A method and system for harmonic suppression on the DC side of a capacitor commutated converter

Abstract

The application relates to the technical field of converter harmonic suppression and discloses a DC side harmonic suppression method and system of a capacitor commutation converter, which comprises the following steps: obtaining an AC voltage value at a grid side of a converter transformer, and calculating an actual DC side voltage value of the converter; according to the difference between a DC side voltage preset value and the actual DC side voltage value of the converter, switching control is performed on non-commutation capacitor modules; according to the periodic voltage variation of the capacitor commutation converter, a first voltage compensation value is calculated, and according to the difference between an actual capacitor voltage value and a rated capacitor voltage value of the capacitor commutation converter, a second voltage compensation value is calculated; and according to the first voltage compensation value and the second voltage compensation value, the DC side voltage preset value is iteratively updated. Through dynamic switching of the capacitor modules, the DC side harmonic voltage can be eliminated, and through feedforward design, the efficiency of the DC side harmonic suppression is improved, so that the safety and stability of the DC power transmission system operation are ensured.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of converter harmonic suppression, in particular to a DC side harmonic suppression method and system of a capacitor commutated converter. BACKGROUND

[0002] Line Commutated Converter (LCC) technology is based on thyristor converter to realize AC / DC energy conversion, which has the advantages of large current carrying capacity, high transmission capacity and strong stability, and has become the core technology of China's UHV DC transmission project.

[0003] The commutation process of LCC technology is essentially to cut off the DC current by thyristor switch, and to guide the current into the three-phase AC grid in time, which results in a staircase waveform of the AC side current, and the DC side voltage is formed by the time domain envelope superposition of different phase AC voltages. This mechanism causes significant harmonics on both AC and DC sides of the LCC system. Due to the existence of harmonics, large-capacity smoothing reactors and multi-tuned DC filters need to be configured to suppress harmonics, which will significantly increase system investment and land area; residual harmonics under transient conditions can easily cause frequent operation of lightning arresters and overload of energy absorption devices; in addition, the harmonic voltage superposition effect also causes prominent insulation coordination and communication interference problems when the converter is operated in parallel, thereby restricting the construction of multi-terminal DC networks.

[0004] In order to solve the above-mentioned limitations of LCC technology, the capacitor commutated converter (CCC) technology is currently mostly used. CCC technology is to use capacitor groups in series between the converter transformer and the converter valve, and to use capacitor voltage to compensate for the reactive power demand in the commutation process, thereby reducing harmonics. However, in the traditional CCC technology, the capacitor groups need to be switched as a whole, which cannot achieve fine control of the commutation process, and the capacitor charging and discharging characteristics are mismatched with the system operating conditions, which can easily lead to voltage imbalance and unstable operation. SUMMARY

[0005] In order to solve the above-mentioned technical problems, the present application provides a DC side harmonic suppression method and system of a capacitor commutated converter, which dynamically compensates harmonics through a modular capacitor switching mechanism, optimizes the overall characteristics of the DC side of the converter, and achieves the technical effect of suppressing DC side harmonics.

[0006] In a first aspect, the present application provides a DC side harmonic suppression method of a capacitor commutated converter, wherein the capacitor in the capacitor commutated converter adopts a modular multilevel converter, and the method comprises:

[0007] The converter transformer grid-side AC voltage value is obtained, the converter transformer grid-side AC voltage value is converted into a valve-side line voltage through a transformation ratio, and the valve-side line voltage is used to calculate an actual DC-side voltage value of the converter;

[0008] The non-commutated capacitor modules are controlled according to the difference between the DC-side voltage preset value and the actual DC-side voltage value of the converter;

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

[0010] The DC-side voltage preset value is iteratively updated according to the first voltage compensation value and the second voltage compensation value, and the step of controlling the non-commutated capacitor modules according to the difference between the DC-side voltage preset value and the actual DC-side voltage value of the converter is performed again 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 comprises:

[0012] 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 is calculated;

[0013] The first voltage compensation value is calculated 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 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 Δt represents the non-commutation stage input time.

[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 comprises:

[0018] calculating a rated voltage difference value between the rated capacitance voltage value and an average voltage of the capacitor phase commutated converter at the end of the current switching period;

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

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

[0021]

[0022] wherein, ΔU 2T represents the second voltage compensation value of the T phase, T represents a phase, N C represents the total number of the capacitor modules in the capacitor phase commutated converter, C represents the capacitance value of the capacitor module, U C1T represents the average voltage of the capacitor module of the T phase at the end of the current switching period, U CN represents the rated capacitance voltage value, I dc represents the DC current, and Δt represents the non-commutation stage input time.

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

[0024] iteratively updating the DC side voltage preset value of the rectification side of the capacitor phase commutated converter according to the first voltage compensation value and the second voltage compensation value, and using a first iteration formula;

[0025] iteratively updating the DC side voltage preset value of the inversion side of the capacitor phase 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] wherein, represents the DC side voltage preset value of the kth switching period, represents the DC side voltage preset value of the k+1th switching period, ΔU 1T represents the first voltage compensation value of the T phase, ΔU 2T represents the second voltage compensation value of the T phase, T represents a phase.

