CN113075440B - Power grid short-circuit current calculation method containing flexible direct current transmission system - Google Patents

Power grid short-circuit current calculation method containing flexible direct current transmission system Download PDF

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CN113075440B
CN113075440B CN202110336123.2A CN202110336123A CN113075440B CN 113075440 B CN113075440 B CN 113075440B CN 202110336123 A CN202110336123 A CN 202110336123A CN 113075440 B CN113075440 B CN 113075440B
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CN113075440A (en
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蔡晖
谢珍建
赵菲菲
许偲轩
彭竹弈
黄成辰
张文嘉
祁万春
徐政
刘昕宇
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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 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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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R19/00—Arrangements for measuring currents or voltages or for indicating presence or sign thereof
    • G01R19/0092—Arrangements for measuring currents or voltages or for indicating presence or sign thereof measuring current only
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/50—Testing of electric apparatus, lines, cables or components for short-circuits, continuity, leakage current or incorrect line connections
    • G01R31/52—Testing for short-circuits, leakage current or ground faults
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
    • H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
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Abstract

The invention discloses a power grid short-circuit current calculation method containing a flexible direct-current transmission system. The algorithm can be used as a calculation method for checking the short-circuit current level of a power grid containing flexible direct current under different control strategies, and has certain significance for evaluating the effects of flexible direct related control protection strategies and flexible direct short-circuit current contribution inhibition strategies.

Description

Power grid short-circuit current calculation method containing flexible direct current transmission system
Technical Field
The invention belongs to the technical field of power systems, and particularly relates to a power grid short-circuit current calculation method with a flexible direct current transmission system.
Background
The high-voltage direct-current transmission based on the modular multilevel converter has the technical characteristics of passive inversion, active and reactive independent regulation, no problem of commutation failure, flexible operation mode and the like, has remarkable technical advantages in the occasions of renewable energy grid connection, long-distance large-capacity transmission, island power supply, asynchronous grid interconnection and the like, and is a key technology for constructing future high-proportion new energy power systems and smart grids. In recent years, flexible direct current transmission projects in China enter a rapid development period, Yubei back-to-back flexible direct current projects, Zhang-North four-end flexible direct current power grids and Wudongde extra-high voltage multi-end direct current projects are put into operation in succession, and extra-high voltage direct current projects of the portions of white crane beach, Jiangsu and the like which adopt flexible direct current technology are in the project planning stage.
At present, the installed capacity of a power grid in China is increased day by day, the interconnection of all areas is also tighter and tighter, and the level check of short-circuit current is more and more important to the safety of the power grid. The short-circuit current level of the accessed system can be changed due to flexible and straight access, and if the flexible and straight access is performed on an alternating current power grid with high short-circuit current level, the short-circuit current level of the alternating current power grid can exceed the standard, so that the short-circuit current level is necessary to be checked in the planning and design stage of flexible and straight engineering.
At present, related research on a flexible direct-current short-circuit current calculation method mainly aims at direct-current side faults, and most of alternating-current side faults are concentrated on a calculation method of short-circuit current during faults in a converter station and analysis of characteristics of output short-circuit current of a converter during faults of PCC points outside the converter station. However, there is little analysis on how to calculate the short-circuit current in the AC Power grid in consideration of the influence of the inverter, and documents [ MAO S R, XU Z, YANG J, et al. The literature [ LI Y S.Fault control and systematic short-circuit calculation for AC/DC hybrid power system [ D ]. Chongqing: Chongqing University,2018] analyzes the control strategy after the fault of the MMC-HVDC system and the operation mode and provides a mechanism and a calculation method for flexibly and directly outputting short-circuit current when the PCC point has the fault. However, there is little literature on methods for analyzing and calculating the short-circuit current characteristics of any node fault of the alternating current system.
