Power grid high-sensitivity protection device and method based on voltage, direction and delay element

Abstract

The invention provides a power grid high-sensitivity protection device and method based on voltage, direction and delay elements, and relates to the power system protection technology. According to the method, faults (phase voltage, negative-sequence voltage and zero-sequence voltage criteria) are sensed through voltage elements, fault directions (adaptive to different fault types) are identified through direction elements, main and backup protection cooperation is achieved through time delay elements, low-current criteria are introduced to avoid misoperation, the problem that traditional current protection sensitivity is insufficient in a high-proportion new energy power grid is solved, and the reliability of current protection is improved. The device has the characteristics of simple configuration, low cost and strong adaptability, can meet the four-property requirements of relay protection, and is suitable for main and backup protection of a power transmission network and a power distribution network.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and in particular to a highly sensitive power grid protection device and method based on voltage, direction and delay elements. Background Art

[0002] With the large-scale integration of renewable energy and distributed generation, the short-circuit capacity of power systems is decreasing, leading to a continuous decline in the amplitude of system short-circuit currents. In particular, in power grids with a high proportion of renewable energy, distribution networks with a high proportion of distributed generation, "high-sea-free" microgrids, and AC grids at the sending end of renewable energy-connected flexible direct current systems (HVDCs), the system power supply equivalent impedance is very high, or even non-existent, with the short-circuit current being entirely provided by renewable energy and other power electronic devices. Since the short-circuit current of renewable energy and other power electronic devices is generally 1.2 times the rated current, and the ratio of this value to the rated current is no more than 1.3 times, this results in insufficient sensitivity of existing current protection and difficulty in setting and coordinating backup current protection. Existing research has proposed high-sensitivity differential protection and high-sensitivity traveling wave protection. However, these protection methods, when applied to power grids and distribution networks with a high proportion of renewable energy, require a large number of terminal devices, communication equipment, and high-precision sampling equipment, resulting in high investment and low economic efficiency. Therefore, simple, efficient, and cost-effective high-sensitivity grid protection methods are urgently needed for power grids with a high proportion of renewable energy, microgrids, and AC grids at the sending end of renewable energy-connected flexible direct current systems.

[0003] Based on this, the present invention addresses the problem of insufficient sensitivity of traditional protection faced by high-proportion new energy power grids, distribution networks with high proportion of distributed power access, microgrids, and AC power grids at the sending end of new energy through flexible DC systems. It proposes a high-sensitivity protection principle based on voltage, direction, and delay elements, which changes the idea based on traditional current protection and meets the protection sensitivity requirements of high-proportion new energy power grids, microgrids, and AC power grids at the sending end of new energy through flexible DC systems. The protection scheme of the invention has high sensitivity, does not require complex setting calculations, and easily meets the coordination requirements between backup protection sensitivities. It effectively solves the problem of difficulty in balancing sensitivity and selectivity faced by high-proportion new energy power grids. It also has the characteristics of simple configuration, easy installation, and low investment cost, and will have broad application prospects in the main and backup protection of transmission and distribution networks. Summary of the Invention

[0004] In response to the above-mentioned defects or improvement needs of the existing technology, and to the problem of lack of sensitivity of protection faced by scenarios such as high-proportion new energy power grids, microgrids, and AC power grids at the sending end of new energy through flexible DC systems, the present invention provides a high-sensitivity power grid protection method based on voltage, direction and delay elements. Its purpose is to address the problem of lack of sensitivity of protection in scenarios such as high-proportion new energy power grids and microgrids. It senses faults through voltage, identifies fault directions through directional elements, and realizes the coordination of main and backup protection through delay elements, changing the traditional idea of ​​current-based protection and effectively guaranteeing the "four properties" requirements of relay protection in scenarios such as high-proportion new energy power grids and microgrids.

[0005] A highly sensitive power grid protection device based on voltage, direction and delay elements, comprising:

[0006] Voltage element, used to sense grid faults through preset criteria of phase voltage, negative sequence voltage and zero sequence voltage, and output fault signals;

[0007] a directional element, the input end of which is electrically connected to the output end of the voltage element, for identifying the fault type according to the fault signal and adaptively selecting a directional criterion and identifying the fault direction;

[0008] A time delay element, the input end of which is electrically connected to the output end of the directional element, for realizing selective action of the main and backup protection by step-by-step coordination of time according to the fault direction identified by the directional element;

[0009] The low-current locking element has an input end connected to the grid line current sampling end, and an output end electrically connected to the locking end of the delay element, and is used to monitor the line current in real time. If the low-current criterion is met, the locking protection action is taken to avoid false tripping of non-fault lines.

