Multi-terminal flexible DC system DC line protection method and system

Abstract

The invention relates to a DC line protection method and system for a multi-terminal flexible DC system, belongs to the technical field of relay protection, and solves the problem that the DC line protection of the multi-terminal flexible DC system in the prior art is difficult to ensure accurate identification of a DC line fault under a low communication requirement. Comprising the steps of collecting voltage and current of current-limiting reactors at two ends of a DC line to be protected in the multi-terminal flexible DC system before and after a fault occurs, and further obtaining voltage break variables and current break variables of the current-limiting reactors at the two ends of the DC line to be protected when the high-frequency component frequency is omegaf; then obtaining the sudden change energy of the current limiting reactors at the two ends of the direct current line needing to be protected when the high-frequency component frequency is omega f; according to the sudden change energy of the current limiting reactors at the two ends of the direct current line needing to be protected when the high-frequency component frequency is omega f and the fault protection criterion, whether an internal fault occurs in the direct current line needing to be protected or not is determined; and if yes, starting a protection action of the direct-current line needing to be protected.

Description

Technical Field

[0001] The present invention relates to the technical field of relay protection, and in particular to a DC line protection method and system for a multi-terminal flexible DC system. Background Art

[0002] In a flexible DC transmission system, when a DC line fails, the current amplitude will rise sharply within 2-5 milliseconds, which will not only damage the power electronic devices in the converter, but may even cause the collapse of the entire power system, causing significant losses to people. A DC line fault in a multi-terminal flexible DC system will generate a larger fault current due to the boost current generated by the interconnection of the DC busbars. Therefore, a multi-terminal flexible DC system needs to identify and accurately remove the fault within a few milliseconds, which places extremely high demands on the speed and selectivity of line protection.

[0003] At present, the DC line protection methods for multi-terminal flexible DC systems can be divided into single-terminal protection and double-terminal protection. The protection method based on single-terminal electrical quantity information is fast and has low communication requirements. However, since the control and other links are not considered to affect the judgment of faults inside and outside the area under the access of new energy, the judgment may be inaccurate; and if it only relies on single-terminal electrical quantities, it is often necessary to select the least important line as the non-cooperative line for the ring network system, which is not conducive to the safe and stable operation of the system. The protection method based on double-terminal electrical quantity information often uses analog quantities for interaction. Due to communication limitations, there is a certain action delay, which requires the installation of PMU and other equipment. It is not only difficult to meet the high requirements of the DC line main protection for speed, but also increases the installation cost.

[0004] Therefore, the existing DC line protection of multi-terminal flexible DC systems is difficult to ensure accurate identification of DC line faults under low communication requirements. Summary of the invention

[0005] In view of the above analysis, an embodiment of the present invention aims to provide a DC line protection method and system for a multi-terminal flexible DC system, so as to solve the problem that the existing DC line protection of a multi-terminal flexible DC system is difficult to ensure accurate identification of DC line faults under low communication requirements.

[0006] On the one hand, an embodiment of the present invention provides a DC line protection method for a multi-terminal flexible DC system, comprising the following steps:

[0007] The voltage and current of the current-limiting reactors at both ends of the DC line to be protected in the multi-terminal flexible DC system before and after the fault occurs are collected, and then the DC line to be protected is obtained at the high frequency component frequency ω f The voltage and current mutation of the current-limiting reactors at both ends;

[0008] Based on the fact that the DC line to be protected has a high frequency component of ω fThe voltage and current mutations of the current-limiting reactors at both ends are obtained, and the DC line to be protected has a high-frequency component frequency of ω f The current limiting reactor at both ends has sudden change energy;

[0009] According to the DC line to be protected, the high frequency component frequency is ω f The sudden change energy of the current-limiting reactors at both ends and the fault protection criterion are used to determine whether an in-zone fault occurs in the DC line to be protected; if so, the protection action of the DC line to be protected is started.

[0010] Furthermore, let one end of the DC line to be protected be the M end and the other end be the N end; let the DC bus point to the DC line as the positive direction, and the DC line to be protected has a high frequency component frequency of ω f The sudden change energy of the current limiting reactor at both ends is expressed as:

[0011]

[0012] In the formula, S m (ω f ), S n (ω f ) represent the DC line to be protected at the high frequency component frequency ω f The sudden change energy of the current limiting reactor at the M and N terminals, H represents the total number of sampling points, h′ is the sampling time interval, T is the integration time, Δu m (ω f ,h),Δu n (ω f ,h) respectively represent the DC line to be protected at the hth sampling point and the high frequency component frequency is ω f When the voltage of the current limiting reactor at the M and N terminals is suddenly changed, Δi m (ω f ,h),Δi n (ω f ,h) respectively represent the DC line to be protected at the hth sampling point and the high frequency component frequency is ω f The current of the current-limiting reactor at the M and N terminals changes suddenly, and t represents time.

[0013] Furthermore, the high frequency component frequency is ω f The fault protection criteria include:

[0014]

[0015] In the formula, S set (ω f ) indicates that the high frequency component frequency is ω f The action threshold value when

[0016] If S m (ωf ) and S n (ω f ) all meet the fault protection criteria, it is determined to be an in-zone fault requiring DC line protection; otherwise, it is determined to be an out-of-zone fault requiring DC line protection.

[0017] Furthermore, when the high frequency component frequency is ω f The action threshold S set (ω f )satisfy:

[0018] S set (ω f )>S 0max (ω f )

[0019] In the formula, S 0max (ω f ) indicates that the multi-terminal flexible DC system is in steady-state operation when the high-frequency component frequency is ω f The maximum current limiting reactor sudden change energy at this time.

[0020] Further, the high frequency component frequency ω is obtained by the following method: f :

[0021] The derivative of the sudden energy difference between the fault reactors inside and outside the DC line to be protected is set to 0, and the frequency domain solutions are obtained;

[0022] Based on each frequency domain solution, the sudden energy difference of the in-zone and out-zone fault reactors of the DC line to be protected under each frequency domain solution is obtained;

[0023] The frequency domain solution that maximizes the sudden energy difference between the fault reactors inside and outside the DC line to be protected is selected as the high-frequency component frequency ω f .

[0024] Furthermore, the sudden change energy difference between the internal and external fault reactors of the DC line to be protected is expressed as:

[0025] ΔS(ω f′ )=S m区内 (ω f′ )-S m区外 (ω f′ )

[0026] In the formula, ΔS(ω f′ ) indicates that the DC line to be protected has a high frequency component frequency of ω f′ The sudden energy difference of the internal and external fault reactors, S m区内 (ω f′ ) indicates that the DC line to be protected has a fault in the fault zone and the high frequency component frequency is ω f′ When the reactor sudden change energy of bus port at M end is m区外 (ωf′ ) indicates that the DC line to be protected is faulty outside the area and the high frequency component frequency is ω f′ The reactor sudden change energy at the bus port at the M end.

[0027] Furthermore, when the DC line to be protected has an in-area fault and the high-frequency component frequency is ω f′ When the reactor sudden change energy S at the bus port at the M end m区内 (ω f′ ) is expressed as:

[0028] S m区内 (ω f′ )=∫|jω f′ L dc i′ mn (ω f′ )|(i′ mn (ω f′ )-i mn0 )

[0029] Where, L dc Represents the inductance of the current limiting reactor, i′ mn Indicates that when the DC line to be protected in the multi-terminal flexible DC system has an in-area fault and the high-frequency component frequency is ω f′ When the line current from the M-terminal converter station to the N-terminal converter station is mn0 It represents the steady-state current amplitude flowing from the M-terminal converter station of the DC line to be protected to the N-terminal converter station in the multi-terminal flexible DC system, and j represents a complex unit.

