A method and system for protecting DC lines in a multi-terminal flexible DC system

Abstract

This invention relates to a method and system for protecting DC lines in a multi-terminal flexible DC system, belonging to the field of relay protection technology. It solves the problem in existing technologies where multi-terminal flexible DC system DC line protection struggles to accurately identify DC line faults under low communication requirements. The method includes 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 a fault occurs, thereby obtaining the high-frequency component frequency of the DC line to be protected at ω. f The voltage and current surges at both ends of the current-limiting reactor are used to determine the frequency of the DC line to be protected at a high-frequency component of ω. f The sudden energy change of the current-limiting reactors at both ends; according to the DC line to be protected at a high frequency component frequency of ω f The sudden energy change of the current-limiting reactors at both ends and the fault protection criteria are used to determine whether a fault has occurred in the DC line to be protected; if so, the protection action of the DC line to be protected is initiated.

Description

Technical Field

[0001] This invention relates to the field of relay protection technology, and in particular to a method and system for protecting DC lines in a multi-terminal flexible DC system. Background Technology

[0002] In flexible DC transmission systems, when a DC line fault occurs, the current amplitude rises sharply within 2-5 milliseconds. This can damage the power electronic components in the converter and even lead to the collapse of the entire power system, causing significant losses. In multi-terminal flexible DC systems, DC line faults can generate even larger fault currents due to the boosted current from the interconnection of DC buses. Therefore, multi-terminal flexible DC systems require fault identification and accurate isolation within milliseconds, placing extremely high demands on the speed and selectivity of line protection.

[0003] Currently, DC line protection methods for multi-terminal flexible DC systems can be divided into single-ended quantity protection and double-ended quantity protection. Protection methods based on single-ended electrical quantity information are fast and have low communication requirements, but because they do not consider the impact of control mechanisms on fault diagnosis within and outside the protection zone under renewable energy integration, the judgment may be inaccurate. Furthermore, relying solely on single-ended electrical quantities often requires selecting the least important line as the non-cooperating line in ring network systems, which is detrimental to the safe and stable operation of the system. Protection methods based on double-ended electrical quantity information often use analog quantities for interaction. Due to communication limitations, there is a certain action delay, necessitating the installation of PMU and other equipment. This not only makes it difficult to meet the high speed requirements of DC line main protection but also increases installation costs.

[0004] Therefore, existing multi-terminal flexible DC system DC line protection cannot guarantee accurate identification of DC line faults under low communication requirements. Summary of the Invention

[0005] Based on the above analysis, the embodiments of the present invention aim to provide a method and system for protecting DC lines in a multi-terminal flexible DC system, in order to solve the problem that existing multi-terminal flexible DC system DC line protection cannot guarantee accurate identification of DC line faults under low communication requirements.

[0006] On one hand, embodiments of the present invention provide a method for protecting DC lines in 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 were collected before and after the fault occurred, and then the high-frequency component frequency of the DC line to be protected was obtained. f The voltage and current surges at both ends of the current-limiting reactor;

[0008] Based on the aforementioned DC line requiring protection, at a high-frequency component frequency of ω fThe voltage and current surges at both ends of the current-limiting reactor are used to obtain the high-frequency component frequency ω of the DC line to be protected. f The sudden energy change of the current-limiting reactor at both ends;

[0009] According to the aforementioned DC line requiring protection, the high-frequency component frequency is ω. f The sudden energy change of the current-limiting reactors at both ends and the fault protection criteria are used to determine whether a fault has occurred in the DC line to be protected; if so, the protection action of the DC line to be protected is initiated.

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

[0011]

[0012] In the formula, S m (ω f ), S n (ω f The numbers ) represent the DC lines requiring protection at high-frequency components with a frequency of ω. f The transient energy of the current-limiting reactor at terminals M and N, where H represents the total number of sampling points, h′ is the sampling time interval, T is the integration time, and Δu m (ω f ,h) Δu n (ω f (h) represent the DC line requiring protection at the h-th sampling point with a high-frequency component frequency of ω. f The voltage fluctuation of the current-limiting reactor at terminals M and N, Δi m (ω f ,h),Δi n (ω f (h) represent the DC line requiring protection at the h-th sampling point with a high-frequency component frequency of ω. f The sudden change in current of the current-limiting reactor at terminals M and N is denoted by t, where t represents time.

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

[0014]

[0015] In the formula, S set (ω f ) indicates that the frequency of the high-frequency component is ω f Action threshold value at that time;

[0016] If S m (ωf ) and S n (ω f If all conditions are met, the fault is determined to be an internal fault of the DC line requiring protection; otherwise, it is determined to be an external fault of the DC line requiring protection.

[0017] Furthermore, at a frequency of ω for the high-frequency component... f Action threshold value S at time set (ω f )satisfy:

[0018] S set (ω f )>S 0max (ω f )

[0019] In the formula, S 0max (ω f This indicates that the multi-terminal flexible DC system operates in steady state at a high-frequency component frequency of ω. f The maximum current-limiting reactor sudden energy at that time.

[0020] Furthermore, the high-frequency component frequency ω is obtained in the following way. f :

[0021] Set the derivative of the sudden energy difference between the fault reactors inside and outside the protected DC line to 0, and obtain the solutions in each frequency domain;

[0022] Based on the solutions in each frequency domain, the energy difference of sudden change of fault reactors inside and outside the protected DC line is obtained under each frequency domain solution.

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

[0024] Furthermore, the sudden 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′ This indicates that the DC line needs protection at a high-frequency component frequency of ω. f′ The energy difference between the internal and external fault reactors during sudden changes, S m区内 (ω f′ This indicates that the DC line needs protection during a fault within the fault zone, and the high-frequency component frequency is ω. f′ The sudden energy change of the reactor at the M-terminal bus port, S m区外 (ωf′ This indicates that the DC line needs protection when a fault occurs outside the protection zone and the high-frequency component frequency is ω. f′ The sudden change in reactor energy at the M-terminal bus port.

[0027] Furthermore, when the DC line to be protected experiences a fault within the fault zone and the high-frequency component frequency is ω f′ The sudden change energy S of the reactor at the M-terminal bus port m区内 (ω f′ ) is represented as:

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

[0029] In the formula, L dc Indicates the inductance of the current-limiting reactor, i′ mn This indicates that in a multi-terminal flexible DC system, protection is required for DC line faults occurring within the fault zone, and the high-frequency component frequency is ω. f′ The line current i flowing from converter station M to converter station N. mn0 This represents the steady-state current amplitude flowing from the M-terminal converter station to the N-terminal converter station of the DC line that needs protection in a multi-terminal flexible DC system, where j represents a complex number unit.

