Island protection method for power distribution network containing distributed power supply based on transient characteristics

Abstract

The invention relates to the field of power system protection, and relates to a transient characteristic-based distributed power supply-containing power distribution network islanding protection method, which comprises the following steps of: acquiring distributed power supply grid-connected point data, and determining a voltage setting value of islanding protection and an impedance setting value of islanding protection; determining an island protection setting value; obtaining harmonic impedance of the grid-connected point, and determining a harmonic voltage effective value according to the harmonic impedance; determining a voltage continuity parameter according to the harmonic voltage effective value and the voltage setting value, and determining an impedance continuity parameter according to the harmonic impedance and the impedance setting value of the grid-connected point; determining an island fusion action value; judging whether an island protection condition is met or not based on the island fusion action value and the island protection setting value, if yes, judging that the distributed power supply has an island, and otherwise, collecting data again; according to the method, the island protection criterion based on voltage and impedance is formed by using the weighting coefficient, the operation mode of the power distribution network is comprehensively judged from two aspects, and the probability of operation refusal and misoperation of island protection is effectively reduced.

Description

Technical Field

[0001] The present invention relates to the field of power system protection, and in particular to an island protection method for a distribution network containing distributed power sources based on transient characteristics. Background Art

[0002] With the widespread application of distributed power sources, islanding has become one of the important issues in power grid operation. Islanding protection aims to detect and isolate power grid islands to ensure the safe operation of the system. The current mainstream islanding protection technologies can be divided into passive protection, active protection, and communication-based protection. However, anti-islanding protection technology still faces many challenges in practical applications.

[0003] First, the balance between protection sensitivity and false alarm rate is a major problem in island protection. Passive protection methods are based on changes in electrical quantities such as voltage and frequency. The protection is simple, but the sensitivity is insufficient and it is easy to miss detection. Active protection improves sensitivity by applying disturbance signals, but it will increase the false alarm rate and affect the stability of the power grid. Secondly, the contradiction between protection time and system stability also poses a challenge to island protection. Island protection requires fast response, usually required to be completed within a few hundred milliseconds, but communication-based protection methods may extend the response time due to communication delays. Although active protection responds quickly, frequent disturbances may threaten the stability of large-scale photovoltaic access systems. The widespread use of distributed power sources such as photovoltaics has caused the nonlinear characteristics of power sources to affect island protection. Photovoltaic power generation relies on power electronic converters to connect to the grid, and the output current is weakly fed and controlled, which makes it difficult for traditional detection methods based on electrical quantities such as voltage and frequency to accurately identify the island state. In addition, the external characteristics of the photovoltaic system in the island state are similar to those in normal operation, which further increases the difficulty of detection.

[0004] In summary, existing island protection research still has problems in terms of sensitivity, response time, nonlinear characteristics, etc., and there is no effective island protection method for distribution networks containing distributed power sources. Therefore, it has become an urgent problem for technicians in this field to develop an island protection method with high sensitivity, rapid and reliable action, and good economy for distribution networks containing distributed power sources. Summary of the invention

[0005] In order to solve the above problems in the prior art, the present invention proposes a method for island protection of a distribution network containing distributed power sources based on transient characteristics, the method comprising:

[0006] Collect data on distributed power grid connection points;

[0007] According to the distributed power grid connection point data, determine whether the island protection is activated. If not, re-collect the distributed power grid connection point data; if activated, determine the voltage setting value of the island protection and the impedance setting value of the island protection;

[0008] Determine the island protection setting value based on the voltage setting value and impedance setting value;

[0009] Obtain the harmonic impedance of the grid connection point and determine the effective value of the harmonic voltage based on the harmonic impedance;

[0010] The voltage continuity parameter is determined according to the harmonic voltage effective value and the voltage setting value, and the impedance continuity parameter is determined according to the harmonic impedance of the grid connection point and the impedance setting value;

[0011] Determine the island fusion action value according to the voltage continuity parameter and the impedance continuity parameter;

[0012] Based on the island fusion action value and the island protection setting value, it is judged whether the island protection conditions are met. If so, it is determined that the distributed power source has an island. Otherwise, the distributed power source grid connection point data is collected again.

