Method for calculating output current and protection setting value by using in-situ information

Abstract

The invention discloses a method for calculating an output current and a protection setting value by using in-situ information, which comprises the following steps of: establishing an equivalent power supply model comprising a system main power supply and a distributed power supply, and determining a sequence network diagram of a fault line according to the position of the fault line in the equivalent power supply model; based on the sequence network diagram of the fault line, calculating the grid-connected point voltage containing the distributed line by using the in-situ measurement information at the protection installation position after the fault, carrying out iterative correction on the grid-connected point voltage by using the output current of the distributed power supply, and calculating the actual output current of all the distributed power supplies related to the fault line; according to the actual output current of all the distributed power supplies related to the fault line and the output current of the system main power supply, calculating the current flowing through the protection installation position when the tail end of the fault line breaks down; calculating a protection constant value corresponding to the fault line according to the current flowing through the protection installation position; according to the invention, protection of different fault scenes can be realized.

Description

Technical Field

[0001] The invention relates to the technical field of current protection of distribution network, and in particular to a method for calculating output current and protection setting value by using local information. Background Art

[0002] The access of large-scale distributed generation (DG) brings new challenges to the current protection of distribution networks. The distribution network has changed from a single power supply structure to a multi-power supply structure. When a fault occurs in the distribution network, the additional current generated by the access of DG will seriously affect the correlation between the short-circuit current and the short-circuit distance, resulting in protection failure or malfunction. As an important part of the relay protection of the power system, it is necessary to study how to improve the current protection action principle and its setting method, and form an adaptive current protection scheme suitable for the access of multiple DGs to the power grid to ensure the normal operation of the distribution network.

[0003] At present, domestic and foreign scholars have carried out a variety of research. The existing adaptive current protection schemes are mainly adaptive current protection setting schemes based on local information and adaptive protection schemes under communication cooperation: (1) The main idea of ​​adaptive current protection based on local information is to analyze the relationship between the local electrical quantities after the fault, so as to determine the setting value of the protection. The document "Adaptive positive sequence current quick-break protection of distribution network containing inverter-type distributed power sources, Chen Xiaolong et al., Automation of Power Systems" proposes an adaptive positive sequence current protection scheme that uses voltage and current positive sequence components to calculate the current protection setting. This scheme only uses the electrical quantities at the protection point to set the protection, but the scheme ignores the output current of the DG when calculating the fault current of the phase-to-phase short circuit, which will cause the protection to refuse to operate when the DG capacity is large. The document "Adaptive current protection of high-proportion new energy power grid based on real-time generation of fixed values, Zhang Zhengwei et al., Electric Power Construction Engineering Technology II" proposes a real-time generation of fixed value adaptive current protection scheme that only uses local information. This scheme distinguishes the output components of the generator and DG models and measures the current at the fault point; but this scheme only considers the situation where the power supply side contains a high proportion of new energy, and does not consider the new energy in the middle of the line. Therefore, the above current protection schemes do not consider the situation of multiple DG access, and the adaptive current protection scheme based on local information still has room for improvement. (2) The adaptive current protection scheme based on communication uses multi-point information sharing to collect information such as DG grid connection and load parameters in a larger area to set the current protection setting value. The document "Research on Fault Characteristics of Photovoltaic Power Sources and Analysis of Influencing Factors, Xu Kehan ​​et al., Transactions of the Chinese Society of Electrical Engineering" proposes to detect the current flow direction based on communication and directional elements to achieve fault location, thereby achieving real-time adjustment of the protection setting value. The above research has high requirements on the reliability of communication and the accuracy of information. Once there is a problem with communication, the protection will also fail; at the same time, the previous current protection scheme did not consider the branch line or only considered the adaptability of protection under a single branch line, and did not consider the scenario of multiple branch lines. Therefore, for multi-branch distribution networks with multiple DG access, it is necessary to study the current protection using local information. Summary of the invention

[0004] The technical problem to be solved by the present invention is how to provide a method for calculating the output current and the protection setting value using local information, which can adapt to changes in the fault location, fault type or DG access capacity, and perform real-time setting to achieve protection in different fault scenarios.

[0005] The present invention solves the above technical problems by the following technical means:

[0006] The present invention proposes a method for calculating output current and protection setting value by using local information, the method comprising:

[0007] Establishing an equivalent power supply model including the main power supply and the distributed power supply of the system, and determining the sequence network diagram of the fault line according to the position of the fault line in the equivalent power supply model;

[0008] Based on the sequence network diagram of the fault line, the grid connection point voltage including the distributed line is calculated using the local measurement information at the protection installation after the fault, and the grid connection point voltage is iteratively corrected using the output current of the distributed power source to calculate the actual output current of all the distributed power sources related to the fault line;

[0009] According to the actual output current of all distributed power sources related to the fault line and the output current of the system main power supply, calculate the current flowing through the protection installation when a fault occurs at the end of the fault line;

[0010] The protection setting corresponding to the faulty line is calculated based on the current flowing through the protection installation.

