Networking type photovoltaic sending-out line pilot protection method, system, equipment and medium

Abstract

The invention relates to the technical field of electric power systems, and discloses a pilot protection method, system and device for a network construction type photovoltaic transmission line and a medium, and the method comprises the steps: obtaining photovoltaic field data, and carrying out the preprocessing of the photovoltaic field data, and obtaining the preprocessed photovoltaic field data; based on the preprocessed photovoltaic field data, according to a fault correlation reference direction, obtaining a peak value and a valley value of in-phase current at the two ends of the network construction type photovoltaic sending-out line; and according to the peak value and the valley value, based on a Jensen-Shannon distance algorithm, constructing a pilot protection judgment strategy, and based on the pilot protection judgment strategy, carrying out pilot protection on a sending-out line area of the network construction type photovoltaic sending-out line. According to the method, a pilot protection judgment strategy is constructed according to the obtained Jensen-Shannon distance result, and the method has relatively high sensitivity and relatively high adaptability and can be better applied to protection of a transmission line of a constructed network type photovoltaic electric field.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and particularly relates to a pilot protection method, system, device and medium for a grid-forming photovoltaic outgoing line. Background Art

[0002] The proportion of new energy will continue to increase. For a long time, China has built a number of large-scale AC synchronous power grids dominated by thermal power, hydropower, etc. For a long time to come, conventional synchronous power sources will still be the main supporting power sources of China's power grid, and coal power remains the mainstay and ballast of the power grid. However, various problems are bound to be faced during the development of new energy, such as power balance problems, transient stability problems, etc. Grid-forming technology can improve system inertia and short-circuit capacity, improve the short-circuit ratio of multiple new energy stations, and improve the impedance characteristics of the power grid, and can effectively reconcile various problems faced by the development of new energy. It is one of the key technologies for the construction of future new power systems, and its importance for the research on relay protection of the converter AC system of the photovoltaic outgoing system is self-evident.

[0003] At present, relay protection devices based on power frequency quantities designed specifically for synchronous machine systems are still widely used for the outgoing lines of new energy AC power grids. However, the external characteristics of grid-forming converters and grid-following converters with current source characteristics are quite different, and there are certain differences in the dynamic response during the fault ride-through period of traditional synchronous machine systems. When a fault occurs in the new energy converter AC system, the fault characteristics of the system will change significantly, resulting in a significant decline in the operating performance of traditional protection methods based on power frequency quantities, and even misoperation of the protection may occur.

[0004] Therefore, it is necessary to design a new protection principle with more accurate discrimination and higher operating efficiency for the new application scenarios of grid-forming photovoltaic outgoing lines to meet the requirements of the reliability of grid-forming photovoltaic station outages. Summary of the Invention

[0005] Embodiments of the present invention provide a pilot protection method, system, device and medium for a grid-forming photovoltaic outgoing line to solve the above technical problems in the prior art.

[0006] To provide a basic understanding of some aspects of the disclosed embodiments, a simple summary is given below. This summary part is not a general review, nor is it intended to identify key / important constituent elements or delineate the protection scope of these embodiments. Its sole purpose is to present some concepts in a simple form as a prelude to the detailed description that follows.

[0007] According to the first aspect of the embodiments of the present invention, a pilot protection method for a grid-forming photovoltaic outgoing line is provided.

[0008] In one embodiment, the pilot protection method for the grid-forming photovoltaic outgoing line includes:

[0009] Obtain photovoltaic field data, and preprocess the photovoltaic field data to obtain preprocessed photovoltaic field data;

[0010] Based on the preprocessed photovoltaic field data, according to the pre-set fault correlation reference direction, obtain the peak and valley values of the in-phase currents at both ends of the grid-connected photovoltaic outgoing line;

[0011] According to the peak and valley values, based on the Jensen-Shannon distance algorithm, construct a pilot protection judgment strategy, and based on the pilot protection judgment strategy, perform pilot protection on the outgoing line area of the grid-connected photovoltaic outgoing line.

[0012] In one embodiment, the photovoltaic field data includes:

[0013] The three-phase current sampling values at both ends of the grid-connected photovoltaic outgoing line and the current data in a predetermined period after a fault occurs.

[0014] In one embodiment, the predetermined period is 20 ms.

[0015] In one embodiment, preprocessing the photovoltaic field data to obtain preprocessed photovoltaic field data includes:

[0016] Remove invalid data, abnormal data, and duplicate data from the photovoltaic field data.

[0017] In one embodiment, the fault correlation reference direction includes: the current direction and waveform difference characteristics when internal and external faults occur in the grid-connected photovoltaic field area.

[0018] In one embodiment, based on the preprocessed photovoltaic field data, according to the pre-set fault correlation reference direction, obtaining the peak and valley values of the in-phase currents at both ends of the photovoltaic outgoing line includes:

[0019] Based on the preprocessed photovoltaic field data, according to the current direction and waveform difference characteristics when internal and external faults occur in the grid-connected photovoltaic field area, determine the fault type and location of internal and external faults in the grid-connected photovoltaic field area;

[0020] According to the determined fault type and location of internal and external faults in the grid-connected photovoltaic field area, obtain the peak and valley values of the in-phase currents at both ends of the grid-connected photovoltaic outgoing line.

