CN110096767B - Cascading failure simulation method for alternating current-direct current series-parallel power grid - Google Patents
Cascading failure simulation method for alternating current-direct current series-parallel power grid Download PDFInfo
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Abstract
The invention discloses a cascading failure simulation method for an alternating current-direct current series-parallel power grid, which comprises the following steps of: s1, determining parameters of a power grid; s2, performing cycle simulation on the power grid, and compared with the previous simulation, increasing the capacity and the load level of the generator according to a certain proportion, and increasing the capacity of the line with the fault in the previous simulation according to a certain proportion; s3, simulating all lines to be disconnected according to a certain probability each time; s4, carrying out power flow optimization on the power grid after disconnection, judging whether a line reaches a heavy load, if the line reaches the heavy load, entering S5, and if not, entering S6; s5, disconnecting the heavy load line according to a certain probability, repeating S4 if a new line is disconnected, and otherwise, entering S6; and S6, judging whether the simulation times reach preset times, if so, ending the simulation, and otherwise, repeating the S2. The cascading failure simulation method provided by the invention can take a direct-current line model and a direct-current protection measure into consideration, and is suitable for cascading failure simulation of an alternating-current and direct-current series-parallel power grid.
Description
Technical Field
The invention relates to the field of electrical engineering, in particular to a cascading failure simulation method for an alternating current-direct current series-parallel power grid.
Background
Under the background of vigorously constructing an AC/DC extra-high voltage interconnected power grid in China at present, along with the increasing large scale of the power grid and the construction of extra-high voltage AC/DC transmission projects, the transmission power of a system is gradually increased, the system is dynamic and complex, and the operation safety of an AC/DC hybrid power grid is greatly threatened. On the other hand, uncertain factors in the system gradually increase, and problems such as local power grid power imbalance, instability, oscillation and the like caused by factors such as faults and weather can be diffused, so that cascading faults are caused, and even major power failure accidents are induced. Researches show that a blackout accident of a power system is generally caused by a chain reaction that a certain element in the system breaks down to cause a series of other elements to stop running, and the chain overload and the operation/misoperation of protection of a transmission line caused by the large-scale transfer of the active power flow of the line are main factors for the occurrence of the chain fault. Therefore, a cascading failure model facing the alternating current-direct current hybrid power grid is established, a cascading failure mode and risks of the system are revealed, guidance is provided for cascading failures of the system, the running risks of the power grid are effectively reduced, and great economic and social benefits are generated. The traditional cascading failure OPA simulation method adopts a direct current power flow optimization model, can only be applied to a pure alternating current system, and cannot analyze cascading failure of an alternating current and direct current series-parallel power system; therefore, the method for researching the cascading failure simulation method of the alternating current-direct current hybrid power grid is of great significance.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to solve the technical problem that the traditional cascading failure OPA simulation method can only be applied to a pure alternating current system and can not analyze cascading failures of an alternating current-direct current hybrid power system.
In order to achieve the purpose, the invention provides a cascading failure simulation method for an alternating current-direct current series-parallel power grid, which comprises the following steps of:
s1, determining an alternating current circuit, a direct current circuit, a tie line, a generator, a converter station and a load of an alternating current-direct current hybrid power grid; the capacity and the load level of the generator can be used for calculating the power flow of the power grid, wherein the power flow comprises active power, reactive power, active loss and reactive loss of each line, and the voltage amplitude and the phase angle of each bus node of the power grid;
s2, performing cyclic simulation on the alternating current-direct current hybrid power grid for preset times, wherein compared with the previous simulation, the capacity and the load level of the generator are improved according to a first preset proportion in each simulation, and the capacity of a line with a fault in the previous simulation is increased according to a second preset proportion;
s3, in each simulation, all lines are disconnected according to a first preset probability;
s4, if the line of the alternating current-direct current hybrid power grid is disconnected, carrying out load flow optimization on the disconnected alternating current-direct current hybrid power grid, distributing the power of the disconnected line to the line which is not disconnected, judging whether the line reaches a heavy load or not after the optimization, if the line reaches the heavy load, entering the step S5, and if not, entering the step S6;
s5, disconnecting the heavy load line according to a second preset probability, then judging whether a new line is disconnected or not, if so, repeating the step S4, otherwise, entering the step S6;
and S6, judging whether the simulation times reach preset times, if so, ending the cascading failure simulation process, and otherwise, repeating the step S2.