[0011] Further, the establishing of an equivalent power supply model including a system main power supply and a distributed power supply, and determining a sequence network diagram of the faulty line according to a position of the faulty line in the equivalent power supply model, includes:

[0012] A voltage source series resistor is used to represent the main power supply of the system, and a controlled current source is used to represent the distributed power supply, and the equivalent power supply model is established;

[0013] According to the equivalent power source model, the location of the fault line, the fault type, the installation location and access capacity of the distributed power source, a sequence network diagram of the fault line is determined.

[0014] Furthermore, the distributed power sources related to the fault line include distributed power sources directly connected to the fault line, distributed power sources upstream of the fault line, and distributed power sources downstream of the fault line.

[0015] Furthermore, the output current expression of the distributed power supply is:

[0016]

[0017] Where: P0 is the active power output of the distributed generation before the fault; is the grid connection point voltage of the distributed power source; is the output current of the distributed power supply.

[0018] Furthermore, for the distributed power source directly connected to the fault line, based on the sequence network diagram of the fault line, the grid connection point voltage including the distributed line is calculated using the local measurement information at the protection installation after the fault, and the grid connection point voltage is iteratively corrected using the output current of the distributed power source, and the actual output current of the distributed power source directly connected to the fault line is calculated, including:

[0019] Determine the grid connection point voltage expression of the T-connected distributed power source according to the sequence network diagram of the fault line;

[0020] Calculating the corrected output current of the distributed power source according to the output current expression of the distributed power source;

[0021] The corrected output current of the distributed power source is substituted into the grid connection point voltage expression of the T-connected distributed power source for iterative calculation until the iterative difference of the grid connection point voltage meets the accuracy of the iterative voltage difference, thereby obtaining the grid connection point voltage and the actual output current of the T-connected distributed power source.

[0022] Furthermore, for the distributed power source upstream of the fault line, based on the sequence network diagram of the fault line, the grid connection point voltage including the distributed line is calculated using the local measurement information at the protection installation after the fault, and the grid connection point voltage is iteratively corrected using the output current of the distributed power source to calculate the actual output current of the distributed power source upstream of the fault line, including:

[0023] According to the actual output current of the T-connected distributed power source and the current measured at the fault line protection installation, Kirchhoff's current law is used to calculate the current flowing through the upstream of the fault line;

[0024] According to the current flowing through the upstream of the fault line and the voltage measured at the fault line protection installation, Kirchhoff's voltage law is used to calculate the output current of the system main power supply;

[0025] Determine the voltage expression of the branch where the distributed power source upstream of the fault line is located according to the current flowing through the upstream of the fault line and the output current of the system main power supply;

[0026] An iterative calculation is performed based on the output current expression of the distributed power source, the grid connection point voltage expression of the T-connected distributed power source, and the voltage expression of the branch where the distributed power source upstream of the fault line is located to obtain the actual output current of the distributed power source upstream of the fault line.

[0027] Furthermore, for the distributed power source downstream of the fault line, based on the sequence network diagram of the fault line, the grid connection point voltage including the distributed line is calculated using the local measurement information at the protection installation after the fault, and the grid connection point voltage is iteratively corrected using the output current of the distributed power source to calculate the actual output current of the distributed power source downstream of the fault line, including:

[0028] The fault location is assumed to occur at the end of the fault line, and the grid connection point voltage expression of the distributed generation downstream of the fault line is obtained according to the sequence network diagram;

[0029] An iterative calculation is performed according to the output current expression of the distributed power source, the grid connection point voltage expression of the T-connected distributed power source and the grid connection point voltage expression of the distributed power source downstream of the fault line to obtain the actual output current of the distributed power source downstream of the fault line.

[0030] Further, the current flowing through the protection installation when a fault occurs at the end of the fault line according to the actual output current of all distributed power sources related to the fault line combined with the output current of the system main power supply includes:

[0031] According to the actual output current of each distributed power source related to the fault line, calculate the current of each distributed power source flowing through the fault line;

[0032] Based on the current flowing through the fault line of each distributed power source and the output current of the system main power supply, the current flowing through the protection installation when a fault occurs at the end of the fault line is calculated.

