A centralized photovoltaic sending-out line single-phase ground fault location method and system

Abstract

The application discloses a centralized photovoltaic sending-out line single-phase grounding fault ranging method and system, after detecting that a power frequency voltage drop exceeds 10% and lasts for 3 ms, the inverter is switched to a low-voltage ride-through control strategy, a positive sequence double-frequency characteristic signal is injected, phase selection is performed by using a phase difference of a power frequency voltage sequence component, voltage and current of a fault phase under the double-frequency signal are collected, a double-frequency composite sequence network equivalent circuit is established based on a boundary condition of a fault point sequence component, a frequency domain equation is listed at a line first end, a frequency domain parameter equation containing a fault distance is derived by combining the boundary condition, the frequency domain equation is split into a real part equation and an imaginary part equation, a nonlinear equation group containing four equations is constructed by using the double-frequency signal, characteristic signal phasors are extracted by using a sliding window, the nonlinear equation group is solved by using an interior point method, and a fault ranging result is output, so that the industry problem of single-phase grounding fault ranging of a photovoltaic sending-out line is solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power system fault detection, and particularly relates to a centralized photovoltaic sending-out line single-phase ground fault ranging method and system. BACKGROUND

[0002] New energy power generation, especially photovoltaic power generation, has been increasing in penetration rate in the power grid. In 2024, the installed capacity of photovoltaic power generation increased by 277.17 GW, an increase of about 28% year-on-year, and the cumulative installed capacity reached 886.66 GW. With the continuous progress of photovoltaic technology, the conversion efficiency of photovoltaic cells is continuously improved, and the production cost is continuously reduced, making photovoltaic power generation more competitive in the energy market.

[0003] However, unlike the traditional synchronous generator-dominated power system, new power systems use a large number of power electronic devices for grid connection, and their fault characteristics are affected by the vulnerability of fully controlled power electronic switches, showing nonlinearity and weak feed, which is significantly different from the fault characteristics of traditional systems.

[0004] Single-phase ground fault is a common fault type in new energy sending-out lines, with a high probability of occurrence. After the connection of new energy systems containing inverters and other power electronic devices, the short-circuit current of single-phase ground fault is often only one to two times the rated short-circuit current; at the same time, the connection of power electronic devices destroys the assumption of the same phase of the fault branch current and the current at the installation site of the distance measuring device based on the traditional power frequency quantity single-end distance measurement method, making it difficult for traditional fault location methods based on power frequency current to accurately determine the fault location. SUMMARY

[0005] The application provides a centralized photovoltaic sending-out line single-phase ground fault ranging method and system, which uses the high controllability of photovoltaic grid-connected inverters to inject positive sequence double-frequency characteristic signals, increases non-power frequency fault information, and realizes single-phase ground fault ranging of photovoltaic sending-out lines in new energy power systems based on consideration of the parameters of the opposite end system and the transition resistance, to solve the technical problems of low ranging accuracy and poor positioning of single-phase ground fault ranging of photovoltaic sending-out lines in new energy power systems.

[0006] The application adopts the following technical solutions:

[0007] A centralized photovoltaic sending-out line single-phase ground fault ranging method, comprising the following steps:

[0008] After the occurrence of a line fault, the normal control strategy is switched to a low-voltage ride-through control strategy according to a fault starting criterion; at the same time of switching to the low-voltage ride-through control strategy, an additional signal injection control strategy is put into operation, and on the basis of the power frequency control component, a positive sequence double-frequency characteristic signal is injected into the sending-out line through the inverter additional control;

[0009] The phase selection is realized according to the phase difference between the positive sequence component of the power frequency voltage and the negative sequence component of the power frequency voltage; meanwhile, the voltage and the current of the fault phase under the positive sequence double-frequency characteristic signal are collected, and based on the sequence component boundary condition of the single-phase grounding fault at the fault branch, a composite sequence network equivalent circuit under the positive sequence double-frequency characteristic signal is established;

[0010] According to the obtained composite sequence network equivalent circuit, the frequency domain network equation of the composite sequence network is written at the head end of the to-be-measured line, and the frequency domain parameter equation containing the fault distance is obtained in combination with the boundary condition of the fault point;

[0011] The frequency domain parameter equation is split into a real part equation and an imaginary part equation; two groups of equations corresponding to the positive sequence double-frequency characteristic signal are used to construct a nonlinear equation group containing four equations;

[0012] The phasor of the positive sequence double-frequency characteristic signal is extracted in a preset data window; the nonlinear equation group is solved by using an interior point method, and the fault distance measurement result of the new energy transmission line is output.

[0013] Preferably, the fault starting criterion is that the power frequency voltage of the transmission line is monitored in real time, and when it is detected that the power frequency voltage drops by more than 10% and lasts for 3 ms, it is determined that a line fault occurs, triggering the distance measurement process.

[0014] Preferably, the positive sequence double-frequency characteristic signal is two different non-power frequency frequencies, and the frequency range is 100-500 Hz.

[0015] Preferably, the low voltage ride through control strategy includes:

[0016] The power frequency electrical quantity at the grid connection point is divided into the positive sequence component of the power frequency voltage and the negative sequence component of the power frequency voltage through a positive and negative sequence separation link, the positive sequence component of the power frequency voltage is controlled by using a reactive power priority control strategy, and the negative sequence component of the power frequency voltage is controlled by using a negative sequence suppression strategy.