[0021] In one embodiment, constructing a pilot protection judgment strategy based on the Jensen-Shannon distance algorithm according to the peak and valley values includes:

[0022] According to the interval of the peak and valley values, construct 15 equally divided sections, and divide the in-phase current sampling data at both ends of the grid-connected photovoltaic outgoing line into each equally divided section;

[0023] Statistically calculate the proportion of the sampled data of the in-phase currents at both ends of the grid-connected photovoltaic outgoing line in the common equal division section to obtain the discrete probability distribution data of the sampled data of the in-phase currents at both ends of the photovoltaic outgoing line;

[0024] Based on the discrete probability distribution data, use the Jensen-Shannon distance algorithm to calculate the Jensen-Shannon distance of the discrete probability distribution; and construct a pilot protection judgment strategy according to the Jensen-Shannon distance.

[0025] In one embodiment, the pilot protection judgment strategy includes:

[0026] When the Jensen-Shannon distance is greater than a predetermined protection threshold, it is determined that an internal fault has occurred in the grid-connected photovoltaic field area, and a protection action command is issued;

[0027] When the Jensen-Shannon distance is less than or equal to the predetermined protection threshold, it is determined that an external fault has occurred in the grid-connected photovoltaic field area, and protection reset processing is performed.

[0028] According to the second aspect of the embodiments of the present invention, a pilot protection system for a grid-connected photovoltaic outgoing line is provided.

[0029] In one embodiment, the pilot protection system for the grid-connected photovoltaic outgoing line includes:

[0030] A data acquisition module, configured to acquire photovoltaic field data and preprocess the photovoltaic field data to obtain preprocessed photovoltaic field data;

[0031] A data processing module, configured to obtain the peak and valley values of the in-phase currents at both ends of the grid-connected photovoltaic outgoing line according to a preset fault correlation reference direction based on the preprocessed photovoltaic field data;

[0032] A judgment protection module, configured to construct a pilot protection judgment strategy based on the peak and valley values and the Jensen-Shannon distance algorithm, and perform pilot protection on the outgoing line area of the grid-connected photovoltaic outgoing line based on the pilot protection judgment strategy.

[0033] In one embodiment, the photovoltaic field data includes:

[0034] The three-phase current sampling values at both ends of the grid-connected photovoltaic outgoing line and the current data within a predetermined period after a fault occurs.

[0035] In one embodiment, the predetermined period is 20 ms.

[0036] In one embodiment, when the data acquisition module preprocesses the photovoltaic field data to obtain the preprocessed photovoltaic field data, it removes invalid data, abnormal data, and duplicate data from the photovoltaic field data.

[0037] In one embodiment, the fault correlation reference direction includes: the current direction and waveform difference characteristics when faults occur inside and outside the network-forming photovoltaic power generation area.

[0038] In one embodiment, when the data processing module obtains the peak and valley values of the in-phase currents at both ends of the photovoltaic transmission line based on the preprocessed photovoltaic field data according to the preset fault correlation reference direction, it determines the fault type and location of the faults inside and outside the network-forming photovoltaic power generation area based on the current direction and waveform difference characteristics when faults occur inside and outside the network-forming photovoltaic power generation area according to the preprocessed photovoltaic field data; and obtains the peak and valley values of the in-phase currents at both ends of the network-forming photovoltaic transmission line according to the determined fault type and location of the faults inside and outside the network-forming photovoltaic power generation area.

[0039] In one embodiment, when the differential protection judgment module constructs a pilot protection judgment strategy based on the peak and valley values using the Jensen-Shannon distance algorithm, it constructs 15 equally divided sections according to the intervals of the peak and valley values, and divides the in-phase current sampling data at both ends of the network-forming photovoltaic transmission line into each equally divided section; counts the proportion of the in-phase current sampling data at both ends of the network-forming photovoltaic transmission line in the common equally divided section to obtain the discrete probability distribution data of the in-phase current sampling data at both ends of the photovoltaic transmission line; calculates the Jensen-Shannon distance of the discrete probability distribution based on the discrete probability distribution data using the Jensen-Shannon distance algorithm; and constructs a pilot protection judgment strategy according to the Jensen-Shannon distance.

[0040] In one embodiment, the pilot protection judgment strategy includes:

[0041] When the Jensen-Shannon distance is greater than a predetermined protection threshold value, it is determined that a fault occurs inside the network-forming photovoltaic power generation area, and a protection action instruction is issued;

[0042] When the Jensen-Shannon distance is less than or equal to the predetermined protection threshold value, it is determined that a fault occurs outside the network-forming photovoltaic power generation area, and a protection reset process is performed.

[0043] According to the third aspect of the embodiments of the present invention, a computer device is provided.

[0044] In some embodiments, the computer device includes a memory and a processor, the memory stores a computer program, and the processor implements the steps of the above method when executing the computer program.

[0045] According to a fourth aspect of embodiments of the present invention, a computer-readable storage medium is provided.

[0046] In one embodiment, the computer-readable storage medium stores a computer program, and the computer program implements the steps of the above method when executed by a processor.

[0047] The technical solution provided by the embodiment of the present invention may have the following beneficial effects:

[0048] The present invention uses the Jensen-Shannon distance algorithm to quantitatively describe the similarity of the distribution of current sampling values ​​at both ends of the line after a fault. The obtained Jensen-Shannon distance result is compared with the set value. When an out-of-zone fault occurs, the present invention can be stable and not erroneous. When an in-zone fault occurs, the fault phase can be accurately determined and the fault can be reliably operated. The protection performance is improved, the present invention has high sensitivity and strong adaptability, and the present invention can be better applied to the protection of grid-type photovoltaic electric field transmission lines.

