A method and system for line protection in a new energy transmission system via flexible direct current transmission.

Abstract

This invention relates to a line protection method and system for a new energy transmission system via flexible DC transmission, belonging to the field of power system relay protection technology. It solves the problems of unreliable operation and frequent false trips and failures to trip in existing line protection systems for new energy transmission systems via flexible DC transmission. The method includes collecting the voltage and current on both sides of the transmission line after a fault occurs, and then extracting the high-frequency component at frequency ω. f The voltage and current at that time; based on the high-frequency component frequency ω on both sides of the transmitting line. f The voltage and current at that time are obtained at a high frequency component frequency of ω. f The high-frequency inductance difference coefficient on both sides of the output line; based on the high-frequency component frequency ω f The high-frequency inductance difference coefficient on both sides of the transmitting line is compared with the fault identification criteria to determine whether an intra-zone fault has occurred in the transmitting line; if so, the protection action of the transmitting line is activated.

Description

Technical Field

[0001] This invention relates to the field of power system relay protection technology, and in particular to a line protection method and system for a new energy transmission system via flexible DC transmission. Background Technology

[0002] Wind power flexible DC transmission technology, as an effective solution to the uneven distribution of wind energy and load in my country, has broad development prospects. Its operational reliability will have a significant impact on the safety and stability of the power grid. The transient process of short-circuit current in wind farms is subject to dual constraints of electromagnetic physics and nonlinear control, and the amplitude of the short-circuit current is reduced due to the overcurrent withstand capability of power electronic equipment. Therefore, how to achieve reliable operation of wind power flexible DC transmission systems and prevent protection maloperation and failure to operate is of great practical significance for the safe operation of actual systems.

[0003] Transient quantity protection mainly utilizes the transient characteristics of faults to construct protection criteria, which can significantly shorten the protection time. Therefore, transient quantity protection has become one of the main research directions in transmission line protection. Transient quantity protection can be mainly divided into three categories: transient traveling wave protection, protection based on single-ended transient quantities, and protection based on double-ended transient quantities. Transient traveling wave protection mainly uses the transient traveling wave generated by the line fault, i.e., the fault component of the traveling wave, to construct protection criteria. Traveling wave protection usually determines the fault direction by comparing the polarity relationship between the initial traveling wave voltage and current or by using the amplitude ratio of the forward and reverse traveling waves. However, the traveling wave front signal has a short existence time, is difficult to acquire, and is easily interfered with, thus having certain limitations. Protection based on single-ended transient quantities utilizes the transient components of the fault to reflect the transient process of the fault. It has a short operating time and ultra-high-speed operating performance, requiring the processor to have a high sampling rate. However, protection based on single-ended transient quantities cannot accurately identify different types of faults. Protection based on two-terminal transient quantities constructs a protection scheme by comparing two-terminal transient information. It has a fast response speed, can identify different types of faults, and has strong resistance to transition resistance and noise. However, it usually requires two-terminal electrical quantity interaction and has high requirements for the synchronization rate of data acquisition devices.

[0004] Therefore, the existing protection system for new energy transmission via flexible direct current is unreliable and prone to malfunctions and failures to operate. Summary of the Invention

[0005] Based on the above analysis, the embodiments of the present invention aim to provide a line protection method and system for new energy transmission via flexible DC transmission, in order to solve the problems of unreliable operation of existing line protection in new energy transmission via flexible DC transmission, which are prone to false tripping and failure to trip.

[0006] On one hand, embodiments of the present invention provide a line protection method for a new energy transmission system via flexible direct current, comprising:

[0007] After a fault occurs, the voltage and current on both sides of the transmission line of the new energy source via the flexible DC transmission system are collected, and then the high-frequency component with frequency ω is extracted. f Voltage and current at that time;

[0008] Based on the high-frequency component frequency ω on both sides of the transmission line f The voltage and current at that time are obtained at a high frequency component frequency of ω. f The high-frequency inductance difference coefficient on both sides of the output line at that time;

[0009] Based on the high-frequency component frequency of ω f The high-frequency inductance difference coefficient on both sides of the transmitting line is compared with the fault identification criteria to determine whether an intra-zone fault has occurred in the transmitting line; if so, the protection action of the transmitting line is activated.

[0010] Furthermore, the wind farm side of the transmission line is designated as the M side, and the flexible DC system side is designated as the N side; the high-frequency inductance difference coefficient between the two sides of the transmission line is obtained in the following manner:

[0011] Based on the system structure and fault analysis of the M and N sides of the transmitting line, the frequency of the high-frequency component during the fault transient process is obtained as ω. f The equivalent total impedance of the wind farm and the equivalent impedance of the MMC converter are obtained, and then the high-frequency component frequency ω is obtained at the M and N sides of the transmission line. f Calculated inductance value at that time;

[0012] Based on the high-frequency component frequency ω on the M and N sides of the aforementioned transmission lines. f The voltage and current at that time are used to obtain the high-frequency component frequency ω on the M and N sides of the transmitting line. f Inductance measurement value at that time;

[0013] Based on the high-frequency component frequency ω on the M and N sides of the aforementioned transmission lines. f The calculated and measured inductance values ​​at time ω are used to obtain the high-frequency component values ​​at the M and N sides of the transmitting line. f The high-frequency inductance difference coefficient at that time.

[0014] Furthermore, the high-frequency component frequency of the transmitting lines M and N is ω. f The high-frequency inductance difference coefficients at different times are expressed as follows:

[0015]

[0016] In the formula, S m (ω f ), S n (ω f ) represent the high-frequency components at the M and N sides of the transmitting lines, respectively, with a frequency of ω. fThe high-frequency inductance difference coefficient, where H represents the number of sampling points in one cycle after the fault occurs, and L... fm,h (ω f L fn,h (ω f ) represent the high-frequency components with frequencies ω at the h-th sampling point on the M and N sides of the transmitting lines, respectively. f The inductance measurement value at that time, L DFIG (ω f L mmc (ω f ) represent the high-frequency components at the M and N sides of the transmitting lines, respectively, with a frequency of ω. f The inductance value at that time.

[0017] Furthermore, the high-frequency component frequency is ω f The fault identification criteria at that time include:

[0018]

[0019] In the formula, S set (ω f ) indicates that the frequency of the high-frequency component is ω f Action threshold value at time;

[0020] If S m (ω f ) and S n (ω f If all of the above conditions are met, the fault is determined to be within the sending line area; otherwise, it is determined to be outside the sending line area.

[0021] Furthermore, if the fault is determined to be outside the transmission line area, then

[0022] If S m (ω f If the fault identification criteria are not met, it is determined that a fault has occurred in the back-side system of the M-side bus of the outgoing line;

[0023] If S n (ω f If the fault identification criteria are not met, the fault is determined to be a fault in the back-side system of the N-side bus of the sending line.

[0024] Furthermore, the frequency of the high-frequency component ω is obtained in the following way. f Action threshold value S at time set (ω f ):

[0025] Let a represent the percentage of fault locations on the transmission line; where a ranges from 0 to 100%.

