CN108134401B - Multi-target power flow optimization and control method for alternating current-direct current hybrid system - Google Patents
Multi-target power flow optimization and control method for alternating current-direct current hybrid system Download PDFInfo
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Abstract
A multi-target power flow optimization and control method for an alternating current and direct current hybrid system belongs to the technical field of electric power. The invention aims to realize multi-target power flow optimization and control method of an alternating current-direct current hybrid system, which is used for carrying out multi-target power flow optimization on an alternating current-direct current power grid containing a direct current power flow controller, converting an optimization result into an upper-layer command of a converter station and the direct current power flow controller and sending the upper-layer command from a power grid dispatching center, and realizing coordinated control of the converter station and the direct current power flow controller. The method comprises the following steps: the dispatching control center receives data such as direct-current voltage, active power and the like obtained by measurement of an alternating-current and direct-current power grid; performing optimal power flow calculation of an alternating current-direct current power grid with a direct current power flow controller by using the measured data; according to the result of the optimal power flow, the dispatching control center sends an instruction to each converter station and the direct current power flow controller; and adjusting the running state of each converter station and the direct current power flow controller according to a pre-selected set control strategy to realize power flow optimization control. The direct current power flow controller is generally arranged at a direct current bus of the junction converter station, and the power flow control function of the direct current power flow controller is effective supplement of the power flow control of the converter station.
Description
Technical Field
The invention belongs to the technical field of electric power.
Background
With the gradual deterioration of global climate, the increasing environmental pollution and the increasing exhaustion of fossil energy, the utilization and development of renewable energy are receiving attention from various countries in the world, and especially the proportion of solar power generation, wind power generation and the like in recent years is gradually increasing. However, wind energy, solar energy and ocean energy all have the characteristics of intermittence, fluctuation, randomness, non-storability and the like, and are used for generating electricity and belong to intermittent power sources. Various large-scale renewable energy sources are gradually connected into a power grid, and various distributed energy collecting and storing devices, plug and play equipment and large-scale urban loads are rapidly increased, so that the requirements of providing multi-power supply and multi-drop point power receiving for the traditional power grid structure, the traditional operation technology, the traditional power equipment and the like are difficult to meet, and a new power grid structure, an advanced technology and a new equipment are required to meet the deep change of a future energy pattern.
Disclosure of Invention
The invention aims to realize multi-target power flow optimization and control method of an alternating current-direct current hybrid system, which is used for carrying out multi-target power flow optimization on an alternating current-direct current power grid containing a direct current power flow controller, converting an optimization result into an upper-layer command of a converter station and the direct current power flow controller and sending the upper-layer command from a power grid dispatching center, and realizing coordinated control of the converter station and the direct current power flow controller.
The method comprises the following steps:
step 1: the dispatching control center receives data such as direct-current voltage, active power and the like obtained by measurement of an alternating-current and direct-current power grid;
step 2: performing optimal power flow calculation of an alternating current-direct current power grid with a direct current power flow controller by using the measured data;
(1) establishing a multi-target power flow optimization mathematical model of an alternating current-direct current system containing a direct current power flow controller;
firstly, establishing an optimal power flow mathematical model, wherein the optimal power flow mathematical model comprises three objective functions: network loss, voltage offset, and quiescent voltage stability margin;
determining a direct current system equation in the optimal power flow mathematical model;
thirdly, determining an alternating current system equation and system constraint conditions in the optimal power flow mathematical model;
(2) solving the optimal power flow introduced by an equivalent injection power method, and performing multi-target power flow optimization on the whole alternating current and direct current power grid;
and step 3: according to the result of the optimal power flow, the dispatching control center sends an instruction to each converter station and the direct current power flow controller;
and 4, step 4: each converter station and the direct current power flow controller adjust the operation state according to a pre-selected set control strategy to realize power flow optimization control;
(1) adjusting the converter station;
(2) and adjusting the direct current power flow controller.