[0033] Further, the calculating of the current of each distributed power source flowing through the fault line according to the actual output current of each distributed power source related to the fault line includes:

[0034] According to the actual output current of the distributed power source and the corresponding line impedance and system internal resistance, the current flowing through the distributed power source through the fault current limiting is calculated.

[0035] Further, the calculating of the protection setting corresponding to the fault line according to the current flowing through the protection installation location includes:

[0036] Calculate the protection setting value of protection stage I according to the current flowing through the protection installation location;

[0037] Calculate the protection range of the lower-level line section I protection, and calculate the current flowing through the protection installation of this level of line when a fault occurs at the maximum protection range;

[0038] The protection setting of protection stage II is calculated based on the current flowing through the line protection installation of this level when a fault occurs at the maximum protection range.

[0039] The advantages of the present invention are:

[0040] (1) While utilizing the measurement information at the protection installation site, the present invention uses the output expression of the distributed power source to iteratively correct the grid connection point voltage of the distributed power source, calculates the actual output current of all the distributed power sources in the faulty line, and sets the protection setting corresponding to the faulty line according to the voltage of the system main power source and the actual output current of all the distributed power sources in the line. Without the need for additional communication equipment, the real-time setting of the protection setting can be achieved during a fault. When different types of faults occur at different locations of the line, the output current of the distributed power source will change, and the protection setting will also change accordingly, thereby ensuring that each section of protection can respond to the fault in the area, greatly improving the sensitivity of the protection, and being suitable for multiple fault scenarios such as simultaneous access of multiple distributed power sources, T-connection of DG lines, direct access of DG to the distribution network, and access of other branch lines to photovoltaic power sources.

[0041] (2) With the large-scale access of distributed power sources to the distribution network, the additional current generated by the access of distributed power sources seriously affects the performance of traditional protection, reducing its protection range. The present invention corrects the output current of the distributed power sources through iterative calculation. At the same time, when setting the protection constant, it takes into account the impact of the access of distributed power sources. Therefore, compared with traditional current protection and traditional adaptive current protection, the present invention has a larger protection range.

[0042] (3) This embodiment does not need to calculate the impedance of the back side of the fault line, nor does it need to perform equivalence on the back side of the fault line, thus simplifying the setting process.

[0043] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is a flow chart of a method for calculating output current and protection setting value by using local information proposed in one embodiment of the present invention;

[0045] Figure 2 It is a structural diagram of a 10kV distribution network including a distributed power source in one embodiment of the present invention;

[0046] Figure 3 is a schematic diagram of a T-connection of a DG line in one embodiment of the present invention;

[0047] Figure 4 1 is a sequence network diagram when a two-phase fault occurs at the BC line in one embodiment of the present invention, wherein (a) is a positive sequence network, (b) is a negative sequence network, and (c) is a composite sequence network;

[0048] Figure 51 is a sequence network diagram when a two-phase fault occurs at the AB line in one embodiment of the present invention, wherein (a) is a positive sequence network, (b) is a negative sequence network, and (c) is a composite sequence network. DETAILED DESCRIPTION

[0049] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0050] like Figure 1 As shown, an embodiment of the present invention provides a method for calculating output current and protection setting value using local information, the method comprising the following steps:

[0051] S10, establishing an equivalent power supply model including a system main power supply and a distributed power supply, and determining a sequence network diagram of the fault line according to a position of the fault line in the equivalent power supply model;

[0052] S20, based on the sequence network diagram of the fault line, using the on-site measurement information at the protection installation after the fault to calculate the grid connection point voltage including the distributed line, and using the output current of the distributed power source to iteratively correct the grid connection point voltage, and calculate the actual output current of all distributed power sources related to the fault line;

[0053] S30, calculating the current flowing through the protection installation when a fault occurs at the end of the fault line according to the actual output current of all distributed power sources related to the fault line and the output current of the system main power supply;

[0054] S40. Calculate a protection setting corresponding to the faulty line according to the current flowing through the protection installation.

[0055] This embodiment uses the voltage and current information measured at the protection installation location and the output current of the distributed power source to iteratively correct the grid connection point voltage, calculate the actual output current of all distributed power sources related to the fault line, and adaptively change the protection setting value to ensure that the current protection can reflect the fault in the line area. This method can adapt to changes in fault location, fault type or DG access capacity, and perform real-time setting to achieve protection in different fault scenarios. It has high selectivity and sensitivity when the line length is short and the line is T-connected.

[0056] As a further preferred technical solution, the step S10: establishing an equivalent power supply model including the system main power supply and the distributed power supply, and determining the sequence network diagram of the fault line according to the position of the fault line in the equivalent power supply model, includes the following steps:

[0057] S11, using a voltage source series resistor to represent the main power supply of the system, using a controlled current source to represent the distributed power supply, and establishing the equivalent power supply model;

[0058] S12. Determine a sequence network diagram of the fault line according to the equivalent power model, the location of the fault line, the fault type, the installation location and access capacity of the distributed power source.

