Positive sequence augmented network analysis-based distributed power supply-containing power distribution network current protection method

Abstract

The invention relates to the technical field of power system relay protection, in particular to a distributed power supply-containing power distribution network current protection method based on positive sequence augmented network analysis, which comprises the following steps of: constructing a distributed power supply-containing power distribution network simulation model; setting simulation fault points and fault types in the power distribution network simulation model, and respectively drawing positive sequence augmented networks under different fault types; and deriving a positive sequence current flowing through protection based on the positive sequence augmented network, and performing protection setting on the power distribution network according to the positive sequence current. According to the invention, the problem that the sensitivity and reliability of traditional relay protection are reduced under the access of the new energy distributed power supply can be solved.

Description

Technical Field

[0001] The present invention relates to the technical field of relay protection of electric power systems, and in particular to a current protection method for a distribution network containing distributed power sources based on positive sequence augmented network analysis. Background Art

[0002] At present, there have been relevant studies on the relay protection methods of distribution networks, but there are still problems such as decreased sensitivity and reliability and complicated setting process after the access of new energy distributed power sources.

[0003] At present, the relay protection methods for distribution networks under the access of new energy sources mainly focus on limiting the access capacity of distributed power sources, timely exiting distributed power sources in case of faults, and improving traditional protection methods. On the one hand, limiting the access capacity of distributed power sources and timely exiting distributed power sources in case of faults are not conducive to the economic and environmental benefits of new energy sources. On the other hand, frequent switching of power sources is not conducive to the safety and stability of the power grid. There are also problems such as complex setting process and high requirements for communication reliability and bandwidth in improving traditional protection methods. The distribution network is the main battlefield for the access of new energy sources and is of great significance to the realization of the "dual carbon" goals. Therefore, it is of great academic and engineering value to study the relay protection technology of distribution networks under the access of distributed power sources. Summary of the invention

[0004] The purpose of the present invention is to provide a current protection method for a distribution network containing distributed power sources based on positive sequence augmented network analysis, so as to solve the problem of decreased sensitivity and reliability of traditional relay protection under the access of new energy distributed power sources.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] A current protection method for a distribution network containing distributed power sources based on positive sequence augmented network analysis, comprising:

[0007] Build a distribution network simulation model including distributed power sources;

[0008] Setting simulated fault points and fault types in the distribution network simulation model, and drawing positive sequence augmented networks under different fault types respectively;

[0009] A positive sequence current flowing through the protection is derived based on the positive sequence augmented network, and protection setting is performed on the distribution network according to the positive sequence current.

[0010] Optionally, the distribution network simulation model adopts constant power control and low voltage ride-through control, wherein the constant power control is used to control the distributed power source to output according to a given active power reference value and reactive power reference value, and the low voltage ride-through control is used to control the distributed power source to continue to operate for a period of time without being disconnected from the grid when the terminal voltage drops to a target range, and inject preset reactive power into the grid connection point.

[0011] Optionally, the output of the constant power control is:

[0012]

[0013] Among them, P o , Q o are respectively the actual output active power and reactive power of distributed power sources; U pcc I is the voltage rotation vector amplitude at the inverter grid connection point; d is the d-axis component of the inverter output current.

[0014] Optionally, setting a simulated fault point and a fault type in the distribution network simulation model includes:

[0015] A two-phase short circuit fault and a three-phase short circuit fault are respectively set within a preset flow-through protection time period at α of line A of the distribution network simulation model, wherein α represents the ratio of the distance from the fault point to the busbar to line A.

[0016] Optionally, drawing positive sequence augmented networks under different fault types respectively includes:

[0017] Obtain additional impedance under different fault types;

[0018] The generating elements and distributed power sources are drawn, and the positive sequence augmented network under different fault types is drawn in combination with the additional impedance under different fault types, wherein the distributed power source is drawn as a current source controlled by the node voltage.

[0019] Optionally, the additional impedance is:

[0020]

[0021] Among them, Z Δ is the additional impedance, Z eq0 , Z eq2 They are the network equivalent zero-sequence impedance and negative-sequence impedance respectively.