[0013] To achieve the above objectives, the present invention also provides a computer-readable storage medium having a computer program stored thereon, and when the computer program is executed by a processor, it implements any of the above-mentioned methods for island protection of a distribution network containing distributed power sources based on transient characteristics.

[0014] To achieve the above-mentioned objectives, the present invention also provides an island protection device for a distribution network containing distributed power sources based on transient characteristics, comprising a processor and a memory; the memory is used to store a computer program; the processor is connected to the memory, and is used to execute the computer program stored in the memory, so that the island protection device for a distribution network containing distributed power sources based on transient characteristics executes any of the above-mentioned island protection methods for a distribution network containing distributed power sources based on transient characteristics.

[0015] Beneficial effects of the present invention:

[0016] 1. The existing island protection technology only uses a single electrical quantity as the protection criterion when making island judgments, which leads to different sensitivities in distribution networks with different structures, and missed detection may occur. The present invention uses weighted coefficients to form an island protection criterion based on voltage and impedance, and makes a comprehensive judgment on the operation mode of the distribution network from two aspects, effectively reducing the probability of island protection refusal or malfunction.

[0017] 2. When the prior art uses transient quantities to make islanding judgments, it does not fully consider the transient changes in electrical quantities caused by grid disturbances, which can easily lead to false operation of islanding protection. The present invention uses the comparison results of voltage, impedance and their set values ​​to calculate the continuity coefficient, which effectively avoids the interference of short-term changes in electrical quantities on islanding protection and improves the operation reliability of islanding protection in distribution networks containing distributed power sources.

[0018] 3. The existing communication-based island protection technology requires the installation of additional communication devices and lines, and is not very applicable in medium and low voltage distribution networks. At the same time, communication delays may cause the island protection to be unable to meet the speed requirements. The data collection, processing and judgment processes of the present invention can all be carried out in the existing island protection device, without the need to communicate with other devices, and have the advantages of good economy and fast action speed. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to make the purpose, technical solution and advantages of the invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings, in which:

[0020] Figure 1 It is a flow chart of the island protection method of the distribution network containing distributed power sources based on transient characteristics of the present invention;

[0021] Figure 2 It is a structural diagram of distributed power grid connection;

[0022] Figure 3 This is a rendering of an embodiment of the present invention. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. The components of the embodiments of the present invention generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but only represents selected embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work belong to the scope of protection of the present invention.

[0024] 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.

[0025] The present invention proposes a method for island protection of a distribution network containing distributed power sources based on transient characteristics. By proposing a method for calculating the island protection setting value and the island fusion action value applicable to distributed power sources, a distributed power source island criterion based on transient characteristics is formed, thereby improving the reliability of island protection in a distribution network containing distributed power sources. The specific process of the method of the present invention is as follows: Figure 1 As shown, the following steps are included:

[0026] S101: Collect the frequency, voltage and current of the distributed power grid connection point;

[0027] S102: judging whether the island protection is activated according to the voltage of the distributed power supply grid connection point, and if activated, executing step S103, otherwise executing step S101;

[0028] S103: Calculating a voltage setting value of island protection and an impedance setting value of island protection;

[0029] S104: Calculate the island protection setting value according to the voltage setting value and the impedance setting value;

[0030] S105: Calculate the voltage continuity parameter according to the harmonic voltage effective value and the voltage setting value, and determine the impedance continuity parameter according to the grid connection point harmonic impedance and the impedance setting value;

[0031] S106: Calculating an island fusion action value according to a voltage continuity parameter and an impedance continuity parameter;

[0032] S107: Determine whether the action criterion is met according to the island fusion action value and the island protection setting value. If so, it is determined that the distributed power source has an island and the island protection is actuated. Otherwise, it is determined that the distributed power source has not an island and the process returns to S101.

