A line differential protection tripping method and system suitable for new energy access

Abstract

The present invention discloses a line differential protection tripping method and system applicable to new energy access, including: on the basis of the basic criterion of conventional line current differential protection, adding the following supplementary criteria: the absolute value of the sum of the current vectors on both sides of the line differential protection is greater than k1 times the differential current setting value, the absolute value of the sum of the current vectors on both sides is greater than k2 times the absolute value of the difference between the current vectors on both sides, the sum of the absolute values of the current vectors on both sides is less than k3 times the rated value of the secondary current, and the voltages on both sides are less than k4 times the rated value of the secondary voltage; k1, k2, k3, and k4 are the action coefficient, braking coefficient, current coefficient, and voltage coefficient respectively. The present invention can effectively solve the problem of insufficient action sensitivity of conventional line differential protection when new energy is connected to the power grid through a flexible DC converter station and a grid-connected AC line fails.

Description

Technical Field

[0001] The invention belongs to the technical field of power systems and relates to a line differential protection tripping method and system suitable for renewable energy access. Background Art

[0002] Most areas rich in new energy sources such as wind power and photovoltaic power are far away from load centers, and the AC power grids are relatively weak. They need to be connected from the end of the regional power grid through long-distance transmission. New energy sources have characteristics such as unsustainability and instability. If they are connected to the power grid on a large scale, the voltage stability will be difficult to meet the requirements of the AC power grid.

[0003] Flexible DC transmission technology can smooth out the volatility and intermittency of renewable energy through flexible and rapid regulation capabilities, achieve large-scale power flow regulation and control, and improve the reliability of renewable energy power generation and grid-connected operation. At present, large-scale renewable energy is mainly connected to the power grid through flexible DC converter stations. The grid-connected lines of new energy stations connected to flexible DC converter stations are AC transmission lines.

[0004] New energy stations are generally inverter-type power sources. During a grid fault, in order to ensure that the inverter has low voltage ride-through capability, the control link needs to limit the current to ensure that the power electronic devices do not over-current. The fault current generally does not exceed about 1.2 times the rated load current of the inverter. In addition, when the new energy station is unloaded, the grid-connected line fails and the fault current provided is very small. Therefore, the new energy access side of wind power and photovoltaic power presents the characteristics of a weak power source.

[0005] When a fault occurs in the grid-connected line, the new energy station will enter the low voltage ride-through mode, and the flexible DC converter station will enter the current limiting mode due to the saturation of the voltage outer loop. If the new energy station is unloaded, the voltage at the fault point is determined by the current output by the flexible DC converter station; if the new energy station has a fault current output, the voltage at the fault point is determined by the sum of the currents output by the new energy station and the flexible DC converter station. The capacities of the new energy station and the flexible DC converter station are matched, and the current amplitudes of the equivalent outputs of the two are close. The phase angle difference is affected by the steady-state and transient regulation processes before the fault. When a two-phase phase-to-phase or symmetrical short-circuit fault occurs, the phase angle difference of the short-circuit current on both sides may exceed 90°. At this time, the sensitivity of the conventional line differential protection is reduced, and even refusal to operate may occur.

[0006] Therefore, there is an urgent need for a line differential protection tripping method suitable for the access of renewable energy. Summary of the invention

[0007] In order to solve the deficiencies in the prior art, the present invention provides a line differential protection tripping method and system suitable for renewable energy access, which solves the problem of insufficient action sensitivity of conventional line differential protection when renewable energy is connected to the power grid through a flexible DC converter station and a grid-connected AC line fails.

