A method and system for protection of a flexible high voltage direct current transmission line

Abstract

The application provides a flexible direct current transmission line protection method and system, which collects voltage modulus component values and current modulus component values on the local side of the flexible direct current transmission line; when a protection device detects a fault and starts, the time when the fault mutation ends is determined according to the voltage modulus component values, the current modulus component values and the load current, and the time when the current mutation quantity changes from positive to negative; then the preset voltage differential accumulation criterion, the negative current mutation quantity integral criterion and the current direction criterion are determined according to the voltage modulus component values, the current modulus component values, the time when the protection device starts, the time when the fault mutation ends and the time when the current mutation quantity changes from positive to negative; finally, the protection action output result of the protection device is determined according to the criterion result. The method and system can reliably and quickly identify and distinguish the fault characteristics of the intra-zone fault and the extra-zone fault, and effectively improve the sensitivity, reliability and selectivity of the main protection of the hybrid direct current transmission line.

Description

Technical Field

[0001] This invention relates to the field of relay protection, and more specifically, to a method and system for protecting flexible DC transmission lines. Background Technology

[0002] To further improve the construction and operation efficiency of flexible DC transmission systems, the academic community has proposed some modification schemes for existing flexible DC transmission line topologies, many of which primarily involve moving the installation location of current-limiting reactors to the DC filter outlet. This approach can significantly reduce the number of current-limiting reactors while maintaining their functionality, effectively reducing the cost and workload of engineering construction and subsequent maintenance. However, the current-limiting reactors in flexible DC transmission lines can directly affect the reflection and refraction characteristics of the forward traveling wave from a fault to the protection installation location. Changes in their installation location can significantly alter the fault characteristics of measured electrical quantities, severely impacting the applicability of the existing main protection principle for flexible DC transmission lines.

[0003] Currently, there are still few fault analysis results available in academia for such protection application scenarios. Traveling wave protection and differential undervoltage protection, inherited from traditional conventional DC transmission systems, not only have deficiencies in key protection performance aspects such as withstand transition resistance, numerical resistance, and external disturbance resistance, but their adaptability also makes it difficult for them to meet the basic requirements of flexible DC lines for main line protection performance in the aforementioned application scenarios. Therefore, it is still necessary to re-analyze the fault characteristics of the electrical quantities measured by line protection under different fault conditions in flexible DC transmission line scenarios where current-limiting reactors are moved externally, and to design a targeted overall main protection scheme that can fully meet actual performance requirements. Summary of the Invention

[0004] To address the problem that existing technologies for flexible DC transmission lines, which involve installing current-limiting reactors at the DC filter outlet, cause significant changes in the fault characteristics of measured electrical quantities, severely affecting the applicability of the main protection principle for flexible DC transmission lines, while lacking an overall main protection scheme that meets actual performance requirements, this invention provides a protection method and system for flexible DC transmission lines.

[0005] According to one aspect of the present invention, a method for protecting flexible DC transmission lines is provided, the method comprising:

[0006] The voltage and current modulus components of the flexible DC transmission line are collected on the local side, wherein the local side of the flexible DC transmission line is any one of the two sides of the line, and the current-limiting reactor of the flexible DC transmission line is installed at the DC filter outlet.

[0007] When the protection device detects a fault and starts, the time when the fault change ends is determined based on the current modulus component value.

[0008] The moment when the current mutation changes from positive to negative is determined based on the current modulus component value, the end time of the fault mutation, and the load current of the local pole of the flexible DC transmission line. The load current of the local pole of the flexible DC transmission line is the load current of any pole among the positive and negative poles of the local pole.

[0009] The first criterion result is determined based on the voltage modulus component value, the start time of the protection device, the end time of the fault change, and a preset voltage difference accumulation criterion; the second criterion result is determined based on the current modulus component value, the time when the current change amount changes from positive to negative, and a preset negative current change amount integration criterion; and the third criterion result is determined based on the current modulus component value, the load current, and a preset current direction criterion.

[0010] The protection action output result of the protection device is determined based on the results of the first criterion, the second criterion, and the third criterion.

[0011] Optionally, after the protection device detects a fault and activates, it determines the end time of the fault abrupt change based on the current modulus component value, including:

[0012] The current surge is calculated based on the current modulus component values, and the calculation formula is as follows:

[0013] ΔI y (t)=I y (t)-I y (tt j )

[0014] In the formula, ΔI y (t) represents the sudden change in current at time t, I y (t) represents the current-mode component at time t, I y (tt j ) for tt j The current modulus component value at time t, where t is the current time. j The data sampling time interval;

[0015] The reference time t for calculating the peak value of the current surge is based on the aforementioned current surge amount. k Its expression is:

[0016] ΔI y (t k )-ΔI y (t k -t j )≤0

[0017] According to t k -t j The sudden change in current ΔI at time 1 y (t k -tj Calculate the fault termination time t F Its expression is:

[0018] ΔI y (t F )>k1×ΔI y (t k -t j )

[0019] k1>0

[0020] In the formula, k1 is a constant value that is set.

[0021] Optionally, the moment when the current mutation changes from positive to negative is determined based on the current modulus component value, the end time of the fault mutation, and the load current of the local pole of the flexible DC transmission line. The expression for this is:

[0022]

[0023] In the formula, t G ΔI is the moment when the current abruptly changes from positive to negative. y (t G ) for t G The sudden change in current at time t, I y (t G ) for t G The current-mode component value at time t, I y (t G -t j ) for t G -t j The current modulus component value at time t, k2 is a set constant value, I N t represents the load current of this pole on this side. setA The time parameter is a preset constant value.

[0024] Optionally, a first criterion result is determined based on the voltage modulus component value, the start time of the protection device, the end time of the fault abrupt change, and a preset voltage differential accumulation criterion; a second criterion result is determined based on the current modulus component value, the load current, and a preset current direction criterion; and a third criterion result is determined based on the current modulus component value, the time when the current abrupt change from positive to negative, and a preset negative current abrupt change integral criterion, including:

[0025] Based on the voltage modulus component value, the start time of the protection device, the end time of the fault abrupt change, and a preset voltage differential accumulation criterion, the first criterion result is determined, wherein the expression for the voltage differential accumulation criterion is:

[0026]

[0027] In the formula, ΔU y (t) represents the voltage fluctuation at time t, U y (t) represents the voltage magnitude component at time t, U y (tt j ) for tt j The voltage modulus component value at time t0, where t0 is the time when the protection device starts, t0≤c≤t, U Δset The inherent parameters of the DC transmission system; when the expression of the voltage differential accumulation criterion is true, the result of the first criterion is that the voltage differential accumulation criterion is satisfied; when the expression of the voltage differential accumulation criterion is not true, the result of the first criterion is that the voltage differential accumulation criterion is not satisfied.

