Transient reactive polarity-based microgrid fault adaptive direction discrimination method and system

Abstract

The invention discloses a micro-grid fault adaptive direction discrimination method and system based on transient reactive polarity, and belongs to the technical field of micro-grids. The method comprises the following steps: carrying out equivalent modeling on a transient fault component network of the micro-grid to obtain an equivalent model of a line and a distributed power supply, and constructing a system broadband transient equivalent model of the micro-grid based on the equivalent model of the line and the distributed power supply; based on the system broadband transient equivalent model, extracting fault transient quantity polarity characteristics, and obtaining transient characteristics and transient reactive polarity characteristics of a sound branch and a fault branch; selecting a characteristic frequency band according to the transient characteristics and the transient reactive polarity characteristics, performing polarity discrimination on the characteristics in the selected characteristic frequency band, and discriminating the fault direction of the micro-grid according to the discriminated polarity. The method solves a problem that a conventional fault direction discrimination principle is liable to be affected by a distributed power supply differentiation control strategy.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of micro-grid, and more particularly, to a micro-grid fault adaptive direction discrimination method and system based on transient reactive polarity. BACKGROUND

[0002] With the rapid development of renewable energy and higher requirements for power supply reliability, micro-grid, which is an integrated structure containing power supply, load and energy storage, as an effective way of new energy consumption and emergency power supply, is occupying an increasingly important position. However, the distributed generators (DG) in the micro-grid are scattered in different locations, and have different control strategies, which produce complex fault characteristics. The off-grid operation mode further amplifies the influence of distributed generator control. Overall, the fault characteristics of micro-grid are basically determined by the control of distributed generators, and there is a feature of bidirectional short-circuit current. The limited overcurrent and low penetration capability of inverters make it more challenging to quickly and accurately isolate faults, and micro-grid protection is facing challenges.

[0003] In the design of micro-grid protection strategy, due to the variability of micro-grid topology and operation mode, and the existence of multi-directional short-circuit current in the fault, accurate judgment of the fault direction is the key to reducing incorrect protection actions, and fast direction discrimination is the key to improving protection action speed and reducing the risk of inverter-type main power off-grid and even micro-grid collapse. However, there are many problems in the application of existing direction discrimination methods to micro-grid. First, the fault characteristics of micro-grid dominated by inverter control are different from traditional distribution networks, which are reflected as additional nonlinear controlled power in the fault component network. The premise of using the direction discrimination principle of fault components is destroyed. Second, under the action of fault ride-through strategy, the angle between line fault current and voltage depends on the control strategy rather than the line impedance angle. Negative sequence suppression makes the difference between the output of distributed generators and traditional power sources obvious during asymmetric faults, and the division of traditional power positive and negative direction action zones is no longer suitable. In addition, the dead zone problem of traditional power direction elements in the protection of near-zone short circuits also exists in micro-grid. The frequency fluctuation after the fault of grid-connected power sources causes deviation in fault phase calculation. The above reasons make it difficult to apply traditional direction discrimination principles to micro-grid.

[0004] In the existing fault direction discrimination methods of microgrid, most of them are designed for the specific control strategy of single power supply to design the direction discrimination principle, but the adaptability to the complex scene of different control strategy distributed power supply and different line parameter combination is insufficient. Secondly, most of the researches do not involve the voltage dead zone problem when the protection near end fault, or only propose to use the memory voltage to solve the problem, but do not discuss the influence of the objective difference of the phase angle between the memory voltage and the actual voltage caused by the inverter control on the direction discrimination. Although the protection using the traveling wave has good performance, it needs high sampling frequency and communication device, which is difficult to be widely used in microgrid at present. SUMMARY

[0005] In view of the above problems, the application provides a microgrid fault adaptive direction discrimination method based on transient reactive polarity, which comprises the following steps:

[0006] The equivalent model of the microgrid transient fault component network is established, the equivalent model of the line and the distributed power supply is obtained, and the system broadband transient equivalent model of the microgrid is constructed based on the equivalent model of the line and the distributed power supply.

[0007] Based on the system broadband transient equivalent model, the transient feature and the transient reactive polarity feature of the healthy branch and the fault branch are extracted.

[0008] The feature frequency band is selected according to the transient feature and the transient reactive polarity feature, the polarity of the features in the selected feature frequency band is discriminated, and the fault direction of the microgrid is discriminated according to the discriminated polarity.

[0009] Optionally, the transient feature and the transient reactive polarity feature of the healthy branch and the fault branch include:

[0010] The transient feature of the healthy branch containing only load, the transient reactive polarity feature of the healthy branch containing load and DG, and the transient reactive polarity feature of the fault branch.

[0011] Optionally, based on the system broadband transient equivalent model, the transient feature and the transient reactive polarity feature of the healthy branch and the fault branch are extracted, including:

[0012] Based on the system broadband transient equivalent model, a frequency calculation model is established for the healthy branch containing only load and the healthy branch containing load and DG, and the transient feature of the healthy branch containing only load and the transient reactive polarity feature of the healthy branch containing load and DG are determined based on the calculation result of the resonance calculation model.

[0013] Based on the directional protection measurement value on the fault branch, the transient reactive polarity characteristics of the fault branch are determined under the frequency of the healthy branch containing only load and the healthy branch containing load and DG.

[0014] Optionally, the selection of the feature frequency band is performed for the transient characteristics and the transient reactive polarity characteristics, including:

[0015] The upper limit and the lower limit of the high-pass filtering frequency are established to have constraint conditions, including:

[0016] The constraint condition of the upper limit of the resonance frequency is as follows:

[0017]

[0018] Wherein, f max1 is the upper limit of the high-pass filtering frequency, L i and C i are the inductance and capacitance values of the i-th line, respectively;

[0019] The constraint condition of the lower limit of the high-pass filtering frequency is as follows:

[0020]

[0021] Wherein, f min is the lower limit of the resonance frequency, C DC,i and L 1,i are the DC side capacitance and the inverter side inductance of the i-th distributed power supply, respectively;

[0022] The constraint condition of the high-pass filtering frequency is set again as follows:

[0023]

[0024] Wherein, f max2 and f max3 are the upper limit of the high-pass filtering frequency, L 2,i , R 1,i and R 2,i are the load equivalent inductance, line resistance and load equivalent resistance in the i-th line ending with load, L 1,j , L 3,j , C 1,j and C 3,j are the line equivalent inductance, distributed power supply equivalent inductance, line-to-ground capacitance and distributed power supply equivalent capacitance in the j-th line ending with distributed power supply, respectively;

[0025] The final high-pass filtering frequency limit value is set as follows:

[0026] f high = min{f max1 , fmax2 f max3}

[0027] Therefore, the feature frequency band is selected, that is, the feature frequency band satisfying f min and less than f high is selected.

