Adaptive low-frequency transmission line current differential protection method and system

Abstract

The present invention discloses an adaptive low-frequency transmission line current differential protection method and system. The technical solution adopted by the present invention is: the protection device collects the current on both sides of the low-frequency transmission line; the protection devices on both sides use channels to exchange information and share the electrical quantities on both sides, and calculate the phase current, phase differential current and phase braking current on both sides of the low-frequency transmission line; combined with the load current information on both sides of the low-frequency transmission line before the fault, the ratio braking coefficient in the ratio braking differential protection is adaptively adjusted to form an adaptive line current differential protection; each phase independently performs fault judgment. The present invention effectively solves the problem that the traditional transmission line current differential protection method is not suitable for low-frequency transmission lines, and improves the sensitivity, reliability and speed of the transmission line current differential protection.

Description

Technical Field

[0001] The invention belongs to the field of electric power systems and relates to a relay protection method, in particular to an adaptive low-frequency transmission line current differential protection and system. Background Art

[0002] In traditional power frequency power systems, fiber optic current differential protection has been widely used as the main protection for line protection. The principle of fiber optic current differential protection is relatively advanced. It uses the electrical quantities at both ends of the line to calculate the differential and braking currents, and determines the internal and external faults by calculating the ratio of the differential current and the braking current.

[0003] Due to the constraints of wind energy, solar energy and land resources, more than 2 / 3 of my country's wind power and photovoltaic power are concentratedly developed and transmitted over long distances at sea and in desertified areas. COSCO's offshore wind power and other scenarios will appear in large numbers, using industrial frequency AC transmission. Large-scale new energy bases have prominent problems such as voltage stability and broadband oscillation, which have become key factors restricting the large-scale development of new energy power generation.

[0004] The low-frequency transmission system uses the law that line impedance decreases proportionally with frequency. When the frequency is reduced, the reactance of the transmission and transformation system can be reduced, the electrical distance can be shortened, and the transmission power limit can be increased. When applied to offshore wind power AC transmission, the charging current along the line is reduced, which can increase the offshore wind power AC transmission distance and save investment in reactive compensation devices.

[0005] When a low-frequency system transmission line fails, the fault current generated by the control strategy of the power electronic device is limited and the phase is controlled, resulting in a small difference in the amplitude of the differential current and the braking current, which is particularly obvious under heavy line conditions, making it difficult to set the ratio braking coefficient of the optical fiber current differential protection. Therefore, it is necessary to study new differential protection principles to improve the protection performance. Summary of the invention

[0006] In order to solve the problem that when there is an internal fault in a low-frequency transmission line, the optical fiber current differential protection may refuse to operate due to the fault current limitation and phase control caused by the control strategy of the power electronic device, the present invention provides an adaptive low-frequency transmission line current differential protection and system.

[0007] A technical solution adopted by the present invention is: an adaptive low-frequency transmission line current differential protection method, which comprises the following steps:

[0008] 1) Collect the instantaneous current on both sides of the low-frequency transmission line and connect it to the protection device;

[0009] 2) The protection devices on both sides use the channel to exchange information and share the electrical quantities on both sides, and calculate the phase current, phase differential current and phase braking current on both sides of the low-frequency transmission line;

[0010] 3) Combined with the load current information on both sides of the low-frequency transmission line before the fault, the ratio braking coefficient in the ratio braking differential protection is adaptively adjusted to form an adaptive line current differential protection;

[0011] 4) Each phase performs fault judgment independently. When the relationship between the phase differential current and the braking current of a phase meets the adaptive line current differential protection criterion, it is judged that a fault has occurred in the line area, the adaptive line current differential protection is activated, and the circuit breakers of the phases on both sides of the low-frequency transmission line are tripped; otherwise, the adaptive line current differential protection does not operate.

[0012] Furthermore, the protection devices on both sides use the phase current, phase differential current and phase braking current calculated by the electrical quantities on both sides.

[0013]

[0014]

[0015] in, is the current on both sides of the line, is the phase sequence, m and n represent the two sides of the line; is the line phase differential current amplitude, is the phase braking current amplitude.

