AC micro-grid protection method and system based on reference point voltage waveform comparison

Abstract

The invention discloses an alternating-current micro-grid protection method and system based on reference point voltage waveform comparison, and the method comprises the steps: setting a first-stage reference point, a second-stage reference point and a third-stage reference point at at least A1, A2 and A3 positions of a protected line respectively; establishing a dual-port network model of a protected line and port electrical quantity, taking one port as a measurement point, and calculating the voltage of each level of reference point at the other port based on the voltage of the measurement point; and calculating the cosine similarity between the voltage waveform of the measurement point and the voltage waveform of each level of reference point, and taking the value of the cosine similarity corresponding to each level of reference point as the action criterion of each level of protection, thereby realizing the multi-level sectional protection of the protected circuit of the AC micro-grid, and determining the fault position at the same time. The method can expand the protection range, is not affected by branch loads or distributed power supply branches, is not affected by a distributed power supply control strategy, and can be suitable for a grid-connected operation mode and an island operation mode at the same time.

Description

Technical Field

[0001] The present invention relates to the technical field of AC microgrid protection, and in particular to an AC microgrid protection method and system based on reference point voltage waveform comparison. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] At present, distributed power generation systems based on renewable energy such as wind power and photovoltaics are developing rapidly, but the integration of a high proportion of distributed new energy into the power grid has brought serious absorption problems. In order to improve the utilization rate and absorption level of new energy, new energy and local loads form a microgrid to achieve local power balance, and on this premise, they are connected to the upper distribution network. The distributed power sources inside the microgrid are mainly clean energy, and the internal electric energy is basically self-balanced.

[0004] There are two typical operating modes of microgrids: grid-connected mode and off-grid mode. Since the structural characteristics and operating characteristics of microgrids are different from those of traditional power grids, the fault characteristics of microgrids also have some particularities. For example, when a fault occurs inside the microgrid, the distribution network can provide a larger short-circuit current during grid-connected operation, while the short-circuit current is only provided by distributed power sources during island operation. Therefore, under different operating modes, the fault characteristics of a fault at the same location are significantly different. The operating characteristics of distributed power sources within the microgrid are affected by the control strategy. Due to the diversity of control strategies for distributed power sources, the fault characteristics of the microgrid are significantly affected by the control strategy.

[0005] Therefore, the microgrid protection strategy should consider both grid-connected and islanded operation modes, and be able to adapt to different types of distributed power sources; in the prior art, the fault detection and protection of AC microgrids are mostly derived from traditional distribution network protection methods, which require switching fixed values ​​or changing protection strategies, and are affected by different types of distributed power sources. The prior art discloses that a reference point is set and a longitudinal comparison protection method is used to perform fault protection on the distribution network. However, during fault protection, two-end communication is required to compare the results of the head end and the end end of the protected line to determine the protection action; this method is subject to the reliability of communication. Once there is a problem with the communication, reliable protection cannot be performed. Summary of the invention

[0006] In order to solve the above problems, the present invention proposes an AC microgrid protection method and system based on reference point voltage waveform comparison. Three reference points are selected at the set positions of the protected lines. By analyzing the changing trends of the voltage waveforms at the measuring points and the voltage waveforms at the reference points, the specific fault location can be determined. The detection result is not affected by the microgrid operation mode, nor is it affected by the distributed power supply control strategy.

[0007] In some embodiments, the following technical solutions are adopted:

[0008] An AC microgrid protection method based on reference point voltage waveform comparison includes:

[0009] At least A1, A2 and A3 positions of the protected line, a primary reference point, a secondary reference point and a tertiary reference point are respectively set; the position of each reference point can respectively form the main protection, near backup protection and far backup protection ranges for the protected line, as well as the near backup protection range for the next adjacent line;

[0010] A two-port network model of the protected line and port electrical quantities is established, one of the ports is used as the measurement point, and based on the voltage at the measurement point, the voltage of each level reference point is calculated at the other port;

[0011] The cosine similarity between the voltage waveform at the measurement point and the voltage waveform at each level of reference point is calculated, and the cosine similarity value corresponding to each level of reference point is used as the action criterion for protection at each level, so as to realize multi-level segmented protection of the protected line of the AC microgrid and determine the fault location at the same time.

[0012] Optionally, the position of the primary reference point is selected specifically as follows:

[0013]

[0014] in, is the line length corresponding to the primary reference point of the protected line, is the reliability coefficient of the primary protection of the protected line, with a value range of [0.9, 0.95], L AB is the length of the protected line.

[0015] Optionally, the location of the secondary reference point is selected specifically as follows:

[0016] Principle 1: Determine the location of the second reference point of the protected line in accordance with the principle of coordination with the primary protection of the next adjacent line:

[0017]

[0018] in, is the line length corresponding to the secondary reference point of the protected line, is the reliability coefficient of the secondary protection of the protected line, and its value range is [0.8, 0.85]; L AB is the length of the protected line, The line length corresponding to the primary reference point of the next adjacent line;

[0019] Principle 2: Determine the second reference point of the protected line by performing sensitivity calibration when a short circuit occurs at the end of the protected line:

[0020]

[0021] in, is the sensitivity coefficient of the secondary protection of the protected line, and its value range is [1.25, 1.4];

[0022] The results obtained by selecting the above two principles The larger one of the two is used as the position of the secondary reference point of the protected line;

[0023] If the secondary protection range of the protected line extends into the secondary protection range of the next adjacent line, a delay is set to avoid conflicting protection actions.

[0024] Optionally, the location of the third-level reference point is selected specifically as follows:

[0025] Principle 1: Determine the location of the third reference point of the protected line in accordance with the principle of coordination with the secondary protection and tertiary protection of the next adjacent line:

[0026]

[0027] in, is the line length corresponding to the third-level reference point of the protected line, is the sensitivity coefficient of the third-level protection of the protected line, and its value range is [0.8, 0.9], L AB is the length of the protected line, The line length corresponding to the secondary reference point of the next adjacent line;

[0028] Principle 2: When used as the primary protection or secondary protection of the protected line, the position of the third reference point of the protected line shall be determined by performing sensitivity calibration when a short circuit occurs at the end of the protected line:

[0029]

[0030] in, is the sensitivity coefficient of the third-level protection of the protected line, and its value range is [1.5, 1.6];

[0031] Principle 3: When acting as remote backup protection for the next adjacent line, the location of the third reference point of the protected line shall be determined by performing sensitivity verification when a short circuit fault occurs at the end of the next adjacent line:

[0032]

[0033] in, L is the sensitivity coefficient of the third-level protection of the protected line, and its value range is [1.2, 1.3]; BC is the length of the next adjacent line;

[0034] The results obtained by selecting the above three principles The largest one among them is taken as the position of the tertiary reference point of the protected line.

