DFIG outgoing line pilot protection method based on ratio of fundamental frequency components at two ends

Abstract

The invention discloses a DFIG outgoing line pilot protection method based on the ratio of fundamental frequency components at two ends, relates to the technical field of fault positioning, and solves the problem that in the prior art, when a traditional protection scheme is applied to a DFIG outgoing line, misoperation or operation refusal of the traditional protection scheme can be caused. According to the method, firstly, different fault ride-through strategies adopted by the DFIG under different voltage sags are analyzed, then specific fault characteristics of the DFIG under all stages are researched, and stator side fault current expressions of different stages are given; introducing a process of extracting a fundamental component value by using traditional Fourier transform in an existing protection device, and applying the process to extraction of fundamental components at two ends of a DFIG outgoing line; according to the DFIG outgoing line pilot protection method, the proportion of the fundamental frequency component values at the two ends is calculated, the DFIG outgoing line pilot protection based on the proportion of the fundamental frequency components at the two ends is provided, accuracy and reliability of existing protection actions are improved, and the method has high engineering applicability.

Description

Technical Field

[0001] The present invention relates to the technical field of fault location, and in particular to a DFIG transmission line longitudinal protection method based on the proportion of fundamental frequency components at both ends. Background Art

[0002] With the rapid growth of global demand for renewable energy, new energy technologies, especially wind power generation technology, have become an important way to alleviate the energy crisis and reduce carbon emissions. Among the many wind turbine types, the doubly-fed induction generator (DFIG) has gradually taken a leading position in the wind power market with its superior variable speed constant frequency operation capability, high energy conversion efficiency and flexible grid access characteristics. At present, DFIG has been commercialized on a large scale around the world and has become an important force in promoting the rapid development of the new energy industry.

[0003] In addition, with the high penetration rate of wind power connected to the grid, the fault characteristics of DFIG are significantly different from those of traditional synchronous motors, and its impact on the transmission line is becoming more and more complex, making traditional protection schemes face challenges when applied to DFIG transmission lines. For example, the short-circuit current characteristics generated by DFIG during a fault may cause the traditional protection scheme to malfunction or fail to operate.

[0004] In view of this, a DFIG transmission line longitudinal protection method based on the proportion of fundamental frequency components at both ends is needed to improve the operation reliability and stability of the existing new energy power grid with DFIG access. Summary of the invention

[0005] In view of the problem that the traditional protection scheme in the prior art may cause false operation or refusal to operate when applied to the DFIG transmission line, the present invention provides a DFIG transmission line longitudinal protection method based on the proportion of the fundamental frequency components at both ends, which can extract the fundamental frequency components of the fault current by Fourier transform (FT), and calculate the ratio of the fundamental frequency components at both ends, compare it with the setting value, and construct a DFIG longitudinal protection with a simple principle and strict judgment of faults inside and outside the zone, thereby improving the reliability of the existing DFIG access system. The specific technical scheme is as follows:

[0006] The present invention provides a DFIG transmission line longitudinal protection method based on the proportion of the fundamental frequency components at both ends, comprising the following steps:

[0007] (1) Analysis of different fault ride-through strategies adopted by DFIG under different voltage drop levels

[0008] When the fault is serious, the voltage drop on the DFIG side is large, and the crowbar protection will be activated. When the fault is minor, the DFIG maintains the power outer loop control, and the output is still controlled by the rotor-side converter (RSC) excitation.

[0009] (2) Calculation of the output fault current value of the DFIG stator side under different fault ride-through strategies

[0010] When the saturation of the magnetic circuit is ignored and the motor convention is adopted, the stator and rotor voltage and flux equations of the doubly fed wind turbine in the dq two-phase rotating coordinate system can be expressed as:

[0011]

[0012] Among them, the subscript s represents the electrical quantity on the stator side, the subscript r represents the electrical quantity on the rotor side, U, i, r represents voltage, current, flux linkage and resistance respectively; L s , L r are the self-inductance values ​​of the stator and rotor sides of the fan respectively; L m is the mutual inductance value of the stator and rotor sides, and the three satisfy the relationship: L s =L m +L σs ,L r =L m +L σr , where L σs , L σr are the leakage inductance values ​​of the stator and rotor sides respectively; ω1 is the synchronous angular velocity value, s is the slip rate when the fan is running, and s=(ω1-ω r ) / ω1, where ω r is the rotor speed value.