[0026] By changing the value of 'a' in the following formula, we obtain the action threshold value set S.s a et :

[0027]

[0028] in,

[0029]

[0030] In the formula, S set,a (ω f ) indicates that the percentage of fault location on the M and N sides of the transmitting line is 'a', and the high-frequency component frequency is ω. f The action threshold value at time, L fm,h′ (ω f ) indicates that on the M side of the transmitting line, at any sampling point h′, the high-frequency component frequency is ω. f Inductance measurement value at that time;

[0031] Selecting a set of action threshold values The maximum or minimum value in the high-frequency component is taken as the value at frequency ω. f Action threshold value S at time set (ω f ).

[0032] Furthermore, the frequency ω of the high-frequency component f Values Determined in the following ways

[0033]

[0034] in,

[0035] ω m ={ω f |f'(ω f )=0,f(ω) f <0}

[0036] f(ω f )=(L mmc (ω f )-L DFIG (ω f )) 2

[0037] In the formula, max() represents taking the maximum value.

[0038] Furthermore, the high-frequency component frequency of the transmitting line M is ω. f Calculated inductance value L at time DFIG (ω f ) is represented as:

[0039]

[0040] In the formula, c9, c7, c5, c3, c1, d8, d6, d4, d2, and d0 represent constants calculated based on the parameters of each component on the M side of the transmission line.

[0041] Furthermore, the N-side of the transmitting line has a high-frequency component frequency of ω. f Calculated inductance value L at time mmc (ω f ) is represented as:

[0042]

[0043] In the formula, a7, a5, a3, a1, b6, b4, b2, and b0 represent constants calculated based on the parameters of each component on the N side of the transmission line.

[0044] On the other hand, embodiments of the present invention provide a line protection system for a new energy transmission system via flexible direct current, comprising:

[0045] The data acquisition module is used to collect the voltage and current on both sides of the transmission line of the new energy source via the flexible DC transmission system after a fault occurs, and then extract the high-frequency component with a frequency of ω. f Voltage and current at that time;

[0046] The high-frequency inductance difference coefficient module is used to base the difference on both sides of the output line at a high-frequency component frequency of ω. f The voltage and current at that time are obtained at a high frequency component frequency of ω. f The high-frequency inductance difference coefficient on both sides of the output line at that time;

[0047] The fault identification module is used to identify faults based on the frequency of the high-frequency component ω. f The high-frequency inductance difference coefficient on both sides of the transmission line is compared with the fault identification criteria to determine whether an intra-zone fault has occurred in the transmission line.

[0048] The action protection module is used to activate the protection action of the transmission line if a fault occurs within the zone of the transmission line.

[0049] Compared with the prior art, the present invention can achieve at least the following beneficial effects:

[0050] This invention provides a method and system for protecting the lines of a new energy transmission system via flexible DC transmission. By collecting data after a fault occurs, the system obtains data at the high-frequency component frequency, and then obtains the high-frequency inductance difference coefficient. Based on the fault identification criteria, it accurately identifies faults inside and outside the transmission area. It only requires the transmission of the fault direction judgment results from both sides of the line, rather than electrical quantity information, and has low requirements for data synchronization. It effectively solves the problem of incorrect operation of the line protection in the new energy transmission system via flexible DC transmission. It has strong tolerance to fault resistance and high sensitivity to high-resistance faults occurring at the end of the line.

[0051] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0052] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0053] Figure 1 This is a schematic flowchart of the line protection method for a new energy transmission system via flexible direct current transmission provided in Embodiment 1 of the present invention;

[0054] Figure 2 This is a schematic diagram of the wind power transmission flexible DC system provided in Embodiment 1 of the present invention;

[0055] Figure 3 This is the MMC converter station topology provided in Embodiment 1 of the present invention;

[0056] Figure 4 This refers to the complex frequency domain operation circuit for short-circuit faults on the AC side of the MMC provided in Embodiment 1 of the present invention;

[0057] Figure 5 This refers to the equivalent fault model of the MMC converter provided in Embodiment 1 of the present invention;

[0058] Figure 6 This is a schematic diagram of the doubly fed wind farm structure provided in Embodiment 1 of the present invention;

[0059] Figure 7 This is the equivalent model of a single doubly fed wind turbine provided in Embodiment 1 of the present invention;

[0060] Figure 8 This is a model of the doubly fed wind farm in Embodiment 1 of the present invention;

[0061] Figure 9This is the equivalent model of the doubly fed wind farm provided in Embodiment 1 of the present invention;

[0062] Figure 10 This is the equivalent frequency domain model of the outgoing line fault system provided in Embodiment 1 of the present invention;

[0063] Figure 11 This refers to the fault component network for intra-regional faults provided in Embodiment 1 of the present invention.

[0064] Figure 12 This refers to the fault component network for external faults provided in Embodiment 1 of the present invention;

[0065] Figure 13 This refers to the fault component network for external faults provided in Embodiment 1 of the present invention;

[0066] Figure 14 (a) and (b) are respectively the high-frequency inductance difference coefficients at the M and N side protection installation locations when a single-phase ground fault occurs in the zone via phase A in Embodiment 3 of the present invention.

[0067] Figure 14 (c) and (d) are respectively the high-frequency inductance difference coefficients at the M and N side protection installation points when there is a phase-to-phase fault in the BC two-phase area provided in Embodiment 3 of the present invention.

[0068] Figure 14 (e) and (f) are respectively the high-frequency inductance difference coefficients at the M and N side protection installation locations when the fault occurs in the ABC three-phase area within the zone, as provided in Embodiment 3 of the present invention.

[0069] Figure 15 (a) and (b) are respectively the high-frequency inductance difference coefficients at the M and N side protection installation locations when there is a phase-to-phase fault in phase BC within the zone provided in Embodiment 3 of the present invention.

[0070] Figure 16 (a) and (b) are respectively the high-frequency inductance difference coefficients at the M and N side protection installation points when the three-phase grounding fault occurs at the M-side bus back-side outlet of Embodiment 3 of the present invention.

[0071] Figure 17 (a) and (b) are the high-frequency inductance difference coefficients at the M and N side protection installation points when there is a single-pole grounding fault of MMC at 50% of the flexible DC line provided in Embodiment 3 of the present invention. Detailed Implementation

[0072] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0073] Example 1

[0074] A specific embodiment of the present invention discloses a line protection method for a new energy transmission system via flexible direct current transmission, such as... Figure 1 As shown, it includes:

[0075] S1. Collect the voltage and current on both sides of the transmission line of the new energy source via the flexible DC transmission system after the fault occurs, and then extract the high-frequency component with a frequency of ω. f The voltage and current at that time.

[0076] Specifically, current and voltage are collected using current transformers installed at the protection points on both sides of the transmission line, and then the high-frequency components at a frequency of ω are extracted using existing technologies. f The voltage and current at that time.