[0030] Furthermore, the DC line to be protected is in the event of a fault outside the fault zone and the high-frequency component frequency is ω. f′ The sudden change energy S of the reactor at the M-terminal bus port m区外 (ω f′ ) is represented as:

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

[0032] In the formula, L dc Indicates the inductance of the current-limiting reactor, i′ m ′ n This indicates that in a multi-terminal flexible DC system, protection is required for DC lines experiencing external faults, and the high-frequency component frequency is ω. f′ The line current i flowing from converter station M to converter station N. mn0This represents the steady-state current amplitude flowing from the M-terminal converter station to the N-terminal converter station of the DC line that needs protection in a multi-terminal flexible DC system, where j represents a complex number unit.

[0033] On the other hand, embodiments of the present invention provide a multi-terminal flexible DC system DC line protection 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 a fault occurs, thereby obtaining the high-frequency component frequency of the DC line to be protected at ω. f The voltage and current surges at both ends of the current-limiting reactor;

[0035] The current-limiting reactor sudden energy calculation module is used to calculate the energy of the DC line to be protected at a high frequency component frequency of ω. f The voltage and current surges at both ends of the current-limiting reactor are used to obtain the high-frequency component frequency ω of the DC line to be protected. f The sudden energy change of the current-limiting reactor at both ends;

[0036] The fault identification and protection module is used to identify the DC line to be protected at a high-frequency component frequency of ω. f The sudden energy change of the current-limiting reactors at both ends and the fault protection criteria are used to determine whether a fault has occurred in the DC line to be protected; if so, the protection action of the DC line to be protected is initiated.

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

[0038]

[0039] In the formula, S m (ω f ), S n (ω f The numbers ) represent the DC lines requiring protection at high-frequency components with a frequency of ω. f The transient energy of the current-limiting reactor at terminals M and N, where H represents the total number of sampling points, h′ is the sampling time interval, T is the integration time, and Δu m (ω f ,h) Δu n (ω f (h) represent the DC line requiring protection at the h-th sampling point with a high-frequency component frequency of ω. f The voltage fluctuation of the current-limiting reactor at terminals M and N, Δi m (ω f ,h),Δi n (ωf (h) represent the DC line requiring protection at the h-th sampling point with a high-frequency component frequency of ω. f The sudden change in current of the current-limiting reactor at terminals M and N is denoted by t, where t represents time.

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

[0041] This invention provides a method and system for protecting DC lines in a multi-terminal flexible DC system. It obtains 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 a fault occurs, thus determining the high-frequency component frequency of the DC line to be protected at ω. f The voltage and current surges at both ends of the current-limiting reactor are used to obtain the high-frequency component frequency ω of the DC line to be protected. f The system detects sudden energy changes in the current-limiting reactors at both ends and then determines, based on fault protection criteria, whether a fault has occurred in the DC line requiring protection. If so, it initiates the protection action for the DC line. This method can accurately and quickly identify faults inside and outside the protection zone, with fast action speed and low requirements for communication devices. The communication devices at both ends of the line only need to transmit real-time logic information and are not affected by synchronization errors at both ends. This solves the problem that existing multi-terminal flexible DC system DC line protection cannot guarantee accurate identification of DC line faults under low communication requirements. It adopts a dual-terminal quantity protection approach, using single-terminal electrical quantities for threshold judgment in the frequency domain under the comprehensive consideration of the converter control influence, and then using logic information for interaction. It also considers the impact of the new energy side on the flexible DC line, ensuring more accurate calculation results. Moreover, the dual-terminal only requires logic information interaction, which can solve the communication requirement problem, has low dependence on communication equipment, and strong resistance to transition resistance.

[0042] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0043] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0044] Figure 1 This is a schematic diagram of the process for protecting DC lines in a multi-terminal flexible DC system provided in Embodiment 1 of the present invention;

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

[0046] Figure 3 This is a schematic diagram of the MMC positive terminal operational circuit under DC-side fault conditions provided in Embodiment 1 of the present invention;

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

[0048] Figure 5 This is a schematic diagram of the transient frequency domain model of the DC-side fault of the MMC converter provided in Embodiment 1 of the present invention;

[0049] Figure 6 This is a schematic diagram of the equivalent model of the positive electrode fault in the multi-terminal flexible DC system under regional fault conditions provided in Embodiment 1 of the present invention;

[0050] Figure 7 This is a schematic diagram of the equivalent model of the positive electrode fault in the multi-terminal flexible DC system under external fault conditions provided in Embodiment 1 of the present invention;

[0051] Figure 8(a) shows the sudden 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 provided in Embodiment 3 of the present invention via different transition resistances. m Schematic diagram;

[0052] Figure 8(b) shows the sudden 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 provided in Embodiment 3 of the present invention via a fault with different transition resistances. n Schematic diagram;

[0053] Figure 9(a) shows the sudden energy S of the current-limiting reactor when a fault occurs through a 120Ω transition resistor at different locations on the positive DC line of the multi-terminal flexible DC system provided in Embodiment 3 of the present invention. m Schematic diagram;

[0054] Figure 9(b) shows the sudden energy S of the current-limiting reactor when a fault occurs through a 120Ω transition resistor at different locations on the positive DC line within the multi-terminal flexible DC system provided in Embodiment 3 of the present invention. n Schematic diagram;

[0055] Figure 10(a) shows the sudden energy S of the current-limiting reactor when a fault occurs at the beginning of an adjacent DC line outside the multi-terminal flexible DC system provided in Embodiment 3 of the present invention via a fault with different transition resistances. m Schematic diagram;

[0056] Figure 10(b) shows the sudden energy S of the current-limiting reactor when a fault occurs at the beginning of an adjacent DC line outside the multi-terminal flexible DC system provided in Embodiment 3 of the present invention via a fault with different transition resistances. n Schematic diagram;

[0057] Figure 11(a) shows the sudden energy S of the current-limiting reactor when a fault occurs on the AC side outside the multi-terminal flexible DC system provided in Embodiment 3 of the present invention via different transition resistances. m Schematic diagram;

[0058] Figure 11(b) shows the sudden energy S of the current-limiting reactor when a fault occurs on the AC side outside the multi-terminal flexible DC system provided in Embodiment 3 of the present invention via different transition resistances. n Schematic diagram. Detailed Implementation

[0059] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0060] Example 1

[0061] A specific embodiment of the present invention discloses a method for protecting DC lines in a multi-terminal flexible DC system, such as... Figure 1 As shown, it includes the following steps:

[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 at ω. f The voltage and current surges at both ends of the current-limiting reactor;

[0063] S2, Based on the DC line requiring protection having a high-frequency component frequency of ω f The voltage and current surges at both ends of the current-limiting reactor are used to obtain the high-frequency component frequency ω of the DC line to be protected. f The sudden energy change of the current-limiting reactor at both ends;

[0064] S3. According to the DC line requiring protection, the high-frequency component frequency is ω. f The sudden energy change of the current-limiting reactors at both ends and the fault protection criteria are used to determine whether a fault has occurred in the DC line to be protected; if so, the protection action of the DC line to be protected is initiated.

