High-capacity new energy transmission line fault property identification method

Abstract

The invention discloses a method for identifying the fault property of a high-capacity new energy transmission line. The method comprises the following steps of: analyzing the voltage specificity time sequence distribution characteristics of a fault phase of a wind-solar energy storage combined new energy station system under different faults; based on fault phase end voltage specificity time sequence distribution characteristics, a full-wave FFT effective value calculation model is adopted to calculate a voltage effective value; constructing a fault property identification starting criterion; and a full-wave FFT effective value is used to calculate a model jitter mechanism to construct a fault property identification criterion. According to the method, subtle changes in the fault occurrence process can be more sensitively captured, and then accurate recognition of fault properties is achieved. The method not only improves the accuracy of fault identification, but also improves the fault response speed and the stability of the power system.

Description

Technical Field

[0001] The present invention relates to the technical field of fault identification of new energy transmission lines, and in particular to a method for identifying the nature of faults of large-capacity new energy transmission lines based on specific voltage jitter identification. Background Art

[0002] As renewable energy generation gradually becomes an important part of the power system, the research on fault identification of renewable energy transmission lines in large-capacity renewable energy transmission lines is currently focused on current and voltage monitoring and relay protection technology. However, when faced with the high volatility and uncertainty of large-capacity renewable energy, existing technologies often have difficulty in effectively identifying the nature of faults, which can easily lead to false operation or missed protection, thus causing more serious power accidents.

[0003] For fault nature identification, one method is to use the harmonic saliency technology of TT transformation to identify the fault nature of the transmission line of the double-fed renewable energy station. For example, Wu Jiayi's method for identifying the fault nature of the transmission line of the double-fed renewable energy station based on harmonic saliency of TT transformation [J]. Guangdong Electric Power, 2024. However, since the TT transformation involves a relatively complex mathematical structure, it may pose certain challenges to practical applications. Another method is to use the regulation characteristics of the controllable series reactor to improve the accuracy and response speed of fault identification, such as Yang Hongyu's method for identifying the fault nature of the renewable energy transmission line based on the controllable series reactor [J]. Modern Electric Power, 2024. However, this method mainly focuses on specific grid structures and conditions, and the applicability of other types of grids needs to be further verified. Summary of the invention

[0004] In order to cope with the shortcomings of traditional fault nature identification methods under the access of large-capacity new energy, such as poor real-time performance, complex model establishment, and large amount of calculation. The present invention provides a method for identifying the fault nature of large-capacity new energy transmission lines based on specific voltage jitter identification. This method constructs a fault nature identification criterion by analyzing the timing characteristics of the voltage at the fault phase end and using the full-wave FFT to calculate the effective value jitter mechanism. This method can more sensitively capture subtle changes in the process of fault occurrence, thereby realizing accurate identification of the fault nature. This method not only improves the accuracy of fault identification, but also improves the fault response speed and the stability of the power system.

[0005] The technical solution adopted by the present invention is:

[0006] A method for identifying the nature of a large-capacity new energy transmission line fault comprises the following steps:

[0007] Step 1: Analyze the specific time series distribution characteristics of the fault phase terminal voltage of the wind-solar-energy-storage combined with new energy station system under different faults;

[0008] Step 2: Based on the specific time series distribution characteristics of the fault phase terminal voltage, the voltage effective value is calculated using the full-wave FFT effective value calculation model;

[0009] Step 3: Construct the fault nature identification start criterion;

[0010] Step 4: Use the full-wave FFT effective value calculation model to determine the jitter mechanism and construct the fault nature identification criteria.

[0011] In step 1, for the wind-solar-energy-storage combined with new energy station system, after a single-phase grounding instantaneous fault occurs in the transmission line and the circuit breaker trips, the arc at the fault point needs to go through a secondary arc stage process of burning-extinguishing-reigniting-extinguishing repeatedly. This process has highly nonlinear characteristics and causes serious disturbances to the fault phase terminal voltage. The fault phase terminal voltage in the secondary arc stage can be expressed as:

[0012]

[0013] In formula (4), V 2arc (t) is the fault phase terminal voltage in the secondary arc stage; ω is the power frequency angular frequency; U 2k-1 ,θ 2k-1 are the amplitude and initial phase angle of the harmonic component of the fault phase terminal voltage respectively; M represents the end term of the summation; k represents the start term of the summation; t represents time.

