Centralized photovoltaic transmission line single-phase earth fault distance measurement method and centralized photovoltaic transmission line single-phase earth fault distance measurement system

Abstract

The invention discloses a centralized photovoltaic transmission line single-phase earth fault location method and system, and the method comprises the steps: switching an inverter to a low-voltage ride-through control strategy after detecting that the power frequency voltage drop exceeds 10% for 3ms, injecting a positive-sequence double-frequency characteristic signal, carrying out the phase selection through employing the phase difference of a power frequency voltage sequence component, and carrying out the fault location of a single-phase earth fault. Collecting voltage and current of a fault phase under a dual-frequency signal, and establishing a dual-frequency composite sequence network equivalent circuit based on a fault point sequence component boundary condition; deriving a frequency domain parameter equation containing a fault distance in combination with boundary conditions on a line head end column frequency domain equation; splitting the frequency domain equation into a real part equation and an imaginary part equation; constructing a nonlinear equation set containing four equations by using a double-frequency signal; extracting a feature signal phasor by using a sliding window; the industrial problem of photovoltaic transmission line single-phase earth fault distance measurement 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 occurrence probability. 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 traditional power frequency quantity single-end ranging method based on the in-phase of the fault branch current and the current at the installation site, 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: A centralized photovoltaic sending-out line single-phase ground fault ranging method, comprising the following steps: 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; The fault 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; 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; 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; The phasor of the positive sequence double-frequency characteristic signal is extracted in a preset data window by sliding window extraction; 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.

[0007] 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.

[0008] Preferably, the positive sequence double-frequency characteristic signal is two different non-power frequency frequencies, and the frequency range is 100-500 Hz.

[0009] Preferably, the low voltage ride through control strategy includes: 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.

[0010] Preferably, the fault 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: 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°, it is determined that the phase is a fault phase, and the phase selection result when a single-phase grounding fault occurs is as follows:

[0011] wherein, , and are the current fault components under the positive sequence, the negative sequence and the zero sequence respectively.

[0012] Preferably, the frequency domain parameter equation containing the fault distance is as follows:

[0013] wherein, , positive sequence of the voltage and current at the protection measuring point at the characteristic frequency, is the transition resistance, , are respectively the voltage and current of phase a at the characteristic frequency, K is the zero sequence compensation coefficient, d is the fault distance, is the zero sequence component of the current at the protection measuring point at the characteristic frequency, is the unit line positive sequence impedance at the characteristic frequency, is the equivalent impedance of the opposite end system at the characteristic frequency.

[0014] Preferably, according to the obtained composite sequence network equivalent circuit, the frequency domain network equation of the composite sequence network is written as follows at the head of the line to be measured:

[0015] wherein, , and are respectively the positive sequence, negative sequence and zero sequence components of the voltage at the fault branch at the characteristic frequency, , and are respectively the positive sequence, negative sequence and zero sequence components of the voltage at the protection measuring point at the characteristic frequency, , and are respectively the positive sequence, negative sequence and zero sequence components of the current at the protection measuring point at the characteristic frequency, , and are respectively the unit line positive sequence, negative sequence and zero sequence impedance of the line at the characteristic frequency.

[0016] Preferably, the two groups of equations corresponding to the positive sequence double-frequency characteristic signal are used to construct a nonlinear equation group containing four equations , , and , and the specific construction is as follows:

[0017] wherein, , , and are respectively the positive sequence voltage and positive sequence current at the protection measuring point at the two injection frequencies; , , and are respectively the voltage and current of phase a at the protection measuring point at the two injection frequencies; and are the zero sequence currents at the protection measuring point at the two injection frequencies, R is the ratio coefficient of two injection frequencies f R is the transition resistance, K R is the zero sequence compensation coefficient, and are the unit line positive sequence impedance at the characteristic frequency and the equivalent impedance of the opposite end system at the characteristic frequency, respectively.

[0018] Preferably, the phasor of the positive sequence double-frequency characteristic signal is extracted in a preset data window by sliding window extraction; the nonlinear equation set is solved by using the interior point method, and the fault distance measurement result of the new energy sending-out line is output, specifically: the phasor of the positive sequence double-frequency characteristic signal is extracted in a preset data window by sliding window extraction; N the phasor of the positive sequence double-frequency characteristic signal at the secondary characteristic frequency is extracted, and the nonlinear equation set is solved by using the interior point method in each data window to measure the result of the fault distance d, thereby obtaining the fault distance measurement result of the new energy sending-out line.

