Alternating current and direct current hybrid power distribution network protection method and system based on current abrupt change direction

Abstract

The invention relates to the technical field of power distribution network operation and maintenance, in particular to an alternating current and direct current hybrid power distribution network protection method and system based on a current abrupt change direction, and the method comprises the steps: obtaining a current value of a current sampling point in an alternating current and direct current hybrid power distribution network and an average value of a set number of sampling data before the current sampling point as a reference value; subtracting the reference value from the current value of the current sampling point to obtain a current break variable; determining a current abrupt change direction according to the value of the current abrupt change quantity, and further determining a change condition of the current abrupt change direction by utilizing a set interval range; and determining a fault type and executing a corresponding protection action according to the voltage, the current, the current abrupt change direction and the corresponding change condition obtained in the AC / DC hybrid power distribution network. According to the characteristics of the AC / DC hybrid power distribution network, the current break variable is calculated by adopting an average difference method, and the direction criterion is improved and optimized, so that a fault interval can be quickly identified, and a DC fault and an AC fault can be correctly distinguished.

Description

Technical Field

[0001] The present invention relates to the technical field of distribution network operation and maintenance, and in particular to an AC / DC hybrid distribution network protection method and system based on current mutation direction. Background Art

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

[0003] An AC / DC hybrid distribution network is a distribution system that mixes AC and DC distribution networks. This system combines the advantages of AC and DC distribution networks and can improve the energy efficiency, reliability and flexibility of the power system.

[0004] A high-penetration distributed power source is connected to the hybrid DC distribution network, and its equipment contains a large number of power electronic devices, so its fault characteristics are very different from those of the traditional AC distribution network. Affected by the characteristics of nonlinear power electronic devices such as converter thyristors and the control method of the converter station, the fault characteristics of the hybrid DC distribution network are complex, rich in harmonics, and difficult to handle.

[0005] For example, when a short circuit occurs in a DC line, due to the low inertia of the DC distribution network, the fault current will rise to dozens of times the maximum normal current in a short time of milliseconds. After the distribution network converter station closes the converter valve, the fault current will not be directly interrupted, but will inject energy into the fault point through the thyristor anti-parallel diode with poor overcurrent capacity.

[0006] In summary, when a fault occurs in an AC / DC hybrid distribution network, the fault characteristics are complex and the harmonic content in the characteristics is rich, making it difficult to handle and protect the fault. Summary of the invention

[0007] In order to solve the technical problems existing in the above-mentioned background technology, the present invention provides an AC / DC hybrid distribution network protection method and system based on current mutation direction. According to the characteristics of the AC / DC hybrid distribution network, the average difference method is adopted to calculate the current mutation amount, and the direction judgment criterion is improved and optimized, so as to quickly identify the fault interval and correctly distinguish DC faults from AC faults.

[0008] In order to achieve the above object, the present invention adopts the following technical solution:

[0009] A first aspect of the present invention provides an AC / DC hybrid distribution network protection method based on current mutation direction, comprising the following steps:

[0010] The current value of the current sampling point in the AC / DC hybrid distribution network is obtained, and the average value of the set number of sampling data before the current sampling point is used as the reference value. The current mutation amount is obtained by subtracting the reference value from the current value of the current sampling point.

[0011] Determine the current mutation direction according to the value of the current mutation amount, and further determine the change of the current mutation direction using the set interval range;

[0012] According to the voltage, current, current mutation direction and corresponding changes obtained in the AC / DC hybrid distribution network, the fault type is determined and the corresponding protection action is performed.

[0013] Furthermore, the set interval range is used to further determine the direction of the current mutation. Specifically, when the current mutation amount is within the set interval range, the current mutation direction remains unchanged. When it exceeds the set interval range, the current mutation direction changes.

[0014] Furthermore, the set interval range is used to further determine the direction of the current mutation, specifically:

[0015]

[0016] Where P dc is the direction of current mutation. Δi is the amount of current mutation, Δi xn·set Indicates the setting value of the current mutation amount. 1 indicates that the detected current direction is positive, 0 indicates that the detected current direction is negative, and X indicates that the value is the same as the previous judgment result, that is, the mutation direction remains unchanged.

[0017] Furthermore, the current mutation setting value is used to avoid the current changes caused by the load shedding operation of the distribution network. The current mutation setting value is 0.1I N The product of the reliability coefficient.

[0018] Furthermore, according to the voltage, current, current mutation direction and corresponding changes obtained in the AC / DC hybrid distribution network, the fault type is determined and the corresponding protection action is performed, including:

[0019] According to the change of current during the capacitor discharge of the converter submodule in the AC / DC hybrid distribution network, a short circuit fault is determined.