[0065] It should be noted that there are multiple DC lines in a multi-terminal flexible DC system. One line can be selected as the DC line to be protected, or several lines can be selected as the DC lines to be protected at the same time. If several 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] In implementation, one end of the DC line to be protected is named end M, and the other end is named end N; the direction from the DC bus to the DC line is taken as positive, and the DC line to be protected is at a high-frequency component frequency of ω. fThe sudden energy change of the current-limiting reactor at both ends is expressed as:

[0067]

[0068] In the formula, S m (ω f ), S n (ω f The numbers ) represent the DC lines requiring protection at high-frequency components with a frequency of ω. f The transient energy of the current-limiting reactor at terminals M and N, where H represents the total number of sampling points, h′ is the sampling time interval, T is the integration time, and Δu m (ω f ,h) Δu n (ω f (h) represent the DC line requiring protection at the h-th sampling point with a high-frequency component frequency of ω. f The voltage fluctuation of the current-limiting reactor at terminals M and N, Δi m (ω f ,h),Δi n (ω f (h) represent the DC line requiring protection at the h-th sampling point with a high-frequency component frequency of ω. f The sudden change in current of the current-limiting reactor at terminals M and N is denoted by t, where t represents time.

[0069] In implementation, the high-frequency component frequency is ω f The fault protection criteria at that time include:

[0070]

[0071] In the formula, S set (ω f ) indicates that the frequency of the high-frequency component is ω f Action threshold value at that time;

[0072] If S m (ω f ) and S n (ω f If all conditions are met, the fault is determined to be an internal fault of the DC line requiring protection; otherwise, it is determined to be an external fault of the DC line requiring protection.

[0073] In practical implementation, the frequency of the high-frequency component is ω f Action threshold value S at time set (ω f )satisfy:

[0074] S set (ω f )>S 0max (ω f )

[0075] In the formula, S 0max (ω f This indicates that the multi-terminal flexible DC system operates in steady state at a high-frequency component frequency of ω. f The maximum current-limiting reactor sudden energy at that time.

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

[0077] Set the derivative of the sudden energy difference between the fault reactors inside and outside the protected DC line to 0, and obtain the solutions in each frequency domain;

[0078] Based on the solutions in each frequency domain, the energy difference of sudden change of fault reactors inside and outside the protected DC line is obtained under each frequency domain solution.

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

[0080] Specifically, the sudden energy difference between 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′ This indicates that the DC line needs protection at a high-frequency component frequency of ω. f′ The energy difference between the internal and external fault reactors during sudden changes, S m区内 (ω f′ This indicates that the DC line needs protection during a fault within the fault zone, and the high-frequency component frequency is ω. f′ The sudden energy change of the reactor at the M-terminal bus port, S m区外 (ω f′ This indicates that the DC line needs protection when a fault occurs outside the protection zone and the high-frequency component frequency is ω. f′ The sudden change in reactor energy at the M-terminal bus port.

[0083] More specifically, when the DC line to be protected experiences a fault within the fault zone and the high-frequency component frequency is ω f′ The sudden change energy S of the reactor at the M-terminal bus port m区内 (ω f′ ) is represented as:

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

[0085] In the formula, L dc Indicates the inductance of the current-limiting reactor, i′ mn This indicates that in a multi-terminal flexible DC system, protection is required for DC line faults occurring within the fault zone, and the high-frequency component frequency is ω. f′ The line current i flowing from converter station M to converter station N. mn0 This represents the steady-state current amplitude flowing from the M-terminal converter station to the N-terminal converter station in a multi-terminal flexible DC system that requires protection.

[0086] More specifically, the DC line to be protected is in the event of a fault outside the fault zone and the high-frequency component frequency is ω. f′ The sudden change energy S of the reactor at the M-terminal bus port m区外 (ω f′ ) is represented as:

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

[0088] In the formula, L dc Indicates the inductance of the current-limiting reactor, i′ m ′ n This indicates that in a multi-terminal flexible DC system, protection is required for DC lines experiencing external faults, and the high-frequency component frequency is ω. f′ The line current i flowing from converter station M to converter station N. mn0 This represents the steady-state current amplitude flowing from the M-terminal converter station to the N-terminal converter station of the DC line that needs protection in a multi-terminal flexible DC system, where j represents a complex number unit.

[0089] To facilitate a better understanding of the formation process of the solution in this embodiment by those skilled in the art, the following will be used as an example. Figure 2 The schematic diagram of the bipolar converter station shown is as follows: Figure 3 Taking the schematic diagram of the MMC positive terminal operational circuit during a DC-side fault as an example, the working principle of the DC line protection method for a multi-terminal flexible DC system provided in this embodiment is explained as follows:

[0090] by Figure 2 Taking the schematic diagram of the bipolar converter station shown as an example for calculation, since there is a grounding point between the positive and negative poles, the positive and negative poles can be regarded as operating independently. Figure 3 The following is a calculation using the schematic diagram of the MMC positive terminal operational circuit under DC-side fault conditions as an example. Figure 3 in,i vj Let u be the phase j current on the AC side of the converter. vj Let u be the phase j voltage on the AC side of the converter. pj u nj R0 and L0 are the voltages across the upper and lower bridge arm modules of phase j, respectively, and the equivalent resistance and inductance of the bridge arm are respectively; where j represents a, b, and c.

[0091] according to Figure 3 The circuit topology can be used to establish equations:

[0092]

[0093]

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

[0095] In the formula, U dc The voltage difference between the positive and negative terminals of the MMC DC side, u pj u nj These are the voltages across the upper and lower bridge arm modules of phase j of the converter, respectively, and i pj i nj These are the upper and lower arm currents of phase j of the converter, respectively, u oo' This is the voltage between the positive and negative balance point of the MMC and the three-phase grounding point on the AC side.

[0096] Combining (1), (2), and (3), considering the three-phase symmetry under steady-state operation or DC-side fault, then u oo' If the fundamental frequency is 0, then we can combine the equations and use the Laplace transform to convert it to the complex frequency domain to obtain:

[0097]

[0098] Considering that the MMC uses nearest-level approximation modulation, the number of submodules n in the upper and lower arms of phase j is... pj n nj They are respectively:

[0099]

[0100] In the formula, U c Here, N represents the voltage corresponding to the capacitor of a single submodule, N is the rated number of MMC submodules, and Round is the mathematical rounding function; u vref.pj u vref.nj These are the reference values ​​for the upper and lower bridge arm voltages of phase j, respectively.

