Busbar DC voltage control method and device for offshore wind power flexible DC transmission system

Abstract

The present invention relates to the technical field of offshore wind power high voltage direct current transmission, and discloses a busbar direct current voltage control method and device for an offshore wind power flexible direct current transmission system. The method sorts the voltage of the bridge arm submodules of each phase according to the voltage size, and sums the voltage of the submodules put into operation according to the arrangement order, so as to obtain the sum of the bridge arm submodule voltages of each phase at the next moment; calculates the phase current prediction value, circulating current prediction value and bridge arm energy prediction value at the next moment; and iterates the cost function using the phase current prediction value, circulating current prediction value and bridge arm energy prediction value at different moments, so as to obtain the number of bridge arm submodules put into operation corresponding to the lowest cost function value. A direct current busbar voltage control system is established by a model prediction algorithm, and a cost function optimization control effect is established by taking the MMC operation system control requirements, circulating current suppression and upper and lower bridge arm energy balance as constraints, so that the implemented control method has good dynamic performance and control stability.

Description

Technical Field

[0001] The present invention relates to the technical field of offshore wind power high voltage direct current transmission, and in particular to a busbar direct current voltage control method and device for an offshore wind power flexible direct current transmission system. Background Art

[0002] The existing technology mainly adopts a dual closed-loop control strategy, using the outer loop to control the DC voltage. In principle, the voltage signal on the DC side is collected and compared with a given DC voltage reference value to obtain a voltage error signal. After the error signal is adjusted by a PI (proportional-integral) controller, the current reference value of the inner loop control loop is output.

[0003] However, the closed-loop PI controller has high control complexity, difficult parameter adjustment, and poor dynamic response. The PI controller mainly adjusts the system through proportional and integral links, and it is necessary to determine the optimal parameters through repeated experiments and experience, which increases the complexity and time cost of debugging. In addition, the PI controller may not be able to effectively compensate for the dynamic errors of the system. Summary of the invention

[0004] In view of this, the present invention provides a bus DC voltage control method and device for an offshore wind power flexible DC transmission system to solve the problems of high debugging complexity, difficult parameter adjustment and poor dynamic response caused by the use of a double closed-loop control strategy.

[0005] In a first aspect, the present invention provides a method for controlling a busbar DC voltage of an offshore wind power flexible DC transmission system, the method comprising:

[0006] Obtain the bridge arm current, bridge arm submodule voltage, phase current and phase voltage of each phase of the converter at the current moment;

[0007] The voltages of the bridge arm submodules of each phase are sorted according to the voltage magnitude, and the voltages of the submodules put into operation are summed according to the arrangement order to obtain the sum of the bridge arm submodules put into operation of each phase at the next moment;

[0008] Based on the DC bus voltage reference value at the next moment, the phase voltage reference value at the next moment, the sum of the bridge arm input submodule voltages of each phase at the next moment, the phase current at the current moment, and the circulating current at the current moment, the phase current prediction value at the next moment, the circulating current prediction value at the next moment, and the bridge arm energy prediction value of each phase at the next moment are calculated;

[0009] The cost function is iterated using the predicted values ​​of phase current, circulating current and bridge arm energy at different times to obtain the number of bridge arm submodules that are invested when the cost function value is the lowest.

[0010] The corresponding sub-modules are invested according to the number of bridge arm sub-modules corresponding to the lowest cost function value.

[0011] The present invention provides a bus DC voltage control method for an offshore wind power flexible DC transmission system. A DC bus voltage control system is established through a model prediction algorithm. A cost function is established to optimize the control effect based on the control requirements of the MMC operation system, circulating current suppression, and energy balance of the upper and lower bridge arms as constraints. The implemented control method has good dynamic performance and control stability.

[0012] In an optional implementation, the bridge arm submodule voltage includes an upper bridge arm submodule voltage and a lower bridge arm submodule voltage, and the bridge arm submodule voltages of each phase are sorted according to voltage magnitudes, including:

[0013] Arrange the upper bridge arm submodule voltages in ascending order and number them to establish an upper bridge arm submodule voltage matrix;

[0014] Arrange the lower bridge arm submodule voltages in ascending order and number them to establish a lower bridge arm submodule voltage matrix.

[0015] In an optional implementation, each phase has N submodules in the upper bridge arm and the lower bridge arm respectively, and the voltages of the submodules put into operation are summed in the order of arrangement to obtain the voltage sum of the submodules put into operation in each phase at the next moment, including:

[0016] Calculate the voltage sum of the first x items of the upper bridge arm submodule voltage matrix to obtain the upper bridge arm submodule voltage sum, and use the upper bridge arm submodule voltage sum as the upper bridge arm submodule voltage sum at the next moment, where x is the number of upper bridge arm submodules put into use, 0≤x≤N;

[0017] The voltage sum of the first y items of the lower bridge arm submodule voltage matrix is ​​calculated to obtain the lower bridge arm submodule voltage sum, and the lower bridge arm submodule voltage sum is used as the lower bridge arm submodule voltage sum at the next moment, y is the number of lower bridge arm submodules put into use, 0≤y≤N.