[0013] According to the rotor flux equation of formula (2), we can get:

[0014]

[0015] Substituting it into the rotor flux equation, we get:

[0016]

[0017] Substituting equation (6) into the rotor voltage equation, the relationship between the current and flux on the stator side is obtained as follows:

[0018]

[0019] From this, it can be deduced that the stator flux value after Crowbar is put into operation in the dq synchronous rotating coordinate system is:

[0020]

[0021] The stator and rotor current time domain expressions can be solved by combining the stator and rotor voltage and flux equations.

[0022] When the fault drop is relatively minor, for example, the voltage value is between 0.4 and 0.9 pu, DFIG still adopts the RSC excitation control strategy. The stator flux after the fault has two components: one is the stator flux steady-state component corresponding to the residual voltage at the machine end. The second is the stator flux transient component corresponding to the voltage drop, which decays with the stator time constant. Then the stator flux in the dq synchronous rotating coordinate is for:

[0023]

[0024] In the formula: sc is the inverse of the stator side decay time constant, and its expression is τ sc =R s L r / L D ; t0 is the time when the fault occurs; U s0 is the fault initial voltage per unit value; ω1 is the synchronous angular velocity value; k c is the fault voltage drop per unit value, ranging from 0 to 1; t is the time; is the stator flux when the crowbar is dropped. According to the stator and rotor flux equation, we can get:

[0025]

[0026] The stator and rotor current time domain expressions can be solved by combining the stator and rotor voltage and flux equations.

[0027] (3) Study the process of extracting the fundamental frequency component value through Fourier transform

[0028] The process of extracting the fundamental frequency component by Fourier transform mainly involves converting the signal from the time domain to the frequency domain and identifying the fundamental frequency component from it. The following are the detailed steps of the process:

[0029] 1. Obtain time domain signal

[0030] First, it is necessary to obtain the time domain signal to be analyzed. This is usually obtained by measuring sensors. For example, in power systems, it may be an electric power signal measured by a current or voltage sensor.

[0031] 2. Perform Fourier Transform

[0032] The time domain signal is input into the Fourier transform algorithm to obtain the frequency domain signal. Fourier transform is a mathematical method to convert a signal from the time domain (or space domain) to the frequency domain, which can reveal the spectral characteristics of the signal.

[0033] 3. Extract the fundamental frequency component

[0034] In the frequency domain signal, the fundamental frequency component corresponds to the component with the lowest frequency in the signal, usually the main component, and its frequency is called the fundamental frequency. In the power system, the fundamental frequency component is usually the same as the power frequency (ie 50Hz).

[0035] Identify the fundamental frequency: In the frequency domain diagram, the fundamental frequency component usually appears as an obvious peak, and its corresponding frequency is the fundamental frequency.

[0036] Extracting the fundamental frequency component: By setting an appropriate frequency range or threshold, the fundamental frequency component can be extracted. Specifically, the frequency components near the fundamental frequency in the frequency domain signal can be screened and integrated to obtain complete information about the fundamental frequency component.

[0037] This process is widely used in signal processing, power system analysis and other fields. Through Fourier transform, the fundamental frequency component in the signal can be efficiently and accurately identified, providing strong support for subsequent analysis and application. Assuming the period of the periodic function u(t) is T, if the periodic function satisfies the Dirichlet condition, the periodic function can be expressed by the Fourier series as follows:

[0038]

[0039] Where: a n 、b n are the sine term and cosine term coefficients of the nth harmonic component respectively. When n is set to different values, the component values ​​of different frequencies can be solved by the above formula. For the fundamental frequency component, n=1 is set.

[0040] (4) Extract the fundamental frequency component of the current under different fault ride-through strategies and calculate the proportion of the fundamental frequency component values ​​on both sides

[0041] According to the process of extracting the power frequency component using Fourier transform mentioned in Section (3), its specific calculation steps can be obtained. Assuming the period of the periodic function u(t) is T, if the periodic function satisfies the Dirichlet condition, then the periodic function can be expressed by the Fourier series as:

[0042]

[0043] Where: a n 、b n are the sine term and cosine term coefficients of the nth harmonic component respectively. When n is set to different values, the component values ​​of different frequencies can be solved by the above formula. For the fundamental frequency component, n=1 is set.

