Four-quadrant power supply AC tie line protection method based on relative distance deviation coefficient

Abstract

With the rapid increase of the installation number and capacity of a four-quadrant power supply mainly comprising a battery energy storage station, a flexible DC power transmission converter station and a modular multi-level matrix converter station in a novel power system, the design of a line protection method capable of adapting to the unique fault characteristics of the four-quadrant power supply is crucial. The invention discloses a differential protection (RDDC-DP) method based on a relative distance deviation coefficient, the RDDC-DP method can accurately identify internal faults, and the method has inherent robustness in the aspect of handling abnormal values and current transformer (CT) measurement errors, and can be used for accurately identifying the internal faults, and the relative distance deviation coefficient based RDDC-DP method has the advantages that the relative distance deviation coefficient based RDDC-DP method can be used for accurately identifying the internal faults, the relative distance deviation coefficient based RDDC-DP method can be used for accurately identifying the internal faults, and the relative distance deviation coefficient based RDDC-DP method can be used for accurately identifying the internal faults, and the relative distance deviation coefficient based RDDC-DP method can be used for accurately identifying the internal faults, and the relative distance deviation coefficient based RDDC-DP method can be used for accurately identifying the internal faults. However, in consideration of the problem of high protection maloperation risk caused by external fault CT saturation, it is determined to introduce Lorentz transform to improve the RDDC-DP, and the adaptive light velocity alpha is ingeniously designed to ensure that the method can still maintain high reliability under non-ideal conditions such as synchronization error and CT saturation.

Description

Technical Field

[0001] The present invention belongs to the technical field of relay protection of power systems, and particularly relates to a four-quadrant power AC tie line protection method based on a relative distance deviation coefficient. Background Art

[0002] Promoting the development of renewable energy, especially wind power and photovoltaic power generation, is crucial for achieving the carbon neutrality goal. In the context of carbon neutrality, the driving force for economic growth is gradually transforming towards clean energy, and the dominant position of thermal power generation will be replaced by renewable energy. As is well known, the Battery Energy Storage System (BESS) plays a crucial role in solving the inherent defects of renewable energy and the challenges it brings to the stability and resilience of the power grid. As a flexible resource with bidirectional charging and discharging capabilities, BESS can achieve the spatio-temporal conversion of energy, not only effectively reducing the power prediction error of renewable energy but also providing peak load transfer support for the power system, thus becoming an important part of modern power systems. Compared with traditional AC transmission systems, Voltage Source Converter-based High Voltage Direct Current (VSC-HVDC) technology has shown significant advantages in scenarios of large-capacity and long-distance power transmission with higher transmission capacity, lower power loss, and better power quality, and has become the mainstream technical solution for long-distance offshore wind power grid connection. At the same time, as a typical topology of high-voltage high-power AC-AC conversion, the Modular Multilevel Matrix Converter (M3C) has been successfully applied to the Flexible Low-Frequency AC Transmission (FLFAC) system, and has broad application prospects in scenarios such as medium-distance offshore wind power grid connection, sending out new energy from deserts, gobi, and wastelands, and asynchronous interconnection of power grids. It will play an important role in energy transformation and building a new power system.

[0003] The rapid development and wide application of power sources with four-quadrant operation capabilities such as BESS, VSC-HVDC, and M3C converter stations have brought new challenges to relay protection. Especially in the AC tie lines connecting four-quadrant power sources, traditional protection methods are difficult to adapt to their fault characteristics, resulting in a significant reduction in reliability and seriously threatening the safety and stability of the power system. Therefore, it is necessary to study a new differential protection technology that can adapt to the fault characteristics of four-quadrant power sources to provide important technical support for ensuring the safe and stable operation of the power grid and supporting the efficient utilization of renewable energy. Summary of the Invention

[0004] Aiming at the problem that the performance of existing traditional relay protection decreases or even refuses to operate after the four - quadrant power source is connected to the grid, the present invention provides a four - quadrant power source AC tie - line protection method based on the relative distance deviation coefficient.

