Directional pilot protection method and device for flexible DC distribution network based on line mode power

Abstract

The present invention discloses a method and device for directional longitudinal protection of a flexible direct current (DC) distribution network based on line-mode power. The method comprises: obtaining voltage and current data at both ends of a flexible DC distribution network line, and calculating the line-mode fault component voltage and line-mode power, respectively; comparing the amplitude of the line-mode fault component voltage with a threshold value to determine whether a fault has occurred; if a fault has occurred, calculating the sum of the line-mode power within the data windows of the line end and the opposite end of the line; determining the line-mode power polarity of the line end and the opposite end of the line based on the line-mode power sum; and determining whether the fault is an internal or external fault based on the line-mode power polarity of the line end and the opposite end of the line. The present invention has the characteristics of fast action, high sensitivity, and strong resistance to transition resistance. In addition, the method is independent of line boundaries, has a moderate sampling frequency, and has good engineering applicability.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible DC distribution network relay protection, and in particular relates to a directional longitudinal protection method and device for a flexible DC distribution network based on line mode power. 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] Currently, when clean energy sources, such as photovoltaic power generation, are connected to AC distribution networks, they require inverters and rectifiers to power DC loads such as consumer electronics, electric vehicles, and battery energy storage stations. Multi-stage commutation results in high energy losses, which DC distribution networks can effectively address. Flexible DC distribution networks based on modular multilevel converters (MMCs) offer advantages such as flexible operation, large power supply capacity, and high reliability. They can save a significant number of commutation steps and hold great promise for application in distribution systems. However, when a bipolar short-circuit fault occurs on the DC side of a flexible DC distribution network, the capacitors in the MMC bridge arm submodules rapidly discharge, resulting in a rapid rise in fault current and a high peak value, which can easily cause serious damage to the system. Protection must identify the fault within milliseconds. Furthermore, distribution networks have short lines and numerous branches, requiring not only rapid fault identification and isolation of the faulty line by circuit breakers, but also the continued operation of non-faulty lines. Therefore, flexible DC distribution networks place extremely high demands on protection performance.

[0004] Two-terminal protection realizes absolute selection of protection by exchanging fault information at both ends of the line. According to different principles, two-terminal protection is mainly divided into current differential pilot protection, directional pilot protection and other pilot protection.

[0005] Existing technologies apply current differential longitudinal protection to DC microgrids. However, current differential protection requires strict data synchronization, and the high prices of GPS receivers and chip-level atomic clocks increase project costs. In addition, since the DC distribution network lines are short, the differential current is less affected by the distributed capacitance of the lines, and the communication time required between the two ends of the line is short. Existing technologies attempt to use current differential protection as the main protection for the distribution network to ensure the speed and selectivity of the protection. At the same time, existing technologies also use current longitudinal differential protection as backup protection and low-voltage overcurrent as the main protection.

[0006] Compared to current differential protection, the biggest advantage of directional pilot protection is that it requires no data synchronization and is simple and easy to implement. Existing technologies identify fault direction by swapping the voltage polarity of the current-limiting reactances at both ends of a DC line. This method is highly sensitive to high-resistance faults, but when there are no current-limiting reactances at either end of the line, protection is no longer applicable. Existing technologies calculate line inductance using the least squares method and propose a directional pilot protection. However, the accuracy of the protection equivalent calculation decreases with increasing line length. Existing technologies analyze the transient high-frequency impedance differences of multiple outgoing lines on the same DC bus and propose a directional pilot protection based on transient high-frequency impedance. However, this method is overly dependent on the line operation mode and is no longer applicable when a branch line is taken out of operation for some reason. To address the problem that the reliability and sensitivity of traditional traveling wave directional protection are significantly affected by the distribution of line parameters and frequency-varying characteristics, existing technologies use the Mallat wavelet algorithm to extract the high-frequency components of the fault voltage and current, and then propose an improved traveling wave directional protection based on high-frequency transient quantities, which improves the reliability and sensitivity of the protection action.

[0007] In addition, other pilot protection methods based on other protection principles have also been widely proposed. Prior art proposes a pilot protection method based on an additional component of the fault current. However, the load current fluctuations and reverse flow problems caused by the integration of renewable energy into the DC grid affect the reliability of this protection. Prior art proposes a pilot protection method based on full current direction characteristics. This protection method can identify the fault section within 1ms and can be configured as the main protection for flexible DC distribution networks. Prior art proposes a pilot protection method based on transient measurement impedance. This method is applied to high-voltage DC transmission systems. However, flexible DC distribution networks have multiple outgoing lines, multiple connections, and uncertain flow, making this protection difficult to apply.