[0020] Furthermore, in case of a short circuit fault, the fault section is determined by comparing the sudden change directions of the current at both ends of the line, and protection is performed through tripping action.

[0021] Furthermore, according to the voltage, current, current mutation direction and corresponding changes obtained in the AC / DC hybrid distribution network, the fault type is determined and the corresponding protection action is performed, including:

[0022] In the AC / DC hybrid distribution network, the sum of the currents of all input buses I SB With set value If the sum is greater than the set value, it is considered a bus fault.

[0023] Furthermore, when a busbar fault occurs, protection is achieved by disconnecting all circuit breakers connected to the busbar.

[0024] Furthermore, based on the least square smoothing filter algorithm, the current and voltage signals collected on the converter side after the short circuit fault are processed to remove the noise signals in the sampled data.

[0025] A second aspect of the present invention provides an AC / DC hybrid distribution network protection system based on current mutation direction, comprising:

[0026] The current mutation amount acquisition module is configured to: obtain the current value of the current sampling point in the AC / DC hybrid distribution network and the average value of the set number of sampling data before the current sampling point as a reference value, and subtract the reference value from the current value of the current sampling point to obtain the current mutation amount;

[0027] The mutation direction determination module is configured to: determine the current mutation direction according to the value of the current mutation amount, and further determine the change of the current mutation direction using a set interval range;

[0028] The fault judgment and action protection module is configured to determine the fault type and perform corresponding protection actions according to the voltage, current, current mutation direction and corresponding changes obtained in the AC / DC hybrid distribution network.

[0029] Compared with the prior art, one or more of the above technical solutions have the following beneficial effects:

[0030] Current mutation direction protection is based on current change rate protection. Judging the direction of current mutation can improve the reliability of fault interval identification. In traditional methods, after obtaining the current mutation amount, the current mutation direction can be judged according to its specific value, but only the positive and negative mutation values ​​are noted. In some faults in AC / DC hybrid distribution networks, such as when a high-resistance fault occurs, the current mutation amount may not be obvious, and the traditional direction judgment is prone to misjudgment. This scheme uses the average difference method to calculate the current mutation amount, and uses the set threshold interval orientation to optimize the direction judgment, determine the change in the current mutation direction, and can quickly identify the fault interval and correctly distinguish DC faults from AC faults. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0032] Figure 1 is a schematic diagram of a topological structure of a hand-in-hand AC / DC hybrid distribution network provided by one or more embodiments of the present invention;

[0033] Figure 2 is a schematic diagram of bipolar short-circuit current in an AC / DC hybrid distribution network provided by one or more embodiments of the present invention;

[0034] Figure 3 is a submodule capacitor discharge equivalent circuit diagram provided by one or more embodiments of the present invention;

[0035] Figure 4 is an equivalent circuit diagram of AC side current feeding when a capacitor is discharged provided by one or more embodiments of the present invention;

[0036] Figure 5 is an AC side current feeding equivalent circuit diagram provided by one or more embodiments of the present invention;

[0037] Figure 6 is a bridge arm reactance freewheeling equivalent circuit diagram provided by one or more embodiments of the present invention;

[0038] Figure 7 It is a schematic diagram of the DC current value of the DC side output response of the MMC in the case of a bipolar short circuit fault provided by one or more embodiments of the present invention;

[0039] Figure 8 It is a schematic diagram of the DC voltage value of the DC side output response of the MMC in the case of a bipolar short circuit fault provided by one or more embodiments of the present invention;

[0040] Fig. 9 is a schematic diagram of output current of a DAB high voltage side output response to a bipolar short circuit fault provided by one or more embodiments of the present invention;

[0041] Fig.10 It is a schematic diagram of the inter-pole voltage of the DAB high-voltage side output response to a bipolar short-circuit fault provided by one or more embodiments of the present invention;

[0042] Fig.11 is a fault additional state equivalent circuit diagram provided by one or more embodiments of the present invention;

[0043] Fig.12 is a busbar protection flow chart provided by one or more embodiments of the present invention;

[0044] Fig.13 is a bipolar short circuit fault protection action flow chart provided by one or more embodiments of the present invention;

[0045] Fig.14 Schematic diagram of voltage waveform before filtering provided by one or more embodiments of the present invention (0-10kV;

[0046] Fig.15 is a schematic diagram of a voltage waveform before filtering provided by one or more embodiments of the present invention (0-15kV;

[0047] Fig.16 is a schematic diagram of a voltage waveform after filtering provided by one or more embodiments of the present invention (0-10kV;

[0048] Fig.17 is a schematic diagram of a voltage waveform after filtering provided by one or more embodiments of the present invention (0-15kV;

