A method and system for longitudinal protection of DC lines considering active current limiting control

Abstract

A method and system for longitudinal protection of DC lines incorporating active current limiting control includes: after a DC fault occurs in the DC grid, obtaining the proportional coefficient K1 of the first-stage current limiting control and the proportional coefficient K2 of the second-stage current limiting control strategy; comparing K1 and K2 and comparing the arm current amplitude with the set value to form the activation criterion for the second-stage current limiting control strategy; determining whether the protection activation condition is met based on the arm current change rate; if met, extracting the current at both ends of the line, calculating the current mutation and the integral value of the current mutation at both ends of the line, and multiplying the integral values ​​of the current mutation at both ends to obtain a product value; processing the integral product value of the current mutation at both ends of the line using a sign function, comparing it with the setting value of 0, and determining whether the fault is inside or outside the zone; and using the ratio of the integral product value of the current mutation at both ends of the positive and negative poles after the fault to determine the fault pole.

Description

Technical Field

[0001] This invention belongs to the field of flexible DC transmission line protection, and specifically relates to a method and system for longitudinal protection of DC lines that takes into account active current limiting control. Background Technology

[0002] After a DC fault occurs in a flexible DC grid, the short-circuit current rises rapidly and has a high peak value. The additional current limiting control strategy of MMC can effectively reduce the rise rate and amplitude of the fault current while also being highly economical. However, the additional current limiting control strategy will change the characteristics of the fault current, causing the DC line protection based on the fault current information to be affected by the current limiting control strategy, resulting in the failure of some protection systems. Summary of the Invention

[0003] The purpose of this invention is to provide a method and system for longitudinal protection of DC lines that takes into account active current limiting control, so as to solve the problem that DC line protection based on fault current information is affected by current limiting control strategy, resulting in partial protection failure.

[0004] To achieve the above objectives, the present invention adopts the following technical solution:

[0005] In a first aspect, the present invention provides a method for longitudinal protection of DC lines that takes into account active current limiting control, comprising:

[0006] After a DC fault occurs in the DC grid, the bridge arm current is detected in real time. The bridge arm current change rate is used to form the proportional coefficient K1 of the first-stage current limiting control, and the bridge arm current amplitude is used to form the proportional coefficient K2 of the second-stage current limiting control strategy.

[0007] The comparison between K1 and K2 and the comparison between the arm current amplitude and the set value constitute the start-up criteria for the two-stage current limiting control strategy, and determine the parameter selection range of the two-stage active current limiting control strategy.

[0008] The protection start-up condition is determined based on the rate of change of the bridge arm current. If the condition is met, the current at both ends of the line is extracted, and the sudden change in current at both ends of the line and the integral value of the sudden change in current are calculated. The integral values ​​of the sudden change in current at both ends are multiplied to obtain the product value. The integral product value of the sudden change in current at both ends of the line is processed using a sign function and compared with the setting value of 0 to determine whether the fault is inside or outside the protection zone.

[0009] The fault pole is determined by the ratio of the product of the integrals of the sudden changes in current at both ends of the positive and negative lines after a fault.

[0010] Optionally, the ratio of the MMC's operational submodule after a fault to that during normal operation is a proportionality coefficient K; the bridge arm current is differentiated to obtain the bridge arm current change rate di. ij / dt, the upper limit of the proportionality coefficient K is 1 minus the differential coefficient K D With the rate of change of bridge arm current diij The product of / dt yields the proportional coefficient K1 of the first-stage current limiting control strategy;

[0011] Its expression and activation criterion are as follows:

[0012]

[0013] In the formula: i ij For the bridge arm currents; Δ1 and I set1 The threshold value for the first-stage current limiting control strategy is Δ1; Δ1 is 1.3 times the peak value of the arm current change rate under normal operation; I set1 It is 1.15 times the maximum value of the bridge arm current during normal operation; if the activation conditions of a current limiting control strategy are met, this current limiting circuit will be activated; otherwise, the detection will continue.

[0014] Optionally, the difference between the arm current and the peak value of the arm current under normal operation is obtained by passing the arm current through a PI circuit, and the rated DC bus voltage U dcn Subtract the bridge arm current difference, then compare it with U. dcn The proportional coefficient K2 of the two-stage current limiting control strategy is obtained by dividing the two; the comparison between K1 and K2 and the comparison between the bridge arm current amplitude and the set value constitute the start-up criterion of the two-stage current limiting control strategy.