[0008] In summary, the busbars of flexible DC distribution networks have multiple outgoing lines and are relatively short. To reduce costs, current-limiting inductors are generally placed at the converter outlet. Existing directional longitudinal protection methods mostly rely on current-limiting inductors at both ends of the DC line. These methods generally have problems such as weak tolerance to transition resistance and susceptibility to the influence of distributed capacitance of the line. This makes two-terminal protection that relies on the boundary characteristics of the line ends unsuitable. Summary of the Invention

[0009] To overcome the shortcomings of the prior art, the present invention provides a method and device for directional pilot protection of flexible DC distribution networks based on line-mode power. This method uses line-mode power to determine fault direction, is independent of line boundaries and frequency, requires a low sampling rate, and eliminates the need for data synchronization, resulting in strong engineering applicability. Furthermore, the method eliminates the need to pre-determine the fault pole before determining the fault section, resulting in rapid and sensitive operation and strong resistance to transient resistance.

[0010] To achieve the above objectives, one or more embodiments of the present invention provide the following technical solutions:

[0011] A directional pilot protection method for a flexible DC distribution network based on line mode power, comprising:

[0012] Obtain voltage and current data at both ends of each line in the flexible DC distribution network, and calculate the line-mode fault component voltage and line-mode power respectively;

[0013] Compare the amplitude of the line mode fault component voltage with the threshold value to determine whether a fault has occurred;

[0014] If a fault occurs, calculate the sum of the line mode power at the sampling points within the data window at both the local and opposite ends of the line;

[0015] Determining the line mode power polarity of the line local end and the line opposite end respectively based on the line mode power sum value;

[0016] Based on the line mode power polarity at the local and opposite ends of the line, determine whether the fault is an internal or external fault.

[0017] As a further solution, the amplitude of the line-mode fault component voltage is compared with a threshold value to determine whether a fault has occurred. Specifically:

[0018] The threshold value is 0.1 times the rated voltage. If the amplitude of the line mode fault component voltage is greater than the threshold value, it is determined that a fault has occurred and protection is initiated.

[0019] As a further solution, based on the line mode power sum value, the line mode power polarity of the line local end and the line opposite end is determined respectively, specifically:

[0020]

[0021] Where R is the logic value, S is the line mode power sum value;

[0022] When the line mode power sum is less than zero, the logic value is 1, indicating that the line mode power polarity is negative;

[0023] When the line mode power sum is greater than zero, the logic value is 0, indicating that the line mode power polarity is positive.

[0024] As a further solution, based on the line mode power polarity at the local and opposite ends of the line, it is determined whether the fault is an internal or external fault. Specifically:

[0025] Both ends of the line send their own logic values ​​to the other end;

[0026] The local end of the line receives the logic value of the other end of the line and performs an AND operation with the logic value of the local end of the line. If the logic value after the operation is 1, it is determined that an internal fault has occurred; if the logic value after the operation is 0, it is determined that an external fault has occurred.

[0027] As a further solution, if an intra-zone fault occurs, a trip signal is sent to the circuit breaker on the fault section.

[0028] Other embodiments of the present invention provide the following technical solutions:

[0029] A directional pilot protection device for a flexible DC distribution network based on line mode power, comprising:

[0030] A data acquisition module is used to obtain voltage and current data at both ends of each line in the flexible DC distribution network and calculate the line-mode fault component voltage and line-mode power respectively;

[0031] A fault occurrence judgment module is used to compare the amplitude of the line mode fault component voltage with a threshold value to determine whether a fault has occurred;

[0032] The fault location identification module is used to calculate the line mode power sum of a set number of sampling points at the line end and the line end when a fault occurs; determine the line mode power polarity of the line end and the line end based on the line mode power sum; and determine whether the fault is an internal fault or an external fault based on the line mode power polarity of the line end and the line end.

[0033] Other embodiments of the present invention provide the following technical solutions:

[0034] A terminal device includes a processor and a memory, wherein the processor is used to implement instructions; the memory is used to store multiple instructions, and the instructions are suitable for being loaded and executed by the processor to implement the above-mentioned flexible DC distribution network directional longitudinal protection method based on line mode power.

[0035] Other embodiments of the present invention provide the following technical solutions:

[0036] A computer-readable storage medium stores a plurality of instructions, wherein the instructions are suitable for being loaded and executed by a processor of a terminal device to implement the above-mentioned flexible direct current distribution network directional longitudinal protection method based on line mode power.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] (1) This paper proposes a directional pilot protection method for borderless flexible DC distribution networks. This method utilizes the characteristic difference between the line mode power polarity at both ends of the line being the same during an intra-zone fault and the line mode power polarity being opposite during an extra-zone fault. A directional pilot protection method based on line mode power polarity comparison is constructed. The proposed protection method has the characteristics of fast action, high sensitivity, and strong resistance to transition resistance. Furthermore, the method is independent of line boundaries, has a moderate sampling frequency, and has good engineering applicability.

