CN112564159A - High-voltage direct-current transmission transmitting end power grid equivalence scheme based on node residual voltage method - Google Patents

High-voltage direct-current transmission transmitting end power grid equivalence scheme based on node residual voltage method Download PDF

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CN112564159A
CN112564159A CN202011598842.3A CN202011598842A CN112564159A CN 112564159 A CN112564159 A CN 112564159A CN 202011598842 A CN202011598842 A CN 202011598842A CN 112564159 A CN112564159 A CN 112564159A
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高本锋
王刚
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North China Electric Power University
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    • 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
    • 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
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
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Abstract

The invention discloses a high-voltage direct-current transmission transmitting end power grid equivalence scheme based on a node residual voltage method. Firstly, according to the number of buses contained in the minimum path from each bus of a backbone network to a converter bus, a power grid phase-change high-voltage direct current transmission (LCC-HVDC) transmitting-end power grid is stepped to reflect the electrical coupling relation between each bus and the converter bus. And secondly, determining the backbone network of the internal system based on the backbone network node residual voltage, thereby greatly simplifying the determination process of the internal system. And thirdly, calculating detailed parameters of each equivalent hydroelectric/thermal power and new energy unit in the internal system. And finally, simplifying an external system based on a multi-port Thevenin equivalent method on the premise of not changing the short circuit capacity of the boundary system node, and finally establishing an LCC-HVDC transmission end power grid equivalent system. The equivalent scheme can greatly reduce the scale of an LCC-HVDC transmitting end power grid equivalent system, fully retains the electrical characteristics of the original LCC-HVDC transmitting end power grid, and has certain practicability and engineering reference value.

Description

High-voltage direct-current transmission transmitting end power grid equivalence scheme based on node residual voltage method
Technical Field
The invention relates to the technical field of power systems, in particular to a high-voltage direct-current transmission transmitting end power grid equivalence scheme based on a node residual voltage method.
Background
After a large-scale wind power, photovoltaic and other new energy source units are connected to a line-commutated-converter based high voltage direct current (LCC-HVDC) transmission end power grid, when the LCC-HVDC has a phase commutation failure or a direct current lockout and other faults, transient overvoltage of the transmission end power grid is easily caused, and therefore the large-scale new energy source units are disconnected. At present, scholars at home and abroad generally adopt a time domain simulation analysis method to carry out research aiming at the overvoltage limitation of an LCC-HVDC transmitting-end power grid. By establishing an electromagnetic transient simulation model of a transmission end power grid including LCC-HVDC, the dynamic response condition of a current converter and a control system thereof in the operation process can be accurately simulated. On the basis, overvoltage characteristics of a transmitting-end power grid are analyzed in detail, and an overvoltage suppression strategy is formulated and optimized so as to improve the LCC-HVDC overvoltage suppression capability. Before the simulation model is established, in order to give consideration to both simulation efficiency and coupling characteristics between alternating current and direct current systems, equivalent simplification needs to be carried out on an LCC-HVDC transmitting-end power grid on the premise of ensuring that the dynamic characteristics of a research system are not distorted.
At present, the existing research on system equivalence of scholars at home and abroad focuses on an alternating current system with a hydro-thermal power generating unit as a main power unit of the system, the occupation ratio of a new energy unit is small, and the influence of overvoltage on the stable operation of an alternating current and direct current system is small. When the effectiveness of an alternating current and direct current equivalent system is evaluated, the previous research focuses on the problem of system power angle stability under the alternating current system fault, and the attention to the voltage response characteristic of the system under the LCC-HVDC fault, particularly the overvoltage problem of a transmitting-end power grid, is little. With the continuous highlighting of the overvoltage problem of the domestic LCC-HVDC transmission end power grid, the research on the equivalent scheme of the LCC-HVDC transmission end power grid is more urgent for the problem.
Aiming at the equivalence problem of an LCC-HVDC transmitting-end power grid, the research focus is on the method for determining the range of an internal system, the detailed parameters of the internal system and simplifying an external system. In the existing equivalent research of the alternating current system, an internal system is usually determined by a researcher according to experience. Because the main dynamic characteristics of the equivalence system are embodied by the internal system, when the internal system is artificially determined according to experience, on one hand, the internal system is possibly too small to lose a large amount of dynamic characteristics, so that a large equivalence error is generated; on the other hand, it may cause the internal system to be too large and waste computing resources. The objective and effective internal system determination method can avoid uncertainty caused by subjective judgment of researchers, thereby greatly improving the accuracy of an advanced system.
