CN107634536B - Method and system for calculating maximum power transmission capacity of alternating current-direct current hybrid system - Google Patents

Method and system for calculating maximum power transmission capacity of alternating current-direct current hybrid system Download PDF

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CN107634536B
CN107634536B CN201710953877.6A CN201710953877A CN107634536B CN 107634536 B CN107634536 B CN 107634536B CN 201710953877 A CN201710953877 A CN 201710953877A CN 107634536 B CN107634536 B CN 107634536B
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耿光飞
魏子睿
陆凌芝
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China Agricultural University
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    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
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    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Abstract

The embodiment of the invention provides a method and a system for calculating the maximum power transmission capacity of an alternating current-direct current hybrid system, wherein the method for calculating the maximum power transmission capacity of the alternating current-direct current hybrid system comprises the following steps: performing inner-layer circulation according to an optimal power flow method, and obtaining the increment of the active load and the reactive load of each load node in the t-th circulation of the outer-layer circulation relative to the increment in the t-1 circulation; and performing outer circulation according to a continuous power flow method, increasing the output of a generator in a power supply area and the active and reactive loads of each load node in a power receiving area, and obtaining the voltage of each node and the power of each branch circuit in the t-th circulation through power flow calculation. The embodiment of the invention calculates the maximum power transmission capacity of the VSC-HVDC alternating current and direct current hybrid system, can quickly and effectively find the maximum TTC between the regions, determines the system state when the TTC reaches the maximum value, and has guiding significance for network planning and VSC control mode parameter determination.

Description

Method and system for calculating maximum power transmission capacity of alternating current-direct current hybrid system
Technical Field
The embodiment of the invention relates to the technical field of power transmission, in particular to a method and a system for calculating the maximum power transmission capacity of an alternating current-direct current hybrid system.
Background
With the construction of smart grids and the competition of power markets becoming more and more intense, power systems are closer to the limit operation, which brings a serious challenge to the safe and stable operation of the power systems. The maximum power transmission capacity (TTC) of the power grid refers to the maximum power which can be transmitted by a system power transmission section or an interconnected power transmission network on the premise of meeting the safe and reliable operation of the system. The TTC is an important technical index in the power market environment, and the accuracy and rapidity of calculation thereof have a crucial influence on the reliable operation of a power system and the benefit distribution of market transaction parties.
The TTC algorithms for the communication system can be classified into a deterministic algorithm and a probabilistic algorithm 2 considering uncertainty factors. The deterministic algorithm comprises the following steps: a direct current power flow method, a repeated power flow method, a continuous power flow method and an optimal power flow method based on power flow calculation; a sensitivity analysis method; genetic algorithm, artificial neural network method, etc. in the intelligent method. The uncertainty algorithm comprises: an enumeration method; the Monte Carlo method; a stochastic linear programming method; self-service methods, etc. With the wide application of large-area power grid interconnection and high-voltage direct-current power transmission, the theory and method about the power transmission capacity of the alternating-current and direct-current hybrid power grid need to be researched urgently.
With the development of fully-controlled power electronic devices, a voltage source converter type high-voltage direct current (VSC-HVDC) transmission technology based on a voltage source converter (VSC for short) and a pulse width modulation control technology is realized. Compared with the traditional phase control converter direct-current transmission technology, the VSC-HVDC has the advantages of flexible and changeable operation control mode, capability of supplying power to a passive receiving end system (or a load center) in a long distance, more economy, flexibility and the like; however, the element characteristics and the mathematical model of the VSC-HVDC are different from those of the conventional direct-current transmission, so that the original TTC calculation model of the alternating-current and direct-current hybrid system cannot be directly applied to the alternating-current and direct-current hybrid system containing the VSC-HVDC.
At present, TTC algorithms suitable for VSC-HVDC alternating current and direct current hybrid systems are relatively few. The Continuous Power Flow (CPF) method calculates TTC by continuously increasing power exchange between regions by step size. The conventional power switching scheme assumes that each load at the receiving end proportionally increases with a constant power factor, that is, if a load increase direction vector is formed by using load increments of all load nodes, the load increase direction vector in each step cycle of the conventional CPF is the same, and only the TTC of the system in a specific load increase direction can be calculated, not necessarily the maximum TTC that the system can reach. On one hand, the TTC calculation is completed by solving an optimization model, and compared with a CPF algorithm, the system state under various load states is not convenient to obtain, the weakest link influencing the TTC is not convenient to find, and the actual guiding significance on the power system planning is relatively weak; on the other hand, the objective function in the solving process does not necessarily have only one extreme value, that is, when the variable range of the independent variable is too large, the TTC to be solved may be a local maximum, and therefore, how to select an appropriate initial value of the independent variable becomes a problem.
Disclosure of Invention
Embodiments of the present invention provide a method and system for maximizing power transfer for a hybrid ac/dc system that overcomes or at least partially solves the above-mentioned problems.
According to an aspect of the present invention, there is provided a method for calculating a maximum power transmission capacity of an ac/dc hybrid system including VSC-HVDC, the method comprising:
performing inner-layer circulation according to an optimal power flow method, and obtaining the increment of the active load and the reactive load of each load node in the t-th circulation of the outer-layer circulation relative to the increment in the t-1 circulation;
performing the outer circulation according to a continuous power flow method, increasing the output of generator nodes in a power supply area and the active and reactive loads of all load nodes in a power receiving area, and obtaining the voltage of each node and the power of each branch circuit in the t circulation through power flow calculation;
and increasing the active load and the reactive load of each load node in the power receiving area according to the obtained increment of the active load and the reactive load on each load node in the t-th cycle of the outer cycle relative to the increment in the t-1 cycle.
Preferably, the method for calculating the maximum power transmission capacity of the ac/dc hybrid system further includes:
and judging and obtaining the t +1 times of circulation according to the fact that the voltage of each node and the power of each branch circuit meet the inequality constraint condition of the alternating current-direct current hybrid system during the t-th circulation.
Preferably, the method for calculating the maximum power transmission capacity of the ac/dc hybrid system further includes:
and obtaining the maximum power transmission capacity of the AC-DC hybrid system according to the node voltage and the branch power which do not meet the inequality constraint condition of the AC-DC hybrid system during the t-th cycle and the branch power during the t-1 cycle.