[0028] Optionally, the feature in the selected feature frequency band is polarized, and the fault direction of the micro-grid is determined according to the polarization.

[0029] Based on the feature in the selected feature frequency band, the fault phase reactive power is calculated, and the fault phase reactive power is taken as a feature quantity, if the feature quantity is positive, the downstream line is a healthy line, and the fault is located on the upstream thereof; if the feature quantity is negative, there is a fault line in the downstream line.

[0030] Optionally, the calculation formula of the fault phase reactive power is as follows:

[0031]

[0032] Wherein, is a feature quantity, is the instantaneous reactive power of the fault phase, t0 is the starting time of the fault, and t0+T needs to be less than the time when the inverter first controls the intervention, and 2ms<T<5ms can be allowed.

[0033] In still another aspect, the present application also provides a micro-grid fault adaptive direction discrimination system based on transient reactive polarity, comprising:

[0034] A modeling unit is configured to perform equivalent modeling on the micro-grid transient fault component network, to obtain an equivalent model of a line and a distributed power supply, and to construct a system broadband transient equivalent model of the micro-grid based on the equivalent model of the line and the distributed power supply.

[0035] A frequency band selection unit is configured to extract fault transient quantity polarity features based on the system broadband transient equivalent model, to obtain transient features and transient reactive polarity features of healthy branches and fault branches.

[0036] A discrimination unit is configured to select a feature frequency band based on the transient features and the transient reactive polarity features, to polarize features in the selected feature frequency band, and to determine the fault direction of the micro-grid according to the polarization.

[0037] Optionally, the transient features and the transient reactive polarity features of the healthy branches and the fault branches include:

[0038] The transient features of the healthy branches containing only loads, the transient reactive polarity features of the healthy branches containing loads and DGs, and the transient reactive polarity features of the fault branches.

[0039] Optionally, based on the system broadband transient equivalent model, the fault transient quantity polarity feature is extracted, and the transient features and transient reactive polarity features of the healthy branch and the fault branch are obtained, including:

[0040] Based on the system broadband transient equivalent model, the frequency calculation model is established for the healthy branch containing only load and the healthy branch containing load and DG, and based on the calculation result of the resonance calculation model, the transient features of the healthy branch containing only load and the transient reactive polarity features of the healthy branch containing load and DG are determined.

[0041] Based on the directional protection measurement value on the fault branch, the transient reactive polarity features of the fault branch are determined under the frequency of the healthy branch containing only load and the healthy branch containing load and DG.

[0042] Optionally, for the transient features and transient reactive polarity features, the feature frequency band is selected, including:

[0043] The upper and lower limits of the high-pass filter frequency are established to have constraint conditions, including:

[0044] The constraint condition of the upper limit of the resonance frequency is as follows:

[0045]

[0046] Wherein, f max1 is the upper limit of the high-pass filter frequency, L i and C i are the inductance and capacitance values of the i-th line, respectively;

[0047] The constraint condition of the lower limit of the high-pass filter frequency is as follows:

[0048]

[0049] Wherein, f min is the lower limit of the resonance frequency, C DC,i and L 1,i are the DC side capacitance and inverter side inductance of the i-th distributed power supply, respectively.

[0050] The constraint condition of the high-pass filter frequency is set again, as follows:

[0051]

[0052] Wherein, f max2 and f max3 are the upper limits of the high-pass filter frequency, L 2,i , R 1,i and R 2,iL 1,j , L 3,j , C 1,j and C 3,j are respectively the line equivalent inductance, the distributed power equivalent inductance, the line-to-ground capacitance and the distributed power equivalent capacitance in the line containing the distributed power at the jth end.

[0053] The final high-pass filtering frequency limit is set as follows:

[0054] f high = min{f max1 , f max2 , f max3}

[0055] Therefore, the characteristic frequency band is selected, i.e., a characteristic frequency band satisfying f min and f high is selected.

[0056] Optionally, the polarity of the characteristic in the selected characteristic frequency band is identified, and the fault direction of the microgrid is identified according to the identified polarity, including:

[0057] Based on the characteristic in the selected characteristic frequency band, the fault phase reactive power is calculated, and the fault phase reactive power is taken as a characteristic quantity, if the characteristic quantity is positive, the downstream line is a healthy line and the fault is located upstream thereof, and if the characteristic quantity is negative, there is a fault line in the downstream line.

[0058] Optionally, the calculation formula of the fault phase reactive power is as follows:

[0059]

[0060] wherein, is the characteristic quantity, is the instantaneous reactive power of the fault phase, t0 is the starting time of the fault, and t0+T needs to be less than the time when the inverter first controls intervention, and 2ms<T<5ms can be allowed.

[0061] In another aspect, the present application further provides a computing device, comprising: one or more processors;

[0062] the processor is used for executing one or more programs;

[0063] When the one or more programs are executed by the one or more processors, the method as described above is implemented.

[0064] In another aspect, the present application further provides a computer readable storage medium, which has a computer program stored thereon, and the computer program is executed to implement the method as described above.

[0065] Compared with the prior art, the present application has the following advantages:

[0066] The present application provides a micro-grid fault adaptive direction discrimination method based on transient reactive polarity, comprising: equivalent modeling for micro-grid transient fault component network, obtaining equivalent model of line and distributed power supply, and based on the equivalent model of line and distributed power supply, constructing system broadband transient equivalent model of micro-grid; based on the system broadband transient equivalent model, extracting fault transient quantity polarity characteristics, obtaining transient characteristics and transient reactive polarity characteristics of healthy branch and fault branch; for the transient characteristics and transient reactive polarity characteristics, selecting characteristic frequency band, discriminating the polarity of the characteristics in the selected characteristic frequency band, and discriminating the fault direction of the micro-grid according to the discriminated polarity. The present application solves the problem that the traditional fault direction discrimination principle is easily affected by the differentiated control strategy of distributed power supply by discriminating the polarity of the characteristics in the selected characteristic frequency band. BRIEF DESCRIPTION OF DRAWINGS

[0067] Figure 1 The flow chart of the method of the present application;

[0068] Figure 2 The uniform transmission line distributed parameter model diagram of the method embodiment of the present application;

[0069] Figure 3 The uniform transmission line g-type equivalent model diagram of the method embodiment of the present application;

