A method for selecting the neutral grounding impedance of a wind farm collector system

Abstract

This invention discloses a method for selecting the neutral point grounding impedance of a wind farm collector system, comprising: determining the first-stage operating current of the zero-sequence overcurrent protection of the collector line and the grounding transformer based on the time-current characteristic curve of the fuse with the largest rated current in the wind farm; determining the second-stage operating current of the zero-sequence overcurrent protection of the collector line based on the maximum capacitive current of the collector line; determining the single-phase ground fault current based on the sensitivity principle of relay protection, the 0.1s fusing current of the fuse, the first and second-stage operating currents of the zero-sequence overcurrent protection of the collector line, and the first-stage operating current of the zero-sequence overcurrent protection of the grounding transformer; and obtaining the optimal solution for the neutral point grounding resistance and the zero-sequence impedance of the grounding transformer based on the single-phase ground fault current and the phase relationship of the fault current distribution in the collector system. This invention, based on the coordination with the operating characteristics of the fuse, ensures that when a single-phase ground fault occurs on the fuse-transformer side, the fuse blows first, followed by the zero-sequence protection of the collector line.

Description

Technical Field

[0001] This invention belongs to the field of power system relay protection technology, specifically relating to a method for selecting the neutral point grounding impedance of a wind farm collector system. Background Technology

[0002] To reduce the duration of single-phase grounding faults in wind farm collection systems, limit transient overvoltages at faulty equipment, and reduce the stress voltage borne by the insulation system of fault-free equipment, NB / T31026-2012 "Code for Electrical Design of Wind Farm Engineering" requires that the neutral point of the 35kV and below low-voltage side of the main transformer in a wind farm should preferably be grounded using a low-resistance grounding method, and should not be grounded without grounding.

[0003] There is currently no definitive method for selecting the neutral point grounding resistance in a wind farm's power collection system. DL / T5222-2005, "Technical Specifications for Conductor and Electrical Equipment Selection and Design," only stipulates that when a low-resistance grounding method is used for the neutral point, the grounding resistance should be selected based on the chosen single-phase grounding current.

[0004]

[0005] Among them, I d For the selected single-phase grounding current, A; U N Let V be the system line voltage.

[0006] The literature, including "Research on Neutral Grounding Methods in Distribution Networks" (Su Jifeng) and "Analysis of Improved Schemes for Grounding Transformers and Neutral Grounding Resistance in Wind Farms" (Lin Feng), proposes a method for selecting the neutral grounding resistance, namely, taking U as the neutral grounding resistance. p (2~3)I C U p The rated phase voltage of the collector system is typically 37kV, I C This represents the total capacitive current of the collector system.

[0007] However, selecting the neutral point resistor using the above method generally makes it difficult to set the zero-sequence overcurrent protection of the collector lines, and it is hard to guarantee both reliability and sensitivity at the same time. For example, in a wind farm, two collector lines on the 35kV busbar frequently trip simultaneously due to a single-phase ground fault in one of the collector lines. This is because the neutral point grounding resistor is chosen to be too large, and reliability has to be sacrificed in order to meet sensitivity requirements.

[0008] Meanwhile, if the zero-sequence overcurrent protection action value of the protection device is set too small, it will lose its coordination with the high-voltage side fuse of the wind turbine generator box-type transformer, thus losing selectivity. To save construction costs, wind farms typically install fuses on the high-voltage side of the box-type transformer to protect it. According to "Configuration and Setting of Distribution Network Branch Line Protection" (Zhang Liang), under low-resistance grounding, the short-circuit current during a single-phase ground fault is relatively small compared to a phase-to-phase short circuit, approximately 300–1000A. The required fusing time of the fuse is relatively long; if it fails to fuse within the zero-sequence protection time limit, it will inevitably cause the zero-sequence protection to trip cascaded over time, expanding the power outage area. Summary of the Invention

[0009] The purpose of this invention is to address the shortcomings of existing methods for selecting the neutral grounding resistance in wind farm power collection systems by proposing a method for selecting the neutral grounding impedance in wind farm power collection systems that can improve the selectivity of relay protection in the power collection system.

