Flexible lcc control method, system, device and medium for suppressing direct current ripple

Abstract

The application discloses a kind of flexible LCC control method, system, equipment and medium of direct current ripple, the method comprises the following steps: control cascaded H bridge in flexible LCC output three-phase amplitude unequal AC capacitive voltage to inhibit the direct current ripple generated by asymmetric grid voltage. The negative sequence component of cascaded H bridge output voltage is equal to the negative sequence component amplitude of AC bus voltage, and the phase difference is 180 degrees, and the positive sequence component meets the requirement of flexible LCC arc extinction angle control. The method gives the calculation method of the effective value of cascaded H bridge three-phase voltage when meeting the above two requirements. By using the control method, the commutation failure of flexible LCC can be prevented while the direct current ripple is suppressed. The method is fast and accurate, and has important significance for improving the operating stability of high-voltage direct current system.

Description

Technical Field

[0001] This invention belongs to the field of high voltage direct current transmission technology, and specifically relates to a control method, system, equipment and medium for suppressing DC ripple using a flexible LCC. Background Technology

[0002] Traditional line commutated converter high-voltage direct current (LCC-HVDC) transmission, based on grid-commutated converters, boasts advantages such as large transmission capacity, surge resistance, high reliability, economic efficiency, and mature technology, making it the primary means of long-distance, high-capacity power transmission and asynchronous grid interconnection. However, traditional HVDC transmission inherently suffers from commutation failure, which can significantly impact the stable operation of the DC transmission system. Methods to address commutation failure fall into three main categories: first, adding reactive power compensation devices to the AC bus to improve bus voltage stability; second, optimizing and modifying the converter's control strategy; and third, adding auxiliary commutation circuits to assist in converter commutation. Research indicates that the last method is most effective at suppressing commutation failure and represents the most promising direction for future development.

[0003] Connecting a cascaded H-bridge circuit in series on the converter valve side, so that the LCC inverter and the cascaded H-bridge together form a flexible LCC, is an effective auxiliary converter scheme. Under the auxiliary voltage output by the cascaded H-bridge, the arc-extinguishing angle of the LCC inverter is effectively increased, and the probability of commutation failure is significantly reduced. However, existing flexible LCC technologies do not consider the problem of increased DC ripple caused by the negative sequence component of the grid voltage when an asymmetrical fault occurs in the AC grid. Generally speaking, DC current ripple increases the current stress on the converter valve, and if the converter valve operates under this condition for a long time, its service life will inevitably be affected. Therefore, it is necessary to further improve the control methods of existing flexible LCC technologies to solve the DC ripple problem and promote the stable operation of the DC system. Summary of the Invention

[0004] The purpose of this invention is to provide a control method, system, device and medium for suppressing DC ripple in flexible LCCs, so as to solve the impact of DC current ripple on LCC systems when AC power grids are unbalanced.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A control method for suppressing DC ripple in a flexible LCC includes the following steps:

[0007] The positive and negative sequence components of the instantaneous three-phase AC bus voltage are extracted to obtain the effective value U of the positive sequence AC bus voltage. s(1)AC bus negative sequence voltage effective value U s(2) And the angle η by which the negative sequence component of the AC bus voltage lags behind the positive sequence component when switching to the valve side;

[0008] Based on the effective value of the positive sequence voltage U of the AC bus s(1) The measured DC current I d Inverted angle β s Solve for the angle λ between the positive sequence voltage of the cascaded H-bridge output and the positive sequence voltage of the AC bus.

[0009] Based on the effective value of the positive sequence voltage U of the AC bus s(1) AC bus negative sequence voltage effective value U s(2) 1. When switching to the valve side, the angle η of the negative sequence component of the AC bus voltage lagging behind the positive sequence component, and the angle λ of the positive sequence voltage output of the cascaded H-bridge lagging behind the positive sequence voltage of the AC bus, calculate the effective value of the unequal amplitude three-phase voltage of the cascaded H-bridge;

[0010] The output voltage of the cascaded H-bridge is controlled based on the effective value of the three-phase voltage.

[0011] Furthermore, based on the effective value U of the positive sequence voltage of the AC bus... s(1) The measured DC current I d and inverse angle β s When solving for the angle λ between the positive sequence voltage of the cascaded H-bridge output and the positive sequence voltage of the AC bus, an iterative calculation method is used.

[0012] Furthermore, based on the effective value of the positive sequence voltage of the AC bus Us(1), the measured DC current Id, and the inverter angle βs, the angle λ of the positive sequence voltage of the cascaded H-bridge output lagging behind the positive sequence voltage of the AC bus includes the following steps:

[0013] Set the effective value of the positive sequence voltage U required for the output of the cascaded H-bridge cp(1) The initial values ​​of and the initial values ​​of the intermediate parameter δ;

[0014] The required positive sequence voltage RMS value U of the cascaded H-bridge output cp(1) Substituting the initial value of and the initial value of the intermediate parameter δ into the system of equations, we obtain the calculated value of the intermediate parameter δ; the system of equations is as follows:

[0015]

[0016] In the system of equations, the DC current I d Inverted angle β s The effective value of the positive sequence voltage U of the AC bus s(1) All values ​​are measurements from a flexible LCC control system; λ is the angle by which the positive-sequence voltage of the cascaded H-bridge output lags behind the positive-sequence voltage of the AC bus, and X... B For commutation reactance, U tIt is the effective value of the combined voltage of the cascaded H-bridge positive sequence voltage and the grid positive sequence voltage, ε is the angle by which the combined voltage lags the grid positive sequence voltage, β is the inverter angle of the LCC with the combined voltage as a reference, γ is the arc extinction angle, and μ is the commutation overlap angle.