[0031] Further, before the step of switching the non-commutation capacitor module according to the difference between the preset value of the DC side voltage and the actual voltage value of the DC side of the converter, the method further comprises the steps of:

[0032] calculating the equivalent DC voltage value in the commutation stage and the equivalent DC voltage value in the non-commutation stage according to the commutation angle and the phase of the line voltage;

[0033] calculating the equivalent DC voltage value of the charge-discharge energy according to the output of the capacitor module in the commutation stage;

[0034] taking 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 taking the equivalent DC voltage value as the preset value of the DC side voltage.

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

[0036]

[0037] In the formula, the equivalent DC voltage value in the commutation stage is expressed by the following formula: the effective unit value of the line voltage U AB , θ AB represents the phase of the line voltage U AB , and μ represents the commutation angle, the effective unit value of the line voltage U CB , θ CB represents the phase of the line voltage U CB .

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

[0039]

[0040] In the formula, the equivalent DC voltage value in the non-commutation stage is expressed by the following formula:

[0041] The equivalent DC voltage value of the charge-discharge energy is expressed by the following formula:

[0042]

[0043] In the formula, the equivalent DC voltage value of the charge-discharge energy is expressed by the following formula: the effective unit value of the line voltage U AC , θ AC represents the phase of the line voltage U AC .

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

[0045]

[0046] In the formula, The equivalent DC voltage value is expressed by the following formula.

[0047] In a second aspect, the application provides a DC side harmonic suppression system of a capacitor commutation converter, wherein the capacitor in the capacitor commutation converter is a modular multi-level converter, and the system comprises:

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

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

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

[0051] an iterative control module configured to iteratively update the DC side voltage preset value according to the first voltage compensation value and the second voltage compensation value, and return to perform the step of performing switching control on the non-commutated capacitor module according to a difference between the DC side voltage preset value and the actual DC side voltage value of the converter until the capacitor commutation converter reaches capacitor voltage balance.

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

[0053] The first voltage compensation value is calculated according to the average voltage difference value.

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

[0055]

[0056] In the formula, ΔU 1T The first voltage compensation value of the T phase is expressed by the following formula, wherein T represents a phase, N C The total number of capacitor modules in the capacitor commutation converter is expressed by the following formula, wherein C represents a capacitor value of the capacitor module, U C1Trepresents 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 direct current, and Δt represents the non-commutation stage input time.

[0057] Further, the compensation value calculation module is further configured to calculate a rated voltage difference value between the rated capacitor voltage value and the average voltage of the capacitor commutation converter at the end of the current switching period.

[0058] According to the rated voltage difference value, a second voltage compensation value is calculated.

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

[0060]

[0061] In the formula, ΔU 2T represents the second voltage compensation value of phase T, T represents a phase, and N C represents the total number of capacitor modules in the capacitor commutation converter, C represents the capacitor 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, U CN represents the rated capacitor voltage value, I dc represents the direct current, and Δt represents the non-commutation stage input time.

[0062] Further, the iteration control module is further configured to perform iterative update on the direct side voltage preset value of the rectification side of the capacitor commutation converter according to the first voltage compensation value and the second voltage compensation value, and by using a first iteration formula.

[0063] According to the first voltage compensation value and the second voltage compensation value, the direct side voltage preset value of the inversion side of the capacitor commutation converter is iteratively updated by using a second iteration formula.

[0064] Further, the first iteration formula is represented by the following formula:

[0065]

[0066] The second iteration formula is represented by the following formula:

[0067]

[0068] In the formula, represents the direct side voltage preset value of the kth switching period, represents the direct side voltage preset value of the k+1th switching period, ΔU 1Ta first voltage compensation value of the T phase, ΔU 2T a second voltage compensation value of the T phase, T represents a phase.

[0069] Further, the initial switching control module is further configured to calculate an equivalent DC voltage value in the commutation phase and an equivalent DC voltage value in the non-commutation phase according to the commutation angle and the phase of the line voltage, respectively.

[0070] According to the output of the capacitor module during commutation, an equivalent DC voltage value of charge and discharge energy is calculated.

[0071] The sum of the equivalent DC voltage value in the commutation phase, the equivalent DC voltage value in the non-commutation phase, and the equivalent DC voltage value of charge and discharge energy is taken as the equivalent DC voltage value of the capacitor commutation converter, and the equivalent DC voltage value is taken as the preset value of the DC side voltage.