[0010] 2. Furthermore, the preset criteria include:

[0011] ;

[0012] Among them, a, b, c represent the three phases; Indicates the phase voltage setting value, its typical value can be set to 0.8 pu; Indicates the negative sequence voltage setting value, and its secondary typical setting value is 4V; Indicates the zero-sequence voltage setting value, and its secondary typical setting value is 6V.

[0013] Furthermore, the directional element includes a fault type identification unit and a criterion selection unit:

[0014] The fault type identification unit is used to determine whether the fault is an asymmetric ground fault, a two-phase phase-to-phase fault or a three-phase symmetrical fault based on the negative sequence voltage and the zero sequence voltage;

[0015] The criterion selection unit is used to output a corresponding direction criterion signal to the delay element according to the fault type.

[0016] Furthermore, the main protection action time of the delay element is instantaneous, and the backup protection action time difference is 0.2s or 0.3s.

[0017] Furthermore, the low current criterion formula is:

[0018] ;

[0019] Where, Indicates the phase current action value; subscript " " indicates separation; represents the reliability coefficient, and its typical value is 1.2; is the current before the phase voltage drops, that is, the memory current; is the actual voltage value at that time; is the voltage of the phase before the fault, that is, the memory voltage.

[0020] A highly sensitive power grid protection method based on voltage, direction and delay elements comprises the following steps:

[0021] Step S1. Voltage sensing: collecting phase voltage, negative sequence voltage and zero sequence voltage through voltage elements

[0022] Voltage, when the preset criteria are met, it is determined that the power grid has failed and a fault signal is output;

[0023] Step S2. Direction determination: Adaptively select the direction criterion and identify the fault direction based on the fault signal to identify the fault type;

[0024] Step S3. Delay coordination: According to the fault direction signal, the main backup protection action is controlled according to the preset time difference;

[0025] Step S4. Low current blocking: The low current blocking element monitors the line current in real time. If the low current criterion is met, the blocking protection is activated to avoid false tripping of non-fault lines.

[0026] Furthermore, in step S1, the preset criteria include:

[0027] ;

[0028] Among them, a, b, c represent the three phases; Indicates the phase voltage setting value, its typical value can be set to 0.8 pu; Indicates the negative sequence voltage setting value, and its secondary typical setting value is 4V; Indicates the zero-sequence voltage setting value, and its secondary typical setting value is 6V.

[0029] Furthermore, the step S2 of direction determination: identifying the fault type according to the fault signal, adaptively selecting a direction criterion and identifying the fault direction, specifically includes:

[0030] Step 1: Identify different fault types based on phase voltage, negative sequence voltage and zero sequence voltage under grid fault conditions:

[0031] If the voltage or current sudden change element is activated and the negative sequence voltage criterion and the zero sequence voltage criterion are satisfied, it is determined that an asymmetric ground fault has occurred and the process goes to step 2.

[0032] If the voltage or current sudden change element is activated, and the negative sequence voltage criterion is satisfied but the zero sequence voltage criterion is not satisfied, it is determined that a two-phase-to-phase fault has occurred, and the process goes to step 6;

[0033] If the voltage or current sudden change element is activated and the voltage element criterion is met, but the negative sequence voltage criterion is not met, it is determined that a three-phase symmetrical fault has occurred and the process goes to step 7.

[0034] Step 2: For an asymmetric ground fault, determine whether the zero-sequence directional element is applicable based on the magnitude of the zero-sequence current. If the magnitude of the zero-sequence current meets the criteria, the zero-sequence directional element is used provided that the zero-sequence component sensitivity meets the conditions. Otherwise, proceed to step 3.

[0035] Step 3: In the case of insufficient sensitivity, determine the fault type and fault phase based on the voltage amplitude characteristics of each phase. If it is a two-phase grounding fault, proceed to step 4; otherwise, it is a single-phase grounding fault and proceed to step 5.

