Differential protection adaptability identification method for wind power transmission alternating current line through flexible direct current

Abstract

The invention discloses a differential protection adaptability identification method for a wind power flexible direct-current transmission AC line, and the method specifically comprises the steps: obtaining all system parameters, and building a simulation model of a wind power flexible direct-current transmission system; respectively simulating the fault working condition of two-phase interphase short circuit at the tail end of the sending-out alternating current line and the normal operation working condition of the system, acquiring the wind field side current and the direct current side current of the sending-out alternating current line required for judging the adaptability of current differential protection, and calculating the phase difference between the wind field side current and the direct current side current; the fundamental wave effective value and the phase difference of the wind field side current and the direct current side current are obtained, the current ratio of the two sides of an alternating current line is solved and sent out, the current ratio is compared with a braking coefficient in a current differential protection criterion, and differential protection adaptability judgment is carried out. The method is suitable for a scene in which large-scale new energy is transmitted out through a flexible direct current power transmission system, can effectively verify the correctness of the current differential protection action, and guarantees the safety and stability of the system.

Description

Technical Field

[0001] The invention belongs to the technical field of relay protection of electric power systems, and in particular relates to a method for identifying adaptability of differential protection of AC lines for wind power transmission via flexible direct current. Background Art

[0002] The consumption of wind energy resources mainly presents the characteristics of "large-scale centralized access, long-distance transmission, and large-scale consumption". The installed capacity of wind power generation shows an increasing trend year by year. It is generally connected to the grid through power electronic devices. The fault current of the wind farm exhibits weak feedback, so it can be regarded as a weak AC power source. The flexible direct current transmission system has the advantages of flexible controllability, low transmission loss, suitable for weak AC system transmission, and no phase change failure problems. Therefore, the flexible direct current transmission system has gradually become the best choice for wind farm grid connection.

[0003] The converters on both sides of the AC line of the wind farm through the flexible direct current transmission system contain a variety of control strategies, which makes the fault current on the wind farm side and the DC side present the characteristics of limited amplitude and controlled phase angle when the AC line fails, and lacks continuous and stable power frequency components. This is quite different from the fault characteristics of the traditional synchronous machine AC power supply, which will bring challenges and threats to the protection scheme based on the power supply characteristics of the traditional synchronous generator. Ratio-restrained current differential protection is generally used as the main protection for AC lines of 110kV and above because of its high reliability. In this scenario, the current differential protection has the risk of reduced sensitivity and weakened speed, which will be unfavorable to the consumption of wind energy resources and may even restrict the development of new power systems. If it can be accurately judged that the current differential protection refuses to operate in the AC line of the wind power through the flexible direct current transmission system, it can provide a theoretical basis for the proposal of a new principle of AC line protection, thereby effectively improving the safety and stability of the wind power through the flexible direct current transmission system and promoting the rapid development of new power systems. Therefore, it is urgent to introduce a method for adaptability identification of ratio-restrained current differential protection of AC lines in wind power transmission systems through flexible direct current transmission. Summary of the invention

[0004] In view of the above problems, the present invention provides a method for identifying the adaptability of differential protection of AC lines for transmitting wind power via flexible direct current.

[0005] A method for identifying adaptability of differential protection of AC lines for wind power transmission via flexible direct current transmission of wind power according to the present invention comprises the following steps:

[0006] Step 1: Obtain various system parameters and build a simulation model of the wind power transmission system via flexible direct current in PSCAD / EMTDC.

[0007] Step 2: Simulate the fault condition of two-phase short circuit at the end of the AC transmission line and the normal operation condition of the system respectively, obtain the wind farm side and DC side currents of the AC transmission line required for the adaptability judgment of the current differential protection, extract the fundamental effective value of the wind farm side and DC side currents, and calculate the phase difference between the wind farm side and DC side currents.

[0008] Step 3: According to the fundamental effective value and phase difference of the current on the wind farm side and the DC side obtained in step 2, calculate the current ratio R on both sides of the AC line, and compare it with the braking coefficient k in the current differential protection criterion; for the fault condition of two-phase short circuit at the end of the AC line, when R is less than the braking coefficient k, it is judged that the protection has the risk of refusal to operate; for the normal operation condition of the AC line, when R is greater than the braking coefficient k, it is judged that the protection has the risk of false operation.

[0009] Furthermore, various system parameters obtained in step 1 include: single-shot rated voltage, frequency, capacity, control parameters of machine-side converter and grid-side converter, impedance of filter and transformer of permanent magnet direct-drive wind turbine; control parameters, rated voltage, submodule capacitance of flexible DC transmission system; equivalent impedance and length of outgoing AC line.

[0010] Furthermore, the specific calculation formula for the phase difference between the fault current on the wind farm side and the DC side of the AC line sent out in step 2 is:

[0011] When an AB phase short circuit occurs in the outgoing AC line,

[0012] When a BC phase short circuit occurs in the outgoing AC line,

[0013] When a phase-to-phase short circuit occurs in the AC transmission line,

[0014] When the system is operating normally,

[0015] Where: i m + 、i m - They are respectively the positive sequence current and negative sequence current on the DC side of the transmitted AC line.

