Method for identifying direction of system fault, directional element and relay protection device

Abstract

The invention relates to a method for identifying the direction of a fault in an electric power system, a sequence impedance directional element and a relay protection device comprising the element. The power system comprises two power supply networks and a power transmission line, and a relay protection device is arranged at a first end point of the line. The method comprises the following steps: measuring three-phase current and three-phase voltage flowing through the relay protection device; determining a sequence voltage and a sequence current at the relay protection device based on the three-phase current and the voltage; calculating a sequence impedance at a predetermined compensation point in the power transmission line based on the determined sequence voltage and sequence current; and comparing the calculated sequence impedance at the predetermined compensation point with a reference threshold, and determining the direction of the fault in the power system relative to the relay protection device based on the comparison result. According to the method and the directional element provided by the invention, the fault direction can still be accurately judged under the condition that the negative sequence voltage on the relay protection device is very small, and the anti-interference capability is good.

Description

Technical Field

[0001] The present invention relates to the field of fault detection in power systems. More specifically, the present invention relates to a method for identifying the fault direction in a power system, a negative-sequence impedance direction element based on compensated voltage, and a relay protection device including the negative-sequence impedance direction element. Background Art

[0002] In a power system, the direction element, as a key component to ensure the stable operation of the system, plays a crucial role in various protective relay protection devices (such as distance protection devices, overcurrent direction protection devices, and pilot protection devices based on direction comparison, etc.). These protection devices rely on the direction element to identify the position relationship of the fault in the system relative to the relay protection device, so as to isolate the fault area in a timely manner.

[0003] The negative-sequence direction element based on the phase angle is a widely used direction element, which usually determines the fault direction based on the phase angle between the negative-sequence voltage and the negative-sequence current. Under normal circumstances, this negative-sequence direction element can work effectively: if it is recognized that the negative-sequence current leads the negative-sequence voltage, the line fault can be identified as a forward fault; otherwise, a reverse line fault can be identified.

[0004] However, this negative-sequence direction element cannot work properly when the negative-sequence voltage is very small. For example, in a super-powerful system or when a high-impedance fault occurs, the negative-sequence voltage on the relay protection device will become very small, even close to zero. In this case, not only is the detection of the negative-sequence voltage difficult, but also due to the noise interference in the signal, this negative-sequence direction element may produce misjudgments, resulting in the protection device being unable to correctly identify the direction of the line fault, thus affecting the safety and reliability of the entire power system. Summary of the Invention

[0005] In order to overcome the defects existing in the traditional direction element, the present invention proposes a negative-sequence impedance direction element based on compensated voltage. This direction element can still accurately judge the fault direction when the negative-sequence voltage on the relay protection device is very small, that is, it still has a high sensitivity to weak negative-sequence voltage signals. In addition, this direction element also has the ability to resist noise interference, thus ensuring reliable fault direction judgment under various complex working conditions.

[0006] According to a first aspect of the present invention, a method for identifying a fault direction in a power system is proposed. The power system includes two power networks and a power transmission line for transmitting three-phase alternating current between the two power networks. A relay protection device is provided at a first end point of the power transmission line. The method aims to determine the direction of the fault occurring in the power system relative to the relay protection device. The method includes the following steps:

[0007] Measure the three-phase current flowing through the relay protection device and the three-phase voltage at the relay protection device;

[0008] Use the symmetrical component method to determine the sequence voltage and sequence current at the relay protection device based on the measured three-phase current and three-phase voltage;

[0009] Calculate the sequence impedance at a predetermined compensation point in the power transmission line based on the determined sequence voltage and sequence current at the relay protection device, where the predetermined compensation point is between the two end points of the power transmission line and the compensation impedance from the predetermined compensation point to the first end point is Z q2 , where the calculation result of the sequence impedance is associated with the compensation impedance Zq2; and

[0010] Compare the calculated sequence impedance at the predetermined compensation point with a reference threshold, and determine the direction of the fault in the power system relative to the relay protection device based on the comparison result.

[0011] According to the above fault identification method defined in the first aspect of the present invention, since the compensation point is inside the transmission line, it means that the compensation voltage at the compensation point will not be zero in the case of a fault on the line. Therefore, this directional element has strong anti-noise ability. In addition, the reference threshold used in this fault identification method is only related to the line impedance, and it will not change with different systems, and customers do not need to set any threshold parameters for this, which further provides convenience for customers to use the product.

[0012] According to an optional embodiment, the method further includes:

[0013] Determine the fault direction in the power system based on the angle information and / or amplitude information of the sequence impedance at the predetermined compensation point.

