Single-terminal protection method and system for multi-terminal DC system based on transient current variance

Abstract

The present invention discloses a single-ended protection method and system for a multi-terminal DC system based on transient current variance. For a multi-terminal DC system, the sampling window of the protection method is determined according to the system parameters based on the principle that the sensitivity of the protection method is as high as possible and the transient variance current error is as small as possible; the protection threshold is set according to the system parameters and the sampling window length based on ensuring selectivity; the high-frequency energy of the current signal is extracted based on the transient variance. If it is greater than the protection threshold, it is judged as an in-zone fault and the protection action is executed; if it is less than the threshold, the protection action is not executed. This method does not rely on simulation, and uses simpler calculations to extract the high-frequency component of the electric quantity. The analytical solution of the protection threshold can be theoretically calculated, which ensures the selectivity of the protection method, has strong resistance to external noise, and has high sensitivity and good resistance to transition resistance.

Description

Technical Field

[0001] The present invention belongs to the field of electric power technology, and in particular relates to a single-ended quantity protection method and system for a multi-terminal DC system based on transient current variance. Background Art

[0002] In recent years, multi-terminal DC systems with offshore wind power access have attracted widespread attention. In response to DC line faults, two-level voltage source converters (TLVSCs) and half-bridge modular multilevel converters (HBMMCs) lack the ability to regulate and control, resulting in rapid locking of the insulated gate bipolar transistors (IGBTs) of the converters, and the power transmission of the entire DC system is forced to be interrupted. With the development of power electronic DC circuit breakers (DCCBs), DC faults can be removed within a few milliseconds after the fault occurs, avoiding converter locking. However, fast and accurate detection and isolation of faulty lines after DC faults is one of the great challenges in protecting multi-terminal DC systems.

[0003] There have been many studies on the fault characteristics and protection methods of current in DC power grids, but these studies have some limitations, as follows:

[0004] 1. Lack of analytical expression of fault current applicable to multi-terminal DC grids;

[0005] 2. The protection method requires a high sampling rate and a large amount of calculation;

[0006] 3. Calculate the protection threshold based on complex numerical simulation;

[0007] 4. The protection method is easily affected by external noise and has poor fault tolerance. Summary of the invention

[0008] The technical problem to be solved by the present invention is to provide a single-ended quantity protection method and system for a multi-terminal DC system based on transient current variance in response to the deficiencies in the above-mentioned prior art. The high-frequency component of the electrical quantity is extracted by simple calculation, and an analytical solution of the protection threshold can be calculated, thereby ensuring the selectivity, sensitivity and speed of the protection method, and having good transient resistance resistance.

[0009] The present invention adopts the following technical solutions:

[0010] A single-ended quantity protection method for a multi-terminal DC system based on transient current variance, with the highest sensitivity and the smallest transient variance current error as the principle, determines the sampling window according to the multi-terminal DC system parameters; based on ensuring selectivity, sets the protection threshold according to the system parameters and the length of the sampling window; extracts the high-frequency energy of the current signal based on the transient variance, if it is greater than the protection threshold, it is judged as an in-zone fault and the protection action is executed, if it is less than the protection threshold, no protection action is executed.

[0011] Specifically, the sensitivity K is defined by the ratio of the transient current variance of line mn to the protection threshold. sen as follows:

[0012]

[0013] Among them, D in is the transient variance of the fault current during the capacitor discharge phase, D set is the protection threshold, K rel is the reliability factor of protection, C n Indicates T n The capacitance of terminal VSC to ground, T represents the calculation time of transient current variance, L mn is the equivalent inductance of the DC transmission line mn, L Tmn , L Tnm and L Tnq They respectively represent the DC compensation reactors close to the m side on the DC line mn, close to the n side on the DC line mn, and close to the n side on the DC line nq.

[0014] Furthermore, the frequency domain expression and time domain expression of the fault component current of line mn are:

[0015]

[0016]

[0017] Among them, I m0 and I mn are the fault component currents of the line when metal faults occur inside and outside the area, respectively; U0 is the voltage at the fault point during normal operation; t is the reference time with the fault moment as time 0; L Tmn , L Tnm and L Tnq They represent the DC compensation reactors on the DC line mn near the m side, the DC line mn near the n side, and the DC line nq near the n side, respectively. mn is the equivalent inductance of the DC transmission line mn, C n Indicates T n The capacitance of terminal VSC to ground.