[0039] Other features and advantages of additional aspects of the present invention will be given in part in the following description and in part will become obvious from the following description or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 Schematic diagram of a multi-terminal flexible DC distribution network model in an embodiment of the present invention;

[0041] Figure 2 (a)-(b) are respectively the protection R when a bipolar short circuit fault occurs at point F1 in the embodiment of the present invention. 12 and R 21 Peterson equivalent circuit diagram at ;

[0042] Figure 3 (a)-(b) are respectively the protection R when a bipolar short circuit fault occurs at point F3 in the embodiment of the present invention. 12 and R 21 Peterson equivalent circuit diagram at ;

[0043] Figure 4 (a)-(b) are respectively the protection R when a bipolar short circuit fault occurs at point F2 in the embodiment of the present invention. 12 and R 21 Peterson equivalent circuit diagram at ;

[0044] Figure 5 (a)-(b) are respectively the protection R when a single-pole grounding fault occurs at point F1 in an embodiment of the present invention. 12 and R 21 Peterson equivalent circuit diagram at ;

[0045] Figure 6 (a)-(b) are respectively the protection R when a single-pole grounding fault occurs at point F3 in an embodiment of the present invention. 12 and R 21 Peterson equivalent circuit diagram at ;

[0046] Figure 7 Flowchart of a directional pilot protection method for a flexible DC distribution network based on line mode power in an embodiment of the present invention;

[0047] Figure 8 This is the simulation result when a bipolar short circuit fault occurs at point F1 in an embodiment of the present invention;

[0048] Figure 9This is the simulation result when a bipolar short circuit fault occurs at point F3 in an embodiment of the present invention;

[0049] Figure 10 This is the simulation result when a bipolar short circuit fault occurs at point F2 in an embodiment of the present invention;

[0050] Figure 11 This is the simulation result when a single-pole grounding fault occurs at point F1 in an embodiment of the present invention;

[0051] Figure 12 This is the simulation result when a single-pole grounding fault occurs at point F3 in an embodiment of the present invention;

[0052] Figure 13 This is the simulation result when a single-pole grounding fault occurs at point F2 in an embodiment of the present invention;

[0053] Figure 14 This is the simulation result of a single-pole grounding fault when the transition resistance is 100Ω in an embodiment of the present invention;

[0054] Figure 15 (a)-(b) are respectively the protection R when the line ends contain inductance boundaries in the embodiment of the present invention. 12 and R 21 Line mode power simulation results at . DETAILED DESCRIPTION

[0055] It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0056] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0057] Example 1

[0058] In one or more embodiments, a directional pilot protection method for a flexible DC distribution network based on line mode power is disclosed, combined with Figure 7 , specifically including the following process:

[0059] (1) During normal system operation, the protection device continuously samples the voltage and current data at both ends of each line in the flexible DC distribution network and calculates the line-mode fault component voltage and line-mode power. At the same time, the line-mode fault component voltage amplitude is compared with the threshold value. If the amplitude is greater than the threshold value, it indicates a fault has occurred, the protection is activated, and the next step is entered. If the amplitude is less than the threshold value, it indicates that no fault has occurred, and the next set of data is determined.

[0060] In this embodiment, based on the characteristic that the line mode fault component voltage amplitude rises rapidly after a fault, the line mode fault component voltage amplitude is constructed as a starting criterion, specifically:

[0061] When the voltage amplitude of the line-mode fault component is greater than the set value, which is set to 0.1 times the rated voltage, a fault is determined to have occurred and the protection is activated.

[0062] The startup criteria are as follows:

[0063] |Δu|>0.1u rate (1)

[0064] Where △u is the line mode fault component voltage detected at the protection installation, u rate is the line mode rated voltage. When the line mode fault component voltage satisfies equation (1), the protection starts.

[0065] (2) The line mode power value is used to calculate the line mode power sum within the data window, and the polarity of the line mode power is determined based on the sum. If the power sum is less than 0, the line mode power polarity is negative, and the logic value at the local end of the line is set to 1. Otherwise, the line mode power polarity is positive, and the logic value is set to 0. At the same time, the logic value at the local end is sent to the other end of the line.

[0066] (3) After receiving the logic value sent from the other end, both ends of the line perform an AND operation on the logic values ​​at both ends. If the logic value after the operation is 1, it is judged as an internal fault and a trip signal is sent to the circuit breaker; if the logic value is 0, the protection is returned.

[0067] In this embodiment, the principle of using line mode power to determine the fault section is as follows:

[0068] Due to the distributed parameter characteristics of transmission lines, a fault will generate a traveling wave at the fault point, propagating toward both ends of the line and undergoing refraction and reflection at the impedance discontinuity. Given that the zero-mode component is significantly affected by frequency, decays more rapidly than the line-mode fault component, and exhibits poor stability, this embodiment utilizes line-mode fault component analysis. Using the symmetrical component method, the line-mode fault component voltage after a line fault can be calculated as shown below.