[0028] The second criterion result is determined based on the current modulus component value, the time when the current abrupt change from positive to negative, and a preset negative current abrupt change integral criterion, wherein the expression for the negative current abrupt change integral criterion is:

[0029]

[0030] In the formula, G(s) is the negative current mutation function, and t G ≤s≤t,ΔI y (s) represents the sudden change in current at time s, and k4 and t set All are constant values. k4 can be set according to the protection sensitivity according to the performance requirements of the protection to reliably distinguish and identify faults outside the zone when the fault is outside the zone outlet through a certain transition resistance. When the expression of the negative current mutation amount integral criterion is true, the result of the second criterion is that the negative current mutation amount integral criterion is satisfied. When the expression of the negative current mutation amount integral criterion is not true, the result of the second criterion is that the negative current mutation amount integral criterion is not satisfied.

[0031] The third criterion result is determined based on the current modulus component value, the load current, and the preset current direction criterion, wherein the expression for the current direction criterion is:

[0032] I y (t)-I N >k3×I N

[0033] In the formula, k3 is a set constant value; when the expression of the current direction criterion is true, the result of the third criterion is that the current direction criterion is satisfied; when the expression of the current direction criterion is false, the result of the third criterion is that the current direction criterion is not satisfied.

[0034] Optionally, determining the protection action output result of the protection device based on the first criterion result, the second criterion result, and the third criterion result includes:

[0035] When the first criterion result satisfies the voltage difference accumulation criterion, the second criterion result does not satisfy the negative current change amount integration criterion, and the third criterion result satisfies the current direction criterion, the protection action output result of the protection device is the protection action output.

[0036] When the first criterion result is not satisfied with the voltage differential accumulation criterion, or the second criterion result is satisfied with the negative current mutation integral criterion, or the third criterion result is not satisfied with the current direction criterion, the protection action output result of the protection device is to lock out the protection device.

[0037] According to another aspect of the present invention, a flexible DC transmission line protection system is provided, the system comprising:

[0038] The data acquisition unit is used to acquire the voltage modulus component value and current modulus component value of the flexible DC transmission line on its own side, wherein the flexible DC transmission line on its own side is any one of the two sides of the line, and the current limiting reactor of the flexible DC transmission line is installed at the DC filter outlet.

[0039] The first calculation unit is used to determine the end time of the fault sudden change based on the current modulus component value after the protection device detects a fault and starts.

[0040] The second calculation unit is used to determine the time when the current mutation amount changes from positive to negative based on the current modulus component value, the time when the fault mutation ends and the load current of the local pole of the flexible DC transmission line. The load current of the local pole of the flexible DC transmission line is the load current of any pole of the positive or negative pole of the local side.

[0041] The criterion result unit is used to determine a first criterion result based on the voltage modulus component value, the start time of the protection device, the end time of the fault change, and a preset voltage difference accumulation criterion; to determine a second criterion result based on the current modulus component value, the time when the current change amount changes from positive to negative, and a preset negative current change amount integration criterion; and to determine a third criterion result based on the current modulus component value, the load current, and a preset current direction criterion.

[0042] The result output unit is used to determine the protection action output result of the protection device based on the first criterion result, the second criterion result, and the third criterion result.

[0043] Optionally, the first computing unit includes:

[0044] The mutation quantum unit is used to calculate the current mutation amount based on the current mode component value, and its calculation formula is as follows:

[0045] ΔI y (t)=I y (t)-I y(tt j )

[0046] In the formula, ΔI y (t) represents the sudden change in current at time t, I y (t) represents the current-mode component at time t, I y (tt j ) for tt j The current modulus component value at time t, where t is the current time. j The data sampling time interval;

[0047] The first time subunit is used to calculate the reference time t of the current surge peak based on the current surge amount. k Its expression is:

[0048] ΔI y (t k )-ΔI y (t k -t j )≤0

[0049] The second time subunit is used to determine the time based on t. k -t j The sudden change in current ΔI at time 1 y (t k -t j Calculate the fault termination time t F Its expression is:

[0050] ΔI y (t F )>k1×ΔI y (t k -t j )

[0051] k1>0

[0052] In the formula, k1 is a constant value that is set.

[0053] Optionally, the second calculation unit determines the moment when the current mutation changes from positive to negative based on the current modulus component value, the end time of the fault mutation, and the load current of the local pole of the flexible DC transmission line. The expression for this is:

[0054]

[0055] In the formula, t G ΔI is the moment when the current abruptly changes from positive to negative. y (t G ) for t G The sudden change in current at time t, I y (t G ) for t GThe current-mode component value at time t, I y (t G -t j ) for t G -t j The current modulus component value at time t, k2 is a set constant value, I N t represents the load current of this pole on this side. setA The time parameter is a preset constant value.

[0056] Optionally, the criterion result unit includes:

[0057] The first result subunit is used to determine a first criterion result based on the voltage modulus component value, the start time of the protection device, the end time of the fault abrupt change, and a preset voltage differential accumulation criterion, wherein the expression for the voltage differential accumulation criterion is:

[0058]

[0059] In the formula, ΔU y (t) represents the voltage fluctuation at time t, U y (t) represents the voltage magnitude component at time t, U y (tt j ) for tt j The voltage modulus component value at time t0, where t0 is the time when the protection device starts, t0≤c≤t, U Δset The inherent parameters of the DC transmission system; when the expression of the voltage differential accumulation criterion is true, the result of the first criterion is that the voltage differential accumulation criterion is satisfied; when the expression of the voltage differential accumulation criterion is not true, the result of the first criterion is that the voltage differential accumulation criterion is not satisfied.

