CN112564159B - High-voltage direct-current transmission end power grid equivalence scheme based on node residual voltage method - Google Patents
High-voltage direct-current transmission end power grid equivalence scheme based on node residual voltage method Download PDFInfo
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Abstract
The invention discloses a high-voltage direct-current transmission 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 main network to a converter bus, a power grid of a power grid phase-change high-voltage direct-current transmission (LCC-HVDC) transmitting end is stepped so as to reflect the electric coupling relation between each bus and the converter bus. And secondly, the backbone network of the internal system is determined based on the residual pressure of the backbone network node, so that the internal system determination flow is greatly simplified. And thirdly, calculating detailed parameters of each equivalent water/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 node short-circuit capacity of the boundary system, and finally establishing an LCC-HVDC power transmission end power grid equivalent system. The equivalent scheme provided by the invention can fully reserve the electrical characteristics of the original LCC-HVDC power transmission network while greatly reducing the scale of the equivalent system of the LCC-HVDC power transmission network, and has certain practicability and engineering reference value.
Description
Technical Field
The invention relates to the technical field of power systems, in particular to a high-voltage direct-current transmission end power grid equivalence scheme based on a node residual voltage method.
Background
After a large-scale wind power generation, photovoltaic and other new energy units are connected to a line-communication-based high-voltage direct current (LCC-HVDC) power transmission end power grid, transient overvoltage of the power transmission end power grid is easily caused when the LCC-HVDC fails to perform phase-communication or is blocked by direct current and other faults, so that the large-scale new energy units are disconnected. At present, students at home and abroad commonly adopt a time domain simulation analysis method to develop researches aiming at overvoltage limitation of an LCC-HVDC power transmission end power grid. By establishing the electromagnetic transient simulation model of the power grid at the transmitting end including LCC-HVDC, the dynamic response condition of the converter and the control system thereof in the running process can be accurately simulated. On the basis, the overvoltage characteristics of the power grid at the power transmission end are analyzed in detail, and an overvoltage suppression strategy is formulated and optimized so as to improve the overvoltage suppression capability of the LCC-HVDC. Before the simulation model is built, in order to give consideration to the simulation efficiency and the coupling characteristic between the AC and DC systems, the LCC-HVDC power transmission end power grid is subjected to equivalent simplification on the premise of ensuring that the dynamic characteristic of the research system is not distorted.
At present, the research on the system equivalence of students at home and abroad is concentrated in an alternating current system with a water/thermal power unit as a system main power unit, the new energy unit has small duty ratio, and the overvoltage problem has small influence on the stable operation of the alternating current and direct current system. When the effectiveness of an AC/DC equivalent system is evaluated, the previous research focuses on the problem of system power angle stability under the fault of an AC system, and the problem of overvoltage of a power grid at a transmitting end is less on the voltage response characteristic of the system under the fault of LCC-HVDC. With the continuous highlighting of the problem of overvoltage of the LCC-HVDC power transmission network in China, the research on the equivalent scheme of the LCC-HVDC power transmission network is more urgent aiming at the problem.
Aiming at the equivalence problem of an LCC-HVDC power transmission end power grid, the internal system range, the internal system detailed parameter determination and the external system simplification method are important to be studied. In the prior art of equivalent studies of communication systems, the internal system is usually empirically determined by researchers. Because the main dynamic characteristics of the equivalent system are reflected by the internal system, when the internal system is manually determined according to experience, on one hand, a large amount of dynamic characteristics of the internal system are possibly lost due to the fact that the internal system is too small, and therefore larger equivalent errors are generated; on the other hand, the internal system may be too large, which may 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 equivalent system.
Whether the method for determining the detailed parameters of each element in the equivalent system is reasonable or not can also influence the accuracy of equivalent work of the whole system. In the existing research work, the traditional synchronous units such as hydroelectric power, thermal power and the like are used as dominant factors influencing the dynamic characteristics of the equivalent system, and only the equivalent method of the synchronous units is researched, and the influence of the dynamic characteristics of the equivalent system of the new energy power supply is ignored. When new energy is accessed into an LCC-HVDC power transmission end grid in a large scale, the dynamic characteristics of an AC-DC system, particularly the transient overvoltage characteristics of the power transmission end grid, are changed to a great extent. Therefore, in the process of power grid equivalence of the LCC-HVDC transmitting end, the dynamic characteristics of the new energy power supply cannot be ignored.
