A method for preventing differential protection malfunction caused by transmission line resonance

Abstract

The present invention belongs to the technical field of power line protection, and specifically relates to a method for preventing differential protection malfunction caused by transmission line resonance. Pre-set fast-segment differential protection and steady-state segment differential protection; obtain differential current sampling channel data and brake current sampling channel data, and calculate the harmonic amplitude and fundamental amplitude; when the protection is started, compare the harmonic amplitude of the differential current with the fundamental amplitude, and construct a differential current harmonic amplitude ratio criterion to identify whether there is resonance in the protected line, which is used as the locking condition for the differential protection action; if the differential current harmonic amplitude ratio criterion condition is met, it is determined that there is resonance influence, the fast-segment differential protection is locked, and the steady-state segment differential protection constant is raised; if the differential current harmonic amplitude ratio criterion condition is not met, the fast-segment differential protection is unlocked, and the steady-state segment differential protection constant is restored to the preset threshold.

Description

Technical Field

[0001] The invention belongs to the technical field of power line protection, and in particular relates to a method for preventing differential protection malfunction caused by transmission line resonance. Background Art

[0002] In high-voltage long-distance transmission lines, there is a large capacitive current to the ground. For this, shunt reactors are generally connected on one or both sides of the line in an under-compensated manner. Connecting shunt reactors can limit overvoltage, eliminate backflow current, and improve the stability, reliability and efficiency of the power grid. At the same time, in terms of relay protection, it can reduce the impact of line distributed capacitance on differential protection.

[0003] Reactors can effectively improve transmission efficiency in steady state. Generally, under-compensation is adopted. Line capacitance and parallel inductance are not completely equivalent. On ultra-high voltage long-distance transmission lines, when the system is severely disturbed or the line operation mode changes, a certain degree of resonance will be exhibited between the line capacitance elements and the system impedance. The amplitude and duration of the resonant current are related to the line capacitance compensation degree and the current and voltage changes, which will cause unbalanced current on both sides of the line, thus affecting the correctness of the differential protection action.

[0004] The following are two situations where resonant current is likely to occur:

[0005] Case 1: When the line is operating under load, the switch on one side is opened, the circuit state changes, the current is cut off instantaneously, and energy is transferred between the ground capacitance in the line and the inductive elements in the system, forming an oscillation process, showing the characteristics of resonant current. The greater the current change before and after the operation, the more obvious the resonant current characteristics.

[0006] Case 2: Under the 3 / 2 wiring mode, two long-distance transmission lines have shunt reactors mounted on each line side. If a phase-to-ground fault occurs near the busbar, causing a sudden drop in the system voltage, the line-to-ground capacitance energy is excited, and energy is transferred to the inductive elements of the adjacent line through the connecting switch, forming an oscillation process.

[0007] Fourier algorithm is a commonly used algorithm for relay protection. In order to calculate the power frequency fault quantity in a timely and rapid manner during the initial period of the fault, a half-cycle Fourier algorithm or a half-cycle integral algorithm is generally used. After the fault data meets one power frequency cycle, it is converted to a full-cycle Fourier calculation. It should be understood that the half-cycle algorithm cannot filter out even harmonic components, and the full-cycle Fourier algorithm is limited in its filtering effect on non-integer harmonics, which will produce certain calculation errors during data processing.

[0008] Under the influence of the resonant current, the differential current calculated using the Fourier algorithm will produce a large error, which may exceed the set value and cause the differential protection to malfunction. Summary of the invention

[0009] In order to overcome the problems in the prior art, the present invention proposes a method for preventing differential protection malfunction caused by transmission line resonance.