[0069]

[0070] Where, △uf1_PPF and △u f1_SPGF are the line mode fault component voltages of bipolar short circuit fault and unipolar ground fault respectively; V p is the rated voltage; Z c0 and Z c1 are zero-mode wave impedance and line-mode wave impedance respectively; R g is the transition resistance. From formula (2), we can see that the polarity of the line mode fault component voltage after the fault is negative.

[0071] This embodiment focuses on analyzing the first traveling wave at the protection points at both ends of the line, and uses the Peterson equivalent circuit to analyze the characteristics of the traveling wave when a fault occurs inside or outside the fault zone.

[0072] (1) Analysis of bipolar short-circuit fault characteristics

[0073] A. Fault characteristics within the area

[0074] Figure 1 This is an example diagram of a multi-terminal flexible DC distribution network model. Figure 1 When a fault occurs at point F1 in the complex frequency domain, the protection R 12 and R 21 The Peterson equivalent circuit at is shown in Figure 2(a)-(b), where L eq and C eq are the equivalent inductance and equivalent capacitance of the MMC converter respectively; C DC / DC is the equivalent capacitance of the DC / DC converter, Z c1 is the line mode wave impedance, L dc is the current limiting inductor.

[0075] Considering the attenuation of traveling waves when propagating on the line, the protection R 12 and R 21 The line mode fault component voltages at can be approximated as:

[0076]

[0077] Among them, L sum =L eq +L dc ;L eq and C eq are the equivalent inductance and equivalent capacitance of the MMC converter, respectively; x is the propagation distance of the traveling wave; τ a1 and k a1 are the dispersion time constant and attenuation coefficient under unit distance respectively, t is the traveling wave transmission time, △u f1_PPF is the line-mode fault component voltage of a bipolar short-circuit fault. The magnitudes of other parameters are shown in Table 1.

[0078] Table 1 Parameters' order of magnitude

[0079]

[0080]

[0081] The order of magnitude of the parameters is shown in Table 1, and the transfer function of the traveling wave satisfies The parameters of the overhead line meet the Considering △u f1_PPF The polarity is negative, and combined with formula (3), we can get △u 12 (t) and ∆u 21 The polarity of (t) is negative.

[0082] The positive direction of current is defined as the flow from the busbar to the line. According to the voltage polarity at both ends of the line, the current direction diagram of Figure 2(a)-(b) can be obtained. Therefore, the protection R 12 and R 21 Line mode fault component current △i 12 (t) and △i 21 The polarity of (t) is always positive.

[0083] The line-mode fault component power △P(t) (referred to as line-mode power) is defined as the product of the line-mode fault component voltage △u(t) and the line-mode fault component current △i(t), that is:

[0084] ΔP(t)=Δu(t)·Δi(t) (4)

[0085] According to the polarity of the voltage and current of the line mode fault component above, when an internal fault occurs, the protection R 12 and R 21 Line mode power △P 12 (t) and ΔP 21 (t) The polarity is negative.

[0086] B. Out-of-area fault analysis

[0087] when Figure 1 After a bipolar short circuit fault occurs at F3 in the figure, the voltage traveling wave first propagates to the protection R 21 Then it passes through the DC line L 12 The attenuation reaches the protection R 12 ; After the fault, protect R 12 and R 21 The Peterson equivalent circuit at L is shown in Figure 3(a)-(b); therefore, considering the traveling wave in line L 12 When F3 fails, the protection R 12 and R 21 The line mode fault component voltages at are:

[0088]

[0089] Where L is the line L 12 The length of △u f1_PPF is the line mode fault component voltage of the bipolar short circuit fault, C DC / DC is the equivalent capacitance of the DC / DC converter, Z c1 is the line mode wave impedance, and t is the traveling wave transmission time.

[0090] Since the equivalent capacitance of the DC / DC converter is generally on the order of 10 -3 F, combined with Table 1, we can get Given that △u f1_PPF The polarity is negative, and from formula (5) we can know that the protection R 12 and R 21 Line mode fault component voltage △u at 12 (t) and ∆u 21 (t) The polarity is negative.

[0091] According to the voltage polarity at both ends of the line, the current direction diagram in Figure 3 (a)-(b) can be obtained. 12 and R 21 The polarity of the line mode fault component voltage at is negative, so the protection R 12 Line mode fault component current △i 12 (t) The polarity is positive, protecting R 21 Line mode fault component current △i 21 (t) polarity is negative, so when an out-of-zone fault F3 occurs, the protection R 12 Line mode power △P 12 The polarity of (t) is negative, protecting R 21 Line mode power △P 21 (t) The polarity is positive.