The optimization of the power flow model is mainly divided into the establishment of a multi-target power flow optimization mathematical model of an alternating current-direct current system containing a direct current power flow controller and the coordination and coordination of the direct current power flow controller and a converter station;
the establishment of the multi-target power flow optimization mathematical model of the alternating current and direct current system containing the direct current power flow controller comprises the following steps: three objective functions, an alternating current system equation and a direct current system equation;
(1) an objective function:
firstly, network loss is minimum:
in the formula fQRepresenting the total loss of the system; pk.lossRepresenting the loss of branch k; pi.lossRepresenting the network loss of the ith VSC; gkDenotes the conductance of branch k, IciThe current of the ith VSC; a. b and c are coefficients for calculating loss of the converter; n is a radical ofBAll the branches are collected; n is a radical ofVSCIs a VSC node set; pDCPFC.lossRepresenting the loss of the direct current power flow controller; n is a radical ofCCollecting all branch circuits connected with a converter station additionally provided with a direct current power flow controller; u shapekThe voltage value of two ends of the current converter in the line is obtained; i iskThe current value of a line connected with a converter station additionally provided with a direct current power flow controller is obtained;
secondly, the system voltage offset is minimum:
in the formula (I), the compound is shown in the specification,
representing the desired voltage value of the node i,
which represents the maximum voltage at the node i,
represents the minimum voltage of node i; n is a radical ofAAll nodes are collected; u shapeilineThe outlet voltage of the power flow controller; after the power flow controller is added, the corresponding node voltage is changed due to the influence of the transformation ratio M;
thirdly, the static voltage stability margin is maximum:
Max.vSM＝δmin (4)
in the formula, deltaminThe minimum singular value of a Jacobian matrix which is the convergence trend;
(2) an alternating system equation:
in the formula, the voltage amplitudes at the two ends of the branch are u respectivelyiAnd uj，θijIs the phase angle difference of the two nodes i and j; pGiAnd PDiRespectively the active power generation power and the load power of the node i; qGiAnd QDiRespectively the reactive power generation power and the load power of the node i; gijAnd BijRespectively representing the real part and the imaginary part of the ith row and the jth column in the node admittance matrix; n is a radical ofPQIs a PQ node set; n is a radical ofiRepresents the set of all nodes connected to node i (including itself); s represents a balanced node;
(3) direct current system equation:
the ring network type direct current power grid after the direct current power flow controller is added is specifically explained as follows:
R12、R15、R23、R25、R35、R45resistance values on five branches are respectively; vM1、VM2、VM3、VM4Respectively obtaining equivalent MMC sub-module voltage values of the direct current power flow controller installed at the converter station 5 in each line;
the direct current power flow controller is replaced by an ideal transformer model which takes the port voltage of a converter station as the input voltage and takes the outlet voltage of the power flow controller as the output voltage; the expression of the transformation ratio M of the ideal transformer is as follows:
in the formula of UlineThe voltage of a VSC converter station port is used as an input voltage, namely U, for the outlet voltage of the power flow controllerVSC；
The branch current is added after the tidal current controller is added:
in the formula, VCRepresenting the voltage across the power flow controller, followed by UCRepresents;
the additional branch transmission power is as follows:
in the formula, P12、P15、P23、P25、P35、P45Are respectively VSC1And VSC2，VSC1And VSC5，VSC2And VSC3，VSC2And VSC5，VSC3And VSC5，VSC4And VSC5The transmission power between;
the injection power of the nodes i and j of the direct current power grid before the power flow controller is not added:
in the formula: pi、PjIs the injected power of node i, j; m and n respectively represent nodes connected with the nodes i and j; y isijIs a branch path LijAdmittance of (a);
the injection power after the power flow controller is added is as follows:
in the formula:
the injection power of the nodes i and j after the power flow controller is added;
calculating the correction quantity of the power flow controller to the injected power of the nodes i and j by an equivalent injected power method, wherein the correction quantity is represented by a formula (12); therefore, the influence of the power flow controller on the system is converted into equivalent additional power added into nodes at two ends;
in the formula: pi'、Pj' is the injected power of the nodes i and j after the power flow controller is added; delta Pi、ΔPjThe correction quantity of the injection power of the nodes i and j after the power flow controller is added;
the above formula is rewritten:
after a multi-target power flow optimization mathematical model of the alternating current-direct current system containing the direct current power flow controller is established, power flow calculation is carried out;
coordination and coordination of the direct current power flow controller and the converter station:
step 1: inputting initial states A of the direct current power flow controller and all converter stations;
step 2: let N equal to 1;
and step 3: adjusting a converter station N;
and 4, step 4: judging whether the states of the converter station and the direct current power flow controller which are not adjusted and are adjusted after the converter station N is adjusted are out of limit, if so, giving up the adjustment of the converter station N, putting the N converter station at the end of an adjustment sequence, and returning to the step 3, wherein N is N + 1; if the current station N is not out of limit, adjusting the current station N, wherein N is N + 1;
and 5: judging whether all the converter stations are completely adjusted, if so, continuing to adjust the direct current power flow controller, and then ending; otherwise, go back to step 3.
The load flow calculation method comprises the following steps:
step 1: inputting original data and setting an initial voltage value;
step 2: correcting the node injection power based on an equivalent injection power method;
and step 3: forming a system admittance parameter matrix;
and 4, step 4: solving the unbalance of active power by a column write power equation;
and 5: judging whether the maximum unbalance is smaller than a set precision value or not, and if so, calculating the output results of the node power and the branch current; and if the active power is greater than the set precision value, forming a Jacobian matrix, solving the voltage correction quantity, correcting the initial value and returning to the step 2 until the active power is less than the precision value.
