Fault protection method for direct-current flexible power distribution network and related device

Abstract

The embodiment of the invention relates to the field of circuit protection, and provides a DC flexible power distribution network fault protection method and a related device, and the method comprises the steps: carrying out the detection of the current of a line through a protection device disposed at the head end of the line, and obtaining a first current curve; if the break variable of the first current curve is greater than a first preset current threshold value, tripping off a direct-current circuit breaker on the line; if the break variable of the first current curve is between a second preset current threshold value and the first preset current threshold value, the energy peak-to-average ratio is obtained according to the instantaneous energy of the current signal when the first current curve suddenly changes, and the first preset current threshold value is larger than the second preset current threshold value; determining fault type information according to the energy peak-to-jun ratio; fault protection is carried out according to the fault type information, fault protection can be carried out according to the peak-to-average ratio calculated according to the current curve obtained at the head end of the line, and the reliability during fault protection is improved.

Description

Technical Field

[0001] The present application relates to the field of circuit protection technology, and in particular to a DC flexible distribution network fault protection method and related devices. Background Art

[0002] When the flexible DC distribution network fails, the converter itself will lock quickly, and there is less fault information available. Therefore, the system protection needs to complete fault detection and action in a very short time. Considering the protection action time, single-ended protection is often used in engineering. This method does not require communication between the two ends and can act quickly. However, most of the existing protection methods are seriously affected by transition resistance, resulting in a small protection range. For high-resistance faults at the far end of the area, the protection may not act. There is still a lot of room for improvement.

[0003] Flexible DC distribution network is a relatively new form of distribution network, and its fault protection method has not been well developed and improved. For the protection of more serious grounding faults in this form of distribution network, the existing methods are difficult to meet the requirements of instantaneous fast action of DC distribution network protection, and there are also problems such as single-end protection is difficult to protect the entire length of the line, is seriously affected by transition resistance, and is easily affected by noise interference. Summary of the invention

[0004] The embodiments of the present application provide a DC flexible distribution network fault protection method and related devices, which can perform fault protection based on the peak-to-peak ratio calculated according to the current curve obtained at the head end of the line, thereby improving the reliability of fault protection.

[0005] A first aspect of an embodiment of the present application provides a DC flexible distribution network fault protection method, the method comprising:

[0006] The current of the line is detected by a protection device installed at the head end of the line to obtain a first current curve;

[0007] If the mutation amount of the first current curve is greater than a first preset current threshold, tripping the DC circuit breaker on the line;

[0008] If the mutation amount of the first current curve is between the second preset current threshold and the first preset current threshold, the energy peak-to-peak ratio is obtained according to the instantaneous energy of the current signal when the first current curve undergoes a mutation, and the first preset current threshold is greater than the second preset current threshold;

[0009] Determining fault type information according to the energy peak-to-peak ratio;

[0010] Fault protection is performed according to the fault type information.

[0011] In this example, the current of the line is detected by a protection device installed at the head end of the line to obtain a first current curve. If the mutation amount of the first current curve is greater than a first preset current threshold, the DC circuit breaker on the line is tripped. If the mutation amount of the first current curve is between a second preset current threshold and the first preset current threshold, the energy peak-to-peak ratio is obtained based on the instantaneous energy of the current signal when the first current curve mutates, the first preset current threshold is greater than the second preset current threshold, the fault type information is determined based on the energy peak-to-peak ratio, and fault protection is performed based on the fault type information. Therefore, fault protection can be performed in combination with the current mutation amount and the energy peak-to-peak ratio, thereby improving the reliability of fault protection.

[0012] In a possible implementation, obtaining the energy peak-to-peak ratio according to the instantaneous energy of the current signal when the first current curve suddenly changes includes:

[0013] Determine the instantaneous energy value at the time of mutation according to the amplitude of the current signal;

[0014] The energy peak-to-peak ratio is calculated based on the instantaneous energy value and the average energy value of the current signal.

[0015] In a possible implementation, determining the fault type information according to the energy peak-to-peak ratio includes:

[0016] If the energy peak-to-peak ratio is greater than the preset energy peak-to-peak ratio threshold, the fault type information is an out-of-zone high-resistance fault;

[0017] If the energy peak-to-peak ratio is less than a preset energy peak-to-peak ratio threshold, the fault type information is an intra-zone low-resistance fault.

[0018] In a possible implementation, the method further includes:

[0019] Obtaining a first preset current threshold;

[0020] The obtaining of the first preset current threshold comprises:

[0021] The first preset current threshold is obtained by the method shown in the following formula:

[0022]

[0023] Among them, λ max is the first preset current threshold, U dc is the DC voltage when the line is in normal operation, L eq is the equivalent reactance of the entire line, L dc is the reactance value of the current limiting reactor, K rel1 is the reliability coefficient, and its value range is 1.1-1.3.

[0024] In a possible implementation, the method further includes:

[0025] Obtaining a second preset current threshold;

[0026] The obtaining of the second preset current threshold comprises:

[0027] The second preset current threshold is obtained by the method shown in the following formula:

[0028]

[0029] Among them, λ min is the second preset current threshold, I 0 It is the load current when the line is operating normally.

[0030] A second aspect of an embodiment of the present application provides a DC flexible distribution network fault protection device, the device comprising:

[0031] A detection unit, used to detect the current of the line through a protection device installed at the head end of the line to obtain a first current curve;

[0032] a judgment unit, configured to trip a DC circuit breaker on the line if the mutation amount of the first current curve is greater than a first preset current threshold; and to obtain an energy peak-to-peak ratio according to the instantaneous energy of the current signal when the first current curve undergoes a mutation if the mutation amount of the first current curve is between a second preset current threshold and the first preset current threshold, and the first preset current threshold is greater than the second preset current threshold;

[0033] A determination unit, configured to determine fault type information according to the energy peak-to-peak ratio;

[0034] A protection unit is used to perform fault protection according to the fault type information.

[0035] In a possible implementation, in terms of obtaining the energy peak-to-peak ratio according to the instantaneous energy of the current signal when the first current curve suddenly changes, the judgment unit is specifically used to:

[0036] Determine the instantaneous energy value at the time of mutation according to the amplitude of the current signal;

[0037] The energy peak-to-peak ratio is calculated based on the instantaneous energy value and the average energy value of the current signal.

[0038] In a possible implementation, the determining unit is specifically configured to:

[0039] If the energy peak-to-peak ratio is greater than the preset energy peak-to-peak ratio threshold, the fault type information is an out-of-zone high-resistance fault;

[0040] If the energy peak-to-peak ratio is less than a preset energy peak-to-peak ratio threshold, the fault type information is an intra-zone low-resistance fault.

[0041] In a possible implementation, the device is further used for:

[0042] Obtaining a first preset current threshold;

[0043] The obtaining of the first preset current threshold comprises:

[0044] The first preset current threshold is obtained by the method shown in the following formula:

[0045]

[0046] Among them, λ max is the first preset current threshold, U dc is the DC voltage when the line is in normal operation, L eg is the equivalent reactance of the entire line, L dc is the reactance value of the current limiting reactor, K rel1 is the reliability coefficient, and its value range is 1.1-1.3.