Whether the determination method of the detailed parameters of each element in the equivalent system is reasonable or not can also influence the accuracy of the equivalent work of the whole system. In the existing research work, the traditional synchronous units such as hydropower/thermal power and the like are taken as leading factors influencing the dynamic characteristics of the equivalent system, and only the equivalent method of the synchronous units is researched, but the influence of a new energy power supply on the dynamic characteristics of the equivalent system is ignored. When a new energy source is connected into an LCC-HVDC transmission end power grid in a large scale, the dynamic characteristics of an alternating current and direct current system, particularly the transient overvoltage characteristics of the transmission end power grid, are changed to a great extent. Therefore, in the equivalent process of an LCC-HVDC transmission end power grid, the dynamic characteristics of the new energy power supply cannot be ignored.
In an LCC-HVDC transmit-end grid equivalent system, the detailed modeled internal system dominates the overall dynamics of the entire equivalent system, while the external system dynamics are negligible. In the process of establishing the equivalent system, the original characteristics of the external system, such as static tide, short-circuit capacity and the like, are generally only required to be reserved, so that the external system can be simplified in a large scale. In the existing research, a WARD equivalent method is mostly adopted when an external system is simplified, but parameters which do not accord with the actual condition of a power system are easy to appear in an equivalent result obtained by adopting the method, the method cannot be realized in electromagnetic transient simulation, and the consistency of the short-circuit capacity of the system before and after the equivalence cannot be ensured.
On the basis of the research, the invention provides a high-voltage direct-current transmission transmitting end power grid equivalent scheme based on a node residual voltage method. Firstly, according to the number of buses contained in the minimum path between each bus of a backbone network and a converter bus, the LCC-HVDC transmitting-end power grid is stepped. Secondly, calculating node residual voltage of each step section point of a main network of the LCC-HVDC transmission end power grid, and determining a main network structure of an internal system by setting a node residual voltage threshold value; thirdly, classifying the water/thermal power and new energy source units according to different voltage grades of the power grid accessed by the units, and respectively providing equivalent methods for the water/thermal power and the new energy source units of different classes; and finally, simplifying the LCC-HVDC transmission end power grid based on a multi-port Thevenin equivalent method, and establishing an LCC-HVDC transmission end power grid equivalent system. The scheme provides a detailed equivalence flow of the LCC-HVDC transmitting end power grid on the basis that the transient characteristic and the overvoltage characteristic of the LCC-HVDC transmitting end power grid are unchanged before and after equivalence. According to the scheme, the LCC-HVDC transmission end power grid is simplified in a large scale, meanwhile, the equivalent system can fully reproduce the transient state characteristic and the overvoltage characteristic of the original LCC-HVDC transmission end power grid, and the equivalent system has certain economic and practical values.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide a high-voltage direct-current transmission transmitting end power grid equivalent scheme based on a node residual voltage method.
In order to solve the technical problems, the technical scheme adopted by the invention is as follows.
A high-voltage direct-current transmission transmitting end power grid equivalence scheme based on a node residual voltage method. Firstly, aiming at the position of a converter bus and the structure of a power grid in an LCC-HVDC (capacitor-high voltage direct current) transmitting end power grid, a method for dividing the LCC-HVDC transmitting end power grid in a cascading manner is provided; secondly, calculating the residual voltage of each bus node of a main network of the LCC-HVDC transmission end power grid, and determining the main network of the internal system by setting a node residual voltage threshold value; thirdly, classifying the water/thermal power generating units and the new energy source units based on the voltage grade of the accessed power grid, and providing corresponding equivalent methods for the water/thermal power generating units and the new energy source units of different types respectively; and finally, simplifying an external system of the LCC-HVDC transmitting-end power grid based on a multi-port Thevenin equivalent method.
