Inversion control method and system of capacitor commutation converter, and capacitor commutation converter

Abstract

The invention relates to the technical field of inverter inverter control, and discloses an inverter control method and system for a capacitor commutation converter and the capacitor commutation converter, and the method comprises the steps: carrying out the PI control of a difference value between a measured voltage value and a target voltage value of an inverter side of the capacitor commutation converter, and obtaining a first control quantity; pI adjustment is carried out on the difference value between the actually measured current value of the inversion side of the capacitor commutation converter and the target current value, a second control quantity is obtained, and the second control quantity is smaller than or equal to the first control quantity; according to the comparison relationship between the second control quantity and the trigger angle limit value, calculating the trigger angle and the DC voltage output of the modular multilevel converter; and controlling the inverter side of the capacitor commutation converter according to the trigger angle and the DC voltage output. According to the invention, all-condition continuous control can be realized, the commutation reliability is improved, and commutation failure is avoided, so that the reactive power consumption of the converter is effectively reduced, and the efficiency of a direct-current power transmission system is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of inverter control of converters, and particularly to an inverter control method, a system and a capacitor commutated converter for a capacitor commutated converter. Background Art

[0002] A line commutated converter (LCC) uses thyristors as the main devices, and has the characteristics of large current carrying capacity, low loss, simple structure, stability and reliability, etc., and plays an important role in large-capacity long-distance energy transmission. Currently, it is widely used in UHV DC projects. During the commutation process of an LCC DC transmission project, a large amount of reactive power is absorbed on the AC side. Since thyristors need a positive voltage to conduct and a negative voltage to turn off, and a certain negative voltage area is required to ensure complete turn-off during turn-off, therefore, on the rectifier side and the inverter side, the phase of the current lags behind the phase of the voltage, which will cause the converter to absorb a large amount of reactive power. This also leads to the problems that the LCC converter station must be equipped with a large number of passive filters and capacitors for reactive power compensation, and at the same time, it also causes problems such as overvoltage during LCC DC blocking and low voltage after DC faults, which limit the access of LCC DC to weak systems.

[0003] In view of the problems existing in the LCC technology, the currently commonly used improvement method is the capacitor commutated converter technology. The capacitor commutated converter technology adds a capacitor between the converter transformer and the converter valve of the LCC converter, and by changing the commutation time of the converter valve, the reactive power consumption of the converter valve is eliminated, which is equivalent to directly compensating the inductive characteristics of the LCC DC with a series capacitor. However, the capacitor used in the capacitor commutated converter is an oil-filled capacitor. When an impact current is injected into the capacitor due to factors such as lightning strikes or line faults, it is difficult to control the capacitor voltage of the capacitor, which easily leads to commutation failure. Summary of the Invention

[0004] In order to solve the above technical problems, the present invention provides an inverter control method, a system and a capacitor commutated converter for a capacitor commutated converter, so as to solve the problem that the capacitor module is easily affected by the impact current and causes commutation failure, and achieve the technical effect of improving the safety and stability of the inverter side control of the converter.

[0005] In a first aspect, the present invention provides an inverter control method for a capacitor commutated converter, wherein the capacitor in the capacitor commutated converter adopts a modular multilevel converter, and the method includes:

[0006] Performing PI control on the difference between the measured voltage value and the target voltage value on the inverter side of the capacitor commutated converter to obtain a first control quantity;

[0007] Perform PI regulation on the difference between the measured current value and the target current value on the inverter side of the capacitor-commutated converter to obtain a second control quantity, where the second control quantity is less than or equal to the first control quantity;

[0008] Calculate the trigger angle and the DC voltage output of the modular multilevel converter according to the comparison relationship between the second control quantity and the trigger angle limit value;

[0009] Control the inverter side of the capacitor-commutated converter according to the trigger angle and the DC voltage output.

[0010] Further, the step of calculating the trigger angle and the DC voltage output of the modular multilevel converter according to the comparison relationship between the second control quantity and the trigger angle limit value includes:

[0011] Judge whether the second control quantity is less than or equal to the trigger angle limit value. If so, use the second control quantity as the trigger angle and use the DC voltage output base value as the DC voltage output;

[0012] Otherwise, use the trigger angle limit value as the trigger angle, and calculate the DC voltage output according to the difference between the second control quantity and the trigger angle limit value and the DC voltage output base value.

[0013] Further, the DC voltage output is expressed by the following formula:

[0014]

[0015] In the formula, represents the DC voltage output, represents the DC voltage output base value, Sigma represents the second control quantity, α max represents the trigger angle limit value, K up represents the proportionality coefficient.

[0016] Further, after the step of calculating the trigger angle and the DC voltage output of the modular multilevel converter according to the comparison relationship between the second control quantity and the trigger angle limit value, it further includes:

[0017] Judge whether the measured current value is less than or equal to the DC current critical value. If so, use the first preset strategy to calculate the first maximum trigger angle, the first maximum control quantity, the minimum DC voltage output and the maximum DC voltage output;

[0018] Use the first maximum trigger angle as the maximum value of the trigger angle, use the first maximum control quantity as the maximum value of the first control quantity, and set the constraint range of the DC voltage output according to the minimum DC voltage output and the maximum DC voltage output;

[0019] If not, adopt the second preset strategy to calculate the second maximum triggering angle, the second maximum control amount, and the fixed value of the DC voltage output.

[0020] Take the second maximum triggering angle as the maximum value of the triggering angle, the second maximum control amount as the maximum value of the first control amount, and set the DC voltage output to the fixed value of the DC voltage output.

[0021] Further, the steps of adopting the first preset strategy to calculate the first maximum triggering angle, the first maximum control amount, the minimum value of the DC voltage output, and the maximum value of the DC voltage output include:

[0022] Calculate the maximum value of the DC voltage output according to the total voltage output capacity of the capacitor module.

[0023] Calculate the minimum value of the DC voltage output according to the total voltage output capacity of the capacitor module and the designed value of the maximum commutation angle.

[0024] Calculate the maximum value of the turn-off angle according to the maximum value of the DC voltage output, and calculate the first maximum triggering angle according to the maximum value of the turn-off angle.

[0025] Calculate the first maximum control amount according to the maximum value of the DC voltage output and the first maximum triggering angle.

