Linear power flow-based static safety analysis method, system, equipment and medium for alternating current and direct current system containing VSC-MTDC (Voltage Source Converter-Multi-Terminal Direct Current)

Abstract

The invention relates to a static safety analysis method, system, equipment and medium for a VSC-MTDC-containing AC / DC system based on linear power flow, and belongs to the technical field of power system engineering. The method comprises the following steps: firstly, on the basis of a conventional AC system electrothermal coupling power flow model, constructing an electrothermal coupling power flow model containing a VSC-MTDC AC / DC system; decoupling calculation of electric and thermal states of the system is realized; the method comprises the following steps of: linearizing alternating-current and direct-current branch power flows by referring to a traditional power flow linearization idea, and constructing a linearized power flow model containing a VSC-MTDC alternating-current and direct-current system by taking an alternating-current node phase angle, a voltage amplitude square and a direct-current node voltage square as state variables; and finally, providing a linear power flow calculation method suitable for the VSC-MTDC-containing AC / DC system and a power flow correction method considering branch loss, rapidly calculating the power flow and improving the precision of the power flow, and outputting system safety analysis early warning information after calculating an approximate analytical solution of the temperature of the AC / DC overhead conductor.

Description

Technical Field

[0001] The invention relates to a static safety analysis method, system, equipment and medium of an AC / DC system containing a VSC-MTDC based on linear power flow, and belongs to the technical field of power system engineering. Background Art

[0002] A multi-terminal direct current transmission system is a direct current transmission system that is connected by three or more converter stations containing voltage source converters (VSC) in series, parallel or hybrid mode. Compared with the traditional two-terminal direct current transmission system, the multi-terminal direct current transmission system (MTDC) can coordinate and control the power between each converter station, has higher flexibility and reliability, and is more suitable for scenarios such as offshore wind power grid connection, urban power supply and asynchronous interconnection of AC systems.

[0003] With the continuous maturity of dynamic thermal rating (DTR) technology in software and hardware, it has gradually become an extension of the SCADA system, providing basic data for simulating the thermal characteristics of transmission components. In this context, some literature combines the transient thermal balance equation of overhead conductors with the power flow model, and proposes an electrothermal coupling power flow model of the time process, which can reveal the true load capacity contained in the thermal inertia process of the conductor. Based on the electrothermal coupling power flow model, some scholars have proposed a safety analysis method considering the transient thermal process of overhead conductors, and the severity of the expected accident is evaluated by the temperature of the overhead conductor and the allowable current over-limit time. Existing research mainly focuses on the fast calculation method of power system power flow and how to comprehensively evaluate the safety of the system, but the safety analysis method using the overhead conductor temperature as the line load limit in the AC / DC system containing VSC-MTDC needs to be studied. In addition, the overhead conductor temperature is used as the state variable of the power flow model, and the network parameters are functions of the temperature. The complexity of the power flow model increases, and multiple iterations are required to solve it. In order to meet the requirements of online safety analysis, it is necessary to further study the optimal power flow model solution method in the accuracy and speed of AC / DC system safety analysis. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention provides a static safety analysis method, system, equipment and medium for an AC / DC system containing VSC-MTDC based on linear power flow. Firstly, on the basis of the electrothermal coupling power flow model of the traditional AC system, an electrothermal coupling power flow model of the AC / DC system containing VSC-MTDC is constructed; then, the decoupling calculation of the electrical and thermal states of the system is realized; the AC and DC branch power flows are linearized by referring to the traditional power flow linearization idea, and a linearized power flow model of the AC / DC system containing VSC-MTDC is constructed with the AC node phase angle and the square of the voltage amplitude and the DC node voltage square as state variables; finally, a linearized power flow calculation method suitable for AC / DC systems containing VSC-MTDC and a power flow correction method taking into account branch losses are proposed, the power flow is quickly calculated and its accuracy is improved, and the system safety analysis warning information is output after the approximate analytical solution of the AC and DC overhead conductor temperatures is calculated.

[0005] The technical solution of the present invention is as follows:

[0006] A static safety analysis method for AC / DC systems containing VSC-MTDC based on linear power flow, the steps are as follows:

[0007] (1) Based on the equivalent circuit of the VSC converter station, a steady-state power model of the VSC converter station is constructed, and the control method of the VSC converter station and the DC power grid is given;

[0008] (2) Based on the traditional AC system electrothermal coupling power flow model, an electrothermal coupling power flow model of the AC / DC system containing VSC-MTDC is constructed, and then based on the degree of electrothermal coupling, the decoupling calculation of the system electrical (Equation (21) below) and thermal (Equation (23) below) states is realized;

[0009] (3) By drawing on the traditional power flow linearization idea, the AC and DC branch power flows are linearized, and the AC node phase angle and voltage amplitude square and DC node voltage square are used as state variables to construct a linear power flow model of the AC and DC system containing VSC-MTDC;

[0010] (4) Design a linearized power flow calculation method suitable for AC and DC systems containing VSC-MTDC and a power flow correction method taking into account branch losses, quickly calculate the power flow and improve its accuracy, calculate the approximate analytical solution of the AC and DC overhead conductor temperatures (Equation (23) below), and output system safety analysis warning information.

[0011] Preferably, according to the present invention, in step (1), the steady-state power model of the VSC converter station includes:

[0012]

[0013] In the formula, The power injected into the converter station for node s, i.e. the power at the head end of branch sf; and are the voltage amplitudes of the AC nodes s and f respectively; and are the corresponding node voltage phase angles,

[0014] 1 / z sf =g sf +jb sf , 1 / z sg =g sg +jb sg , z sf is the equivalent line impedance of the converter, z sg is the equivalent ground impedance of the converter transformer at the node f and the filter impedance z f The sum of g sf and b sf are the real and imaginary parts of the equivalent circuit admittance of the commutation transformer, g sg and b sg are the real and imaginary parts of the corresponding admittance, respectively;

[0015]

[0016] In the formula, Inject the active power of VSC into node c;

[0017] Active power loss of VSC a', b' and c' are the loss coefficients of VSC;

[0018] is the square of the current flowing through the commutation reactor, 1 / z c =g c +jb c is the commutation reactor admittance, z c is the impedance of the commutation reactor, g c and b c are the real and imaginary parts of the commutation reactor admittance, respectively. is the voltage amplitude of the AC node c, is the corresponding node voltage phase angle;

[0019] Active power injected into DC node d by VSC;

[0020]

[0021] Where P sfc,loss is the sum of the active losses of the converter transformer, filter and converter reactor, and is:

[0022]

[0023] In the formula, is the square of the current flowing through the converter transformer;

[0024]

[0025] Preferably, in step (1) of the present invention, the control method of the VSC converter station and the DC power grid is:

[0026] The control methods of VSC converter station are divided into two categories: active power control and reactive power control. Among them, active power control includes fixed AC side ( Figure 3 Active power control at midpoint s, #1 Control method, It is the AC active power control target value;

[0027] Constant DC side ( Figure 3 Point d) voltage control, #2 Control method, is the DC voltage control target value, is the voltage of DC node d;

[0028] DC voltage-power (VP) droop control, #3 control mode;

[0029] DC voltage-current (VI) droop control, #4 control mode;

[0030] The two droop control equations are expressed as:

[0031]

[0032] In the formula, the superscript ^ of the parameter represents the reference value of the corresponding control quantity. k is the DC current injected into the DC node d by the VSC; P and k I They are the droop coefficients of #3 control mode and #4 control mode respectively;

[0033] Reactive power control includes fixed-point reactive power control, #5 Control method, It is the AC reactive power control target value;

[0034] Fixed-point voltage control, #6 Control method, is the AC voltage control target value;

[0035] The DC grid contains multiple VSC converter stations, and its control modes include master-slave control and droop control. Under the two control modes:

[0036] The reactive power control mode of each VSC converter station is the #5 control mode or the #6 control mode. In the master-slave controlled DC power grid, the active power control mode of the VSC master station is set to the #2 control mode, and the active power control mode of other VSC slave stations is set to the #1 control mode. In the droop controlled DC power grid, there is at least one VSC converter station whose active power control mode is the #3 control mode or the #4 control mode, and the active power control mode of other VSC converter stations is selected from the #1 control mode, the #3 control mode or the #4 control mode.