[0035] Optionally, the position of the first reference point corresponds to a primary protection range, the position of the second reference point corresponds to a secondary protection range, and the position of the third reference point corresponds to a tertiary protection range;

[0036] Set delays for each level of protection action, specifically:

[0037] The first level protection is direct action without delay;

[0038] The delay of the secondary protection action is:

[0039] If the secondary protection range of the protected line does not conflict with the secondary protection range of the next adjacent line, then

[0040] If the secondary protection range of the protected line extends to the secondary protection range of the next adjacent line,

[0041] in, is the delay of the secondary protection action of the protected line, is the first-level protection action delay of the next adjacent line, Δt and Δt′ are the set delay times respectively;

[0042] The delay of the third level protection action is:

[0043]

[0044] in, is the delay of the secondary protection action of the protected line, and Δt″ is the set delay time.

[0045] Optionally, the cosine similarity between the voltage at the measurement point and the voltage at each reference point is calculated, specifically:

[0046]

[0047] Wherein, a represents the voltage waveform vector at the measurement point, and b represents the voltage waveform vector at the reference point.

[0048] Optionally, the cosine similarity value corresponding to each level of reference point is used as the criterion for each level of protection action, specifically:

[0049] If cos(θ1)<T, the first-level protection is activated;

[0050] If cos(θ1)>T, and cos(θ2)<T, the secondary protection will be activated;

[0051] If cos(θ1)>T, and cos(θ2)>T, and cos(θ3)<T, the third level protection is activated;

[0052] Among them, cos(θ1), cos(θ2), and cos(θ3) are the cosine similarities between the voltage at the measurement point and the voltage at the first-level reference point, the voltage at the second-level reference point, and the voltage at the third-level reference point, respectively; and T is the set threshold.

[0053] Optionally, the process of determining the fault location is specifically as follows:

[0054] Based on the cosine similarity value between the voltage waveform at the measuring point and the voltage waveform at the primary reference point, determine whether the primary protection is activated; if the primary protection is activated, the fault is located within the length range of the primary reference point of the protected line; if the primary protection is not activated, continue to determine whether the secondary protection is activated based on the cosine similarity value between the voltage waveform at the measuring point and the voltage waveform at the secondary reference point;

[0055] If the second-level protection is activated, the fault is located between the first-level reference point and the second-level reference point of the protected line, or the fault is located within the length range of the first-level reference point but the first-level protection refuses to operate; if the second-level protection does not operate, continue to determine whether the third-level protection is activated based on the value of the cosine similarity between the voltage waveform at the measuring point and the voltage waveform at the third-level reference point; if the third-level protection is activated, the fault is located between the second-level reference point and the third-level reference point of the protected line, or the fault is located within the length range of the first-level reference point or the second-level reference point but the first-level protection and the second-level protection refuse to operate; if the third-level protection does not operate, it is an out-of-zone fault or there is no fault.

[0056] In other embodiments, the following technical solutions are adopted:

[0057] An AC microgrid protection system based on reference point voltage waveform comparison, comprising:

[0058] A reference point selection module is used to set a primary reference point, a secondary reference point and a tertiary reference point at at least A1, A2 and A3 positions of the protected line, respectively; the position of each reference point can respectively form the main protection and backup protection of the protected line, and the main protection and backup protection of the next adjacent line;

[0059] The reference point voltage calculation module is used to establish a two-port network model of the protected line and the port electrical quantity, taking one of the ports as the measurement point and calculating the voltage of each level of reference point at the other port based on the voltage at the measurement point;

[0060] The line protection module is used to calculate the cosine similarity between the voltage waveform of the measurement point and the voltage waveform of each level of reference point, and use the cosine similarity value corresponding to each level of reference point as the action criterion for protection at each level, so as to realize multi-level segmented protection of the protected line of the AC microgrid and determine the fault location at the same time.

[0061] In other embodiments, the following technical solutions are adopted:

[0062] A terminal device includes a processor and a memory, wherein the processor is used to implement instructions; the memory is used to store multiple instructions, and the instructions are suitable for being loaded by the processor and executing the above-mentioned AC microgrid protection method based on reference point voltage waveform comparison.

[0063] Compared with the prior art, the present invention has the following beneficial effects:

[0064] (1) The present invention comprehensively considers the selectivity and reliability of protection, the access of distributed power sources, transition resistance and other influencing factors, sets three-level reference points for the protected lines, and determines the fault location and performs protection actions by comparing the change trends of the voltage waveforms at the measurement points and the reference points at each level. The method can expand the protection range and will not be affected by branch loads or distributed power source branches, nor by distributed power source control strategies. It can be applied to both grid-connected and island operation modes.

[0065] (2) The present invention comprehensively considers different principles to determine the positions of the three-level reference points respectively, and adds different delays to the three-level protection to avoid conflicts between the protection actions of the protected line and the next adjacent line, thereby ensuring the reliable execution of the protection actions; the first-level protection is used as the main protection, the second-level protection and the third-level protection are used as backup protection respectively, and the three-level protection cooperates with each other to accurately give fault judgment results and realize reliable protection of AC microgrid faults.

[0066] (3) The fault protection method of the present invention does not require data communication between the two ends, and can completely get rid of the dependence on communication; it does not require complex fixed value setting calculations, nor does it require fixed value switching operations, and has high reliability and flexibility.