[0026] Further, the maximum value of the DC voltage output is expressed by the following formula:

[0027]

[0028] In the formula, represents the maximum value of the DC voltage output, V mmc represents the total voltage output capacity of the capacitor module, represents the per-unit value of the no-load DC voltage of the converter, ω * represents the per-unit value of the angular frequency, represents the per-unit value of the DC current, d x represents the per-unit value of the equivalent inductive voltage drop of the converter transformer;

[0029] The minimum value of the DC voltage output is expressed by the following formula:

[0030]

[0031] In the formula, represents the minimum value of the DC voltage output, max(*) represents the maximum value function, μ N represents the designed value of the maximum commutation angle;

[0032] The first maximum triggering angle is expressed by the following formula:

[0033] αmax1 = π + γ max

[0034] Wherein, α max1 represents the first maximum triggering angle, and γ max represents the maximum turn-off angle;

[0035] The first maximum control amount is expressed by the following formula:

[0036]

[0037] Wherein, sigma max1 represents the first maximum control amount, represents the base value of the DC voltage output, and K up represents the proportionality coefficient.

[0038] Furthermore, the steps of calculating the second maximum triggering angle, the second maximum control amount, and the fixed value of the DC voltage output by adopting the second preset strategy include:

[0039] Calculate the critical commutation angle according to the designed value of the maximum commutation angle and the calculated value of the maximum commutation angle corresponding to the critical DC current value;

[0040] Calculate the fixed value of the DC voltage output according to the critical commutation angle;

[0041] Calculate the second maximum triggering angle according to the total voltage output capacity of the capacitor module, the critical commutation angle, and the fixed value of the DC voltage output;

[0042] Calculate the second maximum control amount according to the fixed value of the DC voltage output and the second maximum triggering angle.

[0043] Furthermore, the fixed value of the DC voltage output is expressed by the following formula:

[0044]

[0045] Wherein, represents the fixed value of the DC voltage output, μ max1 represents the critical commutation angle, ω * represents the per-unit value of the angular frequency, represents the per-unit value of the DC current, and d x represents the per-unit value of the equivalent inductive voltage drop of the converter transformer;

[0046] The second maximum triggering angle is expressed by the following formula:

[0047]

[0048] Wherein, α max2 represents the second maximum triggering angle, Vmmc represents the total voltage output capacity of the capacitor module, represents the per-unit value of the no-load DC voltage of the converter;

[0049] The second maximum control quantity is represented by the following formula:

[0050]

[0051] In the formula, sigma max2 represents the second maximum control quantity, represents the base value of the DC voltage output, K up represents the proportionality coefficient.

[0052] Furthermore, the DC current critical value is calculated based on the total voltage output capacity of the capacitor module.

[0053] In a second aspect, the present invention provides an inverter control system for a capacitor-commutated converter, wherein the capacitors in the capacitor-commutated converter adopt a modular multilevel converter, and the system includes:

[0054] A voltage control module for performing PI control on the difference between the measured voltage value and the target voltage value on the inverter side of the capacitor-commutated converter to obtain a first control quantity;

[0055] A current control module for performing PI adjustment on the difference between the measured current value and the target current value on the inverter side of the capacitor-commutated converter to obtain a second control quantity, wherein the second control quantity is less than or equal to the first control quantity;

[0056] Calculate the trigger angle and the DC voltage output of the modular multilevel converter according to the comparison relationship between the second control quantity and the trigger angle limit value;

[0057] An inverter control module for controlling the inverter side of the capacitor-commutated converter according to the trigger angle and the DC voltage output.

[0058] Furthermore, the current control module is further configured to determine whether the second control quantity is less than or equal to the trigger angle limit value. If so, use the second control quantity as the trigger angle and use the base value of the DC voltage output as the DC voltage output;

[0059] Otherwise, use the trigger angle limit value as the trigger angle and calculate the DC voltage output according to the difference between the second control quantity and the trigger angle limit value and the base value of the DC voltage output.

[0060] Furthermore, the DC voltage output is represented by the following formula:

[0061]

[0062] Wherein, represents the DC voltage output, represents the base value of the DC voltage output, Sigma represents the second control quantity, and α max represents the trigger angle limit value, and K up represents the proportionality coefficient.

[0063] Furthermore, the current control module is also used to determine whether the measured current value is less than or equal to the DC current critical value. If so, it adopts the first preset strategy to calculate the first maximum trigger angle, the first maximum control quantity, the minimum value of the DC voltage output, and the maximum value of the DC voltage output;

[0064] It takes the first maximum trigger angle as the maximum value of the trigger angle, takes the first maximum control quantity as the maximum value of the first control quantity, and sets the constraint range of the DC voltage output according to the minimum value and the maximum value of the DC voltage output;

[0065] If not, it adopts the second preset strategy to calculate the second maximum trigger angle, the second maximum control quantity, and the fixed value of the DC voltage output;

[0066] It takes the second maximum trigger angle as the maximum value of the trigger angle, takes the second maximum control quantity as the maximum value of the first control quantity, and sets the DC voltage output to the fixed value of the DC voltage output.

[0067] Furthermore, the current control module is also used to calculate the maximum value of the DC voltage output according to the total voltage output capacity of the capacitor module;

[0068] It calculates the minimum value of the DC voltage output according to the total voltage output capacity of the capacitor module and the designed value of the maximum commutation angle;

[0069] It calculates the maximum value of the turn-off angle according to the maximum value of the DC voltage output, and calculates the first maximum trigger angle according to the maximum value of the turn-off angle;

[0070] It calculates the first maximum control quantity according to the maximum value of the DC voltage output and the first maximum trigger angle.

[0071] Furthermore, the maximum value of the DC voltage output is expressed by the following formula:

[0072]

[0073] Wherein, represents the maximum value of the DC voltage output, V mmc represents the total voltage output capacity of the capacitor module, represents the per-unit value of the no-load DC voltage of the converter, ω * represents the per-unit value of the angular frequency, denotes the per-unit value of direct current, d x denotes the per-unit value of the equivalent inductive voltage drop of the converter transformer;

[0074] The minimum value of the direct current voltage output is expressed by the following formula:

[0075]

[0076] In the formula, denotes the minimum value of the direct current voltage output, max(*) denotes the maximum value function, μ N denotes the designed maximum commutation angle;

[0077] The first maximum firing angle is expressed by the following formula:

[0078] α max1 = π + γ max

[0079] In the formula, α max1 denotes the first maximum firing angle, γ max denotes the maximum extinction angle;

[0080] The first maximum control quantity is expressed by the following formula:

[0081]

[0082] In the formula, sigma max1 denotes the first maximum control quantity, denotes the base value of the direct current voltage output, K up denotes the proportionality coefficient.