[0015] Its expression and activation criterion are as follows:

[0016]

[0017] In the formula: i ij1 This represents the peak value of the bridge arm current under normal operating conditions; I set2 The activation threshold for the two-stage current limiting control strategy; I set1 The current limit is three times the maximum value of the bridge arm current during normal operation. If the conditions for starting the two-stage current limiting control strategy are met, the two-stage current limiting control strategy will be activated and the one-stage current limiting control strategy will be deactivated. If the conditions are not met, the detection will continue.

[0018] Optionally, determine the parameter selection range for the two-stage active current limiting control strategy:

[0019] For the proportionality coefficient k in PI p Explanation of parameter selection range:

[0020] If the two-stage current limiting control strategy is to be adaptive, then the expression is:

[0021]

[0022] Where: K min The lower limit of the proportionality coefficient K is set to 0.2; k p Let K be the proportional coefficient in the PI stage. D With kp The selection rules are as follows:

[0023]

[0024] Optionally, the protection activation conditions can be determined based on the rate of change of the bridge arm current:

[0025] When a fault occurs in the DC line, the bridge arm current and the current signal at the DC line protection installation point are collected, with a data window of 3ms. The fault initiation conditions are determined based on the bridge arm current. The initiation conditions are as follows:

[0026]

[0027] In the formula: Δ2 and I set3 The protection activation threshold; Δ2 is 1.5 times the peak rate of change of the bridge arm current under normal operation, I set3 The current is 1.3 times the maximum value of the bridge arm current during normal operation; if the start-up conditions are met, proceed to the next step; otherwise, continue the test.

[0028] Extract the current at both ends of the line, calculate the sudden change in current at both ends and the integral value of the sudden change in current, and multiply the integral values ​​of the sudden changes in current at both ends to obtain the product value:

[0029] When an internal fault occurs, the current changes in both the positive and negative terminals in the same direction. When an external fault occurs, the current changes in both the positive and negative terminals in opposite directions. Therefore, the current change at the installation location of the line protection is calculated as follows:

[0030] Δi(t)=i(t)-i0(t) (6)

[0031] In the formula: Δi(t) is the current surge at the line protection installation point; i(t) is the real-time measured current of the line; i0(t) is the current of the line at the moment before the protection starts; then the integral value of the current surge at the line protection installation point is calculated as follows:

[0032]

[0033] In the formula: N is the number of sampling points; the product of the integral values ​​of the current surge at the protection installation points at both ends of the line can distinguish between faults inside and outside the zone; the polarity is positive when a fault occurs inside the zone; the polarity is negative when a fault occurs outside the zone; the expression for the product value of the integral values ​​of the current surge is:

[0034] Emn = Em × En (8)

[0035] In the formula: Em and En are the integral values ​​of the current mutation at the protection installation points at both ends of the line, respectively.

[0036] Optionally, a sign function can be used to process the integral product of the current surge at both ends of the line, and compared with the setting value of 0 to determine whether the fault is inside or outside the zone.

[0037] The polarity of Emn is described using a sign function, and compared with 0 to determine whether it is inside or outside the region. The criterion is:

[0038] sgn(Emn)>0 (9)

[0039] When Emn is positive, sgn(Emn) is 1, which satisfies the protection criterion and indicates an internal fault in the line; when Emn is negative, sgn(Emn) is -1, indicating an external fault in the line.

[0040] Optionally, the fault pole can be determined by the ratio of the integrals of the sudden changes in current at both ends of the positive and negative lines after a fault:

[0041] Based on the fact that the integrals of the current surges on the positive and negative lines are equal during a bipolar short-circuit fault, and that the integral of the current surge on the faulty line is much larger than that on the non-faulty line during a unipolar ground fault, the faulty pole is determined by the ratio of the product of the integrals of the current surges on the positive and negative lines after the fault, Emn. The selection factor is defined as L, and its expression is:

[0042]

[0043] In the formula: Emnp and Emnn are the product of the integrals of the current abrupt changes at the positive and negative limiting current ends, respectively; then the stage selection criterion is:

[0044]

[0045] In the formula: l set A threshold of 1.2 was selected for the fault pole.

[0046] Secondly, the present invention provides a DC line longitudinal protection system that incorporates active current limiting control, comprising:

[0047] The data acquisition module is used to detect the arm current in real time after a DC fault occurs in the DC grid. The arm current change rate is used to form the proportional coefficient K1 of the first-stage current limiting control, and the arm current amplitude is used to form the proportional coefficient K2 of the second-stage current limiting control strategy.

[0048] The current limiting judgment module is used to compare K1 with K2 and the arm current amplitude with the set value, forming the start criterion for the two-stage current limiting control strategy and determining the parameter selection range of the two-stage active current limiting control strategy.