[0037] Preferably, in step (2), the electrothermal coupled power flow model of the VSC-MTDC AC / DC system includes the heat balance equation of the AC and DC overhead conductors in the kth period, the DC system power flow equation and the AC system power flow equation, wherein the heat balance equation of the AC and DC overhead conductors in the kth period is expressed as:

[0038]

[0039] In the formula, the variable subscripts k and k-1 represent the kth and k-1th time periods respectively. and are the mismatch of the heat balance equation of the AC overhead conductor and the DC overhead conductor in the kth period (℃); are the conductor temperatures of AC overhead wire and DC overhead wire l respectively (℃); Δt is the discretization step size (s); and is the mass per unit length of AC overhead wire and DC overhead wire (kg / m), and is the specific heat capacity of AC overhead wire and DC overhead wire (J / (kg·℃)); is the Joule heat generated by the current carried by the wire per unit length (W / m); is the solar heat absorption per unit length of conductor (W / m); is the convective heat dissipation per unit length of wire (W / m); is the radiation heat dissipation per unit length of wire (W / m); n al and n dl are the numbers of AC overhead conductors and DC overhead lines, respectively; the superscripts ac and bc represent AC overhead conductors and DC overhead conductors, respectively.

[0040] According to the present invention, further preferably, in step (2), the DC system power flow equation is:

[0041]

[0042] In the formula, the number of DC nodes in the AC / DC system is n d , the number of VSC converter stations in #2 control mode is n dv(In droop controlled DC grid n dv =0; in a master-slave controlled DC grid n dv =1); and The set of VSC DC side nodes d in the VSC converter station of control mode #1, control mode #3 and control mode #4 respectively; is the set of non-VSC DC side nodes; is the mismatch of the power balance equation of DC node i in the kth period (pu); and are the generation and load active power (pu) of node i in the kth period, respectively; and are the voltages of nodes i and j at the kth time period (pu); is the element in the i-th row and j-th column of the DC grid node admittance matrix Y (pu). When considering the temperature effect of the conductor resistance, the elements in Y are functions of the conductor temperature. is the temperature vector of the DC overhead wire in the kth period, when i≠j, l∈(i,j), when i=j, T dc represents the temperature vector of the DC overhead wire connected to node i; The active power (pu) injected by VSC into node i in the kth period after the VSC DC side nodes d are uniformly numbered as i; from the perspective of the converter station v where node i is located, That is Figure 3 middle The following uses represents the active power (pu) injected into the DC node d by the VSC in the converter station v during the kth period, and A collection of VSC converter stations using control modes #1, #3 and #4 respectively.

[0043] According to the preferred embodiment of the present invention, in step (2), the AC system power flow equation includes equations (10) and (11):

[0044] Assume the number of communication nodes is n a , where the number of PQ type nodes is n pq , the set is The number of P-type nodes is n p , the set is The number of type 0 nodes is n z , the set is The number of PQV nodes is n pqv , the set is The number of PV type nodes is n pv , the set is

[0045]

[0046] In the formula, is the mismatch (pu) introduced into the active power flow equation for AC node i in time period k; and are the active generation power and active load power (pu) of node i respectively; and are the voltage amplitudes (pu) of AC nodes i and j respectively; and are the voltage phase angles (rad) at AC nodes i and j respectively, and are the corresponding elements (pu) in the ith row and jth column of the real part G and imaginary part B of the AC node admittance matrix respectively; is the AC overhead conductor temperature vector, when i≠j, l∈(i,j); when i=j, represents the temperature vector of the AC overhead line conductor connected to node i;

[0047] In order to uniformly number the VSC AC side nodes c as i, the active power (pu) injected into the VSC by the node i in the kth period, from the perspective of the converter station v where the node i is located, That is Figure 3 middle The following uses represents the active power (pu) injected into the VSC by node c in converter station v during the kth period, In order to clearly express the relationship between the power flow and the node voltage in the converter station, the active power equation of the branch sf at the converter station v on the right side of the third equation is adopted. Figure 3 The node identifiers s and f in the converter station are shown as subscripts instead of node numbers (the following description of the reactive power flow equations involving type 0 and type P nodes in the converter station also adopts a similar expression). is the control target value (pu) of the active power at the head end of branch sf by converter station v in the kth period under the #1 control mode; G sf,v and B sf,v are the corresponding elements (pu) in the real part G and imaginary part B of the AC node admittance matrix respectively;

[0048]

[0049] In the formula, is the mismatch (pu) introduced into the reactive power flow equation for AC node i in time period k; and are the reactive power generation and reactive load power (pu) of AC node i in period k, i is the number of node c in VSC converter station v, It is a VSC converter station set of #5 control mode; is the control target value (pu) of reactive power at the head end of branch sf by converter station v in the kth period under the #5 control mode, It is a collection of VSC converter stations of #6 control mode; is the control target value (pu) of the voltage amplitude of node s at converter station v in the kth period under the #6 control mode;

[0050] Preferably, according to the present invention, in step (3), the AC branch loss and power flow linear expressions are:

[0051]

[0052] Where i and j are the nodes at both ends of the AC branch ij; and are the AC voltage amplitudes (pu) at nodes i and j respectively; and are the AC voltage phase angles (rad) at nodes i and j respectively, g ij +jb ij is the admittance (pu) of branch ij (j represents the symbol of the imaginary part in mathematics); (j represents the imaginary part symbol in mathematics) is the power loss (pu) of branch ij; is the linearized AC branch power in period k (pu); and are the state variables of the AC system and the DC system in period k (pu);

[0053] The linear expressions of DC branch loss and power flow are:

[0054]

[0055]

[0056] Where i and j are the nodes at both ends of the DC branch ij; and are the DC voltages (pu) at nodes i and j respectively; is the resistance of the DC branch ij (pu); is the DC power at the first end of the DC branch ij (pu); is the power loss of the DC branch ij (pu); and They are the lossless power flow of DC branch ij and the linearized power flow (pu) of DC branch ij, respectively, and they are equal; and are the state variables of the AC system and the DC system (pu).