As a preferable technical scheme of the invention, the scheme comprises the following steps:
s1: cascaded LCC-HVDC transmission end power grid backbone network
According to the number of buses contained in the minimum path from each bus of a backbone network of the LCC-HVDC transmission end power grid to a converter bus, the LCC-HVDC transmission end power grid is divided in a cascading manner;
s2: backbone network of internal system is confirmed based on backbone network node residual voltage
In the operation process of an LCC-HVDC transmission end power grid, when a three-phase short-circuit fault occurs in a current conversion bus and the current conversion bus reaches a stable state, determining a backbone network of an internal system according to the distribution characteristics of node residual voltages of each step section point of the backbone network;
s3: calculating detailed operation parameters of the water/thermal power generating unit and the new energy source unit in the internal system;
s4: simplifying an LCC-HVDC transmission end power grid external system based on a multi-port Thevenin equivalence method, and finally finishing equivalence of the LCC-HVDC transmission end power grid.
Step S1 is to carry out the cascade division of the LCC-HVDC transmitting end power grid, and the concrete strategy is as follows:
s21: giving a definition of backbone and path, i.e.
(1) Backbone network: the system is a power grid structure formed by an alternating current bus with the highest voltage level and a power transmission line in an LCC-HVDC transmitting end power grid;
(2) path: the method is characterized in that on the level of a backbone network, a certain backbone network bus starts to reach a first cascade section point through a plurality of power transmission lines and buses, the power transmission lines and the buses can not appear repeatedly, a set formed by all the power transmission lines is called as a path from the bus to the first cascade section point, and the path with the least number of the buses is the shortest path.
On the basis of (1) and (2), determining the voltage grade of a backbone network of the LCC-HVDC transmission end power grid, and defining a conversion bus of the LCC-HVDC transmission end power grid as a first step section point;
s22: taking the first step section point as a division start, and determining a corresponding bus as an nth step section point according to the number n of buses contained in the shortest path from each backbone network bus to the first step section point;
s23: and (c) dividing the power transmission line with two ends respectively connected with the (a) th step section point and the (b) th step section point into the (a) th step section point.
Step S2 is to determine the backbone network of the internal system based on the backbone network node residual voltage, and the specific strategy is:
s31: under the stable operation state of an alternating current-direct current system, when a three-phase short circuit fault occurs to a converter bus of a power grid at a transmitting end of LCC-HVDC and the converter bus reaches a stable state, the voltage value of each bus of a backbone network is the node residual voltage of a corresponding step section point, and the calculation formula of the node residual voltage of each step section point is as follows:
Figure BSA0000229149940000031
in the formula (I), the compound is shown in the specification,
Figure BSA0000229149940000032
a step section point node residual voltage matrix is obtained;
Figure BSA0000229149940000033
injecting a short-circuit current matrix into the node, wherein the short-circuit current matrix injected into the node is formed by short-circuit currents injected into each step section point as shown in formula (2); zFIs a node impedance matrix, Y, of a backbone network of an LCC-HVDC transmission end power gridFAdmittance matrices for respective nodes;
Figure BSA0000229149940000034
s32: calculating node residual pressure of each step section point of a backbone network, sequencing all step section points according to the sequence of the node residual pressure from small to large, determining a node residual pressure threshold value theta, and enabling all step section points corresponding to the node residual pressure to be smaller than theta to form an internal system backbone network of the equal-value system to be established;
s33: and verifying the accuracy of the equivalent system established by the node residual voltage threshold value theta determined in the S32, and when the accuracy of the equivalent system cannot meet the requirements of engineering application, changing the value theta and establishing a new equivalent system until the requirements are met.
Step S3 is to emphasize the equivalence method of the new energy generating set in the transmission-side ac grid in determining the detailed operating parameters of the hydro/thermal power generating set and the new energy generating set in the internal system range, and the detailed parameter determining methods of each electrical element are as follows:
s41: the method for determining the detailed parameters of the water/thermal power equivalent unit comprises the following steps:
a1: the method comprises the following steps of dividing a water/thermal power generating unit into a type a (connected to a backbone network) unit and a type b (connected to a non-backbone network) unit according to the voltage level of a power grid connected to the water/thermal power generating unit;
a2: for the class a unit, a detailed parameter calculation method shown in formula (3) is adopted, and all units in the same power plant are equivalent to one equivalent machine respectively:
Figure BSA0000229149940000041
in the formula, SGFor rated capacity of equivalent unit, SjRated capacity of each unit before equivalence, KGParameters of the polymerization equivalent machine such as inertia constant, motive power, electromagnetic power, dq axis synchronous reactance, transient reactance, sub-transient reactance, gain and time constant of each link of the excitation system, and KjCorresponding parameters of each unit before equivalence;
a3: for the class b unit, a backbone network topology area is divided according to each backbone network node, and on the basis, a detailed parameter calculation method shown in formula (2) is adopted to equate each unit belonging to the same backbone network topology area to an equivalence machine;
s42: the method for determining the detailed parameters of the new energy equivalent unit comprises the following steps:
a1: for the unit connected to the backbone network, a single-machine aggregation equivalent method is adopted to carry out detailed modeling on a new energy unit converter link;
a2: for other types of new energy source units, the dynamic characteristics are ignored, and the dynamic characteristics are equivalent to a load with a negative value.
