Direct current side harmonic suppression method and system of capacitor commutation converter

Abstract

The invention relates to the technical field of converter harmonic suppression, and discloses a direct-current side harmonic suppression method and system for a capacitor commutation converter, and the method comprises the steps: obtaining an alternating-current voltage value of a converter network side, and calculating an actual voltage value of the direct-current side of the converter; according to the difference value between the direct current side voltage preset value and the direct current side actual voltage value of the converter, switching control is carried out on a non-commutation capacitor module; calculating a first voltage compensation value according to the periodic voltage change of the capacitor commutation converter, and calculating a second voltage compensation value according to the difference between the actual capacitor voltage value and the rated capacitor voltage value of the capacitor commutation converter; and iteratively updating the direct current side voltage preset value according to the first voltage compensation value and the second voltage compensation value. According to the invention, through dynamic switching of the capacitor module, the DC side harmonic voltage can be eliminated, and through the feedforward design, the efficiency of DC side harmonic suppression is improved, so that the safety and stability of operation of the DC power transmission system are ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of converter harmonic suppression, and particularly to a method and a system for suppressing DC-side harmonics of a capacitor-commutated converter. Background Art

[0002] Line Commutated Converter (LCC) technology is based on thyristor converters to achieve AC-DC energy conversion. It has advantages such as large current-carrying capacity, high transmission capacity, and strong operation stability, and has become the core technical route for UHV DC transmission projects in China.

[0003] The commutation process of LCC technology essentially truncates the DC current through thyristor switches and conducts the current into the three-phase AC grid in a time-sharing manner, resulting in a stepped waveform of the AC-side current. At the same time, the DC-side voltage is formed by superimposing the time-domain envelopes of different-phase AC voltages. This mechanism causes significant harmonics to be generated on both the AC and DC sides of the LCC system. Due to the existence of harmonics, it is necessary to configure large-capacity smoothing reactors and multi-tuned DC filters to suppress harmonics, which will significantly increase the system investment and floor area; residual harmonics in transient conditions are likely to cause frequent operation of arresters and overload of energy absorption devices; in addition, the harmonic voltage superposition effect will also lead to prominent problems in insulation coordination and communication interference when converters operate in parallel, thus restricting the construction of multi-terminal DC networks.

[0004] To solve the above limitations of LCC technology, currently, Capacitor Commutated Converter (CCC) technology is mostly adopted. CCC technology reduces harmonic generation by connecting capacitor banks in series between the converter transformer and the converter valve and using the capacitor voltage to compensate for the reactive power demand during the commutation process. However, in traditional CCC technology, the capacitor banks need to be switched as a whole, and fine control of the commutation process cannot be achieved. Moreover, the charging and discharging characteristics of the capacitors are mismatched with the system operating conditions, easily leading to voltage imbalance and unstable operation. Summary of the Invention

[0005] To solve the above technical problems, the present invention provides a method and a system for suppressing DC-side harmonics of a capacitor-commutated converter, which dynamically compensate harmonics through a modular capacitor switching mechanism to achieve the technical effect of optimizing the overall characteristics of the DC side of the converter and suppressing DC-side harmonics.

[0006] In a first aspect, the present invention provides a method for suppressing DC-side harmonics of a capacitor-commutated converter, where the capacitors in the capacitor-commutated converter adopt a modular multilevel converter, and the method includes:

[0007] Obtain the AC voltage value on the network side of the converter transformer, convert the AC voltage value on the network side of the converter transformer into the line voltage on the valve side through ratio conversion, and calculate the actual DC side voltage value of the converter according to the line voltage on the valve side;

[0008] Perform switching control on the non-commutating capacitor modules according to the difference between the preset DC side voltage value and the actual DC side voltage value of the converter;

[0009] Calculate the first voltage compensation value according to the periodic voltage change of the capacitor-commutated converter, and calculate the second voltage compensation value according to the difference between the actual capacitor voltage value and the rated capacitor voltage value of the capacitor-commutated converter;

[0010] Iteratively update the preset DC side voltage value according to the first voltage compensation value and the second voltage compensation value, and return to execute the step of performing switching control on the non-commutating capacitor modules according to the difference between the preset DC side voltage value and the actual DC side voltage value of the converter until the capacitor-commutated converter reaches capacitor voltage balance.

[0011] Further, the step of calculating the first voltage compensation value according to the periodic voltage change of the capacitor-commutated converter includes:

[0012] Calculate the average voltage difference between the average voltage of the capacitor-commutated converter at the end of the current switching period and the average voltage of the capacitor-commutated converter at the end of the previous switching period;

[0013] Calculate the first voltage compensation value according to the average voltage difference.

[0014] Further, the first voltage compensation value is represented by the following formula:

[0015]

[0016] In the formula, ΔU 1T represents the first voltage compensation value of phase T, T represents the phase, N C represents the total number of capacitor modules in the capacitor-commutated converter, C represents the capacitance value of the capacitor module, U C1T represents the average voltage of the capacitor modules of phase T at the end of the current switching period, U C0T represents the average voltage of the capacitor modules of phase T at the end of the previous switching period, I dc represents the DC current, and △t represents the input time during the non-commutating stage.

[0017] Further, the step of calculating the second voltage compensation value according to the difference between the actual capacitor voltage value and the rated capacitor voltage value of the capacitor-commutated converter includes:

[0018] Calculate the rated voltage difference between the rated capacitance voltage value and the average voltage of the capacitor commutated converter at the end of the current switching cycle;

[0019] Calculate a second voltage compensation value according to the rated voltage difference.

[0020] Further, the second voltage compensation value is expressed by the following formula:

[0021]

[0022] In the formula, ΔU 2T represents the second voltage compensation value of phase T, T represents the phase, N C represents the total number of capacitor modules in the capacitor commutated converter, C represents the capacitance value of the capacitor module, U C1T represents the average voltage of the capacitor module of phase T at the end of the current switching cycle, U CN represents the rated capacitance voltage value, I dc represents the DC current, and △t represents the on-time during the non-commutation stage.

[0023] Further, the step of iteratively updating the preset DC side voltage value according to the first voltage compensation value and the second voltage compensation value includes:

[0024] Iteratively update the preset DC side voltage value of the rectifier side of the capacitor commutated converter according to the first voltage compensation value and the second voltage compensation value, and using a first iteration formula;

[0025] Iteratively update the preset DC side voltage value of the inverter side of the capacitor commutated converter according to the first voltage compensation value and the second voltage compensation value, and using a second iteration formula.