[0018] Specifically, the ratio of the transient current variance is close to 1, specifically:

[0019]

[0020] in, are the transient variances of the DC line using the distributed parameter model and the lumped parameter model, L0 is the equivalent inductance of the transmission line per unit length, l is the length of the DC transmission line, and Z l is the wave impedance of the transmission line, L Tmn is the DC compensation reactor close to the m side of the DC line mn, and T represents the calculation time of the transient current variance.

[0021] Specifically, the protection threshold after setting is D set for:

[0022] D set =K rel ·D out

[0023] Among them, K rel is the reliability factor of protection, D out is the transient variance of the fault component current of the protected line.

[0024] Furthermore, the corresponding transient current variance D out for:

[0025]

[0026] Where, T represents the calculation time of transient current variance, I mn (t) is the function of the fault component current of the line with respect to time when there is a metallic fault outside the area, L mn is the equivalent inductance of the DC transmission line mn, L Tmn , L Tnm and L Tnq They respectively represent the DC compensation reactors close to the m side on the DC line mn, close to the n side on the DC line mn, and close to the n side on the DC line nq, and U0 is the voltage at the fault point during normal operation.

[0027] Furthermore, the calculation time T of the transient current variance is equal to 2.

[0028] Specifically, the conditions for executing the protection action are as follows:

[0029] D(I(t))>D set

[0030] Where D(I(t)) is the transient variance of the DC line current, D set is the protection threshold.

[0031] Furthermore, the transient variance D(I(t)) of the DC line current is:

[0032]

[0033] Wherein, T is the calculation time of transient current variance, I(τ) is the actual DC line current, t is the reference time with the fault moment as time 0, and τ is the integral variable.

[0034] Another technical solution of the present invention is a single-ended quantity protection system for a multi-terminal DC system based on transient current variance, comprising:

[0035] The sampling module determines the sampling window according to the multi-terminal DC system parameters based on the principle of maximum sensitivity and minimum transient variance current error;

[0036] The setting module sets the protection threshold according to the system parameters and the length of the sampling window to ensure selectivity;

[0037] The protection module extracts the high-frequency energy of the current signal based on the transient variance. If it is greater than the protection threshold, it is judged as an in-zone fault and the protection action is executed. If it is less than the protection threshold, no protection action is executed.

[0038] Compared with the prior art, the present invention has at least the following beneficial effects:

[0039] The present invention discloses a single-ended protection method for a multi-terminal DC system based on transient current variance. By analyzing the fault of the multi-terminal DC system, an analytical expression of the fault current is obtained, and theoretical support is provided for the selection of a circuit breaker, the design of a protection method, and the setting of a protection threshold. The communication problem of the double-ended protection method is avoided, and the fault line can be quickly identified, which is beneficial to improving the quickness of protection. The single-ended protection method for a multi-terminal DC system based on transient current variance proposes an indicator of transient current variance to describe the characteristics of DC fault current, avoids relying on complex numerical simulation to determine the protection threshold, simplifies the design of the protection method, and has high sensitivity and accuracy, thereby realizing rapid detection and isolation of the fault line.

[0040] Furthermore, using the transient current variance D of line mn in and protection threshold D set The ratio of defines the sensitivity, and through appropriate sensitivity calibration, it is ensured that this protection method has sufficient response capability when a fault occurs.

[0041] Furthermore, by solving the frequency domain expression and time domain expression of the fault component current of line mn, it is convenient to quantitatively analyze the influence of line parameters on the fault current, which provides theoretical guidance for the design of protection schemes and the setting of thresholds.

[0042] Furthermore, by defining the transient current variance ratio of different models close to 1, the error of using the lumped parameter model is reduced, making the protection scheme more accurate.