[0059] Among them, the expression of the controlled current source is:

[0060]

[0061] Where: P0 is the active power output of the distributed generation before the fault; is the grid connection point voltage of the distributed power source; is the output current of the distributed power supply, all are per unit values.

[0062] As a further preferred technical solution, all distributed power sources of the fault line include distributed power sources directly connected to the fault line, distributed power sources upstream of the fault line and distributed power sources downstream of the fault line.

[0063] Specifically, the traditional distribution network is generally a radial single-end power supply network, and the fault current flows from the system to the fault point. However, the access of DG turns the distribution network into a multi-end power supply network. In addition to the current provided by the system main power supply, the current flowing through the protection installation also includes the current provided by the DG. The access of DG changes the fault characteristics of the short-circuit current of the distribution network.

[0064] Below Figure 2 The 10kV distribution network shown in the figure is taken as an example to illustrate the T-connection diagram of the DG line. Figure 3 As shown: Establish Figure 2 The PSCAD simulation model of the distribution network system shown has a system base voltage of 10.5 kV, a base capacity of 100 MVA, a system impedance, and line lengths of AB, BC, and CD are all 6 km, and line lengths of AE and EF are 4 km.

[0065] (1) When a line fault occurs downstream of the DG access location, such as the CD line, the DG plays an auxiliary role in increasing the fault current, and the impact on the short-circuit current becomes more obvious as the DG capacity increases and the access location is farther away from the system power supply; (2) When a line fault occurs upstream of the DG access location, such as the AB line, DG1 and DG2 play an external draining role, and the external draining effect becomes stronger as the DG access capacity increases; (3) When DGs are connected both upstream and downstream of the fault point, such as the BC line, the upstream DG1 plays an auxiliary role in increasing the fault current, and the downstream DG2 plays an external draining role. However, the overall change in fault current is affected by factors such as the DG capacity and the fault location; (4) When the DG access location is on other feeders, such as the AE line, DG3 plays an external draining role on the fault current, but distributed power sources such as DG1 and DG2 will increase the fault current. However, since the fault line and the distributed power sources are on different feeders, the auxiliary role is weak.

[0066] Taking the BC line fault as an example, the sequence network diagram for two-phase fault is established as follows: Figure 4 As shown in the figure, the sequence network diagram when a two-phase fault occurs at the AB line is as follows Figure 5 As shown, since DG adopts the negative-sequence current suppression strategy, there is no DG in the negative-sequence network; since the neutral point of the distribution network is not grounded, there is no zero-sequence current path, so there is no zero-sequence network.

[0067] As a further preferred technical solution, the step S20: based on the sequence network diagram of the fault line, the grid connection point voltage including the distributed line is calculated using the local measurement information at the protection installation after the fault, and the grid connection point voltage is iteratively corrected using the output current of the distributed power source, and the actual output current of all the distributed power sources related to the fault line is calculated, including:

[0068] For the distributed power source directly connected to the fault line, determine the grid connection point voltage expression of the T-connected distributed power source according to the sequence network diagram of the fault line, and use the output current of the distributed power source to iteratively correct the grid connection point voltage to calculate the actual output current of the T-connected distributed power source;

[0069] For the distributed generation upstream of the fault line, the short-circuit current provided by the system is calculated based on the actual output current of the T-connected distributed generation and the current measured at the fault line protection installation point, and then based on KCL and KVL. Finally, the actual output current of the remaining upstream DGs is calculated by combining with iteration;

[0070] For the distributed generation downstream of the faulty line, it is assumed that the fault is located at the end of the line. On this basis, the voltage at the downstream DG grid connection point is calculated, and it is iteratively corrected in combination with the output current expression of the distributed generation to calculate the actual output current of the downstream DG.

[0071] Specifically, taking the BC line fault as an example, the DG directly connected to the faulty line is DG1, the DG upstream of the faulty line is DG3, and the DG downstream of the faulty line is DG2. It can be considered that DG1 is connected to the faulty line through line T. Figure 3 shown.

[0072] (1) Calculation of the actual output current of DG1 directly connected to the fault line BC:

[0073] According to the output current expression of distributed power supply, the output current of DG will change with the degree of grid voltage drop when a fault occurs. At the moment of fault, considering the current tracking response delay inside DG and the role of filtering circuit, it can be considered that the output current of DG when a fault occurs Equal to the output current before the fault Voltage at the DG grid connection point at the moment of fault It can be calculated from the grid connection point voltage expression:

[0074]

[0075] In the formula, is the voltage at the DG grid connection point, is the output current of DG1, Z L is the impedance of the line to which DG is connected. When DG is directly connected to the system, Z L is 0, It is the voltage detected at the access point of the DG line.