Optionally, the step S4 specifically includes the following steps:
step S401, in each simulation, all lines are possible to be cut off randomly, and if any line is cut off, a fast dynamic process is started;
step S402, carrying out primary power flow optimization on the new network after disconnection by adopting a branch power flow model, wherein the branch power flow model comprises: the method comprises the following steps of AC node active power constraint, AC node reactive power constraint, AC line active loss constraint, AC line reactive loss constraint, voltage relation constraint at two ends of an AC line, AC network phase angle difference relation constraint, current conversion node power balance constraint, voltage relation constraint at two ends of a DC line, power coupling relation constraint at a current conversion station and current conversion station transmission capacity constraint;
step S403, after the optimized scheduling is finished, judging whether a line reaches a heavy load, and if the line reaches the heavy load, entering step S5; otherwise, the process proceeds to step S6.
Optionally, the active power constraint of the ac node is:
the reactive power constraint of the alternating current node is as follows:
wherein, P Gi And Q Gi Active and reactive power respectively, N, from the ith generator lAC Is the total number of AC lines, P cvi And Q cvi Active and reactive power, P, respectively, injected by the ith converter station rACj And Q rACj Active and reactive power, P, respectively, received at the end of the j-th line lsACj And Q lsACj Active and reactive power respectively lost on the j-th line, B ii Is the total susceptance to ground, W, of the ith AC node ACi Is the square of the voltage at the ith AC node, P cuti And Q cuti Respectively the active and reactive load quantities, M, of the cut-off required due to power imbalance PQAC (i, j) is a coefficient value of active power and reactive power received at the end of the line determined according to the relationship between the node i and the branch j, M lAC (i, j) is the coefficient value of the line active loss and reactive loss determined according to the relation between the node i and the branch j, P Di Is the active load at the ith AC node, Q Di Is the reactive load at the ith ac node.
Optionally, the ac line active loss constraint is:
and the reactive loss of the alternating current line is restrained:
wherein, W rACj Representing the square of the voltage at the end of line j, R lACjj 、X ljj The resistance and reactance of the jth ac line, respectively.
Optionally, the voltage relationship across the ac line is constrained by:
the alternating current network phase angle difference relation constraint is as follows:
wherein, the matrix M WAC Is M PQAC Is a node incidence matrix of the network, C kj Is a matrix of elementary loops which is, nC is the basic number of loops.
Optionally, the commutation node power balance constraint is:
wherein, the matrix M PDC A matrix M representing a coefficient value of active power received at the end of the DC line determined according to the relationship between the node i and the branch j lDC Representing the value of the coefficient of the active loss of the direct current line, P, determined according to the relation between node i and branch j rDCj The power transmitted to the tail end of the jth direct current line; p lsDCj Is the active loss, P, on the jth DC line cvm Injecting active power, N, for a converter station lDC Is the total number of the direct current lines.
Optionally, the voltage relationship between the two ends of the dc line is constrained as:
wherein, the matrix M WDC Is M PDC Transposed matrix of (2), R DCjj Is the resistance of the jth DC line, W DCi Is the square of the voltage at the ith dc node.
Optionally, there is a reactive power coupling relationship constraint at the converter station as:
P cvm ＝|Q cvm tanα|
wherein Q is cvm For the converter station to absorb reactive power, the angle α is a fixed value, determined by the actual mode of operation.
Optionally, the converter station transmission capacity constraint is:
wherein S is max cvm Representing the upper limit of the transmission capacity of the mth converter station.