[0022] Optionally, deriving the positive sequence current flowing through the protection based on the positive sequence augmentation network includes:

[0023] According to the current flow direction and additional impedance distribution in the positive-sequence augmented network under different fault types, a circuit analysis method is used to derive the positive-sequence current flowing through the protection, wherein the circuit analysis method includes Kirchhoff's current law, Kirchhoff's voltage law, and node voltage method.

[0024] Optionally, performing protection setting on the distribution network according to the positive sequence current includes:

[0025] A setting current is obtained based on the positive sequence current, and protection setting is performed on the distribution network using the setting current, wherein the setting current is obtained by multiplying a preset reliability coefficient by the positive sequence current.

[0026] The beneficial effects of the present invention are:

[0027] The present invention takes the DG output current as a factor in setting the protection setting value, so its setting value can change dynamically with the output of the DG, while the traditional setting method is a single fixed value protection, which does not consider the impact of DG on the protection. Therefore, when dealing with distribution networks containing DG, the method proposed by the present invention has better adaptability. In the case of two-phase short circuit and three-phase short circuit, since the protection setting value proposed by the present invention can be dynamically adjusted with the output of the DG, the protection effect is better than that of the traditional protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0029] Figure 1 Schematic diagram of PQ control according to an embodiment of the present invention;

[0030] Figure 2 A schematic diagram of low voltage ride through control according to an embodiment of the present invention;

[0031] Figure 3 Schematic diagram of the structure of a distribution network including DG according to an embodiment of the present invention;

[0032] Figure 4 It is a short-circuit current variation curve diagram of the protection 3 and 4 when the DG is not connected in the embodiment of the present invention;

[0033] Figure 5 It is a short-circuit current variation curve diagram of the protection 4 when the DG is connected in the embodiment of the present invention;

[0034] Figure 6 This is a short-circuit current variation curve diagram of the protection 3 when the DG is connected in the embodiment of the present invention;

[0035] Figure 7 It is a schematic diagram of a positive sequence augmentation network of a two-phase short circuit at a certain point of line CD according to an embodiment of the present invention;

[0036] Figure 8 Schematic diagram of a three-phase short-circuit positive-sequence augmentation network at a certain point in line CD according to an embodiment of the present invention. DETAILED DESCRIPTION

[0037] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only 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.

[0038] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0039] This embodiment provides a current protection method for a distribution network containing distributed power sources based on positive sequence augmented network analysis, including:

[0040] Build a distribution network simulation model including distributed power sources;

[0041] Setting simulated fault points and fault types in the distribution network simulation model, and drawing positive sequence augmented networks under different fault types respectively;

[0042] A positive sequence current flowing through the protection is derived based on the positive sequence augmented network, and protection setting is performed on the distribution network according to the positive sequence current.

[0043] Specifically, this embodiment takes the output current of the distributed generation (DG) as a factor for setting the protection setting value, so its setting value can change dynamically with the output of the DG, while the traditional setting method is a single setting protection, which does not consider the impact of the DG on the protection. Therefore, when dealing with a distribution network containing DG, the method proposed in this embodiment has better adaptability. In the case of two-phase short circuit and three-phase short circuit, since the protection setting value proposed in this embodiment can be dynamically adjusted with the output of the DG, the protection effect is better than that of the traditional protection.

[0044] Furthermore, building a distribution network simulation model including distributed power sources specifically involves building a distribution network simulation model including distributed power sources in the electromagnetic simulation software PSCAD, and setting the parameters of power sources, lines, loads, DGs and other components;

[0045] The distribution network simulation model adopts constant power control and low voltage ride-through control, wherein the constant power control is used to control the distributed power source to output according to a given active power reference value and reactive power reference value, and the low voltage ride-through control is used to control the distributed power source to continue to operate for a period of time without being disconnected from the grid when the terminal voltage drops to a target range, and to inject preset reactive power into the grid connection point.

[0046] Furthermore, setting the simulated fault point and fault type in the distribution network simulation model includes:

[0047] A two-phase short circuit fault and a three-phase short circuit fault are respectively set within a preset flow-through protection time period at α of line A of the distribution network simulation model, wherein α represents the ratio of the distance from the fault point to the busbar to line A.