[0030] Furthermore, when the DC line to be protected has an out-of-area fault and the high-frequency component frequency is ω f′ When the reactor sudden change energy S at the bus port at the M end m区外 (ω f′ ) is expressed as:

[0031] S m区外 (ω f′ )=∫|jω f′ L dc i″ mn (ω f′ )|(i″ mn (ω f′ )-i mn0 )

[0032] Where, L dc Represents the inductance of the current limiting reactor, i′ m ' n Indicates that when the DC line to be protected in the multi-terminal flexible DC system has an out-of-area fault and the high-frequency component frequency is ω f′ When the line current from the M-terminal converter station to the N-terminal converter station is mn0It represents the steady-state current amplitude flowing from the M-terminal converter station of the DC line to be protected to the N-terminal converter station in the multi-terminal flexible DC system, and j represents a complex unit.

[0033] On the other hand, an embodiment of the present invention provides a DC line protection system for a multi-terminal flexible DC system, comprising:

[0034] The data acquisition module is used to collect the voltage and current of the current-limiting reactors at both ends of the DC line to be protected in the multi-terminal flexible DC system before and after the fault occurs, and then obtain the DC line to be protected at the high frequency component frequency ω f The voltage and current mutation of the current-limiting reactors at both ends;

[0035] The current limiting reactor sudden change energy calculation module is used to calculate the sudden change energy of the DC line to be protected based on the high frequency component frequency ω f The voltage and current mutations of the current-limiting reactors at both ends are obtained, and the DC line to be protected has a high-frequency component frequency of ω f The current limiting reactor at both ends has sudden change energy;

[0036] The fault identification protection module is used to detect the fault of the DC line to be protected when the high frequency component frequency is ω f The sudden change energy of the current-limiting reactors at both ends and the fault protection criterion are used to determine whether an in-zone fault occurs in the DC line to be protected; if so, the protection action of the DC line to be protected is started.

[0037] Furthermore, let one end of the DC line to be protected be the M end and the other end be the N end; let the DC bus point to the DC line as the positive direction, and the DC line to be protected has a high frequency component frequency of ω f The sudden change energy of the current limiting reactor at both ends is expressed as:

[0038]

[0039] In the formula, S m (ω f ), S n (ω f ) represent the DC line to be protected at the high frequency component frequency ω f The sudden change energy of the current limiting reactor at the M and N terminals, H represents the total number of sampling points, h′ is the sampling time interval, T is the integration time, Δu m (ω f ,h),Δu n (ω f ,h) respectively represent the DC line to be protected at the hth sampling point and the high frequency component frequency is ω f When the voltage of the current limiting reactor at the M and N terminals is suddenly changed, Δi m (ω f ,h),Δi n (ωf ,h) respectively represent the DC line to be protected at the hth sampling point and the high frequency component frequency is ω f The current of the current-limiting reactor at the M and N terminals changes suddenly, and t represents time.

[0040] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0041] The present invention provides a DC line protection method and system for a multi-terminal flexible DC system, which obtains the DC line protection frequency of the DC line to be protected at a high frequency component frequency of ω by collecting the voltage and current of the current limiting reactors at both ends of the DC line to be protected in the multi-terminal flexible DC system before and after the fault occurs. f The voltage and current mutation of the current-limiting reactor at both ends are obtained, and then the DC line to be protected is obtained when the high-frequency component frequency is ω f The current limiting reactors at both ends suddenly change their energy, and then based on the fault protection criterion, it is determined whether the DC line to be protected has an in-zone fault. If so, the protection action of the DC line to be protected is started. It can accurately and quickly identify faults inside and outside the zone, has a fast action speed, and has low requirements for communication devices. The communication devices at both ends of the line only need to transmit real-time logical quantity information, and are not affected by the synchronization errors at both ends. This solves the problem that the DC line protection of the existing multi-terminal flexible DC system is difficult to ensure accurate identification of DC line faults under low communication requirements; a dual-end protection idea is adopted, and a single-end electrical quantity is used in the frequency domain for threshold judgment under comprehensive consideration of the influence of converter control, and then logical quantity information is used for exchange; and the influence of the new energy side on the flexible straight line is taken into account, ensuring a more accurate calculation result, and only logical quantity information exchange is required at both ends, which can solve the problem of communication requirements, has low dependence on communication equipment, and has strong transition resistance resistance.

[0042] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0044] Figure 1 A schematic flow chart of a DC line protection method for a multi-terminal flexible DC system provided in Embodiment 1 of the present invention;

[0045] Figure 2 A schematic diagram of a bipolar structure converter station provided in Embodiment 1 of the present invention;

[0046] Figure 3 A schematic diagram of an MMC positive electrode operation circuit when a DC side fault occurs provided in Embodiment 1 of the present invention;

[0047] Figure 4 A schematic diagram of a transient frequency domain model of a positive DC side fault of an MMC converter provided in Example 1 of the present invention;

[0048] Figure 5 A schematic diagram of a transient frequency domain model of a negative DC side fault of an MMC converter provided in Example 1 of the present invention;

[0049] Figure 6 A schematic diagram of a positive pole fault equivalent model when a fault occurs in a multi-terminal flexible DC system area provided in Example 1 of the present invention;

[0050] Figure 7 A schematic diagram of an equivalent model of a positive pole fault in a multi-terminal flexible DC system when an out-of-area fault occurs in the embodiment 1 of the present invention;

[0051] FIG8(a) is a diagram showing the sudden change energy S of the current limiting reactor when a fault occurs at 50% of the positive DC line in the multi-terminal flexible DC system area through different transition resistances provided in Example 3 of the present invention. m Schematic diagram;

[0052] FIG8( b ) is a diagram showing the sudden change energy S of the current limiting reactor when a fault occurs at 50% of the positive DC line in the multi-terminal flexible DC system area through different transition resistances provided in Example 3 of the present invention. n Schematic diagram;

[0053] FIG9( a ) is a diagram showing the sudden change energy S of the current limiting reactor when a fault occurs at different locations of the positive DC line through a 120Ω transition resistor in the multi-terminal flexible DC system area provided in Example 3 of the present invention. m Schematic diagram;

[0054] FIG9( b ) is a diagram showing the sudden change energy S of the current limiting reactor when a fault occurs at different locations of the positive DC line through a 120Ω transition resistor in the multi-terminal flexible DC system provided in Example 3 of the present invention. n Schematic diagram;

[0055] FIG10( a ) is a diagram showing the sudden change energy S of the current limiting reactor when a fault occurs at the head end of an adjacent DC line outside the multi-terminal flexible DC system provided in Example 3 of the present invention through different transition resistances. m Schematic diagram;

[0056] FIG10( b ) is a diagram showing the sudden change energy S of the current limiting reactor when a fault occurs at the head end of an adjacent DC line outside the multi-terminal flexible DC system provided in Example 3 of the present invention through different transition resistances. n Schematic diagram;

[0057] FIG. 11( a ) is a diagram showing the sudden change energy S of the current limiting reactor when a fault occurs on the AC side of the multi-terminal flexible DC system outside the area through different transition resistances provided in Example 3 of the present invention. m Schematic diagram;

[0058] FIG. 11( b ) is a diagram showing the sudden change energy S of the current limiting reactor when a fault occurs on the AC side of the multi-terminal flexible DC system outside the area through different transition resistances provided in Example 3 of the present invention. n Schematic diagram. DETAILED DESCRIPTION

[0059] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0060] Example 1

[0061] A specific embodiment of the present invention discloses a DC line protection method for a multi-terminal flexible DC system, such as Figure 1 As shown, the following steps are included:

[0062] S1. Collect the voltage and current of the current-limiting reactors at both ends of the DC line to be protected in the multi-terminal flexible DC system before and after the fault occurs, and then obtain the high-frequency component frequency of the DC line to be protected. f The voltage and current mutation of the current-limiting reactors at both ends;

[0063] S2, based on the DC line to be protected having a high frequency component frequency of ω f The voltage and current mutations of the current-limiting reactors at both ends are obtained, and the DC line to be protected has a high-frequency component frequency of ω f The current limiting reactor at both ends has sudden change energy;

[0064] S3, according to the DC line to be protected, the high frequency component frequency is ω f The sudden change energy of the current-limiting reactors at both ends and the fault protection criterion are used to determine whether an in-zone fault occurs in the DC line to be protected; if so, the protection action of the DC line to be protected is started.