[0060] The second result subunit is used to determine the second criterion result based on the current modulus component value, the time when the current abrupt change from positive to negative, and a preset negative current abrupt change integral criterion, wherein the expression of the negative current abrupt change integral criterion is:

[0061]

[0062] In the formula, G(s) is the negative current mutation function, and t G ≤s≤t,ΔI y (s) represents the sudden change in current at time s, and k4 and t setAll are constant values. k4 can be set according to the protection sensitivity according to the performance requirements of the protection to reliably distinguish and identify faults outside the zone when the fault is outside the zone outlet through a certain transition resistance. When the expression of the negative current mutation amount integral criterion is true, the result of the second criterion is that the negative current mutation amount integral criterion is satisfied. When the expression of the negative current mutation amount integral criterion is not true, the result of the second criterion is that the negative current mutation amount integral criterion is not satisfied.

[0063] The third result subunit is used to determine the third criterion result based on the current modulus component value, the load current, and the preset current direction criterion, wherein the expression of the current direction criterion is:

[0064] I y (t)-I N >k3×I N

[0065] In the formula, k3 is a set constant value; when the expression of the current direction criterion is true, the result of the third criterion is that the current direction criterion is satisfied; when the expression of the current direction criterion is false, the result of the third criterion is that the current direction criterion is not satisfied.

[0066] Optionally, the result output unit determines the protection action output result of the protection device based on the first criterion result, the second criterion result, and the third criterion result, including:

[0067] When the first criterion result satisfies the voltage difference accumulation criterion, the second criterion result does not satisfy the negative current change amount integration criterion, and the third criterion result satisfies the current direction criterion, the protection action output result of the protection device is the protection action output.

[0068] When the first criterion result is not satisfied with the voltage differential accumulation criterion, or the second criterion result is satisfied with the negative current mutation integral criterion, or the third criterion result is not satisfied with the current direction criterion, the protection action output result of the protection device is to lock out the protection device.

[0069] The flexible DC transmission line protection method and system provided by the present invention collects the voltage modulus component value and current modulus component value of the flexible DC transmission line on its own side; when the protection device detects a fault and starts, it determines the fault change ending time based on the current modulus component value; it determines the time when the current change amount changes from positive to negative based on the current modulus component value, the fault change ending time, and the load current of the local pole of the flexible DC transmission line; it determines a first criterion result based on the voltage modulus component value, the protection device start time, the fault change ending time, and a preset voltage difference accumulation criterion; it determines a second criterion result based on the current modulus component value, the time when the current change amount changes from positive to negative, and a preset negative current change amount integration criterion; it determines a third criterion result based on the current modulus component value, the load current, and a preset current direction criterion; and it determines the protection action output result of the protection device based on the first criterion result, the second criterion result, and the third criterion result. The method and system described herein construct a composite protection criterion based on the continuous change trend of the protection measurement current after a sudden change under different fault conditions. This criterion is applicable to flexible DC transmission lines when the current-limiting reactor is moved outward. It can not only withstand a large transition resistance, but also has excellent operating performance. It does not maloperate under various disturbances outside the protection zone. It can reliably and quickly identify and distinguish the fault characteristics of faults inside and outside the protection zone, effectively improving the sensitivity, reliability and selectivity of the main protection of hybrid DC transmission lines. Attached Figure Description

[0070] Exemplary embodiments of the present invention can be more fully understood by referring to the following figures:

[0071] Figure 1 This is a flowchart illustrating a flexible DC transmission line protection method according to a preferred embodiment of the present invention.

[0072] Figure 2 This is a schematic diagram of the structure of a flexible DC transmission system under the condition of outward displacement of current-limiting reactance according to a preferred embodiment of the present invention;

[0073] Figure 3 A waveform diagram illustrating the voltage differential accumulation criterion during an intra-area fault according to a preferred embodiment of the present invention.

[0074] Figure 4 A waveform diagram of the integral criterion for negative current mutation during an intra-regional fault according to a preferred embodiment of the present invention.

[0075] Figure 5 This is a waveform diagram of the voltage differential accumulation during a positive external fault according to a preferred embodiment of the present invention;

[0076] Figure 6This is a waveform diagram of the integral of the negative current surge during a positive external fault according to a preferred embodiment of the present invention.

[0077] Figure 7 This is a schematic diagram of the current mode component waveform during a fault outside the positive region according to a preferred embodiment of the present invention;

[0078] Figure 8 This is a schematic diagram of the structure of a flexible DC transmission line protection system according to a preferred embodiment of the present invention. Detailed Implementation

[0079] Exemplary embodiments of the invention will now be described with reference to the accompanying drawings. However, the invention may be embodied in many different forms and is not limited to the embodiments described herein. These embodiments are provided to fully and completely disclose the invention and to fully convey its scope to those skilled in the art. The terminology used in the exemplary embodiments illustrated in the drawings is not intended to limit the invention. In the drawings, the same units / elements are referred to by the same reference numerals.

[0080] Unless otherwise stated, the terms used herein (including technical terms) have their common meaning as understood by one of ordinary skill in the art. Furthermore, it is understood that terms defined in commonly used dictionaries should be understood to have a meaning consistent with the context of their relevant field, and not to be interpreted as having an idealized or overly formal meaning.

[0081] Figure 1 This is a schematic flowchart of a flexible DC transmission line protection method according to a preferred embodiment of the present invention. Figure 1 As shown, the flexible DC transmission line protection method of this preferred embodiment starts from step 101.

[0082] In step 101, the voltage modulus component and current modulus component values ​​of the flexible DC transmission line are collected on this side, wherein the flexible DC transmission line on this side is any one of the two sides of the line, and the current-limiting reactor of the flexible DC transmission line is installed at the DC filter outlet.

[0083] Figure 2 This is a schematic diagram of a flexible DC transmission system under the condition of outward displacement of the current-limiting reactance according to a preferred embodiment of the present invention. Figure 2 As shown, the flexible DC transmission system is a double-circuit transmission line with bipolar symmetry. The current-limiting reactance L of each pole is... dc It is installed at the DC filter outlet. In this embodiment, MMC #1 is selected as this side, with the positive electrode being the local electrode.

[0084] In step 102, after the protection device detects a fault and starts, the time when the fault change ends is determined based on the current modulus component value.