Preferably, the step of performing the inner-layer cycle according to the optimal power flow method to obtain the increment of the active load and the reactive load on each load node in the t-th cycle of the outer-layer cycle relative to the t-1 cycles comprises:
creating an inner layer circulation model according to an optimal power flow method, wherein the inner layer circulation model comprises an equality constraint condition of an alternating current-direct current hybrid system;
updating the equality constraint condition of the alternating current-direct current hybrid system according to the active power output of each generator node in the t-th cycle;
and solving the updated inner-layer circulation model according to an inner point method to obtain the increment of the active load and the reactive load on each load node relative to the t-1 circulation in the t-th circulation of the outer-layer circulation.
Preferably, the step of performing outer-layer circulation according to a continuous power flow method, increasing the output of the generator node in the power supply area and the active and reactive loads of each load node in the power receiving area, and obtaining the voltage of each node and the power of each branch at the t-th circulation includes:
carrying out outer circulation according to a continuous power flow method by utilizing the increment of the active and reactive loads on each load node in the t-th circulation of the outer circulation relative to the increment in the t-1 circulation, and gradually increasing the active and reactive loads of each load node in a power receiving area to obtain the active and reactive loads of each load node in the t-th circulation;
and substituting the active and reactive loads of each load node and the active output of each generator node in the t-th cycle into an equality constraint condition of the alternating current-direct current hybrid system, and calculating the voltage of each node and the power of each branch circuit in the t-th cycle through alternating current-direct current power flow.
Preferably, the step of obtaining the increment of the active load and the reactive load on each load node in the t-th cycle of the outer cycle relative to the increment in the t-1 cycle further includes:
and gradually increasing the output of the generator in the circulating power supply area according to a continuous power flow method to obtain the active output of each generator node in the t-th circulation.
Preferably, the step of obtaining the active power output of each generator node in the t-th cycle specifically includes:
for any generator node of the alternating current-direct current hybrid system, obtaining the output increment of the generator according to the product of the total output increment of the generator node in the t-th cycle of the outer cycle and the ratio of the active output of the generator node to the total output of the generator;
and obtaining the active output of the generator in the t-th cycle according to the sum of the initial value of the output of the node of the generator and the output increment.
Preferably, the inner-layer cycle model further includes inequality constraint conditions of the ac-dc hybrid system, a maximum transmission capacity objective function, a relation function between an active load of each load node in the t-th cycle and a total output increment of the generator in the t-th cycle, and a relation function between an active load and a reactive load of each load node in the t-th cycle and a load power factor angle;
and the relation function of the active load of each load node in the t-th cycle and the total output increment of the generator in the t-th cycle is as follows:
Figure BDA0001433479500000041
wherein the content of the first and second substances,
Figure BDA0001433479500000051
is the increment of the active load on the load node i in the t-th cycle of the outer cycle relative to the t-1 cycle, lambda(t)The step length of the t-th cycle in the continuous power flow method is the total output increment of the generator in the t-th cycle;
the relation function of the active and reactive loads of each load node and the load power factor angle in the t-th cycle is as follows:
Figure BDA0001433479500000052
wherein the content of the first and second substances,
Figure BDA0001433479500000053
and theta is the increment of the reactive load on the load node i relative to t-1 circulation in the outer circulation for t times, and is a load power factor angle.
Preferably, the equality constraint condition of the alternating current and direct current hybrid system comprises an equality constraint condition of an alternating current subsystem and an equality constraint condition of a direct current subsystem;
the equality constraints of the communication subsystem include:
Figure BDA0001433479500000054
wherein j ∈ i denotesAll nodes j, N connected to node issIs the set of all nodes in the AC subsystem, Gij、BijFor elements Y in the node admittance matrix YijReal and imaginary parts of, deltaij＝δi-δjIs the voltage phase angle difference between nodes i, j, Psi、QsiRespectively active and reactive, U, flowing from the node into the DC systemiIs the voltage of node i, Pi、QiRespectively the active power and the reactive power of a flow node i in the alternating current system;
the equality constraint conditions of the direct current subsystem comprise:
Figure BDA0001433479500000055
Pc＝Pd
Figure BDA0001433479500000056
Figure BDA0001433479500000061
Figure BDA0001433479500000062
wherein, Pdv、UdvNode injection power and node voltage, N, respectively, for DC node vdFor a set of DC nodes, YdvkAdmittance matrix Y for nodes of a DC subnetworkdM is the VSC modulation ratio, deltascFor alternating current fundamental voltage vector at the dividing point of alternating current and direct current systemAC output fundamental voltage vector with converter
Figure BDA0001433479500000064
The phase angle difference between the VSC converter stations, R is the equivalent resistance of all active loss in the VSC converter stationX is the equivalent reactance of the converter reactor, PdActive power flowing out of the converter station at the direct current outlet side of the converter station is shown, subscript s represents a connection point of an alternating current system and the converter station, and subscript c represents a node between equivalent impedance inside the converter station and the converter station;
the inequality constraint conditions of the alternating current and direct current hybrid system comprise inequality constraint conditions of an alternating current subsystem and inequality constraint conditions of a direct current subsystem;
the inequality constraint conditions of the communication subsystem comprise:
wherein N isGFor all generator node sets, NssFor all the AC node sets, the superscripts max and min represent the upper limit and the lower limit of the variable respectively, UsiIs the voltage of node i, SijFor apparent power flowing from node i to node j, QsiFor reactive power flowing on node i, PGi、QGiRespectively sending out active power and reactive power for a generator node i;
the inequality constraint conditions of the direct current subsystem comprise:
Figure BDA0001433479500000071
wherein, subscript N is VSC number, NCFor the set of all VSCs in the system, UdkIs the voltage on the DC node k, UcnFor the voltage on the n AC side of the converter, MnIs the modulation ratio of inverter n, IcnIs a direct current, I, flowing on the converter nkvIs a direct current flowing from node k to node v, NdIs a set of all dc nodes.
According to another aspect of the present invention, there is also provided a computing system for maximum power transmission capacity of a dc-dc hybrid system, the dc-dc hybrid system including VSC-HVDC, the computing system comprising:
the inner-layer circulation module is used for carrying out inner-layer circulation according to an optimal power flow method to obtain the increment of the active load and the reactive load of each load node in the t-th circulation of the outer-layer circulation relative to the increment in the t-1 circulation;
the outer circulation module is used for carrying out outer circulation according to a continuous power flow method, increasing the output of the generator nodes in a power supply area and the active and reactive loads of all load nodes in a power receiving area, and obtaining the voltage of each node and the power of each branch circuit in the t-th circulation;
and increasing the active load and the reactive load of each load node in the power receiving area according to the increment of the active load and the reactive load on each load node in the t-th cycle of the outer-layer cycle relative to the increment in the t-1 cycle.