[0016] Furthermore, the criteria for adaptive line current differential protection are as follows:

[0017]

[0018] in, is the line phase differential current amplitude, is the phase braking current amplitude, k is the current differential protection braking coefficient, which is automatically adjusted according to the load current information on both sides of the line before the fault.

[0019] Furthermore, the current differential protection braking coefficient k is adjusted as follows:

[0020]

[0021] Among them, I load is the line load current amplitude before the fault, that is I set The current differential protection setting set for the protection device; I N is the secondary rated value of CT; k1 and k2 are constants; I load ,I N ,I set All are quadratic values.

[0022] Furthermore, the value range of k1 and k2 is 0.1 to 0.6.

[0023] Another technical solution adopted by the present invention is: an adaptive low-frequency transmission line current differential protection system, which comprises:

[0024] Instantaneous current collection unit: collects instantaneous currents on both sides of the low-frequency transmission line and connects to the protection device;

[0025] Current calculation unit: The protection devices on both sides exchange information through the optical fiber channel to share the electrical quantities on both sides, and calculate the phase current, phase differential current and phase braking current on both sides of the low-frequency transmission line;

[0026] Adaptive line current differential protection component unit: Combined with the load current information on both sides of the low-frequency transmission line before the fault, the ratio braking coefficient in the ratio braking differential protection is adaptively adjusted to form an adaptive line current differential protection;

[0027] Criteria judgment unit: Each phase performs fault judgment independently. When the relationship between the phase differential current and the braking current of a phase meets the adaptive line current differential protection criterion, it is judged that a fault has occurred in the line area, the adaptive line current differential protection is activated, and the circuit breakers of the phases on both sides of the low-frequency transmission line are tripped; otherwise, the adaptive line current differential protection does not operate.

[0028] The invention has the following beneficial effects: the adaptive line current differential protection proposed by the invention has the characteristics of high sensitivity and strong reliability. The invention can solve the problem of differential protection refusal caused by fault current limitation and phase control caused by the control strategy of power electronic devices when a fault occurs in a low-frequency system transmission line, and improve the sensitivity and speed of low-frequency transmission line current differential protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a flow chart of the adaptive low-frequency transmission line current differential protection method of the present invention;

[0030] Figure 2 This is a schematic diagram of a typical 220kV low-frequency power transmission system model in an embodiment of the present invention;

[0031] Figure 3 This is a single-phase grounding fault current waveform diagram under light load in an embodiment of the present invention;

[0032] Figure 4 This is a single-phase grounding fault current waveform diagram under heavy load in an embodiment of the present invention;

[0033] Figure 5 This is a diagram of the braking coefficient of the differential protection during a single-phase grounding fault under light load conditions of the present invention;

[0034] Figure 6 This is a diagram of the braking coefficient of the differential protection during a single-phase grounding fault under heavy load conditions of the present invention;

[0035] Figure 7 It is a structural block diagram of the adaptive low-frequency transmission line current differential protection system of the present invention. DETAILED DESCRIPTION

[0036] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0037] Example 1

[0038] Figure 1 This is a flow chart of the adaptive low-frequency transmission line current differential protection method of the present invention, which mainly includes the following steps:

[0039] (1) Collect the instantaneous current and voltage on both sides of the low-frequency transmission line, including the instantaneous values ​​of the phase current and voltage on the secondary side of the PT and CT on both sides, and connect them to the protection device.

[0040] (2) The protection devices on both sides use the channel to exchange information and share the electrical quantities on both sides. The electrical quantities include the instantaneous values ​​of the phase currents on the secondary sides of the CTs on both sides, and calculate the phase current amplitude, phase angle, differential current, and braking current amplitude on both sides of the low-frequency line:

[0041]

[0042]

[0043] in, is the current on both sides of the line, is the phase sequence, m and n represent the two sides of the line; is the line phase differential current amplitude, is the phase braking current amplitude.

[0044] 3) Combined with the load current information on both sides of the low-frequency transmission line before the fault, the ratio braking coefficient in the ratio braking differential protection is adaptively adjusted to form an adaptive line current differential protection. The criteria for the adaptive line current differential protection are as follows:

[0045]

[0046] Among them, k is the braking coefficient of the current differential protection, which is automatically adjusted according to the load current information on both sides of the line before the fault.