[0083] Furthermore, the current control module is also used to calculate the critical commutation angle according to the designed maximum commutation angle and the calculated commutation angle corresponding to the critical direct current value;

[0084] Calculate the fixed value of the direct current voltage output according to the critical commutation angle;

[0085] Calculate the second maximum firing angle according to the total voltage output capacity of the capacitor module, the critical commutation angle and the fixed value of the direct current voltage output;

[0086] Calculate the second maximum control quantity according to the fixed value of the direct current voltage output and the second maximum firing angle.

[0087] Furthermore, the fixed value of the direct current voltage output is expressed by the following formula:

[0088]

[0089] In the formula, denotes the fixed value of the direct current voltage output, μmax1 Represents the critical value of the commutation angle, ω * Represents the per-unit value of the angular frequency, Represents the per-unit value of the DC current, d x Represents the per-unit value of the equivalent inductive voltage drop of the commutation transformer;

[0090] The second maximum firing angle is represented by the following formula:

[0091]

[0092] In the formula, α max2 Represents the second maximum firing angle, V mmc Represents the total voltage output capacity of the capacitor module, Represents the per-unit value of the no-load DC voltage of the converter;

[0093] The second maximum control quantity is represented by the following formula:

[0094]

[0095] In the formula, sigma max2 Represents the second maximum control quantity, Represents the basic value of the DC voltage output, K up Represents the proportionality coefficient.

[0096] Thirdly, an embodiment of the present invention further provides a capacitor-commutated converter. The capacitors of the capacitor-commutated converter adopt a modular multilevel converter, and the inverter side of the capacitor-commutated converter is controlled by the method described above.

[0097] The present invention provides an inverter control method, system and capacitor-commutated converter for a capacitor-commutated converter. Through the dual control quantity dynamic distribution mechanism and in combination with the two-stage control strategy, the present invention can ensure that the voltage required for commutation is always within the actual capacity range of the capacitor module, significantly improving the reliability of commutation and avoiding commutation failure; through the control strategy switching logic based on the DC current critical value, smooth transition between the two-stage controls can be achieved, thus realizing full-condition continuous control and eliminating control jumps; through the inverter side control based on the dual control quantities, the reactive power consumption of the converter is reduced, and the efficiency of the DC power transmission system is improved; in addition, the control method provided by the present invention can be compatible with the traditional control architecture, effectively reducing the transformation cost and providing an effective solution for improving the reliability of the converter and reducing the operation and maintenance cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0098] Figure 1 Is a schematic flowchart of the inverter control method for a capacitor-commutated converter in an embodiment of the present invention;

[0099] Figure 2It is a schematic diagram of the equivalent circuit structure of the LCC thyristor converter;

[0100] Figure 3 It is a schematic diagram of the equivalent circuit structure of the capacitor-commutated converter in the embodiment of the present invention;

[0101] Figure 4 It is a schematic diagram of the structure of the inverter control system of the capacitor-commutated converter in the embodiment of the present invention;

[0102] Reference numerals:

[0103] 10. Voltage control module; 20. Current control module; 30. Inverter control module. Detailed implementation manners

[0104] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0105] Please refer to Figure 1 , an inverter control method for a capacitor-commutated converter proposed in the first embodiment of the present invention. The capacitor in the capacitor-commutated converter adopts a modular multilevel converter, including steps S10 to S40:

[0106] Step S10, perform PI control on the difference between the measured voltage value and the target voltage value on the inverter side of the capacitor-commutated converter to obtain a first control quantity;

[0107] Step S20, perform PI regulation on the difference between the measured current value and the target current value on the inverter side of the capacitor-commutated converter to obtain a second control quantity, where the second control quantity is less than or equal to the first control quantity;

[0108] Step S30, calculate the trigger angle and the DC voltage output of the modular multilevel converter according to the comparison relationship between the second control quantity and the trigger angle limit value;

[0109] Step S40, control the inverter side of the capacitor-commutated converter according to the trigger angle and the DC voltage output.

[0110] Before explaining the inverter control method for the capacitor-commutated converter provided by the present invention, first, a brief introduction to the LCC thyristor converter in the DC project based on the line-commutated converter (LCC) technology is given. Please refer to Figure 2, the LCC thyristor converter consists of upper and lower half - bridges, and each half - bridge is composed of three thyristors. Among them, Usa, Usb, and Usc respectively represent the voltages of three commutation transformers (conversion transformers), and T i (i = 1,..., 6) represents the i - th thyristor, A, B, and C are respectively the three phases of the transformer, and A`, B`, and C` are respectively the three phases of the transformer. The thyristor T1 of the A - phase of the upper half - bridge can be triggered after the zero - crossing point of the line voltage U AC , and the current is commutated from the C - phase to the A - phase. The angle difference between the triggering angle and the zero - crossing point of U AC is the triggering angle α. The thyristor commutation process requires a certain amount of time, and the electrical angle occupied is the commutation angle μ. The triggering angles and commutation angles of the thyristors in the upper and lower half - bridges of the B - phase and C - phase also have the same definition. Since the thyristor must be triggered under a certain positive voltage and the commutation process requires time, the current angle of the LCC converter is always later than the voltage angle, resulting in reactive power consumption, and commutation must occur within 0° - 180° of the line voltage U ac .

[0111] To solve the problem of reactive power consumption of LCC DC, the traditional capacitor - commutated converter (CCC) technology adds a capacitor between the commutation transformer and the commutation valve of the LCC converter, changes the commutation time of the commutation valve, eliminates the reactive power consumption of the commutation valve, and reduces the AC - DC side harmonics of the commutation valve. However, due to the uncontrollable charging characteristics of the capacitor, the traditional capacitor - commutated converter has great limitations in application scenarios. Based on this, the present invention replaces the capacitor in the capacitor - commutated converter with a modular multilevel converter (MMC). Please refer to Figure 3 . The converter topology structure in this embodiment adds an MMC between the thyristor commutation valve and the commutation transformer of the LCC. Since the MMC includes multiple sub - capacitor modules, represented by three phases ABC, it can be understood that the MMC includes an A - phase capacitor module, a B - phase capacitor module, and a C - phase capacitor module. Therefore, by controlling these capacitor modules, the commutation moment and commutation duration of the thyristor commutation valve can be controlled. The MMC capacitor module can be equivalent to a voltage source, that is, Figure 3 Uva, Uvb, and Uvc in AC . On the premise of maintaining the charge - discharge balance of the capacitor module, various voltage waveforms can be superimposed. Since the capacitor module modifies the commutation voltage of the thyristor commutation valve on the basis of the AC voltage, the thyristor converter can commutate when the triggering angle α < 0 or the turn - off angle γ < 0, that is, start or end commutation when U

[0112] In practical applications, during the commutation from phase C to phase A, the output amplitudes of the phase A capacitor module and the phase C capacitor module are the same but in opposite directions. First, they jointly cancel the difference in the valve-side AC voltage, and then a DC voltage output Up is superimposed to control the commutation process. For the inverter side, assuming the sub-module voltages of phase A and phase C are U cA and U cC , they are respectively shown as follows:

[0113]

[0114] In the formula, U A is the voltage of phase A, U C is the voltage of phase C, and U p is the DC voltage output.