[0072] Further, the equivalent DC voltage value in the commutation phase is represented by the following formula:

[0073]

[0074] In the formula, the equivalent DC voltage value in the commutation phase is represented by ΔU the effective per-unit value of the line voltage U AB , θ AB the phase of the line voltage U AB , and μ represents the commutation angle, the effective per-unit value of the line voltage U CB , θ CB the phase of the line voltage U CB ;

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

[0076]

[0077] In the formula, the equivalent DC voltage value in the non-commutation phase is represented by ΔU

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

[0079]

[0080] In the formula, the equivalent DC voltage value of charge and discharge energy is represented by ΔU the effective per-unit value of the line voltage U AC , θ AC 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, The equivalent DC voltage value is expressed by the following formula:

[0084] The application provides a DC side harmonic suppression method and system of a capacitor commutation converter. The application improves the LCC converter into a capacitor module commutation converter, and dynamically switches the capacitor module based on a 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 feedforward design, the iteration speed of the DC side voltage preset value can be improved, thereby improving the efficiency of DC side harmonic suppression. The application integrates the filtering function in the capacitor module, which can effectively suppress the DC side harmonic voltage of the capacitor commutation converter in steady state, thereby improving the safety and stability of the DC power transmission system. BRIEF DESCRIPTION OF DRAWINGS

[0085] Figure 1 Fig. 1 is a flowchart of the DC side harmonic suppression method of the capacitor commutation converter in the embodiment of the application;

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

[0087] Figure 3 Fig. 3 is a schematic diagram of the equivalent circuit of the capacitor commutation converter in the embodiment of the application;

[0088] Figure 4 Fig. 4 is a structural schematic diagram of the DC side harmonic suppression system of the capacitor commutation converter in the embodiment of the application;

[0089] Reference signs:

[0090] 10, voltage value calculation module; 20, initial switching control module; 30, compensation value calculation module; 40, iteration control module. DETAILED DESCRIPTION

[0091] To make the purpose, technical scheme and advantages of the embodiments of the application clearer, the technical scheme in the embodiments of the application will be described clearly and completely below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are some embodiments of the application, rather than all the embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the application.

[0092] Please refer to Figure 1The first embodiment of the application provides a DC side harmonic suppression method of a capacitor commutation converter, wherein the capacitor in the capacitor commutation converter adopts a modular multi-level converter, and the DC side harmonic suppression method comprises steps S10-S40.

[0093] In step S10, an AC voltage value of a converter transformer side is obtained, the AC voltage value of the converter transformer side is converted into a valve side line voltage through a transformation ratio, and an actual DC voltage value of the converter is calculated according to the valve side line voltage.

[0094] In step S20, a non-commutation capacitor module is controlled to be switched according to a difference between a DC side voltage preset value and the actual DC voltage value of the converter.

[0095] In step S30, a first voltage compensation value is calculated according to a periodic voltage change of the capacitor commutation converter, and a second voltage compensation value is calculated according to a difference between an actual capacitor voltage value and a rated capacitor voltage value of the capacitor commutation converter.

[0096] In step S40, the DC side voltage preset value is iteratively updated according to the first voltage compensation value and the second voltage compensation value, and the step of controlling the non-commutation capacitor module to be switched according to the difference between the DC side voltage preset value and the actual DC voltage value of the converter is performed again until the capacitor commutation converter reaches capacitor voltage balance.

[0097] Before the DC side harmonic suppression method of the capacitor commutation converter provided by the application is described, the LCC thyristor converter in the LCC-based DC project of the grid commutation DC transmission technology LCC is first introduced, please refer to Figure 2 The LCC thyristor converter is composed of upper and lower half bridges, and each half bridge is composed of three thyristors. Among them, Usa, Usb and Usc represent the voltages of three converter transformers (converter transformers), T i (i=1,…,6) represents the i-th thyristor, A, B and C are three phases of the transformer, and A`, B` and C` are three phases of the transformer. The thyristor T1 of the upper half bridge A phase 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 needs a certain time, and the occupied electrical angle is the commutation angle μ. The triggering angle and the commutation angle of the thyristors of the upper and lower half bridges of the B phase and the C phase also have the same definition. Since the thyristor must be triggered under a certain positive voltage, and the commutation process needs time, the current angle of the LCC converter is always later than the voltage angle, resulting in reactive power consumption.

[0098] In order to solve the problem of reactive power consumption of LCC DC, the traditional capacitor commutation converter (CCC) technology is to add a capacitor between the converter transformer and the converter valve of the LCC converter, change the commutation time of the converter valve, eliminate the reactive power consumption of the converter valve, and reduce the AC and DC side harmonics of the converter valve. However, due to the uncontrollable charging characteristics of the capacitor, the traditional capacitor commutation converter has great limitations in application scenarios. Based on this, the capacitor in the capacitor commutation converter is replaced by a modular multilevel converter (MMC), please refer to Figure 3 The converter topology in the embodiment is to add an MMC between the thyristor converter valve and the converter transformer of the LCC. Since the MMC includes a plurality of sub-capacitor modules, the commutation time and duration of the thyristor converter valve can be controlled through the controllable MMC capacitor modules. The MMC capacitor module can be equivalent to a voltage source, that is Figure 3 Uva, Uvb and Uvc in the formula, under the premise of maintaining the charge balance of the capacitor module, various voltage waveforms can be superimposed. 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 alpha is less than 0, that is, U AC is negative. Combined with the commutation time, the commutation current and the AC voltage phase are aligned, thereby eliminating the reactive power consumption.

