CN112054515B - Receiving-end power grid DC receiving capacity evaluation method based on multi-objective optimization - Google Patents
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Abstract
The invention relates to a receiving end power grid DC accepting capability evaluation method based on multi-objective optimization, which comprises the steps of firstly, primarily screening DC falling points and reducing the number of scenes; then, adopting a Generalized Equivalent Short Circuit Ratio (GESCR) index, a transient voltage support strength (TVSI) index and a grid loss index as optimization targets of a direct current drop point and unit output, and considering multi-constraint conditions to establish a multi-target optimization model of the direct current feed-in receiving end power grid; then, performing constant step length increment of direct current feed-in scale and repeating direct current point drop and unit optimization until the safe operation constraint condition is not met; and finally, carrying out N-1 safety and stability verification on the reverse sequence of the calculation and analysis result, and taking the verification result as the maximum direct current bearing scale of the receiving-end power grid. The calculation and analysis method provided by the invention has clear logic, comprehensive consideration factors and easy engineering realization, and can effectively obtain the maximum direct current receiving capacity of the receiving-end power grid.
Description
Technical Field
The invention relates to a receiving-end power grid DC receiving capacity calculation method, in particular to a receiving-end power grid maximum DC receiving capacity analysis and detection method which considers the influence of DC drop points and unit output distribution on the DC carrying capacity of a receiving-end power grid and establishes a multi-objective optimization method and multi-constraint conditions.
Background
The high-voltage direct-current transmission has unique advantages in large-capacity and long-distance transmission and plays a key role in energy optimization configuration in China. For a receiving-end power grid, simultaneous feeding of multiple direct currents easily causes a series of safety and stability problems such as voltage collapse, frequency instability, commutation failure and the like, and particularly, with the continuous increase of the scale of the fed direct currents, the safety and stability problems of the receiving-end power grid are increasingly prominent, so that a scientific and effective analysis method for the maximum direct current carrying capacity of the receiving-end power grid is researched and established, the maximum direct current carrying scale of the receiving-end power grid is obtained, and the method has important reference significance for planning and scheduling of an alternating-current and direct-current hybrid power grid.
At present, an evaluation method for researching the direct current receiving capacity of a receiving-end power grid is mainly based on a multi-feed short circuit ratio (MSCR) index, by continuously increasing the direct current feed scale until the MSCR index is equal to a critical value, and performing some safety and stability checks, the carrying scale which can pass various safety constraints is the maximum direct current receiving capacity of the receiving-end power grid. Therefore, in the existing research, the direct current receiving capacity of the receiving-end power grid is quantitatively evaluated mainly by using MSCR indexes, and some safety and stability auxiliary verifications are carried out, but because the critical value of the multi-feed short-circuit ratio depends on engineering experience, the voltage stability margin of the receiving-end power grid cannot be accurately quantified in practical application, the obtained direct current receiving capacity of the receiving-end power grid can have deviation, and meanwhile, a complete direct current receiving capacity evaluation method system of the receiving-end power grid based on multi-objective optimization is not formed in the existing research. Furthermore, existing studies do not consider and relate to the impact of optimal allocation of genset output on dc-carrying size when assessing dc-acceptance of the receiving grid.
The invention establishes an index system and a calculation method for evaluating the maximum direct current bearing scale of a receiving-end power grid from two layers of direct current drop point and unit output optimization through a loop iteration mode and by considering multi-constraint conditions. Firstly, the primary screening of direct current falling points is carried out, and the number of scenes is reduced. And then establishing a direct current drop point and a unit output as optimization targets by adopting a Generalized Equivalent Short Circuit Ratio (GESCR) index, a Transient Voltage Support Intensity (TVSI) index and a network loss index, and establishing a multi-target optimization model of the direct current feed-in receiving-end power grid by considering multi-constraint conditions. And repeatedly performing drop point and unit output optimization through the cyclic increasing of the direct current feed-in scale until the set safe operation constraint condition is not met, performing N-1 safe and stable verification on the reverse sequence of the calculation analysis result, and taking the verification result as the maximum direct current receiving capacity of the receiving-end power grid. And finally, the IEEE 39 node system is used as a receiving-end power grid fed in by multiple direct currents, the maximum direct current admission capacity of the receiving end is obtained through calculation and analysis, and the feasibility and the effectiveness of the established receiving-end power grid maximum direct current admission capacity analysis method are verified.
