A dual-terminal transient quantity protection method, system and device based on oscillation frequency

Abstract

The present invention discloses a double-terminal transient quantity protection method, system and device based on oscillation frequency, which relates to the field of line protection. By determining the oscillation characteristics of the line system after a fault occurs, the oscillation component and the power frequency quantity are separated in a short time window, and then the fault is identified by their relative relationship and the energy of the power frequency quantity. Under the condition that the fault outside the area is not misoperated, the reliable action of the fault inside the area can be achieved in most cases. The present invention can effectively improve the action speed of the protection and improve the reliability of the protection. At the same time, the present invention has low requirements on the algorithm, strong engineering feasibility, and has broad application prospects.

Description

Technical Field

[0001] The present invention relates to the technical field of line protection, and in particular to a double-terminal transient quantity protection method, system and device based on oscillation frequency. Background Art

[0002] When an AC transmission line in a new power system fails, the fault characteristics change, and the original current protection based on the power frequency current amplitude is affected and no longer applicable. At the same time, the increase in power electronic devices requires faster fault removal. Therefore, it is of great practical significance to study the high-speed dynamic AC line protection principle to quickly identify and isolate faults.

[0003] Existing transient quantity protection schemes mainly include schemes using time domain waveforms and schemes using boundary effects. Among them, the scheme using time domain waveforms actually uses the polarity difference of the current on both sides of the fault inside and outside the zone to distinguish the fault. However, in fact, the fault characteristics of the initial transient state are not only polarity, and the reliability of fault distinction based only on the current polarity difference is often not high; and the scheme using time domain waveforms requires a longer time window to ensure reliability when the waveforms are similar, and the action time is longer. The scheme using boundary effects cannot handle faults with weak boundaries, and requires a very high sampling rate to observe the fault characteristics. At the same time, it has poor anti-interference ability and poor engineering feasibility. It can be seen that the existing transient quantity protection schemes generally have problems such as insufficient action speed and poor reliability. Summary of the invention

[0004] In view of the problems raised in the above background technology, the present invention provides a double-terminal transient quantity protection method, system and device based on oscillation frequency to improve the protection action speed and reliability.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] On the one hand, the present invention provides a two-terminal transient quantity protection method based on oscillation frequency, comprising:

[0007] The protection devices installed on both sides of the line detect the current mutation and determine whether a fault has occurred; the current mutation is the change in the differential current at adjacent moments;

[0008] When it is determined that a fault has occurred, the sampling window length is determined according to the line system structure; the line system structure includes power supply side impedance, line parameters and line topology;

[0009] The differential current signal of the protection line within the sampling window length is decomposed into a high-frequency band and a low-frequency band by a frequency analysis method, and then the high-frequency energy and the low-frequency energy are calculated;

[0010] When the low-frequency energy is greater than the action threshold and the ratio of the low-frequency energy to the high-frequency energy is greater than the ratio threshold, it is determined that an in-zone fault has occurred and the protection action trips.

[0011] Optionally, detecting the current mutation amount and determining whether a fault occurs by means of protection devices installed on both sides of the line specifically includes:

[0012] The protection devices installed on both sides of the line are used to detect and display the phase current waveform, and the vector sum obtained by adding the phase current signals of the lines on both sides is used as the differential current;

[0013] Calculate the change of differential current at adjacent moments As the amount of current mutation;

[0014] When the current mutation is greater than the change threshold Δ set , it is determined that a fault has occurred.

[0015] Optionally, determining the sampling window length according to the line system structure specifically includes:

[0016] Calculate the impedance Z from the fault power supply to the system side according to the line system structure ∑ ;

[0017] According to the impedance Z ∑ Calculate the fault current i 0 (s) and analyze i 0 (s) the frequency of the pole;

[0018] Determine the frequency corresponding to the smallest imaginary part among all poles as the oscillation frequency f ω ;

[0019] Determine the sampling window length

[0020] Optionally, decomposing the differential current signal of the protection line within the sampling window length into a high frequency band and a low frequency band by a frequency analysis method, and then calculating the high frequency energy and the low frequency energy, specifically includes:

[0021] The differential current signal of the protection line within the sampling window length T is decomposed into multiple frequency bands by wavelet transform method, where the kth layer detail coefficient is D k , the approximate coefficient is A k ;

[0022] Using formula Calculate high frequency energy E H ; where N is D k The number of discrete signal points in the frequency band; n represents the nth signal point;

[0023] Using formula Calculate the low frequency energy E L .