[0085] Preferably, after the protection device detects a fault and activates, the time when the fault abruptly ends is determined based on the current modulus component value, including:

[0086] The current surge is calculated based on the current modulus component values, and the calculation formula is as follows:

[0087] ΔI y (t)=I y (t)-I y (tt j )

[0088] In the formula, ΔI y (t) represents the sudden change in current at time t, I y (t) represents the current-mode component at time t, I y (tt j ) for tt j The current modulus component value at time t, where t is the current time. j The data sampling time interval;

[0089] The reference time t for calculating the peak value of the current surge is based on the aforementioned current surge amount. k Its expression is:

[0090] ΔI y (t k )-ΔI y (t k -t j )≤0

[0091] According to t k -t j The sudden change in current ΔI at time 1 y (t k -t j Calculate the fault termination time t F Its expression is:

[0092] ΔI y (t F )>k1×ΔI y (t k -t j )

[0093] k1>0

[0094] In the formula, k1 is a constant value that is set.

[0095] Calculate the fault termination time t F This is the first step in reliably identifying the current development trend after the initial traveling current of the fault precursor arrives at the local side of the line and completes its abrupt change. The above calculation of the fault abrupt change termination time t... FThe function of the expression is as follows: After the line protection on this side is started, the current change ΔI at time t of the current sampling point is calculated at each sampling point using the formula for calculating the current change. y (t); When the sudden current peak occurs, the reference time t for calculating the current sudden peak is used. k The expression yields the reference time for the current surge to reach its peak value; however, the charging time of the forward traveling wave of the fault to the equivalent branch of the current-limiting reactor and converter on this side requires time, and the surge will continue for a certain period of time to decay smoothly and rapidly. Therefore, the fault surge termination time t is calculated. F The expression determines the sampling point where the current mutation steepness drops rapidly, and the time t to which that sampling point belongs. F This is used to determine the moment when the measured current stops abruptly. In one embodiment, the coefficient k1 in this expression is set to 0.5 based on experience, and the choice should be based on the application scenario to quickly select the actual moment when the measured current stops abruptly.

[0096] In step 103, the moment when the current mutation changes from positive to negative is determined based on the current modulus component value, the end time of the fault mutation, and the load current of the local pole of the flexible DC transmission line. The load current of the local pole of the flexible DC transmission line is the load current of any one of the positive or negative poles of the local pole.

[0097] Preferably, the moment when the current mutation changes from positive to negative is determined based on the current modulus component value, the end time of the fault mutation, and the load current of the local pole of the flexible DC transmission line. The expression for this is:

[0098]

[0099] In the formula, t G ΔI is the moment when the current abruptly changes from positive to negative. y (tG) is t G The sudden change in current at time t, I y (t G ) for t G The current-mode component value at time t, I y (t G -t j ) for t G -t j The current modulus component value at time t, k2 is a set constant value, I N t represents the load current of this pole on this side. setA The time parameter is a preset constant value.

[0100] Whether the current surge becomes negative is an important indicator for determining whether a fault has occurred within the zone. The second equation in the above expression for determining the moment when the current surge changes from positive to negative clearly defines the moment when the current surge crosses zero and becomes negative, based on the measured current modulus component. In one embodiment, the coefficient k2 in this expression is set to 0.001 based on experience. The third equation in the expression clearly states that the time elapsed from the end of the current surge to the start of the surge current crossing zero should not be too long; otherwise, the protection device should directly identify the disturbance measured on its own side as a fault or disturbance within the zone, without further determining the subsequent protection criteria, thus saving the hardware resources of the protection device.

[0101] In step 104, a first criterion result is determined based on the voltage modulus component value, the start time of the protection device, the end time of the fault mutation, and a preset voltage differential accumulation criterion; a second criterion result is determined based on the current modulus component value, the time when the current mutation changes from positive to negative, and a preset negative current mutation integral criterion; and a third criterion result is determined based on the current modulus component value, the load current, and a preset current direction criterion.

[0102] Preferably, a first criterion result is determined based on the voltage modulus component value, the start time of the protection device, the end time of the fault abrupt change, and a preset voltage differential accumulation criterion; a second criterion result is determined based on the current modulus component value, the load current, and a preset current direction criterion; and a third criterion result is determined based on the current modulus component value, the time when the current abrupt change from positive to negative, and a preset negative current abrupt change integral criterion, including:

[0103] Based on the voltage modulus component value, the start time of the protection device, the end time of the fault abrupt change, and a preset voltage differential accumulation criterion, the first criterion result is determined, wherein the expression for the voltage differential accumulation criterion is:

[0104]

[0105] In the formula, ΔU y (t) represents the voltage fluctuation at time t, U y (t) represents the voltage magnitude component at time t, U y (tt j ) for tt j The voltage modulus component value at time t0, where t0 is the time when the protection device starts, t0≤c≤t, U ΔsetThe inherent parameters of the DC transmission system are set values ​​based on the premise that the fault can be correctly identified by the criterion under the condition that a 750Ω transition fault occurs in a single pole at the end of the region. When the expression of the voltage differential accumulation criterion is true, the result of the first criterion is that the voltage differential accumulation criterion is satisfied; when the expression of the voltage differential accumulation criterion is not true, the result of the first criterion is that the voltage differential accumulation criterion is not satisfied.

[0106] Because the current-limiting reactor is moved outward, the forward voltage traveling wave before the fault will allow for a large voltage jump when it reaches the protection installation point on this side, and the measured voltage will recover relatively slowly after the jump. Therefore, the voltage accumulation amount is designed according to the above-mentioned voltage differential accumulation criterion expression, which can effectively improve the main protection's ability to cope with transition resistance when identifying faults on this line and adjacent lines. This criterion is applied from the moment the protection device starts (t0) to the moment the sudden current ends (t). F If the condition is met before the mutation, the negative current mutation integral criterion is initiated for the next step of judgment; otherwise, the protection device is directly locked out.

[0107] The second criterion result is determined based on the current modulus component value, the time when the current abrupt change from positive to negative, and a preset negative current abrupt change integral criterion, wherein the expression for the negative current abrupt change integral criterion is:

[0108]

[0109] In the formula, G(s) is the negative current mutation function, and t G ≤s≤t,ΔI y (s) represents the sudden change in current at time s, and k4 and t set All are constant values. k4 can be set according to the protection sensitivity according to the performance requirements of the protection to reliably distinguish and identify faults outside the zone when the fault is outside the zone outlet through a certain transition resistance. When the expression of the negative current mutation integral criterion is true, the result of the second criterion is that the negative current mutation integral criterion is satisfied. When the expression of the negative current mutation integral criterion is not true, the result of the second criterion is that the negative current mutation integral criterion is not satisfied.