The invention has the following beneficial effects: firstly, in an alternating current-direct current system, the number of buses contained in the minimum path from each bus of a main network of an LCC-HVDC transmission end power grid to a conversion bus is calculated, the LCC-HVDC transmission end power grid is divided in a stepping mode, and the electric coupling between each bus of the transmission end power grid and the conversion bus is disclosed. Secondly, on the basis of the stepped division of the transmission end power grid, three-phase short-circuit faults are set at the position of a converter bus to obtain the node residual voltage of each stepped section point of the backbone network of the LCC-HVDC transmission end power grid. The backbone network structure of the internal system is determined by setting the node residual voltage threshold, so that the possible equivalence error caused by human factors in the conventional equivalence method is effectively avoided. And thirdly, according to the difference of the grid-connected voltage grades, classifying the water/thermal power generating units and the new energy source units in the internal system, and adopting different equivalence methods for the units of different classes respectively. And finally, a multi-port Thevenin equivalent method is introduced, so that an external system of the LCC-HVDC transmission end power grid is simplified. According to the simulation analysis results before and after the equivalence of the system, the effectiveness and the accuracy of the scheme are verified.
Thus, the superiority of the present invention can be summarized as follows: firstly, a novel LCC-HVDC transmission end power grid structure definition method, namely a cascading division method is provided, and the method can clearly determine the electrical coupling between each bus in a main network of the LCC-HVDC transmission end power grid and a conversion bus of the LCC-HVDC transmission end power grid. And secondly, the residual voltage of each bus node of the backbone network can reflect the electrical coupling between the corresponding step section point and the current conversion bus, and the range of an internal system can be determined quickly and objectively by setting a node residual voltage threshold value. And thirdly, the importance of the dynamic characteristics of the new energy unit in the equivalent system for reproducing the dynamic characteristics of the original system is fully considered, and the defect that the dynamic characteristics of the new energy are neglected in the equivalent research of the conventional alternating current system is overcome. And finally, a multi-port Thevenin equivalent method is introduced into simplification of an external system of the LCC-HVDC transmitting-end power grid, so that the simulation efficiency of the equivalent system is greatly improved, and the effectiveness of the equivalent system can be ensured.
Drawings
Fig. 1 is a flow chart of an equivalent scheme of a high-voltage direct-current transmission end power grid based on a node residual voltage method provided by the invention.
Fig. 2 is a schematic diagram of the step division of the LCC-HVDC transmission-end grid provided in step S1 of the present invention.
Fig. 3 is a method for representing the node residual voltage of the ac system, and is used to illustrate a specific method and process for calculating the node residual voltage.
Fig. 4 is a detailed flow diagram of the internal system backbone network determination method based on node residual voltage.
Fig. 5 is a structural diagram of an example of an LCC-HVDC transmission grid for verifying the validity of the equivalent scheme of the present invention.
Fig. 6 is a comparison chart of voltage changes of the bus A, B, D before and after a three-phase short circuit fault occurs in a converter bus before and after the equivalent scheme of the present invention is adopted in a sample LCC-HVDC transmission-end power grid. Fig. 6(a) is a comparison graph of the voltage change condition of the bus a, fig. 6(B) is a comparison graph of the voltage change condition of the bus B, and fig. 6(c) is a comparison graph of the voltage change condition of the bus D before and after the equivalence of the LCC-HVDC transmission-side power grid and during the three-phase short-circuit fault of the converter bus.
