CN112310994A - Method for evaluating influence of power grid line outage on harmonic stability of alternating current-direct current system - Google Patents

Method for evaluating influence of power grid line outage on harmonic stability of alternating current-direct current system Download PDF

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CN112310994A
CN112310994A CN202011181483.1A CN202011181483A CN112310994A CN 112310994 A CN112310994 A CN 112310994A CN 202011181483 A CN202011181483 A CN 202011181483A CN 112310994 A CN112310994 A CN 112310994A
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毛炽祖
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Maintenance and Test Center of Extra High Voltage Power Transmission Co
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Abstract

The invention discloses an evaluation method for influence of power grid line outage on harmonic stability of an alternating current-direct current system, which comprises the following steps: taking a node of a flexible direct current system accessed to a transformer substation as a starting point, and layering an alternating current-direct current hybrid power grid; carrying out transfer function derivation on each layer of power grid obtained after layering under the condition of single flexible direct current system access and power grid N-1 faults; and calculating the sensitivity of each line parameter according to the closed loop transfer function expression of the single flexible direct current system accessed to the alternating current network with any port obtained by calculation, and screening out the maximum line link which is the weakest line link after the sensitivity is sorted. The invention has the beneficial effects that: by taking a node of a flexible direct current system accessed to a transformer substation as a starting point, the alternating current-direct current hybrid power grid is layered, the line parameter sensitivity and weak link identification are calculated, the weak link in the alternating current-direct current system is finally screened out, and a theoretical basis is provided for power grid operation mode management and power equipment model selection.

Description

Method for evaluating influence of power grid line outage on harmonic stability of alternating current-direct current system
Technical Field
The invention relates to the technical field of harmonic stability analysis of an alternating current-direct current hybrid power grid, in particular to an evaluation method for influence of power grid line outage on harmonic stability of an alternating current-direct current system.
Background
In recent years, broadband harmonic oscillation occurs in a plurality of flexible direct-current transmission projects based on Modular Multilevel Converters (MMC) at home and abroad in the debugging or running process, and the safe and stable running of an alternating-current and direct-current system is seriously threatened. With the wide-range popularization of flexible direct current engineering in different application occasions, a large number of power electronic devices and equipment are introduced into an alternating current-direct current system. In the current multi-terminal direct current transmission project, a transmitting-end Converter station usually adopts a thyristor-type power grid phase-change Converter (LCC), and a receiving end usually adopts a topological structure of two full-bridge and half-bridge mixed type MMC Converter stations. The MMC converter station comprises hundreds of high-capacity fully-controlled power semiconductor devices and a suspended direct-current energy storage capacitor, and the converter valve generally adopts a nonlinear modulation strategy and capacitor voltage balance based on a sequencing algorithm, so that the flexible direct-current converter station has strong nonlinearity and strong coupling characteristics. Therefore, when the flexible direct current converter station is under a certain control strategy or operation condition, certain specific frequency resonance points may exist with line impedance, series-parallel compensation devices and the like in the alternating current system, so that oscillation of the alternating current-direct current system is caused. Although resonance can be avoided when the flexible circuit is connected into the alternating current power grid, when N-1 disconnection fault occurs in the alternating current power grid to cause parameter change on the alternating current side, resonance can still be caused at certain frequency to threaten safe operation of the power grid.
Disclosure of Invention
The invention provides an evaluation method for the influence of power grid line outage on the harmonic stability of an alternating current-direct current system, which aims at solving the problem of the harmonic stability of the existing alternating current-direct current hybrid power grid and mainly solves the problem of the background technology.
In order to solve the technical problems, the technical scheme of the invention is as follows:
a method for evaluating influence of power grid line outage on harmonic stability of an alternating current-direct current system comprises the following steps:
step S1: taking a node of a flexible direct current system accessed to a transformer substation as a starting point, and layering an alternating current-direct current hybrid power grid;
step S2: carrying out transfer function derivation on each layer of power grid obtained after S1 layering under the condition of single flexible direct current system access and power grid N-1 faults;
step S3: and (4) calculating the sensitivity of each line parameter according to a closed loop transfer function expression of the single flexible direct current system accessed to the alternating current network with any port obtained by calculation of S2, and sorting the sensitivity to obtain the largest link which is the weakest link.