In the LCC-HVDC power transmission end power grid equivalent system, the internal system modeled in detail dominates all dynamic characteristics of the whole equivalent system, and the dynamic characteristics of the external system are negligible. In the process of establishing the equivalent system, the original static tide, short-circuit capacity and other characteristics of the external system are usually only reserved, so that the external system can be simplified on a large scale. The prior research mostly adopts WARD equivalent method when the external system is simplified, but parameters which do not accord with the actual condition of the power system easily appear in the equivalent result obtained by adopting the method, the parameters cannot be realized in electromagnetic transient simulation, and the consistency of the short-circuit capacity of the system before and after the equivalent cannot be ensured.
Based on the research, the invention provides a high-voltage direct-current transmission end power grid equivalence scheme based on a node residual voltage method. Firstly, the LCC-HVDC transmitting end power grid is stepped according to the number of buses contained in the minimum path between each bus of the main network and the converter bus. Secondly, calculating the node residual pressure of each step section point of the main network of the LCC-HVDC power transmission end power grid, and determining the main network structure of the internal system by setting a node residual pressure threshold value; thirdly, classifying the water/thermal power and new energy units according to different voltage levels of the units connected to the power grid, and respectively giving equivalent methods for the water/thermal power and new energy units of different types; and finally, simplifying the LCC-HVDC power transmission end power grid based on a multi-port Thevenin equivalent method, and establishing an LCC-HVDC power transmission end power grid equivalent system. The scheme provides an detailed equivalent flow of the LCC-HVDC power transmission network by taking transient characteristics and overvoltage characteristics of the LCC-HVDC power transmission network before and after the equivalent as principles. According to the scheme, the LCC-HVDC power transmission end power grid is simplified on a large scale, and meanwhile, the equivalent system can fully reproduce transient characteristics and overvoltage characteristics of the original LCC-HVDC power transmission end power grid, so that the method has certain economic and practical values.
Disclosure of Invention
Aiming at the defects existing in the prior art, the invention aims to provide a high-voltage direct-current transmission 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.
And a high-voltage direct-current transmission end power grid equivalence scheme based on a node residual voltage method. Firstly, providing a step division method for the LCC-HVDC power transmission end power grid aiming at the position of a current conversion busbar and the power grid structure in the LCC-HVDC power transmission end power grid; secondly, calculating the residual voltage of each busbar node of the main network of the LCC-HVDC power 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 units and the new energy units based on the voltage level of the connected power grid, and respectively giving corresponding equivalent methods for the water/thermal power units and the new energy units of different types; finally, the LCC-HVDC power transmission end power grid external system is simplified based on the multi-port Thevenin equivalent method.
As a preferred technical solution of the present invention, the solution comprises the following steps:
s1: stepped division LCC-HVDC power transmission end power grid backbone network
According to the number of buses contained in the minimum path from each bus of the main network of the LCC-HVDC power transmission end power grid to the converter bus, the LCC-HVDC power transmission end power grid is divided in a stepped mode;
s2: internal system backbone network determination based on backbone network node residual pressure
In the running process of the LCC-HVDC power transmission end power grid, when a three-phase short circuit fault occurs in a converter bus and reaches a steady state, determining an internal system main network according to the distribution characteristics of residual voltage of each step section point node of the main network;
s3: calculating detailed operation parameters of a water/thermal power unit and a new energy unit in an internal system;
s4: and simplifying an external system of the LCC-HVDC power transmission network based on a multi-port Thevenin equivalent method, and finally completing the equivalent of the LCC-HVDC power transmission network.
Step S1, performing step division on an LCC-HVDC power transmission end power grid, wherein the specific strategy is as follows:
s21: giving definition of backbone and path, i.e
(1) Backbone network: the power grid structure is formed by an alternating current bus with the highest voltage level and a power transmission line in the LCC-HVDC power transmission end power grid;
(2) Path: the method is characterized in that a certain main network bus starts from a main network layer to reach a first step section point through a plurality of power transmission lines and buses, the power transmission lines and the buses cannot repeatedly appear, a set formed by all the power transmission lines passing through is called a path from the bus to the first step section point, and the path with the least buses is the shortest path.
