Method for ac line pilot protection at receiving end based on pseudo-power instantaneous value

Abstract

The application discloses a kind of based on quasi-power instantaneous value's receiving end ac line pilot protection method, according to the mutation of three-phase current sampling value, start protection;Near inverter station side and far inverter station side protection unit respectively intercept 5ms before and after voltage, current sampling value;Extract the 50Hz fundamental component of voltage and current before and after 5ms fault intercepted using narrow band-pass filter, and ensure that phase and amplitude are not changed;Near inverter side and far inverter side two end protection unit calculate quasi-power difference power difference differential, according to the action amount and braking amount of the protection method obtained by quasi-power difference differential calculation;According to the action amount and braking amount of each phase obtained, judge fault interval and fault phase.The application is not affected by the nonlinear time-varying characteristics of converter and commutation failure, has high reliability, fast action speed, and has important significance for improving the safety and stability level of ac-dc hybrid system.

Description

Technical Field

[0001] This invention relates to the field of relay protection technology for AC / DC hybrid power grids in power systems, specifically to a longitudinal protection method for receiving-end AC lines based on simulated instantaneous power values. Background Technology

[0002] The nonlinear response characteristics and commutation failures of DC systems cause fault characteristics in AC systems with DC feeds to differ from those of pure AC systems powered by synchronous machines. In pure AC systems, the phase of the current fault component leads the phase of the voltage fault component by approximately 90°. Compared to pure AC systems, inverter stations lack fixed equivalent impedances and are nonlinear components. Influenced by inverter operating conditions and commutation failures, the phase of the current fault component output by the inverter station exhibits significant oscillations, resulting in an unstable phase relationship with the voltage fault component. When applied to receiving-end AC grids, the performance of current differential longitudinal protection degrades, and directional longitudinal protection may fail. Therefore, to ensure the safe operation of AC / DC hybrid grids, a longitudinal protection method adapted to receiving-end AC lines is needed to reliably isolate line faults.

[0003] In summary, traditional longitudinal protection methods are no longer suitable for receiving-end AC lines. There is an urgent need to develop a longitudinal protection method for receiving-end AC lines that can accurately identify fault sections, is cost-effective, easy to implement, and has strong applicability. Summary of the Invention

[0004] To address the problem that traditional longitudinal protection is no longer applicable or has poor reliability in receiving-end AC lines, the purpose of this invention is to provide a receiving-end AC line longitudinal protection method based on pseudo-power instantaneous values.

[0005] To achieve the above objectives, the present invention employs the following technical solution: A longitudinal protection method for receiving-end AC lines based on pseudo-power instantaneous values ​​includes the following steps: (1) Obtain the instantaneous value change of the phase current, compare the instantaneous value change of the phase current with the set value, and once three consecutive phase current changes exceed the set value, it is determined that a short circuit fault has occurred, and the protection is started. Mark the point where the first change exceeds the set value as the fault start time. (2) After the protection is started, the protection units at both ends of the near inverter station side and the far inverter station side extract a 5 ms data window before the fault start time and extract a 5 ms data window after the fault to obtain the voltage and current sampling values ​​before and after the fault. (3) Narrow bandpass digital filtering is used to filter the voltage and current before and after the fault to obtain the fundamental frequency component; (4) Exchange the filtered voltage and current data before and after the fault for 5 ms at both ends, and use the data to calculate the pseudo-power difference.s qpφ Pseudopower difference differential QPDD φ and the operating quantity based on the pseudo-power instantaneous value longitudinal protection method S a_φ and braking amount S res_φ .

[0006] (5) Based on the obtained phase action and braking quantities, determine the fault range and, based on the judgment results, identify the faulty phase; Preferably, in step (1), the protection activation criterion is: (1) in, i φ ( k () represents the sampled value of the phase current of any one of phases A, B, and C at the current moment. φ To distinguish them, namely A, B, and C. i φ ( k - N T The value represents the sampled value corresponding to the cycle preceding the fault. N T This represents the number of sampling points per cycle. I set The threshold is 0.2 times the rated current.