[0070] Figure 4 (a)-(d) are respectively four typical forms of three-phase short-circuit topology one, three-phase short-circuit topology two, two-phase short-circuit topology one and two-phase short-circuit topology two in the inverter-type power supply high-frequency impedance topology of the method embodiment of the present application,

[0071] Figure 5 (a)-(b) are the micro-grid typical structure and transient equivalent model diagram in the micro-grid broadband transient equivalent model of the method embodiment of the present application;

[0072] Figure 6 (a)-(b) are the healthy branch transient equivalent circuit diagram containing only load and the healthy branch transient equivalent circuit diagram containing distributed power supply and load in the transient equivalent circuit of different types of branches of the method embodiment of the present application;

[0073] Figure 7 The micro-grid simulation model diagram of the method embodiment of the present application;

[0074] Figure 8 (a)-(c) are the F1 fault simulation results of the method embodiment of the present application, including: common bus voltage and protection installation current, filtered voltage and current, and filtered voltage and current diagram;

[0075] Figure 9 F2 fault broadband transient reactive instantaneous value result graph for an embodiment of the method of the present application;

[0076] Figure 10 F3 fault broadband transient reactive instantaneous value result graph for an embodiment of the method of the present application;

[0077] Figure 11 Structure diagram of the system of the present application. DETAILED DESCRIPTION

[0078] Reference will now be made to the drawings to describe the exemplary embodiments of the present application in detail. The present application may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and fully convey the scope of the application to those skilled in the art. Like reference numerals refer to like elements throughout the specification. It will be understood that when an element is referred to as being "on" another element, it can be directly on the element or intervening elements can also be present. In addition, terms such as first and second are used herein when claiming certain embodiments of the present application and should not be construed as limiting the scope of the present application unless otherwise stated.

[0079] Unless otherwise defined, all terms (including 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. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0080] Embodiment 1:

[0081] The present application proposes a microgrid fault adaptive direction discrimination method based on transient reactive polarity, as shown in FIG. 1, comprising: Figure 1

[0082] Step 1, equivalent modeling is performed on the microgrid transient fault component network, equivalent models of lines and distributed power sources are obtained, and a system broadband transient equivalent model of the microgrid is constructed based on the equivalent models of the lines and distributed power sources;

[0083] Step 2, based on the system broadband transient equivalent model, transient feature and transient reactive polarity feature of healthy branches and fault branches are obtained by extracting the fault transient quantity polarity feature;

[0084] Step 3, for the transient feature and transient reactive polarity feature, a feature frequency band is selected, the features in the selected feature frequency band are polarized, and the fault direction of the microgrid is discriminated according to the polarized polarity.

[0085] The transient feature and transient reactive polarity feature of the healthy branches and fault branches comprise:

[0086] ​The transient characteristics of the healthy branch containing only load, the transient reactive polarity characteristics of the healthy branch containing load and DG, and the transient reactive polarity characteristics of the fault branch.

[0087] The transient characteristics of the healthy branch containing only load, the transient reactive polarity characteristics of the healthy branch containing load and DG, and the transient reactive polarity characteristics of the fault branch.

[0088] Based on the system broadband transient equivalent model, a frequency calculation model is established for the healthy branch containing only load and the healthy branch containing load and DG, and based on the calculation results of the resonance calculation model, the transient characteristics of the healthy branch containing only load and the transient reactive polarity characteristics of the healthy branch containing load and DG are determined.

[0089] Based on the directional protection measurement value on the fault branch, the transient reactive polarity characteristics of the fault branch are determined under the frequency of the healthy branch containing only load and the healthy branch containing load and DG.

[0090] The feature frequency band is selected for the transient characteristics and the transient reactive polarity characteristics, including:

[0091] The upper and lower limits of the high-pass filter frequency are established with constraint conditions, including:

[0092] The constraint condition of the upper limit of the resonance frequency is as follows:

[0093]

[0094] Wherein, f max1 is the upper limit of the high-pass filter frequency, L i and C i are the inductance and capacitance values of the i-th line, respectively;

[0095] The constraint condition of the lower limit of the high-pass filter frequency is as follows:

[0096]

[0097] Wherein, f min is the lower limit of the resonance frequency, C DC,i and L 1,i are the DC side capacitance and inverter side inductance of the i-th distributed power source, respectively.

[0098] The constraint condition of the high-pass filter frequency is set again as follows:

[0099]

[0100] Wherein, f max2 and f max3 are the upper limit of the high-pass filter frequency, L2,i , R 1,i , and R 2,i are the load equivalent inductance, line resistance and load equivalent resistance in the line with load at the i-th end, respectively, L 1,j , L 3,j , C 1,j , and C 3,j are the line equivalent inductance, distributed power equivalent inductance, line-to-ground capacitance and distributed power equivalent capacitance in the line with distributed power at the j-th end, respectively;

[0101] The final high-pass filtering frequency limit is set as follows:

[0102] f high = min{f max1 , f max2 , f max3}

[0103] Therefore, the characteristic frequency band is selected, i.e., a characteristic frequency band satisfying f min and f high is selected.

[0104] wherein, the polarity of the characteristic in the selected characteristic frequency band is determined, and the fault direction of the microgrid is determined according to the determined polarity, including:

[0105] Based on the characteristic in the selected characteristic frequency band, the fault phase reactive power is calculated, and the fault phase reactive power is taken as a characteristic quantity, if the characteristic quantity is positive, the downstream line is a healthy line and the fault is located upstream thereof, if the characteristic quantity is negative, there is a fault line in the downstream line.

[0106] wherein, the calculation formula of the fault phase reactive power is as follows:

[0107]

[0108] wherein, is the characteristic quantity, is the instantaneous reactive power of the fault phase, t0 is the starting time of the fault, and t0+T needs to be less than the time when the inverter first controls to intervene, and 2ms<T<5ms can be allowed.

[0109] The specific embodiments of the present application are described in detail below with reference to the accompanying drawings.