[0024] Optionally, decomposing the differential current signal of the protection line within the sampling window length into a high frequency band and a low frequency band by a frequency analysis method, and then calculating the high frequency energy and the low frequency energy, specifically includes:

[0025] The differential current signal of the protection line within the sampling window length T is decomposed into the amplitude A at each frequency point n by the fast Fourier transform method. n ;

[0026] Using formula Calculate high frequency energy E H ; Where N is the number of frequency points after Fourier transform except the 0Hz component;

[0027] Using formula Calculate the low frequency energy E L .

[0028] Optionally, when the low-frequency energy is greater than the action threshold value and the ratio of the low-frequency energy to the high-frequency energy is greater than the ratio threshold value, determining that an in-zone fault occurs and a protection action trip is performed specifically includes:

[0029] Calculate different action thresholds E by using different frequency analysis methods Lset , and set the ratio threshold K that can avoid the weakest transient oscillation process when the fault occurs outside the zone set ;

[0030] When E L >E Lset And E L >K set E H When it is determined that a fault has occurred within the zone, the protection will trip; otherwise, it is determined that a fault has occurred outside the zone, and the protection will be locked and will not operate.

[0031] On the other hand, the present invention also provides a two-terminal transient quantity protection system based on oscillation frequency, comprising:

[0032] A preliminary fault judgment module is used to detect the current mutation through the protection devices installed on both sides of the line and determine whether a fault has occurred; the current mutation is the change in the differential current at adjacent moments;

[0033] A sampling window length calculation module is used to determine the sampling window length according to the line system structure when it is determined that a fault occurs; the line system structure includes power supply side impedance, line parameters and line topology;

[0034] A high- and low-frequency energy calculation module is used to decompose the differential current signal of the protection line within the sampling window length into a high-frequency band and a low-frequency band by a frequency analysis method, and then calculate the high-frequency energy and the low-frequency energy;

[0035] The zone fault judgment module is used to determine that a zone fault has occurred and the protection action trips when the low-frequency energy is greater than the action threshold value and the ratio of the low-frequency energy to the high-frequency energy is greater than the ratio threshold.

[0036] On the other hand, the present invention also provides an electronic device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the two-terminal transient quantity protection method based on oscillation frequency when executing the computer program.

[0037] Optionally, the memory is a non-transitory computer-readable storage medium.

[0038] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0039] The present invention provides a two-terminal transient quantity protection method, system and device based on oscillation frequency, which separates the oscillation component from the power frequency quantity in a short time window by determining the oscillation characteristics of the line system after a fault occurs, and then distinguishes the fault by their relative relationship and the energy of the power frequency quantity, so that the action of the fault in the zone can be realized in most cases while ensuring that the fault outside the zone does not malfunction. The present invention can effectively improve the action speed of the protection and improve the reliability of the protection. At the same time, the present invention has low requirements on the algorithm, strong engineering feasibility and broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0041] Figure 1 It is a flow chart of a double-terminal transient quantity protection method based on oscillation frequency of the present invention;

[0042] Figure 2 It is a schematic diagram of the principle of a double-terminal transient quantity protection method based on oscillation frequency of the present invention;

[0043] Figure 3 A schematic diagram of a protection line system structure model provided by an embodiment of the present invention;

[0044] Figure 4 A schematic diagram of an additional fault network when an out-of-zone fault occurs according to an embodiment of the present invention;

[0045] Figure 5 A schematic diagram of a lumped parameter circuit provided by an embodiment of the present invention;

[0046] Figure 6 A schematic diagram of a curve showing changes in oscillation frequency versus distance provided in an embodiment of the present invention;

[0047] Figure 7 It is a schematic diagram of the action characteristics of the double-terminal transient quantity protection method of the present invention;

[0048] Figure 8 A schematic diagram of the maximum energy of a discrete signal provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0049] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0050] The object of the present invention is to provide a double-terminal transient quantity protection method, system and device based on oscillation frequency, so as to improve the protection action speed and reliability.

[0051] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0052] See also Figure 1 and Figure 2 The core idea of ​​the present invention is to calculate the high-frequency energy and low-frequency energy of the signal, and then perform fault judgment based on the judgment criteria. Therefore, the present invention provides a two-terminal transient quantity protection method based on oscillation frequency, including the following steps 1 to 4.