[0017] Compare the calculated values ​​of intermediate parameters with their initial values:

[0018] If the two are equal, compare the last calculated arc-extinguishing angle γ with the arc-extinguishing angle set value γ*; otherwise, substitute the calculated intermediate parameters into the equation system and recalculate until the calculated intermediate parameters are equal to the intermediate parameters substituted into the equation system.

[0019] When comparing the last calculated arc-extinguishing angle γ with the arc-extinguishing angle set value γ*:

[0020] If the arc extinction angle γ is less than the arc extinction angle set value γ*, or if the arc extinction angle γ is a complex number, then increase the effective value U of the positive sequence voltage required for the output of the cascaded H-bridge. cp(1) The value is iteratively calculated again using intermediate parameters until the arc-extinguishing angle γ is greater than or equal to the arc-extinguishing angle set value γ* and the arc-extinguishing angle γ is a real number. At this point, the required positive sequence voltage RMS value U of the cascaded H-bridge output is... cp(1) This is the final solution value of the arc-extinguishing angle control iterative calculation, where the arc-extinguishing angle λ is the final solution value.

[0021] Furthermore, increasing the effective value U of the positive sequence voltage required for the cascaded H-bridge output cp(1) When the value is less than the effective value U of the positive sequence voltage required by the cascaded H-bridge, the step size of each increment is less than the effective value U of the positive sequence voltage required by the cascaded H-bridge. cp(1) 1% of the maximum allowed value.

[0022] Furthermore, the formula for calculating the effective value of the unequal amplitude three-phase voltage of the cascaded H-bridge is as follows:

[0023]

[0024] Among them, U cpa U cpb U cpc These are the effective values ​​of the voltages of phases a, b, and c of the cascaded H-bridge, respectively, U cp(1) U is the effective value of the positive sequence voltage required for the cascaded H-bridge. s(2) η is the effective value of the negative sequence voltage of the AC bus, η is the angle by which the negative sequence component of the AC bus voltage lags behind the positive sequence component when switching to the valve side, and λ is the angle by which the positive sequence voltage of the cascaded H-bridge output lags behind the positive sequence voltage of the AC bus.

[0025] Furthermore, controlling the output voltage of the cascaded H-bridge based on the effective value of the three-phase voltage includes the following steps:

[0026] The three-phase output voltage of the cascaded H-bridge uses the fundamental AC current as a phase reference and is controlled by the dq axis. The direction in phase with the fundamental current is the d-axis, and the d-axis control quantity is given by the average capacitor voltage u of the cascaded H-bridge submodules. cvav * and feedback value u cvav The difference is obtained through PI control; the direction leading the d-axis by 90 degrees is the q-axis, and the control quantity of the q-axis is the effective value of the three-phase voltage U. cpa U cpb and U cpc The effective values ​​of the three-phase voltages and the d-axis voltages are transformed by dq / abc to obtain the three-phase sinusoidal voltages used for PWM modulation.

[0027] A flexible LCC control system for suppressing DC ripple includes:

[0028] Extraction module: Extracts positive and negative sequence components from the instantaneous values ​​of the three-phase AC bus voltage to obtain the effective value U of the positive sequence AC bus voltage. s(1) AC bus negative sequence voltage effective value U s(2) And the angle η by which the negative sequence component of the AC bus voltage lags behind the positive sequence component when switching to the valve side;

[0029] Solution module: Used to calculate the effective value U of the positive sequence voltage of the AC bus. s(1) The measured DC current I d and inverse angle β s Solve for the angle λ between the positive sequence voltage of the cascaded H-bridge output and the positive sequence voltage of the AC bus.

[0030] Voltage RMS Calculation Module: Used to calculate the effective value of the positive sequence voltage U of the AC bus. s(1) AC bus negative sequence voltage effective value U s(2) 1. When switching to the valve side, the angle η of the negative sequence component of the AC bus voltage lagging behind the positive sequence component, and the angle λ of the positive sequence voltage output of the cascaded H-bridge lagging behind the positive sequence voltage of the AC bus, calculate the effective value of the unequal amplitude three-phase voltage of the cascaded H-bridge;

[0031] Control module: Used to control the output voltage of the cascaded H-bridge based on the effective value of the three-phase voltage.

