Distance protection method for active power distribution network with neutral point of transformer grounded through gap

Abstract

The invention relates to a distance protection method for an active power distribution network with a transformer neutral point grounded through a gap, and the method comprises the steps: firstly, analyzing factors which affect the size of measurement impedance according to the condition that a single load branch and a plurality of load branches are connected to a distributed power supply; thirdly, considering that measured impedance is composed of additional impedance and real short-circuit impedance of a line, solving the real short-circuit impedance based on the relation of impedance phasors in a directional impedance circle and the triangle sine theorem, the real short-circuit impedance being capable of effectively reflecting the position of a fault point and meeting the action criterion of distributed power supply side distance II section protection; and finally, when the real short-circuit impedance is solved, the additional impedance angle needs to be solved by using local information through distributed power supply side protection. According to the distance protection method provided by the invention, the real short-circuit impedance can be effectively solved when the fault occurs in the positive direction, and the protection action criterion is continuously met; when a fault occurs in a reverse direction, the method does not act accurately under different fault transition resistors, which indicates the reliability of the method.

Description

Technical Field

[0001] The invention belongs to the technical field of distribution network relay protection and fault characteristic analysis, and in particular relates to an active distribution network distance protection method with a transformer neutral point grounded via a gap. Background Art

[0002] In order to limit the overvoltage that may occur when the neutral point of the transformer is not grounded, a discharge gap is usually installed at the neutral point of the transformer. When an asymmetric grounding fault occurs in the adjacent transmission line of the transformer, and the low-voltage side of the transformer contains distributed power sources, its fault characteristics will affect the performance of the line relay protection. If the line protection cannot disconnect the faulty line in time, the distributed power source continues to provide short-circuit current, which may cause the neutral point voltage of the transformer to rise, and then break through the gap and change the zero-sequence circuit of the system. Distance protection has the advantages of high sensitivity and little influence of changes in the operation mode of the power grid on the action, and is widely used in line protection of 110kV voltage level. However, the grounding distance protection is greatly affected by the zero-sequence circuit, and the complex transient process after the gap breakdown may affect its action characteristics.

[0003] In the existing research work, there is a lack of research on the distance protection of the distributed power supply side in the neutral point gap grounding system. Considering that when a single-phase grounding fault occurs near the system side, the system side protection will act quickly. If the neutral point gap of the transformer is broken down, a gap zero-sequence current will be generated. Since the measured current of the input impedance relay includes the zero-sequence current flowing through the protection installation, it will affect the measured current of the distributed power supply side protection. In addition, the transient process of gap breakdown is relatively complex, which may cause the measured impedance to fail to continuously meet the protection action criterion or delay the action.

[0004] Based on the transformer neutral point gap system, some scholars proposed an improvement scheme for the grounding distance section II protection by introducing the distance section II self-holding signal and zero-sequence current detection. However, this study simply explained that gap breakdown may cause the distance protection section II to fail to operate. It did not specifically analyze the relationship between distributed power supply access and protection failure, nor did it consider the impact of transition resistance on the distance protection operation performance. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide an active distribution network distance protection method with a transformer neutral point grounded via a gap. First, in a gap grounding system, for the situation where a single load branch and multiple load branches are connected to a distributed power source, the factors affecting the size of the measured impedance are analyzed; then, considering that the measured impedance is composed of an additional impedance and a real short-circuit impedance of the line, in order to eliminate the influence of the additional impedance, the real short-circuit impedance is solved based on the relationship between the impedance phasors in the directional impedance circle and the triangle sine theorem. The real short-circuit impedance can effectively reflect the location of the fault point and meet the action criterion of the distance II section protection on the distributed power source side; finally, when solving the real short-circuit impedance, the additional impedance angle needs to be obtained by the distributed power source side protection using local information.

[0006] The present invention solves the technical problem by the following technical solutions:

[0007] A distance protection method for an active distribution network with a transformer neutral point grounded via a gap, characterized in that the steps of the method are:

[0008] S1. In an active distribution network with the neutral point of the transformer grounded through a gap, when a single-phase grounding fault occurs near the system side, the distance protection on the system side will be activated first, while the distance protection on the distributed power supply side will be activated after a delay of 0.3 to 0.5 seconds. If the neutral point of the transformer is broken down and then grounded during this delay period, the system zero-sequence network will change, which will affect the reliable operation of the distance protection. In a similar fault scenario, assuming two situations: a single load branch is connected to the distributed power supply and multiple load branches are connected to the distributed power supply, the influencing factors of the distance protection operation are analyzed. For a single load branch, the measured impedance on the distributed power supply side is only affected by the fault transition resistance; for multiple load branches, the measured impedance is related to the fault transition resistance and the fault short-circuit current flowing through each protection installation.

[0009] S2. Considering that the measured impedance is generally composed of additional impedance and the real short-circuit impedance of the line, the additional impedance changes with factors such as the fault transition resistance and the short-circuit current at the protection installation, while the real short-circuit impedance is only related to the fault location. In the case where a single load branch and multiple load branches are connected to the distributed power source, in order to eliminate the influence of the additional impedance, the triangle sine theorem is used to solve the expression of the real short-circuit impedance of the line based on the relationship between the impedance phasors in the directional impedance circle;

[0010] S3. Define the ratio of short-circuit currents at the distributed power sources on both sides to represent the measured impedance, and convert the short-circuit current ratio into a relational expression of the ratio of access capacities of the distributed power sources on both sides, and observe whether the measured impedance of the input direction impedance relay meets the distance protection action criterion by changing the capacity ratio and the fault transition resistance;

[0011] S4. Since the additional impedance angle needs to be calculated based on the short-circuit current at the fault point, and the short-circuit current cannot be directly obtained on the distributed power supply side, the negative-sequence current distribution coefficient is used to characterize the short-circuit current at the fault point.