[0014] After the secondary arc is extinguished, the fault phase voltage enters the recovery stage. At this time, the fault phase voltage V recover (t) can be expressed as:

[0015]

[0016] In formula (5), U a ', β are the amplitude and phase angle of the fundamental frequency component of the recovery voltage respectively; C0 and C m They are line-to-ground capacitance and line-to-phase capacitance respectively; U cm (0 - ) represents the initial value of the capacitor voltage of the phase-to-phase capacitor at the moment of stable arc extinguishing at the fault point.

[0017] In step 2, the timing characteristics of the voltage stage under different fault conditions are analyzed by step 2. In the case of a transient ground fault, at the moment of arc extinction, the effective value shows obvious climbing and jittering phenomenon, while permanent faults do not have this characteristic. The full-wave FFT is used to calculate the effective value of the voltage and converted to the frequency domain representation. Through FFT transformation, the signal in the frequency domain is represented as:

[0018]

[0019] In formula (6), V(f) represents the frequency domain voltage value obtained after FFT transformation; v(t) represents the instantaneous voltage value; e -j2πft represents the rotation factor.

[0020] Then the effective value is obtained through the amplitude spectrum in the frequency domain:

[0021]

[0022] In formula (7), V rms represents the effective value of voltage; △f is the frequency interval; N is the number of FFT points; V(f k ) represents the voltage frequency domain sampling value at time k.

[0023] The data window length of the full-wave FFT is the power frequency period. As the data window slides, when the data window slides to the vicinity of the arc extinction point, the instantaneous value of the voltage signal undergoes a sudden change from the secondary arc to the recovery voltage, which will affect the calculation of the current effective value. Before and after the arc extinction, the instantaneous voltage value v(t) can be expressed as:

[0024]

[0025] In the formula, t0 is the arc extinction time. From formula (7) and formula (8), we can see that during the arc extinction process, the instantaneous value of the voltage changes rapidly, from V 2arc (t) mutation to V recover (t), when the data window slides to the arc extinction area, the secondary arc and recovery voltage values ​​are mixed in the data window, and the corresponding voltage effective value obtained by FFT also shows jitter. Only when the data window completely slides into the recovery voltage area will the effective value tend to be stable.

[0026] In step 3, the construction of the fault property identification start criterion includes:

[0027] B(t) represents the circuit breaker action signal, 1 means the circuit breaker has been operated, 0 means it has not been operated; considering the unstable transient process of the primary and secondary arc state conversion, in order to ensure the reliability of the criterion, the fault judgment is started after a delay of Δt=100ms after the circuit breaker is operated, and the start criterion S is comprehensively expressed as:

[0028]

[0029] In the formula, 1 indicates that the criterion is activated, 0 indicates that it is not activated, t0 is the action time of the circuit breaker, t is the current time; B(t0) indicates the action state of the circuit breaker at time t0; Δt indicates the delay time.

[0030] In step 4, the standard deviation is introduced to evaluate the data volatility in the data window. When the data window slides to the arc extinction area, the standard deviation will increase. The characteristic energy density is introduced to enhance the fault characteristics and smooth the data jitter.

[0031] Based on the full-wave effective value calculation model, the effective value climbing and jitter phenomenon is used as the fault nature identification criterion, and the moving standard deviation V of the data in the sliding data window is SD With characteristic energy density V ρstd (i) The characteristics are as follows:

[0032] The standard deviation can evaluate the data volatility in the data window. When the data window slides to the arc extinction area, the standard deviation will increase; the characteristic energy density can improve the fault characteristics and smooth the data jitter. The formulas are as follows:

[0033]

[0034]

[0035] Where V rms is the effective value of the voltage in the data window, is the average voltage in the data window, and N is the length of the data window. Set the threshold V of the fault nature identification criterion set The threshold value must be greater than the maximum characteristic energy density of the voltage in the secondary arc stage max[V 2arc_ρstd ], which is less than the maximum characteristic energy density max[V ρstd ],Pick:

[0036] V set =(max[V 2arc_ρstd ]+max[V ρstd ]) / 2(12);

[0037] The fault nature identification criteria are as follows:

[0038] V ρstd (i)>V set (13);

[0039] It can be seen from formula (13) that if the characteristic energy density exceeds the set threshold, it means that the voltage value change exceeds the normal range, that is, it can be considered that there is an effective value climbing jitter from the secondary arc extinction stage to the recovery voltage stage, and it is determined to be a transient fault. When a permanent fault occurs, this jitter phenomenon will not occur, so the nature of the fault can be identified.

