Correction-type line differential protection method for flexible DC transmission system

Abstract

The topology and control mode of the flexible DC transmission system are significantly different from those of the traditional synchronous power supply, which makes its fault current characteristics quite different from those of the synchronous power supply, resulting in the degradation of the current differential protection performance or even incorrect operation, which seriously threatens the safe and stable operation of the power system. The present invention discloses a correction type line differential protection method adapted to the flexible DC transmission system, which can still accurately distinguish between in-zone and out-of-zone faults when the flexible DC transmission system is connected, and is not affected by abnormal data and current transformer saturation.

Description

Technical Field

[0001] The invention relates to a correction type line differential protection method adapted to a flexible direct current power transmission system, and belongs to the technical field of power system relay protection. Background Art

[0002] Flexible direct current transmission technology, with its high controllability, high flexibility and high stability, has become a key technology in the fields of large-scale renewable energy grid connection, long-distance large-capacity power transmission and island power supply, and has broad application prospects.

[0003] Unlike traditional synchronous power supplies, the fault current of the flexible DC transmission system has a limited amplitude and controllable phase angle due to the limitation of the overcurrent capacity of fully controlled power electronic devices and the high controllability of the converter station. The change in fault current characteristics causes the traditional current differential protection to have low sensitivity or even refuse to operate under intra-zone faults. In addition, affected by abnormal data and current transformer saturation, the current differential protection may malfunction when an out-of-zone fault occurs. As the main protection for high-voltage transmission lines, the incorrect operation of the current differential protection seriously threatens the safe and stable operation of the power system. Therefore, formulating a suitable protection scheme is of great significance and value for accurately identifying intra-zone and out-of-zone faults. Summary of the invention

[0004] In view of the problem that the performance of the existing current differential protection is degraded or even does not operate correctly after the flexible DC system is connected, the present invention provides a correction line differential protection method that is suitable for the flexible DC transmission system.

[0005] The correction type line differential protection method adapted to the flexible direct current transmission system comprises the following steps:

[0006] Step 1: Sample the three-phase current at both ends of the AC line;

[0007] Step 2: Calculate the Abnormal Data Index (ADI). If the sampled current exceeds twice the rated current and the abnormal data index ADI is greater than its threshold ADI thr When abnormal data is identified, step three is executed; otherwise, step four is executed;

[0008] Step 3: Use the first-order Lagrange interpolation formula to calculate interpolation data to replace abnormal data;

[0009] Step 4: Use discrete Fourier algorithm to calculate the current phasors on both sides of the line;

[0010] Step 5: Calculate the three-phase current mutation If the maximum value of the three-phase current mutation amplitude on the converter station side and the grid side is greater than the starting current, it is considered that a fault has occurred and step 6 is executed; otherwise, no fault has occurred on the grid and step 1 is executed;

[0011] Step 6: Calculate whether the current differential protection criterion based on the amplitude and phase angle correction factor is satisfied. If satisfied, execute step 7; otherwise, determine it as an out-of-zone fault;

[0012] Step 7: Calculate the saturation index (SI). If the saturation index SI exceeds its threshold SI thr , it is determined to be an internal fault. Otherwise, it is determined to be an external fault.

[0013] The present invention discloses a correction type line differential protection method adapted to a flexible direct current transmission system. When the flexible direct current transmission system is connected, the method can still accurately distinguish between faults within and outside the area, and is not affected by abnormal data and current transformer saturation, thereby ensuring the safe and stable operation of the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 Schematic diagram of the flexible DC transmission system connected to the power grid;

[0015] Figure 2 Flow chart of the correction line differential protection adapted to the flexible DC transmission system. DETAILED DESCRIPTION

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

[0017] The correction type line differential protection method adapted to the flexible direct current transmission system comprises the following steps:

[0018] Step 1: If Figure 1 and Figure 2 , sampling the three-phase currents at both ends of the AC line;

[0019] Step 2: If Figure 2 , calculate the abnormal data index ADI, if the sampling current exceeds twice the rated current and the abnormal data index ADI is greater than its threshold ADI thr When abnormal data is identified, step three is executed; otherwise, step four is executed;

[0020] The specific calculation formula is as follows:

[0021]

[0022]

[0023] In the formula, k rel is the reliability coefficient, which is 1.5, and N is the number of sampling points in one power frequency cycle. For the power system with a power frequency of 50 Hz, when the sampling frequency is 2 kHz, N = 40.

[0024] Step 3: If Figure 2 , use the first-order Lagrange interpolation formula to calculate the interpolation data to replace the abnormal data;

[0025] The specific formula is as follows:

[0026]

[0027] Wherein, i′(k) is the interpolation data used as a substitute when the k-th sampling point is abnormal data, which is calculated from the sampling currents i(k-1) and i(k-2) of the first two points.

