A single-terminal protection method and system for borderless flexible DC distribution network

Abstract

The present invention discloses a single-ended protection method and system for a borderless flexible DC distribution network, including: judging whether a DC line has a fault according to the DC voltage at the protection installation location in the DC line; after a fault occurs, detecting the change in the amplitude of the positive and negative voltages, and judging the fault pole according to the polarity of the change in the amplitude of the positive and negative voltages; detecting the current fault component of the fault pole, judging the fault direction according to the polarity of the current fault component of the fault pole and the fault pole judgment result, and the obtained fault direction is a forward fault or a reverse fault; determining the first wave amplitude time ratio of the line mode voltage according to the line mode voltage at the protection installation location, determining the fault section according to the first wave amplitude time ratio of the line mode voltage and the set two-stage fault identification criterion, and obtaining the fault section as an internal fault or an external fault. It does not rely on the line boundary, does not need to install current limiting inductors at both ends of the line, and has strong engineering universality; constructs a two-stage protection to achieve protection of the entire length of the line, and has good speed and selectivity.

Description

Technical Field

[0001] The present invention relates to the technical field of relay protection of flexible direct current distribution networks, and in particular to a single-ended quantity protection method and system for a borderless flexible direct current distribution network. Background Art

[0002] The inherent intermittency of renewable energy increases the complexity of grid access. The flexible DC distribution network based on modular multilevel converters (MMCs) has the advantages of flexible operation, large power supply capacity and high operation reliability, which can solve the problem of renewable energy access to the grid. After a short circuit fault occurs on the DC side of the flexible DC distribution network, the discharge of the MMC submodule capacitor causes the fault current to rise quickly and have a high peak value, requiring a DC circuit breaker to quickly isolate the fault. Therefore, it is necessary to reliably identify and isolate the fault through limited fault information.

[0003] In order to limit the rapid rise of fault current, a current limiting inductor is usually added to the DC side. According to the position of the current limiting inductor, the existing protection technology can be divided into two categories: boundary-based protection and boundaryless protection.

[0004] The current limiting inductor placed at both ends of the DC line can be used as a natural boundary for protection. Many single-ended protections identify faults based on this boundary. Single-ended protection is generally used as the main protection in the DC protection scheme, and has high requirements for the speed of protection, while traveling wave protection can meet this requirement. The existing methods for traveling wave protection are as follows:

[0005] (1) Based on The traveling wave protection method of Bayes algorithm combines the main protection and the backup protection to make the protection more rapid. However, this method relies on the backup protection to improve the ability to resist transition resistance.

[0006] (2) The voltage change rate on the line side of the current-limiting inductor is used to determine the fault. This method is also insensitive to high-resistance faults and has limited ability to resist noise interference.

[0007] (3) A protection method based on the voltage first wave is proposed. With the help of the current-limiting reactance at both ends of the line, the fault location can be determined without direction judgment. This method has strong tolerance to transition resistance. However, the voltage first wave is easily affected by boundary refraction and reflection, and the speed of the traveling wave in the propagation medium needs to be considered.

[0008] (4) Based on the traveling wave characteristics of the positive-sequence voltage at both ends of the current-limiting reactance, a single-ended protection method is proposed. This method has a strong ability to withstand transition resistance, but the harmonics in the DC system have a certain impact on its setting value.

[0009] Therefore, the protection method based on traveling wave generally has the problems of poor tolerance of transition resistance, difficulty in identifying near-end faults, and limited anti-interference ability. In order to solve the problems of traveling wave protection, a single-ended quantity protection method based on frequency domain transient signals is proposed. Specifically:

[0010] (1) The fault line is identified by using the difference in the high frequency band between transient voltage and current after passing through the current limiting reactor, and the fault section and fault type are determined based on the line mode voltage and zero mode voltage of the current limiting reactor. However, there is a problem of complex extraction algorithm.

[0011] (2) A directional traveling wave-based longitudinal protection method is proposed for a hybrid multi-terminal DC system. Although this method has high reliability, it still has the common problems of traveling wave protection, such as high dependence on sampling frequency.

[0012] In summary, the sensitivity of the boundary-based protection method is positively correlated with the value of the current-limiting inductor, but the size of the current-limiting inductor affects the operating stability of the DC system, and its value should not be too high. When the current-limiting inductor is not installed at both ends of the line, single-ended protection will encounter great difficulties. Therefore, the protection method that does not rely on the boundary of the current-limiting inductor is mostly double-ended protection. Specifically:

[0013] (1) The longitudinal protection method based on the additional component of the fault current is more reliable than the current differential protection and the longitudinal protection based on the current direction. However, the reverse flow problem caused by the access of new energy sources can easily affect the reliability of the protection.

[0014] (2) The inductance of the fault circuit is estimated by the least squares method, and a directional longitudinal protection method is proposed. However, the protection method is computationally complex, and the accuracy of its equivalent calculation decreases as the DC line grows.

[0015] (3) Current differential protection and fault location methods with high speed. However, the PI model is only applicable to short lines, and the scope of protection application is limited.

[0016] Since there is no current limiting inductor installed at both ends of the line, the available fault information is reduced, and the single-ended protection that does not rely on boundaries is mostly based on frequency domain information. Specifically:

[0017] (1) The fault area is identified by the difference in high-frequency impedance between the fault inside and outside the area on both sides of the busbar. This method does not require setting the protection action value, but its ability to tolerate transition resistance is weak.

[0018] (2) Based on the frequency domain characteristics of the current traveling wave, a single-ended protection method is proposed. The waveform characteristics of the traveling wave are used to identify the fault direction, and the attenuation rate of the frequency component of the current traveling wave is used to identify the fault area.

[0019] (3) The fault area is identified based on the area ratio of the initial reverse first wave. This method has strong dynamism, but it is easily affected by branch line interference.

[0020] In summary, the inventors found that the busbar of the flexible DC distribution network has multiple outgoing lines and the lines are relatively short. In order to reduce costs, the fault current limiting inductor is generally configured at the converter outlet, which makes the protection method that relies on the line end characteristics unable to adapt. Summary of the invention

[0021] In order to solve the above problems, the present invention proposes a single-ended protection method and system for a borderless flexible DC distribution network, which does not rely on line boundaries, does not require the installation of current-limiting inductors at both ends of the line, and has strong engineering applicability. A two-stage protection is constructed to achieve protection of the entire length of the line, and has good speed and selectivity.