FIG. 7 is a comparison chart of the voltage change condition of the bus A, B, D before and after the N-1 fault occurs in the transmission end power grid before and after the equivalent scheme of the present invention is adopted in the sample LCC-HVDC transmission end power grid. Fig. 7(a) is a comparison graph of the voltage change condition of the bus a before and after the equivalence of the transmission-side power grid and during the failure of the transmission-side power grid N-1, fig. 7(B) is a comparison graph of the voltage change condition of the bus B, and fig. 7(c) is a comparison graph of the voltage change condition of the bus D.
Fig. 8 is a comparison graph of power angle swing curves of the unit I1 and W2 in the original system and the equivalent system before and after the three-phase short circuit fault occurs in the converter bus before and after the equivalent scheme of the present invention is adopted in the sample LCC-HVDC transmission-end power grid.
FIG. 9 is a comparison chart of voltage response of the commutation busbars of the original system and the equivalent system before and after the equivalent scheme is adopted and during the commutation failure of the LCC-HVDC in the example LCC-HVDC transmitting-end power grid.
Fig. 10 is a comparison graph of voltage response of a main new energy collection bus in an original system and an equivalent system during a phase commutation failure of an LCC-HVDC transmission-end grid of a calculation example, where fig. 10(a) is a comparison graph of voltage response of a new energy collection bus B, and fig. 10(B) is a comparison graph of voltage response of a new energy collection bus C.
Detailed Description
The invention provides a high-voltage direct-current transmission transmitting end power grid equivalent scheme based on a node residual voltage method, and in order to make the purpose, technical scheme and effect of the invention clearer, the specific implementation scheme of the invention is described in detail below by combining with the attached drawings and examples. The specific examples described herein are intended to be illustrative only and are not intended to be limiting.
1. Description of the preferred embodiments
Fig. 1 is a flow chart of an equivalent scheme of a high-voltage direct-current transmission end power grid based on a node residual voltage method, provided by the invention, fig. 2 is a schematic diagram of the stepped division of an LCC-HVDC transmission end power grid, provided by the invention, fig. 3 is a method for calculating the node residual voltage of an alternating current system, provided by the invention, and fig. 4 is a detailed flow chart of a method for determining an internal system backbone network based on the node residual voltage, provided by the invention. With reference to fig. 1, 2, 3 and 4, an equivalent scheme of an LCC-HVDC transmission-end grid for new energy access described in the present invention includes:
s1: the method comprises the steps of dividing an LCC-HVDC transmission end power grid backbone network in a cascading manner;
s2: determining a backbone network of an internal system based on the backbone network node residual voltage;
s3: calculating detailed operation parameters of the water/thermal power generating unit and the new energy source unit in the internal system;
s4: and the equivalence of an LCC-HVDC transmission end power grid external system based on a multi-port Thevenin equivalence method is simplified.
Further, the method comprises the following specific steps:
s1: cascaded LCC-HVDC transmission end power grid backbone network
Referring to fig. 2, with the LCC-HVDC transmission-end converter bus as a research center, the LCC-HVDC transmission-end grid is divided into stages according to the number of buses included in the minimum path from each bus of the LCC-HVDC transmission-end grid backbone network to the converter bus.
S21: determining the voltage grade of a backbone network of an LCC-HVDC transmitting end power grid, determining the position of an LCC-HVDC transmitting end current conversion bus on the backbone network, and defining the transmitting end current conversion bus as a first step section point;
s22: taking the first step section point as a division start, and determining a corresponding bus as an nth step section point according to the number n of buses contained in the shortest path from each backbone network bus to the first step section point;
s23: and (c) dividing the power transmission line with two ends respectively connected with the (a) th step section point and the (b) th step section point into the (a) th step section point.
Step S1 is described in detail with reference to fig. 2 of the specification. All second step cross-sectional points are indicated in fig. 2, and the first, second, and third step cross-sections are indicated by different line types, respectively. By a ring network4-l5-l6The concrete process of the LCC-HVDC transmitting end power grid stepped division is described. Bus N2To the first step section point T1Has a total of4-l1、l6-l5-l1、l6-l3-l 23 paths are equal, wherein4-l1Is the shortest path including bus N1、N2And T1I.e. N is 3, so N2Is the third step cross-sectional point. Transmission line l4Two ends of the first step are respectively connected with the second step section point N1And third step cross-sectional point N2Are connected to each other so4Cutting into a second step section; transmission line l5Two ends of the first step are respectively connected with the second step section point N1And third step cross-sectional point N3Are connected to each other so5Cutting into a second step section; transmission line l6Are each independently of N2、N3To each other, N2、N3Are all the third step cross-sectional points, so6Cut into the third step section.