Disclosure of Invention
The technical problem of the invention is mainly solved by the following technical scheme:
a receiving end power grid DC receiving capacity evaluation method based on multi-objective optimization is characterized by comprising the following steps:
step 6, executing N dc ＝N dc +1, and set the Nth dc The strip DC transmission power being at a minimum limit value, i.e. P d (N dc )＝P dmin Running, and entering step 3;
In the method, in order to reduce the number of direct current drop point scenes and improve the optimization accuracy, in step 1, a load concentration index and a node degree index are defined to perform preliminary screening on the drop points, as shown in formula one:
in the formula I, F (i) and M (i) are respectively the load concentration ratio of a node i and the index value of the node degree; j is an adjacent node set of the node i; l is a system load node set; p Lj 、P Lk The active loads of the node j and the node k are respectively; l bij Is a branch between the nodes i and j; count is a counting function.
In the method, when the direct current drop point and unit output combined optimization model is established in the step 3, a Generalized Equivalent Short Circuit Ratio (GESCR) index and a transient voltage support intensity index (I) are utilized TVSI ) And constructing an optimization target by the network loss index, wherein the optimization target comprises the following steps:
(1) when the GESCR index quantificationally evaluates the voltage supporting capability of the receiving end power grid, because the theoretical critical value is 1, compared with a multi-feed short circuit ratio (MSCR) index, the GESCR index has better accuracy and practicability, the GESCR index is selected as the index for evaluating the voltage supporting capability of the receiving end power grid, and the GESCR index and the safety margin index are respectively:
in the formula II, I bal (i)、I mar (i) Respectively obtaining the balance and safety margin index values under the ith multiple direct current feed-in schemes; n is a radical of dc Total number of DC feeds; GESCR (j) represents the GESCR corresponding to the j direct current feed-in node; GESCR ave Feeding corresponding GESCR average values for each direct current under the ith scheme; GESCR min For node voltageAnd the size of the GESCR corresponding to the boundary stability is equal to 1, wherein the GESCR index is defined as: for LCC type direct current, when the fixed power of a sending end and the fixed extinction angle of a receiving end are adopted for control, the GESCR indexes corresponding to the jth feed-in direct current are as follows:
in the formula III, U Lj A node voltage of load node j; z eqj After the influence of other direct current feed-in on the direct current is converted to a current conversion bus j in an impedance form, the equivalent impedance of a feed-in point is obtained; s dj Apparent power for the dc feed node j; QVCF is a reactive voltage compensation coefficient, which represents the influence of the dc control characteristic on the GESCR, and the expression is:
in the formula IV, Q dj Absorbing reactive power for the jth direct current conversion bus; theta eqj Is an equivalent impedance Z eqj The impedance angle of (d);
(2) in order to reflect the mutual influence degree among all the commutation buses, a transient voltage support intensity index (I) is adopted TVSI ) As one of the optimization objectives, the average transient voltage support strength index under the ith dc-feed scheme is defined as:
in the formula V, N dc Total number of DC feeds; wherein, I TVSI The index is defined as:
in the formula VI, X ekj Transfer resistance between k and j nodes in equivalent power grid for only reserving k and j of converter busesResisting; x ek The branch impedance between the node k in the equivalent network and the equivalent power supply is obtained;
(3) and (3) ensuring the operation economy of the power grid in the optimization process, and defining the grid loss index under the ith operation scheme as follows:
I loss (i)＝P G_total (i)+P D_total (i)-P L_total (i) Formula seven
In the formula VII, P G_total The total active output of the conventional generator set of the system; p D_total Feeding total power into receiving end direct current; p L_total Is the total load of the system.