Optionally, in the step S402, performing primary power flow optimization on the new network after disconnection by using a branch power flow model, specifically including the following steps:
if the AC line is disconnected, fixing the transmission power of the DC line to be unchanged, regarding the DC line as a fixed load of the AC system, and determining the power flow of the AC-DC hybrid power grid after the AC line is disconnected; if the direct current line is disconnected, the power injected into the alternating current system by the corresponding converter station is 0, the reactive power injection at the converter station still exists, the transmission power of other direct current lines is fixed and is not changed, the other direct current lines are regarded as fixed loads of the alternating current system, and the power flow of the alternating current-direct current series-parallel power grid after the direct current line is disconnected is determined; if no new line is disconnected, the fast dynamic process is exited.
Generally, compared with the prior art, the above technical solution conceived by the present invention has the following beneficial effects:
(1) The alternating current-direct current branch power flow model adopted by the method can calculate reactive power and active and reactive losses in a network, does not fix voltage, is suitable for an alternating current system, and can be expanded to an alternating current-direct current series-parallel system.
(2) The method provided by the invention can consider the control of the direct current line, in the process of optimizing the dispatching, the transmission power of the direct current line can be adjusted, and meanwhile, the direct current line is considered to be possibly disconnected due to heavy load, so that the fault is further caused; and after the direct current line is disconnected due to overload, the capacity of the direct current line can be updated during next simulation.
(3) The method provided by the invention considers commutation failure protection, and considers that direct current protection is to respond after dispatching to disconnect a line or lock a converter. The criterion of the protection action is that the AC voltage of the converter is lower than a given threshold, the converter is locked if the AC voltage is lower than the given threshold, and the fault time is not considered, namely the scheduling is considered to be completed within the setting time.
Drawings
FIG. 1 is an overall flow chart of a cascading failure simulation method for an AC/DC hybrid power grid provided by the invention;
FIG. 2 is a topology diagram of a modified IEEE30 node system in an embodiment of the present invention;
fig. 3 is a modified distribution curve diagram of the power outage scale of the IEEE30 node system according to the embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the respective embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
The invention provides a cascading failure simulation method for an alternating current-direct current hybrid power grid, which aims to obtain the failure times, cascading failure steps, cascading failure scale and risk size of the alternating current-direct current hybrid power grid through load flow optimization calculation based on an alternating current-direct current load flow model, so that a reference basis is provided for cascading failure risk analysis, failure coping and operation planning of an alternating current-direct current power system, the failure risk of the alternating current-direct current hybrid power system is reduced, and the safety and reliability of the operation of the power system are improved.
As shown in fig. 1, the method for simulating cascading failures of an ac/dc hybrid power grid provided by the invention comprises the following steps:
s1, collecting all technical parameters of the alternating current-direct current hybrid power grid where the alternating current-direct current hybrid power grid is located, and calculating the initial power flow of the power grid.
S101, all technical parameters of the alternating current and direct current hybrid power grid comprise: capacity reference values, node data, branch data, generator data, tie line data, direct current line and converter station data.
And S102, setting the capacity reference value baseMVA as 100MVA.
S103, the node data comprises the number N of the nodes of the alternating current system bAC Node type, active load P at the ith AC node Di And reactive load Q Di Ground conduction g at the ith AC node i And ground pair susceptance b i Amplitude V of the voltage at the ith AC node g And phase angle V a And an upper voltage limit V ACi max And lower voltage limit V ACi min Number of nodes N in DC system bDC Upper limit of voltage at ith DC node V DCi max And lower voltage limit V DCi min 。
S104, the branch data comprises the total number N of the alternating current system lines lAC Resistance R of jth AC line lACj Reactance X lj And susceptance b lj Rated capacity f of jth AC line lACj (ii) a Total number N of DC system lines lDC Resistance R of jth DC line lDCj Capacity f of j-th DC line lDCj 。
S105, the generator data comprise the active power output P of the generator g And reactive power Q g Maximum reactive output Q of each generator max And minimum reactive output Q min Maximum active power output P max And minimum active power output P min 。
S106, the junctor data comprise active power P injected by each junctor inj And reactive power Q inj 。
S107, the converter station data comprise bus node numbers of all converter stations m Rated capacity S of converter station cvm Active and reactive coupling coefficient alpha of converter station and active power P injected into converter station cvm 。
And S108, performing load flow calculation by adopting an MATpower software package, and inputting an operation program to obtain an initial load flow state of the power grid.
