Four-quadrant ac tie-line protection method based on hough fault coefficient

Abstract

With the wide access of four-quadrant power supply represented by battery energy storage station and flexible HVDC system into power grid, the complex fault characteristics of the four-quadrant power supply bring challenges to traditional relay protection, resulting in the failure of traditional current differential protection. In order to solve this problem, a protection method based on Hough fault coefficient is disclosed. The protection method uses the improved Hough transform to extract the slope of the two-dimensional current trajectory, and selects three specific slope angles to construct the Hough fault coefficient to quickly and sensitively identify the internal and external faults. The Hough fault coefficient shows excellent reliability when facing various internal fault conditions on the AC tie line of the four-quadrant power supply, and ensures the safety of the protection in the presence of non-ideal working conditions such as CT saturation, CT error, synchronization error, noise and abnormal value.

Description

Technical Field

[0001] This invention relates to a protection method for four-quadrant AC tie lines based on the Hough fault coefficient, belonging to the field of power system relay protection technology. Background Technology

[0002] In the process of accelerating the construction of new power systems, the large-scale application and development of renewable energy provides an important guarantee for achieving the goals of "carbon peaking" and "carbon neutrality" on schedule. However, the inherent volatility and randomness of renewable energy can reduce grid stability, deteriorate frequency quality, and widen the peak-valley difference. Battery energy storage stations (BESS) have functions such as frequency regulation and peak shaving and valley filling, which can effectively alleviate the problems caused by renewable energy grid connection. Therefore, the installed capacity of BESS is steadily increasing year by year globally. Flexible DC transmission systems (VSC-HVDC) have the ability to flexibly access and dispatch renewable energy sources such as wind and solar power, providing important support for promoting the efficient utilization of renewable energy. In addition, in the field of inter-regional grid interconnection, VSC-HVDC, with its efficient and stable transmission performance, is gradually becoming a key means to achieve large-scale, long-distance power transmission, demonstrating broad development potential and application prospects.

[0003] Four-quadrant power sources, represented by battery energy storage stations and flexible DC transmission systems, can achieve bidirectional control of active and reactive power. Compared with traditional synchronous power sources, their flexible operation inevitably alters the fault current characteristics of the AC lines connected to them. Therefore, the performance of traditional relay protection degrades or even fails to operate after four-quadrant power sources are connected to the grid, posing a threat to the safe and stable operation of the power grid. Summary of the Invention

[0004] To address the problem of performance degradation or even failure to operate in existing traditional relay protection systems after four-quadrant power supply grid connection, this invention provides a protection method for four-quadrant power supply AC tie lines based on the Hough fault coefficient.

[0005] The protection method for four-quadrant AC tie lines based on the Hough fault coefficient includes the following steps:

[0006] Step 1: Sample the current on both sides of the protected line to obtain the sampling current point set i. m and i n , for i m and i n Normalization operation is performed to obtain the point set i′ m and in ′;

[0007] Step 2: Calculate the slope indicator coefficient r′(k) using the improved Hough transform. θ＝0° r′(k) θ＝45° r′(k) θ＝90° ;

[0008] Step 3: Set r′(k) θ＝0° r′(k) θ＝45° r′(k) θ＝90° Delaying by a quarter of a power frequency cycle yields r′(kN / 4). θ＝0° r′(kN / 4) θ＝45° r′(kN / 4) θ＝90° ;

[0009] Step 4: Based on r′(k) θ＝0° r′(k) θ＝45° r′(k) θ＝90° and r′(kN / 4) θ＝0° r′(kN / 4) θ＝45° r′(kN / 4) θ＝90° The six Level I fault coefficients α1(k)~α6(k) were calculated.

[0010] Step 5: Arrange and combine the six Level I fault coefficients α1(k) to α6(k) to form four Level II fault coefficients β1(k) to β4(k). Take the maximum value of the four Level II fault coefficients β1(k) to β4(k) to obtain the Level III fault coefficient γ(k).

[0011] Step 6: Classify the Level III fault coefficient γ(k) using the step classification function ε(k). Take ε(k) for one power frequency cycle and calculate their average value to obtain the Hough fault coefficient HFI.

[0012] Step 7: Determine whether the Hough Fault Factor (HFI) is greater than or equal to the protection threshold (THR). If it is greater than or equal to the protection threshold (THR), it is determined to be an in-zone fault; if it is less than the protection threshold (THR), it is determined to be an out-of-zone fault. Taking into account the reliability and safety of the protection, THR is set to 0.5.

[0013] This invention discloses a protection method for AC tie lines of four-quadrant power supplies based on the Hough fault coefficient. This method solves the problem of performance degradation or even failure to operate of traditional relay protection after four-quadrant power supplies are connected to the grid, and can ensure the correct operation of the protection after the four-quadrant power supply is connected to the grid. Attached Figure Description

[0014] Figure 1 A schematic diagram of a four-quadrant power source connected to the power grid;

[0015] Figure 2 This is a flowchart of a four-quadrant AC tie-line protection method based on the Hough fault coefficient. Detailed Implementation

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

[0017] The protection method for four-quadrant AC tie lines based on the Hough fault coefficient includes the following steps:

[0018] Step 1: As Figure 1 and Figure 2 The current on both sides of the protected line is sampled to obtain the point set i of the sampled current. m and i n , for i m and i n Normalization operation is performed to obtain the point set i′ m and i n ′; where the subscripts m and n represent the four-quadrant power supply side and grid side of the protected line, respectively; the specific formula for the normalization operation process is as follows:

[0019]