[0047] In a possible implementation, the device is further used for:

[0048] Obtaining a second preset current threshold;

[0049] The obtaining of the second preset current threshold comprises:

[0050] The second preset current threshold is obtained by the method shown in the following formula:

[0051]

[0052] Among them, λ min is the second preset current threshold, I 0 It is the load current when the line is operating normally.

[0053] A third aspect of an embodiment of the present application provides a terminal, comprising a processor, an input device, an output device and a memory, wherein the processor, input device, output device and memory are interconnected, wherein the memory is used to store a computer program, the computer program comprises program instructions, and the processor is configured to call the program instructions to execute the step instructions in the first aspect of the embodiment of the present application.

[0054] The fourth aspect of the embodiments of the present application provides a computer-readable storage medium, wherein the computer-readable storage medium stores a computer program for electronic data exchange, wherein the computer program enables a computer to execute part or all of the steps described in the first aspect of the embodiments of the present application.

[0055] A fifth aspect of the embodiments of the present application provides a computer program product, wherein the computer program product includes a non-transitory computer-readable storage medium storing a computer program, and the computer program is operable to cause a computer to execute some or all of the steps described in the first aspect of the embodiments of the present application. The computer program product may be a software installation package. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0057] Figure 1 A schematic flow chart of a DC flexible distribution network fault protection method is provided for an embodiment of the present application;

[0058] Figure 2 A schematic diagram of a multi-terminal ring-shaped flexible DC distribution network structure is provided for an embodiment of the present application;

[0059] Figure 3 A fault current waveform diagram of different fault locations is provided for the embodiment of the present application;

[0060] Figure 4 A fault current energy spectrum of different fault locations is provided for the embodiment of the present application;

[0061] Figure 5 A fault current waveform diagram of different fault distances is provided for the embodiment of the present application;

[0062] Figure 6 A fault current energy spectrum with different fault distances is provided for the embodiment of the present application;

[0063] Figure 7 A fault current waveform diagram with different transition resistances is provided for an embodiment of the present application;

[0064] Figure 8 A fault current energy spectrum with different transition resistances is provided for the embodiment of the present application;

[0065] Fig. 9 A fault current waveform diagram of different fault conditions is provided for the embodiment of the present application;

[0066] Fig.10 A fault current mutation diagram for different fault conditions is provided for the embodiment of the present application;

[0067] Fig.11 A schematic diagram of the variation law of the peak-to-average energy ratio and the current mutation amount is provided for the embodiment of the present application;

[0068] Fig.12 A schematic diagram of a DC line bipolar fault is provided for an embodiment of the present application;

[0069] Fig.13 A schematic diagram of the structure of a terminal provided in an embodiment of the present application;

[0070] Fig.14 A structural schematic diagram of a DC flexible distribution network fault protection device is provided for an embodiment of the present application. DETAILED DESCRIPTION

[0071] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0072] The terms "first", "second", etc. in the specification and claims of this application and the above-mentioned drawings are used to distinguish different objects, rather than to describe a specific order. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not limited to the listed steps or units, but optionally includes steps or units that are not listed, or optionally includes other steps or units inherent to these processes, methods, products or devices.

[0073] Reference to "embodiments" in this application means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described in this application may be combined with other embodiments.

[0074] See also Figure 1 , Figure 1 A schematic diagram of a flow chart of a DC flexible distribution network fault protection method is provided for an embodiment of the present application. Figure 1 As shown, the method includes:

[0075] 101. Detect the current of the line through a protection device installed at the head end of the line to obtain a first current curve.

[0076] The protection device may be integrated with a current detection unit, through which current detection is performed to obtain current values ​​at multiple detection moments, and then a first current curve is generated based on the detection moments and the corresponding current values.

[0077] 102. If the mutation amount of the first current curve is greater than a first preset current threshold, trip a DC circuit breaker on the line.

[0078] If the mutation amount of the first current curve is greater than the first preset current threshold, it indicates that the fault is a low-resistance fault within the area, and a converter station locking signal is issued and the DC circuit breaker on the line is tripped.

[0079] 103. If the mutation amount of the first current curve is between the second preset current threshold and the first preset current threshold, the energy peak-to-peak ratio is obtained according to the instantaneous energy of the current signal when the first current curve mutates, and the first preset current threshold is greater than the second preset current threshold.

[0080] If the mutation amount of the first current curve is between the second preset current threshold and the first preset current threshold, it indicates that the fault may be an out-of-zone high resistance fault, etc. At this time, the fault type cannot be accurately determined by the mutation amount of the current alone, so the energy peak-to-peak ratio can be calculated, and then the energy peak-to-peak ratio can be combined to determine the fault type, and finally perform fault protection.

[0081] Specifically, obtaining the energy peak-to-peak ratio according to the instantaneous energy of the current signal when the first current curve suddenly changes includes:

[0082] A1. Determine the instantaneous energy value at the time of mutation according to the amplitude of the current signal;

[0083] A2. Calculate the energy peak-to-peak ratio according to the instantaneous energy value and the average energy value of the current signal.

[0084] A first preset current threshold may also be obtained, specifically:

[0085] The first preset current threshold is obtained by the method shown in the following formula:

[0086]

[0087] Among them, λ max is the first preset current threshold, U dc is the DC voltage when the line is in normal operation, L eq is the equivalent reactance of the entire line, L dc is the reactance value of the current limiting reactor, K rel1 is the reliability coefficient, and its value range is 1.1-1.3.

[0088] A second preset current threshold may also be obtained, specifically:

[0089] The second preset current threshold is obtained by the method shown in the following formula:

[0090]

[0091] Among them, λ min is the second preset current threshold, I 0 It is the load current when the line is operating normally.

[0092] 104. Determine fault type information according to the energy peak-to-peak ratio.

[0093] Specifically, determining the fault type information according to the energy peak-to-peak ratio includes:

[0094] If the energy peak-to-peak ratio is greater than the preset energy peak-to-peak ratio threshold, the fault type information is an out-of-zone high-resistance fault; if the energy peak-to-peak ratio is less than the preset energy peak-to-peak ratio threshold, the fault type information is an in-zone low-resistance fault.

[0095] 105. Perform fault protection according to the fault type information.

[0096] A general fault protection method may be used to perform fault protection based on fault type information.

[0097] In this example, the current of the line is detected by a protection device installed at the head end of the line to obtain a first current curve. If the mutation amount of the first current curve is greater than a first preset current threshold, the DC circuit breaker on the line is tripped. If the mutation amount of the first current curve is between a second preset current threshold and the first preset current threshold, the energy peak-to-peak ratio is obtained based on the instantaneous energy of the current signal when the first current curve mutates, the first preset current threshold is greater than the second preset current threshold, the fault type information is determined based on the energy peak-to-peak ratio, and fault protection is performed based on the fault type information. Therefore, fault protection can be performed in combination with the current mutation amount and the energy peak-to-peak ratio, thereby improving the reliability of fault protection.