[0019] 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: A switching module, after a fault occurs, switches from a normal control strategy to a low-voltage ride-through control strategy according to a starting criterion; simultaneously switches the low-voltage ride-through control strategy and inputs an additional signal injection control strategy, and on the basis of a power frequency control component, injects a positive sequence double-frequency characteristic signal into the sending-out line through inverter additional control; A collection module, realizes fault phase selection according to the phase difference of positive sequence and negative sequence components of a power frequency voltage; simultaneously collects the voltage and current of a fault phase under the positive sequence double-frequency characteristic signal, and establishes a composite sequence network equivalent circuit under the positive sequence double-frequency characteristic signal based on the sequence component boundary condition of a single-phase ground fault at a fault branch; An equation module, according to the obtained composite sequence network equivalent circuit, writes a frequency domain network equation of the composite sequence network at the head of the to-be-measured line, and obtains a frequency domain parameter equation containing a fault distance in combination with a fault point boundary condition; A construction module, splits the frequency domain parameter equation into real part and imaginary part equations; uses two groups of equations corresponding to the double-frequency signal to construct a nonlinear equation set containing four equations; An output module, extracts the phasor of the characteristic signal in a preset data window by sliding window extraction; solves the nonlinear equation set by using the interior point method, and outputs the fault distance measurement result of the new energy sending-out line.

[0020] Compared with the prior art, the present application has at least the following beneficial effects: A method for locating single-phase ground faults in centralized photovoltaic (PV) transmission lines utilizes the active control capability of the PV inverter to inject positive-sequence dual-frequency characteristic signals and addresses the inaccuracy of traditional location methods through frequency domain modeling. Specifically, this method synchronously switches to a low-voltage ride-through control strategy and injects positive-sequence dual-frequency characteristic signals after a fault occurs. Fault phase selection is achieved through component phase differences, an equivalent circuit of a composite-sequence network is established, and finally, a system of nonlinear equations is constructed and solved using the positive-sequence dual-frequency characteristic signals to obtain the fault location. The advantages of this invention are reflected in three aspects: First, it fully utilizes the high controllability of photovoltaic inverters, injecting characteristic signals into the line without the need for additional signal generation devices, thus reducing hardware costs and avoiding the dependence of traditional traveling wave methods on high-precision synchronous clocks. Second, by constructing a composite sequence network, it avoids the impact of the weak feed characteristics of new energy transmission lines on power frequency ranging. Traditional power frequency ranging relies 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 dual-frequency characteristic signal injected by this method enhances the identifiability of fault characteristics. Finally, by forming a redundant equation system through the real and imaginary solutions of the frequency domain parametric equations, it solves the nonlinearity problem caused by the transition resistance and improves ranging accuracy. This invention achieves closed-loop innovation from signal injection and feature extraction to equation solving, solving the problems of weak feed characteristics, the influence of transition resistance, and the uncertainty of the opposite-end parameters in new energy scenarios while ensuring the economy of single-end ranging. Compared with traditional methods, this invention does not require additional signal generation devices and directly utilizes the inverter's own control capabilities to achieve signal injection, reducing hardware costs and system complexity. By injecting and detecting characteristic signals, combined with an improved ranging algorithm, it is expected to significantly improve the ranging accuracy of single-phase ground faults.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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%.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] The technical solutions of the present application will be further described in detail below with the aid of the drawings and examples. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 Equivalent circuit of single-phase grounding fault sequence component considering active injection of detection signal Figure 2 Flow chart of the method of the present application Figure 3 Model of 110kV centralized photovoltaic sending-out line 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 of the measuring point, (b) is the current of the measuring point, (c) is the voltage amplitude-frequency characteristic, and (d) is the current amplitude-frequency characteristic Figure 5 Result of fault location of single-phase grounding fault at 10km from the head of sending-out line DETAILED DESCRIPTION

[0033] 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, but not 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 work fall within the scope of protection of the present application.

[0034] In the description of the present application, it should be understood that the terms "comprise" and "include" indicate the existence of described features, integers, steps, operations, elements and / or components, but do not exclude one or more other features, integers, steps, operations, elements, components and / or sets thereof.

[0035] 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, unless otherwise clearly indicated by the context, the singular forms "a", "an" and "the" are intended to include the plural forms.

[0036] 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.

[0037] 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 "when it is determined" or "in response to determining" or "when [a stated condition or event] is detected" or "in response to detecting [a stated condition or event]."

[0038] 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 of presentation and may be omitted. The shapes of various regions, layers shown in the drawings and their relative sizes and positional relationships are only exemplary, and in actuality, they can be deviated due to manufacturing tolerances or technical limitations, and a person skilled in the art can additionally design regions / layers with different shapes, sizes, relative positions according to actual needs.