[0022] In order to achieve the above object, the present invention adopts the following technical solution:

[0023] In a first aspect, the present invention provides a single-end quantity protection method for a borderless flexible DC distribution network, comprising:

[0024] Determine whether a fault occurs in the DC line based on the DC voltage at the protection installation location in the DC line;

[0025] After determining that a fault has occurred, detecting the change in the positive and negative voltage amplitudes, and determining the fault pole according to the polarity of the change in the positive and negative voltage amplitudes;

[0026] Detecting the current fault component of the fault pole, judging the fault direction according to the polarity of the current fault component of the fault pole and the fault pole judgment result, and the obtained fault direction is a forward fault or a reverse fault;

[0027] The line mode voltage first wave amplitude time ratio is determined according to the line mode voltage at the protection installation location after the fault, and the fault section is determined according to the line mode voltage first wave amplitude time ratio and the set two-stage fault identification criterion, and the fault section is obtained as an internal fault or an external fault.

[0028] As an optional implementation, the DC line fault judgment process includes: taking the voltage fault component gradient as a starting judgment criterion; if the voltage fault component gradient is greater than the fault threshold, judging that the DC line has a fault;

[0029] The voltage fault component gradient is:

[0030]

[0031] Where △u(ki) is the DC voltage sampling signal of the nth sampling point before the current sampling time of the fault component voltage detected at the protection installation, and △U(k) is the voltage fault component gradient.

[0032] As an optional implementation, the fault pole judgment process includes: if the polarity of the positive pole voltage amplitude change is negative, and the polarity of the negative pole voltage amplitude change is positive, it is a positive pole grounding fault; if the polarity of the positive pole voltage amplitude change is positive, and the polarity of the negative pole voltage amplitude change is negative, it is a negative pole grounding fault; if the polarity of the positive pole voltage amplitude change and the polarity of the negative pole voltage amplitude change are both negative, it is a bipolar short circuit fault.

[0033] As an optional implementation, the fault direction determination process includes: for a positive pole grounding fault or a bipolar short circuit fault, if the polarity of the current fault component of the fault pole is positive, it is a forward fault, otherwise it is a reverse fault; for a negative pole grounding fault, if the polarity of the current fault component of the fault pole is negative, it is a forward fault, otherwise it is a reverse fault.

[0034] As an optional implementation, the two-stage fault identification criterion is: setting the protection stage I threshold value, the protection stage II threshold value and the amplitude threshold value; if the line mode voltage first wave amplitude time ratio is greater than the protection stage I threshold value, an in-zone fault occurs; otherwise, if the line mode voltage first wave amplitude time ratio is greater than the protection stage II threshold value, and after the delay set time period, the voltage fault component amplitude is greater than the amplitude threshold value, an in-zone fault occurs; otherwise, an out-of-zone fault occurs.

[0035] As an optional implementation, the protection I threshold value and the protection II threshold value are respectively and Among them, k I r and k II r are reliability coefficients, G I uset and G II uset are the setting value of protection stage I and protection stage II respectively.

[0036] As an optional implementation method, the setting value of protection section I is taken as the time ratio of the first amplitude of the line mode voltage when a high-resistance fault occurs at the end of the zone; the setting value of protection section II is taken as the time ratio of the first amplitude of the line mode voltage when the fault occurs at 50% of the next-level adjacent line.

[0037] In a second aspect, the present invention provides a single-terminal quantity protection system for a borderless flexible DC distribution network, comprising:

[0038] A fault start judgment module is configured to judge whether a fault occurs in the DC line according to a DC voltage at a protection installation location in the DC line;

[0039] The fault pole identification module is configured to detect the change in the positive and negative voltage amplitudes after determining that a fault has occurred, and determine the fault pole according to the polarity of the change in the positive and negative voltage amplitudes;

[0040] A fault direction identification module is configured to detect the current fault component of the fault pole, and determine the fault direction according to the polarity of the current fault component of the fault pole and the fault pole determination result, and the obtained fault direction is a forward fault or a reverse fault;

[0041] The fault section identification module is configured to determine the line mode voltage first wave amplitude time ratio according to the line mode voltage at the protection installation after the fault, determine the fault section according to the line mode voltage first wave amplitude time ratio and the set two-stage fault identification criterion, and obtain whether the fault section is an internal fault or an external fault.

[0042] In a third aspect, the present invention provides an electronic device comprising a memory and a processor, and computer instructions stored in the memory and executed on the processor, wherein when the computer instructions are executed by the processor, the method described in the first aspect is performed.

[0043] In a fourth aspect, the present invention provides a computer-readable storage medium for storing computer instructions, which, when executed by a processor, completes the method described in the first aspect.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] The present invention proposes a single-ended quantity protection method and system for a borderless flexible DC distribution network, which does not rely on the boundary characteristics of the line ends and does not require the placement of current-limiting reactors at both ends of the line, thus saving economic costs and having strong engineering universality.

[0046] The present invention proposes a single-end protection method and system for a borderless flexible DC distribution network, which constructs a two-stage protection criterion based on the time ratio of the first wave amplitude of the line mode voltage, wherein the stage I protection identifies the proximal fault, and the stage II protection identifies the terminal fault within the area. Both stage protections use local end information to achieve protection of the entire length of the line, and have good speed and selectivity.

[0047] The present invention proposes a single-ended protection method and system for a borderless flexible DC distribution network, which utilizes the first wave amplitude time ratio of the line mode voltage to identify faults, has strong resistance to transition resistance, can identify 40dB noise interference, and is suitable for flexible DC systems with inductive boundaries at both ends of the DC line. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0049] Figure 1 A flow chart of a single-terminal protection method for a borderless flexible DC distribution network provided in Example 1 of the present invention;

[0050] Figure 2 A multi-terminal flexible DC distribution network model provided in Example 1 of the present invention;

[0051] Figure 3 The protection R when F1 fails provided in Embodiment 1 of the present invention 12 Peterson equivalent circuit of line mode fault component;

[0052] Figure 4 The protection R when F3 fails provided in Embodiment 1 of the present invention 21 Peterson equivalent circuit of line mode fault component;

[0053] Figure 5 The protection R when F3 fails provided in Embodiment 1 of the present invention 12 Peterson equivalent circuit of line mode fault component;

[0054] Figure 6 The protection R when F2 fails provided in Embodiment 1 of the present invention 12 Peterson equivalent circuit of line mode fault component;

[0055] Figure 7 The protection R when F1 fails provided in Embodiment 1 of the present invention 21 Peterson equivalent circuit of line mode fault component;

[0056] Figure 8 The protection R when F2 fails provided in Embodiment 1 of the present invention 21 Peterson equivalent circuit of line mode fault component;