The invention has the beneficial effects that: by taking a node of a flexible direct current system accessed to a transformer substation as a starting point, the alternating current-direct current hybrid power grid is layered, the line parameter sensitivity and weak link identification are calculated, the weak link in the alternating current-direct current system is finally screened out, and a theoretical basis is provided for power grid operation mode management and power equipment model selection.
Drawings
Fig. 1 is a flowchart of a method for evaluating influence of power grid line outage on harmonic stability of an ac/dc system according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a three-port equivalent circuit according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of an equivalent admittance circuit of the AC/DC system disclosed in the embodiments of the present invention;
FIG. 4 is a schematic diagram of a single flexible line accessing a three-port equivalent circuit according to an embodiment of the present invention;
FIG. 5 is a schematic diagram of a single flexible line access multiport network according to an embodiment of the present invention;
fig. 6 is a schematic diagram of an equivalent circuit of a single flexible line access multiport network according to an embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention clearer and clearer, the following detailed description of the present invention is provided with reference to the accompanying drawings and detailed description. It is to be understood that the specific embodiments described herein are merely illustrative of the invention and are not limiting of the invention. It should be further noted that, for the convenience of description, only some but not all of the relevant aspects of the present invention are shown in the drawings.
As shown in fig. 1, the present embodiment provides a method for evaluating an influence of a power grid line outage on harmonic stability of an ac/dc system, including the following steps:
step S1: taking a node of a flexible direct current system accessed to a transformer substation as a starting point, and layering an alternating current-direct current hybrid power grid;
step S1 specifically includes: taking a node of a flexible direct current system accessed to a transformer substation as a starting point, and taking a network formed by a simplest loop where the node of the transformer substation is located and a tail end node connected to the simplest loop as a first-layer network; then taking the nodes of the first layer network as first layer generalized nodes, a first layer simplest loop where the first layer generalized nodes are located, and a network formed by end nodes connected to the first layer simplest loop as a second layer network; and taking the nodes of the second-layer network as second-layer generalized nodes, the simplest loop of the second layer where the second-layer generalized nodes are located, and a network formed by the end nodes connected to the simplest loop of the second layer as a third-layer network, and so on to obtain the N-layer network.
Step S2: carrying out transfer function derivation on each layer of power grid obtained after S1 layering under the condition of single flexible direct current system access and power grid N-1 faults;
step S2 specifically includes: as shown in fig. 2, the harmonic stability analysis model for accessing the flexible dc system to the ac power grid is equivalent to a three-port equivalent circuit, one port of the three-port equivalent circuit is equivalent to the flexible dc system, and the other two ports are equivalent to the corresponding ac power grid 1 and ac power grid 2, respectively. The other two ports in the power grid can be simply regarded as a Thevenin branch equivalent circuit with a voltage source and impedance connected in series by using an alternating current system equivalent method; the ports for accessing the two alternating current systems are also connected with loads, and the loads are represented in the form of admittances. Meanwhile, in order to better analyze the problem, all impedances and admittances in the three-port equivalent circuit are uniformly expressed in an admittance form by using the KCL, KVL and norton equivalent theorem, so as to obtain the flexible direct current grid-connected alternating current and direct current system equivalent admittance circuit, as shown in fig. 3; in fig. 3, after all impedances in the ac/dc system are converted into admittance forms, the ac/dc system equivalent admittance circuit includes two naturally formed admittance matrices, where the first admittance matrix is a system network side admittance matrix that does not include power supply equivalent admittance, and the second admittance matrix is a total admittance matrix that includes power supply equivalent admittance, and a closed-loop transfer function of the ac/dc system is obtained according to the first admittance matrix and the second admittance matrix; solving an equivalent admittance transfer function at an alternating current side in the alternating current-direct current system, solving a closed-loop transfer function expression of the alternating current-direct current system according to the equivalent admittance transfer function, solving a closed-loop transfer function expression of a single flexible linear circuit accessed to an alternating current network with any port according to the closed-loop transfer function expression of the alternating current-direct current system, and further obtaining an open-loop transfer function expression.