[0039] The application provides a centralized photovoltaic sending line single-phase grounding fault ranging method, only using the control characteristics of the inverter to inject positive sequence double-frequency characteristic signals to the sending line, constructing a fault circuit topology under non-power frequency characteristics after a single-phase grounding fault occurs, further giving a single-end ranging frequency domain equation and algorithm containing fault distance under non-power frequency electrical quantities, realizing fault ranging for centralized new energy sending lines, while considering the parameters of the opposite end 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 an additional control strategy after the fault occurs, and enhancing the identifiable of the fault characteristics.

[0040] Please refer to Figure 2 The centralized photovoltaic sending line single-phase grounding fault ranging method provided by the application comprises the following steps: 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; The specific criterion is to start after detecting that the power frequency voltage drop exceeds 10% and lasts for 3ms; the condition for switching the control strategy is as follows: (1) After the fault occurs, the power frequency electrical quantities at the grid-connected point are respectively divided into power frequency voltage positive sequence components and power frequency voltage negative sequence components through a positive and negative sequence separation link, the power frequency voltage positive sequence components adopt the reactive power priority control strategy in the low voltage ride through control strategy, and the power frequency voltage negative sequence components adopt a negative sequence suppression strategy. Taking the grid-connected point three-phase voltage as an example, the expression of the link for separating the positive and negative sequences is as follows: (2) wherein, u α 、u β are voltage components in two-phase stationary coordinate system respectively, u + α 、u - α are positive and negative sequence components of α-axis respectively, u + β 、u - β are positive and negative sequence components of β-axis respectively, T is a period under power frequency.

[0041] S2, at the same time of fault strategy switching, put in additional signal injection control strategy, superimpose double frequency signal of positive sequence on the basis of power frequency control component; The modulation wave output by the controller during the fault is: (3) wherein, U * 0 is the amplitude of the power frequency voltage modulation wave, ω 0 is the angular frequency of the power frequency, ω k , φ k are the angular frequency and initial phase of the injection signal of characteristic frequency f 1 and f 2 respectively.

[0042] S3, realize fault phase selection according to the phase difference of positive sequence component of power frequency voltage and negative sequence component of power frequency voltage, and collect fault phase voltage and current under two characteristic frequencies at the same time; The phase selection result when single-phase ground fault occurs is as follows: (4) wherein, , and are current fault components under positive sequence, negative sequence and zero sequence respectively.

[0043] S4, when the fault phase voltage and current are collected, obtain the equivalent circuit of composite sequence network under characteristic frequency according to the boundary condition of sequence components at fault branch under single-phase ground fault; Please refer to Figure 1 , , , , and , represent the positive, negative and zero sequence components of the voltage and current at the protection point at the characteristic frequency, respectively; , , , and , represent the positive, negative and zero sequence components of the voltage and current at the faulted branch at the characteristic frequency, respectively; , and represent the positive, negative and zero sequence current phasors flowing towards the opposite end system at the characteristic frequency, respectively; , and represent the positive, negative and zero sequence per-unit length impedance of the line at the characteristic frequency, respectively; , and represent the positive, negative and zero sequence equivalent impedance of the opposite end system at the characteristic frequency, respectively; and represent the negative and zero sequence equivalent impedance of the inverter-based converter at the characteristic frequency, respectively; d is the fault distance; R f is the transition resistance.

[0044] S5, based on the composite sequence network equivalent circuit at the characteristic frequency obtained in step S4, write the network equation at the head of the line to be measured, and finally obtain the frequency domain parameter equation about the fault distance; Write the network equation at the head of the line to be measured, and obtain: (5) According to the boundary conditions at the fault point, we have: (6) Thus, the frequency domain parameter equation about the fault distance is obtained: (7) where, , represent the a-phase voltage and current at the characteristic frequency, respectively, K is the zero sequence compensation coefficient.

[0045] S6, the real part and the imaginary part 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 equation groups about the fault distance are obtained , , and ; Specifically, the four nonlinear equation groups are , , and As follows: (8) wherein, , , and are the positive sequence voltage and the positive sequence current of the protection measuring point under two injection frequencies respectively; , , and are the voltage and the 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.

[0046] S7, in order to improve the stability of solving formula (5), when extracting the non-public frequency electrical quantity in step S3, the phasor of the positive sequence double-frequency characteristic signal under the characteristic frequency is extracted in a data window, and the four nonlinear equation groups obtained in step S6 are solved by using the interior point method in each data window, the result of the fault distance d is measured, and thus the fault distance measurement result of the new energy sending-out line is obtained. N

[0047] In another embodiment of the present application, a centralized photovoltaic sending-out line single-phase ground fault distance measurement system is provided, which can be used to implement the above-mentioned centralized photovoltaic sending-out line single-phase ground fault distance measurement method. Specifically, the centralized photovoltaic sending-out line single-phase ground fault distance measurement system comprises a switching module, an acquisition module, an equation module, a construction module and an output module.