[0077] S2, based on the high-frequency component frequency ω on both sides of the transmission line. f The voltage and current at that time are obtained at a high frequency component frequency of ω. f The high-frequency inductance difference coefficient on both sides of the output line at that time;

[0078] In implementation, in step S2, the wind farm side of the transmission line is designated as side M, and the flexible DC system side is designated as side N; the high-frequency inductance difference coefficient between the two sides of the transmission line is obtained in the following way:

[0079] Based on the system structure and fault analysis of the M and N sides of the transmitting line, the frequency of the high-frequency component during the fault transient process is obtained as ω. f The equivalent total impedance of the wind farm and the equivalent impedance of the MMC converter are obtained, and then the high-frequency component frequency ω is obtained at the M and N sides of the transmission line. f Calculated inductance value at that time;

[0080] Based on the high-frequency component frequency ω on the M and N sides of the aforementioned transmission lines. f The voltage and current at that time are used to obtain the high-frequency component frequency ω on the M and N sides of the transmitting line. f Inductance measurement value at that time;

[0081] Based on the high-frequency component frequency ω on the M and N sides of the aforementioned transmission lines. f The calculated and measured inductance values ​​at time ω are used to obtain the high-frequency component values ​​at the M and N sides of the transmitting line. f The high-frequency inductance difference coefficient at that time.

[0082] In specific implementation, the M and N sides of the transmitting lines have a high-frequency component frequency of ω. f The high-frequency inductance difference coefficients at different times are expressed as follows:

[0083]

[0084] In the formula, S m (ω f ), S n(ω f ) represent the high-frequency components at the M and N sides of the transmitting lines, respectively, with a frequency of ω. f The high-frequency inductance difference coefficient, where H represents the number of sampling points in one cycle after the fault occurs, and L... fm,h (ω f L fn,h (ω f ) represent the high-frequency components with frequencies ω at the h-th sampling point on the M and N sides of the transmitting lines, respectively. f The inductance measurement value at that time, L DFIG (ω f L mmc (ω f ) represent the high-frequency components at the M and N sides of the transmitting lines, respectively, with a frequency of ω. f The inductance value at that time.

[0085] Specifically, the M-side of the transmitting line has a high-frequency component frequency of ω. f Calculated inductance value L at time DFIG (ω f ) is represented as:

[0086]

[0087] In the formula, c9, c7, c5, c3, c1, d8, d6, d4, d2, and d0 represent constants calculated based on the parameters of each component on the M side of the transmission line.

[0088] Specifically, the N-side of the transmitting line has a high-frequency component frequency of ω. f Calculated inductance value L at time mmc (ω f ) is represented as:

[0089]

[0090] In the formula, a7, a5, a3, a1, b6, b4, b2, and b0 represent constants calculated based on the parameters of each component on the N side of the transmission line.

[0091] S3, based on the high-frequency component frequency being ω f The high-frequency inductance difference coefficient on both sides of the transmitting line is compared with the fault identification criteria to determine whether an intra-zone fault has occurred in the transmitting line; if so, the protection action of the transmitting line is activated.

[0092] In implementation, the high-frequency component frequency is ω f The fault identification criteria at that time include:

[0093]

[0094] In the formula, S set (ω f) indicates that the frequency of the high-frequency component is ω f Action threshold value at time;

[0095] If S m (ω f ) and S n (ω f If all of the above conditions are met, the fault is determined to be within the sending line area; otherwise, it is determined to be outside the sending line area.

[0096] Specifically, if the fault is determined to be outside the transmission line area, then

[0097] If S m (ω f If the fault identification criteria are not met, it is determined that a fault has occurred in the back-side system of the M-side bus of the outgoing line;

[0098] If S n (ω f If the fault identification criteria are not met, the fault is determined to be a fault in the back-side system of the N-side bus of the sending line.

[0099] Specifically, the frequency of the high-frequency component ω is obtained in the following way. f Action threshold value S at time set (ω f ):

[0100] Let a represent the percentage of fault locations on the transmission line; where a ranges from 0 to 100%.

[0101] By changing the value of 'a' in the following formula, we obtain the set of action threshold values.

[0102]

[0103] in,

[0104]

[0105] In the formula, S set,a (ω f ) indicates that the percentage of fault location on the M and N sides of the transmitting line is 'a', and the high-frequency component frequency is ω. f The action threshold value at time, L fm,h′ (ω f ) indicates that on the M side of the transmitting line, at any sampling point h′, the high-frequency component frequency is ω. f Inductance measurement value at that time;

[0106] Selecting a set of action threshold values The maximum or minimum value in the high-frequency component is taken as the value at frequency ω. f Action threshold value S at time set (ωf ).

[0107] Preferably, the frequency ω of the high-frequency component f Values Determined in the following ways

[0108]

[0109] in,

[0110] ω m ={ω f |f'(ω f )=0,f(ω) f <0}

[0111] f(ω f )=(L mmc (ω f )-L DFIG (ω f )) 2

[0112] In the formula, max() represents taking the maximum value.

[0113] To facilitate a better understanding of the formation process of the solution in this embodiment by those skilled in the art, the following will be used as an example. Figure 2 The wind power transmission connection to the flexible DC system shown is Figure 3 Taking the MMC converter station topology shown as an example, the working principle of a line protection method for a new energy transmission system via flexible DC transmission provided in this embodiment is explained as follows:

[0114] Figure 3 for Figure 2 The diagram shows the structure of the MMC converter in the wind power transmission to flexible DC system. Figure 3 Middle,U dc For the flexible DC side voltage, i va For the A-phase current on the AC side of the converter, u va i is the voltage at the A-phase port of the converter. pa i na Let LA be the upper and lower arm currents of phase A of the converter, L0 be the converter arm inductance, and R0 be the equivalent resistance of the converter arm. s For the AC side inductance of the converter, SM represents the submodule on the bridge arm, and n1 represents the number of submodules on the upper or lower bridge arm of each phase.

[0115] Assuming a short-circuit fault occurs at the AC side outlet, a Laplace transform is performed to the complex frequency domain, yielding the fault complex frequency domain operational circuit for the AC side fault, as follows: Figure 4 As shown in the figure. L sa For the AC side inductance of phase A of the converter, i va(0) represents the initial value of the phase A current on the AC side of the converter, n pa n na The number of sub-modules deployed in the upper and lower arms of phase A on the AC side of the converter, n pa (0), n na (0) represents the initial number of upper and lower bridge arm submodules deployed on phase A of the AC side of the converter, respectively; C0 represents the bridge arm module capacitance; i pa (0), i na (0) represents the initial current values ​​of the upper and lower arms of phase A on the AC side of the converter, respectively. va (0) represents the initial value of the phase A current on the AC side of the converter, U c This refers to the voltage corresponding to the capacitor in a single submodule.

[0116] Depend on Figure 4 As can be seen, taking phase A as an example, based on the voltage and current relationship between the upper and lower arms of the AC side of the converter, we can obtain:

[0117] u sa (s)+sL sa i va (s)=u va (s)+L sa i va (0) (1)

[0118]

[0119]

[0120] i pa (s)=i va (s)+i na (s) (4)

[0121] In the formula, s represents the Laplace operator, u sa (s) represents the output voltage at the A-phase port of the converter.