In the method, in the first step and the third step, in order to process the inconsistency of the magnitude and the direction of the index, the index under the ith scheme is processed by a range transformation method, the method is shown as the formula eight, and the index is subjected to subjective and objective combination weighting by adopting an analytic hierarchy process and an anti-entropy weight method;
in the formula VIII, I 1 、I 2 Respectively representing a negative index and a positive index;
respectively are indexes after standardized treatment; i is max 、I min The value range after index normalization is shown, in order to avoid normalization to a closed interval [0,1 ]]When the index value is 0, the index value can be taken as I max ＝0.996,I min ＝0.002。
In the method, in step 4, multiple constraint conditions such as system power flow constraint, generator active and reactive power constraint, node voltage constraint, short-circuit current constraint, branch maximum transmission power constraint, GESCR index not less than a critical value, effective direct-current inertia time constant constraint, maximum single feed-in constraint and the like are constructed, as shown in formula nine:
in the formula Jiu, P i And Q i Active and reactive power are injected into a node i; delta ij Is the voltage phase difference between nodes i and j; g ij And B ij Respectively, the mutual conductance and the mutual admittance between the nodes i and j;
and
respectively representing the upper limit and the lower limit of active power output and reactive power output of the generator; n is the total number of system nodes; n is a radical of G The total number of the nodes of the generator of the system is; n is a radical of l Is the total number of system branches;
and
respectively representing the upper limit and the lower limit of the node voltage;
representing the upper limit of node short-circuit current control;
representing the upper limit of the transmission power of the branch; h dc Taking the effective direct current inertia time constant to be more than 2-3; gamma ray dcmax,i The ith direct current is locked due to the fault of a receiving end system, after the system recovers the steady state, in order to ensure that the steady state frequency of the system can be maintained within an allowable deviation range, the lower ith direct current transmission power accounts for the maximum ratio of the output power of all the generator sets of the system, wherein H dc And gamma dcmax,i The calculation expression of (a) is:
formula ten, J A_total Is the total moment of inertia of the receiving end system; p D_total For feeding the receiving end with a straight wireTotal power of the stream; r eq The equivalent difference adjustment coefficient of the system generator set is obtained; d L Adjusting the coefficient for the system load active frequency; gamma is the ratio of the total power of the fed-in direct current to the sum of the active power output of all the generators in the system; Δ f max Maximum allowable steady state frequency deviation for the system; f. of N The system nominal frequency.
In the method, in the step 3, the direct current drop point and unit output combined optimization model is solved by using an MOEA/D optimization algorithm, so that the local search capability of the MEOA/D optimization algorithm is enhanced, the optimization effect is improved, and the population diversity is increased by adopting Gaussian variation after the evolution operation; for the ith element v in the evolved individual i The mutation operation is as follows:
in the eleventh formula, rand is a function for generating random numbers; c is a random number matrix; n is a radical of v Representing the number of independent variables; gussiant (t) denotes a Gaussian function with an argument t;
given variance of gaussian variation; p i The mutation rate is.
Therefore, compared with the prior art, the receiving-end power grid DC receiving capacity analysis method based on multi-objective optimization has the following advantages: based on GESCR indexes, a calculation and analysis method for the maximum direct current receiving capacity of a receiving-end power grid is established systematically, and the ambiguity and inaccuracy of the receiving-end voltage supporting capacity evaluated by MSCR indexes in the traditional method are overcome; the influence of different unit output optimization distribution on the direct current scale borne by the receiving end is considered, the optimal drop point and the unit output distribution change are considered along with the iterative increase of the direct current feed-in scale, the direct current drop point and the unit output are repeatedly optimized, the reasonability is achieved, and compared with the method that the optimal drop point is fixed firstly, the power receiving scale obtained by the iterative direct current scale increasing method is not biased to be conservative; in addition, multi-constraint conditions such as effective direct current inertia time constant constraint and maximum single feed-in constraint after direct current feed-in are considered, safety and stability check is carried out on the result according to a reverse order, and the receiving end maximum direct current receiving capacity obtained through calculation by the method is higher in reliability and accuracy.
Drawings
Fig. 1 is a flow chart of a receiving-end power grid maximum direct current receiving capacity calculation method based on multi-objective optimization.
Fig. 2 is a topology diagram of an IEEE 39 node system.
Detailed Description
The technical scheme of the invention is further specifically described by the following embodiments and the accompanying drawings.
1. And (3) calculating the maximum direct current receiving capacity of the receiving-end power grid based on multi-objective optimization.
The invention provides a method for analyzing the maximum direct current receiving capacity of a receiving-end power grid based on a multi-objective optimization method, the flow of the method is shown as figure 1, and the specific method comprises the following steps:
step 6, executing N dc ＝N dc +1, and set the Nth dc The strip DC transmission power being at a minimum limit value, i.e. P d (N dc )＝P dmin Running, and entering step 3;
2. And (5) primarily screening direct current falling points.
According to the method in 1, in order to reduce the number of scenes and improve the optimization accuracy, in step 1, a load concentration index and a node degree index are defined to perform preliminary screening on the drop points, as shown in formula 1:
in the formula, F (i) and M (i) are respectively the load concentration ratio of the node i and the index value of the node degree; j is an adjacent node set of the node i; l is a system load node set; p Lj 、P Lk The active loads of the node j and the node k are respectively; l. the bij Is a branch between the nodes i and j; count is a counting function.
