AC microgrid protection method and system based on reference point voltage waveform comparison

Abstract

The application discloses an alternating current micro-grid protection method and system based on reference point voltage waveform comparison, comprising the following steps: setting a first reference point, a second reference point and a third reference point at at least A1, A2 and A3 positions of a protected line respectively; establishing a two-port network model of the protected line and port electrical quantity, taking one port as a measurement point, calculating the voltage of each reference point at the other port based on the voltage of the measurement point; calculating the cosine similarity of the voltage waveform of the measurement point and the voltage waveform of each reference point, taking the value of the cosine similarity corresponding to each reference point as the action criterion of each level of protection, and realizing the multi-level sectional protection of the alternating current micro-grid protected line and determining the fault position simultaneously. The application can expand the protection range, is not affected by the branch load or the branch of the distributed power supply, is not affected by the control strategy of the distributed power supply, and can be applied to both grid-connected and island operation modes simultaneously.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of alternating current micro-grid protection, and particularly relates to an alternating current micro-grid protection method and system based on reference point voltage waveform comparison. BACKGROUND

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

[0003] At present, the distributed generation system mainly based on renewable energy such as wind energy and photovoltaic is developing rapidly, but the high proportion of distributed new energy into the power grid brings serious consumption problems. In order to improve the utilization rate and consumption level of new energy, new energy and local load form a micro-grid to realize local power balance, and under this premise, the micro-grid is connected to the upper distribution network. The internal distributed power supply of the micro-grid is mainly clean energy, and the internal electric energy is basically self-balanced.

[0004] There are two typical operation modes of the micro-grid: grid-connected operation mode and off-grid operation mode. Due to the difference between the mechanism characteristics and operation characteristics of the micro-grid and the traditional power grid, the fault characteristics of the micro-grid also have some particularities, such as: when a fault occurs in the micro-grid, the distribution network can provide a larger short-circuit current in the grid-connected operation mode, while the short-circuit current is only provided by the distributed power supply in the island operation mode; therefore, the fault characteristics are obviously different when a fault occurs at the same location in different operation modes. The working characteristics of the distributed power supply in the micro-grid are affected by the control strategy, and due to the diversity of the control strategy of the distributed power supply, the fault characteristics of the micro-grid are obviously affected by the control strategy.

[0005] Therefore, the micro-grid protection strategy should consider both grid-connected and island operation modes, and be able to adapt to different types of distributed power supply; in the prior art, the fault detection and protection of the alternating current micro-grid are mostly derived from the traditional distribution network protection method, which needs to switch the setting value or replace the protection strategy, and is affected by different types of distributed power supply. The prior art discloses a method for protecting the distribution network by setting a reference point and using longitudinal comparison protection, but the results of the first end and the end of the protected line are compared to determine the protection action through double-end communication during fault protection; this method is subject to the reliability of communication, and once the communication fails, reliable protection cannot be performed. SUMMARY

[0006] In order to solve the above problems, the present application provides an alternating current micro-grid protection method and system based on reference point voltage waveform comparison, three reference points are selected at the set positions of the protected line, and by analyzing the change trend of the voltage waveform of the measurement point and the voltage waveform of the reference point, the specific fault position can be determined, the detection result is not affected by the operation mode of the micro-grid, and is also not affected by the control strategy of the distributed power supply.

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

[0008] An AC micro-grid protection method based on reference point voltage waveform comparison, comprising:

[0009] At least A1, A2 and A3 positions of the protected line, a first reference point, a second reference point and a third reference point are respectively arranged; the positions of the reference points can respectively form a main protection range, a near backup protection range and a far backup protection range of the protected line, and a near backup protection range of the next adjacent line;

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

[0011] The cosine similarity of the voltage waveform of the measurement point and the voltage waveform of each reference point is calculated, and the value of the cosine similarity corresponding to each reference point is taken as the action criterion of each level of protection, so as to realize multi-level segmented protection of the AC micro-grid protected line and determine the fault position.

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

[0013]

[0014] wherein, L1 is the line length corresponding to the first reference point of the protected line, L is the length of the protected line, AB L is the length of the protected line.

[0015] Optionally, the position of the second reference point is specifically selected as:

[0016] Principle one: the position of the second reference point of the protected line is determined according to the principle of cooperating with the first protection of the next adjacent line:

[0017]

[0018] wherein, L2 is the line length corresponding to the second reference point of the protected line, L is the length of the protected line, AB L is the length of the protected line, L is the line length corresponding to the first reference point of the next adjacent line;

[0019] Principle two: the second reference point of the protected line is determined according to sensitivity check when a short-circuit fault occurs at the end of the protected line:

[0020]

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

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

[0023] 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.

[0024] Optionally, the position of the tertiary reference point is specifically:

[0025] Principle one: the third reference point of the protected line is determined according to the principle of cooperating with the secondary protection and the tertiary protection of the next adjacent line:

[0026]

[0027] wherein, is the line length corresponding to the tertiary reference point of the protected line, is the sensitivity coefficient of the tertiary protection of the protected line, and the value range is [0.8, 0.9], L AB is the length of the protected line, is the line length corresponding to the secondary reference point of the next adjacent line;

[0028] Principle two: when the protected line is the near backup protection of the primary protection and the secondary protection, the third reference point of the protected line is determined according to sensitivity check when a short-circuit fault occurs at the end of the protected line:

[0029]

[0030] wherein, is the sensitivity coefficient of the tertiary protection of the protected line, and the value range is [1.5, 1.6];

[0031] Principle three: when the protected line is the far backup protection of the next adjacent line, the third reference point of the protected line is determined according to sensitivity check when a short-circuit fault occurs at the end of the next adjacent line:

[0032]

[0033] wherein, ​The sensitivity coefficient of the three-level protection of the protected line is in the range of [1.2, 1.3]; L BC The length of the next adjacent line;

[0034] The maximum of the results obtained by selecting the above three principles is taken as the position of the three-level reference point of the protected line.