[0043] Furthermore, by setting a suitable protection threshold D set , ensuring the selectivity of this protection method. The introduction of the reliability coefficient takes into account the fact that the actual transient current variance may be greater than the calculated value, the actual action value of the protection device may be less than the set value, and certain margin factors.

[0044] Furthermore, the threshold is determined according to the most serious out-of-zone fault, thereby avoiding malfunction of the out-of-zone fault protection.

[0045] Furthermore, when the appropriate time window length is selected, the proposed protection method has higher sensitivity and higher accuracy.

[0046] Furthermore, when the transient current variance of the DC line is greater than the threshold, the protection should act quickly to ensure that the protection of the DC line will never act under normal circumstances, thereby ensuring the reliability of the protection.

[0047] Furthermore, by extracting the high-frequency part of the fault current through the transient current variance, the fault line can be quickly identified and the complex time-frequency domain transformation is avoided.

[0048] In summary, the protection method of the present invention has the advantages of threshold setting not relying on simulation, low sampling rate, strong resistance to external noise, and resistance to transition resistance.

[0049] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is a flow chart of the present invention;

[0051] Figure 2 It is a protection circuit diagram including internal faults and external faults;

[0052] Figure 3 Schematic diagram of the four-terminal DC system under test;

[0053] Figure 4 are simulation diagrams, where (a) is simulation result one, (b) is simulation result two, (c) is simulation result three, (d) is simulation result four, (e) is simulation result five, and (f) is simulation result six. DETAILED DESCRIPTION

[0054] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0055] In the description of the present invention, it should be understood that the terms “include” and “comprises” indicate the presence of described features, wholes, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, wholes, steps, operations, elements, components and / or collections thereof.

[0056] It should also be understood that the terms used in the present specification are only for the purpose of describing specific embodiments and are not intended to limit the present invention. As used in the present specification and the appended claims, unless the context clearly indicates otherwise, the singular forms "a", "an" and "the" are intended to include plural forms.

[0057] It should be further understood that the term "and / or" used in the present description and the appended claims refers to any and all possible combinations of one or more of the associated listed items, and includes these combinations.

[0058] Various structural schematic diagrams of the embodiments disclosed in the present invention are shown in the accompanying drawings. These figures are not drawn to scale, and some details are magnified and some details may be omitted for the purpose of clear expression. The shapes of various regions and layers shown in the figures and the relative sizes and positional relationships therebetween are only exemplary, and may deviate in practice due to manufacturing tolerances or technical limitations, and those skilled in the art may additionally design regions / layers with different shapes, sizes, and relative positions according to actual needs.

[0059] The present invention provides a single-ended quantity protection method for a multi-terminal DC system based on transient current variance. The transient current variance is defined to describe the fault characteristics of a DC line. A suitable sampling window length can be calculated based on sensitivity analysis and error analysis of different models of transmission lines. A method for extracting high-frequency signals based on transient current variance is proposed, and an analytical solution for a protection threshold is calculated. If the protection setting threshold is exceeded, the line is judged to be a faulty line and the protection is initiated.

[0060] See also Figure 1 The present invention provides a single-ended protection method for a multi-terminal DC system based on transient current variance, comprising the following steps:

[0061] S1. For multi-terminal DC systems, the sampling window length of the protection method is determined according to the system parameters;

[0062] See also Figure 2 , is the protected circuit including both internal and external faults. Wherein, line mn is any transmission line in the multi-terminal DC system, and its adjacent lines are pm and nq. For the convenience of calculation, the transmission line adopts the classic RL model. Wherein, R mn and L mn are the resistance and inductance of line mn respectively, R m0 and L m0 are the resistance and inductance of the DC line from the fault point to the m side, C m T m The capacitance of the VSC to ground. In order to limit the rising rate of the DC fault current and suppress the DC ripple current, a compensating reactor is usually installed between the transmission line and the busbar. The compensating reactor near the m side on the line mn is L Tmn For line mn, if line mn fails, it is an internal fault, and other line faults are external faults. If and only if line mn fails, the fault at T m and T n The protective equipment at the end should reliably perform the protection action;

