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CN108829989B - Method for designing parameters of main loop on direct current side of flexible direct current system containing superconducting direct current limiter - Google Patents

Method for designing parameters of main loop on direct current side of flexible direct current system containing superconducting direct current limiter Download PDF

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CN108829989B
CN108829989B CN201810652435.2A CN201810652435A CN108829989B CN 108829989 B CN108829989 B CN 108829989B CN 201810652435 A CN201810652435 A CN 201810652435A CN 108829989 B CN108829989 B CN 108829989B
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CN108829989A (en
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韩笑
孙建龙
李妍
黄俊辉
何大瑞
徐政
张哲任
徐雨哲
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State Grid Jiangsu Electric Power Design Consultation Co ltd
State Grid Corp of China SGCC
State Grid Jiangsu Electric Power Co Ltd
Economic and Technological Research Institute of State Grid Jiangsu Electric Power Co Ltd
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State Grid Jiangsu Electric Power Design Consultation Co ltd
State Grid Corp of China SGCC
State Grid Jiangsu Electric Power Co Ltd
Economic and Technological Research Institute of State Grid Jiangsu Electric Power Co Ltd
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Abstract

本发明公开了一种含有超导直流限流器的柔性直流系统直流侧主回路参数设计方法，包括：(1)确定短路电流峰值、避雷器支路接入时刻和最严酷工况；(2)计算平波电抗与短路电流峰值时刻的关系；(3)求解平波电抗大小；(4)求解限流电阻大小。本发明直流侧主回路参数设计方法能够填补含有超导限流器的直流主回路设计研究的空白，能为未来工程的设计起到一定的指导作用；在保证有效性的前提之下，本发明提出的基于递推法的短路电流计算公式，能够提高计算效率，避免耗时的时域仿真计算，减少直流主回路设计花费的时间。

Figure 201810652435

The invention discloses a parameter design method for the DC side main loop of a flexible DC system including a superconducting DC current limiter, comprising: (1) determining the short-circuit current peak value, the connection time of the arrester branch circuit and the most severe working condition; (2) Calculate the relationship between the smoothing reactance and the peak moment of the short-circuit current; (3) solve the magnitude of the smoothing reactance; (4) solve the magnitude of the current limiting resistance. The parameter design method of the DC side main circuit of the present invention can fill the blank of the design and research of the DC main circuit containing the superconducting current limiter, and can play a certain guiding role for the design of future projects; under the premise of ensuring the effectiveness, the present invention The proposed short-circuit current calculation formula based on the recursion method can improve the calculation efficiency, avoid time-consuming time-domain simulation calculation, and reduce the time spent on the design of the DC main circuit.

Figure 201810652435

Description

Method for designing parameters of main loop on direct current side of flexible direct current system containing superconducting direct current limiter
Technical Field
The invention belongs to the technical field of power transmission and distribution of a power system, and particularly relates to a method for designing parameters of a main loop on a direct current side of a flexible direct current system with a superconducting direct current limiter.
Background
In recent years, with the development of power electronic technology, flexible direct-current transmission technology using a Modular Multilevel Converter (MMC) is rapidly developed, the voltage level and the rated power are close to those of the conventional direct-current transmission technology, but the most important technical bottleneck of the flexible direct-current transmission technology is the rapid isolation technology of direct-current faults at present. In general, the difficulty of the flexible dc power transmission system in dc fault handling is mainly manifested as:
(1) the rising speed of the short-circuit current is high, and the short-circuit current can rise to a peak value within 10ms of failure generally; (2) the steady-state short-circuit current is large and can exceed the rated current by more than 10 times; (3) the short-circuit current has no polarity change in the fault process, zero crossing points do not exist, and the circuit breaker is difficult to extinguish arc; (4) the requirement for rapid fault removal is extremely high, the direct current fault removal time generally needs to be controlled within 5ms, and otherwise, the safety of equipment is seriously threatened.
At present, a flexible direct current system which is put into operation usually adopts a strategy of tripping an alternating current switch to process direct current faults, but the direct current fault processing strategy has the defects of slow action response, large impact on an alternating current and direct current system and the like; in order to solve the problems existing when the tripping alternating current switch is adopted to process the direct current fault, research institutions of various countries in the world are vigorously researched and developed with high voltage direct current circuit breakers. However, the short-circuit current of the high-voltage large-capacity flexible direct-current transmission system can usually reach tens of thousands of amperes, which far exceeds the breaking capability of the current high-voltage direct-current circuit breaker, and in order to reduce the requirement for the breaking current of the circuit breaker, a large impedance needs to be additionally connected in series in the direct-current line of the flexible direct-current transmission system.