[0122] It should be noted that in this embodiment, the parameters in the formula include the suffix (s) to indicate that they are in the complex frequency domain form.

[0123] By combining equations (1) to (4), we can obtain:

[0124]

[0125] In the formula, N represents the sum of the total number of submodules in the upper and lower bridge arms.

[0126] If the MMC converter uses nearest-level approximation modulation, then:

[0127]

[0128]

[0129] In the formula, u va * This represents the reference value of the A-phase port voltage of the converter, which is output through the control loop; round(x) means taking the largest integer less than x.

[0130] By combining equations (1)-(7), we can calculate:

[0131]

[0132] According to equation (8), the equivalent model of the voltage and short-circuit current at the A-phase port of the MMC converter during an AC line fault can be obtained, as follows: Figure 5 As shown:

[0133] u va (s)=U mmc (s)-i va (s)Z mmc (s) (9)

[0134] In the formula, U mmc (s) represents the equivalent voltage source of the MMC converter during the fault transient process in the complex frequency domain, Z mmc (s) represents the equivalent impedance of the MMC converter during the fault transient process in the complex frequency domain, and its expressions are as follows:

[0135]

[0136]

[0137] In this embodiment, the doubly-fed wind farm consists of multiple doubly-fed wind turbine units. Each unit within the wind farm is connected to the wind farm's main bus via a collector line, and the power is stepped up by the wind farm's main transformer before being transmitted. Its structure is as follows: Figure 6 As shown.

[0138] To obtain a detailed expression for the short-circuit current in a wind farm, it is first necessary to derive the relationship between the short-circuit current and the port voltage of a single wind turbine when a short-circuit fault occurs.

[0139] The voltage and flux linkage equations for a single DFIG unit in the wind farm in the dq coordinate system are as follows:

[0140]

[0141]

[0142] Where ω=ω1-ω r .

[0143] In the formula, ω1 represents the synchronous angular velocity, and ω represents the sum of the synchronous angular velocity ω1 and the rotor angular velocity ω. rThe difference; L m L s L r These represent the equivalent magnetizing inductance, stator inductance, and rotor inductance, respectively. sd u sq Let d and q components, u, represent the stator voltage of the doubly-fed generator, respectively. rd u rq Let i represent the d and q components of the rotor voltage of the doubly-fed generator, respectively. sd i sq Let i represent the d and q components of the stator current of the doubly-fed generator, respectively. rd i rq Let d and q represent the rotor current components of the doubly-fed generator, respectively, and ψ sd ψ sq Let d and q be the stator flux linkages of the doubly-fed generator, respectively, and ψ be the d and q components. rd ψ rq Represent the d and q components of the rotor flux linkage of the doubly-fed generator, respectively, and R s R represents the stator-side resistance. r This indicates the rotor-side resistance.

[0144] Considering the time scale of the control loop, the outer loop control can be ignored during the fault transient process, and the given reference value is continuously output. Only the doubly-fed induction generator (DFIG) current loop control loop is considered:

[0145]

[0146] in,

[0147]

[0148] In the formula, These represent the d-axis and q-axis reference values ​​of the rotor voltage of the doubly-fed generator, respectively. rdref i rqref These represent the d-axis and q-axis reference values ​​of the rotor current, respectively, and k p k i These represent the proportional coefficient and integral coefficient on the rotor side of the doubly-fed generator, respectively.

[0149] By combining equations (12) to (15), we can obtain the relationship between the port voltage and the short-circuit current when a single wind turbine experiences a short-circuit fault:

[0150]

[0151] in,

[0152] α1=L r +R r τ s +k p τ s (17)

[0153] α2=R r +k i τ s +k p (18)

[0154]

[0155]

[0156]

[0157]

[0158] In the formula, U sd U sq Representing the d-axis and q-axis components of the fan port voltage, respectively, i s0d i s0q Represent the d-axis and q-axis components of the grid-side current caused by the voltage drop at the wind turbine port, respectively. ss1d i ss1q Let i represent the d-axis and q-axis components of the first grid-side current caused by grid-side converter control, respectively. sr1d i sr1q These represent the d-axis and q-axis components of the first grid-side current caused by rotor-side converter control, respectively; i ss2d i ss2q Let d and q axis components of the second grid-side current caused by grid-side converter control be represented respectively, in the time domain as a function of time constant τ. s 'attenuation; i ss3d i ss3q Let d and q axis components of the third grid-side current caused by grid-side converter control be represented respectively, in the time domain as a function of time constant τ. r 'attenuation; i sr2d i sr2q Let L represent the d-axis and q-axis components of the second grid-side current caused by the rotor-side converter control, respectively, in the time domain as a function of the time constant L. r τ s ' / α1 decay; i sr3d i sr3q i sr3d Let L represent the d-axis and q-axis components of the third grid-side current caused by the rotor-side converter control, respectively, in the time domain as a function of the time constant L. r τ s ' / α2 decay.

[0159] In equation (16), each short-circuit current fault component is expressed as follows:

[0160]

[0161]

[0162]

[0163]

[0164]

[0165]

[0166]

[0167]

[0168]

[0169]

[0170]

[0171]

[0172] in,

[0173] α3=(R r +k p )ω1τ s +ωL r (1-σ) (35)

[0174]

[0175] In the formula, k represents the voltage drop degree, u s0 This indicates the voltage at the wind turbine's port before the fault.

[0176] When a power grid fault occurs, the voltage phase angle will change abruptly. At this time, the PLL control system cannot perfectly track the power grid phase angle, meaning the output reference value will deviate from the actual power grid phase angle. Considering the dynamic error of the phase-locked loop (PLL), i.e., using the actual PLL output phase angle instead of the synchronization speed when calculating the short-circuit current, as shown in the following equation:

[0177]

[0178] In the formula, k pll-p k pll-i These represent the proportional coefficient and integral coefficient of the phase-locked loop control element, respectively.

[0179] By combining equations (16) and (37) and applying the Parker transformation, we can obtain the relationship between fault current and port voltage in the abc three-phase coordinate system.

[0180] I g (s)=Ug (s) / Z g +I μ (s) (38)

[0181] In the formula, I g U represents the feed current of the doubly fed wind turbine. g Z represents the port voltage of the doubly fed wind turbine. g I represents the equivalent impedance of a doubly-fed wind turbine. μ This represents the equivalent current inside the doubly fed fan caused by the control loop during a fault, and its value is independent of the port voltage.