[0110] When an intra-zone fault occurs, ΔI y (s) The rate at which the measured current changes from zero to negative is slower than during faults outside the protection zone, especially under low-resistance fault conditions. In some cases, the measured current may not have crossed zero by the time the reverse traveling wave reaches the protection installation point on this side. However, during faults outside the protection zone, the measured current develops in the negative direction, ΔI... y (s) The speed at which it crosses zero and becomes negative is very fast, and at this time ΔI y (s) will continue to decrease, which is consistent with ΔI during faults within the region. y(s) stabilizes near 0 after crossing zero, forming a contrast. Therefore, using the expression for the above-mentioned integral criterion for negative current abrupt change, it is required that the calculated current abrupt change ΔI after crossing zero is... y (s) remains negative for a period of time, and must continue to develop within that duration. When this condition is met, the fault is identified as an external fault, and the protection device is locked out; otherwise, if the criterion is not met after the criterion is activated until the moment t when the sudden current changes from positive to negative... G If the above conditions are still not met, the fault is identified as an intra-zone fault, and the protection device is allowed to activate. In the formula, the setpoint k4 is a given negative value, which can be adjusted according to the protection sensitivity to reliably distinguish and identify extra-zone faults when the extra-zone outlet passes through a certain transition resistance. In one embodiment, t set The value is 0.8.

[0111] The third criterion result is determined based on the current modulus component value, the load current, and the preset current direction criterion, wherein the expression for the current direction criterion is:

[0112] I y (t)-I N >k3×I N

[0113] In the formula, k3 is a set constant value; when the expression of the current direction criterion is true, the result of the third criterion is that the current direction criterion is satisfied; when the expression of the current direction criterion is false, the result of the third criterion is that the current direction criterion is not satisfied.

[0114] When a fault occurs outside the reverse zone, the direction of the sudden change in current measured on this side is negative, i.e., ΔI y (t) is less than 0. At this time, it can be easily identified by the above current direction criterion, so that the protection device can reliably not operate. Specifically, in one embodiment, k3 is 0.001, that is, starting from the time t0 when the protection device detects the fault, only when the difference between the collected current modulus component and the load current is a positive value greater than 0.001 times the load current, it is determined that the DC transmission line fault occurs in the positive direction with the protection installation point as the reference point, and the protection device is allowed to operate; otherwise, it is determined that the fault is a fault outside the reverse zone, and the protection device is blocked.

[0115] In step 105, the protection action output result of the protection device is determined based on the first criterion result, the second criterion result, and the third criterion result.

[0116] Preferably, determining the protection action output result of the protection device based on the first criterion result, the second criterion result, and the third criterion result includes:

[0117] When the first criterion result satisfies the voltage difference accumulation criterion, the second criterion result does not satisfy the negative current change amount integration criterion, and the third criterion result satisfies the current direction criterion, the protection action output result of the protection device is the protection action output.

[0118] When the first criterion result is not satisfied with the voltage differential accumulation criterion, or the second criterion result is satisfied with the negative current mutation integral criterion, or the third criterion result is not satisfied with the current direction criterion, the protection action output result of the protection device is to lock out the protection device.

[0119] In one embodiment, according to Figure 2 A simulation model of the hybrid DC transmission system shown is built, and the effectiveness of the method described in this embodiment is verified by the simulation waveforms of different fault attributes occurring at various fault points. Figure 2 Of the fault points F1 to F4 shown, F1 and F2 are faults within the zone that the positive double-sided protection should correctly reflect and trip, with F1 being the fault point at the beginning of the positive pole and F2 being the fault point at the end of the positive pole; F3 and F4 are faults outside the zone that the positive double-sided protection should not trip, and depending on the reference point, F3 and F4 will belong to either the reverse outside the zone fault or the forward outside the zone fault, respectively.

[0120] Figure 3 This is a waveform diagram of the voltage differential accumulation during an intra-regional fault according to a preferred embodiment of the present invention. Figure 3 and Figure 4 The embodiment shown simulates the change in voltage differential accumulation on the MMC#1 side over time when a unipolar metallic ground fault occurs from F1 to F4. The simulation is performed under the worst-case scenario, taking a high-resistance fault at the end of the zone as an example. Figure 3 This is the waveform showing the change in voltage differential accumulation over time when a 750Ω ground fault occurs at point F2 at the end of the zone. For example... Figure 3 As shown, the solid line represents the waveform of the voltage differential accumulation over time, and the dashed line represents the constant value U. Δset In the case of a high-resistance fault at the end of the circuit, the voltage drops suddenly upon fault initiation. Due to the slow transition process between the current-limiting reactor and the converter, the voltage recovery takes time. Therefore, the voltage accumulation criterion remains greater than the set value within the integration time window, indicating reliable operation of the criterion. This suggests a possible fault in this line or an adjacent line, thus triggering the negative current mutation integral location criterion to further locate the fault.

[0121] Figure 4 This is a waveform diagram of the integral of negative current mutation during an intra-regional fault according to a preferred embodiment of the present invention. Figure 4 In the diagram, the solid line represents the waveform of the integral of the sudden change in negative current over time, and the dashed line represents a constant value k4, which is less than 0. For example... Figure 4As shown, the sudden change in current will gradually and smoothly decrease under in-zone fault conditions, eventually stabilizing near 0. This means that after the sudden change in current crosses zero and becomes negative, the integral term calculated by the criterion at the current time may have a positive value, thus keeping the final negative current sudden change integral criterion action value near 0. The criterion is not satisfied, i.e., an in-zone fault has occurred. Thus, the fault at the positive terminal of the zone via a 750Ω ground fault was correctly and reliably identified within approximately 1.4ms.