[0033] In this embodiment, judging whether the island protection is activated includes: setting a voltage threshold value, collecting the nth fundamental voltage and the n+1th fundamental voltage of the grid connection point; calculating the difference between the absolute value of the n+1th fundamental voltage and the absolute value of the nth fundamental voltage, and comparing the difference with the voltage threshold value. If the difference is greater than the voltage threshold value, the island protection is activated, otherwise the island protection is not activated. Its expression is "

[0034]

[0035] in, is the nth sampling value of the fundamental voltage of the distributed generation grid connection point, is the n+1th sampling value of the fundamental voltage of the distributed power grid connection point, || represents the amplitude, The voltage threshold value for the start criterion.

[0036] In this embodiment, determining the voltage setting value of the island protection includes: obtaining the reference short-circuit capacity corresponding to the voltage level of the distribution network and the minimum short-circuit capacity at the distributed power grid connection point; calculating the short-circuit capacity as S K1The hth order background harmonic current allowable value at the time of the load is calculated, and the load equivalent harmonic impedance is calculated; the voltage setting value of the island protection is calculated according to the benchmark short-circuit capacity, the minimum short-circuit capacity, the background harmonic current allowable value and the load equivalent harmonic impedance. The expression is:

[0037]

[0038] Among them, U h.set is the hth harmonic voltage setting value, S K1 is the benchmark short-circuit capacity corresponding to the voltage level of the distribution network, S K2 is the minimum short-circuit capacity at the distributed generation grid connection point, δI hP The short-circuit capacity is S K1 hth order background harmonic current allowable value, ω0 is the angular frequency corresponding to the power frequency, L S is the equivalent inductance on the grid side, and h is the harmonic order.

[0039] The settings for harmonic orders are:

[0040]

[0041] Among them, L is the equivalent inductance of the load, and C is the equivalent capacitance of the load.

[0042] Z h is the load equivalent harmonic impedance under the hth harmonic, which is calculated as follows:

[0043]

[0044] Among them, R is the equivalent resistance of the load, / / is the impedance parallel relationship.

[0045] In this embodiment, the impedance setting value of the island protection includes: determining the impedance reliability coefficient and the impedance setting value of the harmonic, and calculating the impedance setting value of the island protection based on the impedance reliability coefficient and the impedance setting value of the harmonic. The calculation formula is:

[0046] Z h.set =K z.rel (Z h / / jhω0L S )

[0047] Among them, Z h.set is the impedance setting value of the hth harmonic, K z.rel is the impedance reliability factor.

[0048] Determining the island protection setting value includes: determining the impedance weighting coefficient and the voltage weighting coefficient, wherein the impedance weighting coefficient and the voltage weighting coefficient are both less than 1, and the sum of the two is equal to 1; performing weighted fusion of the voltage setting value and the impedance setting value according to the impedance weighting coefficient and the voltage weighting coefficient to obtain the island protection setting value. Its expression is:

[0049] F GD =ω Z +ω U

[0050] Among them, F GD is the island protection setting value, ω Z and ω U are the impedance weighting coefficient and the voltage weighting coefficient, respectively, both less than 1, and ω Z +ω U =1.

[0051] In this embodiment, the weights are set as follows: When Z Take 0.3, ω U Take 0.7; When Z and ω U U is 0.5. DGN is the rated voltage of the distributed power supply.

[0052] In this embodiment, determining the voltage continuity parameter includes:

[0053]

[0054] Among them, F U is the voltage continuity parameter, t is the start time of island protection, Δt is the protection judgment duration, which can be taken as 20~50ms, f(U h.PCC ) is a function related to the effective value of the hth harmonic voltage at the distributed generation grid connection point, which can be specifically expressed as:

[0055]

[0056] Among them, U h.PCC It is the effective value of the hth harmonic voltage at the grid connection point of the distributed generation.