[0057] Preferably, according to the present invention, in step (3), the linear power flow expression of the DC system is:

[0058]

[0059] In the formula, is the known power injected into DC node i (pu);

[0060] The linear power flow expressions of the AC system include equations (17) and (18):

[0061]

[0062] In the formula, is the known active power (pu) introduced to AC node i in period k; is the set of balanced nodes of the AC system; G P is the square correlation coefficient matrix of AC voltage amplitude in the linearized AC active power flow equation (pu); B P is the AC voltage phase angle correlation coefficient matrix (pu) in the linearized AC active power flow equation;

[0063]

[0064] In the formula, is the known reactive power (pu) introduced to AC node i during period k; B Q is the AC voltage amplitude square correlation coefficient matrix (pu) in the linearized AC reactive power flow equation; G Q is the AC voltage phase angle correlation coefficient matrix (pu) in the linearized AC reactive power flow equation.

[0065] According to the present invention, further preferably, in step (3), the linearized power flow model of the AC / DC system is uniformly expressed as:

[0066]

[0067] In the formula, Y ac，p is the DC voltage square correlation coefficient matrix in the linearized AC active power flow, with dimension n a ×n d ; Y dc，pis the DC voltage square correlation coefficient matrix in the linearized DC power flow, with dimension n d ×n d ;

[0068] The linearized power flow model of AC / DC system is expressed in blocks as follows:

[0069]

[0070] Wherein, subscript R represents the set of AC balancing nodes; subscript G represents the set of AC generator nodes except AC balancing nodes; subscript L represents the set of AC nodes except AC generator nodes; subscript g represents the set of AC generator nodes and s nodes in the VSC converter station of #6 control mode except AC balancing nodes; subscript l represents the set of other AC nodes except AC balancing nodes and node set g; subscript r represents the set of node d in the VSC converter station of #2 control mode; subscript d represents the set of other DC nodes except node set r;

[0071] The linearized power flow equations of AC and DC systems with small dimension coefficient matrices are expressed as:

[0072]

[0073] According to the preferred embodiment of the present invention, step (4), the power flow correction method is: firstly, the state quantity in the linearized power flow model of the AC / DC system with a small dimension coefficient matrix is ​​used to calculate the loss, then the calculated loss is used as the known injection power to correct the AC / DC linear power flow equation, and finally the linear power flow model of the AC / DC system with the corrected dimension coefficient matrix is ​​solved;

[0074] The approximate model of the overhead conductor thermal balance equation is:

[0075]

[0076] In the formula, K l ′ and K l ″ are the first and second coefficients of the approximate model of the overhead conductor heat balance equation respectively;

[0077]

[0078] T l factor =[(T am +273) 2 +(T l +273) 2 ](T l +T am +546); ignore the changes of meteorological parameters in the thermal dynamic process of overhead wires. ijis the current flowing through the overhead wire ij (i.e. l);

[0079] The analytical solution of the approximate model of the overhead conductor thermal balance equation is:

[0080]

[0081] The static safety analysis system of AC / DC system containing VSC-MTDC based on linear power flow includes:

[0082] A VSC converter station steady-state power model building module is configured to build a VSC converter station steady-state power model according to an equivalent circuit of the VSC converter station, and provide a control method of the VSC converter station and a control method of the DC power grid;

[0083] The electro-thermal coupling power flow model module of the AC / DC system containing VSC-MTDC is configured to construct an electro-thermal coupling power flow model of the AC / DC system containing VSC-MTDC based on the electro-thermal coupling power flow model of the traditional AC system;

[0084] The linearized power flow model module of the AC / DC system containing VSC-MTDC is configured to linearize the AC and DC branch power flows by referring to the traditional power flow linearization idea, and to construct the linearized power flow model of the AC / DC system containing VSC-MTDC by taking the AC node phase angle and the square of the voltage amplitude and the DC node voltage square as state variables;

[0085] The linear power flow calculation and correction module is configured to design a linearized power flow calculation method and a power flow correction method taking into account branch losses suitable for AC and DC systems containing VSC-MTDC, quickly calculate the power flow and improve its accuracy, calculate the approximate analytical solution of the AC and DC overhead conductor temperatures, and then output system safety analysis warning information.

[0086] An electronic device includes a memory and a processor, and computer instructions stored in the memory and running on the processor. When the computer instructions are run by the processor, the above-mentioned static safety analysis method of AC and DC systems containing VSC-MTDC based on linear power flow is completed.

[0087] A computer-readable storage medium is used to store computer instructions. When the computer instructions are executed by a processor, the above-mentioned static safety analysis method of AC and DC systems containing VSC-MTDC based on linear power flow is completed.

[0088] The beneficial effects of the present invention are:

[0089] Due to the asynchronous characteristics of the line current lagging behind the temperature, under certain initial conditions, the line current may exceed the limit while the line temperature does not exceed the limit in a short time. At this time, if the steady-state current of the overhead wire is used as the line load constraint, the output safety alarm information may be conservative and cannot truly reflect the severity of the expected accident. Therefore, using the overhead wire temperature to display the load capacity of the line can provide thermal dynamic information of the overhead wire, which has higher accuracy in evaluating the safety of disturbances in the thermal dynamic process and the overload of the power system.

[0090] The proposed safety analysis method reflects the thermal limitation nature of line overload, comprehensively describes the change process of load state, and more realistically evaluates the severity of anticipated accidents, thereby providing strong support for accurately guiding dispatching to take safety control measures, which is conducive to the safe and economic control of AC / DC systems containing VSC-MTDC. A fast calculation method of electric and thermal state based on linear power flow is proposed to meet the requirements of online safety analysis of AC / DC systems containing VSC-MTDC under anticipated accidents. BRIEF DESCRIPTION OF THE DRAWINGS

[0091] Figure 1 It is a schematic diagram of the process of the present invention;

[0092] Figure 2 A structural diagram of a VSC converter station provided in Example 1 of the present invention;

[0093] Figure 3 An equivalent circuit diagram of a VSC converter station provided in Embodiment 1 of the present invention;

[0094] Figure 4 A static security analysis flow chart provided in Example 1 of the present invention;

[0095] Figure 5 An equivalent circuit diagram of the AC branch power flow provided in Embodiment 1 of the present invention;

[0096] Figure 6 An equivalent circuit diagram of the DC branch power flow provided in Embodiment 1 of the present invention;

[0097] Figure 7 A system diagram of a transformation of an IEEE39 node including an 8-terminal VSC converter station provided in Example 1 of the present invention;

[0098] Figure 8 A DC power grid diagram including an 8-terminal VSC converter station provided in Example 1 of the present invention;

[0099] Fig. 9 Current and temperature diagrams of key circuits under three anticipated accidents provided in Example 1 of the present invention. DETAILED DESCRIPTION

[0100] The present invention will be further described below by way of embodiments in conjunction with the accompanying drawings, but is not limited thereto.