[0067] Other features and advantages of additional aspects of the present invention will be given in part in the following description, and in part will become obvious from the following description, or will be learned through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Figure 1 Schematic diagram of a Π-type circuit model in an embodiment of the present invention;

[0069] Figure 2 A schematic diagram of the network model structure of the protected line and port electrical quantities in an embodiment of the present invention;

[0070] Figure 3 This is a reference voltage waveform diagram of the voltage for measuring the fault voltage outside the forward zone in an embodiment of the present invention;

[0071] Figure 4 This is a reference voltage waveform diagram of a fault voltage measured outside the reverse zone in an embodiment of the present invention;

[0072] Figure 5 This is a reference voltage waveform diagram of the fault measurement voltage within the zone in an embodiment of the present invention;

[0073] Figure 6 A schematic diagram of a protected circuit in an embodiment of the present invention;

[0074] Figure 7 Analysis of the impact of load branch current in the embodiment of the present invention;

[0075] Figure 8 Analysis of the influence of DG branch current in the embodiment of the present invention;

[0076] Fig. 9 A schematic diagram of reference points in an embodiment of the present invention;

[0077] Fig.10 This is a schematic diagram of the coordination of the three-stage protection range in an embodiment of the present invention;

[0078] Fig.11 This is a flow chart of an AC microgrid protection method based on reference point voltage waveform comparison in an embodiment of the present invention;

[0079] Fig.12 It is a 10kV medium voltage AC microgrid simulation model in an embodiment of the present invention;

[0080] Figures 13(a) and 13(b) are respectively the reference voltage waveforms of the measured voltages for the out-of-zone fault and the in-zone fault during the grid-connected operation in Case 1;

[0081] Figure 14(a) and Figure 14(b) are the reference voltage waveforms of the measured voltages for the out-of-zone fault and the in-zone fault during the island operation in Case 1, respectively;

[0082] Figures 15(a) and 15(b) are respectively the reference voltage waveforms of the voltage measured with transition resistance for the out-of-zone fault and the in-zone fault in case 2 during grid-connected operation;

[0083] FIG16(a) and FIG16(b) are the reference voltage waveforms of the voltage measured with transition resistance for the out-of-zone fault and the in-zone fault in case 2, respectively. DETAILED DESCRIPTION

[0084] It should be noted that the following detailed descriptions are illustrative and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

[0085] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0086] Embodiment 1

[0087] In one or more embodiments, an AC microgrid protection method based on reference point voltage waveform comparison is disclosed, combined with Fig.11 , specifically including the following process:

[0088] (1) A primary reference point, a secondary reference point and a tertiary reference point are respectively set at at least A1, A2 and A3 positions of the protected line; the positions of the reference points can respectively form the main protection and backup protection for the protected line, and the main protection and backup protection for the next adjacent line;

[0089] (2) Establish a two-port network model of the protected line and port electrical quantities, use one port as the measurement point, and calculate the voltage of each level reference point at the other port based on the voltage at the measurement point;

[0090] (3) The cosine similarity between the voltage at the measurement point and the voltage at each reference point is calculated, and the cosine similarity value corresponding to each reference point is used as the action criterion for each level of protection, thereby realizing multi-level segmented protection of the protected lines of the AC microgrid and determining the fault location at the same time.

[0091] As a specific implementation method, this embodiment uses Figure 1 The Π line model shown in the figure is taken as an example for analysis, and the following is constructed: Fig.12 The 10 kV microgrid model shown takes line AB as the protected line.

[0092] In a microgrid, the electrical quantities at the line port are subject to dual constraints of the distributed power source and the line equivalent model, and the power supply inside the microgrid generates and maintains the electrical quantities. Although the fault output characteristics of different types of power sources vary greatly, the electrical quantities of the same line model have the same mathematical relationship, which in turn determines the distribution law of the power output characteristics. The mathematical relationship between them has nothing to do with the power supply characteristics and only depends on the line model. Therefore, in a microgrid, once the line model of the protected object is determined, there is a corresponding relationship between the port voltage and the port current.

[0093] Combination Figure 2 , establish a network model of the protected line and the port electrical quantity. According to the dual-port theory, in this embodiment, one port is called the measurement point and the other port is called the reference point. The voltage and current of the measurement point are respectively recorded as u1 and i1, which can be obtained by mutual inductor sampling; the voltage and current of the reference point are respectively recorded as u2 and i2, which can be calculated based on the voltage of the measurement point.

[0094] According to the circuit theorem, when faults occur at different positions of the line, the mathematical equations for the electrical quantities at the measurement points and reference points at both ends of the two-port network model are as follows:

[0095] When the line is operating normally or an out-of-zone fault occurs, that is, there is no fault or the fault is located at the out-of-zone fault location, the protected object is intact. The instantaneous sampling values ​​of the measured voltage u1 and the measured current i1 at the head end of line AB are known. According to the mathematical differential equation, the reference voltage u2 of the reference point is specifically:

[0096]

[0097] Among them, R BR1 , L BR1 are the resistance and inductance of line BR1. Line BR1 is the line between the measurement point and the reference point. The line resistance and inductance corresponding to reference points at different positions are different.

[0098] Based on this analysis, the following is made:

[0099] ① When the system operates normally, the line model is complete, and there is basically no difference in the change trend between the measured voltage and the reference voltage waveforms, and the waveforms are similar.

[0100] ② When a fault occurs outside the forward zone and the fault point is located at K3 (line BC), the protected line model is still intact. Taking a three-phase metallic fault as an example, when a fault occurs outside the forward zone, the relationship between the measured voltage u1 and the reference voltage u2 is as follows:

[0101]

[0102] Among them, R BK3 and L BK3They represent the line impedance between the measurement point and the fault point respectively. The line model is not damaged and the protected object is complete, so the corresponding mathematical relationship still holds, and the expression of the reference voltage is obtained as follows:

[0103]

[0104] Among them, R BR3 and L BR3 Represent the line impedance between the measurement point and the reference point, and R BK3 Greater than R BR3 , comparing formula (2) and formula (3), the reference point voltage u2 will have a voltage drop compared to the measured voltage u1, because the amplitude of u2 is smaller than that of u1, but u1>0, u2>0, the two voltage waveforms have the same change trend; the specific performance is as follows Figure 3 shown.

[0105] ③ When a fault occurs outside the reverse zone and the fault point is located at K1 (line EA), the direction of the assumed measurement current i1 is opposite to the actual measured measurement current. At this time, the expression of the measurement voltage becomes:

[0106]

[0107] Among them, R BK1 and L BK1 Represent the line resistance and inductance between the measurement point and the fault point respectively.