[0005] Step 1: Sample the currents on both sides of the protected line to obtain the point sets \(i_{M}\) M and \(i_{N}\) N . Among them, the subscripts M and N in \(i_{M}\) M and \(i_{N}\) N represent the four - quadrant power source side and the grid side respectively;

[0006] Step 2: Perform operations on the current signals collected in Step 1 to obtain \(i_{M}^{+}\) Δ-abs and \(i_{N}^{+}\), and sort \(i_{M}^{+}\) Δ-abs and \(i_{N}^{+}\) abs-Δ in descending order within one sampling period, and obtain \(i_{M1}^{+}\) and \(i_{N1}^{+}\) and

[0007] Step 3: Calculate the parameter β;

[0008] Step 4: Calculate the adaptive parameter α according to \(i_{M1}^{+}\) and \(i_{N1}^{+}\) and β obtained in Steps 2 and 3;

[0009] Step 5: Perform Lorentz transformation on any one - side current signal in \(i_{M}\) M and \(i_{N}\) N , and draw the \(i'_{M}\) M or \(i'_{N}\) N - \(i_{M}\) M or \(i_{N}\) N - \(i'_{N}\) M N current trajectory;

[0010] Step 6: Calculate the relative distance deviation coefficient RDDC according to the current trajectory obtained in Step 5;

[0011] Step 7: Judge whether the relative distance deviation coefficient RDDC is greater than the protection threshold. If the relative distance deviation coefficient RDDC is greater than or equal to the protection threshold, the protection element trips; otherwise, it does not operate. Considering the reliability, safety and sensitivity of the protection comprehensively, the protection threshold is set to 0.2.

[0012] The present invention discloses a four - quadrant power source AC tie - line protection method based on the relative distance deviation coefficient. This method solves the problem that the performance of traditional relay protection methods decreases or even refuses to operate after the four - quadrant power source is connected, and can ensure the correct operation of the protection after the four - quadrant power source is connected to the grid. Brief Description of the Drawings

[0013]

[0013] Figure 1 Schematic diagram of a four - quadrant power supply connected to the power grid.

[0014] Figure 2 Flowchart of the protection method for the AC tie line of a four - quadrant power supply based on the relative distance deviation coefficient.

[0015] Figure 3 Schematic diagram of the calculation principle of RDDC. Specific implementation mode

[0016] The present invention will be further described below with reference to the accompanying drawings.

[0017] The protection method for the AC tie line of a four - quadrant power supply based on the relative distance deviation coefficient includes the following steps:

[0018] Step 1: As shown in Figure 1 and Figure 2 , sample the currents on both sides of the protected line to obtain the point sets i M and i N , where the subscripts M and N in i M and i N represent the four - quadrant power supply side and the grid side respectively. The point sets i M and i N are shown in Equation (1).

[0019]

[0020] where n represents the number of sampling points in a power frequency cycle, and k represents any integer in the point set.

[0021] Step 2: Perform operations on the current signals collected in Step 1 to obtain i Δ-abs and i abs-Δ , as shown in Equation (2).

[0022]

[0023] To eliminate the influence of abnormal data during external faults, it is necessary to sort i Δ-abs and i abs-Δ , discard the 10 maximum values in i Δ-abs and i abs-Δ , and obtain the point sets and i Δ-abs and i abs-Δ shown in Equation (3). and shown in Equation (3).

[0024]

[0025] Step 3: β is shown in Equation (4).

[0026]

[0027] In Equation (4), max represents taking the maximum value of the point set, and min represents taking the minimum value of the point set.

[0028] Step 4: α is shown in Equation (5).

[0029]

[0030] It should be noted that when calculating α and β, it is necessary to mark the sequence numbers of the first n / 4 largest values of the point set and store the sequence numbers in INDEX = [index(1), …, index(k), …, index(n / 4)]. If k is equal to any element in INDEX, then let |i N (k)| = 0, |i M (k)| = 0.

[0031] Step 5: As Figure 2 , introduce the Lorentz transformation to correct the collected current signal. Only either side of M or N can be selected for improvement here, and it is necessary to ensure that the other side remains unchanged. Taking the improvement of the N side as an example, the specific formula of the Lorentz transformation is as follows:

[0032] i′ N = γ(i N - Δ N-M ) = γ(i N - i N + i M ) = γi M (6)

[0033] In Equation (6), Δ N-M represents the numerical difference between i N and i M , and γ represents the Lorentz factor, which is shown in Equation (7).

[0034]

[0035] In Equation (7), α represents the adaptive parameter, which will change according to the different characteristics of the currents on both sides of the line.

[0036] Step 6: As Figure 2 , combine the i M and i′ N calculated in Step 2. Taking i M as the abscissa and i′ N as the ordinate, map them to the Cartesian coordinate system and plot i M - i′N Current trajectory. As Figure 2 and Figure 3 , according to the obtained i M -i′ N current trajectory, calculate the relative distance deviation coefficient RDDC. The calculation method of the relative distance deviation coefficient RDDC is shown in Equation (8).