[0044] According to the above formula, the fundamental frequency components of the current values ​​at both ends of the DFIG transmission line can be extracted respectively, and the magnitudes of the remaining components can be calculated, so that the ratio of the two can be further calculated.

[0045] (5) A DFIG transmission line longitudinal protection based on the proportion of the fundamental frequency components at both ends is proposed

[0046] The ratio of the fundamental frequency component amplitude to the sum of all component amplitudes is defined as the fundamental frequency ratio (FFR). By comparing the calculated fundamental frequency component ratios at both ends with the set value, it can be determined whether the fault occurs inside or outside the zone.

[0047] The beneficial effects of the present invention are:

[0048] The present invention describes a DFIG transmission line longitudinal protection based on the proportion of fundamental frequency components at both ends. First, different fault ride-through strategies adopted by DFIG under different voltage drops are analyzed, and then the specific fault characteristics of DFIG under each stage are studied, and the stator side fault current expressions at different stages are given; then, the process of extracting fundamental frequency component values ​​using traditional Fourier transform in the existing protection device is introduced, and it is applied to the extraction of fundamental frequency components at both ends of the DFIG transmission line; the proportion of the fundamental frequency component values ​​at both ends is calculated, and a DFIG transmission line longitudinal protection based on the proportion of fundamental frequency components at both ends is proposed, which improves the accuracy and reliability of existing protection actions and has high engineering applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for the specific embodiments or the description of the prior art. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn according to the actual scale.

[0050] Figure 1 It is a DFIG transmission line longitudinal protection based on the proportion of the fundamental frequency components at both ends described in the present invention;

[0051] Figure 2 It is a schematic diagram of the DFIG grid-connected simulation model of the present invention;

[0052] Figure 3 A comparison diagram of the currents at both ends of the fault inside and outside the zone described in the present invention;

[0053] Figure 4 It is the spectrum diagram of the current at both ends after Fourier transformation under the fault in the area. DETAILED DESCRIPTION

[0054] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0055] It should be understood that when used in this specification and the appended claims, the terms "include" and "comprises" indicate the presence of described features, integers, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or combinations thereof.

[0056] It should also be understood that the terms used in the present specification are only for the purpose of describing specific embodiments and are not intended to limit the present invention. As used in the present specification and the appended claims, the singular forms "a", "an" and "the" are intended to include plural forms unless the context clearly indicates otherwise.

[0057] It should be further understood that the term "and / or" used in the present description and the appended claims refers to any and all possible combinations of one or more of the associated listed items, and includes these combinations.

[0058] Combine the following Figure 1-Figure 4 , the implementation mode of the present invention is further described.

[0059] In one embodiment of the present invention, a DFIG transmission line longitudinal protection method based on the proportion of the fundamental frequency components at both ends is provided, comprising the following steps:

[0060] S1: Calculate the output fault current value of the DFIG stator side under different fault ride-through strategies, where the fault ride-through strategies include the crowbar protection strategy and the RSC excitation control strategy;

[0061] S2: Use Fourier transform to convert the signal from time domain to frequency domain to obtain component values ​​of different frequencies, including the fundamental frequency component;

[0062] S3: The ratio of the fundamental frequency component amplitude to the sum of all component amplitudes is defined as the fundamental frequency fraction FFR. Based on the absolute value of the FFR difference between the currents at both ends, the fault is judged as an internal fault or an external fault, thereby determining the fault area.

[0063] S4: The protection device issues a tripping command based on the result of fault location and operates the circuit breaker to cut off the fault circuit.

[0064] The following further describes the above steps:

[0065] 1. DFIG ride-through strategy under different voltage drops:

[0066] When the fault is serious, when the grid fails and the voltage drops, the rotor side of the doubly fed wind turbine may be subjected to excessive current and voltage, thus damaging the equipment. At this time, the crowbar protection will be quickly activated to isolate the rotor side from the grid to protect the stable operation of the wind turbine and the grid. After the crowbar circuit is activated, the current on the rotor side is limited to a safe range, avoiding equipment damage caused by overcurrent.