[0057] Figure 9(a)-Figure 9(c) When a single-pole grounding fault occurs at point F1 provided in Embodiment 1 of the present invention, R 12 The waveforms of the positive and negative voltages, current fault components and line mode voltage at the location;

[0058] Figure 10(a)-Figure 10(c) When a single-pole grounding fault occurs at point F3 provided in Embodiment 1 of the present invention, R 12 The waveforms of the positive and negative voltages, current fault components and line mode voltage at the location;

[0059] Figure 11(a)-Figure 11(c) When a single-pole grounding fault occurs at point F4 provided in Embodiment 1 of the present invention, R 12 The waveforms of the positive and negative voltages, current fault components and line mode voltage at the location;

[0060] Fig.12 When a single-pole grounding fault occurs at point F2 provided in Embodiment 1 of the present invention, R 12 The current fault component at ;

[0061] Figure 13(a)-Figure 13(c)When a bipolar short circuit fault occurs at point F1 provided in Embodiment 1 of the present invention, R 12 The waveforms of the positive and negative voltages, current fault components and line mode voltage at the location;

[0062] Figure 14(a)-Figure 14(c) When a bipolar short circuit fault occurs at point F3 provided in Embodiment 1 of the present invention, R 12 The waveforms of the positive and negative voltages, current fault components and line mode voltage at the location;

[0063] Figure 15(a)-Figure 15(c) When a bipolar short circuit fault occurs at point F4 provided in Embodiment 1 of the present invention, R 12 The waveforms of the positive and negative voltages, current fault components and line mode voltage at the location;

[0064] Fig.16 When a bipolar short circuit fault occurs at point F2 provided in Embodiment 1 of the present invention, R 12 The current fault component at ;

[0065] Fig.17 The circuit provided in Example 1 of the present invention includes an inductor boundary condition R 12 Line mode voltage waveform at . DETAILED DESCRIPTION

[0066] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0067] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0068] It should be noted that the terms used herein are only for describing specific embodiments, and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that the terms "include" and "have" and any of their variations are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device comprising a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0069] In the absence of conflict, the embodiments of the present invention and the features of the embodiments may be combined with each other.

[0070] Example 1

[0071] like Figure 1As shown, this embodiment provides a single-end quantity protection method for a borderless flexible DC distribution network, including:

[0072] Determine whether a fault occurs in the DC line based on the DC voltage at the protection installation location in the DC line;

[0073] After determining that a fault has occurred, detecting the change in the positive and negative voltage amplitudes, and determining the fault pole according to the polarity of the change in the positive and negative voltage amplitudes;

[0074] Detecting the current fault component of the fault pole, judging the fault direction according to the polarity of the current fault component of the fault pole and the fault pole judgment result, and the obtained fault direction is a forward fault or a reverse fault;

[0075] The line mode voltage first wave amplitude time ratio is determined according to the line mode voltage at the protection installation location after the fault, and the fault section is determined according to the line mode voltage first wave amplitude time ratio and the set two-stage fault identification criterion, and the fault section is obtained as an internal fault or an external fault.

[0076] In this embodiment, the DC voltage signal and current signal of the node where the DC line protection is installed are collected in real time; according to the characteristic of the rapid rise of the voltage fault component, the gradient of the voltage fault component is used as the starting criterion to judge whether the voltage fault component meets the protection starting condition, and if so, the DC line fails;

[0077] Specifically: when the voltage fault component gradient value is greater than the fault threshold, it is determined that the DC line has a fault and the protection is activated;

[0078] Among them, the voltage fault component gradient is:

[0079]

[0080] Wherein, △u(ki) is the sampling signal of the fault component voltage detected at the protection installation at the nth sampling point before the current sampling time, and △U(k) represents the voltage fault component gradient;

[0081] In this embodiment, the fault threshold is set to 0.01 times the rated voltage; the specific criteria are as follows:

[0082] ΔU(k)>0.01U dc (2)

[0083] Where U dc is the rated voltage.

[0084] Since most flexible DC distribution networks are connected in a symmetrical unipolar manner, when a unipolar grounding fault occurs, the voltage value of the faulty pole drops to zero, and the voltage amplitude of the non-faulty pole increases to twice the original value; when a bipolar short circuit fault occurs, the voltage values ​​of both poles drop to zero; therefore, this embodiment determines the faulty pole based on the change in the voltage amplitude of the positive and negative poles after the fault.

[0085] In this embodiment, the change in the positive and negative voltage amplitudes is assumed to be:

[0086]

[0087] Among them, the sampling interval between N and N+1 is △T, S P , S N is the change in the positive and negative voltage amplitude, U P , U N is the positive and negative voltage.

[0088] Therefore, when the polarity of the positive and negative voltage amplitude changes is opposite, a single-pole grounding fault occurs, and the faulty pole is the pole whose voltage amplitude change is less than zero; otherwise, it is a bipolar short circuit fault, that is:

[0089]

[0090] Specifically, if the polarity of the change in the positive voltage amplitude is negative, and the polarity of the change in the negative voltage amplitude is positive, the fault is a positive ground fault;

[0091] If the polarity of the positive pole voltage amplitude change is positive, and the polarity of the negative pole voltage amplitude change is negative, the fault is a negative pole grounding fault;

[0092] If the polarities of the positive voltage amplitude change and the negative voltage amplitude change are both negative, the fault is a bipolar short circuit fault.

[0093] In this embodiment, the principle of judging the fault direction based on the polarity of the fault component of the fault pole current is as follows:

[0094] The positive direction of the DC line is defined as the direction in which the busbar flows toward the DC line;

[0095] For a positive grounding fault, when a positive direction fault occurs, the polarity of the current fault component of the fault pole is positive; when a reverse fault occurs, the polarity of the current fault component of the fault pole is negative;

[0096] For a negative pole grounding fault, when a positive direction fault occurs, the polarity of the current fault component of the fault pole is negative; when a reverse direction fault occurs, the polarity of the current fault component of the fault pole is positive;

[0097] For a bipolar short-circuit fault, the polarity of the current fault component of the faulted pole is the same as that of a single-pole grounding fault;

[0098] Therefore, the fault direction can be determined by using the polarity of the current fault component of the fault pole.

[0099] Specifically, for a positive pole grounding fault or a bipolar short circuit fault, if the polarity of the current fault component of the fault pole is positive, it is a forward fault, otherwise it is a reverse fault;

[0100] For a negative pole grounding fault, if the polarity of the current fault component of the fault pole is negative, it is a forward fault, otherwise it is a reverse fault.