Second layer admittance matrix YncComprises the following steps:
Figure BDA0002750318310000031
the result of the conversion is that,
Figure BDA0002750318310000032
first layer admittance matrix YnoComprises the following steps:
Figure BDA0002750318310000033
the result of the conversion is that,
Figure BDA0002750318310000034
and determining the total impedance matrix Znc：
Figure BDA0002750318310000035
Wherein, Yg1For AC systems 1 equivalent admittance, Vg1For an equivalent voltage, V, of the AC system 1b1For the voltage at the 1-port of the AC system, Ig1For an equivalent output current, Y, of the AC system 1l1Load equivalent admittance, Y, for access to port 1 of AC systemg2For AC system 2 equivalent admittance, Vg2For an equivalent voltage, V, of the AC system 2b2For an AC system 2-port voltage, Ig2For an equivalent output current, Y, of the AC system 2l2Equivalent admittance of load, G, for access to port 2 of AC systemclFor a flexible DC closed loop transfer function, IrefIs a reference value of current of a flexible direct current system, Vb3For a flexible DC system port voltage, YinvFor flexible DC system equivalent admittance, Ig3For flexible DC system output current, Yc1For line admittance, Y, between ports of AC system 1 and ports of AC system 2c2For line admittance, Y, between the 2-port and the flexible DC-port of the AC systemc3Is a line admittance between the port of the alternating current system 1 and the flexible direct current port; z11For the bus node self-impedance, Z, of the AC system 112And Z21Is the mutual impedance between the bus node of the alternating current system 1 and the bus node of the alternating current system 2, Z13And Z31Is the mutual impedance between the bus node of the alternating current system 1 and the bus node of the alternating current system 3, Z22For the self-impedance of the bus-bar node 2 of the AC system, Z23And Z32Is the mutual impedance between the bus node of the alternating current system 2 and the bus node of the alternating current system 3.
The method for calculating the closed loop transfer function expression of the flexible direct current system comprises the following steps: the formula (3) is converted into a formula,
Figure BDA0002750318310000041
the closed loop transfer function is obtained according to the equation (6), as shown in the equation (7), and the corresponding equivalent circuit is shown in fig. 4.
Figure BDA0002750318310000042
In the formula, YgridIs the equivalent impedance transfer function on the alternating current side.
The calculation method of the closed-loop transfer function expression comprises the following steps: listing the equivalent impedance Z from the power supply end of the flexible direct system to the AC-DC hybrid system33As can be seen in FIG. 4, Z33Actually, the equivalent impedance from the power supply end of the flexible direct system to the alternating current-direct current hybrid system is YinvAnd YgridInverse of the parallel admittance value of (a):
Figure BDA0002750318310000043
deriving Z from the matrix transformation relationship33The expression of (a) is, in order,
Figure BDA0002750318310000044
in the formula, YnomIs a matrix YncDeterminant of (4);
the expression (7) is calculated to obtain a closed-loop transfer function expression shown in the expression (10),
Figure BDA0002750318310000051
for an alternating current network with any port, a single flexible line access is considered, and when nodes i and j are connectedThe network schematic diagram when the connection line is disconnected is shown in fig. 5, wherein k represents the node number of the flexible line access substation, I and j are the node numbers of two ends of the disconnected line, and Iref,kFor accessing a k-node flexible DC system current reference value, Yinv,kFor accessing k-node flexible DC system equivalent admittance, YcijFor line admittance between nodes i, j, m1+1 to m1+m2Numbering the nodes of the AC substation, Yg,m1+1Is an equivalent admittance, V, of an AC system m1+1g,m1+1Is an equivalent voltage of m1+1 of an AC system, Yg,m1+m2Is an equivalent admittance, V, of an alternating current system m1+ m2g,m1+m2Is the equivalent voltage of an alternating current system m1+ m 2. The equivalent circuit of the multiport network obtained by the same analysis method as that of the three-port network is shown in FIG. 6, in which IgkFor connecting to k node flexible DC system output current, VbkFor accessing the port voltage, Y, of the k-node flexible direct current systemgrid,kAnd connecting the k node into the flexible direct current system and then performing equivalent impedance transfer function on the alternating current side. And when the connection lines of the nodes i and j are disconnected, a single flexible direct access multiport network is established, a multiport network equivalent circuit is obtained by applying the analysis methods from (1) to (10), and a closed-loop transfer function expression of the single flexible direct access arbitrary port alternating-current network is calculated.