[0018] In an optional implementation, the phase current prediction value at the next moment, the circulating current prediction value at the next moment, and the bridge arm energy prediction value of each phase at the next moment are calculated based on the DC bus voltage reference value at the next moment, the phase voltage reference value at the next moment, the voltage sum of the bridge arm input submodules of each phase at the next moment, the phase current at the current moment, and the circulating current at the current moment, including:

[0019] Obtain the DC bus voltage reference value and the phase voltage reference value at the next moment, and obtain the phase current and circulating current at the current moment according to Kirchhoff's current law;

[0020] The predicted phase current value at the next moment is calculated based on the DC bus voltage reference value at the next moment, the voltage sum of the bridge arm input submodules of each phase at the next moment, and the phase current at the current moment;

[0021] The predicted circulating current value at the next moment is obtained based on the phase voltage reference value at the next moment, the voltage sum of the bridge arm input submodules of each phase at the next moment, and the circulating current at the current moment;

[0022] The predicted bridge arm energy value of each phase at the next moment is calculated based on the bridge arm input submodule voltage of each phase at the next moment and the phase current at the current moment.

[0023] In an optional implementation, the cost function is iterated using the phase current prediction value, the circulating current prediction value and the bridge arm energy prediction value at different times to obtain the number of bridge arm submodules invested corresponding to the lowest cost function value, including:

[0024] Construct a cost function, set the minimum value of the cost function, and set x and y at the current moment to 1;

[0025] Substituting the predicted value of the phase current, the predicted value of the circulating current and the predicted value of the bridge arm energy at the next moment into the cost function to calculate the cost function value, and comparing the cost function value with the minimum value;

[0026] When the cost function value is less than the minimum value, the cost function value is used as the minimum value at the next moment, and the number of upper bridge arm submodules invested is determined according to the value of x at the current moment, and the number of lower bridge arm submodules invested is determined according to the value of y at the current moment.

[0027] In an optional implementation, the cost function is iterated using the phase current prediction value, the circulating current prediction value and the bridge arm energy prediction value at different times to obtain the number of bridge arm submodules invested corresponding to the lowest cost function value, and further includes:

[0028] When the cost function value is not less than the minimum value, determining whether y is equal to N at the current moment;

[0029] If y is not equal to N at the current moment, then y is increased by 1, and the step of calculating the sum of the voltages of the submodules put into operation by each phase of the bridge arm at the next moment is returned;

[0030] If y is equal to N at the current moment, then determine whether x is equal to N at the current moment;

[0031] If x is not equal to N at the current moment, then x is increased by 1, and the process returns to the step of calculating the sum of the voltages of the sub-modules of each phase of the bridge arm at the next moment, until x is equal to N at the current moment.

[0032] The number of upper and lower bridge arm submodules that are turned on corresponding to the lowest cost function value is found through a cyclic iteration method. The iterative process has high accuracy, a simple algorithm, and a small error, thereby improving the stability of the calculation process.

[0033] In an optional implementation, investing in corresponding submodules according to the investment quantity of the bridge arm submodule corresponding to the lowest cost function value includes:

[0034] According to the x value corresponding to the lowest cost function value, the first x submodules in the upper bridge arm submodule voltage matrix are controlled to be put into operation;

[0035] According to the y value corresponding to the lowest cost function value, the first y sub-modules in the voltage matrix of the lower bridge arm sub-module are controlled to be put into operation.

[0036] Through the principle of minimizing the cost function, the optimal combination of switch states is selected in each control cycle, so as to achieve comprehensive optimization of the overall performance of the MMC system and realize the effects of stabilizing the DC bus voltage, suppressing current harmonics, and balancing the capacitor voltage of the sub-module.

[0037] In a second aspect, the present invention provides a busbar DC voltage control device for an offshore wind power flexible DC transmission system, the device comprising:

[0038] An acquisition module is used to obtain the bridge arm current, bridge arm submodule voltage, phase current and phase voltage of each phase of the converter at the current moment;

[0039] A summing module is used to sort the voltages of the bridge arm submodules of each phase according to the voltage magnitude, and sum the voltages of the submodules put into operation according to the arrangement order to obtain the sum of the voltages of the bridge arm submodules put into operation of each phase at the next moment;

[0040] A calculation module is used to calculate the predicted phase current value at the next moment, the predicted circulating current value at the next moment, and the predicted bridge arm energy value of each phase at the next moment based on the DC bus voltage reference value at the next moment, the phase voltage reference value at the next moment, the voltage sum of the bridge arm input submodules of each phase at the next moment, the phase current at the current moment, and the circulating current at the current moment;

[0041] A prediction module is used to iterate the cost function using the predicted values ​​of phase current, circulating current and bridge arm energy at different times to obtain the number of bridge arm submodules invested when the cost function value is the lowest;

[0042] The investment module is used to invest the corresponding sub-module according to the investment quantity of the bridge arm sub-module corresponding to the lowest cost function value.

[0043] The present invention provides a bus DC voltage control device for an offshore wind power flexible DC transmission system. A DC bus voltage control system is established through a model prediction algorithm. A cost function is established to optimize the control effect based on the control requirements of the MMC operation system, circulating current suppression, and energy balance of the upper and lower bridge arms as constraints. The implemented control method has good dynamic performance and control stability.

[0044] In a third aspect, the present invention provides a computer device, comprising: a memory and a processor, the memory and the processor being communicatively connected to each other, the memory storing computer instructions, and the processor executing the bus DC voltage control method of an offshore wind power flexible DC transmission system according to the first aspect or any corresponding embodiment thereof by executing the computer instructions.