[0049] Fig.18 It is a schematic diagram of voltage waveform at the IED on the left side of the L1 line (internal fault) provided by one or more embodiments of the present invention;

[0050] Fig.19 1 is a schematic diagram of a voltage waveform at the IED on the left side of the L2 line (out-of-zone fault) provided by one or more embodiments of the present invention;

[0051] Fig. 20 It is a schematic diagram of the waveform of the current change rate at the IED on the left side of the L1 line (intra-area fault) provided by one or more embodiments of the present invention;

[0052] Fig.21 1 is a schematic diagram of a waveform of a current change rate at an IED on the left side of the L2 line (out-of-zone fault) provided by one or more embodiments of the present invention;

[0053] Fig. 22 It is a schematic diagram of the waveform of the current change rate at the IED on the right side of the L1 line (internal fault) provided by one or more embodiments of the present invention;

[0054] Fig.23 1 is a schematic diagram of a current change rate waveform at the IED on the right side of the L2 line (out-of-zone fault) provided by one or more embodiments of the present invention;

[0055] Fig.24 It is a schematic diagram of a DC line double-stage fault through a 0.5 ohm resistor at the installation of L1 line fault protection provided by one or more embodiments of the present invention;

[0056] Fig.25 It is a schematic diagram of a DC line double-stage fault through a 1-ohm resistor at the installation of L1 line fault protection provided by one or more embodiments of the present invention;

[0057] Fig.26 It is a schematic diagram of a DC line double-stage fault through a 3-ohm resistor at the installation of L1 line fault protection provided by one or more embodiments of the present invention;

[0058] Fig. 27 It is a schematic diagram of a DC line double-stage fault through a 5-ohm resistor at the installation of L1 line fault protection provided by one or more embodiments of the present invention;

[0059] Fig.28is a schematic diagram of the voltage value on the DC side when the three-phase short circuit on the AC side is provided by one or more embodiments of the present invention;

[0060] Fig.29 is a schematic diagram of the rate of change of the DC side current when the AC side three-phase short circuit is provided by one or more embodiments of the present invention;

[0061] Fig.30 is a schematic diagram of the voltage value on the DC side when two phases on the AC side are short-circuited, provided by one or more embodiments of the present invention;

[0062] Fig.31 is a schematic diagram of the rate of change of the DC side current when two phases of the AC side are short-circuited, provided by one or more embodiments of the present invention;

[0063] Fig.32 is a schematic diagram of current waveforms of each node when bus B fails, provided by one or more embodiments of the present invention;

[0064] Fig.33 It is a schematic diagram of ISB current waveforms under different faults provided by one or more embodiments of the present invention. DETAILED DESCRIPTION

[0065] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0066] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used in this embodiment have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0067] As introduced in the background technology, in the AC / DC hybrid distribution network, affected by the characteristics of nonlinear power electronic devices such as converter thyristors and the control method of the converter station, the fault characteristics of the hybrid DC distribution network are complex, the harmonic content is rich, and it is difficult to handle.

[0068] Therefore, the following embodiments provide an AC / DC hybrid distribution network protection method and system based on current mutation direction. According to the characteristics of the AC / DC hybrid distribution network, the average difference method is used to calculate the current mutation amount, and the direction judgment criterion is improved and optimized, so that the fault interval can be quickly identified and DC faults and AC faults can be correctly distinguished.

[0069] Embodiment 1:

[0070] The AC / DC hybrid distribution network protection method based on current mutation direction includes the following steps:

[0071] The current value of the current sampling point in the AC / DC hybrid distribution network and the average value of the current values ​​of a set number before the current sampling point are obtained as the reference value, and the current mutation amount is obtained by subtracting the reference value from the current value of the current sampling point;

[0072] The current mutation direction is determined according to the value of the current mutation amount, and the change of the current mutation direction is further determined by using the set interval range;

[0073] According to the voltage, current, current mutation direction and corresponding changes obtained in the AC / DC hybrid distribution network, the fault type is determined and the corresponding protection action is performed.

[0074] This embodiment uses Figure 1 As shown in the figure, a double-end hand-in-hand AC / DC hybrid distribution network model based on a ±10kV voltage level is taken as an example. In the model structure of the distribution network, a branch line of the 110kV AC main power supply on both sides is supplied to the DC distribution network after the transformer steps down. The AC line is first connected to the MMC converter, the core component of energy conversion. The voltage after rectification by the converter is plus or minus 10kV, and the control mode of the converters on both sides is master-slave control. In the figure, L1~L3 are cable lines, which are connected to the converters at both ends. There is a branch line at the end of the L1 line and the beginning of the L3 line connected to the distribution terminal through DBA, which reduces the 10kV DC distribution voltage to a low voltage of 400V. In order to fully reflect the DC load situation as much as possible, the DC load designed in this embodiment is a hybrid DC load, including a 1MW energy storage system, a photovoltaic power generation system rated at 0.54MW and a 1.0MW DC load. The main parameters of the DC distribution system are shown in Table 1.

