Direct-drive wind farm collection line protection method and system based on high-frequency mutation energy

Abstract

The present invention relates to a direct-drive wind farm collection line protection method and system based on high-frequency mutation energy, belonging to the technical field of power system relay protection, and solves the problems that the existing wind farm collection line protection has poor ability to withstand transition resistance during in-zone faults and is prone to misoperation during out-of-zone faults. When a fault occurs in the direct-drive wind farm collection line is monitored, the voltages and currents at the installation location of the collection line protection and at each fan port before and after the fault, as well as the electrical angle after the fault, are obtained; based on the obtained information, the high-frequency mutation energies and action threshold values at the installation location of the collection line protection and at each fan port are obtained, and then the high-frequency mutation energy difference coefficients at the installation location of the collection line protection and at each fan port are obtained; according to the obtained high-frequency mutation energy difference coefficients and fault identification criteria, the fault type of the collection line is judged. If the fault type is an in-zone collection line fault, the protection action is started to realize the protection of the direct-drive wind farm.

Description

Technical Field

[0001] The present invention relates to the technical field of relay protection for power systems, and particularly to a protection method and system for a direct-drive wind farm collection line based on high-frequency sudden change energy. Background Art

[0002] Large-scale wind power is integrated into the power grid through power electronic devices. Affected by the control strategy and parameters of the converter, the fault transient response characteristics change, showing characteristics such as amplitude limitation, non-power frequency, and phase angle control, which are quite different from synchronous power sources. The protection methods for the fault characteristics of pure AC systems are no longer applicable to the direct-drive wind farm collection line. Therefore, studying the protection method for the wind farm to be sent out through the collection line has important practical significance for the safe operation of the actual system.

[0003] Currently, according to whether the electrical quantity used for criterion construction is a frequency-domain quantity or a time-domain quantity, the existing longitudinal protection for wind power transmission lines is divided into longitudinal line protection based on time-domain quantities and longitudinal line protection based on frequency-domain quantities. Among them, the longitudinal protection based on time-domain quantities refers to the method of constructing protection criteria according to the time-domain electrical quantity information collected at the installation location of the line protection and using methods such as model parameter identification or waveform characteristics; the longitudinal protection based on frequency-domain quantities refers to the method of extracting the frequency-domain characteristics of the electrical quantities collected at the installation location of the line protection by using methods such as Fourier transform and wavelet transform, and constructing protection criteria on this basis. However, the above research mainly focuses on the protection of wind farm transmission lines, and the research on the protection of large-scale wind farms sent out through collection lines is relatively scarce. The existing wind farm collection line protection has problems such as poor ability to withstand transition resistance during in-zone faults and easy misoperation during out-of-zone faults. Summary of the Invention

[0004] In view of the above analysis, the embodiments of the present invention aim to provide a protection method and system for a direct-drive wind farm collection line based on high-frequency sudden change energy, so as to solve the problems of poor ability to withstand transition resistance during in-zone faults and easy misoperation during out-of-zone faults in the existing wind farm collection line protection.

[0005] On the one hand, the embodiments of the present invention provide a protection method for a direct-drive wind farm collection line based on high-frequency sudden change energy, including the following steps:

[0006] When a fault occurs in the direct-drive wind farm collection line is monitored, obtain the voltages, currents at the installation location of the collection line protection and at each fan port before and after the fault, and the electrical angle after the fault;

[0007] Based on the obtained information, obtain the high-frequency sudden change energy and the action threshold value at the installation location of the collection line protection and at each fan port, and then obtain the high-frequency mutation energy difference coefficient at the installation location of the collection line protection and at each fan port;

[0008] According to the high-frequency mutation energy difference coefficients and fault identification criteria at the installation location of the collector line protection and at each wind turbine port, determine the fault type of the collector line. If the fault type is an in-zone collector line fault, then initiate the protection action to achieve the protection of the direct-drive wind farm.

[0009] Further, the high-frequency mutation energy difference coefficients at the installation location of the collector line protection and at each wind turbine port are expressed as:

[0010]

[0011] In the formula, s m , s wi respectively represent the high-frequency mutation energy difference coefficients at the installation location of the protection on the collector line and at the port of the i-th wind turbine; h is the sampling time interval, T is the integration time, and N is the total number of sampling points; Δu Mφ (j), Δi Mφ (j) respectively represent the voltage mutation and current mutation at the j-th sampling point of the fault phase φ at the installation location of the protection on the collector line, Δe wiφ (j), Δi wiφ (j) respectively represent the voltage mutation and current mutation at the j-th sampling point of the fault phase φ at the port of the i-th wind turbine on the collector line; W set , W wiset respectively represent the action threshold values at the installation location of the protection on the collector line and at the port of the i-th wind turbine.

[0012] Further, the fault identification criteria include:

[0013] If the high-frequency mutation energy difference coefficient at the installation location of the protection on the collector line is less than 0 and the high-frequency mutation energy difference coefficient at each wind turbine port is greater than 0, then an in-zone collector line fault occurs;

[0014] If the high-frequency mutation energy difference coefficient at the installation location of the protection on the collector line is less than 0 and the high-frequency mutation energy difference coefficient at any wind turbine port is less than 0, then an in-zone wind turbine fault occurs;

[0015] Otherwise, an out-of-zone fault occurs.

[0016] Further, determine the action threshold values W set , W wiset at the installation location of the protection on the collector line and at the port of the i-th wind turbine through the following method:

[0017] W set = max{W nor , W m2max}

[0018] W wiset = max{Wwnor ,W wimax}

[0019] In the formula, W nor represents the port energy in the steady - state operation mode of the direct - drive wind farm, and W m2max represents the sudden - change energy at the installation location of the collector line protection under the maximum operation mode of the short - circuit of the terminal fan in the direct - drive wind farm; W wnor represents the port energy in the steady - state operation mode of the fan unit in the direct - drive wind farm, and W wimax represents the sudden - change energy of the i - th fan unit port fault on the collector line of the direct - drive wind farm.

[0020] Furthermore, the sudden - change energy W m2max at the installation location of the collector line protection under the maximum operation mode of the short - circuit of the terminal fan in the direct - drive wind farm is expressed as:

[0021] W m2max =∫Δu Mmax Δi m2max dt

[0022] In the formula, Δu Mmax is the voltage sudden - change at the installation location of the protection on the collector line under the maximum operation mode of the short - circuit of the terminal fan in the direct - drive wind farm; Δi m2max is the current sudden - change at the installation location of the protection on the collector line under the maximum operation mode of the short - circuit of the terminal fan in the direct - drive wind farm.