[0057] In specific implementation, in step S105, the impedance continuity parameter is calculated according to the following method:

[0058]

[0059] Among them, F Z is the impedance continuity parameter, f(Z h ) is a function related to the load equivalent harmonic impedance under the hth harmonic, which is specifically expressed as:

[0060]

[0061] Determining the island fusion action value includes: determining the impedance continuity parameter weight and the voltage continuity parameter weight, and performing weighted fusion of the impedance continuity parameter and the voltage continuity parameter according to the impedance continuity parameter weight and the voltage continuity parameter weight to obtain the island fusion action value. The calculation formula is:

[0062] F DZ =ω U F U +ω Z F Z

[0063] Among them, F DZ is the island fusion action value.

[0064] The island protection condition is that the island fusion action value is greater than or equal to the island protection setting value. That is:

[0065] F DZ ≥F GD

[0066] The present invention provides an island protection method for a distribution network containing distributed power sources based on transient characteristics. On the basis of proposing an island protection setting value calculation method and an island fusion action value calculation method suitable for distributed power sources, a distributed power source island judgment method based on transient characteristics is proposed, which effectively reduces the problems of low sensitivity, slow action speed, insufficient reliability, etc. in existing island protection.

[0067] To verify the effectiveness of the method of the present invention, Figure 2 The distributed power grid-connected structure diagram shown in the figure is used as an example for analysis. The main grid frequency is 50Hz; the equivalent resistance of the load is 1Ω, and the equivalent inductance is 1×10 -3 H, the equivalent capacitance is 1.034×10 -2 F; the equivalent inductance on the grid side is 2×10 -5 H; The voltage level of the grid-connected point is 380V, the corresponding benchmark short-circuit capacity is 10MVA, and the minimum short-circuit capacity at the distributed power grid-connected point is 3MVA.

[0068] In order to verify the effect of the island protection method for a distribution network containing distributed power sources based on transient characteristics disclosed in the present invention, this embodiment takes the island operation of the distributed power source and the voltage transient disturbance at 0.1s as the scenarios, and calculates and analyzes the setting value, fusion action value and island determination result of the island protection.

[0069] According to the formula in the aforementioned step S102, this embodiment determines whether the island protection process is started at 0.1s. Since the voltage level of the grid-connected point is 380V, 10% of the voltage level, i.e. 38V, is taken as the voltage threshold value of the start-up criterion. Starting from 0.1s, a round of sampling is performed every 0.01s, and the first sampling point of the grid-connected point fundamental voltage amplitude is 372V, the second sampling point is 373V, and the third sampling point is 421V, and the island protection process is started.

[0070] According to the formula in the aforementioned step S103, this embodiment takes h=2nd harmonic as the research object, the corresponding background harmonic current allowable value is 78A, and the equivalent harmonic impedance of the load is calculated to be 0.1998∠-78.48°, and the voltage setting value is 3.527V. Taking the impedance reliability coefficient as 1.1, the impedance setting value is calculated to be 0.0147∠89.23°.

[0071] In this embodiment, according to the formula in the aforementioned step S104, the impedance weighting coefficient and the voltage weighting coefficient are both taken as 0.5, and the island protection setting value is obtained as 1.

[0072] This embodiment calculates the values ​​of the voltage continuity parameter and the impedance continuity parameter when the distributed power source experiences island operation and voltage transient disturbance according to the formula in the aforementioned step S105. The island protection start time is 0.1s, and the determination duration is 40ms to ensure the accuracy of the determination.