[0076] Depend on The voltage drop degree of the grid connection point can be obtained, and the output current of the modified DG can be calculated according to the output current expression of the distributed power source. Will Re-substitute into the grid connection point voltage expression and perform iterative calculation until the grid connection point voltage iterative difference satisfies The iteration is stopped when , so as to obtain the voltage and output current of DG1 grid connection point in, is the k-th iteration result of the grid connection point voltage, is the result of the k-1th iteration of the grid-connected point voltage, ε is the accuracy of the iterative voltage difference of the grid-connected point voltage, and different values ​​can be selected according to different requirements. In this embodiment, it is 0.001.

[0077] (2) Calculation of the actual output current of DG3 upstream of the fault line BC:

[0078] After calculating the output current of DG1, the current flowing through the AB line at this time is calculated according to Kirchhoff's current law KCL:

[0079]

[0080] In the formula, is the current flowing through line AB, is the current measured at the BC line protection installation, is the actual output current of distributed power source DG1.

[0081] Depend on Figure 4 (c) According to Kirchhoff's voltage law KVL, the short-circuit current output by the system is:

[0082]

[0083] In the formula, is the short-circuit current output by the system during a fault. is the system reference voltage, is the voltage at the BC line protection installation location, is the current flowing through the AB line, Z AB is the line impedance of line AB, Z s is the internal resistance of the system.

[0084] The current flowing through the AE line and the voltage expression of the branch where DG3 is located can be further calculated:

[0085]

[0086]

[0087] In the formula, is the current flowing through the AE line, is the voltage of the branch where DG3 is located, is the current flowing through line AB, is the system output current, Z AE is the line impedance of line AE, Z s is the system internal resistance, is the system reference voltage.

[0088] Therefore, the method for calculating the output current of DG1 is the same as that for calculating the output current of DG1. The actual output short-circuit current of DG3 can be iteratively calculated based on the output current expression of the distributed power source, the grid connection point voltage expression, and the voltage expression of the branch where DG3 is located.

[0089] (3) Calculation of the actual output current of DG3 upstream of the fault line BC:

[0090] In order to obtain the current output by DG2, since the fault location is unknown, it is assumed that the fault occurs at the end of the BC line, that is, α = 1. This will cause the voltage at the DG2 grid connection point to decrease, the output current to increase, and the current flowing through the BC line of DG2 to increase accordingly. At this time, the voltage expression at both ends of the branch where DG2 is located is:

[0091]

[0092] In the formula, is the voltage across the branch where DG2 is located, is the current flowing through the BC line, Z BC is the line impedance of line BC, is the current flowing through line AB, is the short-circuit current output by the system during a fault, Z AB is the line impedance of line AB, Z s is the system internal resistance, is the system reference voltage.

[0093] Similarly, the actual output current of DG2 under this assumption can be obtained by iterative calculation based on the output current expression of the distributed power supply, the grid connection point voltage expression, and the voltage expression at both ends of the branch where DG2 is located.

[0094] Specifically, taking the AB line fault as an example, the DG upstream of the fault line is DG3, and the DGs downstream of the fault line are DG1 and DG2. Figure 5 The sequence network diagram when a phase-to-phase fault occurs on the AB line shown in the figure, combined with the voltage and current information measured at the protection installation, first calculate the current provided by the system flowing through the protection installation during the fault:

[0095]

[0096] In the formula, is the short-circuit current output by the system during a fault. is the system reference voltage, Z s is the system impedance.

[0097] The voltage expression of the T-connected line of DG3 is determined as:

[0098]

[0099] In the formula, is the voltage of the T-connected line of DG3, is the current flowing through line AB, is the voltage of AB line, Z AE is the line impedance of line AE.

[0100] Then, the actual output current of DG3 is calculated by iterative correction using the output current expression of the distributed power supply, the grid connection point voltage expression and the voltage expression of the T-connected line of DG3.

[0101] The expression for determining the T-connected line voltage of DG1 is:

[0102]

[0103] In the formula, is the voltage of the T-connected line of DG1.

[0104] Then, the output current of DG1 is calculated by iteratively correcting the output current expression of the distributed power source, the grid connection point voltage expression and the voltage expression of the T-connected line of DG1.