[0014] In this embodiment, it is stipulated that this directional element can be based on the negative sequence impedance Z at the compensation point c2Both the angle information and the amplitude information are used to identify the fault direction. Existing technologies can only use one of the amplitude and angle information to judge the fault direction. For example, some directional elements cannot use the sign of the negative-sequence impedance to judge the fault direction because when the negative-sequence voltage at the relay protection device approaches zero, its negative-sequence impedance also approaches zero. Due to the presence of noise, the judgment based on the sign (angle) of the negative-sequence impedance becomes unreliable. In the method proposed in the present invention, when a line fault occurs, the negative-sequence compensation voltage will never be zero. Therefore, the sign and amplitude information of the angle of Z c2 can both be used to discriminate the fault direction.

[0015] According to an optional embodiment, the sequence voltage includes the negative-sequence voltage at the first end point The sequence current includes the negative-sequence current at the first end point And the sequence impedance includes the negative-sequence impedance Z at the predetermined compensation point c2 .

[0016] Although this article focuses on taking "negative-sequence impedance" as an example to introduce the performance of the directional element according to the present invention under different faults, it can be understood that a similar reasoning process can also be extended to zero-sequence impedance or positive-sequence impedance. For example, a zero-sequence (or positive-sequence) impedance directional element based on the compensation voltage can be designed to identify the fault direction in the system. Such a directional element can also work under the condition that the zero-sequence voltage is very small, and it not only has a simple structure but also has high reliability.

[0017] According to an optional embodiment, the current in the power system flows from the first end point to the second end point of the power transmission line, and the reference threshold includes a forward reference threshold and a reverse reference threshold greater than the forward reference threshold. Wherein, the method further includes:

[0018] When the negative-sequence impedance at the predetermined compensation point is greater than the reverse reference threshold, it is determined that a reverse fault occurs in the power system; and

[0019] When the negative-sequence impedance at the predetermined compensation point is less than the forward reference threshold, it is determined that a forward fault occurs in the power system.

[0020] According to an optional embodiment, the line negative-sequence impedance of the power transmission line is Z L2 , where the reference threshold is set based on the line negative-sequence impedance Z L2 and the position of the predetermined compensation point in the power transmission line.

[0021] According to an optional embodiment, assuming that the predetermined compensation point is set at the midpoint of the power transmission line, the forward reference threshold is set to k1×(-ZL2 / 2), and the reverse reference threshold is set to k2×Z L2 / 2, where k1 and k2 are reliability coefficients.

[0022] According to an optional embodiment, the sequence impedance angle θ at the predetermined compensation point is determined based on the sequence voltage and sequence current at the first end point, and the fault direction in the power system is determined based on the sequence impedance angle θ.

[0023] According to an optional embodiment, the method further includes:

[0024] When the negative sequence impedance angle θ at the predetermined compensation point is within the first reference range, it is determined that a reverse fault has occurred in the power system; and

[0025] When the negative sequence impedance angle θ at the predetermined compensation point is within the second reference range, it is determined that a forward fault has occurred in the power system.

[0026] According to an optional embodiment, the first reference range is -90° to 90°, and the second reference range is 90° to 270°.

[0027] The above embodiments specify that the fault direction can be directly discriminated by using the sign of the negative sequence impedance. This is because the negative sequence impedance angles at the compensation points for forward faults and reverse faults are completely separated. For example, for forward faults, they are always located in the first and second quadrants, while for reverse faults, they are always located in the third and fourth quadrants. Therefore, the fault identification method of the present invention has a large discrimination degree for forward faults and reverse faults.

[0028] According to the second aspect of the present invention, a sequence impedance direction element based on the compensation voltage is also proposed. The sequence impedance direction element is arranged in a power system including two power networks and a power transmission line for transmitting three-phase alternating current between the two power networks. According to an optional embodiment, the direction element is configured to identify the fault direction in the power system by using the method as described above.

[0029] According to the third aspect of the present invention, a relay protection device for a power system is also proposed. The relay protection device includes:

[0030] The sequence impedance direction element as described above;

[0031] A control unit, the control unit is connected to the sequence impedance direction element and is configured to output corresponding control signals to the relay when the sequence impedance direction element identifies a forward fault or a reverse fault; and

[0032] The relay connected to the control unit, which is configured to disconnect the power transmission line when receiving a control signal from the control unit.