[0026] Further, the first iteration formula is expressed by the following formula:

[0027]

[0028] The second iteration formula is expressed by the following formula:

[0029]

[0030] In the formula, represents the preset DC side voltage value of the k-th switching cycle, represents the preset DC side voltage value of the (k + 1)-th switching cycle, ΔU 1T represents the first voltage compensation value of phase T, ΔU 2T represents the second voltage compensation value of phase T, and T represents the phase.

[0031] Further, before the step of controlling the switching of the non-commutating capacitor modules according to the difference between the preset DC side voltage value and the actual DC side voltage value of the converter, the following steps are also included:

[0032] Calculate the equivalent DC voltage value during the commutation stage and the equivalent DC voltage value during the non-commutation stage respectively according to the commutation angle and the line voltage phase;

[0033] Calculate the equivalent DC voltage value of the charging and discharging energy according to the output of the capacitor module during commutation;

[0034] Take the sum of the equivalent DC voltage value during the commutation stage, the equivalent DC voltage value during the non-commutation stage and the equivalent DC voltage value of the charging and discharging energy as the equivalent DC voltage value of the capacitor-commutating converter, and use the equivalent DC voltage value as the preset DC side voltage value.

[0035] Further, the equivalent DC voltage value during the commutation stage is expressed by the following formula:

[0036]

[0037] In the formula, represents the equivalent DC voltage value during the commutation stage, represents the effective per-unit value of the line voltage U AB θ represents the phase of the line voltage U AB represents the effective per-unit value of the line voltage U AB θ represents the phase of the line voltage U represents the effective per-unit value of the line voltage U CB θ represents the phase of the line voltage U CB represents the effective per-unit value of the line voltage U CB θ represents the phase of the line voltage U

[0038] The equivalent DC voltage value during the non-commutation stage is expressed by the following formula:

[0039]

[0040] In the formula, represents the equivalent DC voltage value during the non-commutation stage;

[0041] The equivalent DC voltage value of the charging and discharging energy is expressed by the following formula:

[0042]

[0043] In the formula, represents the equivalent DC voltage value of the charging and discharging energy, represents the effective per-unit value of the line voltage U AC θ represents the phase of the line voltage U AC represents the effective per-unit value of the line voltage U AC θ represents the phase of the line voltage U

[0044] The equivalent DC voltage value is expressed by the following formula:

[0045]

[0046] In the formula, represents the equivalent DC voltage value.

[0047] In a second aspect, the present invention provides a DC-side harmonic suppression system for a capacitor-commutated converter, where the capacitors in the capacitor-commutated converter adopt a modular multilevel converter, and the system includes:

[0048] A voltage value calculation module, configured to obtain the AC voltage value on the network side of the converter transformer, convert the AC voltage value on the network side of the converter transformer into the line voltage on the valve side through ratio conversion, and calculate the actual voltage value on the DC side of the converter according to the line voltage on the valve side;

[0049] An initial switching control module, configured to perform switching control on the non-commutating capacitor modules according to the difference between the preset DC-side voltage value and the actual voltage value on the DC side of the converter;

[0050] A compensation value calculation module, configured to calculate a first voltage compensation value according to the periodic voltage change of the capacitor-commutated converter, and calculate a second voltage compensation value according to the difference between the actual capacitor voltage value and the rated capacitor voltage value of the capacitor-commutated converter;

[0051] An iterative control module, configured to iteratively update the preset DC-side voltage value according to the first voltage compensation value and the second voltage compensation value, and return to execute the step of performing switching control on the non-commutating capacitor modules according to the difference between the preset DC-side voltage value and the actual voltage value on the DC side of the converter, until the capacitor voltages of the capacitor-commutated converter reach balance.

[0052] Further, the compensation value calculation module is further configured to calculate the average voltage difference between the average voltage of the capacitor-commutated converter at the end of the current switching period and the average voltage of the capacitor-commutated converter at the end of the previous switching period;

[0053] Calculate the first voltage compensation value according to the average voltage difference.

[0054] Further, the first voltage compensation value is expressed by the following formula:

[0055]

[0056] In the formula, ΔU 1T represents the first voltage compensation value of phase T, T represents the phase, N C represents the total number of capacitor modules in the capacitor-commutated converter, C represents the capacitance value of the capacitor module, U C1Trepresents the average voltage of the capacitor module of phase T at the end of the current switching cycle, U C0T represents the average voltage of the capacitor module of phase T at the end of the previous switching cycle, I dc represents the DC current, and △t represents the input time during the non-commutation stage.

[0057] Further, the compensation value calculation module is also used to calculate the rated voltage difference between the rated capacitor voltage value and the average voltage of the capacitor commutating converter at the end of the current switching cycle;

[0058] Calculate the second voltage compensation value according to the rated voltage difference.

[0059] Further, the second voltage compensation value is expressed by the following formula:

[0060]

[0061] In the formula, ΔU 2T represents the second voltage compensation value of phase T, T represents the phase, N C represents the total number of capacitor modules in the capacitor commutating converter, C represents the capacitance value of the capacitor module, U C1T represents the average voltage of the capacitor module of phase T at the end of the current switching cycle, U CN represents the rated capacitor voltage value, I dc represents the DC current, and △t represents the input time during the non-commutation stage.

[0062] Further, the iterative control module is also used to iteratively update the preset DC-side voltage value of the rectifier side of the capacitor commutating converter according to the first voltage compensation value and the second voltage compensation value, and using the first iterative formula;

[0063] Iteratively update the preset DC-side voltage value of the inverter side of the capacitor commutating converter according to the first voltage compensation value and the second voltage compensation value, and using the second iterative formula.

[0064] Further, the first iterative formula is expressed by the following formula:

[0065]

[0066] The second iterative formula is expressed by the following formula:

[0067]

[0068] In the formula, represents the preset DC-side voltage value of the kth switching cycle, represents the preset DC-side voltage value of the (k + 1)th switching cycle, ΔU 1TThe first voltage compensation value for the T phase, ΔU 2T The second voltage compensation value for the T phase, where T represents the phase.

[0069] Furthermore, the initial switching control module is further configured to calculate the equivalent DC voltage value during the commutation phase and the equivalent DC voltage value during the non - commutation phase respectively according to the commutation angle and the line - voltage phase;

[0070] Calculate the equivalent DC voltage value of the charge - discharge energy according to the output of the capacitor module during commutation;

[0071] Take the sum of the equivalent DC voltage value during the commutation phase, the equivalent DC voltage value during the non - commutation phase, and the equivalent DC voltage value of the charge - discharge energy as the equivalent DC voltage value of the capacitor commutation converter, and use the equivalent DC voltage value as the preset DC - side voltage value.