S2, giving simulation days k, and increasing the load level and the generator capacity according to a certain proportion and increasing the capacity of the line with the fault in the previous simulation according to a certain proportion compared with the previous simulation in each simulation.
In particular, the load is scaled to the generator output by a growth factor r 1 、r 2 And for the line disconnected due to overload or overload in the last simulation, carrying out capacity updating, and selecting a line capacity increase factor u to improve the line to increase the capacity of the line.
S201, in one example, given simulation days k =2000 days, each simulation represents one day, the power generation capacity and the load level of the system are continuously improved along with the development of the power system, and a load and generator output increase factor r is taken 1 ＝r 2 ＝1.00041。
S202, regarding the line which is cut off due to overload in the last simulation, the line is generally considered to be required to be modified by an electric power planning department, the transmission capacity of the line is increased according to a certain proportion, and a line capacity increase factor u =1.005 is taken.
And S3, in each simulation, all lines can be disconnected according to a certain probability to enter a fast dynamic process.
Specifically, in each simulation, all lines may be randomly disconnected due to weather and the like, a random disconnection probability t =0.0007 may be taken, and if there is a line disconnection, a fast dynamic process is entered.
S4, carrying out primary optimization on the new network after the disconnection, judging whether a line reaches a heavy load or not after the scheduling is finished, if so, entering the step S5, and if not, entering the step S6;
s401, performing primary power flow optimization on the new network after disconnection by adopting a branch power flow model.
Specifically, in each simulation, all lines are likely to be randomly disconnected according to a certain probability due to weather and the like, and if the line is disconnected, a fast dynamic process is started. In the fast dynamic process, a new network needs to be optimized for one time, and the direct current power flow model does not consider the reactive power of a line, so that the direct current power flow model cannot be applied to an alternating current-direct current hybrid power grid. The branch power flow model can be used for calculating reactive power and loss in a network, does not fix voltage, is suitable for an alternating current system, can be expanded to a direct current system, and is suitable for research of the project. The model specifically includes the following constraints:
(a) And (3) carrying out active power and reactive power constraint on the alternating current nodes:
wherein, P Gi And Q Gi Respectively, the i-th generator generates active and reactive power, P cvi And Q cvi Active and reactive power, P, respectively, injected by the ith converter station rACj And Q rACj Active and reactive powers received at the end of the j-th line, respectively, matrix M PQAC A matrix M representing a coefficient value of active power received at the end of the DC line determined according to the relationship between the node i and the branch j lAC Representing the value of the coefficient of the active loss of the direct current line, P, determined according to the relation between node i and branch j lsACj And Q lsACj Active and reactive power respectively lost on the j-th line, B ii Is the total susceptance to ground, W, of the ith AC node ACi Is the square of the voltage at the ith AC node, P cuti And Q cuti Respectively the amount of active and reactive load cut off due to power imbalance.
(b) And (3) constraining the relation between active loss and reactive loss of the alternating current line:
wherein, W rACj Representing the square of the voltage at the j-end of the line, R lACjj 、X ljj The resistance and reactance of the jth ac line, respectively. Prior art W rACj Considered as an invariant value, the present invention can consider W rACj And the active and reactive losses of the line.
(c) The relation constraint of the voltage amplitude values at two ends of the alternating current line and the phase angle difference relation constraint of each independent loop in the alternating current network are as follows:
wherein, the matrix M WAC Is M PQAC The transposed matrix of (2) is a node association matrix of the network.
Wherein the number N of independent basic circuits C ＝N lAC -N bAC +1; the matrix C is a basic loop matrix, C kj Representing the relationship of branch j and loop k.
