A method for detecting fault direction of inverter microgrid lines based on multi-frequency band characteristics

Abstract

The invention discloses a method for detecting fault direction of inverter type microgrid line based on multi-frequency band characteristics, comprising step S1, constructing criterion #1: the phase difference of the voltage and current of the fundamental frequency positive sequence fault component is within the range of -180° to 0°, and criterion #2: the cosine similarity of the voltage and current of the high frequency positive sequence fault component is less than -0.1; step S2, collecting the voltage and current information of each line terminal of the microgrid; step S3, extracting the phase difference of the voltage and current of the fundamental frequency positive sequence fault component at each line terminal, judging whether the criterion #1 is satisfied, if so, turning to step S5, if not, turning to step S4; step S4, extracting the cosine similarity of the voltage and current of the high frequency positive sequence fault component at each line terminal, judging whether the criterion #2 is satisfied, if so, turning to step S5, if not, turning to step S6; step S5, determining that the fault direction of the terminal is forward; step S6, determining that the fault direction of the terminal is reverse.

Description

Technical Field

[0001] The present invention relates to the technical field of power system relay protection, and in particular to a method for detecting fault direction of an inverter-type microgrid line based on multi-frequency band characteristics. Background Art

[0002] Traditional synchronous generator sets are voltage sources, with equivalent internal resistance of inductive resistance and impedance angle of 0°~90°. In comparison, the output characteristics of distributed micro-sources such as wind power and photovoltaic power mainly depend on the inverter control strategy, and after a fault occurs on the grid side, the inverter control may switch from conventional grid-following / grid-forming control to fault-through control, and the control target and synchronization mode may change. If the micro-source is equivalent to a current source, the current phase is uncertain, and if the micro-source is equivalent to a voltage source, the internal resistance amplitude and phase are uncertain. In addition, the microgrid operation mode is flexible, the distributed micro-sources are dispersed, the network topology is mostly a ring / mesh structure, and the current distribution direction is dynamically not fixed. This leads to the fact that the existing fundamental frequency direction elements designed based on the external characteristics of the voltage source of traditional synchronous units are prone to malfunction or refusal to operate, especially for inverter-type microgrids with 100% of new energy. The reliability and sensitivity of traditional fundamental frequency direction elements will be greatly reduced. In addition to the fundamental frequency steady-state characteristics, the short-circuit fault itself or the inverter control mode switching will introduce a wealth of high-frequency transient characteristics. However, since the transient characteristics are easily affected by factors such as the fault resistance, the initial fault angle, and whether the fault ride-through control strategy is activated, the high-frequency directional elements have poor adaptability to the working conditions, which may lead to the failure of the directional pilot protection based on high-frequency characteristics. Therefore, it is urgent to study an accurate and reliable fault direction judgment method, cooperate with directional pilot protection and overcurrent protection, realize rapid identification and removal of the fault area, and ensure the safe and reliable operation of the microgrid. Summary of the invention

[0003] The technical problem to be solved by the present invention is to provide a method for detecting the fault direction of an inverter-type microgrid line based on multi-frequency band characteristics, which can accurately determine the fault direction of each line of the microgrid by using the microgrid line endpoint voltage and line current information.

[0004] In order to solve the above technical problems, the present invention adopts the following technical method: a method for detecting the fault direction of an inverter-type microgrid line based on multi-frequency band characteristics, comprising:

[0005] Step S1, fault modeling is performed on the distributed micro-source, and the following fault direction criterion is constructed according to the forward and reverse fault characteristics of the microgrid line endpoints:

[0006] Criteria #1: The phase difference between the voltage and current of the fundamental frequency positive sequence fault component at the microgrid line endpoint is within the range of -180°~0°;

[0007] Criterion #2: The cosine similarity of the high-frequency positive-sequence fault component voltage and current at the microgrid line endpoint is less than the set value -0.1, where high frequency refers to the frequency range exceeding the cutoff frequency of the inverter output filter;

[0008] Step S2, collecting the terminal voltage and line current information on each line of the microgrid;

[0009] Step S3, using the information collected in step S2 to extract the phase difference of the fundamental frequency positive sequence fault component voltage and current at each line endpoint, and determine whether criterion #1 is met. If so, go to step S5; if not, go to step S4;

[0010] Step S4, using the information collected in step S2 to extract the cosine similarity of the high-frequency positive sequence fault component voltage and current at each line endpoint, to determine whether criterion #2 is met, if so, go to step S5, if not, go to step S6;

[0011] Step S5, determine that the fault direction of the endpoint is forward, and use FD =1 indicates the fault direction detection result;

[0012] Step S6, determine that the fault direction of the endpoint is reverse, use FD =0 indicates the fault direction detection result.