[0010] The technical solution of the present invention to solve the above technical problems is as follows:

[0011] The present invention provides a method for preventing differential protection malfunction caused by transmission line resonance, comprising the following steps:

[0012] Pre-set two-stage differential protection, the two-stage differential protection includes fast-stage differential protection and steady-state stage differential protection;

[0013] Acquire current sampling values ​​of each phase at the same time on both sides of the transmission line, and calculate differential current sampling channel data and braking current sampling channel data based on the current sampling values;

[0014] Based on the differential current sampling channel data, the harmonic amplitude and fundamental amplitude are calculated;

[0015] When the protection is started, the harmonic amplitude of the differential current is compared with the fundamental amplitude, and the differential current harmonic amplitude ratio criterion is constructed to identify whether there is resonance in the protected line, which is used as the locking condition for the differential protection action; if the differential current harmonic amplitude ratio criterion is met, it is determined that there is resonance influence, the fast-section differential protection is locked, and the steady-state section differential protection setting is raised; if the differential current harmonic amplitude ratio criterion is not met, it is determined that there is no resonance influence, the fast-section differential protection is unlocked, and the steady-state section differential protection setting returns to the preset threshold.

[0016] Furthermore, the action criterion of the fast section differential protection is:

[0017]

[0018] In the above formula, is the phase differential current value after current compensation, is the split-phase braking current value after current compensation, K h I is the proportional braking coefficient of the fast section differential protection; Hset It is the differential current setting value of fast section differential protection.

[0019] Furthermore, the action criterion of the steady-state differential protection is:

[0020]

[0021] In the above formula, is the phase differential current value after current compensation, is the split-phase braking current value after current compensation; K l I is the proportional braking coefficient of the steady-state differential protection;set The differential current setting for the steady-state differential protection has a preset threshold value, and the steady-state differential protection is set with a fixed delay time.

[0022] Further, the single-phase current sampling values ​​at the same time on both sides of the line are obtained, and the single-phase current sampling values ​​are synthesized into differential current sampling channel data and braking current sampling channel data, specifically including:

[0023]

[0024] in, is the single-phase current sampling value on both sides of the line at the same time, To correspond to each other, is the differential current sampling value, It is the braking current sampling value.

[0025] Furthermore, the obtaining of harmonic amplitudes and fundamental amplitudes based on the differential current sampling channel data specifically includes:

[0026] Based on the differential current sampling channel data, the Fourier algorithm is used to obtain the fundamental wave amplitude and the 2nd, 3rd, and 4th harmonic amplitudes.

[0027] Furthermore, when the protection is started, the harmonic amplitude of the differential current is compared with the fundamental amplitude, and a differential current harmonic amplitude ratio criterion is constructed to identify whether there is a resonant current in the protected line, which is used as the locking condition for the differential protection action, specifically:

[0028]

[0029] Among them, K ha is the reliability coefficient, is the fundamental amplitude, They are the 2nd, 3rd and 4th harmonic amplitudes respectively.

[0030] Furthermore, the reliability coefficient K ha , with a value ranging from 0.8 to 1.2.

[0031] Furthermore, if the differential current harmonic amplitude ratio criterion is met and it is determined that there is a resonant current influence, the fast-section differential protection is locked and the steady-state section differential setting is raised, specifically including: locking the protection whose action time is less than the first preset time, including fast-section differential protection, variable differential protection, and sampling value differential protection, and at the same time raising the differential current setting of the steady-state section differential protection to twice the preset threshold.

[0032] Furthermore, the first preset time is 20 ms.

[0033] Compared with the prior art, the present invention has the following technical effects:

[0034] This method combines the detection of the proportion of finite-order harmonics with two-stage differential protection. The principle is simple and reliable. When the system disturbance causes current resonance, it can achieve the effect of rapid identification and timely blocking. The full-cycle Fourier algorithm is used as the implementation method of the harmonic criterion, which can more accurately identify the power frequency fundamental component and make up for the error caused by the half-cycle algorithm. It can effectively identify whether the protected line has resonance, and when no short-circuit fault occurs in the area, it can be reliably blocked without mistakenly opening. Once there is a fault in the area, the criterion can be quickly unlocked, and even in the most unfavorable situation, it can still be opened by the steady-state section differential protection without causing protection refusal. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the technical solutions and advantages in the embodiments of the present invention or the prior art, the drawings required for use in the embodiments or the prior art descriptions are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0036] Figure 1 It is a schematic diagram of the process of the present invention. DETAILED DESCRIPTION

[0037] In order to further explain the technical means and effects taken by the present invention to achieve the predetermined invention purpose, the specific implementation methods, structures, features and effects of the technical solutions proposed by the present invention are described in detail below in conjunction with the accompanying drawings and preferred embodiments. The specific features, structures or characteristics in one or more embodiments may be combined in any suitable form. Unless otherwise defined, all technical and scientific terms used in the present invention have the same meaning as those commonly understood by technicians in the technical field of the present invention.