The literature [ LI Y B, LI Y Q, QIN S M, et al.study on the effect of VSC-HVDC on converter output short circuit current level [ J ]. Journal of Global Energy Interconnection,2019,2(6):581-8] analyzes the influence factor of the amplitude of the short circuit current output by the MMC when any node fails and proposes that the PCC point voltage amplitude after the failure occurs should be firstly calculated, then the converter station is equivalent to a three-phase symmetrical current source according to the operation mode and the power level of the MMC to participate in the calculation of the short circuit current of the alternating current system, but the methods of calculating the PCC point voltage and participating in the calculation of the short circuit current of the alternating current system are not analyzed. The documents [ YI Y, SHEN Y, LIN Z S. characteristics and analysis methods of AC short-circuit current distribution by VSC-HVDC [ J ]. High Voltage Engineering,2018,44(7):2150-8] analyze the characteristics and mechanism of soft-direct contribution short-circuit current when any node of an alternating current system fails and propose corresponding inhibition measures, but the calculation methods of MMC controlled internal potential and PCC point Voltage are not given in the text.
In fact, there are three common engineering short-circuit current calculation methods for flexible direct-current access power grid: (1) the influence of flexible direct current is not counted; (2) the short-circuit current provided by the flexible and straight circuit is superposed with the short-circuit current amplitude provided by the alternating current system; (3) the influence of the soft direct current limiting link is considered and the phase relation between the short-circuit current provided by the soft direct current and the short-circuit current provided by the alternating current system is simply considered. Although the third algorithm is obviously improved compared with the first two algorithms, the error of the calculated result is still larger under partial control modes or faults. Therefore, in order to perfect the calculation mechanism of the short-circuit current contributed by the flexible and direct components in the engineering algorithm and improve the accuracy of the algorithm, it is necessary to analyze the characteristic of the short-circuit current output by the MMC when any node of the alternating current system fails and develop a more accurate engineering practical algorithm.
Disclosure of Invention
In view of the above, the invention provides a power grid short-circuit current calculation method including a flexible direct-current power transmission system, which is characterized in that the normal component of each node voltage is calculated by adopting the steady-state short-circuit current output by flexible direct-current after a fault based on the superposition principle, the final result can be obtained by iterative calculation of mutual correction of the normal network voltage and the fault network short-circuit current, and the accuracy of flexible direct-current power grid short-circuit current level check in the engineering can be improved.
A power grid short-circuit current calculation method containing a flexible direct current transmission system is characterized in that when an alternating current system accessed by the flexible direct current transmission system has a short-circuit fault, the fault characteristic of an MMC alternating current side connected with the flexible direct current transmission system is equivalent to a controlled current source, and the output short-circuit current and PCC point voltage are influenced mutually, so that the power grid short-circuit current can be calculated by adopting an iterative algorithm, and the method specifically comprises the following steps:
(1) in consideration of the capacity limit of the MMC and the amplitude limiting and current limiting links of an outer ring controller in an MMC control system, establishing an MMC alternating-current side fault model according to a control strategy adopted by the MMC;
(2) when the alternating current system has a short-circuit fault, obtaining the voltage steady-state value of each node in the alternating current system before the fault according to the load flow calculation result, and further calculating the initial value of the short-circuit current of the fault node by using the voltage steady-state value of the fault node;
(3) calculating the voltage correction quantity of the PCC according to the node voltage equation, and further correcting the voltage of the PCC;
(4) substituting the corrected PCC point voltage into an MMC alternating current side fault model so as to calculate and obtain the short-circuit current I output by the MMC mmc ；
(5) Calculating and correcting the voltage of each node according to a node voltage equation, further calculating and correcting the short-circuit current of the fault node by using the voltage of the fault node, returning to execute the step (3), and repeating iteration until the voltage of each node is converged;
(6) and calculating the short-circuit current of each branch in the alternating current system according to the converged node voltages.
Furthermore, the outer ring controller is formed by sequentially connecting a PI controller, an amplitude limiting link and a current limiter, wherein the amplitude limiting link and the current limiter can be described by the following relational expression;
Figure GDA0003736452380000031
wherein: i.e. i vref Is the output current amplitude, i, of the PI controller vdref And i vqref Respectively d-axis and q-axis components of the PI controller output current,
Figure GDA0003736452380000032
which is the current command of the MMC i.e. the output current of the current limiter,
Figure GDA0003736452380000033
and
Figure GDA0003736452380000034
d-axis component and q-axis component, I, of the MMC current command, respectively vmax Is the current limiting value of the current limiter i vdqmax The current amplitude limiting value of the amplitude limiting link.