[0007] Preferably, in step (2), the protection units at both ends of the near inverter station side and the far inverter station side capture a data window of 5 ms before and after the fault start time to obtain the voltage and current sampling values ​​5 ms before and after the fault. The period of the pseudo power difference differential waveform is 10 ms and the data window is 5 ms, which can reflect the characteristics of the pseudo power difference differential waveform. On the one hand, in order to reduce the damage to the converter equipment and avoid the expansion of the fault, the response speed of the protection scheme should be as fast as possible. On the other hand, in order to eliminate the transient delay of the filter.

[0008] Preferably, in step (3), a narrowband filter is used to extract the 50 Hz fundamental frequency component. The narrowband filter is an infinite impulse response filter, and the difference equation of the filter is: (2) In the formula A =2 a / T s / (4 / T s 2 / b +2 c / T s +d ); B =- A ; C =(2 c -8 b / T s 2 ) / ( 4 / T s 2 / b +2 c / T s + d ); D =(4 b / T s 2 -2 c / T s + d ) / ( 4 / T s 2 / b +2 c / T s + d ); a =8 T s / 78; b = R 2 T s 2 + T s 2 +2 RT s cos(2 πf p T s ); c =4 T s -4 R 2 T s ; d =4 R 2 -8 R cos(2 πf p T s)+4; T s The sampling interval is the sampling rate in practical engineering. f s It is generally 4 kHz, therefore T s =0.00025; f p The extracted frequency is, in this invention, a 50 Hz fundamental frequency component; R The pole location parameter is related to the signal frequency's deviation from the passband center. R =0.987.

[0009] Preferably, the method for calculating the pseudo-power difference in step (4) is as follows: (3) Starting from the week before the failure N Each voltage and current sampling data point is labeled from 1 to 1. N After troubleshooting N Each voltage and current sampling data point is labeled from 1 to 1. N . N This represents the number of sampling points within the data window. i m_φ [ i ]、 i n_φ [ i ]、 and , i =1~ N These are the current sampling data within 5 ms before and after the fault at the near-inverter side and the far-inverter side, respectively. u m_φ [ i ], u n_φ [ i ], and , i =1~ N These are voltage sampling data within 5 ms before and after a fault at the near-inverter side and the far-inverter side, respectively.

[0010] Strictly speaking, the term "differentiation" only applies in the continuous-time domain. For discrete signals, it needs to be properly defined. Step (4) uses... s qp Finite difference approximation s qp Differential QPDD , represented as: (4) The action quantity of the longitudinal protection based on the pseudo-power instantaneous value in step (4) S a_φ and braking amount S res_φ The algorithm is as follows: (5) in ω The fundamental angular frequency; U e This is the rated voltage under normal conditions; K rel The reliability coefficient is set to 0.15; max( X (time series) X The maximum value.

[0011] Preferably, the criterion for identifying the faulty section in step (5) is: (6) Criterion (6) is executed phase by phase. If criterion (6) is satisfied, it is an intra-zone fault, and the protection device of the corresponding phase issues a trip signal. If criterion (6) is not satisfied, it is determined to be an extra-zone fault, and the protection is blocked.

[0012] The present invention has the following advantages over the prior art: The receiving-end AC line longitudinal protection method based on the pseudo-power instantaneous value proposed in this invention is not affected by the nonlinear time-varying characteristics of the converter and commutation failure. It can adapt to different receiving-end AC grid structures, eliminate the hidden danger of the nonlinear power supply on the protection back side affecting the traditional longitudinal protection principle, and meet the protection requirements of AC / DC hybrid systems. It retains the high selectivity, high sensitivity and phase selection capability of traditional current differential longitudinal protection. The present invention proposes a longitudinal protection method for AC lines at the receiving end based on the instantaneous value of pseudo-power. The protection units at both ends utilize the differential of the pseudo-power difference within a data window to eliminate the influence of distributed capacitance and distributed inductance of the transmission line. It has the advantages of being unaffected by distributed capacitance and load current and allowing for larger fault resistance. It can reliably identify faults inside or outside the zone and has good adaptability. At the same time, it utilizes the integral of pseudo-power over a period of time, resulting in high protection reliability. The time window is 5ms, and the protection action speed is fast.

