Frequency reduction effect suppression method and system of MMC-STATCOM with low carrier wave ratio under harmonic compensation

Abstract

The invention discloses a frequency reduction effect suppression method and system for a low-carrier-ratio MMC-STATCOM under harmonic compensation, and the method comprises the steps: firstly, analyzing the modulation wave correction and fifth harmonics brought by a carrier modulation strategy of harmonic compensation, defining the capacitance voltage unbalance degree, and dividing the nonlinear capacitance voltage harmonics into four types for simplified analysis; secondly, determining an optimal carrier wave ratio value interval in which the capacitor voltage harm of the MMC-STATCOM is suppressed under the influence of a fifth harmonic frequency reduction effect, and designing a decimal part of the optimal carrier wave ratio; by analyzing association rules of different label data, the operation state of the MMC-STATCOM is optimized, three evaluation labels of switching loss, harmonic wave THD and capacitance voltage unbalance degree are considered, the switching loss and the THD are reduced, and the capacitance voltage unbalance phenomenon is inhibited. And finally, selecting a specific numerical value of the optimal carrier wave ratio through scoring evaluation. According to the invention, the operation efficiency of the MMC-STATCOM is improved, and the MMC-STATCOM stably operates at a low carrier wave ratio.

Description

Technical Field

[0001] The invention belongs to the technical field of converter control, and relates to a method and system for suppressing frequency reduction effect of a low-carrier ratio MMC-STATCOM under harmonic compensation. Background Art

[0002] The static synchronous compensator (STATCOM) based on modular multilevel converter (MMC), namely MMC-STATCOM, has advantages such as good equipment expansion capability and has become a research hotspot in the field of ultra-high voltage power transmission.

[0003] The carrier phase shift modulation strategy (CPS-PWM) has the advantages of constant switching frequency, high consistency of sub-module switching loss, and excellent harmonic characteristics. At present, in high-power application scenarios, the carrier ratio of CPS-PWM is mostly set to be greater than 10. However, studies have shown that CPS-PWM with a low carrier ratio (no more than 5) can significantly reduce switching losses and cooling requirements, further improve the harmonic characteristics of the MMC system, and has practical significance for the system optimization of MMC-STATCOM. However, there is a lack of clear judgment criteria for the damage of low carrier ratio to the capacitor voltage of MMC-STATCOM.

[0004] The prior art document (CN118381363A) discloses a method and system for suppressing the frequency reduction effect of a low carrier ratio MMC-UPFC. This document ignores the influence of the harmonic injection amount on the switching function and the harmonic compensation current on the bridge arm current during the MMC control process. At the same time, the frequency reduction effect suppression of the label technology adopted only uses a single label to suppress the frequency reduction effect, which cannot make the MMC-STATCOM operate stably under a low carrier ratio. Summary of the invention

[0005] In order to solve the deficiencies in the prior art, the present invention provides a method and system for suppressing the frequency reduction effect of a low-carrier ratio MMC-STATCOM under harmonic compensation under the influence of the fifth harmonic, clustering, classification, prediction, and correlation analysis are performed on the capacitor voltage of the low-carrier ratio MMC-STATCOM under harmonic compensation, and labels are formed by mining capacitor voltage fluctuations. The capacitor voltage balancing strategy is improved according to the labels to better promote the stable operation of the MMC-STATCOM, improve the operating efficiency of the MMC-STATCOM, and make it stably operate under a low carrier ratio.

[0006] The first aspect of the present invention provides a method for suppressing the frequency reduction effect of a low-carrier ratio MMC-STATCOM under harmonic compensation: Introduce the carrier phase-shifted modulation strategy with a low carrier ratio into the MMC-STATCOM for 5th harmonic compensation. Considering the modulation wave correction amount brought by 5th harmonic compensation, the compensation current of the arm current, and the arm current correction amount, determine the analytical function of the nonlinear capacitor voltage harmonic according to the relationship between the frequency reduction effect, the low carrier ratio, and the capacitor voltage. According to the analytical function of the nonlinear capacitor voltage harmonic, quantitatively analyze the influence of the carrier ratio of the MMC-STATCOM on the operating state of the MMC-STATCOM under the frequency reduction effect of harmonic compensation. Based on the analysis results, determine the optimal value range of the carrier ratio where the frequency reduction effect is suppressed. Taking the switching loss, harmonic THD, and capacitor voltage unbalance degree as evaluation labels, calculate the label evaluation values at different carrier ratios in the optimal value range of the carrier ratio. By comparing the label evaluation values at different carrier ratios, determine the specific value of the optimal carrier ratio. For the capacitor voltage fluctuation that still exists after determining the optimal carrier ratio, suppress the frequency reduction effect of the MMC-STATCOM by optimizing the capacitor voltage balance.

[0007] Preferably, the determining the analytical function of the nonlinear capacitor voltage harmonic according to the relationship between the frequency reduction effect, the low carrier ratio, and the capacitor voltage includes: Considering the 5th harmonic current and circulating current of the MMC-STATCOM, determine the upper and lower arm currents of the MMC-STATCOM. Considering the 5th harmonic injection amount in the modulation wave and the nonlinear dynamic switching process of CPS-PWM, use the double Fourier integral to derive the new MMC-STATCOM switching function S ji (t); Substitute the nonlinear switching function S ji (t) and the arm current into the capacitor voltage solving formula to obtain the sub-module capacitor voltage of the MMC-STATCOM, and only retain the harmonic Δu c_i of the capacitor voltage related to FR 5 in the nonlinear capacitor voltage harmonic Δu c_i(m5,n5) ; Simplify the harmonic components of the capacitor voltage harmonic Δu c_i(m5,n5) at different frequencies; Conduct quantitative analysis on the simplified capacitor voltage harmonic Δu c_i(m5,n5) to obtain the analytical function.