[0101] Furthermore, in order to improve reliability, the fault direction is determined by using the current fault component of the fault pole within a period of time after the fault, that is:

[0102]

[0103] Where N represents the number of data samples, and the number of samples in the data window is taken; △i FP (k) is the fault component of the current of the fault pole at sampling point k.

[0104] When the positive pole is the fault pole, λ FP When it is greater than 0, it is a forward fault, otherwise it is a reverse fault; when the negative pole is the fault pole, λ FP When it is less than 0, it is a forward fault, otherwise it is a reverse fault.

[0105] Since both poles of a bipolar short circuit fault are faulty poles, detecting any pole can determine the fault direction. In this embodiment, the fault direction of a bipolar short circuit fault is determined by detecting the current fault component of the positive pole.

[0106] In this embodiment, the fault identification criterion principle based on the first wave amplitude time ratio of the line mode voltage is as follows:

[0107] In view of the distributed parameter characteristics of the line, when a fault occurs in the line, a traveling wave will be generated at the fault point to propagate to both ends of the line, and will be refracted and reflected at the impedance discontinuity point. This embodiment focuses on analyzing the initial traveling wave characteristics at the protection point. Considering the mutual impedance and mutual admittance between the two poles of the DC line, it is very difficult to directly calculate the voltage after the fault, and the voltage needs to be converted into a phase mode, as shown in formula (6):

[0108]

[0109] Where, u0 and u1 are zero-mode voltage and line-mode voltage, u p 、u n is the positive and negative voltage.

[0110] Since the zero-mode component is greatly affected by frequency, decays faster than the line-mode component, and has poor stability, this embodiment selects line-mode voltage analysis.

[0111] The line mode voltage fault components under bipolar short circuit fault and unipolar ground fault are calculated by symmetrical component method as shown in formula (7):

[0112]

[0113] In the formula, △u f1_PPF and △u f1_SPGF are the line-mode voltage fault components of bipolar short-circuit fault and unipolar grounding fault respectively; V p is the rated voltage; Z c0 and Z c1 are zero mode wave impedance and line mode wave impedance respectively; R g is the transition resistor.

[0114] In this embodiment, considering that there are different converters (MMC, DC / DC or DC / AC converter) on the back side of the protection in the flexible DC distribution network, the traveling waves are analyzed in two categories.

[0115] A. Protect the back side with MMC; Figure 2 As shown, to protect R 12 The fault characteristics of the forward zone, the external fault and the reverse fault are analyzed.

[0116] (1) Fault characteristics within the area

[0117] like Figure 2 As shown, when a fault occurs at the fault point F1, the protection R 12 After the fault occurs, the voltage wave moves from the fault point F1 to R 12 Transmission, its Peterson equivalent circuit in the complex frequency domain is as follows Figure 3 As shown, where Z c1 is the wave impedance of the line, △u f1 is the line mode voltage fault component, L dc is the current limiting inductor, L eq and C eq are the equivalent inductance and equivalent capacitance of MMC.

[0118] When the attenuation and dispersion of the traveling wave are not considered, Figure 3 The initial voltage traveling wave is obtained at R 12 The complex frequency domain expression is:

[0119]

[0120] Using Vieta's theorem, we can get:

[0121]

[0122] Among them, L sum =L eq +L dc .

[0123] For a 10kV overhead line, its wave impedance Z c1Generally, it is several hundred ohms, which is on the order of 10 2 For the converter MMC, its equivalent inductance L eq Generally, it is tens of millihenries, that is, on the order of 10 -2 , equivalent capacitance C eq Generally, it is a few millifarads, which is on the order of 10 -3 ; and the line inductance L dc Generally a few millihenries, i.e., on the order of 10 -3 The parameter values ​​of overhead lines and converters are shown in Table 1 and Table 2, from which we can conclude:

[0124]

[0125] Due to the attenuation and dispersion of traveling waves on the transmission line, -r1(s)x The transfer function representing the line mode traveling wave can be equivalent to:

[0126]

[0127] Where x is the propagation distance of the traveling wave, τ a1 and k a1 are the dispersion time constant and attenuation coefficient under unit distance, e -x / v is the time delay of the traveling wave when it propagates a distance x.

[0128] Considering that this embodiment mainly studies the fault characteristics of the first traveling wave, the delay of the traveling wave on the line has no effect on the fault characteristics, and the delay of the traveling wave can be ignored. 12 The line mode voltage at can be transformed into:

[0129]

[0130] Perform an inverse Laplace transform on the above equation and take the derivative to obtain the initial line-mode voltage traveling wave at R 12 The line-mode voltage amplitude-to-time ratio at is:

[0131]

[0132] (2) Fault characteristics outside the forward zone

[0133] like Figure 2 As shown, the fault at F3 is caused by the protection R 12 Forward fault outside the zone, due to the traveling wave on line L 23 The distance between the upper and lower parts is short, and the distortion can be ignored. After the fault, the voltage traveling wave first propagates to R 21 Then through the DC line L 12 The attenuation and dispersion of the 12 , so R is calculated in two steps 12 The voltage at .

[0134] Since the high voltage side of the DC-DC converter can be equivalent to a large capacitor, for protecting R 21 When a fault occurs at point F3, the Peterson equivalent circuit in the complex frequency domain is as follows: Figure 4 As shown, C DC / DC is the equivalent capacitance of the DC / DC converter. Therefore, R 21 The line mode voltage at is:

[0135]

[0136] Performing the inverse Laplace transform on the above equation yields:

[0137]

[0138] In the formula,

[0139]

[0140] When the traveling wave is protected from R 21 Propagate to the protection R 12 When it undergoes a distortion process, its Peterson equivalent circuit is as follows Figure 5 shown.

[0141] The analysis of the traveling wave during the fault in the reference area, taking into account the distortion of the traveling wave during transmission, R 12 The line mode voltage at can be approximated as:

[0142]

[0143] Performing the inverse Laplace transform and taking the derivative of the above equation, we can get R 12 The line mode voltage amplitude to time ratio is:

[0144]

[0145] Take the ratio of equation (12) and equation (17):

[0146]

[0147] Since the equivalent capacitance of a DC / DC converter is generally several thousand microfarads, that is, 10 -3 Order of magnitude, according to the parameters in Table 1 and Table 2, substituting into formula (19) we can get It is obviously greater than 1. Therefore, the time ratio of the first wave amplitude of the line mode voltage during the positive out-of-zone fault is much smaller than the time ratio of the first wave amplitude of the line mode voltage during the in-zone fault.

[0148] (3) Fault characteristics outside the reverse zone

[0149] When F2 fails, the fault location is at R 12 At the busbar outside the reverse region, the Peterson equivalent circuit is as follows Figure 6 shown.