[0067] When the fault is minor, the DFIG keeps the power outer loop control intact, and the output is still controlled by the rotor-side converter (RSC) excitation. By controlling the RSC, the active power and reactive power output of the doubly fed wind turbine can be adjusted to achieve maximum wind energy tracking, unity power factor power generation, or reactive power compensation for the power grid.

[0068] 2. DFIG fault output current value under different fault ride-through strategies:

[0069] Fault characteristics when crowbar protection is enabled:

[0070] When a serious fault occurs on the grid side, the crowbar protection on the DFIG side will be activated. According to the principle of flux conservation, at the moment of fault, the voltage at the DFIG machine end drops sharply, and the stator flux does not change suddenly. Therefore, the stator flux after the fault contains the steady-state component corresponding to the residual voltage at the machine end and the transient component induced in the stator flux by the voltage drop. The stator flux expression after the fault is:

[0071]

[0072] Where: τ sc is the inverse of the stator side decay time constant, and its expression is τ sc =R s L r / L D ; t0 is the time when the fault occurs; U0 is the fault initial voltage per unit value; ω1 is the synchronous angular velocity value. According to the rotor flux equation, we can get:

[0073]

[0074] Substituting it into the rotor flux equation, we get:

[0075]

[0076] Among them, L s , L r are the self-inductance values ​​of the stator and rotor sides of the fan respectively; L m is the mutual inductance value on the stator and rotor sides, L D=L r L s -L m 2 After the crowbar protection is activated, the rotor-side converter will be locked. At this time, the RSC control loses its function and the rotor-side voltage value suddenly drops to 0, that is, U r =0. After the crowbar protection is put into use, the rotor side is equivalent to having an additional resistor value Rc connected in series, and its resistance value will change from the initial value Rr to Rr'=Rr+Rc. Therefore, after the crowbar is put into use, the voltage and resistance value on the rotor side have changed, which will affect the coupling relationship between the stator and rotor sides and the current values ​​on both sides. Substituting formula (6) into the rotor voltage equation, the relationship between the current and flux on the stator side is obtained as follows:

[0077]

[0078] Where: is the rotor flux vector after the crowbar protection is put into use; R r ' is the equivalent resistance value of the rotor side after the crowbar protection is put into use, and its expression is R r '=R r +R c , R c L is the crowbar protection resistor value; D =L s L r -L m 2 Combining equations (3) and (5), we can obtain The expression of stator current and its component coefficients can be further obtained as follows.

[0079]

[0080]

[0081] C=i sc(0) -AB (22)

[0082] Where: i sc(0) is the stator current vector of the wind turbine during normal operation; τ rc is the inverse of the rotor side decay time constant, expressed as R r 'L s / L D Transforming equation (6) to the abc three-phase stationary coordinate system, its time domain expression is:

[0083]

[0084] Where: i A 、i B 、i Care the power frequency, DC and rotor frequency components in the fault current respectively; A1, B1 and C1 are the amplitudes of expressions A, B and C respectively; is the phase value of expressions A, B, and C.

[0085] Fault characteristics under RSC excitation control:

[0086] According to the principle of flux conservation, the stator flux will not change suddenly at the moment of fault. Therefore, the stator flux after the fault has two components: one is the stator flux steady-state component corresponding to the residual pressure at the machine end. The second is the stator flux transient component corresponding to the voltage drop, which decays with the stator time constant. Then the stator flux in the dq synchronous rotating coordinate is for:

[0087]

[0088] Where: τ s_rsc =R s L r / L D According to the stator and rotor flux equation, we can get:

[0089]

[0090] From this, the rotor voltage equation can be obtained. Combining the above equations, the second-order differential equation of the rotor current can be obtained as follows:

[0091]

[0092] Where: β1, β2, β r is the value related to the RSC inner and outer loop control parameters. Solving the last time, the stator current analytical formula under dq synchronous rotating coordinates is:

[0093]

[0094] In the formula, A s_rsc , B s_rsc , C s_rsc It can be seen that in the three-phase stationary coordinate system, the DFIG stator short-circuit current considering RSC control contains forced components, transient DC components and transient natural components; the rotor current contains forced difference frequency components, attenuated speed frequency components and natural components.