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

[0009] A line differential protection tripping method suitable for new energy access, based on the conventional line current differential protection criterion, adds the following supplementary criterion: the absolute value of the sum of the current vectors on both sides of the line differential protection is greater than k1 times the differential current setting value, the absolute value of the sum of the current vectors on both sides is greater than k2 times the absolute value of the current vector difference on both sides, the absolute value of the sum of the current vectors on both sides is less than k3 times the secondary current rated value, and the voltages on both sides are less than k4 times the secondary voltage rated value; k1, k2, k3, k4 are action coefficient, braking coefficient, current coefficient, voltage coefficient respectively;

[0010] The current differential protection allows the line to trip only after the supplementary criterion function is activated and all supplementary criteria are met at the same time and confirmed by delay t.

[0011] The present invention further includes the following preferred embodiments:

[0012] Preferably, the method comprises the following steps:

[0013] Step 1: The current differential protection devices on both sides of the line differential protection collect the three-phase current and three-phase voltage at the protection installation on this side in real time and send them to the current differential protection device on the opposite side;

[0014] Step 2: The current differential protection devices on both sides respectively receive the three-phase current and three-phase voltage collected on the opposite side;

[0015] Step 3: Determine whether the protection on both sides has been activated. If so, go to step 4, otherwise return to step 1;

[0016] Step 4: Determine whether both sides meet the conventional line current differential protection action criteria. If both sides do not meet the criteria, proceed to step 5, otherwise the line differential protection trips;

[0017] Step 5: Determine whether the current differential protection device is in the supplementary criterion function. If so, proceed to step 6, otherwise return to step 1;

[0018] Step 6: Based on the three-phase current and three-phase voltage of the current differential protection devices on both sides, determine whether the various supplementary criteria of the current differential protection are met. If so, the line differential protection is activated after confirmation by delay t, otherwise return to step 1.

[0019] Preferably, in step 1, in the line differential protection system, it is assumed that the M side is a new energy station, which is a wind power or photovoltaic inverter power source, the N side is a flexible DC converter station, and the MN grid-connected line is an AC transmission line. Current differential protection devices are installed on the M side and the N side respectively. The current differential protection devices on both sides of the M side and the N side communicate through dedicated or multiplexed optical fibers, and the protection on both sides transmit three-phase current, three-phase voltage, startup, action, and trip information to each other;

[0020] The current differential protection device collects the three-phase current I at the protection installation location on this side in real time. A ,I B ,I C , three-phase voltage U A , U B , U C .

[0021] Preferably, in step 5, if the protection device puts into use the control word of the supplementary criterion, it means that the current differential protection device puts into use the supplementary criterion function.

[0022] Preferably, in step 6, the supplementary criterion of the current differential protection specifically includes:

[0023]

[0024] in, is the current on the M side, including the three-phase current on the M side That is to say, the corresponding supplementary criteria are supplementary criteria applicable to the three groups A, B and C respectively;

[0025] N-side current, including N-side three-phase current

[0026] I DZ It is the setting value of current differential protection;

[0027] U M Including M side phase voltage U MA , U MB , U MC and phase voltage U MAB , U MBC , U MCA ;

[0028] U N Including N-side phase voltage U NA , U NB , U NC and phase voltage U NAB , U NBC , U NCA ;

[0029] I n is the secondary current rating;

[0030] U n is the secondary voltage rating.

[0031] Preferably, in step 6, the supplementary criterion is calculated using the current data and voltage data at the same time.

[0032] Preferably, in step 6, if any phase among phase A, phase B, and phase C satisfies various current supplementary criterion conditions at the same time, and any phase among the voltages of phase A, phase B, phase C, phase AB, phase BC, and phase CA on the M side and the N side satisfies various voltage supplementary criterion conditions, the current differential protection is activated.

[0033] Preferably, phase voltage is used to distinguish U M , U N When the relevant supplementary criteria are used, the secondary voltage rated value U n 57.7V;

[0034] Use phase-to-phase voltage to determine U M , U N When the relevant supplementary criteria are used, the secondary voltage rated value U n is 100V.

[0035] Preferably, the values ​​of k1, k2, k3, and k4 are 0.2, 0.15, 2.5, and 0.75, respectively;

[0036] The value of t is 40ms.