[0072] Furthermore, the equivalent DC voltage value during the commutation phase is expressed by the following formula:

[0073]

[0074] In the formula, Represents the equivalent DC voltage value during the commutation phase, Represents the effective per - unit value of the line voltage U AB , θ AB Represents the phase of the line voltage U AB , μ represents the commutation angle, Represents the effective per - unit value of the line voltage U CB , θ CB Represents the phase of the line voltage U CB ;

[0075] The equivalent DC voltage value during the non - commutation phase is expressed by the following formula:

[0076]

[0077] In the formula, Represents the equivalent DC voltage value during the non - commutation phase;

[0078] The equivalent DC voltage value of the charge - discharge energy is expressed by the following formula:

[0079]

[0080] In the formula, Represents the equivalent DC voltage value of the charge - discharge energy, Represents the effective per - unit value of the line voltage U AC , θ AC Represents the phase of the line voltage U AC ;

[0081] The equivalent DC voltage value is expressed by the following formula:

[0082]

[0083] In the formula, represents the equivalent DC voltage value.

[0084] The present invention provides a method and a system for suppressing DC-side harmonics of a capacitor phase-shifting converter. The present invention improves the LCC converter into a capacitor module phase-shifting converter, and dynamically switches the capacitor modules based on a preset DC-side voltage value, so that the valve-side voltage value of the converter is the same as the preset DC-side voltage value, thereby eliminating the DC-side harmonic voltage; through feed-forward design, the iteration speed of the preset DC-side voltage value can be increased, thereby improving the efficiency of DC-side harmonic suppression. The present invention integrates the filtering function into the capacitor module, which can effectively suppress the DC-side harmonic voltage of the capacitor phase-shifting converter in the steady state, thereby improving the safety and stability of the operation of the DC power transmission system. BRIEF DESCRIPTION OF THE DRAWINGS

[0085] Figure 1 is a schematic flow chart of the method for suppressing DC-side harmonics of the capacitor phase-shifting converter in the embodiment of the present invention;

[0086] Figure 2 is a schematic equivalent circuit diagram of the LCC thyristor converter;

[0087] Figure 3 is a schematic equivalent circuit diagram of the capacitor phase-shifting converter in the embodiment of the present invention;

[0088] Figure 4 is a schematic structural diagram of the system for suppressing DC-side harmonics of the capacitor phase-shifting converter in the embodiment of the present invention;

[0089] Reference Signs:

[0090] 10. Voltage value calculation module; 20. Initial switching control module; 30. Compensation value calculation module; 40. Iterative control module. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0091] 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.

[0092] Please refer to Figure 1, a method for suppressing DC - side harmonics of a capacitor - commutated converter proposed in the first embodiment of the present invention. In the capacitor - commutated converter, the capacitor adopts a modular multilevel converter. The DC - side harmonic suppression method includes steps S10 to S40:

[0093] Step S10: Obtain the AC voltage value on the grid side of the converter transformer, convert the AC voltage value on the grid side of the converter transformer into the line voltage on the valve side through ratio conversion, and calculate the actual DC - side voltage value of the converter according to the line voltage on the valve side;

[0094] Step S20: Perform switching control on the non - commutation capacitor modules according to the difference between the preset DC - side voltage value and the actual DC - side voltage value of the converter;

[0095] Step S30: Calculate the first voltage compensation value according to the periodic voltage change of the capacitor - commutated converter, and calculate the second voltage compensation value according to the difference between the actual capacitor voltage value and the rated capacitor voltage value of the capacitor - commutated converter;

[0096] Step S40: Iteratively update the preset DC - side voltage value according to the first voltage compensation value and the second voltage compensation value, and return to execute the step of performing switching control on the non - commutation capacitor modules according to the difference between the preset DC - side voltage value and the actual DC - side voltage value of the converter until the capacitor - commutated converter reaches capacitor voltage balance.

[0097] Before explaining the method for suppressing DC - side harmonics of 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 of the power grid is given. Please refer to Figure 2 , the LCC thyristor converter consists of upper and lower half - bridges, and each half - bridge consists of three thyristors. Among them, Usa, Usb, and Usc respectively represent the voltages of three converter transformers (converter transformers), and T i (i = 1,..., 6) represents the i - th thyristor, A, B, and C are the three phases of the transformer respectively, and A`, B`, and C` are the three phases of the transformer respectively. 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 , commutating the current 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.

[0098] To solve the reactive power consumption problem of LCC DC, the traditional capacitor commutated converter (CCC) technology adds a capacitor between the converter transformer and the converter valve of the LCC converter to change the commutation time of the converter valve, eliminate the reactive power consumption of the converter valve, and reduce the AC-DC side harmonics of the converter valve. However, due to the irresistible charging characteristics of the capacitor, the traditional capacitor commutated converter has great limitations in application scenarios. Based on this, the capacitor in the capacitor commutated converter of the present invention is replaced with a modular multilevel converter (Modular Multilevel Converter, MMC). Please refer to Figure 3 , the converter topology structure in this embodiment adds an MMC between the thyristor converter valve and the converter transformer of the LCC. Since the MMC includes multiple sub-capacitor modules, the commutation moment and commutation duration of the thyristor converter valve can be controlled through the controllable MMC capacitor module. The MMC capacitor module can be equivalent to a voltage source, that is Figure 3 Uva, Uvb and Uvc in AC , various voltage waveforms can be superimposed on the premise of maintaining the charge and discharge balance of the capacitor module. Since the capacitor module modifies the commutation voltage of the thyristor converter valve on the basis of the AC voltage, the thyristor converter can start commutation when the trigger angle α < 0, that is, U

[0099] is negative, and combined with the commutation time, the commutation current and the AC voltage phase are aligned, thus eliminating the reactive power consumption. cA and U cC in

[0100]

[0101] 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.

[0102] Finally, the voltage drop on the inductor of the converter transformer is:

[0103]

[0104] The expression based on the commutation current change rate is:

[0105]

[0106] In the formula, L represents the equivalent inductance value of the converter transformer, I arepresents the instantaneous current flowing through the phase A converter valve during commutation, and t represents time. represents the instantaneous change rate of the phase A converter valve current.

[0107] That is to say, during the C / A commutation, the B-phase capacitor module does not output any power. However, if the B-phase capacitor module outputs power, the DC voltage of the converter will change accordingly. After the C / A commutation ends, the A-phase and B-phase capacitor modules do not need to output power, but if they do, the DC voltage of the converter will also change. Based on this principle, the originally AC line voltage on the DC side of the converter can be smoothed by using the capacitor modules that are idle during commutation and operation, and flattened into a DC voltage.