[0036] Step 4: For a two-phase ground fault, construct a directional element criterion based on the non-fault phase voltage and the reverse non-fault phase current;

[0037] Step 5: For single-phase grounding faults, construct a directional element criterion based on the fault phase negative sequence voltage and the fault phase current;

[0038] Step 6: For a two-phase-to-phase fault, construct a directional element criterion based on the negative-sequence voltage and positive-sequence current;

[0039] Step 7: For a three-phase symmetrical fault, the positive-sequence directional element is used when the positive-sequence voltage exceeds the preset threshold; otherwise, it indicates a nearby three-phase short-circuit fault. Considering the passive load branch, the positive-sequence voltage mutation amount and reactive current amplitude are used to form a comprehensive comparison criterion;

[0040] Step 8: End.

[0041] This invention addresses the lack of protection sensitivity faced by scenarios such as power grids with a high proportion of renewable energy, microgrids, and AC grids with renewable energy fed by flexible DC systems. It provides a highly sensitive power grid protection device and method based on voltage, direction, and delay elements. This device leverages voltage to sense faults, direction elements to identify fault direction, and delay elements to coordinate primary and backup protection. This solution offers simple configuration, easy installation, and low investment costs, making it suitable for large-scale application as primary and backup protection for transmission and distribution networks, microgrids, and AC grids with a high proportion of renewable energy, fed by flexible DC systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 Schematic diagram of the structure of a highly sensitive power grid protection device based on voltage, direction and delay elements according to an embodiment of the present invention;

[0043] Figure 2 The present invention is a flowchart of a method for identifying fault types and adaptively selecting direction criteria in a highly sensitive power grid protection method based on voltage, direction and delay elements according to an embodiment of the present invention. DETAILED DESCRIPTION

[0044] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0045] See also Figure 1 The embodiment of the present invention provides a highly sensitive power grid protection device based on voltage, direction and delay elements, including:

[0046] Voltage element, used to sense grid faults through preset criteria of phase voltage, negative sequence voltage and zero sequence voltage, and output fault signals;

[0047] a directional element, the input end of which is electrically connected to the output end of the voltage element, for identifying the fault type according to the fault signal and adaptively selecting a directional criterion and identifying the fault direction;

[0048] A time delay element, the input end of which is electrically connected to the output end of the directional element, for realizing selective action of the main and backup protection by step-by-step coordination of time according to the fault direction identified by the directional element;

[0049] The low-current locking element has an input end connected to the grid line current sampling end, and an output end electrically connected to the locking end of the delay element, and is used to monitor the line current in real time. If the low-current criterion is met, the locking protection action is taken to avoid false tripping of non-fault lines.

[0050] As described above, the voltage element is used to sense grid faults. Its criterion can be set based on phase voltage, negative-sequence voltage, and zero-sequence voltage according to different fault types, thereby achieving highly sensitive perception of various types of faults. In this embodiment, the preset criterion based on phase voltage, negative-sequence voltage, and zero-sequence voltage is shown in Formula (1).

[0051] ;

[0052] Among them, a, b, c represent the three phases; Indicates the phase voltage setting value, its typical value can be set to 0.8 pu; Indicates the negative sequence voltage setting value, and its secondary typical setting value can be taken as 4V; It represents the zero-sequence voltage setting value, and its secondary typical setting value can be taken as 6V.

[0053] As described above, the directional element is used to determine the direction of the fault. Considering the diverse system topology and the impact of distributed power generation, a directional element that is unaffected by distributed power generation and operating mode can be used. The directional element includes a fault type identification unit and a criterion selection unit: the fault type identification unit is used to determine whether the fault is an asymmetric ground fault, a two-phase-to-phase fault, or a three-phase symmetrical fault based on the negative-sequence voltage and zero-sequence voltage; the criterion selection unit is used to output a corresponding directional criterion signal to the delay element based on the fault type.

[0054] As mentioned above, the delay element ensures the selectivity of the protection action by coordinating the time step by step, and its configuration can be configured according to the system topology. In this embodiment, the main protection action time is instantaneous action, ; The backup protection action time difference is , its typical value can be 0.2s or 0.3s.