[0049] It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0051] Figure 1 It is a flow chart of a method for longitudinal protection of a grid-type photovoltaic transmission line according to an exemplary embodiment;

[0052] Figure 2 It is a structural block diagram of a grid-type photovoltaic transmission line longitudinal protection system according to an exemplary embodiment;

[0053] Figure 3 is a diagram of a grid-type inverter control system according to an exemplary embodiment;

[0054] Figure 4 is a schematic diagram of an equivalent circuit of a grid-connected converter transmission system according to an exemplary embodiment;

[0055] Figure 5 is a phasor diagram of a fault current variation according to an exemplary embodiment;

[0056] Figure 6It is a longitudinal protection flow chart of a grid-connected photovoltaic transmission line shown according to an exemplary embodiment;

[0057] Figure 7 It is a schematic diagram of a photovoltaic-grid transmission system shown according to an exemplary embodiment;

[0058] Figure 8 It is a waveform diagram of the A-phase current on both sides of the transmission line during an external AG-phase fault shown according to an exemplary embodiment;

[0059] Figure 9 It is a waveform diagram of the A-phase current on both sides of the transmission line during an internal AG-phase fault shown according to an exemplary embodiment;

[0060] Figure 10 It is a distribution diagram of the sampled values of the A-phase current on both sides of the transmission line during an internal AB-phase fault shown according to an exemplary embodiment;

[0061] Figure 11 It is a distribution diagram of the sampled values of the A-phase current on both sides of the transmission line during an external AB-phase fault shown according to an exemplary embodiment;

[0062] Figure 12 It is a schematic diagram of the structure of a computer device shown according to an exemplary embodiment. Detailed implementation manners

[0063] The following description and drawings fully illustrate the specific implementation manners herein, enabling those skilled in the art to practice them. Parts and features of some embodiments may be included in or substituted for parts and features of other embodiments. The scope of the embodiments herein includes the entire scope of the claims and all available equivalents of the claims. Herein, terms such as "first", "second", etc. are only used to distinguish one element from another, without requiring or implying any actual relationship or order between these elements. In fact, the first element can also be called the second element, and vice versa. Moreover, the term "including", "comprising" or any other variant thereof is intended to cover non-exclusive inclusion, such that a structure, device or equipment including a series of elements not only includes those elements, but also includes other elements not explicitly listed, or elements inherent to such structure, device or equipment. Without further limitation, an element defined by the statement "including one..." does not exclude the existence of another identical element in the structure, device or equipment including the said element. The embodiments herein are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts among the embodiments can be referred to each other.

[0064] As used herein, the terms "longitudinal", "lateral", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings. These are only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation on the present invention. In the description of the present application, unless otherwise specified and defined, the terms "mounted", "connected", and "coupled" should be understood in a broad sense. For example, it may be a mechanical connection or an electrical connection, or it may be the communication inside two elements. It may be directly connected or indirectly connected through an intermediate medium. For those of ordinary skill in the art, the specific meanings of the above terms can be understood according to specific circumstances.

[0065] As used herein, unless otherwise specified, the term "a plurality" means two or more.

[0066] As used herein, the character " / " indicates that the objects before and after are in an "or" relationship. For example, A / B means: A or B.

[0067] As used herein, the term "and / or" is an associative relationship describing an object, indicating that three relationships can exist. For example, A and / or B means: A or B, or, A and B, these three relationships.

[0068] It should be understood that although the steps in the flowchart are shown sequentially according to the indication of the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless there is a clear indication in the present application, the execution of these steps has no strict order limit, and these steps can be executed in other orders. Moreover, at least a part of the steps in the figure may include multiple sub-steps or multiple stages. These sub-steps or stages are not necessarily executed at the same time, but can be executed at different times. The execution order of these sub-steps or stages is not necessarily sequential, but can be executed alternately or in turn with at least a part of other steps or sub-steps or stages of other steps.

[0069] Each module in the device or system of the present application can be implemented in whole or in part by software, hardware, and their combination. The above-mentioned modules can be embedded in the processor of the computer device in hardware form or be independent of it, or can be stored in the memory of the computer device in software form, so as to facilitate the processor to call and execute the operations corresponding to the above-mentioned modules.

[0070] Without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other.

[0071] Figure 1An embodiment of a longitudinal protection method for a network-forming photovoltaic outgoing line of the present invention is shown.

[0072] In this optional embodiment, the longitudinal protection method for the network-forming photovoltaic outgoing line includes:

[0073] Step S101: Obtain photovoltaic field data and preprocess the photovoltaic field data to obtain preprocessed photovoltaic field data;

[0074] Step S102: Based on the preprocessed photovoltaic field data, obtain the peak and valley values of the in-phase currents at both ends of the network-forming photovoltaic outgoing line according to a preset fault correlation reference direction;

[0075] Step S103: According to the peak and valley values, construct a longitudinal protection judgment strategy based on the Jensen-Shannon distance algorithm, and perform longitudinal protection on the outgoing line area of the network-forming photovoltaic outgoing line based on the longitudinal protection judgment strategy.

[0076] Figure 2 An embodiment of a longitudinal protection system for a network-forming photovoltaic outgoing line of the present invention is shown.

[0077] In this optional embodiment, the longitudinal protection system for the network-forming photovoltaic outgoing line includes:

[0078] A data acquisition module 201, configured to obtain photovoltaic field data and preprocess the photovoltaic field data to obtain preprocessed photovoltaic field data;

[0079] A data processing module 202, configured to obtain the peak and valley values of the in-phase currents at both ends of the network-forming photovoltaic outgoing line according to a preset fault correlation reference direction based on the preprocessed photovoltaic field data;

[0080] A judgment and protection module 203, configured to construct a longitudinal protection judgment strategy based on the Jensen-Shannon distance algorithm according to the peak and valley values, and perform longitudinal protection on the outgoing line area of the network-forming photovoltaic outgoing line based on the longitudinal protection judgment strategy.