[0108] Based on the above principle, in this embodiment, the AC voltage value on the network side of the converter transformer is first obtained, that is, the three-phase voltage values U A 、U B and U C of the network side of the converter transformer, and then it is converted into the valve-side line voltage through ratio conversion:

[0109]

[0110] In the formula, k represents the ratio coefficient, U AB is the line voltage between phase A and phase B, U BC is the line voltage between phase B and phase C, U AC is the line voltage between phase A and phase C, U A is the voltage value of phase A, U B is the voltage value of phase B, U C is the voltage value of phase C.

[0111] Taking the C / A commutation as an example, when phase C and phase A are commuted, the actual voltage value U dc_actual on the DC side of the converter is:

[0112]

[0113] After the commutation ends, the actual voltage value on the DC side of the converter is:

[0114] U dc_actual =U AB

[0115] Assuming that the planned DC side voltage value to be controlled is U dco , that is, the DC side voltage preset value is U dco , then the difference △U between the actual DC voltage value calculated from the AC line voltage and the preset value is:

[0116] ΔU=U dco -U dc_actual

[0117] By controlling the switching of the non - output capacitor modules according to the calculated difference △U to make them output power, the DC voltage of the converter can be changed, so that the DC - side voltage exhibits the characteristics of a DC voltage. Still taking the C\A commutation as an example, during the C\A commutation, the capacitor modules of phase B do not output power. At this time, the switching control of the capacitor modules of phase B can be carried out to make the capacitor modules of phase B output power according to △U. After the commutation ends, the capacitor modules of phase A and phase B do not output power. At this time, the switching control of the capacitor modules of phase A or phase B can be carried out to make the capacitor modules of phase A or phase B output power according to △U, where △U in the commutation stage and the non - commutation stage are calculated according to the above formulas respectively.

[0118] During the switching process of the capacitor modules, since the DC current will flow through the capacitor modules, the charging and discharging of the capacitor modules will inevitably occur. For the rectifier side, during the C\A commutation, the DC - side voltage of the converter valve is relatively low. Therefore, the capacitor modules of phase B that flatten the DC voltage output positive voltage, and the current charges the capacitor modules. After the commutation ends, the DC - side voltage of the converter valve is relatively high. Therefore, the capacitor modules of phase B output negative voltage, and the current discharges the capacitor modules. In addition, during the commutation process, the commutation current will also charge and discharge the capacitor modules of the two phases. Therefore, during the commutation process on the rectifier side, the DC current will have an overall charging effect on the capacitor modules of the two phases, and during the commutation process on the inverter side, the DC current will have an overall discharging effect on the capacitor modules of the two phases. During the DC operation process, in order to ensure the voltage balance of the capacitor modules, it is necessary to continuously adjust the preset value of the DC - side voltage until the charge - discharge balance is achieved.

[0119] That is to say, the voltage balance of the capacitor module is a feedback control process. Since the three-phase voltages of the AC power grid are not strictly balanced, the above iterations should be controlled separately for each single phase. For example, the B-phase module starts to control the commutation process from the A / B commutation, controls the DC-side voltage during and after the C / A commutation of the three-phase converter bridge on the opposite side, and controls the commutation process during the B / C commutation of this three-phase converter bridge. This is a complete switching cycle of the commutation module. Therefore, the voltage changes of the capacitor modules at the beginning and end of this cycle should be used to determine how to adjust the preset DC voltage. Similarly, the switching cycle of the C-phase capacitor module starts from the B / C commutation and ends at the C / A commutation. Each fundamental cycle has two such charge-discharge cycles for each phase capacitor module. The duration of each cycle is approximately 1 / 2 of a fundamental cycle. When performing control, a PI controller can be used to control the preset value of the DC-side voltage according to the voltage change of the capacitor module. However, since the voltage control cycle of this capacitor module is relatively long, it is easy for the PI controller to be unstable. Therefore, in a preferred embodiment, the present invention provides a discretized method for controlling the DC-side voltage of a converter, which iteratively updates the preset value of the DC-side voltage through a voltage compensation value, thereby realizing the adjustment of the preset value of the DC-side voltage. Among them, the voltage compensation value includes a first voltage compensation value and a second voltage compensation value. The specific calculation steps of the first voltage compensation value include:

[0120] Calculate the average voltage difference between the average voltage of the capacitor commutation converter at the end of the current switching cycle and the average voltage of the capacitor commutation converter at the end of the previous switching cycle;

[0121] Calculate the first voltage compensation value according to the average voltage difference.

[0122] In this embodiment, taking the rectifier side of the converter as an example, during the process of canceling the DC-side harmonics, the charging energy of the DC current to the capacitor module is the increase in the overall total energy of the capacitor module. If the preset value of the DC-side voltage U dco is increased by ΔU, then the energy of the capacitor module increases by ΔU×I dc ×Δt during this process, where ΔU is the voltage increase amount and I dcis the DC current, and Δt is the time length during which the capacitor module of this phase is connected during the switching period, that is, the time length for canceling the harmonics on the DC side, preferably, this time length is about 1 / 6 of the fundamental wave period. That is to say, the time for the capacitor module to cancel the harmonics on the DC side is during the non-commutation time of the capacitor module, that is, △t represents the input time during the non-commutation stage of the capacitor module. However, the commutation process will also charge and discharge the capacitor module, and the charging of the capacitor module during the commutation process is fixed, while adjusting the preset value of the DC side voltage can adjust the charging energy of the capacitor module in an entire cycle. Therefore, the rise and fall of the average voltage of the capacitor module at the head and tail of a capacitor module switching period can be used to characterize whether the current DC side voltage preset value is the true equivalent DC voltage on the DC side, that is, whether it can reach the DC voltage at which the charging and discharging of the capacitor module are balanced.

[0123] Assume that the current DC side voltage preset value is the DC voltage at which the charging and discharging of the capacitor module can be balanced. Then, at the beginning and end of a charging and discharging cycle of a capacitor module, the voltage of the capacitor module will remain unchanged, that is, the voltage-time area for charging and the voltage-time area for discharging within the cycle are the same. If the voltage of the capacitor module rises within this cycle, it means that the energy charged into the capacitor module is more than the energy released, and the preset value of the DC side voltage needs to be adjusted to restore the balance. That is, the current DC side voltage preset value is lower than the DC voltage at which the charging and discharging can be balanced by ΔU 1T , so it is necessary to increase the preset value of the DC side voltage to prompt the capacitor module to discharge. According to the principle of energy conservation, the power corresponding to this compensation value should be equal to the increased power of the capacitor module voltage rise. Therefore, ΔU 1T can be calculated by the following formula:

[0124]

[0125] In the formula, ΔU 1T represents the first voltage compensation value of phase T, T represents the phase, and its values are phase A, phase B, and phase C, N C represents the total number of capacitor modules in the capacitor commutation converter, C represents the capacitance value of the capacitor module, U C1T represents the average voltage of the capacitor module of phase T at the end of the current switching period, U C0T represents the average voltage of the capacitor module of phase T at the end of the previous switching period, I dc represents the DC current, and △t represents the input time during the non-commutation stage.