[0035] Optionally, 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;

[0036] The delay time is set for each level of protection, specifically:

[0037] The first-level protection is directly acted without delay;

[0038] The delay time of the second-level protection action is:

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

[0040] If the second-level protection range of the protected line extends to the second-level protection range of the next adjacent line, then

[0041] wherein, is the delay time of the second-level protection action of the protected line, is the first-level protection action delay time of the next adjacent line, and Δt and Δt' are the set delay times;

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

[0043]

[0044] wherein, is the delay time of the second-level protection action of the protected line, and Δt" is the set delay time.

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

[0046]

[0047] wherein, a represents a measurement point voltage waveform vector, and b represents a reference point voltage waveform vector.

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

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

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

[0051] If cos(θ1) > T and cos(θ2) > T and cos(θ3) < T, the tertiary protection acts;

[0052] Wherein, cos(θ1), cos(θ2) and cos(θ3) are cosine similarities between the measured point voltage and the primary reference point voltage, the secondary reference point voltage and the tertiary reference point voltage respectively; T is a set threshold value.

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

[0054] Based on the value of the cosine similarity between the measured point voltage waveform and the primary reference point voltage waveform, it is determined whether the primary protection acts; if the primary protection acts, the fault is located within the length range of the primary reference point of the protected line; if the primary protection does not act, it is continued to determine whether the secondary protection acts based on the value of the cosine similarity between the measured point voltage waveform and the secondary reference point voltage waveform;

[0055] If the secondary protection acts, the fault is located between the primary reference point and the secondary reference point of the protected line or the fault is located within the length range of the primary reference point but the primary protection refuses to act; if the secondary protection does not act, it is continued to determine whether the tertiary protection acts based on the value of the cosine similarity between the measured point voltage waveform and the tertiary reference point voltage waveform; if the tertiary protection acts, the fault is located between the secondary reference point and the tertiary reference point of the protected line or the fault is located within the length range of the primary reference point or the secondary reference point but the primary protection and the secondary protection refuse to act, if the tertiary protection does not act, it is an out-of-zone fault or no fault.

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

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

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

[0059] A reference point voltage calculation module is configured to establish a two-port network model of the protected line and port electrical quantities, take one port as a measurement point, calculate the voltage of each reference point at the other port based on the measurement point voltage;

[0060] The line protection module is used for calculating cosine similarity of a measurement point voltage waveform and each level reference point voltage waveform, taking a value of the cosine similarity corresponding to each level reference point as an action criterion of each level protection, and realizing multi-level sectional protection of the protected line of the AC micro-grid and determining the fault position.

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

[0062] A terminal device comprises a processor and a memory, the processor is used for implementing instructions, and the memory is used for storing a plurality of instructions, the instructions are suitable for being loaded and executed by the processor to implement the AC micro-grid protection method based on reference point voltage waveform comparison.

[0063] Compared with the prior art, the beneficial effects of the present application are:

[0064] (1) The present application comprehensively considers the selectivity and reliability of protection, considers the influence factors such as the access of distributed power supply and transition resistance, sets three levels of reference points for the protected line respectively, compares the change trend of the voltage waveforms of the measurement point and each level reference point, and determines the fault position and performs protection action; the method can expand the protection range, will not be affected by the branch load or the distributed power supply branch, will not be affected by the distributed power supply control strategy, and can be applied to both grid-connected and island operation modes.

[0065] (2) The present application comprehensively considers different principles to determine the positions of the three levels of reference points respectively, and adds different time delays in the three levels of protection, avoids the conflict between the protection actions of the protected line and the next adjacent line, and ensures the reliable execution of the protection action; wherein the first level protection is the main protection, the second level protection and the third level protection are backup protections, the three levels of protection cooperate with each other, can accurately give the fault judgment result, and realize reliable protection of the AC micro-grid fault.

[0066] (3) The fault protection method of the present application does not need data communication between the two ends, can completely get rid of the dependence on communication; without complex setting value calculation and setting value switching operation, has high reliability and flexibility.

