

[0106] A fault type identification module, which is used to construct an identification criterion using the mean value of the sum of the squares of the amplitudes of each column of the modulus matrix to identify the fault type, output a closing signal for a transient fault, and output a locking signal for a permanent fault;

[0107] A reclosing control module, which is used to receive the closing or locking signal sent by the fault type identification module and control the DC circuit breaker to operate.

[0108] The fault handling module specifically includes:

[0109] A signal receiving unit, which is used to receive a tripping signal;

[0110] An action starting unit, which is used to control the DC circuit breaker to trip.

[0111] The numerical acquisition and calculation module specifically includes:

[0112] A data acquisition unit, which is used to extract and store voltage data at both ends of the faulty line;

[0113] A digital-to-analog conversion unit, which is used to perform an S-transform on the obtained faulty pole voltage to obtain an S-transform modulus matrix;

[0114] A numerical calculation unit, which calculates the mean value of the sum of the squares of the amplitudes of each column of the S matrix according to Parseval's theorem.

[0115] The fault type identification module specifically includes:

[0116] A setting determination time limit setting unit, which is used to analyze the obtained faulty pole voltage data and set a maximum detection time limit ΔT. In this example, ΔT = 100 ms;

[0117] A fault type identification unit, which is used to construct an identification criterion using the mean value of the sum of the squares of the amplitudes of each column of the modulus matrix and identify the fault type within the maximum detection time limit;

[0118] An action signal sending unit, which is used to calculate the fault arc extinction time when it is identified as a transient fault, and output a closing signal after a fixed deionization; output a locking signal when it is identified as a permanent fault. In this embodiment, when it is identified as a transient fault, the fault duration is 11.2 ms, and a closing signal is output at 111.2 ms after the circuit breaker trips.

[0119] The reclosing control module specifically includes:

[0120] A closing unit, used to receive a closing signal and control the reclosing of the DC circuit breaker;

[0121] The locking unit is used to receive the locking signal and control the DC circuit breaker to lock.

[0122] Example 2: With a flexible DC grid as the background, the line is 200 km long and the voltage level is ±500 kV. The fault is set to occur at the midpoint of the line, the fault type is set to a permanent positive ground fault, the fault duration is infinite, and the sampling rate is 10 kHz.

[0123] The fault type identification flow chart of the S-transformation-based flexible DC grid adaptive reclosing method provided by the present invention is shown in the attached figure. Figure 2 As shown, the specific steps for implementation are:

[0124] Step 1: When a fault occurs in the flexible DC grid transmission line, the DC circuit breakers at both ends of the line are used to cut off the fault line.

[0125] Specifically, after a fault occurs in the flexible DC grid at time t0, the DC circuit breakers installed at both ends of the fault line trip at time t1=6ms to isolate the fault. The DC circuit breaker topology is shown in the attached figure. Figure 1 shown.

[0126] Step 2: Use the data acquisition device to collect voltage data at the measuring points at both ends of the fault line.

[0127] Specifically:

[0128] Step 2.1: Install a high-speed voltage signal acquisition device at both ends of the transmission line;

[0129] Step 2.2: After a line fault occurs, the voltage of the faulty line is continuously sampled using a voltage signal high-speed acquisition device.

[0130] Step 3: Perform S transformation on the fault voltage signal within the preset time window length at the measurement point.

[0131] Specifically:

[0132] Step 3.1: Define the length of each sliding window as T and the sliding factor as a. In this example, T = 10ms and n = 1ms.

[0133] Step 3.2: Perform S transformation on the collected fault line voltage in each time window. The S transformation S(τ, f) of the signal x(t) is defined as:

[0134]

[0135]

[0136] wherein, is a Gaussian window, is the position parameter of the Gaussian window on the time axis ; is the frequency, and j is the imaginary unit;

[0137] For the N discrete signal points x[i], i = 0, 1, …, N−1 collected, perform the S transform using the above formula, and the transform result is a complex time-frequency matrix with n + 1 rows and m columns, denoted as the S matrix. Its columns correspond to the sampling time points, and its rows correspond to the frequencies.

[0138] Step4: Calculate the mean value of the sum of the squared amplitudes of each column of the modulus matrix after the S transform.

[0139] Specifically:

[0140] According to Parseval's theorem, calculate the mean value of the sum of the squared amplitudes of each column of the S matrix X i :

[0141]

[0142] where f k is the frequency corresponding to the nth row of the S matrix.

[0143] Step5: Determine the nature of the fault based on the obtained mean value of the sum of the squared amplitudes. If it is identified as a permanent fault, lock the DC circuit breaker. If it is identified as a transient fault, execute Step6.

[0144] Specifically:

[0145] Step5.1: Set the maximum detection time limit ΔT = 100 ms;

[0146] ΔT = T1 - T0 = 200 - 100 = 100 ms

[0147] where T1 is the fixed time of reclosing at 200 ms, and T0 is the insulation recovery time of the arc path at 100 ms. According to the maximum discrimination time limit, the maximum number of sliding times x is calculated to be 91.