[0037] The present invention also provides a line differential protection tripping system suitable for renewable energy access, the system comprising:

[0038] The signal acquisition module is used for the current differential protection devices on both sides of the line differential protection to collect the three-phase current and three-phase voltage at the protection installation on this side in real time and send them to the current differential protection device on the opposite side;

[0039] A signal receiving module is used for the current differential protection devices on both sides to respectively receive the three-phase current and three-phase voltage collected on the opposite side;

[0040] The protection start-up judgment module is used to judge whether the protection on both sides has been started. If so, it enters the conventional differential protection action judgment module, otherwise it returns to the signal acquisition module;

[0041] Conventional differential protection action judgment module, used to judge whether both sides meet the conventional line current differential protection action judgment criteria. If both sides do not meet the judgment criteria, the supplementary judgment function is put into the judgment module, otherwise the line differential protection trips;

[0042] The supplementary criterion function input determination module is used to determine whether the current differential protection device is in the supplementary criterion function. If so, it enters the supplementary differential protection action determination module, otherwise it returns to the signal acquisition module;

[0043] The supplementary differential protection action judgment module is used to judge whether the various supplementary judgment criteria of current differential protection are met based on the three-phase current and three-phase voltage of the current differential protection devices on both sides. If so, the line differential protection is actuated after confirmation by delay t, otherwise it returns to the signal acquisition module.

[0044] The beneficial effects of the present invention are as follows:

[0045] The present invention adds supplementary criteria based on action coefficient k1, braking coefficient k2, current coefficient k3 and voltage coefficient k4 on the basis of the basic criteria of conventional line current differential protection, wherein the action coefficient k1 is the coefficient of differential current relative to the differential current protection constant, and the smaller the value is, the more sensitive the action is; the braking coefficient k2 is the coefficient of differential current relative to the braking current, and is the slope of the braking curve, and the smaller the value is, the more sensitive the action is; the current coefficient k3 is the ratio of the sum of the current modulus values ​​on both sides to the secondary rated current, and is used to judge when the current on both sides is large enough to put into use the supplementary criteria, and prevent the supplementary criteria from being put into use by mistake; the voltage coefficient k4 is the ratio of the voltage on both sides to the secondary rated voltage, and is used to judge when the voltage is reduced to determine that the system has a fault, and further prevent the supplementary criteria from being put into use by mistake;

[0046] Furthermore, considering that sensitive action can prevent refusal to operate, but there is a risk of false operation, therefore, after the supplementary criterion meets the action condition, the differential protection can only operate and trip after confirmation by delay t;

[0047] Furthermore, by setting the value range and preferred values ​​of the k1, k2, k3, k4 coefficients and time t, the supplementary criteria can be reliably put into use, which can effectively solve the problem of insufficient sensitivity of conventional line differential protection when new energy is connected to the power grid through a flexible DC converter station and the grid-connected AC line fails. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 It is a typical schematic diagram of the action characteristics of conventional current differential protection;

[0049] Figure 2 A wiring diagram of a line differential protection tripping method suitable for access to new energy sources according to the present invention;

[0050] Figure 3 The present invention is a schematic flow chart of the steps of a line differential protection tripping method suitable for renewable energy access. DETAILED DESCRIPTION

[0051] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The embodiments described in this application are only part of the embodiments of the present invention, not all of them. Based on the spirit of the present invention, other embodiments obtained by ordinary technicians in this field without creative work are all within the scope of protection of the present invention.

[0052] First, the following technical terms used in the present invention are explained or defined:

[0053] Figure 1 The following is a typical schematic diagram of the action characteristics of conventional current differential protection. The current differential protection has a proportional braking characteristic, where I D is the differential current, I B is the braking current, I DZ It is the setting value of current differential protection. The braking coefficient K is generally 0.6~0.8. When the action characteristics are met, the conventional differential protection will operate.