[0150] From this we can get that the traveling wave reaches R 12 The line mode voltage at is:

[0151]

[0152] By performing an inverse Laplace transform and taking the derivative of the above equation, we can obtain the line-mode voltage amplitude-to-time ratio:

[0153]

[0154] Take the ratio of equation (13) and equation (21):

[0155]

[0156] By inserting the parameter value, we can get is significantly less than 1, which is due to the out-of-zone fault occurring at R 12 The rear busbar, while the fault in the area occurs at a distance R 12 At 10 km from the zone, the time ratio of the first wave amplitude of the line-mode voltage when a reverse bus fault occurs outside the zone is greater than the time ratio of the first wave amplitude of the line-mode voltage at the fault inside the zone.

[0157] B. The back side of the protection is a DC / DC converter. In the flexible DC distribution network, the load is connected through a DC / DC converter and a DC / AC converter. The DC side can be equivalent to a large capacitor. Therefore, the DC / DC converter is taken as an example for analysis. 21 The proposed method is used as the object to analyze the characteristics of the faults inside and outside the forward zone and the reverse faults, and is also applicable to DC / AC converters.

[0158] (1) Fault characteristics within the area

[0159] Depend on Figure 2 It can be seen that R 21 The back side is the DC / DC converter. When the DC line L 12 When an internal fault occurs, the Peterson equivalent circuit is as follows: Figure 7 As shown. Therefore, R 21 The line mode voltage at can be approximated as:

[0160]

[0161] Perform inverse Laplace transformation on the above equation and take the derivative to obtain the fault time R in the area: 21 The time ratio of the first wave amplitude of the line mode voltage at is:

[0162]

[0163] (2) Fault outside the forward zone

[0164] like Figure 2 As shown in Figure 2, when F2 fails, the fault point is located at the protection R 21 Outside the positive region, R 21 The line-mode voltage at can be analyzed in two steps.

[0165] After the fault, the traveling wave moves from the fault point to the protection R 12 Propagation, the equivalent circuit of this process is the same Figure 6 , so R in the complex frequency domain 12 The line mode voltage at is also the same as that in equation (20).

[0166] The traveling wave passes through the protection R 12 After that, in the DC line L 12 Spread, reach protection R 21 , its Peterson equivalent circuit is as follows Figure 8 As shown, considering the distortion of traveling wave in DC line, the protection R 21 The line mode voltage at can be approximated as:

[0167]

[0168] By performing an inverse Laplace transform on the above equation and taking the derivative, we can obtain the time ratio of the first wave amplitude of the line mode voltage in the time domain:

[0169]

[0170] By taking the ratio of equations (24) and (26), we can obtain:

[0171]

[0172] By inserting the parameters, we can get It is obviously greater than 1, so the time ratio of the first wave amplitude of the line-mode voltage during the intra-zone fault is greater than the time ratio of the first wave amplitude of the line-mode voltage during the positive out-of-zone fault.

[0173] (3) Fault outside the reverse zone

[0174] To protect R 21 When a reverse fault F3 occurs, the traveling wave reaches R 21 The voltage expression at is the same as equation (10), from which the time ratio of the first wave amplitude of the line mode voltage can be obtained as:

[0175]

[0176] By taking the ratio of equations (24) and (28), we can obtain:

[0177]

[0178] Substituting the parameter values, we can get It is about 1, that is, the time ratio of the first wave amplitude of the line mode voltage of the fault in the zone is not significantly greater than that of the first wave amplitude of the line mode voltage of the fault outside the reverse zone. This is because the fault outside the reverse zone is too close to the protection, and the traveling wave does not attenuate and disperse.

[0179] In summary, no matter whether the back side of the protection is MMC or DC-DC converter, when the protection has a forward fault, as the fault distance increases, the line mode voltage first wave amplitude time ratio at the protection gradually decreases, and the line mode voltage first wave amplitude time ratio of the forward out-of-zone fault is significantly smaller than the amplitude time ratio of the in-zone fault, so this difference can be used to identify the forward in-zone and out-of-zone faults. For the reverse proximal fault of the protection, the line mode voltage first wave amplitude time ratio is not much different from that of the in-zone fault, so the line mode voltage first wave amplitude time ratio difference cannot be directly used to identify the in-zone and out-of-zone faults.

[0180] Considering that under weak boundary, the fault near the outside of the zone may cause false operation of the protection, this embodiment designs a two-stage fault identification criterion, and adopts the method of mutual cooperation between protection stage I and stage II to identify the fault inside and outside the zone;

[0181] Specifically: if the time ratio of the first wave amplitude of the line mode voltage is greater than the threshold value of protection section I, an intra-zone fault occurs; otherwise, if the time ratio of the first wave amplitude of the line mode voltage is greater than the threshold value of protection section II, and after the delay setting time period, the amplitude of the voltage fault component is greater than the amplitude threshold value, an intra-zone fault occurs; otherwise, an extra-zone fault occurs.

[0182] (1) Protection stage I:

[0183] Since the time ratio of the first amplitude of the line-mode voltage when a positive out-of-zone fault occurs is smaller than the time ratio of the first amplitude of the line-mode voltage when a fault occurs within the zone, the fault section can be determined based on this. The fault identification criterion for protection section I is that if the time ratio of the first amplitude of the line-mode voltage is greater than the threshold value of protection section I, an internal fault occurs:

[0184]

[0185] Where G is the time ratio of the first wave amplitude of the line mode voltage; k I r is the reliability coefficient, G I uset It is the setting value of protection section I.

[0186] From formula (7), we can know that the line mode voltage fault component △u f1 It is inversely proportional to the transition resistance. At the same time, from the above analysis, it can be seen that the line mode voltage amplitude time ratio is directly proportional to the line mode voltage fault component. Therefore, the line mode voltage amplitude time ratio of the protection is inversely proportional to the transition resistance. The transition resistance affects the sensitivity of the judgment criterion. Here, the protection I section setting value G I usettTake the time ratio of the first wave amplitude of the voltage when a high-resistance fault occurs at the end of the zone. Since the maximum transition resistance in the medium-voltage flexible DC distribution network is about 100Ω, the setting value is the calculated value of the time ratio of the first wave amplitude of the line-mode voltage fault component when the transition resistance is 100Ω. Since the protection section I is under-measurement protection, the influence of transformer error and parameter measurement error is considered at the same time, and the reliability coefficient k I r Take 1.2.