[0049] The fault diagnosis module is used to determine whether the protection start-up conditions are met based on the rate of change of the bridge arm current. If the conditions are met, the current at both ends of the line is extracted, the sudden change in current at both ends of the line and the integral value of the sudden change in current are calculated, and the integral value of the sudden change in current at both ends is multiplied to obtain the product value. The sign function is used to process the integral product value of the sudden change in current at both ends of the line and compared with the setting value of 0 to determine whether the fault is inside or outside the zone. The fault pole is determined by the ratio of the integral product value of the sudden change in current at both ends of the positive and negative poles after the fault.

[0050] Thirdly, the present invention provides a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement the steps of a DC line longitudinal protection method taking into account active current limiting control.

[0051] Fourthly, the present invention provides a computer-readable storage medium storing a computer program that, when executed by a processor, implements the steps of a DC line longitudinal protection method taking into account active current limiting control.

[0052] Compared with the prior art, the present invention has the following technical effects:

[0053] 1. First, based on the bridge arm current information, an adaptive K value is generated to reduce the number of sub-modules activated after a fault, thereby reducing capacitor discharge and suppressing current. Second, this current-limiting control strategy directly acts on the valve control stage, with a short delay control and keeping the K value at a low level, thus achieving good current-limiting effect. 2. The additional current-limiting control strategy changes the DC-side fault current characteristics but does not change its current direction. Based on this characteristic, a longitudinal protection scheme is designed. The current surge after a fault is extracted, and the polarity characteristic is amplified through integration. Fault identification can be achieved by transmitting only logic quantities at both ends, without strict data synchronization. The impact of the current-limiting control strategy on the integral of the current surge is also considered. In principle, it is not affected by the current-limiting control strategy, has strong tolerance to transition resistance and noise, and offers good protection performance. Attached Figure Description

[0054] Figure 1 This is a flowchart of the present invention.

[0055] Figure 2 This is a schematic diagram of the two-stage current limiting control strategy in this invention.

[0056] Figure 3 This is the overall flowchart of the present invention.

[0057] Figure 4 This is the structural topology of a flexible DC transmission system according to an embodiment of the present invention.

[0058] Figure 5This is a schematic diagram of the fault current under the two-stage current limiting control strategy in this invention.

[0059] Figure 6 This is a diagram showing the influence of the current limiting control strategy on the integral value of the current surge in this invention.

[0060] Figure 7 This is a schematic diagram showing the integral value, product value, and polarity of the sudden change in current at both ends of the line during an inter-pole fault in the line occurrence area according to the present invention. Detailed Implementation

[0061] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0062] Please see Figures 1 to 7 A method for longitudinal protection of DC lines that takes into account active current limiting control strategies includes the following steps:

[0063] Step 1: The ratio of the MMC submodule in operation after a fault to the submodule in normal operation is the proportionality coefficient K. The bridge arm current is differentiated to obtain the bridge arm current change rate di. ij / dt proportional coefficient K upper limit value 1 minus differential coefficient K D With the rate of change of bridge arm current di ij The product of / dt yields the proportional coefficient K1 of the first-stage current-limiting control strategy. The rate of change of the bridge arm current di ij The comparison between / dt and the peak value of the bridge arm current change rate under normal operation or the comparison between the bridge arm current amplitude and the set value constitutes the start criterion for the first-stage current limiting control strategy.

[0064] Step 2: Subtract the peak value of the bridge arm current under normal operation from the bridge arm current to obtain the bridge arm current difference through a PI circuit. The rated DC bus voltage U... dcn Subtract the bridge arm current difference, then compare it with U. dcn The proportional coefficient K2 of the two-stage current limiting control strategy is obtained by dividing K1 by K2. The comparison between K1 and K2 and the comparison between the bridge arm current amplitude and the set value constitute the activation criterion of the two-stage current limiting control strategy.

[0065] Step 3: Based on the principle that the active current limiting method has adaptive characteristics, design the parameter selection range of the two-stage active current limiting control strategy;

[0066] Step 4: Determine whether the protection start-up conditions are met based on the rate of change of the bridge arm current;

[0067] Step 5: Extract the current at both ends of the line, calculate the current change at both ends of the line and the integral value of the current change, and multiply the integral values ​​of the current change at both ends to obtain the product value.

[0068] Step 6: Use the sign function to process the integral product of the current surge at both ends of the line, compare it with the setting value of 0, and determine whether the fault is inside or outside the zone.

[0069] Step 7: Determine the fault pole by using the ratio of the product of the integrals of the sudden changes in current at both ends of the positive and negative lines after the fault.