[0126] Therefore, increase the current DC side voltage preset value by ΔU 1T, it is possible to gradually approach the DC voltage that can achieve the charge and discharge balance of the capacitor module, that is, to reach the true equivalent DC voltage of the DC converter. However, in order to control the average voltage of the capacitor module, it is not enough to only compensate the preset DC side voltage with the first voltage compensation value, because the average voltage of the capacitor module is also limited by the rated capacitor voltage. When the average voltage of the capacitor module deviates significantly from the rated capacitor voltage, it is necessary to increase the capacitor voltage through charging. At this time, it is necessary to reduce the preset DC side voltage so that the actual DC voltage is relatively higher and the capacitor module absorbs current for charging. Therefore, a compensation value is also required to control the average voltage of the capacitor module to approach the rated capacitor voltage. The specific calculation steps of this compensation value include:

[0127] Calculate the rated voltage difference between the rated capacitor voltage value and the average voltage of the capacitor commutator at the end of the current switching period;

[0128] Calculate the second voltage compensation value according to the rated voltage difference.

[0129] In this embodiment, consistent with the calculation principle of the first voltage compensation value, based on the principle of energy conservation, according to the principle of energy conservation, the power corresponding to this compensation value should be equal to the power corresponding to the deviation value between the average voltage of the capacitor module and the rated capacitor voltage. Therefore, ΔU 2T Can be calculated by the following formula:

[0130]

[0131] In the formula, ΔU 2T Represents the second voltage compensation value of phase T, T represents the phase, N C Represents the total number of capacitor modules in the capacitor commutator, C represents the capacitance value of the capacitor module, U C1T Represents the average voltage of the capacitor module of phase T at the end of the current switching period, U CN Represents the rated capacitor voltage value, I dc Represents the DC current, and △t represents the input time during the non-commutation stage.

[0132] According to the above embodiment, when the voltage of the capacitor module rises, it is necessary to increase the preset DC side voltage to promote the discharge of the capacitor module. When the average voltage of the capacitor module is less than the rated capacitor voltage, it is necessary to reduce the preset DC side voltage to promote the capacitor module to absorb current for charging. Therefore, on the rectifier side, the iterative formula of the preset DC side voltage can be expressed as:

[0133]

[0134] In the formula, Represents the preset DC side voltage of the kth switching period, Represents the preset DC-side voltage value in the (k + 1)-th switching cycle, ΔU 1T Represents the first voltage compensation value of phase T, ΔU 2T Represents the second voltage compensation value of phase T, where T represents the phase.

[0135] It should be noted that the subscript T in the above formula represents the three phases A, B, and C, that is, T = A, B, C. Therefore, when performing iterative calculations on the preset DC-side voltage value, for the first voltage compensation value and the second voltage compensation value, the corresponding compensation values for the three phases A, B, and C need to be calculated separately, that is:

[0136]

[0137] In the formula, ΔU 1A Represents the first voltage compensation value of phase A, ΔU 2A Represents the second voltage compensation value of phase A, ΔU 1B Represents the first voltage compensation value of phase B, ΔU 2B Represents the second voltage compensation value of phase B, ΔU 1C Represents the first voltage compensation value of phase C, ΔU 2C Represents the second voltage compensation value of phase C.

[0138] Similarly, for the inverter side of the converter, the adjustment amplitude of the preset DC-side voltage value is the same as that of the rectifier side, but the polarity is opposite to that of the rectifier side. Therefore, on the inverter side, the iterative calculation formula for the preset DC-side voltage value is expressed as:

[0139]

[0140] Since the iterative principle of the preset DC-side voltage value on the inverter side is the same as that on the rectifier side, the iterative derivation process on the inverter side can refer to the above rectifier side derivation process and will not be elaborated here.

[0141] According to the above iterative calculation process, it can be seen that the adjustment process of the preset DC-side voltage value is a relatively slow adjustment process. Under certain special working conditions, such as converter startup, DC-side grounding fault, and AC-side grounding fault, this adjustment of the DC voltage often cannot keep up with the change speed of the system, resulting in overcharging or discharging of the capacitor. Severe discharging will cause overvoltage of the capacitor module, thereby causing the capacitor module to drop out.

[0142] To avoid the occurrence of the above situation, in a preferred embodiment, the present invention also designs a feedforward control link to accelerate the iterative speed of the DC-side voltage by directly calculating the equivalent DC voltage of the capacitor module commutation converter. The specific steps include:

[0143] Calculate the equivalent DC voltage value during the commutation stage and the equivalent DC voltage value during the non-commutation stage respectively according to the commutation angle and the line voltage phase;

[0144] Calculate the equivalent DC voltage value of the charge and discharge energy according to the output of the capacitor module during commutation;

[0145] Take the sum of the equivalent DC voltage value during the commutation stage, the equivalent DC voltage value during the non-commutation stage, and the equivalent DC voltage value of the charge and discharge energy as the equivalent DC voltage value of the capacitor-commutated converter, and use the equivalent DC voltage value as the preset DC side voltage value.

[0146] In this embodiment, according to the analysis and calculation process of the actual DC side voltage value of the converter in the above embodiment, it can be known that during commutation, the DC side voltage of the converter is the line voltage U of the AC side AB and U CB sum of 1 / 2, during non-commutation, the DC side voltage of the converter is the line voltage U of the AC side AB , therefore, the equivalent DC voltage of the converter can be obtained by dividing the integral of the voltage with respect to time by the time. In addition, during the commutation process, the capacitor module is charged and discharged, and it is necessary to increase or decrease the equivalent DC voltage at the non-commutation moment to charge or discharge. Therefore, in the calculation of the equivalent voltage, it is also necessary to consider the energy increase / decrease brought by the charge and discharge of the capacitor module during commutation. This energy is divided by the DC current, that is, the equivalent increase / decrease of the DC voltage time integral.