[0053] Step 1: Detect the current mutation through the protection devices installed on both sides of the line and determine whether a fault occurs.

[0054] The present invention detects whether a fault occurs by current mutation (i.e., the change in differential current at adjacent moments). When the change exceeds a threshold, it is considered that a fault may occur in the system and protection is initiated.

[0055] Specifically, the line protection devices (such as relay protection devices) installed on both sides of the line are used to detect and display the phase current waveform, and the vector sum obtained by adding the phase current signals of the lines on both sides is used as the differential current; the change amount of the differential current at adjacent moments is calculated. As the current mutation amount; when the change exceeds the threshold, that is, when When the fault occurs, it is preliminarily determined that a fault has occurred and the line protection device is activated.

[0056] Step 2: When it is determined that a fault has occurred, the sampling window length is determined according to the line system structure.

[0057] After a fault occurs, the present invention determines the sampling window length T according to the system structure, that is, the power supply side impedance, line parameters and line topology.

[0058] Figure 3 Schematic diagram of the protection line system structure model provided by the embodiment of the present invention, in which R 1 , R 2 For protection device; 1 、f 2 There are two fault points, where f 1 is the fault point outside the area, f 2 is the fault point in the area; E M Indicates the M-side power supply voltage, Z M represents the system impedance on the M side, E N Indicates the N-side power supply voltage, Z N Indicates the N-side system impedance.

[0059] The basic principle of the method of the present invention is:

[0060] The difference between the differential current during faults inside and outside the zone is mainly reflected in the power frequency component. The power frequency component accounts for a higher proportion during faults inside the zone:

[0061] i n (t)>i ω (t)+i T (t) (1)

[0062] In formula (1), i n (t) is the power frequency current, i ω (t) is the high frequency oscillating current, i T (t) is the traveling wave reflected current.

[0063] When the transient process is strong, the power frequency component accounts for a low proportion during out-of-zone faults:

[0064] i n (t)<i ω (t)+i T (t) (2)

[0065] The validity of equation (2) does not satisfy all out-of-zone fault situations. When the fault transient process is weak (small initial phase angle, high resistance grounding), the relationship expressed by the equation is not strictly valid. ω (t) and i T (t) is partially filtered out, then the remaining i n (t) shall meet the following requirements:

[0066]

[0067] In formula (3), It represents the steady-state differential current in the faulty additional network when there is an out-of-zone fault.

[0068] When the transient process is strong, the oscillation component occupies the main part of the signal. After filtering, the remaining signal amplitude is very low, and still satisfies equation (3). When there is an internal fault, the oscillation component accounts for a small proportion of the overall signal. Therefore, after filtering this part of the component, the amplitude of the power frequency current signal is still very high. For an intra-zone fault, it is a very small value. In most cases, i n (t) will be much larger than

[0069] Therefore, according to equations (1) to (3), the faults inside and outside the zone can be completely distinguished.

[0070] However, the transient quantity protection window is short and the frequency accuracy cannot reach the power frequency quantity. It is necessary to determine the minimum value of the oscillation frequency in the system through the system structure, and then determine the protection window length to ensure that there is at least one cycle of oscillation in the observation window. In this way, the oscillation quantity can be completely separated during spectrum analysis. The steps to determine the minimum oscillation frequency and the corresponding sampling window length are as follows.

[0071] Step 2.1: Calculate the impedance Z from the fault power source to the system side based on the line system structure ∑ .

[0072] When there is a fault outside the zone, the additional network is as follows Figure 4 As shown, Figure 4 in,i 0 is the current at the power outlet, i M and i N It is the current detected by the protection devices on both sides of the protected line. 0 The oscillation frequency and i M 、i N , that is, the oscillation frequency of the line differential current is consistent, so by analyzing i 0 The spectrum of the oscillation frequency can be obtained.

[0073] right Figure 4 By simplifying the fault component additional network shown in the figure, the fault network represented by lumped lines can be obtained, as shown in Figure 5 shown. Figure 5 In the example, n indicates that the circuit is divided into n π-type circuits of length R 0 The equivalent resistance of this length, Z 0 Indicates the equivalent reactance of this length, C 0 Represents the capacitance equivalent to this length; R N and Z N Indicates the impedance of the system side; U F(s) represents the fault voltage source.