(d) And (3) converter node power balance constraint:
wherein, the matrix M PDC And M lDC The matrix represents the coefficient value of the active power received by the tail end of the direct current line determined according to the relation between the node i and the branch j, the matrix represents the coefficient value of the active loss of the direct current line determined according to the relation between the node i and the branch j, and P rDCj The power transmitted to the tail end of the jth direct current line; p lsDCj Is the active loss on the jth dc line.
(e) And (3) voltage relation constraint at two ends of the direct current line:
wherein, the matrix M WDC Is M PDC Transposed matrix of (2), R DCjj Is the resistance of the jth DC line, W DCi Is the square of the voltage at the ith dc node.
(f) Reactive power coupling relation constraint exists at the converter station:
P cvm ＝|Q cvm tanα|
wherein, the absolute value in the formula can be eliminated according to the operation mode of the converter, and the angle alpha is a fixed value and is determined by the actual operation mode.
(g) And (3) constraint of transmission capacity of the converter station:
wherein S is max cvm Representing the upper limit of the transmission capacity of the mth converter station.
(h) Upper and lower bounds for other variables in the model:
P Gk min ≤P Gk ≤P Gk max ,Q Gk min ≤Q Gk ≤Q Gk max
-f LACj ≤P rACj ≤f LACj
-f LDCj ≤P rDCj ≤f LDCj
wherein, P Gk min Is the active power P generated by the kth generator Gk The minimum value of (d); p Gk max Is P Gk Maximum value of (d); q Gk min Representing the reactive power Q generated by the kth generator Gk The minimum value of (d); q Gk max Represents Q Gk Maximum value of (d); f. of LACj And f LDCj Upper limit of line capacity, V, of the jth AC line and DC line, respectively 2 ACi max And V 2 ACi min The voltage of the ith alternating current node is the square of the maximum value and the square of the minimum value respectively; v 2 DCi max And V 2 DCi min The voltage at the ith ac node is squared at the maximum and squared at the minimum, respectively.
S402, after the optimized scheduling is finished, judging whether a line reaches a heavy load, and if the line reaches the heavy load, entering the step S5; otherwise, the process proceeds to step S6.
Specifically, after the optimized scheduling is completed, a line overloading threshold value a is given to judge whether a line is overloaded or not. In one example, the line reload threshold is taken as a =0.99.
S5, disconnecting the heavy-load line according to a certain probability, then judging whether a new line is disconnected, if so, forming a new network and repeating the process of the step S4, otherwise, entering the step S6;
specifically, the line reaching the heavy load threshold is disconnected according to a given probability b, then it is judged that if a new line is disconnected, an island needs to be divided, island power balance optimization is carried out to form a new grid, and then the step S4 is continuously repeated. If the AC line is disconnected, the transmission power of the fixed DC line is unchanged as an initial value, the fixed load of the AC system is regarded as the fixed load of the AC system, and the load flow of the AC/DC system after the AC line is disconnected is calculated according to the initial load flow state of the power grid; if the direct current line is disconnected, the power injected into the alternating current system by the corresponding converter station is 0, the reactive power injection provided by the reactive power compensation device at the converter station still exists, the transmission power of other direct current lines is fixed to be an initial value and is not changed, the fixed load of the alternating current system is regarded as the fixed load, and the power flow of the alternating current and direct current system after the direct current line is disconnected is calculated according to the initial power flow state of the power grid. If no new line is disconnected, the fast dynamic process is exited.
In one example, take the heavy haul line disconnect probability b =0.999.
And S6, judging whether the simulation days k reach a set maximum value, if so, ending the cascading failure simulation process, and otherwise, repeating the process of the step S2.
Specifically, if the maximum value is reached, the cascading failure simulation process is exited. At this time, the number of faults, the maximum number of line breaks, the maximum number of cascading fault steps, the fault scales at different confidence levels, and the like can be counted according to the cascading fault simulation result.
The method comprises the steps of obtaining a branch power flow model, obtaining a power flow optimization problem, and solving the power flow optimization problem after the power flow is disconnected. When the grid parameters of the AC/DC system are known, considering safety constraints such as the capacity limit of an AC line, the voltage limit of an AC side of a converter station and the like, and operation constraints such as power balance, a reactive power output range of a generator, an injection power range of the converter station and the like, the grid parameters of the AC/DC system can be optimized in terms of power flow of the disconnected AC/DC power system, and then a new grid is formed.