[0110] A) Microgrid transient fault component network equivalent modeling:

[0111] A.1) Establishing a line wide-frequency transient model:

[0112] Based on the distributed parameter model (as shown in Figure 2 ), the power transmission line is modeled, and the input impedance of the line can be represented as:

[0113]

[0114] In the formula, Z C Γ and Γ are the wave impedance and propagation constant of the transmission line, respectively:

[0115]

[0116]

[0117] As can be seen from the input impedance expression, the input impedance characteristics of the distributed parameter circuit model change with frequency, alternately exhibiting capacitive or inductive characteristics. The resonant frequency at which this first change occurs (i.e., the first resonant frequency) is:

[0118]

[0119] In the formula, L = L u l, C = C u l. Below the initial resonant frequency, the equivalent capacitance when the distributed parameter model is open-circuited at its end is:

[0120]

[0121] When using a Γ-type equivalent circuit to model the transmission line (e.g.) Figure 3 As shown), its series inductance, resistance, and capacitance to ground are L, respectively. g R g and C g The input impedance of the Γ-type equivalent circuit is:

[0122]

[0123] Its resonant frequency is:

[0124]

[0125] The input impedance described above is inductive below the resonant frequency and capacitive above the resonant frequency. Below the resonant frequency, its equivalent capacitance is:

[0126]

[0127] The simulation accuracy of the Γ-type equivalent circuit modeling is close to that of the distributed parameter model at the following two levels:

[0128] 1) The initial resonant frequency remains consistent.

[0129] 2) The equivalent impedance comprehensive error is the smallest below the first resonant frequency.

[0130] This can be seen as an optimization problem with the equivalent impedance synthesis error as the objective function, that is, find the coefficients k1, k2 to satisfy:

[0131]

[0132] Solving k1, k2, and substituting the latter two equations, we get:

[0133]

[0134] This formula establishes the inductance and capacitance parameter equivalence between the Γ-type model and the distributed parameter model under the requirements of the equivalent target. The Γ-type equivalent model determined according to the above conditions can ensure the accuracy of the first resonant frequency after equivalence and the minimum impedance synthesis error below the first resonant frequency, which can support the microgrid fault characteristic research.

[0135] A.2) Establish a wideband transient model of distributed power supply:

[0136] The earliest control intervention time of inverter-type distributed power supply is in the order of ten milliseconds. Therefore, the analysis of the initial transient response of the distributed power supply fault can ignore the influence of the control characteristics and consider it as always working in the constant power state. Within this time window, a wideband transient model of the distributed power supply independent of the control characteristics can be established.

[0137] The possible high-frequency impedance topologies when symmetric and asymmetric faults occur are shown in Figs. Figure 4 , where Z2 is the grid-side impedance of the inverter, Z C is the filter capacitance of the inverter, Z1 is the DC-side impedance of the inverter, and Z DC is the DC-side capacitance. Topologies (a) and (b) correspond to the case where the wideband fault current flows through the impedance of the DC-side capacitance Z DC through one or two switch tubes of a single-sided bridge arm when three-phase short-circuit occurs, and topologies (c) and (d) correspond to the case where the short-circuit current flows through the impedance of the DC-side capacitance Z DC through two switch tubes of the same side or through switch tubes of the opposite side when two-phase short-circuit occurs.

[0138] At higher frequencies, the capacitive branch impedance is much smaller than the impedance of the parallel branch, so the inductive branch in the dashed box can be ignored, and the above four topologies can be simplified to the structure of the grid-side inductance Z1 and the filter capacitance Z C in series, without considering the specific conduction of the switch tube.

[0139] At the same frequency, the branch with the smallest impedance of the parallel inductive branch in the four high-frequency impedance topologies determines the frequency limit value of the ignored parallel inductive branch. If the parallel branch is ignored when Z C is not greater than 1 / 10 of the parallel impedance, then:

[0140]

[0141] The three simultaneous equations correspond to the fourth topology, the first topology and the remaining two topologies, respectively. Further simplification of the three simultaneous equations gives:

[0142]

[0143] The smaller part of the interval is only meaningful when C>10C DC , which can be discarded considering that the wideband impedance at high frequencies is of interest. The frequency boundary condition for the negligible parallel inductance branch is obtained as:

[0144]

[0145] That is, when the characteristic frequency band satisfies the frequency requirement in (13), the parallel branch can be neglected, and the distributed generation transient model containing only the grid-side impedance and the filter capacitance is obtained.

[0146] A.3) Establishment of the wideband transient equivalent model of the microgrid:

[0147] Based on the wideband transient model of the line and the distributed generation, the wideband transient equivalent model of the microgrid as a whole can be established. Take Figure 5 (a) as an example of a typical structure of the microgrid, which is in the form of a switching station. The switching station outlet is connected to the external distribution network system on one side and to the downstream radial branch through multiple connection points on the other side. In the short time after the fault occurs, which is the focus of transient wideband analysis, the control strategy response has not yet taken effect, and the distributed generation still works in the constant power mode. In the fault component network, it can be represented by impedance. Using the line impedance lumped parameter model, the wideband transient impedance model of the distributed generation and the load model represented by impedance in the fault component network, the equivalent model of the fault component network suitable for wideband transient analysis is obtained, as shown in Figure 5 (b).

[0148] B) Polar characteristics of fault transient quantities:

[0149] B.1) Transient reactive power polarity characteristics of healthy branch containing only load

[0150] Figure 6 In (a), C1 is the line-to-ground capacitance, R1 and L1 are the line resistance and reactance, and the total length of the line is l. The unit length resistance, capacitance and inductance represented by distributed parameters are R u ,C u ,L uThe capacitance and reactance are determined by equation (10) respectively. In this case, the healthy branch structure containing only the load is approximately LC parallel resonant, and its input impedance is:

[0151]

[0152] where the imaginary part X of the molecule is:

[0153] X = L1 + L2 - C1 (R1 + R2) 2 - ω 2 C (L1 + L2) 2 (15)

[0154] It can be seen that as the frequency increases, the imaginary part sign changes, and the impedance changes from inductive to capacitive. Its resonant frequency is:

[0155]

[0156] Below this frequency, the branch structure is inductive, and above this frequency, it is capacitive. That is, if the selected frequency band is lower than the resonant frequency, the load branch in the fault component network is inductive and absorbs reactive power. For typical line parameters, as the line length increases, the resonant frequency decreases; as the load power increases, the resonant frequency shows a slight upward trend. Generally, for the line length, line parameters and load size commonly seen in engineering, the resonant frequency is between a few hundred hertz and a few thousand hertz. From the above analysis, for a healthy branch containing only a load, if the selected frequency band is lower than the resonant frequency determined according to equation (16), the branch is inductive, and the protection installation will measure inductive instantaneous reactive power.