Further, if the MMC adopts a constant active power and reactive power control strategy or a constant direct current voltage and reactive power control strategy, the alternating current side fault model is as follows:
when the PCC point voltage drops less after the fault
Figure GDA0003736452380000041
Short-circuit current I output by MMC mmc The expression is as follows:
Figure GDA0003736452380000042
Figure GDA0003736452380000043
Figure GDA0003736452380000044
when the voltage drop of the PCC point is larger after the fault
Figure GDA0003736452380000045
Then the short-circuit current I output by the MMC mmc The expression is as follows:
Figure GDA0003736452380000046
Figure GDA0003736452380000047
Figure GDA0003736452380000048
when a PCC point and a near zone thereof have serious faults
Figure GDA0003736452380000049
Then the short-circuit current I output by the MMC mmc The expression is as follows:
Figure GDA00037364523800000410
Figure GDA00037364523800000411
Figure GDA00037364523800000412
Figure GDA00037364523800000413
wherein:
Figure GDA00037364523800000414
and
Figure GDA00037364523800000415
respectively the d-axis component and the q-axis component of the MMC current command,
Figure GDA00037364523800000416
is the positive sequence voltage magnitude of the PCC point,
Figure GDA00037364523800000417
is the positive sequence voltage phase of the PCC point, j is an imaginary unit, P s * And
Figure GDA00037364523800000418
respectively an active power command value and a reactive power command value, I vmax Sig () is a sign function for the current limiting value of the current limiter.
Further, if the MMC adopts a constant active power and alternating voltage control strategy or a constant direct current voltage and alternating voltage control strategy, an alternating current side fault model is as follows:
when the PCC point voltage can reach the command value under the action of the control system after the fault
Figure GDA00037364523800000419
Then the short-circuit current I output by the MMC mmc The expression is as follows:
Figure GDA0003736452380000051
Figure GDA0003736452380000052
Figure GDA0003736452380000053
when the PCC point voltage falls after the fault
Figure GDA0003736452380000054
Then the short-circuit current I output by the MMC mmc The expression is as follows:
Figure GDA0003736452380000055
Figure GDA0003736452380000056
Figure GDA0003736452380000057
when a PCC point and a near zone thereof have serious faults
Figure GDA0003736452380000058
Then the short-circuit current I output by the MMC mmc The expression is as follows:
Figure GDA0003736452380000059
Figure GDA00037364523800000510
Figure GDA00037364523800000511
wherein:
Figure GDA00037364523800000512
and
Figure GDA00037364523800000513
are MMC current commands respectivelyThe d-axis component and the q-axis component of (a),
Figure GDA00037364523800000514
is the positive sequence voltage magnitude of the PCC point,
Figure GDA00037364523800000515
is the positive sequence voltage phase of the PCC point, j is an imaginary unit, P s * For active power command value, U 0 Is a command value of AC voltage, Q 0 Before fault, the control system controls the system to work at the voltage command value of U 0 Reactive power, Z, of the MMC output under the circumstances DD Is the self-impedance of the faulty node, Z mD Is the mutual impedance between the faulty node and the PCC point, I vmax Sig () is a sign function for the current limiting value of the current limiter.
Further, in the step (2), calculating an initial value of the short-circuit current of the fault node by the following formula;
Figure GDA00037364523800000516
wherein: i is D0 As initial value of short-circuit current of the fault node, U D0 Is the steady state value of the voltage of the fault node before the fault, Z DD Is the self-impedance of the failed node.
Further, in the step (3), the PCC point voltage is corrected through the following formula;
Figure GDA0003736452380000061
wherein: delta U pcc Is the voltage correction of the PCC point, Z mD For the mutual impedance between the faulty node and the PCC point, I D Short-circuit current, U, for faulty nodes pcc And U' pcc The PCC point voltages before and after correction are respectively.