Detailed exemplary embodiments are disclosed below, and specific structural and functional details disclosed herein are merely for purposes of describing exemplary embodiments. It should be understood, however, that the intention is not to limit the invention to the particular exemplary embodiments disclosed, but to cover all modifications, equivalents, and alternatives falling within the scope of the disclosure. Like reference numerals refer to like elements throughout the description of the figures.
Fig. 2 is a system diagram of an IEEE30 node test system according to the present invention, and an example is described. In the example, a converter is added to each of the node 2 and the node 8, and a direct current line is added therebetween, so that an alternating current-direct current interconnected power system is formed. Collecting the conventional technical parameters of the AC/DC interconnected power grid, and simultaneously giving out simulation parameter values of a slow dynamic process and a fast dynamic process.
The AC/DC interconnected power system shown in FIG. 2 includes the number N of AC nodes bAC =30, number of ac lines N lAC Number of converter stations N =41 cv Number of nodes N of direct current system =2 bDC =2, total number of lines N of direct current system lDC ＝1。
Specifically, the ac node parameters are as in table 1:
TABLE 1 communication of system node parameters
Ac line parameters are as in table 2:
TABLE 2 AC system line parameters
Generator parameters are as in table 3:
TABLE 3 Generator parameters
The converter parameters are shown in table 4:
TABLE 4 converter parameters
| AC node | Direct current node | Capacity (MW) |
| 2 | 1 | 100 |
| 8 | 2 | 100 |
The direct current node parameters are shown in table 5:
TABLE 5 DC system node parameters
| Numbering | Upper limit of voltage (p.u.) | Lower voltage limit (p.u.) | Reference voltage (kV) |
| 1 | 1.06 | 0.94 | 320 |
| 2 | 1.06 | 0.94 | 320 |
The dc line parameters are shown in table 6:
TABLE 6 DC line parameters
| Head end node | End node | Line resistance (p.u.) | Line capacity (MVA) |
| 1 | 2 | 0.01 | 100 |
In addition, considering the actual operation condition, the active and reactive coupling coefficient α =0.8 of the commutation station is taken. The power transmitted by the direct current line is 27MW, the initial alternating current and direct current system power flow state can be obtained through MATpower calculation, and the voltage of an alternating current node 8 where the inverter station is located is 0.9538p.u.
Given the simulation days k of 2000 days, taking the increase factor u =1.005 of the line capacity and the increase factor r of the load and the generator 1 ＝r 2 =1.00041, line overload threshold a =0.99, overload line disconnection probability b =0.999, and line random disconnection probability t =0.0007 due to weather or the like.
The overall results of the calculations for cascading failures are shown in table 7 below:
TABLE 7 simulation of cascading failures Overall results
The total cascading failure simulation result in table 7 can obtain the random failure times and the heavy-load failure times of the alternating current-direct current hybrid system in all the specified simulation days, the maximum load shedding rate under a certain confidence level, the maximum number of broken lines and the number of cascading failure steps.
The top ten lines with the greatest number of faults are shown in table 8 below:
table 8 fault times sequence table
The maximum fault sizes at different confidence levels are shown in table 9:
TABLE 9 maximum Fault Scale at different confidence levels
Table 8 and table 9 show the first ten lines with the largest number of faults in the specified simulation days and the maximum fault scale under different confidence levels for the ac/dc hybrid system, respectively. The data given in tables 7-9 can provide reference basis for cascading failure risk and planning operation of the alternating current and direct current power system, so that measures can be taken timely to expand the capacity of overload and heavy-load lines, and the occurrence risk of cascading failure is reduced.
Fig. 3 is a graph showing a distribution of the blackout scale after the cascading failure simulation of the ac/dc hybrid system, in which the horizontal axis represents the blackout scale measured by the cut-off load amount x, and the vertical axis represents the distribution of the blackout scale. As can be seen from fig. 3, the power failure scale distribution of the improved ac-dc system cascading failure model approximately presents a power law characteristic, conforms to the actual situation, and can indirectly prove the correctness of the cascading failure simulation method.