[0023] Furthermore, the voltage sudden - change Δu Mmax at the installation location of the protection on the collector line under the maximum operation mode of the short - circuit of the terminal fan in the direct - drive wind farm is expressed as:

[0024]

[0025] Among them,

[0026]

[0027] Z li =R li +jωL li ,

[0028] Z Tw =R Tw +jωL Tw ,Z w =1 / Y w ,

[0029]

[0030]

[0031]

[0032]

[0033]

[0034] In the formula, k Tw is the turn ratio of the converter transformer of the wind turbine generator set, and R li , L li are respectively the resistance and inductance of the i-th wind turbine generator set on the collection line. R Tw , L Tw respectively represent the resistance and inductance of the converter transformer of the wind turbine generator set; ω is the angular frequency, and u f is the voltage at the end wind turbine when the fault point is a short circuit of the end wind turbine. n represents the total number of wind turbine generator sets on the collection line, and n > 3; ω p , ω s are respectively the p-th harmonic angular frequency and the fundamental wave angular frequency; E s is the steady-state voltage amplitude after the fault at the wind turbine port; K ppll , K ipll are respectively the proportional and integral coefficients of the phase-locked loop PI control; θ u is the actual electrical angle of the power grid after the fault; K gp , K gi are respectively the proportional and integral coefficients of the current loop PI control; R g , L g are respectively the equivalent resistance and inductance of the line reactor at the grid side converter inlet; are respectively the reference values of the d-axis and q-axis currents under steady-state operation conditions, are respectively the reference values of the d-axis and q-axis currents under steady-state operation conditions after the fault.

[0035] Furthermore, the sudden change energy W wimax of the i-th wind turbine generator set port on the direct-drive wind farm collection line is expressed as:

[0036] W wimax = ∫Δu wimax Δi wimax dt

[0037] In the formula, Δu wimax is the voltage sudden change at the i-th wind turbine generator set port on the collection line under the maximum operating mode of the short circuit of the end wind turbine in the direct-drive wind farm; Δi wimax is the current sudden change at the i-th wind turbine generator set port on the collection line under the maximum operating mode of the short circuit of the end wind turbine in the direct-drive wind farm.

[0038] Furthermore, the current sudden change Δi wimax at the i-th wind turbine generator set port on the collection line under the maximum operating mode of the short circuit of the end wind turbine in the direct-drive wind farm is expressed as:

[0039] Δiwimax = Y w Δu wimax + ΔI w 。

[0040] On the other hand, an embodiment of the present invention provides a direct-drive wind farm collection line protection system based on high-frequency mutation energy, including:

[0041] A data acquisition module, configured to acquire the voltages, currents at the protection installation location of the collection line and at each fan port before and after a fault, and the electrical angle after a fault when it is detected that a fault occurs in the direct-drive wind farm collection line;

[0042] A high-frequency mutation energy difference coefficient calculation module, configured to obtain the high-frequency mutation energy and the action threshold value at the protection installation location of the collection line and at each fan port based on the acquired information, and further obtain the high-frequency mutation energy difference coefficient at the protection installation location of the collection line and at each fan port;

[0043] A fault identification and protection action module, configured to determine the type of the collection line fault according to the high-frequency mutation energy difference coefficient at the protection installation location of the collection line and at each fan port and the fault identification criterion. If the fault type is an in-zone collection line fault, the protection action is started to realize the protection of the direct-drive wind farm.

[0044] Further, the high-frequency mutation energy difference coefficient at the protection installation location of the collection line and at each fan port in the fault identification and protection action module is expressed as:

[0045]

[0046] In the formula, s m , s wi respectively represent the high-frequency mutation energy difference coefficients at the protection installation location on the collection line and at the port of the i-th fan; h is the sampling time interval, T is the integration time, and N is the total number of sampling points; Δu Mφ (j), Δi Mφ (j) respectively represent the voltage mutation and current mutation at the j-th sampling point of the fault phase φ at the protection installation location on the collection line, and Δe wiφ (j), Δi wiφ (j) respectively represent the voltage mutation and current mutation at the j-th sampling point of the fault phase φ at the port of the i-th fan on the collection line; W set , W wiset respectively represent the action threshold values at the protection installation location on the collection line and at the port of the i-th fan.

[0047] Compared with the prior art, the present invention can at least achieve one of the following beneficial effects:

[0048] The direct-drive wind farm collection line protection method and system based on high-frequency mutation energy proposed by the present invention can calculate the high-frequency mutation energy difference coefficients at the installation location of the collection line protection and at each fan port based on the data collected before and after a fault, and accurately identify internal and external faults according to the fault identification criterion. It has a fast action speed, is not affected by transition resistance, fault location, and fault type, has high sensitivity, and has a low sampling frequency, making it easy to implement in engineering. It fundamentally eliminates the problems of misoperation and refusal to operate of collection line faults. Moreover, the present invention only requires the identification result of the fault direction on one side of the protection installation location, does not need to exchange electrical quantity information at both ends, the fault identification is not affected by synchronization errors, and has low requirements for communication devices.

[0049] In the present invention, the above technical solutions can also be combined with each other to achieve more preferred combination schemes. Other features and advantages of the present invention will be described in the following specification, and some advantages can be made obvious from the specification or understood by implementing the present invention. The objectives and other advantages of the present invention can be achieved and obtained through the content specifically pointed out in the specification and the drawings. Description of the Drawings

[0050] The drawings are only for the purpose of showing specific embodiments and are not considered to be a limitation of the present invention. Throughout the drawings, the same reference signs represent the same components.

[0051] Figure 1 It is a schematic flow chart of the direct-drive wind farm collection line protection method based on high-frequency mutation energy provided in Embodiment 1 of the present invention;

[0052] Figure 2 It is a grid-connected system diagram of a permanent magnet direct-drive wind turbine provided in Embodiment 1 of the present invention;

[0053] Figure 3 It is a control block diagram of the grid-side converter of a permanent magnet direct-drive wind turbine provided in Embodiment 1 of the present invention;

[0054] Figure 4 It is a schematic diagram of the equivalent circuit of the direct-drive wind farm collection line provided in Embodiment 1 of the present invention;

[0055] Figure 5 It is a fault mutation quantity network diagram of each wind farm collection line provided in Embodiment 1 of the present invention;

[0056] Figure 6 It is a fault mutation quantity network diagram of the internal collection line provided in Embodiment 1 of the present invention;

[0057] Figure 7 It is a fault mutation quantity network diagram of the external AC system provided in Embodiment 1 of the present invention;

[0058] Figure 8It is the network diagram of the sudden change quantity of faults on other collecting lines outside the area provided by Embodiment 1 of the present invention;

[0059] Figure 9 It is the schematic structural diagram of the direct-drive wind farm collecting line protection system based on the high-frequency sudden change quantity energy provided by Embodiment 2 of the present invention;

[0060] Figure 10 (a) and 10(b) are respectively s when phase A ground fault occurs through different transition resistances in the area provided by Embodiment 3 of the present invention m and s w2 ;

[0061] Figure 10 (c) and 10(d) are respectively s when BC phase interphase fault occurs through different transition resistances in the area provided by Embodiment 3 of the present invention m and s w2 ;

[0062] Figure 10 (e) and 10(f) are respectively s when ABC three-phase fault occurs through different transition resistances in the area provided by Embodiment 3 of the present invention m and s w2 ;

[0063] Figure 11 (a) and 11(b) are respectively s when phase A ground fault occurs at different positions on the collecting line provided by Embodiment 3 of the present invention m and s w2 ;

[0064] Figure 11 (c) and 11(d) are respectively s when AB phase interphase fault occurs at different positions on the collecting line provided by Embodiment 3 of the present invention m and s w2 ;

[0065] Figure 12 (a) and 12(b) are respectively s when phase A of the AC system outside the collecting line area of Embodiment 3 of the present invention is grounded through different transition resistances m and s w2 ;

[0066] Figure 13 (a) and 13(b) are respectively s when AB phase of other collecting lines outside the collecting line area of Embodiment 3 of the present invention is grounded through different transition resistances m and s w2 . Specific embodiments

[0067] The following specifically describes the preferred embodiments of the present invention with reference to the accompanying drawings. Among them, the accompanying drawings form a part of this application and are used together with the embodiments of the present invention to explain the principle of the present invention, rather than to limit the scope of the present invention.