[0067] Other features and advantages of the additional aspects of the present application will be partially given in the following description, partially become obvious from the following description, or be understood by the practice of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0068] Figure 1 It is a schematic diagram of a Π type line model in the embodiment of the present application;

[0069] Figure 2 It is a schematic diagram of a network model structure of the electrical quantity of the protected line and the port in the embodiment of the present application;

[0070] Figure 3 Fig. 4 is a voltage reference waveform diagram for measuring voltage in a forward external fault in an embodiment of the present application;

[0071] Figure 4 Fig. 5 is a voltage reference waveform diagram for measuring voltage in a reverse external fault in an embodiment of the present application;

[0072] Figure 5 Fig. 6 is a voltage reference waveform diagram for measuring voltage in an internal fault in an embodiment of the present application;

[0073] Figure 6 Fig. 7 is a schematic diagram of a protected line in an embodiment of the present application;

[0074] Figure 7 Fig. 8 is an analysis of load branch current influence in an embodiment of the present application;

[0075] Figure 8 Fig. 9 is an analysis of DG branch current influence in an embodiment of the present application;

[0076] Figure 9 Fig. 10 is a schematic diagram of a reference point in an embodiment of the present application;

[0077] Figure 10 Fig. 11 is a schematic diagram of three-stage protection range cooperation in an embodiment of the present application;

[0078] Figure 11 Fig. 12 is a flow chart of an AC microgrid protection method based on reference point voltage waveform comparison in an embodiment of the present application;

[0079] Figure 12 Fig. 13(a) and Fig. 13(b) are voltage reference waveform diagrams for measuring voltage in external faults and internal faults respectively in a grid-connected operation in Case 1;

[0080] Fig. 14(a) and Fig. 14(b) are voltage reference waveform diagrams for measuring voltage in external faults and internal faults respectively in an island operation in Case 1;

[0081] Fig. 15(a) and Fig. 15(b) are voltage reference waveform diagrams for measuring voltage in external faults and internal faults respectively in a grid-connected operation in Case 2 with transition resistance;

[0082] Fig. 16(a) and Fig. 16(b) are voltage reference waveform diagrams for measuring voltage in external faults and internal faults respectively in an island operation in Case 2 with transition resistance.

[0083] Fig. 16(a) and Fig. 16(b) are voltage reference waveform diagrams for measuring voltage in external faults and internal faults respectively in an island operation in Case 2 with transition resistance. DETAILED DESCRIPTION

[0084] It should be noted that the following detailed description is exemplary in nature and is intended to provide further description of the application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0085] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments in accordance with the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and / or "comprising," when used in this specification, specify the presence of stated features, steps, operations, devices, components and / or combinations thereof, but do not preclude the presence or addition of one or more other features, steps, operations, devices, components and / or combinations thereof.

[0086] Embodiment One

[0087] In one or more embodiments, an AC microgrid protection method based on reference point voltage waveform comparison is disclosed, which combines Figure 11 , and specifically includes the following processes:

[0088] (1) At least A1, A2 and A3 positions of the protected line, a first level reference point, a second level reference point and a third level reference point are respectively set; the positions of the reference points can form primary protection and backup protection for the protected line, and primary protection and backup protection for the next adjacent line, respectively;

[0089] (2) A two-port network model of the protected line and the port electrical quantity is established, one port is taken as a measurement point, and based on the measurement point voltage, the voltage of each level reference point is calculated at the other port;

[0090] (3) The cosine similarity of the measurement point voltage and the voltage of each level reference point is calculated, and the value of the cosine similarity corresponding to each level reference point is taken as the action criterion of each level protection, to realize multi-level segmented protection of the AC microgrid protected line and determine the fault location.

[0091] As a specific embodiment, the present embodiment takes the Π line model shown in Figure 1 as an example for analysis, and constructs a 10kV microgrid model as shown in Figure 12 , taking line AB as the protected line.

[0092] In microgrid, line port electrical quantity is constrained by distributed power and line equivalent model, and the internal power of microgrid generates and maintains electrical quantity. Although the fault output characteristics of different types of power sources are quite different, the electrical quantities of the same line model have the same mathematical relationship, which determines the distribution rule of power output characteristics. The mathematical relationship between them is independent of power characteristics, and only depends on the line model. Therefore, in the microgrid, once the line model of the protected object is determined, the port voltage and port current have a corresponding relationship.

[0093] In combination Figure 2 , a network model of the protected line and the port electrical quantity is established, according to the two-port theory, 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 denoted as u1 and i1, which can be obtained by sampling through the transformer; the voltage and current of the reference point are respectively denoted as u2 and i2, which can be calculated based on the voltage of the measurement point.

[0094] According to the circuit theorem, when the line fails at different positions, the mathematical equation expressions of the electrical quantities of the measurement point and the reference point of the two-port network model are as follows:

[0095] When the line is in normal operation or has an external fault, that is, there is no fault or the fault is located at the external fault position, the protected object is complete, the instantaneous sampling values of the measurement voltage u1 and the measurement current i1 of the line AB first end are known, and the reference voltage u2 of the reference point is specifically as follows according to the mathematical differential equation:

[0096]

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

[0098] According to the analysis as follows:

[0099] ① When the system is in normal operation, the line model is complete, and the measurement voltage and the reference voltage waveform have no difference in the change trend, and the waveforms are similar.

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

[0101]

[0102] Where, R BK3 and L BK3respectively. The line model is not damaged, and the protected object is intact, so the above corresponding mathematical relationship still holds, and the expression of the reference voltage is:

[0103]

[0104] wherein R BR3 and L BR3 represent the line impedance values between the measurement point and the reference point, and R BK3 is greater than R BR3 . Comparing formula (2) and formula (3), the reference point voltage u2 will have a voltage drop compared to the measurement voltage u1, because the amplitude of u2 is less than the amplitude of u1, but u1>0, u2>0, and the change trends of the two voltage waveforms are the same; the specific performance is shown in Figure 3 .