[0016] Furthermore, the calculation steps of the current ratio R on both sides of the AC line in step 3 are as follows:

[0017] When a short circuit fault occurs between phases AB in the outgoing AC line, Calculate R A and R B, and set R to R A and R B The smaller of the two.

[0018] in: Indicates phase A and B, They are the wind farm side and DC side of the AC line respectively. The effective value of the phase current fundamental wave, To send out the AC line when a two-phase short circuit occurs on the wind farm side and the DC side The phase difference between the phase currents.

[0019] When a short circuit fault occurs between two phases BC in the outgoing AC line, Calculate R B and R C , and set R to R B and R C The smaller of the two.

[0020] in: Indicates B and C phases.

[0021] When a CA two-phase short circuit occurs in the AC transmission line, Calculate R C and R A , and set R to R C and R A The smaller of the two.

[0022] in: Indicates C and A phases.

[0023] When the system is operating normally, Calculate R A , R B and R C , and set R to R A , R B , R C The smallest among them.

[0024] in: It represents phases A, B, and C, and Δθ is the phase difference between the current on the wind farm side and the DC side when the system is operating normally.

[0025] The beneficial technical effects of the present invention are:

[0026] The present invention extracts the fundamental effective value of the fault current on the wind farm side and the DC side of the AC transmission line, establishes a two-phase phase-to-phase short-circuit fault composite sequence network, and uses the symmetrical component method to derive the analytical expression of the fault current phase angle on both sides of the AC transmission line. The phase difference between the fault currents on both sides is quantified, and the adaptability of the ratio-braked current differential protection in the AC transmission line of the wind power via the flexible DC transmission system can be obtained by solving the current ratio on both sides and comparing it with the braking coefficient. The adaptability result of the ratio-braked current differential protection in the AC transmission line of the wind power via the flexible DC transmission system can be obtained. The invention is suitable for the transmission scenario of large-scale new energy via the flexible DC transmission system, and can effectively verify the correctness of the current differential protection action, providing a theoretical basis for the subsequent proposal of a new principle for the protection of AC transmission lines, and ensuring the safety and stability of the system, which is of great significance for the consumption of wind energy resources and the development of new power systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The present invention is a flow chart of the adaptability identification method of the AC line differential protection for wind power transmission via flexible direct current.

[0028] Figure 2 It is a structural diagram of the wind power flexible direct current transmission system of the present invention.

[0029] Figure 3 The figure is a curve diagram of the current ratio R on both sides when a BC phase short circuit fault occurs at the end of the AC line sent by the embodiment of the present invention. DETAILED DESCRIPTION

[0030] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0031] The process of the adaptive identification method of the AC line differential protection for wind power transmission via flexible direct current of the present invention is as follows: Figure 1 As shown, the following steps are included:

[0032] Step 1: Obtain various system parameters and build the following in PSCAD / EMTDC: Figure 2 The simulation model of wind power flexible direct current transmission system is shown.

[0033] The system parameters obtained include: single-shot rated voltage, frequency, capacity, control parameters of machine-side converter and grid-side converter, impedance of filter and transformer of permanent magnet direct-drive wind turbine; control parameters, rated voltage, submodule capacitance of flexible DC transmission system; equivalent impedance and length of outgoing AC line.

[0034] Step 2: Simulate the fault condition of two-phase short circuit at the end of the AC transmission line and the normal operation condition of the system respectively, obtain the wind farm side and DC side currents of the AC transmission line required for the adaptability judgment of the current differential protection, extract the fundamental effective value of the wind farm side and DC side currents, and calculate the phase difference between the wind farm side and DC side currents.

[0035] The FFT module in PSCAD / EMTDC is used to extract the effective value of the fundamental wave of the current on the wind farm side and the DC side respectively.

[0036] The specific calculation formula for the phase difference between the fault current on the wind farm side and the DC side of the AC line is:

[0037] When an AB phase short circuit occurs in the outgoing AC line,

[0038] When a BC phase short circuit occurs in the outgoing AC line,

[0039] When a phase-to-phase short circuit occurs in the AC transmission line,

[0040] When the system is operating normally,

[0041] Where: i m + 、i m - They are respectively the positive sequence current and negative sequence current on the DC side of the transmitted AC line.

[0042] Step 3: According to the fundamental effective value and phase difference of the current on the wind farm side and the DC side obtained in step 2, calculate the current ratio R on both sides of the AC line, and compare it with the braking coefficient k in the current differential protection criterion; for the fault condition of two-phase short circuit at the end of the AC line, when R is less than the braking coefficient k, it is judged that the protection has the risk of refusal to operate; for the normal operation condition of the AC line, when R is greater than the braking coefficient k, it is judged that the protection has the risk of false operation.

[0043] The calculation steps of the current ratio R on both sides of the outgoing AC line are:

[0044] When a short circuit fault occurs between phases AB in the outgoing AC line, Calculate R A and R B , and set R to R A and R B The smaller of the two.