[0148] Step5.2: Construct an identification criterion using the mean value of the sum of the squared amplitudes of each column of the S matrix within the maximum detection time limit ΔT, and combine the mean value of the sum of the squared amplitudes of each column of the S matrix of each time window to identify the fault type: If within the (x + 1)th time window, it satisfies X i > Y max , Y max is the setting criterion, then it is determined as a transient fault, otherwise it is determined as a permanent fault. To reserve a certain margin, in the present invention, Y max is taken as 0.5.

[0149] As can be seen from Figure 5 , within the detection time duration ΔT, there is no sudden change in the mean square sum of amplitudes and the time curve within the first time window and the last time window. Therefore, this fault type is judged as a permanent fault, and the circuit breaker should be blocked immediately. In the present invention, the set detection time length is 100 ms after the fault, and the action time of the line protection and the circuit breaker is about 6 ms. Therefore, it is determined whether the fault exists within 94 ms after the circuit breaker is disconnected. So the 86th time window is the last time window.

[0150] Figure 3 The figure is a functional block diagram of an adaptive reclosing system for a flexible DC power grid based on the S transform provided by the present invention, mainly including:

[0151] A fault processing module, used to isolate the faulty line when a fault occurs on the transmission line;

[0152] A numerical acquisition and calculation module, used to obtain the voltage data at the installation location of the protection of the faulty line, perform S transform on the voltage data within each time window according to the set time window, and calculate the mean value X of the sum of the squares of the amplitudes of each column of the transformed modulus matrix i ;

[0153] A fault type identification module, used to construct an identification criterion using the mean value of the sum of the squares of the amplitudes of each column of the modulus matrix to identify the fault type, output a closing signal for an instantaneous fault, and output a blocking signal for a permanent fault;

[0154] A reclosing control module, used to receive the closing or blocking signal sent by the fault type identification module and control the DC circuit breaker to act.

[0155] The fault processing module specifically includes:

[0156] A signal receiving unit, used to receive the tripping signal;

[0157] An action starting unit, used to control the DC circuit breaker to trip.

[0158] The numerical acquisition and calculation module specifically includes:

[0159] A data acquisition unit, used to extract and store the voltage data at both ends of the faulty line;

[0160] A digital-to-analog conversion unit, used to perform S transform on the obtained faulty pole voltage to obtain the S transform modulus matrix;

[0161] A numerical calculation unit, according to Parseval's theorem, calculates the mean value of the sum of the squares of the amplitudes of each column of the S matrix.

[0162] The fault type identification module specifically includes:

[0163] The setting and determination time limit setting unit is used to analyze the obtained faulty pole voltage data and set the maximum detection time limit ΔT. In this example, ΔT = 100 ms;

[0164] The fault type identification unit is used to construct an identification criterion by using the mean value of the sum of the squared amplitudes of each column of the modulus matrix and identify the fault type within the maximum detection time limit;

[0165] The action signal sending unit is used to calculate the fault arc extinction moment when it is identified as a transient fault and send out a closing signal after a fixed deionization; when it is identified as a permanent fault, it sends out a locking signal. In this embodiment, when it is identified as a permanent fault, the circuit breaker should be locked immediately.

[0166] The reclosing control module specifically includes:

[0167] The closing unit is used to receive the closing signal and control the reclosing of the DC circuit breaker;

[0168] The locking unit is used to receive the locking signal and control the locking of the DC circuit breaker.

[0169] Therefore, the method can distinguish between transient faults and permanent faults and immediately lock the DC circuit breaker in case of permanent faults.

[0170] To further highlight the superiority of the proposed adaptive reclosing scheme, the proposed method is objectively compared with the existing methods, and the comparison results are shown in Table 1.

[0171] Table 1 Comparison results of the present invention with other methods

[0172]

[0173] Verification shows that an adaptive reclosing method and system for a flexible DC power grid based on the S transform described in the present invention can accurately identify transient faults, calculate the fault disappearance moment, and have high reliability.

[0174] The specific embodiments of the present invention have been described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the above embodiments, and various changes can be made without departing from the spirit of the present invention within the knowledge scope of those of ordinary skill in the art.

Claims

1. An adaptive reclosing method for a flexible DC power grid based on the S-transform, characterized in that: Step1: When a fault occurs in the transmission line of the flexible DC power grid, the DC circuit breakers at both ends of the line are used to cut off the faulty line; Step2: Use a data acquisition device to collect the voltage data at the measurement points at both ends of the faulty line; Step3: Perform an S-transform on the fault voltage signal within the preset time window length at the measurement point; Step4: Calculate the mean value of the sum of the squares of the amplitudes of each column of the modulus matrix after the S-transform; Step5: Judge the nature of the fault according to the obtained mean value of the sum of the squares of the amplitudes. If it is identified as a permanent fault, lock the DC circuit breaker. If it is identified as a transient fault, execute Step6; Step6: Calculate the arc extinction time of the fault point and reclose the DC circuit breaker.

2. The adaptive reclosing method for a flexible DC power grid based on the S-transform according to claim 1, characterized in that The specific content of Step1 is as follows: After an internal fault occurs in the flexible DC power grid, the DC circuit breakers installed at both ends of the faulty line trip to isolate the fault.