The method and the system for the maximum power transmission capacity of the alternating current-direct current hybrid system provided by the embodiment of the invention are integrated into each cycle of a continuous power flow method by using an optimal power flow method with the maximum power transmission capacity on a transmission section as an objective function to form a double-layer optimization (cycle) TTC calculation method. The method comprises the steps of taking the increment of active load and reactive load on each node in outer circulation relative to the increment in previous circulation as the optimization variable of inner circulation, and continuously adjusting the change direction of the load by adopting an optimal power flow method, namely setting the increment of the load of each node in each step of circulation of the CPF as an independent variable, and taking the maximum transmission power on a transmission section as a target optimization independent variable, so as to realize load optimization distribution.
The method and the system in the embodiment of the invention calculate the maximum power transmission capacity of the VSC-HVDC alternating current and direct current hybrid system, can quickly and effectively find the maximum TTC between the regions, and determine the system state when the TTC reaches the maximum, thereby having guiding significance for network planning and VSC control mode parameter determination.
Drawings
Fig. 1 is a flow chart illustrating a method for calculating a maximum power transmission capacity of an ac/dc hybrid system according to an embodiment of the present invention;
fig. 2 is a schematic flow chart of the process of performing inner-layer circulation according to the optimal power flow method to obtain the increment of the active and reactive loads on each node in the t-th circulation of the outer-layer circulation relative to the increment in the t-1 circulation according to the embodiment of the invention;
fig. 3 is a schematic flow chart illustrating outer-layer circulation performed according to a continuous power flow method to increase the output of a generator in a power supply region and the load in a power receiving region and obtain the voltage of each node and the power of each branch circuit at the t-th circulation according to the embodiment of the present invention;
fig. 4 is a detailed flowchart of a method for calculating a maximum transmission capacity of an ac/dc hybrid system according to an embodiment of the present invention;
fig. 5 is a schematic structural diagram of a hybrid power grid based on an IEEE9 node system according to an embodiment of the present invention;
fig. 6 is a functional block diagram of a computing system for maximum power transfer capability of a ac/dc hybrid system according to an embodiment of the present invention.
Detailed Description
The following detailed description of embodiments of the present invention is provided in connection with the accompanying drawings and examples. The following examples are intended to illustrate the invention but are not intended to limit the scope of the invention.
The embodiment of the present invention provides a method for maximum power transmission capability of an ac/dc hybrid system, and for easy understanding, explanation is first made on related concepts that may be involved in this embodiment and subsequent embodiments:
the term of load flow calculation and electromechanics refers to the calculation of the distribution of active power, reactive power and voltage in the power network under the conditions of given power system network topology, element parameters, power generation parameters and load parameters. The tidal current calculation is a calculation for determining steady-state operation state parameters of each part of the power system according to the given power grid structure, parameters and operation conditions of elements such as a generator and a load. Typically given operating conditions there are power at each source and load point in the system, pivot point voltage, voltage at the balance point and phase angle. The operating state parameters to be solved comprise voltage amplitude and phase angle of each bus node of the power grid, power distribution of each branch circuit, power loss of the network and the like.
A continuous power flow method, also called continuation power flow, is a powerful tool for analyzing the voltage stability of a power system, overcomes the singularity of a Jacobian matrix by introducing a load increase coefficient on the basis of the conventional power flow, thereby overcoming the problem of convergence when the operation state is close to a stable limit operation state and solving the problems that the conventional power flow has no solution outside a collapse point and can not reliably converge near the collapse point. The continuous power flow method is that from an initial stable working point, along with the slow change of the load, the next working point is estimated and corrected along a corresponding PV curve until a complete PV curve is drawn. The basic idea of the continuous power flow method is to continuously solve the power flow (the operation point of the system) by using a prediction/calibration operator from the current working point along with the continuous increase of the load until a voltage collapse point (SNB) is obtained, and obtain the power flow solution (stability margin) of the load critical state while obtaining the whole PV curve.
The optimal power flow method is that when the structural parameters and the load condition of the system are given, available control variables (such as the output power of a generator, a tap of an adjustable transformer and the like) are adjusted to find power flow distribution which can meet all operation constraint conditions and enable a certain performance index (such as power generation cost or network loss) of the system to reach an optimal value. Classical optimal power flows often seek minimum operating costs, or minimum grid loss, minimum load shedding, maximum voltage levels, etc., subject to feasibility and safety constraints.
Fig. 1 is a schematic flow chart of a method for calculating a maximum power transmission capacity of an ac/dc hybrid system according to an embodiment of the present invention, where the ac/dc hybrid system is described as an ac/dc hybrid system including VSC-HVDC, and the method includes:
101. and performing inner-layer circulation according to an optimal power flow method, and obtaining the increment of the active load and the reactive load on each load node relative to the t-1 circulation in the t-th circulation of the outer-layer circulation.
102. And performing outer circulation according to a continuous power flow method, increasing the output of the generator nodes in a power supply area and the active and reactive loads of all load nodes in a power receiving area, and obtaining the voltage of each node and the power of each branch circuit in the t-th circulation through power flow calculation. And increasing the active load and the reactive load of each load node in the power receiving area according to the increment of the active load and the reactive load on each load node in the t-th cycle of the outer-layer cycle relative to the increment in the t-1 cycle.
It should be noted that, in the embodiment of the present invention, an optimal power flow method with the maximum transmission power on the transmission section as an objective function is merged into each cycle of the continuous power flow method to form a double-layer optimization (cycle) TTC calculation method. The method comprises the steps of taking the increment of active load and reactive load on each node in outer circulation relative to the increment in previous circulation as the optimization variable of inner circulation, and continuously adjusting the change direction of the load by adopting an optimal power flow method, namely setting the increment of the load of each node in each step of circulation of the CPF as an independent variable, and taking the maximum transmission power on a transmission section as a target optimization independent variable, so as to realize load optimization distribution.