[0017] Preferably, the phase selection is realized according to the phase difference between the positive sequence component of the power frequency voltage and the negative sequence component of the power frequency voltage, and specifically:

[0018] When the phase difference between the positive sequence component of the power frequency voltage and the negative sequence component of the power frequency voltage of a certain phase voltage is 180°, the phase is determined as a fault phase, and the phase selection result when a single-phase grounding fault occurs is as follows:

[0019]

[0020] Wherein, , and are the current fault components under the positive sequence, the negative sequence and the zero sequence respectively.

[0021] Preferably, the frequency domain parameter equation containing the fault distance is as follows:

[0022]

[0023] in, , This indicates the positive sequence of voltage and current at the protection measuring point under the characteristic frequency. For transition resistance, , These represent the phase voltage and current at the characteristic frequency, respectively. K The zero-order compensation coefficient is... d The distance to the fault. This refers to the zero-sequence component of the current at the protection measuring point at the characteristic frequency. This is the positive-sequence impedance per unit line at the characteristic frequency. This is the equivalent impedance of the system at the opposite end at the characteristic frequency.

[0024] Preferably, based on the obtained equivalent circuit of the composite sequence network, the frequency domain network equation of the composite sequence network is written at the beginning of the line to be measured as follows:

[0025]

[0026] in, , and These represent the positive-sequence, negative-sequence, and zero-sequence components of the voltage at the fault branch at the characteristic frequency, respectively. , and These represent the positive-sequence, negative-sequence, and zero-sequence components of the voltage at the protection measuring point under the characteristic frequency, respectively. , and These represent the positive-sequence, negative-sequence, and zero-sequence components of the current at the protection measuring point under the characteristic frequency, respectively. , and These are the unit line impedances for the positive sequence, negative sequence, and zero sequence at the characteristic frequency, respectively.

[0027] Preferably, a system of four nonlinear equations is constructed using two sets of equations corresponding to the positive-sequence dual-frequency characteristic signals. , , and The details are as follows:

[0028]

[0029] in, , , and These represent the positive sequence voltage and positive sequence current of the protection measuring point under two injection frequencies, respectively. , , and are the voltage and current of phase a of the protection measuring point under two injection frequencies, respectively; and are the zero sequence currents of the protection measuring point under two injection frequencies, is the proportional coefficient of two injection frequencies, R f is the transition resistance, K is the zero sequence compensation coefficient, and are the unit line positive sequence impedance under the characteristic frequency and the equivalent impedance of the opposite end system under the characteristic frequency, respectively.

[0030] Preferably, the phasor of the positive sequence double-frequency characteristic signal is extracted in a preset data window by a sliding window, the nonlinear equation set is solved by an interior point method, and a fault distance measurement result of the new energy sending-out line is output, specifically as follows:

[0031] the phasor of the positive sequence double-frequency characteristic signal is extracted in a preset data window by a sliding window, N the phasor of the positive sequence double-frequency characteristic signal is extracted in a preset data window by a sliding window,

[0032] In a second aspect, an embodiment of the present application provides a centralized photovoltaic sending-out line single-phase ground fault distance measurement system, comprising:

[0033] The switching module switches from a normal control strategy to a low voltage ride through control strategy according to a starting criterion after the fault occurs; the additional signal injection control strategy is put into operation at the same time of switching the low voltage ride through control strategy, and the positive sequence double-frequency characteristic signal is injected into the sending-out line through the inverter additional control on the basis of the power frequency control component;

[0034] The acquisition module realizes fault phase selection according to the phase difference of the positive sequence and negative sequence components of the power frequency voltage; meanwhile, the voltage and current of the fault phase under the positive sequence double-frequency characteristic signal are collected, and a composite sequence network equivalent circuit under the positive sequence double-frequency characteristic signal is established based on the sequence component boundary condition of the single-phase ground fault at the fault branch;

[0035] The equation module writes the frequency domain network equation of the composite sequence network at the head of the to-be-measured line according to the obtained composite sequence network equivalent circuit, and obtains the frequency domain parameter equation containing the fault distance in combination with the boundary condition of the fault point;

[0036] The construction module splits the frequency domain parameter equation into real part and imaginary part equations; the nonlinear equation set containing four equations is constructed by using two groups of equations corresponding to the double-frequency signal;

[0037] The output module extracts the phasor of the feature signal in a preset data window by sliding window, solves the nonlinear equation set by using an interior point method, and outputs the fault location result of the new energy sending-out line.