[0008] Preferably, the simplifying the harmonic components of the capacitor voltage harmonic Δu c_i(m5,n5) at different frequencies includes: Construct the fundamental frequency carrier ratio FR in decimal form and represent it with the following formula:

[0009] Wherein, Round(FR) is the integer part of the fundamental frequency carrier ratio FR, replaced by α; Dec(FR) is the decimal part of the fundamental frequency carrier ratio FR, replaced by β, and the value range is [0, 1); It is set that the value range of α is from 2 to 7, m 5 The value range of is from 1 to 5, n 5 The value range of is from -5 to 5; For the capacitor voltage harmonic Δu c_i(m5,n5)|ω=0 Simplify it, and draw the corresponding table of the harmonic amplitude coefficient H 5 , n 5 under different m 5 (m 5 , n 5 ). Retain the harmonic components of H 5 (m 5 , n 5 ) greater than 1×10 -9 . Further simplify the setting of (m 5 , n 5 ). Set the frequency (m 5 FR / 5 + n 5 ±l / 5)f 5 equal to 0, and obtain the judgment criterion of Δu c_i(m5,n5)|ω=0 :

[0010] According to the judgment criterion and the set (m 5 , n 5 ), the harmonic components with a frequency equal to zero under different fundamental frequency carrier ratio FR settings after simplification can be obtained, Δu c_i(m5,n5)|ω=0，FR=α and Δu c_i(m5,n5)|ω=0，FR=α+β ; For the harmonic components with a frequency less than 5 times the fundamental wave frequency, retain the harmonic components with an amplitude H 5 (m 5 , n 5 ) greater than 1×10 -4 . Draw the frequency value table of Δu c_i(m5,n5)|0<ω≤ω5 , and determine the non-linear harmonic of the capacitor voltage whose frequency can be reduced to greater than 0 and less than or equal to 250 Hz.

[0011] Preferably, the quantization of the simplified capacitor voltage harmonic Δu c_i(m5,n5) to determine the analytical function includes: The analytical function after quantitative analysis of the capacitor voltage harmonic Δu c_i(m5,n5)|ω=0 is expressed by the following formula:

[0012] Wherein, I 0jand φ 0 are the amplitude and phase of the fundamental frequency component of the compensation current respectively; I 5j and φ 5 are the amplitude and phase of the fifth - harmonic compensation current respectively; I z6j and φ 6 are the amplitude and phase of the sixth - harmonic circulating current respectively; C is the capacitance value of the sub - module; t represents time; θ i is the phase - shift angle of the i - th sub - module.

[0013] Preferably, the analytical function after quantitative analysis of the Δu c_i(m5,n5)|0<ω<ω0 is expressed by the following formula:

[0014] In the formula, I 5j / 4ω 5 C is the amplitude of the 5 - times power frequency component of the capacitor voltage when the frequency - reduction effect is not considered, ω 5 is the fifth - harmonic fundamental frequency, and I 5j is the amplitude of the fifth - harmonic compensation current.

[0015] Preferably, the influence of the harmonic compensation frequency - reduction effect on MMC - STATCOM under quantitative analysis includes: When the fundamental - frequency carrier ratio FR is equal to an integer or a set decimal, the ramp harmonic Δu c_i(m5,n5)|ꞷ=0 with a frequency of zero overlaps with the DC component of the capacitor voltage, causing the continuous increase or decrease of the DC component of the capacitor voltage. The charge - discharge state of the capacitor becomes unstable. At the same time, due to different phase - shift angles, the change trends of the capacitor voltage harmonics of different sub - modules are different, resulting in the imbalance of the capacitor voltages between sub - modules, and the harmonic compensation function of MMC - STATCOM cannot be normally executed; When the fundamental - frequency carrier ratio FR changes with the fractional part β, Δu c_i(m5,n5)|0<ω<ω0 is distorted into an ultra - low - frequency harmonic with a frequency mainly of βf 0 , causing ultra - low - frequency fluctuations in the capacitor voltage, Δu c_i(m5,n5)|ꞷ=0 ; Δu c_i(m5,n5)|0<ω<ω0 is distorted into Δu c_i(m5,n5)|ω=ω0 and Δu c_i(m5,n5)|ω=ω5 , which overlap with the fundamental - wave component of the capacitor voltage and the fifth - harmonic component of the capacitor voltage, causing an increase in the fundamental - wave component and the fifth - harmonic component in the compensation current, resulting in a decline in the harmonic control effect of MMC - STATCOM and deterioration of the line THD.

[0016] Preferably, the optimal value range of the carrier ratio for suppressing the frequency - reduction effect determined based on the analysis results includes: Ignoring the phase angle and frequency of Δu c_i(m5,n5)|0<ω≤ω5，FR=5+β and only retaining the sum of the absolute values of the amplitudes of Δu c_i(m5,n5)|0<ω≤ω5，FR=5+β , the absolute value sum of the amplitudes of Δu c_i(m5,n5)|0<ω≤ω5，FR=5+βAmplitude coefficient H c_i (β); By plotting the relationship curve of H c_i (β) and β, select the optimal value range of β where the harmonic amplitude of the capacitor voltage is the smallest. At this time, the influence of the capacitor voltage harmonic on the capacitor voltage of the MMC-STATCOM is the smallest, and the corresponding fundamental frequency carrier ratio FR value range is the optimal carrier ratio value range.

[0017] Preferably, the determination of the optimal carrier ratio includes: Under different carrier ratio operating conditions of the MMC-STATCOM, sort the switching loss, capacitor voltage unbalance degree, and line-side THD evaluation labels according to the actual operating results to obtain the label values and corresponding evaluation values of each evaluation label; By combining carrier ratios with different decimal parts and multiplying the weights by the corresponding evaluation values, obtain the states of the MMC-STATCOM under different carrier ratio values, and use the carrier ratio corresponding value when the MMC-STATCOM is in the best operating state as the optimal carrier ratio.

[0018] Preferably, the capacitor voltage balance optimization includes: Use a proportional regulator to design an equalizing loop, select the proportional regulator according to the evaluation label, output the capacitor voltage balance adjustment amount through the proportional regulator, and then adjust the modulation wave according to the deviation between the sub-module capacitor voltage and the capacitor voltage reference value, and then change the time of the sub-module in two working states to adjust the capacitor voltage. The capacitor voltages of N sub-modules on the bridge arm track their reference values, and suppress the frequency reduction effect of harmonic compensation under the optimal carrier ratio.

[0019] The second aspect of the present invention provides a system for suppressing the frequency reduction effect of a low-carrier-ratio MMC-STATCOM under harmonic compensation, which operates the method for suppressing the frequency reduction effect of a low-carrier-ratio MMC-STATCOM described in the first aspect. The system for suppressing the frequency reduction effect includes: A harmonic analysis module for analyzing the harm of harmonics; A carrier ratio calculation module for determining the optimal carrier ratio range that limits the frequency reduction effect; A carrier ratio optimization module for determining the value of the optimal carrier ratio; A voltage optimization module for optimizing the capacitor voltage balance and suppressing the frequency reduction effect.