[0065] It should be noted that there are multiple DC lines in a multi-terminal flexible DC system. One can be selected as the DC line to be protected, or several can be selected at the same time as the DC lines to be protected. If several DC lines are selected as the DC lines to be protected, each DC line to be protected will be protected according to the above process.

[0066] During implementation, one end of the DC line to be protected is named M-end and the other end is named N-end; the DC bus pointing to the DC line is the positive direction, and the DC line to be protected has a high frequency component of ω fThe sudden change energy of the current limiting reactor at both ends is expressed as:

[0067]

[0068] In the formula, S m (ω f ), S n (ω f ) represent the DC line to be protected at the high frequency component frequency ω f The sudden change energy of the current limiting reactor at the M and N terminals, H represents the total number of sampling points, h′ is the sampling time interval, T is the integration time, Δu m (ω f ,h),Δu n (ω f ,h) respectively represent the DC line to be protected at the hth sampling point and the high frequency component frequency is ω f When the voltage of the current limiting reactor at the M and N terminals is suddenly changed, Δi m (ω f ,h),Δi n (ω f ,h) respectively represent the DC line to be protected at the hth sampling point and the high frequency component frequency is ω f The current of the current-limiting reactor at the M and N terminals changes suddenly, and t represents time.

[0069] When implemented, the high frequency component frequency is ω f The fault protection criteria include:

[0070]

[0071] In the formula, S set (ω f ) indicates that the high frequency component frequency is ω f The action threshold value when

[0072] If S m (ω f ) and S n (ω f ) all meet the fault protection criteria, it is determined to be an in-zone fault requiring DC line protection; otherwise, it is determined to be an out-of-zone fault requiring DC line protection.

[0073] In specific implementation, when the high frequency component frequency is ω f The action threshold S set (ω f )satisfy:

[0074] S set (ω f )>S 0max (ω f )

[0075] In the formula, S 0max (ω f ) indicates that the multi-terminal flexible DC system is in steady-state operation when the high-frequency component frequency is ω f The maximum current limiting reactor sudden change energy at this time.

[0076] Preferably, the high frequency component frequency ω is obtained by the following method: f :

[0077] The derivative of the sudden energy difference between the fault reactors inside and outside the DC line to be protected is set to 0, and the frequency domain solutions are obtained;

[0078] Based on each frequency domain solution, the sudden energy difference of the in-zone and out-zone fault reactors of the DC line to be protected under each frequency domain solution is obtained;

[0079] The frequency domain solution that maximizes the sudden energy difference between the fault reactors inside and outside the DC line to be protected is selected as the high-frequency component frequency ω f .

[0080] Specifically, the sudden change energy difference of the internal and external fault reactors of the DC line to be protected is expressed as:

[0081] ΔS(ω f′ )=S m区内 (ω f′ )-S m区外 (ω f′ )

[0082] In the formula, ΔS(ω f′ ) indicates that the DC line to be protected has a high frequency component frequency of ω f′ The sudden energy difference of the internal and external fault reactors, S m区内 (ω f′ ) indicates that the DC line to be protected has a fault in the fault zone and the high frequency component frequency is ω f′ When the reactor sudden change energy of bus port at M end is m区外 (ω f′ ) indicates that the DC line to be protected is faulty outside the area and the high frequency component frequency is ω f′ The reactor sudden change energy at the bus port at the M end.

[0083] More specifically, when the DC line to be protected has an in-zone fault and the high-frequency component frequency is ω f′ When the reactor sudden change energy S at the bus port at the M end m区内 (ω f′ ) is expressed as:

[0084] S m区内 (ω f′ )=∫|jω f′ L dc i′mn (ω f′ )|(i′ mn (ω f′ )-i mn0 )

[0085] Where, L dc Represents the inductance of the current limiting reactor, i′ mn Indicates that when the DC line to be protected in the multi-terminal flexible DC system has an in-area fault and the high-frequency component frequency is ω f′ When the line current from the M-terminal converter station to the N-terminal converter station is mn0 It represents the steady-state current amplitude flowing from the M-terminal converter station to the N-terminal converter station of the DC line to be protected in the multi-terminal flexible DC system.

[0086] More specifically, when an out-of-area fault occurs on the DC line to be protected and the high-frequency component frequency is ω f′ When the reactor sudden change energy S at the bus port at the M end m区外 (ω f′ ) is expressed as:

[0087] S m区外 (ω f′ )=∫|jω f′ L dc i″ mn (ω f′ )|(i″ mn (ω f′ )-i mn0 )

[0088] Where, L dc Represents the inductance of the current limiting reactor, i′ m ' n Indicates that when the DC line to be protected in the multi-terminal flexible DC system has an out-of-area fault and the high-frequency component frequency is ω f′ When the line current from the M-terminal converter station to the N-terminal converter station is mn0 It represents the steady-state current amplitude flowing from the M-terminal converter station of the DC line to be protected to the N-terminal converter station in the multi-terminal flexible DC system, and j represents a complex unit.

[0089] In order to facilitate those skilled in the art to better understand the formation process of the scheme in this embodiment, Figure 2 The schematic diagram of the bipolar converter station shown in Figure 3 Taking the MMC positive pole operation circuit schematic diagram when the DC side is faulty as an example, the working principle of a DC line protection method for a multi-terminal flexible DC system provided by this embodiment is described as follows:

[0090] by Figure 2 The schematic diagram of the bipolar converter station shown in the figure is used as an example for calculation. Since there is a grounding point between the positive and negative poles, the positive and negative poles can be considered to operate independently of each other. Figure 3 The calculation is performed using the MMC positive pole operation circuit schematic diagram when a DC side fault occurs as an example. Figure 3 in,i vj is the j-phase current on the AC side of the converter, u vj is the j-phase voltage on the AC side of the converter, u pj 、u nj are the voltage and across the upper and lower bridge arm modules of phase j, respectively; R0 and L0 are the equivalent resistance and inductance of the bridge arm, respectively; where j is a, b, and c.

[0091] according to Figure 3 The circuit topology can be established as equation:

[0092]

[0093]

[0094] i vj =i pj -i nj (3)

[0095] Where U dc is the voltage difference between the positive and negative electrodes of the MMC DC side, u pj 、u nj are the voltages at both ends of the upper and lower bridge arm modules of the j-phase converter, and i pj 、i nj are the upper and lower arm currents of the j-phase converter, u oo' It is the voltage between the positive and negative balance points of the MMC and the three-phase grounding point on the AC side.

[0096] Combining (1), (2), and (3), when the three phases are symmetrical in steady state operation or DC side fault occurs, then u oo' The fundamental wave is 0, so we can combine them and use Laplace transform to convert them into the complex frequency domain to get:

[0097]

[0098] Considering that MMC adopts the nearest level approximation modulation, the number of submodules put into the upper bridge arm and lower bridge arm of phase j is n pj 、n nj They are:

[0099]

[0100] Where U c is the voltage corresponding to the capacitance of a single submodule, N is the number of rated submodules of the MMC, and Round is the mathematical rounding function; u vref.pj 、u vref.nj are the voltage reference values ​​of the upper and lower bridge arms of phase j respectively.

[0101] Output through the control link, when taking into account the circulation suppression link u vref.pj 、u vref.nj The expression is as follows:

[0102]

[0103] In the formula, u v_ref.j is the j-phase current loop output reference value, u cir_ref.j It is the output reference value of the j-phase circulating current suppression link.