[0098] In a specific implementation, the present application embodiment provides another DC flexible distribution network fault protection method, which is as follows:

[0099] Step 1: Concept and extraction of fault current energy peak-to-average ratio

[0100] Since the fault current amplitude and the fault current change rate amplitude decrease with the increase of fault distance, and also decrease with the increase of transition resistance, the use of fault current amplitude or fault current change rate as a single-ended protection criterion cannot theoretically protect the entire length of the line, and is greatly affected by the transition resistance, making it difficult to protect high-resistance faults. On the other hand, as the total resistance of the fault circuit increases, the fault current decay rate will also increase; from the energy perspective, it can be seen that the greater the total resistance of the fault circuit, the faster the fault current energy decays, and the duration of this decay process is generally within 1 millisecond. Therefore, by studying the change of fault current energy within a period of time after the fault occurs, it is possible to construct a protection criterion from the energy perspective.

[0101] TEO is a nonlinear operator that can track the instantaneous energy corresponding to each moment in the signal. When studying nonlinear speech modeling, American scientist HMTeager proposed a simple and effective signal analysis method. This method denotes energy as ψ. If there is a signal x(t), the instantaneous energy ψ[x(t)] of the signal is as shown in formula (1):

[0102]

[0103] The vibration displacement of a linear oscillator in a state of undamped free motion can be expressed as:

[0104] x(t)=Acos(w c t+θ) (2)

[0105] Its instantaneous energy is shown in formula (3):

[0106] ψ[x(t)]=ψ(Acos(w c t+θ))=(Aw c ) 2 (3)

[0107] From the perspective of physics, the instantaneous energy of the oscillator in the physical sense is a constant, that is, m is the mass of the oscillator. It can be seen that the calculation result of formula (3) differs from the real energy in the physical sense by only a constant factor Therefore, this ψ operator is called TEO.

[0108] TEO can efficiently calculate the instantaneous energy corresponding to each moment in the signal, has excellent time resolution, and is widely used in the field of signal processing. Therefore, TEO can be used to calculate the instantaneous energy of the current signal when a fault occurs. + When , the instantaneous current is differentiated once and twice with respect to time to obtain:

[0109]

[0110] In formula (4) and (5), R S It is the equivalent resistance in the fault circuit, that is, the sum of the transition resistance at the fault point and the line equivalent resistance. is the first differential value of current with respect to time, is the second derivative of current with respect to time, U dc is the DC voltage rating, L eq , C eq are the equivalent values ​​of inductance and capacitance of the fault circuit, I 0 Indicates the instantaneous value of the current at the protection installation during a fault.

[0111] Substituting equations (4) and (5) into equation (1), we can obtain the maximum instantaneous energy of the fault current at the initial moment of the fault: max As shown in formula (6):

[0112]

[0113] Similarly, after a fault occurs, the instantaneous energy expression of the fault current is shown in formula (7):

[0114]

[0115] It can be seen that the maximum instantaneous energy of the fault current in equations (6) and (7) is max And the instantaneous energy value ψ at any time varies with the equivalent resistance R of the fault circuit S The larger the transition resistance, the smaller the instantaneous energy.

[0116] But it is worth noting that when R S When the fault current increases, the instantaneous energy value of the fault current will decay faster. Assume that t 1 The average value of the instantaneous energy of the fault current during the time period is but The approximate value is shown in formula (8):

[0117]

[0118] The maximum instantaneous fault energy ψ max The instantaneous energy average The ratio K is shown in formula (2.9):

[0119]

[0120] It can be seen that R S The bigger, The smaller the value, the greater the ratio K. The maximum instantaneous energy of the fault ψ max The instantaneous energy average The ratio K is called the peak-to-average energy ratio. This ratio can effectively protect against high-resistance faults as a fault characteristic.

[0121] In practical applications, considering that the sampling frequency of DC distribution network protection devices is mostly 10 kHz to 100 kHz, the collected waveform signal is a discrete signal rather than a continuous signal in the above theoretical analysis. Therefore, the above theoretical analysis can only be used as a qualitative analysis and cannot be a numerical analysis.

[0122] In practical applications, discrete TEO will be used to calculate the peak-to-average energy ratio. Assuming that the actual collected discrete current signal is x(n), its energy value is as shown in formula (10):

[0123] ψ(x(n))=x 2 (n)-x(n+1)x(n-1) (10)

[0124] In the above formula, ψ[x(t)] is the instantaneous energy value of the current signal x(n).

[0125] Considering that the DC power distribution system has not yet been widely used and the construction cost is relatively high, there is no need to be too demanding on the cost of the protection device. The use of sensors with a sampling frequency of 100 kHz will make the protection device have better protection sensitivity and reliability. Therefore, the analysis and simulation verification of this scheme only considers the case of a sampling frequency of 100 kHz.

[0126] Since the DC current remains basically unchanged during the normal operation of the DC distribution network, that is, the instantaneous energy value remains at 0, it is necessary to appropriately adjust the peak-to-average energy ratio. A small amount of bias is added to the numerator and denominator of to keep the peak-to-average energy ratio K close to K=1 when the system is operating normally. Even if it is disturbed by 515616 noise, ψ max and The peak-to-average energy ratio K is proportional to the noise intensity and is theoretically not affected by white noise, so it has strong noise resistance. When applying the peak-to-average energy ratio K to protection, it is only necessary to select a sufficient time window. This ensures that K>>1 in the event of a fault, has a high protection margin, and will not cause false protection even in a high-noise environment.

[0127] Step 2: Boundary characteristics analysis of fault current energy peak-to-average ratio

[0128] The equivalent inductance of a DC line is lower than that of its AC counterpart because it does not have the inductive and capacitive effects caused by the continuously changing electromagnetic field of AC. The inductance of a typical overhead DC transmission line is about 1 to 2 millihenry / kilometer (mH / km), but this value varies depending on the specific design of the line. The reactance value of a current limiting reactor is generally 50mH. It can be seen from this that in a DC power distribution system, the reactance value of a current limiting reactor is usually greater than the reactance value of a 25km line, so the presence of a current limiting reactor will significantly affect the peak-to-average energy ratio characteristics of the fault current.

[0129] When there is an out-of-zone fault, the fault circuit needs to pass through the current limiting reactor, and the total circuit reactance L eq The peak-to-average energy ratio K value of the fault outside the zone will be significantly smaller than that of the fault inside the zone, which can form the basis for judging the fault inside and outside the zone.

[0130] Figure 2 This is a structural diagram of a multi-terminal ring flexible DC distribution network. Figure 2 The 10kV multi-terminal DC distribution system shown in the figure simulates various fault locations for analysis, and detects the fault current signal at the M terminal. The faults are all bipolar short-circuit faults, and the transition resistance is 5Ω. 1 is the fault within the area, f 2 、f 3 For a positive direction out-of-zone fault, f 4 For the reverse direction out-of-zone fault, the measured waveform is as follows Figure 3 The fault current waveforms at different fault locations are shown.

[0131] Figure 3 In the figure, faults f1, f3, and f4 are all set at the midpoints of the corresponding lines in the figure, and fault f2 is a bus fault on the DC side of MMC2; the four faults are all set to occur at the 0th ms, and the sampling frequency is 100 kHz.