[0101] The output from the control loop, when considering the circulating current suppression loop, is u vref.pj u vref.nj The expression is as follows:

[0102]

[0103] In the formula, u v_ref.j The reference value for the j-phase current loop output, u cir_ref.j This is the output reference value for the j-phase circulating current suppression circuit.

[0104] Considering that the MMC control method 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 For the proportional and integral parameters of the inner and outer loops of MMC control, ω pll U is the phase-locked loop angular velocity, L is the AC side line inductance, and u is the phase-locked loop angular velocity. sd u sq These are the d-axis and q-axis voltages at the AC side grid connection point of the converter, respectively. sd i sq These are the d-axis and q-axis currents at the AC side grid connection point of the converter, respectively; i sd_ref i sq_ref These are the d-axis and q-axis current command values ​​at the AC side grid connection point of the converter, respectively, u sd_ref u sq_ref These are the command values ​​for the d-axis and q-axis voltages at the AC side grid connection point of the converter, respectively. vd_ref u vq_ref These are the d-axis and q-axis voltage command values ​​at the AC side ports of the converter, respectively, with s representing the Laplace operator; the superscript "c" indicates the speed ω controlled by the phase-locked loop. pll In the dq coordinate system, the absence of the superscript "c" indicates that the dq coordinate system is in the grid stator speed ω0.

[0108] For ease of calculation, equation (5) assumes that the number of bridge arm modules is large, then u vref.pj u vref.nj It is always divisible, meaning the Round function is no longer involved in the following calculations.

[0109] During the s-domain transformation, the initial excitation sources of the equivalent inductance and capacitance of each phase will be separated. The excitation sources for each phase of the upper and lower bridge arms are as follows:

[0110]

[0111] In the formula, U LCp.j (0), U LCn.j (0) are the excitation sources for the upper and lower bridge arms j phases, respectively, i pj (0), i nj (0) represents the initial values ​​of the current in the upper and lower arms of phase j of the converter at the moment of short circuit, and n represents the initial values ​​of the current in the upper and lower arms of phase j of the converter at the moment of short circuit. pj (0), n nj (0) represents the initial number of sub-modules put into operation on the upper and lower arms of phase j of the converter, respectively.

[0112] Because circulating current exists in the MMC, it can distort the bridge arm current and increase losses. Therefore, it is necessary to consider MMC circulating current suppression measures.

[0113]

[0114] In the formula, u cir_ref k is the voltage command value for the circulating current suppression stage. pcir k icir These are the proportional and integral parameters of the circulation suppression element, I. r2m_ref This is the instruction value input to the circulating current suppression circuit. Generally, it's set to zero to enable circulating current suppression; therefore, this instruction value can be considered as 0. r2m The peak value of the negative sequence second harmonic of the MMC circulating current is determined by the upper and lower arm currents and i. p.j +i n.j This is obtained when transforming into the negative-sequence second harmonic dq coordinate system. Therefore, the d-axis and q-axis components I of the negative-sequence second harmonic peak value of the MMC circulation can be considered as... r2md I r2mq All are constant values, and the specific expressions are as follows:

[0115]

[0116] In the formula, C0 is the rated capacitance of the unit submodule, ω0 is the fundamental angular frequency, and P s The apparent power of the converter is:

[0117]

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

[0119] Combining equations (1)-(12), considering that when a fault occurs on the DC side, the AC side current remains symmetrical, i.e., i va +i vb +i vc =0, so the angular velocity of the power grid in the dq coordinate system can be obtained, as follows: Figure 3 As shown, the DC side voltage U of the converter dc With DC current I dc The relation is:

[0120]

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

[0122]

[0123]

[0124] That is, Figure 4 As shown, Y mmc.dc For the equivalent admittance of DC-side faults in the converter, I mmc.dc It is the equivalent current source for DC-side faults in the converter.

[0125] Equations (14) and (15) are the frequency domain transient models of faults on the positive DC side of the MMC converter. Similarly, the frequency domain transient model of faults on the negative DC side of the MMC converter can be obtained as follows. Figure 5 As shown.

[0126] Based on the above derivation, when a unipolar fault occurs in a 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 terminal. In the figure, α represents the fault location relative to the new energy transmission terminal, with a value ranging from (0 to 1); R f For fault resistor; L dc This refers to the inductance value of the current-limiting reactor. Here, it is assumed that the inductance value of the current-limiting reactor is the same at each terminal on each line. Taking a four-terminal flexible DC power grid as an example, Z... 12 Z 13 Z 24 Z 34 These are the transmission line impedances of converter stations 1 to 2, 1 to 3, 2 to 4, and 3 to 4 in a four-terminal flexible DC power grid. The positive direction of the measured current is defined as from the DC bus to the DC line.

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

[0128]

[0129] In the formula, i 12 For the line current from converter station 1 to station 2, i 31 For the line current from converter station 3 to station 1, i 21 For the line current from converter station 2 to station 1, i 42 For the line current from converter station 4 to station 2, i 34 For the line current from converter station 3 to station 4, i DC1 i DC2 i DC3 i DC4These are the DC-side port currents of converter stations 1, 2, 3, and 4, respectively.

[0130] Representing it in matrix form, we get:

[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 formed, I LN For i 12 i 31 i 21 i 42 i 34 The column matrix formed, A LN It is a matrix of purely numerical coefficients.

[0133] Where, matrix I DCN I LN A LN They are respectively

[0134]

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

[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 formed by Y mmcN For the equivalent admittance of a DC-side fault in a four-terminal flexible DC converter station, Y mmc.dc1 Y mmc.dc2 Y mmc.dc3 Y mmc.dc4 The diagonal matrix formed, U dcN The DC side port voltage u of the four-terminal flexible DC converter station dc1 u dc2 u dc3 u dc4 The column matrix formed, I mmcN For the DC side port current I of the four-terminal flexible DC converter station mmc.dc1 I mmc.dc2 I mmc.dc3 Immc.dc4 The column matrix formed.

[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 equation (22), diag is a mathematical diagonal matrix representation.

[0142] Based on the three non-faulty lines and the faulty line of the four-terminal flexible DC system, the following five loop equations can be derived:

[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 Z is a matrix of purely numerical coefficients. N For α, R f L dc Z 12 Z 13 Z 34 Z 24 The coefficient matrix.