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

[0039] A centralized photovoltaic sending-out line single-phase grounding fault location method, which injects positive sequence double-frequency characteristic signals by using the active control ability of photovoltaic inverters and solves the problem of inaccurate traditional fault location through frequency domain modeling. Specifically, the method switches to a low voltage ride through control strategy after the fault occurs and injects positive sequence double-frequency characteristic signals, realizes fault phase selection through component phase difference, establishes a composite sequence network equivalent circuit, and finally constructs a nonlinear equation set by using the positive sequence double-frequency characteristic signals and solves the fault location. The advantages of the method are reflected in three aspects: first, the high controllability of photovoltaic inverters is fully utilized, and no additional signal generating device is needed to inject characteristic signals into the line, which not only reduces the hardware cost, but also avoids the dependence of traditional traveling wave method on high-precision synchronous clock; second, by constructing a composite sequence network, the influence of weakly fed characteristics of new energy sending-out line on power frequency fault location is avoided, and the traditional power frequency fault location depends on the fault current amplitude, while the output current of the new energy power electronic interface is limited, resulting in weak fault characteristics, the positive sequence double-frequency characteristic signals injected by the method enhance the recognizability of fault characteristics; finally, the real and imaginary parts of the frequency domain parameter equation form a redundant equation set, solving the nonlinear problem caused by the transition resistance and improving the fault location accuracy. The method realizes the closed-loop innovation from signal injection, feature extraction to equation solving, solves the problems of weakly fed characteristics, transition resistance influence and parameter uncertainty at the receiving end in the new energy scene under the premise of ensuring the economy of single-ended fault location; compared with the traditional method, the method does not need additional signal generating device, directly uses the control ability of the inverter itself to realize signal injection, reduces the hardware cost and system complexity. Through the injection and detection of characteristic signals, combined with the improved fault location algorithm, the single-phase grounding fault location accuracy can be significantly improved.

[0040] Further, the criterion reliability and control response real-time are considered. The starting criterion adopts the double conditions of power frequency voltage drop exceeding 10% and lasting for 3 ms. The threshold of 10% is higher than the normal fluctuation range to prevent false start, and the duration of 3 ms ensures to avoid transient disturbance. This strict time window design is derived from the fast response characteristics of power electronic equipment, which improves the fault identification speed by nearly 7 times compared with the weekly criterion of traditional protection. In the control strategy, the power frequency component is controlled differently through the positive and negative sequence separation link: the reactive power priority strategy is adopted for the positive sequence component to dynamically increase the reactive current output during voltage drop and enhance the voltage support capability; the negative sequence suppression strategy is adopted for the negative sequence component to limit the negative sequence current to within 5% through closed-loop control to avoid the thermal shock of unbalanced current to the power device of the inverter. The outstanding advantage of this separation control strategy is that it not only meets the reactive power support requirement of the low voltage ride through control strategy of the grid connection guide, but also ensures the minimization of the background harmonic of the system during the signal injection stage, creating ideal conditions for subsequent feature signal detection. Experimental data shows that the strategy can restore the fault phase voltage to more than 0.85 p.u. within 20 ms, providing a stable electrical environment for the ranging process.

[0041] Further, the frequency range is 100-500 Hz, which achieves the best balance between signal detectability and system safety. The non-power frequency signal avoids confusion with the fundamental component, such as the combination of 200 Hz and 300 Hz, which can utilize the line bandwidth and avoid common harmonic frequency points. The advantage of high frequency band (>100 Hz) signal is that its impedance sensitivity is higher. The inductive reactance ωL in the line per unit length impedance increases linearly with frequency, making the fault distance parameter d more significant in the frequency domain equation. However, it needs to be limited within 500 Hz, because the switching frequency of photovoltaic inverters is usually 2-20 kHz, and 500 Hz signal can be accurately generated through PWM modulation, while too high frequency will cause switching loss and heat dissipation problems. The dual frequency design improves the anti-interference ability through signal redundancy: when one frequency point is affected by system resonance, the other frequency point can still provide effective equation. The test shows that the error of dual frequency ranging under 300Ω transition resistance is reduced by 60% compared with single frequency. In addition, the positive sequence injection mode avoids the impact on the transformer neutral point insulation, which meets the safety specifications of power equipment. The frequency optimization selection makes the signal amplitude only need to be 5%-10% of the power frequency current, which maximizes the occupation of the output capacity of the inverter.

[0042] Further, the low-voltage ride-through control strategy includes reactive power priority control strategy and negative sequence suppression strategy, which has the technical advantage of optimizing the power quality and the reliability of fault location. The reactive power priority control strategy is realized by modifying the d-axis current reference value: the d-axis corresponds to the active component in normal operation, and the d-axis corresponds to the reactive component during fault. This strategy makes the reactive current increase to 130% of the rated value within 100 ms, accelerating the voltage recovery process. The negative sequence suppression strategy uses double-loop control: the inner loop provides high gain at the negative sequence frequency through the controller to suppress the negative sequence current below 0.05 p.u., and the outer loop adjusts the negative sequence voltage to near zero. The core value of this design is to eliminate the interference of negative sequence component on the phase selection link: the traditional method is prone to misjudgment of fault phase in severe asymmetry, while this strategy reduces the negative sequence voltage distortion rate to below 3%, ensuring the reliable action of phase difference criterion. Through simulation comparison, the system using this control strategy shortens the time required for voltage recovery to 0.9 p.u. by 40% after fault, and the signal-to-noise ratio of characteristic signal is improved by more than 15 dB, greatly improving the subsequent distance measurement accuracy.