The method for calculating the closed-loop transfer function expression of the single flexible linear path accessing to the alternating current network with any port specifically comprises the following steps:
when the line between nodes i and j is down, Ycij0, pair admittance matrix YncThe influence of (a) is that,
diagonal line element
Figure BDA0002750318310000052
In the formula: y'iiFor node i self-admittance, Y after disconnectioniiIs self-admittance of node i before disconnection, Y'jjFor node j self-admittance, Y after disconnectionjjFor node j self-admittance, Y before disconnectioncijIs the line admittance between the nodes i and j;
off diagonal elements
Y′ij＝Y′ji＝Yij+Ycij (12)
In the formula: y'ijAnd Y'jiFor mutual admittance between nodes i, j after disconnection, YijMutual admittance between nodes i and j before disconnection;
solving the equivalent admittance transfer function Y of the multiport network equivalent circuitgrid,k，
Figure BDA0002750318310000053
In the formula: y isgrid,kThe k node is connected into the equivalent impedance transfer function, Z, of the alternating current side of the flexible direct current systemkkIs the self-impedance of node k, Yinv,kFor accessing the k-node flexible dc system equivalent admittance, and,
Figure BDA0002750318310000061
in the formula: mkkIs Ync,kMiddle element ZkkThe remainder type of (1);
the expression form of the closed loop transfer function of the single flexible line access to the alternating current network with any port is the same as that of the expression (10), as shown in the expression (15),
Figure BDA0002750318310000062
in the formula, k represents the node number of the flexible line access substation, I and j are the node numbers of two ends of a disconnected line, and Iref,kFor accessing a k-node flexible DC system current reference value, IgkAnd outputting current for connecting a k-node flexible direct current system.
Further obtaining an open-loop transfer function expression of the single flexible line access to the alternating current network with any port, as shown in formula (16),
Figure BDA0002750318310000063
step S3: and (4) calculating the sensitivity of each line parameter according to a closed loop transfer function expression of the single flexible direct current system accessed to the alternating current network with any port obtained by calculation of S2, and sorting the sensitivity to obtain the largest link which is the weakest link.
Step S3 specifically includes: the relative sensitivity of the parameters of the lines connected with the nodes i and j in the system is calculated by adopting the following formula
Figure BDA0002750318310000064
In the formula: rcijIs a line resistance, X, between nodes i, jcijIs a line reactance between nodes i and j, and when a line disconnection accident occurs, the line R is connectedcijAnd XcijThe value of (d) is a positive real number between 600 and 1000;
and (3) calculating the relative sensitivity value of each line after a certain line in the alternating current power grid is disconnected by adopting the formula (17) under different frequencies, sorting the lines from large to small according to the magnitude relation of the relative sensitivity values, and screening the line with the highest sorting.
According to the method, the flexible direct-current system is accessed to the transformer substation node as a starting point, the alternating-current and direct-current series-parallel power grid is layered, the line parameter sensitivity and weak link identification are calculated, the weak link in the alternating-current and direct-current system is finally screened, and a theoretical basis is provided for power grid operation mode management and power equipment model selection.
The above embodiments are only for illustrating the technical concept and features of the present invention, and the purpose thereof is to enable those skilled in the art to understand the contents of the present invention and implement the present invention accordingly, and not to limit the protection scope of the present invention accordingly. All equivalent changes or modifications made in accordance with the spirit of the present disclosure are intended to be covered by the scope of the present disclosure.

Claims (9)

1. A method for evaluating the influence of power grid line outage on harmonic stability of an alternating current-direct current system is characterized by comprising the following steps:
step S1: taking a node of a flexible direct current system accessed to a transformer substation as a starting point, and layering an alternating current-direct current hybrid power grid;
step S2: carrying out transfer function derivation on each layer of power grid obtained after S1 layering under the condition of single flexible direct current system access and power grid N-1 faults;
step S3: and (4) calculating the sensitivity of each line parameter according to a closed loop transfer function expression of the single flexible direct current system accessed to the alternating current network with any port obtained by calculation of S2, and sorting the sensitivity to obtain the largest link which is the weakest link.