[0074] according to Figure 5 The impedance Z from the fault power supply to the system side can be calculated using the line structure shown. Σ , as shown in formula (4)

[0075]

[0076] In the formula, Z n (s) represents the equivalent impedance starting from the system impedance to the nth lumped parameter.

[0077] Step 2.2: Based on the impedance Z ∑ Calculate the fault current i 0 (s) and analyze i 0 The frequency of the pole in (s).

[0078] The fault current can be expressed as:

[0079] i 0 (s)=U F (s) / Z ∑ (s) (5)

[0080] Get i 0 After the transfer function of (s), we analyze i 0 The frequency of the pole in (s).

[0081] Step 2.3: Determine the frequency corresponding to the smallest imaginary part among all poles as the oscillation frequency f ω .

[0082] Among all the poles, the frequency corresponding to the smallest imaginary part has the largest energy and the highest proportion in the spectrum, which can represent the oscillation frequency f ω size.

[0083]

[0084] In the formula, p n Indicates i 0 (s), Im means taking the imaginary part of the pole, and min means taking the minimum value.

[0085] Thus, the oscillation frequency at a certain fault location can be obtained.

[0086] Step 2.4: Determine the sampling window length

[0087] If you want to get the lowest oscillation frequency of the system, you need to set the fault location to traverse each point on the line and take the minimum value. Figure 6 The distance along the x-axis is represented by Figure 5By solving the oscillation frequency of each point in the changeable fault distance, the curve can be obtained, and its minimum value is the lowest oscillation frequency of the entire system. The lowest oscillation frequency corresponds to the longest oscillation period. As long as the sampling window length of the protection is greater than this period, it can be guaranteed that all oscillation quantities are separated from the power frequency quantities during frequency analysis.

[0088] When there are multiple adjacent lines to the protection line, it is necessary to calculate the lowest oscillation frequency in each direction and compare them to select the minimum value.

[0089] Based on this, the minimum oscillation frequency of the protection line can be calculated through line parameters and system parameters, and then the protection scheme can be designed. The sampling window length T of the protection scheme should meet the following requirements:

[0090]

[0091] In addition, for lines with a total length of less than 400 km, the frequency analysis accuracy under the observation window length of 5 ms can completely separate the transient quantity from the power frequency quantity. Therefore, for lines with unclear system structure but a total line length of less than 400 km, a 5 ms window length can be selected as the sampling window length of the protection method.

[0092] Step 3: Decompose the differential current signal of the protection line within the sampling window length into a high-frequency band and a low-frequency band by using a frequency analysis method, and then calculate the high-frequency energy and the low-frequency energy.

[0093] Through spectrum analysis, the differential current signal of the protection line is decomposed into different frequency bands, and the frequency bands below the lowest oscillation frequency f ω The sum of the frequency band values ​​is defined as the low-frequency energy E L , the sum of the remaining frequency band values ​​is defined as the high frequency energy E H According to formula (1), when there is a fault in the area, there should be:

[0094] E L >E H (8)

[0095] Considering the worst case when the initial phase angle of the fault is zero during out-of-zone faults, the protection should be reliably rejected, so the proportional coefficient K is introduced. set , and set the low-frequency energy action threshold E according to the power frequency characteristics Lset , the action criteria can be summarized as:

[0096] E L >K set E H (9)

[0097] E L >E Lset (10)

[0098] Formula (9) is essentially to use the transient characteristics of the differential current to identify faults, and should avoid the worst case of out-of-zone faults, that is, the weakest transient oscillation process. After a large number of simulation tests, it was found that the high-to-low frequency energy ratio K of out-of-zone faults under the worst conditions will not exceed 1.5. After multiplying it by the reliability coefficient 2, K is selected. set Equals 3.

[0099] The essence of formula (10) is to use the steady-state characteristics to distinguish faults. When the low-frequency energy in the calculated result is lower than the threshold, it is considered that the power frequency energy content in the differential current is insufficient. When there is an out-of-zone fault, there is no power supply inside the line to provide power frequency current. In this case, the possibility of an out-of-zone fault is too high and the protection refuses to operate. For different data analysis methods, E Lset There will be different tuning methods, which will be introduced in detail in the following analysis.

[0100] The action characteristics of the protection method are as follows: Figure 7 The protection method of the present invention sets the data window and the set value according to the line structure, and has strong adaptability. If the oscillation frequency in the system is too high, the sampling window length can be set to n times the period to improve the reliability of the protection.