[0105] The expression for determining the T-connected line voltage of DG2 is:

[0106]

[0107] In the formula, is the voltage of the T-connected line of DG2, is the current flowing through the BC line.

[0108] Then, the actual output current of DG2 is calculated by iteratively correcting the output current expression of the distributed power source, the grid connection point voltage expression and the voltage expression of the T-connected line of DG2.

[0109] It should be noted that it is difficult to calculate the output current of renewable energy using only local information. According to the output current expression of distributed power sources, there is a nonlinear relationship between the grid-connected point voltage and the output current of the DG. Therefore, if the linear node voltage equations obtained by the fault composite sequence network are directly solved by linear transformation, the fault current and grid-connected point voltage of the DG cannot be accurately calculated. For this reason, this embodiment adopts an iteratively corrected fault characteristic analysis method. According to the output current expression of distributed power sources, the output current of the DG will vary with the degree of grid-connected point voltage drop during a fault. At the moment of the fault, considering the current tracking response delay inside the DG and the role of the filtering circuit, it can be considered that the output current of the DG at the time of the fault occurs. Equal to the output current before the fault This embodiment uses local information to respectively calculate the output current of the DG directly connected to the fault line, the output current of the DG upstream of the fault line, and the output current of the DG downstream of the fault line.

[0110] As a further preferred technical solution, the step S30: calculating the current flowing through the protection installation when a fault occurs at the end of the fault line according to the actual output current of all distributed power sources related to the fault line combined with the output current of the system main power supply, includes:

[0111] S31, calculating the current of all distributed power sources flowing through the fault line according to the actual output current of each distributed power source related to the fault line;

[0112] S32. Calculate the current flowing through the protection installation when a fault occurs at the end of the fault line based on the current flowing through the fault line of each distributed power source and the output current of the system main power source.

[0113] Specifically, take the BC line failure as an example. Figure 4 As shown, according to the positive sequence equivalent rule, the additional impedance Z in the composite sequence network Δ for:

[0114]

[0115] In the formula, Z Δ is the additional impedance, Z Σ - is the negative sequence impedance, α is the fault location, Z BC , Z CD are the impedances of BC and CD lines respectively, Z LD2 is the equivalent load impedance of S2, Z AB , Z AE , Z EF are the impedances of AB line, AE line, and EF line respectively, Z S , Z LD1 are the system internal resistance and the equivalent load impedance of S1 line respectively.

[0116] When the BC line fails, the actual output current of DG1 And the corresponding line impedance, system internal resistance, calculate the current flowing through the BC line in DG1 for:

[0117]

[0118] In the formula, || represents parallel connection.

[0119] According to the actual output current of DG2 And the corresponding line impedance, system internal resistance, calculate the current flowing through the BC line in DG2 for:

[0120]

[0121] According to the actual output current of DG3 And the corresponding line impedance, system internal resistance, calculate the current flowing through the BC line in DG3 for:

[0122]

[0123] When the BC line fails, the current of the main power supply of the system flowing through the BC line is calculated as:

[0124]

[0125] Then the current flowing through the BC line is calculated as:

[0126]

[0127] Assuming that the fault occurs at the end of the line, the calculated protection setting value becomes smaller, which expands the protection range of the adaptive current protection improvement scheme. Let α = 1, and the current when the fault occurs at the end of the line can be calculated as:

[0128]

[0129] Then the protection setting corresponding to the BC line is:

[0130]

[0131]

[0132] In the formula, The protection setting value of stage I of protection 2 is K according to the requirements of relay protection. I rel is the protection sensitivity coefficient of stage I, taking 1.2-1.3, K is the protection setting value of stage II of protection 2. II rel is the protection sensitivity coefficient of stage II, taking 1.1-1.2, K d is the fault type coefficient, which is 1 for three-phase short circuit and 1 for two-phase short circuit. β CD is the protection range of stage I protection, Z e is the backside line impedance, is the equivalent electromotive force on the back side of the system.

[0133] Specifically, take the AB line failure as an example. Figure 5 As shown, according to the positive sequence equivalent rule, the additional impedance under negative sequence conditions is calculated:

[0134] Z Δ =[(1-α)Z AB +Z BC +ZCD +Z LD2 ]||[αZ AB +Z s ||(Z AE +Z EF +Z LD1 )]

[0135] In the formula, Z Δ is the additional impedance, α is the fault location, Z AB is the impedance of the AB line, Z BC , Z CD are the impedances of BC and CD lines respectively, Z LD1 , Z LD2 is the line load impedance, Z s is the system internal resistance, Z AE , Z EF They are the impedances of AE line and EF line respectively.