[0037]

[0038] In Equation (8), PM represents the projection distance of the current trajectory onto the line y = x, and OP represents the distance of the current trajectory to the origin of coordinates. Combining Figure 3 it can be known that regardless of how the fault conditions and system short-circuit capacity change, RDDC is also less than or equal to 1. Let the coordinates of point P and point M be (x, y) and (x 0 , y 0 ), respectively. According to the Pythagorean theorem, the following relationship can be obtained:

[0039]

[0040] Substitute the relationship of x = i M (k) and y = i′ N (k) into (9), and the following can be obtained for x 0 as follows:

[0041]

[0042] To sum up, RDDC can be shown by Equation (11):

[0043]

[0044] Step 7: As Figure 2 , determine whether the relative distance deviation coefficient RDDC is greater than the protection setting value. If the relative distance deviation coefficient RDDC is greater than or equal to the protection threshold, the protection component trips; otherwise, it does not operate. Considering the reliability, safety, and sensitivity of the protection comprehensively, the protection threshold is set to 0.2.

[0045] The above is only the preferred embodiment of the present invention. It should be noted that: for those of ordinary skill in the art in this technical field, without departing from the principle and purpose of the present invention, several improvements, substitutions, variations, and retouches can still be made, and these improvements, substitutions, variations, and retouches should also be regarded as the protection scope of the present invention.

[0046] The content not described in detail in this specification belongs to the prior art well-known to those of ordinary skill in the art.

Claims

1. A four-quadrant power supply AC tie line protection method based on a relative distance deviation coefficient comprises the following steps: Step 1: Sample the current on both sides of the protected line to obtain the point set i of the sampled current M and i N , where i M with i N The subscripts M and N in the represent the four-quadrant power supply side and the grid side respectively; Step 2: Calculate the current signal collected in step 1 to obtain i Δ-abs with i abs-Δ , and for i within a sampling period Δ-abs with i abs-Δ Sort in descending order and get and Step 3: Calculate parameter β; Step 4: Based on the results obtained in steps 2 and 3 and β to calculate the adaptive parameter α; Step 5: Replace the speed of light c in the Lorentz factor with α, and M with i N The current signal on either side of the Lorentz transformation is carried out, and the i′ obtained after the transformation is M or i′ N Plot i′ M -i N or M -i′ N Current trace; Step 6: Calculate the relative distance deviation coefficient RDDC according to the current trajectory obtained in step 5; Step 7: Determine whether the relative distance deviation coefficient RDDC is greater than the protection threshold. If the relative distance deviation coefficient RDDC is greater than or equal to the protection threshold, the protection element will trip, otherwise it will not operate. Considering the reliability, safety and sensitivity of the protection, the protection threshold is set to 0.

2.

2. The four-quadrant power supply AC tie line protection method of relative distance deviation coefficient RDDC according to claim 1, characterized in that: In step 2, the current signal on either side of M or N is selected for improvement, ensuring that the other side remains unchanged. Here, the improvement of the current signal on the N side is taken as an example. The specific formula of the Lorentz transformation is as follows: i′ N =γ(i N -Δ N-M )=γ(i N -i N +i M )=γi M (1) In formula (1), Δ N-M Indicates i N with i M The numerical difference of , γ represents the Lorentz factor, as shown in formula (2): The design of α in step 4 is shown in formula (3): In the formula, n is the number of sampling points in one power frequency cycle, k represents any integer in the point set, max represents the maximum value of the point set, and min represents the minimum value of the point set. The expression of β is shown in formula (4): i Δ-abs with i abs-Δ Then it is shown in formula (5): Then i Δ-abs with i abs-Δ Arrange in descending order to get and As shown in formula (6): It is worth noting that when calculating α and β, the point set Mark the serial numbers of the first n / 4 largest values ​​and store them in INDEX = [index(1), ..., index(k), ..., index(n / 4)]. If k is equal to any element in INDEX, let |i N (k)|=0,|i M (k)|=0.

3. The four-quadrant power supply AC tie line protection method based on relative distance deviation coefficient according to claim 1, characterized in that: The RDDC calculation method in step 6 is shown in formula (7): In the formula, PM represents the projection distance of the current trajectory onto the straight line y=x, and OP represents the distance from the current trajectory to the origin of the coordinate system. According to formula (7), no matter how the fault conditions and system short-circuit capacity change, RDDC is less than or equal to 1. Let the coordinates of point P and point M be (x, y) and (x0, y0) respectively. According to the Pythagorean theorem, the following relationship can be obtained: Set x=i M (k) and y = i′ N Substituting the relation of (k) into (8), we can get x0 as follows: In summary, RDDC can be expressed as formula (10):

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