[0101] Embodiment 1:

[0102] This embodiment provides a static safety analysis method for AC / DC systems containing VSC-MTDC based on linear power flow. Figure 1 As shown, including:

[0103] (1) Based on the equivalent circuit of the VSC converter station, a steady-state power model of the VSC converter station is constructed, and the control method of the VSC converter station and the control method of the DC power grid are given;

[0104] The steady-state power model of the VSC converter station includes:

[0105]

[0106] In the formula, The power injected into the converter station for node s, i.e. the power at the head end of branch sf; and are the voltage amplitudes of the AC nodes s and f respectively; and are the corresponding node voltage phase angles,

[0107] 1 / z sf =g sf +jb sf , 1 / z sg =g sg +jb sg , z sf is the equivalent line impedance of the converter, z sg is the equivalent ground impedance of the converter transformer at the node f and the filter impedance z f The sum of g sf and b sf are the real and imaginary parts of the equivalent circuit admittance of the commutation transformer, g sg and b sg are the real and imaginary parts of the corresponding admittance, respectively;

[0108]

[0109] In the formula, Inject the active power of VSC into node c;

[0110] Active power loss of VSC a', b' and c' are the loss coefficients of VSC;

[0111] is the square of the current flowing through the commutation reactor, 1 / z c =g c+jb c is the commutation reactor admittance, z c is the impedance of the commutation reactor, g c and b c are the real and imaginary parts of the commutation reactor admittance, respectively. is the voltage amplitude of the AC node c, is the corresponding node voltage phase angle;

[0112] Active power injected into DC node d by VSC;

[0113]

[0114] Where P sfc,loss is the sum of the active losses of the converter transformer, filter and converter reactor, and is:

[0115]

[0116] In the formula, is the square of the current flowing through the converter transformer;

[0117]

[0118] The control method of VSC converter station and DC grid is:

[0119] The control methods of VSC converter station are divided into two categories: active power control and reactive power control. Among them, active power control includes fixed AC side ( Figure 3 Active power control at midpoint s, #1 Control method, It is the AC active power control target value;

[0120] Constant DC side ( Figure 3 Point d) voltage control, #2 Control method, is the DC voltage control target value, is the voltage of DC node d;

[0121] DC voltage-power (VP) droop control, #3 control mode;

[0122] DC voltage-current (VI) droop control, #4 control mode;

[0123] The two droop control equations are expressed as:

[0124]

[0125] In the formula, the superscript ^ of the parameter represents the reference value of the corresponding control quantity. k is the DC current injected into the DC node d by the VSC; P and k I They are the droop coefficients of #3 control mode and #4 control mode respectively;

[0126] Reactive power control includes fixed-point reactive power control, #5 Control method, It is the AC reactive power control target value;

[0127] Fixed-point voltage control, #6 Control method, is the AC voltage control target value;

[0128] The DC grid contains multiple VSC converter stations, and its control modes include master-slave control and droop control. Under the two control modes:

[0129] The reactive power control mode of each VSC converter station is the #5 control mode or the #6 control mode. In the master-slave controlled DC power grid, the active power control mode of the VSC master station is set to the #2 control mode, and the active power control mode of other VSC slave stations is set to the #1 control mode. In the droop controlled DC power grid, there is at least one VSC converter station whose active power control mode is the #3 control mode or the #4 control mode, and the active power control mode of other VSC converter stations is selected from the #1 control mode, the #3 control mode or the #4 control mode.

[0130] (2) Based on the electrothermal coupling power flow model of the traditional AC system, an electrothermal coupling power flow model of the AC / DC system containing VSC-MTDC is constructed, and then the decoupling calculation of the electrical and thermal states of the system is realized based on the degree of electrothermal coupling.

[0131] The electrothermal coupled power flow model of the AC / DC system containing VSC-MTDC includes the thermal balance equation of the AC and DC overhead conductors in the kth period, the DC system power flow equation and the AC system power flow equation. The thermal balance equation of the AC and DC overhead conductors in the kth period is expressed as:

[0132]

[0133] In the formula, the variable subscripts k and k-1 represent the kth and k-1th time periods respectively. and are the mismatch of the heat balance equation of the AC overhead conductor and the DC overhead conductor in the kth period (℃); are the conductor temperatures of AC overhead wire and DC overhead wire l respectively (℃); Δt is the discretization step size (s); and is the mass per unit length of AC overhead wire and DC overhead wire (kg / m), and is the specific heat capacity of AC overhead wire and DC overhead wire (J / (kg·℃)); is the Joule heat generated by the current carried by the wire per unit length (W / m); is the solar heat absorption per unit length of conductor (W / m); is the convective heat dissipation per unit length of wire (W / m); is the radiation heat dissipation per unit length of wire (W / m); n al and n dl are the numbers of AC overhead conductors and DC overhead lines, respectively; the superscripts ac and bc represent AC overhead conductors and DC overhead conductors, respectively.

[0134] The DC system power flow equation is:

[0135]

[0136] In the formula, the number of DC nodes in the AC / DC system is n d , the number of VSC converter stations in #2 control mode is n dv (In droop controlled DC grid n dv =0; in a master-slave controlled DC grid n dv =1); and The set of VSC DC side nodes d in the VSC converter station of control mode #1, control mode #3 and control mode #4 respectively; is the set of non-VSC DC side nodes; is the mismatch of the power balance equation of DC node i in the kth period (pu); and are the generation and load active power (pu) of node i in the kth period, respectively; and are the voltages of nodes i and j at the kth time period (pu); is the element in the i-th row and j-th column of the DC grid node admittance matrix Y (pu). When considering the temperature effect of the conductor resistance, the elements in Y are functions of the conductor temperature. is the temperature vector of the DC overhead wire in the kth period, when i≠j, l∈(i,j), when i=j, T dc represents the temperature vector of the DC overhead wire connected to node i; The active power (pu) injected by VSC into node i in the kth period after the VSC DC side nodes d are uniformly numbered as i; from the perspective of the converter station v where node i is located, That is Figure 3 middle The following uses represents the active power (pu) injected into the DC node d by the VSC in the converter station v during the kth period, and A collection of VSC converter stations using control modes #1, #3 and #4 respectively.

[0137] The AC system power flow equations include equations (10) and (11):

[0138] Assume the number of communication nodes is n a , where the number of PQ type nodes is n pq , the set is The number of P-type nodes is n p , the set is The number of type 0 nodes is n z , the set is The number of PQV nodes is n pqv , the set is The number of PV type nodes is n pv , the set is

[0139]

[0140] In the formula, is the mismatch (pu) introduced into the active power flow equation for AC node i in time period k; and are the active generation power and active load power (pu) of node i respectively; and are the voltage amplitudes (pu) of AC nodes i and j respectively; and are the voltage phase angles (rad) at AC nodes i and j respectively, and are the corresponding elements (pu) in the ith row and jth column of the real part G and imaginary part B of the AC node admittance matrix respectively; is the AC overhead conductor temperature vector, when i≠j, l∈(i,j); when i=j, represents the temperature vector of the AC overhead line conductor connected to node i;

[0141] In order to uniformly number the VSC AC side nodes c as i, the active power (pu) injected into the VSC by the node i in the kth period, from the perspective of the converter station v where the node i is located, That is Figure 3 middle The following uses represents the active power (pu) injected into the VSC by node c in converter station v during the kth period, In order to clearly express the relationship between the power flow and the node voltage in the converter station, the active power equation of the branch sf at the converter station v on the right side of the third equation is adopted. Figure 3 The node identifiers s and f in the converter station are shown as subscripts instead of node numbers (the following description of the reactive power flow equations involving type 0 and type P nodes in the converter station also adopts a similar expression). is the control target value (pu) of the active power at the head end of branch sf by converter station v in the kth period under the #1 control mode; G sf,v and B sf,v are the corresponding elements (pu) in the real part G and imaginary part B of the AC node admittance matrix respectively;

[0142]

[0143] In the formula, is the mismatch (pu) introduced into the reactive power flow equation for AC node i in time period k; and are the reactive power generation and reactive load power (pu) of AC node i in period k, i is the number of node c in VSC converter station v, It is a VSC converter station set of #5 control mode; is the control target value (pu) of reactive power at the head end of branch sf by converter station v in the kth period under the #5 control mode, It is a collection of VSC converter stations of #6 control mode; is the control target value (pu) of the voltage amplitude of node s at converter station v in the kth period under the #6 control mode;

[0144] (3) Drawing on the traditional power flow linearization idea, the AC and DC branch power flows are linearized, and the linearized power flow model of the AC and DC system containing VSC-MTDC is constructed with the AC node phase angle and voltage amplitude square and the DC node voltage square as state variables.