On the basis of (1) and (2), determining the voltage level of a main network of an LCC-HVDC power transmission end power grid, and defining a current conversion busbar of the LCC-HVDC power transmission end power grid as a first step section point;
s22: taking the first step section point as a division start, and determining the corresponding bus as an nth step section point according to the number n of buses contained in the shortest path from each main network bus to the first step section point;
s23: and (3) scribing the power transmission lines with two ends connected with the step section points a and b (a is less than or equal to b) into the step section a.
Step S2, determining an internal system backbone based on the backbone node residual pressure, wherein the specific strategy is as follows:
s31: under the stable running state of the AC-DC system, when the three-phase short circuit fault occurs to the LCC-HVDC power transmission end power grid commutation bus and the stable state is reached, the voltage value of each bus of the main network is the node residual voltage of the corresponding step section point, and the calculation formula of the node residual voltage of each step section point is as follows:
in the method, in the process of the invention,the residual pressure matrix is a step section point node residual pressure matrix; />Injecting a short-circuit current matrix for the node, wherein the node injection short-circuit current matrix is composed of short-circuit currents injected into the section points of each step as shown in the formula (2); z is Z F For LCC-HVDC transmitting end power grid main network node impedance matrix, Y F Is a corresponding node admittance matrix;
s32: calculating the node residual pressure of each step section point of the main network, sequencing all step section points according to the order of the node residual pressures from small to large, determining a node residual pressure threshold value theta, and enabling all step section points corresponding to the node residual pressure less than theta to form an internal system main network of an equivalent system to be built;
s33: and (3) verifying the accuracy of the equivalent system established by the node residual voltage threshold value theta determined in the step (S32), and changing the value theta when the accuracy of the equivalent system cannot meet the requirements of engineering application, and establishing a new equivalent system until the accuracy of the equivalent system meets the requirements.
Step S3, in the detailed operation parameters of the water/thermal power generating unit and the new energy unit in the internal system range, emphasis is placed on an equivalent method of the new energy unit in the sending-end alternating current power grid, and the detailed parameter determination method of each electric element is as follows:
s41: the method for determining the detailed parameters of the water/thermal power equivalent unit comprises the following steps:
a1: dividing the machine components into a class a (connected to a backbone network) machine set and a class b (connected to a non-backbone network) machine set according to the voltage grade of a power grid connected to the water/thermal power machine set;
a2: for a class-a unit, a detailed parameter calculation method shown in a formula (3) is adopted, and all units in the same power plant are respectively equivalent to one equivalent machine:
wherein S is G Is equivalent to rated capacity of a unit, S j Rated capacity of each unit before equivalent, K G Is the parameters such as inertia constant, motive power, electromagnetic power, dq axis synchronous reactance, transient reactance, sub-transient reactance, gain of each link of an excitation system, time constant and the like of the equivalent machine after aggregation, K j Corresponding parameters of each unit before equivalence;
a3: for a class b unit, dividing a main network topological area according to each main network node, and on the basis, adopting a detailed parameter calculation method shown in a formula (2) to make the equivalent of each unit belonging to the same main network topological area into an equivalent machine;
s42: the method for determining the detailed parameters of the new energy equivalent unit comprises the following steps:
a1: for a unit connected with a backbone network, a single-machine aggregation equivalent method is adopted to model a new energy unit converter link in detail;
a2: and for other types of new energy units, the dynamic characteristics of the new energy units are ignored, and the new energy units are equivalent to loads with negative values.
The beneficial effects of the invention are as follows: firstly, in an AC/DC system, the LCC-HVDC power transmission network is divided in a stepped manner by calculating the number of buses contained in the minimum path from each bus of the main network of the LCC-HVDC power transmission network to the converter bus, and electric coupling between each bus of the power transmission network and the converter bus is disclosed. And secondly, on the basis of step division of the power grid at the power transmission end, three-phase short-circuit faults are arranged at the position of the converter bus, so that the residual voltage of each step section point node of the main network of the power grid at the LCC-HVDC power transmission end is obtained. The backbone structure of the internal system is determined by setting the node residual voltage threshold value, so that equivalent errors possibly existing in the prior equivalent method and caused by human factors are effectively avoided. And classifying the water/thermal power units and the new energy units in the internal system according to different grid-connected voltage levels, and respectively adopting different equivalent methods for the units of different categories. Finally, a multi-port Thevenin equivalent method is introduced, and an external system of the LCC-HVDC power transmission end power grid is simplified. According to the simulation analysis results before and after the system equivalence, the effectiveness and the accuracy of the scheme of the invention are verified.