[0105] ③When a reverse external fault occurs, the fault point is located at K1 (line EA), the assumed measurement current i1 direction and the actually measured measurement current are opposite, at this time the expression of the measurement voltage becomes:

[0106]

[0107] wherein R BK1 and L BK1 represent the line resistance and inductance between the measurement point and the fault point.

[0108] The expression of the reference voltage is:

[0109]

[0110] R BR1 and L BR1 represent the line resistance and inductance between the measurement point and the reference point, 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 measurement voltage, and u1<0, u2<0, and the change trends of their waveforms are still the same; the specific performance is shown in Figure 4 .

[0111] ④When an internal fault occurs, the fault point is located at K2 (line AB), at this time the expression of the measurement voltage is still:

[0112]

[0113] but the expression of the reference voltage becomes:

[0114]

[0115] R BR1 and L BR1 R represents the line resistance and inductance between the measurement point and the reference point, respectively, and R BK2 Less than R BR1 Therefore (R) BK2 -R BR1 If L is less than zero, BK2 Less than L BR1 Therefore (L) BK2 -L BR1 The value is less than zero. At this time, u1>0, but u2<0, and the measured voltage waveform changes in the opposite direction to the reference voltage waveform, as shown below. Figure 5 As shown.

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

[0117] When the line is operating normally or a fault occurs outside the fault zone, the waveforms of the reference voltage and the measured voltage show the same trend, and are considered similar. When a fault occurs within the fault zone, the waveforms of the reference voltage and the measured voltage show opposite trends, and are considered inverse. Therefore, by comparing the similarity of the two waveforms, the location of the fault can be accurately determined.

[0118] The process of selecting reference points will be explained in detail below.

[0119] Each reference point corresponds to a protection range. In this embodiment, three reference points at different locations are selected, and a three-level protection system is constructed according to the main protection, near backup protection, and far backup protection, which works together to achieve a complete protection effect. Of course, multi-level protection can also be constructed if needed.

[0120] This embodiment Figure 6 The circuit model is taken from Figure 12 One branch of a 10kV microgrid model, combined with Figure 6 Taking the protection at CB7 at the beginning of line AB as the research object, the first reference point R is selected respectively. 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 represents the range of the main protection, near backup protection, and far backup protection corresponding to the protection at CB1, respectively.

[0121] ① First reference point R Ⅰ Position selection of 1:

[0122] The first level protection is constructed according to the position of the first reference point as the main protection of the protected line AB. In order to meet the speed of the protection, the first level protection is required to have the characteristics of simple and reliable, rapid action, and protect the full length of the line as far as possible. The corresponding relationship between the protection range of the first level protection and the full length of the line is: Wherein is the length of the line corresponding to the reference point of the first level protection, is the reliability coefficient of the first level protection, L AB is the length of the line AB; since the protection method proposed in the embodiment does not depend on the size of the specific value of the electrical quantity, the reliability coefficient can be large, [0.9, 0.95], which can expand the protection range of the first level protection.

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

[0124] ② The position of the second reference point R Ⅱ 1 is selected:

[0125] The second level protection is constructed according to the position of the second reference point as the backup protection, which is required to protect the full length of the protected line AB, and cannot exceed the protection range of the first level protection of the next adjacent line BC.

[0126] The position of the second reference point is mainly selected according to the following two principles:

[0127] Principle one: the protection range of the second level protection of the protected line AB (i.e. the position of the second reference point) is determined according to the principle of cooperating with the first level protection of the next adjacent line.

[0128] First, the influence of the branch load and the access of the distributed power supply on the proposed protection method is analyzed:

[0129] When the branch load exists, the current flowing through the branch load is denoted as I load , and the fault point current is denoted as I fault , as shown in Figure 7 , the line AB is taken as an example for analysis, the intra-zone fault K1 occurs, I load and I fault are in the same direction, and the amplitude of I load is much smaller than I fault ; when the out-of-zone fault K2 occurs, the current flowing through the load branch is also negligible compared with the fault point current. When the branch load exists, it only has a very small influence on the current size at the reference point, so it only affects the amplitude of the reference point voltage, and does not affect the change trend of the calculated voltage.

[0130] When the DG (distributed power supply) branch exists, the current provided by the distributed power supply is denoted as I DG, the fault point current is recorded as I fault In combination Figure 8 , the influence of the distributed power supply on the protection is determined by whether the distributed power supply absorbs power or outputs power. In most cases, the DG acts as a power supply to deliver power to the microgrid. At this time, if an intra-zone fault K1 occurs, I DG and I fault are in opposite directions, but due to the current limiting effect of the internal converter of the distributed power supply, the output current of the DG will not continue to increase unlimitedly, and the maximum value it can reach is usually not 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 supply injects current into the protected line, it will not offset the fault current. When the fault is located outside the zone K2, it is equivalent to the large power supply and the distributed power supply together providing current to the fault point, which is more conducive to the detection of the fault by the method. When the distributed power supply absorbs power, it is equivalent to a load branch, as analyzed above, which will not be repeated here.

[0131] Through the above analysis, it can be seen that the existence of the branch load or the distributed power supply only has the possibility of affecting the size 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 calculation of the voltage direction. The protection method proposed in this embodiment is to measure the voltage and the reference point voltage waveform trend as the standard for judging the fault, so the existence of the branch load and the distributed power supply branch has no adverse effect on the protection method. Unlike traditional distance protection, which needs to consider the influence of branch load to set the branch coefficient, the present technology does not need to consider the branch coefficient when selecting the reference point.