[0048] Furthermore, the positive sequence augmented networks under different fault types are drawn separately, including:

[0049] Obtain additional impedance under different fault types;

[0050] The generating elements and distributed power sources are drawn, and the positive sequence augmented network under different fault types is drawn in combination with the additional impedance under different fault types, wherein the distributed power source is drawn as a current source controlled by the node voltage.

[0051] Further, deriving the positive sequence current flowing through the protection based on the positive sequence augmented network includes:

[0052] According to the current flow direction and additional impedance distribution in the positive-sequence augmented network under different fault types, a circuit analysis method is used to derive the positive-sequence current flowing through the protection, wherein the circuit analysis method includes Kirchhoff's current law, Kirchhoff's voltage law, and node voltage method.

[0053] Further, performing protection setting on the distribution network according to the positive sequence current includes:

[0054] A setting current is obtained based on the positive sequence current, and protection setting is performed on the distribution network using the setting current, wherein the setting current is obtained by multiplying a preset reliability coefficient by the positive sequence current.

[0055] Combine the following Figure 1-Figure 8 A current protection method for a distribution network containing distributed power sources based on positive sequence augmented network analysis proposed in this embodiment is described in detail, which specifically includes the following contents:

[0056] Step 1: Build a distribution network simulation model including DG in the electromagnetic simulation software PSCAD, and set the parameters of power supply, line, load, DG and other components;

[0057] Step 2: Set the simulation fault point and fault type, and draw the positive sequence augmented network under different fault types respectively;

[0058] Step 3: Based on the drawn positive-sequence augmented network, use circuit analysis methods such as Kirchhoff's current law, Kirchhoff's voltage law, and node voltage method to derive the positive-sequence current flowing through the protection, and set the protection according to the positive-sequence current.

[0059] Furthermore, in step 1, the control strategies adopted by the simulation model of the distribution network containing DG are constant power control and low voltage ride-through control, wherein the constant power control is used to control the distributed power source to output according to the given active power reference value and reactive power reference value, and the low voltage ride-through control is used to control the distributed power source to continue to operate for a period of time without being disconnected from the grid when the terminal voltage drops to within the target range, and inject preset reactive power into the grid connection point.

[0060] Specifically, distributed generators (DG), such as photovoltaic and wind power, are usually connected to the grid through converters (mainly rectifiers and inverters). In order to improve their output characteristics, it is often necessary to control the grid-connected converters to which they are connected. Currently, the commonly used control strategies are constant power control / PQ control and low voltage ride-through (LVRT) control, which are respectively Figure 1 and Figure 2 shown.

[0061] 1) PQ control:

[0062] PQ control is the most common control strategy for DG. It takes the active and reactive power output of DG as the control target and intends to make DG output according to the given active and reactive power reference values. PQ control is a dual-loop control structure. The outer layer is the power loop, which is used to control the output power and generate the inner loop reference current. The inner loop is the current loop, which is used to control the actual output current to track the reference current, so as to finally output the reference power. The output power expression of PQ control is as follows:

[0063] P o =P ref =U d I d +U q I q

[0064] Q o =Q ref =U q I d -U d I q (1)

[0065] Where: P o , Qo are the actual output active power and reactive power of DG respectively; P ref , Q ref are the reference values ​​of active power and reactive power given to DG respectively; U d , U q are the d-axis and q-axis components of the inverter grid-connected point voltage respectively; I d ,I q They are the d-axis and q-axis components of the inverter output current respectively.

[0066] Orient the d-axis to the grid-connected voltage rotation vector,

[0067]

[0068] Where: U pcc is the voltage rotation vector amplitude at the inverter grid connection point.

[0069] Substituting formula (2) into formula (1), we get:

[0070]

[0071] When the power grid is operating normally, U pcc Basically, it remains constant, and DG can be equivalent to a current source model controlled by reference power. And during normal operation of the power grid, DG is generally required to only output active power.

[0072] 2) Low voltage ride-through control:

[0073] The national standard GB / T33593-2017 "Technical Requirements for Distributed Generation Grid Connection" clearly stipulates that distributed generation should have a certain low voltage ride-through capability, that is, when the terminal voltage of DG drops to a certain range, it should be able to continue to operate for a period of time without being disconnected from the grid, and even inject a certain amount of reactive power into the grid connection point to help the system restore voltage.