[0157] B.2) Transient reactive polarity characteristics of healthy branch containing load and DG:

[0158] Figure 6 In (b), R1, L1, C1 are the line resistance, inductance and capacitance to ground, R2, L2 are the equivalent resistance and inductance of the load, and L3 and C3 are the equivalent model of the distributed generator, whose values are equivalent to the inductance on the inverter side and the filter capacitance. Note that the filter capacitance and network side inductance commonly used in inverter structures are small, and the impedance of the DG equivalent branch is always much smaller than the impedance value of the load branch at power frequency and above. At this time, the load impedance can be ignored. After simplification, it becomes a Γ type structure with a capacitance at both ends, and its input impedance is:

[0159]

[0160] In the equation,

[0161]

[0162] Note that the sign of the imaginary part of impedance (17) depends on the value of B, let t = ω 2 which can be regarded as a quadratic function of t with opening downward. When the quadratic function has a zero point, the zero point determines the resonance frequency f 1,2 so that the imaginary part of impedance is negative, i.e. the branch impedance is capacitive. When the quadratic function has no zero point, the imaginary part of impedance is always negative, i.e. the branch is always capacitive. For common line parameters, f2 always exists and is positive, so that the interval (f1, f2) can always be found, in which the branch is inductive. According to the solution of the above quadratic function, the frequency can be determined as:

[0163]

[0164] where

[0165] In addition, when the line resistance is ignored, taking R equal to 0 in the above formula, the solution degenerates to:

[0166]

[0167] In actual calculation, the influence of R on the frequency interval is not large, and the line resistance can be ignored to approximately calculate the frequency interval. For common line parameters, the frequency interval in which the line impedance is inductive is in the order of hundreds of hertz to thousands of hertz, and the frequency selected to ensure that the branch is inductive can be taken in actual engineering. 1,2

[0168] B.3) Transient reactive polarity characteristics of the fault branch:

[0169] Finally, the directional protection measurement value located on the fault branch is considered. From the installation of the directional protection, the impedance is equivalent to the parallel connection of a plurality of healthy branches and the transient impedance of the transformer connected with the distribution network, and the transient voltage-current reactive characteristics depend on the capacitive and inductive conditions of the parallel structure. Considering the actual value, at the frequency at which each healthy branch is inductive, the parallel transformer transient impedance can be ignored. Therefore, as long as the frequency band meets the above frequency condition, the fault branch directional protection can measure positive capacitive reactive power.

[0170] C) Adaptive directional discrimination principle based on transient reactive polarity:

[0171] ​According to B), by selecting a suitable frequency, each healthy branch can be ensured to present inductive impedance characteristics, under which, in the frequency band, the inductive reactive power can be regarded as flowing from the fault power source to the micro-grid and injected into each healthy branch, at this time, the directional protection located in the fault branch measures the reverse (flowing to the switch station) inductive reactive power, and the directional protection located in the healthy branch measures the forward (flowing to the branch end load and distributed power) inductive reactive power, according to which, the protection can be designed to perform fault direction discrimination.

[0172] C.1) Selection of frequency band:

[0173] The Γ equivalent model is used for line equivalent, and its effectiveness is ensured below the first resonant frequency, therefore, the first constraint for determining the upper limit of the frequency is:

[0174]

[0175] In the formula, L i and C i are the inductance and capacitance values of the i-th line.

[0176] Next, when the DG is equivalent, in order to linearize the nonlinear structure, a certain frequency needs to be ensured to make the branch parallel to the filter capacitor negligible. When selecting the frequency, this condition needs to be met, therefore, the constraint for determining the lower limit of the frequency is:

[0177]

[0178] In the formula, C DC,i , L 1,i , and C i represent the DC side capacitance, inverter side inductance, and filter capacitance of the i-th distributed power source.

[0179] Subsequently, the selected frequency band needs to meet the two conditions in B.1) and B.2) respectively, so that the healthy branch presents a capacitive characteristic. Further, considering that in the transient response, the proportion of low frequency components is much higher than that of high frequency components, assuming that we perform high-pass filtering on the sampling signal, only the filter frequency needs to be ensured to fall within the range determined by the two conditions in B.1) and B.2), that is, the capacitive component can be ensured to occupy the main component. Therefore, the high-pass filtering frequency needs to be ensured not to exceed:

[0180]

[0181] In the formula, L 1,i , L 2,i , R 1,i , R 2,i , and C iL represents the equivalent inductance of the line, the equivalent inductance of the load, the line resistance, the equivalent resistance of the load, and the line-to-ground capacitance in the i-th line whose end is a load, respectively. 1,j L 3,j C 1,j C 3,j These represent the line equivalent inductance, distributed source equivalent inductance, line-to-ground capacitance, and distributed source equivalent capacitance of the j-th line containing a distributed source at its end, respectively.

[0182] In summary, the final selected filter frequency is:

[0183] f high =min{f max1 ,f max2 ,f max3} (25)

[0184] As long as the filtering frequency f high Greater than the lower limit f min By applying a high-pass filter to the data from each protection measurement point at this frequency, the filtered result will inevitably contain components where the reactive power of healthy branches is inductive and the reactive power of faulty branches is capacitive, with the proportion of these components being significantly higher than other components. Therefore, the filtered data can be used for subsequent transient reactive power polarity determination. On the other hand, the filtered signal can be considered as a fault component within a wide frequency band, and it still has a significant value in near-field faults, without exhibiting a dead zone problem.

[0185] C.2) Fault direction determination criteria:

[0186] After determining the frequency band used for protection, characteristic quantities are needed to characterize the reactive power polarity at the protection installation location. The reactive power of the faulted phase is selected as the characteristic quantity, and its instantaneous value can be obtained from the instantaneous values ​​of the phase current and the phase voltage after Hilbert transformation.

[0187]

[0188] In the formula, It uses the Hilbert transform to Each frequency signal lags behind the signal by 90 degrees in its respective frequency domain, that is:

[0189]

[0190] In the formula, h(t) is the time-domain form of the impulse response of the Hilbert converter. The Hilbert converter can be understood as an all-pass filter with an amplitude-frequency characteristic of 1, a positive frequency component phase shift of -90 degrees, and a negative frequency component phase shift of +90 degrees. Using the above instantaneous reactive power calculation formula, the reactive power of the fault phase in the characteristic interval can be calculated as follows:

[0191]

[0192] For positive inductive reactive power, the reactive power value is calculated as positive; for positive capacitive reactive power, its value is negative. Therefore, the sign of this transient broadband reactive power characteristic can be used to determine the direction of the fault.

[0193] C.2) Fault direction determination process:

[0194] Based on the above definitions, the direction determination scheme can be designed as follows:

[0195] 1) Based on overcurrent or undervoltage criteria, determine the fault and activate the direction discrimination function.