[0115] Finally, the voltage drop across the inductor of the commutation transformer is:

[0116]

[0117] The expression based on the commutation current change rate is:

[0118]

[0119] In the formula, L represents the equivalent inductance value of the commutation transformer, I a represents the instantaneous current flowing through the phase A commutation valve during commutation, t represents time, represents the instantaneous change rate of the phase A commutation valve current.

[0120] It can be seen that the capacitor module cancels the original AC line voltage and superimposes a DC voltage output during commutation. After commutation, to ensure the true turn-off of the thyristor device, in addition to maintaining the cancellation of the original AC line voltage, a negative voltage area is superimposed to ensure the reverse recovery process of the thyristor after commutation, so as to avoid commutation failure. The duration and amplitude of this part of the negative voltage area are generally determined by the principle of not increasing the number of capacitor module configurations. That is to say, the capacitor commutation converter in this embodiment has two fast control variables. One is the trigger angle α, which is the same as that of the LCC converter, and the other is the controllable DC voltage output U p .

[0121] The controllers configured on the inverter side of the traditional LCC DC include a current controller, a voltage controller, and a fixed γ - angle controller. The current controller and the voltage controller are both equipped with a PI regulator for PI control, and the fixed γ - angle controller is a calculation link. Among them, the calculated trigger - angle value output by the fixed γ - angle controller is the maximum limit value of the trigger angle output by the PI regulator in the voltage controller, and the trigger angle output by the voltage controller is the maximum limit value of the trigger angle output by the PI regulator in the current controller, so that the γ - angle, DC voltage output, and DC current are all within the limit range, and the trigger angle output under any working condition is a continuously variable control.

[0122] It can be seen that the control variable on the inverter side of the traditional LCC is the trigger angle. In this embodiment, in addition to the trigger angle, the control variable on the inverter side of the capacitor - commutated converter also includes a DC voltage output. Therefore, in the control design, referring to the control architecture of the traditional LCC, the output value of the current controller can be changed from the trigger angle to a control variable, and then the trigger angle and the DC voltage output of the modular multilevel converter are calculated according to the control variable, so as to realize the control of the inverter side of the converter. And in order to ensure the continuity of the control variable, it is also necessary to change the output value of the voltage controller to the control variable and use this control variable as the maximum limit value of the control variable output by the current controller.

[0123] In this embodiment, the configurations of the voltage controller and the current controller of the capacitor - commutated converter are the same as those of the traditional LCC controller, that is, a PI regulator is configured in both the voltage controller and the current controller. The input of the voltage controller is the difference between the measured voltage value and the target voltage value on the inverter side of the capacitor - commutated converter. The PI regulator performs PI control on this voltage difference, and its output value is the first control variable. The input of the current controller is the difference between the measured current value and the target current value on the inverter side of the capacitor - commutated converter. The PI regulator performs PI control on this current difference to output the second control variable, and the trigger angle and the DC voltage output of the modular multilevel converter are calculated according to the second control variable, and the first control variable is the maximum limit value of the second control variable. The calculation steps of the controller can refer to the conventional calculation steps of PI control and will not be elaborated here.

[0124] After the PI regulator of the current controller outputs the second control variable, it is also necessary to calculate the trigger angle and the DC voltage output of the modular multilevel converter according to the second control variable. The specific calculation steps are as follows:

[0125] Judge whether the second control variable is less than or equal to the trigger - angle limit value. If so, use the second control variable as the trigger angle and use the DC voltage output base value as the DC voltage output;

[0126] Conversely, the triggering angle limit value is used as the triggering angle, and the DC voltage output is calculated based on the difference between the second control quantity and the triggering angle limit value and the basic value of the DC voltage output.

[0127] In this embodiment, to ensure the continuity of control, during dynamic control, in the power rising stage, the DC voltage output can be first maintained at a preset basic value, the triggering angle is pulled up, and then the triggering angle is kept unchanged, and then the DC voltage output is pulled up. Therefore, first, according to the comparison relationship between the second control quantity and the triggering angle limit value, the size of the triggering angle is determined, where the triggering angle limit value refers to the maximum allowable triggering angle of the converter, and this value is a design parameter of the converter. If the second control quantity is less than or equal to the triggering angle limit value, that is, the triggering angle of the converter has not reached the maximum limit value, at this time, the triggering angle α should be equal to the second control quantity Sigma, and the DC voltage output should be set to the preset basic value. Conversely, when the second control quantity is greater than the triggering angle limit value, the triggering angle of the converter should be limited to the triggering angle limit value α max , that is, α = α max , and the DC voltage output is superimposed on the basis value by the part where Sigma exceeds α max , so as to realize the pulling up of the DC voltage output. Based on the above control logic, the DC voltage output can be expressed as:

[0128]

[0129] In the formula, represents the DC voltage output, represents the basic value of the DC voltage output, Sigma represents the second control quantity, α max represents the triggering angle limit value, and K up represents the proportionality coefficient. In order to make the calculated DC voltage output different from the DC voltage output in the above principle introduction in terms of expression, here is used to represent the calculated DC voltage output.

[0130] For the inverter side of the capacitor-commutated converter, whether there is enough capacitor module voltage to complete the entire designed commutation process is far more important than the rectifier side, because if the actual ability of the capacitor module is not enough to generate the DC voltage output that meets the commutation requirements, commutation failure may occur. Therefore, in a preferred embodiment, the present invention also needs to calculate the control quantity Sigma, the triggering angle α, and the limit value of the DC voltage output that meet the commutation requirements according to the actual voltage ability of the capacitor module to prevent the capacitor module voltage ability from being insufficient.