[0063] Since VSC has poor regulation capability and is easily damaged by DC fault current, fault detection must be completed before IGBT lockout. Therefore, the short-circuit current calculation in the capacitor discharge stage a few milliseconds after the fault is the research object of this invention. In the capacitor discharge stage, the fault component current increases linearly in the fault line and increases cubically in the non-fault line. The frequency domain expression and time domain expression of the fault component current of line mn are:

[0064]

[0065]

[0066] Among them, I m0 and I mn are the fault component currents of the line when metal faults occur inside and outside the area, respectively; U0 is the voltage at the fault point during normal operation; t is the reference time with the fault moment as time 0; L Tmn , L Tnm and L Tnq They represent the DC compensation reactors on the DC line mn near the m side, the DC line mn near the n side, and the DC line nq near the n side, respectively. mn is the equivalent inductance of the DC transmission line mn, C n Indicates T n The capacitance of terminal VSC to ground;

[0067] Since the high-frequency part of the DC fault current drops sharply in a normal line, the transient high-frequency component can be used to identify the fault line. Let S be an arbitrary time series consisting of {x(0), x(1), …, x(N-1)}, and the mean value of S is:

[0068]

[0069] The transient variance of S is:

[0070]

[0071] The discrete Fourier transform can be used to realize the transformation of time domain and frequency domain:

[0072]

[0073] Where X(k) is the Fourier transform of the signal x(n), and X'(k) is the discrete Fourier series of the signal;

[0074] According to Parseval's theorem, the integral of the square of a function is equal to the integral of the square of its Fourier transform:

[0075]

[0076] According to formula (4), D(S) is expanded to:

[0077]

[0078] Combining equations (4) and (5), the transient variance of S can be written as:

[0079]

[0080] According to formula (6), the transient variance of S is related to its high-frequency component, so the transient high-frequency energy of S can be calculated without time-frequency domain transformation. Assume that the duration of signal S is T, and the frequency Δf of X(k) is:

[0081]

[0082] Among them, f s is the sampling frequency. Combining equation (6) and equation (7), the frequency calculated by equation (6) includes all frequencies higher than Δf;

[0083] In order to facilitate real-time calculation, when S is a continuous function of time, the transient variance during the sampling window T is:

[0084]

[0085] For the sensitivity of the multi-terminal DC system, the calculation is done for the slightest fault in the protected line area, that is, the metallic fault occurs at the far end of the protected line mn. At this time, the fault component current of the protected line is:

[0086]

[0087] During the capacitor discharge stage, the transient variance of the fault current is:

[0088]

[0089] Wherein, T represents the calculation time of transient current variance;

[0090] Determine the sampling window length of the protection method based on system parameters, and make the protection sensitivity as large as possible:

[0091]

[0092] Among them, D in is the fault current variance during the capacitor discharge phase, D set is the protection threshold, K rel is the reliability factor of protection, C n Indicates T n The capacitance of terminal VSC to ground, T represents the calculation time of transient current variance, L mn is the equivalent inductance of the DC transmission line mn, L Tmn , L Tnm and L Tnq They respectively represent the DC compensation reactors close to the m side on the DC line mn, close to the n side on the DC line mn, and close to the n side on the DC line nq.

[0093] In a multi-terminal DC system, the DC side capacitance of the VSC is usually several hundred μF, and the compensation reactor is several hundred mH. In order to make the protection method have a higher sensitivity, according to the system parameters, when the sensitivity K sen When it is greater than 20, it can be obtained that the sampling window length T should be less than 4ms;

[0094] When selecting an appropriate sampling window length for a protection scheme, it is necessary not only to consider the sensitivity constraint, but also the impact of different models used in the transmission line. The distributed parameter models of the transmission line include the Berelon model line and the frequency-varying parameter line model, which are generally used to describe the transient process of the fault. The proposed protection scheme is based on the lumped parameter model, which ignores the influence of the traveling wave process in the transmission line. Therefore, in the application of this scheme, the errors of the lumped parameter model and the distributed parameter model of the line should be considered. After a fault occurs in a DC transmission line, a fault traveling wave is generated at the fault point and propagates along the line, and the fault traveling wave exhibits step signal characteristics. According to the Peterson law, the first traveling wave surge I on the m side of the transmission line mn after the fault m0,1 :