[0081] In the above embodiment, the photovoltaic field data includes: three-phase current sampling values at both ends of the network-forming photovoltaic outgoing line and current data in a predetermined period after a fault occurs; the predetermined period is 20 ms.

[0082] In the above embodiment, when preprocessing the photovoltaic field data to obtain preprocessed photovoltaic field data, invalid data, abnormal data, and duplicate data in the photovoltaic field data can be removed.

[0083] In the above embodiments, the fault correlation reference direction includes: the current direction and waveform difference characteristics when internal and external faults occur in the network-forming photovoltaic power generation area. Then, when obtaining the peak and valley values of the in-phase currents at both ends of the photovoltaic outgoing line based on the preprocessed photovoltaic power generation area data according to the pre-set fault correlation reference direction, the fault type and location of the internal and external faults in the network-forming photovoltaic power generation area can be determined based on the current direction and waveform difference characteristics when internal and external faults occur in the network-forming photovoltaic power generation area according to the preprocessed photovoltaic power generation area data; according to the determined fault type and location of the internal and external faults in the network-forming photovoltaic power generation area, the peak and valley values of the in-phase currents at both ends of the network-forming photovoltaic outgoing line can be obtained.

[0084] In the above embodiments, when constructing a pilot protection judgment strategy based on the Jensen-Shannon distance algorithm according to the peak and valley values, it is necessary to first perform standardization processing on the processed photovoltaic power generation area data. The so-called standardization processing is to divide the collected photovoltaic-side current data into 15 equal sub-intervals according to the numerical interval of the previously obtained data peak and valley values, and then respectively count the number of sampling points of the photovoltaic-side current data falling into each numerical interval. Generally speaking, it is to see the proportion of the current value falling into each numerical interval, so as to count the proportion of the number of data points in each interval of the current data to the total number of data points, and obtain the so-called "probability distribution"; and re-sample the time-sensitive data or divide it according to a time window. Specifically, 15 equal sub-intervals can be constructed according to the interval of the peak and valley values, and the in-phase current sampling data at both ends of the network-forming photovoltaic outgoing line can be divided into each equal sub-interval; count the proportion of the in-phase current sampling data at both ends of the network-forming photovoltaic outgoing line in the common equal sub-interval to obtain the discrete probability distribution data of the in-phase current sampling data at both ends of the photovoltaic outgoing line; based on the discrete probability distribution data, use the Jensen-Shannon distance algorithm to calculate the Jensen-Shannon distance of the discrete probability distribution; and construct a pilot protection judgment strategy according to the Jensen-Shannon distance.

[0085] Specifically, the Jensen-Shannon distance (JSD) is a statistic used to measure the difference between two probability distributions. It is a concept widely used in information theory and statistics. JSD provides a symmetric and bounded way to compare the similarity between two probability distributions, especially developed on the basis of the Kullback-Leibler divergence (KL divergence) to solve some deficiencies of the KL divergence. In machine learning, JSD can be used for similarity measurement in clustering algorithms or to evaluate the quality of generated samples in generative adversarial networks (GANs). The definition and application method of the Jensen-Shannon distance are as follows:

[0086] For discrete probability distributions P and Q, which are respectively composed of a series of discrete values and their corresponding probabilities, such as P = {p1, p2,... p n} and Q = {q1, q2,... q n}, where p i and q i respectively represent the probabilities of the i-th discrete value, and satisfy

[0087] (1) Calculate the intermediate harmonic distribution M: For discrete distributions, the weighted average of the two distributions can be directly obtained according to the definition to get

[0088] (2) Calculate the KL divergence: For the KL divergences of each discrete probability distribution P and Q from the intermediate harmonic distribution M, calculate according to the discrete form of the KL divergence:

[0089]

[0090] In the formula, p i and q i respectively represent the probabilities of the i-th discrete value of the probability distributions P and Q, and M i is the weighted average of p i and q i ;

[0091] Calculate the Jensen-Shannon divergence JSD(P||Q): Take the average of the above two KL divergences to get:

[0092]

[0093] Calculate the Jensen-Shannon distance D JS (P, Q): Finally, take the square root of half of the Jensen-Shannon divergence as the Jensen-Shannon distance:

[0094]

[0095] In the above embodiment, the pilot protection judgment strategy includes: when the Jensen-Shannon distance is greater than a predetermined protection threshold, it is judged that an internal fault occurs in the network-forming photovoltaic field area, and a protection action instruction is issued; when the Jensen-Shannon distance is less than or equal to the predetermined protection threshold, it is judged that an external fault occurs in the network-forming photovoltaic field area, and a protection reset process is performed.

[0096] Specifically, if the calculation result of the Jensen-Shannon distance of a certain phase satisfies the following formula, it is determined that an internal fault occurs in that phase, and the protection on both sides issues a tripping signal; otherwise, it is determined that an external fault occurs.

[0097] D JS (P, Q) > D set

[0098] In the formula: D set is the protection threshold value, which is set to a small value greater than zero to avoid the influence of external and system interference factors.