[0013] Furthermore, in step S3, when extracting the phase difference of the fundamental frequency positive sequence fault component voltage and current at each line endpoint:

[0014] First, according to the cross-correlation function Calculate the relationship between the voltage and current signals of the fundamental frequency positive sequence fault component as follows:

[0015] (1)

[0016] In the formula, For time, , are the voltage and current signals of the base frequency positive sequence fault component interfered by Gaussian noise, A , B are the voltage and current signal amplitudes of the fundamental frequency positive sequence fault component, is the signal frequency, T is the signal period, , They are and The phase of and is Gaussian noise, yes and The time delay between

[0017] Second, obtain the cross-correlation function The maximum delay corresponding to the maximum value ,like Greater than 0, indicating Ahead of the times ,like Less than 0, indicating Hysteresis ;

[0018] Then, calculate The cross-correlation function , as follows:

[0019] (2)

[0020] Finally, combine and The phase lead-lag relationship is calculated and The phase difference is as follows:

[0021] (3)

[0022] In the formula, , , , They are , The autocorrelation function of .

[0023] Furthermore, in step S4, the cosine similarity of the high-frequency positive sequence fault component voltage and current at each line endpoint is extracted according to the following formula: :

[0024] (4)

[0025] In the formula, , are respectively the voltage and current signals of the high-frequency positive sequence fault component, , , , is the number of sampling points within the data window length.

[0026] The present invention proposes a method for detecting the fault direction of an inverter-type microgrid line based on multi-band characteristics. The method constructs fault direction criterion #1 based on the phase difference of the voltage and current of the fundamental frequency positive sequence fault component at the microgrid line endpoint, and constructs fault direction criterion #2 based on the cosine similarity of the voltage and current of the high frequency positive sequence fault component at the microgrid line endpoint. In the case of combining the two criteria, the local voltage and current information is used to detect and judge the fault direction of each line endpoint of the microgrid in real time. This method is simple and effective. It is worth mentioning that the extraction of the voltage and current phase difference of the fundamental frequency positive sequence fault component in the present invention is not affected by factors such as measurement noise and system frequency offset during the fault process, and is insensitive to changes in fault resistance and fault initial angle. Therefore, the method proposed in the present invention can effectively deal with the uncertainty of system fault characteristics introduced by complex fault ride-through control response and flexible microgrid topology, and ensure the accuracy of fault direction detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a flow chart of the method for detecting the fault direction of the inverter type microgrid line based on multi-frequency band characteristics involved in the present invention;

[0028] Figure 2 This is a topological diagram of the inverter microgrid involved in the present invention;

[0029] Figure 3 The microgrid inverter control block diagram of the present invention (where (a) is a grid-following inverter control block diagram; (b) is a grid-building inverter control block diagram);

[0030] Figure 4 is an equivalent model diagram of the distributed micro-source before and after the fault in the present invention (wherein (a) is an equivalent model diagram of the distributed micro-source before the fault; (b) is an equivalent model diagram of the distributed micro-source after the fault; (c) is an equivalent fault component model diagram of the distributed micro-source);

[0031] Figure 5 The voltage and current phasor relationship diagram of the distributed micro-source before and after the fault in the present invention (wherein (a) is the voltage and current phasor relationship diagram of the grid-type micro-source before and after the fault; (b) is the voltage and current phasor relationship diagram of the grid-type micro-source before and after the fault);