[0196] 2) Extract the electrical quantities of the current and voltage transient phases, and obtain the current and voltage sampling signals of a specific frequency band according to the high-pass filter determined by equation (25).

[0197] 3) Calculate the reactive power of the faulty phase according to formula (28). If the characteristic quantity is positive, the downstream line of the protection is a healthy line and the fault is located upstream of it; if the characteristic quantity is negative, there is a faulty line in the downstream line of the protection.

[0198] The reliability of the proposed protection principle was verified through simulation using the PSCAD platform. The simulation system is as follows: Figure 7 As shown, the system voltage is 10kV, connected to the external distribution network via a 110kV / 10kV transformer, and its on-grid and off-grid status can be switched. The system includes distributed power sources with various control strategies, among which DG1 is a photovoltaic power source controlled by a virtual synchronous machine, and DG2 is a PQ-controlled photovoltaic power source with a low-voltage ride-through control strategy.

[0199] Table 1

[0200]

[0201]

[0202] As shown in Table 1, considering the two parameter setting scenarios in the table above, where scenario 2 has all line lengths as lines...

[0203] Consider the two parameter settings in the table above. In scenario 2, all line lengths are three times that of line 1. This is to verify the applicability of the frequency band determination method to different line lengths. Based on the parameters listed in the table, the filtering frequencies of the characteristic frequency bands in the two scenarios are below 700Hz and 400Hz, respectively. Therefore, 500Hz can be used for filtering in scenario 1 and 300Hz for scenario 2, and the filtered electrical quantities can be used to construct the characteristic quantities.

[0204] The simulation results of the first parameter setting case in the above table are described below, and the simulation results of the other case are similar and will not be described again. In the off-grid state, when F1 fault occurs, the switch station bus voltage and the current measured by each protection Figure 8 (a) is shown. High-pass filtering is performed using the above frequency, and the filtered waveform is 8(b). As can be seen from the figure, the filtered wide-frequency transient voltage and current are concentrated within 5 ms after the fault occurs, and the time for determining the protection criterion is earlier than the time for the DG control system to respond to the fault. Then, the value of the reactive power characteristic quantity is calculated using the filtered electrical quantity, and the instantaneous reactive power at each sampling point is plotted as shown in Figure 8 (c).

[0205] In Figure 8 (c), the calculated values of normal line 1 and normal line 2 are close, and the integral characteristic quantity value is obviously positive, that is, the positive inductive reactive power flows through the protection installation during the wide-frequency transient process; in contrast, the reactive power characteristic quantity of the fault line is negative, that is, the reverse inductive reactive power flows through the protection installation during the wide-frequency transient process. According to the proposed protection principle, it can be accurately determined that the fault is located downstream of the line 2 protection, and the fault location can be accurately determined.

[0206] Similarly, the fault of F2 in the off-grid state is verified, and the wide-frequency transient reactive instantaneous value is plotted as shown in Figure 9 .

[0207] Integrating the instantaneous value gives the reactive characteristic quantity. As can be seen, the calculated wide-frequency transient reactive characteristic quantity is positive for the normal line within a short time after the fault, which represents positive inductive reactive power; for the fault line, it is negative, which represents reverse inductive. According to the characteristic value and criterion, it can be accurately determined that the fault is located in line 3.

[0208] For the near-zone fault F3, the wide-frequency transient instantaneous value of each sampling point is calculated as Figure 10 , which is compared with Figure 9 . It can be seen that when the near-zone fault occurs, the order of magnitude of the fault wide-frequency transient instantaneous value of each sampling point is larger than that of the far zone, which is easier to identify the fault direction. This shows that the proposed principle will not have a dead zone problem due to the near-zone fault.

[0209] Embodiment 2:

[0210] The application also proposes a micro-grid fault adaptive direction discrimination system 200 based on transient reactive polarity, as shown in Figure 11 , comprising:

[0211] The modeling unit 201 is configured to perform equivalent modeling on a micro-grid transient fault component network, to obtain equivalent models of lines and distributed power supplies, and to construct a system-wide broadband transient equivalent model of the micro-grid based on the equivalent models of the lines and the distributed power supplies.

[0212] The frequency band selection unit 202 is configured to extract fault transient quantity polarity characteristics based on the system-wide broadband transient equivalent model, to obtain transient characteristics and transient reactive polarity characteristics of healthy branches and fault branches.

[0213] The discrimination unit 203 is configured to select feature frequency bands for the transient characteristics and the transient reactive polarity characteristics, to discriminate the polarity of the features in the selected feature frequency bands, and to discriminate the fault direction of the micro-grid according to the discriminated polarity.

[0214] The transient characteristics and the transient reactive polarity characteristics of the healthy branches and the fault branches include:

[0215] The transient characteristics of the healthy branches containing only loads, the transient reactive polarity characteristics of the healthy branches containing loads and DGs, and the transient reactive polarity characteristics of the fault branches.

[0216] The extraction of the fault transient quantity polarity characteristics based on the system-wide broadband transient equivalent model to obtain the transient characteristics and the transient reactive polarity characteristics of the healthy branches and the fault branches includes:

[0217] Based on the system-wide broadband transient equivalent model, frequency calculation models are established for the healthy branches containing only loads and the healthy branches containing loads and DGs, and the transient characteristics of the healthy branches containing only loads and the transient reactive polarity characteristics of the healthy branches containing loads and DGs are determined based on the calculation results of the resonance calculation models.

[0218] Based on the direction protection measurement values on the fault branches, the transient reactive polarity characteristics of the fault branches are determined at a frequency that is inductive to the outside in the healthy branches containing only loads and the healthy branches containing loads and DGs.

[0219] The selection of the feature frequency bands for the transient characteristics and the transient reactive polarity characteristics includes:

[0220] Constraint conditions are established for the upper limit and the lower limit of the high-pass filtering frequency, including:

[0221] The constraint condition for the upper limit of the resonance frequency is as follows:

[0222]

[0223] Wherein, f max1 is the upper limit of the high-pass filtering frequency, L i and C iThe inductance and capacitance values of the i-th line, respectively;

[0224] The constraint condition of the lower limit of the high-pass filter frequency is as follows:

[0225]

[0226] Wherein, f min is the lower limit of the resonance frequency, C DC,i and L 1,i are the inductance and capacitance values of the i-th distributed power source, respectively.