[0122] Figure 5 This is a waveform diagram of the voltage differential accumulation during a positive external fault according to a preferred embodiment of the present invention. Figure 5 and Figure 6 The waveform is a simulation verification based on the worst-case scenario, taking a metallic grounding fault at point F4 outside the positive zone as an example. Figure 5 In the diagram, the solid line represents the waveform of the voltage differential accumulation over time, and the dashed line represents the constant value U. Δset ,like Figure 5 As shown, when the first traveling wave of an external fault is refracted into this line and transmitted to the installation location of the protection device on this side, the amplitude of the traveling wave is not significantly weakened. Its abrupt change characteristics are more obvious than those of a high-resistance fault within the zone, satisfying the operating conditions of the voltage differential accumulation criterion, that is, a fault may have occurred on this line or an adjacent line.

[0123] Figure 6 This is a waveform diagram of the integral of the negative current mutation during a positive external fault according to a preferred embodiment of the present invention. Figure 6 In the diagram, the solid line represents the waveform of the integral of the sudden change in negative current over time, and the dashed line represents a constant value k4, which is less than 0. For example... Figure 6 As shown, refer to Figure 5 The period during which the sudden change in medium current continues to decrease after crossing zero and turning negative indicates that the integral of the sudden change in negative current is always close to -1, thus quickly reaching the set value. This is equivalent to satisfying the integral criterion for sudden change in negative current. The output result of the protection device should be to lock out the protection device. This is consistent with the protection being locked out when there is a fault outside the positive zone. The protection is reliable and does not operate, so the verification result is correct.

[0124] Figure 7 This is a schematic diagram of the current mode component waveform during a fault outside the positive region according to a preferred embodiment of the present invention. Figure 7 The simulation verification was conducted based on the worst-case scenario, taking a metallic grounding fault at point F3 outside the reverse zone as an example. Figure 7 In the diagram, the solid line represents the difference between the collected current modulus component and the load current, and the dashed line represents the constant value k3*I. N .like Figure 7 As shown, the current modulus component on this side drops below the set value immediately after the fault starts and remains in this state for a long time. The protection eliminates the fault in the zone through the current direction, thus reliably not operating.

[0125] The simulation experiments described above show that the method described in this embodiment, based on the continuous change trend of the protection measurement current after sudden change under different fault conditions, constructs a composite protection criterion with excellent action performance based on the integral of negative current sudden change and the accumulation of voltage difference. It does not malfunction under various disturbances outside the zone and can reliably and quickly identify and distinguish the fault characteristics of faults inside and outside the zone, effectively improving the sensitivity, reliability and selectivity of the main protection of hybrid DC transmission lines.

[0126] Figure 8 This is a schematic diagram of the structure of a flexible DC transmission line protection system according to a preferred embodiment of the present invention. Figure 8 As shown, the flexible DC transmission line protection system of this preferred embodiment includes:

[0127] The data acquisition unit 801 is used to acquire the voltage modulus component value and current modulus component value of the flexible DC transmission line on its own side, wherein the flexible DC transmission line on its own side is any one of the two sides of the line, and the current limiting reactor of the flexible DC transmission line is installed at the DC filter outlet.

[0128] The first calculation unit 802 is used to determine the end time of the fault sudden change based on the current modulus component value after the protection device detects a fault and starts.

[0129] The second calculation unit 803 is used to determine the time when the current mutation amount changes from positive to negative based on the current modulus component value, the fault mutation end time and the load current of the local pole of the flexible DC transmission line, wherein the load current of the local pole of the flexible DC transmission line is the load current of any pole among the positive and negative poles of the local pole.

[0130] The criterion result unit 804 is used to determine a first criterion result based on the voltage modulus component value, the start time of the protection device, the end time of the fault change, and a preset voltage difference accumulation criterion; to determine a second criterion result based on the current modulus component value, the time when the current change amount changes from positive to negative, and a preset negative current change amount integration criterion; and to determine a third criterion result based on the current modulus component value, the load current, and a preset current direction criterion.

[0131] The result output unit 805 is used to determine the protection action output result of the protection device based on the first criterion result, the second criterion result, and the third criterion result.

[0132] Preferably, the first computing unit includes:

[0133] The mutation quantum unit is used to calculate the current mutation amount based on the current mode component value, and its calculation formula is as follows:

[0134] ΔI y (t)=I y(t)-I y (tt j )

[0135] In the formula, ΔI y (t) represents the sudden change in current at time t, I y (t) represents the current-mode component at time t, I y (tt j ) for tt j The current modulus component value at time t, where t is the current time. j The data sampling time interval;

[0136] The first time subunit is used to calculate the reference time t of the current surge peak based on the current surge amount. k Its expression is:

[0137] ΔI y (t k )-ΔI y (t k -t j )≤0

[0138] The second time subunit is used to determine the time based on t. k -t j The sudden change in current ΔI at time 1 y (t k -t j Calculate the fault termination time t F Its expression is:

[0139] ΔI y (t F )>k1×ΔI y (t k -t j )

[0140] k1>0

[0141] In the formula, k1 is a constant value that is set.

[0142] Preferably, the second calculation unit determines the moment when the current mutation changes from positive to negative based on the current modulus component value, the end time of the fault mutation, and the load current of the local pole of the flexible DC transmission line, and its expression is:

[0143]

[0144] In the formula, t G ΔI is the moment when the current abruptly changes from positive to negative. y (t G ) for t G The sudden change in current at time t, I y (t G) for t G The current-mode component value at time t, I y (t G -t j ) for t G -t j The current modulus component value at time t, k2 is a set constant value, I N t represents the load current of this pole on this side. setA The time parameter is a preset constant value.

[0145] Preferably, the criterion result unit includes:

[0146] The first result subunit is used to determine a first criterion result based on the voltage modulus component value, the start time of the protection device, the end time of the fault abrupt change, and a preset voltage differential accumulation criterion, wherein the expression for the voltage differential accumulation criterion is:

[0147]

[0148] In the formula, ΔU y (t) represents the voltage fluctuation at time t, U y (t) represents the voltage magnitude component at time t, U y (tt j ) for tt j The voltage modulus component value at time t0, where t0 is the time when the protection device starts, t0≤c≤t, U Δset The inherent parameters of the DC transmission system; when the expression of the voltage differential accumulation criterion is true, the result of the first criterion is that the voltage differential accumulation criterion is satisfied; when the expression of the voltage differential accumulation criterion is not true, the result of the first criterion is that the voltage differential accumulation criterion is not satisfied.