[0099] In practical applications, the grid-forming inverter's grid-connection topology and control link structure are as Figure 3 shown. The grid-forming converter realizes the self-synchronization function by simulating the synchronization mechanism and power generation characteristics of a synchronous generator, and outputs a given voltage amplitude and phase. Its typical control architecture includes an outer-loop control link, a virtual impedance link, and an inner-loop control link. The function of the outer-loop control is to simulate the output characteristics of a synchronous machine, enabling the converter to have the grid-forming ability to provide frequency and voltage support; the function of the inner-loop control is to accurately control through current feedback to enhance the current limiting ability of the converter. The active power loop and reactive power loop of the outer-loop control respectively simulate the primary frequency regulation, swing equation, and excitation link of a synchronous generator, control the output reference values of the internal electromotive force phase and amplitude, and then generate a modulation wave through virtual impedance calculation and the current control loop. Among them, u dc is the voltage on the DC side of the three-phase inverter, C d is the DC bus capacitor, i abc is the output current, u gabc is the converter output voltage, PCC (Point of Common Coupling, PCC) is the point of common coupling, u abc is the grid connection point voltage, R c , L c are respectively the equivalent resistance and equivalent inductance between the converter port and the PCC point, R g , L g are respectively the line equivalent resistance and equivalent inductance; P ref and P e are respectively the active reference power and input electromagnetic power, Q ref and Q e are respectively the reactive reference power and input reactive electromagnetic power, ω m , ω and ω s are respectively the angular velocity reference value, the output angular velocity value, and the input angular velocity, J and D are respectively the inertia and damping values of the control link, δ is the output angle value; U o is the input system voltage value, E ref is the output voltage reference value. 1 / s is the operator of the Laplace inverse transform, k is a multiple value without physical meaning. abc, dq0 represent three-phase transformation and dq transformation.

[0100] The network-forming control generates a voltage amplitude reference signal by regulating the active and reactive power of the system. And the phase reference value (θ), which is the synchronization signal. The voltage amplitude and phase are used as the reference values for the inner-loop voltage control. The modulation signal output by the inner-loop current control generates the drive signal for the power switching device through the PWM generator.

[0101] The equivalent circuit of the network-forming converter sending out the system is as Figure 4 shown, where E S is the port voltage phasor of the converter when it is no-load, R S , Xs are the resistance and reactance of the converter respectively, R L , X L are the resistance and reactance of the grid-connected line respectively, X T is the reactance of the step-up transformer, U P , U M , U N are the voltage phasors corresponding to points P, M, and N respectively; J is the mathematical expression form of the phasor. The calculation expression for the voltage corresponding to the point of common coupling P is:

[0102] U p = E s - I p (R s + jX s )

[0103] where I p is the current at point P. And the expression under symmetrical fault is:

[0104] U p = E s - I p (R s + jX s ) = E s - I p (R V + j(X f + X V ))

[0105] where U p is the voltage at point P, R v , X v represent the resistance and reactance of the virtual impedance, X f represents the fault-to-ground reactance. During the fault, an additional virtual impedance is introduced, making the network-forming converter equivalent to a constant internal electromotive force, while the system impedance changes significantly before and after the fault according to the design parameters, and thus there are certain differences in the external electrical characteristics presented.

[0106] Due to the increase in the virtual impedance value when the current limiter measures are input after a fault occurs, the increase in the fault current on the PV side is limited, and there is an obvious difference from the fault current on the large power grid side. Therefore, there are also obvious differences in the distribution of the sampled values of the fault current. In order to efficiently limit the fault current output by the inverter, help the system achieve the fault ride-through (FRT) ability, and for the purpose of improving the system stability and reducing energy consumption, the reactance component of the virtual impedance is generally much larger than the resistance component, which easily causes the change amount formed by the current before and after the fault to not directly reflect the characteristics of the fault component.

[0107] The phasor diagram of the current change amount after a fault occurs is as Figure 5 shown. Among them, E M and E N are the electromotive forces on the power grid side and the PV side respectively, are the amplitudes of the differential and braking currents of each phase change amount respectively. I o is the starting value. are the phasors of the same-name currents at the protection installation locations on the power grid side and the PV source side of the outgoing line respectively; are the phasors of the same-name change amount currents at the protection installation locations on the power grid side and the PV source side of the outgoing line respectively;

[0108] Different from the characteristic that the change amount currents on both sides obtained by the superposition theorem in the synchronous machine system are basically in the same phase, there is a large phase angle difference between the change amount currents on both sides of the grid-forming converter system. In extreme cases, the phase angle difference between the fault current on the PV station side and the pre-fault current on the power grid side is about 90°. It can be seen from this that when an in-zone fault occurs on the outgoing line, due to the low-voltage ride-through control inside the inverter and the current limiter control set to protect the power electronic devices, its fault characteristics are different from the traditional line fault characteristics. The increase in the fault current on the PV side is limited, and there is an obvious difference from the fault current on the large power grid side. Therefore, there are also obvious differences in the distribution of the sampled values of the fault current; due to the characteristics of phase angle distortion and amplitude limitation that the fault current on the PV station side is prone to due to the influence of the virtual impedance characteristics, it is very different from the fault current characteristics on the AC power grid side of the outgoing line. In addition, the existing protection methods for PV outgoing lines may be affected by factors such as changes in fault current characteristics, weak power output on the PV side, and high noise, which may affect the performance of large-scale PV outgoing line protection. There is a possibility that the existing protection fails to operate, malfunctions, or even fails. Therefore, the differential protection action criterion can be constructed by using the difference in the distribution of the current sampled values at both ends of the line during in-zone and out-of-zone faults.