3. And constructing a direct current drop point and unit output optimization target.
According to the method in the step 1, in the direct current drop point and unit output combined optimization model in the step 3, a Generalized Equivalent Short Circuit Ratio (GESCR) index and a transient voltage support intensity index (I) are utilized TVSI ) And constructing an optimization target by the network loss index respectively：
(1) When the GESCR index quantificationally evaluates the voltage supporting capability of the receiving end power grid, because the theoretical critical value is 1, compared with a multi-feed short circuit ratio (MSCR) index, the GESCR index has better accuracy and practicability, the GESCR index is selected as the index for evaluating the voltage supporting capability of the receiving end power grid, and the GESCR index and the safety margin index are respectively:
in the formula I bal (i)、I mar (i) Respectively obtaining the balance and safety margin index values under the ith multiple direct current feed-in schemes; n is a radical of dc The total number of DC feeds; GESCR (j) represents the GESCR corresponding to the j direct current feed-in node; GESCR ave Feeding corresponding GESCR average values for each direct current under the ith scheme; GESCR min The size of the GESCR corresponding to the critical stability of the node voltage is equal to 1, wherein the GESCR index is defined as: for LCC type direct current, when the fixed power of a sending end and the fixed extinction angle of a receiving end are adopted for control, the GESCR indexes corresponding to the jth feed-in direct current are as follows:
in the formula of U Lj A node voltage of load node j; z is a linear or branched member eqj After the influence of other direct current feed-in on the direct current is converted to a current conversion bus j through an impedance form, the equivalent impedance of a feed-in point is obtained; s. the dj Apparent power for the dc feed node j; QVCF is a reactive voltage compensation coefficient, which represents the influence of the dc control characteristic on the GESCR, and the expression is:
in the formula, Q di Absorbing reactive power for the ith direct current conversion bus; theta eqi Is an equivalent impedance Z eqj The impedance angle of (c).
(2) In order to reflect the mutual influence degree among all the commutation buses, a transient voltage support strength index (I) is adopted TVSI ) As one of the optimization objectives, the average transient voltage support strength index under the ith dc-feed scheme is defined as:
in the formula, N dc The total number of DC feeds; wherein, I TVSI The index is defined as:
in the formula, X ekj Only the transfer impedance between the nodes k and j in the equivalent power grid of the converter buses k and j is reserved; x ek Is the branch impedance between node k and the equivalent power source in the equivalent network.
(3) After the direct current drop point and the unit output are combined, the operation economy of the power grid is guaranteed, and the grid loss index under the ith operation scheme is defined as follows:
I loss (i)＝P G_total (i)+P D_total (i)-P L_total (i) (7)
in the formula, P G_total The total active output of the conventional generator set of the system; p D_total Feeding total power into receiving end direct current; p L_total Is the total load of the system.
4. Index processing and index weight determination methods.
According to the method described in the step 1, in the step one and the step three, in order to process the inconsistency of the magnitude and the direction of the index, the index under the ith scheme is processed by a range transform method, the method is shown as the formula (8), and the index is subjectively and objectively weighted by an analytic hierarchy process and an entropy weight resisting method.
In the formula I 1 、I 2 Respectively representing a negative index and a positive index;
respectively are indexes after standardized treatment; i is max 、I min The value range after index normalization is shown, in order to avoid normalization to a closed interval [0,1 ]]When the index value is 0, the index value can be taken as I max ＝0.996,I min ＝0.002。
In summary, the total objective function of the primary screening of the direct current drop point, the total objective function of the direct current drop point and the unit output optimization are respectively as follows:
in the formula (f) D,i 、f i Primarily screening a total target value for the direct current drop point under the ith scheme and optimizing the total target value by combining the drop point and the unit output; lambda [ alpha ] 1 、λ 2 、α 1 、α 2 、α 3 And alpha 4 Are all weight factors, where 1 +λ 2 ＝1，α 1 +α 2 +α 3 +α 4 ＝1。