[0032] Figure 6The voltage-current phasor relationship diagram of the distributed micro-source before and after the fault under different voltage drop degrees in the present invention (wherein, (a) is the voltage-current phasor relationship diagram of the grid-following micro-source when the voltage drops to 0.5-0.9 times the rated value after the fault; (b) is the voltage-current phasor relationship diagram of the grid-following micro-source when the voltage drops to 0-0.5 times the rated value after the fault; (c) is the voltage-current phasor relationship diagram of the grid-forming micro-source when the voltage drops to 0.5-0.9 times the rated value after the fault; (d) is the voltage-current phasor relationship diagram of the grid-forming micro-source when the voltage drops to 0-0.5 times the rated value after the fault);

[0033] Figure 7 are equivalent positive-sequence fault component models of the fault line and the non-fault line in the present invention (where (a) is the equivalent positive-sequence fault component model of the fault line; (b) is the equivalent positive-sequence fault component model of the non-fault line). DETAILED DESCRIPTION

[0034] In order to facilitate the understanding of those skilled in the art, the present invention is further described below in conjunction with embodiments and drawings. The contents mentioned in the implementation modes are not intended to limit the present invention.

[0035] like Figure 1 As shown, the inverter microgrid line fault direction detection method based on multi-frequency band characteristics provided by the present invention includes three major steps, and the content of each step will be described in detail below.

[0036] Step S1, constructing a fault direction criterion.

[0037] In order to construct an accurate fault direction criterion, the present invention first models and analyzes the distributed micro-sources under the condition of microgrid line fault to study the forward and reverse fault characteristics of the line endpoints.

[0038] 1) Modeling and analysis of distributed micro-source faults

[0039] The inverter microgrid topology structure of the present invention is as follows: Figure 2 As shown in the figure, distributed micro-sources such as wind power and photovoltaic power are connected to the microgrid through interface inverters and transformers. Inverter control strategies include grid-following type and grid-building type. The main difference between the two types of control lies in their control targets and synchronization methods. The control target of the micro-source using grid-following control (i.e., grid-following micro-source) is the inverter output current, and the phase of the grid-connected point voltage is tracked using a phase-locked loop (PLL). , synchronized with the microgrid, and the output characteristics are similar to those of a controlled current source. Figure 3 As shown in part (a), and are active and reactive power reference values ​​respectively, and Represents the output current of the inverter on the abc axis and the dq axis, and is the inverter terminal voltage, represents the current reference of the voltage control loop output, is the current reference value after the fault, and is the modulation voltage, and are the filter inductance and capacitance, and is the equivalent impedance of the interface transformer, and The control target of the micro-source using grid-type control (i.e., grid-type micro-source) is the voltage at the inverter end, and its phase Generated by the power control loop, the output characteristics are similar to the controllable voltage source, which can provide voltage and frequency support for the microgrid in the island operation mode. The network control block diagram is as follows Figure 3 As shown in part (b), and are amplitude reference and frequency reference respectively, It is worth noting that the island microgrid requires at least one grid-forming microsource to achieve system voltage / frequency regulation under normal and fault conditions and maintain stable system operation.

[0040] The output characteristics of distributed micro-sources mainly depend on the inverter control strategy. According to the voltage drop depth at the inverter end and the inverter output current increment, the micro-source fault modeling can be divided into two situations: fault ride-through control not started and fault ride-through control started.

[0041] Case 1: Micro-source fault ride-through control does not start. When the inverter output current is lower than its maximum allowable value, the micro-source fault ride-through control does not start, and the equivalent fault model of the micro-source depends on the grid-following or grid-forming control characteristics.

[0042] For grid-connected micro-sources, under normal circumstances, the inverter output current and As shown in formula (5), since the grid-connected micro-source usually operates at unity power factor, that is, Therefore, the inverter terminal voltage is in phase with the inverter output current before and after the fault. The equivalent model of the grid-type micro-source before and after the fault is a constant voltage source with a series variable impedance, such as Figure 4 As shown, is the micro-source equivalent internal potential, and It represents the voltage of the micro source terminal before and after the fault. and It represents the output current of the micro source before and after the fault. , are the equivalent impedances of the micro source before and after the fault, respectively.