[0068] Embodiment 1

[0069] A specific embodiment of the present invention discloses a direct-drive wind farm collection line protection method based on high-frequency mutation energy. As Figure 1 shown, it includes the following steps:

[0070] S1. When it is detected that a fault occurs in the direct-drive wind farm collection line, obtain the voltages, currents at the protection installation location of the collection line and at each fan port before and after the fault, and the electrical angle after the fault;

[0071] Specifically, the collection line fault is a single-phase ground fault, a two-phase ground fault, a two-phase interphase fault or a three-phase fault.

[0072] S2. Based on the obtained information, obtain the high-frequency mutation energy and the action threshold value at the protection installation location of the collection line and at each fan port, and then obtain the high-frequency mutation energy difference coefficient at the protection installation location of the collection line and at each fan port;

[0073] S3. According to the high-frequency mutation energy difference coefficient and the fault identification criterion at the protection installation location of the collection line and at each fan port, judge the type of the collection line fault. If the fault type is an in-zone collection line fault, start the protection action to realize the protection of the direct-drive wind farm.

[0074] Compared with the prior art, this embodiment provides a direct-drive wind farm collection line protection method based on high-frequency mutation energy, which can calculate the high-frequency mutation energy difference coefficient at the protection installation location of the collection line and at each fan port based on the data collected before and after the fault, and accurately identify in-zone and out-of-zone faults according to the fault identification criterion. It has a fast action speed, is not affected by the transition resistance, fault location and fault type, has high sensitivity, and has a low sampling frequency, which is easy to be realized in engineering. Fundamentally, the problems of misoperation and refusal of the collection line fault are eliminated; and the present invention only needs the identification result of the fault direction on one side of the protection installation location, does not need to exchange the electrical quantity information at both ends, the fault identification is not affected by the synchronization error, and the requirement for the communication device is low

[0075] During implementation, in step S3, the high-frequency mutation energy difference coefficient at the protection installation location of the collection line and at each fan port is expressed as:

[0076]

[0077] In the formula, s m and s wi respectively represent the high-frequency mutation energy difference coefficients at the protection installation location on the collection line and at the port of the i-th fan; h is the sampling time interval, T is the integration time, N is the total number of sampling points; Δu Mφ (j), Δi Mφ(j) represent the voltage mutation and current mutation at the j-th sampling point of the fault phase φ at the protection installation location on the collection line, Δe wiφ (j), Δi wiφ (j) represent the voltage mutation and current mutation at the j-th sampling point of the fault phase φ at the port of the i-th wind turbine on the collection line; W set 、W wiset represent the action threshold values at the protection installation location and the port of the i-th wind turbine on the collection line respectively.

[0078] During implementation, in step S3, the fault identification criterion includes:

[0079] If the high-frequency mutation energy difference coefficient at the protection installation location on the collection line is less than 0, and the high-frequency mutation energy difference coefficient at each wind turbine port is greater than 0, then an in-zone collection line fault occurs;

[0080] If the high-frequency mutation energy difference coefficient at the protection installation location on the collection line is less than 0, and the high-frequency mutation energy difference coefficient at any wind turbine port is less than 0, then an in-zone wind turbine unit fault occurs;

[0081] Otherwise, an out-of-zone fault occurs.

[0082] Specifically during implementation, the action threshold values W set 、W wiset at the protection installation location on the collection line and the port of the i-th wind turbine are determined in the following manner:

[0083] W set =max{W nor ,W m2max} (2)

[0084] W wiset =max{W wnor ,W wimax} (3)

[0085] In the formula, W nor represents the port energy in the steady-state operation mode of the direct-drive wind farm, W m2max represents the mutation energy at the protection installation location on the collection line in the maximum operation mode of the short circuit of the terminal wind turbine in the direct-drive wind farm; W wnor represents the port energy in the steady-state operation mode of the wind turbine unit in the direct-drive wind farm, W wimax represents the fault mutation energy at the port of the i-th wind turbine unit on the collection line in the direct-drive wind farm.

[0086] Specifically, the mutation energy W m2max at the protection installation location on the collection line in the maximum operation mode of the short circuit of the terminal wind turbine in the direct-drive wind farm is expressed as:

[0087] Wm2max = ∫Δu Mmax Δi m2max dt (4)

[0088] Wherein, Δu Mmax is the voltage mutation at the protection installation location on the collection line under the maximum operating mode of the short circuit of the terminal fan in the direct-drive wind farm; Δi m2max is the current mutation at the protection installation location on the collection line under the maximum operating mode of the short circuit of the terminal fan in the direct-drive wind farm.

[0089] More specifically, the voltage mutation Δu at the protection installation location on the collection line under the maximum operating mode of the short circuit of the terminal fan in the direct-drive wind farm Mmax , is expressed as:

[0090]

[0091] Wherein,

[0092]

[0093] Z li = R li + jωL li ,

[0094] Z Tw = R Tw + jωL Tw Z w = 1 / Y w ,

[0095]

[0096]

[0097]

[0098]

[0099]

[0100] Wherein, k Tw is the turn ratio of the converter transformer of the wind turbine generator, R li , L li are respectively the resistance and inductance of the i-th fan unit on the collection line, R Tw , L Tw respectively represent the resistance and inductance of the converter transformer of the fan unit; ω is the angular frequency, u f is the voltage at the terminal fan when the fault point is the short circuit of the terminal fan, n represents the total number of fan units on the collection line, n > 3; ω p , ω sare the p - th harmonic angular frequency and the fundamental wave angular frequency respectively; E s is the steady - state voltage amplitude after the fault at the fan port; K ppll 、K ipll are the proportional and integral coefficients of the PLL PI control respectively; θ u is the actual electrical angle of the power grid after the fault; K gp 、K gi are the proportional and integral coefficients of the current loop PI control respectively; R g 、L g are the equivalent resistance and inductance of the line - side converter's incoming line reactor respectively; are the d - axis and q - axis current reference values under steady - state operation conditions respectively, are the d - axis and q - axis current reference values under steady - state operation conditions after the fault respectively.