The direct current power flow controller is generally arranged at a direct current bus of the junction converter station, and the power flow control function of the direct current power flow controller is effective supplement of the power flow control of the converter station. On the premise of considering the economy of a power grid, network loss, static voltage stability indexes and voltage offset are added into an objective function of optimal power flow, according to different features of a static voltage stability analysis method, a minimum mode characteristic value of a Jacobian matrix of convergent power flow is selected to represent a static stability margin of a current system working point, and an alternating current and direct current hybrid system optimization model containing a direct current power flow controller and a control method for improving the running performance of an alternating current and direct current system are provided.
Drawings
FIG. 1 is a control architecture diagram of a DC power grid;
FIG. 2 is a diagram of generalized droop control;
fig. 3 is a topology structure diagram of a dc power flow controller;
FIG. 4 is a graph of line current change for a DC line;
FIG. 5 is an equivalent diagram of a looped network type DC power grid after a DC power flow controller is added;
FIG. 6 is a flow chart of power flow calculation incorporating an equivalent injected power method;
FIG. 7 is a coordination control flow diagram;
FIG. 8 is an equivalent view of the Zhoushan project according to an embodiment of the present invention;
FIG. 9 is a graph comparing the voltage at each node before and after optimization;
FIG. 10 is a comparison graph of objective functions before and after optimization;
FIG. 11 is a graph of power variation for each converter station using coordinated control;
FIG. 12 is a graph showing voltage changes at nodes under coordinated control;
fig. 13 is a diagram of the change in power of the DCPFC when the cooperative control is adopted.
Detailed Description
Through exploration and research, the direct current power grid technology based on flexible direct current power transmission becomes a research hotspot in a plurality of current schemes for solving the problem by virtue of the characteristics of flexibility, controllability, strong adaptability and the like, and the direct current power transmission network technology and construction become an important development direction and a component of a future power grid. The direct-current power grid is an energy transmission system formed by interconnection of a large number of direct-current ends in a direct-current mode, is an important platform for implementing a new energy strategy and optimizing energy resource allocation, and covers the links of power transmission, power transformation, power distribution and the like. By utilizing an advanced direct current transmission technology, the access of large-scale renewable energy power generation and large-capacity long-distance electric energy transmission can be realized, the optimal configuration of resources can be effectively realized, the problems of reliable access caused by large-scale growth of dispersive renewable energy, high-efficiency reliable power supply brought by the growth of modern city economy and the like are solved, the energy utilization efficiency can be improved, and the safe, reliable and high-quality power supply is ensured.
Compared with a two-end direct-current power transmission system, the redundancy and flexibility of the multi-end flexible direct-current power transmission system ensure the continuity of power transmission on a direct-current line and the high quality of electric energy, and the adoption of the multi-end flexible direct-current system not only helps to reduce line loss, but also helps to improve the reliability of the system. However, these advantages can be achieved without leaving many technical challenges, one of which is the control of power flow in a multi-terminal flexible dc system. For a direct current transmission system with two ends, only one transmission line is arranged between two direct current ports, and the power flow distribution is only determined by the voltage difference between two nodes and the line resistance according to ohm's law, so that the control of the power flow on the direct current line can be realized only by controlling the voltage or the current of a converter station. However, for the multi-terminal flexible dc power transmission system, each dc port may be connected with two or more dc lines at the same time, so that the multi-terminal flexible dc power transmission system includes a ring and a mesh structure inside thereof, and the number of controllable branches in the multi-terminal flexible dc system is N-1, and when the number of dc branches exceeds this number, branches with uncontrollable power flow exist in the dc power grid, that is, the degree of freedom of dc power flow control is insufficient. At the moment, only the converter station controls the power flow without adopting other means, the power flow on a direct current line in a multi-end flexible direct current system is not uniformly distributed due to the fact that the power flow of a certain line cannot be effectively controlled, and even the line is overloaded in serious cases, so that a conductor is overheated, the safe operation of the system is endangered, and therefore, the direct current power flow controller is added to improve the degree of freedom of direct current power flow control. In order to control the steady-state power flow of the direct current power grid so as to avoid overload or optimize the internal power of the direct current power grid, a method for optimizing the power flow is needed, but at present, no method for realizing coordinated control and optimization of the power flow by a converter station and a power flow controller exists, so that the research on the direct current power grid power flow optimization control method containing the direct current power flow controller is very important.