The method in the embodiment of the invention is used for calculating the maximum power transmission capacity of the VSC-HVDC alternating current and direct current hybrid system, so that the maximum TTC between the regions can be quickly and effectively found, the system state when the TTC reaches the maximum value is determined, and the method has guiding significance for network planning and VSC control mode parameter determination.
On the basis of the above embodiment, the method for calculating the maximum power transmission capacity of the ac/dc hybrid system according to the embodiment of the present invention further includes:
and judging and obtaining the t +1 times of circulation according to the fact that the voltage of each node and the power of each branch circuit meet the inequality constraint condition of the alternating current-direct current hybrid system during the t-th circulation.
On the basis of the above embodiment, the method for calculating the maximum power transmission capacity of the ac/dc hybrid system according to the embodiment of the present invention further includes:
and obtaining the maximum power transmission capacity of the alternating current-direct current hybrid system according to the power of each branch circuit during the t-1 cycle according to the condition that the voltage of each node and the power of each branch circuit during the t-1 cycle do not meet the inequality constraint condition of the alternating current-direct current hybrid system.
It should be noted that the inequality constraint conditions of the ac/dc hybrid system provide the apparent power of each branch in the system, the reactive power flowing through each node (including the generator node and the load node), the active power and reactive power range of each generator, and the like, and when the load in the ac/dc hybrid system changes, the reactive power output by the generator, the voltage of each load node, and the current flowing in the system all change, so that the transmission capacity of the system can be reflected more truly and accurately by using the inequality constraint conditions of whether the voltage of each node and the power of each branch meet the requirements of the ac/dc hybrid system in the t-th cycle as the judgment conditions of the continuous cycle or the skip cycle. When the voltage of each node and the power of each branch circuit meet the inequality constraint condition of the alternating current-direct current hybrid system in the t-th cycle, it is indicated that the active output of each motor and the load of the load node do not reach the maximum value yet at this time, a new cycle should be performed to continuously adjust the active output of the generator and the load of the load node, and the inequality constraint condition contains a plurality of inequality relations, so that the inequality constraint condition of the alternating current-direct current hybrid system is not met in the embodiment of the invention, namely any one of the inequality relations is not met. When the node voltage and the branch power do not meet the inequality constraint condition of the alternating current-direct current hybrid system, it is indicated that the active power output of each motor and the load of the load node exceed the carrying capacity of the system at this time, new circulation cannot be performed, and the maximum power transmission capacity of the alternating current-direct current hybrid system needs to be obtained according to the node voltage and the branch power obtained at the previous time.
With reference to fig. 2, the flowchart is a schematic flow chart of the embodiment of the present invention that the inner-layer cycle is performed according to the optimal power flow method to obtain the increment of the active and reactive loads on each load node in the t-th cycle of the outer-layer cycle relative to the t-1 cycle, and as shown in the figure, the method of obtaining the increment of the active and reactive loads on each load node in the t-th cycle of the outer-layer cycle relative to the t-1 cycle includes:
201. and creating an inner layer circulation model according to the optimal power flow method, wherein the inner layer circulation model comprises equality constraint conditions of the alternating current-direct current hybrid system.
It should be noted that the equality constraint conditions of the ac/dc hybrid system include equality constraint conditions of the dc subsystem and equality constraint conditions of the ac subsystem, where the equality constraint conditions of the ac subsystem are used to express the relationship between the active power and the reactive power of the outgoing node and the voltage phase angle difference of the branch circuit and the active power and the reactive power of the incoming node into the dc system, and the equality constraint conditions of the dc subsystem are used to express the relationship between the injected power and the voltage of the dc node.
202. And updating the equality constraint conditions of the alternating current-direct current hybrid system according to the active power output of each generator node in the t-th cycle, and specifically updating the power parameters of the generator nodes.
203. And solving the updated inner-layer circulation model according to an inner point method to obtain the increment of the active load and the reactive load on each load node relative to the t-1 circulation in the t-th circulation of the outer-layer circulation. As is common knowledge in the art, the interior point method (interpolation methods) is an algorithm for solving a linear programming or nonlinear convex optimization problem. The increment of the active load and the reactive load on each node relative to the t-1 circulation when the t-th circulation of the outer circulation is obtained by the inner point method is not described any more.
With reference to fig. 3, a schematic flow chart of performing outer-layer circulation according to a continuous power flow method, increasing the output of a generator in a power supply region and the load in a power receiving region, and obtaining voltages of nodes and powers of branches in a t-th circulation according to an embodiment of the present invention is shown in the drawing, where the method includes:
301. and performing outer circulation according to a continuous power flow method by utilizing the increment of the active and reactive loads on each load node in the t-th circulation of the outer circulation relative to the increment in the t-1 circulation, gradually increasing the active and reactive loads of each load node in the power receiving area, and obtaining the active and reactive loads of each load node in the t-th circulation.
Specifically, the method for obtaining the active load and the reactive load of each load node in the t-th cycle comprises the following steps:
Figure BDA0001433479500000131
wherein, PLi、QLiRespectively active and reactive loads on the node i; the superscript "0" represents the initial value; the superscript "t" indicates the number of iterations,and respectively the increment of the active load and the reactive load on the node i when the outer layer is circulated for t times relative to the increment when the outer layer is circulated for t-1 times.
302. And substituting the active and reactive loads of each load node and the active output of each generator in the t-th cycle into an equality constraint condition of the alternating current-direct current hybrid system, and calculating the voltage of each node and the power of each branch in the t-th cycle through alternating current-direct current power flow.
On the basis of the foregoing embodiments, the step of obtaining the increment of the active load and the reactive load on each load node in the t-th cycle of the outer cycle relative to the increment in the t-1 cycle of the outer cycle in this embodiment further includes:
and gradually increasing the output of the generators in the circulating power supply area according to a continuous power flow method to obtain the active output of each generator in the t-th circulation.
On the basis of the above embodiment, the step of obtaining the active power output of each generator in the t-th cycle specifically includes:
for any generator node of the alternating current-direct current hybrid system, obtaining the output increment of the generator according to the product of the total output increment of the generator node in the t-th cycle of the outer cycle and the ratio of the active output of the generator node to the total output of the generator;
and obtaining the active output of the generator in the t-th cycle according to the initial value of the output of the generator node and the sum of the output increments.