[0108] The expression of reference voltage is:

[0109]

[0110] R BR1 and L BR1 Represent the line resistance and inductance between the measurement point and the reference point, respectively, and (R BK1 +R BR1 ) is greater than R BK1 , (L BK1 +L BR1 ) is greater than L BK1 , at this time, the amplitude of the reference voltage is greater than the amplitude of the measured voltage, and u1<0, u2<0, their waveform change trends are still the same; the specific performance is as follows Figure 4 shown.

[0111] ④When an internal fault occurs, the fault point is located at K2 (line AB), and the expression for measuring voltage is still:

[0112]

[0113] But the expression for the reference voltage becomes:

[0114]

[0115] R BR1 and L BR1 Represent the line resistance and inductance between the measurement point and the reference point, respectively, and R BK2 Less than R BR1 , so (R BK2 -R BR1 ) is less than zero, L BK2 Less than L BR1 , so (L BK2 -L BR1 ) is less than zero. At this time, u1>0, but u2<0, the measured voltage and the reference voltage waveform change trend are opposite, such as Figure 5 shown.

[0116] Through the above analysis, we can conclude that:

[0117] When the line is operating normally or an out-of-zone fault occurs, the waveform change trends of the reference voltage and the measured voltage are the same, and the two waveforms are considered similar; when an in-zone fault occurs, the waveform change trends of the reference voltage and the measured voltage are opposite, and the two waveforms are considered opposite. Therefore, by comparing the similarity of the two waveforms, the fault location can be accurately determined.

[0118] The reference point selection process is described in detail below.

[0119] Each time a reference point is determined, it corresponds to a protection range. In this embodiment, three reference points at different positions are selected, and three levels of protection are constructed according to the main protection, near backup protection and far backup protection, which cooperate with each other to achieve a perfect protection effect. Of course, multi-level protection can also be constructed if necessary.

[0120] This embodiment Figure 6 The line model is taken from Fig.12 A branch of the 10kV microgrid model, combined with Figure 6 , taking the protection of CB7 at the head end of line AB as the research object, the first reference point R Ⅰ 1. Second reference point R Ⅱ 1 and the third reference point R ⅡⅠ 1. R Ⅰ 1. R Ⅱ 1. R ⅡⅠ 1 are respectively used as the main protection, near backup protection and far backup protection range corresponding to the protection at CB1.

[0121] ①The first reference point R Ⅰ 1 position selection:

[0122] This embodiment constructs the first-level protection as the main protection of the protected line AB according to the position of the first reference point. In order to meet the rapidity of protection, the first-level protection is required to be simple, reliable, and quick to act, and to protect the entire length of the line as much as possible. Suppose the corresponding relationship between the protection range of the first-level protection and the entire length of the line is: in is the line length corresponding to the primary protection reference point, is the reliability coefficient of the first-level protection, L AB is the length of line AB; since the protection method proposed in this embodiment does not depend on the specific value of the electrical quantity, the reliability coefficient can be relatively large, such as [0.9, 0.95], which can expand the protection range of the first-level protection.

[0123] In this embodiment, the reliability coefficient is 0.9, that is,

[0124] ② Second reference point R Ⅱ 1 position selection:

[0125] According to the position of the second reference point, the secondary protection is constructed as the close backup protection, which is required to protect the full length of the protected line and cannot exceed the primary protection range of the next adjacent line BC.

[0126] The location of the second reference point is selected mainly based on the following two principles:

[0127] Principle 1: Determine the secondary protection range (i.e., the position of the second reference point) of the protected line AB in accordance with the principle of coordination with the primary protection of the next adjacent line.

[0128] Firstly, the influence of the access of branch loads and distributed generation on the proposed protection method is analyzed:

[0129] When there is a load branch, the current flowing through the load branch is recorded as I load , the fault point current is recorded as I fault ,like Figure 7 As shown in the figure, take line AB as the protected line for analysis, and the fault K1 occurs in the area, I load and I fault The same direction, and I load The amplitude is much smaller than I fault When an out-of-zone fault K2 occurs, the current flowing through the load branch can also be ignored compared to the current at the fault point. When there is a branch load, it may only have a very small impact on the current flowing through the reference point, so it will only affect the amplitude of the reference point voltage and will not affect the change trend of the calculated voltage.

[0130] When there is a DG (distributed generation) branch, the current provided by the distributed generation is recorded as I DG, the fault point current is recorded as I fault . Combined Figure 8 The impact of distributed generation on protection depends on whether the distributed generation absorbs power or outputs power. In most cases, DG is used as a power source to transmit power to the microgrid. At this time, if a fault occurs in the area, K1, I DG and I fault The direction is opposite, but due to the current limiting effect of the internal converter of the distributed power source, the output current of the DG will not continue to increase indefinitely. The maximum value that can be reached is usually no more than 1.2 times the rated current of the converter type DG, and will not exceed the size of the short-circuit current. Therefore, even if the distributed power source injects current into the protected line, it will not offset the fault current. When the fault is located outside the area K2, for the fault point, it is equivalent to the large power source and the distributed power source providing current to the fault point together, which is more conducive to the detection of faults by this method. When the distributed power source absorbs power, it is equivalent to a load branch, which is the same as the above analysis and will not be repeated here.

[0131] Through the above analysis, it can be seen that the existence of branch loads or distributed power sources may only affect the magnitude of the current flowing through the reference point, so it will only affect the amplitude of the reference point voltage, and will not affect the calculated voltage direction. The protection method proposed in this embodiment uses the measured voltage and the reference point voltage waveform change trend as the standard for judging faults, so the existence of branch loads and distributed power branches has no adverse effect on the protection method. Different from the traditional distance protection, which needs to consider the influence of branch loads to set the branch coefficient, this technology does not need to consider the branch coefficient when selecting the reference point.

[0132] Therefore, there is no need to consider the branch coefficient when determining the position of the second reference point, so:

[0133]

[0134] in, is the protection range corresponding to the secondary protection of the protected line, is the reliability coefficient of the secondary protection of the protected line, and the value range may be [0.8, 0.9]. In this embodiment, it is taken as 0.8.

[0135] Principle 2: Determine the location of the second reference point by performing a sensitivity check when a short circuit occurs at the end of the protected line.