[0013] The receiving-end AC line longitudinal protection method proposed in this invention, based on the instantaneous value of pseudo-power, uses a narrow bandpass filter to extract the 50 Hz fundamental frequency components of voltage and current after a fault, which has a certain degree of anti-interference capability against noise. The protection units at both ends utilize the absolute value sum of the differentials of pseudo-power difference within a data window, which can withstand different transition resistances, different fault types, and different fault initial angles, and have high reliability. The fast action speed effectively reduces damage to converter equipment and avoids fault expansion, which can improve the stability of AC / DC hybrid power grids to a certain extent. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the data window capture for the receiving-end AC line longitudinal protection method based on the pseudo-power instantaneous value of the present invention; Figure 2 This is a schematic diagram of the receiving-end AC line longitudinal protection method based on the pseudo-power instantaneous value of the present invention; Figure 3 The AC / DC hybrid network in this embodiment of the invention is built based on the Tianzhong DC transmission system; Figure 4 The present invention includes a high-capacity filter for the AC bus of the inverter station and a reactive power compensation device. Figure 5 The amplitude-frequency and phase-frequency characteristics of the narrow bandpass filter in this embodiment of the invention are shown. Figure 6 The pseudo-power difference during a typical fault in the region, as described in this embodiment of the invention. s qpφ ; Figure 7 The pseudo-power difference differential in a typical intra-regional fault scenario in this embodiment of the invention. QPDD φ The waveform; Figure 8 The pseudo-power difference during a typical out-of-area fault in this embodiment of the invention. s qpφ ; Figure 9 This is the pseudo-power difference derivative for a typical out-of-area fault in an embodiment of the present invention. QPDD φ The waveform. Detailed Implementation

[0015] The purpose of this invention is to provide a longitudinal protection method for receiving-end AC lines based on the pseudo-power instantaneous value. The invention will be further described below with reference to specific embodiments. Example

[0016] A longitudinal protection method for receiving-end AC lines based on pseudo-power instantaneous values, such as Figure 2 As shown, it includes the following steps: (1) Obtain the instantaneous value change of the phase current, compare the instantaneous value change of the phase current with the set value, and if three consecutive phase current changes exceed the set value, it is determined that a short circuit fault has occurred, the protection is started, and the point where the first change exceeds the set value is marked as the fault start time. The sampling frequency is set to 4 kHz, consistent with the actual converter stations and substations used in engineering projects. Based on the three-phase current sampling values, the protection start-up criteria are checked: (7) in, i φ ( k () represents the sampled value of the phase current of any one of phases A, B, and C at the current moment. φ To distinguish them, namely A, B, and C. i φ ( k - N T The value represents the sampled value corresponding to the cycle preceding the fault. N T This is the number of sampling points per cycle, which is 80. I set The threshold value is 0.2 times the rated current, and its value is set to 0.24 kA.

[0017] (2) After the protection is started, the protection units at both ends, near the inverter station and far from the inverter station, capture a 5 ms data window before and after the fault start time, such as Figure 1 As shown, voltage and current samples were captured 5 ms before and after the fault (N=20). Protection devices typically store data from several cycles prior to a fault, and this pre-fault data is readily available. This invention retrieves data from the pre-fault period. N Each voltage and current sampling data point is labeled from 1 to 1. N After troubleshooting N Each voltage and current sampling data point is labeled from 1 to 1. N .