Further, in the step (5), the voltage of each node is corrected by calculating through the following formula;
Figure GDA0003736452380000062
wherein: u shape i Is the voltage of the ith node in an AC system, I j For injecting current, Z, into node j in an AC system ij The mutual impedance between the ith node and the node j in the alternating current system is shown, E is a power supply node set in the alternating current system, the power supply node comprises a synchronous generator and an MMC, the injection current of the MMC is the short-circuit current I calculated in the step (4) mmc The injection current of the synchronous generator is
Figure GDA0003736452380000063
U n Is the rated voltage of the synchronous generator, c is the voltage coefficient of the synchronous generator, Z k I is a natural number, i is more than or equal to 1 and less than or equal to N, and N is the number of nodes in the alternating current system.
Further, in the step (5), the short-circuit current of the fault node is corrected by calculating according to the following formula;
Figure GDA0003736452380000064
wherein: i is D Short-circuit current for fault node, Z DD Is the self-impedance of the faulty node, U D Is the fault node voltage.
Further, the convergence condition of the voltage of each node in the step (5) is | U i(k+1) -U ik |<ε，U i(k+1) And U ik And the ith node voltage is respectively obtained by the k +1 th iteration and the k th iteration through calculation, k is a natural number greater than 0, and epsilon is a given convergence threshold value.
Further, in the step (6), for any branch in the ac system, dividing the voltage difference after the node convergence at the two ends of the branch by the line impedance is the short-circuit current of the branch.
Compared with the prior art, the invention has the following beneficial technical effects:
1. the algorithm of the invention can be applied to all common control strategies in the existing flexible direct current transmission project.
2. The algorithm of the invention can improve the accuracy of checking the short-circuit current level of the flexible direct power grid in the engineering.
3. The invention adopts an iterative calculation method, so that the requirement on the accuracy of the load flow calculation result is not high, and the engineering practical value is higher.
Drawings
Fig. 1 is a schematic structural diagram of a control system of a flexible direct current transmission system.
Fig. 2(a) is a schematic structural diagram of an outer-loop power controller of a flexible direct-current transmission system.
Fig. 2(b) is a schematic diagram of the current limiting effect of the outer loop power controller of the flexible dc power transmission system.
Fig. 3 is a schematic diagram of an equivalent calculation model of a short-circuit current of the flexible direct-current power transmission system.
Fig. 4 is a schematic diagram of an iterative calculation process of the short-circuit current of the grid including the flexible direct current in the embodiment of the present invention.
Fig. 5 is a schematic diagram of a receiving-end ring network test system of the flexible direct current transmission system.
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 and straight control system adopts direct current control under dq rotation coordinate system, and the control system mainly comprises an inner and outer ring controller and a phase-locked loop based on a decoupling double-synchronous reference coordinate system. Due to the existence of the converter transformer and the adoption of a negative sequence current suppression strategy, the converter only injects positive sequence current into the power grid during the asymmetric fault, and the output instantaneous power of the converter is also calculated by a positive sequence component.
As shown in fig. 2(a) and 2(b), when the ac system with soft dc access fails, the controlled variables such as the PCC point voltage and the soft dc output power drop instantaneously, and to ensure that the controlled variables reach the command value, the dq-axis current command value i output from the PI element of the outer loop power controller vdqref Will be correspondingly larger, but should be smallerAmplitude limiting value i of dq axis amplitude limiting link of PI controller vdqmax . In order to prevent the converter valve from overloading, the current instruction value i output when the PI link vref Exceeding the limiting value I of the outer loop current limiter vmax Then, by limiting the dq axis component in equal proportion, the current instruction value finally output by the outer loop current limiter
Figure GDA0003736452380000071
I.e. as a command value for the output current of the inner loop current controller.