[0132] from Figure 3 It can be seen intuitively that before the 0ms fault occurs, the DC current remains basically unchanged, and after the 0ms fault occurs, the DC current rises rapidly. That is, when the fault occurs, the current curve undergoes a large mutation, and the degree of mutation caused by faults at different locations is different.

[0133] At the same time, by comparing the four fault current waveforms of f1, f2, f3, and f4, it can be found that when faults occur in different fault sections, the fault current waveforms measured at the M terminal not only differ in amplitude, but also have significant differences in the steepness of the waveform. This is because there are two current limiting reactors in the fault circuits of the out-of-zone faults f3 and f4, and there is one current limiting reactor in the fault circuit of the DC side bus fault f2. Due to the smoothing effect of the current limiting reactor, the fault current waveforms of faults f3 and f4 are relatively gentle, followed by f2, and the fault current waveform of fault f1 is the steepest.

[0134] From the perspective of energy, before the fault occurs in the system at 0ms, the current waveform energy is almost zero (in fact, affected by noise and power electronic devices, the waveform energy is small but not zero). After the fault occurs, the current waveform energy changes dramatically. For a period of time after the fault, the current waveform energy is much higher than normal, and the energy begins to decay gradually after reaching the peak. From the above analysis, it can be seen that the faster the current waveform energy decays, the greater the energy peak-to-average ratio K value, that is, the size of the K value is closely related to the total resistance and total reactance of the fault circuit.

[0135] Using TEO to convert the current curve into a current energy spectrum can more intuitively analyze the current waveform differences caused by different fault locations. The fault current energy spectrum is as follows: Figure 4 As shown, Figure 4 The fault current energy spectra at different fault locations are shown.

[0136] By comparing the current energy spectra of the above four faults, it can be seen that after the fault occurs, the current energy of the in-zone fault f1 is the largest, followed by the DC bus fault f2, and the current energy of the out-of-zone faults f3 and f4 is relatively small. However, due to the presence of a current limiting reactor in the fault circuit of the out-of-zone fault to store part of the energy, the energy of the out-of-zone fault decays slowly. It can be seen that the current energy of the above four fault conditions has decayed to almost the same level when approaching the 2ms, which means that the peak-to-average ratio of the energy of the in-zone fault will be greater than that of the out-of-zone fault.

[0137] The time window of 1ms after the fault is intercepted, with a total of 100 sampling points, and the energy peak-to-average ratio K values ​​of the above four faults are calculated as shown in Table 1. The results are consistent with the theoretical analysis. The energy peak-to-average ratio of the fault within the zone is significantly greater than that of the fault outside the zone.

[0138] Table 1 Peak-to-average energy ratio of four faults at different locations

[0139]

[0140] Step 3: Analysis of distance characteristics of peak-to-average ratio of fault current energy

[0141] In the DC distribution network, the performance of the fault current is completely different from that of the AC distribution network, especially it is not affected by the complex factor of the voltage and current phase difference in the AC system. This is because the DC distribution network itself does not have the concept of phase difference, which makes the analysis of DC fault current more intuitive. In the fault analysis of the DC distribution network, the calculation of the bipolar short-circuit current will be particularly important, and its size is mainly affected by the transition resistance in the system and the equivalent resistance of the line. Since the distance between the fault point and the power supply directly affects the equivalent resistance value in the fault circuit, the bipolar short-circuit current of the DC distribution network shows a more obvious distance dependence characteristic. In short, when the fault point is located at the far end of the system, the equivalent resistance in the entire circuit increases, resulting in a decrease in the short-circuit current; if the fault point is close to the power supply side, the equivalent resistance is relatively small and the short-circuit current is correspondingly large. This characteristic provides a strong basis for fault detection and location, and is also one of the important theoretical foundations for fault analysis of DC distribution networks.

[0142] Since the peak-to-average ratio of fault current energy increases with the increase of fault loop resistance, that is, the farther the fault point is from the power source, the greater the peak-to-average ratio of fault energy. Since DC does not involve the frequency effect of AC, the line will not be affected by the skin effect and proximity effect to increase the resistance. Therefore, in the DC system, the resistance of the conductor is relatively stable and usually lower. For DC transmission lines, the value of equivalent resistance is often between 0.07 and 0.1 ohms / km, so in theory, the influence of fault distance on the peak-to-average ratio of fault current energy is quite small.

[0143] In step 2 Figure 2 In the 10kV multi-terminal DC distribution system shown in the figure, various fault distances are simulated at f1 for analysis, and the fault current signal is detected at the M terminal. The distances from the simulated four faults x1, x2, x3, and x4 to the M terminal are 5km, 10km, 15km, and 20km respectively. The other simulation conditions are the same as step 2. The measured waveform is as follows Figure 5 As shown, Figure 5 The fault current waveforms for different fault distances are shown.

[0144] Figure 5 The current waveform cannot directly show the fault current attenuation at different fault distances. By using TEO to convert the current curve into a current energy spectrum, the fault energy characteristics at different fault distances can be analyzed more intuitively. Figure 6 As shown, Figure 6 The fault current energy spectra for different fault distances are shown.

[0145] From an energy perspective, the greater the fault loop resistance, the faster the energy decays.

[0146] Comparing the current energy spectra of the above four faults, it can be seen that the current energy after the fault occurs is x1>x2>x3>x4, but the difference is not large. When approaching the 2nd ms, the current energy is still x1>x2>x3>x4, and the relative gap is larger, indicating that the energy decays faster when the fault distance is farther, which means that the energy peak-to-average ratio increases with the increase of the fault distance.

[0147] By intercepting a total of 100 sampling points in the 1 ms time window after the fault occurs, the energy peak-to-average ratio K values ​​of the above four faults are calculated as shown in Table 2. The results are consistent with the theoretical analysis. The farther the fault distance is, the larger the energy peak-to-average ratio is. In addition, since the resistance value of the DC distribution line is not high, the fault distance has a relatively small effect on the energy peak-to-average ratio.

[0148] Table 2 Peak-to-average energy ratio for four different fault distances

[0149]

[0150] Step 4: Analysis of the influence of transition resistance on peak-to-average ratio of fault current energy

[0151] High-resistance faults are more common in DC distribution networks than in AC distribution networks, especially when such faults occur due to aging of cable insulation materials, oxidation or corrosion of connection parts, etc. High-resistance faults are usually more difficult to detect because the flowing fault current may be relatively small and insufficient to trigger traditional overcurrent protection devices. Therefore, high-resistance fault conditions are also one of the factors that need to be considered in the protection method proposed in the present invention.

[0152] Through simple theoretical analysis, it can be known that the peak-to-average ratio of the energy of the fault current increases with the increase of the loop resistance, that is, for a bipolar short circuit fault occurring at the same location, its peak-to-average ratio increases with the increase of the transition resistance. When a bipolar short circuit occurs in a DC distribution line, the specific value of the transition resistance depends on the specific conditions at the fault site, such as the size of the contact surface, the corrosion condition, the type of contact material, etc. When discussing "high-resistance faults", this resistance value is usually much higher than that of low-resistance faults, and may range from tens of ohms to thousands of ohms. Therefore, in design and fault analysis, an assumed or typical value is usually considered as the transition resistance. This section sets four fault conditions with different transition resistances for more intuitive analysis and display.