[0033] Compared with traditional solutions, the negative sequence impedance direction element and fault direction identification method based on compensated voltage according to the present invention have the following advantages:

[0034] - First, the direction element can identify the fault direction based on both the angle information and amplitude information of the negative sequence impedance Z c2 at the compensation point. Existing technologies can only use one of the amplitude and angle information to judge the fault direction. For example, some direction elements cannot use the sign of the negative sequence impedance to judge the fault direction because when the negative sequence voltage at the relay protection device is close to zero, its negative sequence impedance is also close to zero. Due to the existence of noise, the judgment based on the sign (angle) of the negative sequence impedance becomes unreliable. In the method proposed by the present invention, when a line fault occurs, the negative sequence compensated voltage is never zero, so both the sign of the angle information and the amplitude information of Z c2 can be used to discriminate the fault direction;

[0035] - The direction element based on the amplitude of Z c2 does not require any setting. The reference threshold of the direction element of the present invention is only related to the line impedance, and it will not change with different systems, and the customer does not need to set any threshold parameters for this direction element;

[0036] - The direction element proposed by the present invention has a greater discrimination degree for the negative sequence impedance shown in forward faults and reverse faults. For the direction element of the present invention, the negative sequence reference threshold at the compensation point is always negative in forward faults and always positive in reverse faults. For existing direction elements, the negative sequence reference threshold is always positive in reverse faults, but its threshold may be negative or positive in forward faults. Therefore, the direction element of the present invention has a greater discrimination degree for forward faults and reverse faults;

[0037] - Compared with traditional direction elements, the direction element proposed by the present invention has a greater safety margin between the forward fault area and the reverse fault area;

[0038] - The direction element proposed by the present invention can directly discriminate the fault direction using the sign of the negative sequence impedance, with high reliability, so its response speed is relatively fast;

[0039] - Since the compensation point is inside the transmission line, it means that the compensated voltage at the compensation point will not be zero in the case of a line fault, so this direction element has strong anti-noise ability.

[0040] - The negative-sequence impedance ranges at the forward fault and reverse fault compensation points are completely separated. For example, for forward faults, they are always in the first and second quadrants, while for reverse faults, they are always in the third and fourth quadrants. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] By incorporating the drawings herein and the following specific embodiments used in conjunction with the Figure 1 to illustrate certain principles of the present invention, other features and advantages of the method of the present invention will become clear or be more specifically illustrated.

[0042] Figure 1 A simplified model diagram of a conventional dual-source power system is shown.

[0043] Figure 2 A schematic diagram of a power system model using a negative-sequence impedance direction element based on the compensated voltage is shown.

[0044] Figure 3 The performance of the direction element based on the compensated voltage when a forward fault occurs is shown.

[0045] Figure 4 The performance of the direction element based on the compensated voltage when a reverse fault occurs is shown.

[0046] Figure 5 The performance of the negative-sequence impedance direction element based on the compensated voltage is shown.

[0047] Figure 6 A schematic model diagram of faults occurring at different positions in a dual-source power system is shown.

[0048] Figures 7A-7D The simulation results of the negative-sequence impedance performance at the compensation point in the case of faults occurring at the fault points K1, K2, K3, and K5 are shown respectively.

[0049] Figure 8 A flowchart of a method for identifying the fault direction in a power system according to an exemplary embodiment of the present invention is shown. DETAILED DESCRIPTION

[0050] The method for identifying the fault direction of a power system based on a compensation voltage and the negative sequence impedance direction element according to the present invention will be described below with reference to the accompanying drawings and by way of examples. In the following description, numerous specific details are set forth in order to provide a more thorough understanding of the present invention to those skilled in the relevant art. However, it is apparent to those skilled in the relevant art that some of these specific details may not be required for the implementation of the present invention. Instead, the present invention can be implemented by considering any combination of the following features and elements, regardless of whether they relate to different embodiments. Therefore, the following aspects, features, embodiments, and advantages are for illustrative purposes only and should not be regarded as elements or limitations of the claims.

[0051] Figure 1 A simplified model diagram of a conventional dual-source power system is shown. As Figure 1 shown, the power sources V S and V R located at both ends of the circuit are electrically connected to each other through a power transmission line with an impedance of Z L . Assume that during the normal operation of the system, the current flows from the power source V S to V R . Z S represents the system impedance at the terminal of the power source V S , and Z R represents the system impedance at the terminal of the power source V R . In order to be able to cut off the transmission line between the two power sources in an emergency to protect the power system, a relay protection device REL1 is provided at the upstream end point of Z L .