[0073] Figure 3 (a) and (b) are the curves of the change of voltage continuity parameter and impedance continuity parameter when the distributed power source is in island operation. In the early stage of island operation, the transient process of the power system causes the continuity parameter to show a small step change; after the island operation enters the steady state, the effective value of the harmonic voltage and the equivalent harmonic impedance of the load at the grid connection point continue to meet the condition that the correlation function takes the value of 1, making the voltage continuity parameter and the impedance continuity parameter grow steadily and linearly, and reach 1 at the end of the judgment.

[0074] Figure 3 (c) and (d) are the curves of the voltage continuity parameter and impedance continuity parameter when the distributed power source experiences voltage transient disturbance. In the early stage of voltage transient disturbance, the transient process of the power system causes the continuity parameter to show a small step change; after the voltage transient disturbance ends, the distributed power source operates normally, and the harmonic voltage effective value and load equivalent harmonic impedance of the grid connection point cannot meet the condition that the correlation function takes a value of 1, so that the voltage continuity parameter and impedance continuity parameter remain unchanged at a value less than 1 after the disturbance.

[0075] According to the formula in the aforementioned step S106, this embodiment calculates that when the distributed power source is operating in an island state, the island fusion action value is 1; when the distributed power source is subjected to a voltage transient disturbance, the island fusion action value is 0.12.

[0076] According to the formula in the aforementioned step S107, this embodiment determines that the distributed power source is in an island operation state when the distributed power source is in island operation, so that the island protection device operates correctly; when the distributed power source is subjected to a voltage transient disturbance, it is determined that the distributed power source is in a grid-connected operation state, so that the island protection device does not operate correctly.

[0077] To sum up, the island protection method for a distribution network containing distributed power sources based on transient characteristics provided by the present invention forms a distributed power source island protection method based on transient characteristics by proposing an island protection setting value calculation method and an island fusion action value calculation method suitable for distributed power sources, thereby effectively improving the reliability of island protection in a distribution network containing distributed power sources.

[0078] In one embodiment of the present invention, the present invention also includes a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements any of the above-mentioned methods for island protection of a distribution network containing distributed power sources based on transient characteristics.

[0079] Those skilled in the art can understand that all or part of the steps of implementing the above-mentioned method embodiments can be completed by hardware related to the computer program. The aforementioned computer program can be stored in a computer-readable storage medium. When the program is executed, the steps of the above-mentioned method embodiments are executed; and the aforementioned storage medium includes: ROM, RAM, magnetic disk or optical disk and other media that can store program codes.

[0080] A device for island protection of a distribution network containing distributed power sources based on transient characteristics, comprising a processor and a memory; the memory is used to store a computer program; the processor is connected to the memory and is used to execute the computer program stored in the memory, so that the device for island protection of a distribution network containing distributed power sources based on transient characteristics executes any of the above-mentioned methods for island protection of a distribution network containing distributed power sources based on transient characteristics.

[0081] Specifically, the memory includes: ROM, RAM, magnetic disk, USB flash drive, memory card or optical disk, etc., which can store program codes.

[0082] Preferably, the processor can be a general-purpose processor, including a central processing unit (CPU), a network processor (NP), etc.; it can also be a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components.

[0083] The above embodiments further illustrate the purpose, technical solutions and advantages of the present invention in detail. It should be understood that the above embodiments are only preferred implementation modes of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made to the present invention within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for island protection of a distribution network containing distributed power sources based on transient characteristics, characterized in that: include: Collect data on distributed power grid connection points; According to the distributed power grid connection point data, determine whether the island protection is activated. If not, re-collect the distributed power grid connection point data; If started, the voltage setting value of island protection and the impedance setting value of island protection are determined; Determine the island protection setting value based on the voltage setting value and impedance setting value; Obtain the harmonic impedance of the grid connection point and determine the effective value of the harmonic voltage based on the harmonic impedance; The voltage continuity parameter is determined according to the harmonic voltage effective value and the voltage setting value, and the impedance continuity parameter is determined according to the harmonic impedance of the grid connection point and the impedance setting value; Determine the island fusion action value according to the voltage continuity parameter and the impedance continuity parameter; Based on the island fusion action value and the island protection setting value, it is judged whether the island protection conditions are met. If so, it is determined that the distributed power source has an island. Otherwise, the distributed power source grid connection point data is collected again.