[0131] Similarly referring to the control architecture of the traditional LCC, the traditional LCC is controlled by the firing angle, and the calculated value of the firing angle output by the fixed γ angle controller is used as the maximum limit value of the firing angle output in the voltage controller. In this embodiment, the inverter side is controlled by the firing angle and the DC voltage output, and the voltage controller outputs a control quantity. Therefore, in this embodiment, a fixed capacitor voltage controller can be designed to calculate the control quantity Sigma, the firing angle α, and the DC voltage output that meet the commutation requirements according to the actual voltage capacity of the capacitor module. The limit value of the control quantity is used as the maximum limit value of the first control quantity output by the voltage controller, and the limit value of the firing angle and the limit value of the DC voltage output are used as the limit ranges of the firing angle and the DC voltage output output by the current controller, thereby preventing the occurrence of commutation failure due to insufficient voltage capacity of the capacitor module. Among them, the calculation steps of these limit values include:

[0132] Judge whether the measured current value is less than or equal to the DC current critical value. If so, adopt the first preset strategy to calculate the first maximum firing angle, the first maximum control quantity, the minimum DC voltage output, and the maximum DC voltage output.

[0133] Take the first maximum firing angle as the maximum value of the firing angle, take the first maximum control quantity as the maximum value of the first control quantity, and set the constraint range of the DC voltage output according to the minimum DC voltage output and the maximum DC voltage output.

[0134] If not, adopt the second preset strategy to calculate the second maximum firing angle, the second maximum control quantity, and the fixed value of the DC voltage output.

[0135] Take the second maximum firing angle as the maximum value of the firing angle, take the second maximum control quantity as the maximum value of the first control quantity, and set the DC voltage output to the fixed value of the DC voltage output.

[0136] In this embodiment, first analyze the voltage capacity of the capacitor module. When the turn-off angle γ < 0, in order to avoid having two steady-state operating points under the same DC power, it is necessary to avoid -γ > π - α. When the control quantity is increased to increase the DC voltage output, -γ should operate at π - α, and at the same time, -γ is also restricted by the commutation angle μ: -γ = μ / 2. Therefore, the firing angle α is symmetric at both ends of 180° at the commutation start time and the commutation end time, and the limit value of the firing angle is π - μ / 2.

[0137] In the capacitor module commutation converter, since the commutation speed is controllable, the commutation angle μ can be calculated:

[0138]

[0139] Wherein, represents the DC voltage output, and ω * represents the per-unit value of angular frequency, represents the per-unit value of DC current, and d x represents the per-unit value of the equivalent inductive voltage drop of the converter transformer.

[0140] Under this symmetrical control, when the DC current remains unchanged, as the control quantity increases, the DC voltage output and the firing angle will continuously increase. If the control quantity continues to increase, the calculated DC voltage output will exceed the limit that the capacitor module can provide. At this time, if the firing angle calculated is still used for triggering, commutation failure will occur. Obviously, the moment when the requirements for the voltage output of the capacitor module are the most stringent is near the end of commutation when γ < 0. At this time, the output of the capacitor module needs to offset the AC line voltage and generate a sufficiently large controllable DC voltage output.

[0141] Under the state of symmetrical control, the total voltage output capacity V mmc of the capacitor module has the following expression:

[0142]

[0143] Substituting the commutation angle expression, we can get:

[0144]

[0145] Wherein, represents the per-unit value of the no-load DC voltage of the converter. Under rated conditions, this value is 1.

[0146] For a fixed DC current V mmc is a hyperbolic function with respect to or μ. As increases from small to large, the required voltage output of the capacitor module first decreases and then increases, and the same is true for μ.

[0147] By taking the derivative and calculating the extreme value of the above expression, the minimum value of the required output of the capacitor module can be found. Specifically, by taking the derivative of V m with respect to and setting the derivative to zero. To simplify the calculation, during the calculation of the minimum value, sinθ is equivalent to θ, and cosθ is equivalent to 1. After the derivative calculation, we can obtain:

[0148]

[0149] The above value corresponds to the minimum value of the required DC output of the capacitor module as:

[0150]

[0151] Combining the above value expression with the commutation angle expression, the commutation angle corresponding to the minimum DC output required by the capacitor module can be obtained as:

[0152]

[0153] For the minimum DC output required by the capacitor module, for a certain DC current there exists a maximum DC voltage output and a minimum DC voltage output within the control range between them. Only when is within this range, the voltage capacity of the capacitor module, that is, the total output capacity, can meet the minimum output required for commutation. When the DC current is too large, there is no real solution for the expression of V mmc at all, that is, the current is too large and the capacity of the capacitor module is too small to commutate at all. That is to say, there exists a critical DC current value When the DC current exceeds the voltage capacity of the capacitor module can no longer support commutation in the symmetric state. That is, the minimum output required by the capacitor module, V The corresponding DC current is the critical DC current value mmcmin Converting the above expression, we can get:

[0154]

[0155] Since the commutation angle is an expression based on the DC current, therefore, the calculated value μ of the maximum commutation angle corresponding to the critical DC current value max is:

[0156]

[0157] For the maximum and minimum DC voltage outputs, they can be obtained by solving the above expression of V mmc Specifically, according to the small-angle approximation, the sin function is simplified, both sides of the equation are squared and the terms are moved, so as to organize the expression of V mmc into a binary linear equation based on and solve this equation to obtain two roots for Among them, the larger root is the maximum DC voltage output

[0158]

[0159] In the formula, represents the maximum DC voltage output, Vmmc Indicates the total voltage output capacity of the capacitor module. Indicates the per unit value of the converter no-load DC voltage, ω * represents the per-unit value of angular frequency, Indicates the DC current per unit value, d x It represents the per unit value of equivalent inductive voltage drop of the commutation transformer.