[0054] Current differential protection device: It is the main protection element of the line protection device, including phase current differential protection and zero-sequence current differential protection. With the help of optical fiber channels, the current, voltage, start, action, jump and other information are transmitted to each other, and the differential current and braking current are calculated. After the conditions for differential action are met, the differential protection is activated. The phase current differential protection of phase A, phase B or phase C is instantaneous, and the zero-sequence current differential protection is delayed by 100ms.

[0055] Differential current: includes phase differential current and zero-sequence differential current. Among them, the phase differential current includes phase A differential current, phase B differential current and phase C differential current. The differential current is the vector addition of the same-phase current measured by the protection on both sides of the line, and then the absolute value is taken.

[0056] Braking current: includes phase braking current and zero-sequence braking current. The phase braking current includes phase A braking current, phase B braking current and phase C braking current. The braking current is the vector subtraction of the same-phase current measured by the protection on both sides of the line, and then the absolute value is taken.

[0057] Current differential protection setting value: It is the setting value of the protection device. If the differential current is less than this setting value, the current differential protection cannot operate.

[0058] Protection start: The protection start element is used to monitor whether the power system has a fault. If a fault is confirmed, the power supply of the protection trip output relay is opened and the protection fault handling procedure is started. The protection start element includes current sudden change start, zero sequence auxiliary start, static stability destruction start, weak feed start and other elements. After any start element is activated, the protection function is opened and maintained until the entire group of the device is reset.

[0059] Figure 2 The present invention is a wiring diagram of a line differential protection tripping method suitable for new energy access. Figure 2 In the figure, the M side is a new energy station, which is an inverter power source such as wind power or photovoltaic power, the N side is a flexible DC converter station, and the MN grid-connected line is an AC transmission line. Current differential protection devices are installed on the M and N sides respectively. The current differential protections on both sides communicate through dedicated or multiplexed optical fibers. The protections on both sides transmit three-phase current, three-phase voltage and other data to each other. Point F is a fault in the grid-connected line area.

[0060] The present invention is applicable to a line differential protection tripping method for access to new energy sources. On the basis of conventional line current differential protection criteria, the following supplementary criteria are added: the absolute value of the sum of the current vectors on both sides of the line differential protection is greater than k1 times the differential current constant value, the absolute value of the sum of the current vectors on both sides is greater than k2 times the absolute value of the current vector difference on both sides, the absolute value of the sum of the current vectors on both sides is less than k3 times the secondary current rated value, and the voltages on both sides are less than k4 times the secondary voltage rated value; k1, k2, k3, and k4 are action coefficient, braking coefficient, current coefficient, and voltage coefficient, respectively;

[0061] The current differential protection allows the line to trip only after the supplementary criterion function is activated and all supplementary criteria are met at the same time and confirmed by delay t.

[0062] Figure 3 FIG. 1 is a schematic flow chart of the steps of the line differential protection tripping method applicable to the access of new energy sources according to the present invention, as shown in FIG. Figure 3 As shown, the method comprises the following steps 1-6:

[0063] Step 1: The current differential protection devices on both sides collect the three-phase current and three-phase voltage at the protection installation in real time.

[0064] The current differential protection device collects the three-phase current I at the protection installation in real time A ,I B ,I C , three-phase voltage U A , U B , U C , and sent to the opposite side.

[0065] The three-phase voltage collected in step 1 is used as the supplementary criterion U in step 6 M <k4U n and U N <k4U n .

[0066] For phase voltage, for example, U N (U NA , U NB , U NC), which is the three-phase voltage collected in step 1;

[0067] For the phase-to-phase voltage U NAB , U NBC , U NCA It is necessary to perform a simple calculation based on the three-phase voltage collected in step 1, such as U NAB =U NA -U NB .

[0068] Step 2: The current differential protection devices on both sides receive the three-phase current and three-phase voltage on the opposite side.

[0069] Step 3: Determine whether the protection start element is activated. If both sides of the protection are activated, go to step 4, otherwise return to step 1.