[0099] In fact, in engineering applications, when C\A commutates, the A-phase capacitor module and the C-phase capacitor module have the same output amplitude and opposite directions, which first collectively offset the difference between the AC voltages, and then superimpose a DC voltage for controlling the commutation process. That is, the sub-module voltages of the A-phase and the C-phase are U cA and U cC , as shown in the following formula:

[0100]

[0101] In the formula, U A is the A-phase voltage, U C is the C-phase voltage, and U p is the DC voltage.

[0102] The voltage drop on the inductance of the final 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 A-phase converter valve during commutation, t represents time, represents the instantaneous rate of change of the A-phase converter valve current.

[0107] That is, during C\A commutation, the B-phase capacitor module does not contribute anything, but if the B-phase capacitor module contributes, the DC voltage of the converter will change. After the end of C\A commutation, the A-phase and B-phase capacitor modules do not need to contribute, but if they contribute, the DC voltage of the converter will also change. According to this principle, the original AC line voltage on the DC side of the converter can be peak-clipped and valley-filled to be flattened to a DC voltage by using the capacitor modules that are idle during commutation and operation.

[0108] Based on the above principle, the embodiment first acquires the converter transformer grid-side AC voltage value, i.e., the converter transformer grid-side three-phase voltage values U A , U B , and U C , and then converts them to valve-side line voltages through a transformation ratio conversion:

[0109]

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

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

[0112]

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

[0114] U dc_actual = U AB

[0115] Suppose the planned control DC side voltage value is U dco , i.e., the DC side voltage preset value is U dco , then the difference ΔU 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] According to the calculated difference △U, the non-power output capacitor modules are controlled to be switched out of power, so as to change the DC voltage of the converter, so that the DC side voltage presents the characteristics of the DC voltage. Still taking the C\A commutation as an example, during the C\A commutation, the capacitor modules of the B phase do not output power, at this time, the capacitor modules of the B phase can be controlled to be switched, so that the capacitor modules of the B phase output power according to △U, after the commutation ends, the capacitor modules of the A phase and the B phase do not output power, at this time, the capacitor modules of the A phase or the B phase can be controlled to be switched, so that the capacitor modules of the A phase or the B phase output power according to △U, wherein △U in the commutation stage and the non-commutation stage is calculated according to the above formula 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 rectification side, during the C\A commutation, the DC side voltage of the converter valve is low, so the power output of the capacitor modules of the B phase for flattening the DC voltage is positive voltage, and the current charges the capacitor modules. After the commutation ends, the DC side voltage of the converter valve is high, so the power output of the B phase capacitor modules is negative voltage, and the current discharges the capacitor modules. In addition, during the commutation process, the commutation current also charges and discharges the capacitor modules of the two phases, so during the commutation process of the rectification side, the DC current will have a charging effect on the capacitor modules of the two phases as a whole, and during the commutation process of the inverter side, the DC current will have a discharging effect on the capacitor modules of the two phases as a whole. During the DC operation, 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 charging and discharging balance.

[0119] That is, the voltage balance of the capacitor module is a feedback control process, since the three-phase voltage of the alternating current grid is not strictly balanced, the above iterations should be controlled separately in single phase, for example, the B-phase module starts to control the commutation process from A / B commutation, and controls the DC side voltage during and after C / A commutation of the opposite three-phase converter bridge, and controls the commutation process during B / C commutation of the current three-phase converter bridge, which is a complete converter module switching cycle, so the voltage change of the capacitor module at the beginning and end of the cycle should be used to determine how to adjust the preset DC voltage. Similarly, the switching cycle of the C-phase capacitor module also starts from B / C commutation and ends at C / A commutation. Each fundamental cycle has two such charging and discharging cycles for each phase capacitor module. Each cycle lasts about 1 / 2 of a fundamental cycle. When controlling, a PI controller can be used to control the DC side voltage preset value according to the voltage change of the capacitor module. However, since the capacitor module voltage control cycle is relatively long, the use of a PI controller is prone to instability, so in a preferred embodiment, the present application provides a discrete converter DC side voltage control method, which updates the DC side voltage preset value iteratively through a voltage compensation value to adjust the DC side voltage preset value, wherein the voltage compensation value includes a first voltage compensation value and a second voltage compensation value, and the specific calculation steps of the first voltage compensation value include:

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

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

[0122] In this embodiment, taking the converter rectifier side as an example, in the process of canceling the DC side harmonics, the charging energy of the capacitor module by the DC current is the increase of the total energy of the capacitor module as a whole. If the DC side voltage preset value U dco is increased by ΔU, then the energy of the capacitor module in this process will increase ΔU×I dc ×Δt, ΔU is the voltage increase, I dcis the DC current, and Δt is the length of time that the capacitor module is put into use during a switching period when the phase is not commutated, i.e., the length of time that the capacitor module is put into use for offsetting the DC side harmonic. Preferably, the length of time is about 1 / 6 of a fundamental period. That is, the length of time that the capacitor module is put into use for offsetting the DC side harmonic is during the non-commutation time of the capacitor module, i.e., Δt represents the length of time that the capacitor module is put into use during the non-commutation stage. However, the commutation process also charges and discharges the capacitor module, and the charging of the capacitor module by the commutation process is fixed, while adjusting the preset value of the DC side voltage can adjust the charging energy of the capacitor module during an entire period, and thus the increase or decrease of the average voltage of the capacitor module at the beginning and end of a capacitor module switching period can represent whether the preset value of the DC side voltage is the equivalent DC voltage of the DC side, i.e., whether the preset value of the DC side voltage is the DC voltage that can achieve the balance of the charging and discharging of the capacitor module.