[0147] Wherein, coefficient matrix B N Z N They are respectively

[0148]

[0149]

[0150] Using the system of equations (17), (20), and (25) above, the fault line current when a fault f1 occurs can be solved as follows:

[0151]

[0152] in,

[0153]

[0154] a = sL dc +(1-α)Z 12 +R f

[0155] In the formula, E (4×4) It is a fourth-order identity matrix. Matrix A is... + For the line parameters and matrix Y mmcN The generalized inverse of matrix A.

[0156] When a fault occurs within the protection zone, taking the line from station 1 to station 2 as an example of a protected DC line, under the condition that the positive direction is from the DC bus to the DC line, since the fault occurs within the protection zone, the fault current will all flow from the DC bus to the DC line. Therefore, according to the magnitude and corresponding positive / negative of the fault current obtained from equation (28), the amplitude of the fault current is greater than the amplitude of the steady-state current, i.e., the fault current i 12 The magnitude of the steady-state current will be greater than i. 120 i 21 Similarly, we can obtain:

[0157]

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

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

[0160] In the formula, ΔU12 is the sudden voltage of the current-limiting reactor near station 1, and ΔI12 is the sudden 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 For the sudden voltage change of the current-limiting reactor near station 2, ΔI 21 This refers to the sudden change in line current from station 2 to station 1.

[0164] Similarly, it can be deduced that,

[0165] Taking the direction from the DC bus to the DC line as positive, in a multi-terminal flexible DC system, the sudden energy S of the current-limiting reactor at the M-terminal bus port needs to be protected against when the DC line experiences a fault within the fault zone. m区内 satisfy:

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

[0167] In the formula, s represents the Laplace operator, L dc Indicates the inductance of the current-limiting reactor, ΔU mn ΔI represents the voltage surge from the M-terminal converter station to the N-terminal converter station in a multi-terminal flexible DC system that requires protection. mn i represents the sudden change in current flowing from the M-terminal converter station to the N-terminal converter station in a multi-terminal flexible DC system that needs protection. mn This represents the line current flowing from the M-terminal converter station to the N-terminal converter station in a multi-terminal flexible DC system that needs protection. mn0 This represents the steady-state current amplitude flowing from the M-terminal converter station to the N-terminal converter station in a multi-terminal flexible DC system that requires protection.

[0168] Taking the direction from the DC bus to the DC line as positive, in a multi-terminal flexible DC system, the sudden energy S of the current-limiting reactor at the N-terminal bus port needs to be protected against when the DC line experiences a fault within the fault zone. n区内 satisfy:

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

[0170] In the formula, s represents the Laplace operator, L dc Indicates the inductance of the current-limiting reactor, ΔU nm ΔI represents the voltage surge from the N-terminal converter station to the M-terminal converter station in a multi-terminal flexible DC system that needs protection. nm i represents the sudden change in current flowing from the N-terminal converter station to the M-terminal converter station in a multi-terminal flexible DC system that needs protection. nm This represents the line current flowing from the N-terminal converter station to the M-terminal converter station in a multi-terminal flexible DC system that needs protection. mn0 This represents the steady-state current amplitude flowing from the N-terminal converter station to the M-terminal converter station in a multi-terminal flexible DC system that needs protection.

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

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

[0173] Since this is for cases with the same operating state but different fault locations, the matrix needs to be modified and the system of equations resolved. According to Kirchhoff's current law, we have:

[0174]

[0175] Its matrix form can be listed as follows:

[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, based on the three non-faulty lines and the faulty line of the multi-terminal flexible DC system, five loop equations can be listed as follows:

[0182]

[0183] It can be converted into matrix form as follows:

[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 system of equations (22), (37) and (42) above, the fault line current when a fault f2 occurs can be solved as follows:

[0190]

[0191] Wherein, matrix B + For the line parameters and matrix Y mmcN The generalized inverse of the B matrix.

[0192] When an external fault occurs, the line from station 1 to station 2 is considered the DC line requiring protection, and the line from station 1 to station 2 is considered within the protection zone. Taking the external fault of the line from station 1 to station 3 as an example, under the condition that the positive direction is from the DC bus to the DC line, since the fault occurs outside the protection zone, there will inevitably be a fault current from the DC line to the DC bus. Therefore, according to the magnitude and corresponding positive and negative values ​​of the fault current obtained from equation (43), the fault current i can be obtained. 12 The amplitude is less 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 Then we can get:

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

[0194] i 21 -i 210 >0

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

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

[0197] Similarly, line 12 The sudden energy of the current-limiting reactor measured near the protection terminals 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 deduced that,

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

[0201]

[0202] In the formula, s represents the Laplace operator, L dc Indicates the inductance of the current-limiting reactor, ΔU mn ΔI represents the voltage surge from the M-terminal converter station to the N-terminal converter station in a multi-terminal flexible DC system that requires protection. mn i represents the sudden change in current flowing from the M-terminal converter station to the N-terminal converter station in a multi-terminal flexible DC system that needs protection. mn This represents the line current flowing from the M-terminal converter station to the N-terminal converter station in a multi-terminal flexible DC system that needs protection. mn0 This represents the steady-state current amplitude flowing from the M-terminal converter station to the N-terminal converter station in a multi-terminal flexible DC system that requires protection; ΔU nm ΔI represents the voltage surge from the N-terminal converter station to the M-terminal converter station in a multi-terminal flexible DC system that needs protection. nm i represents the sudden change in current flowing from the N-terminal converter station to the M-terminal converter station in a multi-terminal flexible DC system that needs protection. nmThis represents the line current flowing from the N-terminal converter station to the M-terminal converter station in a multi-terminal flexible DC system that needs protection. mn0 This represents the steady-state current amplitude flowing from the N-terminal converter station to the M-terminal converter station in a multi-terminal flexible DC system that needs protection.

[0203] According to equation (47), when a DC line needs to be protected against an external fault, there will be a situation where the current-limiting reactor at one end of the DC line has a sudden energy of less than 0, while the current-limiting reactor at the other end has a sudden energy of greater than 0.

[0204] In summary, when a fault occurs within the DC line, the protection devices at both ends of the DC line measure the voltage and current surges of the current-limiting reactors, and the calculated surge energy of the current-limiting reactors is greater than 0. When a fault occurs outside the DC line, the protection devices at both ends of the DC line measure the voltage and current surges of the current-limiting reactors, and the surge energy of the current-limiting reactors measured at both ends is less than 0, while the surge energy of the current-limiting reactors measured at the other end is greater than 0.