[0070] Both sides must be judged to start and act. For the judgment of starting, both sides are required to start, not to start at the same time. That is, in case of a fault, the protection start elements on both sides may not start at the same time, but both sides are required to start before entering the action judgment, which can prevent incorrect actions in the case of secondary current circuit disconnection.

[0071] Step 4: Determine whether both sides meet the conventional line current differential protection action criteria. If both sides do not meet the criteria, proceed to step 5, otherwise the line differential protection trips;

[0072] For the action, the two sides are judged separately, and each judges its own. If the current differential protection of the conventional lines on both sides does not operate, then enter the supplementary judgment criteria.

[0073] The action criteria of conventional current differential protection are:

[0074]

[0075] Among them, I DZ It is the setting value of current differential protection;

[0076] K is the braking coefficient, which is generally between 0.6 and 0.8.

[0077] Step 5: Check whether the current differential protection has been activated for the supplementary criterion function. If so, proceed to step 6; otherwise, return to step 1.

[0078] The supplementary criterion is only used when new energy is connected to the power grid through a flexible DC converter station. If the protection device is put into the control word of the supplementary criterion, the supplementary criterion function is effective. It is used to solve the problem of insufficient action sensitivity of conventional line differential protection.

[0079] Step 6: According to the three-phase current and three-phase voltage of the current differential protection devices on both sides, determine whether the various supplementary criteria of the current differential protection are met. If so, the line differential protection is activated after confirmation by delay t, otherwise return to step 1;

[0080] Supplementary criteria for current differential protection include:

[0081]

[0082] in, is the current on the M side, including the three-phase current That is to say, the corresponding supplementary criteria are supplementary criteria applicable to the three groups A, B and C respectively;

[0083] is the N-side current, including the three-phase current

[0084] I DZ It is the setting value of current differential protection, which is generally set at a primary value of 600A to 1000A;

[0085] U M is the phase voltage U on the M side MA , U MB , U MC and phase voltage U MAB , U MBC , U MCA ;

[0086] U N is the N-side phase voltage U NA , U NB , U NC and phase voltage U NAB , U NBC , U NCA ;

[0087] In specific implementation, the M side and the N side do not need to be selected synchronously;

[0088] I n The secondary current rating is 1A or 5A;

[0089] U n The secondary voltage rating is 57.7V or 100V;

[0090] k1, k2, k3, and k4 are coefficients respectively;

[0091] t is the delay time.

[0092] k1 is the action coefficient, which is the coefficient of the differential current relative to the differential current protection constant. The smaller the value, the more sensitive the action; k2 is the braking coefficient, which is the coefficient of the differential current relative to the braking current, and is the slope of the braking curve. The smaller the value, the more sensitive the action; k3 is the current coefficient, which is the ratio of the sum of the current modulus values ​​on both sides to the secondary rated current. It is used to determine when the current on both sides is large enough to put the supplementary criterion into operation to prevent the supplementary criterion from being mistakenly put into operation; k4 is the voltage coefficient, which is the ratio of the voltage on both sides to the secondary rated voltage. It is used to determine how much the voltage is reduced to determine that the system has a fault, and further prevent the supplementary criterion from being mistakenly put into operation. Sensitive action can prevent refusal to operate, but there is a risk of false operation. Therefore, after the supplementary criterion meets the action conditions, it needs to be confirmed after a delay t before the differential protection can operate and trip. k1, k2, k3, k4, and t need to be limited to a reasonable value range to correctly put the supplementary criterion into operation to ensure the correct operation of the current differential protection.