The method comprises the following steps:
step 1: the dispatching control center receives data such as direct-current voltage, active power and the like obtained by measurement of an alternating-current and direct-current power grid;
step 2: performing optimal power flow calculation of an alternating current-direct current power grid with a direct current power flow controller by using the measured data;
(1) establishing a multi-target power flow optimization mathematical model of an alternating current-direct current system containing a direct current power flow controller;
firstly, establishing an optimal power flow mathematical model, wherein the optimal power flow mathematical model comprises three objective functions: network loss, voltage offset, and quiescent voltage stability margin;
determining a direct current system equation in the optimal power flow mathematical model;
thirdly, determining an alternating current system equation and system constraint conditions in the optimal power flow mathematical model;
(2) solving the optimal power flow introduced by an equivalent injection power method, and performing multi-target power flow optimization on the whole alternating current and direct current power grid;
and step 3: according to the result of the optimal power flow, the dispatching control center sends an instruction to each converter station and the direct current power flow controller;
and 4, step 4: each converter station and the direct current power flow controller adjust the operation state according to a pre-selected set control strategy to realize power flow optimization control;
(1) adjusting the converter station;
(2) and adjusting the direct current power flow controller.
And in the step 2, the control strategy of the converter station selects a generalized droop control mode, which is a unified form of three control methods of constant power, droop and constant voltage, and the generalized droop control is realized by changing a control coefficient. The direct current power flow controller adopts a novel power flow controller based on MMC, and the control of power flow is realized by changing the voltage value of a current converter serially connected into a direct current circuit to change the circuit current. The minimum network loss, the minimum voltage offset (namely the best voltage level) and the maximum static voltage stability margin of the system are simultaneously taken as optimization targets; the alternating current system equation mainly adopts an alternating current node power equation in a polar coordinate form, a new node can be added in the system by additionally arranging the direct current power flow controller, an equivalent power injection method is adopted, and the change of the direct current power flow controller on the power flow can be converted into the correction of the node injection power by the equivalent injection power method.
In the step 4, the sequence of adjusting the direct current power flow controller and the converter station is as follows: firstly, the converter station 1 is adjusted from an initial state to a next state, whether the direct current power grid is out of limit after the converter station 1 is adjusted is checked, and if the direct current power grid is not out of limit, the adjustment of the next converter station is continued. And if the limit is out of limit, abandoning to adjust the converter station, placing the converter station at the end of the sequence of adjusting the converter stations, adjusting the next converter station 2, and so on until all the converter stations are adjusted. Finally, the DC current controller is adjusted.
The specific implementation of the method is mainly divided into three parts, namely a direct current power grid control strategy based on optimal power flow, optimization of a power flow model and example analysis;
the first part is: and (4) controlling a direct current power grid strategy based on the optimal power flow. The part mainly comprises a control framework of a direct current power grid, a control strategy of a current exchange station and a control strategy of a direct current power flow controller.
Fig. 1 shows a control architecture diagram of a dc power grid, and the specific control method is as follows:
(1) a direct current power flow controller is added in a direct current network, and direct current voltage and active power obtained by measurement of an alternating current and direct current power grid are taken as reference values to be transmitted to a dispatching center.
(2) The direct current power flow controller is equivalent to a voltage source in a circuit, and the measured voltage of each MMC sub-module of the direct current power flow controller and the measured current of the circuit are used as reference values to be transmitted to a dispatching center.
(3) And performing multi-target power flow optimization on the whole alternating current-direct current hybrid power grid according to the measured reference value.
(4) And according to the result of the multi-objective optimization, the dispatching center sends out an instruction to adjust the measured values of the converter station and the direct current power flow controller.
(5) And the data is retransmitted to the AC/DC power grid, so that the converter station and the DC power flow controller are matched with each other to control the power flow of the system.
(6) The steps are repeated in a circulating mode until the system runs in an optimal state.
A control strategy of the converter station adopting the generalized droop control mode is shown in fig. 2, and is specifically described as follows:
the generalized droop control mode is divided into three control modes:
(1) the constant voltage control is to make the dc voltage of a certain converter station a given initial value, and the converter station needs to achieve the task of maintaining power balance.
(2) The droop control does not need real-time communication between the converter stations, and each converter station is coordinated to control the power flow of the system through local control, but the direct-current voltage of the converter stations is not controlled.
(3) The power received and transmitted by the converter station with constant power control keeps constant, and the adjustment of the input and output current is realized according to different voltage values.
The three control modes have respective applicable conditions, and the invention adopts a unified form of the three control methods to realize that the system operates in an optimal state when the power flow of the system changes.
These three common control modes can be represented by a straight line:
αVDC+βP+γ＝0 (1)
in the formula, VDCIs the direct current voltage of the converter station; p is the converter station active power.