[0047]

[0048] Among them, I load is the line load current amplitude before the fault, that is I set The current differential protection setting set for the protection device; I Nis the CT rated value; k1 and k2 are constants, ranging from 0.1 to 0.6, with a typical value of k1 of 0.2 and a typical value of k2 of 0.4. load ,I N ,I set All are quadratic values.

[0049] 4) Each phase performs fault judgment independently. When the relationship between the phase differential current and the braking current of a phase meets the adaptive line current differential protection criterion, it is judged that a fault has occurred in the line area, the current differential protection is activated, and the circuit breakers of the phase on both sides of the line are tripped; otherwise, the current differential protection does not operate.

[0050] The effectiveness of the differential protection method provided by the embodiment of the present invention is illustrated below in combination with a light-load single-phase grounding fault of a line and a heavy-load single-phase grounding fault of a line.

[0051] Figure 2 It is a typical 220kV low-frequency transmission system model. It simulates a single-phase metallic grounding fault at the midpoint F1 of the line under light load and heavy load conditions. The primary CT sampling values ​​and differential currents on both sides are obtained as follows: Figure 3 and Figure 4 As shown in the figure, the fault start time is 250ms, the fault duration is 100ms, the CT ratio is 800:1, and I N 1A, I set It is 0.4A. After calculation, the load current is 0.96A under heavy load and 0.35A under light load. The following formula shows that the differential protection braking coefficient is 0.21 under heavy load and 0.4 under light load.

[0052]

[0053] After the current sampling value is calculated by Fourier, the differential current and braking current of each phase are calculated according to the following formula, and finally the ratio of differential current to braking current is obtained. Among them, Figure 5 is the ratio of phase A differential current to braking current under light load conditions, Figure 6 It is the ratio of phase A differential current to braking current under heavy load conditions.

[0054]

[0055]

[0056] like Figure 5 As shown in the figure, one cycle after the metallic ground fault of phase A occurs under light load, the ratio of the differential current of phase A to the braking current is 0.66, which is greater than the differential protection braking coefficient of 0.4 under light load, and the protection can operate correctly; Figure 6As shown, one cycle after the metallic grounding fault of phase A occurs under heavy load conditions, the ratio of the differential current of phase A to the braking current is 0.34, which is greater than the differential protection braking coefficient of 0.21 under heavy load conditions, and the protection can operate correctly.

[0057] Example 2

[0058] This embodiment provides an adaptive low-frequency transmission line current differential protection system, which is composed of an instantaneous current acquisition unit, a current calculation unit, an adaptive line current differential protection forming unit and a criterion judgment unit.

[0059] Instantaneous current collection unit: collects instantaneous current on both sides of the low-frequency transmission line and connects to the protection device.

[0060] Current calculation unit: Use the optical fiber differential channels of the protection devices on both sides to exchange information and share the electrical quantities on both sides, and calculate the phase current, phase differential current and phase braking current on both sides of the low-frequency transmission line.

[0061] Adaptive line current differential protection constituent unit: Combined with the load current information on both sides of the low-frequency transmission line before the fault, the braking coefficient in the ratio braking differential protection is adaptively adjusted to form an adaptive line current differential protection.

[0062] Logic judgment unit: Each phase performs fault judgment independently. When the relationship between the phase differential current and the braking current of a phase meets the adaptive line current differential protection criterion, the adaptive line current differential protection is activated, and the circuit breakers on both sides of the low-frequency transmission line are tripped according to the phases; otherwise, the adaptive line current differential protection does not operate.

[0063] The criteria for adaptive line current differential protection are as follows:

[0064]

[0065] in, is the line phase differential current amplitude, is the phase-splitting braking current amplitude, k is the current differential protection braking coefficient, which is automatically adjusted according to the load current information on both sides of the line before the fault.