[0153] In step 2 Figure 2 In the 10kV multi-terminal DC distribution system shown in the figure, the section f1 where the fault is located is simulated with different transition resistances for analysis. The distance from the fault point to the M terminal is 10km. The fault current signal is detected at the M terminal. The transition resistances of the simulated four faults r1, r2, r3, and r4 are 1Ω, 5Ω, 20Ω, and 200Ω respectively. The other simulation conditions are the same as before. The measured waveform is as follows Figure 7 As shown, Figure 7The fault current waveforms for different transition resistances are shown.

[0154] from Figure 7 It can be seen that as the transition resistance increases, the fault current amplitude decreases significantly, which makes it difficult for traditional overcurrent protection to be applied to high-resistance fault scenarios. However, it is worth noting that in the case of r1 fault with a transition resistance of 1Ω, the oscillation attenuation of the fault current can be clearly seen, and as the transition resistance increases, the oscillation attenuation of the fault current becomes increasingly difficult to observe. This is because the increase in transition resistance causes the fault current to decay faster.

[0155] From the previous theoretical analysis, it can be seen that the peak-to-average ratio of fault current energy increases with the increase of transition resistance. Converting the current curve into a current energy spectrum can more intuitively analyze the effect of transition resistance on the energy decay rate. The fault current energy spectrum is shown in Figure 8 As shown, Figure 8 The fault current energy spectra for different transition resistances are shown.

[0156] Depend on Figure 8 It can be seen that the transition resistance has a great influence on the fault current energy. By intercepting 100 sampling points in the 1ms time window after the fault occurs, the energy peak-to-average ratio K values ​​of the above four faults r1, r2, r3, and r4 are calculated as shown in Table 3.

[0157] Since the signal sampled by the sensor is not a continuous signal, and the DC distribution network itself contains a small amount of harmonics and noise during operation, the simulation result of the K value cannot completely correspond to the theoretical value in terms of numerical value. However, it can be clearly observed that the K value increases with the increase of the transition resistance, which further shows that the energy peak-to-average ratio theory proposed in the present invention can indeed be used for the qualitative analysis of the fault energy of the DC distribution network, and the energy peak-to-average ratio as a protection criterion is more suitable for high-resistance fault conditions in the protection zone.

[0158] It is worth noting that the peak-to-average energy ratio K value of the in-zone fault r1 when the transition resistance is 1Ω and the out-of-zone fault f3 when the transition resistance is 5Ω are relatively close, indicating that the peak-to-average energy ratio mentioned in this chapter as a protection criterion is difficult to protect low-resistance faults and needs to be used in combination with other protection criteria to ensure reliability.

[0159] Table 3 Energy peak-to-average ratio under four different transition resistances

[0160]

[0161] Step 5: Method of strengthening fault protection by combining fault current mutation and energy peak-to-average ratio

[0162] by Figure 3Taking the fault simulation current waveform of the 10kV flexible DC distribution network as an example, when there is no fault, the DC distribution network operates normally and the DC current remains basically unchanged; when a bipolar short circuit fault occurs, the DC current rises rapidly. From the perspective of energy, it can be seen that the occurrence of the fault causes the originally stable waveform energy to mutate, and the degree of mutation is closely related to the fault circuit parameters; the fault within the zone has a larger amplitude of energy fluctuation and decays faster than the fault outside the zone, and the low-resistance fault has a larger amplitude of energy fluctuation and decays slower than the high-resistance fault.

[0163] From the analysis of signing steps 2 to 4, it can be seen that the fault current energy peak-to-average ratio K value has the following characteristics:

[0164] (1) When the transition resistance is the same, the K value of the fault within the zone is greater than that of the fault outside the zone.

[0165] (2) The K value is less affected by the fault distance.

[0166] (3) When the fault location is the same, the K value increases with the increase of transition resistance.

[0167] From the above characteristics, it can be seen that the peak-to-average ratio of energy for high-resistance faults within the zone is the largest, and the peak-to-average ratio of energy for low-resistance faults outside the zone is the smallest. Therefore, the peak-to-average ratio of energy is more suitable for identifying high-resistance faults within the zone. However, for low-resistance faults within the zone and high-resistance faults outside the zone, the peak-to-average ratios of these two faults may be relatively close, which is not conducive to the setting of protection. Therefore, the protection method that simply uses the peak-to-average ratio of energy still has defects, and its ability to identify low-resistance faults within the zone needs to be further strengthened.

[0168] In step 1, formula (4) is the expression of the DC fault current mutation. It can be seen that in this formula, the current mutation With the loop resistance R S decreases with the increase of the equivalent reactance L eq decreases with the increase of .

[0169] Based on the above characteristics, it can be seen that the current mutation amount, as a fault characteristic amount, decreases rapidly with the increase of fault distance and transition resistance, and the equivalent reactance L eq Increase, the current mutation will be smaller than the fault within the zone. Therefore, the low-resistance fault within the zone is the case with the smallest current mutation, and the high-resistance fault outside the zone is the case with the largest current mutation.

[0170] From the above analysis, it can be seen that the use of current mutation as a protection criterion is more suitable for identifying low-resistance faults in the area. In theory, reliable protection of the entire line length can be achieved in the case of low-resistance faults. However, since the current mutation of high-resistance faults in the area and low-resistance faults outside the area are relatively close, it is difficult to protect high-resistance faults.

[0171] Considering that the current mutation protection based on formula (4) is widely used in DC distribution systems and has good speed, a simulation analysis is carried out in a 10kV multi-terminal DC distribution system, where the four types of faults y1, y2, y3, and y4 are low-resistance faults within the zone (10km, 1Ω), high-resistance faults within the zone (10km, 50Ω), low-resistance faults outside the positive zone (35km, 1Ω), and high-resistance faults outside the positive zone (35km, 50Ω). The current waveform changes with the fault location and fault resistance as shown in the following figure. Fig. 9 As shown, Fig. 9 The fault current waveforms of different fault conditions are shown.

[0172] Depend on Fig. 9 It can be clearly seen that the current mutation of the faults y1 and y2 within the zone is more drastic, while the current mutation of the faults y3 and y4 outside the zone is relatively gentle, and the current amplitude of the low-resistance faults y1 and y3 is significantly greater than that of the high-resistance faults y2 and y4, which is consistent with the theoretical analysis above.

[0173] Since the analysis in this solution is based on the case where the sampling rate is 100 kHz, the calculation formula for the current mutation is the current difference between each sampling point and the previous sampling point divided by the sampling interval 0.01 ms, which is slightly different from the current mutation of the continuous signal in the theoretical analysis. Its expression is shown in formula (11):

[0174]

[0175] In the above formula, i dc (n) is the DC current signal collected at the sampling point, and Δt is the sampling interval, which is 0.01 ms in this solution description.