[0028] Step 4: If Figure 2 The discrete Fourier algorithm is used to calculate the current phasors on the flexible DC converter station side and the grid side respectively.

[0029] The discrete Fourier algorithm is:

[0030]

[0031] in,

[0032]

[0033] In the formula, is the current phasor, I Re and I Im They represent the real and imaginary parts of the current phasor respectively, N is the number of sampling points in one power frequency cycle, and i(k) is the sampling current at the kth sampling point.

[0034] Step 5: If Figure 2 , calculate the three-phase current mutation If the maximum value of the three-phase current mutation amplitude on the converter station side and the grid side is greater than the starting current, it is considered that a fault has occurred and step 6 is executed; otherwise, no fault has occurred on the grid and step 1 is executed;

[0035] The three-phase current mutation is:

[0036] Δi x (k) = i x (k)-i x (k-nN) (x=a,b,c) (6)

[0037] Where N is the number of sampling points in one power frequency cycle, i x (k) is the sampling current of the kth sampling point, i x (k-nN) is the sampled current n cycles ago. a, b, c represent phase A, phase B, and phase C respectively.

[0038] The criteria for determining whether a fault has occurred are:

[0039]

[0040] In the formula, I start is the starting current, I N is the rated current.

[0041] Step 6: If Figure 2 , calculate whether the current differential protection criterion based on the amplitude and phase angle correction factor is satisfied, if satisfied, execute step 7; otherwise, determine it as an out-of-zone fault;

[0042] The criterion for current differential protection based on amplitude and phase angle correction factors is:

[0043]

[0044] In the formula, and are the current phasors on the converter station side and the grid side, respectively. ε and δ represent the amplitude and phase angle correction factors, respectively:

[0045]

[0046]

[0047] In the formula,

[0048] Step 7: If Figure 2 , calculate the saturation index SI, if the saturation index SI exceeds its threshold SI thr , it is determined to be an internal fault. Otherwise, it is determined to be an external fault.

[0049] Saturation index SI and its threshold SI thr The formula is as follows:

[0050]

[0051] Where t0 represents the time when the fault is detected.

[0052] SI thr =0.0014I N (12)

[0053] I N is the rated current.

[0054] The above description is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements, substitutions, modifications and embellishments can be made without departing from the principle and purpose of the present invention. These improvements, substitutions, modifications and embellishments should also be regarded as the scope of protection of the present invention.

Claims

1. A correction type line differential protection method adapted to a flexible DC transmission system comprises the following steps: Step 1: Sample the three-phase current at both ends of the AC line; Step 2: Calculate the abnormal data index ADI. If the sampling current exceeds twice the rated current and the abnormal data index ADI is greater than its threshold ADI thr When abnormal data is identified, step three is executed; Otherwise, go to step 4; Step 3: Use the first-order Lagrange interpolation formula to calculate interpolation data to replace abnormal data; Step 4: Use discrete Fourier algorithm to calculate the current phasors on both sides of the line; Step 5: Calculate the three-phase current mutation If the maximum value of the three-phase current mutation amplitude on the converter station side and the grid side is greater than the starting current, it is considered that a fault has occurred and step 6 is executed; otherwise, no fault has occurred on the grid and step 1 is executed; Step 6: Calculate whether the current differential protection criterion based on the amplitude and phase angle correction factor is satisfied. If satisfied, execute step 7; otherwise, determine it as an out-of-zone fault; Step 7: Calculate the saturation index SI. If the saturation index SI exceeds its threshold SI thr , it is determined to be an internal fault; otherwise, it is determined to be an external fault; The abnormal data identification method described in step 2, the abnormal data index ADI and its threshold ADI thr The formula is as follows: In the formula, k rel is the reliability coefficient, which is 1.5, and N is the number of sampling points in one power frequency cycle. is the initial phase angle; for a power system with an industrial frequency of 50 Hz, when the sampling frequency is 2 kHz, N = 40; The saturation index SI and its threshold SI thr The formula is as follows: Where t0 represents the time when the fault is detected; SI thr =0.0014I N (7) In the formula, I N is the rated current.

2. The correction type line differential protection method adapted to the flexible DC transmission system according to claim 1 is characterized in that: The current differential protection criterion based on amplitude and phase angle correction factor described in step 6 is specifically formulated as follows: In the formula, and are the current phasors at the converter station side and the grid side respectively; ε and δ represent the amplitude and phase angle correction factors respectively: In the formula,

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