[0227] The constraint condition of the high-pass filter frequency is set again as follows:

[0228]

[0229] Wherein, f max2 and f max3 are the upper limits of the high-pass filter frequency, L 2,i , R 1,i and R 2,i are the equivalent inductance of the load, the line resistance and the equivalent resistance of the load in the i-th line ending with the load, respectively, L 1,j , L 3,j , C 1,j and C 3,j are the line equivalent inductance, the distributed power source equivalent inductance, the line-to-ground capacitance and the distributed power source equivalent capacitance in the j-th line ending with the distributed power source.

[0230] The final high-pass filter frequency limit is set as follows:

[0231] f high = min{f max1 , f max2 , f max3}

[0232] Therefore, the characteristic frequency band is selected, that is, the characteristic frequency band satisfying greater than f min and less than f high is selected.

[0233] Wherein, the polarity of the characteristics in the selected characteristic frequency band is identified, and the fault direction of the microgrid is identified according to the identified polarity, including:

[0234] Based on the characteristics in the selected characteristic frequency band, the fault phase reactive power is calculated, and the fault phase reactive power is taken as a characteristic quantity, if the characteristic quantity is positive, the downstream line is a healthy line and the fault is located upstream thereof; if the characteristic quantity is negative, there is a fault line in the downstream line.

[0235] Wherein, the calculation formula of the fault phase reactive power is as follows:

[0236]

[0237] wherein, is a characteristic quantity, is the instantaneous reactive power of the fault phase, t0 is the starting moment of the fault, t0+T needs to be less than the moment when the inverter first controls intervention, and 2ms<T<5ms can be allowed.

[0238] The application solves the problem that the traditional fault direction discrimination principle is easily affected by the differentiated control strategy of the distributed power supply by discriminating the polarity of the selected characteristics in the characteristic frequency band.

[0239] Embodiment 3:

[0240] Based on the same inventive concept, the application further provides a computer device, which comprises a processor and a memory, the memory is used for storing a computer program, the computer program comprises program instructions, and the processor is used for executing the program instructions stored in the computer storage medium. The processor can be a central processing unit (CPU), and can also be other general-purpose processors, digital signal processors (DSP), application specific integrated circuits (ASIC), field-programmable gate arrays (FPGA) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components and the like. It is the computing core and control core of the terminal, and is suitable for implementing one or more instructions. Specifically, it is suitable for loading and executing one or more instructions in the computer storage medium to implement a corresponding method process or a corresponding function, so as to implement the steps of the method in the above embodiments.

[0241] Embodiment 4:

[0242] Based on the same inventive concept, the present application also provides a storage medium, specifically a computer readable storage medium (Memory), which is a memory device in a computer device, used for storing programs and data. It can be understood that the computer readable storage medium here can include the built-in storage medium in the computer device, and of course can also include the extended storage medium supported by the computer device. The computer readable storage medium provides a storage space, which stores the operating system of the terminal. In addition, one or more instructions suitable for being loaded and executed by the processor are also stored in the storage space, and these instructions can be one or more computer programs (including program codes). It should be noted that the computer readable storage medium here can be a high-speed RAM memory, or a non-volatile memory such as at least one disk memory. One or more instructions stored in the computer readable storage medium can be loaded and executed by the processor to realize the steps of the method in the above embodiments.

[0243] Those skilled in the art should understand that the embodiments of the present application can be provided as a method, a system, or a computer program product. Therefore, the present application can take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can take the form of a computer program product implemented on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROMs, optical storage, etc.) containing computer-usable program code. The solutions in the embodiments of the present application can be implemented in various computer languages, such as the object-oriented programming language Java and the interpreted scripting language JavaScript.

[0244] The present application is described with reference to flowcharts and / or block diagrams of the method, device (system), and computer program product according to the embodiments of the present application. It should be understood that each flow and / or block in the flowcharts and / or block diagrams, and the combination of the flows and / or blocks in the flowcharts and / or block diagrams can be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device produce a device that implements the functions specified in the flowcharts and / or block diagrams. Figure 1 The functions specified in one flow or multiple flows and / or blocks Figure 1 The device that implements the functions specified in one flow or multiple flows and / or blocks.

[0245] These computer program instructions can also be stored in a computer- readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instructions which implement the Figure 1 function specified in the flow or flows and / or blocks Figure 1 of the block or blocks.

[0246] The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions that are executed on the computer or other programmable apparatus provide steps for implementing the Figure 1 function specified in the flow or flows and / or blocks Figure 1 of the block or blocks.

[0247] While the preferred embodiments of the application have been described, additional variations and modifications can be made to the embodiments by those of skill in the art once they have the benefit of the present disclosure. Therefore, the appended claims are intended to encompass within their scope all possible variations and modifications of the preferred embodiments. 1

[0248] It is apparent that a person skilled in the art can make various changes and modifications to the application without departing from the spirit and scope thereof. Therefore, if these modifications and changes fall within the scope of the claims and their equivalents, it is intended to include them in the application.

Claims

1. A microgrid fault adaptive direction discrimination method based on transient reactive polarity, characterized in that, Comprise: For micro-grid transient fault component network equivalent modeling, get the equivalent model of line and distributed power supply, and based on the equivalent model of line and distributed power supply, build the system wideband transient equivalent model of micro-grid; Based on the system wideband transient equivalent model, the polarity feature of fault transient quantity is extracted, and the transient feature and transient reactive polarity feature of healthy branch and fault branch are obtained; For the transient feature and transient reactive polarity feature, the feature frequency band is selected, the polarity of the feature in the selected feature frequency band is judged, and the fault direction of the micro-grid is judged according to the judged polarity.

2. The method of claim 1, wherein, The transient feature and transient reactive polarity feature of the healthy branch and the fault branch include: Only containing load healthy branch transient feature, containing load and DG healthy branch transient reactive polarity feature and fault branch transient reactive polarity feature.

3. The method of claim 1, wherein, Based on the system wideband transient equivalent model, the polarity feature of fault transient quantity is extracted, and the transient feature and transient reactive polarity feature of healthy branch and fault branch are obtained, including: Based on the system wideband transient equivalent model, for only containing load healthy branch and containing load and DG healthy branch, the frequency calculation model is established, and based on the calculation result of the resonance calculation model, the transient feature of only containing load healthy branch and the transient reactive polarity feature of containing load and DG healthy branch are determined; Based on the direction protection measurement value on the fault branch, under the inductive frequency of only containing load healthy branch and containing load and DG healthy branch, the transient reactive polarity feature of fault branch is determined.