As shown in fig. 3, when an asymmetric short-circuit fault occurs in the ac power grid, it can be known from the analysis of the principle of the flexible-direct control system that: the flexibility and the straightness in the positive sequence network can be externally equivalent to a positive sequence voltage control current source; in a negative sequence network, since the negative sequence current suppression control strategy is adopted flexibly, the command value of the negative sequence current is usually set to zero, i.e. the command value of the negative sequence current is set to zero
Figure GDA0003736452380000081
Therefore, no negative sequence current flows in the branch where the flexible direct current is located, namely, the flexible direct negative sequence equivalent calculation model is a controlled voltage source and is completely consistent with the magnitude and the phase of the negative sequence voltage of the PCC point, which is equivalent to the open circuit of the negative sequence loop of the current converter; in the zero sequence network, because the converter transformer blocks a zero sequence path between the flexible direct current and the alternating current network, the zero sequence current provided by the alternating current system on the network side can only flow through the transformer grounding branch, but the flexible direct current does not provide the zero sequence current, so the port current of the flexible direct current network side only contains the zero sequence current provided by the alternating current system, and the flexible direct current in the zero sequence network is equivalent to an open circuit.
As shown in fig. 4, the method for calculating the short-circuit current of the power grid including the flexible direct current transmission system of the present invention includes the following steps:
(1) and considering the capacity limit of the converter, the amplitude limiting link of the outer ring dq axis and the current limiting function of the current limiter, and establishing a fault model on the AC side of the converter according to a control strategy adopted by the converter.
The outer loop dq axis clipping element and current limiting effect of the current limiter can be described as:
Figure GDA0003736452380000082
wherein: i.e. i vdqmax Amplitude limiting value of dq axis amplitude limiting link, I vmax For the amplitude limit of the outer ring current limiter, i vref A current instruction value output by the PI link of the outer loop controller,
Figure GDA0003736452380000083
and the current command value is finally output by the outer loop current limiter.
When the current converter adopts a fixed active power and reactive power control strategy or a fixed direct current voltage and reactive power control strategy, the current converter can be discussed according to three conditions of the voltage drop degree of a PCC point:
PCC point voltage drop is less after failure
Figure GDA0003736452380000084
When the current converter outputs the active power and the reactive power, both the active power and the reactive power can reach the instruction values, and the short-circuit current I output by the current converter at the moment mmc Comprises the following steps:
Figure GDA0003736452380000085
wherein:
Figure GDA0003736452380000086
is the positive sequence voltage amplitude of the PCC point,
Figure GDA0003736452380000087
is the PCC point positive sequence voltage phase.
PCC point voltage drop is larger after fault
Figure GDA0003736452380000088
In the process, under the action of an outer ring current limiting link, the active power and the reactive power output by the current converter can reach the instruction values only when the instruction values are smaller. Because the rectifier station absorbs power and the inverter station outputs power during the fault under the constant active power and reactive power control strategies, the invention adopts the sign function sig to express the powerThe relation between the rate flow and the short-circuit current output from the converter station, i.e. I mmc Can be expressed as:
Figure GDA0003736452380000091
when PCC point and its near zone have more serious fault
Figure GDA0003736452380000095
When the voltage of the PCC point drops to zero or is slightly higher than zero, the active power and the reactive power output by the current converter cannot reach the instruction value under the action of an amplitude limiting link. Also, I when considering the relation between the power flow direction and the short-circuit current output from the converter station mmc Can be expressed as:
Figure GDA0003736452380000092
when the converter adopts a constant active power and alternating voltage control strategy or a constant direct current voltage and alternating voltage control strategy, the output short-circuit current of the converter is also discussed in three cases according to different PCC point voltage drop degrees.
When the PCC point voltage reaches the command value under the action of the control system after the fault
Figure GDA0003736452380000093
In the process, the active power output by the converter can also reach the instruction value, and the reactive power sent by the converter cannot be accurately judged, so that the magnitude of the reactive current output by the converter at the moment can be approximately judged through the electrical distance (the ratio of the mutual impedance to the self impedance) between the PCC and the fault point. In summary, the short-circuit current I outputted by the inverter mmc Comprises the following steps:
Figure GDA0003736452380000094
wherein: q 0 For the voltage command value to be equal to U 0 Time-fault front converter transmissionOutput reactive power, Z DD And Z mD Respectively the self-impedance of the faulty node and the mutual impedance between it and the PCC point.