[0032] Furthermore, the solution module includes:

[0033] The initialization module is used to set the effective value U of the positive sequence voltage required for the output of the cascaded H-bridge. cp(1) The initial values ​​of and the initial values ​​of the intermediate parameter δ;

[0034] The intermediate parameter calculation module is used to calculate U cp(1) Substituting the initial value of and the initial value of the intermediate parameter δ into the system of equations, we obtain the calculated value of the intermediate parameter δ; the system of equations is as follows:

[0035]

[0036] In the system of equations, the DC current I d Inverted angle β s The effective value of the positive sequence voltage U of the AC bus s(1) All values ​​are measurements from a flexible LCC control system; λ is the angle by which the positive-sequence voltage of the cascaded H-bridge output lags behind the positive-sequence voltage of the AC bus, and X... B For commutation reactance, U t It is the effective value of the combined voltage of the cascaded H-bridge positive sequence voltage and the grid positive sequence voltage, ε is the angle by which the combined voltage lags the grid positive sequence voltage, β is the inverter angle of the LCC with the combined voltage as a reference, γ is the arc extinction angle, and μ is the commutation overlap angle.

[0037] The first comparison module is used to compare the calculated values ​​of intermediate parameters with their initial values:

[0038] If the two are equal, compare the last calculated arc-extinguishing angle γ with the arc-extinguishing angle set value γ*; otherwise, recalculate the intermediate parameters, substitute the recalculated intermediate parameters into the equation system and calculate again until the calculated intermediate parameters are equal to the intermediate parameters substituted into the equation system.

[0039] The second comparison module is used to compare the arc-extinguishing angle γ obtained from the last calculation with the arc-extinguishing angle set value γ*:

[0040] If the arc extinction angle γ is less than the arc extinction angle set value γ*, or if the arc extinction angle γ is a complex number, then increase the effective value U of the positive sequence voltage required for the output of the cascaded H-bridge. cp(1) The value is iteratively calculated again using intermediate parameters until the arc-extinguishing angle γ is greater than or equal to the arc-extinguishing angle set value γ* and the arc-extinguishing angle γ is a real number. At this point, the required positive sequence voltage RMS value U of the cascaded H-bridge output is... cp(1) This is the final solution value of the arc-extinguishing angle control iterative calculation, where the arc-extinguishing angle λ is the final solution value.

[0041] A computer device includes an electrically connected memory and a processor, the memory storing a computer program executable on the processor, wherein when the processor executes the computer program, it implements the steps of the method described above.

[0042] A computer-readable storage medium storing a computer program, characterized in that, when executed by a processor, the computer program implements the steps of the above-described method.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] In this invention, the cascaded H-bridge outputs three-phase capacitive AC voltages with unequal amplitudes, all lagging the fundamental current by 90 degrees. The positive-sequence component of the output voltage satisfies the requirements of flexible LCC arc-extinguishing angle control, while the negative-sequence component has the same amplitude as the negative-sequence component of the AC bus voltage, but a phase difference of 180 degrees. This invention provides a formula for calculating the effective value of the three-phase voltage of the cascaded H-bridge under the condition of satisfying the above two requirements. This formula can be used to achieve unequal amplitude control of the three phases of the cascaded H-bridge. Compared with existing schemes that control the output of cascaded H-bridges to have equal amplitude capacitive AC voltages in three phases, this invention can cancel the negative-sequence component of the AC grid voltage and reduce DC current ripple caused by grid voltage asymmetry, while ensuring that the LCC does not fail to commutate.

[0045] Furthermore, this invention employs an iterative calculation method to calculate the positive sequence voltage required for the output of the cascaded H-bridge and directly solves for the three-phase output voltage of the cascaded H-bridge based on the AC bus voltage. This can effectively improve the speed and accuracy of control and help to respond more quickly to various fault changes in the power grid. Attached Figure Description

[0046] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art are briefly introduced below; obviously, the drawings described below are some embodiments of the present invention, and those skilled in the art can obtain other drawings based on these drawings without creative effort.

[0047] Figure 1 It is a flexible LCC topology diagram;

[0048] Figure 2 This is a flowchart of the iterative calculation for controlling the arc extinction angle of a cascaded H-bridge;

[0049] Figure 3 This is a block diagram of the control strategy for implementing three-phase unequal amplitude control using cascaded H-bridges;

[0050] Figure 4 This is the phasor diagram of the flexible LCC valve side voltage when using unequal amplitude control;

[0051] Figure 5 This is a schematic diagram of the module structure of a flexible LCC control system for suppressing DC ripple provided by the present invention;

[0052] Figure 6 This is a schematic diagram of the solution module structure. Detailed Implementation

[0053] To make the objectives, technical effects, and technical solutions of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention. Based on the embodiments disclosed in the present invention, other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0054] Please see Figure 1 , Figure 1 This is a schematic diagram of a flexible LCC topology. A cascaded H-bridge circuit is connected in series on the converter valve side, and the LCC and the cascaded H-bridge together constitute the flexible LCC. The cascaded H-bridge contains a large number of series H-bridge submodules to achieve a higher output voltage. Different control strategies can be applied to the cascaded H-bridge to achieve various control objectives, such as suppressing commutation failure in the LCC and reducing DC current ripple.