[0012] Moreover, S1 is specifically as follows: considering that the distributed power source is randomly connected to the 110kV low-voltage transformer side according to the power demand, assuming that a single load branch and multiple load branches are connected to the distributed power source, after a single-phase grounding fault occurs on the 110kV line and the system side protection is activated, the measured impedance on the distributed power source side is analyzed:

[0013] (1) A single load branch is connected to a distributed power source

[0014] After the system side protection is activated, during the delay period of the N side protection distance protection stage II, the system changes from a dual power supply system to a single power supply system, and the measured impedance at the N side protection is:

[0015]

[0016] in, Measure the voltage at the N-side protection installation; Z N is the measured impedance value at the N-side protection; It is the fault phase short-circuit current measured at the N-side protection; is the fault phase short-circuit current at the fault point; Z MN(1) is the positive sequence impedance from point N to the fault point; k MN =(Z MN(0) -Z MN(1) ) / 3Z MN(1) is the zero-sequence current compensation coefficient, which is 2 / 3, Z MN(1) and Z MN(0) They are the positive-sequence and zero-sequence impedances of the 110 kV line MN respectively;

[0017] The measured impedance is mainly composed of the real short-circuit impedance of the line and the additional impedance. The real short-circuit impedance of the line remains unchanged, and the measured impedance is affected by the additional impedance. and R f The impact of

[0018] (2) Multiple load branches connected to distributed power sources

[0019] Assume that the 110kV low-voltage transformers of the two load branches are connected to the distributed power supply, and the fault setting is the same as when a single load branch is connected. At this time, the measured impedances of the two distributed power supply sides are expressed as:

[0020]

[0021] in, Measure the voltage at the D-side protection installation; It is the zero sequence current at the D side protection installation location; is the short-circuit current of the fault phase at point D; Z MD(1) is the positive sequence impedance from point D to the fault point; k MD is the zero-sequence current compensation coefficient, which takes a value of 2 / 3;

[0022] After the system side protection is activated, if the neutral point gap is not broken down, which is equivalent to the neutral point being ungrounded, then the zero-sequence current will not flow through the protection points, the short-circuit current of the fault phase is small, and the distance protection may not be activated; if the gap is broken down, a zero-sequence loop is formed on the 110kV high-voltage side, and the zero-sequence current flows through the protection point on the distributed power supply side. The measured impedance is related to the zero-sequence current flowing through the protection point, the short-circuit current of the fault phase, and the transition resistance.

[0023] Moreover, the S2 is specifically:

[0024] Define the short-circuit current ratio at the protection installation And k MN =2 / 3 Substituting into the measured impedance we get:

[0025]

[0026] Among them, (1), (2) and (0) are the positive, negative and zero-sequence components of the fault phase short-circuit current respectively;

[0027] Measure the impedance first and the short circuit current ratio Assuming that the short-circuit current characteristics of the distributed power outputs on both sides are the same, then It can be considered as a real number; secondly, the measured impedance is also related to the ratio of the zero-sequence current at the protection installation to the short-circuit current of the fault phase;

[0028] Therefore, a specific analysis is conducted on each sequence current at the protection installation. If the impact of the short-circuit current output by the distributed power supply on the other side on the short-circuit current at the protection installation on this side is not considered, each sequence current can be expressed as:

[0029]

[0030]

[0031] At this time, assuming that the short-circuit current characteristics of the two distributed power sources are the same and they are both in the state of outputting the maximum short-circuit current, the short-circuit current ratio It is expressed as:

[0032]

[0033] Where, m is the maximum short-circuit current coefficient; and are the short-circuit currents of the distributed power supplies on both sides; PDG1 and P DG2 They are the access capacity of distributed power sources on both sides; and They are the rated voltages of the distributed power sources on both sides during normal operation; and They are the rated currents of the distributed power sources on both sides during normal operation;

[0034] The ratio of the short-circuit currents at the protection installations on both sides is related to the ratio of the capacity of the distributed power sources connected on both sides. When the capacity of the distributed power source on the D side continues to increase, and the measurement impedance also increases continuously; however, the measurement impedance on the N side is also related to the ratio of the zero-sequence current at the N side protection installation to the fault phase short-circuit current. The ratio of the zero-sequence current at the N side protection to the fault phase short-circuit current is:

[0035]

[0036] Since the line impedance and load impedance on the 10kV side are much larger than the 110kV line impedance when converted to the 110kV side, and the transformer zero-sequence impedance is generally much larger than the 110kV line impedance, assuming that the downstream impedances on the N and D sides are the same, the above formula values ​​can be considered to be close to real numbers, which are:

[0037]

[0038] Substituting the above results into the formula for measuring impedance, we get:

[0039]

[0040] Where Re(·) represents the real part of the complex number, and Im(·) represents the imaginary part of the complex number. When the gap breaks down, the measured impedance at the protection point is related to the transition resistance, the short-circuit current output by the distributed power supply, and the fault location, and it is easy to cause the action criterion of the distance protection to fail.