[0149] The second result subunit is used to determine the second criterion result based on the current modulus component value, the time when the current abrupt change from positive to negative, and a preset negative current abrupt change integral criterion, wherein the expression of the negative current abrupt change integral criterion is:

[0150]

[0151] In the formula, G(s) is the negative current mutation function, and t G ≤s≤t,ΔI y (s) represents the sudden change in current at time s, and k4 and t setAll are constant values. k4 can be set according to the protection sensitivity according to the performance requirements of the protection to reliably distinguish and identify faults outside the zone when the fault is outside the zone outlet through a certain transition resistance. When the expression of the negative current mutation amount integral criterion is true, the result of the second criterion is that the negative current mutation amount integral criterion is satisfied. When the expression of the negative current mutation amount integral criterion is not true, the result of the second criterion is that the negative current mutation amount integral criterion is not satisfied.

[0152] The third result subunit is used to determine the third criterion result based on the current modulus component value, the load current, and the preset current direction criterion, wherein the expression of the current direction criterion is:

[0153] I y (t)-I N >k3×I N

[0154] In the formula, k3 is a set constant value; when the expression of the current direction criterion is true, the result of the third criterion is that the current direction criterion is satisfied; when the expression of the current direction criterion is false, the result of the third criterion is that the current direction criterion is not satisfied.

[0155] Preferably, the result output unit 805 determines the protection action output result of the protection device based on the first criterion result, the second criterion result, and the third criterion result, including:

[0156] When the first criterion result satisfies the voltage difference accumulation criterion, the second criterion result does not satisfy the negative current change amount integration criterion, and the third criterion result satisfies the current direction criterion, the protection action output result of the protection device is the protection action output.

[0157] When the first criterion result is not satisfied with the voltage differential accumulation criterion, or the second criterion result is satisfied with the negative current mutation integral criterion, or the third criterion result is not satisfied with the current direction criterion, the protection action output result of the protection device is to lock out the protection device.

[0158] The steps of the flexible DC transmission line protection system described in this preferred embodiment to construct a composite protection criterion based on the continuous change trend of the protection measurement current after a sudden change under different fault conditions, and to determine the operation of the protection device, are the same as those taken by the flexible DC transmission line protection system, and the technical effects achieved are also the same. Therefore, they will not be repeated here.

[0159] The invention has been described with reference to a few embodiments. However, as will be known to those skilled in the art, and as defined in the appended claims, other embodiments besides those disclosed above fall equivalently within the scope of the invention.

[0160] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the art, unless otherwise expressly defined herein. All references to “a / the / the [device, component, etc.]” ​​are openly interpreted as at least one instance of said device, component, etc., unless otherwise expressly stated. The steps of any method disclosed herein need not be performed in the exact order disclosed unless explicitly stated otherwise.

[0161] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application 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.

[0162] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. 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... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0163] 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.

[0164] 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.

[0165] 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 protection method for flexible DC transmission lines, characterized in that, The method includes: The voltage and current modulus components of the flexible DC transmission line are collected on the local side, wherein the local side of the flexible DC transmission line is any one of the two sides of the line, and the current-limiting reactor of the flexible DC transmission line is installed at the DC filter outlet. When the protection device detects a fault and activates, it determines the end time of the fault abrupt change based on the current modulus component value, including: The current surge is calculated based on the current modulus component values, and the calculation formula is as follows: In the formula, ΔI y (t) represents the sudden change in current at time t, I y (t) represents the current-mode component at time t, I y (tt j ) for tt j The current modulus component value at time t, where t is the current time. j The data sampling time interval; The reference time t for calculating the peak value of the current surge is based on the aforementioned current surge amount. k Its expression is: According to t k -t j The sudden change in current ΔI at time 1 y (t k -t j Calculate the time t when the fault suddenly ends. F Its expression is: In the formula, k1 is a constant value that is set; The moment when the current mutation changes from positive to negative is determined based on the current modulus component value, the end time of the fault mutation, and the load current of the local pole of the flexible DC transmission line. Here, the load current of the local pole of the flexible DC transmission line is the load current of either the positive or negative pole on this side. The expression for the moment when the current mutation changes from positive to negative is: In the formula, t G ΔI is the moment when the current abruptly changes from positive to negative. y (t G ) for t G The sudden change in current at time t, I y (t G ) for t G The current-mode component value at time t, I y (t G -t j ) for t G -t j The current modulus component value at time t, k2 is a set constant value, I N t represents the load current of this pole on this side. setA To limit the time parameter, it is a preset constant value; The first criterion result is determined based on the voltage modulus component value, the start time of the protection device, the end time of the fault change, and a preset voltage difference accumulation criterion; the second criterion result is determined based on the current modulus component value, the time when the current change amount changes from positive to negative, and a preset negative current change amount integration criterion; and the third criterion result is determined based on the current modulus component value, the load current, and a preset current direction criterion. The protection action output result of the protection device is determined based on the results of the first criterion, the second criterion, and the third criterion.

2. The method according to claim 1, characterized in that, A first criterion result is determined based on the voltage modulus component value, the start time of the protection device, the end time of the fault abrupt change, and a preset voltage differential accumulation criterion; a second criterion result is determined based on the current modulus component value, the time when the current abrupt change from positive to negative, and a preset negative current abrupt change integral criterion; and a third criterion result is determined based on the current modulus component value, the load current, and a preset current direction criterion, including: Based on the voltage modulus component value, the start time of the protection device, the end time of the fault abrupt change, and a preset voltage differential accumulation criterion, the first criterion result is determined, wherein the expression for the voltage differential accumulation criterion is: In the formula, ΔU y (t) represents the voltage fluctuation at time t, U y (t) represents the voltage magnitude component at time t, U y (tt j ) for tt j The voltage modulus component value at time t0, where t0 is the moment the protection device starts, t0≤c≤t, U Δset These are inherent parameters of the DC transmission system; when the expression of the voltage differential accumulation criterion is true, the first criterion result is that the voltage differential accumulation criterion is satisfied; when the expression of the voltage differential accumulation criterion is not true, the first criterion result is that the voltage differential accumulation criterion is not satisfied. The second criterion result is determined based on the current modulus component value, the time when the current abrupt change from positive to negative, and a preset negative current abrupt change integral criterion, wherein the expression for the negative current abrupt change integral criterion is: In the formula, G(s) is the negative current mutation function, and t G ≤s≤t,ΔI y (s) represents the sudden change in current at time s, and k4 and t set All are set constant values; when the expression of the negative current mutation integral criterion is true, the result of the second criterion is that the negative current mutation integral criterion is satisfied; when the expression of the negative current mutation integral criterion is not true, the result of the second criterion is that the negative current mutation integral criterion is not satisfied. The third criterion result is determined based on the current modulus component value, the load current, and the preset current direction criterion, wherein the expression for the current direction criterion is: In the formula, k3 is a set constant value; when the expression of the current direction criterion is true, the result of the third criterion is that the current direction criterion is satisfied; when the expression of the current direction criterion is false, the result of the third criterion is that the current direction criterion is not satisfied.