[0075] Table 1 Main parameters of DC power distribution system

[0076]

[0077]

[0078] Take the DC line bipolar short circuit fault as an example. Assume that a bipolar short circuit fault occurs in the distribution line L2. Figure 2 As shown in the figure, the DC line voltage drops rapidly to 0 potential, the current increases rapidly, and the MMC converters and DABs on both sides of the line feed energy to the fault point. The fault current is divided into two parts according to the feed type, which can be solved separately using the superposition theorem.

[0079] MMC converter fault response. When the MMC converter acts alone, when a bipolar short circuit occurs in the DC line, the current surges. When the MMC detects that the current exceeds the threshold, it will immediately lock the converter valve. Depending on whether the submodule is locked, the MMC fault transient process can be divided into two stages: before and after locking.

[0080] Before the converter is locked: After the bipolar short circuit, the line voltage drops rapidly, and the converter station still switches the submodules under normal operation. At this time, the energy stored in the capacitors of the submodules in the switched-on state will be immediately released to the fault point. In addition, the AC power supply will also feed energy to the fault point through the diodes in the converter valve. The discharge of the submodule capacitors accounts for the majority of the two, so the fault transient process before the converter is locked is also called the capacitor discharge stage. Figure 3-Figure 4 shown.

[0081] When estimating the fault current at this stage, the secondary AC side feed current is ignored to simplify the calculation. Figure 3 The transient process of this stage can be expressed as follows by topologically writing differential equations:

[0082]

[0083] In the formula,

[0084] Since the transition resistance of the inter-electrode short-circuit fault is very small, that is, Therefore, the submodule capacitor discharge stage is a second-order underdamped oscillation process. Assuming that the DC side voltage before the fault is U0 and the current is I0, substitute into equation (1) and solve for the submodule capacitor voltage u c and the fault current i dc They are:

[0085]

[0086] Where σ = R eq 2L eq , θ=arcsin(U0 / A),

[0087] After the converter station is locked: After a DC fault occurs, as the current flowing through the converter arm increases, the converter's own protection starts, locking all SM modules of the converter. In the initial stage after locking, due to the existence of the bridge arm reactor, the freewheeling diode will always be turned on, the AC power will still feed current to the fault point through the diode, and the freewheeling current in the bridge arm reactor will also flow to the fault point. The equivalent circuit diagram is as follows Figure 5-Figure 6 In this stage, the AC power supply provides short-circuit current to the DC side through the AC impedance and the bridge arm impedance. The DC line is equivalent to a conductor, and the DC current is only provided by the bridge arm reactor follow-up current and attenuates in the form of RL.

[0088] Assume that at the moment of MMC locking, the DC side current is I0' and the AC side phase voltage is Ignoring the attenuated DC component in the AC feed, substituting it into equation (2-6) yields the AC feed current i s and DC fault current idc are:

[0089]

[0090] In the formula,

[0091]

[0092] From this, the short-circuit currents of the upper and lower bridge arms are:

[0093]

[0094] When a bipolar short circuit fault occurs in the DC line, the high-voltage side of the DAB is equivalent to being shorted through a small resistor. The high-voltage side capacitor discharges to the fault point through the DC line. The fault characteristics of this stage are similar to the first-stage fault characteristics of the MMC. After the capacitor energy is released, the anti-parallel diode of the thyristor in the DAB is turned on, the DC load side is short-circuited, and the load side inductor forms a second-order discharge loop with the fault point through the freewheeling diode. Due to the working characteristics and control strategy of the DAB, the low-voltage side voltage remains constant and the load system can still work normally.

[0095] The above situation is simulated. In the simulation diagram, in the case of blue and red double lines, the blue line is defined to represent the positive pole and the red line to represent the negative pole. Assume that a bipolar permanent short circuit fault occurs in the DC line L2 at t = 2s. The fault characteristics of the converters MMC1 and DAB1 are as follows: Figure 7-10 As shown in the figure, after the DC bipolar fault, the MMC voltage drops sharply and the current rises sharply, reaching nearly thirty times the rated current. The current on the DAB high-voltage side decreases at the moment of the fault, then the oscillation decays, and finally stabilizes to a constant. The fault voltage on the DAB high-voltage side drops and quickly stabilizes.