Thus, the superiority of the present invention can be summarized as follows: firstly, a new LCC-HVDC power transmission end power grid structure definition method, namely a stepped dividing method, is provided, and the method can clear the electric coupling between each busbar in the main network of the LCC-HVDC power transmission end power grid and the conversion busbar of the LCC-HVDC power transmission end power grid. And secondly, the residual voltage of each bus node of the main network can reflect the electric coupling between the corresponding step section point and the converter bus, and the internal system range can be rapidly and objectively determined by setting the threshold value of the residual voltage of the node. And thirdly, the importance of the dynamic characteristic reservation of the new energy unit in the equivalent system on the dynamic characteristic of the repeated original system is fully considered, and the defect that the dynamic characteristic of the new energy is ignored in the equivalent research of the traditional alternating current system is overcome. Finally, the multi-port Thevenin equivalent method is introduced into the simplification of an external system of the LCC-HVDC power transmission end power grid, so that the effectiveness of an equivalent system can be ensured while the simulation efficiency of the equivalent system is greatly improved.
Drawings
Fig. 1 is a flow chart of a high-voltage direct-current transmission power grid equivalent scheme based on a node residual voltage method.
Fig. 2 is a schematic diagram of a stepped division of an LCC-HVDC power transmission end network provided in step S1 of the present invention.
Fig. 3 is a schematic diagram of a method for representing the residual voltage of a node in an ac system, which is used for explaining a specific calculation method and procedure of the residual voltage of the node.
Fig. 4 is a detailed flowchart of a method for determining an internal system backbone network based on node residual pressure.
Fig. 5 is a schematic diagram of an example of an LCC-HVDC power transmission network, for verifying the effectiveness of the equivalence scheme according to the present invention.
Fig. 6 is a comparison chart of voltage change conditions of a busbar A, B, D before and after a three-phase short-circuit fault occurs on a converter busbar before and after the equivalent scheme of the invention is adopted in an LCC-HVDC power transmission network of a calculation example. Fig. 6 (a) is a comparison chart of voltage change conditions of a bus a during a period of three-phase short-circuit fault of a converter bus before and after the power grid equivalent of an LCC-HVDC power transmission end, fig. 6 (B) is a comparison chart of voltage change conditions of a bus B, and fig. 6 (c) is a comparison chart of voltage change conditions of a bus D.
Fig. 7 is a graph showing comparison of voltage change conditions of a busbar A, B, D before and after an N-1 fault occurs in a transmission end power grid before and after the equivalent scheme of the invention is adopted in an LCC-HVDC transmission end power grid of a calculation example. Fig. 7 (a) is a diagram showing the voltage change of the bus a in comparison with the voltage change of the bus B in comparison with the voltage change of the bus D in comparison with the voltage change of the bus B during the period of the failure of the power grid N-1.
Fig. 8 is a graph showing comparison of power angle swing curves of units I1 and W2 in an original system and an equivalent system before and after a three-phase short-circuit fault occurs on a converter bus before and after the equivalent scheme of the invention is adopted in an LCC-HVDC power transmission end power grid of a calculation example.
Fig. 9 is a graph showing the comparison of the voltage response conditions of the converter bus of the original system and the equivalent system during the period of failure of the commutation of the LCC-HVDC before and after the equivalent scheme of the invention is adopted in the power grid of the LCC-HVDC transmitting end of the calculation example.
Fig. 10 is a graph showing a comparison of voltage response conditions of main new energy collection buses in an original system and an equivalent system during a commutation failure of LCC-HVDC in an LCC-HVDC power transmission network according to an example, fig. 10 (a) is a comparison of voltage response conditions of new energy collection buses B, and fig. 10 (B) is a comparison of voltage response conditions of new energy collection buses C.