[0045] In a fourth aspect, the present invention provides a computer-readable storage medium having computer instructions stored thereon, the computer instructions being used to enable a computer to execute the bus DC voltage control method for an offshore wind power flexible DC transmission system of the above-mentioned first aspect or any corresponding embodiment thereof. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0047] Figure 1 It is a flow chart of a busbar DC voltage control method of an offshore wind power flexible DC transmission system according to an embodiment of the present invention;

[0048] Figure 2 : is an equivalent circuit diagram of the upper and lower bridge arms of the MMC converter according to an embodiment of the present invention;

[0049] Figure 3 is a control block diagram of a busbar DC voltage control method for an offshore wind power flexible DC transmission system according to an embodiment of the present invention;

[0050] Figure 4 It is a flow chart of another method for controlling busbar DC voltage of an offshore wind power flexible DC transmission system according to an embodiment of the present invention;

[0051] Figure 5 is a structural block diagram of a busbar DC voltage control device for an offshore wind power flexible DC transmission system according to an embodiment of the present invention;

[0052] Figure 6 It is a schematic diagram of the hardware structure of a computer device according to an embodiment of the present invention. DETAILED DESCRIPTION

[0053] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0054] In order to solve the problems of high debugging complexity, difficult parameter adjustment and poor dynamic response caused by the double closed-loop control strategy, this embodiment provides a busbar DC voltage control method for an offshore wind power flexible DC transmission system. This embodiment takes a three-phase converter as an example. Figure 1 As shown in the figure, the busbar DC voltage control of the offshore wind power flexible DC transmission system includes:

[0055] Step S11, obtaining the bridge arm current, bridge arm submodule voltage, phase current and phase voltage of each phase of the converter at the current moment.

[0056] Specifically, the upper arm current at the current moment (i.e., moment K) is obtained by sampling the voltage and current sensors. i ju ( k ), lower bridge arm current i jl ( k ), phase current i js (k) , upper arm submodule voltage U u1 ...U un , lower bridge arm submodule voltage U l1 ...U ln And the phase voltage at time K U j ( k ), providing a basis for calculation in subsequent steps.

[0057] Establish the time domain model of the converter, and the equivalent circuits of the upper and lower bridge arms of the MMC converter are as follows: Figure 2 As shown. Among them, , , express A , B , C Phase voltage, , , express A , B , C Phase current, is the DC bus voltage, , Indicates the sum of the upper and lower bridge arm input submodule voltages, represents the bridge arm inductance, represents the bridge arm resistance, , Represents the upper and lower bridge arm currents, represents circulation, R 0 represents the impedance on the load side of a single-phase circuit ，L 0 represents the inductive reactance on the load side in a single-phase circuit.

[0058] Step S12, sorting the voltages of the bridge arm submodules of each phase according to the voltage magnitude, and summing the voltages of the submodules put into operation according to the arrangement order, to obtain the sum of the bridge arm submodule voltages of each phase at the next moment.

[0059] Specifically, it is assumed that each phase has N submodules in the upper bridge arm and the lower bridge arm, respectively. After the voltages of the upper bridge arm submodules of each phase are arranged in ascending order, the sum of the capacitor voltages of the first x submodules of the upper bridge arm of each phase is obtained, and the sum of the voltage is used as the sum of the voltages of the upper bridge arm input submodules of each phase at the next moment, where x is the number of upper bridge arm submodules input. Similarly, after the voltages of the lower bridge arm submodules of each phase are arranged in ascending order, the sum of the capacitor voltages of the first y submodules of the lower bridge arm of each phase is obtained, and the sum of the voltage is used as the sum of the voltages of the lower bridge arm input submodules of each phase at the next moment, where y is the number of lower bridge arm submodules input.

[0060] Step S13, based on the DC bus voltage reference value at the next moment, the phase voltage reference value at the next moment, the sum of the bridge arm input sub-module voltages of each phase at the next moment, the phase current at the current moment, and the circulating current at the current moment, the phase current prediction value at the next moment, the circulating current prediction value at the next moment, and the bridge arm energy prediction value of each phase at the next moment are calculated.

[0061] Specifically, the DC bus voltage reference value at the next moment (i.e., moment K+1) and the phase voltage reference value at the next moment are read, and the above reference values ​​are brought into the prediction model to calculate the phase current prediction value at the next moment, the circulating current prediction value at the next moment, and the bridge arm energy prediction value of each phase at the next moment.

[0062] Step S14, using the phase current prediction values, circulating current prediction values ​​and bridge arm energy prediction values ​​at different times to iterate the cost function, and obtain the number of bridge arm sub-modules corresponding to the lowest cost function value.

[0063] Specifically, the predicted value of the phase current at the next moment, the predicted value of the circulating current at the next moment, and the predicted value of the bridge arm energy of each phase at the next moment are substituted into the cost function to calculate the cost function value. After cyclic processing and judgment, the number x of upper bridge arm sub-modules and the number y of lower bridge arm sub-modules put into use when the cost function value is the smallest are obtained.

[0064] Step S15 , investing corresponding sub-modules according to the investment quantity of the bridge arm sub-modules corresponding to the lowest cost function value.

[0065] Specifically, according to the values ​​of x and y corresponding to the minimum value of the cost function, the first x submodules of each phase upper bridge arm and the first y submodules of each phase lower bridge arm are controlled to be turned on, and the remaining submodules are turned off. By minimizing the cost function, the corresponding submodules are controlled to ensure the balance of energy between the upper and lower bridge arms and the suppression of circulating current. For the specific control block diagram, see Figure 3 .