[0132] Therefore, the branch coefficient also does not need to be considered when determining the position of the second reference point, so we have:

[0133]

[0134] wherein, 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, which can be in the range of [0.8, 0.9], and in this embodiment, it is taken as 0.8.

[0135] Principle two: the position of the second reference point is determined according to the sensitivity check when a short-circuit fault 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 preliminarily set as

[0138] Considering other adverse factors such as transition resistance, the sensitivity coefficient needs to be improved, and in the embodiment, the sensitivity coefficient is adjusted to 1.4, so that:

[0139]

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

[0141] 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 protection action conflict, and the specific delay setting method is described in detail below.

[0142] In the embodiment, the larger one of the results obtained by selecting the above two principles is selected as the position of the secondary reference point of the protected line.

[0143] The position of the third reference point R ⅡⅠ 1 is selected as follows:

[0144] According to the position of the third reference point, the tertiary protection range of the protected line is constructed, which is the near backup protection of the primary protection and the secondary protection of the protected line, and the far backup protection of the next adjacent line, which not only effectively protects the full length of the line, but also protects the full length of the next line.

[0145] The position of the third reference point is mainly determined according to the following three principles:

[0146] Principle one: the protection range of the tertiary protection of the current protected line, i.e. the position of the third reference point, is determined according to the principle of cooperation with the secondary protection and the tertiary protection of the next adjacent line:

[0147]

[0148] Among them, The reliability coefficient of the tertiary protection of the protected line is in the range of [0.8, 0.9], and in the embodiment, it is 0.8. AB L is the length of the protected line AB, L is the position of the second reference point of the next adjacent line, BC L is the length of the next adjacent line.

[0149] Principle two: when acting as the near backup protection of the primary protection and the secondary protection of the current protected line, the sensitivity check is performed when a short-circuit fault occurs at the end of the current protected line to determine the position of the third reference point.

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

[0151]

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

[0153] (3-3) When serving as the far back-up protection of the next adjacent line, the sensitivity check is performed when the short-circuit fault occurs at the end of the next adjacent line, and the position of the third reference point is determined:

[0154] The sensitivity coefficient of the three-level protection is defined as: Therefore, according to the sensitivity check of the far back-up protection, the protection range of the three-level protection of the protected line is:

[0155]

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

[0157] The maximum of the results obtained by finally selecting the above three principles is: as the position of the three-level 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 size of (12) and (13), the selectivity of the action can be ensured by setting the three-level protection as a stepped action delay, and therefore, in the embodiment, the protection range of the three-level protection is set as 1.2 times the full length of the current protected line plus the full length of the next adjacent line, which can ensure that the next adjacent line is also protected on the basis of protecting the full length of the current protected line.

[0159] After the position of the three-level reference point and the three-level protection range are determined, the action delay of each level of protection action needs to be set, in combination with Figure 10 , which is as follows:

[0160] ① The first-level protection is a non-delay instantaneous protection, that is, the delay is

[0161] The first-level protection cannot protect the full length of the line, and therefore, when the fault occurs at the end of the line, the first-level protection cannot detect the fault.

[0162] ② The second-level protection and the first-level protection cooperate with each other, and can realize the protection of the whole line.

[0163] ​In order to prevent the conflict in action with the first protection of the next adjacent line, the second protection of the current protected line needs to add an action delay of Δt, and the embodiment sets Δt = 0.5s, that is: Wherein, is the action delay of the first protection of the next adjacent line.

[0164] According to the selection principle of the second reference point, if the length of the lower line is very short, the action conflict between the second protection of the current protected line and the second protection of the next adjacent line may occur, so the action delay of the second protection needs to be considered in the following two cases:

[0165] (2-1) If the second protection range of the protected line and the second protection range of the next adjacent line have no conflict, that is, 1.4L AB <L AB +0.9L BC , the second protection range of the current protected line does not exceed the second protection range of the next adjacent line, and the fixed delay Δt is set for the second protection between the protections, that is as shown in (b) of Figure 10 .

[0166] (2-2) If there is a conflict between the second protection range of the protected line and the second protection range of the next adjacent line, that is, 1.4L AB >L AB +0.9L BC , the second protection range of the current protected line extends to the second protection range of the next adjacent line, then the action delay of the second protection of the current protected line needs to be added to the action delay of the second protection of the next adjacent line, and the embodiment selects Δt' = 2s, at this time, as shown in (c) of Figure 10 , that is, the selectivity of the protection action can be ensured.

[0167] ③ The third protection is the first protection and the second protection of the current protected line, the near backup protection and the far backup protection of the next adjacent line:

[0168] When the first protection or the second protection of the protected line fails, the third protection delays Δt" as the near backup protection of the protected line, and in the embodiment, Δt" = 0.5s, that is:

[0169]

[0170] When it is the far backup protection of the next adjacent line, the action delay is set to 1.5s.

[0171] Through the mutual cooperation of the third protection and the protection action delay, the ideal fault protection effect can be achieved.

[0172] In this embodiment, the cosine similarity of the measurement point voltage and the reference point voltage of each level is calculated, the value of the cosine similarity corresponding to each reference point is taken as the action criterion of each level of protection, and the multi-level sectional protection of the protected line of the AC microgrid is realized while the fault location is determined.