Control strategies of the dc power flow controller are shown in fig. 3 and 4, which are a topological structure diagram of the dc power flow controller and a line current variation diagram of the dc line, respectively, and are specifically described as follows.
The direct current power flow controller adopted by the invention is a novel power flow controller based on MMC, and is an expansion on the basis of a voltage source type direct current power flow controller. The power flow controller has the advantages of small rated capacity, no need of being connected with an external power supply, convenience in expansion and the like. The direct current system is assumed to be a three-terminal ring network structure, the wiring mode is a pseudo-bipolar type, the position of the direct current power flow controller is located at a converter station 1, the direct current power flow controller mainly comprises two MMC sub-modules and an alternating current transformer, and the topology of the direct current power flow controller is provided with a public alternating current bus. The dc parts of MMC1 and MMC2 are connected in series in the legs of converter station 1, 2 and in the legs of converter station 1, 3, respectively. The AC parts of the MMC1 and the MMC2 are connected with an AC transformer together to realize self power balance. The MMC1 and the MMC2 are respectively composed of 6 bridge arms, each bridge arm is composed of full-bridge sub-modules in the same number in a cascading mode, and positive and negative voltages can be output.
By changing the voltage V of MMC1 and MMC2 in the direct current power flow controller which are connected into a line in seriesM1、VM2To control V2And V3To V1To change the line current I12、I13、I23And finally realizing the control of the power flow. The line current I can be derived from known dc system parameters12、I13、I23And VM1-VM2The relationship (2) of (c). According to FIG. 4, I12And VM1-VM2Is a negative correlation, I13、I23And VM1-VM2Is positively correlated. I is12At VM1-V M20, I at 2.25kV13At VM1-V M20 at-4.05 kV, when I12And I13When the values are all 0, circulation current is generated in the system, which affects the safety and reliability of the system, so that the DC power flow controller needs to operate normally, and V is used as a referenceM1-VM2Should be in the range of-4.05-2.25 kV.
The optimization of the power flow model of the second part of the invention is mainly divided into the establishment of a multi-target power flow optimization mathematical model of an alternating current and direct current system containing a direct current power flow controller and the coordination and coordination of the direct current power flow controller and a converter station.
The establishment of the multi-target power flow optimization mathematical model of the alternating current and direct current system containing the direct current power flow controller comprises the following steps: three target functions, an alternating current system equation and a direct current system equation.
(1) An objective function:
firstly, network loss is minimum:
in the formula fQRepresenting the total loss of the system; pk.lossRepresenting the loss of branch k; pi.lossRepresenting the network loss of the ith VSC; gkDenotes the conductance of branch k, IciThe current of the ith VSC; a. b and c are coefficients for calculating loss of the converter; n is a radical ofBAll branches are collected; n is a radical ofVSCIs a VSC node set; pDCPFC.lossRepresenting the loss of the direct current power flow controller; n is a radical ofCCollecting all branches connected with a converter station provided with a direct current tidal current controller; u shapekThe voltage value of two ends of the current converter in the line is obtained; i iskThe current value of the line to which the converter station is connected is the dc current controller.
Secondly, the system voltage offset is minimum:
in the formula (I), the compound is shown in the specification,
representing the desired voltage value of the node i,
which represents the maximum voltage at the node i,
represents the minimum voltage of node i; n is a radical ofAAll nodes are collected; u shapeilineThe outlet voltage of the power flow controller; after the power flow controller is added, the corresponding node voltage changes due to the influence of the transformation ratio M.
Thirdly, the static voltage stability margin is maximum:
Max.vSM＝δmin (4)
in the formula, deltaminThe minimum singular value of a Jacobian matrix which is the convergence trend;
(2) an alternating system equation:
in the formula, the voltage amplitudes at the two ends of the branch are u respectivelyiAnd uj，θijIs the phase angle difference of the two nodes i and j; pGiAnd PDiRespectively the active power generation power and the load power of the node i; qGiAnd QDiRespectively the reactive power generation power and the load power of the node i; gijAnd BijRespectively representing the real part and the imaginary part of the ith row and the jth column in the node admittance matrix; n is a radical ofPQIs a PQ node set; n is a radical ofiRepresents the set of all nodes connected to node i (including itself); s denotes a balance node.
(3) Direct current system equation:
the equivalent diagram of the ring network type direct current power grid after the direct current power flow controller is added is shown in fig. 5, and is specifically described as follows: r12、R15、R23、R25、R35、R45Resistance values on five branches are respectively; the direction of current flow in DC system is shown as I in FIG. 512、I15、I23、I25、I35、I45Shown; vM1、VM2、VM3、VM4Respectively, the equivalent MMC sub-module voltage values in each line of the dc power flow controller installed at the converter station 5.