[0147] Taking the rectifier side of the converter as an example, assume that at the start of triggering, that is, when the phase of the line voltage U between phase A and phase C AC is the trigger angle α of the DC system, the phase of U AB is θ AB , the phase of U CB is θ CB , in the capacitor-commutated converter, since the commutation speed is controllable, the commutation angle μ can be calculated. Then, during the commutation process, the voltage time integral S AB corresponding to U AB can be expressed as:

[0148] S AB =U AB ×(cos(θ AB ) - cos(θ AB +μ)) / ω

[0149] In the formula, ω is the angular frequency, μ is the commutation angle, U AB is the line voltage between phase A and phase B, S AB is the voltage time integral, θ AB is the phase of U AB .

[0150] The corresponding equivalent DC voltage can be expressed as:

[0151]

[0152] For a DC system, the rated no-load DC voltage U dio for:

[0153] U dio =U ABN ×(cos(π / 3)-cos(2×π / 3)) / ω N / (π / 3 / ω N )=U ABN / ω N / (π / 3 / ω N )

[0154] Where U ABN is the line voltage U AB The reference voltage, ω N is the reference angular frequency.

[0155] Therefore, the equivalent DC voltage The effective per unit value It can be expressed as:

[0156]

[0157] In the formula, Indicates line voltage U AB The effective per unit value of .

[0158] Since during commutation, the DC side voltage of the converter is the AC side line voltage U AB and U CB According to the calculation formula of the above effective per unit value, the equivalent DC voltage value in the commutation stage can be expressed as:

[0159]

[0160] In the formula, Indicates the equivalent DC voltage value during the commutation phase, Indicates line voltage U AB The effective per unit value, θ AB Indicates line voltage U AB The phase of μ represents the commutation angle. Indicates line voltage U CB The effective per unit value, θ CB Indicates line voltage U CB phase.

[0161] When not commutating, the DC side voltage of the converter is the line voltage U AB , and the corresponding angle range is (θ AB +μ) to (θAB If it is +π / 3), the equivalent DC voltage value in the non-commutation stage can be expressed as:

[0162]

[0163] In the formula, represents the equivalent DC voltage value in the non-commutation stage.

[0164] According to the output of the capacitor module during commutation and its charge and discharge process, it can be known that the charge and discharge energy of the capacitor module during commutation is equivalent to the voltage increment. Therefore, during the commutation process on the rectifier side, the equivalent DC voltage corresponding to the charging of the capacitor module increases.

[0165] During the commutation process, the current in phase A increases from 0 to the DC current I dc , and the current in phase C decreases from the DC current I dc to 0. The voltage output U cA of the capacitor module in phase A is:

[0166] U cA =-U AC / 2 + U p

[0167] In the formula, U AC is the line voltage between phase A and phase C, and U p is the DC voltage.

[0168] The voltage output U cC of the capacitor module in phase C is:

[0169]

[0170] Then the charging energy W is:

[0171]

[0172] In the formula, μ is the commutation angle, I a represents the instantaneous current flowing through the thyristor valve in phase A during commutation, I c represents the instantaneous current flowing through the thyristor valve in phase C during commutation, I dc is the DC current, t represents time, α is the trigger angle, ω is the angular frequency, U AC is the line voltage between phase A and phase C, and U p is the DC voltage.

[0173] The increased average DC voltage, that is, the equivalent DC voltage value of the charge and discharge energy, can be expressed as:

[0174]

[0175] In the formula, Represents the equivalent DC voltage value of charge and discharge energy, Represents the line voltage U AC The effective per-unit value of, θ AC Represents the line voltage U AC The phase of, U ABN Is the line voltage U AB The reference voltage of, ω N Is the reference angular frequency, W is the charging energy.

[0176] And since the input time of the non-commutating capacitor module is about 1 / 6 of a fundamental wave period, therefore, for the commutation process within a certain period, the equivalent DC voltage value of about 1 / 6 of a fundamental wave period on the DC side of the converter can be expressed as:

[0177]

[0178] In the formula, Represents the equivalent DC voltage value.

[0179] This equivalent DC voltage value can be directly obtained by measuring and phase-locking the AC voltage, and used as the feed-forward value for the iterative calculation of the equivalent voltage on the DC side of the capacitor module commutation converter, that is, using it as the initial DC side voltage preset value, and then performing iterative calculation based on the equivalent DC voltage value, which can accelerate the iterative speed of the DC side voltage preset value, thereby improving the efficiency of DC side harmonic suppression.

[0180] For the inverter side of the converter, the calculation formula of its equivalent DC voltage value is the same as that of the rectifier side above. It should be noted that both the equivalent DC voltage value during the commutation stage and the equivalent DC voltage value during the non-commutation stage on the inverter side are negative numbers.

[0181] A method for suppressing DC side harmonics of a capacitor commutation converter provided in this embodiment. The present invention improves the LCC converter into a capacitor module commutation converter, and dynamically switches the capacitor module based on the DC side voltage preset value, so that the valve side voltage value of the converter is the same as the DC side voltage preset value, thereby eliminating the DC side harmonic voltage; through feed-forward design, the iterative speed of the DC side voltage preset value is improved, thereby improving the efficiency of DC side harmonic suppression. The present invention integrates the filtering function into the capacitor module, realizes the effective suppression of the DC side harmonic voltage of the capacitor commutation converter in the steady state, reduces the investment and floor area of the DC filter, and improves the safety and stability of the operation of the DC transmission system.

[0182] Please refer to Figure 4 , based on the same inventive concept, a system for suppressing DC side harmonics of a capacitor commutation converter proposed in the second embodiment of the present invention includes:

[0183] The voltage value calculation module 10 is configured to obtain the AC voltage value on the network side of the converter transformer, convert the AC voltage value on the network side of the converter transformer into the line voltage on the valve side through ratio conversion, and calculate the actual DC-side voltage value of the converter according to the line voltage on the valve side;

[0184] The initial switching control module 20 is configured to perform switching control on the non-commutating capacitor modules according to the difference between the preset DC-side voltage value and the actual DC-side voltage value of the converter;

[0185] The compensation value calculation module 30 is configured to calculate a first voltage compensation value according to the periodic voltage change of the capacitor-commutated converter, and calculate a second voltage compensation value according to the difference between the actual capacitor voltage value and the rated capacitor voltage value of the capacitor-commutated converter;

[0186] The iterative control module 40 is configured to iteratively update the preset DC-side voltage value according to the first voltage compensation value and the second voltage compensation value, and return to execute the step of performing switching control on the non-commutating capacitor modules according to the difference between the preset DC-side voltage value and the actual DC-side voltage value of the converter, until the capacitor-commutated converter reaches capacitor voltage balance.