[0095] 3. The process of extracting the fundamental frequency component through Fourier transform:

[0096] Fourier transform (FT) is an important mathematical tool, widely used in signal processing, image processing, physics, engineering and other fields. Through Fourier transform, any signal that meets the conditions can be decomposed into a linear combination of sine and / or cosine functions of different frequencies. These sine and cosine functions represent the various frequency components in the signal, including the fundamental frequency component and the higher harmonic components. The main process is as follows:

[0097] Assume that the period of the periodic function u(t) is T. If the periodic function satisfies the Dirichlet condition, then the periodic function can be expressed by the Fourier series as follows:

[0098]

[0099] Where: a n 、b n are the sine term and cosine term coefficients of the nth harmonic component respectively. When n is set to different values, the component values ​​of different frequencies can be solved by the above formula. For the fundamental frequency component, n=1 is set.

[0100] 4. The process of extracting the fundamental frequency component of the current under different fault ride-through conditions:

[0101] According to the introduction in (2) and (3) above, it can be seen that the fault current of DFIG under both the crowbar protection and RSC excitation control fault ride-through strategies contains non-power frequency component values, which will affect the fundamental frequency component value extracted by Fourier transform in the existing protection. However, since Fourier transform can extract multi-frequency component values, it can be used to analyze the frequency distribution on the DFIG transmission line, thereby constructing protection to judge the fault inside and outside the zone.

[0102] In addition, since one side of the DFIG transmission line is DFIG, the fault current has a more complex component value, which is also related to its specific fault ride-through strategy, and one side is the system side, the current mainly contains the fundamental frequency 50Hz component and the DC component value. For the DFIG side, the ratio of the fundamental frequency component of the current to other frequency components (excluding the DC component, that is, the component value of n=0) under different fault ride-through strategies is small, and for the system side, the fundamental frequency component value of the current is larger than that of other frequency components. Therefore, the proportion of the fundamental frequency components at both ends will be greatly different when there is a fault in the area, and the longitudinal differential protection of the DFIG transmission line can be constructed based on this.

[0103] 5. DFIG pilot protection based on the proportion of fundamental frequency components at both ends:

[0104] Based on the above analysis, it can be seen that when an in-zone fault occurs, the current on the DFIG side can be divided into crowbar protection input and RSC excitation control according to different fault ride-through characteristics. The fault output current values ​​of the two will also be different, but both contain more harmonic components, and the ratio of the fundamental frequency component value to the sum of all components (except the DC component) is small; for the system side, the fault current mainly contains fundamental frequency components and DC components, and the ratio of the fundamental frequency component value to the sum of all components (except the DC component) is large. The ratio of the amplitude of the fundamental frequency component to the amplitude of the sum of all components is defined as the fundamental frequency ratio (FFR), and the expression is:

[0105]

[0106] Therefore, when there is a fault in the area, the FFR values ​​of the currents at both ends differ greatly.

[0107] When an out-of-zone fault occurs, the currents at both ends of the non-fault line are the same, and the ratio of the fundamental frequency component of the current on both sides obtained after Fourier transform to the sum of all components is the same. The FFR values ​​of the currents at both ends are slightly different when an out-of-zone fault occurs.

[0108] From this, we can construct a protection criterion that satisfies within a certain period of time:

[0109] |FFR 差 |>FFR set (32)

[0110] In the present invention, FFR is set set The value is 5%.

[0111] In order to further illustrate a DFIG transmission line longitudinal protection based on the proportion of fundamental frequency components at both ends provided by an embodiment of the present invention, the present invention is further described below in conjunction with an embodiment.

[0112] As Appendix Figure 2 In the DFIG grid-connected system shown in the figure, a three-phase short circuit occurs in Line 1 in the setting area. The current comparison between protection 1 and protection 2, and the current comparison between protection 3 and protection 4 are measured in Appendix Figure 3 .

[0113] according to Figure 3 It can be seen that the current waveforms on both sides of the fault line are quite different, and the currents at both ends of the non-fault line are equal in magnitude and opposite in direction. Therefore, after using Fourier transform to perform spectrum analysis on the waveforms on both sides, the current spectrum difference between the two ends of the fault zone is large, and the current spectrum difference between the two ends of the fault zone is small. The fundamental frequency ratio FFR value is used to quantify the difference, and the spectrum analysis of the two parts of current I1 and I2 (I4, -I3) is performed separately, and the spectrum diagram can be obtained as shown in the appendix. Figure 4shown.