[0092] when Figure 1 After a bipolar short circuit fault occurs at F2 in the figure, the voltage traveling wave first propagates from the fault point F2 to the protection R 12 Then it passes through the DC line L 12 The attenuation reaches the protection R 21 ; After the fault, protect R 12 and R 21 The Peterson equivalent circuit at is shown in Figure 4(a)-(b); From Table 1, we can see that the parameters satisfy Therefore, the protection R 12 and R 21 The line mode fault component voltage at can be approximated as:

[0093]

[0094] Because △u f1_PPF The polarity of is negative. From formula (6), it can be concluded that when an out-of-zone fault F2 occurs, the protection R12 and R 21 The polarity of the line mode fault component voltage at is negative; thus, the current direction diagram in Figure 4 (a)-(b) can be obtained, and the protection R 12 Line mode fault component current △i 12 (t) The polarity is negative, protecting R 21 Line mode fault component current △i 21 (t) polarity is positive; therefore, when an out-of-zone fault F2 occurs, the protection R 12 Line mode power △P 12 (t) The polarity is positive, protecting R 21 Line mode power △P 21 (t) The polarity is negative.

[0095] In summary, when a bipolar short circuit fault occurs within the zone, the line mode power polarities at both ends are the same and both are negative; while when an out-of-zone fault occurs, the line mode power polarities at both ends are opposite.

[0096] (2) Analysis of single-pole grounding fault characteristics

[0097] Since the system adopts the DC side grounding through a clamped large resistor, when a single-pole grounding fault occurs on the DC side, a loop is formed between the fault point and the grounding point of the clamped large resistor. Similar to a bipolar short-circuit fault, after a single-pole grounding fault, a traveling wave is generated from the fault point and transmitted to both ends. The line-mode fault component voltage at the fault point is shown in Equation (2).

[0098] A. Analysis of faults within the area

[0099] When a single-pole ground fault occurs at F1 in the zone, its Peterson equivalent circuit is shown in Figure 5(a)-(b), where R g C is the large resistor for ground clamping. line is the line-to-ground capacitance; therefore, when a single-pole ground fault occurs at F1, the protection R 12 and R 21 The line mode fault component voltage at is approximately:

[0100]

[0101] in,

[0102]

[0103] Combining formula (2), we can get the protection R in the time domain: 12 and R 21The polarity of the line-mode fault component voltage at both ends of the line is negative; thus, the current direction diagram of Figure 5(a)-(b) can be obtained. It can be seen from the figure that the polarity of the line-mode fault component current at both ends of the line is positive when the fault occurs in the zone; therefore, when a single-pole grounding fault occurs in the zone, the polarity of the line-mode power at the protection points at both ends of the line is the same, and the polarity is both negative.

[0104] B. Out-of-area fault analysis

[0105] when Figure 1 When a single-pole ground fault occurs at F3 in the circuit, the protection R 12 and R 21 The Peterson equivalent circuit at the position is shown in Figure 6(a)-(b); therefore, the protection R 12 and R 21 The line mode fault component voltages at are:

[0106]

[0107] in,

[0108]

[0109] Considering the line mode fault component voltage △u at the fault point f1_SPGF The polarity is negative, and the protection R can be obtained from formula (8) 12 and R 21 The polarity of the line mode fault component voltage at F3 is negative; thus, the current direction diagram in Figure 6(a)-(b) can be obtained. Therefore, when a single-pole grounding fault occurs at F3, the protection R 12 Line mode fault component current △i 12 (t) The polarity is positive, protecting R 21 Line mode fault component current △i 21 (t) The polarity is negative; based on this, when a single-pole grounding fault occurs outside the zone of F3, the protection R 12 Line mode power △P 12 (t) The polarity is negative, protecting R 21 Line mode power △P 21 (t) The polarity is positive.

[0110] Similarly, it is also possible to analyze the Figure 1 When a single-pole ground fault occurs in F2, the protection R 12 and R 21 The line mode power polarity at is opposite; the analysis method is the same as when F3 fails, so it will not be repeated here.

[0111] In summary, regardless of whether a bipolar short circuit fault or a unipolar grounding fault occurs on the line, when an intra-zone fault occurs, the polarity of the line mode power at both ends of the line is negative, while when an out-of-zone fault occurs, the polarity of the line mode power at both ends of the line is opposite.

[0112] Based on this, a directional longitudinal protection criterion based on line mode power is established. To improve the reliability of protection, the sum of the line mode power is used to determine the polarity, as shown in formula (10).

[0113]

[0114] Where △P is the line mode power, S is the sum of △P, and N is the number of sampling points.

[0115]

[0116] The polarity criterion is shown in (11), where R is the logical value of the line mode power polarity; when S is less than 0, the logical value of R is 1, indicating that the polarity of the line mode power is negative; and when S is greater than 0, the logical value of R is 0, indicating that the polarity of the line mode power is positive.