5. And constructing multiple constraints.
According to the method in 1, in step 4, multiple constraint conditions such as system power flow constraint, generator active and reactive power constraint, node voltage constraint, short-circuit current constraint, branch maximum transmission power constraint, GESCR index not less than a critical value, effective direct current inertia time constant constraint, maximum single feed-in constraint and the like are constructed, as shown in formula (10):
in the formula, P i And Q i Active and reactive power are injected into the node i; delta ij Is the voltage phase difference between nodes i and j; g ij And B ij Between nodes i and j, respectivelyMutual conductance and mutual admittance of (a);
and
respectively representing the upper limit and the lower limit of active power output and reactive power output of the generator; n is the total number of system nodes; n is a radical of G The total number of the nodes of the generator of the system is; n is a radical of l The total number of the system branches;
and
respectively representing the upper limit and the lower limit of the node voltage;
representing the upper limit of node short-circuit current control;
representing the upper limit of the transmission power of the branch; h dc An effective direct current inertia time constant, generally greater than 2-3; gamma ray dcmax,i When the ith direct current is locked due to system faults and the steady-state frequency of the system is ensured to be maintained within an allowable deviation range when the system recovers the steady state, the lower ith direct current transmission power accounts for the maximum ratio of the output power of all the generator sets of the system, wherein H dc And gamma dcmax,i The calculation expression of (a) is:
in the formula, J A_total Is the total moment of inertia of the receiving end system; r eq The equivalent difference adjustment coefficient of the system generator set is obtained; d L Adjusting the coefficient for the active frequency of the system load; gamma is the ratio of the total power of the fed-in direct current to the sum of the active power output of all the generators in the system; Δ f max Maximum allowable steady state frequency deviation for the system; f. of N For rating the systemAnd (4) rate.
6. And (6) solving an optimization model.
According to the method in the step 1, in the step 3, the direct current drop point and unit output combined optimization model is solved by using an MOEA/D optimization algorithm, so that the local search capability of the MEOA/D optimization algorithm is enhanced, the optimization effect is improved, and the population diversity is increased by adopting Gaussian variation after the evolution operation. For element v in evolved individuals i The mutation operation is as follows:
wherein rand is a function for generating random numbers; c is a random number matrix; n is a radical of v Representing the number of independent variables; gussiant (t) denotes a Gaussian function with an argument t;
given variance of gaussian variation; p i The variation rate is shown.
7. Based on the IEEE 39 receiver net rack embodiment.
7.1, setting parameters.
And (3) adopting an IEEE 39 node system as a receiving-end power grid fed with multiple direct currents to carry out maximum direct current bearing scale analysis. The system topology structure is shown in fig. 2, the system includes 29 load nodes and 10 generator nodes, where the node 31 is a generator balancing node, the total load of the system is 6254.23MW, and the reference value is 100MW.
The fed-in direct currents are all LCC type direct currents, and the sending end fixed power and the receiving end fixed extinction angle control mode is adopted. Without loss of generality, the fed direct currents have the same parameters, and the inversion sides are all formed by 12-pulse inverters, wherein the main direct current parameters are shown in table 1.
TABLE 1 feeding in DC control parameters
In addition, the maximum DC load to the receiving end networkIn the process of scale evaluation and calculation, the system node voltage U is set i The allowable range of (1) is 0.94 pu-1.06 pu, and the rated value is 1.0pu; upper limit of node short circuit current control
Is 63kA; maximum transmission power of branch
Is 1000MW; the total rotational inertia of the system is 15000MW & S; setting the equivalent difference adjustment coefficient R of the system eq Is 0.036, and has a load active frequency regulation coefficient D L Is 1.5, the rated frequency f of the system N At 50Hz, the maximum steady state frequency deviation Δ f allowed for the system max Is 0.5Hz; the generator output allowable range is shown in table 3 below.
7.2, maximum admission capacity calculation.
In order to avoid the mutual influence between the converter station and the power plant, when direct current is fed in, direct current falling into the generator nodes is not considered temporarily, so that 29 load nodes are possible direct current falling points, screening and sorting are carried out on all the falling points after the load concentration index and the degree standardization processing of the nodes, the first 15 nodes in the ranking are selected as alternative falling point sets, and sorting results and corresponding index values are shown in a table 2.
TABLE 2 alternative set of DC drop points
Setting the direct current increasing step length to be delta P =0.5pu, and setting the initial direct current feed-in number to be N 0 ＝1，P dmax 、P dmin 1000MW and 500MW respectively. The maximum iteration number of the MOEA/D algorithm is set to be 100, and the Gaussian variation rate is 0.1. For the loop iteration result, starting from the maximum feed-in direct current scale, performing N-1 safety and stability verification in reverse order and performing proper adjustment until a scheme which can pass the safety and stability verification is selected, wherein the final result is as follows: DC transmission power P d1 、P d2 And P d3 Respectively 1000MW, 1000MW and 650MW, and is optimally straightThe current drop points are nodes 16, 2 and 9, and the generator active and reactive power output allowable ranges and the terminal voltage are shown in table 3.