[0145] The linear expressions of AC branch loss and power flow are:

[0146]

[0147] Where i and j are the nodes at both ends of the AC branch ij; and are the AC voltage amplitudes (pu) at nodes i and j respectively; and are the AC voltage phase angles (rad) at nodes i and j respectively, g ij +jb ijis the admittance (pu) of branch ij (j represents the symbol of the imaginary part in mathematics); (j represents the imaginary part symbol in mathematics) is the power loss (pu) of branch ij; is the linearized AC branch power in period k (pu); and are the state variables of the AC system and the DC system in period k (pu);

[0148] The linear expressions of DC branch loss and power flow are:

[0149]

[0150] Where i and j are the nodes at both ends of the DC branch ij; and are the DC voltages (pu) at nodes i and j respectively; is the resistance of the DC branch ij (pu); is the DC power at the first end of the DC branch ij (pu); is the power loss of the DC branch ij (pu); and They are the lossless power flow of DC branch ij and the linearized power flow (pu) of DC branch ij, respectively, and they are equal; and are the state variables of the AC system and the DC system (pu).

[0151] The linear power flow expression of DC system is:

[0152]

[0153] In the formula, is the known power injected into DC node i (pu); the linear power flow expression of the AC system includes equations (17) and (18):

[0154]

[0155] In the formula, is the known active power (pu) introduced to AC node i in period k; is the set of balanced nodes of the AC system; G P is the square correlation coefficient matrix of AC voltage amplitude in the linearized AC active power flow equation (pu); B P is the AC voltage phase angle correlation coefficient matrix (pu) in the linearized AC active power flow equation;

[0156]

[0157] In the formula, is the known reactive power (pu) introduced to AC node i in period k, B Q is the AC voltage amplitude square correlation coefficient matrix (pu) in the linearized AC reactive power flow equation; G Q is the AC voltage phase angle correlation coefficient matrix (pu) in the linearized AC reactive power flow equation.

[0158] The linearized power flow model of AC / DC system is uniformly expressed as:

[0159]

[0160] In the formula, Y ac，p is the DC voltage square correlation coefficient matrix in the linearized AC active power flow, with dimension n a ×n d ; Y dc，p is the DC voltage square correlation coefficient matrix in the linearized DC power flow, with dimension n d ×n d ;

[0161] The linearized power flow model of AC / DC system is expressed in blocks as follows:

[0162]

[0163] Wherein, subscript R represents the set of AC balancing nodes; subscript G represents the set of AC generator nodes except AC balancing nodes; subscript L represents the set of AC nodes except AC generator nodes; subscript g represents the set of AC generator nodes and s nodes in the VSC converter station of #6 control mode except AC balancing nodes; subscript l represents the set of other AC nodes except AC balancing nodes and node set g; subscript r represents the set of node d in the VSC converter station of #2 control mode; subscript d represents the set of other DC nodes except node set r;

[0164] The linearized power flow equations of AC and DC systems with small dimension coefficient matrices are expressed as:

[0165]

[0166] (4) A linearized power flow calculation method suitable for AC and DC systems containing VSC-MTDC and a power flow correction method taking into account branch losses are proposed to quickly calculate the power flow and improve its accuracy. After calculating the approximate analytical solution of the AC and DC overhead conductor temperatures, system safety analysis warning information is output.

[0167] The power flow correction method is: firstly, the state quantity in the linearized power flow model of the AC / DC system with a small dimension coefficient matrix is ​​used to calculate the loss, then the calculated loss is used as the known injection power to correct the AC / DC linear power flow equation, and finally the linear power flow model of the AC / DC system with the corrected dimension coefficient matrix is ​​solved;

[0168] The approximate model of the overhead conductor thermal balance equation is:

[0169]

[0170] In the formula, K l ′ and K l ″ are the first and second coefficients of the approximate model of the overhead conductor heat balance equation respectively;

[0171]

[0172] T l factor =[(T am +273) 2 +(T l +273) 2 ](T l +T am +546); ignore the changes of meteorological parameters in the thermal dynamic process of overhead wires. ij is the current flowing through the overhead wire ij (i.e. l);

[0173] The analytical solution of the approximate model of the overhead conductor thermal balance equation is:

[0174]

[0175] Static safety analysis process of AC and DC systems:

[0176] The static safety analysis process of AC / DC system including VSC-MTDC considering the thermal characteristics of overhead conductors is shown in Figure 4 ;

[0177] exist Figure 4 In the example, w is the expected accident number, n is outageis the number of expected accidents. Steps ① to ② are the initialization modules of static safety analysis, which provide data basis for power flow calculation. Step ① obtains the topology and parameters of the power system, as well as the initial state, including the parameters of each branch, node number, grounding admittance, node type, node voltage and overhead conductor temperature at the initial time. The overhead conductor temperature can be obtained through DTR equipment or calculated by regional weather forecast data. Step ② obtains meteorological and load forecast data, as well as power generation plans. The load forecast data and power generation plans can be directly obtained through the load forecast and automatic power generation control function modules in the energy management system. The meteorological forecast data can be obtained according to the weather forecast provided by the regional meteorological station. Step ③ provides data support for static safety analysis by quickly calculating the power system power flow and overhead conductor thermal state under the expected accidents during the study period. The fast calculation method is the fast calculation method of electric and thermal state based on linear power flow proposed in this example. Finally, it is determined whether there is a line load or voltage over limit in the power system, and an alarm message is output. Repeat the above steps until all expected accidents are traversed.

[0178] Fast calculation method of electric and thermal state based on linear power flow:

[0179] The AC / DC power flow distribution including VSC-MTDC is limitedly affected by the temperature of overhead conductors. The temperature of overhead conductors has a great impact on the power grid loss, but the loss of overhead conductors is still much smaller than the power flow it transmits. The influence of overhead conductor temperature on the power flow of AC / DC systems is limited. In summary, under the premise of meeting the data accuracy of engineering applications, the AC / DC power flow model and the AC / DC overhead conductor thermal balance equation can be solved independently, and then the power flow linearization technology, the power flow correction technology taking into account line losses and the approximate calculation method of the thermal state of overhead conductors are used to quickly calculate the electrical and thermal states of the AC / DC systems to meet the speed requirements of online safety assessment.