[0055] Furthermore, to prevent tripping of adjacent load lines, a low-current criterion can be set to avoid malfunctioning of adjacent line protection, taking into account the fact that rotating motors typically provide short-circuit current, constant-power loads typically lock out during voltage dips, and the fact that the load current of resistive and impedance loads is proportional to the voltage dip. In this embodiment, the low-current blocking element constructs a low-current criterion based on the load current before the voltage dip and the voltage dip extent, taking into account the fact that the load current of resistive and impedance loads is proportional to the voltage dip, as shown in Equation (2).

[0056]

[0057] Where, Indicates the phase current action value; subscript " " indicates separation; It represents the reliability coefficient, and its typical value can be taken as 1.2; is the current before the phase voltage drops, that is, the memory current; is the actual voltage value at that time; is the voltage of the phase before the fault, that is, the memory voltage.

[0058] At the same time, in order to improve the sensitivity, voltage mutation or current mutation can be used as the starting element. The logic diagram of this protection principle is as follows Figure 1 shown.

[0059] See also Figure 2 The embodiment of the present invention further provides a highly sensitive power grid protection method based on voltage, direction and delay elements, comprising the following steps:

[0060] S1. Voltage sensing: This system uses voltage sensors to collect phase, negative-sequence, and zero-sequence voltages. When preset criteria are met, it determines that a grid fault has occurred and outputs a fault signal.

[0061] S2 direction determination: According to the fault signal to identify the fault type adaptive selection direction criterion and identify the fault direction; the specific steps of step S2 are as follows:

[0062] S21: Under grid fault conditions, identify different fault types. Asymmetric faults are identified based on the negative-sequence voltage. An asymmetric fault is determined to have occurred when the negative-sequence voltage exceeds the negative-sequence voltage secondary value by 4V. Furthermore, a ground fault is identified based on the zero-sequence voltage. A ground fault is determined to have occurred when the zero-sequence voltage exceeds the zero-sequence voltage secondary value by 6V, and the process proceeds to step S22. Otherwise, the process proceeds to step S26.

[0063] S22: For asymmetric ground fault, the zero-sequence directional element is used when the zero-sequence component sensitivity meets the condition; when the zero-sequence current is greater than the zero-sequence current setting value, it indicates that the zero-sequence current sensitivity meets the condition, that is,

[0064]

[0065] Where, Represents the zero sequence current, Indicates the zero-sequence current setting value. At this time, the zero-sequence direction element can be judged by the following formula:

[0066]

[0067] Where, Indicates zero-sequence voltage; represents the zero-sequence current; the typical value of the sensitivity angle is 90°. If Equation (4) satisfies the conditions, it means that the fault occurs in the positive direction; otherwise, the fault occurs in the negative direction.

[0068] S23: In the case of insufficient sensitivity, that is:

[0069]

[0070] At this time, the fault type and fault phase can be determined based on the voltage amplitude characteristics of each phase. The voltage phase selection criterion that is not affected by the distributed power supply can be used to correctly determine the fault type and fault phase.

[0071] S24: For a two-phase ground fault, considering the case where the distributed generation adopts a negative-sequence current suppression strategy, the non-fault phase voltage has the same phase relationship with the non-fault phase zero-sequence voltage, and the reverse non-fault phase current has the same phase relationship with the non-fault phase zero-sequence current. Based on the non-fault phase voltage and the reverse non-fault phase current, a directional element criterion is constructed, as shown in Equation (6).

[0072]

[0073] Where, Indicates the non-fault phase voltage; Indicates the non-fault phase current; the subscript "x" indicates the non-fault phase; the superscript " " represents the sensitive angle, and its typical value can be taken as 90°. If the equation (6) satisfies the conditions, it means that the fault occurs in the positive direction; otherwise, the fault occurs in the negative direction.

[0074] S25: For a single-phase grounding fault, the fault phase negative-sequence voltage and the fault phase zero-sequence voltage have the same phase relationship, and the fault phase current and the fault phase zero-sequence current have the same phase relationship. Therefore, the directional element criterion is constructed based on the fault phase negative-sequence voltage and the fault phase current, as shown in Equation (7).

[0075]

[0076] Where, “s” represents the fault phase; Indicates the non-fault phase voltage; represents the non-fault phase current; if equation (7) satisfies the conditions, it means that the fault occurs in the forward direction; otherwise, the fault occurs in the reverse direction.

[0077] Based on the above criteria, the fault direction of two-phase grounding fault and single-phase grounding fault can be effectively identified when the sensitivity of the zero-sequence directional element is insufficient.