4. The method of claim 1, wherein, The selection of feature frequency band for the transient feature and transient reactive polarity feature includes: The upper and lower limits of high-pass filtering frequency are established, including: The upper limit of resonance frequency constraint condition is as follows: where f max1 is the upper limit of the high-pass filter frequency, L i and C i are the inductance and capacitance values of the i-th line, respectively. The lower limit of high-pass filtering frequency constraint condition is as follows: Wherein, f min is the lower limit of the resonance frequency, C DC,i and L 1,i respectively, the DC side capacitance of the i-th distributed power supply, the inverter side inductance; The constraint condition of high-pass filtering frequency is set again, as follows: wherein f max2 and f max3 are upper limits of the high-pass filter frequency, L 2,i , R 1,i and R 2,i are the equivalent inductance, resistance and resistance of the load in the i-th line with a load at the end, L 1,j , L 3,j , C 1,j and C 3,j are the equivalent inductance of the line, the equivalent inductance of the distributed power supply, the line-to-ground capacitance and the equivalent capacitance of the distributed power supply in the j-th line with a distributed power supply at the end. Set the final high-pass filtering frequency limit value, as follows: f high = min{f max1 ,f max2 ,f max3} Therefore, the selection of the characteristic frequency band, i.e., the selection of the characteristic frequency band satisfying f min and less than f high is performed.

5. The method of claim 1, wherein, The polarity of the feature in the selected feature frequency band is judged, and the fault direction of the micro-grid is judged according to the judged polarity, including: Based on the feature in the selected feature frequency band, the fault phase reactive power is calculated, and the fault phase reactive power is taken as the feature quantity. If the feature quantity is positive, the downstream line of the protection is healthy line, and the fault is located on the upstream thereof; If the feature quantity is negative, there is a fault line in the protection downstream line.

6. The method of claim 1, wherein, The calculation formula of the fault phase reactive power is as follows: wherein, is a characteristic quantity, is the instantaneous reactive power of the faulty phase, t0is the time instant of the fault initiation, t0+ Tneed is less than the time instant of the first intervention of the inverter control, which can be set to 2 ms < T < 5 ms.

7. A microgrid fault adaptive direction discrimination system based on transient reactive polarity, characterized in that, Comprise: Modeling unit, for micro-grid transient fault component network equivalent modeling, get the equivalent model of line and distributed power supply, and based on the equivalent model of line and distributed power supply, build the system wideband transient equivalent model of micro-grid; Frequency band selection unit, for extracting the polarity feature of fault transient quantity based on the system wideband transient equivalent model, obtaining the transient feature and transient reactive polarity feature of healthy branch and fault branch; The discrimination unit is configured to select a feature frequency band according to the transient feature and the transient reactive polarity feature, discriminate the feature in the selected feature frequency band, and discriminate the fault direction of the micro-grid according to the discriminated polarity.

8. The microgrid fault adaptive direction discrimination system of claim 7, wherein, The transient feature and the transient reactive polarity feature of the healthy branch and the fault branch include: The transient feature of the healthy branch only containing the load, the transient reactive polarity feature of the healthy branch containing the load and the DG, and the transient reactive polarity feature of the fault branch.

9. The microgrid fault adaptive direction discrimination system of claim 7, wherein, The method for extracting the fault transient quantity polarity feature based on the system broadband transient equivalent model to obtain the transient feature and the transient reactive polarity feature of the healthy branch and the fault branch includes: Based on the system broadband transient equivalent model, a frequency calculation model is established for the healthy branch only containing the load and the healthy branch containing the load and the DG, and the transient feature of the healthy branch only containing the load and the transient reactive polarity feature of the healthy branch containing the load and the DG are determined based on the calculation result of the resonance calculation model; Based on the directional protection measurement value on the fault branch, the transient reactive polarity feature of the fault branch is determined under the frequency that is inductive to the outside for the healthy branch only containing the load and the healthy branch containing the load and the DG.

10. The microgrid fault adaptive direction discrimination system of claim 7, wherein, The method for selecting the feature frequency band according to the transient feature and the transient reactive polarity feature includes: Constraint conditions are established for the upper limit and the lower limit of the high-pass filtering frequency, including: The constraint condition for the upper limit of the resonance frequency is as follows: where f max1 is the upper limit of the high-pass filter frequency, L i and C i are the inductance and capacitance values of the i-th line, respectively. The constraint condition for the lower limit of the high-pass filtering frequency is as follows: wherein f min is the lower limit of the resonant frequency, C DC,i and L 1,i are the DC-side capacitance and the inverter-side inductance of the i-th distributed power source, respectively. The constraint condition for the high-pass filtering frequency is set again, and is as follows: wherein f max2 and f max3 are upper limits of the high-pass filter frequency, L 2,i , R 1,i and R 2,i are the equivalent inductance, resistance and resistance of the load in the i-th line with a load at the end, L 1,j , L 3,j , C 1,j and C 3,j are the equivalent inductance of the line, the equivalent inductance of the distributed power supply, the line-to-ground capacitance and the equivalent capacitance of the distributed power supply in the j-th line with a distributed power supply at the end. The final high-pass filtering frequency limit value is set, and is as follows: f high = min{f max1 ,f max2 ,f max3} Therefore, the selection of the characteristic frequency band, i.e., the selection of the characteristic frequency band satisfying f min and less than f high .

11. The microgrid fault adaptive direction discrimination system of claim 7, wherein, The method for discriminating the polarity of the feature in the selected feature frequency band and discriminating the fault direction of the micro-grid according to the discriminated polarity includes: Based on the feature in the selected feature frequency band, the fault phase reactive power is calculated, and the fault phase reactive power is taken as a feature quantity. If the feature quantity is positive, the downstream line of the protection is a healthy line, and the fault is located on the upstream thereof. If the feature quantity is negative, there is a fault line in the downstream line of the protection.

12. The microgrid fault adaptive direction discrimination system of claim 7, wherein, The calculation formula of the fault phase reactive power is as follows: wherein, is a characteristic quantity, is the instantaneous reactive power of the faulty phase, t0is the time instant of the fault initiation, t0+ Tneed is less than the time instant of the first intervention of the inverter control, which can be set to 2 ms < T < 5 ms.

13. A computer device, comprising: including: One or more processors; The processor is configured to execute one or more programs; When the one or more programs are executed by the one or more processors, the method in any one of claims 1-6 is implemented.

14. A computer-readable storage medium, characterized in that, The computer program is stored thereon, and when the computer program is executed, the method in any one of claims 1-6 is implemented.

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