[0018] (3) Narrow bandpass digital filtering is used to filter the voltage and current before and after the fault to obtain the fundamental frequency component; The 50 Hz fundamental frequency component is extracted using a narrowband filter. The difference equation of the filter is: (8) in A = 0.0128; B =-0.0128; C =1.9679; D =-0.9742. y ( n() represents the filtered data. x ( n The data before filtering is shown below, representing the voltage and current samples from the extracted data window. Its amplitude-frequency and phase-frequency characteristics are as follows: Figure 5 As shown. Digital filters have transient delay effects. In order to quickly extract the 50 Hz fundamental frequency component after a fault, when using a narrow bandpass digital filter for filtering, the least squares method is used to estimate the initial value of the filter to speed up the transient process of digital filtering. For the estimation of the initial value of the filter using the least squares method, please refer to the article entitled "Phasor Extraction Algorithm Based on Narrowband Filter and Variable Length Data Window" published by Liu Yiqing et al. in the journal "Automation of Electric Power Systems" in 2015. This article proposes a filter initial value estimation based on the principle of least squares.

[0019] (4) Exchange the filtered voltage and current data before and after the fault for 5 ms at both ends, and use this data to calculate the pseudo-power difference. s qpφ Pseudopower difference differential QPDD φ and the operating quantity based on the pseudo-power instantaneous value longitudinal protection method S a_φ and braking amount S res_φ ; As a longitudinal protection system, optical fiber with strong anti-interference capabilities and high transmission speed is selected for information exchange. Protection unit R1 transmits its filtered voltage and current samples (within 5 ms before and after the fault) to protection unit R2 via the optical fiber, and also receives the filtered voltage and current samples (within 5 ms before and after the fault) from protection unit R2. Similarly, protection unit R2 transmits its filtered voltage and current samples (within 5 ms before and after the fault) to protection unit R1 via the optical fiber, and also receives the filtered voltage and current samples (within 5 ms before and after the fault) from protection unit R1.

[0020] After receiving data from the other end, calculate the pseudo-power difference. s qpφ Pseudopower difference differential QPDD φ and the operating quantity based on the pseudo-power instantaneous value longitudinal protection method S a_φ and braking amount S res_φ Pseudo-power difference s qpφ The algorithm is as follows: (9) pseudo-power difference derivative QPDD φ The algorithm is as follows: (10) Sampling rate in this embodiment f s It is 4000 Hz.

[0021] Motion volume S a_φ and braking amount S res_φ The algorithm is (11) in ω The fundamental angular frequency is 100. π ; U e The rated voltage under normal conditions is 400 kV; K rel The reliability coefficient is set to 0.15; max(X) is the time series. X The maximum value; N =20.

[0022] like Figure 3 The typical AC / DC hybrid power grid structure shown is as follows: the DC transmission system is a ±800kV LCC-HVDC system, and the receiving-end AC power grid is a 500kV AC system.

[0023] Figure 4 The large-capacity filter and reactive power compensation device installed on the AC bus of the LCC-HVDC inverter side represent the AC boundary conditions of the system in this embodiment. Line MN is a double-circuit line (Zhonghui I and II), with a length of 62.5 km; line MP is a double-circuit line (Zhongguan I and II), with a length of 21.27 km; line MS is a triple-circuit line (Zhongju I, II, and III), with Zhongju I and II lines being 15.87 km long and Zhongju III line being 16.87 km long. The line model uses a frequency-dependent model. The inverter at Zhongzhou station employs a strategy combining constant current control and constant turn-off angle control, and is also equipped with low-voltage current limiting control and current deviation control. C s For stray capacitance of the busbar, take 0.01. μ F. f 1: Fault within the area; f 2: Fault outside the N-side zone. (By...) Figure 4 As can be seen from the boundary conditions of the DC transmission system in the embodiment, this solution is not affected by the system type. Figure 5 The amplitude-frequency and phase-frequency characteristics of the narrow bandpass filter used in this embodiment show that the amplitude and phase remain unchanged at 50Hz, while the amplitude attenuates significantly in other frequency ranges.

[0024] by Figure 3 fExample 1 illustrates a ground fault occurring at the midpoint of the line, where phase A is connected to a ground fault via a 100 Ω transition resistor. Figure 6 The pseudo-power difference during a typical fault zone s qpφ , Figure 7 The pseudo-power difference derivative for a typical fault within the region QPDD φ The waveforms, after calculation, show the following operating and braking quantities for each phase: S a_A =5937.75 MVA / ms S res_A = 1093.46 MVA / ms; S a_B = 37.10 MVA / ms S res_B = 420.35 MVA / ms; S a_C = 8.80 MVA / ms S res_C = 443.35 MVA / ms.