[0020] Step Two: As Figure 2 The slope indicator coefficient r′(k) is calculated using the improved Hough transform. θ＝0° r′(k) θ＝45° r′(k) θ＝90 Where k represents any point in the sampling current point set, and the subscripts θ = 0°, θ = 45°, and θ = 90° represent three angles in Hough space, respectively; the specific formula for the improved Hough transform is as follows:

[0021] r′(k) θ =i′ m (k)cosθ+i n ′(k)sinθ (2)

[0022] Step 3: As Figure 2 , r′(k) θ＝0° r′(k) θ＝45° r′(k) θ＝90° Delaying by a quarter of a power frequency cycle yields r′(kN / 4). θ＝0° r′(kN / 4) θ＝45° r′(kN / 4) θ＝90° Where N represents the number of sampling points within one power frequency cycle;

[0023] Step Four: As Figure 2According to r′(k) θ＝0° r′(k) θ＝45° r′(k) θ＝90° and r′(kN / 4) θ＝0° r′(kN / 4) θ＝45° r′(kN / 4) θ＝90 The six Level I fault coefficients α1(k) to α6(k) are calculated; the specific formulas for calculating the six Level I fault coefficients α1(k) to α6(k) are as follows:

[0024]

[0025] Step 5: As Figure 2 The six Level I fault coefficients are arranged and combined to form four Level II fault coefficients. The maximum value of the four Level II fault coefficients is taken to obtain the Level III fault coefficient. The specific formulas for calculating the four Level II fault coefficients β1(k)~β4(k) and the Level III fault coefficient γ(k) are as follows:

[0026]

[0027] γ(k)=MAX{β1(k),β2(k),β3(k),β4(k)} (5)

[0028] Step Six: As Figure 2 The level III fault coefficient γ(k) is classified using the step classification function ε(k). The average value of ε(k) over one power frequency cycle is calculated to obtain the Hough fault coefficient HFI. The specific formulas for the step classification function ε(k) and the Hough fault coefficient HFI are as follows:

[0029]

[0030] Step Seven: As Figure 2 The system determines whether the Hough fault factor (HFI) is greater than or equal to the protection threshold (THR). If it is greater than or equal to the protection threshold (THR), it is determined to be an in-zone fault; if it is less than the protection threshold (THR), it is determined to be an out-of-zone fault. Taking into account the reliability and safety of the protection, the THR is set to 0.5.

[0031] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements, substitutions, modifications and refinements can be made without departing from the principles and spirit of the present invention, and these improvements, substitutions, modifications and refinements should also be considered within the scope of protection of the present invention.

[0032] The contents not described in detail in this specification are existing technologies known to those skilled in the art.

Claims

1. A protection method for four-quadrant AC tie lines based on Hough fault coefficients, comprising the following steps: Step 1: Sample the current on both sides of the protected line to obtain the sampling current point set i. m and i n , for i m and i n Normalization operation is performed to obtain the point set i′ m and i′ n The subscripts m and n represent the four-quadrant power supply side and grid side of the protected line, respectively. Step 2: Calculate the slope indicator coefficient r′(k) using the improved Hough transform. θ＝0° r′(k) θ＝45° r′(k) θ＝90° ; where k represents any point in the sampling current point set, and the subscripts θ = 0°, θ = 45°, and θ = 90° represent three angles in Hough space, respectively; Step 3: Set r′(k) θ＝0° r′(k) θ＝45° r′(k) θ＝90° Delaying by a quarter of a power frequency cycle yields r′(kN / 4). θ＝0° r′(kN / 4) θ＝45° r′(kN / 4) θ＝90° Where N represents the number of sampling points within one power frequency cycle; Step 4: Based on r′(k) θ＝0° r′(k) θ＝45° r′(k) θ＝90° and r′(kN / 4) θ＝0° r′(kN / 4) θ＝45° r′(kN / 4) θ＝90° The six Level I fault coefficients α1(k)~α6(k) were calculated. Step 5: Arrange and combine the six Level I fault coefficients α1(k) to α6(k) to form four Level II fault coefficients β1(k) to β4(k). Take the maximum value of the four Level II fault coefficients β1(k) to β4(k) to obtain the Level III fault coefficient γ(k). Step 6: Classify the Level III fault coefficient γ(k) using the step classification function ε(k). Take ε(k) for one power frequency cycle and calculate their average value to obtain the Hough fault coefficient HFI. Step 7: Determine whether the Hough Fault Factor (HFI) is greater than or equal to the protection threshold (THR). If it is greater than or equal to the protection threshold (THR), it is determined to be an in-zone fault; if it is less than the protection threshold (THR), it is determined to be an out-of-zone fault. Taking into account the reliability and safety of the protection, THR is set to 0.

5.

2. The four-quadrant AC tie-line protection method based on Hough fault coefficient according to claim 1, characterized in that: The specific formula for the normalization operation described in step one is as follows:

3. The four-quadrant AC tie-line protection method based on Hough fault coefficient according to claim 1, characterized in that: The specific formula for the improved Hough transform described in step two is as follows: r′(k) θ =i′ m (k)cosθ+i′ n (k)sinθ (2) 4. The four-quadrant AC tie-line protection method based on Hough fault coefficient according to claim 1, characterized in that: The specific formulas for calculating the six Level I fault coefficients α1(k) to α6(k) in step four are as follows:

5. The four-quadrant AC tie-line protection method based on Hough fault coefficient according to claim 1, characterized in that: The specific formulas for calculating the four Level II fault coefficients β1(k)~β4(k) and the Level III fault coefficient γ(k) in step five are as follows: γ(k)=MAX{β1(k),β2(k),β3(k),β4(k)} (5) 6. The four-quadrant AC tie-line protection method based on Hough fault coefficient according to claim 1, characterized in that: The specific formulas for the step classification function ε(k) and the Hough fault coefficient HFI mentioned in step six are as follows:

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