[0066] The present invention provides a bus DC voltage control method for an offshore wind power flexible DC transmission system. A DC bus voltage control system is established through a model prediction algorithm. A cost function is established to optimize the control effect based on the control requirements of the MMC operation system, circulating current suppression, and energy balance of the upper and lower bridge arms as constraints. The implemented control method has good dynamic performance and control stability.

[0067] In an optional implementation, the bridge arm submodule voltage includes an upper bridge arm submodule voltage and a lower bridge arm submodule voltage, and the bridge arm submodule voltages of each phase are sorted according to voltage magnitude, including:

[0068] Step S121 , arranging and numbering the upper bridge arm submodule voltages in ascending order, and establishing an upper bridge arm submodule voltage matrix.

[0069] Specifically, the upper bridge arm submodule voltages of each phase are sorted and arranged in ascending order to form an upper bridge arm submodule voltage matrix: U u , and put the corresponding submodule numbers into the upper bridge arm submodule number matrix in order N u .

[0070] Step S122 , arranging the lower bridge arm submodule voltages in ascending order and numbering them, and establishing a lower bridge arm submodule voltage matrix.

[0071] Specifically, the lower bridge arm submodule voltages of each phase are sorted and arranged in ascending order to form a lower bridge arm submodule voltage matrix: U l , and put the corresponding submodule numbers into the lower bridge arm submodule number matrix in order N l .

[0072] In an optional implementation, each phase has N submodules in the upper bridge arm and the lower bridge arm respectively, and the voltages of the submodules put into operation are summed in the order of arrangement to obtain the voltage sum of the submodules put into operation in each phase at the next moment, including:

[0073] Step S123, calculating the voltage sum of the first x items of the upper arm submodule voltage matrix, obtaining the upper arm submodule voltage sum, and using the upper arm submodule voltage sum as the upper arm submodule voltage sum at the next moment, where x is the number of upper arm submodules put into use, 0≤x≤N.

[0074] Specifically, read the upper bridge arm submodule number matrix of each phase N u The serial number in N u (i) (i.e. the number of upper bridge arm submodules put into use is x), and based on the serial number N u (i) Read the upper bridge arm submodule voltage matrix U u The corresponding first x upper bridge arm submodule voltages in the upper bridge arm submodule voltage matrix is ​​calculated. U u The voltage sum of the first x items of the upper bridge arm submodule is obtained U uj (k +1)， and will U uj (k+1) As the sum of the upper bridge arm input submodule voltages at the next moment.

[0075] Step S124, calculate the voltage sum of the first y items of the lower bridge arm submodule voltage matrix to obtain the lower bridge arm submodule voltage sum, and use the lower bridge arm submodule voltage sum as the lower bridge arm submodule voltage sum at the next moment, y is the number of lower bridge arm submodules put into use, 0≤y≤N.

[0076] Specifically, read the lower bridge arm submodule number matrix of each phase N l The serial number in N l (i) (i.e. the number of lower bridge arm submodules put into use is y), and based on the serial number N l (i) Read the voltage matrix of the lower bridge arm submodule U l The voltage matrix of the lower bridge arm submodule is calculated by the first y items of the corresponding lower bridge arm submodule voltage. U l The voltage sum of the first y items of the lower bridge arm submodule is obtained U lj (k +1)， and will U lj (k+1)As the sum of the lower bridge arm input submodule voltages at the next moment.

[0077] In an optional implementation, the above step S13 includes:

[0078] Step S131, obtaining a DC bus voltage reference value at the next moment and a phase voltage reference value at the next moment, and obtaining the phase current and circulating current at the current moment according to Kirchhoff's current law.

[0079] Specifically, according to Kirchhoff's voltage law, the upper and lower bridge arms of each phase have the following formula:

[0080]

[0081] in, U jo is the j-phase voltage ，i ju is the j-phase upper arm current, i jl is the lower arm current of phase j ,j=A,B,C .

[0082] According to Kirchhoff's current law, the current in each phase has the following formula:

[0083]

[0084] in, i js is the j-phase current ,j=A,B,C .

[0085] Combining (1) and (2) we get:

[0086] (3)

[0087] (4)

[0088] The phase current and circulating current differential equations are obtained:

[0089] (5)

[0090] (6)

[0091] Step S132, based on the DC bus voltage reference value at the next moment, the sum of the bridge arm input submodule voltages of each phase at the next moment, and the phase current at the current moment, the phase current prediction value at the next moment is calculated.

[0092] Specifically, the predicted value of the phase current at the next moment is obtained according to the backward Euler formula:

[0093]

[0094] in, i js (k+1) is the predicted value of phase j current at time k+1, T s is a control cycle time, U dc * (k+1) is the DC bus voltage reference value at time k+1, U u (k+1) is the sum of the upper bridge arm input submodule voltages of each phase at time k+1, U l (k+1) is the sum of the submodule voltages of the lower bridge arm of each phase at time k+1, i js (k) is the predicted value of phase j current at the current moment.

[0095] According to the phase current prediction formula (7), the phase current prediction value is determined by the DC bus voltage reference value , the AC current value obtained by sampling at time k And the predicted voltage values ​​of the upper bridge arm and lower bridge arm submodules at time k+1 , It is obtained that the predicted value of the AC side phase current and the voltage of the upper and lower bridge arm sub-modules under stable control are very close to the actual value corresponding to the cycle. The DC bus voltage can be stabilized by accurately sampling and controlling the AC side current value and the upper and lower bridge arm voltage values.

[0096] Step S133, based on the phase voltage reference value at the next moment, the sum of the bridge arm input submodule voltages of each phase at the next moment, and the circulating current at the current moment, the circulating current prediction value at the next moment is calculated.