[0093] After simulation tests, the value ranges of k1, k2, k3, k4, and t are determined as follows:

[0094] k1 is the action coefficient, with a range of 0.1 to 0.3, i.e., the minimum value is 60A for the primary current, which can avoid the minimum unbalanced current and transmission error. The preferred value is 0.2, i.e., when the primary current is 120A, it can further avoid the differential current when the characteristics of the current transformers on both sides are seriously inconsistent;

[0095] k2 is the braking coefficient, ranging from 0.1 to 0.2, with a preferred value of 0.15, which can ensure sensitive fault phase selection capability and prevent wrong phase selection when there is an out-of-zone fault;

[0096] k3 is the current coefficient, with a value range of 2 to 3, and a preferred value of 2.5, which is consistent with the characteristics of the small fault current provided by the new energy station and the flexible DC converter station, and is a supplementary criterion for preventing mis-operation;

[0097] k4 is the voltage coefficient, with a value range of 0.5 to 0.9, and a preferred value of 0.75. It can be judged that a fault has occurred in the system, resulting in a voltage drop, to prevent the system from being mistakenly put into operation during normal operation.

[0098] t is the delay time, ranging from 30ms to 50ms, with an optimal value of 40ms. For weakly fed lines, when a fault occurs outside the power supply side, it can prevent the transient capacitive current discharge of the line from causing false operation of the differential protection.

[0099] The first three items in the supplementary criteria are current supplementary criteria, and the last two items are voltage supplementary criteria.

[0100] The supplementary criteria are calculated using data at the same time.

[0101] If the current differential protection is to be activated, first of all, any one of phases A, B, and C must simultaneously meet the current supplementary criteria. If only one phase current meets the phase current differential protection action conditions, for the single-phase mode, the protection will trip once, and the phase will trip; if the two-phase or three-phase current meets the phase current differential protection action conditions, the protection will trip three times. Secondly, any one of the A, B, C, AB, BC, and CA voltages on the M and N sides must meet the voltage supplementary criteria.

[0102] In specific implementation, among the five supplementary criteria, three current equations must be satisfied at the same time; the two voltage equations can be satisfied between 10ms before and 10ms after the current is satisfied at the same time. After the supplementary criteria are satisfied, they need to be confirmed by a delay t before the current differential protection can operate and trip.

[0103] Specifically, if the phase voltage of phase A, phase B or phase C meets the voltage supplement criteria, U n The secondary voltage rating is 57.7V;

[0104] If the phase-to-phase voltage AB, BC or CA meets the voltage supplementary criteria, U n The secondary voltage rating is 100V;

[0105] Secondary current rating I n 1A or 5A, determined by the CT model selected on site.

[0106] The line differential protection tripping system applicable to the access of new energy sources of the present invention comprises:

[0107] The signal acquisition module is used for the current differential protection devices on both sides of the line differential protection to collect the three-phase current and three-phase voltage at the protection installation on this side in real time and send them to the current differential protection device on the opposite side;

[0108] A signal receiving module is used for the current differential protection devices on both sides to respectively receive the three-phase current and three-phase voltage collected on the opposite side;

[0109] The protection start-up judgment module is used to judge whether the protection on both sides has been started. If so, it enters the conventional differential protection action judgment module, otherwise it returns to the signal acquisition module;

[0110] Conventional differential protection action judgment module, used to judge whether both sides meet the conventional line current differential protection action judgment criteria. If both sides do not meet the judgment criteria, the supplementary judgment function is put into the judgment module, otherwise the line differential protection trips;

[0111] The supplementary criterion function input determination module is used to determine whether the current differential protection device is in the supplementary criterion function. If so, it enters the supplementary differential protection action determination module, otherwise it returns to the signal acquisition module;

[0112] The supplementary differential protection action judgment module is used to judge whether the various supplementary judgment criteria of current differential protection are met based on the three-phase current and three-phase voltage of the current differential protection devices on both sides. If so, the line differential protection is actuated after confirmation by delay t, otherwise it returns to the signal acquisition module.