[0025] by Figure 3 f Example 2 illustrates a metallic fault in phase A outside the 2-point zone. Figure 8 The pseudo-power difference during a typical out-of-area fault s qpφ , Figure 9 The pseudo-power difference derivative for typical out-of-area faults QPDD φ The waveforms, after calculation, show the following operating and braking quantities for each phase: S a_A =249.83 MVA / ms S res_A = 4608.5 MVA / ms; S a_B = 517.0 MVA / ms S res_B = 900.21 MVA / ms; S a_C = 477.61 MVA / ms S res_C = 1465.50 MVA / ms.

[0026] by Figure 3 fExample 3 shows a commutation failure caused by a fault in phase A at point 1. The calculated action and braking quantities for each phase are as follows: S a_A =45878.0 MVA / ms S res_A = 937.7 MVA / ms; S a_B = 92.6 MVA / ms S res_B = 442.8 MVA / ms; S a_C = 92.6 MVA / ms S res_C = 312.4 MVA / ms.

[0027] Will Figure 3 The Zhongju I, II, III, Zhongguan I, II, and Zhonghui I lines of the AC / DC hybrid system have been taken out of operation, with only the Zhonghui II line in operation. f Example 4 uses a 40Ω transition resistance for phase A at a location 10 km from point M via a ground fault. The calculated operating and braking quantities for each phase are as follows: S a_A =14208.9 / ms, S res_A = 4444.9 MVA / ms; S a_B = 6.91 MVA / ms S res_B = 117.52 MVA / ms; S a_C = 6.91 MVA / ms S res_C = 371.50 MVA / ms.

[0028] (5) Based on the obtained phase action and braking quantities, determine the fault range and determine the fault phase based on the judgment results.

[0029] The criterion is: (12) Criterion (12) is executed phase by phase. If criterion (12) is satisfied, it is an intra-zone fault, and the protection device of the corresponding phase issues a trip signal. If criterion (12) is not satisfied, it is determined to be an extra-zone fault, and the protection is blocked.

[0030] for Figure 6 and Figure 7 The fault in the area shown satisfies the condition in formula (12). S a_A > S res_A , S a_B < S res_B , S a_C < S res_C The protection device correctly identified a fault in phase A within the zone, while phases B and C were non-faulty phases. The protection device for phase A then issued a trip signal.

[0031] for Figure 8 and Figure 9 The fault outside the zone shown in formula (12) satisfies S a_A < S res_A , S a_B < S res_B , S a_C < S res_C The protection system correctly identified the fault as an external fault.

[0032] For Example 3, where the inverter station experiences commutation failure after a fault, formula (12) satisfies... S a_A > S res_A , S a_B < S res_B , S a_C < S res_C The protection device correctly identified a fault in phase A within the zone, while phases B and C were non-faulty phases. The protection device for phase A then issued a trip signal.

[0033] For Example 4, where the receiving-end AC line structure is a single-circuit single-transmission corridor, formula (12) satisfies S a_A > S res_A , S a_B < S res_B , S a_C < S res_CThe protection device correctly identified a fault in phase A within the zone, while phases B and C were non-faulty phases. The protection device for phase A then issued a trip signal.