[0074] Furthermore, in step 2, setting the simulated fault point and fault type in the distribution network simulation model includes: setting a two-phase short circuit fault and a three-phase short circuit fault respectively within the preset flow protection time period at α of line A in the distribution network simulation model, wherein α represents the ratio of the distance of the fault point to the busbar to line A.

[0075] As an optional embodiment, the distribution network structure including DG is as follows: Figure 3 As shown in the figure, bus D is set to be three-phase short-circuited during 1 to 1.1 seconds, and the short-circuit currents flowing through protections 3 and 4 before and after DG is put into operation are compared. The results are as follows: Figure 4 , 5 , as shown in Figure 6.

[0076] After DG is connected to the distribution network, the distribution network changes from the original single-ended power supply network to a complex network with multiple power supplies at the same time. The distribution and size of short-circuit current change significantly, causing a huge impact on traditional protection. Figure 4 , 5 , 6 can be concluded that: after DG is connected, due to the auxiliary effect, the short-circuit current flowing through the downstream protection of DG becomes larger, thereby expanding the protection range; similarly, due to the external drain effect, the short-circuit current flowing through the upstream protection of DG becomes smaller, thereby reducing the protection range.

[0077] Furthermore, in step 2, drawing the positive-sequence augmented network under different fault types respectively includes: obtaining the additional impedance under different fault types; drawing the generating elements and the distributed power sources, and combining the additional impedance under different fault types to complete the drawing of the positive-sequence augmented network under different fault types, wherein the distributed power source is drawn as a current source controlled by the node voltage.

[0078] Specifically include:

[0079] 1) Calculate the additional impedance Z of the positive sequence augmentation network Δ

[0080] Additional impedance Z Δ As shown in formula (4):

[0081]

[0082] Where: Z eq0 , Z eq2 are the network equivalent zero-sequence and negative-sequence impedances respectively.

[0083] 2) Drawing of generators and lines in the positive sequence network:

[0084] The positive sequence network of these two components can be drawn according to the given parameters.

[0085] 3) Drawing of DG in positive sequence network:

[0086] DG is affected by the control strategy and should be drawn as a current source controlled by the node voltage in the positive sequence network.

[0087] Furthermore, in step 3, deriving the positive-sequence current flowing through the protection based on the positive-sequence augmented network includes: deriving the positive-sequence current flowing through the protection using a circuit analysis method according to the current flow direction and the additional impedance distribution in the positive-sequence augmented network under different fault types, wherein the circuit analysis method includes Kirchhoff's current law, Kirchhoff's voltage law, and a node voltage method.

[0088] Performing protection setting on the distribution network according to the positive sequence current includes: obtaining a setting current based on the positive sequence current, and performing protection setting on the distribution network by the setting current, wherein the setting current is obtained by multiplying a preset reliability coefficient by the positive sequence current.

[0089] The following provides some calculation examples to illustrate the content in step 3:

[0090] At a certain point α on line CD, two-phase and three-phase short-circuit faults are set at 1 to 1.1 s. α represents the ratio of the distance between the fault point and busbar C to the line CD. The positive sequence augmented network is as follows: Figure 7 , 8 shown.

[0091] according to Figure 7 and Figure 8 , the positive sequence current of protection 4 in the case of two-phase short circuit and three-phase short circuit can be derived as formula (5) and formula (6) respectively:

[0092]

[0093]

[0094] Based on this, it can be concluded that when two-phase and three-phase short circuit occurs, the setting formula of protection 4 is as follows:

[0095]

[0096] In formulas (5), (6), (7), and (8), represents the equivalent potential of the system, Indicates DG output current, Z s , Z AC , Z CD They represent the system equivalent impedance, line AC impedance, and line CD impedance respectively, and K rel Represents the reliability coefficient, usually taken as 1.1.

[0097] Comparative analysis:

[0098] use Figure 1 The simulation data analysis of the network is performed, and the distribution network parameters are shown in Table 1. Two-phase and three-phase short circuits are set at bus D for 1 to 1.1 seconds, and the action performance comparisons of the protection method proposed in this embodiment and the traditional protection method under two-phase and three-phase short circuit conditions are shown in Tables 2 and 3 respectively.