[0048] The switching module is used 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; 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 a power frequency control component, a positive sequence double-frequency characteristic signal is injected into the sending-out line through an inverter additional control; The acquisition module is used to realize fault phase selection 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 acquired, and based on the sequence component boundary condition of the single-phase ground fault at the fault branch, a composite sequence network equivalent circuit under the positive sequence double-frequency characteristic signal is established; The equation module is used to write 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 combine the fault point boundary condition to obtain a frequency domain parameter equation containing the fault distance; ​The construction module splits the frequency domain parameter equation into a real part equation and an imaginary part equation; and constructs a nonlinear equation group containing four equations by using two groups of equations corresponding to the positive sequence double-frequency characteristic signal; The output module extracts the phasor of the positive sequence double-frequency characteristic signal in a preset data window by sliding window extraction; solves the nonlinear equation group by using an interior point method, and outputs the fault distance measurement result of the new energy sending-out line.

[0049] 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.

[0050] To verify the correctness of the proposed permanent fault identification method, a simulation model as shown in Figure 3 is established based on the Bergeron model of the electromagnetic transient simulation software, taking a 110 kV power transmission network sending-out line as an example. The main parameters are shown in Table 1.

[0051] Table 1 Main parameters of the simulation model

[0052] Among them, the section L1 is the power transmission line for fault distance measurement, and the single-ended electrical quantity is collected through the measuring point on the power transmission line.

[0053] The length of the section L1 is 60 km. Different fault types and transition resistances are set in the middle section L1 to verify the accuracy of the single-ended fault distance measurement algorithm realized by using the positive sequence double-frequency characteristic signal injection.

[0054] Please refer to the sending-out line model shown in Figure 3 , different fault types and transition resistances are set in the middle section L1 to verify the accuracy of the permanent fault identification criterion under different neutral point grounding modes.

[0055] After setting a single-phase grounding fault at 10 km in the sending-out line, the fault is 10 km away from the head end, and the electrical quantity waveform diagram of the protection measuring point is as shown in Figure 4 , and the fault distance measurement result is as shown in Figure 5 .

[0056] Among them, as shown in Figure 4As shown, after the fault occurs, the inverter starts the additional control strategy, and injects the positive sequence double-frequency detection signal with the frequency of 200Hz and 300Hz into the fault line.

[0057] As shown in the formula (1), the fault distance can be calculated by the formula (1) based on the double-frequency signal. Figure 5 As shown, the fault distance solving algorithm based on the double-frequency signal is not affected by the power frequency control characteristics of the converter, and can accurately and stably solve the fault distance.

[0058] The simulation results of different fault positions and different fault types are shown in Table 2.

[0059] Table 2 Simulation results of different fault positions and fault types

[0060] As shown in Table 2, when the single-phase ground fault occurs in different positions of the outgoing line, the above method can correctly calculate the fault distance.

[0061] In summary, the centralized photovoltaic outgoing line single-phase ground fault distance measurement method and system of the application utilizes the control characteristics of the photovoltaic inverter itself, injects the positive sequence double-frequency characteristic signal of 100-500Hz into the fault line, avoids the interference of power electronic equipment on the power frequency quantity, significantly improves the fault feature signal-to-noise ratio, establishes the double-frequency composite sequence network model based on the sequence component boundary condition of the fault branch, deduces the frequency domain parameter equation containing the fault distance, splits the complex equation into real part / imaginary part, constructs the nonlinear equation set containing four equations combined with the positive sequence double-frequency characteristic signal, effectively suppresses the influence of the transition resistance, adopts the interior point method for iterative optimization, ensures the solving stability under complex working conditions, provides the core technical support for the fault rapid disposal of the high-proportion new energy power grid, and improves the system safety and power supply reliability.

[0062] The above content only illustrates the technical idea of the application, and cannot limit the protection scope of the application, and any modification made according to the technical idea of the application on the basis of the technical solution falls within the protection scope of the claims of the application.

Claims

1. A centralized photovoltaic transmission line single-phase ground fault location method, characterized in that, The method comprises the following steps: After the line fault occurs, the normal control strategy is switched to the 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 line through additional control of the inverter; Fault 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 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 grounding fault at the fault branch; According to the obtained composite sequence network equivalent circuit, a frequency domain network equation of the composite sequence network is written at the head end of the line to be measured, and a frequency domain parameter equation containing the fault distance is obtained in combination with the boundary condition of the fault point; 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; Phase quantities of the positive sequence double-frequency characteristic signal are extracted in a preset data window through sliding window extraction; The nonlinear equation group is solved by using an interior point method, and a fault distance measurement result of the new energy sending line is output.