[0113] The beneficial effects of the present invention are as follows:

[0114] The present invention adds supplementary criteria based on action coefficient k1, braking coefficient k2, current coefficient k3 and voltage coefficient k4 on the basis of the basic criteria of conventional line current differential protection, wherein the action coefficient k1 is the coefficient of differential current relative to the differential current protection constant, and the smaller the value is, the more sensitive the action is; the braking coefficient k2 is the coefficient of differential current relative to the braking current, and is the slope of the braking curve, and the smaller the value is, the more sensitive the action is; the current coefficient k3 is the ratio of the sum of the current modulus values ​​on both sides to the secondary rated current, and is used to judge when the current on both sides is large enough to put into use the supplementary criteria, and prevent the supplementary criteria from being put into use by mistake; the voltage coefficient k4 is the ratio of the voltage on both sides to the secondary rated voltage, and is used to judge when the voltage is reduced to determine that the system has a fault, and further prevent the supplementary criteria from being put into use by mistake;

[0115] Furthermore, considering that sensitive action can prevent refusal to operate, but there is a risk of false operation, therefore, after the supplementary criterion meets the action condition, the differential protection can only operate and trip after confirmation by delay t;

[0116] Furthermore, by setting the value range and preferred values ​​of the k1, k2, k3, k4 coefficients and time t, the supplementary criteria can be reliably put into use, which can effectively solve the problem of insufficient sensitivity of conventional line differential protection when new energy is connected to the power grid through a flexible DC converter station and the grid-connected AC line fails.

[0117] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents, and any modifications or equivalent replacements 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 line differential protection tripping method suitable for new energy access, characterized in that: On the basis of the conventional line current differential protection criterion, the following supplementary criterion is added for calculation using data at the same time: the absolute value of the total vector sum of the currents on both sides of the line differential protection is greater than k1 times the differential current setting value, the absolute value of the total vector sum of the currents on both sides is greater than k2 times the absolute value of the total vector difference of the currents on both sides, the sum of the absolute values ​​of the current vectors on each side is less than k3 times the secondary current rated value, and the voltages on both sides are less than k4 times the secondary voltage rated value; k1, k2, k3, k4 are action coefficient, braking coefficient, current coefficient, and voltage coefficient respectively. The value ranges of k1, k2, k3, and k4 are 0.1-0.3, 0.1-0.2, 2-3, and 0.5-0.9 respectively; The current differential protection allows the line to trip only after the supplementary criterion function is activated and all supplementary criteria are met at the same time and confirmed by delay t.

2. A line differential protection tripping method suitable for new energy access according to claim 1, characterized in that: The method comprises the following steps: Step 1: The current differential protection devices on both sides of the line differential protection collect the three-phase current and three-phase voltage at the protection installation on this side in real time and send them to the current differential protection device on the opposite side; Step 2: The current differential protection devices on both sides respectively receive the three-phase current and three-phase voltage collected on the opposite side; Step 3: Determine whether the protection on both sides has been activated. If so, go to step 4, otherwise return to step 1; Step 4: Determine whether both sides meet the conventional line current differential protection action criteria. If both sides do not meet the criteria, proceed to step 5, otherwise the line differential protection trips; Step 5: Determine whether the current differential protection device is in the supplementary criterion function. If so, proceed to step 6, otherwise return to step 1; Step 6: Based on the three-phase current and three-phase voltage of the current differential protection devices on both sides, determine whether the supplementary criteria of the current differential protection are met. If so, perform line differential protection after confirmation after a delay of t, otherwise return to step 1.

3. A line differential protection tripping method suitable for new energy access according to claim 2, characterized in that: In step 1, in the line differential protection system, it is assumed that the M side is a new energy station, which is a wind power or photovoltaic inverter power source, the N side is a flexible DC converter station, and the MN grid-connected line is an AC transmission line. Current differential protection devices are installed on the M side and the N side respectively. The current differential protection devices on the M side and the N side communicate with each other through dedicated or multiplexed optical fibers, and the protection on both sides transmits three-phase current, three-phase voltage, startup, action, and trip information to each other; The current differential protection device collects the three-phase current I at the protection installation location on this side in real time. A ,I B ,I C , three-phase voltage U A , U B , U C .