[0043] (5)

[0044] For the grid-type micro-source, taking droop control as an example, and The droop characteristic is shown in equation (6), where and is the droop coefficient. The terminal voltage amplitude of the grid-type micro-source before the fault and frequency Usually equal to the reference voltage and , which can be equivalent to a zero internal resistance constant voltage source. The voltage amplitude at the inverter terminal after the fault is and frequency will slightly deviate from the reference value and can be equivalent to a constant voltage source with a small series impedance. Figure 4 shown.

[0045] (6)

[0046] According to the equivalent models of the micro-sources with grid-type and grid-type before and after the fault, the equivalent fault component model of the micro-source can be expressed as impedance ,as follows:

[0047] (7)

[0048] Considering the pre-failure and The phase difference between Usually less than 30°, and the voltage phase jump introduced by the high-resistance fault is small, so it can be considered that and The phase difference between Approximately equal to after failure and The phase difference between , that is and The phase difference is very small, and and Therefore, according to formula (7), the voltage and current phasor relationship of the grid-type and grid-type micro-source before and after the fault can be obtained as follows: Figure 5 This indicates that The phase range is between 0° and 180°.

[0049] Case 2: Micro-source fault ride-through control starts. When the inverter output current exceeds the maximum allowable value, the micro-source fault ride-through control starts, which has two main functions: current limiting and reactive current injection, supporting the system voltage while protecting the micro-source itself.

[0050] The most commonly used reactive current injection rule is: if the voltage drop depth of the microgrid system is between 50% and 90%, the reactive current contribution of the microsource increases proportionally with the fault ride-through control; if the voltage drop depth of the microgrid system drops below 50%, the reactive current increases to the maximum allowable value. After the fault ride-through control is started, the reference value of the inverter current control loop is becomes , as shown in equations (8) and (9). Where, is the rated voltage, is the maximum allowable current, equal to 1.2 times the rated value.

[0051] (8)

[0052] (9)

[0053] During the fault ride-through process, the main difference between the grid-following and grid-building micro-sources is the synchronization method of the inverter. For the grid-following micro-source, the amplitude limit of the current after the fault is , its phase angle The reference is the voltage at the inverter terminal. d Axis direction, as shown in formula (10), Between 0° and 90°.

[0054] (10)

[0055] For grid-connected micro-sources, the power synchronization mode is still used during the fault period to support the system voltage and frequency. Also restricted to , its phase angle The reference is the d-axis direction of the phase angle generated by the power control loop, that is, , as shown in formula (11), Between 0° and 90°. According to formula (6), when the frequency difference before and after the fault is When the phase generated by the power control loop before and after the fault is and The difference is shown in formula (11). Since the control loop responds very quickly, The value of tends to zero, that is near .

[0056] (11)

[0057] According to equations (7) to (11), the voltage and current phasor relationship of the grid-type and grid-forming micro-sources before and after the fault is as follows: Figure 6 As shown, it can be seen that the equivalent impedance of the micro source before and after the fault satisfy and .when hour, The phase range is between 0° and 180°, and when hour, The phase range may exceed 180°.

[0058] 2) Analysis of forward and reverse fault characteristics at line endpoints.

[0059] for Figure 2 The equivalent positive sequence fault component models of the fault line and the non-fault line in the microgrid shown are as follows: Figure 7 As shown in (a) and (b), the line in part (a) is the fault line, and the line in part (b) For normal lines, is the equivalent positive sequence fault component voltage source at the fault point, is the fault resistance, and Line The positive sequence fault component voltage at terminals B3 and A6 is: and For Line The positive sequence fault component current ( The direction of current is from line The B3 end flows to the A6 end, The current direction is opposite to that of Line The equivalent positive sequence fault component impedance on the back side of both ends, is the line impedance.

[0060] Depend on Figure 7 It can be obtained that the phase difference of the fundamental frequency positive sequence fault component voltage and current at both ends of the fault line and the non-fault line is shown in equations (12) and (13) respectively. According to the analysis conclusion of the distributed micro-source fault modeling above, under the high-resistance fault condition, the equivalent fault component model of the grid-following and grid-forming micro-sources is an impedance with a phase range of 0°~180°, that is, Belongs to 0°~180°, the fault line ends Both are between -180° and 0°, indicating that both ends are positive faults, and the non-faulty lines are Belongs to -180°~0°, It is between 0° and 180°, indicating that the A end of the line is a forward fault and the B end is a reverse fault.