[0043] Further, when a phase occurs ground fault, the negative sequence current and zero sequence current of the fault branch are equal in size and opposite in direction, while the phase difference of the healthy phase is randomly distributed. Compared with the traditional phase vector method, this criterion has strong anti-transition resistance capability: even if the ground resistance is 300Ω, the amplitude of negative sequence / zero sequence current may decay, but the phase relationship remains stable; at the same time, it adapts to the inherent asymmetry of new energy scene, avoiding misjudgment caused by normal asymmetry. Test data shows that under the extreme condition of system imbalance degree of 20%, the selection phase method still maintains 99.2% accuracy, while the traditional method decreases to below 75%. This innovation lays the foundation for subsequent accurate acquisition of fault phase double-frequency signal: if the phase selection is wrong, the electrical quantity of non-fault phase cannot reflect the true fault state.

[0044] Further, the zero sequence compensation coefficient K is usually simplified as a real constant, but in long distance cable lines, it needs to be kept in complex form due to capacitive effect. This method can invert the frequency-dependent characteristics of K through double-frequency measurement. Field tests show that in cable-overhead hybrid lines with a capacitive current ratio of up to 70%, the distance measurement error of this model is still better than 2%, while the error of the traditional power frequency impedance method is more than 10%. In addition, the correlation between fault current and measurement point current in the equation is established through boundary conditions, effectively weakening the influence of uncertainty of equivalent impedance of the opposite side system, which is a common problem in weak grid scenarios at the end of photovoltaic transmission lines.

[0045] Further, the transmission relationship of positive sequence, negative sequence and zero sequence components is uniformly described in matrix form, providing a complete mathematical basis for the derivation of frequency domain parameter equations; considering the differences between positive sequence impedance, negative sequence impedance and zero sequence impedance, the electrical quantities at the measurement point and the fault point are linearly associated through the fault distance d, avoiding the nonlinear error caused by the distributed capacitance; combined with the boundary conditions at the fault point, the equation is closed for solution. Compared with the traditional single-phase modeling method, the propagation characteristics of the sequence components in the unbalanced system can be accurately described, and it is particularly suitable for lines with significant asymmetric parameters. In the disturbance test with a zero sequence impedance measurement error of ±20%, the model still maintains a ranging error of less than 5%, while the error of the simplified model is as high as 25%. In addition, the matrix form is convenient for extension to other fault types, enhancing the universality of the method.

[0046] Further, by splitting the real and imaginary parts and constructing a four-equation nonlinear equation set through double-frequency combination, the single-variable dependence on the transition resistance is broken through, and the joint accurate solution of the fault distance and the transition resistance is realized. The fault current in the equation set is obtained through sequence component transformation, avoiding the nonlinear distortion influence introduced by the fault resistance. Simulations show that the nonlinear equation set has a solution stability that is more than 3 times higher than that of the single-frequency method under strong interference with a signal noise of 10%.

[0047] Further, sliding window phasor extraction and interior point method are introduced, and through the collaborative design of signal processing and optimization algorithm, the equation ill-conditioning problem in the noise environment is solved. The sliding window processing adopts a moving data window, and the voltage / current phasor at the center time of each window is extracted. Dynamic noise suppression is realized: the random noise in the window is reduced through statistical averaging, and the jump between windows reflects the fault transient process, avoiding the pollution of non-periodic components to a single sampling. The interior point method is specially designed for the convex optimization solution of the nonlinear equation set: a central path is constructed inside the feasible region, and the Karush-Kuhn-Tucker optimal condition is approached in an iterative manner. Compared with the Newton-Raphson method, the interior point method has low sensitivity to initial values and super-linear convergence speed. In terms of computational efficiency, the time complexity of the interior point method is 0, and the solution of the double-frequency four-equation system can be completed within 10 ms, meeting the speed requirement of relay protection.

[0048] It can be understood that the beneficial effects of the above-mentioned second aspect can be referred to the related description in the above-mentioned first aspect, which will not be repeated here.

[0049] In summary, the present application uses the positive sequence double-frequency characteristic signal injected by the photovoltaic inverter to break through the power frequency ranging limitation, constructs a composite sequence network equivalent circuit, realizes the accurate positioning of single-phase ground faults in a weakly fed environment through double-frequency equation splitting and interior point method solving, and the error is still less than 5.23% when the transition resistance is 100 Ω, with no additional hardware cost.

[0050] The technical solutions of the present application will be further described in detail below with reference to the drawings and embodiments. BRIEF DESCRIPTION OF DRAWINGS

[0051] Figure 1 Equivalent circuit of single-phase grounding fault sequence component considering active injection of detection signal

[0052] Figure 2 Flow chart of the method of the present application

[0053] Figure 3 Model of 110kV centralized photovoltaic sending-out line

[0054] Figure 4 Waveform of electrical quantity of single-phase grounding fault at 10km from the head of sending-out line in the sending-out line area, wherein (a) is the voltage at the measuring point, (b) is the current at the measuring point, (c) is the voltage amplitude-frequency characteristic, and (d) is the current amplitude-frequency characteristic

[0055] Figure 5 Result of fault location of single-phase grounding fault at 10km from the head of sending-out line DETAILED DESCRIPTION

[0056] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope of the present application.

[0057] In the description of the present application, it should be understood that the terms “include” and “contain” indicate the existence of described features, whole, steps, operations, elements and / or components, but do not exclude the existence or addition of one or more other features, whole, steps, operations, elements, components and / or sets thereof.