[0052] In a power system, when an asymmetrical fault occurs, negative sequence current and negative sequence voltage will be generated. The direction element based on the negative sequence impedance can determine on which side of the system the fault occurs (i.e., determine the direction of the line fault relative to the relay protection device REL1) by analyzing the phase relationship (i.e., angle information) between the negative sequence current and the negative sequence voltage, thereby realizing the identification of the fault direction (i.e., forward fault or reverse fault). For Figure 1 the system model in, the negative sequence impedance Z2 at the relay protection device REL1 can be calculated according to the formula , where and are the negative sequence voltage and negative sequence current at the relay protection device respectively. This element determines the fault direction based on the phase angle between the negative sequence voltage and the negative sequence current , where θ1 and θ2 are set phase angle thresholds. In the case of a forward fault, in the case of a reverse fault, Here Z S2 and ZR2 respectively represent the system impedances at both ends of the line. In Figure 1 , "X2" refers to the imaginary part of the negative-sequence impedance Z2 (i.e., X2 = imag(Z2)). Since the reactance in the line is usually much larger than its resistance, ideally, it is assumed that the magnitude range of the negative-sequence impedance is mainly determined by its imaginary part.

[0053] In this paper, the "forward fault" and "reverse fault" of the system are defined based on the position of the line fault relative to the protective relay device. Among them, the "forward fault" is defined as the fault occurring downstream of the protective relay device, that is, from the perspective of the relay protection device, the fault point is in front of the relay protection device along the current direction. A forward fault means that the fault occurs in the line section protected by the protective relay device. On the contrary, the "reverse fault" is defined as the fault occurring upstream of the protective relay device, that is, from the perspective of the relay protection device, the fault point is behind the relay protection device. A reverse fault means that the fault occurs in the upstream section of the line protected by the protective relay device.

[0054] However, there are many defects in this conventional negative-sequence impedance direction element. For example, it can be understood from the above derivation formula of the negative-sequence impedance that the negative-sequence impedance calculated by this direction element is always positive under reverse faults and always negative under forward faults. Therefore, it seems that the forward and reverse faults can be clearly distinguished by the positive and negative signs of the negative-sequence impedance. However, for a super-strong system, since the negative-sequence voltage at the relay protection device is close to zero during a forward fault (due to Z S2 ≈0, so ), considering the possible noise in the signal, the finally obtained negative-sequence impedance at the relay protection device may be positive. That is to say, in this case, the fault direction cannot be determined based on the sign of the negative-sequence impedance, which will lead to misidentification results of the direction element.

[0055] In addition, there is also a known negative-sequence impedance direction element based on amplitude information in the prior art. For this negative-sequence impedance direction element, in order to determine the fault direction, it is necessary to set a forward reference threshold Z 2F and a reverse reference threshold Z 2R , and these two thresholds are related to the impedances Z S2 and Z R2 of the upstream and downstream power supply systems. When Z2 < Z 2F , it means that a forward fault occurs; when Z2 > Z 2R , it means that a reverse fault occurs. However, this direction element also has many defects. In particular, it is necessary to set two reference thresholds Z 2F and Z 2R, especially for ultra-strong systems, the thresholds of forward faults must be carefully selected to ensure the reliability of the recognition results, which brings unnecessary trouble to customers using this relay protection device product. In addition, the settings of these two thresholds are related to the system impedance, and the negative-sequence impedance safety margin [Z 2F ,Z 2R varies with different systems. When the system impedance changes during operation (generator connection / disconnection), the sensitivity or reliability of the directional element may decrease.

[0056] Aiming at the defects existing in the existing directional elements, the present invention proposes a negative-sequence impedance directional element based on compensated voltage and a fault direction determination method. This directional element does not need to set reference thresholds according to the system impedance upstream and downstream of the relay protection device, and can accurately judge the fault direction even under extremely low negative-sequence voltages.

[0057] Figure 2 The schematic diagram of a power system model using a negative-sequence impedance directional element based on compensated voltage is shown. As Figure 2 shown, the line negative-sequence impedance of the power transmission line is Z L2 , and three-phase alternating current can be transmitted between two power systems through this transmission line. Z S2 and Z S2_R represent the system impedances at both ends of the line respectively. Assuming that the current flows from Z S2 to Z S2_R , u2 is the voltage value at the protection relay protection device set in the power transmission line, and i2 is the current value in the power transmission line. The special feature of the negative-sequence impedance directional element based on compensated voltage is that a compensation point is selected between the two endpoints of the power transmission line, and the line negative-sequence impedance between this compensation point and the relay protection device can be recorded as Z q2 .

[0058] Based on Figure 2 the system model in, the negative-sequence compensated voltage at the compensation point can be deduced as:

[0059]

[0060] Here, the negative-sequence voltage and negative-sequence current at the relay protection device can be deduced from the three-phase current and three-phase voltage flowing through the transmission line, for example, using the symmetrical component method. The specific derivation process will not be elaborated in detail in this article.