S2: backbone network of internal system is confirmed based on backbone network node residual voltage
Referring to fig. 3, a specific calculation method of residual voltage of each bus node of a main network of an LCC-HVDC transmission end power grid; referring to fig. 4, a detailed flow diagram of the internal system backbone network determination method based on node residual voltage is shown.
S31: under the stable operation state of an alternating current-direct current system, when a three-phase short circuit fault occurs to a converter bus of a power grid at a transmitting end of LCC-HVDC and the converter bus reaches a stable state, the voltage value of each bus of a backbone network is the node residual voltage of the corresponding bus, and the calculation formula of the node residual voltage of each step section point is as follows:
Figure BSA0000229149940000061
in the formula (I), the compound is shown in the specification,
Figure BSA0000229149940000062
a step section point node residual voltage matrix is obtained;
Figure BSA0000229149940000063
the short-circuit current matrix is formed by short-circuit current injected into each step section point as shown in formula (5); zFFor LCC-HVDC transmitting end power grid backbone network impedance matrix, YFIs a corresponding admittance matrix;
Figure BSA0000229149940000064
s32: calculating node residual pressure of each step section point of a backbone network, sequencing all step section points according to the sequence of the node residual pressure from small to large, determining a node residual pressure threshold value theta, and enabling all step section points corresponding to the node residual pressure to be smaller than theta to form an internal system backbone network of the equal-value system to be established;
s33: and verifying the accuracy of the equivalent system established by the node residual voltage threshold value theta determined in the S32, and when the accuracy of the equivalent system cannot meet the requirements of engineering application, changing the value theta and establishing a new equivalent system until the requirements are met. The specification figure 4 shows a detailed determination process of the specific node residual voltage threshold value theta.
S3: calculating detailed operation parameters of water/thermal power generating unit and new energy source unit in internal system
S41: the method for determining the detailed parameters of the water/thermal power equivalent unit comprises the following steps:
a1: the method comprises the following steps of dividing a water/thermal power generating unit into a type a (connected to a backbone network) unit and a type b (connected to a non-backbone network) unit according to the voltage level of a power grid connected to the water/thermal power generating unit;
a2: for the class a unit, a detailed parameter calculation method shown in formula (6) is adopted, and all units in the same power plant are equivalent to one equivalent machine respectively:
Figure BSA0000229149940000071
in the formula, SGFor rated capacity of equivalent unit, SjRated capacity of each unit before equivalence, KGParameters of the polymerization equivalent machine such as inertia constant, motive power, electromagnetic power, dq axis synchronous reactance, transient reactance, sub-transient reactance, gain and time constant of each link of the excitation system, and KjCorresponding parameters of each unit before equivalence;
a3: for the class b unit, a backbone network topology area is divided according to each step section point, and on the basis, a detailed parameter calculation method shown in a formula (6) is adopted to equate each unit belonging to the same backbone network topology area to one equivalence machine;
s42: the method for determining the detailed parameters of the new energy equivalent unit comprises the following steps:
a1: for the unit connected to the backbone network, a single-machine aggregation equivalent method is adopted to carry out detailed modeling on a new energy unit converter link;
a2: for other types of new energy source units, the dynamic characteristics are ignored, and the dynamic characteristics are equivalent to a load with a negative value.
S4: and simplifying an external system of the direct current transmission end alternating current power grid based on a multi-port Thevenin equivalence method, and finally completing equivalence of the LCC-HVDC transmission end alternating current power grid.
2. Technical feasibility verification of the invention
The engineering simulation example system model is shown in fig. 5, and the simulation results are shown in fig. 6, 7, 8, 9 and 10, so as to verify the effectiveness of the invention.
FIG. 6 is a comparison graph of voltage changes of a bus A, B, D before and after a three-phase short circuit fault occurs in a converter bus before and after an equivalent LCC-HVDC transmitting-end power grid. FIG. 7 is a comparison graph of the voltage change of the bus A, B, D before and after an N-1 fault occurs in the transmission-end power grid before and after equivalence of the LCC-HVDC transmission-end power grid. FIG. 8 is a comparison graph of power angle swing curves of an original system and an equivalent system unit I1 and W2 before and after equivalence of a LCC-HVDC transmission-end power grid and before and after a three-phase short-circuit fault occurs in a transmission-end power grid conversion bus. FIG. 9 is a comparison graph of voltage response of the commutation busbars of the original system and the equivalent system before and after equivalence of an LCC-HVDC transmission-end power grid and during commutation failure of the LCC-HVDC. FIG. 10 is a comparison graph of main new energy collection bus voltage response conditions in an original system and an equivalent system before and after equivalence of an LCC-HVDC transmission-end power grid and during a commutation failure of the LCC-HVDC.