[0070] Specifically:

[0071] In step 1: The bridge arm current is detected in real time, and the proportional coefficient K1 of the first-stage current limiting control strategy is constructed using the bridge arm current change rate. Its expression and start-up criterion are as follows:

[0072]

[0073] In the formula: i ij For the bridge arm currents; Δ1 and I set1 This is the activation threshold for the first-stage current-limiting control strategy. Δ1 is 1.3 times the peak rate of change of the arm current under normal operation; I set1 It is 1.15 times the maximum value of the bridge arm current during normal operation. If the activation conditions of a current limiting control strategy are met, this current limiting circuit will be activated; otherwise, the detection will continue.

[0074] Furthermore, the bridge arm current is detected in real time, and the proportional coefficient K2 of the two-stage current limiting control strategy is constructed using the amplitude of the bridge arm current. Its expression and start-up criterion are as follows:

[0075]

[0076] In the formula: i ij1 This represents the peak value of the bridge arm current under normal operating conditions; I set2 This is the activation threshold for the two-stage flow limiting control strategy. set2 It is three times the maximum value of the bridge arm current during normal operation. If the activation conditions of the two-stage current limiting control strategy are met, the two-stage current limiting control strategy is activated and the one-stage current limiting control strategy is deactivated; otherwise, the detection continues.

[0077] Furthermore, based on the adaptive nature of the current limiting control strategy, the parameter selection range for the two-stage current limiting control strategy is explained. Considering that proportional control plays a decisive role in the two-stage current limiting control strategy, only the proportional coefficient k in PI is addressed. p Explain the range of parameter selection.

[0078] If the two-stage current limiting control strategy is to be adaptive, then the expression is:

[0079]

[0080] Where: K minThis is the lower limit of the proportionality coefficient K, which is generally 0, but is set to 0.2 to consider system stability. p Let K be the proportional coefficient in the PI stage. D With k p The selection rules are as follows:

[0081]

[0082] Furthermore, when a fault occurs in the DC line, the bridge arm current and the current signal at the DC line protection installation point are collected, with a data window of 3ms. The bridge arm current is used to determine whether the fault initiation conditions are met. The initiation conditions are:

[0083]

[0084] In the formula: Δ2 and I set3 The protection activation threshold. Δ2 is 1.5 times the peak rate of change of the bridge arm current under normal operation, I set3 It is 1.3 times the maximum value of the bridge arm current during normal operation. If the start-up conditions are met, proceed to the next step; otherwise, continue the test.

[0085] Furthermore, when an internal fault occurs, the current changes in both the positive and negative terminals are in the same direction; when an external fault occurs, the current changes in both the positive and negative terminals are in opposite directions. Therefore, the current change at the installation location of the line protection is calculated as follows:

[0086] Δi(t)=i(t)-i0(t) (6)

[0087] In the formula: Δi(t) represents the current surge at the line protection installation point; i(t) represents the real-time measured current of the line; and i0(t) represents the line current at the moment before protection activation. By integrating to enhance the polarity characteristic, the polarity of the current surge volume at a certain moment can be incorrectly extracted due to current fluctuations. Therefore, the integral value of the current surge at the line protection installation point is calculated as follows:

[0088]

[0089] In the formula: N represents the number of sampling points. The product of the integral values ​​of the current surge at both ends of the line can distinguish between faults within and outside the protection zone. The product polarity is positive for faults within the zone and negative for faults outside the zone. The expression for the product of the integral values ​​of the current surge is:

[0090] Emn = Em × En (8)

[0091] In the formula: Em and En are the integral values ​​of the current mutation at the protection installation points at both ends of the line, respectively.

[0092] Furthermore, a sign function is used to describe the polarity of Emn, and it is compared with 0 to determine whether it is inside or outside the region. The criterion is:

[0093] sgn(Emn)>0 (9)

[0094] When Emn is positive, sgn(Emn) is 1, which satisfies the protection criterion and indicates an internal fault in the line; when Emn is negative, sgn(Emn) is -1, indicating an external fault in the line.

[0095] Furthermore, during a bipolar short-circuit fault, the integrals of the current surges on the positive and negative lines are equal; however, during a unipolar ground fault, the integral of the current surge on the faulty pole line is much larger than that on the non-faulty pole line. The faulty pole is determined by the ratio of the product of the integrals of the current surges on the positive and negative lines after the fault, Emn. The selection factor is defined as L, and its expression is:

[0096]

[0097] In the formula, Emnp and Emnn are the product of the integrals of the current abrupt changes at the positive and negative limiting current ends, respectively. The stage selection criterion is then:

[0098]

[0099] In the formula: l set A threshold value is selected for the fault pole, which is generally slightly greater than 1, and 1.2 is used.