[0010] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0011] A method for selecting the neutral point grounding impedance of a wind farm power collection system is proposed. This method is based on the zero-sequence overcurrent protection of the wind farm power collection line to inversely deduce the neutral point grounding resistance value and the zero-sequence impedance of the grounding transformer in the power collection system.

[0012] A further improvement of the present invention is that the method specifically includes the following steps:

[0013] 1) Count the models and rated currents of the high-voltage side fuses of the box-type transformers connected to the collector lines of the wind farm's power collection system, and find the fuse with the largest rated current.

[0014] 2) Based on the fuse model and rated current, find the time-current characteristic curve of the corresponding fuse model, and determine the fusing current I when the fuse fusing time is 0.1s and 0.3s, respectively. F0.1s and I F0.3s ;

[0015] 3) Based on the selective principle of relay protection, the zero-sequence overcurrent protection of the collector wire has a first-stage operating current I. op0I =I F0.3s The action time is 0.3 seconds;

[0016] 4) When a single-phase ground fault occurs on the collector line with the largest capacitance to ground in the current collector system, the capacitive current flowing out of this collector line is I. C The zero-sequence overcurrent protection of the collector wire has a two-stage operating current I. op0II ;

[0017] 5) Based on the selective principle of relay protection, the operating current I of the zero-sequence overcurrent protection stage of the grounding transformer is...g0I ;

[0018] 6) The sum of the ground capacitance currents of all collector lines in the collector system is I. C∑ ;

[0019] 7) Based on the sensitivity principle of relay protection, that is, ensuring that the sensitivity coefficient of the first stage of the zero-sequence overcurrent protection of the collector line is not less than 2, and the sensitivity coefficient of the first stage of the zero-sequence overcurrent protection of the grounding transformer is not less than 2, therefore, the fault current I during a single-phase ground fault at the end of the collector line is... d ;

[0020] 8) The neutral point grounding resistance of the collector system is R N The zero-sequence impedance of the grounding transformer is X. N The following equation relationship is satisfied. U N Given the rated voltage of the collector system, the neutral point grounding resistance and the zero-sequence impedance of the grounding transformer in the collector system are obtained by solving the simultaneous equations.

[0021] A further improvement of the present invention is that, in step 4), I op0II =1.5×I C .

[0022] A further improvement of the present invention is that, in step 4), the action time is 0.6s.

[0023] A further improvement of the present invention is that, in step 5), I g0I =1.15×I op0I .

[0024] A further improvement of the present invention is that, in step 5), the action time is 0.9s.

[0025] A further improvement of this invention is that, in step 5), the zero-sequence overcurrent protection principle of the grounding transformer is as follows:

[0026] The zero-sequence current flowing through the grounding transformer is I0, the zero-sequence voltage on the high-voltage side of the grounding transformer is U0, and the vector angle between I0 and U0 is θ. Then, when I0cosθ + I0sinθ is greater than the zero-sequence overcurrent protection operating current of the grounding transformer, I... 0dz At that time, the zero-sequence overcurrent protection of the grounding transformer will activate.

[0027] A further improvement of the present invention is that, in step 7), I d =max(1.3×I) F0.1s ,2×I op0I ,4×I op0II ,2×I g0I -I C∑ ).

[0028] The present invention has at least the following beneficial technical effects:

[0029] 1. This invention proposes a method for selecting the neutral point grounding impedance of a wind farm collector system. Based on the coordination with the operating characteristics of fuses, when a single-phase ground fault occurs on the fuse box-type transformer side, it ensures that the fuse blows first, followed by the zero-sequence protection of the collector line. According to GB / T15166.6-2008 "High Voltage AC Fuses Part 6: Selection Guidelines for Fuse Elements of High Voltage Fuses for Transformer Circuits", the pre-arc time-current characteristics of fuse elements used in high voltage AC fuses in transformer circuits should have a high operating current within a range of less than 0.1s to withstand transformer inrush current and provide good coordination with secondary side protection devices.