[0180] The following is a specific example analysis:

[0181] This section includes the IEEE39 node transformation system of the 8-terminal VSC converter station (see Figure 7 ) is taken as an example to verify the effectiveness of the safety control method proposed in this embodiment, where a DC power grid with 8-terminal VSC converter stations is shown in Figure 8 , the DC power grid adopts the master-slave control mode, the control parameters are shown in Table 1, t = 0min, 34 AC overhead wire branches and 12 DC overhead wire branches and 7 VSC converter stations (except VSC7 converter station) are disconnected one by one (fault-cutting protection action) in turn, a total of 53 expected accidents are analyzed for the system under the above expected accidents, and the effectiveness of the method proposed in this embodiment is verified. The research period of this section is 30min, and the environmental parameters are shown in Table 2; the IEEE 39-node AC system topology is shown in Figure 7 ,in The icons of VSC1~VSC8 converter stations, the DC grid topology is shown in Figure 8 ;

[0182] Table 1: VSC converter station control parameters under master-slave control

[0183]

[0184] Table 2: Meteorological environment data from 0 to 30 minutes

[0185]

[0186] 1) Algorithm Analysis

[0187] First, the accuracy of the algorithm proposed in this example is verified. Different algorithms (Table 3 is a description of different algorithms) are used to solve the power flow and overhead conductor thermal state under all expected accidents in the study period. The state solved by algorithm I is used as the reference state, and compared with the states solved by algorithms II and III. Table 7 shows the errors between the node voltage and overhead conductor temperature of all expected accidents in the study period and the reference state.

[0188] Table 3: Description of different algorithms

[0189]

[0190] Table 4: Errors of node voltage and overhead conductor temperature under anticipated accidents compared with the reference state

[0191]

[0192]

[0193] As shown in Table 4, the average error and maximum error of the node voltage amplitude solved by Algorithm II are both more than 10 -2 pu, while the order of magnitude of the solutions obtained by Algorithm III is 10 -3 pu is one order of magnitude smaller than the former. This is because Algorithm II ignores the line loss, of which the maximum percentage of the reactive power loss ignored by the line is 16.8%, and the maximum percentage of the active power loss ignored by the line is 5.2%; the maximum error of the overhead conductor temperature solved by Algorithm II is 6.2% of the base state (the temperature of the overhead conductor is 74.1℃), while that solved by Algorithm III is 1.3% of the base state. Compared with the base state, the safety analysis results solved by Algorithm II have a large deviation, while Algorithm III, which takes into account the branch loss, significantly improves the calculation results of the power flow and the thermal state of the overhead conductor, and has a higher accuracy:

[0194] Table 5: Average time for safety analysis of a single anticipated accident

[0195]

[0196] Table 5 shows the average calculation time of different algorithms for solving the power flow and overhead conductor thermal state under each expected accident. In a period of time, algorithm I needs to solve the nonlinear electrothermal coupling power flow equation multiple times until it converges to a given accuracy. Algorithm III needs to solve the linearized AC / DC power flow twice and the overhead conductor thermal state once. Algorithm III needs to solve the linearized AC / DC power flow once and the overhead conductor thermal state once, so their average calculation time decreases successively. The average calculation time of algorithm III is reduced by 97% compared with algorithm I, accounting for 217.2% of algorithm II, but algorithm III can provide higher-precision safety analysis results than algorithm II, avoiding the output of conservative warning information.

[0197] In summary, under the premise of ensuring the accuracy of the calculation results of the AC and DC system power flow and the thermal state of the overhead conductors under the anticipated accidents, Algorithm III solves the equations less times, which improves the speed of safety analysis. Therefore, the calculation results of Algorithm III can be used instead of those of Algorithm I to meet the speed requirements of online safety analysis.

[0198] 2) Safety analysis results

[0199] The safety analysis results of this example are obtained by solving Algorithm III. The results of three expected accidents, namely, disconnecting the AC system branch 5-8, decommissioning the VSC6 converter station, and disconnecting the DC branch 7-8, are analyzed. In the safety analysis results, except for the voltage amplitude of the AC generator node 36 being 1.063pu, no voltage over-limit situation occurs in all the expected accidents. Fig. 9 are the current and temperature of the key lines under three anticipated accidents, where the key lines are the lines with the most serious load in the AC and DC systems during the study period;

[0200] analyze Fig. 9It can be seen that after disconnecting the AC system branch 5-8, the key line AC overhead conductor 6-7 was overloaded: the current exceeded its maximum allowable current during 15 to 30 minutes, and the temperature of the key line AC overhead conductor 6-7 lagged behind the change of its current. It exceeded its maximum allowable temperature of 70°C about 6 minutes after the current exceeded the limit (i.e. 21 minutes, when the temperature was 70.3°C). This is because after the line was disconnected, the power flow of AC branch 5-8 was transferred to AC branch 5-6, and then passed through AC branches 6-7 and 7-8 to supply power to AC load node 8. At this time, the load of AC branch 6-7 was superimposed on the original load flow transfer after the line was disconnected, which increased the load of AC branch 6-7. The control parameters of VSC6 from the converter station (Pac, ref s = 0.5pu, Qac, ref s = 0.06pu) are small in value. Even if it is out of operation, it will not cause excessive power transfer and overload. After disconnecting the DC system branch 7-8, the photovoltaic power originally transmitted by the DC overhead wires 7-8 and 7-10 to the VSC7 main converter station for power balance was changed to only the DC overhead wire 7-10 for transmission. In addition, the photovoltaic power generation was large during this period, resulting in a high load degree of the DC overhead wire 7-10. Even during the period of 10 to 15 minutes, its current of 4.15pu exceeded the maximum allowable current of 4.06pu. However, due to the characteristic that the current lags behind the temperature of the overhead wire, its temperature gradually approached 70℃ during this period and did not exceed its maximum allowable temperature. In addition, the subsequent photovoltaic power generation decreased, and the line overcurrent and temperature limit did not occur. It can be seen that in the safety analysis, whether the line is overloaded can be determined by the temperature or current of the overhead wire, which can reflect the load constraint of the line. However, determining whether the line is overloaded by the temperature of the overhead wire and utilizing the thermal inertia of the overhead wire (the characteristic of temperature lags behind the current) can provide a more realistic safe operation period and overload situation within the safety analysis time window for safety control, so as to prevent conservative safety control decisions, which is conducive to the economic safety control of the power system.

[0201] Example 2

[0202] This embodiment provides a safety control system for an AC / DC system including a VSC-MTDC under electric-heat coordination, including:

[0203] A VSC converter station steady-state power model building module is configured to build a VSC converter station steady-state power model according to an equivalent circuit of the VSC converter station, and provide a control method of the VSC converter station and a control method of the DC power grid;

[0204] A VSC converter station steady-state power model building module is configured to build a VSC converter station steady-state power model according to an equivalent circuit of the VSC converter station, and provide a control method of the VSC converter station and a control method of the DC power grid;

[0205] The electro-thermal coupling power flow model module of the VSC-MTDC AC / DC system is configured to construct an electro-thermal coupling power flow model of the VSC-MTDC AC / DC system based on the electro-thermal coupling power flow model of the traditional AC system, and then realize the decoupling calculation of the electrical and thermal states of the system based on the degree of electro-thermal coupling.

[0206] The linearized power flow model module of the AC / DC system containing VSC-MTDC is configured to linearize the AC and DC branch power flows based on the traditional power flow linearization idea, and to construct a linearized power flow model of the AC / DC system containing VSC-MTDC using the AC node phase angle and the square of the voltage amplitude and the DC node voltage square as state variables.

[0207] The linear power flow calculation and correction module is configured to propose a linearized power flow calculation method suitable for AC and DC systems containing VSC-MTDC and a power flow correction method taking into account branch losses, quickly calculate the power flow and improve its accuracy, calculate the approximate analytical solution of the AC and DC overhead conductor temperatures, and then output system safety analysis warning information.