At present, a superconducting current limiter based on a high-temperature superconducting technology is a more ideal solution. When the power grid is in a normal operation state, the impedance of the superconducting current limiter is close to zero, and the operation of the power grid is hardly influenced; after the short-circuit fault occurs to the power grid, the superconducting current limiter can be changed from a low-impedance state to a high-impedance state in a short time, so that the short-circuit current is limited; after the fault is eliminated, the superconducting current limiter automatically restores to a near-zero impedance state. When the flexible direct current system has a short-circuit fault, the superconducting direct current limiter and the direct current breaker can be used in a matched mode, the on-off current of the breaker is reduced through the superconducting current limiter, the influence of the short-circuit fault on the direct current system is weakened, and the practical problem that the existing flexible direct current transmission project is weak in short-circuit impact resistance is hopefully solved.
Fig. 1 shows a flexible dc system for handling dc faults using a dc breaker and a superconducting dc current limiter, where a converter station is connected to a dc line via the superconducting dc current limiter and the dc breaker in sequence. Fig. 2 is a structure of a hybrid dc circuit breaker with the most promising future, in which dc current only flows through a normal dc branch circuit in a normal state; when the direct current side short circuit fault is detected to occur, the main circuit breaker is switched on, then the load transfer switch is switched off, and the short circuit current is transferred to the main circuit breaker part; after the time delay of about a few milliseconds, after the ultra-fast mechanical switch is successfully switched off, the main circuit breaker is switched off, the short-circuit current loop is connected to the lightning arrester branch of the circuit breaker, and the short-circuit current is changed from continuous rising to continuous attenuation to zero; the superconducting direct current limiter mainly plays a role in restraining a short-circuit current peak value at the opening moment of a main breaker (namely, at the moment that a lightning arrester branch is connected into a short-circuit current loop).
The parameter design of the direct current side main loop of the flexible direct current system is an important component of the whole direct current system design, for the flexible direct current system containing the superconducting direct current limiter, the most important content of the parameter design of the direct current side main loop becomes the parameter matching problem of the superconducting direct current limiter and the direct current breaker, however, no document provides a corresponding design principle at present.
Disclosure of Invention
In view of the above, the invention provides a method for designing parameters of a direct-current side main loop of a flexible direct-current system containing a superconducting direct-current limiter, and the method has the advantages of clear physical significance, strong applicability and higher use value in engineering design.
A method for designing parameters of a main circuit at a direct current side of a flexible direct current system containing a superconducting direct current limiter comprises the following steps that a converter station in the flexible direct current system is connected to a direct current transmission line sequentially through the superconducting direct current limiter and a hybrid direct current breaker, the superconducting direct current limiter is formed by connecting a smoothing reactor and a current limiting resistor in series, the hybrid direct current breaker is formed by connecting a normal current branch, a main breaker and a lightning arrester branch in parallel, and the converter station adopts MMC;
(1) according to the design requirements of the system, determining the direct-current side short-circuit current peak value of the system before and after the installation of the superconducting direct-current limiter and the moment t when the lightning arrester branch is connected into the short-circuit current loopCBWorking conditions corresponding to the maximum direct current side short-circuit current;
(2) calculating a relation curve between the reactance value of the smoothing reactor and the peak moment of the short-circuit current based on the criterion of the peak moment of the short-circuit current, and then according to the time t that the peak moment of the short-circuit current must be later than the time t that the branch of the lightning arrester is connected into the short-circuit current loopCBIntercepting an interval lambda meeting the principle from a normal value range of the flat wave reactance;
(3) determining the magnitude of a reactance value of the smoothing reactor through adjustment in the interval Lambda according to a direct-current side short-circuit current peak value of a system before the superconducting direct-current limiter is installed;
(4) and adjusting and determining the resistance value of the current-limiting resistor based on a recursion method according to the DC side short-circuit current peak value of the system provided with the superconducting DC current limiter.
Further, the system design requirement in the step (1) is to set the peak value I of the short-circuit current on the DC side of the system after the superconducting DC current limiter is installedpkAnd the short-circuit current peak value suppression rate eta, so that the short-circuit current peak value on the direct current side of the system before the superconducting direct current limiter is arranged is Ipk/(1-η)。
Further, the moment t when the lightning arrester branch circuit is connected into the short-circuit current loop in the step (1)CB3 ms-10 ms after the system has short-circuit fault.