[0136] Combined with the voltage and current information measured at the protection installation, first calculate the current provided by the system flowing through the protection installation during a fault:

[0137]

[0138] In the formula, I s is the short-circuit current output by the system during a fault, E is the system reference voltage, Z s is the system impedance. Then calculate the additional impedance under negative sequence conditions:

[0139] Z Δ =[(1-α)Z AB +Z BC +Z CD +Z LD2 ]||[αZ AB +Z s ||(Z AE +Z EF +Z LD1 )]

[0140] Then the voltage of the T-connected line of DG3 is:

[0141]

[0142] Then the output current of DG3 is calculated by iterative correction method.

[0143] Then the T-connected line voltage of DG1 is:

[0144]

[0145] Then the output current of DG1 is calculated by iterative correction method.

[0146] Then the T-connected line voltage of DG2 is:

[0147]

[0148] Then the output current of DG1 is calculated by iterative correction method.

[0149] The current flowing through the protection installation when a fault occurs at the end of the line and the corresponding protection stage I setting value can be calculated based on the current output by the distributed photovoltaic system at the time of the fault:

[0150]

[0151] Calculate the protection range β of the lower-level line section I protection. When the fault occurs at β of the AB lower-level line (i.e., BC line), there exists:

[0152]

[0153] In the formula, I I set.2 is the setting current of the I-stage protection at the BC line protection installation, is the fault current flowing through the protection installation when a fault occurs at β on the BC line. Let I I set.2 Multiplying by the II protection coefficient is the corresponding II stage protection setting:

[0154] I ΙΙ set.1 =K ΙΙ rel.1 I Ι set.2

[0155] In the formula, I I set.2 ,I II set.1 They are the protection setting value of section I of protection 2 and the protection setting value of section II of protection 1; Z AB is the AB line impedance, K I rel.1 is the reliability coefficient, with a value of 1.2-1.3, K II rel.1 is the reliability matching coefficient, and its value is 1.1-1.2.

[0156] Implementation case: Figure 2 As shown in the figure, the distribution network with multiple DGs is taken as an example. The system base voltage is 10.5 kV, the base capacity is 100 MVA, and the system impedance is Z s=j1Ω, the length of AB, BC, CD lines is 6km, the length of AE, EF lines is 4km, the unit impedance of the lines is 0.27+j0.34Ω / km, and the load power is S LD1 =S LD2 =6+j0.3MVA.

[0157] When DGs of different capacities are connected to the BC line, the data of fault current, traditional current protection, traditional adaptive current protection, and real-time setting current protection when phase-to-phase faults occur at different locations are as follows:

[0158] Table 1 Current data when a phase-to-phase fault occurs on the BC line

[0159]

[0160] The action status is determined by comparing the measured value with the set value. Through the above calculation method, the set value of the improved adaptive current protection is calculated in real time to achieve setting and protection for different fault scenarios.

[0161] This embodiment uses the information from the protection installation location and does not require additional communication equipment to achieve real-time setting of protection settings during faults, meeting the requirements for instantaneous protection action when a fault occurs within 70% of the line and for stage II protection action after 0.5s when a fault occurs at other locations on the line. This is of great significance for improving current protection performance and ensuring stable operation of distribution network lines.

[0162] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0163] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0164] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A method for calculating output current and protection setting value using local information, It is characterized in that The method comprises: Establishing an equivalent power supply model including the main power supply and the distributed power supply of the system, and determining the sequence network diagram of the fault line according to the position of the fault line in the equivalent power supply model; Based on the sequence network diagram of the fault line, the grid connection point voltage including the distributed line is calculated using the local measurement information at the protection installation after the fault, and the grid connection point voltage is iteratively corrected using the output current of the distributed power source to calculate the actual output current of all the distributed power sources related to the fault line; According to the actual output current of all distributed power sources related to the fault line and the output current of the system main power supply, calculate the current flowing through the protection installation when a fault occurs at the end of the fault line; The protection setting corresponding to the faulty line is calculated based on the current flowing through the protection installation.

2. The method for calculating output current and protection setting value using local information as claimed in claim 1, It is characterized in that The establishing of an equivalent power supply model including a system main power supply and a distributed power supply, and determining a sequence network diagram of a fault line according to a position of the fault line in the equivalent power supply model, comprises: A voltage source series resistor is used to represent the main power supply of the system, and a controlled current source is used to represent the distributed power supply, and the equivalent power supply model is established; According to the equivalent power source model, the location of the fault line, the fault type, the installation location and access capacity of the distributed power source, a sequence network diagram of the fault line is determined.

3. The method for calculating output current and protection setting value using local information as claimed in claim 1, It is characterized in that The distributed power sources related to the fault line include the distributed power sources directly connected to the fault line, the distributed power sources upstream of the fault line and the distributed power sources downstream of the fault line.