[0040] The present invention relates to a method for identifying the nature of a large-capacity new energy transmission line fault, and the technical effects are as follows:

[0041] 1) The present invention combines the full-wave FFT calculation effective value jitter mechanism to more accurately capture the fault signal characteristics and improve the accuracy of fault property identification.

[0042] 2) The present invention is highly efficient in processing real-time data, can quickly respond to and identify faults, and shorten fault handling time.

[0043] 3) This method can cope with the instability and complexity of renewable energy power generation, and can more accurately capture the characteristics of fault signals, improve the accuracy of fault nature identification, is efficient in processing real-time data, can quickly respond to and identify faults, and shorten fault handling time.

[0044] 4) The present invention can better adapt to complex and changeable fault environments such as the intermittent and volatile characteristics of new energy, improve recognition accuracy and reliability, and the full-wave FFT algorithm is very mature in the field of digital signal processing and has high computational efficiency. Therefore, the invention has high feasibility and ease of implementation in practical applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The present invention will be further described below in conjunction with the accompanying drawings and examples;

[0046] Figure 1 This is the system structure topology diagram.

[0047] Figure 2 This is a flow chart for determining the nature of a fault.

[0048] Figure 3 The waveform diagram of the instantaneous value of the fault phase terminal voltage under different types of system faults;

[0049] Figure 3 In the figure: a) represents a transient ground fault; b) represents a permanent ground fault.

[0050] Figure 4 It is the waveform diagram of the effective value of the fault phase terminal voltage under different types of system faults;

[0051] Figure 4 In the figure: a) indicates a transient ground fault; b) indicates a permanent ground fault;

[0052] Figure 5 The reliability verification result of the judgment criteria under 0° closing angle fault;

[0053] Figure 5 In the middle: part a) shows the instantaneous fault with 0° closing angle at 30% of the line;

[0054] b) Partially indicates a permanent fault at 0° closing angle at 30% of the line;

[0055] c) Part indicates a transient fault with a closing angle of 0° at 50% of the line;

[0056] Part d) indicates a permanent fault with a closing angle of 0° at 50% of the line;

[0057] e) Part indicates a transient fault with a closing angle of 0° at 70% of the line;

[0058] Part f) indicates a permanent fault with a closing angle of 0° at 70% of the line.

[0059] Figure 6 The reliability verification result of the judgment criteria under the 30° closing angle fault is shown;

[0060] Figure 6 In the middle: part a) shows a transient fault at 30% of the line with a closing angle of 30°;

[0061] b) Partially indicates a permanent fault at 30° closing angle at 30% of the line;

[0062] c) Part indicates a transient fault at 30° closing angle at 50% of the line;

[0063] Part d) indicates a permanent fault at 30° closing angle at 50% of the line;

[0064] e) indicates a transient fault at 30° closing angle at 70% of the line;

[0065] Part f) indicates a permanent fault at 30° closing angle at 70% of the line.

[0066] Figure 7 The reliability verification result of the judgment criteria under the 60° closing angle fault;

[0067] Figure 7 In the middle: part a) shows a transient fault at 60° closing angle at 30% of the line;

[0068] b) Partially indicates a permanent fault at 60° closing angle at 30% of the line;

[0069] c) Partially indicates a transient fault at 60° closing angle at 50% of the line;

[0070] Part d) indicates a permanent fault at 60° closing angle at 50% of the line;

[0071] e) indicates a transient fault at 60° closing angle at 70% of the line;

[0072] Part f) indicates a permanent fault at 60° closing angle at 70% of the line.