[0101] Specifically, the sudden - change energy W wimax of the i - th fan unit port on the direct - drive wind farm collection line is expressed as:

[0102] W wimax =∫Δu wimax Δi wimax dt (6)

[0103] In the formula, Δu wimax is the voltage sudden - change at the i - th fan unit port on the collection line under the maximum operating mode of the direct - drive wind farm terminal fan short - circuit; Δi wimax is the current sudden - change at the i - th fan unit port on the collection line under the maximum operating mode of the direct - drive wind farm terminal fan short - circuit.

[0104] More specifically, the current sudden - change Δi wimax at the i - th fan unit port on the collection line under the maximum operating mode of the direct - drive wind farm terminal fan short - circuit is expressed as:

[0105] Δi wimax =Y w Δu wimax +ΔI w (7)

[0106] It should be noted that the protection method for the direct - drive wind farm collection line based on high - frequency sudden - change energy in this embodiment is obtained based on the following derivation:

[0107] First, based on the direct - drive wind turbine control system and electrical parameters, the sudden - change impedance model of the wind turbine in the frequency domain is derived. Combining with the topological structure of the wind farm collection line system, the inductive frequency band of the wind farm is extracted to obtain the equivalent resistance and inductance of the wind farm.

[0108] The permanent - magnet direct - drive wind turbine grid - connected system is as Figure 2As shown in the figure, the permanent magnet direct drive wind turbine generator set consists of a mechanical system, a permanent magnet synchronous generator, and a full-power wind power converter. Among them, the permanent magnet synchronous generator (PMSG) uses permanent magnets for excitation, and there is no excitation coil winding on the stator, resulting in low copper loss and high power generation efficiency. The full-power wind power converter decouples the machine-side converter RSC and the grid-side converter GSC through a DC link, and the DC bus capacitor is large enough, so the dynamic changes of the machine-side system have little impact, and it is simplified to a constant power source.

[0109] According to the Park transformation principle and performing Laplace transformation on the time-domain voltage equation, the complex frequency domain form of the output voltage of the grid-side converter in the synchronous rotating coordinate system can be obtained as follows:

[0110]

[0111] In the formula, s represents the Laplace operator, u gd (s), u gq (s), i gd (s), i gq (s) are the d-axis and q-axis components of the voltage and current of the grid-side converter in the complex frequency domain respectively; R g , L g are the equivalent resistance and inductance of the grid-side converter's incoming line reactor respectively; ω s is the grid angular velocity; e d (s), e q (s) are the d-axis and q-axis voltages of the wind turbine generator set port in the complex frequency domain respectively. It should be noted that the formulas in this embodiment are all represented in the complex frequency domain form, and the suffix (s) will not be elaborated further.

[0112] The control system of the grid-side converter of the wind turbine generator set is as Figure 3 shown. The grid-side converter uses grid voltage-oriented vector control to stabilize the DC voltage, control the input power factor, and transfer the active power to the grid immediately. When the wind turbine generator set operates in a steady state, the phase angle output by the phase-locked loop can accurately track the actual grid phase angle, making the d-axis voltage oriented to the grid voltage.

[0113] It can be obtained that the output voltage of the grid-side converter can also be expressed as:

[0114]

[0115] In the formula, are the reference values of the d-axis and q-axis currents under steady-state operation respectively; K gp , K gi are the proportional and integral coefficients of the current loop PI control respectively.

[0116] For the convenience of establishing the impedance model, the voltage and current of the grid-side converter are represented in the following complex form:

[0117]

[0118] By simultaneously solving the above equations (8)-(10), the complex frequency-domain output current of the wind turbine under steady-state operation can be obtained:

[0119]

[0120] wherein,

[0121] After performing the inverse coordinate transformation, the output current of the wind turbine in the three-phase stationary coordinate system in the frequency domain is obtained:

[0122]

[0123] In the formula, ω p and ω s are the angular frequencies of the pth harmonic and the fundamental wave respectively.

[0124] At the moment when the power grid fails, due to the change in the power grid topology, the amplitude and phase of the grid-connected point voltage will change suddenly, and the coordinate axes of the phase-locked loop cannot immediately follow the coordinate transformation of the grid-connected point voltage. Among them, the grid-connected point, i.e., the PCC point, refers to the wind farm bus in the power system. Therefore, considering the dynamic characteristics of the phase-locked loop after the fault, there is an error Δθ pll between the phase detected by the phase-locked loop and the actual terminal voltage phase, which is expressed as:

[0125] Δθ pll = θ u - θ pll (13)

[0126] wherein,

[0127]

[0128] In the formula, θ pll is the output electrical angle of the phase-locked loop; θ u is the actual electrical angle of the power grid after the fault; K ppll and K ipll are the proportional and integral coefficients of the PI control of the phase-locked loop respectively.

[0129] The parameters of the actual power grid and the phase-locked loop satisfy the following formula:

[0130]

[0131] In the formula, x d s1 and x q s1 are the d-axis and q-axis parameters in the input coordinate system of the actual power grid respectively; x d p and x qp They are the d-axis and q-axis parameters in the PLL coordinate system respectively.

[0132] At this time, the vector control equation of the grid-side converter is as follows:

[0133]

[0134] In the formula, u′ gd and u′ gq , i′ gd and i′ gq are the d-axis and q-axis components of the voltage and current of the grid-side converter after the fault respectively; e′ d and e′ q are the d-axis and q-axis voltages at the fan port after the fault respectively; are the d-axis and q-axis current reference values under the steady-state operation condition after the fault respectively; ω pll is the angular velocity output by the PLL after the fault.

[0135] By combining equations (8), (13) to (15), the short-circuit current of the wind turbine in the complex frequency domain form in the rotating coordinate system after the fault is obtained:

[0136]

[0137] Among them, i′ gdq = i′ gd + ji′ gq , e′ dq = e′ d + je′ q ;

[0138] In the formula, E s is the amplitude of the steady-state voltage at the fan port after the fault.

[0139] By performing the Park inverse transformation on equation (16) to transform the rotating coordinate system into the stationary coordinate system, the output short-circuit current of the wind turbine in the three-phase stationary coordinate system after the fault is as follows:

[0140] i′ gabc = Y w e′ gabc + I sabc (17)

[0141] In the formula, e′ gabc is the three-phase voltage in the stationary coordinate system at the grid connection point of the fan port after the fault; Y w is the equivalent admittance of the wind turbine after the fault; I sabc is the equivalent current source of the wind turbine after the fault.

[0142] They are respectively expressed as:

[0143]

[0144]

[0145] Construct a fault mutation model in the frequency domain of a wind turbine generator set:

[0146] Δi gabc = i′ gabc - i gabc = Y w e′ gabc + ΔI w

[0147]

[0148] Among them, ΔI w is the equivalent mutation current source of the wind turbine generator set.