[0038] In one embodiment of the present invention, referring to Figure 1 , provides a method for preventing differential protection malfunction caused by transmission line resonance, comprising the following steps:

[0039] Pre-set two-stage differential protection, the two-stage differential protection includes fast-stage differential protection and steady-state stage differential protection;

[0040] Acquire single-phase current sampling values ​​at the same time on both sides of the transmission line, and calculate differential current sampling channel data and braking current sampling channel data based on the single-phase current sampling values;

[0041] Based on the differential current sampling channel data, the harmonic amplitude and fundamental amplitude are calculated;

[0042] When the protection is started, the harmonic amplitude of the differential current is compared with the fundamental amplitude, and the differential current harmonic amplitude ratio criterion is constructed to identify whether there is resonance in the protected line, which is used as the locking condition for the differential protection action; if the differential current harmonic amplitude ratio criterion is met, it is determined that there is resonance influence, and the fast-section differential protection is immediately locked, and the steady-state section differential protection setting is raised; if the differential current harmonic amplitude ratio criterion is not met, it is determined that there is no resonance influence, and the fast-section differential protection is unlocked, and the steady-state section differential protection setting returns to the preset threshold.

[0043] The following is a detailed explanation of each of the above steps:

[0044] In some embodiments, this step may include the following steps:

[0045] Step 1. Pre-set two-stage differential protection, which includes fast-stage differential protection and steady-state stage differential protection.

[0046] The action criterion of the fast section differential protection is:

[0047]

[0048] In the above formula, is the phase differential current value after current compensation, is the split-phase braking current value after current compensation; K h I is the proportional braking coefficient of the fast section differential protection, which can be 0.6 to 0.85; Hset For the differential current setting of the fast section differential protection, take twice the differential current setting.

[0049] The action criterion of the steady-state differential protection is:

[0050]

[0051] In the above formula, is the phase differential current value after current compensation, is the split-phase braking current value after current compensation; K l is the proportional braking coefficient of the steady-state differential protection, which can be 0.5 to 0.7, and the delay time is 25ms to 50ms; I set It is the differential current setting value of the steady-state differential protection, which is given externally and is generally 1 to 2 times greater than the line capacitance current during normal operation.

[0052] The fast-section differential protection improves the reliability of fault identification through a high threshold, without delay, to ensure that the protection can act quickly when short-circuit characteristics such as metallic faults at the near end of the line are obvious; while the steady-state section differential protection is a confirmation mechanism through a certain period of time, which can reliably identify complex faults in the system while ensuring sensitivity and selectivity, so a low threshold plus short delay is adopted.

[0053] The two-stage differential protection is configured to improve the comprehensive performance of the protection device's fault handling through differential coordination and floating threshold adjustment, preventing the differential protection from malfunctioning. When a short-circuit fault occurs in the system, even in the most unfavorable circumstances, the protection will not fail to operate.

[0054] Step 2. Obtain the phase current sampling values ​​at the same time on both sides of the transmission line; based on the phase current sampling values, calculate the differential current sampling channel data and the braking current sampling channel data.

[0055] The sampling channel data of differential current and braking current are specifically differential current sampling value and braking current sampling value:

[0056]

[0057] in, is the single-phase current sampling value on both sides of the line at the same time, To correspond to each other, is the differential current sampling value, It is the braking current sampling value.

[0058] Step 3. Based on the differential current and brake current sampling channel data, calculate the phase differential current value after current compensation and the phase brake current value after current compensation.

[0059] The Fourier algorithm is used to obtain the fundamental wave value of the differential current and the braking current, and the phase differential current value after current compensation is obtained through the π-type equivalent circuit steady-state compensation or transient compensation method. And phase braking current value Used for differential protection.