[0104] Considering that the MMC control mode at the new energy transmission end is constant AC voltage control, the control equation is:

[0105]

[0106]

[0107] In the formula, k np , k ni , k wp , k wi are the proportional and integral parameters of the inner and outer loops of MMC control, ω pll is the angular velocity of the phase-locked loop, L is the line inductance on the AC side, u sd 、u sq are the d-axis and q-axis voltages of the grid-connected points on the AC side of the converter, respectively. sd 、i sq are the d-axis and q-axis currents of the grid-connected point on the AC side of the converter respectively; i sd_ref 、i sq_ref are the d-axis and q-axis current command values ​​of the grid-connected points on the AC side of the converter, u sd_ref 、u sq_ref are the d-axis and q-axis voltage command values ​​of the grid-connected points on the AC side of the converter, u vd_ref 、u vq_ref are the voltage command values ​​of the d-axis and q-axis of the converter AC side ports, s represents the Laplace operator; the superscript “c” represents the speed control ω in the phase-locked loop. pll The dq coordinate system without superscript “c” indicates the dq coordinate system of the stator speed ω0 of the power grid.

[0108] In order to facilitate calculation, formula (5) assumes that the number of bridge arm modules is large, then u vref.pj 、u vref.nj It is always divisible, that is, the Round function is no longer involved in the following calculations.

[0109] When performing s-domain transformation, the initial value excitation sources of equivalent inductance and capacitance of each phase are separated. The excitation sources of each phase of the upper and lower bridge arms are:

[0110]

[0111] Where U LCp.j (0) U LCn.j (0) are the j-phase excitation sources of the upper and lower bridge arms, respectively, i pj (0),i nj (0) are the initial values ​​of the upper and lower bridge arm currents of the j-phase converter at the moment of short circuit, n pj (0), n nj (0) are the initial values ​​of the number of submodules put into operation in the upper and lower bridge arms of the j-phase of the converter.

[0112] Since there is circulating current in the MMC, the bridge arm current will be distorted and the loss will increase, so the MMC circulating current suppression link needs to be considered:

[0113]

[0114] In the formula, u cir_ref is the voltage command value of the circulating current suppression link, k pcir , k icir are the proportional parameter and integral parameter of the circulation suppression link, I r2m_ref It is the input circuit instruction value of the circulating current suppression link. Generally, it is to make the circulating current suppression 0, so this instruction value can be considered as 0. r2m is the negative sequence double frequency peak of the MMC circulating current, which is determined by the upper and lower bridge arm currents and i p.j +i n.j When entering the negative sequence double frequency dq coordinate system transformation, it is obtained, so it can be considered that the d-axis and q-axis components I of the negative sequence double frequency peak of the MMC circulating current are r2md ,I r2mq All are fixed values, and the specific expressions are:

[0115]

[0116] Where C0 is the rated capacitance of the unit submodule, ω0 is the fundamental angular frequency, P s is the apparent power of the converter, that is:

[0117]

[0118] In the formula, is the power factor angle.

[0119] Combine equations (1)-(12), considering that when a fault occurs on the DC side, the current on the AC side is still symmetrical, that is, i va +i vb +i vc = 0, the grid angular velocity dq coordinate system can be obtained, such as Figure 3 As shown, the converter DC side voltage U dc With DC current I dc The relationship is:

[0120]

[0121] According to formula (13), the equivalent fault model of the DC side of the MMC sending-end converter can be obtained:

[0122]

[0123]

[0124] That is Figure 4 As shown, Y mmc.dc is the fault equivalent admittance of the converter DC side, I mmc.dc is the fault equivalent current source on the DC side of the converter.

[0125] Equations (14) and (15) are the transient frequency-domain models of the positive DC side fault of the MMC converter. The transient frequency-domain model of the negative DC side fault of the MMC converter is similar, and the following can be obtained: Figure 5 shown.

[0126] According to the above derivation, when a single-pole fault occurs in the DC line, taking the positive pole as an example, the fault network is as follows: Figure 6 As shown, converter station 1 is the new energy transmission end. In the figure, α is the fault position based on the new energy transmission end, and its value is (0~1); R f is the fault resistance; L dc It is the inductance value of the current limiting reactor. Here, it is assumed that the inductance value of the current limiting reactor at each end of each line is the same. Taking the four-terminal flexible DC power grid as an example, Z 12 , Z 13 , Z 24 , Z 34 They are the transmission line impedances from converter station 1 to 2, converter station 1 to 3, converter station 2 to 4, and converter station 3 to 4 in the four-terminal flexible DC grid. It is stipulated that the positive direction of the measured current is from the DC bus to the DC line.

[0127] According to Kirchhoff's current law, we have:

[0128]

[0129] In the formula, i 12 is the line current from converter station 1 to 2, i 31 is the line current from converter station 3 to station 1, i 21 is the line current from converter station 2 to station 1, i 42 is the line current from converter station 4 to station 2, i 34 is the line current from converter station 3 to 4, i DC1 、i DC2 、i DC3 、i DC4They are the DC side port currents of converter stations 1, 2, 3 and 4 respectively.

[0130] Writing it in matrix form gives:

[0131] I DCN =A LN I LN (17)

[0132] In the formula, I DCN for i DC1 、i DC2 、i DC3 、i DC4 The column matrix I LN for i 12 、i 31 、i 21 、i 42 、i 34 The column matrix A LN is a pure digital coefficient matrix.

[0133] Among them, the matrix I DCN ,I LN , A LN They are

[0134]

[0135] From the equivalent circuit of the DC side of the four-terminal flexible DC converter station, we can get:

[0136] I DCN =Y mmcN ·U dcN +I mmcN (20)

[0137] In the formula, I DCN for i DC1 、i DC2 、i DC3 、i DC4 The column matrix, Y mmcN is the fault equivalent admittance of the DC side of the four-terminal flexible DC converter station, Y mmc.dc1 , Y mmc.dc2 , Y mmc.dc3 , Y mmc.dc4 The diagonal matrix composed of dcN is the voltage u at the DC side of the four-terminal flexible DC converter station dc1 、u dc2 、u dc3 、u dc4 The column matrix I mmcN is the DC side port current I of the four-terminal flexible DC converter station mmc.dc1 ,I mmc.dc2 ,I mmc.dc3 ,Immc.dc4 A column matrix.

[0138] U dcN =[u dc1 u dc2 u dc3 u dc4 ] T (twenty one)

[0139] Y mmcN =diag(Y mmc.dc1 ,Y mmc.dc2 ,Y mmc.dc3 ,Y mmc.dc4 ) (twenty two)

[0140] I mmcN =[I mmc.dc1 I mmc.dc2 I mmc.dc3 I mmc.dc4 ] T (twenty three)

[0141] In formula (22), diag is a mathematical diagonal matrix expression.

[0142] According to the three non-fault lines and the fault line of the four-terminal flexible DC system, five loop equations can be listed as follows:

[0143]

[0144] Its matrix equation can be expressed as:

[0145] B N ·U dcN =Z N I LN (25)

[0146] In the formula, B N is a pure digital coefficient matrix, Z N For α, R f , L dc , Z 12 , Z 13 , Z 34 , Z 24 The coefficient matrix of .

[0147] Among them, the coefficient matrix B N , Z N They are

[0148]

[0149]

[0150] Using the equations connected by equations (17), (20) and (25), when the f1 fault occurs, the fault line current can be solved as:

[0151]

[0152] in,

[0153]

[0154] a=sL dc +(1-α)Z 12 +R f

[0155] In the formula, E (4×4) is a fourth-order unit matrix. Among them, the matrix A + is the relationship between line parameters and matrix Y mmcN The generalized inverse matrix of the A matrix.