[0097] Specifically, similarly, according to the backward Euler formula, the predicted value of the circulation at the next moment is:

[0098] (8)

[0099] in, i c (k+1) is the circulation prediction value at time k+1, U jo * (k+1) is the voltage reference value of phase j at time k+1, i c (k) is the circulation at the current moment.

[0100] For circulating current suppression, it can be achieved by formula (8), where the phase voltage reference value at time k+1 is , Voltage prediction values ​​of upper and lower bridge arm submodules , And the bridge arm current sampled at time k Substitute into formula (8) to calculate the circulation prediction value.

[0101] Step S134, based on the bridge arm input submodule voltage of each phase at the next moment and the phase current at the current moment, the bridge arm energy prediction value of each phase at the next moment is calculated.

[0102] Specifically, according to the energy balance principle of the upper and lower bridge arms, there is an expression:

[0103] (9)

[0104] (10)

[0105] in, is the energy prediction value of the upper and lower bridge arms of phase j at time k+1, It represents the submodule voltage and predicted value of the upper and lower bridge arms of phase j at time k+1. It represents the sum of the submodule voltages of the upper and lower bridge arms of phase j at time k, Indicates the number of submodules put into use in the upper and lower bridge arms of phase j, It represents the upper and lower bridge arm currents of phase j at time k obtained by sampling, i=u,l , C is the submodule capacitance value, and N is the number of upper / lower bridge arm submodules.

[0106] In an optional implementation, the above step S14 includes:

[0107] Step S141, construct a cost function, set the minimum value of the cost function, and set x and y at the current moment to 1.

[0108] Specifically, the cost function is established as follows:

[0109] (11)

[0110] in, i js * ( K+ 1) is the j-phase current reference value at time K+1, i jc * ( K+ 1) is the reference value of the j-phase circulating current at time K+1, i jc (k+1) is the predicted value of the j-phase circulation at time K+1, λ 1. λ 2. λ 3 are prediction coefficients, which can be set manually.

[0111] For further reference, Figure 4 , set K moment to the current moment, and set the initial cost function minimum value to J min =10 8 , set x to cycle from 1 to N, and y to cycle from 1 to N, that is, at time K, the first x submodules in the voltage matrix of the upper bridge arm submodule of each phase are put into operation, and the first y submodules in the voltage matrix of the lower bridge arm submodule of each phase are put into operation, and the upper bridge arm submodule voltage matrix is ​​calculated respectively U u The first x voltages and U uj (k+1) And the lower bridge arm submodule voltage matrix U l The voltage of the first y terms and U lj (k+1) .

[0112] Step S142, substitute the predicted value of the phase current, the predicted value of the circulating current and the predicted value of the bridge arm energy at the next moment into the cost function to calculate the cost function value, and compare the cost function value with the minimum value.

[0113] Specifically, refer to Figure 4 , the predicted phase current value at the next moment i js (k+1) , the circulation prediction value at the next moment i jc (k+1) And the predicted value of the bridge arm energy of each phase at the next moment W ji (k+1) Substitute the cost function into the cost function to calculate the cost function value and compare the cost function value with the minimum value J min Make a comparison.

[0114] Step S143, when the cost function value is less than the minimum value, the cost function value is used as the minimum value of the next moment, and the number of upper bridge arm submodules invested is determined according to the value of x at the current moment, and the number of lower bridge arm submodules invested is determined according to the value of y at the current moment.

[0115] Specifically, refer to Figure 4 , if the calculated cost function value J Less than J min , then the current cost function value JAs the minimum value of the cost function at the next moment J min , and determine the number of upper bridge arm submodules according to the current value of x, and determine the number of lower bridge arm submodules according to the current value of y. J Greater than or equal to the minimum cost function value J min , then go to step S144.

[0116] Step S144, when the cost function value is not less than the minimum value, determine whether y is equal to N at the current moment.

[0117] Step S145, if y is not equal to N at the current moment, y is increased by 1, and the process returns to the step of calculating the sum of the voltages of the submodules put into operation in each phase of the bridge arm at the next moment.

[0118] Specifically, if the number y of lower bridge arm submodules put into use at the current moment is not equal to N, the number y of lower bridge arm submodules put into use is increased by 1 at the next moment, and the process returns to step S12 to start the iterative calculation at the next moment.

[0119] Step S146: if the current time y is equal to N, determine whether the current time x is equal to N.

[0120] Specifically, after all submodules of the lower bridge arm are cyclically calculated, all submodules of the upper bridge arm are cyclically processed and judged.

[0121] Step S147, if x is not equal to N at the current moment, then increase x by 1, and return to the step of calculating the sum of the voltages of the sub-modules of each phase at the next moment, until x is equal to N at the current moment.

[0122] Specifically, if the number x of upper bridge arm submodules at the current moment is not equal to N, it indicates that all upper bridge arm submodules have not been put into the loop calculation at this moment, and the number of lower bridge arm submodules put into the loop is controlled to remain unchanged, and the number x of upper bridge arm submodules put into the loop is increased by 1 at the next moment, and the process returns to step S12 to start the iterative calculation at the next moment until x is equal to N. At this point, the loop processing and judgment of all upper bridge arm submodules and all lower bridge arm submodules are completed, and the iteration ends.

[0123] The number of upper and lower bridge arm submodules that are turned on corresponding to the lowest cost function value is found through a cyclic iteration method. The iterative process has high accuracy, a simple algorithm, and a small error, thereby improving the stability of the calculation process.