[0060] Phase differential current value Phase braking current value Different algorithms are selected according to the current protection startup stage, specifically including: judging the startup time range of the relay protection device according to the startup time of the relay protection device, if the startup time of the relay protection device is greater than the first threshold and less than the second threshold, the half-cycle algorithm of the 10ms data window is used, otherwise the full-cycle Fourier algorithm of the 20ms data window is used for calculation. Among them, the first threshold is 10ms and the second threshold is 30ms.

[0061] Step 4: Based on the differential current sampling channel data, calculate the harmonic amplitude and fundamental amplitude.

[0062] according to Differential current sampling channel data, using the full-cycle Fourier algorithm to directly obtain the fundamental amplitude No current compensation processing is required for harmonic criterion use.

[0063] When the ultra-high voltage transmission line is in normal operation, a capacitor is formed between the earth and each phase line, and there is a capacitive current. The specific size is related to the capacitive reactance of the line to the ground. Therefore, the current flowing into one side of the line is not equal to the current flowing out of the end of the line. Therefore, when calculating the differential current, the size of the line capacitance to the ground current must be considered. There is a general calculation method. Fixed use of full-cycle Fourier acquisition Amplitude is because the full-cycle algorithm has a better filtering effect, which is consistent with the harmonic amplitude calculation and can reduce calculation errors.

[0064] Extract the 2nd, 3rd, and 4th harmonic amplitudes: Obtained through the full-cycle Fourier algorithm corresponding to the harmonic frequency.

[0065] The Fourier algorithm for the power frequency fundamental wave and nth harmonic components is:

[0066]

[0067] in, is the phasor of the sampling channel, A n is the real part of the phasor, B n is the imaginary part of the phase, N is the number of sampling points in the power frequency cycle, n is the harmonic number, x k is a continuous sampling value. is the fundamental component value.

[0068] Step 5. When the protection is started, the harmonic amplitude of the differential current is compared with the fundamental amplitude, and a differential current harmonic amplitude ratio criterion is constructed to identify whether there is resonance in the protected line, which is used as the locking condition for the differential protection action.

[0069] 10ms after the protection is started, the harmonic amplitude of the differential current is compared with the fundamental amplitude, and the differential current harmonic amplitude ratio criterion is constructed to identify whether there is resonance in the protected line, which is used as the locking condition for the differential protection action:

[0070]

[0071] Among them, K ha is the reliability coefficient, and its value ranges from 0.8 to 1.2.

[0072] If the differential current harmonic amplitude ratio criterion condition is met, it is judged that there is resonance influence, the fast section differential protection is blocked, and the steady-state section differential setting is raised to the high threshold. The protection with blocking action time less than 20ms includes fast section differential protection, variable differential protection, and sampling value differential protection. At the same time, the differential current setting of the steady-state section differential protection is raised to the high threshold, which is twice the preset threshold value.

[0073] If the differential current harmonic amplitude ratio criterion is not met, it is determined that there is no resonance influence, and the fast-stage differential is immediately unlocked, and the steady-state stage differential protection setting is restored to the preset threshold.

[0074] Furthermore, if an intra-zone fault occurs, in the early stage of the fault, due to the influence of the fault data not satisfying the half-cycle window and the non-periodic components, the differential current harmonic amplitude ratio criterion condition may be established for a short time. Thereafter, when the fault data is greater than a certain number (not more than 20ms), the fundamental content of the differential current will increase rapidly, and the reliability coefficient in the differential current harmonic amplitude ratio criterion condition is set high. Even if the back side is a weak power supply, the condition will not be established. At this time, the fast-section differential protection can be unlocked, and after the action criterion is met, the fast-section differential protection can be immediately operated.

[0075] On the other hand, if there is a fault, even if the differential current harmonic amplitude ratio criterion condition continues to hold under certain circumstances, the steady-state section differential protection can still be operated when the differential current continues to be greater than 2 times the setting value and meets the action criterion of the steady-state section differential protection.