[0187] In a preferred embodiment, the compensation value calculation module 30 is further configured to calculate the average voltage difference between the average voltage of the capacitor-commutated converter at the end of the current switching period and the average voltage of the capacitor-commutated converter at the end of the previous switching period;

[0188] Calculate the first voltage compensation value according to the average voltage difference.

[0189] Further, the first voltage compensation value is represented by the following formula:

[0190]

[0191] In the formula, ΔU 1T represents the first voltage compensation value of phase T, T represents the phase, N C represents the total number of capacitor modules in the capacitor-commutated converter, C represents the capacitance value of the capacitor module, U C1T represents the average voltage of the capacitor module of phase T at the end of the current switching period, U C0T represents the average voltage of the capacitor module of phase T at the end of the previous switching period, I dc represents the DC current, and △t represents the input time during the non-commutating stage.

[0192] In a preferred embodiment, the compensation value calculation module 30 is further configured to calculate the rated voltage difference between the rated capacitor voltage value and the average voltage of the capacitor-commutated converter at the end of the current switching period;

[0193] Calculate a second voltage compensation value according to the rated voltage difference.

[0194] Further, the second voltage compensation value is expressed by the following formula:

[0195]

[0196] In the formula, ΔU 2T represents the second voltage compensation value of phase T, T represents the phase, N C represents the total number of capacitor modules in the capacitor-commutated converter, C represents the capacitance value of the capacitor module, U C1T represents the average voltage of the capacitor modules of phase T at the end of the current switching period, U CN represents the rated capacitor voltage value, I dc represents the DC current, and △t represents the input time during the non-commutation stage.

[0197] In another preferred embodiment, the iterative control module 40 is further configured to perform iterative update on the preset DC-side voltage value of the rectifier side of the capacitor-commutated converter according to the first voltage compensation value and the second voltage compensation value, and by using a first iterative formula;

[0198] Perform iterative update on the preset DC-side voltage value of the inverter side of the capacitor-commutated converter according to the first voltage compensation value and the second voltage compensation value, and by using a second iterative formula.

[0199] Further, the first iterative formula is expressed by the following formula:

[0200]

[0201] The second iterative formula is expressed by the following formula:

[0202]

[0203] In the formula, represents the preset DC-side voltage value of the kth switching period, represents the preset DC-side voltage value of the (k + 1)th switching period, ΔU 1T represents the first voltage compensation value of phase T, ΔU 2T represents the second voltage compensation value of phase T, and T represents the phase.

[0204] In another preferred embodiment, the initial switching control module 20 is further configured to calculate an equivalent DC voltage value during the commutation stage and an equivalent DC voltage value during the non-commutation stage respectively according to the commutation angle and the line voltage phase;

[0205] Calculate the equivalent DC voltage value of the charging and discharging energy according to the output of the capacitor module during commutation;

[0206] Take the sum of the equivalent DC voltage value in the commutation stage, the equivalent DC voltage value in the non - commutation stage, and the equivalent DC voltage value of the charge - discharge energy as the equivalent DC voltage value of the capacitor commutation converter, and use the equivalent DC voltage value as the preset value of the DC - side voltage.

[0207] Furthermore, the equivalent DC voltage value in the commutation stage is expressed by the following formula:

[0208]

[0209] In the formula, represents the equivalent DC voltage value in the commutation stage, represents the effective per - unit value of the line voltage U AB , θ AB represents the phase of the line voltage U AB , μ represents the commutation angle, represents the effective per - unit value of the line voltage U CB , θ CB represents the phase of the line voltage U CB ;

[0210] The equivalent DC voltage value in the non - commutation stage is expressed by the following formula:

[0211]

[0212] In the formula, represents the equivalent DC voltage value in the non - commutation stage;

[0213] The equivalent DC voltage value of the charge - discharge energy is expressed by the following formula:

[0214]

[0215] In the formula, represents the equivalent DC voltage value of the charge - discharge energy, represents the effective per - unit value of the line voltage U AC , θ AC represents the phase of the line voltage U AC ;

[0216] The equivalent DC voltage value is expressed by the following formula:

[0217]

[0218] In the formula, represents the equivalent DC voltage value.

[0219] The technical features and technical effects of the DC-side harmonic suppression 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 DC-side harmonic suppression system of the capacitor-commutated converter can be implemented in whole or in part by software, hardware, and their combination. The above-mentioned modules can be embedded in or independent of the processor in the computer device in the form of hardware, or stored in the memory of the computer device in the form of software, so that the processor can call and execute the operations corresponding to each of the above modules.

[0220] In summary, the embodiments of the present invention propose a method and a system for suppressing DC-side harmonics of a capacitor-commutated converter. The method includes obtaining the AC voltage value on the network side of the converter transformer, converting the AC voltage value on the network side of the converter transformer into the line voltage on the valve side through ratio conversion, and calculating the actual DC-side voltage value of the converter according to the line voltage on the valve side; controlling the switching of the non-commutating capacitor modules according to the difference between the preset DC-side voltage value and the actual DC-side voltage value of the converter; calculating a first voltage compensation value according to the periodic voltage change of the capacitor-commutated converter, and calculating a second voltage compensation value according to the difference between the actual capacitor voltage value and the rated capacitor voltage value of the capacitor-commutated converter; iteratively updating the preset DC-side voltage value according to the first voltage compensation value and the second voltage compensation value, and returning to execute the step of controlling the switching of the non-commutating capacitor modules according to the difference between the preset DC-side voltage value and the actual DC-side voltage value of the converter until the capacitor-commutated converter reaches capacitor voltage balance. The present invention improves the LCC converter into a capacitor-module commutated converter, and dynamically switches the capacitor modules based on the preset DC-side voltage value, so that the valve-side voltage value of the converter is the same as the preset DC-side voltage value, thereby eliminating the DC-side harmonic voltage. At the same time, through feedforward design, the iteration speed of the preset DC-side voltage value is increased, thereby improving the efficiency of DC-side harmonic suppression.

[0221] 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 the relevant parts can refer to the partial description of the method embodiment. 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 there is no contradiction in the combination of these technical features, it should be considered as the scope recorded in this specification.