[0176] Using formula (11) Figure 8 By calculating the current waveform in the figure, we can get the current mutation amount under different fault conditions as follows: Fig.10 As shown, Fig.10 The fault current mutation diagram for different fault conditions is shown. Fig.10 The maximum current mutation values ​​of the four fault conditions are shown in Table 4:

[0177] Table 4 Maximum current mutation values ​​under four different fault conditions

[0178]

[0179] Depend on Fig.10It can be seen from Table 4 that the current mutation of the fault within the zone is greater than that of the fault outside the zone, and the current mutation of the low-resistance fault is greater than that of the high-resistance fault, which is consistent with the theoretical analysis above. However, it is worth noting that even with the existence of a current-limiting reactor, the current mutation is still difficult to distinguish between the high-resistance fault within the zone and the low-resistance fault outside the zone, and in theory, the forward bus fault only needs to pass through one current-limiting reactor, and its current mutation should be between the fault within the zone and the fault outside the zone. Therefore, the setting value of the protection should not only avoid the low-resistance fault outside the zone, but also avoid the bus fault.

[0180] According to the above analysis, in order to avoid out-of-zone faults and busbar faults with low transition resistance, the current mutation can only operate correctly when the transition resistance of the fault in the zone is low, and cannot protect the high-resistance fault in the zone, taking into account a certain protection margin.

[0181] The comparison of the change rules of energy peak-to-average ratio and current mutation is as follows: Fig.11 As shown, Fig.11 A schematic diagram showing the variation of the peak-to-average energy ratio and the amount of current mutation is shown.

[0182] Fig.11 The approximate values ​​of the above two fault characteristic quantities under different fault distances and transition resistances are shown. It can be seen that if the characteristic quantity value represented by the green dotted line in the figure is used as the protection criterion, the energy peak-to-average ratio protection can correctly identify the fault of the entire line in the area when the transition resistance is R3, R4 and higher resistance values, and the current mutation quantity can correctly identify the fault of the entire line in the area when the transition resistance is R1, R2 and lower resistance values.

[0183] From the above analysis, it can be seen that the peak-to-average energy ratio has the following characteristics: the peak-to-average energy ratio K value under high-resistance fault in the zone is greater than the K value under high-resistance fault outside the zone, and much greater than the K value under low-resistance fault outside the zone. The current mutation has the following characteristics: the absolute value of the current mutation during low-resistance fault in the zone is greater than that during low-resistance fault outside the zone, and much greater than that during high-resistance fault inside and outside the zone. In other words, the peak-to-average energy ratio is easy to identify high-resistance faults in the zone, and the current mutation is easier to identify low-resistance faults in the zone. The two characteristic quantities have strong complementarity.

[0184] Therefore, based on the above derivation and analysis of the current waveform energy characteristics, the present invention proposes a protection method that combines the energy peak-to-average ratio with the current mutation amount, and realizes single-ended instantaneous fast-acting protection for the entire length of the line through the complementary characteristics of the two.

[0185] Step 6: Design of new protection scheme for flexible DC distribution network

[0186] Based on the strong complementarity of energy peak-to-average ratio and current mutation, the present invention proposes a new protection scheme suitable for flexible DC distribution network as follows:

[0187] The protection device installed at the head end of the line is used to continuously monitor the current and voltage signals. The voltage signal constitutes the starting criterion. After the fault direction identification criterion and the fault type identification criterion are used to identify the forward bipolar short-circuit fault, the current mutation amount and the energy peak-to-average ratio are used as the protection criterion to identify whether the fault occurs inside or outside the zone. If it is determined to be an internal fault, the protection will be activated immediately. The protection scheme flow chart is shown in the figure below. Fig.11 shown.

[0188] Theoretically, without considering power transfer, the DC current change rate before the fault occurs remains near zero. When a fault occurs, if the current mutation di / dt is greater than the set value λ max , it means that the fault is a low-resistance fault in the area, and the converter station should immediately send a blocking signal and trip the DC circuit breaker on this line. If di / dt is within the set value λ max With λ min If the fault occurs in the system and the fault circuit resistance is large (the fault distance is far or the transition resistance is large), the peak-to-average energy ratio is used to determine whether the fault occurs inside or outside the zone, which complements the current mutation di / dt.

[0189] In summary, the protection scheme proposed in the present invention based on the combination of transient current energy peak-to-average ratio K and current mutation di / dt can effectively protect faults in the zone. The current mutation protects low-resistance faults, and the energy peak-to-average ratio protects high-resistance faults. The scheme is almost unaffected by the fault transition resistance, and can maintain a high sensitivity even when a high-resistance grounding fault occurs at the end of the zone. At the same time, the proposed method, as a single-ended protection, does not require communication between the two ends, and can meet the protection speed requirements of the DC distribution system.

[0190] Step 7: Research and design of protection criteria and setting methods

[0191] 1. Protection start criteria

[0192] When the traveling wave of the bipolar short-circuit fault reaches the end of the line, the port voltage will drop significantly, so this principle is used as the protection start criterion. Since the protection start criterion is generally set to a small value, it is easily affected by noise. Considering this characteristic of the common Gaussian white noise with a zero average value, the protection start criterion proposed in the present invention uses the port voltage integral value to reduce the possibility of protection misoperation caused by noise. Its criterion is shown in formula (12):

[0193] |∫Δu t dt|>m set (12)

[0194] In the above formula, u t is the line port voltage, Δu t is the port voltage change value, mset It is the setting value of the protection start judgment. The schematic diagram of the DC line bipolar fault is as follows Fig.12 As shown, Fig.12 A schematic diagram of a DC line bipolar fault is shown.

[0195] 2. Direction recognition criteria

[0196] When a fault occurs, the converter station will feed the fault current to the fault point. Therefore, the present invention uses the polarity of the port current change as the direction identification criterion to distinguish the forward and reverse faults. t The positive direction is already in Fig.12 Marked. Considering the influence of noise, the integral form is adopted. The forward fault identification criterion is shown in formula (13):

[0197] ∫Δi t dt>ε set (13)

[0198] In the formula, Δi t is the port current change, ε set It is the setting value of the fault direction identification criterion.

[0199] 3. Fault type identification criteria

[0200] When a bipolar short circuit fault occurs, it can be distinguished from a ground fault based on whether there is a ground mode traveling wave. The criterion is shown in formula (14):

[0201] ∫Δu 0 dt <n set (14)

[0202] In the formula, Δu 0 is the variation of the traveling wave of ground mode voltage, n set It is the setting value of ground fault identification criterion.

[0203] 4. Protection setting

[0204] The protection scheme proposed in the present invention protects low-resistance faults by current mutation and protects high-resistance faults by energy peak-to-average ratio. The setting values ​​of the two need to be coordinated to achieve the overall protection effect. The protection setting method is as follows:

[0205] (1) Protection scheme based on current mutation

[0206] After the protection is started, the current mutation within the 1ms time window is used for preliminary judgment. Two setting values ​​need to be set to cooperate with the energy peak-to-average ratio protection. max is a fixed value, which is used to make protection judgment directly based on the current mutation under low-resistance fault conditions; minis an adaptive value. When the fault current mutation is between the two set values, it can be determined that a fault has occurred. However, the current mutation alone cannot be used as a protection criterion to identify whether the fault occurs inside or outside the zone. Therefore, it is necessary to start the energy peak-to-average ratio algorithm.