[0020] Compared with the prior art, the beneficial effects of the present invention at least include: The present invention introduces CPS-PWM with a low carrier ratio into MMC-STATCOM, reduces the switching loss of MMC-STATCOM, improves the efficiency of MMC-STATCOM. By introducing the concept of non-linear capacitor voltage harmonics and reducing the carrier ratio of MMC-STATCOM, the relationship among the low carrier ratio, frequency reduction effect and the capacitor voltage of MMC-STATCOM is explored, and a quantitative analysis of the operation of the capacitor voltage unbalance degree label of MMC-STATCOM under harmonic compensation is constructed to determine the optimal carrier ratio for the stable operation of MMC-STATCOM, which can avoid the harm caused by the low carrier ratio to the capacitor voltage of MMC-STATCOM under harmonic compensation, reduce the harm of the frequency reduction effect and obtain a stable capacitor voltage waveform, ensuring the stable operation of MMC-STATCOM with a low carrier ratio under harmonic compensation.

[0021] When clustering the capacitor voltage unbalance degree at different moments under the operation of the optimal carrier ratio, the selection of the clustering center and the value of K is set, the label of the capacitor voltage unbalance degree is set, and based on the clustering result, a direct bias component injection is used to modify the modulation wave to improve the capacitor voltage equalization strategy, further reducing the interference of the frequency reduction effect, more intuitively describing the important operation characteristics of MMC-STATCOM under a low carrier ratio, completing the stable control of MMC-STATCOM under a low carrier ratio, and realizing the efficient operation of MMC-STATCOM under a low carrier ratio. Description of the Drawings

[0022] Figure 1 is the flow chart of the method of the present invention; Figure 2 is the amplitude coefficient H c_i (β) and the relationship diagram of β; Figure 3 is the label association system diagram of the present invention; Figure 4 is the state comparison under different FR values; Figure 5 is the capacitor voltage equalization optimization strategy based on label technology feedback regulation. Detailed Embodiment

[0023] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The embodiments described in this application are only a part of the embodiments of the present invention, rather than all embodiments. Based on the spirit of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the protection scope of the present invention.

[0024] Such as Figure 1As shown, Embodiment 1 of the present invention provides a method for suppressing the frequency reduction effect of a low carrier ratio MMC-STATCOM under 5th harmonic compensation. In a preferred but non-limiting embodiment of the present invention, the method includes the following steps: S1: Introduce CPS-PWM with a low carrier ratio into the MMC-STATCOM to correct the modulation wave of the carrier modulation strategy and the harmonic current caused by 5th harmonic current compensation, and analyze the relationship between the frequency reduction effect, the low carrier ratio, and the capacitor voltage under harmonic compensation. Further preferably, reduce the carrier ratio of the carrier phase-shifted modulation strategy of the MMC-STATCOM, calculate the non-linear harmonics in the CPS-PWM modulation process, obtain the analytical function of the capacitor voltage harmonics of the MMC-STATCOM, and quantitatively analyze the relationship between the three hazards of the frequency reduction effect, namely, the low-frequency fluctuation of the capacitor voltage, the unstable charging and discharging state of the sub-module capacitor, and the imbalance of the capacitor voltage between sub-modules, and the low carrier ratio by changing the carrier ratio value in the analytical function. Divide the non-linear capacitor voltage harmonics into four types according to the frequency size, simplify the four harmonics, and analyze their capacitor voltage hazards.

[0025] In a preferred but non-limiting embodiment of the present invention, S1 specifically includes: S1.1: Considering that the proportion of the 5th current harmonic content in the six-pulse current generated by the non-linear load can reach about 20%, taking the 5th current harmonic compensation as an example, calculate the 5th carrier ratio FR 5 .

[0026] The 5th harmonic compensation of the MMC-STATCOM includes: the harmonic current detection module extracts the 5th current harmonic component and converts it into the reference component of the bridge arm current control, generates the 5th harmonic correction amount u 1 and injects it into the modulation wave. The modulation wave enters the CPS-PWM strategy to generate the sub-module drive signal, and finally realizes the accurate tracking of i c to complete the grid current harmonic compensation. L

[0027] At this time, the upper and lower bridge arm currents i pj , i nj of the MMC-STATCOM appear with 5th harmonic compensation current. At the same time, through the cyclic coupling of the capacitor voltage and the bridge arm current, the double-frequency circulating current can be corrected to a six-frequency circulating current. Considering the circulating current and the 5th harmonic compensation current of the MMC-STATCOM, modify the upper and lower bridge arm currents of the MMC-STATCOM after 5th harmonic compensation. Therefore, the upper and lower bridge arm currents can be expressed by the following formula:

[0028] Where, ω 0 is the fundamental angular frequency; I dcI / 3 is the DC current input from the DC side to each phase arm; I 0j and φ 0 are respectively the amplitude and phase of the fundamental frequency component i 0j of the compensation current; I 5j and φ 5 are respectively the amplitude and phase of the fifth - harmonic compensation current; I z6j and φ 6 are respectively the amplitude and phase of the sixth - harmonic circulating current.

[0029] Taking into account the 5th - harmonic injection amount in the modulation wave and the non - linear dynamic switching process of CPS - PWM, the new MMC - STATCOM switching function S ji (t) is derived using the double Fourier integral and is expressed by the following formula:

[0030] In the formula, the four parts respectively represent the DC bias, the 50Hz fundamental component, the 250Hz harmonic component, and the side - band harmonics and carrier harmonics generated by the 250Hz harmonic injection; M, M 5 are respectively the modulation ratios of the fundamental wave and the fifth - harmonic; ΔS ji (m 5 ,n 5 )(t) is the non - linear harmonic after the 250Hz injection, which consists of the carrier harmonic and the side - band harmonic components; ω 0 is the fundamental angular frequency; ω 5 is the 5th - harmonic angular frequency; t represents time; θ i is the phase - shift angle of the i - th sub - module, equal to 2π(i - 1) / N, where N represents the number of sub - modules; the coefficient H(m 5 ,n 5 ) is the harmonic amplitude related to the fundamental - harmonic order m 5 and the carrier - harmonic order n 5 and can be expressed by the following formula:

[0031] In the formula, J n is the Bessel function.