[0096] Analyze the principle of current mutation direction. Current mutation direction protection is based on current change rate protection, and adds direction discrimination elements to improve the reliability of fault interval identification. The DC line is analyzed using the superposition theorem in AC system fault analysis. The fault state can be equivalent to the normal operation state superimposed on the fault additional state. Taking the positive pole fault as an example, when a fault occurs in the area, the DC voltage decreases, which is equivalent to superimposing a negative voltage source u on the fault point. f ,like Fig.11 shown.

[0097] Z R1 and Z L1 are the line impedances from the fault point to the protection measurement points at both ends of the section; Z2 is the line impedance at the protection measurement point in the normal section; R f is the grounding resistance of the fault branch; Z S is the equivalent resistance of the converter station; Δi R1 and Δi L1are the current mutations measured at both ends of the positive line fault section; Δi R2 and Δi L2 It is the current mutation measured by the protection at both ends of the line in the normal section. As can be seen from the figure, when there is a fault in the zone, the current directions at both ends are pointing to the line, and when there is a fault outside the zone, the current directions at both ends are the same.

[0098] The most direct way to calculate the direction of the current mutation is to subtract the current value at the previous moment from the current value detected at the current moment, and determine the mutation direction based on the positive or negative of the differential current. However, the AC / DC hybrid distribution line is a dynamically stable system. Normal operating conditions such as load switching and changes in operating mode will also cause current mutations, and the current and voltage waveforms collected by the relay protection equipment will also be affected by noise, clutter, and harmonic disturbances caused by AC faults. Therefore, the reference value of the current mutation needs to be more stable to improve anti-interference capabilities. In this embodiment, the average value of the 10 sampling data before the current sampling point is selected as the reference value of the mutation, and the current mutation is obtained by subtracting the reference from the current sampling value. The specific formula is as follows:

[0099]

[0100] In the formula, i xn is the current value detected at present, i (δ) is the sampling value before the current moment, Δi xn is the desired current mutation amount.

[0101] After obtaining the current mutation amount, the current mutation direction can be determined according to its specific value. The traditional mutation direction only pays attention to the positive and negative of the mutation value, and uses this as a criterion to obtain the mutation direction parameter. From the previous analysis, it can be seen that when a transition resistance fault occurs, the current mutation amount may not be obvious. At this time, the direction criterion is prone to misjudgment. Based on this, this embodiment selects a threshold to optimize the direction criterion: when the value of the current mutation amount is higher than the maximum threshold value or lower than the minimum threshold value, the current direction changes; and when the current mutation amount is within the threshold range, the current mutation direction remains unchanged.

[0102] In this embodiment, the selected threshold is the reliability coefficient obtained in the following formula (6). In order to optimize the transition resistance grounding situation, the traditional criterion is prone to inaction or detection failure. In the AC / DC hybrid distribution network, the fault on the AC side should be removed by the AC side, and the DC side protection needs to ensure that no false operation occurs. Taking the three-phase short circuit that is most prone to false operation as an example, if the AC three-phase grounding occurs, the DC side voltage oscillates but does not change much, and the current decreases rapidly. Since the current mutation caused by the AC fault is relatively gentle and the mutation amount is less than the threshold, the protection will not operate.

[0103] In order to avoid the current changes caused by normal operations such as load shedding in the distribution network, the setting value of the sudden change is 0.1IN , and multiplied by the reliability factor to ensure protection reliability, as shown in the following formula: Rated

[0104] Δi xn·set =K sen ×0.1I N (6)

[0105] In the formula, Δi xn·set Indicates the setting value of the current mutation; K sen It indicates the reliability coefficient, and its value range is generally between 1.1 and 1.3. The current mutation direction is represented by parameter P dc Indicates that I N is the rated current, and its criterion is as follows:

[0106]

[0107] In the formula, 1 represents that the detected current direction is positive, 0 represents that the detected current direction is negative, and X represents that the value is the same as the previous judgment result, that is, the mutation direction remains unchanged.

[0108] Protection start. In order to improve the sensitivity of DC protection and cut off DC faults in a timely and reliable manner, DC protection generally needs to add start-up criteria. When an AC fault occurs, the DC side current will fluctuate greatly. AC side faults should be caused by AC protection tripping the relevant circuit breakers according to the principle of zoning action, but if current protection start-up criteria are used, DC side protection is prone to misoperation. Compared with the current waveform, the DC side voltage fluctuation is relatively weak when the AC side fault occurs, and it basically remains unchanged. Therefore, the use of voltage start-up criteria can better avoid AC side faults.