[0061] It can be seen from the above formula (1) that the negative-sequence compensated voltage It has two characteristics: First, as long as the compensation point is within the transmission line protected by the relay protection device, the situation where is zero will never occur; in addition, even when a fault occurs, the negative-sequence voltage at the relay protection device is very small (for example, in an ultra-strong system), it still has a relatively large amplitude. Therefore, the directional element based on the compensation voltage can operate under the condition of very small negative-sequence voltage. For example, in an ultra-strong system, or under fault conditions with very high ground impedance.

[0062] Figure 3 Shows the performance of the directional element based on the compensation voltage when a forward fault occurs. As can be seen from Figure 3 , when a forward fault occurs, u q2 >> u2. Even when u2 is zero, u q2 still has a relatively large amplitude. When u2 is zero,

[0063] Figure 4 Shows the performance of the directional element based on the compensation voltage when a reverse fault occurs. As can be seen from Figure 4 , when a reverse fault occurs, u q2 is not zero and remains relatively high (only at node "O", the compensation voltage u q2 is zero).

[0064] Returning to Figure 2 , the negative-sequence impedance Z c2 at the compensation point can be calculated as:

[0065]

[0066] Assume that the compensation point is set at the midpoint of the transmission line, and the negative-sequence impedance of the entire transmission line is Z L2 , then the negative-sequence impedance at the compensation point is Z q2 = Z L2 / 2. Combining with Formula 2, it can be inferred that the negative-sequence impedance Z c2 at the compensation point is:

[0067]

[0068] When a forward fault occurs (as shown in Figure 3 ), it can be obtained that:

[0069]

[0070] When a forward fault occurs, the angle of Z c2 is -90°, and Z c2 is always less than -ZL2 / 2

[0071] When a reverse fault occurs (as Figure 4 shown), it can be obtained that:

[0072]

[0073] When a reverse fault occurs, the angle of Z c2 is 90°, and Z c2 is always greater than Z L2 / 2.

[0074] From the above derivation process, it can be seen that when a forward fault occurs, Z c2 = -Z S2 -Z L2 / 2, and the negative-sequence impedance Z c2 is always less than -Z L2 / 2; when a reverse fault occurs, Z c2 = Z L2 / 2 + Z SR2 , and Z c2 is always greater than Z L2 / 2.

[0075] Therefore, the threshold of the directional element can be set as follows:

[0076] Forward fault: Z c2 < -Z L2 / 2 —— (Formula 6) Reverse fault: Z c2 > Z L2 / 2 —— (Formula 7)

[0077] Figure 5 shows the performance of the negative-sequence impedance directional element based on the compensated voltage. As can be seen from Figure 5 , the safety margin between the forward fault and the reverse fault is relatively sufficient. Therefore, even when the negative-sequence voltage sensed by the relay protection device is very small, it is unlikely that the directional element will misjudge.

[0078] In addition, to improve the reliability of the system, a reliability coefficient k1 and k2 can be added to the directional element for the forward fault and the reverse fault respectively:

[0079] Forward fault: Z c2 < k1×(-Z L2 / 2)) —— (Formula 8)

[0080] Reverse fault: Z c2 > k2×Z L2 / 2) —— (Formula 9)

[0081] For example, k1 and k2 can be set to 0.8 or 0.9.

[0082] Considering that the negative sequence impedance angle of the line is not 90 degrees, Z can be calculated through the following formula: c2 for the angle of:

[0083]

[0084] where, RCA is the negative sequence impedance angle of the line.

[0085] In addition, in addition to the above settings, the angle information or amplitude information of Z c2 can also be used to design the directional element. Among them, the directional element based on the angle information of Z c2 can be defined as follows:

[0086]

[0087] In the above formula 11, δ is the angle of the negative sequence impedance of the line, that is, δ = arg Z L2 , θ1, θ2 are predetermined angle thresholds, and the two are Z for forward faults and reverse faults respectively c2 Set different threshold ranges.

[0088] For reverse faults, θ1 = -90° and θ2 = 90° can be set, so that (assuming that the angle δ is 90° in the following formula):

[0089]

[0090] When the above conditions are met, it can be determined as a reverse fault.

[0091] For forward faults, θ1 = 90° and θ2 = 270° can be set, so that:

[0092]

[0093] When the above conditions are met, it can be determined as a forward fault.

[0094] Although this article focuses on taking "negative sequence impedance" as an example to introduce the performance of the directional element according to the present invention under different faults, those skilled in the art can understand that a similar reasoning process can also be extended to zero sequence impedance or positive sequence impedance. For example, a zero sequence impedance directional element based on the compensated voltage can be designed to identify the fault direction in the system. This kind of directional element can also work under the condition that the zero sequence voltage is very small, and it not only has a simple structure but also has high reliability. In addition, a positive sequence directional element can also be designed to identify the fault direction in the system. All these variants fall within the protection scope of the present invention.