[0101] Here we consider two commonly used signal frequency analysis methods as data analysis methods: wavelet transform and Fourier transform.

[0102] 3.1 Protection method based on discrete wavelet transform

[0103] Wavelet transform is a spectrum analysis method that can solve the signal of each frequency band. It can be applied in conjunction with the idea of ​​the protection method of the present invention to calculate the high and low frequency energy according to the frequency bands of different frequencies.

[0104] Discrete wavelet transform can well reflect the time-frequency characteristics of the signal, decompose the differential current signal into multiple frequency bands, and the k-th layer detail coefficient D k With the approximation coefficient A k The frequency band is:

[0105] D k :[2 -(k+1) f s ,2 -k f s ] (11)

[0106] A k :[0,2 -(k+1) f s ] (12)

[0107] Where f s is the sampling frequency. Figure 4The minimum oscillation frequency of the protection line in is 220Hz, so the fault window length is set to 5ms (including a whole oscillation cycle). The signal is transformed by k=6 layers of discrete wavelet transform, and the detail coefficients D of the first six layers are 1 ~D 6 For frequency components greater than 156 Hz, the sixth layer approximation coefficient A 6 It is a frequency component of 0 to 156 Hz.

[0108] The energy under wavelet transform is defined as the sum of the squares of the values ​​of each point in each frequency band after reconstruction. The energy of high and low frequency bands can be expressed as:

[0109]

[0110]

[0111] N is D k The number of discrete signal points within the frequency band; n represents the nth signal point.

[0112] E Lset The setting of needs to analyze the expression of the maximum power frequency current in the differential current signal after the out-of-zone fault is steady-state, so as to calculate the maximum value of the current under the energy calculation method of the present invention.

[0113] E Lset The setting of needs to obtain the maximum steady-state differential current amplitude I in the fault additional network d , in order to calculate the maximum value of the current under this energy calculation method. Obviously, when there is a three-phase fault outside the zone and the fault power source is closest to the system impedance, the amplitude of the differential current reaches the maximum, which can be obtained through mathematical calculation or modeling simulation analysis.

[0114] Figure 8 This is a schematic diagram of how to obtain the maximum energy of a discrete signal under a fixed window length. The maximum value is achieved when the peak value of the discrete signal is reached in the middle of the sampling window.

[0115] The square sum of the power frequency signal within the 5ms time window is calculated. The maximum value is when the initial phase angle of the power frequency is 45°. The maximum power frequency energy at this time can be obtained:

[0116]

[0117] In the formula, f s is the sampling frequency, N is the sampling point, K rel is the reliability coefficient. Considering that there is an edge effect in the process of wavelet transform in extracting low-frequency signals, and there is a certain error in the extraction of low-frequency signals, which may cause the amplitude of the extracted signal to be larger than the theoretical value, K is taken. rel2 Take 2 as the reliability coefficient of current amplitude, and take K rel1The reliability coefficient of the energy value is 1.5.

[0118] The high-frequency and low-frequency energy calculated by wavelet transform is shown in Table 1. The protection action condition is that the ratio K is greater than 3 and the low-frequency energy E L Greater than 34.48.

[0119] Table 1 Calculation results under the wavelet transform framework

[0120] Failure scenarios <![CDATA[E L ]]> <![CDATA[E H ]]> K Action Result Out-of-area fault 11.7 56.4 0.21 No accidental action Out-of-area fault (worst case) 0.56 0.40 1.40 No accidental action Fault in the area 2652.9 29.0 91.50 action In-area fault (worst case) 191.4 35.6 5.37 action

[0121] In the table, K is the ratio of low-frequency energy to high-frequency energy.

[0122] It can be seen that the way of reconstructing signals through wavelet transform can well separate signals of different frequency bands, and the high and low frequency band energies calculated based on this can be used to reliably identify faults inside and outside the area. However, the time domain characteristics in the transformation results of wavelet transform are actually redundant. This scheme only cares about the characteristic quantity of the signal in the frequency domain, and redundant information will increase the burden of calculation.

[0123] 3.2 Protection method based on fast Fourier transform

[0124] Fourier transform is another spectrum analysis method, which can solve the amplitude of each frequency and can also be applied in combination with the concept of the protection method of the present invention.