3. A flexible DC grid adaptive reclosing method based on S-transform according to claim 1, characterized in that The specific content of Step2 is as follows: Step2.1: Install high-speed voltage signal acquisition devices at both ends of the transmission line; Step2.2: After a fault occurs on the line, use the high-speed voltage signal acquisition device to continuously sample the voltage of the faulty line.

4. A flexible DC grid adaptive reclosing method based on the S-transform according to claim 1, characterized in that, The specific content of Step3 is as follows: Step3.1: Define each sliding time window length as T and the sliding factor as a; Step3.2: Perform an S-transform on the voltage of the faulty line collected within each time window. The S-transform S(τ, f) of the signal x(t) is defined as: ； ； Among them, is a Gaussian window, is the position parameter of the Gaussian window on the time axis and is the frequency, and j is the imaginary unit; For the N discrete signal points x[i], i = 0, 1,..., N - 1, collected, perform the S-transform using the above formula. The transformation result is a complex time-frequency matrix with n + 1 rows and m columns, denoted as the S matrix. Its columns correspond to the sampling time points and its rows correspond to the frequencies.

5. The adaptive reclosing method for a flexible DC power grid based on the S transform according to claim 1, wherein The specific content of Step4 is as follows: According to Parseval's theorem, calculate the mean value X of the sum of the squared magnitudes of each column of the S matrix i : ； where f k is the frequency corresponding to the n-th row of the S matrix.

6. The adaptive reclosing method for a flexible DC power grid based on the S-transform according to claim 1, characterized in that The specific content of Step5 is as follows: Step5.1: Set the maximum detection time limit ΔT: ΔT = T1 - T0; In the formula, T1 is the fixed time of reclosing and T0 is the insulation recovery time of the arc path; Step 5.2: Within the maximum detection time limit ΔT, construct an identification criterion using the mean of the sum of the squared amplitudes of each column of the S matrix, and combine the mean of the sum of the squared amplitudes of each column of the S matrix for each time window to identify the fault type: If it satisfies X i >Y max , Y max is the setting criterion, then it is judged as an instantaneous fault; otherwise, it is judged as a permanent fault.

7. A flexible DC grid adaptive reclosing method based on S - transform according to claim 1, characterized in that, The specific content of Step6 is as follows: Step 6.1: Calculate the arc extinction time t of the fault point end : t end =t1 + x·a + q / f s ; where t1 is the tripping time of the circuit breaker, x is the number of times the time window slides, q is the mutation point within the (x + 1)-th time window, and f s is the signal sampling frequency; Step6.2: According to the calculated arc extinction time t end , wait for 100 ms for deionization and the outlet closing signal, then the closing time is t out =t end +100, and the DC circuit breakers at both ends of the protection are closed.

8. A flexible DC grid adaptive reclosing system based on S-transform, characterized in that, It includes: A fault processing module for isolating the faulty line when a fault occurs in the transmission line; The numerical acquisition and calculation module is used to obtain the voltage data at the protection installation of the faulty line, perform S-transform on the voltage data within each time window according to the set time window, and calculate the mean value X of the sum of the squares of the amplitudes of each column of the transformed modulus matrix i ; A fault type identification module for constructing an identification criterion using the mean value of the sum of the squares of the amplitudes of each column of the modulus matrix to identify the fault type, outputting a closing signal for a transient fault and a locking signal for a permanent fault; A reclosing control module for receiving the closing or locking signal sent by the fault type identification module and controlling the DC circuit breaker to act.

9. The adaptive reclosing system for a flexible DC power grid based on the S-transform according to claim 8, characterized in that, The specific content of the fault processing module includes: A signal receiving unit for receiving the trip signal; An action starting unit for controlling the DC circuit breaker to trip.

10. A flexible DC grid adaptive reclosing system based on S-transform according to claim 8, characterized in that, The specific content of the numerical acquisition and calculation module includes: A data acquisition unit for extracting and storing the voltage data at both ends of the faulty line; A digital-to-analog conversion unit for performing an S-transform on the obtained faulty pole voltage to obtain the S-transform modulus matrix; A numerical calculation unit for calculating the mean value of the sum of the squares of the amplitudes of each column of the S matrix according to Parseval's theorem.

11. A flexible DC grid adaptive reclosing system based on S-transform according to claim 8, characterized in that, The specific content of the fault type identification module includes: A setting and determination time limit setting unit for analyzing the obtained faulty pole voltage data and setting the maximum detection time limit ΔT; A fault type identification unit, which is used to construct an identification criterion by using the mean value of the sum of the squared amplitudes of each column of the modulus matrix, and identify the fault type within the maximum detection time limit; An action signal sending unit, which is used to calculate the fault arc extinction time when it is identified as a transient fault, and send out a closing signal after a fixed deionization; when it is identified as a permanent fault, it sends out a locking signal.

12. The adaptive reclosing system for a flexible DC power grid based on the S-transform according to claim 8, wherein The reclosing control module specifically includes: A closing unit, which is used to receive a closing signal and control the reclosing of the DC circuit breaker; A locking unit, which is used to receive a locking signal and control the locking of the DC circuit breaker.

Images