3. The method according to claim 2, characterized in that, The protective action output result of the protective device is determined based on the results of the first criterion, the second criterion, and the third criterion, including: When the first criterion result satisfies the voltage difference accumulation criterion, the second criterion result does not satisfy the negative current change amount integration criterion, and the third criterion result satisfies the current direction criterion, the protection action output result of the protection device is the protection action output. When the first criterion result is not satisfied with the voltage differential accumulation criterion, or the second criterion result is satisfied with the negative current mutation integral criterion, or the third criterion result is not satisfied with the current direction criterion, the protection action output result of the protection device is to lock out the protection device.

4. A flexible DC transmission line protection system, characterized in that, The system includes: The data acquisition unit is used to acquire the voltage modulus component value and current modulus component value of the flexible DC transmission line on its own side, wherein the flexible DC transmission line on its own side is any one of the two sides of the line, and the current limiting reactor of the flexible DC transmission line is installed at the DC filter outlet. The first calculation unit is used to determine the end time of the fault abrupt change based on the current modulus component value after the protection device detects a fault and starts, including: The mutation quantum unit is used to calculate the current mutation amount based on the current mode component value, and its calculation formula is as follows: In the formula, ΔI y (t) represents the sudden change in current at time t, I y (t) represents the current-mode component at time t, I y (tt j ) for tt j The current modulus component value at time t, where t is the current time. j The data sampling time interval; The first time subunit is used to calculate the reference time t of the current surge peak based on the current surge amount. k Its expression is: The second time subunit is used to determine the time based on t. k -t j The sudden change in current ΔI at time 1 y (t k -t j Calculate the time t when the fault suddenly ends. F Its expression is: In the formula, k1 is a constant value that is set; The second calculation unit is used to determine the moment when the current mutation changes from positive to negative based on the current modulus component value, the end time of the fault mutation, and the load current of the local pole of the flexible DC transmission line. The load current of the local pole of the flexible DC transmission line is the load current of any one of the positive or negative poles on this side. The expression for the moment when the current mutation changes from positive to negative is: In the formula, t G ΔI is the moment when the current abruptly changes from positive to negative. y (t G ) for t G The sudden change in current at time t, I y (t G ) for t G The current-mode component value at time t, I y (t G -t j ) for t G -t j The current modulus component value at time t, k2 is a set constant value, I N t represents the load current of this pole on this side. setA To limit the time parameter, it is a preset constant value; The criterion result unit is used to determine a first criterion result based on the voltage modulus component value, the start time of the protection device, the end time of the fault change, and a preset voltage difference accumulation criterion; to determine a second criterion result based on the current modulus component value, the time when the current change amount changes from positive to negative, and a preset negative current change amount integration criterion; and to determine a third criterion result based on the current modulus component value, the load current, and a preset current direction criterion. The result output unit is used to determine the protection action output result of the protection device based on the first criterion result, the second criterion result, and the third criterion result.

5. The system according to claim 4, characterized in that, The criterion result unit includes: The first result subunit is used to determine a first criterion result based on the voltage modulus component value, the start time of the protection device, the end time of the fault abrupt change, and a preset voltage differential accumulation criterion, wherein the expression for the voltage differential accumulation criterion is: In the formula, ΔU y (t) represents the voltage fluctuation at time t, U y (t) represents the voltage magnitude component at time t, U y (tt j ) for tt j The voltage modulus component value at time t0, where t0 is the moment the protection device starts, t0≤c≤t, U Δset These are inherent parameters of the DC transmission system; when the expression of the voltage differential accumulation criterion is true, the first criterion result is that the voltage differential accumulation criterion is satisfied; when the expression of the voltage differential accumulation criterion is not true, the first criterion result is that the voltage differential accumulation criterion is not satisfied. The second result subunit is used to determine the second criterion result based on the current modulus component value, the time when the current abrupt change from positive to negative, and a preset negative current abrupt change integral criterion, wherein the expression of the negative current abrupt change integral criterion is: In the formula, G(s) is the negative current mutation function, and t G ≤s≤t,ΔI y (s) represents the sudden change in current at time s, and k4 and t set All are set constant values; when the expression of the negative current mutation integral criterion is true, the result of the second criterion is that the negative current mutation integral criterion is satisfied; when the expression of the negative current mutation integral criterion is not true, the result of the second criterion is that the negative current mutation integral criterion is not satisfied. The third result subunit is used to determine the third criterion result based on the current modulus component value, the load current, and the preset current direction criterion, wherein the expression of the current direction criterion is: In the formula, k3 is a set constant value; when the expression of the current direction criterion is true, the result of the third criterion is that the current direction criterion is satisfied; when the expression of the current direction criterion is false, the result of the third criterion is that the current direction criterion is not satisfied.

6. The system according to claim 5, characterized in that, The result output unit determines the protection action output result of the protection device based on the first criterion result, the second criterion result, and the third criterion result, including: When the first criterion result satisfies the voltage difference accumulation criterion, the second criterion result does not satisfy the negative current change amount integration criterion, and the third criterion result satisfies the current direction criterion, the protection action output result of the protection device is the protection action output. When the first criterion result is not satisfied with the voltage differential accumulation criterion, or the second criterion result is satisfied with the negative current mutation integral criterion, or the third criterion result is not satisfied with the current direction criterion, the protection action output result of the protection device is to lock out the protection device.

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