Further, the maximum direct current side short-circuit current in the step (1) is the maximum direct current of the system before the short-circuit fault, and the steady-state direct current of the system converter station under the corresponding working condition is the maximum and has the same direction as the short-circuit current after the fault.
Further, the specific implementation process of calculating the relationship curve between the reactance value of the smoothing reactor and the peak time of the short-circuit current based on the criterion of the peak time of the short-circuit current in the step (2) is as follows:
2.1 sampling in normal value range of smoothing reactor with a certain unit width, and obtaining any reactance value L by sampling smoothing reactordcCalculating the approximate time t corresponding to the occurrence of the peak of the short-circuit current by the following relational expressionp0：
Figure BDA0001705165520000031
Wherein: n is the cascading number of bridge arm sub-modules of the MMC adopted by the converter station, C0Is the capacitance value, L, in the MMC sub-module0Bridge arm reactance value, R, for MMC0The on-state resistance value of the MMC sub module is obtained;
2.2 at approximate time tp0Nearby, the following relational expression is solved by Newton-Raphson methodObtaining the precise short-circuit current peak value moment tp：
Figure BDA0001705165520000041
Wherein: i.e. idc0Direct current, U, of converter station at the instant of system short-circuit faultdc0For the direct voltage of the converter station at the moment of short circuit failure of the system,
Figure BDA0001705165520000042
θdc＝arctan(τdcωdc)，
Figure BDA0001705165520000043
t represents a time;
2.3 traversing each reactance value L in the normal value range of the smoothing reactance according to the steps 2.1-2.2dcTo obtain each reactance value LdcCorresponding short-circuit current peak time tpThereby drawing the reactance value L of the smoothing reactordcAnd the time t of the peak value of the short-circuit currentpThe relationship of (1).
Further, the specific implementation method of the step (3) is as follows:
3.1 calculating t according to the following relationCBShort-circuit current i of time systemdc(tCB) And a DC voltage Udc(tCB)：
Figure BDA0001705165520000044
Wherein: INT () is a rounding function, Δ t is a set step size, idc0Direct current, U, of converter station at the instant of system short-circuit faultdc0For the direct voltage of the converter station at the moment of short circuit failure of the system,
Figure BDA0001705165520000045
n is the cascading number of bridge arm sub-modules of the MMC adopted by the converter station, C0Is the capacitance value, L, in the MMC sub-module0Bridge arm reactance value, R, for MMC0Is the on-state resistance value, L, of the MMC sub-moduledcThe reactance value of the smoothing reactor is shown;
3.2 continuous adjustment of reactance value L within interval LambdadcMagnitude and calculating corresponding short-circuit current i according to step 3.1dc(tCB) Until it reaches the peak value of short-circuit current on the DC side of the system before installing the superconducting DC current limiter, at which time LdcNamely the finally determined reactance value of the smoothing reactor.
Further, the specific implementation method of the step (4) is as follows:
4.1 by INT (t) according to the following formulaCBT) can be calculated by recursion times of/delta t)CBShort-circuit current i of time systemdc(tCB)：
Xk+1＝(I-Δt·Hk)-1Xk
Figure BDA0001705165520000051
Wherein: xkAnd Xk+1Respectively the system direct current voltage current vectors of the kth iteration and the system direct current voltage current vectors of the (k + 1) th iteration, INT () is a rounding function, delta t is a set step length, N is the cascade number of bridge arm sub-modules of the MMC adopted by the converter station, and C0Is the capacitance value, L, in the MMC sub-module0Bridge arm reactance value, R, for MMC0Is the on-state resistance value, L, of the MMC sub-moduledcIs a reactance value of a smoothing reactor, Rdc(t) is the resistance value of the current-limiting resistor at the time t, wherein t is k delta t, k is a natural number,
Figure BDA0001705165520000052
idc0direct current, U, of converter station at the instant of system short-circuit faultdc0Direct current voltage of the converter station at the moment of short circuit fault of the system;
4.2 by continuously adjusting the resistance Rdc(t) and calculating the corresponding t according to step 4.1CBShort-circuit current i of time systemdc(tCB) Until it reaches the DC side short-circuit current peak of the system equipped with superconducting DC current limiter, at which time RdcAnd (t) is the finally determined resistance value of the current limiting resistor.
Further, the resistance value R of the current-limiting resistordc(t) Kf (t), f (t) is a constant or a time-domain function which monotonically increases from 0 to 1, K is a proportionality coefficient and is a real number greater than or equal to 0, and the resistance value R is adjusteddc(t) adjusting the proportionality coefficient K, the larger the proportionality coefficient K is, the larger the short-circuit current idc(tCB) The smaller.