2. The method for evaluating the influence of the outage of the power grid line on the harmonic stability of the alternating current-direct current system according to claim 1, wherein the step S1 specifically comprises: taking a node of the flexible direct current system accessed to a transformer substation as a starting point, and taking a network formed by a simplest loop where the node of the transformer substation is located and a tail end node connected to the simplest loop as a first-layer network; then taking the nodes of the first-layer network as first-layer generalized nodes, a first-layer simplest loop where the first-layer generalized nodes are located, and a network formed by end nodes connected to the first-layer simplest loop as a second-layer network; and taking the nodes of the second layer network as second layer generalized nodes, taking a second layer simplest loop where the second layer generalized nodes are located and a network formed by end nodes connected on the second layer simplest loop as a third layer network, and so on to obtain the N-layer network.
3. The method for evaluating the influence of the grid line outage on the harmonic stability of the ac-dc system according to claim 2, wherein the step S2 specifically comprises: the harmonic stability analysis model of the flexible direct current system accessed to the alternating current power grid is equivalent to a three-port equivalent circuit, one port of the three-port equivalent circuit is equivalent to the flexible direct current system, and the other two ports are equivalent to the corresponding alternating current power grid 1 and the corresponding alternating current power grid 2 respectively; uniformly expressing all impedance and admittance in the three-port equivalent circuit into an admittance form to obtain an AC/DC system equivalent admittance circuit of a flexible DC grid connection; the alternating current and direct current system equivalent admittance circuit comprises two layers of admittance matrixes which are naturally formed, wherein the first layer of admittance matrix is a system network side admittance matrix which does not contain power supply equivalent admittance, the second layer of admittance matrix is a total admittance matrix which contains power supply equivalent admittance, and a closed-loop transfer function of the alternating current and direct current system is obtained according to the first layer of admittance matrix and the second layer of admittance matrix; and solving an equivalent admittance transfer function at the AC side in the AC-DC system, solving a closed-loop transfer function expression of the AC-DC system according to the equivalent admittance transfer function, and solving a closed-loop transfer function expression of a single flexible linear path accessed to an AC network with any port according to the closed-loop transfer function expression of the AC-DC system, so as to obtain an open-loop transfer function expression.
4. The method of claim 3, wherein the second layer admittance matrix Y is used for evaluating the effect of grid line outage on harmonic stability of the AC/DC systemncComprises the following steps:
Figure FDA0002750318300000011
the result of the conversion is that,
Figure FDA0002750318300000021
the first layer admittance matrix YnoComprises the following steps:
Figure FDA0002750318300000022
the result of the conversion is that,
Figure FDA0002750318300000023
and determining the total impedance matrix Znc：
Figure FDA0002750318300000024
Wherein, Yg1For AC systems 1 equivalent admittance, Vg1For an equivalent voltage, V, of the AC system 1b1For the voltage at the 1-port of the AC system, Ig1For an equivalent output current, Y, of the AC system 1l1Load equivalent admittance, Y, for access to port 1 of AC systemg2For AC system 2 equivalent admittance, Vg2For an equivalent voltage, V, of the AC system 2b2For an AC system 2-port voltage, Ig2For an equivalent output current, Y, of the AC system 2l2Equivalent admittance of load, G, for access to port 2 of AC systemclFor a flexible DC closed loop transfer function, IrefIs a reference value of current of a flexible direct current system, Vb3For a flexible DC system port voltage, YinvFor flexible DC system equivalent admittance, Ig3For flexible DC system output current, Yc1For line admittance, Y, between ports of AC system 1 and ports of AC system 2c2For line admittance, Y, between the 2-port and the flexible DC-port of the AC systemc3Is a line admittance between the port of the alternating current system 1 and the flexible direct current port; z11For the bus node self-impedance, Z, of the AC system 112And Z21Is the mutual impedance between the bus node of the alternating current system 1 and the bus node of the alternating current system 2, Z13And Z31Is the mutual impedance between the bus node of the alternating current system 1 and the bus node of the alternating current system 3, Z22For the self-impedance of the bus-bar node 2 of the AC system, Z23And Z32Is the mutual impedance between the bus node of the alternating current system 2 and the bus node of the alternating current system 3.