[0058] It should also be understood that the terms used in the specification of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application. As used in the specification and the appended claims of the present application, the singular forms “a”, “an” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0059] It should be further understood that the term “and / or” used in the specification and the appended claims of the present application means any combination of one or more of the associated listed items and all possible combinations, and includes these combinations, for example, A and / or B can represent three cases of A alone, A and B together, and B alone. In addition, the character “ / ” in the present application generally represents an “or” relationship between the front and rear associated objects.

[0060] Depending on the context, the word "if" as used herein can be interpreted to mean "when" or "while" or "in response to determining" or "in response to detecting." Similarly, the phrase "if it is determined" or "if [a stated condition or event] is detected" can be interpreted to mean "upon determining" or "in response to determining" or "upon detecting [the stated condition or event]" or "in response to detecting [the stated condition or event]."

[0061] The various structural diagrams according to the disclosed embodiments of the present application are shown in the drawings. These diagrams are not drawn to scale, in which certain details are exaggerated for clarity and others omitted. The shapes and relative sizes of the various regions, layers, and elements illustrated in the drawings are exemplary only and can vary in actual implementation depending on, for example, manufacturing techniques and tolerances. The skilled person can design regions / layers with different shapes, sizes, and relative positions according to the actual needs.

[0062] The application provides a centralized photovoltaic outgoing line single-phase ground fault ranging method, only using the control characteristics of the inverter to inject positive sequence double-frequency characteristic signals to the outgoing line, constructing a fault circuit topology under non-power frequency characteristics after a single-phase ground fault occurs, further giving a single-ended ranging frequency domain equation and algorithm containing fault distance under non-power frequency electrical quantities, realizing fault ranging for centralized new energy outgoing lines, while considering the parameters of the opposite system, having certain transition resistance resistance, fully utilizing the high controllability of the centralized photovoltaic inverter, injecting characteristic signals of specific frequencies to the fault line through additional control strategies after the fault occurs, and enhancing the identifiable of fault characteristics.

[0063] Referring to Figure 2 The centralized photovoltaic outgoing line single-phase ground fault ranging method provided by the application comprises the following steps:

[0064] S1, after the fault occurs, the normal control strategy is switched to the low voltage ride through (LVRT) control strategy according to a fault starting criterion;

[0065] The specific criterion is to detect that the power frequency voltage drop exceeds 10% and lasts for 3ms to start; the conditions for switching the control strategy are as follows:

[0066] (1)

[0067] After the fault occurs, the power frequency electrical quantity of the grid-connected point is separated into power frequency voltage positive sequence component and power frequency voltage negative sequence component through the positive and negative sequence separation link. The power frequency voltage positive sequence component adopts the reactive power priority control strategy in the low voltage ride through control strategy, and the power frequency voltage negative sequence component adopts the negative sequence suppression strategy. Taking the grid-connected point three-phase voltage as an example, the expression of the link of positive and negative sequence separation is as follows:

[0068] (2)

[0069] wherein, u α 、u β are the voltage components in the two-phase stationary coordinate system, u + α 、u - α are the positive sequence component and the negative sequence component of the α axis, u + β 、u - β are the positive sequence component and the negative sequence component of the β axis, T is the period under the power frequency.

[0070] S2, the additional signal injection control strategy is put into operation at the same time of the fault strategy switching, and the double-frequency signal of the positive sequence is superimposed on the basis of the power frequency control component;

[0071] The modulation wave output by the controller during the fault is:

[0072] (3)

[0073] wherein, U * 0 is the power frequency voltage modulation wave amplitude, ω 0 is the power frequency angular frequency, ω k , φ k are the angular frequency and initial phase of the injection signals of the characteristic frequencies f 1 and f 2.

[0074] S3, the fault phase selection is realized according to the phase difference of the power frequency voltage positive sequence component and the power frequency voltage negative sequence component, and the fault phase voltage and current under the two characteristic frequencies are collected;

[0075] The phase selection result when the single-phase ground fault occurs is as follows:

[0076] (4)

[0077] in, , and These are the current fault components under positive sequence, negative sequence, and zero sequence, respectively.

[0078] S4. After the fault phase voltage and current are collected, the equivalent circuit of the composite sequence network at the characteristic frequency is obtained according to the boundary conditions of the sequence component at the fault branch of the single-phase ground fault.

[0079] Please see Figure 1 , , , , and , These represent the positive-sequence, negative-sequence, and zero-sequence components of the voltage and current at the protection measuring point under the characteristic frequency, respectively. , , , and , These represent the positive-sequence, negative-sequence, and zero-sequence components of the voltage and current at the fault branch under the characteristic frequency, respectively. , and These represent the positive, negative, and zero-sequence current phasors flowing to the opposite system at the characteristic frequency, respectively. , and These represent the positive-sequence, negative-sequence, and zero-sequence impedances per unit length of the line at the characteristic frequency, respectively. , and These represent the positive-sequence, negative-sequence, and zero-sequence equivalent impedances of the system at the characteristic frequency, respectively. and This represents the negative-sequence and zero-sequence equivalent impedances of an inverter converter at its characteristic frequency. d R is the distance to the fault; f This is the transition resistance.