[0109] The present invention analyzes with the Figure 7 shown PV station outgoing system model. The PV new energy station is located on the M side of the 220 kV outgoing line and is connected to the AC power grid via the outgoing line. Three fault points are set in the line, where F1 and F3 are out-of-zone faults, F2 is an in-zone fault, E M is the PV power source, I Mis the fault current on the PV side, I G is the fault current on the grid side; M and G are the names of the nodes on both sides. The M side is the PV side and the G side is the grid side.

[0110] Since the positive direction of the current is usually defined as pointing from the bus to the line, after an external fault occurs, the fault current I M measured by the current transformers on both sides on the PV side, and the fault current I G on the grid side satisfy I M = -I G , which means that the waveforms of I M and -I G are almost exactly the same. When an internal fault occurs, according to the aforementioned fault current analysis, the frequency and phase of the fault current I M on the PV side are distorted and do not completely present a power-frequency sine form. Moreover, due to the current-limiting control of the virtual impedance, the magnitude of the fault current I M on the PV side is much smaller than the fault current I G on the grid side. Therefore, there are significant differences in the fault current waveforms on both sides of the outgoing line, and this feature can be used to identify external faults.

[0111] The Jensen-Shannon distance has many useful properties. D JS (I M , -I G ) is symmetric, that is, D JS (I M , -I G ) = JSD(Q||P); The value range of D JS (I M , -I G ) is fixed between [0, 1], where 0 means that the two distributions are exactly the same and 1 means that the two distributions are completely different; D JS (I M , -I G ) is always non-negative and takes the zero value only when P = Q. This means that when an external fault occurs on the line, the two current waveforms are almost exactly the same, and the distributions of their current sampling values also tend to be consistent. At this time, the calculated value of D JS (I M , -I G ) approaches 0; conversely, when the difference between the two current waveforms is large, the difference in the distributions of the current sampling values is large, and the calculated value of D JS (I M , -I G ) will be much greater than 0, and the greater the difference, the closer the result is to 1. Therefore, by obtaining D JS (I M , -I G)It can realize the quantification of the distribution similarity of the current sampling values at both ends of the line and construct the discrimination criteria for internal and external faults.

[0112] (1) Starting criterion

[0113] The starting criterion of the pilot protection is as follows:

[0114] |i ph (k)-i ph (k-N)|>I set

[0115] Where N is the number of sampling points per cycle, I set is the threshold value of the starting criterion, i ph is the zero-sequence current sampling value of phase ph, i ph (k-N) is the zero-sequence current sampling value N sampling points before for phase ph. When the continuous three sampling values of any phase current or zero-sequence current satisfy the above formula, the protection starts and continues with the subsequent steps.

[0116] (2) Discrimination criteria for internal and external faults

[0117] During normal operation and after an external fault occurs, the currents I W , -I G on both sides of the outgoing line are almost exactly the same, so the result of calculating D JS (I M , -I G ) is close to zero; while when an internal fault occurs, the waveforms of the currents I W , -I G on both sides of the outgoing line are quite different, and the corresponding result of D JS (I M , -I G ) is much greater than zero. Therefore, by using the MMD algorithm to quantify the waveform similarity, the following criterion can be obtained:

[0118] D JS (P, Q)>D set

[0119] In the formula: D set is the protection threshold value. After the protection starts, if the calculated D JS (I M , -I G ) satisfies the above formula, it is determined as an internal fault. Considering that D JS (I M , -I G ) is close to zero during an external fault, to avoid the influence of system and external interference factors (such as noise), the protection threshold D set should be set to a relatively small value.

[0120] The operation flowchart of the longitudinal protection method for the grid-forming PV outgoing line based on Jensen-Shannon distance proposed by the present invention is as follows Figure 6 As shown. In summary, after the protection criterion is started, if the continuously collected three-phase current sampling values meet the longitudinal protection start criterion, the protection is started. The discrete probability distributions of the sampling values at both ends of the line are obtained according to the proposed technical solution, and the Jensen-Shannon distance D JS (I M , -I G ) between them is calculated. If D JS (I M , -I G ) is greater than the set protection threshold value D set , it indicates that a fault occurs in the line area and the protection operates; otherwise, it is determined as an external fault and the protection is reset.

[0121] Referring to Figures 8 to 11 , in order to verify the beneficial effects of the present invention, scientific demonstrations are carried out through economic benefit calculations and simulation experiments. In this embodiment, experiments are respectively carried out on the existing traditional method and the method of this embodiment.

[0122] A grid-forming PV outgoing system as shown in Figure 7 is built in PSCAD / EMTDC. The voltage level of the outgoing line is 220 kV, the line length is 50 km, the unit impedance parameter is z = 0.0178 + j0.314 (Ω / km), and the PV side capacity is 80 MW. In order to improve the longitudinal protection efficiency, the data window is selected as 20 ms. The sampling frequency of the protection is set to 1 kHz, and the protection criterion D set is set to 0.3. A total of 3 fault points are set in the simulation model. To verify the performance of the proposed longitudinal protection, the following fault scenarios are considered: three fault points, including an external fault F1 occurring on the PV side, an internal fault F2 occurring 25 km away from the PV side on the outgoing line, and an external fault F3 occurring on the large power grid side; four fault types, including A-phase single-phase grounding fault AG, AB-phase interphase fault AB, AB two-phase grounding fault ABG, and three-phase fault ABC; two fault transition resistances Rf, including 0.01 Ω and 100 Ω. The fault time is 0 ms. The waveforms of the A-phase currents at both ends of the line when the AG fault occurs outside and inside the area are shown in Figure 8 , Figure 9 ; the distribution of the A-phase current sampling values at both ends of the line when the AB fault occurs outside and inside the area is shown in Figure 10 , Figure 11 .