[0123] Assuming that the preset value of the DC side voltage is the DC voltage that can achieve the balance of the charging and discharging of the capacitor module, at the beginning and end of a charging and discharging period of the capacitor module, the voltage of the capacitor module remains unchanged, i.e., the voltage time area of the charging during the period is the same as the voltage time area of the discharging. If the voltage of the capacitor module rises during the period, it indicates that the capacitor module charges more energy than discharges, and thus the preset value of the DC side voltage needs to be adjusted to restore the balance, i.e., the preset value of the DC side voltage is lower than the DC voltage that can achieve the balance of the charging and discharging by ΔU 1T Therefore, the preset value of the DC side voltage needs to be increased to promote the discharging of the capacitor module, and according to the principle of energy conservation, the power corresponding to the compensation value should be equal to the power that increases due to the rise of the voltage of the capacitor module, and thus ΔU 1T The compensation value 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 the value of T is A, B, or C, N C represents the total number of capacitor modules in the capacitor commutated converter, C represents the capacitance 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 Δt represents the length of time that the capacitor module is put into use during the non-commutation stage.

[0126] Therefore, the preset value of the DC side voltage is increased by ΔU 1T, which is the real equivalent DC voltage of the DC converter. However, in order to control the average voltage of the capacitor module, it is not enough to compensate the DC side voltage preset value by 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 from the rated capacitor voltage, the capacitor voltage needs to be raised by charging, at which time the DC side voltage preset value needs to be lowered so that the actual DC voltage is relatively higher, and the capacitor module absorbs current to charge. Therefore, a compensation value is also needed to control the average voltage of the capacitor module to approach the rated capacitor voltage, and the specific calculation steps of the compensation value include:

[0127] calculating a rated voltage difference value between the rated capacitor voltage value and the average voltage of the capacitor phase converter at the end of the current switching period;

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

[0129] In this embodiment, the calculation principle of the first voltage compensation value is consistent, and based on the principle of energy conservation, according to the principle of energy conservation, the power corresponding to the 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, so ΔU 2T which 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 phase 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 CN represents the rated capacitor voltage value, I dc represents the DC current, and △t represents the non-commutation stage input time.

[0132] According to the above embodiment, when the voltage of the capacitor module rises, the DC side voltage preset value needs to be increased to promote the discharge of the capacitor module, and when the average voltage of the capacitor module is less than the rated capacitor voltage, the DC side voltage preset value needs to be lowered to promote the capacitor module to absorb current to charge. Therefore, the iteration formula of the DC side voltage preset value on the rectifier side can be represented as:

[0133]

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

[0135] It should be noted that the subscript T in the above formula represents A, B and C three phases, that is, T = A, B, C, so when the DC side voltage preset value is iteratively calculated, the first voltage compensation value and the second voltage compensation value need to be calculated respectively for the compensation values of A, B and C three phases, that is:

[0136]

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

[0138] Similarly, for the inverter side of the converter, the adjustment amplitude of the DC side voltage preset value is the same as that of the rectifier side, but the polarity is opposite to that of the rectifier side, so in the inverter side, the iterative calculation formula of the DC side voltage preset value is represented as:

[0139]

[0140] Since the iteration principle of the DC side voltage preset value of the inverter side is the same as that of the rectifier side, the iteration derivation process of the inverter side can refer to the derivation process of the rectifier side, which will not be described here.

[0141] According to the above iterative calculation process, it can be known that the adjustment process of the DC side voltage preset value is a relatively slow adjustment process, and in some special working conditions, such as converter starting, DC side ground fault and AC side ground fault, the adjustment of the DC voltage often cannot keep up with the speed of system change, resulting in overcharge or discharge of the capacitor, and serious discharge can cause overvoltage of the capacitor module, thereby causing the capacitor module to be taken out.

[0142] In order to avoid the occurrence of the above situation, in a preferred embodiment, the present application also designs a feedforward control link to accelerate the iteration speed of the DC side voltage by directly calculating the equivalent DC voltage of the capacitor module phase-change converter, and the specific steps include:

[0143] According to the commutation angle and the phase of the line voltage, the equivalent DC voltage value in the commutation stage and the equivalent DC voltage value in the non-commutation stage are calculated respectively;

[0144] According to the output of the capacitor module during commutation, the charge and discharge energy equivalent DC voltage value is calculated;

[0145] The sum of the commutation stage equivalent DC voltage value, the non-commutation stage equivalent DC voltage value and the charge and discharge energy equivalent DC voltage value is taken as the equivalent DC voltage value of the capacitor commutation converter, and the equivalent DC voltage value is taken as the DC side voltage preset value.

