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 PDFInfo
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Abstract
The invention discloses a method for designing parameters of a main loop on the direct current side of a flexible direct current system containing a superconducting direct current limiter, which comprises the following steps: (1) determining a short-circuit current peak value, an access time of a lightning arrester branch and the harshest working condition; (2) calculating the relation between the smoothing reactance and the peak time of the short-circuit current; (3) solving the magnitude of the smoothing reactance; (4) and solving the size of the current limiting resistor. The method for designing the parameters of the direct-current side main loop can fill the blank of the design research of the direct-current main loop containing the superconducting current limiter and can play a certain guiding role in the design of future engineering; on the premise of ensuring effectiveness, the short-circuit current calculation formula based on the recurrence method can improve calculation efficiency, avoid time-consuming time domain simulation calculation and reduce time spent on designing a direct current main loop.
Description
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：
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：
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,
θdc＝arctan(τdcωdc)，
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)：
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,
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
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,
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
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:
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:
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,
θdc＝arctan(τdcωdc)，
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:
wherein INT () represents the nearest integer, and the other symbols are defined as follows:
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:
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:
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:
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. 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.
2. The method for designing the parameters of the main circuit on the direct current side of the flexible direct current system according to claim 1, wherein the method comprises the following steps: the system design requirement in the step (1) is to set the DC side short-circuit current peak value I of the system provided with the superconducting DC current limiterpkAnd 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)。
3. The method for designing the parameters of the main circuit on the direct current side of the flexible direct current system according to claim 1, wherein the method comprises the following steps: the moment t when the lightning arrester branch is connected into the short-circuit current loop in the step (1)CB3 ms-10 ms after the system has short-circuit fault.
4. The method for designing the parameters of the main circuit on the direct current side of the flexible direct current system according to claim 1, wherein the method comprises the following steps: and (2) 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.
5. The method for designing the parameters of the main circuit on the direct current side of the flexible direct current system according to claim 1, wherein the method comprises the following steps: 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 judgment 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：
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 through a Newton-Raphson method to obtain accurate short-circuit current peak value time tp：
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,
θdc＝arctan(τdcωdc)，
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).
6. The method for designing the parameters of the main circuit on the direct current side of the flexible direct current system according to claim 1, wherein the method comprises the following steps: 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)：
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,
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.
7. The method for designing the parameters of the main circuit on the direct current side of the flexible direct current system according to claim 1, wherein the method comprises the following steps: 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
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,
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.
8. The method for designing the parameters of the main circuit on the direct current side of the flexible direct current system according to claim 7, wherein the method comprises the following steps: the resistance value R of the current limiting resistor is Kf (t), f (t) is constant or time domain which monotonically increases from 0 to 1A function, K is a proportionality coefficient and is a real number greater than or equal to 0, the proportionality coefficient K is adjusted by adjusting the resistance value R, and the larger the proportionality coefficient K is, the larger the short-circuit current i isdc(tCB) The smaller.
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| CN110829480A (en) | 2019-11-07 | 2020-02-21 | 国网江苏省电力有限公司经济技术研究院 | Control strategy suitable for LCC-MMC hybrid cascade converter station parallel MMC unit |
| CN110829396B (en) * | 2019-11-18 | 2022-04-15 | 武汉大学 | Hybrid direct-current transmission fault processing system and method based on current limiter and circuit breaker |
| CN111953221A (en) * | 2020-08-04 | 2020-11-17 | 中国长江三峡集团有限公司 | Modular multilevel converter and converter station |
| CN113162010B (en) * | 2021-03-19 | 2022-06-24 | 国网宁夏电力有限公司电力科学研究院 | Short-circuit current amplitude and direct-current component suppression method based on resistance type superconducting current limiter |
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