[0149] According to the different numbers of wind turbine generator sets connected, the connection method of the wind farm can be divided into dedicated line sending and multi-point T-shaped sending. The dedicated line sending only connects a single wind turbine generator set, and the multi-point T-shaped sending connects multiple wind turbine generator sets, that is, the chain connection method. The mutation impedance model of the wind turbine generator set constructed in this embodiment is applicable to any connection method, and the wind farm model is obtained according to the change of the wind farm topology. In this embodiment, the chain connection of the wind farm is taken as an example for illustration:

[0150] The wind farm adopts the chain connection. The fan and the box transformer are connected in a one-to-one manner. A certain number of wind turbine generator sets are connected to a collecting line, and the entire wind farm is composed of several strings of wind turbine generator sets. On the same collecting line, multiple wind turbine generator sets operate in parallel; for the entire wind farm, multiple strings of wind turbine generator sets still satisfy the parallel relationship. Among them, the protection installation location is at the port of each collecting line, and the voltage at the protection installation location of each collecting line is equal to the voltage at the PCC point.

[0151] The equivalent circuit diagram of the direct-drive wind farm is as Figure 4 shown. It is assumed that the parameters and steady-state operating conditions of the direct-drive wind turbine generator sets on the feeder are the same, and there are n PMSGs operating in parallel on each collecting line. The fault mutation network of each collecting line is as Figure 5 shown. According to Figure 5 solve the short-circuit current of the PMSG under the parallel operation of multiple machines on any collecting line.

[0152] Considering the conversion ratio conversion of the converter transformer, the voltage and current equations of the wind farm collecting line are:

[0153]

[0154] Among them,

[0155] Z li = R li + jωL lii = 1, …, n

[0156] Z Tw = R Tw + jωL Tw ,Z w = 1 / Y w ,

[0157] where Δu M is the voltage at the Point of Common Connection (PCC) of the wind farm in the fault mutation network; Δi m1 is the current flowing into this collection line at the PCC of the wind farm; e i is the voltage at the outlet of the i-th wind turbine on the collection line, i = 1, ..., n; k Tw is the turn ratio of the converter transformer of the wind turbine generator.

[0158] Furthermore, the wind farm mutation impedance model in the frequency domain is obtained as:

[0159] Δu M = Z fc Δi m1 + ΔU fc (20)

[0160] where

[0161]

[0162] ΔU fc = (k Tw 2 Z w + Z Tw ) ΔI w

[0163] where Z fc is the equivalent mutation impedance of the wind farm; ΔU fc is the equivalent mutation voltage source of the wind farm.

[0164] It can be seen from Equation (20) that the frequency characteristic equation of the wind turbine generator mutation impedance is:

[0165]

[0166] where a k (k = 1, ..., 6), b k (k = 1, ..., 7), c k (k = 1, ..., 6) are constants calculated according to the parameters of the wind turbine generator, and a6, b7, and c6 are all greater than zero.

[0167] According to Equation (21), the mutant impedance of the wind turbine shows inductive or capacitive characteristics at different angular frequencies. Considering that the output characteristic harmonics of the PWM modulation of the grid-side converter of the wind farm are the 6k±1 (k = 1, 2,...) harmonics, and combining with Equation (21), the frequency band when the impedance of the wind turbine is inductive can be obtained as follows:

[0168]

[0169] where ω 1.i (i = 1, 2, 3) are the solutions of b7ω 7 +b5ω 5 +b3ω 3 +b1ω = 0, and ω 2.i (i = 1, 2, 3) are the solutions of c6ω 6 +c4ω 4 +c2ω 2 +c0 = 0.

[0170] At this time, the mutant impedance equivalent of the wind farm is a resistive-inductive element, that is:

[0171] Z fc = R fc +jω fcL L fc (23)

[0172] where R fc and L fc are the equivalent resistance and inductance of the wind farm collection line respectively.

[0173] Thus, the equivalent mutant impedance model of the direct-drive wind farm after a fault on any collection line, as well as the equivalent resistance and inductance, are obtained.

[0174] Second, based on the equivalent mutant impedance model of the wind farm after the fault, analyze the resistive-inductive high-frequency mutant energy at the installation location of the collection line outlet protection and each fan port under different faults respectively, and determine the difference in the flow direction of the mutant energy in the mutant networks inside and outside the zone.

[0175] In the first step, the mutant impedance model of the wind turbines on each collection line is analyzed and established. Using the fault transient component to construct the protection can greatly shorten the protection time.

[0176] Fully considering the topological changes of the wind farm fault mutant network under different fault scenarios, use Equation (22) to analyze the voltage and current information in the inductive frequency band at the installation location of the line protection and each fan terminal in the fault mutant network, and determine the difference in the flow direction of the mutant energy in the mutant networks inside and outside the zone.

[0177] When a fault occurs in the collection line inside the wind farm, the topological structure inside the wind farm changes, while the topological structure on the back side of the wind farm remains unchanged. At this time, the fault mutation network is as shown in Figure 6 . Among them, the collection line fault is a single-phase ground fault, a two-phase ground fault, a two-phase interphase fault or a three-phase fault.

[0178] Taking the single-phase fault of the φ (φ = a, b, c) phase of the second collection line as an example, analyze the high-frequency mutation energy at the installation location of the outlet protection of the collection line and the ports of each wind turbine:

[0179] According to Figure 6 , the following relationships exist between the voltages and currents of the AC system and the non-faulty collection lines:

[0180]

[0181] In the formula, Δu Mφ is the mutation of the voltage of the φ phase at the PCC point of the wind farm in the fault mutation network, that is, the mutation of the voltage of the φ phase at the installation location of the protection on the collection line; Δi sφ is the current of the φ phase of the fault flowing into the AC system at the PCC point of the wind farm; Δi m1φ is the mutation of the current of the φ phase of the fault flowing into the non-faulty collection line at the PCC point of the wind farm; R T , L T are the resistance and inductance of the connection transformer respectively; k T is the transformation ratio of the connection transformer; R s , L s are the equivalent resistance and inductance of the AC system respectively; ΔU fc is the equivalent mutation voltage source of the wind farm.

[0182] The high-frequency mutation energy W sφ of the AC system and the high-frequency mutation energy W 1φ of the non-faulty collection line are respectively:

[0183]

[0184]

[0185] According to the law of conservation of energy, the high-frequency mutation energy W 2φ of the faulty collection line is:

[0186]

[0187] The high-frequency mutation energy of each wind turbine port is:

[0188]

[0189] In the formula, Δu wiφis the phase φ voltage at the port of the i-th wind turbine in the fault mutation network; Δi wiφ is the phase φ current of the i-th wind turbine in the fault mutation network, that is, the mutation of the phase φ current of the i-th wind turbine on the collection line; R w , L w are the equivalent resistance and inductance of the wind turbine generator set respectively; ΔU w is the equivalent mutation voltage source of the wind turbine generator set.

[0190] From the above analysis, when a fault occurs in the collection line within the area, the high-frequency mutation energy at the installation location of the collection line outlet protection is less than 0, and the high-frequency mutation energy at each wind turbine port is greater than 0.