[0055] Please see Figure 2 , Figure 2 This is a flowchart of the iterative calculation for the arc extinction angle control of the flexible LCC. Based on this flowchart, the effective value of the positive sequence voltage U required for the output of the cascaded H-bridge can be calculated. cp(1) And the angle λ between the positive sequence voltage output of the cascaded H-bridge and the positive sequence voltage of the AC bus.

[0056] The specific process of iterative operation is as follows:

[0057] Step 1: Assume the required positive sequence voltage RMS value U of the cascaded H-bridge output. cp(1) The value is 0, and the intermediate parameter δ (fundamental current hysteresis angle) is assumed to be 10 degrees;

[0058] Step 2: Calculate the effective value U of the positive sequence voltage required for the cascaded H-bridge output. cp(1) Substitute the intermediate parameter δ into the system of equations to obtain a new calculated value for δ.

[0059]

[0060] In the system of equations, the DC current I d Inverted angle β s The effective value of the positive sequence voltage U of the AC bus s(1) All values ​​are measurements from a flexible LCC control system. λ represents the angle by which the positive-sequence voltage of the cascaded H-bridge output lags behind the positive-sequence voltage of the AC bus, and X... B For commutation reactance, U tThis is the effective value of the combined voltage of the cascaded H-bridge positive-sequence voltage and the grid positive-sequence voltage. ε is the angle by which the combined voltage lags behind the grid positive-sequence voltage, β is the inverter angle of the LCC with the combined voltage as a reference, γ is the arc extinction angle, and μ is the commutation overlap angle. Step 3: Compare the new calculated value with the initial δ substituted into the equation system. If they are equal, it means that the parameters in the equation system are within the specified U. cp(1) The true value is obtained; if they are not equal, the newly calculated δ is substituted into the system of equations and the calculation is repeated until the calculated δ is equal to the δ substituted into the system of equations.

[0061] Step 4: Compare the arc-extinguishing angle γ calculated in the last step with the target value of the arc-extinguishing angle control, i.e., the arc-extinguishing angle setpoint γ*. If γ is less than γ* or γ is a complex number, increase the effective value U of the positive sequence voltage required for the output of the cascaded H-bridge. cp(1) The value of δ is iterated again until γ satisfies greater than or equal to γ* and γ is a real number. Then, the required positive sequence voltage effective value U of the cascaded H-bridge output is... cp(1) This is the final solution value of the iterative calculation for the arc extinction angle control. Correspondingly, the final λ obtained from the equation system is also the target solution value. In the above process, to ensure the control accuracy of the arc extinction angle, the cascaded H-bridge requires a positive-sequence voltage RMS value U. cp(1) The increment should be set relatively small each time, for example, the effective value of the positive sequence voltage U required for the cascaded H-bridge output. cp(1) 1% of the maximum allowed value.

[0062] Please see Figure 3 , Figure 3 This is a control method for suppressing DC ripple in flexible LCCs using cascaded H-bridges. The core of this control method is to solve for the effective value of the three-phase unequal amplitude voltage, which includes the following steps:

[0063] S1, instantaneous value of three-phase AC bus voltage u sa u sb u sc Positive and negative sequence components are extracted to obtain the effective value U of the positive sequence voltage of the AC bus. s(1) Negative sequence voltage RMS value U s(2) And the angle η by which the negative sequence component of the AC bus voltage lags behind the positive sequence component when switching to the valve side. Common methods for extracting positive and negative sequence components include the virtual orthogonal signal construction method and the PLL detection method.

[0064] S2, extract the U s(1) The measured DC current I d Inverted angle β s Iterative calculation for arc extinction angle control (flowchart shown) Figure 2 As shown, the effective value of the positive sequence voltage U required for the output of the cascaded H-bridge can be obtained. cp(1)And the angle λ between the positive sequence voltage output of the cascaded H-bridge and the positive sequence voltage of the AC bus. Then, the effective value U of the obtained positive sequence voltage of the AC bus is... s(1) AC bus negative sequence voltage effective value U s(2) The angle η of the phasor lag between the negative sequence voltage phasor of phase a of the AC bus and the positive sequence voltage phasor of the AC bus, and the angle λ of the positive sequence voltage lag between the output positive sequence voltage of the cascaded H-bridge and the positive sequence voltage of the AC bus, are used to solve for the effective value of the unequal amplitude three-phase voltage of the cascaded H-bridge. The specific calculation formula is as follows:

[0065]

[0066] Among them, U cpa U cpb U cpc These are the effective values ​​of the voltages of phases a, b, and c of the cascaded H-bridge, respectively.

[0067] S3. Control the output voltage of the cascaded H-bridge based on the effective value of the three-phase voltage.

[0068] The three-phase output voltage of the cascaded H-bridge uses the fundamental AC current as a phase reference and is controlled by the dq-axis. The direction in phase with the fundamental current is the d-axis, and the d-axis control quantity is given by the average capacitor voltage u of the cascaded H-bridge submodules. cvav * and feedback value u cvav The difference is obtained through PI control and used to stabilize the capacitor voltage of the submodule. Under steady-state conditions, the d-axis control value is relatively small. The direction leading the d-axis by 90 degrees is the q-axis, and the q-axis control value is the effective value U of the three-phase voltage obtained above. cpa U cpb and U cpc The three-phase sinusoidal voltage u for PWM modulation is obtained by performing dq / abc transformations on the effective values ​​of each phase voltage and the d-axis voltage, respectively. ai *、u bi *、u ci * Wherein, the reference phase θ for the dq / abc transformation of phase a voltage. i with i a Synchronization is based on the three-phase alternating current i a i b i c The phase is obtained via a phase-locked loop (PLL), and the reference phases for the dq / abc transformation of phases b and c are θ, respectively. i -120°, θ i +120°.