[0124] In an optional implementation, the above step S15 includes:

[0125] Step S151 , according to the x value corresponding to the lowest cost function value, controlling the first x submodules in the upper bridge arm submodule voltage matrix to be put into operation.

[0126] Step S152 , according to the y value corresponding to the lowest cost function value, the first y sub-modules in the lower bridge arm sub-module voltage matrix are controlled to be put into operation.

[0127] Specifically, refer to Figure 4 According to the x value corresponding to the lowest cost function value, the first x sub-modules in the upper bridge arm sub-module voltage matrix are put into operation, and according to the y value corresponding to the lowest cost function value, the first y sub-modules in the lower bridge arm sub-module voltage matrix are put into operation. Other sub-modules remain in the off state. The current sub-module input state corresponds to the lowest cost function value, that is, the highest efficiency and the lowest power consumption.

[0128] The present invention provides a busbar DC voltage control method for an offshore wind power flexible DC transmission system, which optimizes the control effect through model predictive control, and can directly handle MMC systems with multiple variables, multiple system constraints and nonlinearity. By designing a system objective function containing multiple control objectives, this embodiment can select the optimal switch state combination in each control cycle, thereby achieving comprehensive optimization of the overall performance of the MMC system, and achieving the effects of stabilizing the DC busbar voltage, suppressing current harmonics, balancing the submodule capacitor voltage, etc. Compared with the traditional PI control strategy, it has the advantages of fast dynamic response, superior control performance and simple control structure.

[0129] In this embodiment, a busbar DC voltage control device for an offshore wind power flexible DC transmission system is also provided, which is used to implement the above-mentioned embodiments and preferred implementation modes, and will not be repeated hereafter. As used below, the term "module" can be a combination of software and / or hardware that implements a predetermined function. Although the devices described in the following embodiments are preferably implemented in software, the implementation of hardware, or a combination of software and hardware, is also possible and conceivable.

[0130] This embodiment provides a busbar DC voltage control device for an offshore wind power flexible DC transmission system, such as Figure 5 As shown, including:

[0131] The acquisition module 51 is used to acquire the bridge arm current, bridge arm submodule voltage, phase current and phase voltage of each phase of the converter at the current moment.

[0132] The summing module 52 is used to sort the bridge arm submodule voltages of each phase according to the voltage magnitude, and sum the voltages of the submodules put into operation according to the arrangement order, so as to obtain the sum of the bridge arm submodule voltages of each phase at the next moment.

[0133] The calculation module 53 is used to calculate the phase current prediction value at the next moment, the circulating current prediction value at the next moment and the bridge arm energy prediction value of each phase at the next moment based on the DC bus voltage reference value at the next moment, the phase voltage reference value at the next moment, the voltage sum of the bridge arm input submodules of each phase at the next moment, the phase current at the current moment, and the circulating current at the current moment.

[0134] The prediction module 54 is used to iterate the cost function using the phase current prediction values, circulating current prediction values ​​and bridge arm energy prediction values ​​at different times to obtain the number of bridge arm sub-modules invested corresponding to the lowest cost function value.

[0135] The input module 55 is used to input corresponding sub-modules according to the input quantity of the bridge arm sub-module corresponding to the lowest cost function value.

[0136] The further functional description of each of the above modules and units is the same as that of the above corresponding embodiments and will not be repeated here.

[0137] The bus DC voltage control device of the offshore wind power flexible DC transmission system in this embodiment is presented in the form of a functional unit, where the unit refers to an ASIC (Application Specific Integrated Circuit) circuit, a processor and memory that executes one or more software or fixed programs, and / or other devices that can provide the above functions.

[0138] The embodiment of the present invention also provides a computer device having the above Figure 5 The busbar DC voltage control device of the offshore wind power flexible DC transmission system is shown.

[0139] See also Figure 6 , Figure 6 is a schematic diagram of the structure of a computer device provided by an optional embodiment of the present invention, such as Figure 6 As shown, the computer device includes: one or more processors 10, a memory 20, and interfaces for connecting various components, including high-speed interfaces and low-speed interfaces. Various components are connected to each other using different buses for communication, and can be installed on a common mainboard or installed in other ways as needed. The processor can process the instructions executed in the computer device, including instructions stored in or on the memory to display the graphical information of the GUI on an external input / output device (such as, a display device coupled to the interface). In some optional embodiments, if necessary, multiple processors and / or multiple buses can be used together with multiple memories and multiple memories. Similarly, multiple computer devices can be connected, and each device provides some necessary operations (for example, as a server array, a group of blade servers, or a multi-processor system). Figure 6 A processor 10 is taken as an example.

[0140] The processor 10 may be a central processing unit, a network processor or a combination thereof. The processor 10 may further include a hardware chip. The hardware chip may be a dedicated integrated circuit, a programmable logic device or a combination thereof. The programmable logic device may be a complex programmable logic device, a field programmable gate array, a general purpose array logic or any combination thereof.

[0141] The memory 20 stores instructions executable by at least one processor 10, so that the at least one processor 10 executes the method shown in the above embodiment.

[0142] The memory 20 may include a program storage area and a data storage area, wherein the program storage area may store an operating system, an application required for at least one function; the data storage area may store data created according to the use of the computer device, etc. In addition, the memory 20 may include a high-speed random access memory, and may also include a non-transient memory, such as at least one disk storage device, a flash memory device, or other non-transient solid-state storage device. In some optional embodiments, the memory 20 may optionally include a memory remotely arranged relative to the processor 10, and these remote memories may be connected to the computer device via a network. Examples of the above-mentioned network include, but are not limited to, the Internet, an intranet, a local area network, a mobile communication network, and combinations thereof.