[0114] It can be seen that the fault current on the DFIG side is quite different from that on the system side. The spectrum of both sides can be obtained by Fourier transform, from which the ratio of the fundamental frequency component to other subharmonic components, i.e., the FFR value, can be calculated. According to the following table, the result can be further solved as follows: FFR (I1) = 1.009197 / 1.97338538 = 51.14%, FFR (I2) = 0.824137 / 1.288639789 = 63.954%. The difference between the two is 63.954% - 51.14% = 12.814% > 5%, so the protection can ensure the correct operation in the area. In addition, it can be seen that the current I1 is the current value on the fan side. Due to the special fault ride-through strategy of the fan, this side is quite different from the current I2 on the system side. The FFR (I1) value on the fan side is small, and the FFR (I2) value on the system side is large, so the DFIG transmission line longitudinal protection proposed in the present invention can ensure correct operation within the zone and no operation for faults outside the zone.

[0115] Table 1 Amplitude of frequency components on the fan and system side

[0116]

[0117]

[0118] For out-of-zone faults, the current values ​​at both ends only differ in positive and negative signs. Therefore, the frequency spectrum diagrams extracted at both ends by Fourier transform are slightly different, and the difference in the calculated FFR values ​​at both ends is also small, thereby ensuring the correct non-action of out-of-zone faults. In summary, the DFIG refined fault calculation model proposed in the present invention, based on the consideration of different fault crossing strategies of the wind turbine and the GSC feed current, derives in detail the relationship between the DFIG output current and the voltage value at each stage. The iterative values ​​and simulation values ​​based on each model are compared. The results show that the refined fault calculation model proposed in this article has higher accuracy in iterative calculation of faulty power grids, improves the fault calculation accuracy of existing new energy power grids with DFIG access, and illustrates the practical engineering significance of the invention proposed in this article.

[0119] Those of ordinary skill in the art will appreciate that the units of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, computer software, or a combination of the two. In order to clearly illustrate the interchangeability of hardware and software, the composition of each example has been generally described in terms of function in the above description. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of the present invention.

[0120] In the embodiments provided by the present invention, it should be understood that the division of units is only a logical function division, and there may be other division methods in actual implementation, for example, multiple units can be combined into one unit, one unit can be split into multiple units, or some features can be ignored, etc.

[0121] In addition, each functional unit in each embodiment of the present invention may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit. The above-mentioned integrated unit may be implemented in the form of hardware or in the form of software functional units.

[0122] If the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or all or part of the technical solution can be embodied in the form of a software product, which is stored in a storage medium and includes several instructions for a computer device (which can be a personal computer, a server or a network device, etc.) to perform all or part of the steps of the method described in each embodiment of the present invention. The aforementioned storage medium includes: U disk, read-only memory (ROM, Read-0nlyMemory), random access memory (RAM, RandomAccessMemory), mobile hard disk, magnetic disk or optical disk, etc., which can store program code.

[0123] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents. These modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be included in the scope of the claims and specification of the present invention.

Claims

1. A DFIG transmission line longitudinal protection method based on the proportion of the fundamental frequency components at both ends, characterized in that: The following steps are involved: Calculate the output fault current value of the DFIG stator side under different fault ride-through strategies, where the fault ride-through strategies include the crowbar protection strategy and the RSC excitation control strategy; Using Fourier transform, the signal is converted from the time domain to the frequency domain to obtain the component values ​​of different frequencies, including the fundamental frequency component; The ratio of the fundamental frequency component amplitude to the sum of all component amplitudes is defined as the fundamental frequency fraction FFR. Based on the absolute value of the FFR difference between the currents at both ends, the fault is judged as an internal fault or an external fault, thereby determining the fault area. The protection device issues a tripping command based on the result of fault location and operates the circuit breaker to cut off the fault circuit.

2. A DFIG transmission line longitudinal protection method based on the proportion of the fundamental frequency components at both ends according to claim 1, characterized in that: The crowbar protection strategy is specifically as follows: when a fault occurs in the power grid and causes a voltage drop, the crowbar protection is activated to isolate the rotor side from the power grid. After the crowbar circuit is activated, the current on the rotor side is limited to a safe range.