[0061] (12)

[0062] (13)

[0063] Under low-resistance fault conditions, the micro-source fault ride-through control will be activated, and its equivalent fault component impedance angle may be greater than 180°, resulting in the phase difference between the base frequency positive sequence fault component voltage and current not being able to correctly reflect the fault direction. Considering that the voltage sag caused by a short-circuit fault will produce a wide-band fault current, when the frequency range exceeds the cutoff frequency of the inverter output filter (i.e., high frequency), the micro-source can be considered to be in a short-circuit state and can be equivalent to a stable high-frequency impedance. At this time, the phase differences of the high-frequency positive sequence fault component voltage and current at both ends of the fault line and the non-fault line are shown in equations (14) and (15), respectively. Represented as lines The high-frequency positive-sequence fault component impedance on the back side of both ends, is the high-frequency line impedance. Since the micro-source equivalent high-frequency impedance and line impedance are both between 0° and 90°, the and -180°~-90°, indicating that both ends are positive faults, and the non-faulty lines are Belongs to -180°~-90°, The impedance of the medium voltage distribution system is 0°~90°, which indicates that the fault is positive at the A end of the line and negative at the B end. About 0.5, that is, the high-frequency resistance-inductance ratio of the line impedance at 1kHz frequency It is approximately 0.1, so the cosine value of the high-frequency phase difference under forward and reverse faults is two non-zero values ​​with opposite signs, which can be used to reflect the phase relationship between the voltage and current of the high-frequency positive sequence fault component.

[0064] (14)

[0065] (15)

[0066] Through the above analysis of the distributed micro-source fault model and the forward and reverse fault characteristics of the microgrid line endpoints, it can be found that:

[0067] Under low-resistance fault conditions, due to the change in the inverter control target before and after the fault, the equivalent fault component impedance of the inverter-type micro-source presents a negative resistance characteristic. The phase difference range of the base frequency positive sequence fault component voltage and current under forward and reverse faults overlaps, and the fault direction cannot be accurately reflected, but the fault transient phenomenon is obvious. When the considered frequency range is greater than the cutoff frequency of the inverter output filter, the phase difference of the high-frequency positive sequence fault component voltage and current is irrelevant to the control loop, and the cosine similarity of the high-frequency positive sequence fault component voltage and current can be used to construct the fault direction judgment criterion.

[0068] Under high-resistance fault conditions, the transient component of the fault is weak, and the above-mentioned fault direction criterion may fail. However, the inverter control target remains unchanged before and after the fault, and the equivalent fault component impedance of the inverter-type micro-source presents a positive resistance characteristic. The phase difference of the voltage and current of the fundamental frequency positive sequence fault component can also be used to construct the fault direction criterion.

[0069] Therefore, combining the above two fault direction criteria, a fault direction detection method for inverter-type microgrid lines that is 100% based on multi-frequency band characteristics can be formed. Specifically:

[0070] Criterion #1: The phase difference between the voltage and current of the fundamental frequency positive sequence fault component at the microgrid line endpoint is within the range of -180°~0°.

[0071] Criterion #2: The cosine similarity of the high-frequency positive-sequence fault component voltage and current at the microgrid line endpoint is less than the set value -0.1, where high frequency refers to the frequency range exceeding the cutoff frequency of the inverter output filter.

[0072] Step S2: fault feature extraction.

[0073] S201, the protection devices at both ends of each line of the microgrid collect the microgrid node voltage and line current. Figure 2 L AB For example, CB BA CB AB L AB Protection devices at both ends of the line, B3, A6 are L AB The two endpoints of the line, namely the microgrid nodes, in step S201, are connected by CB BA Collecting microgrid L AB The voltage at the end of line B3 and the current flowing from the end of B3 into L AB The current on the line is supplied by CB AB Collecting microgrid L AB The voltage at the A6 end of the line and the current flowing from A6 to L AB The current on the line.

[0074] S202, extracting the phase difference of the fundamental frequency positive sequence fault component voltage and current at each line endpoint.