4. A line differential protection tripping method suitable for new energy access according to claim 2, characterized in that: In step 5, if the protection device is activated with the control word of the supplementary criterion, it means that the current differential protection device is activated with the supplementary criterion function.

5. A line differential protection tripping method suitable for new energy access according to claim 2, characterized in that: In step 6, the supplementary criteria for current differential protection specifically include: in, is the current on the M side, including the three-phase current on the M side That is to say, the corresponding supplementary criteria are supplementary criteria applicable to the three groups A, B and C respectively; N-side current, including N-side three-phase current I DZ It is the setting value of current differential protection; U M Including M side phase voltage U MA , U MB , U MC and phase voltage U MAB , U MBC , U MCA ; U N Including N-side phase voltage U NA , U NB , U NC and phase voltage U NAB , U NBC , U NCA ; I n is the secondary current rating; U n is the secondary voltage rating.

6. A line differential protection tripping method suitable for new energy access according to claim 5, characterized in that: In step 6, the supplementary criterion is calculated using the current data and voltage data at the same time.

7. A line differential protection tripping method suitable for new energy access according to claim 5, characterized in that: In step 6, if any phase among phase A, phase B, and phase C meets various current supplementary judgment conditions at the same time, and any phase among the voltages of phase A, phase B, phase C, phase AB, phase BC, and phase CA on the M side and the N side meets various voltage supplementary judgment conditions, the current differential protection is performed.

8. A line differential protection tripping method suitable for new energy access according to claim 5, characterized in that: Using phase voltage to determine U M , U N When the relevant supplementary criteria are used, the secondary voltage rated value U n 57.7V; Use phase-to-phase voltage to determine U M , U N When the relevant supplementary criteria are used, the secondary voltage rated value U n is 100V.

9. A line differential protection tripping method suitable for new energy access according to claim 5, characterized in that: The values ​​of k1, k2, k3, and k4 are 0.2, 0.15, 2.5, and 0.75 respectively; The value of t is 40ms.

10. A line differential protection tripping system suitable for new energy access, characterized in that: The system comprises: The signal acquisition module is used for the current differential protection devices on both sides of the line differential protection to collect the three-phase current and three-phase voltage at the protection installation on this side in real time and send them to the current differential protection device on the opposite side; A signal receiving module is used for the current differential protection devices on both sides to respectively receive the three-phase current and three-phase voltage collected on the opposite side; The protection start-up judgment module is used to judge whether the protection on both sides has been started. If so, it enters the conventional differential protection action judgment module, otherwise it returns to the signal acquisition module; Conventional differential protection action judgment module, used to judge whether both sides meet the conventional line current differential protection action judgment criteria. If both sides do not meet the judgment criteria, the supplementary judgment function is put into the judgment module, otherwise the line differential protection trips; The supplementary criterion function input determination module is used to determine whether the current differential protection device is in the supplementary criterion function. If so, it enters the supplementary differential protection action determination module, otherwise it returns to the signal acquisition module; The supplementary differential protection action judgment module is used to judge whether the following supplementary judgment criteria of current differential protection are met according to the three-phase current and three-phase voltage of the current differential protection devices on both sides at the same time: The absolute value of the total vector sum of the currents on both sides of the line differential protection is greater than k1 times the differential current setting, the absolute value of the total vector sum of the currents on both sides is greater than k2 times the absolute value of the total vector difference of the currents on both sides, the sum of the absolute values ​​of the current vectors on each side is less than k3 times the secondary current rating, and the voltages on both sides are less than k4 times the secondary voltage rating; k1, k2, k3, k4 are action coefficient, braking coefficient, current coefficient, and voltage coefficient respectively. The value ranges of k1, k2, k3, and k4 are 0.1-0.3, 0.1-0.2, 2-3, and 0.5-0.9 respectively; If it is satisfied, the line differential protection action is performed after confirmation by delay t, otherwise it returns to the signal acquisition module.

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