The invention discloses an alternating current-direct current series-parallel power grid cascading failure simulation method, and belongs to the field of electrical engineering. The method is divided into a fast dynamic process and a slow dynamic process, wherein the fast dynamic process is used for simulating the cascading failure of the power system, and the slow dynamic process is used for simulating the evolution of the power system. The idea of the fast dynamic process is: when a certain line is randomly disconnected due to weather and other reasons, other lines may be sequentially disconnected due to heavy load or overload, so that a power grid cascading failure is caused; the idea of the slow dynamic process is: with the development of power systems, the power generation capacity and load level of the system are continuously improved, the line flow is correspondingly increased, when the power generation capacity and load level of the system reach the limit of the transmission capacity of the line, the line is disconnected, and the lines disconnected due to overload are generally considered to need to be modified by a power planning department, for example, the transmission capacity of the lines is increased, so that the safety of a power transmission system is improved.
By the cascading failure simulation method, a direct-current line model and direct-current protection measures can be taken into consideration, the cascading failure simulation method is suitable for cascading failure simulation of an alternating-current and direct-current hybrid power grid, and reference basis can be provided for failure risk and operation planning of an alternating-current and direct-current power system, so that measures can be taken to reduce the risk of cascading failure occurrence and improve the safety and reliability of operation of the power system.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (9)
1. A cascading failure simulation method for an alternating current-direct current hybrid power grid is characterized by comprising the following steps:
s1, determining an alternating current circuit, a direct current circuit, a tie line, a generator, a converter station and a load of an alternating current-direct current hybrid power grid; the capacity and the load level of the generator can be used for calculating the power flow of the power grid, wherein the power flow comprises active power, reactive power, active loss and reactive loss of each line, and the voltage amplitude and the phase angle of each bus node of the power grid;
s2, performing cyclic simulation on the alternating current-direct current hybrid power grid for preset times, wherein compared with the previous simulation, the capacity and the load level of the generator are improved according to a first preset proportion in each simulation, and the capacity of a line with a fault in the previous simulation is increased according to a second preset proportion;
s3, in each simulation, all lines are disconnected according to a first preset probability;
s4, if the line of the alternating current-direct current hybrid power grid is disconnected, performing power flow optimization on the disconnected alternating current-direct current hybrid power grid by adopting a branch power flow model, distributing the power of the disconnected line to the line which is not disconnected, judging whether the line reaches a heavy load or not after the optimization, if the line reaches the heavy load, entering S5, and if not, entering S6;
s5, disconnecting the heavy load line according to a second preset probability, then judging whether a new line is disconnected or not, if so, repeating the step S4, otherwise, entering the step S6;
s6, judging whether the simulation times reach preset times, if so, ending the cascading failure simulation process, otherwise, repeating the step S2;
wherein, the step S4 specifically includes the following steps:
step S401, in each simulation, all lines are possible to be cut off randomly, and if any line is cut off, a fast dynamic process is started;
step S402, carrying out primary power flow optimization on the new network after disconnection by adopting a branch power flow model, wherein the branch power flow model comprises: the method comprises the following steps of AC node active power constraint, AC node reactive power constraint, AC line active loss constraint, AC line reactive loss constraint, voltage relation constraint at two ends of an AC line, AC network phase angle difference relation constraint, current conversion node power balance constraint, voltage relation constraint at two ends of a DC line, power coupling relation constraint at a current conversion station and current conversion station transmission capacity constraint;
step S403, after the optimized scheduling is finished, judging whether a line reaches a heavy load, and if the line reaches the heavy load, entering step S5; otherwise, the process proceeds to step S6.