[0207] Since the DC line has a current limiting reactor as the line boundary, the fault current of the out-of-zone fault needs to pass through the current limiting reactor, resulting in an increase in the equivalent reactance. Therefore, the current mutation can be used as a protection criterion to protect the entire length of the line in the event of a low-resistance fault. The maximum protection setting value is the case where the out-of-zone current mutation is the largest (a metallic fault occurs on the busbar), as shown in formula (15);

[0208]

[0209] In the above formula, U dc is the DC voltage during normal operation, L eq is the equivalent reactance of the entire line, L dc is the reactance value of the current limiting reactor, K rel1 is the reliability coefficient, which is generally between 1.1 and 1.3. Here, it is recommended to take a value of 1.2. Since the distribution line is usually short, the line equivalent reactance L eq Less than the current limiting reactor L dc Therefore, this setting value can achieve full-line fast-acting protection in the event of a low-resistance fault while ensuring that the protection does not malfunction.

[0210] Since the inductor current (load current during normal operation) is affected by the system operation mode and has uncertainty, the transition resistance value is also uncertain. Therefore, the minimum setting value is an adaptive value. When setting, the system operation mode and the maximum transition resistance value required for protection need to be considered. The present invention takes into account the high-resistance fault situation (200Ω), and the minimum setting value of the current mutation is shown in formula (16):

[0211]

[0212] In the above formula, I 0 is the load current during normal operation.

[0213] 2) Protection scheme based on current energy peak-to-average ratio

[0214] Since the peak-to-average ratio of fault current energy increases with the increase of transition resistance, the peak-to-average ratio of current energy of fault outside the zone is significantly smaller than that of fault inside the zone. Therefore, the peak-to-average ratio of high-resistance fault outside the zone is closer to that of low-resistance fault inside the zone. For the fault with the highest peak-to-average ratio outside the zone, the peak-to-average ratio K of the energy detected by the protection is 200Ω. 0 As the threshold setting, as shown in formula (17):

[0215] K max …Kset =K rel2 K 0 (17)

[0216] In the above formula, K max K is the maximum value of the peak-to-average energy ratio within the 1ms time window after the fault occurs. rel2 is the reliability coefficient. Since the peak-to-average energy ratio is less affected by noise and is not prone to false operation, this scheme recommends K rel2 The value is 1.1.

[0217] 5. Protection action time analysis

[0218] In actual engineering applications, the sampling frequency of DC protection is generally 10kHZ or 100kHZ. Taking 100kHZ as an example, a data window length of 1ms is used for protection calculation. With the computing speed of existing FPGA technology, the protection calculation time does not exceed 0.3ms. Since the length of a single distribution line is usually no more than 30km, the line transmission delay of the fault current traveling wave does not exceed 0.1ms, and the protection start delay is generally 15μs.

[0219] In summary, considering a certain margin, the protection method proposed in the present invention should be able to act within 1.5ms after the fault occurs, meeting the requirement of instantaneous fast action within 3ms for DC protection.

[0220] For the above embodiments, please refer to Fig.13 , Fig.13 A schematic diagram of the structure of a terminal provided in an embodiment of the present application, such as Fig.13 As shown, it includes a processor, an input device, an output device and a memory, which are connected to each other, wherein the memory is used to store a computer program, the computer program includes program instructions, the processor is configured to call the program instructions, and the program includes instructions for executing the following steps;

[0221] The current of the line is detected by a protection device installed at the head end of the line to obtain a first current curve;

[0222] If the mutation amount of the first current curve is greater than a first preset current threshold, tripping the DC circuit breaker on the line;

[0223] If the mutation amount of the first current curve is between the second preset current threshold and the first preset current threshold, the energy peak-to-peak ratio is obtained according to the instantaneous energy of the current signal when the first current curve undergoes a mutation, and the first preset current threshold is greater than the second preset current threshold;

[0224] Determining fault type information according to the energy peak-to-peak ratio;

[0225] Fault protection is performed according to the fault type information.

[0226] The above mainly introduces the scheme of the embodiment of the present application from the perspective of the execution process on the method side. It is understandable that in order to realize the above functions, the terminal includes a hardware structure and / or software module corresponding to the execution of each function. Those skilled in the art should easily realize that, in combination with the units and algorithm steps of each example described in the embodiments provided herein, the present application can be implemented in the form of hardware or a combination of hardware and computer software. Whether a function is executed in the form of hardware or computer software driving hardware depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0227] The embodiment of the present application can divide the terminal into functional units according to the above method example. For example, each functional unit can be divided according to each function, or two or more functions can be integrated into one processing unit. The above integrated unit can be implemented in the form of hardware or in the form of software functional units. It should be noted that the division of units in the embodiment of the present application is schematic and is only a logical function division. There may be other division methods in actual implementation.

[0228] In line with the above, see Fig.14 , Fig.14 The present invention provides a schematic diagram of a DC flexible distribution network fault protection device. Fig.14 As shown, the device comprises:

[0229] The detection unit 301 is used to detect the current of the line through a protection device installed at the head end of the line to obtain a first current curve;

[0230] A judgment unit 302 is configured to trip a DC circuit breaker on the line if the mutation amount of the first current curve is greater than a first preset current threshold; and to obtain an energy peak-to-peak ratio according to the instantaneous energy of the current signal when the first current curve mutates if the mutation amount of the first current curve is between a second preset current threshold and the first preset current threshold, and the first preset current threshold is greater than the second preset current threshold;

[0231] A determination unit 303, configured to determine fault type information according to the energy peak-to-peak ratio;

[0232] The protection unit 304 is configured to perform fault protection according to the fault type information.

[0233] In a possible implementation, in terms of obtaining the energy peak-to-peak ratio according to the instantaneous energy of the current signal when the first current curve suddenly changes, the judgment unit 302 is specifically used to:

[0234] Determine the instantaneous energy value at the time of mutation according to the amplitude of the current signal;

[0235] The energy peak-to-peak ratio is calculated based on the instantaneous energy value and the average energy value of the current signal.

[0236] In a possible implementation, the determining unit 303 is specifically configured to:

[0237] If the energy peak-to-peak ratio is greater than the preset energy peak-to-peak ratio threshold, the fault type information is an out-of-zone high-resistance fault;

[0238] If the energy peak-to-peak ratio is less than a preset energy peak-to-peak ratio threshold, the fault type information is an intra-zone low-resistance fault.

[0239] In a possible implementation, the device is further used for:

[0240] Obtaining a first preset current threshold;

[0241] The obtaining of the first preset current threshold comprises:

[0242] The first preset current threshold is obtained by the method shown in the following formula:

[0243]

[0244] Among them, λ max is the first preset current threshold, U dc is the DC voltage when the line is in normal operation, L eq is the equivalent reactance of the entire line, L dc is the reactance value of the current limiting reactor, K rel1 is the reliability coefficient, and its value range is 1.1-1.3.