[0191] When a fault occurs in the wind turbine generator set inside the wind farm, the fuse on the high-voltage side of the converter transformer of the wind turbine generator set should disconnect the faulty branch. At this time, the mutation energy at the installation location of the collection line outlet protection and each wind turbine port is:

[0192]

[0193]

[0194] From the above analysis, when a fault occurs in the wind turbine generator set within the area, the high-frequency mutation energy at the installation location of the collection line outlet protection is less than 0, and the high-frequency mutation energy at any wind turbine port is less than 0.

[0195] When a fault occurs in the AC system outside the area, it will cause a certain degree of voltage drop at the machine terminal voltage of the units in the wind farm. The transient current flows through the collection line and is fed into the fault point. At this time, the topological structure of the fault network is quite different from that of the fault within the area. According to the physical topological structure in this fault situation and combined with the transient model of the wind turbine, the electrical quantity topological structure diagram in this scenario can be obtained, and its network topological structure is as Figure 7 shown.

[0196] According to Figure 7 it can be known that the voltage, and current relationship of the protected collection line is:

[0197]

[0198] At this time, the high-frequency mutation energy at the protection installation location is:

[0199]

[0200] When a fault occurs outside the wind farm collection line area, the wind turbine current is fed into the fault point, and the mutation energy at the protection installation location is greater than 0.

[0201] When a fault occurs in other collection lines outside the area, the topological structure in this fault scenario is as Figure 8 shown, and the mutation energy of the protected line is analyzed as:

[0202]

[0203] When a fault occurs in other collection lines outside the wind farm collection line area, the sudden change energy at the protection installation location is greater than 0.

[0204] From the above analysis, it can be seen that when an external fault occurs, the high-frequency sudden change energy at the collection line outlet protection installation location is greater than 0.

[0205] Third, based on the differences in the inductive and resistive high-frequency sudden change energies at the protection installation location of the protected collection line and each fan port during internal and external faults, a fault identification criterion is constructed to identify whether the fault on the collection line is an internal collection line fault.

[0206] In the second step, a fault mutation network of the wind farm collection line containing multiple fault additional sources was constructed, and a sudden change energy equation including fault parameters was established. According to the analysis of the sudden change energies at the protection installation location and each fan port after faults occur at different positions, comparing the direction differences of the sudden change energies at the protection installation location and each fan port can determine the fault location.

[0207] Considering the influence of factors such as measurement errors, the action threshold value of the high-frequency mutation energy difference coefficient at the protection installation location and each fan port on the collection line is set by the following formula:

[0208] W set =max{W nor ,W m2max}

[0209] W wiset =max{W wnor ,W wimax}

[0210] Among them,

[0211] W m2max =∫Δu Mmax Δi m2max dt,

[0212] W wimax =∫Δu wimax Δi wimax dt,

[0213] Δi wimax =Y w Δu wimax +ΔI w ,

[0214] In the formula, W nor represents the port energy in the steady-state operation mode of the direct-drive wind farm, and W m2max represents the sudden change energy at the protection installation location of the collection line in the maximum operation mode of the short circuit of the terminal fan in the direct-drive wind farm; W wnorIndicates the port energy of the direct-drive wind farm fan unit under steady-state operation mode, W wimax Indicates the port fault mutation energy of the i-th fan unit in the direct-drive wind farm; Δu Mmax Is the voltage mutation at the protection installation location on the collection line under the maximum operating mode of the short circuit of the end fan in the direct-drive wind farm; Δi m2max Is the current mutation at the protection installation location on the collection line under the maximum operating mode of the short circuit of the end fan in the direct-drive wind farm; Δu wimax Is the voltage mutation at the port of the i-th fan unit on the collection line under the maximum operating mode of the short circuit of the end fan in the direct-drive wind farm; Δi wimax Is the current mutation at the port of the i-th fan unit on the collection line under the maximum operating mode of the short circuit of the end fan in the direct-drive wind farm.

[0215] Specifically, the mutation energy W at the protection installation location on the collection line under the maximum operating mode of the short circuit of the end fan in the direct-drive wind farm m2max , is obtained according to the following derivation:

[0216] When the collection line is short-circuited at the end, the voltage at each fan port is expressed as:

[0217]

[0218] In the formula, u f Is the voltage at the end fan when the fault point is the short circuit of the end fan.

[0219] Iterate successively to get:

[0220]

[0221] At this time, the mutation energy obtained at the protection installation location of the collector line bus is:

[0222] W m2max = ∫Δu Mmax Δi m2max dt.

[0223] It should be noted that the voltage mutation Δu at the port of the i-th fan unit on the collection line under the maximum operating mode of the short circuit of the end fan in the direct-drive wind farm wimax , the current mutation Δi at the protection installation location on the collection line under the maximum operating mode of the short circuit of the end fan in the direct-drive wind farm m2max Is obtained through acquisition.

[0224] Based on the action threshold value, the high-frequency mutation energy difference coefficients at the protection installation location on the collection line and at each fan port are obtained, expressed as:

[0225]

[0226] Furthermore, the fault identification criterion includes:

[0227] If the high-frequency mutation energy difference coefficient at the protection installation location on the collection line is less than 0, and the high-frequency mutation energy difference coefficient at each fan port is greater than 0, a fault occurs in the in-zone collection line;

[0228] If the high-frequency mutation energy difference coefficient at the protection installation location on the collection line is less than 0, and the high-frequency mutation energy difference coefficient at any fan port is less than 0, a fault occurs in the in-zone wind turbine;

[0229] Otherwise, an out-of-zone fault occurs.

[0230] Specifically, if it is identified as an in-zone collection line fault, the protection acts; if it is identified as an in-zone wind turbine fault, the fuse on the high-voltage side of the converter transformer of the wind turbine disconnects the faulty branch; if it is identified as an out-of-zone fault of the collection line, the protection does not act.

[0231] Embodiment 2

[0232] A specific embodiment 2 of the present invention provides a direct-drive wind farm collection line protection system based on high-frequency mutation energy, as Figure 9 shown, including:

[0233] A data acquisition module, configured to obtain the voltage, current at the protection installation location of the collection line and at each fan port before and after the fault, and the electrical angle after the fault when it is detected that a fault occurs in the direct-drive wind farm collection line;

[0234] A high-frequency mutation energy difference coefficient calculation module, configured to obtain the high-frequency mutation energy and the action threshold value at the protection installation location of the collection line and at each fan port based on the acquired information, and further obtain the high-frequency mutation energy difference coefficient at the protection installation location of the collection line and at each fan port;

[0235] A fault identification and protection action module, configured to determine the type of the collection line fault according to the high-frequency mutation energy difference coefficient at the protection installation location of the collection line and at each fan port and the fault identification criterion. If the fault type is an in-zone collection line fault, the protection action is started to realize the protection of the direct-drive wind farm.