5. The method for evaluating the influence of the line outage of the power grid on the harmonic stability of the alternating current-direct current system according to claim 4, wherein the calculation method of the closed-loop transfer function expression of the flexible direct current system comprises the following steps: the formula (3) is converted into a formula,
Figure FDA0002750318300000025
obtaining the closed loop transfer function according to the formula (6), as shown in the formula (7)
Figure FDA0002750318300000031
In the formula, YgridIs the equivalent impedance transfer function on the alternating current side.
6. The method for evaluating the influence of the line outage of the power grid on the harmonic stability of the alternating current-direct current system according to claim 5, wherein the closed-loop transfer function expression is calculated by the following method: listing the equivalent impedance Z from the power supply end of the flexible direct system to the AC-DC hybrid system33：
Figure FDA0002750318300000032
Deriving Z from the matrix transformation relationship33The expression of (a) is, in order,
Figure FDA0002750318300000033
in the formula, YnomIs a matrix YncDeterminant of (4);
the expression (7) is calculated to obtain a closed-loop transfer function expression shown in the expression (10),
Figure FDA0002750318300000034
7. the method for evaluating the influence of the outage of the power grid line on the harmonic stability of the alternating current-direct current system according to claim 6, wherein a single flexible line access is considered for the alternating current network with any port, a single flexible line access multi-port network is established when the connection lines of the nodes i and j are disconnected, the multi-port network equivalent circuit is obtained by applying the analysis methods from (1) to (10), and a closed-loop transfer function expression of the single flexible line access alternating current network with any port is calculated.
8. The method for evaluating the influence of the line outage of the power grid on the harmonic stability of the alternating current-direct current system according to claim 7, wherein the calculation method of the closed-loop transfer function expression of the single flexible line accessing the alternating current network with any port specifically comprises the following steps:
when the line between nodes i and j is down, Ycij0, pair admittance matrix YncThe influence of (a) is that,
diagonal line element
Figure FDA0002750318300000035
In the formula: y'iiFor node i self-admittance, Y after disconnectioniiIs self-admittance of node i before disconnection, Y'jjFor node j self-admittance, Y after disconnectionjjFor node j self-admittance, Y before disconnectioncijIs the line admittance between the nodes i and j;
off diagonal elements
Y′ij＝Y′ji＝Yij+Ycij (12)
In the formula: y'ijAnd Y'jiFor mutual admittance between nodes i, j after disconnection, YijMutual admittance between nodes i and j before disconnection;
solving an equivalent admittance transfer function Y of the multiport network equivalent circuitgrid,k，
Figure FDA0002750318300000041
In the formula: y isgrid,kThe k node is connected into the equivalent impedance transfer function, Z, of the alternating current side of the flexible direct current systemkkIs the self-impedance of node k, Yinv,kFor accessing the k-node flexible dc system equivalent admittance, and,
Figure FDA0002750318300000042
in the formula: mkkIs Ync,kMiddle element ZkkThe remainder type of (1);
the expression form of the closed loop transfer function of the single flexible line access to the alternating current network with any port is the same as that of the expression (10), as shown in the expression (15),
Figure FDA0002750318300000043
in the formula, k represents the node number of the flexible line access substation, I and j are the node numbers of two ends of a disconnected line, and Iref,kFor accessing a k-node flexible DC system current reference value, IgkOutputting current for accessing a k-node flexible direct current system;
further obtaining an open-loop transfer function expression of the single flexible line access to the alternating current network with any port, as shown in formula (16),
Figure FDA0002750318300000044
9. the method for evaluating the influence of the outage of the power grid line on the harmonic stability of the ac-dc system according to claim 8, wherein the step S3 specifically comprises: the relative sensitivity of the parameters of the lines connected with the nodes i and j in the system is calculated by adopting the following formula
Figure FDA0002750318300000045
In the formula: rcijIs a line resistance, X, between nodes i, jcijIs a line reactance between nodes i and j, and when a line disconnection accident occurs, the line R is connectedcijAnd XcijThe value of (d) is a positive real number between 600 and 1000;
and calculating the relative sensitivity value of each line after a certain line in the alternating current power grid is disconnected by adopting the formula (17) under different frequencies, sorting the lines from large to small according to the magnitude relation of the relative sensitivity values, and screening the line with the highest sorting.
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