As can be seen from FIG. 6, the equivalence scheme of the invention is adopted to conduct equivalence on the LCC-HVDC transmitting-end power grid of the example, the voltage response characteristics of the bus A, B, D are basically consistent before and after the three-phase short-circuit fault occurs on the equivalent system and the original system conversion bus, and the static stability characteristics of the system after the three-phase short-circuit fault is cut off are the same. As can be seen from FIG. 7, when the equivalence scheme provided by the invention is adopted to conduct equivalence on an LCC-HVDC transmitting-end power grid of an example, voltage response characteristics of a bus A, B, D are basically consistent before and after an N-1 fault occurs in an equivalence system and an original system, and static stability characteristics of the system after the N-1 fault is removed are the same. As shown in the figure 8, the equivalence scheme is adopted to conduct equivalence on the LCC-HVDC transmitting-end power grid of the calculation example, and the power angle swing curves of the unit I1 and the unit W2 are basically fitted before and after three-phase short circuit faults occur on the LCC-HVDC conversion bus in the equivalent system and the original system. The simulation result shows that when the equivalence scheme is adopted to conduct equivalence on the LCC-HVDC transmitting-end power grid, the alternating current fault response characteristics of an equivalence system and an original system are not obviously changed.
As shown in FIG. 9, the equivalence scheme of the invention is adopted to conduct equivalence on the LCC-HVDC transmitting end power grid of the calculation example, the maximum overvoltage values of the LCC-HVDC transmitting end power grid current conversion buses are approximately equal before and after the phase conversion failure of the LCC-HVDC in the equivalent system and the original system, and the overvoltage response curves are basically fitted. As shown in fig. 10(a) and 10(b), the equivalence scheme of the invention is adopted to perform equivalence on the sample LCC-HVDC transmission-end power grid, and the overvoltage response process of each main new energy bus bar in the LCC-HVDC transmission-end power grid is basically consistent before and after the commutation failure of the LCC-HVDC in the equivalent system and the original system. The simulation result can show that when the equivalence scheme is adopted to conduct equivalence on the LCC-HVDC transmitting-end power grid, the direct current fault response characteristics of an equivalence system and an original system are not obviously changed.
The example simulation results corresponding to fig. 6, fig. 7, fig. 8, fig. 9 and fig. 10 show that the transient characteristics of the equivalence system established by using the equivalence scheme of the invention are highly consistent with those of the original system, and the simulation results verify the validity and feasibility of the equivalence scheme of the high-voltage direct-current transmission end power grid based on the node residual voltage method.
In conclusion, the high-voltage direct-current transmission transmitting end power grid equivalent scheme based on the node residual voltage method can fully reproduce the transient characteristic and the overvoltage characteristic of the original LCC-HVDC transmitting end power grid while greatly reducing the simulation scale of the LCC-HVDC transmitting end power grid. Before the backbone network of the internal system is determined, the LCC-HVDC transmitting-end power grid is layered in a cascading division mode so as to clearly reflect the electrical coupling of each backbone network bus and the LCC-HVDC. The backbone network of the internal system is determined based on the residual voltage of the backbone network nodes, so that the determination process of the internal system can be simplified, and equivalent errors are reduced. On the basis of an internal system backbone network, the calculation scale of an equivalent system is reduced by establishing an equivalent hydroelectric/thermal power and new energy unit. And finally, simplifying an external system based on a multi-port Thevenin equivalent method, and greatly reducing the complexity of an LCC-HVDC transmission end power grid equivalent system. The equivalent scheme of the LCC-HVDC transmitting end power grid for new energy access provided by the invention can fully reproduce the transient characteristic and the overvoltage characteristic of the original LCC-HVDC transmitting end power grid while greatly reducing the scale of the LCC-HVDC transmitting end power grid and improving the simulation calculation efficiency.