Based on the technical scheme, the invention has the following beneficial technical effects:
(1) for the MMC flexible direct-current transmission system containing the superconducting current limiter, the invention fills the blank of the design research of the direct-current main loop, and can play a certain guiding role in the design of future engineering.
(2) On the premise of ensuring effectiveness, the invention provides a short-circuit current calculation formula based on a recurrence method, which can improve calculation efficiency, avoid time-domain simulation calculation consuming time and reduce time spent on designing a direct current main loop.
Drawings
Fig. 1 is a schematic diagram of a structure of a flexible direct current transmission system with a superconducting current limiter.
Fig. 2 is a schematic structural diagram of the hybrid dc circuit breaker.
FIG. 3 is a flow chart illustrating the steps of the method of the present invention.
Fig. 4 is a diagram illustrating the relationship between the smoothing reactance and the peak time of the short-circuit current.
Fig. 5 is a diagram illustrating the relationship between the smoothing reactance and the peak value of the short-circuit current.
Fig. 6 is a diagram showing the relationship between the maximum resistance and the peak value of the short-circuit current of the superconducting current limiter.
FIG. 7 is a diagram illustrating comparison between results of calculating the short-circuit current by simulation and by the method of the present invention.
Detailed Description
In order to more specifically describe the present invention, the following detailed description is provided for the technical solution of the present invention with reference to the accompanying drawings and the specific embodiments.
As shown in fig. 1, the flexible dc power transmission system according to the present embodiment assumes that the current limiting resistance of the superconducting current limiter after the fault is in accordance with Rsup＝Rmax(1-e-t/t0) Change of regularity of, t0Taken as 1ms, the remaining system parameters are shown in table 1:
TABLE 1
Figure BDA0001705165520000061
As shown in fig. 3, the parameters of the dc-side main loop of the flexible dc system including the superconducting dc current limiter are designed by the following method (taking the MMC2 side as an example):
(1) respectively determining the short-circuit current at the direct current side of the front and rear flexible direct current systems provided with the superconducting current limiter according to the design requirements of the systems (considering the short-circuit faults of the point A and the point B); determining the moment when a lightning arrester branch of the circuit breaker is connected into a short-circuit current loop; and determining the working condition (the harshest working condition) corresponding to the maximum short-circuit current on the direct-current side.
The system design requirement is short-circuit current peak I of the direct current system after installing the superconducting current limiterpkAnd short-circuit current peak suppression ratio eta, so that the short-circuit current peak value before installing the superconducting current limiter is IpkV (1-. eta.); moment t of connecting lightning arrester branch of circuit breaker to short-circuit current loopCB(assuming that the fault occurs at 0s, generally 3ms to 10ms after the fault); under the most severe working condition, the steady-state direct current of the converter is the largest and has the same direction as the short-circuit current after the fault.
According to the design requirement of the short-circuit current, the short-circuit current peak values of the front and rear direct current systems provided with the superconducting current limiter are respectively 2.1kA and 3.5kA, and the lightning arrester branch of the direct current circuit breaker is connected to a short-circuit current loop 5ms after the fault. Steady-state direct current i of MMC2 before fault in direct current reference direction shown in FIG. 1dc0Is 0.4 kA.
(2) And calculating the relation between the smoothing reactance and the peak moment of the short-circuit current based on the criterion of the peak moment of the short-circuit current. The calculated peak moment of the short-circuit current must be later than the moment when the branch of the arrester of the circuit breaker is connected into the short-circuit current loop.
Suppose N is the cascade number of the bridge arm converter sub-modules of MMC, C0For the capacitance in the converter submodule, L0Bridge arm reactance, R, being MMC0Is the on-resistance, L, of the converter submoduledcIs a smoothing reactor, RdcIs the equivalent resistance of the direct current side.
2.1 reactance L for smoothing wavedcFirst, an approximate expression t of the peak time of the short-circuit current is calculatedp0＝π/(2*ωdc) Wherein:
Figure BDA0001705165520000071
2.2 at tp0Nearby, calculating the precise short-circuit current peak value moment t by a Newton-Raphson method according to the criterion of short-circuit current peak value momentpAnd the short-circuit current judgment criterion is as follows:
Figure BDA0001705165520000072
wherein: i.e. idc0Direct current, U, of MMC at the instant of occurrence of a faultdc0The dc voltage of the MMC at the moment of the fault occurrence,
Figure BDA0001705165520000081
θdc＝arctan(τdcωdc)，
Figure BDA0001705165520000082
Figure BDA0001705165520000083
and 2.3, repeating the two steps for other values of the smoothing reactance to obtain a relation curve of the smoothing reactance and the peak moment of the short-circuit current.