[0030] 2. The present invention proposes a method for selecting the neutral point grounding impedance of a wind farm power collection system. The zero-sequence overcurrent protection operating current of the grounding transformer is set in coordination with the first-stage operating current of the zero-sequence overcurrent protection of the power collection line, with a coordination coefficient of 1.15, which ensures selectivity. At the same time, the operating quantity of the zero-sequence overcurrent protection of the grounding transformer becomes I0cosθ+I0sinθ, which improves the sensitivity of the zero-sequence overcurrent protection of the grounding transformer.

[0031] Compared with existing technologies, the present invention has the following significant advantages:

[0032] 1. This invention proposes a method for selecting the neutral point grounding impedance in a wind farm power collection system. This method involves inversely calculating the neutral point grounding resistance value and the zero-sequence impedance of the grounding transformer based on the zero-sequence overcurrent protection of the power collection line. This differs from the current method that typically uses R... N =U p / (2~3)I C Compared to other selection methods, this approach is more conducive to improving the selectivity and sensitivity of relay protection for collector lines.

[0033] 2. This invention proposes a method for selecting the neutral point grounding impedance of a wind farm collector system. Based on the coordination with the operating characteristics of fuses, when a single-phase ground fault occurs on the fuse-type transformer side, it ensures that the fuse blows first, followed by the zero-sequence protection of the collector line, thus guaranteeing the selectivity of the relay protection. If the zero-sequence overcurrent protection operating value of the protection device is set too small, it will lose its coordination with the high-voltage side fuse of the wind turbine generator box-type transformer. To save construction costs, wind farms typically install fuses on the high-voltage side of the box-type transformer to protect it. According to "Configuration and Setting of Distribution Network Branch Line Protection" (Zhang Liang), under low-resistance grounding, the short-circuit current during a single-phase ground fault is relatively small compared to a phase-to-phase short circuit, approximately 300–1000A. The required melting time of the fuse is relatively long; if it fails to melt within the zero-sequence protection time limit, it will inevitably cause the zero-sequence protection to trip cascaded over, expanding the power outage area.

[0034] 3. The present invention proposes a method for selecting the neutral point grounding impedance of a wind farm collector system. According to the zero-sequence network impedance model of the wind farm collector system, the zero-sequence current flowing through the fault collector line protection device is equal to the vector sum of the neutral point current flowing through the grounding transformer and the single-phase metallic grounding capacitance current of the non-faulty line on the bus. At the same time, the zero-sequence impedance of the grounding transformer and the grounding resistance of the wind farm tower base will also reduce the zero-sequence current. When a single-phase ground fault occurs in the collector line, the zero-sequence current flowing through the collector line is less than the selected single-phase grounding current. The zero-sequence overcurrent protection action value of the protection device must be set very small to ensure sensitivity. Attached Figure Description

[0035] Figure 1 This is a flowchart illustrating the method of the present invention.

[0036] Figure 2 This is a time-current characteristic diagram of a fuse. Detailed Implementation

[0037] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the disclosure to those skilled in the art. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0038] like Figure 1 As shown, this invention provides a method for selecting the neutral grounding impedance of a wind farm power collection system. This method, based on the zero-sequence overcurrent protection of the wind farm's power collection lines, inversely calculates the neutral grounding resistance value of the power collection system and the zero-sequence impedance of the grounding transformer. Specifically, it includes the following steps:

[0039] 1) Count the models and rated currents of the high-voltage side fuses of the box-type transformers connected to the collector lines of the wind farm's power collection system, and find the fuse with the largest rated current.

[0040] 2) Based on the fuse model and rated current, find the time-current characteristic curve of the corresponding fuse model, and determine the fusing current I when the fuse fusing time is 0.1s and 0.3s, respectively. F0.1s and I F0.3s ;

[0041] 3) Based on the selective principle of relay protection, the zero-sequence overcurrent protection of the collector wire has a first-stage operating current I. op0I =I F0.3s The action time is 0.3 seconds;

[0042] 4) When a single-phase ground fault occurs on the collector line with the largest capacitance to ground in the current collector system, the capacitive current flowing out of this collector line is I. C The zero-sequence overcurrent protection of the collector wire has a two-stage operating current I. op0II =1.5×I C The action time is 0.6 seconds;