[0095]

[0096] Among them, L0 and C0 are the inductance and capacitance of the line per unit length, respectively, and Z l is the wave impedance of the transmission line, and τ is the time constant:

[0097]

[0098] When L Tmn =100mH, τ is 0.2ms. Assuming that a metallic fault occurs at the end of the protected line, the first wave surge of the current represented by equation (12) has been completed. The corresponding transient variance is:

[0099]

[0100] The sampling window length T should be greater than the time of the next injection of the current traveling wave at the starting point of the longest transmission line:

[0101]

[0102] Where v is the speed of the traveling wave, which is equal to (L0C0) -1 / 2 Substitute equation (15) into equation (14):

[0103]

[0104] For the lumped parameter model, the transient current variance of the fault line is obtained based on equation (10):

[0105]

[0106] The sampling window length of the protection method is determined according to the system parameters. It is necessary to make the error of the analysis and calculation using the lumped parameter line model as small as possible, even if the transient variance ratio when using the distributed parameter model and the lumped parameter model is close to 1:

[0107]

[0108] in, are the transient variances of the line model using the distributed parameter model and the lumped parameter model, L0 is the equivalent inductance of the unit length DC transmission line, l is the length of the DC line, Z l is the wave impedance of the transmission line.

[0109] According to formula (18), if the sampling window length T increases, the transient variance ratio of the distributed parameter model and the lumped parameter model will decrease. For any multi-terminal DC system, given L Tmn and Z lIn the case of , the size of T can be determined when the transient variance ratio of different models is set to 1. In this protection scheme, in order to ensure the accuracy of the proposed scheme when the line uses a lumped parameter model, it is recommended that T equals 2, and the ratio is 0.9665.

[0110] S2. Based on ensuring selectivity, the protection threshold is set according to system parameters and sampling window length;

[0111] In order to avoid misoperation of protection equipment during out-of-zone faults, the most serious out-of-zone fault should be considered, that is, a metal fault at the beginning of the lower line. According to formula (20), the fault component current of the protection line is:

[0112]

[0113] The corresponding transient current variance is:

[0114]

[0115] The specific values ​​of the protection threshold are:

[0116]

[0117] Among them, I mn is the fault component current of the protection line when a metal fault occurs at the outlet of the lower line of the protection line, and the corresponding transient variance is expressed as D out Indicates that U0 is the voltage at the fault point during normal operation, K rel is the reliability factor of protection, D set is the protection threshold.

[0118] S3. Determine whether the calculated transient current variance exceeds the protection threshold. If D(I(t))>D set , it is determined that the line has an in-area fault, and a protection action is executed, a tripping command is issued, the DC circuit breaker trips, and the faulty line is cut off. If not, return to step S2.

[0119] The conditions for executing the protection are:

[0120]

[0121] D(I(t))>D set

[0122] Where S(t) is a specific signal that changes with time, x(τ) is a continuous function of the signal S(t), T represents the calculation time of the transient current variance, and D set is the protection threshold.

[0123] In another embodiment of the present invention, a multi-terminal DC system single-ended quantity protection system based on transient current variance is provided. The system can be used to implement the above-mentioned multi-terminal DC system single-ended quantity protection method based on transient current variance. Specifically, the multi-terminal DC system single-ended quantity protection system based on transient current variance includes a sampling module, a setting module and a protection module.

[0124] Among them, the sampling module determines the sampling window according to the multi-terminal DC system parameters based on the principle of maximum sensitivity and minimum transient variance current error;

[0125] The setting module sets the protection threshold according to the system parameters and the length of the sampling window to ensure selectivity;

[0126] The protection module extracts the high-frequency energy of the current signal based on the transient variance. If it is greater than the protection threshold, it is judged as an in-zone fault and the protection action is executed. If it is less than the protection threshold, no protection action is executed.