[0136] The sensitivity coefficient of the secondary protection is defined as:

[0137] The protection range of the secondary protection of the protected line is initially set as

[0138] Considering other adverse factors such as transition resistance, it is necessary to increase the sensitivity coefficient. In this embodiment, the sensitivity coefficient is adjusted to 1.4, so:

[0139]

[0140] The results obtained by selecting the above two principles The larger one of the two is used as the location of the secondary reference point of the protected line.

[0141] If the secondary protection range of the protected line extends to the secondary protection range of the next adjacent line, a delay is set to avoid protection action conflict. The specific delay setting method is described in detail below.

[0142] In this embodiment, select

[0143] ③The third reference point R ⅡⅠ 1 position selection:

[0144] The three-level protection range of the protected line is constructed according to the position of the third reference point. The three-level protection serves as the near backup protection of the primary protection and the secondary protection of the protected line, as well as the far backup protection of the next adjacent line. It can not only effectively protect the entire length of the line, but also play a protective role for the entire length of the next section of the line.

[0145] The location of the third reference point is selected mainly based on the following three principles:

[0146] Principle 1: Determine the protection range of the third-level protection of the current protected line in accordance with the principle of coordination with the second-level protection and third-level protection of the next adjacent line, that is, the location of the third reference point:

[0147]

[0148] in, The reliability coefficient of the protected line's third-level protection has a value range of [0.8, 0.9], and is 0.8 in this embodiment. AB is the length of the protected line AB, is the position of the second reference point of the next adjacent line, L BC is the length of the next adjacent line.

[0149] Principle 2: When serving as the primary protection or secondary protection near backup protection for the currently protected line, a sensitivity check shall be performed according to the short circuit fault at the end of the currently protected line to determine the position of the third reference point.

[0150] The sensitivity coefficient of the third-level protection is defined as: Therefore, according to the sensitivity check of the near backup protection, the protection range of the third-level protection of the current protected line is:

[0151]

[0152] in, is the sensitivity coefficient of the third-level protection of the protected line, and its value range is [1.5, 1.6]; in this embodiment, it is taken as 1.5.

[0153] (3-3) When used as the remote backup protection for the next adjacent line, a sensitivity check is performed according to the short circuit fault at the end of the next adjacent line to determine the position of the third reference point:

[0154] The sensitivity coefficient of the third-level protection is defined as: Therefore, according to the sensitivity check of remote backup protection, the protection range of the third-level protection of the protected line is:

[0155]

[0156] in, is the sensitivity coefficient of the third-level protection of the protected line, and its value range is [1.2, 1.3]. In this embodiment, it is taken as 1.2, L BC is the length of the next adjacent line.

[0157] The final result of selecting the above three principles is The largest one among them, namely: Serves as the location of the tertiary reference point of the protected line.

[0158] Obviously, the protection range determined by formula (11) is smaller than the protection range determined by formula (12). Although it is impossible to compare the sizes of (12) and (13), the selectivity of the action can be ensured by setting the three-level protection to a step-by-step action delay. Therefore, in this embodiment, the protection range of the three-level protection is set to the total length of the current protected line plus 1.2 times the total length of the next adjacent line, which can ensure that the next adjacent line is protected on the basis of protecting the full length of the current line.

[0159] After the three-level reference point position and the three-level protection range are determined, it is necessary to set the action delay for each level of protection action. Fig.10 , as follows:

[0160] ① The first level protection is quick-acting protection without delay, i.e.

[0161] The primary protection cannot protect the entire length of the line, so when a fault occurs at the end of the line, the primary protection cannot detect the fault.

[0162] ②The secondary protection and primary protection work together to protect the entire line.

[0163] In order to prevent the conflict of action with the primary protection of the next adjacent line, the secondary protection of the current protected line needs to add an action delay of Δt. In this embodiment, Δt=0.5S is set, that is: in, Delay the first-level protection action of the next adjacent line.

[0164] According to the above principle for selecting the second reference point, if the length of the lower-level line is very short, there may be a conflict between the secondary protection of the current protected line and the secondary protection of the next adjacent line. Therefore, the action delay of the secondary protection should be considered in the following two situations:

[0165] (2-1) If the secondary protection range of the protected line does not conflict with the secondary protection range of the next adjacent line, that is: 1.4L AB <L AB +0.9L BC When the secondary protection range of the current protected line does not exceed the secondary protection range of the next adjacent line, the secondary protection between each protection is set with a fixed delay Δt, that is, like Fig.10 As shown in (b) in .

[0166] (2-2) If there is a conflict between the secondary protection range of the protected line and the secondary protection range of the next adjacent line, that is: 1.4L AB >L AB +0.9L BC When the secondary protection range of the current protected line extends to the secondary protection range of the next adjacent line, the secondary protection action delay of the current protected line should be based on the secondary protection action delay of the next adjacent line plus an action delay Δt′. In this embodiment, Δt′=2s is selected. At this time, like Fig.10 As shown in (c) in the figure, the selectivity of the protection action can be guaranteed.

[0167] ③ The three-level protection serves as the primary protection of the currently protected line, the near backup protection of the secondary protection, and the far backup protection of the next adjacent line:

[0168] When the primary protection or secondary protection of the protected line fails, the third-level protection is used as the near backup protection of the protected line after a delay of Δt″. In this embodiment, Δt″=0.5s, that is:

[0169]

[0170] When used as remote backup protection for the next adjacent line, the action delay is set to 1.5S.

[0171] Through the mutual coordination of three-level protection and protection action delay, an ideal fault protection effect can be achieved.

[0172] In this embodiment, by calculating the cosine similarity between the voltage at the measurement point and the voltage at each reference point, the cosine similarity value corresponding to each reference point is used as the action criterion for protection at each level, thereby realizing multi-level segmented protection of the protected line of the AC microgrid and determining the fault location at the same time.

[0173] Cosine similarity is the cosine of the angle between two n-dimensional vectors in n-dimensional space. From a mathematical point of view, it is equal to the vector product between the two vectors divided by the product of the lengths of the two vectors, and thus reflects the similarity between the vectors. Its expression is as follows:

[0174]

[0175] Wherein, a represents the voltage waveform vector at the measurement point, and b represents the voltage waveform vector at the reference point.

[0176] The cosine similarity between two vectors in the same direction is 1; the cosine similarity between two vectors in opposite directions is -1; the cosine similarity between two vectors at a certain angle is between -1 and 1.