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

[0147] Taking the rectifier side of the converter as an example, it is assumed that, at the triggering start, the phase of the line voltage U AC between the A phase and the C phase is the triggering angle α of the DC system, the phase of U AB is θ AB , and the phase of U CB is θ CB . In the capacitor commutation converter, since the commutation speed is controllable, the commutation angle μ can be calculated, and then, during the commutation process, the corresponding voltage time integral S AB of U AB can be represented 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 the A phase and the B phase, S AB is the voltage time integral, and θ AB is the phase of U AB .

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

[0151]

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

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

[0154] In the formula, U ABN is the reference voltage of the line voltage U AB , and ω N is the reference angular frequency.

[0155] Therefore, the effective value of the equivalent DC voltage U can be expressed as:

[0156]

[0157] In the formula, U AB represents the effective value of the line voltage U AB . Since at the commutation, the DC side voltage of the converter is 1 / 2 of the sum of the AC side line voltages U CB and U AB , according to the calculation formula of the effective value, the equivalent DC voltage value at the commutation stage can be expressed as:

[0158]

[0160] In the formula, U represents the equivalent DC voltage value at the commutation stage, U represents the effective value of the line voltage U AB , θ AB represents the phase of the line voltage U CB , and μ represents the commutation angle. represents the effective value of the line voltage U CB , and θ CB represents the phase of the line voltage U AB .

[0161] At the non-commutation stage, the DC side voltage of the converter is the line voltage U AB , and the corresponding angle range is (θ AB + μ) to (θ dc - μ).AB If the phase angle is equal to π / 3, the non-commutation stage equivalent DC voltage value can be expressed as:

[0162]

[0163] In the formula, The non-commutation stage equivalent DC voltage value is represented.

[0164] According to the output of the capacitor module and its charging and discharging process during commutation, it can be known that the charging and discharging energy of the capacitor module during commutation is equivalent to the voltage increment, so during the commutation process at the rectifier side, the charging of the capacitor module corresponds to the increased equivalent DC voltage.

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

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

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

[0168] The C-phase capacitor module voltage output U cC is:

[0169]

[0170] The charging energy W is:

[0171]

[0172] In the formula, μ is the commutation angle, I a represents the instantaneous current flowing through the A-phase converter valve during the commutation process, I c represents the instantaneous current flowing through the C-phase converter valve during the commutation process, 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 the A-phase and the C-phase, and U p is the DC voltage.

[0173] The increased average DC voltage, i.e., the charging and discharging energy equivalent DC voltage value, can be expressed as:

[0174]

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

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

[0177]

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

[0179] The equivalent DC voltage value can be directly obtained by measuring and phase-locking the AC voltage, and is used as the feedforward value of the iterative calculation of the equivalent DC voltage of the capacitor module commutated converter DC side, that is, it is used as the initial DC side voltage preset value, and then the equivalent DC voltage value is used for iterative calculation, which can accelerate the iteration speed of the DC side voltage preset value, thereby improving the efficiency of the DC side harmonic suppression.

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

[0181] The DC side harmonic suppression method of the capacitor commutated converter provided in the embodiment, the LCC converter is improved into a capacitor module commutated converter, and the capacitor module is dynamically switched 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 the feedforward design, the iteration speed of the DC side voltage preset value is improved, thereby improving the efficiency of the DC side harmonic suppression. The filtering function is integrated in the capacitor module, which realizes the effective suppression of the DC side harmonic voltage of the capacitor commutated converter in the steady state, reduces the investment and land occupation of the DC filter, and improves the safety and stability of the operation of the DC power transmission system.

[0182] Referring to Figure 4 , based on the same inventive concept, the second embodiment of the capacitor commutated converter DC side harmonic suppression system provided by the present application comprises:

[0183] The voltage value calculation module 10 is configured to obtain a converter transformer grid-side AC voltage value, convert the converter transformer grid-side AC voltage value into a valve-side line voltage through a transformation ratio, and calculate an actual DC-side voltage value of the converter according to the valve-side line voltage.

[0184] The initial switching control module 20 is configured to perform switching control on the non-commutated capacitor modules according to a difference between the DC-side voltage preset 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 a periodic voltage change of the capacitor commutated converter, and calculate a second voltage compensation value according to a difference between an actual capacitor voltage value and a rated capacitor voltage value of the capacitor commutated converter.

[0186] The iteration control module 40 is configured to iteratively update the DC-side voltage preset value according to the first voltage compensation value and the second voltage compensation value, and return to perform the switching control on the non-commutated capacitor modules according to a difference between the DC-side voltage preset 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 an average voltage difference between an average voltage of the capacitor commutated converter at the end of a current switching period and an average voltage of the capacitor commutated converter at the end of a previous switching period.

[0188] The first voltage compensation value is calculated according to the average voltage difference.

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

[0190]

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

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

[0193] According to the rated voltage difference, a second voltage compensation value is calculated.

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

[0195]

[0196] wherein ΔU 2T represents the second voltage compensation value of phase T, T represents a 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 CN represents the rated capacitor voltage value, I dc represents the DC current, and Δt represents the non-commutation stage input time.

[0197] In another preferred embodiment, the iteration control module 40 is further configured to update the DC side voltage preset 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 iteration formula.