[0156] Then, when an intra-area fault occurs, taking the line from station 1 to station 2 as the DC line to be protected, under the condition that the positive direction is from the DC bus to the DC line, since the fault occurs in the protected line area, the fault current will be directed from the DC bus to the DC line. According to the magnitude of the fault current and the corresponding positive and negative properties obtained from formula (28), the amplitude of the fault current is greater than the amplitude of the steady-state current, that is, the fault current i 12 will be greater than the steady-state current amplitude i 120 ,i 21 Similarly, we can get:

[0157]

[0158] According to equations (28) and (29), in order to expand the difference between fault and non-fault conditions, the line from station 1 to station 2 can be defined as 12 , the sudden change energy of the current-limiting reactor measured near the protection installation at end 1 of the converter station, is the sudden change energy of the current-limiting reactor at the bus port at end M:

[0159] S 12 =∫|ΔU 12 |ΔI 12 =∫|sL dc i 12 |(i 12 -i 120 )>0 (30)

[0160] Wherein, ΔU12 is the sudden change voltage of the current-limiting reactor near station 1, and ΔI12 is the sudden change current of the line from station 1 to station 2.

[0161] Similarly, the sudden change energy of the current limiting reactor at the N-terminal bus port can be defined as:

[0162] S21 =∫|ΔU 21 |ΔI 21 =∫|sL dc i 21 |(i 21 -i 210 )>0 (31)

[0163] In the formula, ΔU 21 is the sudden voltage of the current limiting reactor near the 2nd station, ΔI 21 The line from station 2 to station 1 has a sudden change in current.

[0164] Similarly, it can be inferred that

[0165] Taking the DC bus pointing to the DC line as the positive direction, the sudden change energy S of the current limiting reactor at the bus port at the M end when the DC line in the multi-terminal flexible DC system needs to be protected when an in-area fault occurs m区内 satisfy:

[0166] S m区内 =∫|ΔU mn |ΔI mn =∫|sL dc i mn |(i mn -i mn0 )>0 (32)

[0167] Where s represents the Laplace operator, L dc Indicates the inductance of the current-limiting reactor, ΔU mn It represents the voltage mutation amount flowing from the M-terminal converter station of the DC line to the N-terminal converter station in the multi-terminal flexible DC system, ΔI mn It represents the sudden change of current flowing from the M-terminal converter station to the N-terminal converter station of the DC line to be protected in the multi-terminal flexible DC system, i mn Indicates the line current flowing from the M-terminal converter station to the N-terminal converter station of the DC line to be protected in the multi-terminal flexible DC system, i mn0 It represents the steady-state current amplitude flowing from the M-terminal converter station to the N-terminal converter station of the DC line to be protected in the multi-terminal flexible DC system.

[0168] Taking the DC bus pointing to the DC line as the positive direction, the sudden change energy S of the current limiting reactor at the N-terminal bus port when the DC line in the multi-terminal flexible DC system needs to be protected when an in-area fault occurs n区内 satisfy:

[0169] S n区内 =∫|ΔU nm |ΔI nm =∫|sL dc i nm |(i nm -i nm0 )>0 (33)

[0170] Where s represents the Laplace operator, L dc Indicates the inductance of the current-limiting reactor, ΔU nm It represents the voltage mutation amount flowing from the N-terminal converter station of the DC line to the M-terminal converter station in the multi-terminal flexible DC system, ΔI nm It represents the sudden change of current flowing from the converter station at the N end of the DC line to the converter station at the M end in the multi-terminal flexible DC system. nm Indicates the line current flowing from the converter station at the N end of the DC line to the converter station at the M end in the multi-terminal flexible DC system. mn0 It represents the steady-state current amplitude flowing from the N-terminal converter station of the DC line to the M-terminal converter station in the multi-terminal flexible DC system.

[0171] Combining equations (32) and (33), it can be seen that when a fault occurs in the DC line to be protected, the sudden change energy of the current limiting reactors at both ends of the line is greater than 0.

[0172] Similarly, the equivalent model for faults outside the positive region can be obtained, such as Figure 7 shown.

[0173] Since the fault location is different for the same operating state, the matrix needs to be modified to solve the equations again. According to Kirchhoff's current law, we have:

[0174]

[0175] Its matrix form can be listed as:

[0176] I dcW =A LW I LW (35)

[0177] in,

[0178]

[0179] I dcW =[i DC1 i DC2 i DC3 i DC4 ] T (37)

[0180] I LW =[i 12 i 31 i 13 i 42 i 34 ] T (38)

[0181] Similarly, according to the three non-fault lines and the fault line of the multi-terminal flexible DC system, five loop equations can be listed as follows:

[0182]

[0183] Converting it into matrix form can be written as:

[0184] B W ·u dcW =Z W I LW (40)

[0185] In the formula, u dcW =[u dc1 u dc2 u dc3 u dc4 ] T ,

[0186] in,

[0187]

[0188]

[0189] Using the equations connected by equations (22), (37) and (42), when the f2 fault occurs, the fault line current can be solved as:

[0190]

[0191] Among them, the matrix B + is the relationship between line parameters and matrix Y mmcN The generalized inverse matrix of the B matrix.

[0192] Then, when an out-of-zone fault occurs, the line from station 1 to station 2 is the DC line to be protected, and the line from station 1 to station 2 is within the zone. Taking the out-of-zone fault on the line from station 1 to station 3 as an example, when the positive direction is from the DC bus to the DC line, since the fault occurs outside the zone, there must be a fault current at one end that points from the DC line to the DC bus. According to the magnitude of the fault current and the corresponding positive and negative properties obtained from formula (43), the fault current i 12 The amplitude is smaller than the steady-state current amplitude i 120 ,i 21 , that is, i 12 The amplitude is greater than the steady-state current amplitude i 210 , we can get:

[0193] i 12 -i 120 <0 (44)

[0194] i 21 -i 210 >0

[0195] At this time line12 The sudden change energy of the current limiting reactor measured near the protection of the converter station 1 is:

[0196] S 12 =∫|ΔU 12 |ΔI 12 =∫|sL dc i 12 |(i 12 -i 120 )<0 (45)

[0197] Similarly, line 12 The sudden change energy of the current limiting reactor measured at the protection point near the two ends of the converter station is:

[0198] S 21 =∫|ΔU 21 |ΔI 21 =∫|sL dc i 21 |(i 21 -i 210 )>0 (46)

[0199] Similarly, it can be inferred that

[0200] Taking the DC bus pointing to the DC line as the positive direction, the current limiting reactor at the M bus port that needs to protect the DC line in the multi-terminal flexible DC system will have a sudden change energy S when an out-of-zone fault occurs. m区外 The sudden change energy S of the current limiting reactor at the N bus port n区外 satisfy:

[0201]

[0202] Where s represents the Laplace operator, L dc Indicates the inductance of the current-limiting reactor, ΔU mn It represents the voltage mutation amount flowing from the M-terminal converter station of the DC line to the N-terminal converter station in the multi-terminal flexible DC system, ΔI mn It represents the sudden change of current flowing from the M-terminal converter station to the N-terminal converter station of the DC line to be protected in the multi-terminal flexible DC system, i mn Indicates the line current flowing from the M-terminal converter station to the N-terminal converter station of the DC line to be protected in the multi-terminal flexible DC system, i mn0 It represents the steady-state current amplitude flowing from the M-terminal converter station of the DC line to be protected in the multi-terminal flexible DC system to the N-terminal converter station; ΔU nm It represents the voltage mutation amount flowing from the N-terminal converter station of the DC line to the M-terminal converter station in the multi-terminal flexible DC system, ΔI nm It represents the sudden change of current flowing from the converter station at the N end of the DC line to the converter station at the M end in the multi-terminal flexible DC system. nmIndicates the line current flowing from the converter station at the N end of the DC line to the converter station at the M end in the multi-terminal flexible DC system. mn0 It represents the steady-state current amplitude flowing from the N-terminal converter station of the DC line to the M-terminal converter station in the multi-terminal flexible DC system.

[0203] Combining formula (47), it can be seen that when an out-of-zone fault occurs in the DC line to be protected, the DC line will inevitably have a situation where the sudden change energy of the current limiting reactor obtained at one end is less than 0, while the sudden change energy of the current limiting reactor obtained at the other end is greater than 0.