[0125] Fourier transform can calculate the spectrum distribution of the signal. The resolution of the spectrum depends on the length of the sampling window. Under the premise that the lowest oscillation frequency of the system is known to be 220Hz, a sampling window length of 5ms is selected, that is, the frequency resolution is 200Hz, and the spectrum decomposition results are the amplitude components corresponding to 0Hz, 200Hz, 400Hz...10kHz:

[0126]

[0127] When m=0, the calculation result is

[0128]

[0129] That is, the value at 0 Hz is essentially the sum of all values ​​of the signal within the time window.

[0130] Where m represents the sequence number after decomposition, A m represents the amplitude at the frequency point m after decomposition; x(n) represents the discrete signal, n represents the sampling point of the discrete signal in the sampling window, and N represents the total number of points of the discrete signal.

[0131] Now the transient characteristics have been separated into the remaining frequency points. In most cases of fault initial phase angles, the 0Hz component will still maintain a high value when the fault is within the zone, while the amplitude of the 0Hz component is low when the fault is outside the zone because the time window contains a complete oscillation cycle component and the sum of the signal values ​​is equal to zero. Therefore, the 0Hz component is regarded as a component that characterizes the power frequency characteristics, but the disadvantage of this approximate treatment is that it does not have sufficient accuracy. In the worst case of fault within the zone, the protection may refuse to operate.

[0132] The energy under Fourier transform is defined as the sum of the values ​​of each point in the frequency band. The energy of high and low frequency bands can be expressed as:

[0133]

[0134]

[0135] The setting of the action threshold is based on the properties of Fourier transform. The low-frequency energy is the sum of the current amplitudes. This is used to calculate the maximum value of the maximum power frequency current under this energy calculation method:

[0136]

[0137] Substituting the value of the line system into the calculation, the action threshold value E can be obtained. Lset =124.2. The calculation results are shown in Table 2.

[0138] Table 2 Calculation results under Fourier transform framework

[0139]

[0140] It can be seen that the ratio K of low-frequency energy to high-frequency energy can also be calculated by calculating the spectrum through Fourier transform, which can reliably refuse to operate in the event of an out-of-zone fault. However, for an in-zone fault, there is a dead zone in the protection. This is because the Fourier transform does not have sufficient accuracy to represent the power frequency characteristics at this time. However, the algorithm has a small amount of calculation, and the CPU can complete the calculation quickly, thereby improving the speed of the protection.

[0141] Step 4: When the low-frequency energy is greater than the action threshold and the ratio of the low-frequency energy to the high-frequency energy is greater than the ratio threshold, it is determined that an in-zone fault has occurred and the protection action trips.

[0142] Different action thresholds E are calculated by different frequency analysis methods (such as wavelet transform and Fourier transform) Lset And set the ratio threshold K set , ratio threshold K set The worst case of out-of-zone faults, that is, the weakest case of transient oscillation, should be avoided. In the embodiment of the present invention, the ratio threshold K set Set to 3. Set the low-frequency band energy EL and action threshold E Lset Compare, if it is greater than the threshold value, that is, E L >E Lset , then proceed to the next step of determination; calculate the ratio of low-frequency band energy to high-frequency band energy K = E L / E H , if the ratio K is greater than the ratio threshold K set , that is, E L >K set E H , it is determined that a fault has occurred within the zone and the protection will trip; in other cases, it is considered to be an out-of-zone fault and the protection will not be locked.

[0143] The protection method of the present invention determines the oscillation characteristics of the system after the fault occurs, separates the oscillation component from the power frequency quantity in a short time window, and then distinguishes the fault through their relative relationship and the energy of the power frequency quantity. Under the condition that the fault outside the zone is not misoperated, the action of the fault inside the zone can be realized in most cases. The protection method of the present invention can effectively improve the action speed of the protection and improve the reliability of the protection. At the same time, the protection method has low requirements on the algorithm and has strong engineering feasibility.

[0144] Compared to the protection method that utilizes the boundary effect caused by the bus capacitance of the AC line, the method of the present invention does not rely on high bus capacitance values ​​and has better adaptability. Compared to the protection method that utilizes the similarity of time domain waveforms, the waveform similarity requires a longer time window to ensure reliability, while the present invention utilizes the transient characteristics within a short time window, and the time window required for observation is not long, so the method of the present invention has a faster action speed. Compared to the traveling wave protection method, the method of the present invention has low requirements for sampling, is not affected by abnormal data, and can accurately identify faulty lines. Therefore, the two-terminal transient quantity protection method based on oscillation frequency proposed in the present invention is a transient quantity protection scheme with a faster action speed and higher reliability and feasibility.