[0099] Table 1

[0100]

[0101] Table 2

[0102]

[0103]

[0104] Table 3

[0105]

[0106] Based on the above simulation comparison and analysis, the advantage of the DG-containing distribution network protection method based on positive sequence network analysis proposed in this embodiment is that the DG output current is taken into consideration for setting the protection constant, so its setting value can change dynamically with the output of the DG, while the traditional setting method is a single constant protection, which does not consider the impact of DG on the protection. Therefore, when dealing with a distribution network containing DG, the method proposed in this embodiment has better adaptability. For a two-phase short circuit, the traditional protection method can no longer operate when a fault occurs at 80% of the line, while the method proposed in the embodiment can still operate. For a three-phase short circuit, although the traditional protection can continue to operate, it is not difficult to find that when a fault occurs at 80% of the line, the traditional protection is close to the critical value of the action, which is also a disadvantage of the fixed value protection. The protection constant proposed in this embodiment can be dynamically adjusted with the output of the DG, so it is better than the traditional protection.

[0107] The embodiments described above are only descriptions of the preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A current protection method for a distribution network containing distributed power sources based on positive sequence augmented network analysis, characterized in that: include: Build a distribution network simulation model including distributed power sources; Setting simulated fault points and fault types in the distribution network simulation model, and drawing positive sequence augmented networks under different fault types respectively; A positive sequence current flowing through the protection is derived based on the positive sequence augmented network, and protection setting is performed on the distribution network according to the positive sequence current.

2. The current protection method for a distribution network containing distributed power sources based on positive sequence augmented network analysis according to claim 1 is characterized in that: The distribution network simulation model adopts constant power control and low voltage ride-through control, wherein the constant power control is used to control the distributed power source to output according to a given active power reference value and reactive power reference value, and the low voltage ride-through control is used to control the distributed power source to continue to operate for a period of time without being disconnected from the grid when the terminal voltage drops to a target range, and to inject preset reactive power into the grid connection point.

3. The current protection method for a distribution network containing distributed power sources based on positive sequence augmented network analysis according to claim 2 is characterized in that: The output of the constant power control is: in, P o , Q o They are respectively the actual output active power and reactive power of distributed power sources; U pcc is the voltage rotation vector amplitude at the inverter grid connection point; I d is the d-axis component of the inverter output current.

4. The current protection method for a distribution network containing distributed power sources based on positive sequence augmented network analysis according to claim 1 is characterized in that: Setting the simulated fault point and fault type in the distribution network simulation model includes: A two-phase short circuit fault and a three-phase short circuit fault are respectively set within a preset flow-through protection time period at α of line A of the distribution network simulation model, wherein α represents the ratio of the distance from the fault point to the busbar to line A.

5. The current protection method for a distribution network containing distributed power sources based on positive sequence augmented network analysis according to claim 4 is characterized in that: The positive sequence augmented networks under different fault types are drawn separately, including: Obtain additional impedance under different fault types; The generating elements and distributed power sources are drawn, and the positive sequence augmented network under different fault types is drawn in combination with the additional impedance under different fault types, wherein the distributed power source is drawn as a current source controlled by the node voltage.

6. The current protection method for a distribution network containing distributed power sources based on positive sequence augmented network analysis according to claim 5 is characterized in that: The additional impedance is: in, ZΔ is the additional impedance, Z eq 0 , Z eq 2 They are the network equivalent zero-sequence impedance and negative-sequence impedance respectively.

7. The current protection method for a distribution network containing distributed power sources based on positive sequence augmented network analysis according to claim 1 is characterized in that: The positive sequence current flowing through the protection is derived based on the positive sequence augmented network, including: According to the current flow direction and additional impedance distribution in the positive-sequence augmented network under different fault types, a circuit analysis method is used to derive the positive-sequence current flowing through the protection, wherein the circuit analysis method includes Kirchhoff's current law, Kirchhoff's voltage law, and node voltage method.

8. The current protection method for a distribution network containing distributed power sources based on positive sequence augmented network analysis according to claim 7 is characterized in that: The protection setting of the distribution network according to the positive sequence current includes: A setting current is obtained based on the positive sequence current, and protection setting is performed on the distribution network using the setting current, wherein the setting current is obtained by multiplying a preset reliability coefficient by the positive sequence current.

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