[0041] Moreover, the S3 is specifically:

[0042] (1) Forward fault

[0043] Assuming that a single-phase grounding fault occurs near the system side, the impedance phasor relationships of the two networks with a single load branch and multiple load branches are analyzed respectively:

[0044] 1) A single load branch is connected to a distributed power source

[0045] In the process of measuring impedance change, only the real resistance changes, and the imaginary reactance remains unchanged. Based on the triangle sine theorem, the expression for the real short-circuit impedance reactance value is obtained:

[0046] |Im(Z real )|=|Z m |×sinβ

[0047] Among them, Im(Z real ) is the imaginary part of the real impedance of the line; |Z m | is the modulus of the measured impedance, assuming that the unit impedance of the 110kV line MN is z MN , then the real short-circuit impedance of the line is written as:

[0048]

[0049] Among them, Im(z MN ) is the complex part of unit impedance; |Z real | is the modulus of the real impedance of the line. When a fault occurs in the forward zone, since the measured impedance angle is the internal angle of a triangle, sinβ>0 and |Z real |>0 is always established;

[0050] 2) Multiple load branches are connected to distributed power sources

[0051] If multiple load branches are connected to distributed power sources, there will be a certain error when the short-circuit current output by the distributed power sources on both sides is gathered at the fault point. When the error cannot be ignored, the additional impedance angle will affect the solution of the real short-circuit impedance. Therefore, it is necessary to discuss the situation where there is an error. After establishing the triangle relationship, according to the triangle sine theorem, the following related expressions are obtained:

[0052]

[0053] For the distributed power supply side protection, when a fault occurs in the forward zone, the real short-circuit impedance is expressed as:

[0054]

[0055] Among them, K real is the distance protection factor;

[0056] (2) Reverse fault

[0057] Assuming that a single-phase grounding fault occurs at the high-voltage side bus of the 110 / 10kV transformer, that is, the back bus where the distributed power supply protection is installed, the impedance phasor relationship under the two networks of single load branch and multiple load branches is analyzed respectively:

[0058] 1) A single load branch is connected to a distributed power source

[0059] For the case where a single load branch is connected to a distributed power source, when a reverse fault occurs, the magnitude of the fault phase short-circuit current flowing through the protection installation on the distributed power source side is 0. The identification of the reverse fault can be determined by whether there is a fault phase short-circuit current.

[0060] 2) Multiple load branches are connected to distributed power sources

[0061] For the case where multiple load branches are connected to distributed power sources, K r ' eal The distance protection factor is expressed as:

[0062]

[0063] Moreover, S4 is specifically as follows: for the case where only one branch is connected to the distributed power source, the short-circuit current at the fault point and the short-circuit current flowing through the protection installation should be approximately in phase, and the additional impedance angle γ=0° at this time; for the case where multiple branches are connected to the distributed power source, the amplitude of the short-circuit current of the fault phase at the fault point is the sum of the short-circuit currents of the fault phases at the protection installations, and the phase is also obtained by superimposing the short-circuit currents at the protections on the distributed power source side; considering that there is generally no communication mechanism between protections, each protection installation needs to use local information to achieve the purpose of calculating the additional impedance angle,

[0064] When a single-phase grounding fault occurs, according to the boundary conditions of the single-phase grounding fault, the following relationship exists:

[0065]

[0066] Among them, C N(1) and C N(2) They are positive sequence current distribution coefficient and negative sequence current distribution coefficient respectively;

[0067] The negative sequence current distribution coefficient is used to solve the additional impedance angle. The load impedance is much larger than other types of impedances, and the power factor is large, which leads to the real part of the load impedance being much larger than the imaginary part. N(2) It can be further expressed as:

[0068]

[0069] Where y is a real number, then the additional impedance angle is written as:

[0070]

[0071] Using the above formula, the additional impedance angle can be obtained from the local information at the protection installation.

[0072] The positive effects that the present invention can produce are:

[0073] 1. The present invention analyzes the factors affecting the measured impedance on the distributed power supply side for the active distribution network with gap grounding system, and theoretically proves that the measured impedance is related to the fault transition resistance and the access capacity of the distributed power supply under different networks;

[0074] 2. The present invention uses the triangle sine theorem in the directional impedance circle to provide a relationship between the measured impedance, the additional impedance and the real short-circuit impedance for the case where a single load branch or multiple load branches of the distribution network are connected to the distributed power source, so as to achieve the purpose of eliminating the additional impedance;

[0075] 3. In view of the problem that it is difficult to use the positive sequence current distribution coefficient to characterize the short-circuit current at the fault point at the protection installation on the distributed power supply side, the present invention adopts the negative sequence current distribution coefficient to calculate the additional impedance angle so that it is not affected by the distributed power supply. BRIEF DESCRIPTION OF THE DRAWINGS

[0076] Figure 1 This is the wiring diagram of the 110kV active distribution network with the transformer neutral point through the gap;

[0077] Figure 2 It is the short-circuit current distribution diagram when a single load branch is connected to a distributed power source;

[0078] Figure 3 It is the short-circuit current distribution diagram when multiple load branches are connected to distributed power sources;

[0079] Figure 4 It is the system composite sequence network diagram after the transformer neutral point gap breakdown;

[0080] Figure 5 It is the phasor relationship diagram of each impedance of the system when a single load branch is connected to the distributed power source;

[0081] Figure 6 It is the phasor relationship diagram of each impedance of the system when multiple load branches are connected to the distributed power source;

[0082] Figure 7 It is the phasor relationship diagram of each impedance of the system during reverse fault;

[0083] Figure 8 It is a flow chart of the distance protection method of active distribution network with transformer grounded through gap;

[0084] Fig. 9 It is the distance protection action performance diagram under different transition resistances;

[0085] Fig.10 This is the fault phase short-circuit current diagram at the N-side protection during reverse fault. DETAILED DESCRIPTION

[0086] The present invention is further described in detail below through specific examples. The following examples are only illustrative and not restrictive, and the protection scope of the present invention cannot be limited thereto.