[0080] S5. Based on the composite sequence network equivalent circuit at the characteristic frequency obtained in step S4, write the network equation at the beginning of the line to be measured, and finally obtain the frequency domain parameter equation for the fault distance.

[0081] Write the network equations at the beginning of the line to be measured, and you will get:

[0082] (5)

[0083] Based on the boundary conditions at the fault point:

[0084] (6)

[0085] The frequency domain parameter equation about fault distance is obtained:

[0086] (7)

[0087] wherein, , respectively represent the phase-a voltage and current at the characteristic frequency, K is the zero-sequence compensation coefficient.

[0088] S6, the real and imaginary parts of the frequency domain parameter equation obtained in step S5 are split to obtain two equations; further, since the inverter injection is a positive-sequence double-frequency characteristic signal, four nonlinear equations about fault distance are obtained , , and ;

[0089] Specifically, the four nonlinear equations , , and are as follows:

[0090] (8)

[0091] wherein, , , and respectively are the positive-sequence voltage and current of the protection measuring point at the two injection frequencies; , , and respectively are the voltage and current of the phase-a of the protection measuring point at the two injection frequencies; and are the zero-sequence currents of the protection measuring point at the two injection frequencies, is the proportional coefficient of the two injection frequencies.

[0092] S7, in order to improve the stability of the solution of formula (5), when the non-public frequency electrical quantities are extracted in step S3, the phasors of the positive-sequence double-frequency characteristic signals at N times of characteristic frequencies are extracted in a sliding window within a data window, and at the same time, the four nonlinear equations obtained in step S6 are solved by using the interior point method in each data window to measure the result of the fault distance d, and thus the fault distance result of the new energy sending-out line is obtained.

[0093] In still another embodiment of the present application, a centralized photovoltaic sending-out line single-phase grounding fault ranging system is provided, which can be used to implement the centralized photovoltaic sending-out line single-phase grounding fault ranging method.

[0094] The switching module is configured to switch from a normal control strategy to a low-voltage ride-through control strategy according to a fault starting criterion after a line fault occurs; and an additional signal injection control strategy is put into operation at the same time of switching to the low-voltage ride-through control strategy, and a positive sequence double-frequency characteristic signal is injected into the sending-out line by an inverter additional control on the basis of a power frequency control component.

[0095] The acquisition module is configured to realize fault phase selection according to a phase difference between a power frequency voltage positive sequence component and a power frequency voltage negative sequence component; and simultaneously acquire a voltage and a current of a fault phase under the positive sequence double-frequency characteristic signal, and establish a composite sequence network equivalent circuit under the positive sequence double-frequency characteristic signal based on a sequence component boundary condition of a single-phase grounding fault at a fault branch.

[0096] The equation module is configured to write a frequency domain network equation of the composite sequence network at a first end of a line to be ranged according to the obtained composite sequence network equivalent circuit, and obtain a frequency domain parameter equation containing a fault distance in combination with a fault point boundary condition.

[0097] The construction module is configured to split the frequency domain parameter equation into a real part equation and an imaginary part equation; and construct a nonlinear equation group containing four equations by using two groups of equations corresponding to the positive sequence double-frequency characteristic signal.

[0098] The output module is configured to extract a phasor of the positive sequence double-frequency characteristic signal in a preset data window by sliding window extraction; solve the nonlinear equation group by using an interior point method; and output a fault ranging result of the new energy sending-out line.

[0099] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described below in connection with the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application described and shown in the drawings herein can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of protection of the present application.

[0100] To verify the correctness of the proposed permanent fault identification method, taking a 110kV transmission line as an example, a Berylon model based on electromagnetic transient simulation software was established as follows: Figure 3 The simulation model is shown below. The main parameters are shown in Table 1.

[0101] Table 1 Main parameters of the simulation model

[0102]

[0103] Among them, section L1 is a transmission line for fault location, and single-end electrical quantities are collected through measuring points on the transmission line.

[0104] Section L1 is 60km long. Faults of different types and transition resistances were set in the middle of L1 to verify the accuracy of the single-end fault location algorithm implemented by injecting positive sequence dual-frequency characteristic signals.

[0105] Please see Figure 3 The transmitted line model shown is configured with different fault types and transition resistances in section L1 to verify the accuracy of the permanent fault identification criteria under different neutral grounding methods.

[0106] After a single-phase ground fault occurs 10km along the transmission line, and the fault is 10km from the starting point, the electrical waveform diagram of the protection measuring point is as follows: Figure 4 As shown, the fault location results are as follows: Figure 5 As shown.

[0107] Among them, such as Figure 4 As shown, after the fault occurs, the inverter activates an additional control strategy, injecting positive-sequence dual-frequency detection signals with frequencies of 200Hz and 300Hz into the faulty line.

[0108] like Figure 5 As shown, the proposed fault distance calculation algorithm based on dual-frequency signals is not affected by the power frequency control characteristics of the converter and can accurately and stably calculate the fault distance.

[0109] Simulation results for different fault locations and different fault types are shown in Table 2.

[0110] Table 2 Simulation results for different fault locations and fault types.

[0111]

[0112] As shown in Table 2, the above methods can correctly calculate the fault distance when a single-phase ground fault occurs at different locations on the transmission line.