4. The method for calculating output current and protection setting value using local information as claimed in claim 1, It is characterized in that The output current expression of the distributed power supply is: Where: P 0 It is the active power output by the distributed generation before the fault; is the grid connection point voltage of the distributed power source; is the output current of the distributed power supply.

5. The method for calculating output current and protection setting value using local information as claimed in claim 1, It is characterized in that For the distributed power source directly connected to the fault line, based on the sequence network diagram of the fault line, the grid connection point voltage including the distributed line is calculated using the local measurement information at the protection installation after the fault, and the grid connection point voltage is iteratively corrected using the output current of the distributed power source, and the actual output current of the distributed power source directly connected to the fault line is calculated, including: Determine the grid connection point voltage expression of the T-connected distributed power source according to the sequence network diagram of the fault line; Calculating the corrected output current of the distributed power source according to the output current expression of the distributed power source; The corrected output current of the distributed power source is substituted into the grid connection point voltage expression of the T-connected distributed power source for iterative calculation until the iterative difference of the grid connection point voltage meets the accuracy of the iterative voltage difference, thereby obtaining the grid connection point voltage and the actual output current of the T-connected distributed power source.

6. The method for calculating output current and protection setting value using local information as claimed in claim 5, It is characterized in that For the distributed power source upstream of the fault line, based on the sequence network diagram of the fault line, the grid connection point voltage including the distributed line is calculated using the local measurement information at the protection installation after the fault, and the grid connection point voltage is iteratively corrected using the output current of the distributed power source to calculate the actual output current of the distributed power source upstream of the fault line, including: According to the actual output current of the T-connected distributed power source and the current measured at the fault line protection installation, Kirchhoff's current law is used to calculate the current flowing through the upstream of the fault line; According to the current flowing through the upstream of the fault line and the voltage measured at the fault line protection installation, Kirchhoff's voltage law is used to calculate the output current of the system main power supply; Determine the voltage expression of the branch where the distributed power source upstream of the fault line is located according to the current flowing through the upstream of the fault line and the output current of the system main power supply; An iterative calculation is performed based on the output current expression of the distributed power source, the grid connection point voltage expression of the T-connected distributed power source, and the voltage expression of the branch where the distributed power source upstream of the fault line is located to obtain the actual output current of the distributed power source upstream of the fault line.

7. The method for calculating output current and protection setting value using local information as claimed in claim 5, It is characterized in that For the distributed power source downstream of the fault line, based on the sequence network diagram of the fault line, the grid connection point voltage including the distributed line is calculated using the local measurement information at the protection installation after the fault, and the grid connection point voltage is iteratively corrected using the output current of the distributed power source to calculate the actual output current of the distributed power source downstream of the fault line, including: The fault location is assumed to occur at the end of the fault line, and the grid connection point voltage expression of the distributed generation downstream of the fault line is obtained according to the sequence network diagram; An iterative calculation is performed according to the output current expression of the distributed power source, the grid connection point voltage expression of the T-connected distributed power source and the grid connection point voltage expression of the distributed power source downstream of the fault line to obtain the actual output current of the distributed power source downstream of the fault line.

8. The method for calculating output current and protection setting value using local information as claimed in claim 1, It is characterized in that The method of calculating the current flowing through the protection installation when a fault occurs at the end of the fault line based on the actual output current of all distributed power sources related to the fault line combined with the output current of the system main power supply includes: According to the actual output current of each distributed power source related to the fault line, calculate the current of each distributed power source flowing through the fault line; Based on the current flowing through the fault line of each distributed power source and the output current of the system main power supply, the current flowing through the protection installation when a fault occurs at the end of the fault line is calculated.

9. The method for calculating output current and protection setting value using local information as claimed in claim 8, It is characterized in that The step of calculating the current of each distributed power source flowing through the fault line according to the actual output current of each distributed power source related to the fault line includes: According to the actual output current of the distributed power source and the corresponding line impedance and system internal resistance, the current flowing through the distributed power source through the fault current limiting is calculated.

10. The method for calculating output current and protection setting value using local information as claimed in claim 1, It is characterized in that The calculating, according to the current flowing through the protection installation, a protection setting corresponding to the fault line comprises: Calculate the protection setting value of protection stage I according to the current flowing through the protection installation location; Calculate the protection range of the lower-level line section I protection, and calculate the current flowing through the protection installation of this level of line when a fault occurs at the maximum protection range; The protection setting of protection stage II is calculated based on the current flowing through the line protection installation of this level when a fault occurs at the maximum protection range.

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