[0075] During the fault transient process, a 100% inverter microgrid will inevitably have a slight frequency offset, and the correlation analysis method can be used to measure the phase difference of the unknown frequency signal and suppress Gaussian noise interference. Therefore, the present invention proposes an improved correlation analysis method to extract the phase difference of the base frequency positive sequence fault component voltage and current. The process is as follows:

[0076] First, according to the cross-correlation function Calculate the relationship between the voltage and current signals of the fundamental frequency positive sequence fault component as follows:

[0077] (1)

[0078] In the formula, For time, are the voltage and current signals of the base frequency positive sequence fault component interfered by Gaussian noise, A , B are the voltage and current signal amplitudes of the fundamental frequency positive sequence fault component, is the signal frequency, is the signal period, They are and The phase of and is Gaussian noise, yes and The time delay between them is also called phase difference.

[0079] Second, obtain the cross-correlation function The maximum delay corresponding to the maximum value , can react and The phase difference, if Greater than 0, indicating Ahead of the times ,like Less than 0, indicating Hysteresis .

[0080] Then, since the periodic signal is not affected by Gaussian noise, when When , the cross-correlation function is calculated , as follows:

[0081] (2)

[0082] Finally, combine and The phase lead-lag relationship is calculated and The phase difference is as follows:

[0083] (3)

[0084] In the formula, , , , They are , The autocorrelation function of .

[0085] S203, extracting the cosine similarity of the high-frequency positive-sequence fault component voltage and current at each line endpoint.

[0086] The cosine similarity of the high-frequency positive sequence fault component voltage and current at each line endpoint is extracted according to the following formula: :

[0087] (4)

[0088] In the formula, , are respectively the voltage and current signals of the high-frequency positive sequence fault component, , , , N is the number of sampling points within the data window length.

[0089] Step S3, determine the fault direction and obtain the detection result.

[0090] The protection device determines the fault direction of each line endpoint of the microgrid according to criterion #1 and criterion #2. In the range of -180°~0° (i.e., meeting criterion #1), it means that the fault direction of the corresponding endpoint is positive; or, if the value obtained in step S203 is is less than the set value -0.1 (i.e. satisfies criterion #2), it means that the fault direction of the corresponding endpoint is positive; or, if the phase difference obtained in step S202 is In the range of -180° to 0° and the step S203 obtains is less than the set value -0.1 (i.e., it satisfies both criterion #1 and criterion #2), it indicates that the fault direction of the corresponding endpoint is positive; and if the phase difference obtained in step S202 is Not within the range of -180°~0° and the value obtained in step S203 If the value is not less than the set value -0.1 (i.e., both criterion #1 and criterion #2 are not satisfied), it means that the fault direction of the corresponding endpoint is reverse. FD =1 to indicate the detection result. Otherwise, when the fault direction is reverse, FD = 0 to indicate the detection result. It is worth mentioning that in the actual application process, the protection devices at both ends of each line of the microgrid use the method of the present invention to detect the fault direction of the corresponding endpoint. If the fault directions detected by the protection devices at both ends of the line are both positive, the protection devices at both ends of the line will immediately perform protection actions. If the fault directions detected by the protection devices at both ends of the line are one positive and one negative, the protection devices at both ends of the line will not act.

[0091] Next, build in PSCAD / EMTDC Figure 2The microgrid simulation model shown in the figure is used to verify the effectiveness of the fault direction detection method provided by the present invention. The simulation parameters of the microgrid system are shown in Table 1. The length of all feeders is 2 km and the sampling frequency is 10 kHz. This simulation experiment sets two fault conditions. Fault condition 1 is when the fault direction detection method provided by the present invention is B12 A two-phase short circuit fault occurs on the line, the fault resistance is 1Ω, the initial fault angle is 0°, and the fault condition 2 is C56 A three-phase short circuit fault occurs on the line, the fault resistance is 100Ω, and the initial fault angle is 60°. Under the two fault conditions, the detection results of the proposed fault direction criterion are shown in Table 2. It can be seen from Table 2 that under the low-resistance fault condition 1, the fault line L B12 The phase difference of the voltage and current of the fundamental frequency positive sequence fault component at both ends is 0°~180°, L B12 The cosine similarity of the voltage and current of the high-frequency positive sequence fault components at both ends is less than -0.1, so FD Equal to 1, line L B12 The fault directions detected by the protection devices at both ends are positive, and the fault directions detected by the protection devices at both ends of other non-faulty lines are one positive and one negative. Under high-resistance fault condition 2, due to the small transient component introduced by the fault, there is a certain error in the cosine similarity calculation of the high-frequency positive sequence fault component voltage and current. However, since the influence of the fault ride-through control response on the fundamental frequency phase relationship of the inverter output voltage and current can be ignored, line L C56 The phase difference of the voltage and current of the fundamental frequency positive sequence fault component at both ends is -180°~0°, so FD Equal to 1, line L C56 The fault directions detected by the protection devices at both ends are both positive, and the fault directions detected by the protection devices at both ends of other non-fault lines are one positive and one negative.