[0073] Figure 8 The reliability verification results of the judgment criteria under different transition resistances are shown;

[0074] Figure 8 In the figure: a) part shows a 50Ω instantaneous ground fault;

[0075] b) Partially indicates a 50Ω permanent ground fault;

[0076] c) Partially indicates a 100Ω instantaneous ground fault;

[0077] Part d) indicates a 100Ω permanent ground fault;

[0078] e) Partially indicates a 300Ω instantaneous ground fault;

[0079] f) Partially indicates a 300Ω permanent ground fault;

[0080] g) Part indicates a 500Ω instantaneous ground fault;

[0081] h) Part indicates a 500Ω permanent ground fault

[0082] i) Part indicates a 1000Ω instantaneous ground fault;

[0083] Part j) represents a 1000Ω permanent ground fault.

[0084] Fig. 9 The reliability verification results of the judgment criteria under the access of new energy with different capacities are provided;

[0085] Fig. 9 Middle: Part a) shows the verification results when the photovoltaic capacity is 30MW and the wind farm capacity is 30MW;

[0086] Part b) shows the verification results when the photovoltaic capacity is 50MW and the wind farm capacity is 50MW;

[0087] Part c) shows the verification results under the conditions of 70MW photovoltaic capacity and 70MW wind farm capacity;

[0088] Part d) shows the verification results when the photovoltaic capacity is 100MW and the wind farm capacity is 100MW. DETAILED DESCRIPTION

[0089] A method for identifying the fault nature of large-capacity renewable energy transmission lines based on specific voltage jitter identification. This method aims to solve the shortcomings of traditional fault nature identification methods under large-capacity renewable energy access, such as poor real-time performance, complex model establishment, and large amount of calculation. Based on the analysis of the time series characteristics of the fault phase voltage, the full-wave FFT is used to calculate the effective value jitter mechanism, and the fault nature identification criterion is constructed. This method fully considers the feed-in characteristics and impact after large-capacity renewable energy access, targets the time series distribution characteristics of the fault phase voltage under different faults, calculates the effective value jitter mechanism through full-wave FFT, applies the voltage standard deviation to evaluate the volatility of the data in the data window, introduces characteristic energy density to improve the fault characteristics, and constructs a fault nature identification method under renewable energy transmission lines. This method can cope with the instability and complexity of renewable energy power generation, and can more accurately capture the characteristics of fault signals, improve the accuracy of fault nature identification, and is efficient in processing real-time data. It can quickly respond to and identify faults and shorten fault handling time.

[0090] The method for identifying the fault nature of a large-capacity renewable energy transmission line based on specific voltage jitter recognition specifically comprises the following steps:

[0091] Step 1: Analysis of fault feed-out characteristics of wind, solar and storage combined with new energy stations. When a fault occurs in the system, low voltage ride-through control is not performed and conventional steady-state power control is used. The power control model can be expressed by formula (1):

[0092] P total =P wind +P solar +P storage (1);

[0093] Among them, P total is the total system power, P wind is the wind power generation power, P solar is the photovoltaic power generation power, P storage is the power of the energy storage system. When a fault occurs, the output power after the fault can be described by formula (2):

[0094] P out =P total -P loss (2);

[0095] Among them, P out is the output power after the fault, P loss is the power loss caused by the fault. After the fault is removed, the steady-state power control can be expressed as:

[0096] P control =K p *(P set -P out )+K i *∫(P set -P out )dt(3);

[0097] Among them, P control To control power, P set is the set target power, K p and K i is the proportional-integral control gain.

[0098] Step 2: Identification of the specific time-series distribution characteristics of the fault phase terminal voltage. In PSCAD, a simulation model is built for a single-phase grounding instantaneous fault and a permanent fault in a transmission line without a shunt reactor. The time-series distribution characteristics of the fault voltage under different fault conditions are analyzed. The time-series voltage characteristics of instantaneous and permanent faults can be distinguished by the voltage characteristics of the secondary arc and the recovery voltage. In the event of a single-phase grounding instantaneous fault, due to the lack of a reliable grounding point, after the circuit breaker cuts off the fault, the remaining energy in the line maintains the secondary arc to burn and extinguish repeatedly. After the secondary arc is extinguished, the fault current and voltage suddenly change, causing the system voltage to show obvious jitter on the waveform. In the event of a transient grounding fault, obvious voltage jitter occurs after the secondary arc stage ends; in the event of a permanent fault, due to the presence of a single-phase grounding fault, there is a reliable grounding point. In the primary arc stage, the energy decays to zero through the line grounding point, so there is no voltage jitter. The voltage at the fault phase terminal in the secondary arc stage can be expressed as:

[0099]

[0100] Where V 2arc (t) is the fault phase terminal voltage in the secondary arc stage; ω is the power frequency angular frequency; U 2k-1 ,θ 2k-1 are the amplitude and initial phase angle of the harmonic component of the fault phase voltage respectively; M represents the end term of the summation; k represents the start term of the summation; the fault phase voltage V recover (t) can be expressed as:

[0101]

[0102] Where U a ', β are the amplitude and phase angle of the fundamental frequency component of the recovery voltage respectively; C0 and C m They are line-to-ground capacitance and line-to-phase capacitance respectively; U cm (0 - ) represents the initial value of the capacitor voltage when the phase-to-phase capacitor is stable and arc extinguished at the fault point.

[0103] Step 3: The timing characteristics of the voltage stage under different fault conditions are obtained by analyzing step 2. In the case of a transient ground fault, at the moment when the arc is extinguished, the effective value shows obvious climbing and jittering, while permanent faults do not have this characteristic. The full-wave FFT is used to calculate the effective value of the voltage and converted to the frequency domain. Through FFT transformation, the signal in the frequency domain is represented as follows:

[0104]

[0105] Where V(f) represents the frequency domain voltage value obtained after FFT transformation; v(t) represents the instantaneous voltage value; e -j2πft represents the rotation factor.

[0106] Then the effective value is obtained through the amplitude spectrum in the frequency domain:

[0107]

[0108] Where △f is the frequency interval and N is the number of FFT points. The data window length of the full-wave FFT is the power frequency period. As the data window slides, when the data window slides to the vicinity of the arc extinction point, the instantaneous value of the voltage signal undergoes a sudden change from the secondary arc to the recovery voltage, which will affect the calculation of the current effective value. Before and after the arc extinction, the instantaneous voltage value v(t) can be expressed as:

[0109]

[0110] In the formula, t0 is the arc extinction time. From formulas (7) and (8), we can see that during the arc extinction process, the instantaneous value of the voltage changes rapidly, from V 2arc (t) mutation to V recover (t), when the data window slides to the arc extinction area, the secondary arc and recovery voltage values ​​are mixed in the data window, and the corresponding voltage effective value obtained by FFT also shows jitter. Only when the data window completely slides into the recovery voltage area will the effective value tend to be stable.

[0111] Step 4: Construction of fault nature identification start criteria:

[0112] B(t) represents the circuit breaker action signal (1 means the circuit breaker has been operated, 0 means it has not been operated). Considering the unstable transient process of the primary and secondary arc state conversion, in order to ensure the reliability of the criterion, the circuit breaker is operated and the fault is judged after a delay of Δt=100ms. The comprehensive expression of the starting criterion S is:

[0113]

[0114] In the formula, 1 indicates that the criterion is activated; 0 indicates that it is not activated; t0 is the action time of the circuit breaker; t is the current time; B(t0) indicates the action state of the circuit breaker at time t0; Δt indicates the delay time.

[0115] Step 5: Use the full-wave FFT effective value calculation model jitter mechanism to build a fault nature identification criterion. Based on the full-wave effective value calculation model, the effective value climbing jitter phenomenon is used as the fault nature identification criterion, and the moving standard deviation V of the data in the sliding data window is SD With characteristic energy density V ρstd (i) The characteristics are as follows: the standard deviation can evaluate the data volatility in the data window. When the data window slides to the arc extinction area, the standard deviation will increase; the characteristic energy density can improve the fault characteristics and smooth the data jitter. The formulas are as follows:

[0116]

[0117] Where V rms is the effective value of the voltage in the data window, is the average voltage in the data window, and N is the length of the data window. Set the threshold V of the fault nature identification criterion set The threshold value must be greater than the maximum characteristic energy density of the voltage in the secondary arc stage max[V 2arc_ρstd ], which is less than the maximum characteristic energy density max[V ρstd ],Pick:

[0118] V set =(max[V 2arc_ρstd ]+max[V ρstd ]) / 2 (12)

[0119] The fault nature identification criteria are as follows:

[0120] V ρstd (i)>V set (13)

[0121] It can be seen from formula (13) that if the characteristic energy density exceeds the set threshold, it means that the voltage value change exceeds the normal range. It can be considered that there is an effective value climbing jitter from the secondary arc extinction stage to the recovery voltage stage, and it is determined to be a transient fault. When a permanent fault occurs, this jitter phenomenon will not occur, so the nature of the fault can be identified.