[0034] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

[0035] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A longitudinal protection method for receiving-end AC lines based on pseudo-power instantaneous values, characterized in that: Includes the following steps: (1) Obtain the instantaneous value change of the phase current, compare the instantaneous value change of the phase current with the set value, and if three consecutive phase current changes exceed the set value, it is determined that a short circuit fault has occurred, the protection is started, and the point where the first change exceeds the set value is marked as the fault start time. (2) After the protection is started, the protection units at both ends of the near inverter station side and the far inverter station side capture a data window of 5 ms before and after the fault start time to obtain the voltage and current sampling values ​​5 ms before and after the fault; the period of the pseudo power difference differential waveform is 10 ms and the data window is 5 ms; (3) Narrow bandpass digital filtering is used to filter the voltage and current before and after the fault to obtain the fundamental frequency component; In step (3), the narrow bandpass filter extracts the 50 Hz component, and the difference equation is: (2) In the formula A =2 a / T s / (4 / T s 2 / b +2 c / T s + d ); B =- A ; C =(2 c -8 b / T s 2 ) / ( 4 / T s 2 / b +2 c / T s + d ); D =(4 b / T s 2 -2 c / T s + d ) / ( 4 / T s 2 / b +2 c / T s + d ); a =8 T s / 78; b = R 2 T s 2 + T s 2 +2 RT s cos(2 πf p T s ); c =4 T s -4 R 2 T s ; d =4 R 2 -8 R cos(2 πf p T s )+4; T s The sampling interval; f p The extracted frequency is 50 Hz; R The pole location parameter is related to the signal frequency's deviation from the passband center. R =0.987; (4) Exchange the filtered voltage and current data before and after the fault for 5 ms at both ends, and use the data to calculate the pseudo-power difference. s qpφ Pseudopower difference differential QPDD φ and the operating quantity based on the pseudo-power instantaneous value longitudinal protection method S a_φ and braking amount S res_φ ; (5) Based on the obtained phase action and braking quantities, determine the fault range and determine the fault phase based on the judgment results.

2. The receiving-end AC line longitudinal protection method based on pseudo-power instantaneous value according to claim 1, characterized in that: In step (1), the protection activation criterion is: (1) in, i φ ( k () represents the sampled value of the phase current of any one of phases A, B, and C at the current moment. φ To distinguish them, namely A, B, and C; i φ ( k - N T The value represents the sampled value corresponding to the cycle preceding the fault. N T This represents the number of sampling points per wave. I set The threshold is 0.2 times the rated current.

3. The receiving-end AC line longitudinal protection method based on pseudo-power instantaneous value as described in claim 1, characterized in that: In step (4), after the receiving protection unit receives the voltage and current sample values ​​before and after the fault (5 ms after filtering by the other protection unit), the pseudo-power difference is calculated. s qpφ The algorithm is as follows: (3) Before taking the fault N Each voltage and current sampling data point is labeled from 1 to 1. N After troubleshooting N Each voltage and current sampling data point is labeled from 1 to 1. N , N The number of sampling points within a 5 ms data window; where i m_φ [ i ]、 i n_φ [ i ]、 and , i =1~ N These are the current sampling data within 5 ms before and after the fault at the near-inverter side and the far-inverter side, respectively. u m_φ [ i ], u n_φ [ i ], and , i =1~ N These are voltage sampling data within 5 ms before and after a fault at the near-inverter side and the far-inverter side, respectively.

4. The receiving-end AC line longitudinal protection method based on pseudo-power instantaneous value according to claim 1, characterized in that: In step (4), the pseudo-power difference derivative is... QPDD The algorithm is as follows: (4) In the formula f s The sampling frequency.

5. The receiving-end AC line longitudinal protection method based on pseudo-power instantaneous value according to claim 1, characterized in that: In step (4), the action quantity based on the pseudo-power instantaneous value longitudinal protection method S a_φ and braking amount S res_φ The algorithm is as follows: (5) in ω The fundamental angular frequency; U e This is the rated voltage under normal conditions; K rel The reliability coefficient is set to 0.15; max( X (time series) X The maximum value.

6. The receiving-end AC line longitudinal protection method based on pseudo-power instantaneous value according to claim 1, characterized in that: In step (5), the criterion for the longitudinal protection method based on the pseudo-power instantaneous value is: (6) Criterion (6) is executed in phases. If the condition (6) is met, it is an intra-zone fault, and the protection device of the corresponding phase sends a trip signal. If criterion (6) is not met, it is determined to be an extra-zone fault, and the protection is blocked.

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