[0066] The adjustment method for the current differential protection braking coefficient k is:

[0067]

[0068] Among them, I load is the line load current amplitude before the fault, that is I set The current differential protection setting set for the protection device; I N is the CT rated value; k1 and k2 are constants, ranging from 0.1 to 0.6, IN 1A, I set 0.4A; I load ,I N ,I set All are quadratic values.

[0069] Those skilled in the art to which the present invention relates may make various modifications or additions to the specific embodiments described or replace them in a similar manner, but they will not deviate from the spirit of the present invention or exceed the scope defined by the attached claims.

Claims

1. An adaptive low-frequency transmission line current differential protection method, characterized in that: The following steps are included: 1) Collect the instantaneous current on both sides of the low-frequency transmission line and connect it to the protection device; 2) The protection devices on both sides use the channel to exchange information and share the electrical quantities on both sides, and calculate the phase current, phase differential current and phase braking current on both sides of the low-frequency transmission line; 3) Combined with the load current information on both sides of the low-frequency transmission line before the fault, the ratio braking coefficient in the ratio braking differential protection is adaptively adjusted to form an adaptive line current differential protection; 4) Each phase independently performs fault judgment. When the relationship between the phase differential current and the braking current of a certain phase meets the adaptive line current differential protection criterion, it is judged that a fault has occurred in the line area, the adaptive line current differential protection is activated, and the circuit breakers of the phases on both sides of the low-frequency transmission line are tripped; Otherwise, the adaptive line current differential protection does not operate; The protection devices on both sides use the phase current, phase differential current and phase braking current calculated by the electrical quantities on both sides. in, is the current on both sides of the line, are phases A, B, and C, and m and n represent the two sides of the line; is the line phase differential current amplitude, is the phase braking current amplitude; The criterion for adaptive line current differential protection is: in, is the line phase differential current amplitude, is the phase braking current amplitude, k is the current ratio differential protection braking coefficient, which is automatically adjusted according to the load current information on both sides of the line before the fault; The adjustment method for the current ratio differential protection braking coefficient k is: Among them, I load is the line load current amplitude before the fault, that is I set The current differential protection setting set for the protection device; I N is the secondary rated value of CT; k1 and k2 are constants; I load ,I N ,I set All are quadratic values.

2. The adaptive low-frequency transmission line current differential protection method according to claim 1, characterized in that: The value range of k1 and k2 is 0.1 to 0.

6.

3. Adaptive low-frequency transmission line current differential protection system, characterized in that: Include: Instantaneous current collection unit: collects instantaneous currents on both sides of the low-frequency transmission line and connects to the protection device; Current calculation unit: uses the protection devices on both sides to exchange information and share the electrical quantities on both sides, and calculates the phase current, phase differential current and phase braking current on both sides of the low-frequency transmission line; Adaptive line current differential protection component unit: Combined with the load current information on both sides of the low-frequency transmission line before the fault, the ratio braking coefficient in the ratio braking differential protection is adaptively adjusted to form an adaptive line current differential protection; Judgment unit: Each phase performs fault judgment independently. When the relationship between the phase differential current and the braking current of a certain phase meets the adaptive line current differential protection judgment criterion, it is judged that a fault has occurred in the line area, the adaptive line current differential protection is activated, and the circuit breakers of the same phase on both sides of the low-frequency transmission line are tripped; Otherwise, the adaptive line current differential protection does not operate; The protection devices on both sides use the phase current, phase differential current and phase braking current calculated by the electrical quantities on both sides. in, is the current on both sides of the line, is the phase sequence, m and n represent the two sides of the line; is the line phase differential current amplitude, is the phase braking current amplitude; The criteria for adaptive line current differential protection are as follows: in, is the line phase differential current amplitude, is the phase braking current amplitude, k is the current differential protection braking coefficient, which is automatically adjusted according to the load current information on both sides of the line before the fault; The adjustment method for the current differential protection braking coefficient k is: Among them, I load is the line load current amplitude before the fault, that is I set The current differential protection setting set for the protection device; I N is the CT rated value; k1 and k2 are constants; I load ,I N ,I set All are quadratic values.

4. The adaptive low-frequency transmission line current differential protection system according to claim 3, characterized in that: The value range of k1 and k2 is 0.1 to 0.6.

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