[0095] Figure 6 Shows a schematic model of a fault occurring at different positions in a dual-source power system. Assume that the voltage level of this power system model is 245 kV and the total length of the transmission line is 100 km. The positive-sequence system impedances of the two power sources are Z s = 8.23e j89° and Z R = 28.1e j89° . The positive-sequence impedance of the entire transmission line is 42.33 ohms and the sampling rate is 1 kHz. K1, K2, K3, and K5 are different fault points in the system. K1 is located at the starting end of the transmission line, K2 is located on the upstream side of the bus, K3 is located in the middle of the transmission line, and K5 is located at the end of the transmission line. K2 represents a reverse fault, and K1, K3, and K5 represent forward faults.

[0096] Figures 7A-7D Show the simulation results of the negative-sequence impedance performance at the compensation point in the case of faults occurring at fault points K1, K2, K3, and K5 respectively. Before presenting Figures 7A-7D the simulation results, it is worth noting that since the reactance in the line is usually much larger than its resistance, the imaginary part of the negative-sequence impedance Z c2 (i.e., X C2 = imag(Z C2 )) is used to describe the simulation results at the compensation point in this paper.

[0097] Figure 7A , 7C , 7D show the simulation results under fault points K1, K3, and K5 (i.e., forward faults). Under these fault points, the imaginary part of the negative-sequence impedance Z c2 obtained at the compensation point is -29.4 ohms (imag(Z s + Z L / 2) = 29.4). According to formula 6, it can be determined as a forward fault, where Z L is the line impedance.

[0098] Figure 7B Shows the simulation results under fault point K2 (i.e., reverse fault). Under this fault point, the imaginary part of the negative-sequence impedance Zc2 obtained at the compensation point is 49.3 ohms (imag(Z R + Z L / 2) = 49.3). According to formula 7, it can be determined as a reverse fault.

[0099] Figure 8 Shows a flowchart of a method for identifying the fault direction in a power system according to an exemplary embodiment of the present invention. The simplified model of this power system is as Figure 1As shown, it may include two power networks V S , V R , and a power transmission line for transmitting three-phase alternating current between the two power networks.

[0100] At the first end point of the power transmission line (the first end point is located on the upstream side, for example), a relay protection device REL1 may be provided. This method can be implemented by using the negative sequence impedance direction element introduced above with reference to Figures 1-7D , and its purpose is to determine the direction of the fault occurring in the power system relative to the relay protection device REL1. The operation process of this fault direction identification method will be introduced in detail below with reference to Figure 8 .

[0101] First, in step S101, the three-phase current flowing through the relay protection device REL1 and the three-phase voltage at the relay protection device are measured. Subsequently, in step S102, the sequence voltage and sequence current at the relay protection device can be determined based on the measured three-phase current and three-phase voltage. This process can be implemented by using the conventional symmetrical component method, for example. Using the symmetrical component method, the three-phase voltage and current can be decomposed into three symmetrical components: positive sequence component, negative sequence component, and zero sequence component. In this article, the "negative sequence component" (including negative sequence voltage negative sequence current and negative sequence impedance Z c2 ) will be taken as an example to introduce the method for identifying the fault direction in the power system according to the present invention and the direction element for implementing this method.

[0102] Based on the determined negative sequence voltage and negative sequence current at the relay protection device, in step S103, the negative sequence impedance Z c2 at a predetermined compensation point in the power transmission line can be further calculated. Among them, the compensation point can be set between the two end points of the power transmission line, and it is assumed that the compensation impedance between the compensation point and the first end point located upstream is Z q2 . From the above derivation formula 2, it can be seen that the calculation result of the negative sequence impedance Z c2 is associated with the compensation impedance Z q2 .

[0103] After determining the negative sequence impedance Z c2 at the compensation point, in step S104, the negative sequence impedance Z c2 can be compared with a reference threshold, and in step S105, the result of this comparison determines the specific direction of the fault in the power system relative to the relay protection device. The special feature of this embodiment is that the fault direction in the power system can be determined not only based on the angle information of the sequence impedance at the compensation point but also based on its amplitude information.