The present embodiment uses LdcAs an example of 10mH, the following,firstly, an approximate expression t at the moment of the peak value of the short-circuit current is calculatedp09.5 ms; then at tp0Nearby, calculating the precise short-circuit current peak value moment t by a Newton-Raphson method according to the criterion of short-circuit current peak value momentpCalculating to obtain the precise short-circuit current peak value moment t according to the following formulapIs 9.0 ms.
F(t)＝-66.1356cos(ωdct-1.5381)+2.1662sin(ωdct-1.5381)+1353.2cos(ωdct)-44.3230sin(ωdct)＝0
Finally, the value of the smoothing reactor is changed within the range of 0 mH-60 mH, and the two steps are repeated to obtain a relation curve of the smoothing reactor and the peak moment of the short-circuit current, wherein the relation curve is shown in figure 4; as can be seen from fig. 4, the short-circuit current peak timing increases as the smoothing reactance increases. For a calculation system, no matter what the value of the smoothing reactor is, the requirement that the peak time of the short-circuit current is later than the time when the branch of the lightning arrester of the circuit breaker is connected into the short-circuit current loop can be met.
(3) And determining the size of the smoothing reactor according to the short-circuit current required by a system before the superconducting current limiter is installed on the basis of a short-circuit current calculation formula.
First, it is determined that the step size is dt, then tCBThe short-circuit current and the direct-current voltage at a moment can be calculated according to the following formulas:
Figure BDA0001705165520000084
wherein INT () represents the nearest integer, and the other symbols are defined as follows:
Figure BDA0001705165520000085
then changing LdcIn the case of the magnitude, the short-circuit current is calculated by the above formula until the short-circuit current idc(tCB) Is equal to Ipk/(1-η)。
In the present embodiment, the step size is 2 × 10-5s, then the short-circuit current and the dc voltage at the time of 5ms can be calculated according to the following equations:
Figure BDA0001705165520000091
the value of the smoothing reactor is changed within the range of 0 mH-60 mH, the short-circuit current at the moment of 5ms is calculated by using the formula, and the calculation result is shown in figure 5; from FIG. 5, it can be seen that when the smoothing reactance L is equal todcWhen 24.5mH is taken, the short-circuit current is 3.5kA, so the smoothing reactor should be selected to be 24.5 mH.
(4) And determining the current limiting resistance of the superconducting current limiter according to the short-circuit current required by the system after the superconducting current limiter is installed by adopting a short-circuit current calculation formula based on a recurrence method.
First, the step length is determined to be dt, and the calculation result X in the k stepkIn known cases, the result of step k +1 can be calculated by the following recursion formula:
Xk+1＝(I-dt·A)-1Xk
wherein R isdc(k) The change characteristic of the current limiting resistance of the superconducting current limiter along with time is shown, and the rest symbols are defined as follows:
Figure BDA0001705165520000092
then through INT (t)CBDt) recursion times, calculating tCBA time short circuit current; repeating the above steps until the calculated short-circuit current i is changed under the condition of changing the magnitude of the current limiting resistance of the superconducting current limiterdc(tCB) Is equal to Ipk。
In the present embodiment, the step size is 2 × 10-5s at smoothing reactance LdcAssuming a maximum resistance R of the superconducting current limiter to be 24.5mHmaxIs 6 omega. In the case that the calculation result of the k-th step is known, the result of the k + 1-th step can be calculated by the following recursion formula:
Figure BDA0001705165520000093
calculating short-circuit current i at 5ms by recursion for 250 timesdc(5ms) 2.18 kA; making superconducting current limiter maximum resistance RmaxThe value of (1) is changed within the range of 0-9 omega, the steps are repeated to calculate the short-circuit current at the moment of 5ms, and the calculation result is shown in fig. 6; it can be seen from FIG. 6 that when the maximum resistance R of the superconducting current limiter is reachedmaxWhen the resistance is 6.5 omega, the short-circuit current is 2.1kA, so the maximum resistance R of the superconducting current limitermaxShould be selected to be 6.5 omega, the resistance R of the superconducting current limiter at 5ms after the faultsupAnd 6.4 omega.
FIG. 7 shows LdcIs 24.5mH and RmaxAt 6.5 Ω, the comparison between the short-circuit current calculated by the conventional simulation calculation and the short-circuit current calculated by the embodiment within 0-5 ms after the fault is shown schematically, and it can be seen from fig. 7 that the calculation accuracy of the embodiment is very close to the simulation calculation result.