[0109] When a single-pole or double-pole ground fault occurs in a DC line, the voltage will drop significantly. In view of the need for high sensitivity and reliability of the start-up criterion, this embodiment adopts the low voltage criterion as the protection start-up criterion. At the same time, in order to ensure the selectivity of the protection and avoid misidentification of the protection caused by normal operating conditions such as noise, load changes, and equipment switching, the average voltage is selected when performing low voltage judgment. The protection start-up criterion is shown in the following formula:

[0110]

[0111] In the formula, u k is the voltage value of the kth sampling point; m represents the number of sampling points for the startup criterion, and its value is related to the sampling frequency. After comprehensively considering the action requirements and disturbance conditions, combined with the AC low voltage criterion setting requirements, the setting value u se The value of t is selected to be 0.6 to 0.8 times the rated voltage, and the value of m is 10.

[0112] Line protection. When the protection determines that a fault has occurred in the DC line, the protection should quickly determine the fault type based on the change characteristics of the electrical quantity. For inter-pole short-circuit faults, the current rapidly increases during the discharge of the MMC submodule capacitor, and the current change rate protection criterion can be used directly to identify the fault. The protection criterion is shown in the following formula:

[0113]

[0114] In the formula, Δi xn set =K set ·Δi lmax ; Δi xn set is the setting value of the current mutation, K set is the reliability coefficient, Δi lmax P is the maximum current change value of the line during normal operation; dc =1 indicates that the direction detection element detects a positive direction.

[0115] Busbar protection. The busbar in the DC distribution line is connected to the other branch lines. When a DC busbar fails, it will have a serious impact on the distribution lines connected to the busbar. At this time, the relay protection device needs to trip all the circuit breakers connected to the busbar. When a distribution line busbar fails, if the busbar is regarded as a generalized node, the current flowing into and out of the node will be unequal. Based on this, the sum of all the currents input to the busbar, I SB With set value If the sum is greater than the set value, it is considered a bus fault and triggers a trip. The bus fault judgment criterion is as follows:

[0116]

[0117] In the formula, I i is the measured current flowing through the lines on both sides of the busbar, The value of is the value of the maximum bus unbalanced current, and the influence of the transformer error and measurement noise should be considered. The general value range is 0.05I N To 0.1I N .

[0118] The action strategy for triggering protection is as follows:

[0119] (1) The intelligent terminal on the DC distribution network detects the current and voltage in real time. When the DC voltage drops, the protection start criterion starts to be checked. If the start criterion detects that the voltage is lower than the set value for three consecutive times, the protection fault identification program is started.

[0120] (2) When it is detected that the sum of the inflow and outflow current vectors of a DC bus is greater than the set value, the bus is judged to have a fault. The protection device immediately sends a trip signal to the relevant circuit breaker to trip all circuit breakers connected to the bus. The fault action flow chart is as follows: Fig.12 shown.

[0121] (3) When the DC side current change rate is detected to be greater than a certain threshold, the bipolar short-circuit fault action judgment criteria are immediately activated. The fault interval is determined by comparing the direction of the current mutation at both ends of the line, and the tripping is performed. Fig.13 shown.

[0122] Filtering algorithm. The electrical quantities collected by the protection device are often accompanied by a large amount of noise signals, which will not only affect the accuracy of the sampling, but also the noise may be amplified when the hardware system is not designed properly, affecting the detection of normal electrical quantities. Therefore, it is necessary to filter the sampled signal to improve the reliability of the device. Least squares smoothing filtering is an adaptive filtering algorithm in the time domain with a fast calculation speed, which can meet the requirements of relay protection for rapidity. The specific process is as follows:

[0123] (1) Select the signal (x i ,y i ),i=1,2,...,n to establish the polynomial fitting formula (11):

[0124]

[0125] Where: β i0 ...β id For x i The corresponding coefficient; ε i is the corresponding random error.

[0126] (2) Calculate x i Distances to other points:

[0127] d ij =|x i -x j | i=j=1,2,...,n (13)

[0128] D ij Arrange in ascending order, the value of the i-th position is h i , and i is the distance from f n The nearest integer, i = f n is the window length for regression processing of local data, f∈(0,1] is the influencing point y i x j scope.

[0129] (3) Set the weight function W(x) for each x in the window k,k=1,2,...,nCalculate weights:

[0130]

[0131] (4) β in formula (3-7) ik The estimation formula for (i=1,2,...,n;k=1,2,...,d) is:

[0132]

[0133] Combining equations (14) and (15), we can get y i The fitted value of is:

[0134]

[0135] The current and voltage signals collected on the MMC1 side after the DC double-stage short circuit fault are processed using the least squares-based smoothing filter algorithm. The results are as follows: Figure 14-17 shown.