[0177] The similarity between two vectors can be determined by the calculation results of cosine similarity. When the cos(θ) value is -1, it can be considered that the overall change trends of the two waveforms are opposite; when the cos(θ) value is 1, it can be considered that the overall change trends of the two waveforms are the same; when cos( θ ) When the value is close to 0, it can be considered that the overall change trends of the two waveforms are quite different and have weak similarities.

[0178] Since the measured voltage and the reference voltage sometimes have a large amplitude difference, the conventional similarity calculation method may cause large errors. This embodiment uses cosine similarity to characterize the similarity of the changing trends of the two voltage waveforms. It is not affected by the difference in waveform amplitude and only reflects the similarity of the waveform changing trends.

[0179] If the cosine similarity method is used to determine the similarity between the measured voltage and the reference voltage, ideally, when the line is operating normally or an out-of-zone fault occurs, the waveform similarity should be close to 1; when an in-zone fault occurs, the waveform similarity should be close to -1.

[0180] In this embodiment, a protection threshold T is set, and the cosine similarity between the voltage at the measurement point and the voltage at each reference point is calculated respectively to construct a three-level protection action criterion, which is as follows:

[0181] If cos(θ1)<T, the first-level protection is activated;

[0182] If cos(θ1)>T, and cos(θ2)<T, the secondary protection will be activated;

[0183] If cos(θ1)>T, and cos(θ2)>T, and cos(θ3)<T, the third level protection is activated;

[0184] Among them, cos(θ1), cos(θ2), and cos(θ3) are the cosine similarities between the measurement point voltage and the primary reference point voltage, the secondary reference point voltage, and the tertiary reference point voltage, respectively; T is the set threshold value. By comprehensively considering factors such as phase difference, this embodiment sets the protection threshold value T to 0.6.

[0185] The process of determining the fault location is as follows:

[0186] Based on the cosine similarity value between the voltage waveform at the measuring point and the voltage waveform at the primary reference point, determine whether the primary protection is activated; if the primary protection is activated, the fault is located within the length range of the primary reference point of the protected line; if the primary protection is not activated, continue to determine whether the secondary protection is activated based on the cosine similarity value between the voltage waveform at the measuring point and the voltage waveform at the secondary reference point;

[0187] If the second-level protection is activated, the fault is located between the first-level reference point and the second-level reference point of the protected line, or the fault is located within the length range of the first-level reference point but the first-level protection refuses to operate; if the second-level protection does not operate, continue to determine whether the third-level protection is activated based on the value of the cosine similarity between the voltage waveform at the measuring point and the voltage waveform at the third-level reference point; if the third-level protection is activated, the fault is located between the second-level reference point and the third-level reference point of the protected line, or the fault is located within the length range of the first-level reference point or the second-level reference point but the first-level protection and the second-level protection refuse to operate; if the third-level protection does not operate, it is an out-of-zone fault or there is no fault.

[0188] The method of this embodiment does not require two-end communication, does not require the coordination of fixed values, and does not require adaptive correction of fixed values. It only requires the three levels of protection to cooperate with each other in combination with delays. The protection method is simple and reliable.

[0189] Combination Fig. 9 , taking AB line as the protected line as an example for analysis, K1 and K2 are internal faults, K3 and K4 are external faults, and K1, K2, K3, and K4 are located within 0.9 times, within 0.9 to 1 times, within 1 to 1.4 times, and outside 1.4 times of AB line respectively.

[0190] When a short circuit occurs at K1, the primary protection of the protected line will instantly cut off the fault. If the primary protection fails to operate, the secondary and tertiary protections can serve as backup for the primary protection.

[0191] When a short circuit fault occurs at K2 at the end of the AB line, it is not within the primary protection range of the protected line, and the primary protection cannot detect the fault, but the secondary protection can detect it. After a delay of 0.5S, the secondary protection clears the fault; at this time, if the secondary protection refuses to operate, the third-level protection will still start to clear the fault after a delay, serving as a near backup protection for the secondary protection of the protected line.

[0192] When a short circuit occurs at K3 at the head end of the lower line BC, the first-level protection of the next adjacent line detects the fault and starts the first-level protection to clear the fault. In the most extreme case, if the circuit breakers of the next adjacent line fail to operate and the third-level protection of the next adjacent line cannot operate, the third-level protection of the protected line will operate reliably after a sufficiently long delay to clear the fault, thus achieving the role of remote backup protection.

[0193] A specific implementation case is given below to verify the implementation effect of the solution of this embodiment.

[0194] Using PSCAD / EMTC to build a 10kV medium voltage AC microgrid simulation model Fig.12 As shown. It contains 4 bus nodes, 4 lines, 4 inverter-type distributed power sources and 4 loads. The overall control method adopts master-slave control. DG4 is the master control unit, and DG1, DG2, and DG3 are slave control units.

[0195] The system is connected to the grid through a 110 / 10kV transformer. When the system is connected to the grid, the control strategy of DG1 to DG3 is grid-following PQ control with low voltage ride-through, and the control strategy of DG4 is grid-forming VSG control with low voltage ride-through, in which the control strategy of low voltage ride-through adds reactive power compensation. When the system is islanded, the control strategy of DG1 and DG2 is grid-following PQ control with low voltage ride-through, and the control strategy of DG3 is grid-forming VSG control with low voltage ride-through, in which the control strategy of low voltage ride-through also adds reactive power compensation. DG4 is VF control, providing voltage and frequency support points for the system.

[0196] The active power settings of the four distributed power sources are set to 0.4MW, 5MW, 0.3MW, and 0.15MW respectively, and the reactive power is set to 0. The distributed power sources are all equipped with current limiting modules, and the maximum short-circuit current is limited to 1.2 times the rated current in case of faults. The load is a resistive and inductive load, and the load sizes are S Load1 =0.76+j0.25MVA, S Load2 =0.54+j0.26MVA, S Load3 =0.233+j0.092MVA,

[0197] S Load1=0.425+j0.263MVA. The line is an RL model, the line unit impedance is Z=(0.45+j0.35814)Ω / km, the line length is L1=4km, L2=1km, L3=4km, L4=4km, L5=2.5km, L pcc =2km.