[0127] In another embodiment of the present invention, a terminal device is provided, the terminal device includes a processor and a memory, the memory is used to store a computer program, the computer program includes program instructions, and the processor is used to execute the program instructions stored in the computer storage medium. The processor may be a central processing unit (CPU), or other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), field-programmable gate arrays (FPGA) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components, etc. It is the computing core and control core of the terminal, which is suitable for implementing one or more instructions, and is specifically suitable for loading and executing one or more instructions to implement the corresponding method flow or corresponding function; the processor described in the embodiment of the present invention can be used for the operation of a single-ended quantity protection method for a multi-terminal DC system based on transient current variance, including:

[0128] The sampling window is determined according to the multi-terminal DC system parameters based on the principle of maximum sensitivity and minimum transient variance current error. The protection threshold is set according to the system parameters and the length of the sampling window based on ensuring selectivity. The high-frequency energy of the current signal is extracted based on the transient variance. If it is greater than the protection threshold, it is judged as an in-zone fault and the protection action is executed. If it is less than the protection threshold, the protection action is not executed.

[0129] In another embodiment of the present invention, the present invention further provides a storage medium, specifically a computer-readable storage medium (Memory), which is a memory device in a terminal device for storing programs and data. It can be understood that the computer-readable storage medium here can include both the built-in storage medium in the terminal device and the extended storage medium supported by the terminal device. The computer-readable storage medium provides a storage space, which stores the operating system of the terminal. In addition, one or more instructions suitable for being loaded and executed by the processor are also stored in the storage space, and these instructions can be one or more computer programs (including program codes). It should be noted that the computer-readable storage medium here can be a high-speed RAM memory, or a non-volatile memory (non-volatile memory), such as at least one disk storage.

[0130] The processor may load and execute one or more instructions stored in a computer-readable storage medium to implement the corresponding steps of the multi-terminal DC system single-terminal quantity protection method based on transient current variance in the above embodiment; the processor may load and execute the following steps of one or more instructions in the computer-readable storage medium:

[0131] The sampling window is determined according to the multi-terminal DC system parameters based on the principle of maximum sensitivity and minimum transient variance current error. The protection threshold is set according to the system parameters and the length of the sampling window based on ensuring selectivity. The high-frequency energy of the current signal is extracted based on the transient variance. If it is greater than the protection threshold, it is judged as an in-zone fault and the protection action is executed. If it is less than the protection threshold, the protection action is not executed.

[0132] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. The components of the embodiments of the present invention described and shown in the drawings here can usually be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0133] See also Figure 3 , is a schematic diagram of the four-terminal DC system under test, and the naming method of electrical component parameters is the same as Figure 2Similarly, the converter stations at T1, T3 and T4 adopt constant power control, and the converter station at T2 adopts constant voltage control. The fault location and the size of the transition resistance are changed respectively to verify the effectiveness and transition resistance tolerance of the protection method provided by the present invention.

[0134] See also Figure 4 , Figure 4 (a) shows the simulation results and calculation results of the fault component current of different models when a metallic fault occurs at the end of the DC line 12. It can be seen that the fault currents obtained by the RL model and the proposed model are almost the same, and have a similar growth rate to the frequency-variable model. This shows that the proposed model is accurate in analyzing the multi-terminal DC system based on the VSC converter.

[0135] Figure 4 (b) is the transient current variance of the fault line when a metallic fault occurs at the end of the DC line 12. It can be seen that the protection can be started correctly when the threshold is exceeded about 1ms after the fault, which verifies the quickness of the protection scheme.

[0136] Figure 4 (c) and Figure 4 (d) are the fault component current and transient current variance of line 24 when a metallic fault occurs at the end of DC line 12. It can be seen that the fault component current waveforms of the non-fault line of the three models are similar, and the transient variances of the three models do not exceed the threshold within 3ms after the fault, which verifies the selectivity of the proposed scheme.

[0137] Figure 4 (e) and Figure 4 (f) are the fault component currents and their transient variances at different fault distances when the transition resistance is 200Ω. Due to the traveling wave effect, the transient current variance of the fault line of the frequency-variable model oscillates around the transient current variance of the RL model. It can be seen that the transient variances of the RL model and the proposed model are not much different, which shows the accuracy of the proposed protection scheme.