[0069] Please see Figure 4 , Figure 4 This is a phasor diagram of the three-phase voltage and positive / negative sequence voltage on the flexible LCC valve side when a cascaded H-bridge uses three-phase unequal amplitude control. This diagram explains the source of the calculation formula for the effective value of the unequal amplitude three-phase voltage in step S2 above. In the diagram, U...cpa U cpb U cpc These are the three-phase voltage phasors output from the cascaded H-bridge, U cpa(1) U cpa(2) For the output phase a positive-sequence and negative-sequence voltage phasors of the cascaded H-bridge, U sa(1) U sa(2) Let U be the positive-sequence and negative-sequence voltage phasors of phase a of the AC bus. As mentioned earlier, the negative-sequence voltage phasor U of phase a of the AC bus... sa(2) Lagging AC bus positive sequence voltage phasor U sa(1) The angle is η, and the cascaded H-bridge outputs the a-phase positive sequence voltage phasor U. cpa(1) Lagging AC bus positive sequence voltage phasor U sa(1) The angle is λ. To cancel the negative sequence component of the grid voltage, the cascaded H-bridge outputs the a-phase negative sequence voltage phasor U. cpa(2) and AC bus negative sequence voltage phasor U sa(2) Equal amplitude, opposite direction, cascaded H-bridge outputs phase a positive sequence voltage phasor U cpa(1) The effective value is obtained through iterative calculation controlled by the arc extinction angle. According to the symmetric component method, it can be known that...

[0070]

[0071] Where α is the 120-degree rotation factor and α = e j120° .

[0072] The phase voltages of the cascaded H-bridge remain capacitive under three-phase unequal amplitude control, thus maintaining 120-degree phase symmetry. Therefore, according to the positive sequence component formula in equation (1), the effective values ​​of the three-phase voltages of the cascaded H-bridge satisfy...

[0073]

[0074] Take U sa(1) The direction is the positive X-axis direction, and the direction leading the X-axis by 90 degrees is the positive Y-axis direction. According to the formula for the negative order component in equation (1), U cp(2) and 1 / 3(U cpa +α 2 U cpb +αU cpc The projections of the three phase voltages onto the X and Y axes are equal. Therefore, the effective values ​​of the three-phase voltages also satisfy...

[0075]

[0076] Equations (2) and (3) simultaneously contain U cp(2) =U s(2) The formula for the effective value of the three-phase voltage of the cascaded H-bridge as described in step S2 can be obtained.

[0077] Example 2

[0078] This invention provides a flexible LCC control system for suppressing DC ripple, such as... Figure 5 As shown, it includes:

[0079] Extraction module: Extracts positive and negative sequence components from the instantaneous values ​​of the three-phase AC bus voltage to obtain the effective value U of the positive sequence AC bus voltage. s(1) AC bus negative sequence voltage effective value U s(2) And the angle η by which the negative sequence component of the AC bus voltage lags behind the positive sequence component when switching to the valve side;

[0080] Solution module: Used to calculate the effective value U of the positive sequence voltage of the AC bus. s(1) The measured DC current I d Inverted angle β s Solve for the angle λ between the positive sequence voltage of the cascaded H-bridge output and the positive sequence voltage of the AC bus.

[0081] Voltage RMS Calculation Module: Used to calculate the effective value of the positive sequence voltage U of the AC bus. s(1) AC bus negative sequence voltage effective value U s(2) 1. When switching to the valve side, the angle η of the negative sequence component of the AC bus voltage lagging behind the positive sequence component, and the angle λ of the positive sequence voltage output of the cascaded H-bridge lagging behind the positive sequence voltage of the AC bus, calculate the effective value of the unequal amplitude three-phase voltage of the cascaded H-bridge;

[0082] Control module: Used to control the output voltage of the cascaded H-bridge based on the effective value of the three-phase voltage.

[0083] like Figure 6 As shown, the solution module includes:

[0084] The initialization module is used to set the effective value U of the positive sequence voltage required for the output of the cascaded H-bridge. cp(1) The initial value of the intermediate parameter δ is set.

[0085] The intermediate parameter calculation module is used to calculate U cp(1) Substituting the initial value of and the initial value of the intermediate parameter δ into the system of equations, we obtain the calculated value of the intermediate parameter δ; the system of equations is as follows:

[0086]

[0087] In the system of equations, the DC current I d Inverted angle β s The effective value of the positive sequence voltage U of the AC bus s(1) All values ​​are measurements from a flexible LCC control system; λ is the angle by which the positive-sequence voltage of the cascaded H-bridge output lags behind the positive-sequence voltage of the AC bus, and X... B For commutation reactance, U tIt is the effective value of the combined voltage of the cascaded H-bridge positive sequence voltage and the grid positive sequence voltage, ε is the angle by which the combined voltage lags the grid positive sequence voltage, β is the inverter angle of the LCC with the combined voltage as a reference, γ is the arc extinction angle, and μ is the commutation overlap angle.