[0182] Analysis shows that the current fault component of a doubly-fed induction generator (DFIG) consists of a fault component caused by a voltage drop at the port and a fault component determined by the controlled circuit (manifesting as a current source). An equivalent model of a single DFIG can be obtained, such as... Figure 7 As shown. According to Figure 6 The wind field structure shown can be used to obtain a model of a doubly-fed wind field, such as... Figure 8 As shown in the figure. R i Let L be the line resistance value of the collector line containing the i-th fan, where i = 1, 2, ..., n, and n represents the total number of fans; i U is the line reactance value of the collector line where the i-th wind turbine is located. M I is the grid connection point voltage of the PCC. m To supply the line current, L T U is the reactance value of the main transformer in the wind farm. m For the voltage of the wind farm side bus of the transmission line, Z gi Let I be the equivalent impedance of the i-th doubly-fed wind turbine. μi The current is the equivalent current inside the i-th doubly fed fan caused by the control loop during a fault.

[0183] It should be noted that the collector wires and the doubly fed wind turbines are one-to-one correspondences.

[0184] for Figure 8 The doubly fed wind field model shown has the following:

[0185]

[0186] in,

[0187] I m =I g1 +I g2 +…+I gn (40)

[0188] In the formula, U gi I represents the port voltage of the i-th wind turbine; gi This represents the feed current of the i-th doubly fed wind turbine.

[0189] By combining equations (38) to (40), we can obtain:

[0190]

[0191] In the formula, L l R represents the line reactance. l This indicates the line resistance.

[0192] The simplified equivalent wind field model can be obtained from equation (41), such as Figure 9 As shown:

[0193] Depend on Figure 9 It can be seen that,

[0194]

[0195]

[0196] In the formula, U DFIG Z represents the equivalent current source of a wind farm. DFIG This represents the equivalent total impedance of a wind farm.

[0197] Based on the above analysis, the complex frequency domain models of the wind farm and the flexible DC converter under transmission line fault conditions are obtained. Therefore, the system model can be obtained, such as... Figure 10 As shown.

[0198] When a fault occurs in the transmission line, according to equation (11), s=jω f Substituting into equation (11), the frequency ω of the high-frequency component is extracted. f By using coefficients of the same degree, we can obtain the impedance expression Z of the MMC converter. mmc (ω f )for:

[0199]

[0200] In the formula, a7, a6, a5, a4, a3, a2, a1, b6, b4, b2 and b0 are constants calculated based on the parameters of each component on the N side of the transmission line. a7, a6 and b6 are all greater than zero, that is, they are obtained based on formula (11).

[0201] Its inductance expression L mmc (ω f )for:

[0202]

[0203] According to equation (45), the frequency band ω when the fault equivalent impedance of the MMC converter station is inductive and capacitive can be obtained. mmcL ω mmcC They are respectively:

[0204]

[0205]

[0206] In the formula, ω 1.1 ω 1.2 ω 1.3 ω 1.4 a7ω 7 +a5ω 5 +…+a1ω=0 non-negative solutions; ω 2.1 ω 2.2 ω 2.3 a6ω 6 +a4ω 4 The non-negative solution of +…+a0=0.

[0207] When a fault occurs in the transmission line, according to equation (43), s=jω f Substituting into equation (43) and extracting the coefficients of the same degree ω, we can obtain the equivalent total impedance expression Z of the wind field. DFIG (ω f )for:

[0208]

[0209] In the formula, c9, c8, c7, c6, c5, c4, c3, c2, c1, c0, d8, d7, d6, d5, d4, d3, d2, d1, and d0 are constants calculated based on the parameters of each component on the M side of the transmission line. c9, c8, and d8 are all greater than zero, which can be obtained according to formula (43).

[0210] Its inductance expression L DFIG (ω f )for:

[0211]

[0212] According to equation (49), the frequency band ω when the equivalent impedance of the wind farm fault is inductive and capacitive can be obtained. DFIGC ω DFIGL They are respectively:

[0213]

[0214]

[0215] In the formula, ω 3.1 ω 3.2 ω 3.3 ω 3.4 c9ω 9 +c7ω 7 +c5ω 5 +…+c1ω=0 non-negative solutions; ω4.1 ω 4.2 ω 4.3 c8ω 8 +c6ω 6 +c4ω 4 The non-negative solution of +…+c0=0.

[0216] According to equations (46) and (50), the maximum range of the wind field and the inductive frequency band is positive infinity, while the range of the capacitive frequency band is finite. Therefore, compared to the information in the capacitive frequency band, the inductive frequency band is wider, and more voltage and current information from different frequency bands can be utilized. Combining equations (50) and (51), the maximum range of the frequency band of the larger transient component is also positive infinity. Therefore, the overlap area between the inductive frequency band and the frequency band of the larger transient component is larger. Thus, in this embodiment, an appropriate frequency harmonic is selected, and the protection principle is constructed using a fault component network composed of transient models exhibiting resistivity and inductance.

[0217] It is understood that, in this embodiment, based on the electrical parameters and control links of the wind turbine generator and the MMC converter, a transient frequency domain model of the wind turbine generator fault and a transient frequency domain model of the MMC converter fault were established, and the characteristic frequency bands of the wind turbine generator and the MMC converter were accurately analyzed.

[0218] When a fault occurs within the transmission line area, the fault component network of the wind farm's flexible direct transmission system is as follows: Figure 11 As shown in the figure, Z DFIG Z is the transient equivalent impedance of the wind field; mmc Z is the transient equivalent impedance of MMC; mn The output line impedance is denoted by 'a', which represents the percentage of the fault location. R represents the output line impedance. f This is the transition resistance.

[0219] according to Figure 11 It can be seen that the voltage and current on the M side of the transmitting line have the following relationship:

[0220]

[0221] In the formula, I′ m 、I′ n These represent the current measurement values ​​at the protection installation points on the M and N sides of the transmitting line, respectively; U′ m 、U′ n These represent the voltage measurement values ​​at the protection installation points on the M and N sides of the transmitting line, respectively.

[0222] Therefore, the expression for the inductance measurement value L on the M side of the transmitting line is defined. fm for:

[0223]

[0224] In the formula, R DFIGLet represent the fault equivalent resistance of the wind farm, and Im represent the imaginary part.

[0225] Define the expression for the inductance measurement value L on the N side of the output line. fn for:

[0226]

[0227] In the formula, R mmc L represents the fault equivalent resistance of the MMC converter station. mmc This refers to the fault equivalent reactance of the MMC converter station.

[0228] When a fault occurs within the transmission line area, the following applies:

[0229]

[0230] In the formula, aL mn (ω f (1-a)L represents the difference between the measured and calculated inductance values ​​on the M side of the transmitting line. mn (ω f The value is the difference between the measured and calculated inductance values ​​on the N side of the transmission line, and is related to the fault location a.

[0231] When a fault occurs outside the M-side zone of the outgoing line, taking a fault at the back-side outlet of the M-side bus as an example, the fault component network of the wind farm's flexible direct transmission system is as follows: Figure 12 As shown.

[0232] according to Figure 12 It can be seen that the voltage and current on the M side of the transmitting line have the following relationship:

[0233]

[0234] In the formula, I f For the transition resistor R f Fault current flowing to the grounding point.

[0235] When an external fault occurs on the M side of the transmitting line, the following applies:

[0236]

[0237] When a fault occurs at the back-side outlet of the busbar on the M side of the transmitting line, the measured inductance value on the M side of the transmitting line deviates from the calculated value by a large margin, while the measured inductance value on the N side of the transmitting line is close to the calculated value with a small margin.