[0160] The smaller root should be the minimum DC voltage output:

[0161]

[0162] However, the capacitor-commutated converter has a maximum commutation angle design value μ when it is designed. N , which represents the maximum angle allowed by the commutation angle, so the commutation angle μ also needs to satisfy: μ≤μ N According to the above calculation formula for the commutation angle, replace μ in the formula with μ N , and convert it, we can get:

[0163]

[0164] Therefore, the minimum DC voltage output should be the maximum of the above two current values:

[0165]

[0166] That is:

[0167]

[0168] The upper and lower limits of the cut-off angle γ max and γ min and and Correspondingly, this is also a restriction on the angle. Specifically, γ = -μ / 2, and the calculation expression of μ also exists Then, by combining the calculation expression of μ and the expression of γ, we can get the value of γ based on expression:

[0169]

[0170] Will and Substituting into the above expressions, we can get the upper limit value of the cut-off angle γ max and the lower limit of the cut-off angle γ min :

[0171]

[0172] Since the limit value of the firing angle is π - μ / 2, the first maximum firing angle can be expressed as:

[0173] α max1 = π + γ max

[0174] The first maximum control amount can then be expressed as:

[0175]

[0176] That is to say, the first preset strategy is the calculation strategy for the symmetric control stage. The first preset strategy ensures that the voltage of the capacitor module can meet the commutation requirements by restricting parameters such as the firing angle, turn-off angle, and DC voltage output. When the DC current on the inverter side is greater than the critical value of the DC current, the second preset strategy needs to be adopted to limit the control angle. That is, the second preset strategy is the calculation strategy for the fixed-angle control stage. Specifically, as the DC current increases, the distance between the upper and lower limits of the DC voltage output and the turn-off angle will gradually decrease. When γ min = γ max a critical value μ of the commutation angle can be obtained max1 , compared with μ max calculated completely according to the voltage capacity of the capacitor module of the multi-module level converter max1 , μ

[0177] is also limited by the design value of the maximum commutation angle, that is: max1 μ max = min(μ N )

[0178] At this time, in order to allow the commutation to continue, the symmetric control cannot be maintained anymore. Then the control principle is changed to control the commutation angle at the critical value of the commutation angle, and the voltage capacity of the capacitor module can meet the commutation requirements. Since the commutation angle is controlled at the critical value of the commutation angle, the DC voltage output will also be a fixed value. The fixed value of the DC voltage output is expressed as:

[0179]

[0180] In the formula, represents the fixed value of the DC voltage output, μ max1 represents the critical value of the commutation angle, ω * represents the per-unit value of the angular frequency, represents the per-unit value of the DC current, d x represents the per-unit value of the equivalent inductive voltage drop of the commutation transformer.

[0181] The remaining voltage capacity of the capacitor module is used to offset the AC line voltage. Therefore, the limit on γ is:

[0182]

[0183] In the formula, γ min2 represents the minimum value of the second turn-off angle.

[0184] At this time, the firing angle will be restricted within the range of the second maximum firing angle, that is:

[0185]

[0186] In the formula, α max2 represents the second maximum firing angle, V mmc represents the total voltage output capacity of the capacitor module, represents the per-unit value of the no-load DC voltage of the converter.

[0187] The first control quantity is then restricted within the range of the second maximum control quantity, that is:

[0188]

[0189] In the formula, sigma max2 represents the second maximum control quantity, represents the base value of the DC voltage output, K up represents the proportionality coefficient.

[0190] According to the above embodiments, the fixed-capacitance voltage controller adopts a two-stage control strategy and divides the stages based on the magnitude of the DC current. The first stage is the stage with a small DC current. The capacitor-commutated converter based on the capacitor module can maintain symmetric control. At this time, the turn-off angle γ < 0, and the commutation start angle and the commutation end angle are symmetric around 180°. There are maximum and minimum limits on the commutation DC voltage output and the commutation angle. The second stage is the stage with a large DC current. The voltage capacity of the capacitor module is not sufficient to maintain symmetric control. At this time, the target is to control the commutation angle to the critical angle, and the DC voltage output is increased as the DC current increases. In order to ensure the cancellation of the original AC commutation voltage, the firing angle is gradually reduced.

[0191] In this embodiment, the control of the inverter side of the converter can be understood as adopting a control architecture of a constant capacitor voltage controller, a voltage controller, and a current controller. Among them, the input of the voltage controller is the difference between the measured voltage value and the target voltage value on the inverter side of the capacitor commutated converter, and its output value is the first control quantity; the input of the current controller is the difference between the measured current value and the target current value on the inverter side of the capacitor commutated converter, obtaining a second control quantity, and calculating the trigger angle and the DC voltage output according to the second control quantity. That is, the output value of the current controller includes the second control quantity, the trigger angle, and the DC voltage output; the output of the constant capacitor voltage controller is the limit value of the trigger angle, the limit value of the control quantity, and the limit value of the DC voltage output. Among them, the limit value of the control quantity is the limit value of the first control quantity output by the voltage controller, and the first control quantity is the limit value of the second control quantity output by the current controller. The limit value of the trigger angle and the limit value of the DC voltage output are used as the limit values of the trigger angle and the DC voltage output output by the current controller. Finally, the limited trigger angle and DC voltage output are used to realize the inverter control of the capacitor commutated converter.

[0192] The inverter control method of a capacitor commutated converter provided by this embodiment significantly improves the reliability of commutation and avoids commutation failure through a dual control quantity dynamic distribution mechanism combined with a two-stage control strategy; through a control strategy switching logic based on the DC current critical value, full-condition continuous control is realized, eliminating control jumps; through inverter side control based on dual control quantities, the reactive power consumption of the converter is reduced, and the efficiency of the DC power transmission system is improved; in addition, the control method provided by the present invention can be compatible with the traditional control architecture, can effectively reduce the transformation cost, and provides an effective solution for improving the reliability of the converter and reducing the operation and maintenance cost.

[0193] Please refer to Figure 4 , based on the same inventive concept, an inverter control system of a capacitor commutated converter proposed in the second embodiment of the present invention, where the capacitor in the capacitor commutated converter adopts a modular multilevel converter, and the system includes:

[0194] A voltage control module 10 for performing PI control on the difference between the measured voltage value and the target voltage value on the inverter side of the capacitor commutated converter to obtain a first control quantity;

[0195] A current control module 20 for performing PI adjustment on the difference between the measured current value and the target current value on the inverter side of the capacitor commutated converter to obtain a second control quantity, where the second control quantity is less than or equal to the first control quantity;

[0196] Calculating the trigger angle and the DC voltage output of the modular multilevel converter according to the comparison relationship between the second control quantity and the trigger angle limit value;

[0197] The inversion control module 30 is used to control the inversion side of the capacitor commutated converter according to the trigger angle and the DC voltage output.

[0198] In a preferred embodiment, the current control module 20 is further configured to determine whether the second control quantity is less than or equal to the trigger angle limit value. If so, the second control quantity is used as the trigger angle, and the DC voltage output base value is used as the DC voltage output.

[0199] Otherwise, the trigger angle limit value is used as the trigger angle, and the DC voltage output is calculated according to the difference between the second control quantity and the trigger angle limit value and the DC voltage output base value.

[0200] Further, the DC voltage output is expressed by the following formula:

[0201]

[0202] In the formula, represents the DC voltage output, represents the DC voltage output base value, Sigma represents the second control quantity, α max represents the trigger angle limit value, K up represents the proportionality coefficient.