[0032] The 5th - carrier ratio FR 5 , the fundamental - carrier ratio FR and their relationship can be expressed by the following formula:

[0033] In the formula, f c and ω c are the switching frequency and the switching angular frequency, f 0 and ω 0 are the fundamental frequency and the fundamental angular frequency; FR / 5 can be used to replace FR5 。

[0034] S1.2: Select any upper-bridge-arm sub-module SM of phase a of the MMC-STATCOM api (i = 1, 2…N) as an example for analysis, substitute the non-linear switching function S ji (t) and the arm current into the capacitance voltage solution formula to obtain the capacitance voltage and capacitance voltage harmonics of the MMC-STATCOM sub-module:

[0035] In the formula, i ci , u c_i , Δu c_i are respectively the capacitance current, capacitance voltage and capacitance voltage harmonics of the i-th sub-module; i j According to the sub-module position, represent i pj , i nj ; C is the capacitance value of the sub-module.

[0036] Only retain the non-linear capacitance voltage harmonics Δu c_i in the capacitance voltage harmonics that are related to FR 5 , and Δu c_i(m5,n5) , Δu c_i(m5,n5) can be expressed by the following formula:

[0037] Δu c_i(m5,n5) contains harmonics with frequencies of (m 5 FR 5 + n 5 ± l / 5)f 5 . These harmonics can find different m 5 , n 5 values when the carrier ratio decreases, so that the frequency decreases to below five times the fundamental frequency ω 5 ; and the amplitude of Δu c_i(m5,n5) is inversely proportional to FR. When FR decreases, it will cause Δu c_i(m5,n5) to show an excessive increase, resulting in abnormal overvoltage of the capacitance voltage. In summary, the definition of the frequency reduction effect of the MMC-STATCOM under 5th harmonic compensation can be obtained: when the carrier ratio FR of the carrier phase-shifted modulation strategy decreases, after FR 5 , m 5 , n 5 take appropriate ranges, the capacitance voltage harmonics frequency of the MMC-STATCOM is significantly reduced to below five times the fundamental frequency, that is, the phenomenon that the harmonic frequency is reduced to 250 Hz and below.

[0038] S1.3: Based on the frequency reduction effect of harmonic compensation in S1.2, for different m c_i(m5,n5) in Δu 5, n 5 According to the variation law of 5 and the variation laws of different FRs, the non-linear harmonics of the capacitor voltage can be divided into four types of harmonics, namely Δu c_i(m5,n5)|ω=0 , Δu c_i(m5,n5)|0<ω<ω0 , Δu c_i(m5,n5) |ω = ω 0 , Δu c_i(m5,n5)|ω0<ω<ω5 (where ω is the harmonic angular frequency); Δu c_i(m5,n5)|ω=0 represents the non-linear harmonics with a frequency equal to 0. The distortion of this part of the harmonic components into ramp harmonics causes the capacitor voltage to continuously increase or decrease; Δu c_i(m5,n5)|0<ω<ω0 represents the non-linear harmonics below 50 Hz. The distortion of this part of the harmonic components causes ultra-low frequency fluctuations in the capacitor voltage; Δu c_i(m5,n5) |ω = ω 0 and Δu c_i(m5,n5)|ω0<ω<ω5 represent the harmonic component with a frequency equal to 50 Hz and the harmonic component with a frequency greater than 50 Hz and less than 250 Hz, respectively.

[0039] S1.4: Since the variation ranges of FR, m 5 , n 5 are relatively large and Δu c_i(m5,n5) contains many harmonic components, it is necessary to simplify Δu c_i(m5,n5) . The simplification steps include: (1) Due to the injection of the 5th harmonic, FR 5 causes the frequency of the non-linear harmonics of the capacitor voltage to decrease, resulting in more non-linear harmonics. Therefore, FR is constructed as the sum of the integer part Round(FR) and the decimal part Dec[FR], which is expressed by the following formula:

[0040] In the formula, Round(FR) is equal to α; Dec[FR] is equal to β, and the value range is set to [0, 1).

[0041] (2) To ensure that the MMC-STATCOM has sufficient power quality and reduce the distortion of the compensation current on the AC side, and make the MMC-STATCOM operate stably, the set value of α is set to be not less than 2; to ensure that Δu c_i(m5,n5)|ω0<ω<ω5 can cause large fluctuations in the capacitor voltage, α is set to be not greater than 7. Finally, the value range of FR is from 2 to 7.

[0042] (3) Simplify the harmonic content at different frequencies: To ensure that the harmonic amplitude coefficient H(m 5 , n 5 ) is large enough and the harmonic frequency can be less than the fundamental frequency, m is set to 1 to 5, and n is set to -5 to 5. Simplify each m5 , n 5 Harmonic content under the value of: i. Simplify Δu c_i(m5,n5)|ω=0 including: First, according to different m 5 , n 5 the harmonic amplitude coefficient H 5 (m 5 , n 5 ) value correspondence table: Table 1 H 5 , n 5 values under different m 5 (m 5 , n 5 ) values

[0043] According to Table 1, retain the harmonic components of H 5 (m 5 , n 5 ) greater than 1×10 -9 , and further simplify the value range of m 5 , n 5 to 1 to 5 and 1 to 4.

[0044] Set the frequency (m 5 FR / 5 + n 5 ±l / 5)f 5 equal to 0 to obtain the judgment criterion of Δu c_i(m5,n5)|ω=0 , expressed by the following formula:

[0045] In the formula, represents rounding down (∓l - 5n 5 ) / m 5 .

[0046] (m 5 , n 5 ) is equal to (2, -1), (4, -3) and (5, -2), the harmonic component judgment conditions of Δu c_i(m5,n5)|ω=0 are shown in Table 2.

[0047] Table 2 Harmonic component judgment conditions of Δu 5 , n 5 ) equal to (2, -1), (4, -3) and (5, -2) c_i(m5,n5)|ω=0

[0048] According to the judgment criterion and the selected (m 5 , n 5 ​)The harmonic component Δu with a frequency equal to zero under different simplified FR settings can be obtained. c_i(m5,n5)|ω=0 , FR = α and Δu c_i(m5,n5)|ω=0 , FR = α + β.