[0136] It can be found that after filtering, the voltage and current amplitudes basically do not have high-frequency fluctuations. This verifies that the smoothing filter based on least squares can effectively remove burr signals in the sampled data.

[0137] Simulation. An AC / DC hybrid distribution network model was built on PSCAD software to simulate various faults in the DC distribution network and verify the effectiveness of the protection scheme proposed in this embodiment. The topology diagram corresponding to the simulation model diagram is shown in Figure 1. Figure 1 shown.

[0138] Simulate a bipolar short-circuit fault in the DC line. When t=2.0s, bipolar short-circuit faults occur inside and outside the DC line L1 area, respectively. The setting value of the current mutation is set to 3. The simulation results are shown in Table 2.

[0139] Table 2 Current characteristics of different lines corresponding to different fault locations

[0140]

[0141] It can be seen from Table 2 that when a bipolar short circuit fault occurs inside the line, the current flow direction at both ends is positive, and the mutation criterion meets the fixed value. However, when a fault occurs outside the zone, the current flow direction at both ends of the line is one positive and one negative, which does not meet the start condition, and at the end of the line, the current mutation criterion does not meet. This verifies that the protection can effectively identify faults within the zone.

[0142] Figure 18-Figure 23The voltage waveform and current mutation waveform felt by the head end of each line after the L1 fault occurs are shown. As can be seen from the figure, after the bipolar short circuit fault occurs in line L1, the voltage drop at both ends of the line is obvious, and the protection can be activated in time. The current mutation value detected by the protection device at both ends of the L1 line is 11.86, which is much larger than the protection setting value 3, and the protection is reliably activated. The farther away from the fault point, the lower the current mutation value and the smaller the amplitude of the voltage drop.

[0143] Simulate a DC line resistance fault. When t=2.0s, a bipolar transition resistance short circuit fault occurs at the end of DC line L1. Figure 24-Figure 27 The waveforms of current mutation under different transition resistance conditions are given.

[0144] As can be seen from the figure, when the DC line passes through a resistance fault of less than three ohms, the protection criterion proposed in this embodiment can effectively identify the occurrence of the fault. Fig. 27 As shown, the protection can still identify the fault condition, which shows that this protection scheme has a certain tolerance to excessive resistance.

[0145] Simulate a symmetrical short circuit fault in the AC line. When t=2s, a three-phase short circuit fault occurs on the AC valve side. Fig.28 and Fig.29 The current waveform and current mutation waveform after the fault are given respectively. As can be seen from the figure, the voltage on the DC side will not drop suddenly due to the support of the power supply on the other side. The protection detects an abnormality 0.1 seconds after the fault occurs, and the start criterion is met. At this time, the protection action criterion starts to work. Since the current mutation value detected by the line protection is far below 5, the protection will not malfunction when a fault occurs on the AC side.

[0146] The current and voltage waveforms of the DC side after a two-phase short circuit occurs on the AC side are as follows: Fig.30 and Fig.31 As shown in the figure, the fault current and voltage waveforms are similar to those of a three-phase short circuit on the AC side. When a two-phase short circuit occurs, the DC side voltage rises slightly and then falls back. The DC side protection start-up criteria are not met, and the current mutation amount does not meet the action requirements, so the protection will not act.

[0147] Simulate a DC bus fault. Assume that at t = 2s, a short-circuit fault occurs at the bus at the end of the L1 line. Fig.32 The current waveforms flowing through the three circuit breakers on both sides of the bus after a fault are given.

[0148] After the fault occurs, the current flowing through the busbar surges, and its direction is the same as the set positive direction. The fault current direction peaks flowing through the line circuit breaker are 31.998kA and 12.187kA respectively.

[0149] The differential current waveforms when the busbar is faulty and the differential current waveforms when the line is faulty are as follows: Fig.33 As shown in Figure 2, it can be seen that when a busbar fault occurs, |ISB| is significantly greater than 0, while during a line fault, the current remains close to 0. Therefore, even a small setting value IBset can be used to distinguish between a busbar fault and a line fault.

[0150] This embodiment proposes a protection scheme based on the analysis of the complex fault characteristics of the DC distribution network, and performs simulation at the same time. In view of the rapid development of faults in the DC distribution network and the high speed of protection, a protection based on current mutation is proposed; in order to eliminate the influence of AC side faults on DC side faults, the average difference method is used to calculate the current mutation, and the direction judgment criteria are improved and optimized. Considering the sampling error that may occur during the sampling process of the actual protection device, a least squares filtering algorithm is proposed. The simulation results show that the algorithm can quickly identify the fault interval and correctly distinguish DC faults from AC faults; it has a certain tolerance to transition resistance, which verifies the reliability of the protection scheme.