[0203] In a preferred embodiment, the current control module 20 is further configured to determine whether the measured current value is less than or equal to the DC current critical value. If so, the first preset strategy is used to calculate the first maximum trigger angle, the first maximum control quantity, the minimum DC voltage output, and the maximum DC voltage output.

[0204] The first maximum trigger angle is used as the maximum value of the trigger angle, the first maximum control quantity is used as the maximum value of the first control quantity, and the constraint range of the DC voltage output is set according to the minimum DC voltage output and the maximum DC voltage output.

[0205] If not, the second preset strategy is used to calculate the second maximum trigger angle, the second maximum control quantity, and the fixed DC voltage output value.

[0206] The second maximum trigger angle is used as the maximum value of the trigger angle, the second maximum control quantity is used as the maximum value of the first control quantity, and the DC voltage output is set to the fixed DC voltage output value.

[0207] In a preferred embodiment, the current control module 20 is further configured to calculate the maximum DC voltage output according to the total voltage output capacity of the capacitor module.

[0208] Calculate the minimum DC voltage output according to the total voltage output capacity of the capacitor module and the maximum commutation angle design value.

[0209] Calculate the maximum turn-off angle according to the maximum DC voltage output, and calculate the first maximum firing angle according to the maximum turn-off angle.

[0210] Calculate the first maximum control quantity according to the maximum DC voltage output and the first maximum firing angle.

[0211] Furthermore, the maximum DC voltage output is expressed by the following formula:

[0212]

[0213] In the formula, represents the maximum DC voltage output, V mmc represents the total voltage output capacity of the capacitor module, represents the per-unit value of the no-load DC voltage of the converter, ω * represents the per-unit value of the angular frequency, represents the per-unit value of the DC current, d x represents the per-unit value of the equivalent inductive voltage drop of the converter transformer;

[0214] The minimum DC voltage output is expressed by the following formula:

[0215]

[0216] In the formula, represents the minimum DC voltage output, max(*) represents the maximum value function, μ N represents the designed value of the maximum commutation angle;

[0217] The first maximum firing angle is expressed by the following formula:

[0218] α max1 = π + γ max

[0219] In the formula, α max1 represents the first maximum firing angle, γ max represents the maximum turn-off angle;

[0220] The first maximum control quantity is expressed by the following formula:

[0221]

[0222] In the formula, sigma max1 represents the first maximum control quantity, represents the base value of the DC voltage output, K up represents the proportionality coefficient.

[0223] In another preferred embodiment, the current control module 20 is further configured to calculate a commutation angle critical value according to the maximum designed commutation angle value and the calculated value of the maximum commutation angle corresponding to the DC current critical value;

[0224] Calculate a fixed value of the DC voltage output according to the commutation angle critical value;

[0225] Calculate a second maximum trigger angle according to the total voltage output capacity of the capacitor module, the commutation angle critical value, and the fixed value of the DC voltage output;

[0226] Calculate a second maximum control quantity according to the fixed value of the DC voltage output and the second maximum trigger angle.

[0227] Further, the fixed value of the DC voltage output is represented by the following formula:

[0228]

[0229] In the formula, represents the fixed value of the DC voltage output, μ max1 represents the commutation angle critical value, ω * represents the per-unit value of the angular frequency, represents the per-unit value of the DC current, d x represents the per-unit value of the equivalent inductive voltage drop of the commutation transformer;

[0230] The second maximum trigger angle is represented by the following formula:

[0231]

[0232] In the formula, α max2 represents the second maximum trigger angle, V mmc represents the total voltage output capacity of the capacitor module, represents the per-unit value of the no-load DC voltage of the converter;

[0233] The second maximum control quantity is represented by the following formula:

[0234]

[0235] In the formula, sigma max2 represents the second maximum control quantity, represents the basic value of the DC voltage output, K up represents the proportionality coefficient.

[0236] The technical features and effects of the inverter control system of the capacitor-commutated converter proposed in the embodiments of the present invention are the same as those of the method proposed in the embodiments of the present invention, and will not be elaborated here. Each module in the above inverter control system of the capacitor-commutated converter can be implemented in whole or in part by software, hardware, or a combination thereof. The above modules can be embedded in the processor in the computer device in the form of hardware or be independent of it, or can be stored in the memory in the computer device in the form of software, so as to facilitate the processor to call and execute the operations corresponding to each of the above modules.

[0237] In addition, an embodiment of the present invention also proposes a capacitor-commutated converter. The capacitor of the capacitor-commutated converter adopts a modular multilevel converter, and the inverter side of the capacitor-commutated converter is controlled by the method described above.

[0238] In summary, an inverter control method, system, and capacitor-commutated converter proposed in the embodiments of the present invention. The method performs PI control on the difference between the measured voltage value and the target voltage value on the inverter side of the capacitor-commutated converter to obtain a first control quantity; performs PI adjustment on the difference between the measured current value and the target current value on the inverter side of the capacitor-commutated converter to obtain a second control quantity, and the second control quantity is less than or equal to the first control quantity; calculates the trigger angle and the DC voltage output of the modular multilevel converter according to the comparison relationship between the second control quantity and the trigger angle limit value; controls the inverter side of the capacitor-commutated converter according to the trigger angle and the DC voltage output. The present invention significantly improves the reliability of commutation and avoids commutation failure through a dual control quantity dynamic distribution mechanism combined with a two-stage control strategy; realizes full-condition continuous control and eliminates control jumps through a control strategy switching logic based on the critical value of DC current; reduces the reactive power consumption of the converter and improves the efficiency of the DC power transmission system through inverter side control based on dual control quantities.

[0239] Each embodiment in this specification is described in a progressive manner. For the parts that are the same or similar in each embodiment, reference can be made to each other. Each embodiment focuses on the differences from other embodiments. In particular, for the system embodiment, since it is basically similar to the method embodiment, the description is relatively simple, and reference can be made to the partial description of the method embodiment for the relevant parts. It should be noted that the technical features of the above embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as the combinations of these technical features do not conflict, they should all be considered as the scope described in this specification.

[0240] The above-described embodiments merely represent several preferred embodiments of the present application. The description thereof is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the technical principles of the present invention, several improvements and substitutions can be made, and these improvements and substitutions should also be regarded as the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the protection scope of the claims described above.