[0198] ①Case 1: Faults in the middle area of ​​the line and outside the area

[0199] The length of this level line AB is 4km, and the length of the upper level line EA is 4km.

[0200] A metallic three-phase short-circuit fault is set at 2 km in the middle of line AB. The fault occurs at 0.5S, lasts for 0.2S, and has a sampling frequency of 10kHz. The accuracy of the protection method proposed by this technology is verified in different fault location scenarios in the two operating modes of the microgrid, namely, on-grid and off-grid.

[0201] Still combined Figure 6 The fault occurred 2 km in the middle of the AB line. It was an out-of-area fault for the EA line and an in-area fault for the AB line.

[0202] During grid-connected operation, the grid-connected operation results of the out-of-zone fault and the in-zone fault are shown in Table 1 and Table 2, respectively, and the measured voltage reference voltage waveforms of the out-of-zone fault and the in-zone fault are shown in Figure 13(a) and Figure 13(b), respectively.

[0203] Table 1 Grid-connected operation results for out-of-region faults

[0204]

[0205] Table 2 Results of grid-connected operation for faults within the area

[0206]

[0207] When operating in an isolated island, the grid-connected operation results of the out-of-zone fault and the in-zone fault are shown in Table 3 and Table 4, respectively. The measured voltage reference voltage waveforms of the out-of-zone fault and the in-zone fault are shown in Figure 14(a) and Figure 14(b), respectively.

[0208] Table 3 Out-of-zone fault island operation results

[0209]

[0210]

[0211] Table 4 Results of fault island operation within the area

[0212]

[0213] It can be seen that the similarity between the measured voltage and the calculated voltage at the first fault identification point of the fault line AB is less than 0.6, and the fault in the area is identified. In both the grid-connected and islanded operation modes, the three-level comparison protection of the fault line AB and the non-fault adjacent line EA can give accurate judgment results.

[0214] Case 2: There is a fault in the middle of line AB, and there is a transition resistance of 10 ohms.

[0215] During grid-connected operation, the grid-connected operation results of the out-of-zone fault and the in-zone fault are shown in Table 5 and Table 6, respectively. The measured voltage reference voltage waveforms of the out-of-zone fault and the in-zone fault are shown in Figure 15(a) and Figure 15(b), respectively.

[0216] Table 5 Grid-connected operation results with transition resistance for out-of-zone faults

[0217]

[0218] Table 6 Grid-connected operation results with transition resistance in fault area

[0219]

[0220] When operating in an isolated island, the grid-connected operation results of the out-of-zone fault and the in-zone fault are shown in Table 7 and Table 8, respectively. The measured voltage reference voltage waveforms of the out-of-zone fault and the in-zone fault are shown in Figure 16(a) and Figure 16(b), respectively.

[0221] Table 7 Off-grid operation results with transition resistance for out-of-zone faults

[0222]

[0223]

[0224] Table 8 Off-grid operation results with transition resistance in fault area

[0225]

[0226] It can be seen that the similarity calculation values ​​at the first fault identification point of line AB are 0.3741 and 0.4612 when connected to the grid and off the grid, respectively. Although there is no strong negative correlation, the above analysis shows that the protection threshold has a certain margin and is still lower than 0.6. The first-level protection of protection 7 can correctly judge the fault in the area. Therefore, it can be considered that the protection method is not affected by the smaller transition resistance when a phase-to-phase short circuit occurs.

[0227] The protection method of this embodiment can be applied to both grid-connected and off-grid operation modes. There is no need to switch the protection method when the microgrid is switched between grid-connected and off-grid modes, and the method has good adaptability. The method is not affected by the characteristics of the distributed power source inside the microgrid, and has good adaptability to different control strategies. The method does not require complex fixed value setting calculations or fixed value switching operations, and the method has high reliability and flexibility.

[0228] Embodiment 2

[0229] In one or more embodiments, an AC microgrid protection system based on reference point voltage waveform comparison is disclosed, specifically comprising:

[0230] A reference point selection module is used to set a primary reference point, a secondary reference point and a tertiary reference point at at least A1, A2 and A3 positions of the protected line, respectively; the position of each reference point can respectively form the main protection and backup protection of the protected line, and the main protection and backup protection of the next adjacent line;

[0231] The reference point voltage calculation module is used to establish a two-port network model of the protected line and the port electrical quantity, taking one of the ports as the measurement point and calculating the voltage of each level of reference point at the other port based on the voltage at the measurement point;

[0232] The line protection module is used to calculate the cosine similarity between the voltage waveform of the measurement point and the voltage waveform of each level of reference point, and use the cosine similarity value corresponding to each level of reference point as the action criterion for protection at each level, so as to realize multi-level segmented protection of the protected line of the AC microgrid and determine the fault location at the same time.

[0233] The specific implementation methods of the above modules are exactly the same as those in Example 1 and will not be described in detail.

[0234] Embodiment 3

[0235] In one or more embodiments, a terminal device is disclosed, which includes a processor and a memory, the processor is used to implement instructions; the memory is used to store multiple instructions, and the instructions are suitable for being loaded by the processor and executing the AC microgrid protection method based on reference point voltage waveform comparison in Example 1.

[0236] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. An AC microgrid protection method based on reference point voltage waveform comparison, characterized in that: include: At least A1, A2 and A3 positions of the protected line, a primary reference point, a secondary reference point and a tertiary reference point are respectively set; The location of each reference point can respectively form the main protection, near backup protection and far backup protection range for the protected line, as well as the near backup protection range for the next adjacent line; A two-port network model of the protected line and port electrical quantities is established, one of the ports is used as the measurement point, and based on the voltage at the measurement point, the voltage of each level reference point is calculated at the other port; The cosine similarity between the voltage waveform at the measurement point and the voltage waveform at each level of reference point is calculated, and the cosine similarity value corresponding to each level of reference point is used as the action criterion for protection at each level, so as to realize multi-level segmented protection of the protected line of the AC microgrid and determine the fault location at the same time.

2. The AC microgrid protection method based on reference point voltage waveform comparison according to claim 1, characterized in that: The location of the first-level reference point is specifically selected as follows: in, is the line length corresponding to the primary reference point of the protected line, is the reliability coefficient of the primary protection of the protected line, with a value range of [0.9, 0.95], L AB is the length of the protected line.