[0204] From the above analysis, it can be seen that when an in-zone fault occurs in the DC line, the protection devices at both ends of the DC line measure the voltage mutation and current mutation of the current-limiting reactor, and the calculated mutation energy of the current-limiting reactor is greater than 0; when an out-of-zone fault occurs, the protection devices at both ends of the DC line measure the voltage mutation and current mutation of the current-limiting reactor, and the mutation energy of the current-limiting reactor measured at both ends is less than 0, and the mutation energy of the current-limiting reactor measured at the other end is greater than 0.

[0205] Therefore, this embodiment uses the sudden change energy of the current limiting reactor measured at both ends of the DC line to identify the fault location, with the DC bus pointing to the DC line as the positive direction, and the DC line to be protected has a high frequency component frequency of ω f The sudden change energy of the current limiting reactor at both ends is expressed as:

[0206]

[0207] In the formula, S m (ω f ), S n (ω f ) represent the DC line to be protected at the high frequency component frequency ω f The sudden change energy of the current limiting reactor at the M and N terminals, H represents the total number of sampling points, h′ is the sampling time interval, T is the integration time, Δu m (ω f ,h),Δu n (ω f ,h) respectively represent the DC line to be protected at the hth sampling point and the high frequency component frequency is ω f When the voltage of the current limiting reactor at the M and N terminals is suddenly changed, Δi m (ω f ,h),Δi n (ω f ,h) respectively represent the DC line to be protected at the hth sampling point and the high frequency component frequency is ω f The current of the current-limiting reactor at the M and N terminals changes suddenly, and t represents time.

[0208] Through the above analysis, the fault protection criterion can be constructed as follows:

[0209]

[0210] In the formula, S set (ω f ) indicates that the high frequency component frequency is ω f The action threshold value when

[0211] If S m (ω f ) and S n (ω f ) all meet the fault protection criteria, it is determined to be a fault within the DC line area to be protected; otherwise, it is determined to be a fault outside the DC line area to be protected.

[0212] Specifically, if it is identified as a fault within the line area, the line protection is activated; otherwise, if it is identified as a fault outside the line area, the line protection is not activated.

[0213] It should be noted that there are multiple DC lines in a multi-terminal flexible DC system. One can be selected as the DC line to be protected, or several can be selected at the same time as the DC lines to be protected. Each DC line to be protected is protected according to the above process.

[0214] Taking into account the influence of measurement error and noise, a certain margin needs to be set based on the current limiter sudden change energy criterion to ensure reliability. Therefore, the action threshold value needs to be set:

[0215] S set (ω f )>S 0max (ω f ) (50)

[0216] In the formula, S 0max (ω f ) indicates that the multi-terminal flexible DC system is in steady-state operation when the high-frequency component frequency is ω f The maximum current limiting reactor sudden change energy at this time.

[0217] Preferably, when the multi-terminal flexible direct current system is in steady-state operation, the high-frequency component frequency is ω f Maximum current limiting reactor sudden change energy S 0max (ω f ) shall not exceed 100.

[0218] From the above analysis, it can be seen that the sudden change energy of the current limiting reactor will change with the change of frequency. It is necessary to weigh the pros and cons of various aspects and select the fault high-frequency components of different frequencies in the appropriate characteristic frequency band to construct the protection criterion. In this embodiment, the high-frequency component frequency ω is obtained by the following method f :

[0219] The derivative of the sudden energy difference between the fault reactors inside and outside the DC line to be protected is set to 0, and the frequency domain solutions are obtained;

[0220] Based on each frequency domain solution, the sudden energy difference of the in-zone and out-zone fault reactors of the DC line to be protected under each frequency domain solution is obtained;

[0221] The frequency domain solution that maximizes the sudden energy difference between the fault reactors inside and outside the DC line to be protected is selected as the high-frequency component frequency ω f .

[0222] Specifically, the sudden change energy difference of the internal and external fault reactors of the DC line to be protected is expressed as:

[0223] ΔS(ω f′ )=S m区内 (ω f′ )-S m区外 (ω f′ ) (51)

[0224] In the formula, ΔS(ω f′ ) indicates that the DC line to be protected has a high frequency component frequency of ω f′ The sudden energy difference of the internal and external fault reactors, S m区内 (ω f′ ) indicates that the DC line to be protected has a fault in the fault zone and the high frequency component frequency is ω f′ When the reactor sudden change energy of bus port at M end is m区外 (ω f′ ) indicates that the DC line to be protected is faulty outside the area and the high frequency component frequency is ω f′ The reactor sudden change energy at the bus port at the M end.

[0225] More specifically, when the DC line to be protected has an in-zone fault and the high-frequency component frequency is ω f′ When the reactor sudden change energy S at the bus port at the M end m区内 (ω f′ ) is expressed as:

[0226] S m区内 (ω f′ )=∫|jω f′ L dc i′ mn (ω f′ )|(i′ mn (ω f′ )-i mn0 ) (52)

[0227] Where, L dc Represents the inductance of the current limiting reactor, i′ mn Indicates that when the DC line to be protected in the multi-terminal flexible DC system has an in-area fault and the high-frequency component frequency is ωf′ When the line current from the M-terminal converter station to the N-terminal converter station is mn0 It represents the steady-state current amplitude flowing from the M-terminal converter station to the N-terminal converter station of the DC line to be protected in the multi-terminal flexible DC system.

[0228] More specifically, when an out-of-area fault occurs on the DC line to be protected and the high-frequency component frequency is ω f′ When the reactor sudden change energy S at the bus port at the M end m区外 (ω f′ ) is expressed as:

[0229] S m区外 (ω f′ )=∫|jω f′ L dc i′ m ' n (ω f′ )|(i′ m ' n (ω f′ )-i mn0 ) (53)

[0230] In the formula, j represents a complex unit, L dc Represents the inductance of the current limiting reactor, i′ m ' n Indicates that when the DC line to be protected in the multi-terminal flexible DC system has an out-of-area fault and the high-frequency component frequency is ω f′ When the line current from the M-terminal converter station to the N-terminal converter station is mn0 It represents the steady-state current amplitude flowing from the M-terminal converter station to the N-terminal converter station of the DC line to be protected in the multi-terminal flexible DC system.

[0231] It should be noted that in the multi-terminal flexible DC system, when the DC line to be protected has an internal or external fault and the high-frequency component frequency is ω f′ The line current flowing from the M-end converter station to the N-end converter station is essentially a circuit calculation, which can be solved by those skilled in the art.

[0232] Compared with the prior art, this embodiment provides a DC line protection method for a multi-terminal flexible DC system, which obtains the DC line protection frequency of the DC line to be protected at a high frequency component frequency of ω by collecting the voltage and current of the current-limiting reactors at both ends of the DC line to be protected in the multi-terminal flexible DC system before and after the fault occurs. f The voltage and current mutation of the current-limiting reactor at both ends are obtained, and then the DC line to be protected is obtained when the high-frequency component frequency is ω fThe current limiting reactors at both ends suddenly change their energy, and then based on the fault protection criterion, it is determined whether the DC line to be protected has an in-zone fault. If so, the protection action of the DC line to be protected is started. It can accurately and quickly identify faults inside and outside the zone, has a fast action speed, and has low requirements for communication devices. The communication devices at both ends of the line only need to transmit real-time logical quantity information, and are not affected by the synchronization errors at both ends. This solves the problem that the DC line protection of the existing multi-terminal flexible DC system is difficult to ensure accurate identification of DC line faults under low communication requirements; a dual-end protection idea is adopted, and a single-end electrical quantity is used in the frequency domain for threshold judgment under comprehensive consideration of the influence of converter control, and then logical quantity information is used for exchange; and the influence of the new energy side on the flexible straight line is taken into account, ensuring a more accurate calculation result, and only logical quantity information exchange is required at both ends, which can solve the problem of communication requirements, has low dependence on communication equipment, and has strong transition resistance resistance.