[0208] It should be noted that the above modules correspond to the steps described in Example 1, and the examples and application scenarios implemented by the above modules and the corresponding steps are the same, but are not limited to the contents disclosed in the above Example 1. It should be noted that the above modules, as part of the system, can be executed in a computer system such as a set of computer executable instructions.

[0209] In further embodiments, there is also provided:

[0210] An electronic device includes a memory and a processor, and computer instructions stored in the memory and executed on the processor, wherein when the computer instructions are executed by the processor, the method described in Embodiment 1 is performed. For the sake of brevity, it will not be described in detail here.

[0211] It should be understood that in this embodiment, the processor may be a central processing unit CPU, and the processor may also be other general-purpose processors, digital signal processors DSP, application-specific integrated circuits ASIC, off-the-shelf programmable gate arrays FPGA or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor may be a microprocessor or the processor may also be any conventional processor, etc.

[0212] The memory may include a read-only memory and a random access memory, and provide instructions and data to the processor. A portion of the memory may also include a non-volatile random access memory. For example, the memory may also store information about the device type.

[0213] A computer-readable storage medium is used to store computer instructions, and when the computer instructions are executed by a processor, the method described in Example 1 is completed.

[0214] The method in Example 1 can be directly embodied as a hardware processor, or a combination of hardware and software modules in the processor. The software module can be located in a mature storage medium in the field such as a random access memory, a flash memory, a read-only memory, a programmable read-only memory, or an electrically erasable programmable memory, a register, etc. The storage medium is located in the memory, and the processor reads the information in the memory and completes the steps of the above method in combination with its hardware. To avoid repetition, it is not described in detail here.

[0215] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. A static safety analysis method for AC / DC systems containing VSC-MTDC based on linear power flow, characterized in that: Here are the steps: (1) Based on the equivalent circuit of the VSC converter station, a steady-state power model of the VSC converter station is constructed, and the control method of the VSC converter station and the DC power grid is given; (2) Based on the traditional AC system electrothermal coupling power flow model, an electrothermal coupling power flow model of the AC / DC system containing VSC-MTDC is constructed; (3) By drawing on the traditional power flow linearization idea, the AC and DC branch power flows are linearized, and the AC node phase angle and voltage amplitude square and DC node voltage square are used as state variables to construct a linear power flow model of the AC and DC system containing VSC-MTDC; (4) Design a linearized power flow calculation method and a power flow correction method taking into account branch losses applicable to AC and DC systems containing VSC-MTDC, calculate the approximate analytical solution for the AC and DC overhead conductor temperatures, and output system safety analysis warning information.

2. The method for static safety analysis of AC / DC systems containing VSC-MTDC based on linear power flow according to claim 1 is characterized in that: In step (1), the steady-state power model of the VSC converter station includes: In the formula, The power injected into the converter station for node s, i.e. the power at the head end of branch sf; and are the voltage amplitudes of the AC nodes s and f respectively; and are the corresponding node voltage phase angles, 1 / z sf =g sf +jb sf , 1 / z sg =g sg +jb sg , z sf is the equivalent line impedance of the converter, z sg is the equivalent ground impedance of the converter transformer at the node f and the filter impedance z f The sum of g sf and b sf are the real and imaginary parts of the equivalent circuit admittance of the commutation transformer, g sg and b sg are the real and imaginary parts of the corresponding admittance, respectively; In the formula, Inject the active power of VSC into node c; Active power loss of VSC a', b' and c' are the loss coefficients of VSC; is the square of the current flowing through the commutation reactor, 1 / z c =g c +jb c is the commutation reactor admittance, z c is the impedance of the commutation reactor, g c and b c are the real and imaginary parts of the commutation reactor admittance, respectively. is the voltage amplitude of the AC node c, is the corresponding node voltage phase angle; Active power injected into DC node d by VSC; Where P sfc,loss is the sum of the active losses of the converter transformer, filter and converter reactor, and is: In the formula, is the square of the current flowing through the converter transformer; The control method of VSC converter station and DC grid is: The control methods of VSC converter station are divided into two categories: active power control and reactive power control. Among them, active power control includes active power control on the fixed AC side. #1 Control method, It is the AC active power control target value; Constant DC side voltage control, #2 Control method, is the DC voltage control target value, is the voltage of DC node d; DC voltage-power droop control, #3 control mode; DC voltage-current droop control, #4 control mode; The two droop control equations are expressed as: In the formula, the superscript ^ of the parameter represents the reference value of the corresponding control quantity. k is the DC current injected into the DC node d by the VSC; P and k I They are the droop coefficients of #3 control mode and #4 control mode respectively; Reactive power control includes fixed-point reactive power control, #5 Control method, It is the AC reactive power control target value; Fixed-point voltage control, #6 Control method, is the AC voltage control target value; The DC grid contains multiple VSC converter stations, and its control modes include master-slave control and droop control. Under the two control modes: The reactive power control mode of each VSC converter station is the #5 control mode or the #6 control mode. In the master-slave controlled DC power grid, the active power control mode of the VSC master station is set to the #2 control mode, and the active power control mode of other VSC slave stations is set to the #1 control mode. In the droop controlled DC power grid, there is at least one VSC converter station whose active power control mode is the #3 control mode or the #4 control mode, and the active power control mode of other VSC converter stations is selected from the #1 control mode, the #3 control mode or the #4 control mode.

3. The static safety analysis method of AC / DC system containing VSC-MTDC based on linear power flow according to claim 2 is characterized in that: In step (2), the electrothermal coupled power flow model of the AC / DC system containing the VSC-MTDC includes the heat balance equation of the AC and DC overhead conductors in the kth period, the DC system power flow equation and the AC system power flow equation, wherein the heat balance equation of the AC and DC overhead conductors in the kth period is expressed as: In the formula, the variable subscripts k and k-1 represent the kth and k-1th time periods respectively. and are the mismatches of the heat balance equations of the AC overhead conductor and the DC overhead conductor in the kth period respectively; are the conductor temperatures of AC overhead conductor and DC overhead conductor l respectively; Δt is the discretization step size; and is the mass per unit length of AC overhead wire and DC overhead wire, and is the specific heat capacity of AC overhead wire and DC overhead wire; The Joule heat generated by the current carried by the wire per unit length; The amount of heat absorbed by the conductor per unit length due to sunlight; is the convective heat dissipation per unit length of wire; is the radiation heat dissipation per unit length of wire; n al and n dl are the numbers of AC overhead conductors and DC overhead lines, respectively; the superscripts ac and bc represent AC overhead conductors and DC overhead conductors, respectively.

4. The method for static safety analysis of AC / DC systems containing VSC-MTDC based on linear power flow according to claim 3 is characterized in that: In step (2), the DC system power flow equation is: In the formula, the number of DC nodes in the AC / DC system is n d , the number of VSC converter stations in #2 control mode is n dv ; and The set of VSC DC side nodes d in the VSC converter station of control mode #1, control mode #3 and control mode #4 respectively; is the set of non-VSC DC side nodes; is the mismatch of the power balance equation of DC node i in the kth period (pu); and are the generation and load active powers of node i in the kth period respectively; and are the voltages of nodes i and j at the kth time period respectively; is the element in the i-th row and j-th column of the DC grid node admittance matrix Y; is the temperature vector of the DC overhead wire in the kth period, when i≠j, When i=j, T dc represents the temperature vector of the DC overhead wire connected to node i; The active power injected by VSC into node i in the kth period after the VSC DC side node d is uniformly numbered as i; represents the active power (pu) injected into the DC node d by the VSC in the converter station v during the kth period, and A collection of VSC converter stations using control modes #1, #3 and #4 respectively.