[0087] The present invention provides an active distribution network distance protection method with a transformer neutral point grounded via a gap, comprising:

[0088] Taking into account that distributed power sources are randomly connected to the 110kV transformer side according to power demand, assuming that a single load branch and multiple load branches are connected to the distributed power source, the measured impedance on the distributed power source side is analyzed after a single-phase grounding fault occurs on the 110kV line and the system side protection is activated.

[0089] (1) A single load branch is connected to a distributed power source

[0090] exist Figure 1 In the active distribution network model shown in the figure, the distributed generation is connected to the branch of load 1. Assume that the fault occurs at the end of the M side of the MN line, the fault point is outside the protection range of the N side protection distance I, the fault phase is phase A, and the transition resistance is R f , the short-circuit current distribution is as follows Figure 2 As shown. Among them, It is the short-circuit current output by the distributed power source; is the measured current at the N-side protection; It is the short-circuit current at the fault point.

[0091] Depend on Figure 2 It can be seen that after the system side protection is activated, during the delay period of the N side protection distance protection stage II, the system changes from a dual power supply system to a single power supply system, and the measured impedance at the N side protection is:

[0092]

[0093] in, Measure the voltage at the N-side protection installation; Z N is the measured impedance value at the N-side protection; It is the fault phase short-circuit current measured at the N-side protection; is the fault phase short-circuit current at the fault point; Z MN(1) is the positive sequence impedance from point N to the fault point; k MN =(Z MN(0) -Z MN(1) ) / 3Z MN(1) is the zero-sequence current compensation coefficient, which is generally a certain value (here it is taken as 2 / 3), Z MN(1) and Z MN(0)They are respectively the positive sequence and zero sequence impedance of the 110kV line MN. It can be seen that the measured impedance is mainly composed of the real short-circuit impedance of the line and the additional impedance. The real short-circuit impedance of the line is generally unchanged, and the measured impedance is mainly affected by the additional impedance, while the additional impedance is mainly affected by and R f For a single power supply system after the system side protection is activated, based on the boundary conditions of a single-phase grounding fault, it can be known that the relationship between the fault phase short-circuit current at the protection installation, the zero-sequence current at the protection installation, and the fault phase short-circuit current at the fault point is:

[0094]

[0095] Fault point short circuit current Fault phase short-circuit current at the protection installation location and 3 times the zero sequence current The amplitude and phase should be equal. Therefore, for a single load branch connected to a distributed power supply network, if the neutral point is broken, the measured impedance is mainly affected by the transition resistance, the additional impedance is mainly resistive, and the protection may fail to operate due to the influence of the transition resistance.

[0096] (2) Multiple load branches connected to distributed power sources

[0097] Assumptions Figure 1 The 110kV transformer low sides of the two load branches are connected to the distributed power supply, and the fault setting is the same as when a single load branch is connected. The short-circuit current distribution is as follows: Figure 3 As shown. Among them, and It is the short-circuit current output by the distributed power sources on both sides; is the measured current at the D side protection. At this time, the measured impedances of the two distributed power supply sides are expressed as:

[0098]

[0099] in, Measure the voltage at the D-side protection installation; It is the zero sequence current at the D side protection installation location; is the short-circuit current of the fault phase at point D; Z MD(1) is the positive sequence impedance from point D to the fault point; k MD is the zero-sequence current compensation coefficient (taken as 2 / 3). Since the short-circuit current at the fault point is provided by the distributed power sources at the connection points of the two load branches at the same time, after the gap is broken down, the additional impedances at the protection points on the two distributed power supply sides are:

[0100]

[0101] It can be seen that after the system side protection is activated, if the neutral point gap is not broken down, which is equivalent to the neutral point being ungrounded, then the zero-sequence current will not flow through the protection points, the short-circuit current of the fault phase is small, and the distance protection may not be activated; if the gap is broken down, a zero-sequence loop is formed on the 110kV high-voltage side, and the zero-sequence current flows through the protection point on the distributed power supply side. The measured impedance is related to the zero-sequence current flowing through the protection point, the short-circuit current of the fault phase, and the transition resistance.

[0102] Taking the measured impedance at the N-side protection as an example, define the short-circuit current ratio at the protection installation And k MN =2 / 3 Substituting into the measured impedance, we get:

[0103]

[0104] Among them, (1), (2) and (0) are the positive, negative and zero sequence components of the fault phase short-circuit current respectively. Assuming that the short-circuit current characteristics of the distributed power outputs on both sides are the same, then can be considered as a real number; secondly, the measured impedance is also related to the ratio of the zero-sequence current at the protection installation to the short-circuit current of the fault phase. Therefore, it is necessary to conduct a specific analysis of the sequence currents at the protection installation. Figure 4 Shown is the system composite sequence network diagram when the gap breaks down.

[0105] If the impact of the short-circuit current output by the distributed power supply on the other side on the short-circuit current at the protection installation on this side is not considered, the sequence currents can be expressed as:

[0106]

[0107]

[0108]

[0109] Among them, the system comprehensive negative sequence impedance Z Σ(0) and zero sequence impedance Z Σ(2) It can be expressed as:

[0110]

[0111] At this time, the short-circuit current ratio It can be expressed as:

[0112]

[0113] Assuming that the short-circuit current characteristics of the two distributed power sources are the same and they are both in the state of outputting the maximum short-circuit current, equation (9) can be further transformed into:

[0114]

[0115] Where, m is the maximum short-circuit current coefficient; and are the short-circuit currents of the distributed power supplies on both sides; P DG1 and P DG2 They are the access capacity of distributed power sources on both sides; and They are the rated voltages of the distributed power sources on both sides during normal operation; and They are the rated currents of the distributed power sources on both sides during normal operation.