The calculation formula of the active power output of each cycle generator is as follows:
Figure BDA0001433479500000141
wherein λ is(t)The step length of the t-th cycle in the continuous power flow method is the total output increment of the generator in the t-th cycle; kpiThe ratio of the active output of the generator i to the total output of the generator is a fixed value determined by the capacity of the generator i; pGiThe active power output of the generator i is obtained; the superscript "0" represents the initial value; the superscript t denotes the number of iterations.
On the basis of the above embodiments, the inner-layer cycle model further includes inequality constraint conditions of the ac-dc hybrid system, a maximum transmission capacity objective function, a relation function between the active load of each load node in the t-th cycle and the total output increment of the generator in the t-th cycle, and a relation function between the active load and the reactive load of each load node in the t-th cycle and the load power factor angle. The method specifically comprises the following steps:
Figure BDA0001433479500000142
in the formula:
Figure BDA0001433479500000143
and respectively optimizing variables of the active load and the reactive load on the node i in the outer layer circulation for t times of circulation relative to the increment in the t-1 circulation and the inner layer optimization of the active load and the reactive load. Theta is a load power factor angle; g is equality constraints (formulas (2) - (6)) in the mathematical model established in the step one, and h is inequality constraints (formulas (7) - (8)) in the mathematical model established in the step one; x is all variables dependent on optimization in formulas (2) to (8)
Figure BDA0001433479500000144
The dependent variable which changes through change comprises reactive power output by the generator, voltage of each load node and power flow flowing in the system; the subscripts "max", "min" denote the variables respectivelyAn upper limit and a lower limit.
On the basis of the above embodiments, the equality constraint condition of the ac-dc hybrid system includes an equality constraint condition of the ac subsystem and an equality constraint condition of the dc subsystem;
the equality constraints of the communication subsystem include:
Figure BDA0001433479500000151
wherein j e i represents all nodes j, N connected with the node issIs the set of all nodes in the AC subsystem, Gij、BijFor elements Y in the node admittance matrix YijReal and imaginary parts of, deltaij＝δi-δjIs the voltage phase angle difference between nodes i, j, Psi、QsiRespectively active and reactive, U, flowing from the node into the DC systemiIs the voltage of node i, Pi、QiRespectively the active power and the reactive power of a flow node i in the alternating current system; if the i node is not directly connected with VSC, Psi＝Qsi＝0，UiIs node i voltage, if node i is a generator node, then Pi、QiAvailable PGi、QGiIndicating that the i node is a load node Pi、QiAvailable PLi、QLiAnd (4) showing.
The equality constraint conditions of the direct current subsystem comprise:
Pc＝Pd
Figure BDA0001433479500000154
Figure BDA0001433479500000155
wherein, Pdv、UdvNode injection power and node voltage, N, respectively, for DC node vdFor a set of DC nodes, YdvkAdmittance matrix Y for nodes of a DC subnetworkdM is the VSC modulation ratio, deltascFor alternating current fundamental voltage vector at the dividing point of alternating current and direct current system
Figure BDA0001433479500000156
AC output fundamental voltage vector with converter
Figure BDA0001433479500000157
The phase angle difference between the VSC converter station and the VSC converter station is that R is the equivalent resistance of all active power losses in the VSC converter station, X is the equivalent reactance of the converter reactor, and P is the equivalent reactance of the converter reactordFor the active power flowing out of the converter station at the dc outlet side of the converter station, the subscript s indicates the connection point of the ac system to the converter station and the subscript c indicates the node between the converter station internal equivalent impedance and the converter.
The inequality constraint conditions of the alternating current and direct current hybrid system comprise inequality constraint conditions of an alternating current subsystem and inequality constraint conditions of a direct current subsystem;
the inequality constraint conditions of the communication subsystem comprise:
Figure BDA0001433479500000161
wherein j e i represents all nodes j, N connected with the node iGFor all generator node sets, NssFor all AC node sets, the superscripts "max", "min" represent the upper and lower values of the variable, UsiIs the voltage of node i, SijFor apparent power flowing from node i to node j, QsiFor reactive power flowing on node i, PGi、QGiRespectively generating active power and reactive power for a generator i;
the inequality constraint conditions of the direct current subsystem comprise:
Figure BDA0001433479500000162
wherein the subscript "N" is the VSC number, NCFor the set of all VSCs in the system, UdkIs the voltage on the DC node k, UcnFor the voltage on the n AC side of the converter, MnIs the modulation ratio of inverter n, IcnIs a direct current, I, flowing on the converter nkvIs a direct current flowing from node k to node v, NdIs a set of all dc nodes.
The maximum transmission capacity objective function is expressed as:
Figure BDA0001433479500000163
wherein, PijActive power is transmitted between nodes i and j in the alternating current system; pvkActive power is transmitted between nodes v and k in the direct current system; a and B represent the power supply area and the power receiving area, respectively, and if all the tie lines between the power supply area and the power receiving area are AC lines, P isvk＝0。
Fig. 4 shows a specific flowchart of a method for calculating maximum transmission capacity of an ac/dc hybrid system according to an embodiment of the present invention, where the method includes:
401. establishing an objective function containing the maximum transmission capacity of the VSC-HVDC alternating current and direct current hybrid system:
Figure BDA0001433479500000171
in the above formula: pijActive power is transmitted between nodes i and j in the alternating current system; pvkActive power is transmitted between nodes v and k in the direct current system; a and B denote a power feeding area and a power receiving area, respectively. If all the connecting lines between the power supply area and the power receiving area are AC lines, Pvk＝0。
402. Establishing equality constraints, including:
the equation constraint of the alternating current system is the power flow equation of the alternating current subsystem, and the specific formula is as follows:
Figure BDA0001433479500000172
in the formula: j epsilon i represents all nodes j, N connected with the node issIs the set of all nodes in the AC subsystem, Gij、BijFor elements Y in the node admittance matrix YijReal and imaginary parts of, deltaij＝δi-δjIs the voltage phase angle difference between nodes i, j, Psi、QsiRespectively active and reactive power flowing into the DC system from the node, if the node I is not directly connected with VSC, Psi＝Qsi＝0，UiIs the voltage of node i, Pi、QiRespectively the active power and the reactive power of a flow node i in an alternating current system, wherein if the node i is a generator node Pi、QiAvailable PGi、QGiIndicating that the i node is a load node Pi、QiAvailable PLi、QLiAnd (4) showing.