5. The method for static safety analysis of AC / DC systems containing VSC-MTDC based on linear power flow according to claim 4 is characterized in that: In step (2), the AC system power flow equation includes equations (10) and (11): Assume the number of communication nodes is n a , where the number of PQ type nodes is n pq , the set is The number of P-type nodes is n p , the set is The number of type 0 nodes is n z , the set is The number of PQV nodes is n pqv , the set is The number of PV type nodes is n pv , the set is In the formula, The mismatch amount introduced into the active power flow equation for AC node i in time period k; and are the active generation power and active load power of node i respectively; and are the voltage amplitudes of AC nodes i and j respectively; and are the voltage phase angles of AC nodes i and j respectively, and are the corresponding elements of row i and column j in the real part G and imaginary part B of the AC node admittance matrix respectively; is the AC overhead conductor temperature vector, when i≠j, When i=j, represents the temperature vector of the AC overhead line conductor connected to node i; The active power (pu) injected into the VSC by node i in the kth period after the VSC AC side node c is uniformly numbered as i; represents the active power (pu) injected into the VSC by node c in converter station v during the kth period, G is the control target value of the active power at the head end of branch sf at converter station v in the kth period under the #1 control mode; sf,v and B sf,v are the corresponding elements (pu) in the real part G and imaginary part B of the AC node admittance matrix respectively; In the formula, The mismatch amount introduced into the reactive power flow equation for AC node i in time period k; and are the reactive power generation and reactive load power of AC node i in period k, i is the number of node c in VSC converter station v, It is a VSC converter station set of #5 control mode; is the control target of reactive power at the head end of branch sf by converter station v in the kth period under the #5 control mode, It is a collection of VSC converter stations of #6 control mode; is the control target value of the voltage amplitude of node s at converter station v in the kth period under the #6 control mode; 6. The method for static safety analysis of AC / DC systems containing VSC-MTDC based on linear power flow according to claim 5 is characterized in that: In step (3), the linear expressions of AC branch loss and power flow are: Where i and j are the nodes at both ends of the AC branch ij; and are the AC voltage amplitudes at nodes i and j respectively; and are the AC voltage phase angles (rad) at nodes i and j respectively, g ij +jb ij is the admittance of branch ij; is the power loss of branch ij; is the linearized AC branch power in time period k; and are the state variables of the AC system and the DC system during period k; The linear expressions of DC branch loss and power flow are: Where i and j are the nodes at both ends of the DC branch ij; and are the DC voltages at nodes i and j respectively; is the resistance of the DC branch ij; is the DC power at the first end of the DC branch ij; is the power loss of the DC branch ij; and P They are the lossless power flow of DC branch ij and the linearized power flow of DC branch ij, which are equal; and are the state variables of the AC system and the DC system; Preferably, in step (3), the linear power flow expression of the DC system is: In the formula, is the known power injected into DC node i; The linear power flow expressions of the AC system include equations (17) and (18): In the formula, is the known active power introduced to AC node i in period k; is the set of balanced nodes of the AC system; G P B is the square correlation coefficient matrix of AC voltage amplitude in the linearized AC active power flow equation; P is the AC voltage phase angle correlation coefficient matrix in the linearized AC active power flow equation; In the formula, is the known reactive power (pu) introduced to AC node i during period k; B Q is the AC voltage amplitude square correlation coefficient matrix (pu) in the linearized AC reactive power flow equation; G Q is the AC voltage phase angle correlation coefficient matrix (pu) in the linearized AC reactive power flow equation; Preferably, in step (3), the linearized power flow model of the AC / DC system is uniformly expressed as: In the formula, Y ac，p is the DC voltage square correlation coefficient matrix in the linearized AC active power flow, with dimension n a ×n d ; Y dc，p is the DC voltage square correlation coefficient matrix in the linearized DC power flow, with dimension n d ×n d ; The linearized power flow model of AC / DC system is expressed in blocks as follows: Wherein, subscript R represents the set of AC balancing nodes; subscript G represents the set of AC generator nodes except AC balancing nodes; subscript L represents the set of AC nodes except AC generator nodes; subscript g represents the set of AC generator nodes and s nodes in the VSC converter station of #6 control mode except AC balancing nodes; subscript l represents the set of other AC nodes except AC balancing nodes and node set g; subscript r represents the set of node d in the VSC converter station of #2 control mode; subscript d represents the set of other DC nodes except node set r; The linearized power flow equations of AC and DC systems with small dimension coefficient matrices are expressed as:

7. The method for static safety analysis of AC / DC systems containing VSC-MTDC based on linear power flow according to claim 6 is characterized in that: Step (4), the power flow correction method is: firstly, the state quantity in the AC / DC system linear power flow model with a small dimension coefficient matrix is ​​used to calculate the loss, then the calculated loss is used as the known injection power to correct the AC / DC linear power flow equation, and finally, the AC / DC system linear power flow model with the corrected dimension coefficient matrix is ​​solved; The approximate model of the overhead conductor thermal balance equation is: In the formula, K l ′ and K l ″ are the first and second coefficients of the approximate model of the overhead conductor heat balance equation respectively; T l factor =[(T am +273) 2 +(T l +273) 2 ](T l +T am +546); among which, I ij is the current flowing through the overhead wire ij; The analytical solution of the approximate model of the overhead conductor thermal balance equation is:

8. A static safety analysis system for AC / DC systems containing VSC-MTDC based on linear power flow, characterized in that: include: A VSC converter station steady-state power model building module is configured to build a VSC converter station steady-state power model according to an equivalent circuit of the VSC converter station, and provide a control method of the VSC converter station and a control method of the DC power grid; The electro-thermal coupling power flow model module of the AC / DC system containing VSC-MTDC is configured to construct an electro-thermal coupling power flow model of the AC / DC system containing VSC-MTDC based on the electro-thermal coupling power flow model of the traditional AC system; The linearized power flow model module of the AC / DC system containing VSC-MTDC is configured to linearize the AC and DC branch power flows by referring to the traditional power flow linearization idea, and to construct the linearized power flow model of the AC / DC system containing VSC-MTDC by taking the AC node phase angle and the square of the voltage amplitude and the DC node voltage square as state variables; The linear power flow calculation and correction module is configured to design a linearized power flow calculation method and a power flow correction method taking into account branch losses suitable for AC and DC systems containing VSC-MTDC, quickly calculate the power flow and improve its accuracy, calculate the approximate analytical solution of the AC and DC overhead conductor temperatures, and then output system safety analysis warning information.

9. An electronic device, comprising a memory and a processor, and computer instructions stored in the memory and executed on the processor, wherein when the computer instructions are executed by the processor, the static safety analysis method of an AC / DC system containing a VSC-MTDC based on linear power flow as described in claims 1-7 is completed.

10. A computer-readable storage medium for storing computer instructions, wherein when the computer instructions are executed by a processor, the static safety analysis method of an AC / DC system containing a VSC-MTDC based on linear power flow described in claims 1-7 is completed.

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