[0222] The above-described embodiments merely represent several preferred embodiments of the present application. The description 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. A method for suppressing DC-side harmonics of a capacitor phase-shifting converter, characterized in that, The capacitors in the capacitor-commutated converter adopt a modular multilevel converter, and the method includes: Obtain the AC voltage value on the grid side of the converter transformer, convert the AC voltage value on the grid side of the converter transformer into the line voltage on the valve side through ratio conversion, and calculate the actual DC voltage value on the DC side of the converter according to the line voltage on the valve side; Perform switching control on the non-commutating capacitor modules according to the difference between the preset DC side voltage value and the actual DC side voltage value of the converter; Calculate the first voltage compensation value according to the periodic voltage change of the capacitor-commutated converter, and calculate the second voltage compensation value according to the difference between the actual capacitor voltage value and the rated capacitor voltage value of the capacitor-commutated converter; Iteratively update the preset DC side voltage value according to the first voltage compensation value and the second voltage compensation value, and return to execute the step of performing switching control on the non-commutating capacitor modules according to the difference between the preset DC side voltage value and the actual DC side voltage value of the converter until the capacitor-commutated converter reaches capacitor voltage balance.

2. The DC-side harmonic suppression method of the capacitor commutation converter according to claim 1, characterized in that The step of calculating the first voltage compensation value according to the periodic voltage change of the capacitor-commutated converter includes: Calculate the average voltage difference between the average voltage of the capacitor-commutated converter at the end of the current switching period and the average voltage of the capacitor-commutated converter at the end of the previous switching period; Calculate the first voltage compensation value according to the average voltage difference.

3. The method for suppressing DC-side harmonics of the capacitor commutation converter according to claim 2, wherein The first voltage compensation value is expressed by the following formula: where ΔU 1T represents the first voltage compensation value of phase T, T represents the phase, and N C represents the total number of capacitor modules in the capacitor-commutated converter, C represents the capacitance value of the capacitor module, and U C1T represents the average voltage of the capacitor module of phase T at the end of the current switching period, and U C0T represents the average voltage of the capacitor module of phase T at the end of the previous switching period, and I dc represents the DC current, and △t represents the input time during the non-commutation stage.

4. The DC-side harmonic suppression method of the capacitor commutation converter according to claim 1, characterized in that The step of calculating the second voltage compensation value according to the difference between the actual capacitor voltage value and the rated capacitor voltage value of the capacitor-commutated converter includes: Calculate the rated voltage difference between the rated capacitor voltage value and the average voltage of the capacitor-commutated converter at the end of the current switching period; Calculate the second voltage compensation value according to the rated voltage difference.

5. The method for suppressing DC-side harmonics of the capacitor commutation converter according to claim 4, characterized in that, The second voltage compensation value is expressed by the following formula: where ΔU 2T represents the second voltage compensation value of phase T, T represents the phase, and N C represents the total number of capacitor modules in the capacitor-commutated converter, C represents the capacitance value of the capacitor module, and U C1T represents the average voltage of the capacitor module of phase T at the end of the current switching period, and U CN represents the rated capacitor voltage value, I dc represents the DC current, and Δt represents the input time during the non-commutation stage.

6. The method for suppressing DC-side harmonics of a capacitor commutation converter according to claim 1, wherein The step of iteratively updating the preset DC side voltage value according to the first voltage compensation value and the second voltage compensation value includes: Iteratively update the preset DC side voltage value on the rectifier side of the capacitor-commutated converter according to the first voltage compensation value and the second voltage compensation value, and using the first iteration formula; Iteratively update the preset DC side voltage value on the inverter side of the capacitor-commutated converter according to the first voltage compensation value and the second voltage compensation value, and using the second iteration formula.

7. The DC-side harmonic suppression method for the capacitor commutation converter according to claim 6, characterized in that, The first iteration formula is expressed by the following formula: The second iteration formula is expressed by the following formula: wherein, represents the preset DC-side voltage value in the k-th switching period, represents the preset DC-side voltage value in the (k + 1)-th switching period, ΔU 1T represents the first voltage compensation value of phase T, ΔU 2T represents the second voltage compensation value of phase T, and T represents the phase.

8. The method for suppressing DC side harmonics of the capacitor commutation converter according to claim 1, wherein Before the step of performing switching control on the non-commutating capacitor modules according to the difference between the preset DC side voltage value and the actual DC side voltage value of the converter, it further includes: Calculate the equivalent DC voltage value during the commutation stage and the equivalent DC voltage value during the non-commutation stage respectively according to the commutation angle and the line voltage phase; Calculate the equivalent DC voltage value of the charge and discharge energy according to the output of the capacitor module during commutation; Take the sum of the equivalent DC voltage value during the commutation stage, the equivalent DC voltage value during the non-commutation stage, and the equivalent DC voltage value of the charge and discharge energy as the equivalent DC voltage value of the capacitor-commutated converter, and use the equivalent DC voltage value as the preset DC side voltage value.

9. The DC-side harmonic suppression method for the capacitor commutation converter according to claim 8, characterized in that, The equivalent DC voltage value in the commutation stage is expressed by the following formula: In the formula, represents the equivalent DC voltage value during the commutation stage, represents the effective per-unit value of the line voltage U AB , θ AB represents the phase of the line voltage U AB , μ represents the commutation angle, represents the effective per-unit value of the line voltage U CB , θ CB represents the phase of the line voltage U CB ; The equivalent DC voltage value in the non-commutation stage is expressed by the following formula: In the formula, represents the equivalent DC voltage value in the non-commutation stage; The equivalent DC voltage value of the charge and discharge energy is expressed by the following formula: In the formula, represents the equivalent DC voltage value of charge and discharge energy, represents the effective per-unit value of line voltage U AC , and θ AC represents the phase of line voltage U AC . The equivalent DC voltage value is expressed by the following formula: In the formula, represents the equivalent DC voltage value.

10. A DC-side harmonic suppression system for a capacitor phase-shifting converter, characterized in that, The capacitors in the capacitor commutation converter adopt a modular multilevel converter, and the system includes: A voltage value calculation module, configured to obtain the AC voltage value on the grid side of the converter transformer, convert the AC voltage value on the grid side of the converter transformer into the line voltage on the valve side through ratio conversion, and calculate the actual DC voltage value on the DC side of the converter according to the line voltage on the valve side; An initial switching control module, configured to perform switching control on the non-commutation capacitor modules according to the difference between the preset DC side voltage value and the actual DC side voltage value of the converter; A compensation value calculation module, configured to calculate a first voltage compensation value according to the periodic voltage change of the capacitor commutation converter, and calculate a second voltage compensation value according to the difference between the actual capacitor voltage value and the rated capacitor voltage value of the capacitor commutation converter; An iterative control module, configured to iteratively update the preset DC side voltage value according to the first voltage compensation value and the second voltage compensation value, and return to execute the step of performing switching control on the non-commutation capacitor modules according to the difference between the preset DC side voltage value and the actual DC side voltage value of the converter, until the capacitor voltage of the capacitor commutation converter reaches balance.

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