[0116] It can be seen that the ratio of the short-circuit currents at the protection installations on both sides is mainly related to the ratio of the capacity of the distributed power sources connected on both sides. When the capacity of the distributed power source on the D side continues to increase, The measurement impedance also increases. However, the measurement impedance on the N side is also related to the ratio of the zero-sequence current at the N side protection installation to the fault phase short-circuit current. The ratio of the zero-sequence current at the N side protection to the fault phase short-circuit current is:

[0117]

[0118] According to the above analysis, and Respectively expressed as:

[0119]

[0120] Since the line impedance and load impedance on the 10kV side are much larger than the 110kV line impedance when converted to the 110kV side, and the transformer zero-sequence impedance is generally much larger than the 110kV line impedance, assuming that the downstream impedances on the N and D sides are the same, the above formula values ​​can be considered to be close to real numbers, which are:

[0121]

[0122] Substituting the above results into the formula for measuring impedance, we get:

[0123]

[0124] Among them, Re(·) represents the real part of the complex number, and Im(·) represents the imaginary part of the complex number. It can be seen that when the gap breaks down, the measured impedance at the protection point is related to the transition resistance, the short-circuit current output by the distributed power supply, and the fault location, and it is easy to cause the action criterion of the distance protection to fail.

[0125] In order to eliminate the influence of additional impedance in the measured impedance, this method proposes a distance protection scheme based on the real short-circuit impedance of the line in two networks with a single branch and multiple branches connected to distributed power sources.

[0126] (1) Forward fault

[0127] Assuming that a single-phase grounding fault occurs near the system side, the phasor relationships of various impedances in two networks, a single load branch and multiple load branches, are analyzed respectively.

[0128] 1) A single load branch is connected to a distributed power source

[0129] For a single load branch connected to a distributed power source, in the directional impedance circle, the phasor relationship of each impedance is as follows: Figure 5 As shown. Among them, the measured impedance Z m , the real short-circuit impedance Z real and additional impedance Z ad They are:

[0130]

[0131] In addition, α represents the line impedance angle, which is generally determined based on the line impedance parameters; β represents the measured impedance angle, which can be measured at the protection installation location. Figure 5 From equation (15), we can see that as the transition resistance changes, Z m It will keep changing and may fall outside the action area of ​​the impedance complex plane, causing the protection to fail to operate. Since the additional impedance is purely resistive, and the measured impedance is composed of the real resistance and the imaginary reactance, this part of the reactance is completely provided by the real short-circuit impedance. It can be observed that in the process of measuring impedance changes, only the real resistance changes, and the imaginary reactance remains unchanged.

[0132] Based on this principle, in order to obtain the reactance value of the real short-circuit impedance, two right triangles OEH and OFG are constructed in the impedance circle. It is relatively easy to obtain that the length of the line segment EH is equal to GF. Based on the triangle sine theorem, the expression for the reactance value of the real short-circuit impedance can be obtained:

[0133] |Im(Z real )|=|Z m |×sinβ (16)

[0134] Among them, Im(Z real ) is the imaginary part of the real impedance of the line; |Z m | is the modulus of the measured impedance. Assume that the unit impedance of line MN is z MN , then the real short-circuit impedance of the line can be written as:

[0135]

[0136] Among them, Im(z MN ) is the complex part of unit impedance; |Z real | is the modulus of the real impedance of the line. It can be seen that when a fault occurs in the forward zone, since the measured impedance angle is the internal angle of the triangle, sinβ>0 and |Z real |>0 is always true.

[0137] 2) Multiple load branches are connected to distributed power sources

[0138] If multiple load branches are connected to distributed power sources, there may be a certain error when the short-circuit current output by the distributed power sources on both sides is gathered at the fault point. When the error cannot be ignored, the additional impedance angle will affect the solution of the real short-circuit impedance. Therefore, it is necessary to discuss the situation where there is an error.

[0139] Assuming that the phase difference between the short-circuit current at the fault point and the short-circuit current at the protection installation is (-90°, 90°), then Figure 6 (a) It can be seen that after establishing the triangle relationship, according to the triangle sine theorem, the following related expressions can be obtained:

[0140]

[0141] After simplifying the above formula, we can get:

[0142]

[0143] Similarly, for Figure 6 (b), we can get the relevant formula:

[0144]

[0145] Similarly, after simplifying the above formula, the same expression of the real short-circuit impedance of the line can be obtained. In summary, the real short-circuit impedance can be expressed as:

[0146]

[0147] Among them, K real is the distance protection coefficient. Formula (21) can be used to obtain the real impedance of the line when multiple branches have distributed power sources connected. In addition, for the possible transient overrun phenomenon (misoperation of the distance protection stage I on the distributed power source side), the real impedance of the line is obtained by formula (21) as the near-system side, and its size should be significantly greater than the setting value of the distance protection stage I.

[0148] (2) Reverse fault

[0149] Assuming that a single-phase grounding fault occurs at the high-voltage side bus of the 110 / 10kV transformer (i.e., the back bus where the distributed power supply protection is installed), the phasor relationships of various impedances in two networks, a single load branch and multiple load branches, are analyzed respectively.

[0150] 1) A single load branch is connected to a distributed power source

[0151] For the case where a single load branch is connected to a distributed power source, when a reverse fault occurs, the magnitude of the fault phase short-circuit current flowing through the protection installation on the distributed power source side is 0. Therefore, in this type of network, the identification of a reverse fault can be determined by whether there is a fault phase short-circuit current, and no further analysis will be performed here.