2. The cascading failure simulation method for the alternating current-direct current series-parallel power grid according to claim 1, wherein active power constraints of the alternating current nodes are as follows:
the reactive power constraint of the alternating current node is as follows:
wherein, P Gi And Q Gi Active and reactive power respectively, N, from the ith generator lAC Is the total number of AC lines, P cvi And Q cvi Active and reactive power, P, respectively, injected by the ith converter station rACj And Q rACj Active and reactive power, P, respectively, received at the j-th line end lsACj And Q lsACj Active and reactive power respectively lost on the j-th line, B ii Is the total susceptance to ground, W, of the ith AC node ACi Is the square of the voltage at the ith AC node, P cuti And Q cuti Respectively the active and reactive load quantities, M, of the cut-off required due to power imbalance PQAC (i, j) is a coefficient value of active power and reactive power received at the end of the line determined according to the relationship between the node i and the branch j, M lAC (i, j) is the coefficient value of the line active loss and reactive loss determined according to the relation between the node i and the branch j, P Di Is the active load at the ith AC node, Q Di Is the reactive load at the ith ac node.
3. The cascading failure simulation method for the AC-DC series-parallel power grid according to claim 2, wherein the active loss constraint of the AC line is as follows:
and the reactive loss of the alternating current line is restrained:
wherein, W rACj Representing the square of the voltage at the j-end of the line, R lACjj 、X ljj The resistance and reactance of the jth ac line, respectively.
4. The cascading failure simulation method for the alternating-current and direct-current hybrid power grid according to claim 3, wherein the voltage relation constraint at two ends of the alternating-current line is as follows:
the AC network phase angle difference relation constraint is as follows:
wherein, the matrix M WAC Is M PQAC Is a node incidence matrix of the network, C kj Is a matrix of elementary loops which is, nC is the basic number of loops, N bAC The number of nodes of the alternating current system.
5. The cascading failure simulation method for the alternating current-direct current series-parallel power grid according to claim 3, wherein the power balance constraint of the converter node is as follows:
wherein, the matrix M PDC A matrix M representing a coefficient value of active power received at the end of the DC line determined according to the relationship between the node i and the branch j lDC Representing the value of the coefficient of the active loss of the direct current line, P, determined according to the relation between node i and branch j rDCj The power transmitted to the tail end of the jth direct current line; p lsDCj Is the active loss, P, on the jth DC line cvm Injecting active power, N, for a converter station lDC Is the total number of the direct current lines.
6. The cascading failure simulation method for the alternating current-direct current series-parallel power grid according to claim 5, wherein the voltage relation constraint at two ends of the direct current line is as follows:
wherein, the matrix M WDC Is M PDC Transposed matrix of (2), R DCjj Is the resistance of the jth DC line, W DCi Is the square of the voltage at the ith DC node, N bDC The number of the nodes of the direct current system.
7. The method for simulating cascading failure of alternating current-direct current hybrid power grid according to claim 5, wherein reactive power coupling relation constraints at the converter stations are as follows:
P cvm ＝|Q cvm tanα|
wherein Q is cvm For the converter station to absorb reactive power, the angle α is a fixed value, determined by the actual mode of operation.
8. The cascading failure simulation method for the alternating current-direct current hybrid power grid according to claim 7, wherein the constraint of the transmission capacity of the converter station is as follows:
wherein S is maxcvm Representing the upper limit of the transmission capacity of the m-th converter station.
9. The method for simulating cascading failures of the alternating current-direct current hybrid power grid according to any one of claims 1 to 8, wherein the step S402 is to perform primary power flow optimization on the new network after disconnection by using a branch power flow model, and specifically comprises the following steps:
if the AC line is disconnected, the transmission power of the fixed DC line is unchanged, the DC line is regarded as the fixed load of the AC system, and the tide of the AC-DC hybrid power grid after the AC line is disconnected is determined; if the direct current line is disconnected, the power injected into the alternating current system by the corresponding converter station is 0, the reactive power injection at the converter station still exists, the transmission power of other direct current lines is fixed and is not changed, the other direct current lines are regarded as fixed loads of the alternating current system, and the power flow of the alternating current-direct current series-parallel power grid after the direct current line is disconnected is determined; if no new line is disconnected, the fast dynamic process is exited.
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