[0123] From Figure 8 , Figure 9It can be seen that the fault current waveforms at both ends of the line corresponding to internal and external faults are significantly different. During external faults, the waveforms are axisymmetric about the horizontal axis, while during internal faults, the current waveform directions on the PV side and the grid side are significantly different. It can be seen that the simulation results are consistent with the aforementioned theoretical analysis.

[0124] From Figure 10 , Figure 11 the distribution of current sampling values, it can be known that by dividing the current sampling values at both ends into 15 equal partition segments and counting the proportion in the common interval, the discrete probability distribution visually presents the current waveform differences in a data form, and the simulation waveform results are quantified.

[0125] It can be learned from Table 1 that when an internal fault occurs, regardless of the fault type and transition resistance, the action quantity D JS (I M , -I G ) is significantly greater than the set protection threshold D set . The proposed protection method can ensure reliable operation, and the action phase is consistent with the fault. When an external fault occurs, the D JS (I M , -I G ) of each phase is less than the protection threshold D set , and it is not affected by the fault type and transition resistance, which means that the protection method proposed in the present invention does not operate reliably during external faults.

[0126] Table 1 is the simulation result of pilot protection

[0127]

[0128]

[0129] Figure 12 An embodiment of a computer device of the present invention is shown. The computer device can be a server, and the computer device includes a processor, a memory, and a network interface connected through a system bus. Among them, the processor of the computer device is used to provide computing and control capabilities. The memory of the computer device includes a non-volatile storage medium and an internal memory. The non-volatile storage medium stores an operating system, a computer program, and a database. The internal memory provides an environment for the operation of the operating system and the computer program in the non-volatile storage medium. The database of the computer device is used to store static information and dynamic information data. The network interface of the computer device is used to communicate with external terminals through a network connection. When the computer program is executed by the processor, the steps in the above method embodiment are implemented.

[0130] Those skilled in the art can understand, Figure 12The structure shown is only a block diagram of some structures related to the solution of the present invention, and does not constitute a limitation on the computer device to which the solution of the present invention is applied. The specific computer device may include more or fewer components than those shown in the figure, or combine certain components, or have a different component arrangement.

[0131] In addition, a computer device provided by the present invention includes a memory and a processor. A computer program is stored in the memory, and when the processor executes the computer program, the steps in the above method embodiments are implemented.

[0132] In addition, the present invention also provides a computer-readable storage medium, on which a computer program is stored, and when the computer program is executed by the processor, the steps in the above method embodiments are implemented.

[0133] Those of ordinary skill in the art can understand that all or part of the processes in the above method embodiments can be completed by instructing relevant hardware through a computer program. The computer program can be stored in a non-volatile computer-readable storage medium. When the computer program is executed, it can include the processes of the above method embodiments. Among them, any reference to a memory, storage, database, or other medium used in the embodiments provided by the present invention can include at least one of non-volatile and volatile memories. Non-volatile memory may include read-only memory (ROM), magnetic tape, floppy disk, flash memory, or optical memory, etc. Volatile memory may include random access memory (RAM) or external cache memory. By way of illustration and not limitation, RAM can be in various forms, such as static random access memory (SRAM) or dynamic random access memory (DRAM), etc.

[0134] The present invention is not limited to the structure already described and shown in the drawings, and various modifications and changes can be made without departing from its scope. The scope of the present invention is only limited by the appended claims.

Claims

1. A grid-type photovoltaic transmission line longitudinal protection method, characterized in that: include: Acquiring photovoltaic field data, and preprocessing the photovoltaic field data to obtain preprocessed photovoltaic field data; Based on the pre-processed photovoltaic field data, according to the preset fault-related reference direction, the peak and valley values ​​of the same-phase current at both ends of the grid-type photovoltaic transmission line are obtained; According to the peak value and the valley value, based on the Jensen-Shannon distance algorithm, a longitudinal protection judgment strategy is constructed, and based on the longitudinal protection judgment strategy, longitudinal protection is performed on the transmission line area of ​​the grid-type photovoltaic transmission line.

2. The grid-type photovoltaic transmission line longitudinal protection method according to claim 1 is characterized in that: The photovoltaic field data includes: The three-phase current sampling values ​​at both ends of the grid-connected photovoltaic transmission line and the current data of the predetermined period after the fault occurs.

3. The grid-type photovoltaic transmission line longitudinal protection method according to claim 2 is characterized in that: The predetermined period is 20 ms.

4. The grid-type photovoltaic transmission line longitudinal protection method according to claim 1 is characterized in that: The photovoltaic field data is preprocessed to obtain the preprocessed photovoltaic field data including: Invalid data, abnormal data and duplicate data in the photovoltaic field data are removed.

5. The grid-type photovoltaic transmission line longitudinal protection method according to claim 1 is characterized in that: The fault-related reference direction includes: current direction and waveform difference characteristics when faults occur inside and outside the grid-type photovoltaic field; Moreover, based on the preprocessed photovoltaic field data and according to the preset fault-related reference direction, the peak and valley values ​​of the same-phase current at both ends of the photovoltaic transmission line are obtained, including: Based on the preprocessed photovoltaic field data, the fault type and location of the internal and external faults of the grid-type photovoltaic field area are determined according to the current direction and waveform difference characteristics when the internal and external faults of the grid-type photovoltaic field area occur; According to the determined fault types and locations of the faults inside and outside the grid-forming photovoltaic field area, the peak values ​​and valley values ​​of the same-phase currents at both ends of the grid-forming photovoltaic transmission line are obtained.