3. A DFIG transmission line longitudinal protection method based on the proportion of the fundamental frequency components at both ends according to claim 2, characterized in that: The calculation process of the stator side output fault current value under the crowbar protection strategy is as follows: The stator flux expression after the fault is: Among them, τ sc is the inverse of the stator side decay time constant, and its expression is τ sc =R s L r / L D ; t0 is the time when the fault occurs; U s0 is the fault initial voltage per unit value; ω1 is the synchronous angular velocity value; k c is the fault voltage drop per unit value, ranging from 0 to 1; t is the time; The stator flux is the one under the crowbar; According to the rotor flux equation, we can get: Among them, L s is the self-inductance value of the fan stator and rotor side, L m is the mutual inductance value on the stator and rotor sides, i r is the rotor current, i s is the rotor current, is the stator flux; Substituting the rotor flux equation into the rotor flux equation, we can get: Among them, L r are the self-inductance values ​​on the fan rotor side; L D =L r L s -L m 2 ; is the rotor flux; After the crowbar protection is put into use, the rotor side is equivalent to having an additional resistor Rc connected in series, and its resistance value will change from the initial value Rr to Rr'=Rr+Rc; Therefore, the relationship between the current and the flux on the stator side is obtained as follows: Where: R r ' is the equivalent resistance value of the rotor side after the crowbar protection is put into use, and its expression is R r '=R r +R c , R c is the crowbar protection resistor value; i sc is the stator current value after the crowbar is put into operation; The stator and rotor current time domain expressions are solved by combining the stator and rotor voltage and flux equations.

4. A DFIG transmission line longitudinal protection method based on the proportion of the fundamental frequency components at both ends according to claim 3, characterized in that: The RSC excitation control strategy is specifically: by controlling the rotor-side converter, the active power and reactive power output of the doubly-fed wind turbine are adjusted to achieve maximum wind energy tracking, unity power factor power generation or reactive power compensation for the power grid.

5. A DFIG transmission line longitudinal protection method based on the proportion of the fundamental frequency components at both ends according to claim 4, characterized in that: The calculation process of the stator side output fault current value under the RSC excitation control strategy is as follows: Stator flux linkage under RSC excitation control in dq synchronous rotating coordinates for: Where: τ s_rsc =R s L r / L D ; R s is the stator side resistance value; It is the stator side flux linkage under RSC excitation control strategy; According to the stator and rotor flux equation, we can get: In the formula, is the rotor side flux under RSC excitation control strategy; The stator and rotor current time domain expressions are solved by combining the stator and rotor voltage and flux equations.

6. A DFIG transmission line longitudinal protection method based on the proportion of the fundamental frequency components at both ends according to claim 1, characterized in that: The calculation process of the component values ​​at different frequencies is as follows: Assume that the period of the periodic function u(t) is T. If the periodic function satisfies the Dirichlet condition, then the periodic function can be expressed by the Fourier series as follows: Where: a n 、b n are the sine and cosine coefficients of the nth harmonic component respectively. When n is set to different values, the component values ​​of different frequencies can be solved by the above formula. For the fundamental frequency component, n=1 is set, and ω1 is the synchronous angular velocity value.

7. A DFIG transmission line longitudinal protection method based on the proportion of the fundamental frequency components at both ends according to claim 1, characterized in that: The steps to determine the fault area based on the fundamental frequency ratio FFR are as follows: The ratio of the fundamental frequency component amplitude to the sum of all component amplitudes is defined as the fundamental frequency fraction FFR, which is expressed as: Calculate the absolute value of the FFR difference between the two currents 差 , If FFR 差 Less than the set threshold FFR set , it is judged as an out-of-area fault, otherwise it is an in-area fault.

8. A computer-readable storage medium, characterized in that: The computer-readable storage medium includes a stored program, wherein when the program is running, the device where the computer-readable storage medium is located is controlled to execute the DFIG transmission line longitudinal protection method based on the proportion of the fundamental frequency components at both ends as described in any one of claims 1 to 7.

9. A processor, characterized in that: The processor is used to run a program, wherein when the program is run, the DFIG transmission line longitudinal protection method based on the proportion of the fundamental frequency components at both ends as described in any one of claims 1 to 7 is executed.

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