Detailed Description
The invention provides a high-voltage direct-current transmission end power grid equivalent scheme based on a node residual voltage method, and in order to make the purposes, the technical scheme and the effects of the invention clearer, the specific embodiments of the invention are described in detail below with reference to the accompanying 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 invention
Fig. 1 is a flow chart of a high-voltage direct-current transmission end power grid equivalent scheme based on a node residual voltage method provided by the invention, fig. 2 is a schematic diagram of stepped division of an LCC-HVDC transmission end power grid provided by the invention, fig. 3 is a calculation method of node residual voltage of an alternating-current system provided by the invention, and fig. 4 is a detailed flow chart of a determination method of an internal system backbone network based on node residual voltage provided by the invention. Referring to fig. 1, fig. 2, fig. 3, fig. 4, an equivalent scheme of an LCC-HVDC power transmission network for new energy access according to the present invention includes:
s1: step division LCC-HVDC power transmission end power grid backbone network;
s2: determining an internal system backbone based on backbone node residual pressure;
s3: calculating detailed operation parameters of a water/thermal power unit and a new energy unit in an internal system;
s4: LCC-HVDC power transmission end power grid external system equivalence simplification based on multiport Thevenin equivalence method.
Further, the method comprises the following specific steps:
s1: stepped division LCC-HVDC power transmission end power grid backbone network
Referring to fig. 2, with LCC-HVDC power transmission end converter buses as a research center, the LCC-HVDC power transmission end network is stepwise divided according to the number of buses included in the minimum path from each bus of the main network to the converter bus of the LCC-HVDC power transmission end network.
S21: determining a voltage grade of a main network of an LCC-HVDC power transmission end power grid, determining a position of an LCC-HVDC power transmission end converter busbar on the main network, and defining the power transmission end converter busbar as a first step section point;
s22: taking the first step section point as a division start, and determining the corresponding bus as an nth step section point according to the number n of buses contained in the shortest path from each main network bus to the first step section point;
s23: and (3) scribing the power transmission lines with two ends connected with the step section points a and b (a is less than or equal to b) into the step section a.
Step S1 is described in detail with reference to fig. 2 of the specification. In fig. 2, all second step sections are indicated, and the first, second and third step sections are indicated by different lines, respectively. By ring main body 4 -l 5 -l 6 The step division of the internal bus and the power transmission line is exemplified, and the specific process of the step division of the LCC-HVDC power transmission end power grid is described. Bus N 2 To the first step section point T 1 Common L 4 -l 1 、l 6 -l 5 -l 1 、l 6 -l 3 -l 2 Equal 3 paths, wherein l 4 -l 1 Is the shortest path, which includes bus N 1 、N 2 And T 1 I.e. n=3, so N 2 Is the third step breakpoint. Transmission line l 4 Two ends are respectively connected with a second step section point N 1 And a third step section point N 2 Is connected to each other so that l 4 Drawing in a second step section; transmission line l 5 Two ends are respectively connected with a second step section point N 1 And a third step section point N 3 Is connected to each other so that l 5 Drawing in a second step section; transmission line l 6 Respectively with N 2 、N 3 Connected to N 2 、N 3 All are third step section points, so l 6 And drawing in a third step section.
S2: internal system backbone network determination based on backbone network node residual pressure
Referring to fig. 3, a specific calculation method of residual voltage of each busbar node of a main network of an lcc-HVDC power transmission end network is provided; referring to fig. 4, a detailed flowchart of a method for determining an internal system backbone based on node residual pressure is shown.
S31: under the stable running state of the AC-DC system, when the three-phase short circuit fault occurs to the current converting buses of the LCC-HVDC power transmission end power grid and the voltage reaches a stable state, the voltage value of each bus of the main 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:
in the method, in the process of the invention,the residual pressure matrix is a step section point node residual pressure matrix; />The short-circuit current matrix is shown in the formula (5) and consists of short-circuit currents injected into the section points of each step; z is Z F For LCC-HVDC transmitting end power grid main network impedance matrix, Y F Is a corresponding admittance matrix;
s32: calculating the node residual pressure of each step section point of the main network, sequencing all step section points according to the order of the node residual pressures from small to large, determining a node residual pressure threshold value theta, and enabling all step section points corresponding to the node residual pressure less than theta to form an internal system main network of an equivalent system to be built;
s33: and (3) verifying the accuracy of the equivalent system established by the node residual voltage threshold value theta determined in the step (S32), and changing the value theta when the accuracy of the equivalent system cannot meet the requirements of engineering application, and establishing a new equivalent system until the accuracy of the equivalent system meets the requirements. Detailed determination of the specific node residual voltage threshold value theta is given in fig. 4 of the specification.