[0100] Example 1: Create as shown in the attached document Figure 3 The flexible DC transmission system shown is used as a simulation model. The DC line adopts the RL model. All converters are half-bridge MMC converters. MMC4 uses constant DC voltage control, and the other MMCs use constant power control. The four converters have the same parameters. The rated capacity of the MMC is 100MVA; the number of submodules is 200; the submodule capacitance is 2500uF; the bridge arm inductance is 50mH; and the current-limiting reactor is 150mH. An inter-pole metallic fault is set at the midpoint f1 of the line within the zone to verify the current-limiting effect of the proposed current-limiting strategy.

[0101] To evaluate the current limiting effect, a current threshold was set. The current threshold was selected based on the DC component of the arm current's blocking value. In this simulation system, the arm current blocking value is 3kA, and the DC component accounts for approximately 95% of the arm current during a fault; therefore, the threshold was set at 2.85kA. Under the current limiting control strategy, the later the fault current reaches the threshold, the better the current limiting effect. During the fault, the half-bridge MMC is not locked, and the DCCB does not operate. Based on the system without current limiting, the fault current reaches the threshold 0.8ms after the fault. The two-stage current limiting control strategy proposed in this paper reaches the threshold at 1.2ms, effectively limiting the fault current. A schematic diagram of the fault current is attached. Figure 4 As shown.

[0102] Example 2: Create as shown in the attached document Figure 3 The flexible DC transmission system shown is used as a simulation model. The DC line adopts the RL model. All converters are half-bridge MMC converters. MMC4 uses constant DC voltage control, and the other MMCs use constant power control. The four converters have the same parameters. The rated capacity of the MMC is 100MVA; the number of submodules is 200; the submodule capacitance is 2500uF; the bridge arm inductance is 50mH; and the current-limiting reactor is 150mH. An inter-pole fault is set at the midpoint f1 of the line within the zone to verify the effect of the current-limiting control strategy on the product of the integrals of the current mutations at both ends of the line.

[0103] When a fault occurs within the fault zone, the polarity of E(i) is the same at both ends of the line. Secondly, although the current-limiting control strategy reduces the magnitude of E(i), it does not change its polarity characteristic. When a fault occurs outside the fault zone, the polarity of E(i) at both ends of the line is opposite. Similarly, although the current-limiting control strategy reduces the magnitude of E(i), it does not change its polarity characteristic. The effect of the current-limiting control strategy on the integral value of the current surge is shown in the appendix. Figure 5 As shown.

[0104] Example 3: Create as shown in the attached document Figure 3 The flexible DC transmission system shown is used as a simulation model. The DC line adopts the RL model. All converters are half-bridge MMC converters. MMC4 uses constant DC voltage control, and the other MMCs use constant power control. The four converters have the same parameters. The rated capacity of the MMC is 100MVA; the number of submodules is 200; the submodule capacitance is 2500uF; the bridge arm inductance is 50mH; and the current-limiting reactor is 150mH. An inter-pole fault is set at the midpoint f1 of the line within the area.

[0105] When an inter-pole metallic fault occurs at point f1, the DC current is collected to calculate the current surge. The product of the integral values ​​of the current surges at both ends of the line and the integrals of the current surges is obtained. Then, a sign function is used to characterize the polarity of the product of the integrals of the current surges at both ends of the line. This polarity can effectively distinguish between faults within and outside the fault zone, as shown in the appendix. Figure 6 As shown.

[0106] Example 4: Create as shown in the attached document Figure 3 The flexible DC transmission system shown is used as a simulation model. The DC lines adopt the RL model. All converters are half-bridge MMC converters. MMC4 uses constant DC voltage control, and the other MMCs use constant power control. The four converters have the same parameters. The rated capacity of the MMC is 100MVA; the number of submodules is 200; the submodule capacitance is 2500uF; the bridge arm inductance is 50mH; and the current-limiting reactor is 150mH. Positive ground faults are set in lines f2, f1, and f3 within the area, and the transition resistances are set to 0.01Ω, 150Ω, and 300Ω, respectively.

[0107] The impact of transition resistance on line longitudinal protection based on current surge is shown in Table 1.

[0108] Table 1. Performance verification of transition resistance for longitudinal protection of lines based on current mutation.

[0109]

[0110]

[0111] As shown in Table 1, the proposed longitudinal protection method can correctly identify faults and has good resistance to transition resistance when different fault locations and transition resistances are set in the DC line.