[0187] (2) Protection Section II:

[0188] When the fault occurs at the end of the zone or near the end outside the zone, the above criteria cannot identify such faults, and protection section II needs to cooperate in identification; that is:

[0189]

[0190] In the formula, k II r is the reliability coefficient, G II uset It is the setting value of protection stage II.

[0191] Protection stage II setting value G II uset Take the time ratio of the first wave amplitude of the line mode voltage when the fault occurs in 50% of the adjacent lines at the next level, and the reliability coefficient k II r Take 1.1.

[0192] The protection range of protection section II extends to the next level adjacent line. When a fault occurs in the next level line, it is necessary to ensure that the protection section I of the next level line will give priority to clearing the fault to meet the protection selectivity requirements. Therefore, it is necessary to delay protection section II with the next level adjacent line protection section I to achieve protection selectivity. The action delay of protection section II should be one time level greater than the action time of the adjacent line protection. Considering the data window time, circuit breaker action time and other times that affect the protection execution, the delay t of protection section II is II set Set to 15ms.

[0193] Considering that after adding a 15ms delay to the protection stage II, the first wave criterion will not be able to judge the fault state, this embodiment adds a supplementary criterion to judge whether the lower-level circuit breaker is actuated. After the fault, the voltage fault component amplitude will rise to the rated voltage. If the lower-level line clears the fault, the voltage fault component amplitude will return to 0. Therefore, the additional criterion is set as:

[0194] |Δu|>0.9U N (32)

[0195] Where △u is the voltage fault component, U Nis the rated voltage.

[0196] Therefore, when the first wave amplitude ratio of the line mode voltage after the fault meets the protection criterion of protection stage II, and if it satisfies formula (32) after a time delay, it means that the lower-level line circuit breaker refuses to operate, causing an internal fault, and the line circuit breaker at this level trips to prevent the fault range from expanding, otherwise it returns reliably.

[0197] In this embodiment, a flexible DC distribution network simulation model is constructed using PSCAD to simulate and verify the proposed method:

[0198] (1) Build Figure 2 The ring flexible DC distribution network model shown in the figure. Considering the cost factor, the current limiting inductor is usually configured at the MMC outlet. It mainly includes MMC converter, DC / DC converter, DC line and load. The rated voltage on the DC side is ±10kV. The main wiring method of the line adopts symmetrical single-pole wiring. In order to maintain the symmetrical operation of the positive and negative poles of the DC bus voltage, the MMC converter adopts a small current grounding method of DC side clamping resistor grounding. The topology adopts a single-point voltage control method based on master-slave control, in which MMC1 adopts fixed DC voltage and fixed reactive power control as the voltage balance node of the entire system, and MMC2 and MMC3 adopt fixed active power control and reactive power control as power nodes. The specific parameters of the MMC converter are shown in Table 1. The DC distribution line adopts the overhead line model, and its parameters are shown in Table 2. The line is divided into 8 sections, and DC circuit breakers are equipped at both ends to quickly isolate faults. R ij It is line protection.

[0199] Table 1MMC converter simulation model parameters

[0200]

[0201] Table 2 DC overhead line parameters

[0202]

[0203] use Figure 2 The multi-terminal flexible DC distribution network model shown in the figure selects the DC line L 12 Protection R 12 and R 21 The feasibility of the proposed protection scheme is verified by setting various fault conditions. Considering that the DC distribution network line is short and the traveling wave transmission is fast, the sampling frequency of the protection is selected as 100kHz. According to equations (13) and (24), the setting value of the protection can be obtained. 12 For example, when a single-pole short circuit occurs, the protection R 12 and R 21 The I-stage setting value G I usetThey are 0.068kV / us and 0.047kV / us respectively. The setting value of protection stage II is G II uset 0.043kV / us and 0.018kV / us respectively; when a bipolar short circuit fault occurs, the protection R 12 and R 21 The setting value of the I segment G I uset They are 0.14kV / us and 0.11kV / us respectively. The setting value of protection stage II is G II uset They are 0.12kV / us and 0.08kV / us respectively.

[0204] (2) Typical internal and external fault simulation

[0205] A. Single-pole grounding fault. Considering the symmetry of the positive and negative poles of the system, the positive pole metallic grounding fault is used as an example to set the internal and external faults. Figure 2 As shown, the fault in the area is set on line L 12 The midpoint F1 of the zone is set at the far end fault in the zone at F4, the forward zone fault is set at F3, and the reverse zone fault is set at F2. 12 The waveform at Figure 9(a)-Figure 9(c) , Figure 10(a)-Figure 10(c) and Figure 11(a)-Figure 11(c) As shown in the figure, the fault occurrence time is abbreviated as 0s, and the actual occurrence time is 3s.

[0206] It can be seen that when the fault occurs at F1, F3 and F4, the change in fault current is positive, so these faults can be judged as forward faults, and when the fault occurs at F2, the change in fault current is negative, so the fault can be judged as a reverse fault.

[0207] It can also be seen that the change in the positive voltage amplitude is less than zero, while the change in the negative voltage amplitude is greater than 0, so it can be determined that the fault is a positive grounding fault. 12 When the fault is within 80% of the line L, the protection stage I can identify the fault and operate; 12 When the fault is at F4 outside the positive zone, the protection stage II criterion is met. At the same time, after waiting for 15m, the fault component voltage meets the additional criterion, so the protection judges that the fault is an internal fault and operates. When the fault is located at F3 outside the positive zone, the line mode voltage first wave amplitude time ratio does not meet the protection stage I and II criterion, so the protection does not operate.

[0208] For the reverse fault F2, the current fault waveform is as follows: Fig.12 As shown in the figure, since the polarity of the fault component of the current after the fault is negative, it can be judged that the fault is a reverse fault. 21The simulation results at are shown in Table 3, and it can be seen that the protection scheme can correctly identify the fault. Therefore, the proposed protection scheme can correctly identify the single-pole grounding fault inside and outside the zone, and the introduced protection method will not malfunction.

[0209] Table 3 R when single-pole ground fault 21 Simulation results

[0210]

[0211] B. Bipolar short circuit fault. Assume that bipolar short circuit fault occurs at F1, F2, F3 and F4. 12 The simulation waveforms at Figure 13(a)-Figure 13(c) , Figure 14(a)-Figure 14(c) and Figure 15(a)-Figure 15(c) As shown in the figure, it can be seen that in the case of a forward fault, the polarity of the current fault component is positive, and the direction criterion can correctly determine the fault direction; the positive and negative voltages are both less than zero, satisfying the bipolar short-circuit fault criterion; when a fault occurs at the midpoint F1 in the zone, the protection stage I can identify the fault and operate. For the remote fault F4 in the zone, the protection stage II and the supplementary criterion identify the fault and operate. For the near-end fault F3 outside the zone, the first wave amplitude time ratio of the line mode voltage does not meet the R 12 In the judgment criteria of the protection sections I and II, the protection judges the fault as a forward out-of-zone fault.