2. The centralized photovoltaic sending-out line single-phase ground fault location method according to claim 1, characterized in that, The fault starting criterion is that the power frequency voltage of the sending 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, and the distance measurement process is triggered.

3. The centralized photovoltaic sending-out line single-phase ground fault location method according to claim 1, characterized in that, The positive sequence double-frequency characteristic signal is two different non-power frequency frequencies, and the frequency range is 100-500 Hz.

4. The centralized photovoltaic sending-out line single-phase ground fault location method according to claim 1, characterized in that, The low-voltage ride-through control strategy comprises: 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.

5. The centralized photovoltaic sending-out line single-phase ground fault location method according to claim 1, characterized in that, Fault 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: 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°, it is determined that the phase is a fault phase, and the phase selection result when a single-phase grounding fault occurs is as follows: wherein, , and are the current fault components in positive, negative and zero sequence, respectively.

6. The centralized photovoltaic sending-out line single-phase ground fault location method according to claim 1, characterized in that, The frequency domain parameter equation containing the fault distance is as follows: wherein, , denotes the positive sequence of the protection measuring point voltage and current at the characteristic frequency, is the transition resistance, , denotes the a-phase voltage and current at the characteristic frequency, respectively, K is the zero sequence compensation factor, d is the fault distance, is the zero sequence component of the protection measuring point current at the characteristic frequency, is the unit line positive sequence impedance at the characteristic frequency, is the equivalent impedance of the opposite end system at the characteristic frequency.

7. The centralized photovoltaic send-out line single-phase ground fault location method according to claim 6, characterized in that, 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 line to be measured as follows: wherein, , and denote the positive, negative and zero sequence components of the voltage at the faulted branch at the characteristic frequency, , and denote the positive, negative and zero sequence components of the voltage at the protection measuring point at the characteristic frequency, , and denote the positive, negative and zero sequence components of the current at the protection measuring point at the characteristic frequency, , and are the positive, negative and zero sequence unit line impedances of the line at the characteristic frequency.

8. The centralized photovoltaic sending out line single-phase ground fault location method according to claim 1, characterized in that, A nonlinear equation set containing four equations is constructed by using two groups of equations corresponding to the positive sequence double-frequency characteristic signals , , and , specifically as follows: wherein, , , and are the positive sequence voltage and current at the protection measurement point at the two injected frequencies, respectively; , , and are the voltage and current at phase a of the protection measurement point at the two injected frequencies, respectively; and are the zero sequence current at the protection measurement point at the two injected frequencies, is the proportional factor at the two injected frequencies, R f is the transition resistance, K is the zero sequence compensation factor, and are the unit line positive sequence impedance at the characteristic frequency and the equivalent impedance of the opposite end system at the characteristic frequency, respectively.

9. The centralized photovoltaic sending out line single-phase ground fault location method according to claim 1, characterized in that, The phase quantities of the positive sequence double-frequency characteristic signal are extracted in a preset data window through sliding window extraction; The nonlinear equation group is solved by using an interior point method, and a fault distance measurement result of the new energy sending line is output. Sliding window extraction within a preset data window N The positive sequence double-frequency characteristic signal under the secondary characteristic frequency is phasor, and the nonlinear equation set is solved by using the interior point method in each data window to measure the fault distance d. Thus, the fault distance measurement result of the new energy sending-out line is obtained.

10. A centralized photovoltaic send-out line single-phase ground fault location system, characterized in that, It comprises: A switching module, after the fault occurs, the normal control strategy is switched to the low-voltage ride-through control strategy according to the 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 line through additional control of the inverter; A collection module, fault 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 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 grounding fault at the fault branch; An equation module writes a frequency domain network equation of the composite sequence network at a head end of a line to be measured according to the obtained equivalent circuit of the composite sequence network, and obtains a frequency domain parameter equation containing a fault distance in combination with a boundary condition of a fault point; An establishing module splits the frequency domain parameter equation into a real part equation and an imaginary part equation, and establishes a nonlinear equation group containing four equations by using two groups of equations corresponding to the positive sequence double-frequency characteristic signal; An output module extracts a phasor of the positive sequence double-frequency characteristic signal in a preset data window by sliding window extraction; An inner point method is used to solve the nonlinear equation group, and a fault distance measurement result of the new energy transmission line is output.

Images