[0045] in: Indicates phase A and B, They are the wind farm side and DC side of the AC line respectively. The effective value of the phase current fundamental wave, To send out the AC line when a two-phase short circuit occurs on the wind farm side and the DC side The phase difference between the phase currents.

[0046] When a short circuit fault occurs between two phases BC in the outgoing AC line, Calculate R B and R C , and set R to R B and R C The smaller of the two.

[0047] in: Indicates B and C phases.

[0048] When a CA two-phase short circuit occurs in the AC transmission line, Calculate R C and R A , and set R to R C and R A The smaller of the two.

[0049] in: Indicates C and A phases.

[0050] When the system is operating normally, Calculate R A , R B and R C , and set R to R A , R B , R C The smallest among them.

[0051] in: It represents phases A, B, and C, and Δθ is the phase difference between the current on the wind farm side and the DC side when the system is operating normally.

[0052] Simulation experiment:

[0053] In order to verify the identification method of the present invention, a simulation model of a wind power transmission system via flexible direct current was built in PSCAD / EMTDC. A BC two-phase short circuit fault was set to occur at the end of the AC transmission line at the 2nd second, and the fault duration was 0.06 seconds, to verify the adaptability of the current differential protection AC transmission line.

[0054] When a BC phase short circuit fault occurs at the end of the AC transmission line, the current ratio R curve on both sides is as follows: Figure 3 As shown, it can be seen that the curve of the B-phase current ratio R on the wind farm side and the DC side of the AC transmission line is continuously greater than the braking coefficient k, so it can be judged that the current differential protection has not failed to operate; the curve of the C-phase current ratio R on the wind farm side and the DC side of the AC transmission line fluctuates within a very small range above and below the braking coefficient k, so it can be judged that the current differential protection has failed to operate.

Claims

1. A method for identifying the adaptability of differential protection of AC lines for wind power transmission via flexible direct current, characterized in that: The following steps are involved: Step 1: Obtain various system parameters and build a simulation model of the wind power transmission system via flexible direct current in PSCAD / EMTDC; Step 2: Simulate the fault condition of two-phase short circuit at the end of the AC transmission line and the normal operation condition of the system respectively, obtain the wind farm side and DC side currents of the AC transmission line required for the adaptability judgment of the current differential protection, extract the fundamental effective value of the wind farm side and DC side currents, and calculate the phase difference between the wind farm side and DC side currents; Step 3: According to the fundamental effective value and phase difference of the current on the wind farm side and the DC side obtained in step 2, calculate the current ratio R on both sides of the AC line, and compare it with the braking coefficient k in the current differential protection criterion; for the fault condition of two-phase short circuit at the end of the AC line, when R is less than the braking coefficient k, it is judged that the protection has the risk of refusal to operate; for the normal operation condition of the AC line, when R is greater than the braking coefficient k, it is judged that the protection has the risk of false operation.

2. A method for identifying adaptability of differential protection of AC line for wind power transmission via flexible direct current according to claim 1, characterized in that: The various system parameters obtained in step 1 include: single-shot rated voltage, frequency, capacity, control parameters of the machine-side converter and the grid-side converter, impedance of the filter and the transformer of the permanent magnet direct-drive wind turbine; control parameters, rated voltage, submodule capacitance of the flexible direct current transmission system; equivalent impedance and length of the output AC line.

3. The method for identifying adaptability of differential protection of AC line for wind power transmission via flexible direct current according to claim 1 is characterized in that: The specific calculation formula for the phase difference between the fault current on the wind farm side and the DC side of the AC line sent in step 2 is: When an AB phase short circuit occurs in the outgoing AC line, When a BC phase short circuit occurs in the outgoing AC line, When a phase-to-phase short circuit occurs in the AC transmission line, When the system is operating normally, Where: i m + 、i m - They are respectively the positive sequence current and negative sequence current on the DC side of the transmitted AC line.

4. A method for identifying adaptability of differential protection of AC line for wind power transmission via flexible direct current according to claim 1, characterized in that: The calculation steps of the current ratio R on both sides of the AC line in step 3 are as follows: When a short circuit fault occurs between phases AB in the outgoing AC line, Calculate R A and R B , and set R to R A and R B The smaller of in: Indicates phase A and B, They are the wind farm side and DC side of the AC line respectively. The effective value of the phase current fundamental wave, To send out the AC line when a two-phase short circuit occurs on the wind farm side and the DC side Phase difference between phase currents; When a short circuit fault occurs between two phases BC in the outgoing AC line, Calculate R B and R C , and set R to R B and R C The smaller of in: Indicates B and C phases; When a CA two-phase short circuit occurs in the AC transmission line, Calculate R C and R A , and set R to R C and R A The smaller of in: Indicates C and A phases; When the system is operating normally, Calculate R A , R B and R C , and set R to R A , R B , R C the smallest among them; in: It represents phases A, B, and C, and Δθ is the phase difference between the current on the wind farm side and the DC side when the system is operating normally.

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