[0145] Based on the method provided by the present invention, the present invention also provides a two-terminal transient quantity protection system based on oscillation frequency, comprising:

[0146] A preliminary fault judgment module is used to detect the current mutation through the protection devices installed on both sides of the line and determine whether a fault has occurred; the current mutation is the change in the differential current at adjacent moments;

[0147] A sampling window length calculation module is used to determine the sampling window length according to the line system structure when it is determined that a fault occurs; the line system structure includes power supply side impedance, line parameters and line topology;

[0148] A high- and low-frequency energy calculation module is used to decompose the differential current signal of the protection line within the sampling window length into a high-frequency band and a low-frequency band by a frequency analysis method, and then calculate the high-frequency energy and the low-frequency energy;

[0149] The zone fault judgment module is used to determine that a zone fault has occurred and the protection action trips when the low-frequency energy is greater than the action threshold value and the ratio of the low-frequency energy to the high-frequency energy is greater than the ratio threshold.

[0150] Furthermore, the present invention also provides an electronic device, which may include: a processor, a communication interface, a memory and a communication bus. The processor, the communication interface and the memory communicate with each other via the communication bus. The processor may call a computer program in the memory to execute the two-terminal transient quantity protection method based on oscillation frequency.

[0151] In addition, when the computer program in the above-mentioned memory is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a non-transitory computer-readable storage medium. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art or the part of the technical solution, can be embodied in the form of a software product, which is stored in a storage medium and includes several instructions for 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 invention. The aforementioned storage medium includes: various media that can store program codes, such as USB flash drives, mobile hard disks, read-only memories, random access memories, magnetic disks or optical disks.

[0152] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other. For the system disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part.

[0153] The principles and implementation methods of the present invention are described in this article using specific examples. The description of the above embodiments is only used to help understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A two-terminal transient quantity protection method based on oscillation frequency, characterized in that: include: The protection devices installed on both sides of the line detect the current mutation and determine whether a fault has occurred; the current mutation is the change in the differential current at adjacent moments; When it is determined that a fault has occurred, the sampling window length is determined according to the line system structure; The line system structure includes power supply side impedance, line parameters and line topology; Determining the sampling window length according to the line system structure specifically includes: Calculate the impedance Z from the fault power supply to the system side according to the line system structure Σ ; According to the impedance Z Σ Calculate the fault current i0(s) and analyze the frequency of the poles in i0(s); Determine the frequency corresponding to the smallest imaginary part among all poles as the oscillation frequency f ω ; In the formula, p n represents the pole of i0(s), Im represents the imaginary part of the pole, and min represents the minimum value; Determine the sampling window length The differential current signal of the protection line within the sampling window length is decomposed into a high-frequency band and a low-frequency band by a frequency analysis method, and then the high-frequency energy and the low-frequency energy are calculated; The method of decomposing the differential current signal of the protection line within the sampling window length into a high frequency band and a low frequency band by a frequency analysis method, and then calculating the high frequency energy and the low frequency energy, specifically includes: The differential current signal of the protection line within the sampling window length T is decomposed into multiple frequency bands by wavelet transform method, where the kth layer detail coefficient is D k , the approximate coefficient is A k ; Using formula Calculate high frequency energy E H ; where N is D k The number of discrete signal points in the frequency band; n represents the nth signal point; Using formula Calculate the low frequency energy E L ; or, The differential current signal of the protection line within the sampling window length T is decomposed into the amplitude A at each frequency point n by the fast Fourier transform method. n ; Using formula Calculate high frequency energy E H ; Where N is the number of frequency points after Fourier transform except the 0Hz component; Using formula Calculate the low frequency energy E L ; When the low-frequency energy is greater than the action threshold and the ratio of the low-frequency energy to the high-frequency energy is greater than the ratio threshold, it is determined that an internal fault has occurred and the protection action trips; When the low-frequency energy is greater than the action threshold value and the ratio of the low-frequency energy to the high-frequency energy is greater than the ratio threshold value, it is determined that an in-zone fault occurs and the protection action trips, specifically including: Calculate different action thresholds E by using different frequency analysis methods Lset , and set the ratio threshold K that can avoid the weakest transient oscillation process when the fault occurs outside the zone set ;in In the formula, f s is the sampling frequency, N is the sampling point, K re1l is the reliability coefficient of energy value, K rel2 is the reliability coefficient of the current amplitude, I d is the amplitude of the maximum steady-state differential current in the fault-attached network; When E L >E Lset And E L >K set E H When it is determined that a fault has occurred within the zone, the protection will trip; otherwise, it is determined that a fault has occurred outside the zone, and the protection will be locked and will not operate.