[0104] Assume that the current in the power system flows from the first end point to the second end point of the power transmission line. Then the reference thresholds include a positive reference threshold and a negative reference threshold greater than the positive reference threshold. Assume that the negative sequence impedance of the power transmission line is Z L2 , then the reference thresholds can be set based on the negative sequence impedance Z L2 of the line and the position of the compensation point in the power transmission line. For example, when the compensation point is located at the midpoint of the transmission line, the positive reference threshold can be set to -Z L2 / 2, and the negative reference threshold can be set to Z L2 / 2. The specific derivation process can refer to Formulas 6 and 7 mentioned above. In addition, reliability coefficients k1 and k2 can also be set for these two thresholds, referring to Formulas 8 and 9 mentioned above.

[0105] When the negative sequence impedance Z c2 at the compensation point is greater than the negative reference threshold Z L2 / 2, it indicates that a reverse fault occurs in the power system; when the negative sequence impedance Z c2 at the compensation point is less than the positive reference threshold -Z L2 / 2, it is determined that a forward fault occurs in the power system.

[0106] In addition, the method may further include determining the sequence impedance angle θ at a predetermined compensation point based on the sequence voltage and sequence current at the first end point, and determining the fault direction in the power system based on the sequence impedance angle θ. For example, when the negative sequence impedance angle θ at the predetermined compensation point is between -90° and 90°, it can be determined that a reverse fault occurs in the power system; when the negative sequence impedance angle θ at the predetermined compensation point is between 90° and 270°, it can be determined that a forward fault occurs in the power system.

[0107] After determining the specific fault direction, corresponding countermeasures can be triggered in step S106, such as disconnecting the relay protection device to protect the circuit from damage.

[0108] An exemplary embodiment of the present invention also proposes a sequence impedance direction element based on the compensation voltage. The sequence impedance direction element is arranged in a power system including two power networks and a power transmission line for transmitting three-phase alternating current between the two power networks. The special feature of this direction element is that it can be configured to identify the fault direction in the power system by using the method described above with reference Figure 8 to.

[0109] Another exemplary embodiment of the present invention also provides a relay protection device for a power system, which includes: the sequence impedance direction element as described above; a control unit connected to the sequence impedance direction element and configured to output corresponding control signals to the relay when the sequence impedance direction element identifies a forward fault or a reverse fault; and a relay connected to the control unit, which is configured to disconnect the power transmission line when receiving a control signal from the control unit.

[0110] Compared with the traditional solutions, the negative sequence impedance direction element and the fault direction identification method based on the compensated voltage according to the present invention have the following advantages:

[0111] 1) First, the direction element can identify the fault direction based on both the angle information and the amplitude information of the negative sequence impedance Z c2 at the compensation point. Existing technologies can only use one of the amplitude and angle information to judge the fault direction. For example, some direction elements cannot use the sign of the negative sequence impedance to judge the fault direction because when the negative sequence voltage at the relay protection device is close to zero, its negative sequence impedance is also close to zero. Due to the presence of noise, the judgment based on the sign (angle) of the negative sequence impedance becomes unreliable. In the method proposed by the present invention, when a line fault occurs, the negative sequence compensated voltage is never zero, so the sign of the angle information and the amplitude information of Z c2 can both be used to discriminate the fault direction;

[0112] 2) The direction element based on the amplitude of Z c2 does not require any setting. As can be seen from the above formulas 6 and 7, the reference threshold of the direction element of the present invention is only related to the line impedance. The line impedance is an internal parameter of the system, which does not change with different systems, and customers do not need to set any threshold parameters for this direction element;

[0113] 3) The direction element proposed by the present invention has a greater discrimination degree for the negative sequence impedance exhibited by forward faults and reverse faults. For the direction element of the present invention, the negative sequence reference threshold at the compensation point is always negative under forward faults and always positive under reverse faults. For existing direction elements, the negative sequence reference threshold is always positive under reverse faults, but its threshold may be negative or positive under forward faults. Therefore, the direction element of the present invention has a greater discrimination degree for forward faults and reverse faults;

[0114] 4) Compared with traditional direction elements, the direction element proposed by the present invention has a greater safety margin between the forward fault area and the reverse fault area. The safety margin between the forward fault and the reverse fault of the proposed direction element is [-Z L2 / 2, Z L2 / 2], while the safety margin of traditional directional elements is generally less than [0, Z L2 / 2];

[0115] 5) The directional element proposed by the present invention can utilize the sign of Z c2 to directly determine the fault direction, with relatively high reliability, and thus its response speed is relatively fast. For example, when Z c2 > 0, it can be directly determined as a reverse fault. When Z c2 < 0, it can be determined as a forward fault. By the thresholds -Z L2 / 2 and Z L2 / 2 in Formulas 6 and 7, the reliability of fault direction judgment can be further improved;

[0116] 6) The compensated voltage U q2 will not be zero under any circumstances (the compensation point is inside the transmission line), so this directional element has strong anti-noise ability; and

[0117] 7) The negative sequence impedance ranges at the compensation points for forward faults and reverse faults are completely separated. For example, for forward faults, it is always located in the first and second quadrants, while for reverse faults, it is always located in the third and fourth quadrants.