The embodiments described above are presented to enable a person having ordinary skill in the art to make and use the invention. It will be readily apparent to those skilled in the art that various modifications to the above-described embodiments may be made, and the generic principles defined herein may be applied to other embodiments without the use of inventive faculty. Therefore, the present invention is not limited to the above embodiments, and those skilled in the art should make improvements and modifications to the present invention based on the disclosure of the present invention within the protection scope of the present invention.

Claims (8)

1.一种含有超导直流限流器的柔性直流系统直流侧主回路参数设计方法，包括如下步骤，所述柔性直流系统中换流站依次通过超导直流限流器和混合式直流断路器连接至直流输电线路，所述超导直流限流器由平波电抗器和限流电阻串联构成，所述混合式直流断路器由正常通流支路、主断路器以及避雷器支路并联构成，所述换流站采用MMC；1. A method for designing DC side main loop parameters of a flexible DC system containing a superconducting DC current limiter, comprising the steps of: in the flexible DC system, a converter station sequentially passes the superconducting DC current limiter and a hybrid DC circuit breaker Connected to the DC transmission line, the superconducting DC current limiter is composed of a smoothing reactor and a current limiting resistor in series, and the hybrid DC circuit breaker is composed of a normal flow branch, a main circuit breaker and an arrester branch in parallel, The converter station adopts MMC; (1)根据系统设计要求，确定装设超导直流限流器前后系统的直流侧短路电流峰值、避雷器支路接入短路电流回路的时刻tCB、最大直流侧短路电流对应的工况；(1) According to the system design requirements, determine the DC side short-circuit current peak value of the system before and after installing the superconducting DC current limiter, the time tCB when the arrester branch is connected to the short-circuit current loop, and the working conditions corresponding to the maximum DC side short-circuit current; (2)基于短路电流峰值时刻判别准则计算出平波电抗器的电抗值与短路电流峰值时刻的关系曲线，进而根据短路电流峰值时刻必须晚于避雷器支路接入短路电流回路的时刻tCB的原则，从平波电抗正常取值范围内截取满足该原则的区间Λ；(2) Calculate the relationship curve between the reactance value of the smoothing reactor and the short-circuit current peak time based on the criterion of short-circuit current peak time, and then according to the short-circuit current peak time must be later than the time tCB when the arrester branch is connected to the short-circuit current loop principle, intercept the interval Λ that satisfies this principle from the normal value range of the smoothing reactance; (3)根据装设超导直流限流器前系统的直流侧短路电流峰值在所述区间Λ内通过调节确定平波电抗器的电抗值大小；(3) according to the DC side short-circuit current peak value of the system before the superconducting DC current limiter is installed, the reactance value of the smoothing reactor is determined by adjustment in the interval Λ; (4)根据装设超导直流限流器后系统的直流侧短路电流峰值基于递推法调节确定限流电阻的阻值大小。(4) According to the DC side short-circuit current peak value of the system after the superconducting DC current limiter is installed, the resistance value of the current limiting resistor is adjusted and determined based on the recursive method. 2.根据权利要求1所述的柔性直流系统直流侧主回路参数设计方法，其特征在于：所述步骤(1)中系统设计要求会给定装设超导直流限流器后系统的直流侧短路电流峰值Ipk和短路电流峰值抑制率η，故装设超导直流限流器前系统的直流侧短路电流峰值为Ipk/(1-η)。2. The method for designing DC side main loop parameters of a flexible DC system according to claim 1, wherein the system design requirement in the step (1) will set the DC side of the system after the superconducting DC current limiter is installed The short-circuit current peak value I pk and the short-circuit current peak suppression rate η, so the DC side short-circuit current peak value of the system before the superconducting DC current limiter is installed is I pk /(1-η). 3.根据权利要求1所述的柔性直流系统直流侧主回路参数设计方法，其特征在于：所述步骤(1)中避雷器支路接入短路电流回路的时刻tCB为系统发生短路故障后的3ms～10ms。3. The method for designing DC side main loop parameters of a flexible DC system according to claim 1, characterized in that: in the step (1), the moment t CB when the arrester branch is connected to the short-circuit current loop is after the short-circuit fault occurs in the system. 