[0152] 2) Multiple load branches are connected to distributed power sources

[0153] When multiple load branches are connected to distributed power sources, the relationship between the impedance phasors in the directional impedance circle can be expressed as follows: Figure 7 As shown. At this time, the range of the additional impedance angle is γ∈(α-π / 2,π / 2), then K′ real The distance protection factor can be expressed as:

[0154]

[0155] Compared with the distance protection factor for forward faults, the following conclusions can be drawn:

[0156]

[0157] (3) Determination of additional impedance angle

[0158] In the case where only one branch is connected to the distributed power source, the short-circuit current at the fault point and the short-circuit current flowing through the protection installation should be approximately in phase, and the additional impedance angle γ can be considered to be 0°. In the case where multiple branches are connected to the distributed power source, the amplitude of the short-circuit current of the fault phase at the fault point is the sum of the short-circuit currents of the fault phases at each protection installation, and the phase is also obtained by superimposing the short-circuit currents at the protections on the distributed power source side. However, considering that there is generally no communication mechanism between protections, each protection installation needs to use local information to achieve the purpose of calculating the additional impedance angle.

[0159] When a single-phase grounding fault occurs, according to the boundary conditions of the single-phase grounding fault, the following relationship exists:

[0160]

[0161] Among them, C N(1) and C N(2)They are the positive sequence current distribution coefficient and the negative sequence current distribution coefficient. In the network connected to the distributed generation, since the positive sequence current distribution coefficient is difficult to directly express using local information, this scheme uses the negative sequence current distribution coefficient to solve the additional impedance angle:

[0162]

[0163] Because the load impedance is much larger than other types of impedance, and the power factor is generally large, the real part of the load impedance should also be much larger than the imaginary part. Therefore, C N(2) It can be further expressed as:

[0164]

[0165] Where y is a real number, then the additional impedance angle is written as:

[0166]

[0167] Using formula (27), the additional impedance angle can be obtained from the local information at the protection installation.

[0168] The specific implementation flow chart of this plan is as follows Figure 8 As shown, the steps are as follows:

[0169] 1) Collect the zero-sequence voltage at the protection installation. If it is greater than 15% of the rated voltage at the busbar, the starting criterion of the distance protection is met and the system-side protection operates quickly;

[0170] 2) Observe the change of transformer neutral point voltage. If the neutral point gap is not broken down, the protection on the distributed power supply side will refuse to operate. Otherwise, go to 3);

[0171] 3) For a network with a single branch connected to a distributed power source, if the short-circuit current of the fault phase at the protection installation point on the distributed power source side is 0, it is judged as a reverse fault and enters 1); otherwise, the real short-circuit impedance of the line can be obtained by formula (17) and then enter 5). For a network with multiple branches connected to distributed power sources, if the distance protection coefficient is less than 0, enter 1); otherwise, the real short-circuit impedance of the line can be obtained by formula (21) and then enter 4);

[0172] 4) Considering the transient overrun, if the real impedance of the line is less than the setting value of the distance protection stage I, the protection on the distributed power supply side will be activated; otherwise, go to 5);

[0173] 5) If the real impedance of the line meets the protection action criterion of the directional impedance circle, the distributed power supply side protection is allowed to operate; otherwise, go to 1).

[0174] Simulation verification:

[0175] In order to verify the effectiveness of the protection scheme proposed in this invention, a simulation platform is built based on PSCAD / EMTDC. Figure 1 The simulation model of the distribution network with distributed generation access is shown in the figure. The simulation cases are all based on the action of the N-side protection. The single-phase grounding fault is set at 2.0s, the system-side protection is activated at 2.1s, and the simulation is performed in two networks with a single load branch and multiple load branches connected to the distributed generation.

[0176] (1) A single load branch is connected to a distributed power source

[0177] 1) Forward fault

[0178] Assuming the fault transition resistance R f The distributed power supply is connected to the load 1 branch, P DG For 10MW, P L1 and P L1 Both are 2MW, and the fault point is the outlet of the system side protection installation. "1" indicates protection action and "0" indicates protection rejection action.

[0179] like Fig. 9 As shown in the figure, for different transition resistances, the proposed method can effectively solve the real short-circuit impedance of the line, the protection action criteria of the directional impedance circle can be met, and the distance protection coefficient is always greater than 0, indicating that the fault is a positive direction fault and the distributed power supply side protection can be reliably operated.

[0180] 2) Reverse fault

[0181] The single-phase metallic ground fault is set at the N-side protection back bus of the load 1 branch, such as Fig.10 The figure shows the short-circuit current of the fault phase flowing through the N-side protection installation. It can be seen that when the fault just occurs,

[0182] The maximum amplitude can reach 10kA, and when the system side protection QF1 is activated, It quickly decreases to 0, indicating the accuracy of fault direction identification.

[0183] (2) Multiple load branches connected to distributed power sources

[0184] 1) Forward fault

[0185] Assuming the fault transition resistance R f is 100Ω, and the distributed power supply is connected to the load 1 and 2 branches respectively, P DG1 4MW, P DG2 They are 12MW, 14MW, 16MW and 18MW respectively, P L1 4MW, P L2The power supply is 2MW, and the fault point is the outlet of the system side protection installation.

[0186] Table 1 Protection action performance under different distributed power generation capacities

[0187]

[0188] As shown in Table 1, "×" indicates protection refusal and "√" indicates protection action. For different distributed generation access capacities, the action criteria of the distance protection section I of the distributed generation side protection are not met, and the real short-circuit impedance value of the line is close to the actual value, indicating the accuracy of the solved real short-circuit impedance of the line, while the action criteria of the distance protection section II can be effectively met, the protection is reliably acted, and the distance protection coefficient is greater than 0, and the fault direction is correctly judged.