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

[0206]

[0207] In the formula, S m (ω f ), S n (ω f The numbers ) represent the DC lines requiring protection at high-frequency components with a frequency of ω. f The transient energy of the current-limiting reactor at terminals M and N, where H represents the total number of sampling points, h′ is the sampling time interval, T is the integration time, and Δu m (ω f ,h) Δu n (ω f (h) represent the DC line requiring protection at the h-th sampling point with a high-frequency component frequency of ω. f The voltage fluctuation of the current-limiting reactor at terminals M and N, Δi m (ω f ,h),Δi n (ω f (h) represent the DC line requiring protection at the h-th sampling point with a high-frequency component frequency of ω. f The sudden change in current of the current-limiting reactor at terminals M and N is denoted by t, where t represents time.

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

[0209]

[0210] In the formula, S set (ω f ) indicates that the frequency of the high-frequency component is ω f Action threshold value at that time;

[0211] If S m (ω f ) and S n (ω f If all of the above conditions are met, the fault is determined to be a fault within the DC line protection zone; otherwise, it is determined to be a fault outside the DC line protection zone.

[0212] Specifically, if the fault is identified as being within the line area, the line protection will activate; otherwise, if the fault is identified as being being outside the line area, the line protection will not activate.

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

[0214] Considering the influence of measurement errors and noise, a certain margin needs to be set for the current limiter's sudden energy criterion to ensure reliability. Therefore, an 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 This indicates that the multi-terminal flexible DC system operates in steady state at a high-frequency component frequency of ω. f The maximum current-limiting reactor sudden energy at that time.

[0217] Preferably, during steady-state operation of the multi-terminal flexible DC system, the high-frequency component frequency is ω. f Maximum current-limiting reactor sudden energy S 0max (ω f (Not exceeding 100)

[0218] As the above analysis shows, the sudden energy of the current-limiting reactor changes with frequency. It is necessary to weigh the advantages and disadvantages of various factors and select high-frequency fault components of different frequencies within a suitable characteristic frequency band to construct the protection criterion. In this embodiment, the high-frequency component frequency ω is obtained through the following method. f :

[0219] Set the derivative of the sudden energy difference between the fault reactors inside and outside the protected DC line to 0, and obtain the solutions in each frequency domain;

[0220] Based on the solutions in each frequency domain, the energy difference of sudden change of fault reactors inside and outside the protected DC line is obtained under each frequency domain solution.

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

[0222] Specifically, the sudden energy difference between 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′ This indicates that the DC line needs protection at a high-frequency component frequency of ω. f′ The energy difference between the internal and external fault reactors during sudden changes, S m区内 (ω f′ This indicates that the DC line needs protection during a fault within the fault zone, and the high-frequency component frequency is ω. f′ The sudden energy change of the reactor at the M-terminal bus port, S m区外 (ω f′ This indicates that the DC line needs protection when a fault occurs outside the protection zone and the high-frequency component frequency is ω. f′ The sudden change in reactor energy at the M-terminal bus port.

[0225] More specifically, when the DC line to be protected experiences a fault within the fault zone and the high-frequency component frequency is ω f′ The sudden change energy S of the reactor at the M-terminal bus port m区内 (ω f′ ) is represented as:

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

[0227] In the formula, L dc Indicates the inductance of the current-limiting reactor, i′ mn This indicates that in a multi-terminal flexible DC system, protection is required for DC line faults occurring within the fault zone, and the high-frequency component frequency is ω.f′ The line current i flowing from converter station M to converter station N. mn0 This represents the steady-state current amplitude flowing from the M-terminal converter station to the N-terminal converter station in a multi-terminal flexible DC system that requires protection.

[0228] More specifically, the DC line to be protected is in the event of a fault outside the fault zone and the high-frequency component frequency is ω. f′ The sudden change energy S of the reactor at the M-terminal bus port m区外 (ω f′ ) is represented 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 the complex unit, L dc Indicates the inductance of the current-limiting reactor, i′ m ′ n This indicates that in a multi-terminal flexible DC system, protection is required for DC lines experiencing external faults, and the high-frequency component frequency is ω. f′ The line current i flowing from converter station M to converter station N. mn0 This represents the steady-state current amplitude flowing from the M-terminal converter station to the N-terminal converter station in a multi-terminal flexible DC system that requires protection.

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

[0232] Compared with existing technologies, this embodiment provides a method for protecting DC lines in a multi-terminal flexible DC system. It obtains 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 a fault occurs, based on the high-frequency component frequency ω of the DC line to be protected. f The voltage and current surges at both ends of the current-limiting reactor are used to obtain the high-frequency component frequency ω of the DC line to be protected. fThe system detects sudden energy changes in the current-limiting reactors at both ends and then determines, based on fault protection criteria, whether a fault has occurred in the DC line requiring protection. If so, it initiates the protection action for the DC line. This method can accurately and quickly identify faults inside and outside the protection zone, with fast action speed and low requirements for communication devices. The communication devices at both ends of the line only need to transmit real-time logic information and are not affected by synchronization errors at both ends. This solves the problem that existing multi-terminal flexible DC system DC line protection cannot guarantee accurate identification of DC line faults under low communication requirements. It adopts a dual-terminal quantity protection approach, using single-terminal electrical quantities for threshold judgment in the frequency domain under the comprehensive consideration of the converter control influence, and then using logic information for interaction. It also considers the impact of the new energy side on the flexible DC line, ensuring more accurate calculation results. Moreover, the dual-terminal only requires logic information interaction, which can solve the communication requirement problem, has low dependence on communication equipment, and strong resistance to transition resistance.

[0233] Example 2

[0234] A specific embodiment of the present invention discloses a multi-terminal flexible DC system DC line protection 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 a fault occurs, thereby obtaining the high-frequency component frequency of the DC line to be protected at ω. f The voltage and current surges at both ends of the current-limiting reactor;

[0236] The current-limiting reactor sudden energy calculation module is used to calculate the energy of the DC line to be protected at a high frequency component frequency of ω. f The voltage and current surges at both ends of the current-limiting reactor are used to obtain the high-frequency component frequency ω of the DC line to be protected. f The sudden energy change of the current-limiting reactor at both ends;

[0237] The fault identification and protection module is used to identify the DC line to be protected at a high-frequency component frequency of ω. f The sudden energy change of the current-limiting reactors at both ends and the fault protection criteria are used to determine whether a fault has occurred in the DC line to be protected; if so, the protection action of the DC line to be protected is initiated.