[0118] Those skilled in the art can understand that the various steps of the method according to the present invention are not limited to being implemented in the order listed above. In addition, in the present invention, terms such as "comprising" and "including" mean that in addition to the steps directly and clearly stated in the specification and claims, the technical solutions of this application do not exclude the situation of having other steps not directly or clearly stated.

[0119] Although the present invention has been disclosed above with preferred embodiments, the present invention is not limited thereto. Any changes and modifications made by those skilled in the art without departing from the spirit and scope of the present invention should be incorporated into the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the scope defined by the claims.

Claims

1. A method for identifying the fault direction in a power system, the power system comprising two power networks and a power transmission line for transmitting three-phase alternating current between the two power networks, a relay protection device being provided at a first end point of the power transmission line, the method being intended to determine the direction of a fault occurring in the power system relative to the relay protection device, characterized in that The method includes the following steps: Measure the three-phase current flowing through the relay protection device and the three-phase voltage at the relay protection device; Determine the sequence voltage and sequence current at the relay protection device based on the measured three-phase current and three-phase voltage; Calculate the sequence impedance at a predetermined compensation point in the power transmission line based on the determined sequence voltage and sequence current at the relay protection device, where the predetermined compensation point is between two endpoints of the power transmission line and the compensation impedance between the predetermined compensation point and the first endpoint is Z q2 , where the calculation result of the sequence impedance is associated with the compensation impedance Z q2 ; and Compare the sequence impedance at the calculated predetermined compensation point with a reference threshold, and determine the direction of the fault in the power system relative to the relay protection device based on the comparison result.

2. The method according to claim 1, wherein The method further includes: Determine the fault direction in the power system based on the angular information and / or amplitude information of the sequence impedance at the predetermined compensation point.

3. The method according to claim 1 or 2, characterized in that, The sequence voltage includes the negative-sequence voltage at the first end point The sequence current includes the negative-sequence current at the first end point And the sequence impedance includes the negative-sequence impedance Z at the predetermined compensation point c2 .

4. The method according to claim 3, wherein The current in the power system flows from the first end point to the second end point of the power transmission line, and the reference threshold includes a forward reference threshold and a reverse reference threshold greater than the forward reference threshold, wherein the method further includes: When the negative sequence impedance at the predetermined compensation point is greater than the reverse reference threshold, determine that a reverse fault occurs in the power system; and When the negative sequence impedance at the predetermined compensation point is less than the forward reference threshold, determine that a forward fault occurs in the power system.

5. The method according to claim 4, characterized in that, The negative sequence impedance of the power transmission line is Z L2 , where the reference threshold is based on the negative sequence impedance Z of the line L2 and the position of the predetermined compensation point in the power transmission line.

6. The method according to claim 5, wherein Assuming that the predetermined compensation point is set at the midpoint of the power transmission line, the forward reference threshold is set to k1×(-Z L2 / 2), and the reverse reference threshold is set to k2×Z L2 / 2, where k1 and k2 are reliability coefficients.

7. The method according to any one of claims 4 to 6, characterized in that Determine the sequence impedance angle θ at the predetermined compensation point based on the sequence voltage and sequence current at the first end point, and determine the fault direction in the power system based on the sequence impedance angle θ.

8. The method according to claim 7, wherein The method further includes: When the negative sequence impedance angle θ at the predetermined compensation point is within a first reference range, determine that a reverse fault occurs in the power system; and When the negative sequence impedance angle θ at the predetermined compensation point is within a second reference range, determine that a forward fault occurs in the power system.

9. The method according to claim 8, characterized in that, The first reference range is -90° to 90°, and the second reference range is 90° to 270°.

10. A sequence impedance direction element based on a compensation voltage, the sequence impedance direction element being arranged in a power system including two power networks and a power transmission line for transmitting three-phase alternating current between the two power networks, characterized in that, The direction element is configured to identify the fault direction in the power system by using the method according to any one of claims 1 to 9.

11. A relay protection device for use in a power system, characterized in that, The relay protection device includes: A sequence impedance direction element according to claim 10; A control unit, the control unit is connected to the sequence impedance direction element, and is configured to output a corresponding control signal to the relay when the sequence impedance direction element identifies a forward fault or a reverse fault; and The relay connected to the control unit, the relay is configured to disconnect the power transmission line when receiving a control signal from the control unit.

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