[0238] When a fault occurs outside the N-side zone of the transmission line, taking a fault at the back-side outlet of the N-side busbar of the transmission line as an example, the fault component network of the wind farm's flexible direct transmission system is as follows: Figure 13 As shown.

[0239] according to Figure 13It can be seen that the voltage and current on the M side of the transmitting line have the following relationship:

[0240]

[0241] In the formula, I f This is the fault current flowing to the grounding point through the transition resistor Rf.

[0242] When a fault occurs outside the N-side zone of the transmitting line, the following applies:

[0243]

[0244] When a fault occurs at the back-side outlet of the N-side busbar of the transmitting line, the measured inductance value of the N-side of the transmitting line deviates from the calculated value by a large margin, while the measured inductance value of the M-side of the transmitting line is close to the calculated value by a small margin.

[0245] Based on the above analysis, when a fault occurs within the transmission line zone, the measured inductance values ​​of the protection on both sides of the line match the calculated values, with a theoretical error of 0. However, when a fault occurs outside the system zone on the back side of the transmission line busbar, the measured inductance values ​​of the protection on that busbar side do not match the actual values, showing a significant difference.

[0246] Therefore, fault identification is performed by using the high-frequency inductance difference coefficient on the M and N sides of the transmission line. The high-frequency inductance difference coefficient on both sides is expressed as:

[0247]

[0248] In the formula, S m (ω f ), S n (ω f ) represent the high-frequency components at the M and N sides of the transmitting lines, respectively, with a frequency of ω. f The high-frequency inductance difference coefficient, where H represents the number of sampling points in one cycle after the fault occurs, and L... fm,h (ω f L fn,h (ω f ) represent the high-frequency component with frequency ω at the h-th sampling point on the M and N sides of the transmitting lines, respectively. f The inductance measurement value at that time, L DFIG (ω f L mmc (ω f ) represent the high-frequency components at the M and N sides of the transmitting lines, respectively, with a frequency of ω. f The inductance value at that time.

[0249] The inductance measurements on the M and N sides of the transmitting line are calculated based on the voltage and current data on the M and N sides; L mmc L is calculated from equation (45). DFIG It is calculated from equation (49).

[0250] In summary, the fault identification criteria that can be constructed include:

[0251]

[0252] In the formula, S set (ω f ) indicates that the frequency of the high-frequency component is ω f Action threshold value at time;

[0253] If S m (ω f ) and S n (ω f If all conditions are met, the fault is determined to be within the sending line area; otherwise, it is determined to be outside the sending line area.

[0254] If S m (ω f If the fault identification criteria are not met, it is determined that a fault has occurred in the back-side system of the M-side bus of the outgoing line;

[0255] If S n (ω f If the fault identification criteria are not met, the fault is determined to be a fault in the back-side system of the N-side bus of the sending line.

[0256] Furthermore, the action threshold value is determined through the following derivation:

[0257] When a fault occurs within the transmission line area, the fault identification criterion values ​​for the M and N sides of the transmission line are:

[0258]

[0259] When a fault occurs within the sending line area, the maximum possible value of the fault identification criterion is:

[0260]

[0261] When a fault occurs outside the M-side zone of the outgoing line, the following applies:

[0262]

[0263] When a fault occurs outside the N-side zone of the transmitting line, the following applies:

[0264]

[0265] Analysis of equations (63) and (64) shows that when a fault occurs outside the transmission line area, the protection criterion on one side is close to 0, while the protection criterion on the other side is larger.

[0266] When a fault occurs outside the transmission line area, the fault identification criterion value of the side with the larger fault identification criterion value is:

[0267]

[0268] Let the maximum possible value S of the fault identification criterion within the area be... in max If the value is less than the action threshold, the protection criterion value Sex on the side with the larger value of the external fault protection criterion value is greater than the action threshold.

[0269] Therefore, based on the fault identification criteria values ​​within and outside the zone, the action threshold value is determined in the following way:

[0270] Let a represent the percentage of fault locations on the transmission line; where a ranges from 0 to 100%.

[0271] By changing the value of 'a' in the following formula, we obtain the set of action threshold values.

[0272]

[0273] in,

[0274]

[0275] In the formula, S set,a (ω f ) indicates that the percentage of fault location on the M and N sides of the transmitting line is 'a', and the high-frequency component frequency is ω. f The action threshold value at time, L fm,h′ (ω f ) indicates that on the M side of the transmitting line, at any sampling point h′, the high-frequency component frequency is ω. f Inductance measurement value at that time;

[0276] Selecting a set of action threshold values The maximum or minimum value in the high-frequency component is taken as the value at frequency ω. f Action threshold value S at time set (ω f ).

[0277] It should be noted that both the maximum and minimum values ​​can be used to identify faults. The choice between the maximum and minimum values ​​as the action threshold can be determined based on specific needs, i.e., by balancing the fault identification requirements inside and outside the area. Specifically, choosing the maximum value as the action threshold results in more comprehensive fault identification coverage within the area, while choosing the minimum value results in more comprehensive fault identification coverage outside the area.

[0278] Alternatively, L can also be obtained by the following formula: mn (ω f ):

[0279]

[0280] In the formula, L fm,h′ (ω f ) indicates that on the N side of the transmitting line, at any sampling point h′, the high-frequency component frequency is ω. f The inductance measurement value at that time.

[0281] Based on the above analysis, it can be seen that the values ​​in the fault identification criteria are related to the selected high-frequency component frequency. In this embodiment, when the fault is inside or outside the zone, the difference in the fault identification criteria reaches its maximum value, and the proposed protection method is more effective. The value of the high-frequency component frequency is determined in the following way:

[0282] Construct a protection criterion as a function f(ω) with respect to the frequency of the high-frequency component. f ):

[0283] f(ω f )=(L mmc (ω f )-L DFIG (ω f )) 2 (68)

[0284] Meanwhile, the selected high-frequency component frequencies should meet the requirements of the wind field and the presentation of the flexible straight-line model. Therefore, the function in equation (62) is analyzed, and its maximum points are taken as follows:

[0285] ω m ={ω f |f'(ω f )=0,f(ω) f <0} (69)

[0286] Calculate and compare ω m The maximum value among the high-frequency components in the set is selected. As the frequency value of the high-frequency component, that is

[0287]

[0288] In the formula, max() represents taking the maximum value.

[0289] For example, considering the influence of various factors, select

[0290] It is understandable that the frequency of the high-frequency component is [value missing]. This allows the difference in fault identification criteria to reach its maximum value when there is a fault inside or outside the zone, resulting in a better protection method in this embodiment.

[0291] Preferably, in this embodiment, considering the influence of factors such as measurement error and noise drying, the actual selected action threshold value can be slightly larger than the result calculated by equation (66).

[0292] More preferably, in this embodiment, the action threshold value is set to 500.