3ms～10ms. 4.根据权利要求1所述的柔性直流系统直流侧主回路参数设计方法，其特征在于：所述步骤(1)中最大直流侧短路电流即为短路故障前系统的最大直流电流，其对应工况下系统换流站的稳态直流电流最大且与故障后短路电流方向相同。4. The method for designing DC side main loop parameters of a flexible DC system according to claim 1, wherein the maximum DC side short-circuit current in the step (1) is the maximum DC current of the system before the short-circuit fault, which corresponds to the The steady-state DC current of the system converter station is the largest and in the same direction as the short-circuit current after the fault. 5.根据权利要求1所述的柔性直流系统直流侧主回路参数设计方法，其特征在于：所述步骤(2)中基于短路电流峰值时刻判别准则计算平波电抗器电抗值与短路电流峰值时刻关系曲线的具体实现过程如下：5. The method for designing DC side main loop parameters of a flexible DC system according to claim 1, wherein: in the step (2), the reactance value of the smoothing reactor and the short-circuit current peak moment are calculated based on the short-circuit current peak moment judgment criterion The specific implementation process of the relationship curve is as follows: 2.1以一定单位宽度在平波电抗正常取值范围内进行取样，对于平波电抗器取样得到的任一电抗值Ldc，通过以下关系式计算对应出现短路电流峰值的近似时刻tp0：2.1 Sampling within the normal value range of the smoothing reactance with a certain unit width. For any reactance value L dc obtained by sampling the smoothing reactor, calculate the approximate time t p0 corresponding to the short-circuit current peak value by the following relationship:
Figure FDA0003416437060000021
Figure FDA0003416437060000021
 其中：N为换流站所采用MMC的桥臂子模块级联个数，C0为MMC子模块中的电容值，L0为MMC的桥臂电抗值，R0为MMC子模块的通态电阻值；Among them: N is the cascade number of the bridge arm sub-modules of the MMC used in the converter station, C 0 is the capacitance value in the MMC sub-module, L 0 is the bridge arm reactance value of the MMC, and R 0 is the on-state of the MMC sub-module resistance; 2.2在近似时刻tp0附近，通过牛顿-拉夫逊法求解以下关系式得到精确的短路电流峰值时刻tp：2.2 In the vicinity of the approximate time t p0 , the exact short-circuit current peak time t p is obtained by solving the following relationship by the Newton-Raphson method:
Figure FDA0003416437060000022
Figure FDA0003416437060000022
 其中：idc0为系统发生短路故障瞬间换流站的直流电流，Udc0为系统发生短路故障瞬间换流站的直流电压，
Figure FDA0003416437060000023
θdc＝arctan(τdcωdc)，
Figure FDA0003416437060000024
t表示时刻；Among them: i dc0 is the DC current of the converter station at the moment when the system short-circuit fault occurs, U dc0 is the DC voltage of the converter station at the moment when the system short-circuit fault occurs,
Figure FDA0003416437060000023
θ dc = arctan(τ dc ω dc ),
Figure FDA0003416437060000024
t represents time; 2.3根据步骤2.1～2.2遍历平波电抗正常取值范围内的每一个电抗值Ldc，得到每个电抗值Ldc对应的短路电流峰值时刻tp，从而绘制出平波电抗器电抗值Ldc与短路电流峰值时刻tp的关系曲线。2.3 Traverse each reactance value L dc within the normal value range of the smoothing reactance according to steps 2.1 to 2.2 to obtain the short-circuit current peak time t p corresponding to each reactance value L dc , thereby drawing the smoothing reactor reactance value L dc The relationship curve with the short-circuit current peak time t p . 6.根据权利要求1所述的柔性直流系统直流侧主回路参数设计方法，其特征在于：所述步骤(3)的具体实现方法如下：6. The method for designing DC side main loop parameters of a flexible DC system according to claim 1, wherein the specific implementation method of the step (3) is as follows: 3.1根据以下关系式计算tCB时刻系统的短路电流idc(tCB)和直流电压Udc(tCB)：3.1 Calculate the short-circuit current i dc (t CB ) and the DC voltage U dc (t CB ) of the system at time t CB according to the following relations:
Figure FDA0003416437060000031
Figure FDA0003416437060000031
 其中：INT()为四舍五入取整函数，Δt为设定的步长，idc0为系统发生短路故障瞬间换流站的直流电流，Udc0为系统发生短路故障瞬间换流站的直流电压，
Figure FDA0003416437060000032
N为换流站所采用MMC的桥臂子模块级联个数，C0为MMC子模块中的电容值，L0为MMC的桥臂电抗值，R0为MMC子模块的通态电阻值，Ldc为平波电抗器的电抗值；Among them: INT() is the rounding function, Δt is the set step size, i dc0 is the DC current of the converter station at the moment of the short-circuit fault of the system, U dc0 is the DC voltage of the converter station at the moment of the short-circuit fault of the system,