S3: calculating detailed operation parameters of water/thermal power unit and new energy unit in internal system
S41: the method for determining the detailed parameters of the water/thermal power equivalent unit comprises the following steps:
a1: dividing the machine components into a class a (connected to a backbone network) machine set and a class b (connected to a non-backbone network) machine set according to the voltage grade of a power grid connected to the water/thermal power machine set;
a2: for a class-a unit, a detailed parameter calculation method shown in a formula (6) is adopted, and all units in the same power plant are respectively equivalent to one equivalent machine:
wherein S is G Is equivalent to rated capacity of a unit, S j Rated capacity of each unit before equivalent, K G Is the parameters such as inertia constant, motive power, electromagnetic power, dq axis synchronous reactance, transient reactance, sub-transient reactance, gain of each link of an excitation system, time constant and the like of the equivalent machine after aggregation, K j Corresponding parameters of each unit before equivalence;
a3: dividing a main network topology region according to each step section point for a class b unit, and adopting a detailed parameter calculation method shown in a formula (6) on the basis, wherein the equivalence of each unit belonging to the same main network topology region is an equivalence machine;
s42: the method for determining the detailed parameters of the new energy equivalent unit comprises the following steps:
a1: for a unit connected with a backbone network, a single-machine aggregation equivalent method is adopted to model a new energy unit converter link in detail;
a2: and for other types of new energy units, the dynamic characteristics of the new energy units are ignored, and the new energy units are equivalent to loads with negative values.
S4: and simplifying an external system of the direct current end alternating current power grid based on a multi-port Thevenin equivalent method, and finally completing the equivalent of the LCC-HVDC end alternating current power grid.
2. Technical feasibility verification of the invention
The system model of the engineering simulation example is shown in fig. 5, and the simulation results are shown in fig. 6, 7, 8, 9 and 10, so that the effectiveness of the invention is verified.
Fig. 6 is a comparison chart of voltage change conditions of a busbar A, B, D before and after a three-phase short-circuit fault occurs on a converter busbar before and after the equivalent value of an LCC-HVDC power transmission end power grid. Fig. 7 is a comparison chart of voltage change conditions of the busbar A, B, D before and after the N-1 fault occurs in the power transmission network before and after the equivalent of the LCC-HVDC power transmission network. Fig. 8 is a graph comparing the power angle rocking curves of the original system and the equivalent system units I1 and W2 before and after the equivalent of the LCC-HVDC power transmission network, before and after the three-phase short circuit fault of the power transmission network converter bus. Fig. 9 is a graph comparing the voltage response conditions of the converter bus of the original system and the equivalent system during the period of failure of the commutation of the LCC-HVDC before and after the equivalent of the power grid of the LCC-HVDC transmitting end. Fig. 10 is a graph comparing the voltage response conditions of main new energy collecting buses in the original system and the equivalent system during the period of commutation failure of the LCC-HVDC before and after the equivalent of the LCC-HVDC power transmission network.
As can be seen from fig. 6, the equivalence scheme of the present invention is adopted to perform equivalence on the LCC-HVDC power transmission end network of the calculation 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 current conversion bus of the original system, and the static stability characteristics of the system after the three-phase short-circuit fault is removed are the same. As can be seen from fig. 7, the equivalent scheme of the present invention is adopted to perform equivalent on the LCC-HVDC power transmission network of the calculation example, the voltage response characteristics of the busbar A, B, D are basically consistent before and after the N-1 fault occurs in the equivalent system and the original system, and the static stability characteristics of the system after the N-1 fault is removed are the same. And as shown in fig. 8, the equivalence scheme of the invention is adopted to carry out equivalence on the power grid at the LCC-HVDC power transmission end of the calculation example, and the equivalent system is basically fitted with the power angle rocking curves of the units I1 and W2 before and after the three-phase short-circuit fault occurs on the LCC-HVDC converter bus in the original system. The simulation result shows that when the equivalence scheme disclosed by the invention is adopted to carry out equivalence on the LCC-HVDC power transmission end power grid, the alternating current fault response characteristics of the equivalence system and the original system are not obviously changed.