[0143] The memory 20 may include a volatile memory, such as a random access memory; the memory may also include a non-volatile memory, such as a flash memory, a hard disk or a solid state drive; the memory 20 may also include a combination of the above types of memory.

[0144] The computer device further comprises a communication interface 30 for the computer device to communicate with other devices or a communication network.

[0145] The embodiment of the present invention also provides a computer-readable storage medium. The method according to the embodiment of the present invention can be implemented in hardware, firmware, or can be implemented as a computer code that can be recorded in a storage medium, or can be implemented as a computer code that is originally stored in a remote storage medium or a non-temporary machine-readable storage medium and will be stored in a local storage medium through a network download, so that the method described herein can be stored in such software processing on a storage medium using a general-purpose computer, a dedicated processor, or programmable or dedicated hardware. Among them, the storage medium can be a magnetic disk, an optical disk, a read-only storage memory, a random access memory, a flash memory, a hard disk or a solid-state hard disk, etc.; further, the storage medium can also include a combination of the above types of memories. It can be understood that a computer, a processor, a microprocessor controller, or programmable hardware includes a storage component that can store or receive software or computer code. When the software or computer code is accessed and executed by a computer, a processor, or hardware, the method shown in the above embodiment is implemented.

[0146] Although the embodiments of the present invention have been described in conjunction with the accompanying drawings, those skilled in the art may make various modifications and variations without departing from the spirit and scope of the present invention, and such modifications and variations are all within the scope defined by the appended claims.

Claims

1. A busbar DC voltage control method for an offshore wind power flexible DC transmission system, characterized in that: The method comprises: Obtain the bridge arm current, bridge arm submodule voltage, phase current and phase voltage of each phase of the converter at the current moment; The voltages of the bridge arm submodules of each phase are sorted according to the voltage magnitude, and the voltages of the submodules put into operation are summed according to the arrangement order to obtain the sum of the bridge arm submodules put into operation of each phase at the next moment; Based on the DC bus voltage reference value at the next moment, the phase voltage reference value at the next moment, the sum of the bridge arm input submodule voltages of each phase at the next moment, the phase current at the current moment, and the circulating current at the current moment, the phase current prediction value at the next moment, the circulating current prediction value at the next moment, and the bridge arm energy prediction value of each phase at the next moment are calculated; The cost function is iterated using the predicted values ​​of phase current, circulating current and bridge arm energy at different times to obtain the number of bridge arm submodules that are invested when the cost function value is the lowest. Invest in the corresponding sub-modules according to the number of bridge arm sub-modules corresponding to the lowest cost function value; Each phase has N submodules in the upper bridge arm and the lower bridge arm respectively. The voltage sum of the submodules put into operation is performed in the order of arrangement to obtain the voltage sum of the submodules put into operation in each phase at the next moment, including: Calculate the voltage sum of the first x items of the upper bridge arm submodule voltage matrix to obtain the upper bridge arm submodule voltage sum, and use the upper bridge arm submodule voltage sum as the upper bridge arm submodule voltage sum at the next moment, where x is the number of upper bridge arm submodules put into use, 0≤x≤N; Calculate the voltage sum of the first y items of the voltage matrix of the lower bridge arm submodule to obtain the voltage sum of the lower bridge arm submodule, and use the voltage sum of the lower bridge arm submodule as the voltage sum of the lower bridge arm submodule at the next moment, where y is the number of lower bridge arm submodules put into use, 0≤y≤N; The method of calculating the predicted phase current value at the next moment, the predicted circulating current value at the next moment, and the predicted bridge arm energy value of each phase at the next moment based on the DC bus voltage reference value at the next moment, the phase voltage reference value at the next moment, the voltage sum of the bridge arm input submodules of each phase at the next moment, the phase current at the current moment, and the circulating current at the current moment includes: Obtain the DC bus voltage reference value and the phase voltage reference value at the next moment, and obtain the phase current and circulating current at the current moment according to Kirchhoff's current law; The predicted phase current value at the next moment is calculated based on the DC bus voltage reference value at the next moment, the voltage sum of the bridge arm input submodules of each phase at the next moment, and the phase current at the current moment; The predicted circulating current value at the next moment is obtained based on the phase voltage reference value at the next moment, the voltage sum of the bridge arm input submodules of each phase at the next moment, and the circulating current at the current moment; The predicted bridge arm energy value of each phase at the next moment is calculated based on the bridge arm input submodule voltage of each phase at the next moment and the phase current at the current moment.

2. The method for controlling busbar DC voltage of an offshore wind power flexible DC transmission system according to claim 1, characterized in that: The bridge arm submodule voltages include an upper bridge arm submodule voltage and a lower bridge arm submodule voltage, and the bridge arm submodule voltages of each phase are sorted according to voltage magnitudes, including: Arrange the upper bridge arm submodule voltages in ascending order and number them to establish an upper bridge arm submodule voltage matrix; Arrange the lower bridge arm submodule voltages in ascending order and number them to establish a lower bridge arm submodule voltage matrix.