[0238] During implementation, in the current-limiting reactor's sudden energy calculation module, one end of the DC line to be protected is designated as terminal M, and the other end as terminal N; with the DC bus pointing towards the DC line as the positive direction, the DC line to be protected is defined at a high-frequency component frequency of ω. f The sudden energy change of the current-limiting reactor at both ends is expressed as:

[0239]

[0240] In the formula, S m (ωf ), S n (ω f The numbers ) represent the DC lines requiring protection at high-frequency components with a frequency of ω. f The transient energy of the current-limiting reactor at terminals M and N, where H represents the total number of sampling points, h′ is the sampling time interval, T is the integration time, and Δu m (ω f ,h) Δu n (ω f (h) represent the DC line requiring protection at the h-th sampling point with a high-frequency component frequency of ω. f The voltage fluctuation of the current-limiting reactor at terminals M and N, Δi m (ω f ,h),Δi n (ω f (h) represent the DC line requiring protection at the h-th sampling point with a high-frequency component frequency of ω. f The sudden change in current of the current-limiting reactor at terminals M and N is denoted by t, where t represents time.

[0241] The specific implementation process of this invention can be found in the above method embodiments, and will not be repeated here.

[0242] Since this embodiment is based on the same principle as the above method embodiments, this system also has the corresponding technical effects of the above method embodiments.

[0243] Example 3

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

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

[0246]

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

[0248]

[0249]

[0250] In this embodiment, scenario 1 is set as follows: Assume a multi-terminal flexible DC transmission system DC line... 12 A positive-to-ground fault occurs at 50% of the rated current, with varying transition resistances ranging from 0 to 300 Ω. The sudden energy change S of the current-limiting reactor under this fault condition is... m S n As shown in Figure 8(a) and Figure 8(b).

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

[0252] Therefore, it can be concluded that the protection methods and systems of Examples 1 and 2 can be identified as faults within the zone and operate correctly when faults with different sizes of transition resistance occur in the DC lines of multi-terminal flexible DC systems, regardless of the fault type, and have good high resistance performance.

[0253] Scenario 2 in this embodiment is: in a multi-terminal flexible DC transmission system Line 12 Positive grounding faults are set at different locations, and the transition resistance is set to 120Ω. S in this fault scenario... m S n As shown in Figure 9(a) and Figure 9(b) respectively.

[0254] As shown in Figures 9(a) and 9(b), when the fault location changes, the sudden change in energy S of the current-limiting reactor... m It will decrease, S n There were some fluctuations, however S m and S nThe minimum values ​​are all positive, and the protection identifies a fault as occurring within the DC line area. As can be seen from Figure 9(a), when a positive line ground fault occurs at a distance of 60% from the beginning, S... m It reaches its minimum value of 2035.62 at t = 2.0 ms. As can be seen from Figure 9(b), when the positive line ground fault occurs at a distance of 40% from the beginning, S... n It reaches its minimum value of 945.17 at t = 0.3 ms. Because S m and S n The minimum value of all values ​​is greater than the threshold, satisfying S. m >S set >0,S n >S set Since the value is >0, all of the above types of faults can be identified as faults within the zone.

[0255] Therefore, it can be concluded that the protection methods and systems of Examples 1 and 2 can all be identified as faults within the zone and operate correctly when the DC line of the multi-terminal flexible DC system is faulty at different locations.

[0256] In this embodiment, scenario 3 is set as follows: [The following text appears to be unrelated and possibly from a different source:] ...in a multi-terminal flexible DC transmission system DC line Line... 12 When a fault occurs at the beginning of an adjacent DC line and a three-phase ground fault occurs on the AC side, the transition resistance varies from 0 to 300 Ω. When a fault occurs at the beginning of an adjacent DC line, the S value under fault conditions... m S n As shown in Figures 10(a) and 10(b) respectively, when a three-phase ground fault occurs on the AC side, S under the fault condition... m S n As shown in Figure 11(a) and Figure 11(b) respectively.

[0257] As shown in Figures 10(a) and 10(b), under different fault conditions, the larger the transition resistance, the greater the S value at the same time section. m The larger S is n The smaller the value, the more S will be under different transition resistance conditions. m S n Comparing the protection criteria, it can be seen that the protection operation conditions are not met, and the protection identifies it as an external fault. As shown in Figure 10(a), when a double-pole ground fault occurs on the line and the transition resistance is 300Ω, S... m It reaches its maximum value at t = 2.9 ms, which is -3561.9. As can be seen from Figure 10(b), when the transition resistance is 300 Ω, S... n It reaches its minimum value of 3555.44 at t = 2.9 ms. That is, S m S n If the value is less than 0, the protection criterion is not met, and the fault is determined to be outside the protection zone. The protection is reliable and will not operate.

[0258] As shown in Figures 11(a) and 11(b), under different fault conditions, as the transition resistance changes, the S value at the same time section... m S n Not much has changed, S m As the fault duration increases, S n The value decreases as the fault duration increases. S is calculated for different transition resistance conditions. m S n Comparing with the protection criteria, it can be seen that the protection operation conditions are not met, and the protection identifies it as an external fault. Observing Figure 11(a), it can be seen that when a ground fault occurs on the positive line and the transition resistance is 240Ω, S... m It reaches its maximum value of -1467.4 at t = 3.0 ms. As can be seen from Figure 11(b), when the transition resistance is 240 Ω, S... n It reaches its minimum value of 1464.5 at t = 3.0 ms. That is, S m S n If the value is less than 0, the protection criterion is not met, and the fault is determined to be outside the protection zone, so the protection will not operate.

[0259] Based on the above analysis, it can be seen that the protection methods and systems 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 zone through different transition resistances. They can accurately identify different sizes of transition resistances and different fault types as faults outside the zone, and have high sensitivity and high resistance resistance.

[0260] Those skilled in the art will understand that all or part of the processes of the methods described in the above embodiments can be implemented by a computer program instructing related hardware, and the program can be stored in a computer-readable storage medium. The computer-readable storage medium may be a disk, optical disk, read-only memory, or random access memory, etc.