[0088] The first comparison module is used to compare the calculated values ​​of intermediate parameters with their initial values:

[0089] If the two are equal, compare the last calculated arc-extinguishing angle γ with the arc-extinguishing angle set value γ*; otherwise, recalculate the intermediate parameters, substitute the recalculated intermediate parameters into the equation system and calculate again until the calculated intermediate parameters are equal to the intermediate parameters substituted into the equation system.

[0090] The second comparison module is used to compare the arc-extinguishing angle γ obtained from the last calculation with the arc-extinguishing angle set value γ*:

[0091] If the arc extinction angle γ is less than the arc extinction angle set value γ*, or if the arc extinction angle γ is a complex number, then increase the effective value U of the positive sequence voltage required for the output of the cascaded H-bridge. cp(1) The value is iteratively calculated again using intermediate parameters until the arc-extinguishing angle γ is greater than or equal to the arc-extinguishing angle set value γ* and the arc-extinguishing angle γ is a real number. At this point, the required positive sequence voltage RMS value U of the cascaded H-bridge output is... cp(1) This is the final solution value obtained from the iterative calculation of the arc extinction angle control, where the arc extinction angle λ is the target solution value.

[0092] Example 3

[0093] This invention provides an electronic device comprising: a processor, a memory, and a bus. The memory stores computer-executed instructions, and the processor is connected to the memory via the bus. When the electronic device is running, the processor executes the computer-executed instructions stored in the memory, thereby enabling the processor to run a program corresponding to the computer-executed instructions by reading the computer-executed instructions stored in the memory. This program is used to execute the flexible LCC control method for suppressing DC ripple as described in this invention. The bus can be a peripheral component interconnect (PCI) bus or an extended industry standard architecture (EISA) bus, etc. The bus can be divided into address bus, data bus, control bus, etc.

[0094] To implement the above embodiments, the present invention also proposes a computer-readable storage medium storing computer instructions, wherein the computer instructions are used to cause a computer to execute the control method for suppressing DC ripple in a flexible LCC proposed in Embodiment 1 of the present invention.

[0095] Example 4

[0096] To achieve the above embodiments, the present invention also proposes a computer program product, including a computer program that, when executed by a processor, implements the control method for suppressing DC ripple in a flexible LCC as proposed in Embodiment 1 of the present invention.

[0097] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0098] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0099] Any process or method description in the flowchart or otherwise herein can be understood as representing a module, segment, or portion of code comprising one or more executable instructions for implementing custom logic functions or processes, and the scope of preferred embodiments of the invention includes additional implementations in which functions may be performed not in the order shown or discussed, including substantially simultaneously or in reverse order depending on the functions involved, as should be understood by those skilled in the art to which embodiments of the invention pertain.

[0100] The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as a sequenced list of executable instructions for implementing logical functions, and can be embodied in any computer-readable medium for use by, or in conjunction with, an instruction execution system, apparatus, or device (such as a computer-based system, a processor-included system, or other system that can fetch and execute instructions from, an instruction execution system, apparatus, or device). For the purposes of this specification, "computer-readable medium" can be any means that can contain, store, communicate, propagate, or transmit programs for use by, or in conjunction with, an instruction execution system, apparatus, or device. More specific examples (a non-exhaustive list) of computer-readable media include: an electrical connection having one or more wires (electronic device), a portable computer disk drive (magnetic device), random access memory (RAM), read-only memory (ROM), erasable and editable read-only memory (EPROM or flash memory), fiber optic devices, and portable optical disc read-only memory (CDROM). Alternatively, the computer-readable medium may be paper or other suitable media on which the program can be printed, since the program can be obtained electronically, for example, by optically scanning the paper or other medium, followed by editing, interpreting, or otherwise processing as necessary, and then stored in a computer memory.

[0101] It should be understood that various parts of the present invention can be implemented in hardware, software, firmware, or a combination thereof. In the above embodiments, multiple steps or methods can be implemented in software or firmware stored in memory and executed by a suitable instruction execution system. For example, if implemented in hardware as in another embodiment, it can be implemented using any of the following techniques known in the art, or a combination thereof: discrete logic circuits having logic gates for implementing logical functions on data signals, application-specific integrated circuits (ASICs) having suitable combinational logic gates, programmable gate arrays (PGAs), field-programmable gate arrays (FPGAs), etc.

[0102] Those skilled in the art will understand that all or part of the steps of the methods in the above embodiments can be implemented by a program instructing related hardware. The program can be stored in a computer-readable storage medium, and when executed, the program includes one or a combination of the steps of the method embodiments.

[0103] Furthermore, the functional units in the various embodiments of the present invention can be integrated into a processing module, or each unit can exist physically separately, or two or more units can be integrated into a module. The integrated module can be implemented in hardware or as a software functional module. If the integrated module is implemented as a software functional module and sold or used as an independent product, it can also be stored in a computer-readable storage medium.