TABLE 3 Unit output Range and optimization results
At the DC feed scale, the allowable frequency deviation Δ f of the system max When the frequency is not less than 0.5Hz, the maximum allowable single-direct-current transmission power is 1095MW which is more than P dmax =1000MW, the maximum dc carrying size of the receiving-end grid is 3 times at this time, which is 2650MW.
The specific embodiments described in this application are merely illustrative of the spirit of the invention. Various modifications or additions may be made to the described embodiments or alternatives may be employed by those skilled in the art without departing from the spirit or ambit of the invention as defined in the appended claims.
Claims (6)
1. A receiving-end power grid DC receiving capacity evaluation method based on multi-objective optimization is characterized by comprising the following steps:
step 1, preliminarily screening all possible direct current drop points of a receiving end power grid based on a determined receiving end power grid structure and a direct current feed-in typical operation mode to obtain an alternative drop point set D;
step 2, defining the direct current feed-in number N of the initial receiving end power grid dc ＝N 0 And setting the maximum transmission power P of each direct current transmission line dmax Running;
step 3, determining an optimal drop point and an active and reactive power output scheme of the unit through the established direct current drop point and unit output optimization model;
step 4, under the optimized optimal operation mode, judging whether various safe operation constraint conditions are met, if so, entering step 5, and if not, entering step 7;
step 5, N dc Strip DC increases the transmitted power by a step size Δ P, i.e. P d (N dc )＝P d (N dc ) +. DELTA P, with judgment of P d (N dc ) And P dmax Size, if P d (N dc )≤P dmax Entering step 3, otherwise, entering step 6;
step 6, executing N dc ＝N dc +1, and set the Nth dc The strip DC transmission power being at a minimum limit value, i.e. P d (N dc )＝P dmin Running, and entering step 3;
step 7, performing N-1 safety and stability verification on the obtained iterative optimization result in a reverse mode according to the size of the total direct current feed power until a direct current feed scheme without safety and stability problems of voltage overrun, branch power flow overrun and power flow non-convergence in the N-1 verification is obtained, taking the direct current feed scheme as the maximum direct current receiving scale of a receiving end, and entering step 8;
step 8, determining the final fed direct current number N according to the result in step 7 dc And counting the maximum receiving DC scale P of the receiving-end power grid D_total I.e. by
2. The method of claim 1, wherein in step 1, a load concentration index and a node degree index are defined to perform a preliminary screening on the drop points for reducing the number of direct current drop point scenes and improving the optimization accuracy, as shown in formula one:
in the formula I, F (i) and M (i) are respectively the load concentration ratio of a node i and the index value of the node degree; j is an adjacent node set of the node i; l is a system load node set; p Lj 、P Lk The active loads of the node j and the node k are respectively; l. the bij Is a branch between the nodes i and j; count is a counting function.