[0245] In a possible implementation, the device is further used for:

[0246] Obtaining a second preset current threshold;

[0247] The obtaining of the second preset current threshold comprises:

[0248] The second preset current threshold is obtained by the method shown in the following formula:

[0249]

[0250] Among them, λ min is the second preset current threshold, I 0It is the load current when the line is operating normally.

[0251] An embodiment of the present application also provides a computer storage medium, wherein the computer storage medium stores a computer program for electronic data exchange, and the computer program enables a computer to execute part or all of the steps of any one of the DC flexible distribution network fault protection methods recorded in the above method embodiments.

[0252] An embodiment of the present application also provides a computer program product, which includes a non-transitory computer-readable storage medium storing a computer program, and the computer program enables a computer to execute part or all of the steps of any one of the DC flexible distribution network fault protection methods recorded in the above method embodiments.

[0253] It should be noted that, for the aforementioned method embodiments, for the sake of simplicity, they are all expressed as a series of action combinations, but those skilled in the art should be aware that the present application is not limited by the described order of actions, because according to the present application, certain steps can be performed in other orders or simultaneously. Secondly, those skilled in the art should also be aware that the embodiments described in the specification are all preferred embodiments, and the actions and modules involved are not necessarily required by the present application.

[0254] In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0255] In the several embodiments provided in the present application, it should be understood that the disclosed device can be implemented in other ways. For example, the device embodiments described above are only schematic, such as the division of the units, which is only a logical function division. There may be other division methods in actual implementation, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, and the indirect coupling or communication connection of the device or unit can be electrical or other forms.

[0256] The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0257] In addition, the functional units in the various embodiments of the application may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit. The above-mentioned integrated units may be implemented in the form of hardware or in the form of software program modules.

[0258] If the integrated unit is implemented in the form of a software program module and sold or used as an independent product, it can be stored in a computer-readable memory. Based on this understanding, the technical solution of the present application is essentially or the part that contributes to the prior art or all or part of the technical solution can be embodied in the form of a software product, and the computer software product is stored in a memory, including a number of instructions to enable a computer device (which can be a personal computer, server or network device, etc.) to perform all or part of the steps of the method described in each embodiment of the present application. The aforementioned memory includes: U disk, read-only memory (ROM), random access memory (RAM), mobile hard disk, disk or optical disk and other media that can store program codes.

[0259] A person of ordinary skill in the art can understand that all or part of the steps in the various methods of the above embodiments can be completed by instructing related hardware through a program, and the program can be stored in a computer-readable memory, which can include: a flash drive, a read-only memory, a random access memory, a magnetic disk or an optical disk, etc.

[0260] The embodiments of the present application are introduced in detail above. Specific examples are used in this article to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the method and core idea of ​​the present application. At the same time, for general technical personnel in this field, according to the idea of ​​the present application, there will be changes in the specific implementation method and application scope. In summary, the content of this specification should not be understood as a limitation on the present application.

Claims

1. A DC flexible distribution network fault protection method, characterized in that: The method comprises: The current of the line is detected by a protection device installed at the head end of the line to obtain a first current curve; If the mutation amount of the first current curve is greater than a first preset current threshold, tripping the DC circuit breaker on the line; If the mutation amount of the first current curve is between the second preset current threshold and the first preset current threshold, the energy peak-to-peak ratio is obtained according to the instantaneous energy of the current signal when the first current curve undergoes a mutation, and the first preset current threshold is greater than the second preset current threshold; Determining fault type information according to the energy peak-to-peak ratio; Fault protection is performed according to the fault type information.

2. The DC flexible distribution network fault protection method according to claim 1, characterized in that: The step of obtaining the energy peak-to-peak ratio according to the instantaneous energy of the current signal when the first current curve suddenly changes comprises: Determine the instantaneous energy value at the time of mutation according to the amplitude of the current signal; The energy peak-to-peak ratio is calculated based on the instantaneous energy value and the average energy value of the current signal.

3. The DC flexible distribution network fault protection method according to claim 1 or 2, characterized in that: The determining of fault type information according to the energy peak-to-peak ratio includes: If the energy peak-to-peak ratio is greater than the preset energy peak-to-peak ratio threshold, the fault type information is an out-of-zone high-resistance fault; If the energy peak-to-peak ratio is less than a preset energy peak-to-peak ratio threshold, the fault type information is an intra-zone low-resistance fault.

4. The DC flexible distribution network fault protection method according to claim 3 is characterized in that: The method further comprises: Obtaining a first preset current threshold; The obtaining of the first preset current threshold comprises: The first preset current threshold is obtained by the method shown in the following formula: Among them, λ max is the first preset current threshold, U dc is the DC voltage when the line is in normal operation, L eq is the equivalent reactance of the entire line, L dc is the reactance value of the current limiting reactor, K rel1 is the reliability coefficient, and its value range is 1.1-1.

3.

5. The DC flexible distribution network fault protection method according to claim 4, characterized in that: The method further comprises: Obtaining a second preset current threshold; The obtaining of the second preset current threshold comprises: The second preset current threshold is obtained by the method shown in the following formula: Among them, λ min is the second preset current threshold, and I0 is the load current when the line operates normally.

6. A DC flexible distribution network fault protection device, characterized in that: The device comprises: A detection unit, used to detect the current of the line through a protection device installed at the head end of the line to obtain a first current curve; a judgment unit, configured to trip a DC circuit breaker on the line if the mutation amount of the first current curve is greater than a first preset current threshold; and to obtain an energy peak-to-peak ratio according to the instantaneous energy of the current signal when the first current curve undergoes a mutation if the mutation amount of the first current curve is between a second preset current threshold and the first preset current threshold, and the first preset current threshold is greater than the second preset current threshold; A determination unit, configured to determine fault type information according to the energy peak-to-peak ratio; A protection unit is used to perform fault protection according to the fault type information.

7. The DC flexible distribution network fault protection device according to claim 6, characterized in that: In terms of obtaining the energy peak-to-peak ratio according to the instantaneous energy of the current signal when the first current curve suddenly changes, the judgment unit is specifically used to: Determine the instantaneous energy value at the time of mutation according to the amplitude of the current signal; The energy peak-to-peak ratio is calculated based on the instantaneous energy value and the average energy value of the current signal.

8. The DC flexible distribution network fault protection device according to claim 6 or 7, characterized in that: The determining unit is specifically used for: If the energy peak-to-peak ratio is greater than the preset energy peak-to-peak ratio threshold, the fault type information is an out-of-zone high-resistance fault; If the energy peak-to-peak ratio is less than a preset energy peak-to-peak ratio threshold, the fault type information is an intra-zone low-resistance fault.

9. A terminal, characterized in that: It includes a processor, an input device, an output device and a memory, which are interconnected, wherein the memory is used to store a computer program, the computer program includes program instructions, and the processor is configured to call the program instructions to execute the DC flexible distribution network fault protection method according to any one of claims 1 to 5.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores a computer program, wherein the computer program includes program instructions, and when the program instructions are executed by a processor, the processor executes the DC flexible distribution network fault protection method according to any one of claims 1 to 5.

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