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

[0174]

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

[0176] Two vectors in the same direction have a cosine similarity of 1 between them. Two vectors in opposite directions have a cosine similarity of -1 between them. Two vectors with a certain angle have a cosine similarity between them ranging from -1 to 1.

[0177] The calculation result of the cosine similarity can be used to judge the similarity between the two vectors. When the value of cos(θ) is -1, it can be considered that the overall trend of the two waveforms is opposite. When the value of cos(θ) is 1, it can be considered that the overall trend of the two waveforms is the same. When the value of cos(θ) is close to 0, it can be considered that the overall trend of the two waveforms is greatly different and has weak similarity. cos( θ )

[0178] Because there is sometimes a large amplitude difference between the measurement voltage and the reference voltage, the conventional similarity calculation method may cause a large error. In this embodiment, the cosine similarity is used to represent the similarity degree of the change trend of the two voltage waveforms, which is not affected by the amplitude difference of the waveforms and only reflects the similarity of the change trend of the waveforms.

[0179] If the cosine similarity method is used to judge the similarity of the measurement voltage and the reference voltage. Ideally, when the line is normally operating 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, the cosine similarity of the measurement point voltage and the reference point voltage of each level is calculated, and a three-level protection action criterion is constructed, which is as follows:

[0181] If cos(θ1) < T, the first level protection acts.

[0182] ​If cos(θ1) > T and cos(θ2) < T, the secondary protection acts;

[0183] If cos(θ1) > T and cos(θ2) > T and cos(θ3) < T, the tertiary protection acts;

[0184] Wherein, cos(θ1), cos(θ2), cos(θ3) are cosine similarity between the measured 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 the phase difference and other factors, the protection threshold value T is set to 0.6 in the embodiment.

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

[0186] Based on the value of the cosine similarity between the measured point voltage waveform and the primary reference point voltage waveform, it is determined whether the primary protection acts; if the primary protection acts, the fault is located within the length range of the primary reference point of the protected line; if the primary protection does not act, it is continued to determine whether the secondary protection acts based on the value of the cosine similarity between the measured point voltage waveform and the secondary reference point voltage waveform;

[0187] If the secondary protection acts, the fault is located between the primary reference point and the secondary reference point of the protected line or the fault is located within the length range of the primary reference point but the primary protection refuses to act; if the secondary protection does not act, it is continued to determine whether the tertiary protection acts based on the value of the cosine similarity between the measured point voltage waveform and the tertiary reference point voltage waveform; if the tertiary protection acts, the fault is located between the secondary reference point and the tertiary reference point of the protected line or the fault is located within the length range of the primary reference point or the secondary reference point but the primary protection and the secondary protection refuse to act, if the tertiary protection does not act, it is an out-of-area fault or no fault.

[0188] The method of the embodiment does not need double-end communication, does not need the cooperation of the setting value, and does not need to modify the setting value adaptively, only needs the cooperation of the three protections in combination with the delay, and the protection method is simple and reliable.

[0189] In combination with Figure 9 Taking the AB line as the protected line as an example, K1 and K2 are in-area faults, and K3 and K4 are out-of-area faults, K1, K2, K3 and K4 are located within 0.9 times, within 0.9-1 times, within 1-1.4 times and outside 1.4 times of the AB line respectively.

[0190] When a short-circuit fault occurs at K1, the primary protection of the protected line instantaneously removes the fault, if the primary protection refuses to act, the secondary protection and the tertiary protection can act as backup protection of the primary protection.

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

[0192] When a short-circuit fault occurs at the first end K3 of the lower line BC, the primary protection of the next adjacent line detects the fault and starts to remove the fault by the primary protection. In the most extreme case, if the breakers of the next adjacent line all fail to refuse to act, the tertiary protection of the next adjacent line cannot act, and then the tertiary protection of the protected line acts after a long enough delay to reliably remove the fault, achieving the role of the remote backup protection.

[0193] The following gives a specific implementation case to verify the implementation effect of the embodiment scheme.

[0194] A 10kV medium-voltage alternating current microgrid simulation model is constructed by using PSCAD / EMTC as shown in Figure 12 . It contains four bus nodes, four lines, four inverter-type distributed power sources and four loads, and the overall control mode 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 power grid through a 110 / 10kV transformer. When the system is connected to the grid, the control strategy of DG1-DG3 is PQ control with low-voltage ride-through, and the control strategy of DG4 is VSG control with low-voltage ride-through, in which the low-voltage ride-through control strategy adds reactive power compensation. When the system is in island operation, the control strategy of DG1 and DG2 is PQ control with low-voltage ride-through, the control strategy of DG3 is VSG control with low-voltage ride-through, and the low-voltage ride-through control strategy also adds reactive power compensation. DG4 is VF control, which provides a voltage and frequency support point for the system.

[0196] The active power of the four distributed power sources is set to 0.4MW, 5MW, 0.3MW and 0.15MW respectively, and the reactive power is set to 0. The distributed power sources all have current limiting modules, so the maximum short-circuit current is limited to 1.2 times the rated current. The load is a resistance-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 + j0.263 MVA. The line is R-L model, the line unit impedance is Z = (0.45 + j0.35814) Ω / km, the line length is L1 = 4 km, L2 = 1 km, L3 = 4 km, L4 = 4 km, L5 = 2.5 km, L pcc = 2 km.