2. The method for island protection of a distribution network containing distributed power sources based on transient characteristics according to claim 1, characterized in that: Determining whether the island protection is activated includes: setting a voltage threshold value, collecting the nth fundamental voltage and the n+1th fundamental voltage of the grid connection point; calculating the difference between the absolute value of the n+1th fundamental voltage and the absolute value of the nth fundamental voltage, and comparing the difference with the voltage threshold value. If it is greater than the voltage threshold value, the island protection is activated, otherwise the island protection is not activated.

3. The method for island protection of a distribution network containing distributed power sources based on transient characteristics according to claim 1 is characterized in that: Determine the voltage setting value of island protection including: Obtain the benchmark short-circuit capacity corresponding to the voltage level of the distribution network and the minimum short-circuit capacity at the distributed generation grid connection point; Calculate the short-circuit capacity as S K1 The hth order background harmonic current allowable value at the time of load saturation is calculated, and the load equivalent harmonic impedance is calculated; The voltage setting value of island protection is calculated based on the benchmark short-circuit capacity, minimum short-circuit capacity, allowable background harmonic current value and load equivalent harmonic impedance.

4. The method for island protection of a distribution network containing distributed power sources based on transient characteristics according to claim 1, characterized in that: The impedance setting value of the island protection includes: determining the impedance setting value of the impedance reliability coefficient and the harmonic impedance setting value, and calculating the impedance setting value of the island protection based on the impedance reliability coefficient and the harmonic impedance setting value.

5. The method for island protection of a distribution network containing distributed power sources based on transient characteristics according to claim 1, characterized in that: Determining the island protection setting value includes: determining an impedance weighting coefficient and a voltage weighting coefficient, wherein the impedance weighting coefficient and the voltage weighting coefficient are both less than 1, and the sum of the two is equal to 1; weightedly fusing the voltage setting value and the impedance setting value according to the impedance weighting coefficient and the voltage weighting coefficient to obtain the island protection setting value.

6. The method for island protection of a distribution network containing distributed power sources based on transient characteristics according to claim 1, characterized in that: Determining the effective value of the harmonic voltage includes: obtaining the effective value of the h-th harmonic voltage at the current distributed power grid-connected point, comparing the effective value of the h-th harmonic voltage at the current distributed power grid-connected point with the voltage setting value, if it is less than the voltage setting value, the harmonic voltage is invalid, if it is greater than the voltage setting value, the harmonic voltage is valid.

7. The method for island protection of a distribution network containing distributed power sources based on transient characteristics according to claim 1, characterized in that: Determining the island fusion action value includes: determining the impedance continuity parameter weight and the voltage continuity parameter weight, and performing weighted fusion on the impedance continuity parameter and the voltage continuity parameter according to the impedance continuity parameter weight and the voltage continuity parameter weight to obtain the island fusion action value.

8. The method for island protection of a distribution network containing distributed power sources based on transient characteristics according to claim 1, characterized in that: The island protection condition is that the island fusion action value is greater than or equal to the island protection setting value.

9. A computer-readable storage medium having a computer program stored thereon, characterized in that: The computer program is executed by a processor to implement any one of claims 1 to 8 of the method for island protection of a distribution network containing distributed power sources based on transient characteristics.

10. An island protection device for a distribution network containing distributed power sources based on transient characteristics, characterized in that: It comprises a processor and a memory; the memory is used to store a computer program; the processor is connected to the memory and is used to execute the computer program stored in the memory, so that the island protection device for a distribution network containing distributed power sources based on transient characteristics executes the island protection method for a distribution network containing distributed power sources based on transient characteristics according to any one of claims 1 to 6.

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