[0212] For the reverse fault F2 outside the zone, the current fault component waveform is as follows: Fig.16 As shown in the figure, the polarity of the fault component of the current after the fault is negative, so it can be judged that the fault is a reverse fault. 21 The simulation results at are shown in Table 4, and it can be seen that the protection scheme can correctly identify the fault. Therefore, the proposed protection scheme can correctly identify the bipolar short-circuit fault inside and outside the zone, and the proposed protection method will not malfunction.

[0213] Table 4 Bipolar short circuit fault R 21 Simulation results

[0214]

[0215] C. Analysis of protection action results for line faults at different locations and types. In order to further verify the effectiveness of the designed protection scheme, more fault points are set at different locations on the DC line. The fault points are located at line L 23 and L 18 The fault types include positive grounding fault and bipolar short circuit fault. According to the action of each protection under different positions and different fault types, the corresponding statistical results are shown in Table 5. The percentage in Table 5 represents the ratio of the distance from the fault point to the adjacent busbar on the left to the total length of the line. √ indicates that the protection is activated, and × indicates that the protection is not activated. The judgment results shown in Table 5 show that no matter the fault occurs on line L 23 and L18 ,Whether the fault type is a positive grounding fault or a bipolar short circuit fault, the proposed protection method can correctly identify it.

[0216] Table 5 Protection action under different fault locations and fault types

[0217]

[0218] (3) Protection performance analysis

[0219] A. Analysis of Transition Resistance Capability

[0220] In order to verify the ability of the proposed protection scheme to resist transition resistance, a simulation analysis is performed when the transition resistance is large. The negative grounding fault is set to occur at F1, F3 and F4. When the transition resistance at the fault point is 20Ω, 50Ω and 100Ω respectively, R 12 The simulation data and discrimination results at are shown in Table 6. As the transition resistance increases, the amplitude-to-time ratio of the line mode voltage gradually decreases, but the amplitude-to-time ratio of the fault F1 in the zone is always greater than the setting value of the protection stage I; and the fault at the end of the zone F4 can still be identified by the protection stage II when the fault resistance is as high as 100Ω. It can be seen that the protection has a strong ability to resist transition resistance.

[0221] Table 6 Simulation results of single-pole grounding fault with different transition resistances

[0222]

[0223] B. Simulation of anti-noise interference capability

[0224] In order to verify the anti-noise interference ability of the proposed protection scheme, Gaussian white noise is added to the fault. The fault type is set to a bipolar short circuit fault, the fault is located at F1-F4, and the noise is 40dB and 70dB respectively. The simulation results are shown in Table 7. It can be seen that under 40dB noise, the protection can still accurately identify the fault. Therefore, the noise has little effect on the proposed protection scheme.

[0225] Table 7 Simulation results of bipolar short circuit fault under different noise interference

[0226]

[0227] C. Applicability simulation with inductive boundaries

[0228] In order to verify whether the proposed protection method will be affected by the current limiting inductor boundary at both ends of the line, Figure 2 A current limiting inductor is added at both ends of the DC line. The current limiting inductor value is 8mH, and other parameters remain unchanged. When a bipolar short circuit occurs at F1, F2, and F3, the protection R 12 The line mode fault component voltage at Fig.17 shown.

[0229] Comparing the simulation results without inductor boundaries, it can be seen that after adding the current limiting inductor, the voltage first wave amplitude time ratio amplitude at the time of the fault outside the zone is significantly smaller than the voltage first wave amplitude time ratio amplitude at the time of the fault inside the zone, and the voltage first wave amplitude time ratio amplitude of the reverse fault outside the zone is significantly smaller than the voltage first wave amplitude time ratio amplitude of the fault inside the zone, and the fault characteristics are obvious. Therefore, the proposed protection method is still applicable after adding the current limiting inductor at both ends of the line, and under the action of the boundary, the voltage first wave amplitude time ratio can be used to directly judge the fault inside and outside the zone without the direction criterion, simplifying the protection scheme.

[0230] Considering the topology of the DC distribution network and the economic cost of the current-limiting reactance, the current-limiting reactance in the flexible DC distribution network is generally placed at the converter outlet, resulting in the single-ended quantity protection that relies on the boundary characteristics of the line end being unable to adapt. Based on this, this embodiment proposes a single-ended quantity protection scheme suitable for a flexible DC distribution network without boundaries. By analyzing the amplitude-time ratio of the first wave of the line-mode voltage when the DC line is faulted inside and outside the fault zone, a direction criterion based on the current fault component and a fault type criterion based on the voltage conversion quantity are constructed. The amplitude-time ratio of the first wave of the line-mode voltage is used to construct a two-stage fault identification criterion and form a protection scheme. There is no need to install current-limiting inductors at both ends of the line, and the engineering universality is strong. At the same time, a two-stage protection is constructed, in which the I-stage protection identifies the proximal fault and the II-stage protection identifies the end fault in the zone, so as to realize the protection of the entire length of the line, and has good speed and selectivity. In addition, it has strong anti-transition resistance capability, can identify 40dB noise interference, and can be applied to flexible DC systems with inductive boundaries at both ends of the DC line.

[0231] Example 2

[0232] This embodiment provides a single-terminal protection system for a borderless flexible DC distribution network, including:

[0233] A fault start judgment module is configured to judge whether a fault occurs in the DC line according to a DC voltage at a protection installation location in the DC line;

[0234] The fault pole identification module is configured to detect the change in the positive and negative voltage amplitudes after determining that a fault has occurred, and determine the fault pole according to the polarity of the change in the positive and negative voltage amplitudes;

[0235] A fault direction identification module is configured to detect the current fault component of the fault pole, and determine the fault direction according to the polarity of the current fault component of the fault pole and the fault pole determination result, and the obtained fault direction is a forward fault or a reverse fault;

[0236] The fault section identification module is configured to determine the line mode voltage first wave amplitude time ratio according to the line mode voltage at the protection installation after the fault, determine the fault section according to the line mode voltage first wave amplitude time ratio and the set two-stage fault identification criterion, and obtain whether the fault section is an internal fault or an external fault.

[0237] It should be noted that the above modules correspond to the steps described in Example 1, and the examples and application scenarios implemented by the above modules and the corresponding steps are the same, but are not limited to the contents disclosed in the above Example 1. It should be noted that the above modules, as part of the system, can be executed in a computer system such as a set of computer executable instructions.