The direct current system equality constraint comprises a direct current network equation and a VSC converter equation, wherein the specific formula of the direct current network mode is as follows:
Figure BDA0001433479500000181
in the formula: pdv、UdvNode injection power and node voltage, N, respectively, for DC node vdFor a set of DC nodes, YdvkAdmittance matrix Y for nodes of a DC subnetworkdRow v, column k.
The VSC-converter equations include the following equations:
Pc＝Pd(4)
Figure BDA0001433479500000182
Figure BDA0001433479500000183
where M is the VSC modulation ratio, δscFor alternating current fundamental voltage vector at the dividing point of alternating current and direct current system
Figure BDA0001433479500000184
AC output fundamental voltage vector with converter
Figure BDA0001433479500000185
The phase angle difference between the two points is shown in the specification, R is equivalent resistance of all active power loss in the VSC converter station, X is equivalent reactance of a converter reactor, subscript s represents a connection point of an alternating current system and the converter station, subscript c represents a node between equivalent impedance in the converter station and a converter, and P isdThe active power flowing out of the converter station is the dc outlet side of the converter station.
403. Establishing inequality constraints, including:
and (3) communicating system inequality constraint conditions:
wherein N isGFor all generator node sets, NssFor all AC node sets, the superscripts "max", "min" represent the upper and lower values of the variable, UsiIs the voltage of node i, SijFor apparent power flowing from node i to node j, QsiFor reactive power flowing on node i, PGi、QGiRespectively the active power and the reactive power sent out by the generator node i.
The inequality constraints of the direct current system include: the method comprises the following steps of direct-current node voltage constraint, converter alternating-current side voltage constraint, modulation ratio constraint, VSC heat capacity constraint and direct-current line maximum allowable current constraint, namely:
Figure BDA0001433479500000191
wherein the subscript "N" is the VSC number, NCFor the set of all VSCs in the system, UdkIs the voltage on the DC node k, UcnFor the voltage on the n AC side of the converter, MnIs the modulation ratio of inverter n, IcnIs a direct current, I, flowing on the converter nkvIs a direct current flowing from node k to node v, NdFor all DC node sets, NcIs aggregated for all VSCs.
404. And setting a cycle mark t to be 0, establishing an outer cycle of an improved continuous power flow method based on the continuous power flow method, gradually increasing the output of the generators in a power supply area, and solving the active output of each generator of the system during the t-th cycle. The specific power increase method in each cycle is as follows
Wherein λ is(t)The step length of the t-th cycle in the continuous power flow method is the total output increment of the generator in the t-th cycle; kpiThe ratio of the active output of the generator i to the total output of the generator is a fixed value determined by the capacity of the generator i; pGiThe active power output of the generator i is obtained; the superscript "0" represents the initial value; the superscript t denotes the number of iterations.
The inner layer optimization of the improved continuous power flow method established based on the optimal power flow method is as follows:
and respectively optimizing variables of the active load and the reactive load on the node i in the outer layer circulation for t times of circulation relative to the increment in the t-1 circulation and the inner layer optimization of the active load and the reactive load. Theta is a load power factor angle; g is equality constraints (formulas (2) - (6)) in the mathematical model established in the step one, and h is inequality constraints (formulas (7) - (8)) in the mathematical model established in the step one; x is all variables dependent on optimization in formulas (2) to (8)The dependent variable which changes through change comprises reactive power output by the generator, voltage of each load node and power flow flowing in the system; the subscripts "max", "min" represent the upper and lower values of the variable, respectively.
405. Obtained in step 404
Figure BDA0001433479500000204
Substituting into equation (10), the parametric generator node power P in the equality constraint g (x) is updatedGi(if node i is a generator node, then Pi、QiAvailable PGi、QGiExpressed), then the equation (10) is solved by the interior point method to calculate
Figure BDA0001433479500000205
And
406. obtained in step 405
Figure BDA0001433479500000207
And
Figure BDA0001433479500000208
and substituting the load into an outer circulation to gradually increase the load in the power receiving area, and solving the active and reactive loads of each load node in the system during the t-th circulation. The specific power increase method in each cycle is as follows:
Figure BDA0001433479500000209
wherein, PLi、QLiRespectively active and reactive loads on the node i; the superscript "0" represents the initial value; the superscript t denotes the number of iterations.
407. P obtained in step 406Li、QLiAnd obtained in step 404
Figure BDA00014334795000002010
Substituting the formula 2-6 to obtain the voltage of each node and the power of each branch circuit when the alternating current-direct current system circulates for the t time.
408. Judging whether the voltage of each node and the power of each branch meet all inequality constraints in the step 403 when the alternating current-direct current system circulates for the t time, if so, making t equal to t +1, and returning to the step 404 to enter the next circulation of outer layer optimization; if there is at least one inequality constraint that is not satisfied, then step 409 is performed.
409. Stopping circulation, extracting the system state in t-1 times of circulation, calculating an objective function formula (1) under the system state, and outputting a calculation result as the maximum power transmission capacity of the VSC-HVDC-containing alternating current and direct current hybrid system.
Fig. 5 shows a schematic structural diagram of a hybrid grid based on an IEEE9 node system according to an embodiment of the present invention, and the validity of the algorithm proposed in the embodiment of the present invention is verified by using the structure of the hybrid grid, as shown in the figure, the system is a multi-terminal VSC-HVDC system, VSC1, VSC3, and VSC2 connected to ac nodes 4, 5, and 6, respectively, form a 4-node dc subsystem, generators 1, 2, 3, and 5 are connected to nodes 1, 2, 3, and 5, respectively, the remaining ac system data is the same as the standard calculation example of the IEEE9 node, and the dc system data is shown in tables 1 and 2. DC system UdAC voltage measurement U of current convertercThe per unit values of the current converter modulation ratio M constraint range are [1.0,1.1 ] respectively]、[0.85,1.1]And [0,1 ]]The current allowed by the converter does not exceed 1.2 p.u.. Reference value of AC voltageReference value of DC voltage
Figure BDA0001433479500000213
The ac nodes 4, 5, 6 together with the dc subsystem form a power receiving area (inside the dashed line in fig. 5) and the remaining nodes form a power supply area (outside the dashed line in fig. 5), where the TTC calculation from outside the dashed line to inside the dashed line shown in fig. 5 is discussed.
TABLE 1 DC LINE RESISTANCE METER
Line numbering Number of head and end nodes Resistance/p.u.