[0198] Case 1: Zone-in fault in the middle of the line

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

[0200] A metallic three-phase short-circuit fault is set at the middle position of 2 km in the line AB, and the fault occurs at 0.5 s, with a duration of 0.2 s and a sampling frequency of 10 kHz. The accuracy of the protection method proposed by the technology is verified under different fault position scenarios in the grid-connected and off-grid operation modes of the microgrid.

[0201] Still in conjunction with Figure 6 The fault occurs at the middle position of 2 km in the line AB, which is a zone-out fault for the line EA and a zone-in fault for the line AB.

[0202] When the grid is connected, the grid-connected operation results of the zone-out fault and the zone-in fault are shown in Tables 1 and 2, respectively, and the measured voltage reference voltage waveform diagrams of the zone-out fault and the zone-in fault are shown in Figs. 13(a) and 13(b), respectively.

[0203] Table 1 Grid-connected operation result of zone-out fault

[0204]

[0205] Table 2 Grid-connected operation result of zone-in fault

[0206]

[0207] When the grid is connected, the grid-connected operation results of the zone-out fault and the zone-in fault are shown in Tables 3 and 4, respectively, and the measured voltage reference voltage waveform diagrams of the zone-out fault and the zone-in fault are shown in Figs. 14(a) and 14(b), respectively.

[0208] Table 3 Island operation result of zone-out fault

[0209]

[0210]

[0211] Table 4 Island operation result of zone-in fault

[0212]

[0213] It can be seen that the similarity of 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 intra-zone fault is identified. In the grid-connected and island operation modes, the three-stage comparison protection of the fault line AB and the non-fault adjacent line EA can give accurate judgment results.

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

[0215] When the grid is connected, the grid-connected operation results of the external zone fault and the internal zone fault are shown in Tables 5 and 6, respectively, and the measured voltage reference voltage waveform diagrams of the external zone fault and the internal zone fault are shown in Figures 15(a) and 15(b), respectively.

[0216] Table 5 Grid-connected operation results of external zone fault with transition resistance

[0217]

[0218] Table 6 Grid-connected operation results of internal zone fault with transition resistance

[0219]

[0220] When the island is operated, the grid-connected operation results of the external zone fault and the internal zone fault are shown in Tables 7 and 8, respectively, and the measured voltage reference voltage waveform diagrams of the external zone fault and the internal zone fault are shown in Figures 16(a) and 16(b), respectively.

[0221] Table 7 Off-grid operation results of external zone fault with transition resistance

[0222]

[0223]

[0224] Table 8 Off-grid operation results of internal zone fault with transition resistance

[0225]

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

[0227] The protection method of the embodiment can be applied to both grid-connected and off-grid operation modes, does not need to switch the protection method when the microgrid switches between grid-connected and off-grid, has good adaptability, is not affected by the characteristics of the distributed power sources in the microgrid, and has good adaptability to different control strategies. No complex setting calculation is needed, and no setting switching operation is needed, so the protection method has high reliability and flexibility.

[0228] Embodiment two

[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 configured to set a first reference point, a second reference point and a third reference point at at least A1, A2 and A3 positions of the protected line respectively, and the positions of the reference points can form main protection and backup protection for the protected line and main protection and backup protection for the next adjacent line respectively;

[0231] A reference point voltage calculation module is configured to establish a two-port network model of the protected line and the port electrical quantity, take one port as a measurement point, calculate the voltage of each reference point at the other port based on the voltage of the measurement point, and

[0232] A line protection module is configured to calculate the cosine similarity of the voltage waveform of the measurement point and the voltage waveform of each reference point, take the value of the cosine similarity corresponding to each reference point as the action criterion of each protection level, and realize multi-level segmented protection of the AC microgrid protected line and determine the fault location.

[0233] The specific implementation of each module is exactly the same as that in Embodiment One, and will not be described in detail.

[0234] Embodiment three

[0235] In one or more embodiments, a terminal device is disclosed, which includes a processor and a memory, the processor is configured to implement instructions, and the memory is configured to store a plurality of instructions, the instructions are adapted to be loaded and executed by the processor to implement the AC microgrid protection method based on reference point voltage waveform comparison in Embodiment One.

[0236] Although the specific embodiments of the application are described above with reference to the drawings, the description is not a limitation on the scope of protection of the application, and those skilled in the art should understand that various modifications or changes made by those skilled in the art on the basis of the technical solutions of the application without creative labor are still within the scope of protection of the application.

Claims

1. A protection method for AC microgrids based on reference point voltage waveform comparison, characterized in that, include: At least at locations A1, A2, and A3 of the protected line, primary reference points, secondary reference points, and tertiary reference points shall be set respectively. 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; Establish a two-port network model of the electrical quantities of the protected line and port, take one port as the measurement point, and calculate the voltage of each reference point on the other port based on the voltage of the measurement point. The cosine similarity between the voltage waveform at the measurement point and the voltage waveform at each reference point is calculated. The cosine similarity value corresponding to each reference point is used as the action criterion for each level of protection, 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. The process of determining the location of the fault is as follows: Based on the cosine similarity between the voltage waveform at the measurement point and the voltage waveform at the primary reference point, it is determined whether the primary protection has been activated. If the primary protection has been activated, the fault is located within the length range of the primary reference point of the protected line. If the primary protection has not been activated, the cosine similarity between the voltage waveform at the measurement point and the voltage waveform at the secondary reference point is used to determine whether the secondary protection has been activated. If the secondary protection operates, the fault is located between the primary reference point and the secondary reference point of the protected line, or the fault is located within the length range of the primary reference point but the primary protection fails to operate. If the secondary protection does not operate, the cosine similarity between the voltage waveform at the measurement point and the voltage waveform at the tertiary reference point is used to determine whether the tertiary protection operates. If the tertiary protection operates, the fault is located between the secondary and tertiary reference points of the protected line, or the fault is located within the length of the primary or secondary reference points, but the primary and secondary protections fail to operate. If the tertiary protection does not operate, the fault is outside the zone or there is no fault.