[0112] Example 5: Create as shown in the attached document Figure 3 The flexible DC transmission system shown is used as a simulation model. The DC line adopts the RL model. All converters are half-bridge MMC converters. MMC4 uses constant DC voltage control, and the other MMCs use constant power control. The four converters have the same parameters. The rated capacity of the MMC is 100MVA; the number of submodules is 200; the submodule capacitance is 2500uF; the bridge arm inductance is 50mH; and the current-limiting reactor is 150mH. An inter-pole fault occurs on line f2 in the simulation area. Gaussian white noise with a signal-to-noise ratio of 10dB, 20dB, and 30dB is added to the sampling current. The impact of noise on the line longitudinal protection based on current mutation is shown in Table 2.

[0113] Table 2. Verification of the performance of line longitudinal protection based on current mutation caused by noise.

[0114]

[0115] As shown in Table 2, when noise interference is added to the sampling current of the DC line, the proposed longitudinal protection method can correctly identify the fault and has good noise resistance performance.

[0116] In another embodiment of the present invention, a DC line longitudinal protection system incorporating active current limiting control is provided, which can be used to implement the above-mentioned DC line longitudinal protection method incorporating active current limiting control. Specifically, the system includes:

[0117] The data acquisition module is used to detect the arm current in real time after a DC fault occurs in the DC grid. The arm current change rate is used to form the proportional coefficient K1 of the first-stage current limiting control, and the arm current amplitude is used to form the proportional coefficient K2 of the second-stage current limiting control strategy.

[0118] The current limiting judgment module is used to compare K1 with K2 and the arm current amplitude with the set value, forming the start criterion for the two-stage current limiting control strategy and determining the parameter selection range of the two-stage active current limiting control strategy.

[0119] The fault diagnosis module is used to determine whether the protection start-up conditions are met based on the rate of change of the bridge arm current. If the conditions are met, the current at both ends of the line is extracted, the sudden change in current at both ends of the line and the integral value of the sudden change in current are calculated, and the integral value of the sudden change in current at both ends is multiplied to obtain the product value. The sign function is used to process the integral product value of the sudden change in current at both ends of the line and compared with the setting value of 0 to determine whether the fault is inside or outside the zone. The fault pole is determined by the ratio of the integral product value of the sudden change in current at both ends of the positive and negative poles after the fault.

[0120] The module division in this embodiment of the invention is illustrative and represents only one logical functional division. In actual implementation, other division methods may be used. Furthermore, the functional modules in the various embodiments of the invention can be integrated into a single processor, exist as separate physical entities, or be integrated into a single module. The integrated modules described above can be implemented in hardware or as software functional modules.

[0121] In another embodiment of the present invention, a computer device is provided, comprising a processor and a memory. The memory stores a computer program, which includes program instructions. The processor executes the program instructions stored in the computer storage medium. The processor may be a Central Processing Unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. It is the computing and control core of the terminal, suitable for implementing one or more instructions, specifically suitable for loading and executing one or more instructions in the computer storage medium to achieve a corresponding method flow or corresponding function. The processor described in this embodiment of the present invention can be used in the operation of a DC line longitudinal protection method considering active current limiting control.

[0122] In another embodiment of the present invention, a storage medium is provided, specifically a computer-readable storage medium (Memory), which is a memory device in a computer device used to store programs and data. It is understood that the computer-readable storage medium here can include both the built-in storage medium in the computer device and extended storage media supported by the computer device. The computer-readable storage medium provides storage space that stores the terminal's operating system. Furthermore, the storage space also stores one or more instructions suitable for loading and execution by a processor. These instructions can be one or more computer programs (including program code). It should be noted that the computer-readable storage medium here can be a high-speed RAM memory or a non-volatile memory, such as at least one disk storage device. The processor can load and execute one or more instructions stored in the computer-readable storage medium to implement the corresponding steps of the DC line longitudinal protection method considering active current limiting control in the above embodiments.