2. The double-terminal transient quantity protection method based on oscillation frequency according to claim 1 is characterized in that: The method of detecting the current mutation amount and determining whether a fault occurs by using the protection devices installed on both sides of the line specifically includes: The protection devices installed on both sides of the line are used to detect and display the phase current waveform, and the vector sum obtained by adding the phase current signals of the lines on both sides is used as the differential current; Calculate the change of differential current at adjacent moments As the current mutation amount; When the current mutation is greater than the change threshold Δ set , it is determined that a fault has occurred.

3. A two-terminal transient quantity protection system based on oscillation frequency, characterized in that: include: A preliminary fault judgment module is used to detect the current mutation through the protection devices installed on both sides of the line and determine whether a fault has occurred; the current mutation is the change in the differential current at adjacent moments; A sampling window length calculation module, used to determine the sampling window length according to the line system structure when a fault is determined to have occurred; The line system structure includes power supply side impedance, line parameters and line topology; Determining the sampling window length according to the line system structure specifically includes: Calculate the impedance Z from the fault power supply to the system side according to the line system structure Σ ; According to the impedance Z Σ Calculate the fault current i0(s) and analyze the frequency of the poles in i0(s); Determine the frequency corresponding to the smallest imaginary part among all poles as the oscillation frequency f ω ; In the formula, p n represents the pole of i0(s), Im represents the imaginary part of the pole, and min represents the minimum value; Determine the sampling window length A high- and low-frequency energy calculation module is used to decompose the differential current signal of the protection line within the sampling window length into a high-frequency band and a low-frequency band by a frequency analysis method, and then calculate the high-frequency energy and the low-frequency energy; The method of decomposing the differential current signal of the protection line within the sampling window length into a high frequency band and a low frequency band by a frequency analysis method, and then calculating the high frequency energy and the low frequency energy, specifically includes: The differential current signal of the protection line within the sampling window length T is decomposed into multiple frequency bands by wavelet transform method, where the kth layer detail coefficient is D k , the approximate coefficient is A k ; Using formula Calculate high frequency energy E H ; where N is D k The number of discrete signal points in the frequency band; n represents the nth signal point; Using formula Calculate the low frequency energy E L ; or, The differential current signal of the protection line within the sampling window length T is decomposed into the amplitude A at each frequency point n by the fast Fourier transform method. n ; Using formula Calculate high frequency energy E H ; Where N is the number of frequency points after Fourier transform except the 0Hz component; Using formula Calculate the low frequency energy E L ; The internal fault judgment module is used to determine that an internal fault has occurred and the protection action trips when the low-frequency energy is greater than the action threshold and the ratio of the low-frequency energy to the high-frequency energy is greater than the ratio threshold; When the low-frequency energy is greater than the action threshold value and the ratio of the low-frequency energy to the high-frequency energy is greater than the ratio threshold value, it is determined that an in-zone fault occurs and the protection action trips, specifically including: Calculate different action thresholds E by using different frequency analysis methods Lset , and set the ratio threshold K that can avoid the weakest transient oscillation process when the fault occurs outside the zone set ;in In the formula, f s is the sampling frequency, N is the sampling point, K re1l is the reliability coefficient of energy value, K rel2 is the reliability coefficient of the current amplitude, I d is the amplitude of the maximum steady-state differential current in the fault-attached network; When E L >E Lset And E L >K set E H When it is determined that a fault has occurred within the zone, the protection will trip; otherwise, it is determined that a fault has occurred outside the zone, and the protection will be locked and will not operate.

4. An electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the computer program, the two-terminal transient quantity protection method based on oscillation frequency as described in any one of claims 1 to 2 is implemented.

5. The electronic device according to claim 4, characterized in that: The memory is a non-transitory computer-readable storage medium.

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