[0151] Embodiment 2:

[0152] The AC / DC hybrid distribution network protection system based on the current mutation direction includes:

[0153] The current mutation amount acquisition module is configured to: obtain the current value of the current sampling point in the AC / DC hybrid distribution network and the average value of the set number of sampling data before the current sampling point as a reference value, and subtract the reference value from the current value of the current sampling point to obtain the current mutation amount;

[0154] The mutation direction determination module is configured to: determine the current mutation direction according to the value of the current mutation amount, and further determine the change of the current mutation direction using a set interval range;

[0155] The fault judgment and action protection module is configured to determine the fault type and perform corresponding protection actions according to the voltage, current, current mutation direction and corresponding changes obtained in the AC / DC hybrid distribution network.

[0156] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A protection method for AC / DC hybrid distribution network based on current mutation direction, characterized in that: The following steps are involved: The current value of the current sampling point in the AC / DC hybrid distribution network is obtained, and the average value of the set number of sampling data before the current sampling point is used as the reference value. The current mutation amount is obtained by subtracting the reference value from the current value of the current sampling point. The current mutation direction is determined according to the value of the current mutation amount, and the change of the current mutation direction is further determined by using the set interval range; According to the voltage, current, current mutation direction and corresponding changes obtained in the AC / DC hybrid distribution network, the fault type is determined and the corresponding protection action is performed.

2. The AC / DC hybrid distribution network protection method based on current mutation direction according to claim 1 is characterized in that: The set interval range is used to further determine the direction of the current mutation. Specifically, when the current mutation amount is within the set interval range, the current mutation direction remains unchanged. When it exceeds the set interval range, the current mutation direction changes.

3. The AC / DC hybrid distribution network protection method based on current mutation direction according to claim 1 is characterized in that: The set interval range is used to further determine the direction of the current mutation, specifically: Where P dc is the direction of current mutation. Δi is the amount of current mutation, Δi xn·set Indicates the setting value of the current mutation amount. 1 indicates that the detected current direction is positive, 0 indicates that the detected current direction is negative, and X indicates that the value is the same as the previous judgment result, that is, the mutation direction remains unchanged.

4. The AC / DC hybrid distribution network protection method based on current mutation direction according to claim 1 is characterized in that: The current mutation setting value is used to avoid the current change caused by the load shedding operation of the distribution network. The current mutation setting value is 0.1I N The product of the reliability coefficient.

5. The AC / DC hybrid distribution network protection method based on current mutation direction according to claim 1, characterized in that: According to the voltage, current, current mutation direction and corresponding changes obtained in the AC / DC hybrid distribution network, the fault type is determined and the corresponding protection action is performed, including: according to the change of current during the discharge of the converter submodule capacitor in the AC / DC hybrid distribution network, a short circuit fault is determined.

6. The AC / DC hybrid distribution network protection method based on current mutation direction according to claim 5 is characterized in that: In case of a short circuit, the fault section is determined by comparing the direction of the current changes at both ends of the line, and protection is performed through tripping.

7. The AC / DC hybrid distribution network protection method based on current mutation direction according to claim 1, characterized in that: According to the voltage, current, current mutation direction and corresponding changes obtained in the AC / DC hybrid distribution network, the fault type is determined and the corresponding protection action is performed, which also includes: using the sum of the currents of all input buses in the AC / DC hybrid distribution network I SB With set value If the sum is greater than the set value, it is considered a bus fault.

8. The AC / DC hybrid distribution network protection method based on current mutation direction according to claim 7, characterized in that: When a busbar fails, protection is achieved by disconnecting all circuit breakers connected to the busbar.

9. The AC / DC hybrid distribution network protection method based on current mutation direction according to claim 1, characterized in that: Based on the least square smoothing filter algorithm, the current and voltage signals collected on the converter side after the short circuit fault are processed to remove the noise signals in the sampled data.

10. An AC / DC hybrid distribution network protection system based on current mutation direction, characterized in that: include: The current mutation amount acquisition module is configured to: obtain the current value of the current sampling point in the AC / DC hybrid distribution network and the average value of the set number of sampling data before the current sampling point as a reference value, and subtract the reference value from the current value of the current sampling point to obtain the current mutation amount; The mutation direction determination module is configured to: determine the current mutation direction according to the value of the current mutation amount, and further determine the change of the current mutation direction using a set interval range; The fault judgment and action protection module is configured to determine the fault type and perform corresponding protection actions according to the voltage, current, current mutation direction and corresponding changes obtained in the AC / DC hybrid distribution network.

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