[0122] Case analysis:

[0123] The fault nature is determined and the reliability of the judgment criteria under different faults is verified. A double-fed wind turbine and photovoltaic power generation system model is built in PSCAD, as shown in Figure (1). S It is a traditional power supply. On the right are the double-fed wind turbine power supply G DFIG and photovoltaic power source G PV , the effective value of the fault phase terminal voltage is imported into MATLAB for criterion reliability verification. The total length of the line is set to 60km, the system 0° closing angle time is 2s, the instantaneous ground fault duration is 0.3s, the circuit breaker one-time action time is 2.1s, and the reclosing time is 2.6s.

[0124] According to the full-wave FFT algorithm proposed in the present invention, the effective value of the fault phase terminal voltage is calculated, the effective value of the fault phase terminal voltage in PSCAD is imported into MATLAB, and the moving standard deviation and characteristic energy density of the fault phase terminal voltage are obtained by formula (10) and formula (11), and then the fault type is judged based on formula (13). The fault property identification flow chart is as follows: Figure 2 shown.

[0125] The patent of this invention mainly verifies the reliability of the fault nature judgment criteria from three aspects:

[0126] (1) Reliability verification of the judgment criteria under different closing angle faults

[0127] (2) Reliability verification of the judgment criteria under different transition resistances

[0128] (3) Reliability verification of the judgment criteria under different capacities of new energy access

[0129] The instantaneous value and effective value waveforms of the fault phase terminal voltage are as follows: Figure 3 , Figure 4 As shown in the figure: when a single-phase grounding instantaneous fault occurs, due to the lack of a reliable grounding point, after the circuit breaker cuts off the fault, the remaining energy in the line maintains the repeated burning and extinguishing of the secondary arc. After the secondary arc is extinguished, the fault current and voltage suddenly change, which will cause obvious jitter in the system voltage waveform; on the contrary, in the case of a permanent fault, due to the presence of a reliable grounding point for a single-phase grounding fault, the energy decays to zero through the line grounding point during the primary arc stage, so there will be no voltage jitter. The full-wave FFT effective value calculation model is characterized in that as the data window slides, when the data window slides to the vicinity of the arc extinguishing point, the instantaneous value of the voltage signal undergoes a sudden change from the secondary arc to the recovery voltage, which will affect the calculation of the current effective value.

[0130] 1. Considering the impact of different closing angle faults on electrical quantities, the reliability of the judgment criteria under different closing angle faults is first verified. The parameter settings are shown in Table 1:

[0131] Table 1 Faults at different closing angles

[0132]

[0133] The simulation results are as follows Figure 5 , Figure 6 , Figure 7As shown, it can be seen that for instantaneous single-phase grounding faults, different closing angles have a significant impact on the voltage jitter phenomenon caused by instantaneous single-phase grounding faults. Therefore, the characteristic energy density under different closing angle faults is also very different. This is because under different closing angles, the charging and discharging process of capacitors and inductors, the frequency and amplitude of electromagnetic oscillations, etc. during the system recovery process will be different, which will affect the voltage jitter phenomenon and the change of characteristic energy density; on the contrary, for permanent grounding faults, the fault point persists, and the influence of the closing angle is relatively small. At this time, the characteristic energy density is also relatively stable. Under the same closing angle, if it is an instantaneous fault, the closer the fault location is to the power supply side, the larger the fault current, the faster the system responds to the fault, and the more obvious the voltage jitter. On the contrary, the farther the fault location is from the power supply side, the attenuation of the fault current makes the voltage jitter phenomenon less obvious; if it is a permanent fault, no matter whether the fault location is far away from or close to the power supply side, the voltage jitter phenomenon is relatively small due to the persistence of the fault caused by the reliable grounding point of permanent grounding.