The direct current power flow controller is replaced by an ideal transformer model which takes the port voltage of a converter station as the input voltage and takes the outlet voltage of the power flow controller as the output voltage; the expression of the transformation ratio M of the ideal transformer is as follows:
in the formula of UlineThe voltage of the VSC converter station port is used as the outlet voltage of the power flow controllerIs an input voltage, namely UVSC。
The branch current is added after the tidal current controller is added:
in the formula, VCRepresenting the voltage across the power flow controller, followed by UCAnd (4) showing.
The additional branch transmission power is as follows:
in the formula, P12、P15、P23、P25、P35、P45Are respectively VSC1And VSC2，VSC1And VSC5，VSC2And VSC3， VSC2And VSC5，VSC3And VSC5，VSC4And VSC5The transmission power in between.
The addition of the direct current power flow controller can add new nodes in the system, and a complex direct current power grid containing a plurality of power flow controllers or a multi-ring network introduces a large amount of additional calculation to power flow calculation, such as the problems of increase of orders of system node admittance matrixes and Jacobian matrixes, element modification and the like. To solve the above problem, an equivalent injection power method is adopted.
The injection power of the nodes i and j of the direct current power grid before the power flow controller is not added:
in the formula: pi、PjIs the injected power of node i, j; m and n respectively represent nodes connected with the nodes i and j; y isijIs a branch path LijThe admittance of (1).
The injection power after the power flow controller is added is as follows:
in the formula:
is the injected power of the nodes i, j after the power flow controller is added.
Calculating the correction quantity of the power flow controller to the injected power of the nodes i and j by an equivalent injected power method, wherein the correction quantity is represented by a formula (12); therefore, the influence of the power flow controller on the system is converted into equivalent additional power added into nodes at two ends;
in the formula:
the injection power of the nodes i and j after the power flow controller is added; delta Pi、ΔPjIs the injected power correction of the nodes i and j after the power flow controller is added.
In addition, if the series voltage source type power flow controller is expressed by using an equivalent ideal transformer, the following formula is rewritten:
and after a multi-objective power flow optimization mathematical model of the alternating current-direct current system containing the direct current power flow controller is established, power flow calculation is carried out.
The invention introduces an equivalent power injection method to carry out load flow calculation, and the specific steps are shown in fig. 6.
And the dispatching center sends out a control command to correspondingly adjust the converter station and the direct current power flow controller according to the result of the optimal power flow, and the direct current power grid can operate in an optimal state after adjustment. These control commands will be fed back to each controllable commutation station or dc power flow controller at regular intervals. However, it should be noted that the coordination control of the dc power flow controller and the converter station cannot be completed instantaneously, and the adjustment needs to be performed step by step for the stability of the system.
Secondly, the invention introduces coordination control to adjust the direct current power flow controller and the converter station, and the specific steps are shown in fig. 7.
Step 1: inputting initial states A of the direct current power flow controller and all converter stations;
step 2: let N equal to 1;
and step 3: adjusting a converter station N;
and 4, step 4: judging whether the states of the converter station and the direct current power flow controller which are not adjusted and are adjusted after the converter station N is adjusted are out of limit, if so, giving up the adjustment of the converter station N, putting the N converter station at the end of an adjustment sequence, and returning to the step 3, wherein N is N + 1; if the current station N is not out of limit, adjusting the current station N, wherein N is N + 1;
and 5: judging whether all the converter stations are completely adjusted, if so, continuing to adjust the direct current power flow controller, and then ending; otherwise, go back to step 3.
The load flow calculation method comprises the following steps:
step 1: inputting original data and setting an initial voltage value;
step 2: correcting the node injection power based on an equivalent injection power method;
and step 3: forming a system admittance parameter matrix;
and 4, step 4: solving the unbalance of active power by a column write power equation;
and 5: judging whether the maximum unbalance is smaller than a set precision value or not, and if so, calculating the output results of the node power and the branch current; and if the active power is greater than the set precision value, forming a Jacobian matrix, solving the voltage correction quantity, correcting the initial value and returning to the step 2 until the active power is less than the precision value.
And a third part: example analysis
Fig. 8-11 show the specific application of the present invention in MATLAB software, and the equivalent diagram of the navian project is shown in fig. 8 and described in detail below.
In fig. 8, island 5 is an island of oceanic mountain, the island converter station operates in a constant dc voltage mode to maintain the voltage of the dc power grid constant, and islands 1, 2, 3 and 4 are respectively an island of navian, daishan, quaishan and Sireeshi, all of which operate in a constant active power mode.
The ac-dc hybrid system is optimized, and a comparison line graph of the voltages of the nodes after optimization and the voltages of the nodes before optimization is obtained as shown in fig. 9, and a comparison graph of the objective functions after optimization and before optimization is obtained as shown in fig. 10.