[0104] The storage medium mentioned above can be a read-only memory, a disk, or an optical disk, etc. Although embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

[0105] This invention can be applied to high-voltage direct current (HVDC) power systems. By employing the flexible LCC control method proposed in this invention to suppress DC ripple, it can suppress DC current ripple caused by AC grid voltage asymmetry when an asymmetrical voltage drop occurs in the AC grid, ensuring that the LCC does not fail to commutate. This is of great significance for improving the operating conditions of the LCC converter valve, preventing DC overcurrent from affecting the operating life of the converter valve, and improving the operational stability of the DC system.

[0106] The above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can still make modifications or equivalent substitutions to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention are within the protection scope of the claims of the present invention pending approval.

Claims

1. A control method for suppressing DC ripple in a flexible LCC, characterized in that, Includes the following steps: The positive and negative sequence components of the instantaneous three-phase AC bus voltage are extracted to obtain the effective value U of the positive sequence AC bus voltage. s(1) AC bus negative sequence voltage effective value U s(2) And the angle η by which the negative sequence component of the AC bus voltage lags behind the positive sequence component when switching to the valve side; Based on the effective value of the positive sequence voltage U of the AC bus s(1) The measured DC current I d Inverted angle β s Solve for the angle λ between the positive sequence voltage of the cascaded H-bridge output and the positive sequence voltage of the AC bus. Based on the effective value of the positive sequence voltage U of the AC bus s(1) AC bus negative sequence voltage effective value U s(2) 1. When switching to the valve side, the angle η of the negative sequence component of the AC bus voltage lagging behind the positive sequence component, and the angle λ of the positive sequence voltage output of the cascaded H-bridge lagging behind the positive sequence voltage of the AC bus, calculate the effective value of the unequal amplitude three-phase voltage of the cascaded H-bridge; The output voltage of the cascaded H-bridge is controlled based on the effective value of the three-phase voltage. Based on the effective value of the AC bus positive sequence voltage Us(1), the measured DC current Id, and the inverter angle βs, the following steps are taken to solve for the angle λ of the cascaded H-bridge output positive sequence voltage lagging behind the AC bus positive sequence voltage: Set the effective value of the positive sequence voltage U required for the output of the cascaded H-bridge cp(1) The initial values ​​of and the initial values ​​of the intermediate parameter δ; The required positive sequence voltage RMS value U of the cascaded H-bridge output cp(1) Substituting the initial value of and the initial value of the intermediate parameter δ into the system of equations, we obtain the calculated value of the intermediate parameter δ; the system of equations is as follows: In the system of equations, the DC current I d Inverted angle β s The effective value of the positive sequence voltage U of the AC bus s(1) All values ​​are measurements from a flexible LCC control system; λ is the angle by which the positive-sequence voltage of the cascaded H-bridge output lags behind the positive-sequence voltage of the AC bus, and X... B For commutation reactance, U t It is the effective value of the combined voltage of the cascaded H-bridge positive sequence voltage and the grid positive sequence voltage, ε is the angle by which the combined voltage lags the grid positive sequence voltage, β is the inverter angle of the LCC with the combined voltage as a reference, γ is the arc extinction angle, and μ is the commutation overlap angle. Compare the calculated values ​​of intermediate parameters with their initial values: If the two are equal, the arc-extinguishing angle γ obtained from the last calculation will be compared with the arc-extinguishing angle set value γ. Compare them; otherwise, substitute the calculated intermediate parameters into the system of equations and calculate again until the calculated intermediate parameters are equal to the intermediate parameters substituted into the system of equations. The last calculated extinction angle γ is compared with the extinction angle set value γ. When comparing: If the arc-extinguishing angle γ is less than the set value of the arc-extinguishing angle γ If the arc-extinguishing angle γ is a complex number, then increasing the effective value of the positive-sequence voltage U of the cascaded H-bridge requires... cp(1) The value is then iteratively calculated again using intermediate parameters until the arc-extinguishing angle γ is greater than or equal to the set value γ of the arc-extinguishing angle. Furthermore, since the arc-extinguishing angle γ is a real number, the required effective value of the positive-sequence voltage U of the cascaded H-bridge at this time is... cp(1) This is the final solution value of the arc-extinguishing angle control iterative calculation, where the arc-extinguishing angle λ is the final solution value.

2. The control method for suppressing DC ripple in a flexible LCC according to claim 1, characterized in that, The required positive sequence voltage RMS value U of the increased cascaded H-bridge output cp(1) When the value is less than the effective value U of the positive sequence voltage required by the cascaded H-bridge, the step size of each increment is less than the effective value U of the positive sequence voltage required by the cascaded H-bridge. cp(1) 1% of the maximum allowed value.