[0198] The DC side voltage preset value of the inverter side of the capacitor commutated converter is updated according to the first voltage compensation value and the second voltage compensation value, and by using a second iteration formula.

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

[0200]

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

[0202]

[0203] wherein, represents the DC side voltage preset value of the kth switching period, represents the DC side voltage preset value of the k+1th switching period, ΔU 1T represents the first voltage compensation value of phase T, ΔU 2T represents the second voltage compensation value of phase T, T represents a phase.

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

[0205] The charge-discharge energy equivalent DC voltage value is calculated according to the output of the capacitor module during commutation.

[0206] The sum of the commutation phase equivalent DC voltage value, the non-commutation phase equivalent DC voltage value and the charge-discharge energy equivalent DC voltage value is taken as the equivalent DC voltage value of the capacitor commutation converter, and the equivalent DC voltage value is taken as the preset value of the DC side voltage.

[0207] Further, the commutation phase equivalent DC voltage value is expressed by the following formula:

[0208]

[0209] In the formula, U represents the commutation phase equivalent DC voltage value, represents the effective per-unit value of the line voltage U AB , θ AB represents the phase of the line voltage U AB , and 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 non-commutation phase equivalent DC voltage value is expressed by the following formula:

[0211]

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

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

[0214]

[0215] In the formula, U represents the charge-discharge energy equivalent DC voltage value, 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, U represents the equivalent DC voltage value.

[0219] The technical features and technical effects of the DC side harmonic suppression system of the capacitor commutation converter proposed in the embodiments of the present application are the same as the method proposed in the embodiments of the present application, and will not be repeated here. Each module in the DC side harmonic suppression system of the capacitor commutation converter can be realized by software, hardware, or a combination thereof, in whole or in part. Each module described above can be embedded in or independent of the processor in the computer device in hardware form, or can be stored in the memory in the computer device in software form, so that the processor can call and execute the operations corresponding to each module.

[0220] In summary, the method and system for suppressing DC side harmonics of a capacitor commutation converter proposed in the embodiments of the present application, the method obtains the converter transformer side AC voltage value, converts the converter transformer side AC voltage value into a valve side line voltage through a transformation ratio, and calculates an actual DC side voltage value of the converter according to the valve side line voltage; controls the switching of non-commutated capacitor modules according to the difference between the DC side voltage preset value and the actual DC side voltage value of the converter; calculates a first voltage compensation value according to the periodic voltage change of the capacitor commutation converter, and calculates 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; iteratively updates the DC side voltage preset value according to the first voltage compensation value and the second voltage compensation value, and returns to the step of controlling the switching of non-commutated capacitor modules according to the difference between the DC side voltage preset value and the actual DC side voltage value of the converter until the capacitor commutation converter reaches capacitor voltage balance. The LCC converter is improved to a capacitor module commutation converter, and the capacitor modules are dynamically switched based on the DC side voltage preset value, so that the valve side voltage value of the converter and the DC side voltage preset value are the same, thereby eliminating the DC side harmonic voltage. At the same time, through the feedforward design, the iteration speed of the DC side voltage preset value is improved, thereby improving the efficiency of DC side harmonic suppression.

[0221] Each embodiment in the specification is described in a progressive manner, and the same or similar parts of each embodiment can be referred to each other. Each embodiment focuses on the differences from other embodiments. In particular, for system embodiments, since they are basically similar to method embodiments, the description is relatively simple, and the relevant parts can be referred to the part of the method embodiment. It should be noted that each technical feature of the above embodiments can be combined arbitrarily, and in order to make the description simple, not all possible combinations of the technical features of the above embodiments are described, but as long as the combination of the technical features does not exist contradictory, it should be considered as the scope of the specification.

[0222] The above embodiments only express several preferred embodiments of the present application, which are described in a more specific and detailed manner, but cannot be understood as a limitation to the patent scope of the application. It should be pointed out that, for ordinary skilled in the art, several improvements and replacements can be made without departing from the technical principles of the present application, and these improvements and replacements should also be considered as the protection scope of the present application. Therefore, the protection scope of the present application patent should be subject to the protection scope of the claims.