6. The grid-type photovoltaic transmission line longitudinal protection method according to claim 1 is characterized in that: According to the peak value and the valley value, based on the Jensen-Shannon distance algorithm, a longitudinal protection judgment strategy is constructed, including: According to the intervals between the peak value and the valley value, 15 equally divided segments are constructed, and the same-phase current sampling data at both ends of the grid-forming photovoltaic transmission line are divided into each equally divided segment; The proportion of the same-phase current sampling data at both ends of the grid-type photovoltaic transmission line in the common average-divided section is counted to obtain the discrete probability distribution data of the same-phase current sampling data at both ends of the photovoltaic transmission line; Based on the discrete probability distribution data, the Jensen-Shannon distance of the discrete probability distribution is calculated using the Jensen-Shannon distance algorithm; and according to the Jensen-Shannon distance, the longitudinal protection judgment strategy is constructed.

7. The grid-type photovoltaic transmission line longitudinal protection method according to claim 6 is characterized in that: The longitudinal protection judgment strategy includes: When the Jensen-Shannon distance is greater than a predetermined protection threshold value, it is determined that an internal fault occurs in the grid-type photovoltaic field area, and a protection action instruction is issued; When the Jensen-Shannon distance is less than or equal to the predetermined protection threshold value, it is determined that an out-of-area fault occurs in the grid-type photovoltaic field area, and protection restoration processing is performed.

8. A grid-type photovoltaic transmission line longitudinal protection system, characterized in that: include: A data acquisition module is used to acquire photovoltaic field data and preprocess the photovoltaic field data to obtain preprocessed photovoltaic field data; A data processing module is used to obtain the peak value and valley value of the same-phase current at both ends of the grid-type photovoltaic transmission line based on the pre-processed photovoltaic field data and the preset fault-related reference direction; The judgment protection module is used to construct a longitudinal protection judgment strategy according to the peak value and the valley value based on the Jensen-Shannon distance algorithm, and perform longitudinal protection on the transmission line area of ​​the grid-type photovoltaic transmission line based on the longitudinal protection judgment strategy.

9. The grid-type photovoltaic transmission line longitudinal protection system according to claim 8 is characterized in that: The photovoltaic field data includes: The three-phase current sampling values ​​at both ends of the grid-connected photovoltaic transmission line and the current data of the predetermined period after the fault occurs.

10. The grid-type photovoltaic transmission line longitudinal protection system according to claim 9, characterized in that: The predetermined period is 20 ms.

11. The grid-type photovoltaic transmission line longitudinal protection system according to claim 8, characterized in that: When the data acquisition module pre-processes the photovoltaic field data to obtain the pre-processed photovoltaic field data, it removes invalid data, abnormal data and duplicate data in the photovoltaic field data.

12. The grid-type photovoltaic transmission line longitudinal protection system according to claim 8, characterized in that: The fault-related reference direction includes: current direction and waveform difference characteristics when faults occur inside and outside the grid-type photovoltaic field; Moreover, when the data processing module obtains the peak and valley values ​​of the same-phase current at both ends of the photovoltaic transmission line based on the preprocessed photovoltaic field data and according to the preset fault-associated reference direction, the data processing module determines the fault type and location of the fault inside and outside the grid-type photovoltaic field area based on the preprocessed photovoltaic field data and according to the current direction and waveform difference characteristics when the fault occurs inside and outside the grid-type photovoltaic field area; and obtains the peak and valley values ​​of the same-phase current at both ends of the grid-type photovoltaic transmission line based on the determined fault type and location of the fault inside and outside the grid-type photovoltaic field area.

13. The grid-type photovoltaic transmission line longitudinal protection system according to claim 8, characterized in that: When the judgment protection module constructs a longitudinal protection judgment strategy based on the peak value and the valley value and the Jensen-Shannon distance algorithm, it constructs 15 equally divided segments according to the interval of the peak value and the valley value, and divides the same-phase current sampling data at both ends of the grid-forming photovoltaic transmission line into each equally divided segment; the proportion of the same-phase current sampling data at both ends of the grid-forming photovoltaic transmission line in the common equally divided segment is counted to obtain the discrete probability distribution data of the same-phase current sampling data at both ends of the photovoltaic transmission line; Based on the discrete probability distribution data, the Jensen-Shannon distance of the discrete probability distribution is calculated using the Jensen-Shannon distance algorithm; and according to the Jensen-Shannon distance, the longitudinal protection judgment strategy is constructed.

14. The grid-type photovoltaic transmission line longitudinal protection system according to claim 13, characterized in that: The longitudinal protection judgment strategy includes: When the Jensen-Shannon distance is greater than a predetermined protection threshold value, it is determined that an internal fault occurs in the grid-type photovoltaic field area, and a protection action instruction is issued; When the Jensen-Shannon distance is less than or equal to the predetermined protection threshold value, it is determined that an out-of-area fault occurs in the grid-type photovoltaic field area, and protection restoration processing is performed.

15. A computer device comprising a memory and a processor, wherein the memory stores a computer program, wherein: When the processor executes the computer program, the steps of the method according to any one of claims 1 to 7 are implemented.

16. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the method according to any one of claims 1 to 7 are implemented.

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