[0134] Based on the above analysis, although there are differences in the electrical quantities in the system when faults occur at different closing angles and different positions, the method for identifying the nature of faults based on specific voltage jitter characteristics proposed in the patent of this invention accurately meets the identification requirements, indicating that the method proposed in the patent of this invention is not affected by the fault position and the fault closing angle.

[0135] 2. In a single-phase grounding fault, the change of transition resistance will affect the flow characteristics of the fault current, thereby affecting the voltage fluctuation. Therefore, the reliability of the judgment criteria under different transition resistances is verified, and the parameter settings are shown in Table 2:

[0136] Table 2 Faults under different transition resistances

[0137]

[0138]

[0139] The simulation results are as follows Figure 8 As shown in the figure, for instantaneous faults, under different transition resistance conditions, the jitter phenomenon of the effective value of the fault phase terminal voltage is not affected by the transition resistance. The fundamental reason is that there are differences between the stages before and after the arc is extinguished, so the jitter phenomenon is bound to exist; for permanent faults, under different transition resistance conditions, the characteristic energy density basically maintains a constant value. This is because in permanent grounding faults, due to the continuous existence of the fault point, the transition resistance mainly affects the steady-state value of the fault current rather than its dynamic characteristics, and the characteristics of permanent faults are relatively consistent (that is, the fault point continues to exist), so the system's response to the fault energy also tends to be consistent, resulting in the characteristic energy density remaining at a constant value.

[0140] Based on the above analysis, under different transition resistance conditions, transient fault voltage jitter phenomenon is bound to exist while permanent fault has no jitter phenomenon, indicating that the method proposed in the patent of this invention is not affected by transition resistance.

[0141] 3. Due to the intermittent and volatile characteristics of new energy, when these new energy sources are connected to the power system on a large scale, the power structure, power flow distribution and dynamic response of the system will change. Therefore, the reliability of the judgment criteria under the connection of new energy sources of different capacities is verified. The parameter settings are shown in Table 3:

[0142] Table 3 Faults under different new energy access conditions

[0143]

[0144] The simulation results are as follows Fig. 9 As shown in the figure, for instantaneous ground faults, the larger the new energy access capacity, the smaller the characteristic energy density. The reasons for this result are as follows: new energy power generation equipment (such as wind power and photovoltaic power) is usually connected to the power grid through power electronic converters. These converters may exhibit impedance characteristics different from traditional synchronous generators when they fail. After the new energy is connected, the dynamic characteristics of the fault current will change. New energy power generation equipment often has fast response and adjustment capabilities, which can compensate for the unstable factors of the system to a certain extent.

[0145] Based on the above analysis and taking into account the impact of various differences on the system electrical quantities when a fault occurs, the method for identifying the nature of a fault based on specific voltage jitter characteristics proposed in the present invention can effectively, accurately and quickly complete fault identification under different fault conditions, verifying the reliability and advantages of the fault nature judgment criterion.

Claims

1. A method for identifying the nature of faults in a large-capacity renewable energy transmission line, characterized in that The following steps are involved: Step 1: Analyze the specific time series distribution characteristics of the fault phase terminal voltage of the wind-solar-energy-storage combined with new energy station system under different faults; Step 2: Based on the specific time series distribution characteristics of the fault phase terminal voltage, the voltage effective value is calculated using the full-wave FFT effective value calculation model; Step 3: Construct the fault nature identification start criterion; Step 4: Use the full-wave FFT effective value calculation model to determine the jitter mechanism and construct the fault nature identification criteria.