[0261] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for protecting DC lines in a multi-terminal flexible DC system, characterized in that, Includes the following steps: 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 were collected before and after the fault occurred, and then the high-frequency component frequency of the DC line to be protected was obtained. f The voltage and current surges at both ends of the current-limiting reactor; Based on the aforementioned DC line requiring protection, at a high-frequency component frequency of ω f The voltage and current surges at both ends of the current-limiting reactor are used to obtain the high-frequency component frequency ω of the DC line to be protected. f The sudden energy change of the current-limiting reactor at both ends; According to the aforementioned DC line requiring protection, the high-frequency component frequency is ω. f The sudden energy change of the current-limiting reactors at both ends and the fault protection criteria are used to determine whether a fault has occurred in the DC line to be protected; if so, the protection action of the DC line to be protected is initiated. The high-frequency component frequency ω is obtained in the following way. f : Set the derivative of the sudden energy difference between the fault reactors inside and outside the protected DC line to 0, and obtain the solutions in each frequency domain; Based on the solutions in each frequency domain, the energy difference of sudden change of fault reactors inside and outside the protected DC line is obtained under each frequency domain solution. The frequency domain solution that maximizes the energy difference between the fault reactors inside and outside the protected DC line is selected as the high-frequency component frequency ω. f ; The sudden energy difference between 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′ This indicates that the DC line needs protection at a high-frequency component frequency of ω. f′ The energy difference between the internal and external fault reactors during sudden changes, S m区内 (ω f′ This indicates that the DC line needs protection during a fault within the fault zone, and the high-frequency component frequency is ω. f′ The sudden energy change of the reactor at the M-terminal bus port, S m区外 (ω f′ This indicates that the DC line needs protection when a fault occurs outside the protection zone and the high-frequency component frequency is ω. f′ The sudden energy change of the reactor at the M-terminal bus port.

2. The method for protecting DC lines in a multi-terminal flexible DC system according to claim 1, characterized in that, Let one end of the DC line to be protected be terminal M, and the other end be terminal N; taking the direction from the DC bus to the DC line as the positive direction, the DC line to be protected has a high-frequency component frequency of ω. f The sudden energy change of the current-limiting reactor at both ends is expressed as: In the formula, S m (ω f ), S n (ω f The numbers ) represent the DC lines requiring protection at high-frequency components with a frequency of ω. f The transient energy of the current-limiting reactor at terminals M and N, where H represents the total number of sampling points, h′ is the sampling time interval, T is the integration time, and Δu m (ω f ,h) Δu n (ω f (h) represent the DC line requiring protection at the h-th sampling point with a high-frequency component frequency of ω. f The voltage fluctuation of the current-limiting reactor at terminals M and N, Δi m (ω f ,h),Δi n (ω f (h) represent the DC line requiring protection at the h-th sampling point with a high-frequency component frequency of ω. f The sudden change in current of the current-limiting reactor at terminals M and N is denoted by t, where t represents time.

3. The method for protecting DC lines in a multi-terminal flexible DC system according to claim 2, characterized in that, The high-frequency component frequency is ω f The fault protection criteria at that time include: In the formula, S set (ω f ) indicates that the frequency of the high-frequency component is ω f Action threshold value at time; If S m (ω f ) and S n (ω f If all conditions are met, the fault is determined to be an internal fault of the DC line requiring protection; otherwise, it is determined to be an external fault of the DC line requiring protection.

4. The method for protecting DC lines in a multi-terminal flexible DC system according to claim 3, characterized in that, At a high frequency component frequency of ω f Action threshold value S at time set (ω f )satisfy: S set (oh f )>S 0max (oh f ) In the formula, S 0max (ω f This indicates that the multi-terminal flexible DC system operates in steady state at a high-frequency component frequency of ω. f The maximum current-limiting reactor sudden energy at that time.

5. The method for protecting DC lines in a multi-terminal flexible DC system according to claim 3, characterized in that, When the DC line to be protected experiences a fault within the fault zone and the high-frequency component frequency is ω f′ The sudden change energy S of the reactor at the M-terminal bus port m区内 (ω f′ ) is represented as: WITH m区内 (ω f′ )=∫|jω f′ L dc and' mn (ω f′ )|(i′ mn (ω f′ )-and mn0 ) In the formula, L dc Indicates the inductance of the current-limiting reactor, i′ mn This indicates that in a multi-terminal flexible DC system, protection is required for DC line faults occurring within the fault zone, and the high-frequency component frequency is ω. f′ The line current i flowing from converter station M to converter station N. mn0 This represents the steady-state current amplitude flowing from the M-terminal converter station to the N-terminal converter station of the DC line that needs protection in a multi-terminal flexible DC system, where j represents a complex number unit.

6. The method for protecting DC lines in a multi-terminal flexible DC system according to claim 3, characterized in that, The DC line requiring protection is affected when an external fault occurs and the high-frequency component frequency is ω. f′ The sudden change energy S of the reactor at the M-terminal bus port m区外 (ω f′ ) is represented as: WITH m区外 (ω f′ )=∫|jω f′ L dc and" mn (ω f′ )|(i″ mn (ω f′ )-and mn0 ) In the formula, L dc Indicates the inductance of the current-limiting reactor, i″ mn This indicates that in a multi-terminal flexible DC system, protection is required for DC lines experiencing external faults, and the high-frequency component frequency is ω. f′ The line current i flowing from converter station M to converter station N. mn0 This represents the steady-state current amplitude flowing from the M-terminal converter station to the N-terminal converter station of the DC line that needs protection in a multi-terminal flexible DC system, where j represents a complex number unit.

7. A multi-terminal flexible DC system DC line protection system based on the multi-terminal flexible DC system DC line protection method according to any one of claims 1-6, 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 a fault occurs, thereby obtaining the high-frequency component frequency of the DC line to be protected at ω. f The voltage and current surges at both ends of the current-limiting reactor; The current-limiting reactor sudden energy calculation module is used to calculate the energy of the DC line to be protected at a high frequency component frequency of ω. f The voltage and current surges at both ends of the current-limiting reactor are used to obtain the high-frequency component frequency ω of the DC line to be protected. f The sudden energy change of the current-limiting reactor at both ends; The fault identification and protection module is used to identify the DC line to be protected at a high-frequency component frequency of ω. f The sudden energy change of the current-limiting reactors at both ends and the fault protection criteria are used to determine whether a fault has occurred in the DC line to be protected; if so, the protection action of the DC line to be protected is initiated.

8. The multi-terminal flexible DC system DC line protection system according to claim 7, characterized in that, Let one end of the DC line to be protected be terminal M, and the other end be terminal N; taking the direction from the DC bus to the DC line as the positive direction, the DC line to be protected has a high-frequency component frequency of ω. f The sudden energy change of the current-limiting reactor at both ends is expressed as: In the formula, S m (ω f ), S n (ω f The numbers ) represent the DC lines requiring protection at high-frequency components with a frequency of ω. f The transient energy of the current-limiting reactor at terminals M and N, where H represents the total number of sampling points, h′ is the sampling time interval, T is the integration time, and Δu m (ω f ,h) Δu n (ω f (h) represent the DC line requiring protection at the h-th sampling point with a high-frequency component frequency of ω. f The voltage fluctuation of the current-limiting reactor at terminals M and N, Δi m (ω f ,h),Δi n (ω f (h) represent the DC line requiring protection at the h-th sampling point with a high-frequency component frequency of ω. f The sudden change in current of the current-limiting reactor at terminals M and N is denoted by t, where t represents time.

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