[0043] 5) Based on the selective principle of relay protection, the operating current I of the zero-sequence overcurrent protection stage of the grounding transformer is... g0I =1.15×I op0I The operating time is 0.9s; the zero-sequence overcurrent protection principle of the grounding transformer is as follows:

[0044] The zero-sequence current flowing through the grounding transformer is I0, the zero-sequence voltage on the high-voltage side of the grounding transformer is U0, and the vector angle between I0 and U0 is θ. Then, when I0cosθ + I0sinθ is greater than the zero-sequence overcurrent protection operating current of the grounding transformer, I... 0dz When the zero-sequence overcurrent protection of the grounding transformer is activated;

[0045] 6) The sum of the ground capacitance currents of all collector lines in the collector system is I. C∑ ;

[0046] 7) Based on the sensitivity principle of relay protection, that is, ensuring that the sensitivity coefficient of the first stage of the zero-sequence overcurrent protection of the collector line is not less than 2, and the sensitivity coefficient of the first stage of the zero-sequence overcurrent protection of the grounding transformer is not less than 2, therefore, the fault current I during a single-phase ground fault at the end of the collector line is... d =max(1.3×I) F0.1s ,2×I op0I ,4×I op0II ,2×I g0I -I C∑ );

[0047] 8) The neutral point grounding resistance of the collector system is R N The zero-sequence impedance of the grounding transformer is X. N The following equation relationship is satisfied. By solving the simultaneous equations, the resistance value of the neutral point grounding in the collector system and the zero-sequence impedance of the grounding transformer can be obtained.

[0048] Example:

[0049] A wind farm has a 2.2MW wind turbine generator connected to a 2400kVA box-type transformer in its power collection system. The high-voltage side of the transformer is equipped with an SKQDJ-40.5KV / 50A high-voltage fuse as its protection device. According to the fuse's instruction manual, it is a transformer protection fuse with a fuse element diameter of 76mm, a fuse element length of 537mm, a spring striker, a rated voltage of 40.5kV, and a rated current of 50A. In a recent incident, a ground fault occurred on the high-voltage side of the 2400kVA box-type transformer, causing the zero-sequence overcurrent protection stage I of the collector line connected to the transformer to trip, disconnecting the entire collector line and expanding the fault area.

[0050] After analyzing the accident, it was found that the neutral point grounding resistance of the current collector system was 71.3Ω, which was selected according to DL / T5222-2005 "Technical Specifications for Conductor and Electrical Appliance Selection Design". The selected single-phase grounding current I... d If it is 300A, then

[0051]

[0052] The actual ground fault current is 230A. This is because the zero-sequence impedance of the grounding transformer and the grounding resistance of the wind farm tower base are not considered in the above formula. As a result, when a single-phase ground fault occurs in the collector line, the zero-sequence current flowing through the collector line is less than the selected single-phase ground fault current. The zero-sequence overcurrent protection action value of the protection device must be set very small to ensure reliable operation.

[0053] If the zero-sequence overcurrent protection action value of the protection device is set too small, it will lose its coordination with the high-voltage side fuse of the wind turbine generator box-type transformer. To save construction costs, wind farms typically install fuses on the high-voltage side of the box-type transformer to protect it. The time-current characteristics of the fuse are as follows: Figure 2 As shown. Based on the time-current characteristics of the high-voltage fuse, it can be seen that the fusing current is 500A in 0.1s and 350A in 0.3s.

[0054] Therefore, when a ground fault occurs on the high-voltage side of the box-type transformer, the fuse will not blow, causing the collector line to be disconnected. Simultaneously, the fuse on the high-voltage side of the box-type transformer is damaged due to the short-circuit current, but it does not blow, creating a potential hazard for future fuse blowouts. Analyzing the above incidents, it can be seen that because the neutral point grounding resistance of the collector system was chosen to be too large, and the effects of the zero-sequence impedance of the grounding transformer and collector line, as well as the tower base grounding resistance, were not considered, the fault current after a ground fault occurs on the high-voltage side of the box-type transformer is too small to cause the high-voltage side fuse to blow quickly.

[0055] The method for selecting the neutral grounding resistance in the wind farm collector system of the present invention, such as... Figure 1 As shown.