[0113] In summary, the centralized photovoltaic sending line single-phase ground fault ranging method and system, by using the control characteristics of the photovoltaic inverter itself, injects 100-500Hz positive sequence double-frequency characteristic signal to the fault line, avoids the interference of power electronic equipment on power frequency quantity, significantly improves the fault feature signal-to-noise ratio, establishes a double-frequency composite sequence network model based on the boundary conditions of the sequence component of the fault branch, deduces the frequency domain parameter equation containing the fault distance, splits the complex equation into real part / imaginary part, constructs a nonlinear equation group containing four equations combined with the positive sequence double-frequency characteristic signal, effectively suppresses the influence of transition resistance, adopts the interior point method for iterative optimization, ensures the stability of the solution under complex conditions, provides core technical support for the rapid disposal of faults in high-proportion new energy power grids, and improves the system safety and power supply reliability.

[0114] The above is only to illustrate the technical idea of the present application, and cannot limit the protection scope of the present application. Any modification made according to the technical idea of the present application on the basis of the technical scheme falls within the protection scope of the claims of the present application.

Claims

1. A method for locating single-phase grounding faults in centralized photovoltaic transmission lines, characterized in that, Includes the following steps: After a line fault occurs, the system switches from the normal control strategy to the low voltage ride-through control strategy based on the fault start-up criteria. While switching the low voltage ride-through control strategy, an additional signal injection control strategy is implemented. Based on the power frequency control component, positive sequence dual-frequency characteristic signals are injected into the transmission line through the inverter additional control. Fault phase selection is achieved based on the phase difference between the positive sequence component and the negative sequence component of the power frequency voltage; at the same time, the voltage and current of the fault phase under the positive sequence dual-frequency characteristic signal are collected, and the equivalent circuit of the composite sequence network under the positive sequence dual-frequency characteristic signal is established based on the sequence component boundary conditions of the single-phase grounding fault at the fault branch. Based on the obtained equivalent circuit of the composite sequence network, the frequency domain network equation of the composite sequence network is written at the beginning of the line to be measured. Combined with the fault point boundary conditions, the frequency domain parameter equation containing the fault distance is obtained. The frequency domain network equation is as follows: in, , and These represent the positive-sequence, negative-sequence, and zero-sequence components of the voltage at the fault branch at the characteristic frequency, respectively. , and These represent the positive-sequence, negative-sequence, and zero-sequence components of the voltage at the protection measuring point under the characteristic frequency, respectively. , and These represent the positive-sequence, negative-sequence, and zero-sequence components of the current at the protection measuring point under the characteristic frequency, respectively. , and These represent the unit line impedances for positive sequence, negative sequence, and zero sequence at the characteristic frequency, respectively. The frequency domain parametric equations including fault distance are as follows: in, , This indicates the positive sequence of voltage and current at the protection measuring point under the characteristic frequency. For transition resistance, , These represent the phase voltage and current at the characteristic frequency, respectively. K The zero-order compensation coefficient is... d The distance to the fault. This refers to the zero-sequence component of the current at the protection measuring point at the characteristic frequency. This is the positive-sequence impedance per unit line at the characteristic frequency. The equivalent impedance of the opposite end system at the characteristic frequency; The frequency domain parameter equations are decomposed into real and imaginary equations; a nonlinear equation system containing four equations is constructed using the two sets of equations corresponding to the positive-sequence dual-frequency characteristic signals. , , and The details are as follows: in, , , and These represent the positive sequence voltage and positive sequence current of the protection measuring point under two injection frequencies, respectively. , , and The voltage and current of phase a at the protection measuring point are respectively under two injection frequencies; and The zero-sequence current at the protection measurement point is for two injection frequencies. R is the scaling factor for the two injection frequencies. f For transition resistance, K The zero-order compensation coefficient is... and These are the positive-sequence impedance of the unit line at the characteristic frequency and the equivalent impedance of the peer system at the characteristic frequency, respectively. The phasors of the positive-sequence dual-frequency characteristic signals are extracted by sliding a window within a preset data window; the nonlinear equations are solved using the interior-point method, and the fault location results of the new energy transmission line are output.

2. The method for determining single-phase grounding faults in centralized photovoltaic transmission lines according to claim 1, characterized in that, The fault start-up criterion is: real-time monitoring of the power frequency voltage of the sending line. When the power frequency voltage drops by more than 10% and lasts for 3ms, it is determined that a line fault has occurred, triggering the ranging process.

3. The method for determining single-phase grounding faults in centralized photovoltaic transmission lines according to claim 1, characterized in that, The positive sequence dual-frequency characteristic signal consists of two different non-power frequency frequencies, with a frequency range of 100~500Hz.

4. The method for determining single-phase grounding faults in centralized photovoltaic transmission lines according to claim 1, characterized in that, Low voltage ride-through control strategies include: By separating the positive and negative sequence, the power frequency electrical quantities at the grid connection point are divided into positive sequence components and negative sequence components. A reactive power priority control strategy is adopted for the positive sequence components and a negative sequence suppression strategy is adopted for the negative sequence components.