1 1d-2d 0.031
2 1d-3d 0.029
3 2d-3d 0.019
4 2d-4d 0.015
5 3d-4d 0.01
TABLE 2 VSC parameter Table
Figure BDA0001433479500000211
Figure BDA0001433479500000221
The method comprises the following specific steps:
step 1, according to table 1, table 2 and the above description, load flow calculation is performed on the hybrid system first, and then a mathematical model for calculating the maximum power transmission capacity of the hybrid system is established.
And 2, solving the mathematical model established in the step 1 by applying a traditional continuous power flow method, increasing the load of the power receiving area and the power generation of the power supply area in equal proportion, and calculating results are shown in table 3.
Step 3, solving the mathematical model established in the step 1 by applying an improved continuous power flow method: the power generation of the power supply area is increased in equal proportion, the load increment of each cycle is determined by using an optimal power flow method, and the calculation result is shown in table 4.
TABLE 3 TTC calculation result table of conventional continuous power flow method
Table 4 TTC calculation result table of the embodiment of the present invention
Figure BDA0001433479500000223
Figure BDA0001433479500000231
As can be seen from tables 3 and 4, a greater system TTC can be obtained by applying the modified continuous flow method.
According to another aspect of the present invention, there is also provided a system for calculating the maximum power transmission capacity of an ac/dc hybrid system, which is used for calculating the maximum power transmission capacity in the foregoing embodiments, with reference to fig. 6, so that the descriptions and definitions in the calculation methods in the foregoing embodiments can be used for understanding the execution modules in the embodiments of the present invention. Wherein the alternating current-direct current hybrid system is for containing VSC-HVDC alternating current-direct current hybrid system, computing system includes:
the inner-layer circulation module 601 is used for performing inner-layer circulation according to an optimal power flow method to obtain the increment of active and reactive loads on each load node in the t-th circulation of the outer-layer circulation relative to the increment in the t-1 circulation;
an outer circulation module 602, configured to perform outer circulation according to a continuous power flow method, increase the power output of a generator node in a power supply area and the active and reactive loads of each load node in a power receiving area, and obtain the voltage of each node and the power of each branch in the t-th circulation;
and increasing the active load and the reactive load of each load node in the power receiving area according to the increment of the active load and the reactive load on each load node in the t-th cycle of the outer-layer cycle relative to the increment in the t-1 cycle.
The above-described embodiments of the apparatus are merely illustrative, and the units described as separate parts may or may not be physically separate, and parts displayed as units may or may not be physical units, may be located in one place, or may be distributed on a plurality of network units. Some or all of the modules may be selected according to actual needs to achieve the purpose of the solution of the present embodiment. One of ordinary skill in the art can understand and implement it without inventive effort.
Through the above description of the embodiments, those skilled in the art will clearly understand that each embodiment can be implemented by software plus a necessary general hardware platform, and certainly can also be implemented by hardware. With this understanding in mind, the above-described technical solutions may be embodied in the form of a software product, which can be stored in a computer-readable storage medium such as ROM/RAM, magnetic disk, optical disk, etc., and includes instructions for causing a computer device (which may be a personal computer, a server, or a network device, etc.) to execute the methods described in the embodiments or some parts of the embodiments.
Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.

Claims (9)

1. A method for calculating the maximum transmission capacity of an AC/DC hybrid system, wherein the AC/DC hybrid system is a VSC-HVDC-containing AC/DC hybrid system, and the method comprises the following steps:
performing inner-layer circulation according to an optimal power flow method, and obtaining the increment of the active load and the reactive load of each load node in the t-th circulation of the outer-layer circulation relative to the increment in the t-1 circulation;
performing the outer circulation according to a continuous power flow method, increasing the output of generator nodes in a power supply area and the active and reactive loads of all load nodes in a power receiving area, and obtaining the voltage of each node and the power of each branch circuit in the t circulation through power flow calculation;
increasing the active load and the reactive load of each load node in the power receiving area according to the obtained increment of the active load and the reactive load on each load node in the t-th cycle of the outer cycle relative to the increment in the t-1 cycle;
the inner-layer circulation model further comprises inequality constraint conditions of the alternating current-direct current hybrid system, a maximum transmission capacity objective function, a relation function of active load of each load node in the t-th circulation and total output increment of the generator in the t-th circulation, and a relation function of active and reactive load of each load node in the t-th circulation and a load power factor angle;
and the relation function of the active load of each load node in the t-th cycle and the total output increment of the generator in the t-th cycle is as follows:
Figure FDA0002278387290000011
wherein the content of the first and second substances,is the increment of the active load on the load node i in the t-th cycle of the outer cycle relative to the t-1 cycle, lambda(t)The step length of the t-th cycle in the continuous power flow method is the total output increment of the generator in the t-th cycle; a is a power supply area;
the relation function of the active and reactive loads of each load node and the load power factor angle in the t-th cycle is as follows:
Figure FDA0002278387290000021
wherein the content of the first and second substances,
Figure FDA0002278387290000022
and theta is the increment of the reactive load on the load node i relative to t-1 circulation in the outer circulation for t times, and is a load power factor angle.
2. The method of claim 1, further comprising:
and judging and obtaining the t +1 times of circulation according to the fact that the voltage of each node and the power of each branch circuit meet the inequality constraint condition of the alternating current-direct current hybrid system during the t-th circulation.
3. The method of claim 1, further comprising:
and obtaining the maximum power transmission capacity of the AC-DC hybrid system according to the node voltage and the branch power which do not meet the inequality constraint condition of the AC-DC hybrid system during the t-th cycle and the branch power during the t-1 cycle.
4. The method for calculating the maximum transmission capacity of the ac/dc hybrid system according to claim 1, wherein the step of performing the inner-layer cycle according to the optimal power flow method to obtain the increment of the active and reactive loads on each load node at the t-th cycle of the outer-layer cycle relative to the t-1 cycles comprises:
creating an inner layer circulation model according to an optimal power flow method, wherein the inner layer circulation model comprises an equality constraint condition of an alternating current-direct current hybrid system;
updating the equality constraint condition of the alternating current-direct current hybrid system according to the active power output of each generator node in the t-th cycle;
and solving the updated inner-layer circulation model according to an inner point method to obtain the increment of the active load and the reactive load on each load node relative to the t-1 circulation in the t-th circulation of the outer-layer circulation.