[0236] During implementation, the high-frequency mutation energy difference coefficient at the protection installation location of the collection line and at each fan port in the fault identification and protection action module is expressed as:

[0237]

[0238] In the formula, s m 、s wi respectively represent the high-frequency mutation energy difference coefficients at the protection installation location on the collection line and at the port of the i-th fan; h is the sampling time interval, T is the integration time, N is the total number of sampling points; ΔuMφ (j) and Δi Mφ (j) represent the voltage mutation and current mutation at the j-th sampling point of the fault phase φ at the protection installation location on the collection line respectively. Δe wiφ (j) and Δi wiφ (j) represent the voltage mutation and current mutation at the j-th sampling point of the fault phase φ at the port of the i-th wind turbine on the collection line respectively; W set and W wiset represent the action threshold values at the protection installation location and the port of the i-th wind turbine on the collection line respectively.

[0239] For the specific implementation process of the embodiments of the present invention, please refer to the above method embodiments, and this embodiment will not be elaborated here.

[0240] Since the principle of this embodiment is the same as that of the above method embodiment, this system also has the corresponding technical effects of the above method embodiment.

[0241] Embodiment 3

[0242] To verify the correctness of Embodiment 1 and Embodiment 2 of the present invention, the solutions in the above embodiments are experimentally verified. The main parameters of the direct-drive wind farm collection line system are shown in Table 1.

[0243] Table 1 Main parameters of the direct-drive wind farm collection line system

[0244]

[0245] In this embodiment, taking the fault occurrence moment as the zero moment and considering the influence of the fault occurrence location and transition resistance, the simulation verification is divided into the following three parts:

[0246] The first part: Simulation verification results of different transition resistance faults occurring in the wind farm collection line area

[0247] When A-phase ground fault, BC-phase interphase fault, and ABC-phase fault occur at 50% of the collection line respectively, the change range of the transition resistance is 0 - 300 Ω. The mutation energy difference coefficient s m at the protection installation location and s w2 at each wind turbine port (taking the second wind turbine on the collection line as an example in this embodiment), as shown in Figure 10 (a), Figure 10 (b), Figure 10 (c), Figure 10 (d), Figure 10 (e) and Figure 10 (f).

[0248] From Figure 10 (a), Figure 10 (b), Figure 10(c), Figure 10 (d), Figure 10 (e) and Figure 10 (f) shows that under different fault types, the smaller the transition resistance, the longer the fault time, and the more serious the fault. At the same time section, s m and s w2 with larger amplitudes, the protection identifies that a fault has occurred in the in-zone collecting line. From Figure 10 it can be seen that when a ground fault occurs in phase A and the line transition resistance is 300 Ω, at t = 0.1 ms, s m has a maximum value, which is -228.661 and is still less than zero. When the transition resistance of the phase A fault is 300 Ω, at t = 0.1 ms, s w2 has a minimum value, which is 35.435 and is still greater than zero. According to the above analysis, it can be known that the methods and systems provided in Embodiment 1 and Embodiment 2 can correctly operate the protection when a high-resistance fault occurs in the in-zone of the line, are not affected by the fault type, and have high sensitivity and fast recognition ability.

[0249] Part Two: Simulation Verification of Faults Occurring at Different Positions in the In-zone of the Wind Farm Collecting Line

[0250] Suppose a ground fault in phase A and a ground fault in phases AB occur at different positions in the in-zone of the collecting line, and the transition resistance is 100 Ω. The difference coefficient s m of the sudden change energy at the protection installation location and the difference coefficient s w2 of the sudden change energy at the fan port under this fault condition are as shown in Figure 11 (a), Figure 11 (b), Figure 11 (c), Figure 11 (d).

[0251] From Figure 11 (a), Figure 11 (b), Figure 11 (c), Figure 11 (d) it can be seen that when a fault occurs at different positions on the line, under the same time section for a ground fault in phase A, the fluctuation degree of s m and s w2 is relatively small, and under the same time section for a fault in phases AB, the fluctuation degree of s m and s w2 is relatively large. Under different fault conditions, both satisfy s m < 0, s w2 > 0, and the protection identifies that a fault has occurred in the in-zone collecting line. When a line fault occurs at the position 1% away from the M end, at t = 2 ms, s m is the maximum value, which is -261.776, but is still much less than zero. When a line fault occurs at the position 1% away from the M end, at t = 1.2 ms, s w2is the minimum value, which is 228.45, but still greater than zero. According to the analysis of the simulation results, it can be seen that the fault identification criteria proposed by the methods and systems provided in Embodiment 1 and Embodiment 2 are not affected by the fault location. When a high-resistance fault occurs at the end of the line, it still has high sensitivity.

[0252] Part III: Simulation verification of faults occurring through different transition resistances outside the wind farm collection line area

[0253] When a fault occurs outside the collection line area, it is only necessary to identify the difference in the sudden change energy direction at the protection installation location at the M end. Assume that an A-phase ground fault occurs in the AC system outside the collection line area and in other collection lines respectively, with the transition resistance varying from 0 to 300 Ω, and a BC-phase ground fault occurs, with the transition resistance varying from 0 to 100 Ω. The simulation results under the above fault conditions are as Figure 12 (a), Figure 12 (b), Figure 13 (a), Figure 13 (b) shown.

[0254] From Figure 12 (a), Figure 12 (b), it can be seen that when an A-phase ground fault occurs in the AC system on the back side of the M end, with the transition resistance ranging from 0 to 300 Ω, and when a BC-phase fault occurs, with the transition resistance ranging from 0 to 100 Ω, the sudden change energy difference coefficient s m is greater than zero, and it is determined as an external fault. When an A-phase ground fault occurs, with the transition resistance of 300 Ω and t = 0.1 ms, s m has a minimum value of 912.37, indicating that no fault occurs in the line area and the protection does not operate reliably.

[0255] From Figure 13 (a), Figure 13 (b), it can be seen that when an A-phase ground fault and a BC-phase fault occur in other collection lines on the back side of the M end, the sudden change energy difference coefficient s m is greater than zero, and it is determined as an external fault in the collection line area. When an A-phase ground fault occurs, with the transition resistance of 300 Ω and t = 0.1 ms, s m has a minimum value of 547.5745, indicating that no fault occurs in the line area and the protection does not operate reliably.

[0256] Those skilled in the art can understand that all or part of the processes for implementing the methods of the above embodiments can be completed by instructing relevant hardware through a computer program, and the program can be stored in a computer-readable storage medium. Among them, the computer-readable storage medium is a magnetic disk, an optical disk, a read-only memory, or a random access memory, etc.