[0233] Example 2

[0234] A specific embodiment of the present invention discloses a DC line protection system for a multi-terminal flexible DC system, comprising:

[0235] The data acquisition module is used to collect the voltage and current of the current-limiting reactors at both ends of the DC line to be protected in the multi-terminal flexible DC system before and after the fault occurs, and then obtain the DC line to be protected at the high frequency component frequency ω f The voltage and current mutation of the current-limiting reactors at both ends;

[0236] The current limiting reactor sudden change energy calculation module is used to calculate the sudden change energy of the DC line to be protected based on the high frequency component frequency ω f The voltage and current mutations of the current-limiting reactors at both ends are obtained, and the DC line to be protected has a high-frequency component frequency of ω f The current limiting reactor at both ends has sudden change energy;

[0237] The fault identification protection module is used to detect the fault of the DC line to be protected when the high frequency component frequency is ω f The sudden change energy of the current-limiting reactors at both ends and the fault protection criterion are used to determine whether an in-zone fault occurs in the DC line to be protected; if so, the protection action of the DC line to be protected is started.

[0238] During implementation, in the current limiting reactor sudden change energy calculation module, one end of the DC line to be protected is set as the M end and the other end is set as the N end; the DC bus pointing to the DC line is taken as the positive direction, and the DC line to be protected has a high frequency component frequency of ω f The sudden change energy of the current limiting reactor at both ends is expressed as:

[0239]

[0240] In the formula, S m (ωf ), S n (ω f ) represent the DC line to be protected at the high frequency component frequency ω f The sudden change energy of the current limiting reactor at the M and N terminals, H represents the total number of sampling points, h′ is the sampling time interval, T is the integration time, Δu m (ω f ,h),Δu n (ω f ,h) respectively represent the DC line to be protected at the hth sampling point and the high frequency component frequency is ω f When the voltage of the current limiting reactor at the M and N terminals is suddenly changed, Δi m (ω f ,h),Δi n (ω f ,h) respectively represent the DC line to be protected at the hth sampling point and the high frequency component frequency is ω f The current of the current-limiting reactor at the M and N terminals changes suddenly, and t represents time.

[0241] The specific implementation process of the embodiment of the present invention can be found in the above method embodiment, which will not be described in detail in this embodiment.

[0242] Since the principles of this embodiment are the same as those of the above method embodiment, the system also has the corresponding technical effects of the above method embodiment.

[0243] Example 3

[0244] In order to verify the correctness of embodiments 1 and 2 of the present invention, this embodiment conducts experimental verification on the schemes in the above embodiments. The main parameters of the direct-drive wind farm access multi-terminal flexible direct model system in this embodiment are shown in Table 1, the wind farm side parameters are shown in Table 2, and the sampling frequency is selected as 20kHz. According to the protection design scheme, ω1=1200Hz is selected as the high-frequency component frequency extracted in the protection scheme, and the protection action threshold value can be set to 100. In this embodiment, Line 12 The protection at both ends is analyzed, and simulation experiments of different types of faults inside and outside the area are carried out respectively. The time when the fault occurs is taken as the zero time.

[0245] Table 1 Main parameters of multi-terminal flexible DC transmission system

[0246]

[0247] Table 2 Main parameters of wind farm access multi-terminal flexible DC system

[0248]

[0249]

[0250] Scenario 1 set in this embodiment is: Assume that when the DC line Line of the multi-terminal flexible DC transmission system 12 A positive grounding fault occurs at 50% of the total current through different transition resistances, and the transition resistance ranges from 0 to 300Ω. The sudden change energy S of the current limiting reactor under this fault condition m , S n , as shown in Figure 8(a) and Figure 8(b).

[0251] As shown in Figure 8(a) and Figure 8(b), under different fault conditions, as the transition resistance changes, the sudden change energy S of the current limiting reactor m , S n There will be fluctuations, S m and S n The minimum value of is positive, and the protection identifies that a fault occurs in the DC line area. As shown in Figure 8(a), when a fault occurs in the positive line and the transition resistance is 120Ω, S m It reaches the minimum value of 17649.1 at t = 0.5ms. As can be seen from Figure 8(b), when the transition resistance is 240Ω, S n The minimum value is 2451.64 at t = 0.3ms. m and S n The minimum value of is greater than the threshold, satisfying S m >S set >0,S n >S set >0, so the above-mentioned types of faults can be identified as intra-zone faults.

[0252] It can be concluded that the protection methods and systems of Examples 1 and 2 can be judged as intra-zone faults and operate correctly when the DC lines of the multi-terminal flexible DC system experience transition resistance faults of different sizes, are not affected by the fault type, and have good high-resistance resistance.

[0253] Scenario 2 set in this embodiment is: In the multi-terminal flexible direct current transmission system DC line Line 12 The positive grounding fault is set at different positions of the fault scene, and the transition resistance is set to 120Ω. m , S n As shown in Figure 9(a) and Figure 9(b) respectively.

[0254] As shown in Figure 9(a) and Figure 9(b), when the fault location changes, the sudden change energy S of the current limiting reactor m Will be reduced, S n However, S m and S nThe minimum value of is positive, and the protection identifies that a fault occurs in the DC line area. As shown in Figure 9(a), when the positive line grounding fault occurs at 60% of the distance from the head end, S m It reaches the minimum value at t = 2.0ms, which is 2035.62. As shown in Figure 9(b), when the positive line grounding fault occurs at 40% of the distance from the head end, S n The minimum value is 945.17 at t = 0.3 ms. m and S n The minimum value of is greater than the threshold, satisfying S m >S set >0,S n >S set >0, so the above-mentioned types of faults can be identified as intra-zone faults.

[0255] It can be concluded that the protection methods and systems of Examples 1 and 2 can determine that faults in different locations of the DC lines of the multi-terminal flexible DC system are internal faults and operate correctly.

[0256] Scenario 3 set in this embodiment is: respectively on the DC line Line of the multi-terminal flexible DC transmission system 12 The transition resistance changes in the range of 0-300Ω. When the adjacent DC line head fails, S m , S n As shown in Figure 10(a) and Figure 10(b) respectively. When a three-phase ground short circuit fault occurs on the AC side, S m , S n As shown in Figure 11(a) and Figure 11(b) respectively.

[0257] It can be seen from Figure 10(a) and Figure 10(b) that under different fault conditions, the larger the transition resistance, the greater the S m The larger the S n The smaller the S under different transition resistance conditions, the m , S n By comparing the protection criteria, it can be seen that the protection action conditions are not met and the protection is identified as an out-of-zone fault. As shown in Figure 10(a), when a bipolar grounding fault occurs in the line and the transition resistance is 300Ω, S m It reaches the maximum value at t = 2.9ms, which is -3561.9. As can be seen from Figure 10(b), when the transition resistance is 300Ω, S n The minimum value is 3555.44 at t = 2.9 ms. m S n <0, the protection criteria are not met and it is judged as an out-of-zone fault. The protection is reliable and does not operate.

[0258] As shown in Figure 11(a) and Figure 11(b), under different fault conditions, as the transition resistance changes, S m , S n Not much change, S m As the fault time increases, S n As the fault time increases, it decreases. m , S n Compared with the protection criterion, it can be seen that the protection action condition is not met and the protection is identified as an out-of-zone fault. By observing Figure 11(a), it can be seen that when a ground fault occurs in the positive line and the transition resistance is 240Ω, S m The maximum value is -1467.4 at t = 3.0ms. By observing Figure 11(b), we can see that when the transition resistance is 240Ω, S n The minimum value is 1464.5 at t = 3.0 ms. m S n <0, the protection criteria are not met, it is judged as an out-of-zone fault and the protection does not operate.

[0259] According to the above analysis, the protection method and system of Examples 1 and 2 are less affected by the size of the transition resistance when facing different types of faults occurring outside the DC line area through different transition resistances. Different sizes of transition resistances and different fault types can be accurately identified as out-of-zone faults, and have high sensitivity and high resistance capability.

[0260] Those skilled in the art will appreciate that all or part of the processes of the above-mentioned embodiments can be implemented by instructing related hardware through a computer program, and the program can be stored in a computer-readable storage medium, wherein the computer-readable storage medium is a disk, an optical disk, a read-only storage memory, or a random access memory, etc.