[0117] The fault identification criteria for the structural longitudinal protection are as follows:

[0118]

[0119] Among them, R M and R N They are the R values ​​of the protection at the M and N ends of the line respectively.

[0120] When the protection determines that the fault is an internal fault, it sends a trip signal to the circuit breaker; otherwise, the protection returns.

[0121] Next, a flexible DC distribution network simulation model is constructed using PSCAD to simulate and verify the method proposed in this embodiment.

[0122] (1) Model building

[0123] Drawing on the 9-terminal MMC-HVDC model proposed by the B4-58 working group of the International Conference on Large Power Systems, this embodiment extracts some components and builds the following Figure 1 The circular flexible DC distribution network model shown in the figure is shown in the figure. The DC side is interconnected with the AC power supply through three MMC converters, and the rated voltage of the DC side is ±10kV. The three converter stations adopt a single-point voltage control method based on master-slave control, and the main line connection method adopts symmetrical single-pole connection. To maintain the symmetrical operation of the DC bus voltage, the MMC converter adopts a low-current grounding method with DC-side clamping resistor grounding, and a current-limiting inductor L is configured at the outlet. dc The specific parameters of the converter are shown in Table 2. The DC distribution line adopts the overhead line model, and its parameters are shown in Table 3. The line is divided into 8 sections, and both ends are equipped with DC circuit breakers to quickly isolate faults. ij It is line protection.

[0124] Table 2MMC converter simulation model parameters

[0125]

[0126] Table 3 DC line parameters

[0127]

[0128] Considering the short DC distribution network line and fast traveling wave transmission, the protection sampling frequency is selected as 100kHz and the sampling time window is 1ms. Figure 1 The multi-terminal flexible DC distribution network model shown in the figure is based on the DC line L 12 Protection R 12 and R 21 As an example, various fault conditions are set to verify the feasibility of the proposed protection scheme, where the fault in the area is set on line L 12 The midpoint F1 of the zone, and the out-of-zone faults are set at F3 and F2 respectively.

[0129] (2) Typical internal and external fault simulation

[0130] A. Bipolar short circuit fault

[0131] Set F1 to F3 to have a metallic bipolar short circuit fault, such as Figures 8-10 The figure shows the simulation results, and the fault occurs at 2s.

[0132] like Figure 8 As shown in the figure, when a bipolar short circuit fault occurs at F1 in the zone, the protection R 12 and R 21 The polarity of the line mode power at is negative, which is consistent with the theoretical analysis of the bipolar short circuit fault in the line generation area of ​​Section 2. Therefore, it can be judged that the line L 12 A bipolar short circuit fault has occurred.

[0133] When a bipolar short circuit fault occurs at F3 outside the zone, the line mode power waveform is as follows: Figure 9 As shown; due to the fault at F3, the protection R 12 is a forward fault, and for protection R 21 It is a reverse fault, so R 12 The line mode power polarity at is negative, R 21 The polarity of the line mode power at is positive. In view of this, the protection R 12 Determine the fault is a forward fault, protect R 21 The fault is judged to be a reverse fault and can be judged to be an out-of-zone fault through polarity comparison.

[0134] Similarly, when a bipolar short circuit fault occurs at F2, the simulation results are as follows: Figure 10 As shown; it can be seen that the protection R 12 The polarity of the line mode power at is positive, R 21The line mode power polarity at is negative, which can be judged as an out-of-zone fault.

[0135] From the above, it can be seen that the simulation results of bipolar short-circuit fault are consistent with the theoretical analysis, and the proposed directional longitudinal protection can correctly identify bipolar short-circuit fault.

[0136] B. Single-pole ground fault

[0137] Assume that a metallic single-pole grounding fault occurs at F1 to F3 and the fault occurs at 2s. The simulation results are as follows: Figures 11-13 shown.

[0138] from Figure 11 It can be seen that when a single-pole grounding fault occurs in the area, the protection R 12 and R 21 The detected line mode power polarity is negative, which is consistent with the theoretical analysis of the previous single-pole grounding fault. Therefore, it can be determined that the line L 12 A fault occurs within the area; similarly, Figure 12 and Figure 13 It can be seen that when out-of-zone faults F2 and F3 occur, protection R 12 and R 21 The fault direction can be correctly determined and the fault can be identified as out-of-zone by polarity comparison.

[0139] Based on this, it can be concluded that the proposed protection method is also applicable to single-pole grounding faults, and the protection can correctly identify single-pole grounding faults.