5. The method for calculating the maximum transmission capacity of an ac/dc hybrid system according to claim 1, wherein the step of performing outer loop according to the continuous power flow method to increase the output of the generator node in the power supply area and the active and reactive loads of the load nodes in the power receiving area, and obtaining the voltage of each node and the power of each branch at the t-th loop comprises:
carrying out outer circulation according to a continuous power flow method by utilizing the increment of the active and reactive loads on each load node in the t-th circulation of the outer circulation relative to the increment in the t-1 circulation, and gradually increasing the active and reactive loads of each load node in a power receiving area to obtain the active and reactive loads of each load node in the t-th circulation;
and substituting the active and reactive loads of each load node and the active output of each generator node in the t-th cycle into an equality constraint condition of the alternating current-direct current hybrid system, and calculating the voltage of each node and the power of each branch circuit in the t-th cycle through alternating current-direct current power flow.
6. The method of claim 1, wherein the step of obtaining the increment of the active and reactive loads at each load node in the t-th cycle of the outer cycle relative to the t-1 cycle further comprises:
and gradually increasing the output of the generator in the circulating power supply area according to a continuous power flow method to obtain the active output of each generator node in the t-th circulation.
7. The method according to claim 6, wherein the step of obtaining the active output of each generator node at the t-th cycle comprises:
for any generator node of the alternating current-direct current hybrid system, obtaining the output increment of the generator according to the product of the total output increment of the generator node in the t-th cycle of the outer cycle and the ratio of the active output of the generator node to the total output of the generator;
and obtaining the active output of the generator in the t-th cycle according to the sum of the initial value of the output of the node of the generator and the output increment.
8. The method according to any of claims 3-7, wherein the equality constraints of the AC/DC hybrid system include equality constraints of the AC subsystem and equality constraints of the DC subsystem;
the equality constraints of the communication subsystem include:
wherein j e i represents all nodes j, N connected with the node issIs the set of all nodes in the AC subsystem, Gij、BijFor elements Y in the node admittance matrix YijReal and imaginary parts of, deltaij＝δi-δjIs the voltage phase angle difference between nodes i, j, Psi、QsiRespectively active and reactive, U, flowing from the node into the DC systemiIs the voltage of node i, Pi、QiRespectively the active power and the reactive power of a flow node i in the alternating current system;
the equality constraint conditions of the direct current subsystem comprise:
Figure FDA0002278387290000042
Pc＝Pd
Figure FDA0002278387290000043
Figure FDA0002278387290000045
wherein, Pdv、UdvNode injection power and node voltage, N, respectively, for DC node vdFor a set of DC nodes, YdvkAdmittance matrix Y for nodes of a DC subnetworkdM is the VSC modulation ratio, deltascFor alternating current fundamental voltage vector at the dividing point of alternating current and direct current system
Figure FDA0002278387290000046
AC output fundamental voltage vector with converter
Figure FDA0002278387290000047
The phase angle difference between the VSC converter station and the VSC converter station is that R is the equivalent resistance of all active power losses in the VSC converter station, X is the equivalent reactance of the converter reactor, and P is the equivalent reactance of the converter reactordActive power flowing out of the converter station at the direct current outlet side of the converter station is shown, subscript s represents a connection point of an alternating current system and the converter station, and subscript c represents a node between equivalent impedance inside the converter station and the converter station;
the inequality constraint conditions of the alternating current and direct current hybrid system comprise inequality constraint conditions of an alternating current subsystem and inequality constraint conditions of a direct current subsystem;
the inequality constraint conditions of the communication subsystem comprise:
Figure FDA0002278387290000051
wherein N isGFor all generator node setsAnd N isssFor all the AC node sets, the superscripts max and min represent the upper limit and the lower limit of the variable respectively, UsiIs the voltage of node i, SijFor apparent power flowing from node i to node j, QsiFor reactive power flowing on node i, PGi、QGiRespectively sending out active power and reactive power for a generator node i;
the inequality constraint conditions of the direct current subsystem comprise:
Figure FDA0002278387290000052
wherein, subscript N is VSC number, NCFor the set of all VSCs in the system, UdkIs the voltage on the DC node k, UcnFor the voltage on the n AC side of the converter, MnIs the modulation ratio of inverter n, IcnIs a direct current, I, flowing on the converter nkvIs a direct current flowing from node k to node v, NdIs a set of all dc nodes.
9. A computing system of maximum transmission capacity of an AC-DC hybrid system, the AC-DC hybrid system being a VSC-HVDC-containing AC-DC hybrid system, the computing system comprising:
the inner-layer circulation module is used for carrying out inner-layer circulation according to an optimal power flow method to obtain the increment of the active load and the reactive load of each load node in the t-th circulation of the outer-layer circulation relative to the increment in the t-1 circulation;
the outer circulation module is used for carrying out outer circulation according to a continuous power flow method, increasing the output of the generator nodes in a power supply area and the active and reactive loads of all load nodes in a power receiving area, and obtaining the voltage of each node and the power of each branch circuit in the t-th circulation;
increasing the active and reactive loads of each load node in the power receiving area according to the increment of the active and reactive loads on each load node in the t-th cycle of the outer cycle relative to the increment in the t-1 cycle;
the inner-layer circulation model further comprises inequality constraint conditions of the alternating current-direct current hybrid system, a maximum transmission capacity objective function, a relation function of active load of each load node in the t-th circulation and total output increment of the generator in the t-th circulation, and a relation function of active and reactive load of each load node in the t-th circulation and a load power factor angle;
and the relation function of the active load of each load node in the t-th cycle and the total output increment of the generator in the t-th cycle is as follows:
Figure FDA0002278387290000061
wherein the content of the first and second substances,
Figure FDA0002278387290000062
is the increment of the active load on the load node i in the t-th cycle of the outer cycle relative to the t-1 cycle, lambda(t)The step length of the t-th cycle in the continuous power flow method is the total output increment of the generator in the t-th cycle; a is a power supply area;
the relation function of the active and reactive loads of each load node and the load power factor angle in the t-th cycle is as follows:
Figure FDA0002278387290000063
wherein the content of the first and second substances,
Figure FDA0002278387290000064
and theta is the increment of the reactive load on the load node i relative to t-1 circulation in the outer circulation for t times, and is a load power factor angle.
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