[0293] Compared with existing technologies, the new energy transmission line protection method provided in this embodiment obtains data at high frequency components by collecting data after a fault occurs, and then obtains the high frequency inductance difference coefficient. Then, it uses the fault identification criteria to accurately identify faults inside and outside the area. It only needs to transmit the judgment results of the fault direction on both sides of the line instead of electrical quantity information, and has lower requirements for data synchronization. It effectively solves the problem of incorrect operation of the line protection in the new energy transmission line protection system. It has strong tolerance to fault resistance and high sensitivity to high resistance faults occurring at the end of the line.

[0294] Example 2

[0295] A specific embodiment 2 of the present invention provides a line protection system for a new energy transmission system via flexible direct current, comprising:

[0296] The data acquisition module is used to collect the voltage and current on both sides of the transmission line of the new energy source via the flexible DC transmission system after a fault occurs, and then extract the high-frequency component with a frequency of ω. f Voltage and current at that time;

[0297] The high-frequency inductance difference coefficient module is used to base the difference on both sides of the output line at a high-frequency component frequency of ω. f The voltage and current at that time are obtained at a high frequency component frequency of ω. f The high-frequency inductance difference coefficient on both sides of the output line at that time;

[0298] The fault identification module is used to identify faults based on the frequency of the high-frequency component ω. f The high-frequency inductance difference coefficient on both sides of the transmission line is compared with the fault identification criteria to determine whether an intra-zone fault has occurred in the transmission line.

[0299] The action protection module is used to activate the protection action of the transmission line if a fault occurs within the zone of the transmission line.

[0300] The specific implementation process of this invention can be found in the above method embodiments, and will not be repeated here.

[0301] Since this embodiment is based on the same principle as the above method embodiments, this system also has the corresponding technical effects of the above method embodiments.

[0302] Example 3

[0303] To verify the correctness of Embodiments 1 and 2 of the present invention, this embodiment conducts experimental verification of the solutions in the above embodiments. The system structure diagram used in this embodiment is as follows. Figure 2 As shown in Table 1, the main parameters of the wind farm flexible direct current transmission system are as follows. In the simulation verification, ω is selected... f =950Hz.

[0304] Table 1 Main parameters of the wind farm flexible direct current transmission system

[0305]

[0306] Scenario 1 in this embodiment is as follows: Assume that a phase-A ground fault, a phase-B / C two-phase fault, and a three-phase (A, B, C) fault occur at 50% of the transmission line area, with a transition resistance of 0–300Ω. The high-frequency inductance difference coefficient S at the protection installation points on the M and N sides of the transmission line is given under each fault condition. m S n like Figure 14 As shown in the figure. Fault Resistance represents the transition resistance.

[0307] Depend on Figure 14 (a) Figure 14 (b) Figure 14 (c) Figure 14 (d) Figure 14 (e) and Figure 14 (f) It can be seen that under different fault types, S m and S n All values ​​are below the action threshold, indicating a fault in the area's aggregation line. Figure 14 (a) Figure 14 (b) It can be seen that when a phase A ground fault occurs, S m It reaches its maximum value of 22.55806 S at t = 1.5 ms and a transition resistance of 300 Ω. n The value reaches its maximum at t = 1.1 ms with a transition resistance of 300 Ω, which is 24.46. (From...) Figure 14 (c) Figure 14 (d) It can be seen that S m The transition resistance reaches its maximum value of 23.58 Ω at t = 3.4 ms and a transition resistance of 75 Ω. n It reaches its maximum value of 22.62 at t = 1.7 ms and a transition resistance of 300 Ω. (From...) Figure 14 (e) Figure 14 (f) It can be seen that S m It reaches its maximum value of 23.00 Ω at t = 1.8 ms and a transition resistance of 0 Ω; S n It reaches its maximum value of 25.21 at t = 1.1 ms and a transition resistance of 0 Ω. Because S m and Sn The maximum values ​​are all less than the action threshold, satisfying S. m set ,S n set Therefore, all of the above types of faults can be identified as faults within the area.

[0308] As can be seen from the above analysis, the method proposed in the above embodiments can accurately identify the occurrence of faults in the transmission line when faults occur through different transition resistances, and has a strong ability to withstand high resistance.

[0309] Scenario 2 in this implementation is as follows: Two-phase ground faults (BC and P) occur at different locations within the transmission line area, with a transition resistance of 150Ω. The high-frequency inductance difference coefficient S at the protection installation points on the M and N sides of the transmission line under this fault condition is... m S n ,like Figure 15 As shown.

[0310] Depend on Figure 15 (a) Figure 15 (b) It can be seen that when a two-phase ground fault (BC) occurs, S m The maximum value of 74.1 is reached at t = 9.9 ms when a fault occurs at the end of the transmitting line; S n The maximum value of 77.52 was obtained when the first segment of the transmission line failed at t=3.6ms. This value is due to S... m and S n The maximum values ​​are all less than the action threshold, satisfying S. m set ,S n set Therefore, all of the above types of faults can be identified as faults within the area.

[0311] As can be seen from the above analysis, the method proposed in the above embodiments can accurately identify faults within the area when faults occur at different locations on the transmission line, and still has high sensitivity when a high-resistance fault occurs at the end of the line.

[0312] Scenario 3 in this implementation is: [Settings] Figure 2 At point f2, a three-phase ground fault occurs at the back-side outlet of the M-side busbar, with the transition resistance varying from 0 to 300Ω. The high-frequency inductance difference coefficient S at the protection installation points on the M and N sides of the transmitting line under this fault condition is... m S n ,like Figure 16 As shown.

[0313] Depend on Figure 16 (a) Figure 16 (b) It can be seen that when a phase A ground fault occurs at the back-side outlet of the M-side busbar, S m ​​​​The value was consistently higher than the action threshold, reaching its minimum at t = 9.6 ms and a transition resistance of 300 Ω, with a value of 4280; S n The value is consistently below the action threshold, reaching its minimum at t = 3.5 ms with a transition resistance of 0 Ω, at which point it reaches 13.67. Because S m >S set ,S n set The fault was determined to be outside the designated area, and the protection system did not activate.

[0314] The above analysis shows that when a fault occurs outside the wind farm side of the transmission line through different transition resistors, the method proposed in this paper can accurately identify the occurrence of the fault outside the zone and prevent the protection from maloperating.

[0315] Scenario 4 in this implementation is: [Settings would be here] Figure 2 At point f3, which is 50% of the way down the flexible DC line, faults occur through different transition resistances, with the transition resistance varying from 0 to 300Ω. The high-frequency inductance difference coefficient S at the protection installation points on the M and N sides of the transmitting line under this fault condition is... m S n ,like Figure 17 As shown.

[0316] Depend on Figure 17 (a) Figure 17 (b) It can be seen that when a fault occurs at 50% of the points of the flexible DC line, S m The value is consistently below the action threshold, reaching its maximum value of 7.07 at t = 10.0 ms and a transition resistance of 225 Ω; S n The value is consistently higher than the action threshold, reaching its minimum at t = 9.0 ms and a transition resistance of 300 Ω, with a value of 3137.87. Because S m set ,S n >S set The fault was determined to be outside the designated area, and the protection system did not activate.