[0140] C. Simulation analysis of different fault scenarios

[0141] In order to further verify the effectiveness of the proposed protection scheme, more fault points are set at different locations on different DC lines; 12 and R 21 For example, the simulation results are shown in Table 4. The percentage in the table represents the ratio of the distance between the fault point and the busbar to the total length of the line. 12 and S 21 Represents protection R 12 and R 21 The power and value of the line are in kVA. The other tables are similar. 34 , L 23 When a double-pole short-circuit fault or a single-pole grounding fault occurs, the protection R 12 The line mode power polarity is negative, and the protection R 21 The polarity of the line mode power is positive, so it can be judged as an out-of-area fault; when line L 18 , L 78 In case of a fault, the protection R 12 and protect R 21The polarity of the line mode power is opposite, and it can also be judged as an out-of-area fault; when the line L 12 When a fault occurs at different locations of the protection R 12 and protect R 21 The line mode power polarity is negative, which can be judged as an internal fault. Therefore, the simulation results in Table 4 show that the proposed protection method can correctly identify different locations and different fault types in the topology.

[0142] Table 4 Protection judgment under different fault locations and fault types

[0143]

[0144] (3) Protection performance analysis

[0145] A. Analysis of Transition Resistance Capability

[0146] In order to verify the ability of the proposed protection method to resist transition resistance, a single-pole grounding fault with a transition resistance of 100Ω is set at F1. The simulation waveform is as follows: Figure 14 Table 5 shows the simulation results of the two ends of the line when the single-pole grounding fault occurs inside and outside the area, and the transition resistance is 50Ω and 100Ω respectively; Figure 14 It can be seen that the line-mode power amplitude decreases due to the influence of the transition resistance, but its polarity is always negative. At the same time, it can be seen from Table 5 that under different transition resistances, the polarity of the line-mode power is not affected, and the protection can still accurately determine the fault section. The simulation results show that although the transition resistance affects the line-mode power amplitude, the protection proposed in this paper only requires the power value to be able to reliably determine the polarity. Therefore, the power value has little impact on the protection. At the same time, the protection judgment criterion proposed in this paper is based on the sum of the power within a period of time after the fault. Therefore, even if the transition resistance is large, the protection can accurately determine the fault.

[0147] Table 5 Simulation results of single-pole grounding fault under different transition resistances

[0148]

[0149] B. Simulation of anti-noise interference capability

[0150] To verify the proposed protection scheme's noise immunity, Gaussian white noise was added to the fault. The fault type was set to a bipolar short circuit, the faults were located at positions F1 to F3, and the noise levels were 30 dB and 70 dB, respectively. The simulation results are shown in Table 6. As can be seen from the table, the protection scheme can still accurately identify the fault under varying noise levels. Therefore, noise has little impact on the proposed protection scheme.

[0151] Table 6 Simulation results of bipolar short circuit fault under different noise interference

[0152]

[0153] C. Effect of current limiting inductor

[0154] In the above simulation model, there is no current limiting inductor at both ends of the line, and the protection can correctly identify the fault inside and outside the zone. In order to verify whether the proposed protection will be affected by the current limiting inductor at both ends of the line, Figure 1 A current limiting inductor is added at both ends of the DC line. The current limiting inductor value is 5mH, and other parameters remain unchanged. When a bipolar short circuit fault occurs at F1 and F3, the protection R 12 and R 21 The line-mode power at is shown in Figure 15. It can be seen that after adding the current-limiting inductor, the polarity of the line-mode fault component power during the intra-zone fault remains the same, while the polarity of the line-mode fault component power during the extra-zone fault remains opposite. Therefore, the proposed protection method is still applicable after adding the current-limiting inductor at both ends of the line.

[0155] Example 2

[0156] In one or more embodiments, a flexible DC distribution network directional pilot protection system based on line mode power is disclosed, specifically comprising:

[0157] A data acquisition module is used to obtain voltage and current data at both ends of each line in the flexible DC distribution network and calculate the line-mode fault component voltage and line-mode power respectively;

[0158] A fault occurrence judgment module is used to compare the amplitude of the line mode fault component voltage with a threshold value to determine whether a fault has occurred;

[0159] The fault location identification module is used to calculate the line mode power sum of a set number of sampling points at the line end and the line end when a fault occurs; determine the line mode power polarity of the line end and the line end based on the line mode power sum; and determine whether the fault is an internal fault or an external fault based on the line mode power polarity of the line end and the line end.

[0160] It should be noted that the specific implementation of each of the above modules has been described in Example 1 and will not be described in detail here.

[0161] Example 3

[0162] In one or more embodiments, a terminal device is disclosed, including a server. The server includes a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the program, the method for directional pilot protection of a flexible DC distribution network based on line mode power in Example 1 is implemented. For the sake of brevity, this description is omitted here.

[0163] It should be understood that in this embodiment, the processor may be a central processing unit (CPU), or may be other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), off-the-shelf field-programmable gate arrays (FPGA), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor may be a microprocessor or any conventional processor, etc.