[0257] As described above, it is only the preferred specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A direct-drive wind farm collection line protection method based on high-frequency mutation energy, characterized in that Including the following steps: When it is detected that a fault occurs in the collection line of the direct-drive wind farm, obtain the voltages, currents at the protection installation location of the collection line and at each fan port before and after the fault, and the electrical angle after the fault; Based on the obtained information, obtain the high-frequency sudden change energy and the action threshold value at the protection installation location of the collection line and at each fan port, and then obtain the high-frequency mutation energy difference coefficient at the protection installation location of the collection line and at each fan port; According to the high-frequency mutation energy difference coefficient at the protection installation location of the collection line and at each fan port and the fault identification criterion, judge the type of the collection line fault. If the fault type is an in-zone collection line fault, start the protection action to protect the direct-drive wind farm; The high-frequency mutation energy difference coefficient at the protection installation location of the collection line and at each fan port is expressed as: where s m and s wi respectively represent the high-frequency mutation energy difference coefficients at the protection installation location on the collection line and at the port of the i-th wind turbine; h is the sampling time interval, T is the integration time, and N is the total number of sampling points; Δu Mφ (j) and Δi Mφ (j) respectively represent the voltage mutation and current mutation at the j-th sampling point of the fault phase φ at the protection installation location on the collection line, and Δe wiφ (j) and Δi wiφ (j) respectively represent the voltage mutation and current mutation at the j-th sampling point of the fault phase φ at the port of the i-th wind turbine on the collection line; W set and W wiset respectively represent the operating threshold values at the protection installation location on the collection line and at the port of the i-th wind turbine. The fault identification criterion includes: If on the collection line, it is satisfied that the high-frequency mutation energy difference coefficient at the protection installation location is less than 0, and the high-frequency mutation energy difference coefficient at each fan port is greater than 0, then an in-zone collection line fault occurs; If on the collection line, it is satisfied that the high-frequency mutation energy difference coefficient at the protection installation location is less than 0, and the high-frequency mutation energy difference coefficient at any one fan port is less than 0, then an in-zone wind turbine fault occurs; Otherwise, an out-of-zone fault occurs.

2. The method for protecting a direct-drive wind farm collection line based on high-frequency mutation energy according to claim 1, characterized in that Determine the operating threshold values \(W\) at the protection installation location on the aggregation line and at the port of the \(i\)-th fan in the following manner set and \(W\) wiset as follows: W set = max{W nor , W m2max} W wiset = max{W wnor , W wimax} Where, W nor represents the port energy under the steady-state operation mode of the direct-drive wind farm, and W m2max represents the sudden change energy at the installation location of the collector line protection under the maximum operation mode of the short circuit of the end wind turbines in the direct-drive wind farm; W wnor represents the port energy under the steady-state operation mode of the wind turbine groups in the direct-drive wind farm, and W wimax represents the sudden change energy of the i-th wind turbine group port on the collector line of the direct-drive wind farm.

3. The method for protecting a direct-drive wind farm collection line based on high-frequency mutation energy according to claim 2, wherein The sudden change energy W at the installation location of the collector line protection under the maximum short-circuit operating mode of the direct-drive wind farm terminal fan m2max , is expressed as: W m2max = ∫Δu Mmax Δi m2max dt where Δu Mmax is the voltage mutation at the protection installation point on the collection line under the maximum operating mode of short circuit of the wind turbines at the end of the direct-drive wind farm; Δi m2max is the current mutation at the protection installation point on the collection line under the maximum operating mode of short circuit of the wind turbines at the end of the direct-drive wind farm.

4. The method for protecting a direct-drive wind farm collection line based on high-frequency mutation energy according to claim 3, wherein The voltage mutation Δu at the protection installation location on the collection line under the maximum short-circuit operating mode of the direct-drive wind farm end fan Mmax , is expressed as: Wherein, Z li = R li + jωL li , Z Tw = R Tw + jωL Tw ,Z w = 1 / Y w , where k Tw is the turns ratio of the converter transformer of the wind turbine generator, R li , L li are respectively the resistance and inductance of the i-th wind turbine generator set on the collection line, R Tw , L Tw respectively represent the resistance and inductance of the converter transformer of the wind turbine generator set; ω is the angular frequency, u f is the voltage at the terminal wind turbine when the fault point is a short circuit at the terminal wind turbine, n represents the total number of wind turbine generator sets on the collection line, n > 3; ω p , ω s are respectively the p-th harmonic angular frequency and the fundamental angular frequency; E s is the steady-state voltage amplitude after the fault at the wind turbine port; K ppll , K ipll are respectively the proportional and integral coefficients of the phase-locked loop PI control; θ u is the actual electrical angle of the power grid after the fault; K gp , K gi are respectively the proportional and integral coefficients of the current loop PI control; R g , L g are respectively the equivalent resistance and inductance of the line reactor on the grid side converter; are respectively the d-axis and q-axis current reference values under steady-state operation conditions, are respectively the d-axis and q-axis current reference values under steady-state operation after the fault.

5. The method for protecting a direct-drive wind farm collection line based on high-frequency mutation energy according to claim 4, characterized in that The fault mutation energy W of the i-th wind turbine set port on the direct-drive wind farm collection line wimax is expressed as: W wimax = ∫Δu wimax Δi wimax dt Where, Δu wimax is the voltage mutation at the port of the i-th wind turbine unit on the collection line under the maximum operating mode of short circuit of the wind turbines at the end of the direct-drive wind farm; Δi wimax is the current mutation at the port of the i-th wind turbine unit on the collection line under the maximum operating mode of short circuit of the wind turbines at the end of the direct-drive wind farm.

6. The method for protecting a direct-drive wind farm collection line based on high-frequency mutation energy according to claim 5, wherein The current mutation Δi at the port of the i-th wind turbine group on the collection line under the maximum short-circuit operating mode of the direct-drive wind farm terminal wind turbine wimax , is expressed as: Δi wimax = Y w Δu wimax + ΔI w .

7. A direct-drive wind farm collection line protection system based on high-frequency mutation energy for the direct-drive wind farm collection line protection method based on high-frequency mutation energy according to any one of claims 1-6, characterized in that, Including: A data acquisition module, configured to obtain the voltages, currents at the protection installation location of the collection line and at each fan port before and after the fault, and the electrical angle after the fault when it is detected that a fault occurs in the collection line of the direct-drive wind farm; A high-frequency mutation energy difference coefficient calculation module, configured to obtain the high-frequency sudden change energy and the action threshold value at the protection installation location of the collection line and at each fan port based on the obtained information, and then obtain the high-frequency mutation energy difference coefficient at the protection installation location of the collection line and at each fan port; A fault identification and protection action module, configured to judge the type of the collection line fault according to the high-frequency mutation energy difference coefficient at the protection installation location of the collection line and at each fan port and the fault identification criterion. If the fault type is an in-zone collection line fault, start the protection action to protect the direct-drive wind farm.

8. The direct-drive wind farm collector line protection system based on high-frequency mutation energy according to claim 7, wherein The high-frequency mutation energy difference coefficient at the protection installation location of the collection line and at each fan port in the fault identification and protection action module is expressed as: where s m and s wi respectively represent the high-frequency mutation energy difference coefficients at the protection installation location on the collection line and at the port of the i-th wind turbine; h is the sampling time interval, T is the integration time, and N is the total number of sampling points; Δu Mφ (j) and Δi Mφ (j) respectively represent the voltage mutation and current mutation at the j-th sampling point of the fault phase φ at the protection installation location on the collection line, and Δe wiφ (j) and Δi wiφ (j) respectively represent the voltage mutation and current mutation at the j-th sampling point of the fault phase φ at the port of the i-th wind turbine on the collection line; W set and W wiset respectively represent the action threshold values at the protection installation location on the collection line and at the port of the i-th wind turbine.

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