3. The method of claim 1, wherein the direct current drop point and unit output combined optimization model is established in step 3 by using a Generalized Equivalent Short Circuit Ratio (GESCR) index and a transient voltage support strength index I TVSI And constructing an optimization target by the network loss index, wherein the optimization target comprises the following steps:
(1) when the GESCR index quantificationally evaluates the voltage supporting capability of the receiving end power grid, because the theoretical critical value is 1, compared with the MSCR index with the multi-feed short circuit ratio, the GESCR index has better accuracy and practicability, the GESCR index is selected as the index for evaluating the voltage supporting capability of the receiving end power grid, and the GESCR index and the safety margin index are respectively:
in the formula II, I bal (i)、I mar (i) Respectively obtaining the balance and safety margin index values under the ith multiple direct current feed-in schemes; n is a radical of dc Total number of DC feeds; GESCR (j) represents the GESCR corresponding to the j direct current feed-in node; GESCR ave Feeding corresponding GESCR average values for each direct current under the ith scheme; GESCR min The size of the GESCR corresponding to the critical stability of the node voltage is equal to 1, wherein the GESCR index is defined as: for LCC type direct current, when the fixed power of a sending end and the fixed extinction angle of a receiving end are adopted for control, the GESCR indexes corresponding to the jth feed-in direct current are as follows:
in the formula III, U Lj A node voltage of load node j; z eqj After the influence of other direct current feed-in on the direct current is converted to a current conversion bus j through an impedance form, the equivalent impedance of a feed-in point is obtained; s dj Apparent power for the dc feed node j; compensation system for QVCF reactive voltageThe number, which represents the influence of the DC control characteristic on GESCR, is expressed as:
in the formula IV, Q dj Absorbing reactive power for the jth direct current conversion bus; theta eqj Is an equivalent impedance Z eqj The impedance angle of (d);
(2) in order to reflect the mutual influence degree among all the commutation buses, a transient voltage support strength index I is adopted TVSI As one of the optimization objectives, the average transient voltage support strength index under the ith dc-feeding scheme is defined as:
in the formula V, N dc Total number of DC feeds; wherein, I TVSI The index is defined as:
in the formula VI, X ekj Only the transfer impedance between the nodes k and j in the equivalent power grid of the current conversion buses k and j is reserved; x ek The branch impedance between the node k in the equivalent network and the equivalent power supply is obtained;
(3) and (3) ensuring the operation economy of the power grid in the optimization process, and defining the grid loss index under the ith operation scheme as follows:
I loss (i)＝P G_total (i)+P D_total (i)-P L_total (i) Formula seven
In the formula VII, P G_total The total active output of the conventional generator set of the system; p D_total Feeding total power into receiving end direct current; p L_total Is the total load of the system.
4. The method as claimed in claim 1, wherein in the first step and the third step, in order to process the inconsistency of the magnitude and direction of the index, the index under the ith scheme is processed by a range transform method, which is shown in the formula eight, and the index is subjected to subjective and objective combination weighting by an analytic hierarchy process and an entropy weight resisting method;
in the formula VIII, I 1 、I 2 Respectively representing a negative index and a positive index;
respectively are indexes after standardized treatment; i is max 、I min And the value range of the normalized index is represented.
5. The method of claim 1, wherein multiple constraints are constructed in step 4, such as system power flow constraint, generator active and reactive power constraint, node voltage constraint, short circuit current constraint, branch maximum transmission power constraint, GESCR indicator not less than a critical value, active DC inertia time constant constraint, and maximum single feed-in constraint, as shown in formula nine:
in the formula Jiu, P i And Q i Active and reactive power are injected into a node i; delta. For the preparation of a coating ij Is the voltage phase difference between nodes i and j; g ij And B ij Respectively, the mutual conductance and the mutual admittance between the nodes i and j;
and
respectively representing electricity generationUpper and lower limits of active and reactive power output of the machine; n is the total number of system nodes; n is a radical of G The total number of the nodes of the generator of the system is; n is a radical of l The total number of the system branches;
and
respectively representing the upper limit and the lower limit of the node voltage;
representing the upper limit of node short-circuit current control;
representing the upper limit of the transmission power of the branch; h dc Taking the effective direct current inertia time constant to be more than 2-3; gamma ray dcmax,i The ith direct current is locked due to the fault of a receiving end system, after the system recovers the steady state, in order to ensure that the steady state frequency of the system can be maintained within an allowable deviation range, the lower ith direct current transmission power accounts for the maximum ratio of the output power of all the generator sets of the system, wherein H dc And gamma dcmax,i The calculation expression of (a) is:
formula ten, J A_total Is the total moment of inertia of the receiver system; p D_total Feeding DC total power to a receiving end; r eq The equivalent difference adjustment coefficient of the system generator set is obtained; d L Adjusting the coefficient for the system load active frequency; gamma is the ratio of the total power of the fed-in direct current to the sum of the active power output of all the generators in the system; Δ f max Maximum allowable steady state frequency deviation for the system; f. of N The system nominal frequency.
6. The method of claim 1, wherein in step 3, MOEA/D optimization is usedThe algorithm solves the direct current drop point and unit output combined optimization model, in order to enhance the local search capability of the MEOA/D optimization algorithm and improve the optimization effect, gaussian variation is adopted after evolution operation to increase population diversity; for the ith element v in the evolved individual i The mutation operation is as follows:
in the eleventh formula, rand is a function for generating random numbers; c is a random number matrix; n is a radical of hydrogen v Representing the number of independent variables; gussiant (t) denotes a Gaussian function with an argument t;
given variance of gaussian variation; p i The mutation rate is.
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