[0238] In further embodiments, there is also provided:

[0239] An electronic device includes a memory and a processor, and computer instructions stored in the memory and executed on the processor, wherein when the computer instructions are executed by the processor, the method described in Embodiment 1 is performed. For the sake of brevity, it will not be described in detail here.

[0240] It should be understood that in this embodiment, the processor may be a central processing unit CPU, and the processor may also be other general-purpose processors, digital signal processors DSP, application-specific integrated circuits ASIC, off-the-shelf programmable gate arrays FPGA or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor may be a microprocessor or the processor may also be any conventional processor, etc.

[0241] The memory may include a read-only memory and a random access memory, and provide instructions and data to the processor. A portion of the memory may also include a non-volatile random access memory. For example, the memory may also store information about the device type.

[0242] A computer-readable storage medium is used to store computer instructions, and when the computer instructions are executed by a processor, the method described in Example 1 is completed.

[0243] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. A single-terminal quantity protection method for a borderless flexible DC distribution network, characterized in that: include: Determine whether a fault occurs in the DC line based on the DC voltage at the protection installation location in the DC line; After determining that a fault has occurred, detecting the change in the positive and negative voltage amplitudes, and determining the fault pole according to the polarity of the change in the positive and negative voltage amplitudes; Detecting the current fault component of the fault pole, judging the fault direction according to the polarity of the current fault component of the fault pole and the fault pole judgment result, and the obtained fault direction is a forward fault or a reverse fault; Determine the time ratio of the first amplitude of the line mode voltage according to the line mode voltage at the protection installation after the fault, determine the fault section according to the time ratio of the first amplitude of the line mode voltage and the set two-stage fault identification criterion, and obtain whether the fault section is an internal fault or an external fault; wherein, the time ratio of the first amplitude of the line mode voltage is obtained by performing an inverse Laplace transform and deriving the line mode voltage fault component at the protection installation in the complex frequency domain; The two-stage fault identification criteria are: setting the protection stage I threshold value, protection stage II threshold value and amplitude threshold value; if the line mode voltage first wave amplitude time ratio is greater than the protection stage I threshold value, an in-zone fault occurs; otherwise, if the line mode voltage first wave amplitude time ratio is greater than the protection stage II threshold value, and after the delay setting time period, the voltage fault component amplitude is greater than the amplitude threshold value, an in-zone fault occurs; otherwise, an out-of-zone fault occurs.

2. A single-ended quantity protection method for a borderless flexible DC distribution network as claimed in claim 1, characterized in that: The DC line fault judgment process includes: taking the voltage fault component gradient as a starting judgment criterion; if the voltage fault component gradient is greater than the fault threshold, it is judged that the DC line has a fault; The voltage fault component gradient is: In the formula, △ u ( k - n ) is the DC voltage sampling signal of the nth sampling point before the current sampling moment of the fault component voltage detected at the protection installation, △ U ( k ) is the voltage fault component gradient.

3. A single-ended quantity protection method for a borderless flexible DC distribution network as claimed in claim 1, characterized in that: The fault pole judgment process includes: if the polarity of the positive pole voltage amplitude change is negative, and the polarity of the negative pole voltage amplitude change is positive, it is a positive pole grounding fault; if the polarity of the positive pole voltage amplitude change is positive, and the polarity of the negative pole voltage amplitude change is negative, it is a negative pole grounding fault; if the polarity of the positive pole voltage amplitude change and the polarity of the negative pole voltage amplitude change are both negative, it is a bipolar short circuit fault.

4. A single-ended quantity protection method for a borderless flexible DC distribution network as claimed in claim 1, characterized in that: The fault direction determination process includes: for a positive pole grounding fault or a bipolar short circuit fault, if the polarity of the current fault component of the fault pole is positive, it is a forward fault, otherwise it is a reverse fault; for a negative pole grounding fault, if the polarity of the current fault component of the fault pole is negative, it is a forward fault, otherwise it is a reverse fault.

5. A single-ended quantity protection method for a borderless flexible DC distribution network as claimed in claim 1, characterized in that: The protection thresholds for stage I and stage II are and ; in, k I r and k II r are reliability coefficients, G I uset and G II uset are the setting value of protection stage I and protection stage II respectively.

6. A single-ended quantity protection method for a borderless flexible DC distribution network as claimed in claim 5, characterized in that: The setting value of protection stage I is the time ratio of the first amplitude of the line mode voltage when a high-resistance fault occurs at the end of the zone; the setting value of protection stage II is the time ratio of the first amplitude of the line mode voltage when the fault occurs at 50% of the next-level adjacent line.

7. A single-terminal protection system for a borderless flexible DC distribution network, characterized in that: include: A fault start judgment module is configured to judge whether a fault occurs in the DC line according to a DC voltage at a protection installation location in the DC line; The fault pole identification module is configured to detect the change in the positive and negative voltage amplitudes after determining that a fault has occurred, and determine the fault pole according to the polarity of the change in the positive and negative voltage amplitudes; A fault direction identification module is configured to detect the current fault component of the fault pole, and determine the fault direction according to the polarity of the current fault component of the fault pole and the fault pole determination result, and the obtained fault direction is a forward fault or a reverse fault; The fault section identification module is configured to determine the time ratio of the first wave amplitude of the line mode voltage according to the line mode voltage at the protection installation after the fault, determine the fault section according to the time ratio of the first wave amplitude of the line mode voltage and the set two-stage fault identification criterion, and obtain whether the fault section is an internal fault or an external fault; wherein the time ratio of the first wave amplitude of the line mode voltage is obtained by performing an inverse Laplace transform and deriving the line mode voltage fault component at the protection installation in the complex frequency domain; The two-stage fault identification criteria are: setting the protection stage I threshold value, protection stage II threshold value and amplitude threshold value; if the line mode voltage first wave amplitude time ratio is greater than the protection stage I threshold value, an in-zone fault occurs; otherwise, if the line mode voltage first wave amplitude time ratio is greater than the protection stage II threshold value, and after the delay setting time period, the voltage fault component amplitude is greater than the amplitude threshold value, an in-zone fault occurs; otherwise, an out-of-zone fault occurs.

8. An electronic device, characterized in that: The method comprises a memory and a processor and computer instructions stored in the memory and executed on the processor, wherein when the computer instructions are executed by the processor, the method according to any one of claims 1 to 6 is completed.

9. A computer-readable storage medium, characterized in that: Used to store computer instructions, which, when executed by a processor, complete the method described in any one of claims 1 to 6.

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