As shown in fig. 9, the equivalence scheme of the invention is adopted to carry out equivalence on the LCC-HVDC power transmission end power grid of the calculation example, the equivalence system is approximately equal to the maximum overvoltage value of the current conversion busbar of the LCC-HVDC power transmission end power grid before and after the current conversion failure of the LCC-HVDC in the original system, and the overvoltage response curve is basically fitted. As shown in fig. 10 (a) and fig. 10 (b), the equivalence scheme of the invention is adopted to perform equivalence on the LCC-HVDC power transmission end power grid of the calculation example, and the overvoltage response process of each main new energy bus in the LCC-HVDC power transmission end power grid is basically consistent with that before and after the commutation failure of the LCC-HVDC in the original system. The simulation result can show that when the equivalence scheme disclosed by the invention is adopted to conduct equivalence on the LCC-HVDC power transmission end power grid, the direct current fault response characteristics of the equivalence system and the original system are not obviously changed.
The simulation results of the examples corresponding to fig. 6, 7, 8, 9 and 10 show that the transient characteristics of the equivalent system established by adopting the equivalent scheme of the invention are highly consistent with those of the original system, and the simulation results verify the effectiveness and feasibility of the equivalent scheme of the high-voltage direct-current transmission end power grid based on the node residual voltage method.
In summary, the node residual voltage method-based high-voltage direct current transmission power transmission end power grid equivalent scheme can fully reproduce transient characteristics and overvoltage characteristics of an LCC-HVDC power transmission end power grid while greatly reducing the simulation scale of the LCC-HVDC power transmission end power grid. Before the main network of the internal system is determined, the LCC-HVDC power transmission end power grid is layered in a stepped division mode, so that the electric coupling between each main network busbar and the LCC-HVDC is clearly reflected. And the backbone network of the internal system is determined based on the residual pressure of the backbone network node, so that the determination flow of the internal system can be simplified, and the equivalent error is reduced. On the basis of an internal system backbone network, the calculation scale of the equivalent system is reduced by establishing equivalent water/thermal power and new energy units. Finally, the external system is simplified based on the multi-port Thevenin equivalent method, and the complexity of the LCC-HVDC power transmission end power grid equivalent system can be greatly reduced. According to the equivalence scheme of the LCC-HVDC power transmission end power grid for new energy access, the scale of the LCC-HVDC power transmission end power grid is greatly reduced, the simulation calculation efficiency is improved, and the transient characteristic and the overvoltage characteristic of the LCC-HVDC power transmission end power grid can be fully reproduced.
Finally, it should be noted that the above examples of the present invention are merely illustrative of the present invention and are not limiting of the embodiments of the present invention. While the invention has been described in detail with reference to the preferred embodiments, it will be apparent to one skilled in the art that various other changes and modifications can be made therein. Not all embodiments are exhaustive. Obvious changes and modifications which are extended by the technical proposal of the invention are still within the protection scope of the invention.