Drop of PCC point voltage after fault
Figure GDA0003736452380000101
That is, the active power output by the inverter can reach the command value when the active power cannot reach the command value. Under the action of an outer ring current limiting link, in order to provide voltage support as much as possible, the residual capacity of the converter outputs reactive power. No matter the alternating current system on the rectifying side or the inverting side fails during the fault period, the converter station injects reactive power into the fault point, so I mmc Can be expressed as:
Figure GDA0003736452380000102
when PCC point and near zone have serious fault
Figure GDA0003736452380000103
Under the action of the equal-proportion amplitude limiting link, the active power output by the current converter and the PCC point voltage can not reach the instruction value. At this time, I is taken into account the relation between the power flow direction and the short-circuit current output by the converter station mmc Can be expressed as:
Figure GDA0003736452380000104
(2) when the AC system connected with the converter has short-circuit fault, the steady state value of the voltage of each node before the fault is obtained according to the load flow calculation result, wherein the steady state value of the voltage of the fault point before the fault is U D0 Using the steady state value U of the fault point voltage before the fault D0 Calculating the initial value I of the short-circuit current of the fault node according to the following formula D0 ；
Figure GDA0003736452380000105
Wherein: z DD Is the self-impedance of the failed node.
(3) Obtaining the PCC point voltage correction quantity delta U according to the node voltage equation pcc And obtaining the corrected PCC point voltage U' pcc ；
Figure GDA0003736452380000106
Wherein: i is D Is the short circuit current of the failed node.
(4) Substituting the corrected PCC point voltage into an MMC alternating current side fault model so as to calculate and obtain the short-circuit current I output by the MMC mmc 。
(5) Calculating and obtaining corrected node voltage U according to a node voltage equation i Using corrected fault node voltage U D Calculating short-circuit current correction value I of fault node D And the step (3) is carried out;
Figure GDA0003736452380000111
Figure GDA0003736452380000112
wherein: u shape i Is the voltage of the ith node in an AC system, I j For injecting current, Z, into node j in an AC system ij The mutual impedance between the ith node and the j node in the alternating current system is shown, E is a power supply node set in the alternating current system, the power supply node comprises a synchronous generator and an MMC, the injection current of the MMC is the short-circuit current I calculated in the step (4) mmc The injection current of the synchronous generator is
Figure GDA0003736452380000113
U n Is the rated voltage of the synchronous generator, c is the voltage coefficient of the synchronous generator, Z k Is the short circuit impedance of the synchronous generator.
By analogy, the iteration is repeated until the difference between the voltages of the nodes before and after the voltage correction is very small, namely | U i(k+1) -U ik |<ε, convergence error ε is taken to be 10 -6 。
(6) And calculating to obtain the short-circuit current of each branch circuit according to the convergence result of the voltage of each node.
Taking the receiving-end ring network test system of the flexible direct current transmission system as shown in fig. 5 as an example, the main loop parameters and the receiving-end alternating current system parameters of the converter station are respectively shown in tables 1 and 2:
TABLE 1
Figure GDA0003736452380000114
TABLE 2
Figure GDA0003736452380000115
Figure GDA0003736452380000121
The validity of the short-circuit current algorithm of the present invention is verified below under simulation of three-phase metallic short-circuit fault and single-phase metallic short-circuit fault, respectively.
The first condition is as follows: assume that a three-phase metallic short-circuit fault occurs at node D when t is 1s, and the fault is completely cleared at 1.1 s.
When four different control strategies are adopted for the soft-dc, the effective values of the short-circuit current output by the soft-dc and the short-circuit current at the fault point are shown in tables 3 and 4.
TABLE 3
Figure GDA0003736452380000122
TABLE 4
Figure GDA0003736452380000123
As can be seen from tables 3 and 4, when the calculation is performed by selecting a corresponding fault model according to the control strategy adopted by the soft and straight circuit, and taking into account the mutual influence relationship among the capacity of the inverter, the current limiting link, the PCC point voltage, and the short-circuit current output by the inverter, the calculation results are all within the acceptable error range.