[0056] Step S11: First, collect the model and rated current of the high-voltage side fuses of the box-type transformers connected to the collector lines of the wind farm's power collection system. The fuse with the highest rated current is 50A. Then, based on the fuse model and rated current, find the corresponding fuse's time-current characteristic curve to determine the fusing current I when the fusing time is 0.1s. F0.1s =500A, the fusing current when the fusing time is 0.3s is I F0.3s =350A.

[0057] To ensure the selectivity of relay protection, the operating current of the first stage of zero-sequence overcurrent protection for the collector wire is I. op0 =I F0.3s =350A, operating time is 0.3s. When a single-phase ground fault occurs on the collector line with the largest capacitance to ground in the current collector system, the capacitive current flowing out of this collector line is I. C The zero-sequence overcurrent protection of the collector cable has a two-stage operating current of 40A. op0II =60A, operating time is 0.6s. Zero-sequence overcurrent protection of the grounding transformer, first-stage operating current I. g0I =1.15×I F0.3s = 402.5A, operating time is 0.9s. The sum of the ground capacitance currents of all collector lines in the collector system is I. C∑ =120A.

[0058] Step S12, based on the sensitivity principle of relay protection, the sensitivity coefficient is not less than 2, and the fault current at the end of the collector line during a single-phase ground fault is...

[0059] I d =max(1.3×I) F0.1s ,2×I op0I ,4×I op0II ,2×I g0I -I C∑ )=max(650,700,240,685)=700A.

[0060] Step S13, then the neutral point grounding resistance R of the collector system N The zero-sequence impedance of the grounding transformer can be selected as X. N The following system of equations is satisfied:

[0061] Solving the system of equations yields R. N =29.65Ω, X N =15.25Ω.

[0062] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for selecting a neutral grounding impedance in a wind farm collection system, characterized in that, The method reverses the neutral point grounding resistor value and the zero sequence impedance of the grounding transformer in the power collection system according to the zero sequence overcurrent protection of the power collection line of the wind farm. The method specifically includes the following steps: 1) count the type and rated current of the high-voltage side fuse of the box-type transformer connected to the power collection line of the wind farm collection system, and find the fuse with the maximum rated current; 2) According to the model and rated current of the fuse, the time-current characteristic curve of the corresponding model fuse is found, and the fuse current when the fuse time is 0.1s and 0.3s is determined as and ; 3) According to the principle of selectivity of relay protection, the zero sequence overcurrent protection of the collector line has a segment action current , and the action time is 0.3 s; 4) When the external single-phase ground fault occurs in the maximum ground capacitance of the collector line in the collector system, the capacitive current flowing out of the collector line is , and the two-section action current of the zero sequence overcurrent protection of the collector line is , ; 5) According to the selectivity principle of relay protection, the zero sequence overcurrent protection of the grounding transformer , ; 6) The sum of the ground capacitance currents of all the power collection lines of the power collection system is ; 7) According to the sensitivity principle of the relay protection, that is, to ensure that the sensitivity coefficient of the first action current of the zero sequence overcurrent protection of the collector line is not less than 2, and the sensitivity coefficient of the first action current of the zero sequence overcurrent protection of the grounding transformer is not less than 2, the fault current of the single-phase grounding fault at the end of the collector line , ; 8) the neutral grounding resistance of the power collection system is , the zero sequence impedance of the grounding transformer is , and the following equation relationship is satisfied , , is the rated voltage of the power collection system, and the neutral grounding resistance value of the power collection system and the zero sequence impedance of the grounding transformer are obtained by simultaneous solution.

2. The method of claim 1, wherein the method is characterized by: In step 4), the action time is 0.6s.

3. The method of claim 1, wherein the method further comprises: In step 5), the action time is 0.9s.

4. The method of claim 1, wherein, In step 5), the zero sequence overcurrent protection principle of the grounding transformer is: The zero sequence current flowing through the grounding transformer is , the zero sequence voltage at the high voltage side of the grounding transformer is , and The vector angle between , when is greater than the zero sequence overcurrent protection action current of the grounding transformer, the zero sequence overcurrent protection of the grounding transformer acts.

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