[0123] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0124] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0125] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0126] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0127] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for longitudinal protection of DC lines considering active current limiting control, characterized in that, include: After a DC fault occurs in the DC power grid, the bridge arm current is monitored in real time, and the proportional coefficient of the first-stage current limiting control is used to determine the rate of change of the bridge arm current. K 1. The proportional coefficient of the two-stage current limiting control strategy is constructed using the amplitude of the bridge arm current. K 2; K 1 and K The comparison between the phase 2 and the comparison between the arm current amplitude and the set value constitute the start-up criterion for the two-stage current limiting control strategy, and determine the parameter selection range of the two-stage active current limiting control strategy. The protection start-up condition is determined based on the change rate of the bridge arm current. If it is met, the current at both ends of the line is extracted, the current change at both ends of the line and the integral value of the current change are calculated, and the integral value of the current change at both ends is multiplied to obtain the product value. The integral product of the current surge at both ends of the line is processed using a symbolic function and compared with the setting value of 0 to determine whether the fault is inside or outside the zone. The fault pole is determined by the ratio of the integral of the sudden change in current at both ends of the positive and negative lines after a fault. The ratio of the MMC's operational submodules after a failure to the submodules during normal operation is a proportionality coefficient. K ; The rate of change of the bridge arm current is obtained by differentiating the bridge arm current. proportionality coefficient K Upper limit 1 minus differential coefficient K D With the rate of change of bridge arm current The product of these factors yields the proportional coefficient of the first-stage current-limiting control strategy. K 1; Its expression and activation criterion are as follows: （1） In the formula: i ij For the bridge arm current; Δ1 and I set1 The threshold value for the first-stage current limiting control strategy is Δ1; Δ1 is 1.3 times the peak value of the arm current change rate under normal operation. I set1 It is 1.15 times the maximum value of the bridge arm current during normal operation; if the activation conditions of a current limiting control strategy are met, this current limiting circuit will be activated; otherwise, the detection will continue. The difference between the arm current and the peak value of the arm current under normal operation is as follows: PI The circuit obtains the bridge arm current difference and the rated DC bus voltage. U dcn Subtract the bridge arm current difference, then combine with U dcn Dividing yields the proportional coefficient of the two-stage current limiting control strategy. K 2; K 1 and K The comparison between the phase 2 and the comparison between the arm current amplitude and the set value constitute the start criterion for the two-stage current limiting control strategy. Its expression and activation criterion are as follows: （2） In the formula: i ij1 This represents the peak value of the bridge arm current under normal operating conditions. I set2 The threshold for initiating the two-stage flow limiting control strategy; I set2 It is 3 times the maximum value of the bridge arm current during normal operation; if the activation conditions of the two-stage current limiting control strategy are met, the two-stage current limiting control strategy is activated and the one-stage current limiting control strategy is deactivated; if the conditions are not met, the detection continues. Determine the parameter selection range for the two-stage active current limiting control strategy: right PI Medium proportion coefficient k p Explanation of parameter selection range: If the two-stage current limiting control strategy is to be adaptive, then the expression is: （3） In the formula: K min proportionality coefficient K The lower limit is set to 0.2; k p for PI The proportional coefficient in the process, then K D and k p The selection rules are as follows: （4）。 2. The method for longitudinal protection of DC lines considering active current limiting control according to claim 1, characterized in that, Determine whether the protection start-up conditions are met based on the rate of change of the bridge arm current: When a fault occurs in the DC line, the bridge arm current and the current signal at the DC line protection installation point are collected, with a data window of 3ms. The fault initiation conditions are determined based on the bridge arm current. The initiation conditions are as follows: （5） In the formula: Δ2 and I set3 The protection activation threshold; Δ2 is 1.5 times the peak value of the arm current change rate under normal operation. I set3 The current is 1.3 times the maximum value of the bridge arm current during normal operation; if the start-up conditions are met, proceed to the next step; otherwise, continue the test. Extract the current at both ends of the line, calculate the sudden change in current at both ends and the integral value of the sudden change in current, and multiply the integral values ​​of the sudden changes in current at both ends to obtain the product value: When an internal fault occurs, the current changes in both the positive and negative terminals in the same direction. When an external fault occurs, the current changes in both the positive and negative terminals in opposite directions. Therefore, the current change at the installation location of the line protection is calculated as follows: （6） In the formula: Δ i (t) represents the sudden change in current at the installation point of the line protection; i (t) represents the real-time measured current of the line; i Let 0(t) be the current of the line at the moment before the protection is activated; then the integral value of the current surge at the installation point of the line protection is calculated as follows: （7） In the formula: N The number of sampling points; the product of the integral values ​​of the current surge at the protection installation points at both ends of the line can distinguish between faults inside and outside the protection zone; the polarity is positive when a fault occurs inside the protection zone and negative when a fault occurs outside the protection zone; the expression for the product of the integral values ​​of the current surge is: （8） In the formula: Em and En These are the integral values ​​of the current surge at the protection installation points at both ends of the line.

3. The method for longitudinal protection of DC lines considering active current limiting control according to claim 1, characterized in that, The integral product of the current surge at both ends of the line is processed using a symbolic function and compared with the setting value of 0 to determine whether the fault is inside or outside the zone. Using symbolic functions to describe Emn The polarity is compared with 0 to determine whether it is inside or outside the region. The criterion is: (9) when Emn For the correct time sgn(Emn) A value of 1 satisfies the protection criterion, indicating an internal fault in the line; when Emn When negative sgn(Emn) A value of -1 indicates an external fault has occurred on the line.