Case two: assume that a single-phase metallic short-circuit fault occurs at node D when t is 1s, and the fault is completely cleared at 1.1 s.
When four different control strategies are adopted for the soft-dc, the effective values of the short-circuit current output by the soft-dc and the short-circuit current at the fault point are shown in tables 5 and 6.
TABLE 5
Figure GDA0003736452380000131
TABLE 6
Figure GDA0003736452380000132
As can be seen from tables 5 and 6, the short-circuit current calculation method of the present invention is also well applicable to the calculation of the asymmetric short-circuit current of the grid including the flexible dc power grid.
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 power grid short-circuit current calculation method containing a flexible direct current transmission system is characterized in that when an alternating current system accessed by the flexible direct current transmission system has a short-circuit fault, the fault characteristic of an MMC alternating current side connected with the flexible direct current transmission system is equivalent to a controlled current source, and the output short-circuit current and PCC point voltage are influenced mutually, so that the power grid short-circuit current can be calculated by adopting an iterative algorithm, and the method specifically comprises the following steps:
(1) considering MMC capacity limitation and the amplitude limiting and current limiting links of an outer ring controller in an MMC control system, establishing an MMC alternating-current side fault model according to a control strategy adopted by the MMC, specifically:
if the MMC adopts a constant active power and reactive power control strategy or a constant direct current voltage and reactive power control strategy, the alternating current side fault model is as follows:
when the PCC point voltage drops less after a fault
Figure FDA0003736452370000011
Then the short-circuit current I output by the MMC mmc The expression is as follows:
Figure FDA0003736452370000012
Figure FDA0003736452370000013
Figure FDA0003736452370000014
when the voltage drop of the PCC point is larger after the fault
Figure FDA0003736452370000015
Short-circuit current I output by MMC mmc The expression is as follows:
Figure FDA0003736452370000016
Figure FDA0003736452370000017
Figure FDA0003736452370000018
when a PCC point and a near zone thereof have serious faults
Figure FDA0003736452370000019
Then the short-circuit current I output by the MMC mmc The expression is as follows:
Figure FDA00037364523700000110
Figure FDA00037364523700000111
Figure FDA00037364523700000112
Figure FDA00037364523700000113
wherein:
Figure FDA0003736452370000021
and
Figure FDA0003736452370000022
respectively the d-axis component and the q-axis component of the MMC current command,
Figure FDA0003736452370000023
is the positive sequence voltage magnitude at the PCC point,
Figure FDA0003736452370000024
is the positive sequence voltage phase of the PCC point, j is an imaginary unit, P s * And
Figure FDA0003736452370000025
respectively an active power command value and a reactive power command value, I vmax Sig () is a sign function for the current limiting value of the current limiter;
if the MMC adopts a control strategy of constant active power and alternating voltage or a control strategy of constant direct current voltage and alternating voltage, an alternating current side fault model is as follows:
when the PCC point voltage can reach the command value under the action of the control system after the fault
Figure FDA0003736452370000026
Then the short-circuit current I output by the MMC mmc The expression is as follows:
Figure FDA0003736452370000027
Figure FDA0003736452370000028
Figure FDA0003736452370000029
when the PCC point voltage falls after the fault
Figure FDA00037364523700000210
Then the short-circuit current I output by the MMC mmc The expression is as follows:
Figure FDA00037364523700000211
Figure FDA00037364523700000212
Figure FDA00037364523700000213
when a PCC point and a near zone thereof have serious faults
Figure FDA00037364523700000214
Then the short-circuit current I output by the MMC mmc The expression is as follows:
Figure FDA00037364523700000215
Figure FDA00037364523700000216
Figure FDA00037364523700000217
wherein: u shape 0 Is a command value of AC voltage, Q 0 For the control system before fault when the voltage command value is U 0 Reactive power of MMC output under the circumstances, Z DD Is the self-impedance of the faulty node, Z mD Is the mutual impedance between the faulty node and the PCC point;
(2) when the alternating current system has a short-circuit fault, obtaining the voltage steady-state value of each node in the alternating current system before the fault according to the load flow calculation result, and further calculating the initial value of the short-circuit current of the fault node by using the voltage steady-state value of the fault node;
(3) calculating the voltage correction quantity of the PCC according to the node voltage equation, and further correcting the voltage of the PCC;
(4) substituting the corrected PCC point voltage into an MMC alternating current side fault model so as to calculate and obtain the short-circuit current I output by the MMC mmc ；
(5) Calculating and correcting the voltage of each node according to a node voltage equation, further calculating and correcting the short-circuit current of the fault node by using the voltage of the fault node, and returning to the step (3), and repeating the iteration until the voltage of each node is converged;
(6) and calculating the short-circuit current of each branch in the alternating current system according to the converged node voltages.