[0261] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A DC line protection method for a multi-terminal flexible DC system, characterized in that: The following steps are involved: The voltage and current of the current-limiting reactors at both ends of the DC line to be protected in the multi-terminal flexible DC system before and after the fault occurs are collected, and then the DC line to be protected is obtained at the high frequency component frequency ω f The voltage and current mutation of the current-limiting reactors at both ends; Based on the fact that the DC line to be protected has a high frequency component of ω f The voltage and current mutations of the current-limiting reactors at both ends are obtained, and the DC line to be protected has a high-frequency component frequency of ω f The current limiting reactor at both ends has sudden change energy; According to the DC line to be protected, the high frequency component frequency is ω f The sudden change energy of the current-limiting reactors at both ends and the fault protection criterion are used to determine whether an in-zone fault occurs in the DC line to be protected; if so, the protection action of the DC line to be protected is started.

2. The DC line protection method of a multi-terminal flexible DC system according to claim 1, characterized in that: Let one end of the DC line to be protected be the M end and the other end be the N end; let the DC bus point to the DC line as the positive direction, and the DC line to be protected has a high frequency component of ω f The sudden change energy of the current limiting reactor at both ends is expressed as: In the formula, S m (ω f ), S n (ω f ) represent the DC line to be protected at the high frequency component frequency ω f The sudden change energy of the current limiting reactor at the M and N terminals, H represents the total number of sampling points, h′ is the sampling time interval, T is the integration time, Δu m (ω f ,h),Δu n (ω f ,h) respectively represent the DC line to be protected at the hth sampling point and the high frequency component frequency is ω f When the voltage of the current limiting reactor at the M and N terminals is suddenly changed, Δi m (ω f ,h),Δi n (ω f ,h) respectively represent the DC line to be protected at the hth sampling point and the high frequency component frequency is ω f The current of the current-limiting reactor at the M and N terminals changes suddenly, and t represents time.

3. The DC line protection method of a multi-terminal flexible DC system according to claim 2 is characterized in that: The frequency of the high frequency component is ω f The fault protection criteria include: In the formula, S set (ω f ) indicates that the high frequency component frequency is ω f The action threshold value when If S m (ω f ) and S n (ω f ) all meet the fault protection criteria, it is determined to be an in-zone fault requiring DC line protection; otherwise, it is determined to be an out-of-zone fault requiring DC line protection.

4. The DC line protection method of a multi-terminal flexible DC system according to claim 3 is characterized in that: The high frequency component frequency is ω f The action threshold S set (ω f )satisfy: S set (oh f )>S 0max (oh f ) In the formula, S 0max (ω f ) indicates that the multi-terminal flexible DC system is in steady-state operation when the high-frequency component frequency is ω f The maximum current limiting reactor sudden change energy at this time.

5. The DC line protection method of a multi-terminal flexible DC system according to claim 3 is characterized in that: The high-frequency component frequency ω is obtained by f : The derivative of the sudden energy difference between the fault reactors inside and outside the DC line to be protected is set to 0, and the frequency domain solutions are obtained; Based on each frequency domain solution, the sudden energy difference of the in-zone and out-zone fault reactors of the DC line to be protected under each frequency domain solution is obtained; The frequency domain solution that maximizes the sudden energy difference between the fault reactors inside and outside the DC line to be protected is selected as the high-frequency component frequency ω f .

6. The DC line protection method of a multi-terminal flexible DC system according to claim 5, characterized in that: The sudden energy difference of the internal and external fault reactors of the DC line to be protected is expressed as: ΔS(ω f′ )=S m区内 (oh f′ )-S m区外 (oh f′ ) In the formula, ΔS(ω f′ ) indicates that the DC line to be protected has a high frequency component frequency of ω f′ The sudden energy difference of the internal and external fault reactors, S m区内 (ω f′ ) indicates that the DC line to be protected has a fault in the fault zone and the high frequency component frequency is ω f′ When the reactor sudden change energy of bus port at M end is m区外 (ω f′ ) indicates that the DC line to be protected is faulty outside the area and the high frequency component frequency is ω f′ The reactor sudden change energy at the bus port at the M end.

7. The DC line protection method of a multi-terminal flexible DC system according to claim 6, characterized in that: When the DC line to be protected has an in-area fault and the high-frequency component frequency is ω f′ When the reactor sudden change energy S at the bus port at the M end m区内 (ω f′ ) is expressed as: WITH m区内 (ω f′ )=∫|jω f′ L dc and' mn (ω f′ )|(i′ mn (ω f′ )-and mn0 ) Where, L dc Represents the inductance of the current-limiting reactor, i′ mn Indicates that when the DC line to be protected in the multi-terminal flexible DC system has an in-area fault and the high-frequency component frequency is ω f′ When the line current from the M-terminal converter station to the N-terminal converter station is mn0 It represents the steady-state current amplitude flowing from the M-terminal converter station of the DC line to be protected to the N-terminal converter station in the multi-terminal flexible DC system, and j represents a complex unit.

8. The DC line protection method of a multi-terminal flexible DC system according to claim 6, characterized in that: When the DC line to be protected has an out-of-area fault and the high-frequency component frequency is ω f′ When the reactor sudden change energy S at the bus port at the M end m区外 (ω f′ ) is expressed as: WITH m区外 (ω f′ )=∫|jω f′ L dc and' m ′ n (ω f′ )|(i′ m ′ n (ω f′ )-and mn0 ) Where, L dc Represents the inductance of the current-limiting reactor, i′ m ' n Indicates that when the DC line to be protected in the multi-terminal flexible DC system has an out-of-area fault and the high-frequency component frequency is ω f′ When the line current from the M-terminal converter station to the N-terminal converter station is mn0 It represents the steady-state current amplitude flowing from the M-terminal converter station of the DC line to be protected to the N-terminal converter station in the multi-terminal flexible DC system, and j represents a complex unit.

9. A multi-terminal flexible DC system DC line protection system, characterized in that: include: The data acquisition module is used to collect the voltage and current of the current-limiting reactors at both ends of the DC line to be protected in the multi-terminal flexible DC system before and after the fault occurs, and then obtain the DC line to be protected at the high frequency component frequency ω f The voltage and current mutation of the current-limiting reactors at both ends; The current limiting reactor sudden change energy calculation module is used to calculate the sudden change energy of the DC line to be protected based on the high frequency component frequency ω f The voltage and current mutations of the current-limiting reactors at both ends are obtained, and the DC line to be protected has a high-frequency component frequency of ω f The current limiting reactor at both ends has sudden change energy; The fault identification protection module is used to detect the fault of the DC line to be protected when the high frequency component frequency is ω f The sudden change energy of the current-limiting reactors at both ends and the fault protection criterion are used to determine whether an in-zone fault occurs in the DC line to be protected; if so, the protection action of the DC line to be protected is started.

10. The DC line protection system for a multi-terminal flexible DC system according to claim 9, characterized in that: Let one end of the DC line to be protected be the M end and the other end be the N end; let the DC bus point to the DC line as the positive direction, and the DC line to be protected has a high frequency component of ω f The sudden change energy of the current limiting reactor at both ends is expressed as: In the formula, S m (ω f ), S n (ω f ) represent the DC line to be protected at the high frequency component frequency ω f The sudden change energy of the current limiting reactor at the M and N terminals, H represents the total number of sampling points, h′ is the sampling time interval, T is the integration time, Δu m (ω f ,h),Δu n (ω f ,h) respectively represent the DC line to be protected at the hth sampling point and the high frequency component frequency is ω f When the voltage of the current limiting reactor at the M and N terminals is suddenly changed, Δi m (ω f ,h),Δi n (ω f ,h) respectively represent the DC line to be protected at the hth sampling point and the high frequency component frequency is ω f The current of the current-limiting reactor at the M and N terminals changes suddenly, and t represents time.

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