Figure FDA0003416437060000032
N is the cascade number of the bridge arm sub-modules of the MMC used in the converter station, C 0 is the capacitance value in the MMC sub-module, L 0 is the bridge arm reactance value of the MMC, and R 0 is the on-state resistance value of the MMC sub-module , L dc is the reactance value of the smoothing reactor; 3.2在区间Λ内连续调节电抗值Ldc大小并根据步骤3.1计算对应的短路电流idc(tCB)，直至其达到装设超导直流限流器前系统的直流侧短路电流峰值，此时的Ldc即为平波电抗器最终确定的电抗值大小。3.2 Continuously adjust the reactance value L dc in the interval Λ and calculate the corresponding short-circuit current i dc (t CB ) according to step 3.1, until it reaches the DC side short-circuit current peak value of the system before installing the superconducting DC current limiter, at this time The L dc is the reactance value finally determined by the smoothing reactor. 7.根据权利要求1所述的柔性直流系统直流侧主回路参数设计方法，其特征在于：所述步骤(4)的具体实现方法如下：7. The method for designing DC side main loop parameters of a flexible DC system according to claim 1, wherein the specific implementation method of the step (4) is as follows: 4.1根据以下公式通过INT(tCB/Δt)次递推即可计算出tCB时刻系统的短路电流idc(tCB)：4.1 According to the following formula, the short-circuit current i dc (t CB ) of the system at the time of t CB can be calculated by INT(t CB /Δt) recursion: Xk+1＝(I-Δt·Hk)-1Xk X k+1 =(I-Δt·H k ) -1 X k
Figure FDA0003416437060000033
Figure FDA0003416437060000033
 其中：Xk和Xk+1分别为第k次和第k+1次迭代的系统直流电压电流向量，INT()为四舍五入取整函数，Δt为设定的步长，N为换流站所采用MMC的桥臂子模块级联个数，C0为MMC子模块中的电容值，L0为MMC的桥臂电抗值，R0为MMC子模块的通态电阻值，Ldc为平波电抗器的电抗值，Rdc(t)为t时刻限流电阻的阻值，t＝kΔt，k为自然数，
Figure FDA0003416437060000034
idc0为系统发生短路故障瞬间换流站的直流电流，Udc0为系统发生短路故障瞬间换流站的直流电压；Where: X k and X k+1 are the system DC voltage and current vectors of the kth and k+1th iterations, respectively, INT() is the rounding function, Δt is the set step size, and N is the commutation The number of cascaded bridge arm sub-modules of the MMC used in the station, C 0 is the capacitance value in the MMC sub-module, L 0 is the bridge arm reactance value of the MMC, R 0 is the on-state resistance value of the MMC sub-module, and L dc is The reactance value of the smoothing reactor, R dc (t) is the resistance value of the current limiting resistor at time t, t=kΔt, k is a natural number,
Figure FDA0003416437060000034
i dc0 is the DC current of the converter station at the moment when the system short-circuit fault occurs, U dc0 is the DC voltage of the converter station at the moment when the system short-circuit fault occurs; 4.2通过连续调节阻值Rdc(t)的大小并根据步骤4.1计算对应的tCB时刻系统的短路电流idc(tCB)，直至其达到装设超导直流限流器后系统的直流侧短路电流峰值，此时的Rdc(t)即为限流电阻最终确定的阻值大小。4.2 Calculate the short-circuit current i dc (t CB ) of the system at the corresponding time t CB by continuously adjusting the resistance value R dc (t) according to step 4.1 until it reaches the DC side of the system after the superconducting DC current limiter is installed The peak value of short-circuit current, R dc (t) at this time is the final resistance value of the current limiting resistor. 8.根据权利要求7所述的柔性直流系统直流侧主回路参数设计方法，其特征在于：所述限流电阻的阻值R＝Kf(t)，f(t)为常数或为从0单调上升至1的时域函数，K为比例系数且为大于等于0的实数，调节阻值R即调节该比例系数K，比例系数K越大则短路电流idc(tCB)越小。8 . The method for designing DC side main loop parameters of a flexible DC system according to claim 7 , wherein the resistance value of the current limiting resistor is R=Kf(t), and f(t) is constant or monotonic from 0. 9 . The time domain function rising to 1, K is the proportional coefficient and is a real number greater than or equal to 0, adjusting the resistance value R is to adjust the proportional coefficient K, the larger the proportional coefficient K, the smaller the short-circuit current i dc (t CB ) is.
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