2. The grid short-circuit current calculation method according to claim 1, characterized in that: the outer ring controller is formed by sequentially connecting a PI controller, an amplitude limiting link and a current limiter, wherein the amplitude limiting link and the current limiter can be described by the following relational expression;
Figure FDA0003736452370000031
wherein: i.e. i vref Is the output current amplitude, i, of the PI controller vdref And i vqref Respectively d-axis component and q-axis component of the PI controller output current,
Figure FDA0003736452370000032
which is the current command of the MMC i.e. the output current of the current limiter,
Figure FDA0003736452370000033
and
Figure FDA0003736452370000034
d-axis component and q-axis component, I, of the MMC current command, respectively vmax Is the current limiting value, i, of the current limiter vdqmax The current amplitude limiting value of the amplitude limiting link.
3. The grid short-circuit current calculation method according to claim 1, characterized in that: calculating the initial value of the short-circuit current of the fault node in the step (2) by the following formula;
Figure FDA0003736452370000035
wherein: I.C. A D0 As initial value of short-circuit current of the fault node, U D0 Is the steady state value of the voltage of the fault node before the fault, Z DD Is the self-impedance of the failed node.
4. The grid short-circuit current calculation method according to claim 1, characterized in that: correcting the PCC point voltage through the following formula in the step (3);
Figure FDA0003736452370000041
wherein: delta U pcc Is the voltage correction amount of the PCC point, Z mD Is the mutual impedance between the faulty node and the PCC point, I D Short-circuit current, U, for faulty nodes pcc And U' pcc The PCC point voltages before and after correction are respectively.
5. The grid short-circuit current calculation method according to claim 1, characterized in that: in the step (5), the voltage of each node is calculated and corrected through the following formula;
Figure FDA0003736452370000042
wherein: u shape i Is the voltage of the ith node in an AC system, I j For injecting current, Z, into node j in an AC system ij The mutual impedance between the ith node and the node j in the alternating current system is shown, E is a power supply node set in the alternating current system, the power supply node comprises a synchronous generator and an MMC, the injection current of the MMC is the short-circuit current I calculated in the step (4) mmc Injection current of synchronous generatorIs composed of
Figure FDA0003736452370000043
U n Is the rated voltage of the synchronous generator, c is the voltage coefficient of the synchronous generator, Z k I is a natural number, i is more than or equal to 1 and less than or equal to N, and N is the number of nodes in the alternating current system.
6. The grid short-circuit current calculation method according to claim 1, characterized in that: in the step (5), the short-circuit current of the fault node is corrected by calculation through the following formula;
Figure FDA0003736452370000044
wherein: i is D Short-circuit current for fault node, Z DD Is the self-impedance of the faulty node, U D Is the fault node voltage.
7. The grid short-circuit current calculation method according to claim 1, characterized in that: the convergence condition of the voltage of each node in the step (5) is | U i(k+1) -U ik |<ε，U i(k+1) And U ik And the ith node voltage is respectively obtained by the k +1 th iteration and the k th iteration, k is a natural number greater than 0, and epsilon is a given convergence threshold value.
8. The grid short-circuit current calculation method according to claim 1, characterized in that: in the step (6), for any branch in the ac system, dividing the voltage difference after the node convergence at the two ends of the branch by the line impedance is the short-circuit current of the branch.
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