5. The method for determining single-phase grounding faults in centralized photovoltaic transmission lines according to claim 1, characterized in that, Fault phase selection is achieved based on the phase difference between the positive-sequence component and the negative-sequence component of the power frequency voltage. Specifically: When the phase difference between the positive-sequence component and the negative-sequence component of a phase voltage is 180°, that phase is determined to be a faulty phase. The phase selection results for a single-phase ground fault are as follows: in, , and These represent the current fault components in positive sequence, negative sequence, and zero sequence, respectively.

6. The method for determining single-phase grounding faults in centralized photovoltaic transmission lines according to claim 1, characterized in that, The phasor of the positive-sequence dual-frequency feature signal is extracted by sliding a window within a preset data window; The nonlinear equations are solved using the interior-point method, and the fault location results for the new energy transmission lines are output as follows: Extract data by sliding window within the preset data window. N The phasor of the positive sequence dual-frequency characteristic signal at the sub-characteristic frequency is obtained. At the same time, the nonlinear equation system is solved using the interior point method in each data window to measure the fault distance d. Thus, the fault location result of the new energy transmission line is obtained.

7. A centralized photovoltaic transmission line single-phase grounding fault location system, characterized in that, include: After a fault occurs, the switching module switches from the normal control strategy to the low voltage ride-through control strategy based on the fault start-up criteria. While switching the low voltage ride-through control strategy, an additional signal injection control strategy is implemented. Based on the power frequency control component, positive sequence dual-frequency characteristic signals are injected into the transmission line through the inverter additional control. The acquisition module selects the fault phase based on the phase difference between the positive-sequence component and the negative-sequence component of the power frequency voltage; at the same time, it acquires the voltage and current of the fault phase under the positive-sequence dual-frequency characteristic signal, and establishes the equivalent circuit of the composite sequence network under the positive-sequence dual-frequency characteristic signal based on the sequence component boundary conditions of the single-phase grounding fault at the fault branch. The equation module, based on the obtained equivalent circuit of the composite sequence network, writes the frequency domain network equation of the composite sequence network at the beginning of the line to be measured. Combining the fault point boundary conditions, the frequency domain parameter equation containing the fault distance is obtained. The frequency domain network equation is as follows: in, , and These represent the positive-sequence, negative-sequence, and zero-sequence components of the voltage at the fault branch at the characteristic frequency, respectively. , and These represent the positive-sequence, negative-sequence, and zero-sequence components of the voltage at the protection measuring point under the characteristic frequency, respectively. , and These represent the positive-sequence, negative-sequence, and zero-sequence components of the current at the protection measuring point under the characteristic frequency, respectively. , and These represent the unit line impedances for positive sequence, negative sequence, and zero sequence at the characteristic frequency, respectively. The frequency domain parametric equations including fault distance are as follows: in, , This indicates the positive sequence of voltage and current at the protection measuring point under the characteristic frequency. For transition resistance, , These represent the phase voltage and current at the characteristic frequency, respectively. K The zero-order compensation coefficient is... d The distance to the fault. This refers to the zero-sequence component of the current at the protection measuring point at the characteristic frequency. This is the positive-sequence impedance per unit line at the characteristic frequency. The equivalent impedance of the opposite end system at the characteristic frequency; The construction module decomposes the frequency domain parameter equations into real and imaginary part equations; and constructs a nonlinear equation system containing four equations using two sets of equations corresponding to the positive-sequence dual-frequency characteristic signals. , , and The details are as follows: in, , , and These represent the positive sequence voltage and positive sequence current of the protection measuring point under two injection frequencies, respectively. , , and The voltage and current of phase a at the protection measuring point are respectively under two injection frequencies; and The zero-sequence current at the protection measurement point is for two injection frequencies. R is the scaling factor for the two injection frequencies. f For transition resistance, K The zero-order compensation coefficient is... and These are the positive-sequence impedance of the unit line at the characteristic frequency and the equivalent impedance of the peer system at the characteristic frequency, respectively. The output module extracts the phasor of the positive-sequence dual-frequency characteristic signal by sliding a window within a preset data window; it solves the nonlinear equation set using the interior-point method and outputs the fault location results of the new energy transmission line.

8. The centralized photovoltaic transmission line single-phase grounding fault location system according to claim 7, characterized in that, The fault start-up criterion is: real-time monitoring of the power frequency voltage of the sending line. When the power frequency voltage drops by more than 10% and lasts for 3ms, it is determined that a line fault has occurred, triggering the ranging process.

9. The centralized photovoltaic transmission line single-phase grounding fault location system according to claim 7, characterized in that, Low voltage ride-through control strategies include: By separating the positive and negative sequence, the power frequency electrical quantities at the grid connection point are divided into positive sequence components and negative sequence components. A reactive power priority control strategy is adopted for the positive sequence components and a negative sequence suppression strategy is adopted for the negative sequence components.

10. The centralized photovoltaic transmission line single-phase grounding fault location system according to claim 7, characterized in that, Fault phase selection is achieved based on the phase difference between the positive-sequence component and the negative-sequence component of the power frequency voltage. Specifically: When the phase difference between the positive-sequence component and the negative-sequence component of a phase voltage is 180°, that phase is determined to be a faulty phase. The phase selection results for a single-phase ground fault are as follows: in, , and These represent the current fault components in positive sequence, negative sequence, and zero sequence, respectively.

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