2. The AC microgrid protection method based on reference point voltage waveform comparison as described in claim 1, characterized in that, The location selection of the primary reference point is specifically as follows: ； in, The length of the line corresponding to the primary reference point of the protected line. The reliability coefficient of the primary protection of the protected line is defined, with a value ranging from [0.9, 0.95]. The length of the protected line.

3. The AC microgrid protection method based on reference point voltage waveform comparison as described in claim 1, characterized in that, The location selection of the secondary reference point is specifically as follows: Principle 1: Determine the location of the second reference point for the protected line according to the principle of coordination with the primary protection of the next adjacent line: ； in, The length of the line corresponding to the secondary reference point of the protected line. The reliability coefficient of the secondary protection of the protected line is taken as [0.8, 0.85]. The length of the protected line. The length of the line corresponding to the first-level reference point of the next adjacent line; Principle Two: Determine the location of the second reference point for the protected line by performing sensitivity verification when a short-circuit fault occurs at the end of the protected line. ； in, The sensitivity coefficient of the secondary protection of the protected line is taken in the range of [1.25, 1.4]. The results obtained by selecting the above two principles The larger one 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 as described in claim 1, characterized in that, The specific location selection of the third-level reference points is as follows: Principle 1: The location of the third reference point for the protected line shall be determined according to the principle of coordination with the secondary and tertiary protection of the next adjacent line. ； in, The length of the line corresponding to the third-level reference point of the protected line. The sensitivity coefficient for the third-level protection of the protected line is defined, with a value range of [0.8, 0.9]. The length of the protected line. The length of the line corresponding to the secondary reference point of the next adjacent line; Principle Two: When used as a near-backup protection for the primary and secondary protection of the protected line, the position of the third reference point of the protected line shall be determined by sensitivity verification when a short-circuit fault occurs at the end of the protected line. ； in, The sensitivity coefficient for the third-level protection of the protected line is [1.5, 1.6]. Principle 3: When used as a remote backup protection for the next adjacent line, the position 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, The sensitivity coefficient for the third-level protection of the protected line is defined as [1.2, 1.3]. The length of the next adjacent line; The results obtained by selecting the above three principles The largest one in the three is used as the position of the third-level reference point for the protected line.

5. The AC microgrid protection method based on reference point voltage waveform comparison as described in claim 4, characterized in that, The location of the first reference point corresponds to the first-level protection range, the location of the second reference point corresponds to the second-level protection range, and the location of the third reference point corresponds to the third-level protection range. Delays are set for each level of protective action, specifically as follows: Level 1 protection is a direct action with no time delay; The delay time for 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 into the secondary protection range of the next adjacent line, then ; in, For the delay of the secondary protection action of the protected line, Delay for the first-level protection action of the next adjacent line. and These are the set delay times; The delay time for Level 3 protection action is: ； in, For the delay of the secondary protection action of the protected line, The set delay time.

6. The AC microgrid protection method based on reference point voltage waveform comparison as described in claim 1, characterized in that, Calculate the cosine similarity between the measured point voltage and the reference point voltage at each level, specifically as follows: ； Where 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 as described in claim 1, characterized in that, The cosine similarity value corresponding to each reference point is used as the criterion for each level of protection action, specifically: like Then, a level one protective action will be taken; like ,and Then the secondary protection action will be activated; like ,and ,and Then, a level three protective action will be taken; in, , , These are the cosine similarities between the measured point voltage and the voltages of the first-level, second-level, and third-level reference points, respectively; T is the set threshold.

8. An AC microgrid protection system based on reference point voltage waveform comparison, employing the AC microgrid protection method based on reference point voltage waveform comparison as described in any one of claims 1-7, characterized in that, include: The reference point selection module is used to set primary reference points, secondary reference points, and tertiary reference points at at least positions A1, A2, and A3 of the protected line, respectively. The positions 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 electrical quantities of the protected line and port. One port is used as the measurement point, and the voltage of each reference point is calculated on the other port based on the voltage of the measurement point. The line protection module is used to calculate the cosine similarity between the voltage waveform at the measurement point and the voltage waveform at each reference point. The cosine similarity value corresponding to each reference point is used as the action criterion for each level of protection, realizing multi-level segmented protection for the protected lines of the AC microgrid, and determining the fault location.

9. A terminal device comprising a processor and a memory, the processor for implementing instructions; the memory for storing multiple instructions, characterized in that, The instructions are adapted to be loaded by a processor and executed as described in any one of claims 1-7, for the protection of an AC microgrid based on reference point voltage waveform comparison.

Images