[0138] The simulation results verify that the protection method proposed in the present invention has good selectivity and good transition resistance resistance.

[0139] In summary, the present invention provides a single-ended protection method and system for a multi-terminal DC system based on transient current variance. For a multi-terminal DC system, the transient current variance is defined to describe the DC fault current characteristics, and the analytical solution of the protection threshold is calculated to achieve protection threshold setting and ensure the selectivity of the protection scheme. This method does not rely on simulation, but uses simpler calculations to extract high-frequency components of electrical quantity, has strong resistance to external noise, and has high sensitivity and good resistance to transient resistance.

[0140] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the present application may adopt the form of a computer program product implemented in one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that include computer-usable program code.

[0141] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0142] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0143] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0144] The above contents are only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A single-ended protection method for a multi-terminal DC system based on transient current variance, characterized in that: The sampling window is determined according to the multi-terminal DC system parameters based on the principle of maximum sensitivity and minimum transient variance current error; the protection threshold is set according to the system parameters and the length of the sampling window based on ensuring selectivity; the high-frequency energy of the current signal is extracted based on the transient variance. If it is greater than the protection threshold, it is judged as an in-zone fault and the protection action is executed; if it is less than the protection threshold, the protection action is not executed; Through the line mn The sensitivity is defined by the ratio of the transient current variance to the protection threshold as follows: in, D in is the transient variance of the fault current during the capacitor discharge phase, D set is the protection threshold, K rel is the reliability factor of the protection, C n Indicates T n The capacitance of terminal VSC to ground, T Indicates the calculation time of transient current variance, L mn For DC transmission lines mn The equivalent inductance, L Tmn , L Tnm and L Tnq Respectively represent DC lines mn Close to m Side, DC line mn Close to n side, and DC lines nq Close to n DC compensation reactor on the side; line mn The frequency domain expression and time domain expression of the fault component current are: in, I m0 and I mn are the fault component currents of the line when metal faults occur inside and outside the area, respectively. U 0 is the voltage at the fault point during normal operation, t is the reference time with the fault moment as time 0, L Tmn , L Tnm and L Tnq They represent the DC line mn near the m side and the DC line mn Close to n Side and DC lines nq Close to n The DC compensation reactor on the side, L mn is the equivalent inductance of the DC transmission line mn, C n Indicates T n The capacitance of terminal VSC to ground; The ratio of the transient current variance is close to 1, specifically: in, D , D are the transient variances of the DC line using the distributed parameter model and the lumped parameter model, L 0 is the equivalent inductance per unit length of the transmission line, l is the length of the DC transmission line, Z l is the wave impedance of the transmission line, L Tmn For DC lines mn Close to m The DC compensation reactor on the side, T Indicates the calculation time of transient current variance; Protection threshold after setting for: in, K rel is the reliability factor of protection, is the transient variance of the fault component current of the protected line; The corresponding transient current variance for: in, T Indicates the calculation time of transient current variance, is the time function of the fault component current of the line when there is a metal fault outside the zone, L mn For DC transmission lines mn The equivalent inductance of L Tmn , L Tnm and L Tnq Respectively represent DC lines mn Close to m Side, DC line mn Close to n side, and DC lines nq Close to n The DC compensation reactor on the side, is the voltage at the fault point during normal operation; Calculation duration of transient current variance T is equal to 2; The conditions for executing protection actions are as follows: in, is the transient variance of the DC line current, D set is the protection threshold, for: in, is the calculation time of transient current variance, is the actual DC line current, is the reference time with the fault moment as time 0, τ is the integration variable.

2. A single-ended protection system for a multi-terminal DC system based on transient current variance, characterized in that: The method according to claim 1 comprises: The sampling module determines the sampling window according to the multi-terminal DC system parameters based on the principle of maximum sensitivity and minimum transient variance current error; The setting module sets the protection threshold according to the system parameters and the length of the sampling window to ensure selectivity; The protection module extracts the high-frequency energy of the current signal based on the transient variance. If it is greater than the protection threshold, it is judged as an in-zone fault and the protection action is executed. If it is less than the protection threshold, no protection action is executed.

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