Finally, it should be noted that the above-mentioned examples of the present invention are only examples for illustrating the present invention, and are not intended to limit the embodiments of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, it will be apparent to those skilled in the art that other variations and modifications can be made based on the above description. Not all embodiments are exhaustive. All obvious changes and modifications of the present invention are within the scope of the present invention.

Claims (3)

1. A high-voltage direct-current transmission (LCC-HVDC) transmitting end power grid equivalence scheme based on a node residual voltage method is characterized by comprising the following steps:
s1: cascaded LCC-HVDC transmission end power grid backbone network
According to the number of buses contained in the minimum path from each bus of a backbone network of the LCC-HVDC transmission end power grid to a converter bus, the LCC-HVDC transmission end power grid is divided in a cascading manner;
s2: backbone network of internal system is confirmed based on backbone network node residual voltage
In the operation process of an LCC-HVDC transmission end power grid, when a three-phase short-circuit fault occurs in a current conversion bus and the current conversion bus reaches a stable state, determining a backbone network of an internal system according to the distribution characteristics of node residual voltages of each step section point of the backbone network;
s3: calculating detailed operation parameters of the water/thermal power generating unit and the new energy source unit in the internal system;
s4: simplifying an LCC-HVDC transmission end power grid external system based on a multi-port Thevenin equivalence method, and finally finishing equivalence of the LCC-HVDC transmission end power grid.
2. The equivalent scheme of the high-voltage direct-current transmission end power grid based on the node residual voltage method according to claim 1, wherein step S1 is to divide the LCC-HVDC transmission end power grid in a cascaded manner, and the specific strategy is as follows:
s21: giving a definition of backbone and path, i.e.
(1) Backbone network: the system is a power grid structure formed by an alternating current bus with the highest voltage level and a power transmission line in an LCC-HVDC transmitting end power grid;
(2) path: the method is characterized in that on the level of a backbone network, a certain backbone network bus starts to reach a first cascade section point through a plurality of power transmission lines and buses, the power transmission lines and the buses can not appear repeatedly, a set formed by all the power transmission lines is called as a path from the bus to the first cascade section point, and the path with the least number of the buses is the shortest path.
On the basis of (1) and (2), determining the voltage grade of a backbone network of the LCC-HVDC transmission end power grid, and defining a conversion bus of the LCC-HVDC transmission end power grid as a first step section point;
s22: taking the first step section point as a division start, and determining a corresponding bus as an nth step section point according to the number n of buses contained in the shortest path from each backbone network bus to the first step section point;
s23: and (c) dividing the power transmission line with two ends respectively connected with the (a) th step section point and the (b) th step section point into the (a) th step section point.
3. The equivalent scheme of the high-voltage direct-current transmission end power grid based on the node residual voltage method according to claim 1, wherein step S2 is to determine an internal system backbone network based on the cascade section point node residual voltage, and the specific strategy is as follows:
s31: under the stable operation state of an alternating current-direct current system, when a three-phase short circuit fault occurs to a converter bus of a power grid at a transmitting end of LCC-HVDC and the converter bus reaches a stable state, the voltage value of each bus of a backbone network is the residual voltage of a node of a corresponding step section point, and the calculation formula of the residual voltage of each step section point node is as follows:
Figure FSA0000229149930000011
in the formula (I), the compound is shown in the specification,
Figure FSA0000229149930000012
a step section point node residual voltage matrix is obtained;
Figure FSA0000229149930000013
injecting a short-circuit current matrix into the node, wherein the short-circuit current matrix injected into the node is formed by short-circuit currents injected into each step section point as shown in formula (2); zFIs a node impedance matrix, Y, of a backbone network of an LCC-HVDC transmission end power gridFAdmittance matrices for respective nodes;
Figure FSA0000229149930000014
s32: calculating node residual pressure of each step section point of a backbone network, sequencing all step section points according to the sequence of the node residual pressure from small to large, determining a node residual pressure threshold value theta, and enabling all step section points corresponding to the node residual pressure to be smaller than theta to form an internal system backbone network of the equal-value system to be established;
s33: and verifying the accuracy of the equivalent system established by the node residual voltage threshold value theta determined in the S32, and when the accuracy of the equivalent system cannot meet the requirements of engineering application, changing the value theta and establishing a new equivalent system until the requirements are met.
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