4. The method for longitudinal protection of DC lines considering active current limiting control according to claim 1, characterized in that, The fault pole is determined by the ratio of the integrals of the sudden changes in current at both ends of the positive and negative lines after a fault. According to the principle that when a bipolar short-circuit fault occurs, the integral of the sudden current change on the positive and negative lines is equal; When a single-pole grounding fault occurs, the integral of the current change on the faulty pole line is much greater than that on the non-faulty pole. Integral product of the sudden changes in current on the positive and negative lines after a fault Emn The ratio is used to determine the fault pole, and the pole selection factor is defined as follows: L Its expression is: (10) In the formula: Emnp and Emnn Let be the product of the integrals of the current abrupt changes at the positive and negative limiting current ends, respectively; then the pole selection criterion is: (11) In the formula: l set A threshold of 1.2 was selected for the fault pole.

5. A DC line longitudinal protection system incorporating active current limiting control, characterized in that, include: The data acquisition module is used to detect the bridge arm current in real time after a DC fault occurs in the DC power grid, and uses the rate of change of the bridge arm current to form the proportional coefficient for the first-stage current limiting control. K 1. The proportional coefficient of the two-stage current limiting control strategy is constructed using the amplitude of the bridge arm current. K 2; The rate limiting judgment module is used for K 1 and K The comparison between the phase 2 and the comparison between the arm current amplitude and the set value constitute the start-up criterion for the two-stage current limiting control strategy, and determine the parameter selection range of the two-stage active current limiting control strategy. The fault judgment module is used to determine whether the protection start-up conditions are met based on the change rate of the bridge arm current. If they are met, the current at both ends of the line is extracted, the current change at both ends of the line and the integral value of the current change are calculated, and the integral value of the current change at both ends is multiplied to obtain the product value. The integral product of the current surge at both ends of the line is processed using a symbolic function and compared with the setting value of 0 to determine whether the fault is inside or outside the zone. The fault pole is determined by the ratio of the integral of the sudden change in current at both ends of the positive and negative lines after a fault. The ratio of the MMC's operational submodules after a failure to the submodules during normal operation is a proportionality coefficient. K ; The rate of change of the bridge arm current is obtained by differentiating the bridge arm current. proportionality coefficient K Upper limit 1 minus differential coefficient K D With the rate of change of bridge arm current The product of these factors yields the proportional coefficient of the first-stage current-limiting control strategy. K 1; Its expression and activation criterion are as follows: （1） In the formula: i ij For the bridge arm current; Δ1 and I set1 The threshold value for the first-stage current limiting control strategy is Δ1; Δ1 is 1.3 times the peak value of the arm current change rate under normal operation. I set1 It is 1.15 times the maximum value of the bridge arm current during normal operation; if the activation conditions of a current limiting control strategy are met, this current limiting circuit will be activated; otherwise, the detection will continue. The difference between the arm current and the peak value of the arm current under normal operation is as follows: PI The circuit obtains the bridge arm current difference and the rated DC bus voltage. U dcn Subtract the bridge arm current difference, then combine with U dcn Dividing yields the proportional coefficient of the two-stage current limiting control strategy. K 2; K 1 and K The comparison between the phase 2 and the comparison between the arm current amplitude and the set value constitute the start criterion for the two-stage current limiting control strategy. Its expression and activation criterion are as follows: （2） In the formula: i ij1 This represents the peak value of the bridge arm current under normal operating conditions. I set2 The threshold for initiating the two-stage flow limiting control strategy; I set2 It is 3 times the maximum value of the bridge arm current during normal operation; if the activation conditions of the two-stage current limiting control strategy are met, the two-stage current limiting control strategy is activated and the one-stage current limiting control strategy is deactivated; if the conditions are not met, the detection continues. Determine the parameter selection range for the two-stage active current limiting control strategy: right PI Medium proportion coefficient k p Explanation of parameter selection range: If the two-stage current limiting control strategy is to be adaptive, then the expression is: （3） In the formula: K min proportionality coefficient K The lower limit is set to 0.2; k p for PI The proportional coefficient in the process, then K D and k p The selection rules are as follows: （4）。 6. A computer device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the computer program, it implements the steps of the DC line longitudinal protection method that includes active current limiting control as described in any one of claims 1 to 4.

7. A computer-readable storage medium storing a computer program, characterized in that, When the computer program is executed by the processor, it implements the steps of the DC line longitudinal protection method that takes into account active current limiting control as described in any one of claims 1 to 4.

Images