3. The control method for suppressing DC ripple in a flexible LCC according to claim 1, characterized in that, The formula for calculating the effective value of the unequal amplitude three-phase voltage of the cascaded H-bridge is as follows: Among them, U cpa U cpb U cpc These are the effective values ​​of the voltages of phases a, b, and c of the cascaded H-bridge, respectively, U cp(1) U is the effective value of the positive sequence voltage required for the cascaded H-bridge. s(2) η is the effective value of the negative sequence voltage of the AC bus, η is the angle by which the negative sequence component of the AC bus voltage lags behind the positive sequence component when switching to the valve side, and λ is the angle by which the positive sequence voltage of the cascaded H-bridge output lags behind the positive sequence voltage of the AC bus.

4. The control method for suppressing DC ripple in a flexible LCC according to claim 1, characterized in that, The step of controlling the output voltage of the cascaded H-bridge based on the effective value of the three-phase voltage includes the following steps: The three-phase output voltage of the cascaded H-bridge uses the fundamental AC current as a phase reference and is controlled by the dq axis. The direction in phase with the fundamental current is the d-axis, and the d-axis control quantity is given by the average capacitor voltage u of the cascaded H-bridge submodules. cvav and feedback value u cvav The difference is obtained through PI control; the direction leading the d-axis by 90 degrees is the q-axis, and the control quantity of the q-axis is the effective value of the three-phase voltage U. cpa U cpb and U cpc The effective values ​​of the three-phase voltages and the d-axis voltages are transformed by dq / abc to obtain the three-phase sinusoidal voltages used for PWM modulation.

5. A flexible LCC control system for suppressing DC ripple, characterized in that, include: Extraction module: Extracts positive and negative sequence components from the instantaneous values ​​of the three-phase AC bus voltage to obtain the effective value U of the positive sequence AC bus voltage. s(1) AC bus negative sequence voltage effective value U s(2) And the angle η by which the negative sequence component of the AC bus voltage lags behind the positive sequence component when switching to the valve side; Solution module: Used to calculate the effective value U of the positive sequence voltage of the AC bus. s(1) The measured DC current I d and inverse angle β s Solve for the angle λ between the positive sequence voltage of the cascaded H-bridge output and the positive sequence voltage of the AC bus. Voltage RMS Calculation Module: Used to calculate the effective value of the positive sequence voltage U of the AC bus. s(1) AC bus negative sequence voltage effective value U s(2) 1. When switching to the valve side, the angle η of the negative sequence component of the AC bus voltage lagging behind the positive sequence component, and the angle λ of the positive sequence voltage output of the cascaded H-bridge lagging behind the positive sequence voltage of the AC bus, calculate the effective value of the unequal amplitude three-phase voltage of the cascaded H-bridge; Control module: Used to control the output voltage of the cascaded H-bridge based on the effective value of the three-phase voltage; The solution module includes: The initialization module is used to set the effective value U of the positive sequence voltage required for the output of the cascaded H-bridge. cp(1) The initial values ​​of and the initial values ​​of the intermediate parameter δ; The intermediate parameter calculation module is used to calculate U cp(1) Substituting the initial value of and the initial value of the intermediate parameter δ into the system of equations, we obtain the calculated value of the intermediate parameter δ; the system of equations is as follows: In the system of equations, the DC current I d Inverted angle β s The effective value of the positive sequence voltage U of the AC bus s(1) All values ​​are measurements from a flexible LCC control system; λ is the angle by which the positive-sequence voltage of the cascaded H-bridge output lags behind the positive-sequence voltage of the AC bus, and X... B For commutation reactance, U t It is the effective value of the combined voltage of the cascaded H-bridge positive sequence voltage and the grid positive sequence voltage, ε is the angle by which the combined voltage lags the grid positive sequence voltage, β is the inverter angle of the LCC with the combined voltage as a reference, γ is the arc extinction angle, and μ is the commutation overlap angle. The first comparison module is used to compare the calculated values ​​of intermediate parameters with their initial values: If the two are equal, the arc-extinguishing angle γ obtained from the last calculation will be compared with the arc-extinguishing angle set value γ. Compare; otherwise, recalculate the intermediate parameters, substitute the recalculated intermediate parameters into the system of equations and recalculate until the calculated intermediate parameters are equal to the intermediate parameters substituted into the system of equations. The second comparison module is used to compare the arc-extinguishing angle γ obtained from the last calculation with the arc-extinguishing angle setpoint γ. In comparison: If the arc-extinguishing angle γ is less than the set value of the arc-extinguishing angle γ If the arc-extinguishing angle γ is a complex number, then increasing the effective value of the positive-sequence voltage U of the cascaded H-bridge requires... cp(1) The value is then iteratively calculated again using intermediate parameters until the arc-extinguishing angle γ is greater than or equal to the set value γ of the arc-extinguishing angle. Furthermore, since the arc-extinguishing angle γ is a real number, the required effective value of the positive-sequence voltage U of the cascaded H-bridge at this time is... cp(1) This is the final solution value of the arc-extinguishing angle control iterative calculation, where the arc-extinguishing angle λ is the final solution value.

6. A computer device, characterized in that, The method includes an electrically connected memory and a processor, wherein the memory stores a computer program that can run on the processor, and when the processor executes the computer program, it implements the steps of the method according to any one of claims 1-4.

7. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program, characterized in that, when the computer program is executed by a processor, it implements the steps of the method according to any one of claims 1-4.

Images