[0317] As can be seen from the above analysis, the method proposed in the above embodiments can accurately identify the fault as an external fault when the flexible DC line passes through different transition resistances, thus preventing the protection from maloperating, and has a strong ability to withstand high resistance.

[0318] Those skilled in the art will understand that all or part of the processes of the methods described in the above embodiments can be implemented by a computer program instructing related hardware, and the program can be stored in a computer-readable storage medium. The computer-readable storage medium may be a disk, optical disk, read-only memory, or random access memory, etc.

[0319] ​​The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for line protection in a new energy transmission system via flexible direct current transmission, characterized in that, include: After a fault occurs, the voltage and current on both sides of the transmission line of the new energy source via the flexible DC transmission system are collected, and then the high-frequency component frequency is extracted. Voltage and current at that time; Based on the high-frequency component frequency on both sides of the transmission line, The voltage and current at that time are obtained at the high-frequency component frequency of . The high-frequency inductance difference coefficient on both sides of the transmission line; the wind farm side of the transmission line is designated as side M, and the flexible DC system side is designated as side N; the high-frequency components on sides M and N of the transmission line are at frequencies of... The high-frequency inductance difference coefficients at different times are expressed as follows: ； In the formula, , These represent the high-frequency components at the M and N sides of the transmitting line, respectively. High-frequency inductance difference coefficient, This indicates the number of sampling points within one period after the fault occurred. , These represent the M and N sides of the transmitting line at the th... The frequency of the high-frequency component at each sampling point is Inductance measurement value at that time , These represent the high-frequency components at the M and N sides of the transmitting line, respectively. Calculated inductance value at that time; Based on the frequency of the high-frequency component... The high-frequency inductance difference coefficient on both sides of the transmission line is compared with the fault identification criteria to determine whether an intra-zone fault has occurred in the transmission line; if so, the protection action of the transmission line is activated. The frequency of the high-frequency component is The fault identification criteria at that time include: ； In the formula, This indicates that the frequency of the high-frequency component is... Action threshold value at time; like and If all fault identification criteria are met, the fault is determined to be within the sending line area; otherwise, it is determined to be outside the sending line area.

2. The method for line protection of a new energy transmission system via flexible direct current transmission according to claim 1, characterized in that, The high-frequency inductance difference coefficient between the two sides of the output line is obtained in the following way: Based on the system structure and fault analysis of the M and N sides of the transmitting line, the frequency of the high-frequency component during the fault transient process is obtained as follows: The equivalent total impedance of the wind farm and the equivalent impedance of the MMC converter are obtained, and then the high-frequency component frequencies of the M and N sides of the transmission line are obtained. Calculated inductance value at that time; Based on the high-frequency component frequency of the M and N sides of the aforementioned transmission lines, The voltage and current at that time are obtained at the M and N sides of the transmitting line at a high frequency component frequency of . Inductance measurement value at that time; Based on the high-frequency component frequency of the M and N sides of the aforementioned transmission lines, The calculated and measured inductance values ​​at the time are used to obtain the high-frequency component frequencies of the M and N sides of the transmitting line. The high-frequency inductance difference coefficient at that time.

3. The method for line protection of a new energy transmission system via flexible direct current transmission according to claim 1, characterized in that, If the fault is determined to be outside the transmission line area, then like If the fault identification criteria are not met, the fault is determined to be a fault in the back-side system of the M-side bus of the outgoing line; like If the fault identification criteria are not met, the fault is determined to be a fault in the back-side system of the N-side bus of the sending line.

4. The line protection method for a new energy transmission system via flexible direct current transmission according to claim 1, characterized in that, The high-frequency component frequency is obtained in the following way. Action threshold value at time : Let the percentage of fault locations on the transmission line be expressed as ;in, The value ranges from 0 to 100%; By changing the following formula The value is used to obtain the action threshold value set. : ； in, ； In the formula, This indicates the percentage of fault locations on the M and N sides of the transmitting line. The frequency of the high-frequency component is Action threshold value at time, This indicates that at any sampling point on the M side of the sending line. And the frequency of the high-frequency component is Inductance measurement value at that time; Selecting a set of action threshold values The maximum or minimum value in the high-frequency component is used as the value at a frequency of 0. Action threshold value at time .

5. The method for line protection of a new energy transmission system via flexible direct current transmission according to claim 1, characterized in that, The high-frequency component frequency Values ; Determined in the following ways : ； in, ； ； In the formula, max() represents taking the maximum value.

6. The method for line protection of a new energy transmission system via flexible direct current transmission according to claim 5, characterized in that, The transmitting line M side has a high-frequency component frequency of Inductance calculation value at time Represented as: ； In the formula, , , , , , , , , , This represents a constant calculated based on the parameters of each component on the M side of the transmission line.

7. The method for line protection of a new energy transmission system via flexible direct current transmission according to claim 5, characterized in that, The N-side of the transmitting line has a high-frequency component frequency of Inductance calculation value at time Represented as: ； In the formula, , , , , , , , This represents a constant calculated based on the parameters of each component on the N side of the transmission line.

8. A line protection system for a new energy transmission system via flexible direct current transmission, characterized in that, include: The data acquisition module is used to collect the voltage and current on both sides of the transmission line of the new energy source via the flexible DC transmission system after a fault occurs, and then extract the high-frequency component frequency. Voltage and current at that time; The high-frequency inductance difference coefficient module is used to determine the inductance difference coefficient between the two sides of the output line at a high-frequency component frequency of [missing information]. The voltage and current at that time are obtained at the high-frequency component frequency of . The high-frequency inductance difference coefficient on both sides of the transmission line; the wind farm side of the transmission line is designated as side M, and the flexible DC system side is designated as side N; the high-frequency components on sides M and N of the transmission line are at frequencies of... The high-frequency inductance difference coefficients at different times are expressed as follows: ； In the formula, , These represent the high-frequency components at the M and N sides of the transmitting line, respectively. High-frequency inductance difference coefficient, This indicates the number of sampling points within one period after the fault occurred. , These represent the M and N sides of the transmitting line at the th... The frequency of the high-frequency component at each sampling point is Inductance measurement value at that time , These represent the high-frequency components at the M and N sides of the transmitting line, respectively. Calculated inductance value at that time; The fault identification module is used to identify faults based on the frequency of the high-frequency component. The high-frequency inductance difference coefficient on both sides of the transmission line is compared with the fault identification criteria to determine whether an in-zone fault has occurred in the transmission line; the high-frequency component frequency is... The fault identification criteria at that time include: ； In the formula, This indicates that the frequency of the high-frequency component is... Action threshold value at time; like and If all fault identification criteria are met, the fault is determined to be within the transmission line area; otherwise, it is determined to be outside the transmission line area. The action protection module is used to activate the protection action of the transmission line if a fault occurs within the zone of the transmission line.

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