Claims

1. A method of harmonic suppression on a DC side of a capacitive phase commutated converter, characterized by The capacitor in the capacitor commutated converter adopts a modular multilevel converter, and the method comprises: obtaining a converter transformer grid-side AC voltage value, converting the converter transformer grid-side AC voltage value into a valve-side line voltage through a transformation ratio conversion, and calculating an actual DC side voltage value of the converter according to the valve-side line voltage; controlling switching of non-commutated capacitor modules according to a difference between the DC side voltage preset value and the actual DC side voltage value of the converter; calculating a first voltage compensation value according to a periodic voltage change of the capacitor commutated converter, and calculating a second voltage compensation value according to a difference between an actual capacitor voltage value and a rated capacitor voltage value of the capacitor commutated converter; iteratively updating the DC side voltage preset value according to the first voltage compensation value and the second voltage compensation value, and returning to the step of controlling switching of non-commutated capacitor modules according to a difference between the DC side voltage preset value and the actual DC side voltage value of the converter until the capacitor commutated converter reaches capacitor voltage balance; wherein the step of calculating the first voltage compensation value according to the periodic voltage change of the capacitor commutated converter comprises: calculating an average voltage difference value between an average voltage of the capacitor commutated converter at the end of a current switching period and an average voltage of the capacitor commutated converter at the end of a previous switching period; calculating the first voltage compensation value according to the average voltage difference value; 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 comprises: calculating a rated voltage difference value between the rated capacitor voltage value and the average voltage of the capacitor commutated converter at the end of the current switching period; calculating the second voltage compensation value according to the rated voltage difference value; the step of iteratively updating the DC side voltage preset value according to the first voltage compensation value and the second voltage compensation value comprises: iteratively updating the DC side voltage preset 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; iteratively updating the DC side voltage preset 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; wherein the first iteration formula is represented by the following formula: the second iteration formula is represented by the following formula: In the formula, represents the preset value of the DC side voltage of the kth switching cycle, represents the preset value of the DC side voltage of the k+1th switching cycle, represents the first voltage compensation value of the T phase, represents the second voltage compensation value of the T phase, and T represents a phase.

2. The DC side harmonic suppression method of a capacitor commutated converter according to claim 1, characterized by, the first voltage compensation value is represented by the following formula: wherein V1(T) represents a first voltage compensation value of the T phase, T represents a phase, N C C represents a total number of the capacitor modules in the capacitor commutated converter, C represents a capacitance value of the capacitor module, V1(T) represents a first voltage compensation value of the T phase, T represents a phase, N V1(T) represents a first voltage compensation value of the T phase, T represents a phase, N I represents a direct current, and Δt represents a non-commutation phase input time.

3. The DC side harmonic suppression method of a capacitor commutated converter according to claim 2, characterized in that, the second voltage compensation value is represented by the following formula: wherein Vc2 represents a second voltage compensation value of the T phase, T represents a phase, N C C represents a total number of the capacitor modules in the capacitor commutated converter, C represents a capacitance value of the capacitor module, Vc2 represents a second voltage compensation value of the T phase, T represents a phase, N Vc2 represents a second voltage compensation value of the T phase, T represents a phase, N I represents a direct current, and Δt represents a non-commutation phase input time.

4. The method of harmonic suppression on the DC side of a capacitor commutated converter according to claim 1, characterized in that, before the step of controlling switching of non-commutated capacitor modules according to a difference between the DC side voltage preset value and the actual DC side voltage value of the converter, the method further comprises: calculating an equivalent DC voltage value in a commutation phase and an equivalent DC voltage value in a non-commutation phase respectively according to a commutation angle and a line voltage phase; calculating a charge-discharge energy equivalent DC voltage value according to an output of the capacitor module during commutation; summing the commutation phase equivalent DC voltage value, the non-commutation phase equivalent DC voltage value and the charge-discharge energy equivalent DC voltage value to obtain an equivalent DC voltage value of the capacitor commutated converter, and taking the equivalent DC voltage value as the DC side voltage preset value.

5. The DC side harmonic suppression method of a capacitor commutated converter according to claim 4, characterized by, The equivalent DC voltage value of the commutation phase is expressed by the following formula: + ) / 2 wherein represents the equivalent DC voltage value in commutation phase, represents the effective value of the line voltage U AB in per unit, represents the phase of the line voltage U AB , and represents the commutation angle, represents the effective value of the line voltage U CB in per unit, represents the phase of the line voltage U CB . The equivalent DC voltage value of the non-commutation phase is expressed by the following formula: In the formula, denotes the non-commutation phase equivalent DC voltage value; The equivalent DC voltage value of the charge-discharge energy is expressed by the following formula: In the formula, represents the equivalent direct current voltage value of the charge and discharge energy, represents the effective unit value of the line voltage U AC , represents the phase of the line voltage U AC ; The equivalent DC voltage value is expressed by the following formula: + + In the formula, represents the equivalent DC voltage value.

6. A DC side harmonic suppression system for a capacitor commutated converter, characterized by, The system is applied to the method of any one of claims 1 to 5, the capacitor in the capacitor commutation converter adopts a modular multi-level converter, and the system comprises: a voltage value calculation module, configured to acquire a converter transformer grid-side AC voltage value, convert the converter transformer grid-side AC voltage value into a valve-side line voltage through a transformation ratio, and calculate an actual DC side voltage value of the converter according to the valve-side line voltage; an initial switching control module, configured to perform switching control on the non-commutation capacitor module according to a difference between the DC side voltage preset 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 a periodic voltage change of the capacitor commutation converter, and calculate a second voltage compensation value according to a difference between an actual capacitor voltage value and a rated capacitor voltage value of the capacitor commutation converter; an iterative control module, configured to iteratively update the DC side voltage preset value according to the first voltage compensation value and the second voltage compensation value, and return to perform the step of performing switching control on the non-commutation capacitor module according to a difference between the DC side voltage preset value and the actual DC side voltage value of the converter until the capacitor commutation converter reaches capacitor voltage balance.

Images