3. The method for controlling busbar DC voltage of an offshore wind power flexible DC transmission system according to claim 1, characterized in that: The cost function is iterated by using the phase current prediction value, circulating current prediction value and bridge arm energy prediction value at different times to obtain the number of bridge arm submodules invested corresponding to the lowest cost function value, including: Construct a cost function, set the minimum value of the cost function, and set x and y at the current moment to 1; Substituting the predicted value of the phase current, the predicted value of the circulating current and the predicted value of the bridge arm energy at the next moment into the cost function to calculate the cost function value, and comparing the cost function value with the minimum value; When the cost function value is less than the minimum value, the cost function value is used as the minimum value at the next moment, and the number of upper bridge arm submodules invested is determined according to the value of x at the current moment, and the number of lower bridge arm submodules invested is determined according to the value of y at the current moment.

4. The busbar DC voltage control method of the offshore wind power flexible DC transmission system according to claim 3 is characterized in that: The cost function is iterated by using the phase current prediction value, the circulating current prediction value and the bridge arm energy prediction value at different times to obtain the number of bridge arm submodules invested corresponding to the lowest cost function value, and further includes: When the cost function value is not less than the minimum value, determining whether y is equal to N at the current moment; If y is not equal to N at the current moment, then y is increased by 1, and the step of calculating the sum of the voltages of the submodules put into operation of each phase at the next moment is returned; If y is equal to N at the current moment, then determine whether x is equal to N at the current moment; If x is not equal to N at the current moment, then x is increased by 1, and the step of calculating the sum of the voltages of the sub-modules of each phase at the next moment is returned to until x is equal to N at the current moment.

5. The method for controlling busbar DC voltage of an offshore wind power flexible DC transmission system according to claim 4, characterized in that: According to the number of bridge arm submodules corresponding to the lowest cost function value, the corresponding submodules are invested, including: According to the x value corresponding to the lowest cost function value, the first x submodules in the upper bridge arm submodule voltage matrix are controlled to be put into operation; According to the y value corresponding to the lowest cost function value, the first y sub-modules in the voltage matrix of the lower bridge arm sub-module are controlled to be put into operation.

6. A busbar DC voltage control device for an offshore wind power flexible DC transmission system, characterized in that: The device comprises: An acquisition module is used to obtain the bridge arm current, bridge arm submodule voltage, phase current and phase voltage of each phase of the converter at the current moment; A summing module is used to sort the voltages of the bridge arm submodules of each phase according to the voltage magnitude, and sum the voltages of the submodules put into operation according to the arrangement order to obtain the sum of the voltages of the bridge arm submodules put into operation of each phase at the next moment; A calculation module is used to calculate the predicted phase current value at the next moment, the predicted circulating current value at the next moment, and the predicted bridge arm energy value of each phase at the next moment based on the DC bus voltage reference value at the next moment, the phase voltage reference value at the next moment, the voltage sum of the bridge arm input submodules of each phase at the next moment, the phase current at the current moment, and the circulating current at the current moment; A prediction module is used to iterate the cost function using the predicted values ​​of phase current, circulating current and bridge arm energy at different times to obtain the number of bridge arm submodules invested when the cost function value is the lowest; An investment module is used to invest corresponding sub-modules according to the investment quantity of the bridge arm sub-module corresponding to the lowest cost function value; Each phase has N submodules in the upper bridge arm and the lower bridge arm respectively. The voltage sum of the submodules put into operation is performed in the order of arrangement to obtain the voltage sum of the submodules put into operation in each phase at the next moment, including: Calculate the voltage sum of the first x items of the upper bridge arm submodule voltage matrix to obtain the upper bridge arm submodule voltage sum, and use the upper bridge arm submodule voltage sum as the upper bridge arm submodule voltage sum at the next moment, where x is the number of upper bridge arm submodules put into use, 0≤x≤N; Calculate the voltage sum of the first y items of the voltage matrix of the lower bridge arm submodule to obtain the voltage sum of the lower bridge arm submodule, and use the voltage sum of the lower bridge arm submodule as the voltage sum of the lower bridge arm submodule at the next moment, where y is the number of lower bridge arm submodules put into use, 0≤y≤N; The method of calculating the predicted phase current value at the next moment, the predicted circulating current value at the next moment, and the predicted bridge arm energy value of each phase at the next moment based on the DC bus voltage reference value at the next moment, the phase voltage reference value at the next moment, the voltage sum of the bridge arm input submodules of each phase at the next moment, the phase current at the current moment, and the circulating current at the current moment includes: Obtain the DC bus voltage reference value and the phase voltage reference value at the next moment, and obtain the phase current and circulating current at the current moment according to Kirchhoff's current law; The predicted phase current value at the next moment is calculated based on the DC bus voltage reference value at the next moment, the voltage sum of the bridge arm input submodules of each phase at the next moment, and the phase current at the current moment; The predicted circulating current value at the next moment is obtained based on the phase voltage reference value at the next moment, the voltage sum of the bridge arm input submodules of each phase at the next moment, and the circulating current at the current moment; The predicted bridge arm energy value of each phase at the next moment is calculated based on the bridge arm input submodule voltage of each phase at the next moment and the phase current at the current moment.

7. A computer device, characterized in that: include: A memory and a processor, wherein the memory and the processor are communicatively connected to each other, the memory stores computer instructions, and the processor executes the bus DC voltage control method of the offshore wind power flexible DC transmission system according to any one of claims 1 to 5 by executing the computer instructions.

8. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores computer instructions, and the computer instructions are used to enable a computer to execute the bus DC voltage control method for an offshore wind power flexible DC transmission system according to any one of claims 1 to 5.

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