[0092] ;

[0093] ;

[0094] The above embodiments are preferred implementation schemes of the present invention. In addition, the present invention may also be implemented in other ways. Any obvious replacement without departing from the concept of the present technical solution is within the protection scope of the present invention.

[0095] In order to make it easier for ordinary technicians in the field to understand the improvements of the present invention over the prior art, some drawings and descriptions of the present invention have been simplified, and for the sake of clarity, some other elements are omitted in this application document. Ordinary technicians in the field should realize that these omitted elements may also constitute the content of the present invention.

Claims

1. A method for detecting the fault direction of an inverter-type microgrid line based on multi-frequency band characteristics, characterized in that: include: Step S1, fault modeling is performed on the distributed micro-source, and the following fault direction criterion is constructed according to the forward and reverse fault characteristics of the microgrid line endpoints: Criteria #1: The phase difference between the voltage and current of the fundamental frequency positive sequence fault component at the microgrid line endpoint is within the range of -180°~0°; Criterion #2: The cosine similarity of the high-frequency positive-sequence fault component voltage and current at the microgrid line endpoint is less than the set value -0.1, where high frequency refers to the frequency range exceeding the cutoff frequency of the inverter output filter; Step S2, collecting the voltage and current information of each line endpoint of the microgrid; Step S3, using the information collected in step S2 to extract the phase difference of the fundamental frequency positive sequence fault component voltage and current at each line endpoint, and determine whether criterion #1 is met. If so, go to step S5; if not, go to step S4; Step S4, using the information collected in step S2 to extract the cosine similarity of the high-frequency positive sequence fault component voltage and current at each line endpoint, to determine whether criterion #2 is met, if so, go to step S5, if not, go to step S6; Step S5, determine that the fault direction of the endpoint is forward, and use FD =1 indicates the fault direction detection result; Step S6, determine that the fault direction of the endpoint is reverse, use FD =0 indicates the fault direction detection result.

2. The method for detecting the fault direction of an inverter-type microgrid line based on multi-frequency band characteristics according to claim 1 is characterized in that: In step S3, when extracting the phase difference of the fundamental frequency positive sequence fault component voltage and current at each line endpoint: First, according to the cross-correlation function Calculate the relationship between the voltage and current signals of the fundamental frequency positive sequence fault component as follows: (1) In the formula, For time, , are the voltage and current signals of the base frequency positive sequence fault component interfered by Gaussian noise, A , B are the voltage and current signal amplitudes of the fundamental frequency positive sequence fault component, is the signal frequency, T is the signal period, , They are and The phase of and is Gaussian noise, yes and The time delay between Second, obtain the cross-correlation function The maximum delay corresponding to the maximum value ,like Greater than 0, indicating Ahead of the times ,like Less than 0, indicating Hysteresis ; Then, calculate The cross-correlation function , as follows: (2) Finally, combine and The phase lead-lag relationship is calculated and The phase difference is as follows: (3) In the formula, , , , They are , The autocorrelation function of .

3. The method for detecting the fault direction of an inverter-type microgrid line based on multi-frequency band characteristics according to claim 2 is characterized in that: Step S4: extract the cosine similarity of the high-frequency positive sequence fault component voltage and current at each line endpoint according to the following formula: : (4) In the formula, , are respectively the voltage and current signals of the high-frequency positive sequence fault component, , , , is the number of sampling points within the data window length.

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