[0164] The memory may include a read-only memory and a random access memory, and provides instructions and data to the processor. A portion of the memory may also include a non-volatile random access memory. For example, the memory may also store information about the device type.

[0165] During implementation, each step of the above method may be completed by an integrated logic circuit of hardware in a processor or by instructions in the form of software.

[0166] Example 4

[0167] In one or more embodiments, a computer-readable storage medium is disclosed, storing a plurality of instructions, wherein the instructions are suitable for being loaded and executed by a processor of a terminal device for the line-mode power-based flexible DC distribution network directional longitudinal protection method described in Example 1.

[0168] Although the above describes the specific embodiments of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without any creative work are still within the scope of protection of the present invention.

Claims

1. A directional pilot protection method for a flexible DC distribution network based on line mode power, characterized in that: include: Obtain voltage and current data at both ends of each line in the flexible DC distribution network, and calculate the line-mode fault component voltage and line-mode power respectively; Compare the amplitude of the line mode fault component voltage with the threshold value to determine whether a fault has occurred; If a fault occurs, calculate the line mode power and value in the data window at the local end and the opposite end of the line respectively; Determining the line mode power polarity of the line local end and the line opposite end respectively based on the line mode power sum value; Determine whether the fault is an internal or external fault based on the line mode power polarity at the local and opposite ends of the line; The line mode power polarities of the local end and the opposite end of the line are determined based on the line mode power and value, specifically: Where R is the logic value, S is the line mode power sum value; When the line mode power sum is less than zero, the logic value is 1, indicating that the line mode power polarity is negative; When the line mode power sum is greater than zero, the logical value is 0, indicating that the line mode power polarity is positive; The fault is determined to be an internal or external fault based on the line mode power polarity at the local and opposite ends of the line. Specifically: Both ends of the line send their own logic values ​​to the other end; The local end of the line receives the logic value of the other end of the line and performs an AND operation with the logic value of the local end of the line. If the logic value after the operation is 1, it is determined that an internal fault has occurred; if the logic value after the operation is 0, it is determined that an external fault has occurred.

2. A method for directional pilot protection of a flexible DC distribution network based on line mode power according to claim 1, characterized in that: Compare the amplitude of the line mode fault component voltage with the threshold value to determine whether a fault has occurred. Specifically: The threshold value is 0.1 times the rated voltage. If the amplitude of the line mode fault component voltage is greater than the threshold value, it is determined that a fault has occurred and protection is initiated.

3. The method for directional pilot protection of a flexible DC distribution network based on line mode power according to claim 1, characterized in that: If an intra-zone fault occurs, a trip signal is sent to the circuit breaker on the fault section.

4. The method for directional pilot protection of a flexible DC distribution network based on line mode power according to claim 1, characterized in that: The line mode power is the product of the line mode voltage and the line mode current.

5. A directional pilot protection device for a flexible DC distribution network based on line mode power, characterized in that: include: A data acquisition module is used to obtain voltage and current data at both ends of each line in the flexible DC distribution network and calculate the line-mode fault component voltage and line-mode power respectively; A fault occurrence judgment module is used to compare the amplitude of the line mode fault component voltage with a threshold value to determine whether a fault has occurred; A fault location identification module is configured to calculate the sum of the line mode powers in the data windows of the local end and the opposite end of the line, respectively, when a fault occurs; determine the line mode power polarity of the local end and the opposite end of the line based on the sum of the line mode powers; and determine whether the fault is an internal fault or an external fault based on the line mode power polarity of the local end and the opposite end of the line; The line mode power polarities of the local end and the opposite end of the line are determined based on the line mode power and value, specifically: Where R is the logic value, S is the line mode power sum value; When the line mode power sum is less than zero, the logic value is 1, indicating that the line mode power polarity is negative; When the line mode power sum is greater than zero, the logical value is 0, indicating that the line mode power polarity is positive; The fault is determined to be an internal or external fault based on the line mode power polarity at the local and opposite ends of the line. Specifically: Both ends of the line send their own logic values ​​to the other end; The local end of the line receives the logic value of the other end of the line and performs an AND operation with the logic value of the local end of the line. If the logic value after the operation is 1, it is determined that an internal fault has occurred; if the logic value after the operation is 0, it is determined that an external fault has occurred.

6. A terminal device comprising a processor and a memory, wherein the processor is used to implement instructions; the memory is used to store multiple instructions, characterized in that: The instructions are suitable for being loaded by a processor and executing the flexible DC distribution network directional longitudinal protection method based on line mode power according to any one of claims 1 to 4.

7. A computer-readable storage medium storing a plurality of instructions, characterized in that: The instruction is suitable for being loaded by a processor of a terminal device and executing the flexible DC distribution network directional longitudinal protection method based on line mode power according to any one of claims 1 to 4.

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