[0076] The present invention converts the differential current from the time domain to the frequency domain for waveform analysis and extracts key identification factors. When the fault occurs within the area, the fundamental content of the fault current accounts for a high proportion. When the fault occurs outside the area or a large disturbance causes line resonance, the current waveform characteristics are analyzed, showing that the harmonic component is high, the frequency domain is mainly concentrated in the 2nd, 3rd, and 4th harmonics, and the fundamental content is small. The magnitude of the resonant current is proportional to the line capacitance value and the current change, and decays quickly, and decays quickly to less than 2 times the power frequency capacitance current after a certain period of time. When the differential current exceeds the threshold, the harmonic amplitude of the differential current is compared with the fundamental amplitude, and the differential current harmonic amplitude ratio criterion is constructed to identify whether there is a resonance in the protected line, which is used as the locking condition for the differential protection action. At the same time, two-stage differential protection is configured, and through differential matching and floating threshold adjustment, the reliability of the fault handling of the protection device can be improved, and the differential protection can be prevented from malfunctioning. When a short-circuit fault occurs in the system, even in the most unfavorable situation, it will not cause the protection to refuse to operate.

[0077] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. Such modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the protection scope of the present invention.

Claims

1. A method for preventing differential protection malfunction caused by transmission line resonance, characterized in that: The following steps are involved: Pre-set two-stage differential protection, the two-stage differential protection includes fast-stage differential protection and steady-state stage differential protection; The action criterion of the fast section differential protection is: ； In the above formula, is the phase differential current value after current compensation, is the split-phase braking current value after current compensation, is the proportional braking coefficient of fast section differential protection; It is the differential current setting value of fast section differential protection; The action criterion of the steady-state differential protection is: ； In the above formula, is the phase differential current value after current compensation, is the split-phase braking current value after current compensation; is the proportional braking coefficient of the steady-state differential protection; The differential current setting value of the steady-state differential protection is preset with a threshold value, and the steady-state differential protection is set with a fixed delay time; Acquire the current sampling values ​​of each phase at the same time on both sides of the transmission line, and calculate the differential current sampling channel data and the braking current sampling channel data based on the current sampling values; specifically include: ； in, , is the single-phase current sampling value on both sides of the line at the same time, To correspond to each other, is the differential current sampling value, is the braking current sampling value; Based on the differential current sampling channel data, the harmonic amplitude and the fundamental amplitude are calculated, specifically including: based on the differential current sampling channel data, the Fourier algorithm is used to obtain the fundamental amplitude and the 2nd, 3rd, and 4th harmonic amplitudes; When the protection is started, the harmonic amplitude of the differential current is compared with the fundamental amplitude, and the differential current harmonic amplitude ratio criterion is constructed to identify whether there is resonance in the protected line, which is used as the locking condition of the differential protection action, specifically: ； in, is the reliability coefficient, is the fundamental amplitude, , , They are the 2nd, 3rd and 4th harmonic amplitudes respectively; If the differential current harmonic amplitude ratio criterion condition is met, it is determined that there is a resonance influence, the fast-section differential protection is locked, and the steady-state section differential protection setting is raised; if the differential current harmonic amplitude ratio criterion condition is not met, it is determined that there is no resonance influence, the fast-section differential protection is unlocked, and the steady-state section differential protection setting is restored to the preset threshold.

2. A method for preventing differential protection malfunction caused by transmission line resonance according to claim 1, characterized in that: The reliability coefficient , with a value of 0.8~1.

2.

3. A method for preventing differential protection malfunction caused by transmission line resonance according to claim 1, characterized in that: If the differential current harmonic amplitude ratio criterion is met, it is determined that there is a resonant current influence, the fast-section differential protection is locked, and the steady-state section differential setting is raised, specifically including: locking the protection whose action time is less than the first preset time, including fast-section differential protection, variable differential protection, and sampling value differential protection, and at the same time raising the differential current setting of the steady-state section differential protection to twice the preset threshold.

4. A method for preventing differential protection malfunction caused by transmission line resonance according to claim 3, characterized in that: The first preset time is 20ms.

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