Claims

1. An inversion control method for a capacitor phase-shifting converter, characterized in that, The capacitors in the capacitor-commutated converter adopt a modular multilevel converter, and the method includes: Performing PI control on the difference between the measured voltage value and the target voltage value on the inverter side of the capacitor-commutated converter to obtain a first control quantity; Performing PI regulation on the difference between the measured current value and the target current value on the inverter side of the capacitor-commutated converter to obtain a second control quantity, where the second control quantity is less than or equal to the first control quantity; Calculating the trigger angle and the DC voltage output of the modular multilevel converter according to the comparison relationship between the second control quantity and the trigger angle limit value; Controlling the inverter side of the capacitor-commutated converter according to the trigger angle and the DC voltage output.

2. The inverter control method of the capacitor commutation converter according to claim 1, characterized in that The step of calculating the trigger angle and the DC voltage output of the modular multilevel converter according to the comparison relationship between the second control quantity and the trigger angle limit value includes: Judging whether the second control quantity is less than or equal to the trigger angle limit value. If so, using the second control quantity as the trigger angle and using the DC voltage output base value as the DC voltage output; Otherwise, using the trigger angle limit value as the trigger angle and calculating the DC voltage output according to the difference between the second control quantity and the trigger angle limit value and the DC voltage output base value.

3. The inverter control method of the capacitor commutation converter according to claim 2, characterized in that The DC voltage output is expressed by the following formula: In the formula, represents the DC voltage output, represents the base value of the DC voltage output, Sigma represents the second control quantity, α max represents the trigger angle limit value, K up represents the proportionality coefficient.

4. The inverter control method of the capacitor commutation converter according to claim 1, characterized in that After the step of calculating the trigger angle and the DC voltage output of the modular multilevel converter according to the comparison relationship between the second control quantity and the trigger angle limit value, it further includes: Judging whether the measured current value is less than or equal to the DC current critical value. If so, using a first preset strategy to calculate a first maximum trigger angle, a first maximum control quantity, a minimum DC voltage output value, and a maximum DC voltage output value; Using the first maximum trigger angle as the maximum value of the trigger angle, using the first maximum control quantity as the maximum value of the first control quantity, and setting the constraint range of the DC voltage output according to the minimum DC voltage output value and the maximum DC voltage output value; If not, using a second preset strategy to calculate a second maximum trigger angle, a second maximum control quantity, and a fixed DC voltage output value; Using the second maximum trigger angle as the maximum value of the trigger angle, using the second maximum control quantity as the maximum value of the first control quantity, and setting the DC voltage output to the fixed DC voltage output value.

5. The inverter control method of the capacitor commutation converter according to claim 4, characterized in that The step of using the first preset strategy to calculate a first maximum trigger angle, a first maximum control quantity, a minimum DC voltage output value, and a maximum DC voltage output value includes: Calculating the maximum DC voltage output according to the total voltage output capacity of the capacitor modules; Calculating the minimum DC voltage output according to the total voltage output capacity of the capacitor modules and the designed maximum commutation angle value; Calculating the maximum turn-off angle according to the maximum DC voltage output, and calculating the first maximum trigger angle according to the maximum turn-off angle; Calculating the first maximum control quantity according to the maximum DC voltage output and the first maximum trigger angle.

6. The inverter control method of the capacitor commutation converter according to claim 5, characterized in that, The maximum DC voltage output is expressed by the following formula: In the formula, represents the maximum DC voltage output, V mmc represents the total voltage output capacity of the capacitor module, represents the per-unit value of the no-load DC voltage of the converter, ω * represents the per-unit value of the angular frequency, represents the per-unit value of the DC current, d x represents the per-unit value of the equivalent inductive voltage drop of the converter transformer; The minimum DC voltage output is expressed by the following formula: Wherein, represents the minimum value of the DC voltage output, max(*) represents the maximum value function, and μ N represents the designed value of the maximum commutation angle; The first maximum trigger angle is expressed by the following formula: α max1 = π + γ max Where α max1 represents the first maximum firing angle, and γ max represents the maximum turn-off angle; The first maximum control quantity is expressed by the following formula: where, sigma max1 represents the first maximum control amount, represents the DC voltage output base value, K up represents the proportionality coefficient.

7. The inverter control method of the capacitor commutation converter according to claim 4, characterized in that The steps of adopting the second preset strategy to calculate the second maximum trigger angle, the second maximum control quantity and the fixed value of DC voltage output include: Calculating a commutation angle critical value according to the designed maximum commutation angle value and the calculated maximum commutation angle value corresponding to the DC current critical value; Calculating the fixed value of DC voltage output according to the commutation angle critical value; Calculating the second maximum trigger angle according to the total voltage output capacity of the capacitor module, the commutation angle critical value and the fixed value of DC voltage output; Calculating the second maximum control quantity according to the fixed value of DC voltage output and the second maximum trigger angle.

8. The inverter control method of the capacitor commutation converter according to claim 7, characterized in that The fixed value of DC voltage output is expressed by the following formula: Wherein, represents the fixed value of DC voltage output, and μ max1 represents the critical value of commutation angle, and ω * represents the per-unit value of angular frequency, represents the per-unit value of DC current, and d x represents the per-unit value of equivalent inductive voltage drop of converter transformer; The second maximum trigger angle is expressed by the following formula: where α max2 represents the second largest trigger angle, V mmc represents the total voltage output capacity of the capacitor module, represents the per-unit value of the no-load DC voltage of the converter; The second maximum control quantity is expressed by the following formula: where, sigma max2 represents the second largest control amount, represents the base value of the DC voltage output, K up represents the proportionality coefficient.

9. An inverter control system for a capacitor phase-shifting converter, characterized in that The capacitors in the capacitor-commutated converter adopt a modular multilevel converter, and the system includes: A voltage control module for performing PI control on the difference between the measured voltage value and the target voltage value on the inverter side of the capacitor-commutated converter to obtain a first control quantity; A current control module for performing PI adjustment on the difference between the measured current value and the target current value on the inverter side of the capacitor-commutated converter to obtain a second control quantity, wherein the second control quantity is less than or equal to the first control quantity; Calculating the trigger angle and the DC voltage output of the modular multilevel converter according to the comparison relationship between the second control quantity and the trigger angle limit value; An inverter control module for controlling the inverter side of the capacitor-commutated converter according to the trigger angle and the DC voltage output.

10. A capacitive phase-commutated converter, characterized in that, The capacitors of the capacitor-commutated converter adopt a modular multilevel converter, and the inverter side of the capacitor-commutated converter is controlled by the method according to any one of claims 1 to 8.

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