[0049] ii. Simplify Δu c_i(m5,n5)|0<ω≤ω5 , including: For components with a frequency less than 5 times the fundamental frequency, to ensure that the non - linear harmonic of the capacitor voltage Δu c_i(m5,n5)|0<ω≤ω5 can generate a large enough fluctuation in the capacitor voltage, select harmonic components with a large enough amplitude, and retain the harmonics in Table 1 where H 5 (m 5 , n 5 ) is greater than 1×10 -4 . Simplify the value ranges of m 5 and n 5 further to 1 to 5 and 1 to 3.

[0050] In this value range, taking FR = 5 + β as an example, plot the frequency coefficient values of ω / ω 5 for different (m 5 , n 5 ) values.

[0051] Table 3 Frequency coefficient values of ω / ω 5 for different (m 5 , n 5 ) values when FR = 5 + β

[0052] Substitute the value of β into Table 3 and find the corresponding (m 5 , n 5 ) values when the frequency is lower than 250 Hz, and determine the corresponding Δu c_i(m5,n5)|0<ω≤ω5 harmonic components.

[0053] S1.5: Conduct a quantitative analysis of the simplified Δu c_i(m5,n5)|ω=0 and Δu c_i(m5,n5)|0<ω≤ω5 to determine the analytical functions of Δu c_i(m5,n5)|ω=0 and Δu c_i(m5,n5)|0<ω≤ω5 .

[0054] (1) Conduct a quantitative analysis of Δu c_i(m5,n5)|ω=0 to obtain the analytical function of Δu c_i(m5,n5)|ω=0 , which is expressed by the following formula:

[0055]

[0056] (2) Conduct a quantitative analysis of Δu c_i(m5,n5)|0<ω≤ω5 to obtain Δu c_i(m5,n5)|0<ω≤ω5The analytical function.

[0057] In view of the severe capacitor voltage harmonics when FR = 5 and the increasing number of harmonics with frequencies of (m 5 FR 5 + n 5 ± l / 5)f 5 when FR decreases, therefore, taking the Δu generated when FR = 5 + β as an example for quantitative analysis. Δu c_i(m5,n5)|0<ω≤ω5，FR=5+β For example, the analytical function of Δu c_i(m5,n5)|0<ω≤ω5，FR=5+β can be expressed by the following formula:

[0058] (3) According to the above formula, determine the analytical function of the ultra-low frequency harmonics Δu with frequencies lower than 50 Hz, which is expressed by the following formula: c_i(m5,n5)|0<ω<ω0 The analytical function of Δu

[0059] In the formula, I 5j / 4ω 5 C is the amplitude of the 5 times power frequency component of the capacitor voltage without considering the frequency reduction effect.

[0060] (4) Under different β settings, for Δu c_i(m5,n5) |ω 0 ≤ ω ≤ ω 5 , as FR = 5 + β approaches Δu c_i(m5,n5)|ω=ω0 , Δu c_i(m5,n5)|ω=ω5 distorts; among them, when β = 0.5, the amplitude of the Δu c_i(m5,n5)|ω=ω0 , Δu c_i(m5,n5)|ω=ω5 component is the most obvious. Now, taking Δu when FR = 5.5 c_i(m5,n5)|ω=ω0 , FR = 5.5 and Δu c_i(m5,n5)|ω=ω5，FR=5.5 as an example for quantitative analysis. The analytical functions of Δu c_i(m5,n5) |ω = ω 0，FR=5.5 and Δu c_i(m5,n5)|ω=ω5，FR=5.5 can be expressed by the following formulas respectively:

[0061] S2: Quantitatively analyze the influence of the carrier ratio of MMC-STATCOM on the operating state of MMC-STATCOM under the harmonic compensation frequency reduction effect, and propose the optimal value range of the carrier ratio to suppress the frequency reduction effect. Finally, set the switching loss, harmonic THD, and capacitor voltage unbalance degree as the three evaluation labels for optimizing the operating state of MMC-STATCOM under the influence of the harmonic compensation frequency reduction effect.

[0062] In the preferred but non-limiting embodiment of the present invention, S2 specifically includes: S2.1: Quantitatively analyze the influence of the carrier ratio of MMC-STATCOM on its operating state under the harmonic compensation frequency reduction effect, including: (1) Quantitatively analyze the influence on the harmonic and THD of the capacitor voltage.

[0063] According to the above formula, when FR is equal to the integer part or a special β value appears in the fractional part, Δu c_i(m5,n5)|ω=0，FR=α , Δu c_i(m5,n5)|ω=0，FR=α+β both distort into ramp harmonics proportional to time t, overlapping with the DC component of the capacitor voltage, causing the capacitor voltage to continuously increase or decrease, and ultimately leading to an unstable charge and discharge state of the capacitor; at the same time, due to different phase shift angles, with the change of θ i , the values of Δu c_i(m5,n5)|ω=0，FR=α and Δu c_i(m5,n5)|ω=0，FR=α+β change, which means that for different sub-modules, the corresponding change trends of the sub-module capacitor voltages are different, resulting in an imbalance in the capacitor voltages between different sub-modules; Δu c_i(m5,n5)|ω=0，FR=α , Δu c_i(m5,n5)|ω=0，FR=α+β is positively correlated with I 5j . Facing a line with a large degree of harmonic pollution, the harmonic compensation needs to control the generated I 5j to increase, and the capacitor voltage will become more unstable. Among them, when FR = 5, the harmonic of the capacitor voltage generated by the coupling of S ih (t) and 5 times the power frequency compensation current of i pj overlaps with the DC component (when the switching frequency is equal to the frequency of 5 times the power frequency harmonic), and the amplitude of the generated capacitor voltage harmonic is the largest. At this time, the change amplitude of the capacitor voltage is the largest, and the harm of the harmonic compensation frequency reduction effect is the most serious. The harmonic compensation of MMC-STATCOM cannot be normally executed, and the THD of the line cannot be suppressed.