[0078] S26: For two-phase faults, considering the case where the distributed generation adopts the control strategy of suppressing the negative sequence current component, according to the characteristic that the positive sequence current and negative sequence current of the non-fault phase are basically in the same direction under the two-phase fault condition, the direction element judgment criterion is constructed based on the negative sequence voltage and positive sequence current, as shown in formula (8).

[0079]

[0080] Where, "x" represents the three phases a, b, and c.

[0081] S27: For three-phase symmetrical faults, taking into account the severe voltage drop, a direction identification method based on a comprehensive comparison of the memory voltage, positive sequence voltage mutation and reactive current amplitude is adopted;

[0082] like , the positive sequence direction element can be used to determine the fault direction. At this time, the positive judgment criterion is:

[0083]

[0084] in, is the positive sequence voltage amplitude, its typical value can be 0.2 rated voltage. The sensitive angle φm1 can be taken as 90°.

[0085] like , indicating that a three-phase short-circuit fault has occurred in the vicinity. Considering the situation where the passive branch current flows from the system to the load point, the positive sequence voltage mutation and reactive current amplitude are used to form a comprehensive comparison criterion.

[0086]

[0087] Where, It is the memory voltage before the fault; is the positive sequence voltage; is the positive sequence current; is the reactive current. Considering that the reactive current will not exceed 0.1 pu under normal circumstances, the reliability coefficient is set to 2. This ensures that the non-faulty passive load branch may meet the phase comparison criterion in equation (10), but because it is not a faulty branch, its reactive current will not be higher than the reactive current value under normal circumstances. At the same time, for the faulty branch, the generator and new energy units provide reactive support during the fault period, and the reactive current will be greater than 0.2 pu.

[0088] S28: End.

[0089] Step S3. Delay coordination: According to the fault direction signal, the main backup protection action is controlled according to the preset time difference;

[0090] Step S4. Low current blocking: The low current blocking element monitors the line current in real time. If the low current criterion is met, the blocking protection is activated to avoid false tripping of non-fault lines.

[0091] The highly sensitive protection method based on voltage elements, directional elements and delay elements proposed in the embodiments of the present invention can avoid the serious lack of sensitivity faced by traditional current protection in AC grids with a high proportion of new energy grids, microgrids or new energy flexible direct current transmission systems at the sending end, and meet the protection sensitivity requirements of scenarios such as high proportion of new energy grids and microgrids.

[0092] Specifically, the present invention, based on the proposed voltage element, directional element, and time delay element, fully utilizes the voltage element to sense faults, the directional element to accurately identify the fault direction, and the time delay element to achieve coordination between the main and backup protections. Furthermore, the low-current criterion prevents malfunction of non-fault load lines, thereby meeting the "four properties" requirements of relay protection: rapidity, reliability, selectivity, and sensitivity, in high-proportion renewable energy power grids, microgrids, and renewable energy-transmitting flexible direct current systems at the sending end of AC power grids. In particular, with the continuous construction of new power systems and the increasing number of renewable energy and distributed power sources connected, the sensitivity and selectivity of traditional protection systems face severe challenges. The protection method proposed in the present invention is highly sensitive, does not require complex setting calculations, and easily meets the coordination requirements between the backup protection sensitivities, thus having broad application prospects.

[0093] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A highly sensitive power grid protection device based on voltage, direction and delay elements, characterized in that: include: Voltage element, used to sense grid faults through preset criteria of phase voltage, negative sequence voltage and zero sequence voltage, and output fault signals; a directional element, the input end of which is electrically connected to the output end of the voltage element, for identifying the fault type according to the fault signal and adaptively selecting a directional criterion and identifying the fault direction; A time delay element, the input end of which is electrically connected to the output end of the directional element, for realizing selective action of the main and backup protection by step-by-step coordination of time according to the fault direction identified by the directional element; The low-current locking element has an input end connected to the grid line current sampling end, and an output end electrically connected to the locking end of the delay element, and is used to monitor the line current in real time. If the low-current criterion is met, the locking protection action is taken to avoid false tripping of non-fault lines.