[0189] 2) Reverse fault

[0190] Similarly, in order to verify that the proposed protection method does not malfunction in the event of a reverse fault, a single-phase grounding fault is set at the back bus of the N-side protection.

[0191] Table 2 Protection performance in case of reverse fault

[0192]

[0193] As shown in Table 2, for different fault transition resistances, the actual short-circuit impedance of the line obtained by solution all falls in the fourth quadrant of the directional impedance circle, and the distance protection coefficient is less than 0, indicating that the fault direction is correctly identified and QF2 on the distributed power supply side refuses to operate. Therefore, the effectiveness and accuracy of the proposed distance protection method are verified.

[0194] Although the embodiments and drawings of the present invention are disclosed for illustrative purposes, those skilled in the art will appreciate that various substitutions, changes and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A distance protection method for an active distribution network with a transformer neutral point grounded via a gap, characterized in that: The steps of the method are: S1. In an active distribution network with the neutral point of the transformer grounded through a gap, when a single-phase grounding fault occurs near the system side, the distance protection on the system side will be activated first, while the distance protection on the distributed power supply side will be activated after a delay of 0.3 to 0.5 seconds. If the neutral point of the transformer is broken down and then grounded during this delay period, the system zero-sequence network will change, which will affect the reliable operation of the distance protection. In a similar fault scenario, assuming two situations: a single load branch is connected to the distributed power supply and multiple load branches are connected to the distributed power supply, the influencing factors of the distance protection operation are analyzed. For a single load branch, the measured impedance on the distributed power supply side is only affected by the fault transition resistance; for multiple load branches, the measured impedance is related to the fault transition resistance and the fault short-circuit current flowing through each protection installation. S2. Considering that the measured impedance is generally composed of additional impedance and the real short-circuit impedance of the line, the additional impedance changes with factors such as the fault transition resistance and the short-circuit current at the protection installation, while the real short-circuit impedance is only related to the fault location. In the case where a single load branch and multiple load branches are connected to the distributed power source, in order to eliminate the influence of the additional impedance, the triangle sine theorem is used to solve the expression of the real short-circuit impedance of the line based on the relationship between the impedance phasors in the directional impedance circle; S3. Define the ratio of short-circuit currents at the distributed power sources on both sides to represent the measured impedance, and convert the short-circuit current ratio into a relational expression of the ratio of access capacities of the distributed power sources on both sides, and observe whether the measured impedance of the input direction impedance relay meets the distance protection action criterion by changing the capacity ratio and the fault transition resistance; S4. Since the additional impedance angle needs to be calculated based on the short-circuit current at the fault point, and the short-circuit current cannot be directly obtained on the distributed power supply side, the negative-sequence current distribution coefficient is used to characterize the short-circuit current at the fault point.

2. The active distribution network distance protection method with transformer neutral point grounded via gap according to claim 1 is characterized in that: Specifically, S1 is as follows: considering that the distributed power source is randomly connected to the 110kV transformer-lower side according to the power demand, assuming that a single load branch and multiple load branches are connected to the distributed power source, after a single-phase grounding fault occurs on the 110kV line and the system-side protection is activated, the measured impedance on the distributed power source side is analyzed: (1) A single load branch is connected to a distributed power source After the system side protection is activated, during the delay period of the N side protection distance protection stage II, the system changes from a dual power supply system to a single power supply system, and the measured impedance at the N side protection is: in, Measure the voltage at the N-side protection installation; Z N is the measured impedance value at the N-side protection; It is the fault phase short-circuit current measured at the N-side protection; is the fault phase short-circuit current at the fault point; Z MN(1) is the positive sequence impedance from point N to the fault point; k MN =(Z MN(0) -Z MN(1) ) / 3Z MN(1) is the zero-sequence current compensation coefficient, which is 2 / 3, Z MN(1) and Z MN(0) They are the positive-sequence and zero-sequence impedances of the 110 kV line MN respectively; The measured impedance is mainly composed of the real short-circuit impedance of the line and the additional impedance. The real short-circuit impedance of the line remains unchanged, and the measured impedance is affected by the additional impedance. and R f The impact of (2) Multiple load branches connected to distributed power sources Assume that the 110kV low-voltage transformers of the two load branches are connected to the distributed power supply, and the fault setting is the same as when a single load branch is connected. At this time, the measured impedances of the two distributed power supply sides are expressed as: in, Measure the voltage at the D-side protection installation; It is the zero sequence current at the D side protection installation location; is the short-circuit current of the fault phase at point D; Z MD(1) is the positive sequence impedance from point D to the fault point; k MD is the zero-sequence current compensation coefficient, which takes a value of 2 / 3; After the system side protection is activated, if the neutral point gap is not broken down, which is equivalent to the neutral point being ungrounded, then the zero-sequence current will not flow through the protection points, the short-circuit current of the fault phase is small, and the distance protection may not be activated; if the gap is broken down, a zero-sequence loop is formed on the 110kV high-voltage side, and the zero-sequence current flows through the protection point on the distributed power supply side. The measured impedance is related to the zero-sequence current flowing through the protection point, the short-circuit current of the fault phase, and the transition resistance.