Referring to fig. 9 and 10, the total network loss in the multi-objective function is changed from 842.65MW before optimization to 385.45MW after optimization, the voltage offset is changed from 3.70 before optimization to 3.66 after optimization, and the static voltage stability margin is changed from 3.60 before optimization to 3.69 after optimization, which shows that the total network loss in the multi-objective function is greatly reduced after optimization compared with the previous one, the voltage offset is reduced, the static voltage stability margin is increased, and the five-terminal ac/dc system operates in an optimal state.
The power variation diagram of the present invention using cooperative control is shown in fig. 11 and described in detail below.
Referring to fig. 11, at 0.8s, the converter station 1 is adjusted first, the injection power of the converter station 1 from the initial state is adjusted to 60MW, and the converter station 1 can be adjusted if the system is checked to have no abnormal operation condition. And then, adjusting the converter station 2 at 1.2s, adjusting the injection power 160MW of the converter station 2 from the initial state to 180MW, and checking to find that the system does not have abnormal operation conditions, so that the converter station 2 can be adjusted. Then, the converter station 3 is adjusted at 2.1s, the power emitted by the converter station 3 from the initial state is adjusted to be 30MW to 40MW, and the abnormal operation state of the system does not occur after the system is checked, so that the converter station 3 can be adjusted. At 3.1s, the converter station is adjusted to 50MW from the initial state of 40MW, and the converter station 4 can be adjusted if the system is not in abnormal operation after the inspection. The converter station 5 is adjusted at 4.3s, the power emitted by the converter station 5 from the initial state is adjusted to be 150MW to 130MW, and the abnormal operation condition of the system still does not occur after the inspection, so the converter stations can be adjusted in sequence. And finally, adjusting the direct current power flow controller, and changing an initial control value of the direct current power flow controller.
According to fig. 8-11, it is demonstrated that the method can overcome the problem of uneven power flow distribution in the dc line, improve the degree of freedom of power flow control of the dc power grid, reduce network loss, make the ac/dc system operate in an optimal state, and realize the coordination control of the converter station and the dc power flow controller.
Claims (3)
1. A multi-target power flow optimization and control method for an alternating current-direct current hybrid system is characterized by comprising the following steps:
step 1: the method comprises the steps that a dispatching control center receives direct-current voltage and active power data obtained by measurement of an alternating-current and direct-current power grid;
step 2: performing optimal power flow calculation of an alternating current-direct current power grid with a direct current power flow controller by using the measured data;
(1) establishing a multi-target power flow optimization mathematical model of an alternating current-direct current system containing a direct current power flow controller;
firstly, establishing an optimal power flow mathematical model, wherein the optimal power flow mathematical model comprises three objective functions: network loss, voltage offset, and quiescent voltage stability margin;
determining a direct current system equation in the optimal power flow mathematical model;
thirdly, determining an alternating current system equation and system constraint conditions in the optimal power flow mathematical model;
(2) solving the optimal power flow introduced by an equivalent injection power method, and performing multi-target power flow optimization on the whole alternating current and direct current power grid;
and step 3: according to the result of the optimal power flow, the dispatching control center sends an instruction to each converter station and the direct current power flow controller;
and 4, step 4: adjusting the running state of each converter station and the direct current power flow controller according to a pre-selected set control strategy to realize power flow optimization control;
(1) adjusting the converter station;
(2) and adjusting the direct current power flow controller.