2. According to claim 1, a method for identifying the fault nature of a large-capacity renewable energy transmission line is characterized by: In step 1, for the wind-solar-energy-storage combined with new energy station system, after a single-phase grounding instantaneous fault occurs in the transmission line and the circuit breaker trips, the arc at the fault point needs to go through a secondary arc stage process of burning-extinguishing-reigniting-extinguishing repeatedly. This process has highly nonlinear characteristics and causes serious disturbances to the fault phase terminal voltage. The fault phase terminal voltage in the secondary arc stage is expressed as: In formula (4), V 2arc (t) is the fault phase terminal voltage in the secondary arc stage; ω is the power frequency angular frequency; U 2k-1 ,θ 2k-1 are the amplitude and initial phase angle of the harmonic component of the fault phase terminal voltage respectively; M represents the end term of the summation; k represents the start term of the summation; t represents time; After the secondary arc is extinguished, the fault phase voltage enters the recovery stage. At this time, the fault phase voltage V recover (t) can be expressed as: In formula (5), U a ', β are the amplitude and phase angle of the fundamental frequency component of the recovery voltage respectively; C0 and C m They are line-to-ground capacitance and line-to-phase capacitance respectively; U cm (0 - ) represents the initial value of the capacitor voltage of the phase-to-phase capacitor at the moment of stable arc extinguishing at the fault point.

3. According to claim 1, a method for identifying the fault nature of a large-capacity new energy transmission line is characterized by: In step 2, full-wave FFT is used to calculate the effective value of the voltage and converted into a frequency domain representation. Through FFT transformation, the signal is represented in the frequency domain as follows: In formula (6), V(f) represents the frequency domain voltage value obtained after FFT transformation; v(t) represents the instantaneous voltage value; e -j2πft represents the rotation factor; Then the effective value is obtained through the amplitude spectrum in the frequency domain: In formula (7), V rms represents the effective value of voltage; △f is the frequency interval; N is the number of FFT points; V(f k ) represents the voltage frequency domain sampling value at time k; The data window length of the full-wave FFT is the power frequency period. As the data window slides, when the data window slides to the vicinity of the arc extinction point, the instantaneous value of the voltage signal undergoes a sudden change from the secondary arc to the recovery voltage. This sudden change will affect the calculation of the current effective value. Before and after the arc extinction, the instantaneous voltage value v(t) is expressed as: In the formula, t0 is the arc extinction time. From formula (7) and formula (8), we can see that during the arc extinction process, the instantaneous value of the voltage changes rapidly, from V 2arc (t) mutation to V recover (t), when the data window slides to the arc extinction area, the secondary arc and recovery voltage values ​​are mixed in the data window, and the corresponding voltage effective value obtained by FFT also shows jitter.

4. According to claim 1, a method for identifying the fault nature of a large-capacity new energy transmission line is characterized by: In step 3, the construction of the fault property identification start criterion includes: B(t) represents the circuit breaker action signal, 1 means the circuit breaker has been operated, 0 means it has not been operated; considering the unstable transient process of the primary and secondary arc state conversion, in order to ensure the reliability of the criterion, the fault judgment is started after a delay of Δt=100ms after the circuit breaker is operated, and the start criterion S is comprehensively expressed as: In the formula, 1 indicates that the criterion is activated, 0 indicates that it is not activated, t0 is the action time of the circuit breaker, t is the current time; B(t0) indicates the action state of the circuit breaker at time t0; Δt indicates the delay time.

5. According to claim 1, a method for identifying the fault nature of a large-capacity new energy transmission line is characterized by: In step 4, the effective value climbing jitter phenomenon based on the full-wave effective value calculation model is used as the fault property identification criterion, and the moving standard deviation V of the data in the sliding data window is SD With characteristic energy density V ρstd (i) The characteristics are as follows: The standard deviation can evaluate the data volatility in the data window. When the data window slides to the arc extinction area, the standard deviation will increase. The characteristic energy density can improve the fault characteristics and smooth the data jitter. The formulas are as follows: Where V rms is the effective value of the voltage in the data window, is the average voltage in the data window, N is the length of the data window; set the threshold V of the fault nature identification criterion set The threshold value must be greater than the maximum characteristic energy density of the voltage in the secondary arc stage max[V 2arc_ρstd ], which is less than the maximum characteristic energy density max[V ρstd ],Pick: V set =(max[V 2arc_ρstd ]+max[V ρstd ]) / 2(12); The fault nature identification criteria are as follows: V ρstd (i)>V set (13); It can be seen from formula (13) that if the characteristic energy density exceeds the set threshold, it means that the voltage value change exceeds the normal range, that is, it can be considered that there is an effective value climbing jitter from the secondary arc extinction stage to the recovery voltage stage, and it is determined to be a transient fault. When a permanent fault occurs, this jitter phenomenon will not occur, so the nature of the fault can be identified.

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