2. The device according to claim 1, characterized in that The preset criteria include: ； Among them, a, b, c represent the three phases; Indicates the phase voltage setting value, its typical value can be set to 0.8 pu; Indicates the negative sequence voltage setting value, and its secondary typical setting value is 4V; Indicates the zero-sequence voltage setting value, and its secondary typical setting value is 6V.

3. The device according to claim 1, characterized in that The directional element includes a fault type identification unit and a criterion selection unit: The fault type identification unit is used to determine whether the fault is an asymmetric ground fault, a two-phase phase-to-phase fault or a three-phase symmetrical fault based on the negative sequence voltage and the zero sequence voltage; The criterion selection unit is used to output a corresponding direction criterion signal to the delay element according to the fault type.

4. The device according to claim 1, characterized in that The main protection action time of the delay element is instantaneous, and the backup protection action time difference is 0.2s or 0.3s.

5. The device according to claim 1, characterized in that The low current criterion formula is: ； Where, Indicates the phase current action value; subscript " " indicates separation; It represents the reliability coefficient, and its typical value is 1.2; is the current before the phase voltage drops, that is, the memory current; is the actual voltage value at that time; is the voltage of the phase before the fault, that is, the memory voltage.

6. A highly sensitive power grid protection method based on voltage, direction and delay elements, characterized in that: The following steps are involved: Step S1. Voltage sensing: collecting phase voltage, negative sequence voltage and zero sequence voltage through voltage elements Voltage, when the preset criteria are met, it is determined that the power grid has failed and a fault signal is output; Step S2. Direction determination: Adaptively select the direction criterion and identify the fault direction based on the fault signal to identify the fault type; Step S3. Delay coordination: According to the fault direction signal, the main backup protection action is controlled according to the preset time difference; Step S4. Low current blocking: The low current blocking element monitors the line current in real time. If the low current criterion is met, the blocking protection is activated to avoid false tripping of non-fault lines.

7. The method according to claim 6, characterized in that In step S1, the preset criteria include: ； Among them, a, b, c represent the three phases; Indicates the phase voltage setting value, its typical value can be set to 0.8 pu; Indicates the negative sequence voltage setting value, and its secondary typical setting value is 4V; Indicates the zero-sequence voltage setting value, and its secondary typical setting value is 6V.

8. The method according to claim 6, characterized in that Step S2 direction determination: identifying the fault type according to the fault signal, adaptively selecting a direction criterion and identifying the fault direction, specifically includes: Step 1: Identify different fault types based on phase voltage, negative sequence voltage and zero sequence voltage under grid fault conditions: If the voltage or current sudden change element is activated and the negative sequence voltage criterion and the zero sequence voltage criterion are satisfied, it is determined that an asymmetric ground fault has occurred and the process goes to step 2. If the voltage or current sudden change element is activated, and the negative sequence voltage criterion is satisfied but the zero sequence voltage criterion is not satisfied, it is determined that a two-phase-to-phase fault has occurred, and the process goes to step 6; If the voltage or current sudden change element is activated and the voltage element criterion is met, but the negative sequence voltage criterion is not met, it is determined that a three-phase symmetrical fault has occurred and the process goes to step 7. Step 2: For an asymmetric ground fault, determine whether the zero-sequence directional element is applicable based on the magnitude of the zero-sequence current. If the magnitude of the zero-sequence current meets the criteria, the zero-sequence directional element is used provided that the zero-sequence component sensitivity meets the conditions. Otherwise, proceed to step 3. Step 3: In the case of insufficient sensitivity, determine the fault type and fault phase based on the voltage amplitude characteristics of each phase. If it is a two-phase grounding fault, proceed to step 4; otherwise, it is a single-phase grounding fault and proceed to step 5. Step 4: For a two-phase ground fault, construct a directional element criterion based on the non-fault phase voltage and the reverse non-fault phase current; Step 5: For single-phase grounding faults, construct a directional element criterion based on the fault phase negative sequence voltage and the fault phase current; Step 6: For a two-phase-to-phase fault, construct a directional element criterion based on the negative-sequence voltage and positive-sequence current; Step 7: For a three-phase symmetrical fault, the positive-sequence directional element is used when the positive-sequence voltage exceeds the preset threshold; otherwise, it indicates a nearby three-phase short-circuit fault. Considering the passive load branch, the positive-sequence voltage mutation amount and reactive current amplitude are used to form a comprehensive comparison criterion; Step 8: End.

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