3. The active distribution network distance protection method with transformer neutral point grounded via gap according to claim 1 is characterized in that: The S2 is specifically: Define the short-circuit current ratio at the protection installation And k MN =2 / 3 Substituting into the measured impedance we get: Among them, (1), (2) and (0) are the positive, negative and zero-sequence components of the fault phase short-circuit current respectively; Measure the impedance first and the short circuit current ratio Assuming that the short-circuit current characteristics of the distributed power outputs on both sides are the same, then It can be considered as a real number; secondly, the measured impedance is also related to the ratio of the zero-sequence current at the protection installation to the short-circuit current of the fault phase; Therefore, a specific analysis is conducted on each sequence current at the protection installation. If the impact of the short-circuit current output by the distributed power supply on the other side on the short-circuit current at the protection installation on this side is not considered, each sequence current can be expressed as: At this time, assuming that the short-circuit current characteristics of the two distributed power sources are the same and they are both in the state of outputting the maximum short-circuit current, the short-circuit current ratio It is expressed as: Where, m is the maximum short-circuit current coefficient; and are the short-circuit currents of the distributed power supplies on both sides; P DG1 and P DG2 They are the access capacity of distributed power sources on both sides; and They are the rated voltages of the distributed power sources on both sides during normal operation; and They are the rated currents of the distributed power sources on both sides during normal operation; The ratio of the short-circuit currents at the protection installations on both sides is related to the ratio of the capacity of the distributed power sources connected on both sides. When the capacity of the distributed power source on the D side continues to increase, and the measurement impedance also increases continuously; however, the measurement impedance on the N side is also related to the ratio of the zero-sequence current at the N side protection installation to the fault phase short-circuit current. The ratio of the zero-sequence current at the N side protection to the fault phase short-circuit current is: Since the line impedance and load impedance on the 10kV side are much larger than the 110kV line impedance when converted to the 110kV side, and the transformer zero-sequence impedance is generally much larger than the 110kV line impedance, assuming that the downstream impedances on the N and D sides are the same, the above formula values ​​can be considered to be close to real numbers, which are: Substituting the above results into the formula for measuring impedance, we get: Where Re(·) represents the real part of the complex number, and Im(·) represents the imaginary part of the complex number. When the gap breaks down, the measured impedance at the protection point is related to the transition resistance, the short-circuit current output by the distributed power supply, and the fault location, and it is easy to cause the action criterion of the distance protection to fail.

4. The active distribution network distance protection method with transformer neutral point grounded via gap according to claim 1 is characterized in that: The S3 is specifically: (1) Forward fault Assuming that a single-phase grounding fault occurs near the system side, the impedance phasor relationships of the two networks with a single load branch and multiple load branches are analyzed respectively: 1) A single load branch is connected to a distributed power source In the process of measuring impedance change, only the real resistance changes, and the imaginary reactance remains unchanged. Based on the triangle sine theorem, the expression for the real short-circuit impedance reactance value is obtained: |Im(Z real )|=|Z m |×sinβ Among them, Im(Z real ) is the imaginary part of the real impedance of the line; |Z m | is the modulus of the measured impedance, assuming that the unit impedance of the 110kV line MN is z MN , then the real short-circuit impedance of the line is written as: Among them, Im(z MN ) is the complex part of unit impedance; |Z real | is the modulus of the real impedance of the line. When a fault occurs in the forward zone, since the measured impedance angle is the internal angle of a triangle, sinβ>0 and |Z real |>0 is always established; 2) Multiple load branches are connected to distributed power sources If multiple load branches are connected to distributed power sources, there will be a certain error when the short-circuit current output by the distributed power sources on both sides is gathered at the fault point. When the error cannot be ignored, the additional impedance angle will affect the solution of the real short-circuit impedance. Therefore, it is necessary to discuss the situation where there is an error. After establishing the triangle relationship, according to the triangle sine theorem, the following related expressions are obtained: For the distributed power supply side protection, when a fault occurs in the forward zone, the real short-circuit impedance is expressed as: Among them, K real is the distance protection factor; (2) Reverse fault Assuming that a single-phase grounding fault occurs at the high-voltage side bus of the 110 / 10kV transformer, that is, the back bus where the distributed power supply protection is installed, the impedance phasor relationship under the two networks of single load branch and multiple load branches is analyzed respectively: 1) A single load branch is connected to a distributed power source For the case where a single load branch is connected to a distributed power source, when a reverse fault occurs, the magnitude of the fault phase short-circuit current flowing through the protection installation on the distributed power source side is 0. The identification of the reverse fault can be determined by whether there is a fault phase short-circuit current. 2) Multiple load branches are connected to distributed power sources For the case where multiple load branches are connected to distributed power sources, K r ' eal The distance protection factor is expressed as:

5. The active distribution network distance protection method with transformer neutral point grounded via gap according to claim 1 is characterized in that: S4 is specifically: for the case where only one branch is connected to the distributed power source, the short-circuit current at the fault point and the short-circuit current flowing through the protection installation should be approximately in phase, and the additional impedance angle γ=0°. For the case where multiple branches are connected to the distributed power source, the short-circuit current amplitude of the fault phase at the fault point is the sum of the short-circuit currents of the fault phases at the protection installations, and the phase is also obtained by superimposing the short-circuit currents at the protections on the distributed power source side. Considering that there is generally no communication mechanism between protections, each protection installation needs to use local information to achieve the purpose of calculating the additional impedance angle. When a single-phase grounding fault occurs, according to the boundary conditions of the single-phase grounding fault, the following relationship exists: Among them, C N(1) and C N(2) They are positive sequence current distribution coefficient and negative sequence current distribution coefficient respectively; The negative sequence current distribution coefficient is used to solve the additional impedance angle. The load impedance is much larger than other types of impedances, and the power factor is large, which leads to the real part of the load impedance being much larger than the imaginary part. N(2) It can be further expressed as: Where y is a real number, then the additional impedance angle is written as: Using the above formula, the additional impedance angle can be obtained from the local information at the protection installation.

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