[0064] According to the analytical function, the frequency of Δu c_i(m5,n5)|0<ω<ω0 is reduced to β*f 0 , 2β*f 0 and 3β*f 0 . Since the amplitude of Δu c_i(m5,n5)|0<ω<ω0 is inversely proportional to the frequency, when FR changes with the fractional part β, Δu c_i(m5,n5)|0<ω<ω0 can be distorted into ultra-low frequency harmonics mainly with a frequency of βf 0 , causing ultra-low frequency fluctuations Δu c_i(m5,n5)|ω=0 of the capacitor voltage; Δu c_i(m5,n5)|0<ω<ω0 can be distorted into Δu c_i(m5,n5)|0<ω<ω0 , Δu c_i(m5,n5)|ꞷ=ω5 , overlapping with the fundamental component of the capacitor voltage and the fifth harmonic component of the capacitor voltage, resulting in an increase in the fundamental component and the fifth harmonic component in the compensation current, leading to a decline in the harmonic control effect of MMC-STATCOM and deterioration of the line THD; at this time, the frequency is β*f0 The harmonic components of c_i(m5,n5)|0<ω<ω0 account for the largest proportion in Δu. When β is too low, Δu c_i(m5,n5)|0<ω<ω0 brings extremely low-frequency harmonics with extremely large amplitudes to the capacitor voltage, and even brings the risk of overvoltage to the sub-module.

[0065] According to Δu c_i(m5,n5) |ω=ω 0，FR=5.5 and Δu c_i(m5,n5)|ω=ω5，FR=5.5 The analytical function of, Δu c_i(m5,n5)|ω=ω0，ω=ω5，FR=5.5 The frequency at which the capacitor voltage harmonics appear is equal to f 0 or f 5 The harmonic components of cause an increase in the fundamental frequency component and the five-times power frequency compensation component in the compensation current, resulting in a decline in the harmonic compensation control performance and deteriorating the line THD.

[0066] In addition, the switching loss of the MMC-STATCOM decreases as FR decreases, and the system efficiency increases accordingly.

[0067] (2) Conduct a quantitative analysis of the impact on the capacitor voltage unbalance.

[0068] The capacitor voltage unbalance degree ε comprehensively reflects the dispersion degree of the capacitor voltage of each sub-module and the average capacitor voltage. The ratio with the DC-side voltage U dc more realistically shows the dispersion degree and can intuitively display the harm of the capacitor voltage harmonics; When FR is equal to the integer part or a special β value appears in the decimal part, Δu c_i(m5,n5)|ω=0，FR=α，∆uc_i(m5,n5)|ω=0，FR=α+β causes the capacitor voltages of different sub-modules to continuously increase or decrease, the capacitor voltage unbalance degree ε continuously increases, and the MMC-STATCOM cannot operate normally. When FR changes with the decimal part β, Δu c_i(m5,n5)|0<ω<ω0 causes different sub-modules to have ultra-low-frequency harmonics with different amplitudes, and the capacitor voltage unbalance degree ε is relatively large.

[0069] Among them, the capacitor voltage unbalance degree can be expressed by the following formula:

[0070] In the formula, U dc is the DC-side voltage of the MMC-STATCOM; N is the number of sub-modules; u c_i is the capacitor voltage of the i-th sub-module; u c_avg is the average value of the capacitor voltages of the sub-modules.

[0071] (3) Conduct a quantitative analysis of the impact on the switching loss.

[0072] The switching loss can be expressed by the following formula:

[0073] Wherein, E SWT1_i , E SWT2_i , E SWD1_i , E SWD2_i are respectively the switching losses of IGBT 1 , IGBT 2 , VD 1 , VD 2 . U t , U avg are respectively the IGBT test voltage and the average capacitor voltage of the bridge arm; E oniT1_i (t j ) and E offiT1_i (t j ) are respectively the turn-on and turn-off losses of the first power device IGBT 1 ; E oniT1_i (t j ) and E offiT2_i (t j ) are respectively the turn-on and turn-off losses of the second power device IGBT 2 ; E r e ciD1_i (t j ) and E offiD2_i (t j ) are respectively the reverse recovery energies of the first diode and the second diode.

[0074] It can be seen from the switching loss formula that the switching loss of the power device is related to the average capacitor voltage of the bridge arm and the frequency f s of the triangular carrier wave, and is positively correlated with f s . Then: when f s increases, the switching loss increases and the efficiency of MMC-STATCOM decreases; when f s decreases, the switching loss decreases and the efficiency of MMC-STATCOM increases. Therefore, it is necessary to set an appropriate carrier ratio to achieve the efficient operation of MMC-STATCOM.

[0075] S2.2: Quantitatively analyze the optimal value range of the carrier ratio through the amplitude coefficient of the capacitor voltage harmonic to suppress the frequency reduction effect.

[0076] According to S2.1, when FR is set to an integer or β is the set fractional part, it will cause the charging and discharging state of the capacitor voltage to be unstable and the imbalance of the capacitor voltage, resulting in a large variable ε of the capacitor voltage unbalance degree. When β avoids these components, these greater hazards can be avoided, and ε decreases, but it will also cause extremely large low-frequency fluctuations to appear in the capacitor voltage, instead increasing ε. Therefore, in order to suppress the capacitor voltage hazards caused by the frequency reduction effect of harmonic compensation, the present invention designs the carrier ratio in decimal form starting from the amplitude coefficient of the capacitor voltage harmonics, and reduces u by reducing the amplitude of the capacitor voltage harmonics c_i the fluctuation amplitude of, thereby reducing ε, realizing harmonic compensation of MMC-STATCOM at a low carrier ratio, and suppressing the frequency reduction effect of harmonic compensation.

[0077] When considering FR = 5, the capacitor voltage hazard caused by Δu c_i(m5,n5)|ω=0，FR=5 is the greatest, ε rises rapidly, and the influence of the frequency reduction effect of harmonic compensation is the greatest. Therefore, taking Δu c_i(m5,n5)|0<ω<ω5，FR=5+β as an example, ignoring the phase angle and frequency of Δu c_i(m5,n5)|0<ω<ω5，FR=5+β and only retaining the absolute value of the amplitude of each low-frequency harmonic in Δu c_i(m5,n5)|0<ω<ω5，FR=5+β , the amplitude H c_i (β) of each low-frequency harmonic is obtained, as shown in the following expression:

[0078] Determine I dc , I z6j , I 0j , I 5j in the actual operation of MMC-STATCOM under 5th harmonic compensation. Substitute these current data into H c_i (β) for calculation, draw the relationship diagram of the amplitude coefficient H Figure 2 (β) and β as shown in c_i . Select the optimal value range of β with the smallest amplitude of the capacitor voltage harmonics. At this time, the influence of the capacitor voltage harmonics on the capacitor voltage of MMC-STATCOM is the smallest, ε is smaller, and the deterioration of THD caused by the capacitor voltage harmonics can also be effectively suppressed. The corresponding FR value range is the optimal carrier ratio value range.