Claims (3)
1. The high-voltage direct current transmission (LCC-HVDC) power transmission end power grid equivalence scheme based on the node residual voltage method is characterized by comprising the following steps of:
s1: stepped division LCC-HVDC power transmission end power grid backbone network
According to the number of buses contained in the minimum path from each bus of the main network of the LCC-HVDC power transmission end power grid to the converter bus, the LCC-HVDC power transmission end power grid is divided in a stepped mode;
s2: internal system backbone network determination based on backbone network node residual pressure
In the running process of the LCC-HVDC power transmission end power grid, when a three-phase short circuit fault occurs in a converter bus and reaches a steady state, determining an internal system main network according to the distribution characteristics of residual voltage of each step section point node of the main network;
s3: calculating detailed operation parameters of a water/thermal power unit and a new energy unit in an internal system;
the detailed parameter determining method of each electrical element is as follows:
s41: the method for determining the detailed parameters of the water/thermal power equivalent unit comprises the following steps:
a1: dividing the machine components into a class a machine set and a class b machine set according to the voltage grade of a power grid accessed by the water/thermal power machine set, wherein the class a machine set is accessed to a main network and the class b machine set is accessed to a non-main network;
a2: for a class-a unit, a detailed parameter calculation method shown in the following expression is adopted, and all units in the same power plant are respectively equivalent to one equivalent machine:
wherein S is G Is equivalent to rated capacity of a unit, S j Rated capacity of each unit before equivalent, K G Is the parameters such as inertia constant, motive power, electromagnetic power, dq axis synchronous reactance, transient reactance, sub-transient reactance, gain of each link of an excitation system, time constant and the like of the equivalent machine after aggregation, K j Corresponding parameters of each unit before equivalence;
a3: for a class b unit, dividing a main network topological area according to each main network node, and on the basis, adopting a detailed parameter calculation method shown by the following expression to make the equivalent of each unit belonging to the same main network topological area into an equivalent machine:
in the method, in the process of the invention,injecting a short-circuit current matrix for the node, wherein the node injection short-circuit current matrix is composed of short-circuit currents injected into the step section points;
s42: the method for determining the detailed parameters of the new energy equivalent unit comprises the following steps:
a1: for a unit connected with a backbone network, a single-machine aggregation equivalent method is adopted to model a new energy unit converter link in detail;
a2: for other types of new energy units, the dynamic characteristics of the new energy units are ignored, and the new energy units are equivalent to loads with negative values;
s4: and simplifying an external system of the LCC-HVDC power transmission network based on a multi-port Thevenin equivalent method, and finally completing the equivalent of the LCC-HVDC power transmission network.
2. The high-voltage direct-current transmission end power grid equivalence scheme based on the node residual voltage method according to claim 1, wherein step S1 is used for stepwise dividing an LCC-HVDC transmission end power grid, and the specific strategy is as follows:
s21: giving definition of backbone and path, i.e
Backbone network: the power grid structure is formed by an alternating current bus with the highest voltage level and a power transmission line in the LCC-HVDC power transmission end power grid;
path: the method is characterized in that a certain main network bus starts from a main network layer to reach a first step section point through a plurality of power transmission lines and buses, the power transmission lines and the buses cannot repeatedly appear, a set formed by all the power transmission lines passing through is called a path from the bus to the first step section point, and the path with the least buses is the shortest path.
On the basis of a main network and a path, determining the voltage level of the main network of the LCC-HVDC power transmission end power grid, and defining a current conversion busbar of the LCC-HVDC power transmission end power grid as a first step section point;
s22: taking the first step section point as a division start, and determining the corresponding bus as an nth step section point according to the number n of buses contained in the shortest path from each main network bus to the first step section point;
s23: and (3) scribing the power transmission lines with two ends connected with the points of the step section a and the step section b into the step section a, wherein a is less than or equal to b.
3. The high-voltage direct-current transmission end power grid equivalence scheme based on the node residual voltage method according to claim 1, wherein step S2 is based on step breakpoint node residual voltage to determine an internal system backbone, and the specific strategy is as follows:
s31: under the stable running state of the AC-DC system, when the three-phase short circuit fault occurs to the LCC-HVDC power transmission end power grid commutation bus and the stable state is reached, the voltage value of each bus of the main network is the residual voltage of the corresponding step section point node, and the calculation formula of the residual voltage of each step section point node is as follows:
in the method, in the process of the invention,the residual pressure matrix is a step section point node residual pressure matrix; z is Z F For LCC-HVDC transmitting end power grid main network node impedance matrix, Y F Is a corresponding node admittance matrix;
s32: calculating the node residual pressure of each step section point of the main network, sequencing all step section points according to the order of the node residual pressures from small to large, determining a node residual pressure threshold value theta, and enabling all step section points corresponding to the node residual pressure less than theta to form an internal system main network of an equivalent system to be built;
s33: and (3) verifying the accuracy of the equivalent system established by the node residual voltage threshold value theta determined in the step (S32), and changing the value theta when the accuracy of the equivalent system cannot meet the requirements of engineering application, and establishing a new equivalent system until the accuracy of the equivalent system meets the requirements.
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