3. The AC microgrid protection method based on reference point voltage waveform comparison according to claim 1, characterized in that: The location of the secondary reference point is specifically selected as follows: Principle 1: Determine the location of the second reference point of the protected line in accordance with the principle of coordination with the primary protection of the next adjacent line: in, is the line length corresponding to the secondary reference point of the protected line, is the reliability coefficient of the secondary protection of the protected line, and its value range is [0.8, 0.85]; L AB is the length of the protected line, The line length corresponding to the primary reference point of the next adjacent line; Principle 2: Determine the second reference point of the protected line by performing sensitivity calibration when a short circuit occurs at the end of the protected line: in, is the sensitivity coefficient of the secondary protection of the protected line, and its value range is [1.25, 1.4]; The results obtained by selecting the above two principles The larger one of the two is used as the position of the secondary reference point of the protected line; If the secondary protection range of the protected line extends into the secondary protection range of the next adjacent line, a delay is set to avoid conflicting protection actions.

4. The AC microgrid protection method based on reference point voltage waveform comparison according to claim 1, characterized in that: The location of the three-level reference point is specifically selected as follows: Principle 1: Determine the location of the third reference point of the protected line in accordance with the principle of coordination with the secondary protection and tertiary protection of the next adjacent line: in, is the line length corresponding to the third-level reference point of the protected line, is the sensitivity coefficient of the third-level protection of the protected line, and its value range is [0.8, 0.9], L AB is the length of the protected line, The line length corresponding to the secondary reference point of the next adjacent line; Principle 2: When used as the primary protection or secondary protection of the protected line, the position of the third reference point of the protected line shall be determined by performing sensitivity calibration when a short circuit occurs at the end of the protected line: in, is the sensitivity coefficient of the third-level protection of the protected line, and its value range is [1.5, 1.6]; Principle 3: When acting as remote backup protection for the next adjacent line, the location of the third reference point of the protected line shall be determined by performing sensitivity verification when a short circuit fault occurs at the end of the next adjacent line: in, L is the sensitivity coefficient of the third-level protection of the protected line, and its value range is [1.2, 1.3]; BC is the length of the next adjacent line; The results obtained by selecting the above three principles The largest one among them is taken as the position of the tertiary reference point of the protected line.

5. The AC microgrid protection method based on reference point voltage waveform comparison according to claim 4, characterized in that: The position of the first reference point corresponds to the first-level protection range, the position of the second reference point corresponds to the second-level protection range, and the position of the third reference point corresponds to the third-level protection range; Set delays for each level of protection action, specifically: The first level protection is direct action without delay; The delay of the secondary protection action is: If the secondary protection range of the protected line does not conflict with the secondary protection range of the next adjacent line, then If the secondary protection range of the protected line extends to the secondary protection range of the next adjacent line, in, is the delay of the secondary protection action of the protected line, is the first-level protection action delay of the next adjacent line, Δt and Δt′ are the set delay times respectively; The delay of the third level protection action is: in, is the delay of the secondary protection action of the protected line, and Δt″ is the set delay time.

6. The AC microgrid protection method based on reference point voltage waveform comparison according to claim 1, characterized in that: Calculate the cosine similarity between the voltage at the measurement point and the voltage at each reference point, specifically: Wherein, a represents the voltage waveform vector at the measurement point, and b represents the voltage waveform vector at the reference point.

7. The AC microgrid protection method based on reference point voltage waveform comparison according to claim 1, characterized in that: The cosine similarity value corresponding to each level of reference point is used as the criterion for protection actions at each level, specifically: If cos(θ1)<T, the first-level protection is activated; If cos(θ1)>T, and cos(θ2)<T, the secondary protection will be activated; If cos(θ1)>T, and cos(θ2)>T, and cos(θ3)<T, the third level protection is activated; Among them, cos(θ1), cos(θ2), and cos(θ3) are the cosine similarities between the voltage at the measurement point and the voltage at the first-level reference point, the voltage at the second-level reference point, and the voltage at the third-level reference point, respectively; and T is the set threshold.

8. The AC microgrid protection method based on reference point voltage waveform comparison according to claim 1, characterized in that: The process of determining the fault location is as follows: Based on the cosine similarity value between the voltage waveform at the measuring point and the voltage waveform at the primary reference point, determine whether the primary protection is activated; if the primary protection is activated, the fault is located within the length range of the primary reference point of the protected line; if the primary protection is not activated, continue to determine whether the secondary protection is activated based on the cosine similarity value between the voltage waveform at the measuring point and the voltage waveform at the secondary reference point; If the secondary protection is activated, the fault is located between the primary reference point and the secondary reference point of the protected line, or the fault is within the length range of the primary reference point but the primary protection refuses to operate; If the secondary protection does not operate, continue to determine whether the third-level protection is operated based on the cosine similarity value between the voltage waveform at the measuring point and the voltage waveform at the third-level reference point; if the third-level protection operates, the fault is located between the secondary reference point and the third-level reference point of the protected line, or the fault is within the length range of the first reference point or the second reference point but the first and second protections refuse to operate; if the third-level protection does not operate, it is an out-of-zone fault or there is no fault.

9. An AC microgrid protection system based on reference point voltage waveform comparison, characterized in that: include: A reference point selection module is used to respectively set a primary reference point, a secondary reference point and a tertiary reference point at at least A1, A2 and A3 positions of the protected line; The location of each reference point can respectively form the main protection and backup protection for the protected line, as well as the main protection and backup protection for the next adjacent line; The reference point voltage calculation module is used to establish a two-port network model of the protected line and the port electrical quantity, taking one of the ports as the measurement point and calculating the voltage of each level of reference point at the other port based on the voltage at the measurement point; The line protection module is used to calculate the cosine similarity between the voltage waveform of the measurement point and the voltage waveform of each level of reference point, and use the cosine similarity value corresponding to each level of reference point as the action criterion for protection at each level, so as to realize multi-level segmented protection of the protected line of the AC microgrid and determine the fault location at the same time.

10. A terminal device, comprising a processor and a memory, wherein the processor is used to implement instructions; and the memory is used to store multiple instructions, characterized in that: The instructions are suitable for being loaded by a processor and executing the AC microgrid protection method based on reference point voltage waveform comparison as described in any one of claims 1-8.

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