[0079] Specifically, in the present invention, it is set that I dc / 3 is equal to 0.2I 5j , I 0j = 0.04I 5j , M5 = 0.09. Considering the circulating current suppression strategy, it is set that I z6j is equal to 0.02I 5j , I 5j is set to 145.5 A, and the relationship diagram of the amplitude coefficient H c_i (β) and β is drawn, as shown inFigure 2 As shown. Observe the figure to find H c_i (β) The optimal value range of the fractional part of the minimum carrier ratio. When FR = 5 + β, the optimal value range of β is [0.82, 0.9].

[0080] In summary, considering the relationship between the carrier ratio, voltage harmonics, total harmonic distortion (THD), and capacitor voltage imbalance under the harmonic compensation operating state of MMC - STATCOM, form the labels of switching loss, harmonic THD, and capacitor voltage unbalance degree according to the improved K - means clustering method, and optimize the operating state of MMC - STATCOM by finding the optimal carrier ratio through label technology and evaluation, as Figure 3 shown.

[0081] S3: Considering the three evaluation labels of switching loss, harmonic THD, and capacitor voltage unbalance degree, calculate the evaluation value for the operating label results under different carrier ratios. By comparing the label evaluation values of FR under different fractional parts, determine the fractional part of the optimal low carrier ratio, thereby determining the specific value of the optimal carrier ratio. Propose a capacitor voltage equalization strategy based on label technology for the remaining capacitor voltage fluctuations, and further realize the suppression of the frequency reduction effect of MMC - STATCOM under harmonic compensation.

[0082] In the preferred but non - restrictive embodiment of the present invention, S3 specifically includes:[[]] S3.1: Considering the three evaluation labels of switching loss, harmonic THD, and capacitor voltage unbalance degree, calculate the evaluation value for the operating label results under different carrier ratios. By comparing the label evaluation values of FR under different fractional parts in the optimal carrier ratio range, determine the fractional part of the optimal low carrier ratio, thereby determining the specific value of the optimal carrier ratio.

[0083] (1) Optimized value selection of the carrier ratio combined with multiple evaluation labels.

[0084] Preferably, to overcome the shortcomings of the single weighting method in the evaluation process, this design uses a combined weighting method to determine the weights of the evaluation labels. Specifically, the combined weight of the labels is synthesized by the subjective weight and the objective weight of the labels, including:[[]] i. Select the analytic hierarchy process to determine the subjective weight of the label.

[0085] Make pairwise comparisons for each label to establish the following judgment matrix. By analyzing the judgment matrix through the following formula, the weight of each label can be obtained, that is, the importance of each label in the entire hierarchical structure.

[0086]

[0087] In the formula,[[]] is the arithmetic mean of the subjective weights of the i - th label, is the subjective weight value of the i-th label, a ij is the comparison result of the relative importance of label i to label j, and n is the number of labels.

[0088] ii. Select the entropy weight method to determine the objective weights of the labels.

[0089] The entropy weight method uses the concept of entropy to assign weights to labels. Entropy is used to measure uncertainty. The greater the degree of dispersion of the index, the smaller the entropy value, indicating that the more information the index value provides, and the greater the weight of this index should be. The following formula is used to dimensionless process the original data to make the data of different labels comparable.

[0090]

[0091] In the formula: X ij is the data matrix of m samples and n labels. Substitute the dimensionless data into the following formula to calculate the objective weight W of the j-th evaluation label j-EWM .

[0092]

[0093] In the formula: P ij is the proportion of each index, e j is the entropy value of each of the n indexes.

[0094] iii. Determine the combined weight according to the subjective weight and the objective weight.

[0095] Substitute the calculated subjective and objective weights into the combined weight calculation formula of the following formula to obtain the combined weight of the i-th evaluation label.

[0096]

[0097] In order to comprehensively evaluate the influence of different carrier ratio values of MMC-STATCOM on the harmonic compensation operating conditions, this paper determines three evaluation labels: switching loss, capacitor voltage unbalance degree, and line THD. Under different carrier ratio value conditions, the label values of each evaluation label are shown in Table 4. Sort each evaluation label according to the actual operation results. According to the nature of the negative type label, the label corresponding to the lowest percentage in the label is assigned 100, the second lowest label is assigned 90, and so on, to obtain the label values and corresponding scores of each evaluation label.

[0098] Table 4 Label values of each evaluation label

[0099] Table 4 shows the label values of three evaluation labels of the MMC-STATCOM system including 60 sub-modules under 5th harmonic compensation at different carrier ratio values.

[0100] The subjective weights for evaluating the tag switching loss, capacitor voltage unbalance degree, and AC side current harmonics are 42.74%, 49.89%, and 7.37% respectively, and the corresponding objective weights are 34.69%, 64.42%, and 0.89% respectively. The calculated combined weights of the evaluation tags are 0.869%, 91.629%, and 7.289% respectively.

[0101] Combined with the above analysis and calculation of the tag carrier ratio value, multiplying the weights corresponding to the carrier ratios with different decimal parts by the tag values, we can obtain the states of the MMC-STATCOM under different carrier ratio values. The specific results are as Figure 4 shown. It can be seen that when the carrier ratio value is 5.85, the evaluation value calculated by the evaluation system combining the three tags is the highest, that is, the MMC-STATCOM under 5th harmonic compensation is in a better operating state.

[0102] As one of the prominent substantive features of the present invention and a significant improvement over the prior art, the present invention takes into account the switching function and arm current changes of the MMC-STATCOM under 5th harmonic compensation. In addition, by combining the weighting and evaluation of multiple evaluation tags to determine the optimized value of the carrier ratio, the switching frequency of the MMC-STATCOM is 282.5 Hz during stable operation, while significantly suppressing the line THD and low-frequency capacitor voltage harmonics. S3.2 Based on the tag technology feedback regulation, the capacitor voltage is balanced and optimized, including: The low tag value of the carrier ratio used to reduce the switching loss will generate a frequency reduction effect on the capacitor voltage. By optimizing the value of the low carrier ratio, the influence of the frequency reduction effect is minimized, but at this time, the unbalance degree of the capacitor voltage is still not zero. In order to further reduce the capacitor voltage unbalance degree on the basis of suppressing the frequency reduction effect, the present invention proposes as Figure 5 shown the capacitor voltage balance optimization strategy based on the tag technology feedback regulation.