2. The multi-objective power flow optimization and control method for the alternating current-direct current hybrid system according to claim 1, characterized in that: the optimization of the power flow model is mainly divided into the establishment of a multi-target power flow optimization mathematical model of an alternating current-direct current system containing a direct current power flow controller and the coordination and coordination of the direct current power flow controller and a converter station;
the establishment of the multi-target power flow optimization mathematical model of the alternating current and direct current system containing the direct current power flow controller comprises the following steps: three objective functions, an alternating current system equation and a direct current system equation;
(1) an objective function:
firstly, network loss is minimum:
in the formula fQRepresenting the total loss of the system; pk.lossRepresenting the loss of branch k; pi.lossRepresenting the network loss of the ith VSC; gkDenotes the conductance of branch k, IciThe current of the ith VSC; a. b and c are coefficients for calculating loss of the converter; n is a radical ofBAll branches are collected; n is a radical ofVSCIs a VSC node set; pDCPFC.lossRepresenting the loss of the direct current power flow controller; n is a radical ofCCollecting all branch circuits connected with a converter station additionally provided with a direct current power flow controller; u shapekThe voltage value of two ends of the current converter in the line is obtained; i iskThe current value of a line connected with a converter station additionally provided with a direct current power flow controller is obtained;
secondly, the system voltage offset is minimum:
in the formula (I), the compound is shown in the specification,
representing the desired voltage value of the node i,
which represents the maximum voltage at the node i,
represents the minimum voltage of node i; n is a radical ofAAll nodes are collected; u shapeilineThe outlet voltage of the power flow controller; after the power flow controller is added, the corresponding node voltage is changed due to the influence of the transformation ratio M;
thirdly, the static voltage stability margin is maximum:
Max.vSM＝δmin (4)
in the formula, deltaminThe minimum singular value of a Jacobian matrix which is the convergence trend;
(2) an alternating system equation:
in the formula, the voltage amplitudes at the two ends of the branch are u respectivelyiAnd uj，θijIs the phase angle difference of the two nodes i and j; pGiAnd PDiRespectively the active power generation power and the load power of the node i; qGiAnd QDiRespectively the reactive power generation power and the load power of the node i; gijAnd BijRespectively representing the real part and the imaginary part of the ith row and the jth column in the node admittance matrix; n is a radical ofPQIs a PQ node set; n is a radical ofiRepresents a set of all nodes connected to node i; s represents a balanced node;
(3) direct current system equation:
the ring network type direct current power grid after the direct current power flow controller is added is specifically explained as follows:
R12、R15、R23、R25、R35、R45resistance values on five branches are respectively; vM1、VM2、VM3、VM4Are respectively mountedThe equivalent MMC sub-module voltage value of the direct current power flow controller at the converter station 5 in each line;
the direct current power flow controller is replaced by an ideal transformer model which takes the port voltage of a converter station as the input voltage and takes the outlet voltage of the power flow controller as the output voltage; the expression of the transformation ratio M of the ideal transformer is as follows:
in the formula of UlineThe voltage of a VSC converter station port is used as an input voltage, namely U, for the outlet voltage of the power flow controllerVSC；
The branch current is added after the tidal current controller is added:
in the formula, VCRepresenting the voltage across the power flow controller, followed by UCRepresents;
the additional branch transmission power is as follows:
in the formula, P12、P15、P23、P25、P35、P45Are respectively VSC1And VSC2，VSC1And VSC5，VSC2And VSC3，VSC2And VSC5，VSC3And VSC5，VSC4And VSC5The transmission power between;
the injection power of the nodes i and j of the direct current power grid before the power flow controller is not added:
in the formula: pi、PjIs the injected power of node i, j; m and n respectively represent nodes connected with the nodes i and j; y isijIs branch LijAdmittance of (a);
the injection power after the power flow controller is added is as follows:
in the formula: pi'、Pj' is the injected power of the nodes i and j after the power flow controller is added;
calculating the correction quantity of the power flow controller to the injected power of the nodes i and j by an equivalent injected power method, wherein the correction quantity is represented by a formula (12); therefore, the influence of the power flow controller on the system is converted into equivalent additional power added into nodes at two ends;
in the formula: pi'、P'jThe injection power of the nodes i and j after the power flow controller is added; delta Pi、ΔPjThe correction quantity of the injection power of the nodes i and j after the power flow controller is added;
the above formula is rewritten:
after a multi-target power flow optimization mathematical model of the alternating current-direct current system containing the direct current power flow controller is established, power flow calculation is carried out;
coordination and coordination of the direct current power flow controller and the converter station:
step 1: inputting initial states A of the direct current power flow controller and all converter stations;
step 2: let N equal to 1;
and step 3: adjusting a converter station N;
and 4, step 4: judging whether the states of the converter station and the direct current power flow controller which are not adjusted and are adjusted after the converter station N is adjusted are out of limit, if so, giving up to adjust the converter station N, putting the N converter station at the end of an adjustment sequence, and returning to the step 3, wherein N is N + 1; if the current station N is not out of limit, adjusting the current station N, wherein N is N + 1;
and 5: judging whether all the converter stations are completely adjusted, if so, continuing to adjust the direct current power flow controller, and then ending; otherwise, go back to step 3.
3. The multi-objective power flow optimization and control method for the alternating current-direct current hybrid system according to claim 2, characterized in that: the load flow calculation steps are as follows:
step 1: inputting original data and setting an initial voltage value;
step 2: correcting the node injection power based on an equivalent injection power method;
and step 3: forming a system admittance parameter matrix;
and 4, step 4: solving the unbalance of active power by a column write power equation;
and 5: judging whether the maximum unbalance is smaller than a set precision value, and if so, calculating the node power and branch current output result; and if the active power is greater than the set precision value, forming a Jacobian matrix, solving the voltage correction quantity, correcting the initial value and returning to the step 2 until the active power is less than the precision value.
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