[0103] The function of this strategy is to make the capacitor voltages of the N sub-modules on the arm track their reference values. The voltage equalizing loop uses a proportional regulator. According to the tag technology, the proportional regulator is selected to be connected. Through the proportional regulator, the capacitor voltage balance adjustment amount is output, and then according to the deviation between the sub-module capacitor voltage and the capacitor voltage reference value, the modulation wave is adjusted, and further the time of the sub-module in two working states is changed to adjust the capacitor voltage.

[0104] The specific process of the capacitor voltage balance optimization strategy based on the tag technology feedback regulation is as Figure 5 shown.

[0105] Taking the upper arm of phase A as an example, at this time, u up_i is the capacitor voltage of the upper arm. Set the average value of the capacitor voltage of the upper arm uup_avg As the reference value of u up_i After passing through the proportional regulator, the output equalization control modulates the regulation amount. The equalization control of the capacitor voltage is based on the direction of the arm current. If u up_i <u up_avg , when i pj >0, the converter should absorb energy from the DC side to charge the sub-module capacitor of the upper arm. At this time, Δu ref1 is a positive value. If i pj <0 in this voltage condition, then Δu ref1 is a negative value; if u up_i >u up_avg , when i pj >0, the converter should supply energy to the DC side. At this time, the sub-module capacitor of the upper arm discharges, and Δu ref1 is a negative value. If i pj <0 in this voltage condition, then Δu ref is a positive value. The relationships between relevant variables can be obtained from the above analysis and are expressed by the following formula:

[0106] In the formula, i pj is the upper-arm current of phase j(a,b,c), and K p_uc is the proportional coefficient of the capacitor voltage equalization optimization strategy.

[0107] Similarly, the expression of the capacitor voltage regulation amount of the lower arm can be obtained and is expressed by the following formula:

[0108] In the formula, u c_avg is the average value of the lower-arm capacitor voltage, and u c_i is the lower-arm capacitor voltage.

[0109] Taking into account that the low carrier ratio used to reduce the switching frequency may cause harmonic components in the capacitor voltage, the value of the proportional regulator K p_uc should be a suitable value. Otherwise, the harmonics in the capacitor voltage multiplied by a large proportional coefficient and superimposed on the modulation wave will distort the modulation wave, further threatening the stable operation of the STATCOM system. Therefore, the present invention designs to use the capacitor voltage unbalance degree label to further clarify the corresponding range of other labels and select the access proportional regulator according to the label range.

[0110] The second embodiment of the present invention provides a system for suppressing the frequency reduction effect of a low carrier ratio MMC-STATCOM under harmonic compensation, including: A harmonic analysis module for analyzing the harm of harmonics; The carrier ratio calculation module is used to determine the optimal carrier ratio range that limits the frequency reduction effect; The carrier ratio optimization module is used to determine the value of the optimal carrier ratio to suppress the frequency reduction effect; The voltage optimization module is used to optimize the capacitor voltage balance to further reduce the frequency reduction effect uniformly.

[0111] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: the specific implementation manners of the present invention can still be modified or equivalently replaced, and any modification or equivalent replacement without departing from the spirit and scope of the present invention shall be covered by the protection scope of the claims of the present invention.

Claims

1. A method for suppressing the frequency reduction effect of a low-carrier ratio MMC-STATCOM under harmonic compensation, characterized in that: The following steps are involved: The carrier phase shift modulation strategy with low carrier ratio is introduced into the MMC-STATCOM that performs 5th harmonic compensation. The modulation wave correction brought by the 5th harmonic compensation, the compensation current of the bridge arm current and the bridge arm current correction are taken into account. According to the relationship between the frequency reduction effect, low carrier ratio and capacitor voltage, the analytical function of the nonlinear capacitor voltage harmonic is determined. According to the analytical function of nonlinear capacitor voltage harmonics, the influence of the carrier ratio of MMC-STATCOM on the operating state of MMC-STATCOM under the harmonic compensation frequency reduction effect is quantitatively analyzed, and the optimal value range of the carrier ratio under which the frequency reduction effect is suppressed is determined based on the analysis results; Using switching loss, harmonic THD, and capacitor voltage imbalance as evaluation labels, the label evaluation values ​​under different carrier ratios in the optimal value range of the carrier ratio are calculated. By comparing the label evaluation values ​​under different carrier ratios, the specific value of the optimal carrier ratio is determined. In view of the capacitor voltage fluctuation that still exists after determining the optimal carrier ratio, the frequency reduction effect of MMC-STATCOM is suppressed by optimizing the capacitor voltage balance.

2. The method for suppressing the frequency reduction effect of a low-carrier ratio MMC-STATCOM under harmonic compensation according to claim 1 is characterized in that: The method of determining the analytical function of the nonlinear capacitor voltage harmonic according to the relationship between the frequency reduction effect, the low carrier ratio and the capacitor voltage comprises: Taking into account the fifth harmonic current and circulating current of MMC-STATCOM, determine the upper and lower arm currents of MMC-STATCOM; Taking into account the injection of the fifth harmonic in the modulation wave and the nonlinear dynamic switching process of CPS-PWM, a new MMC-STATCOM switching function S is derived using double Fourier integration. ji (t); The nonlinear switching function S ji (t) and the bridge arm current are substituted into the capacitor voltage solution formula to obtain the MMC-STATCOM submodule capacitor voltage, and only the capacitor voltage harmonic Δu is retained c_i The nonlinear capacitor voltage harmonic Δu related to FR5 c_i(m5,n5) ; Capacitor voltage harmonics Δu at different frequencies c_i(m5,n5) The harmonic components of For the simplified capacitor voltage harmonic Δu c_i(m5,n5) Carry out quantitative analysis and obtain the analytical function.

3. The method for suppressing the frequency reduction effect of a low-carrier ratio MMC-STATCOM under harmonic compensation according to claim 2 is characterized in that: The capacitor voltage harmonics Δu at different frequencies c_i(m5,n5) The harmonic components are simplified to include: The baseband carrier ratio FR is constructed as a decimal expression and expressed as follows: Wherein, Round(FR) is the integer part of the baseband carrier ratio FR replaced by α; Dec(FR) is the decimal part of the baseband carrier ratio FR replaced by β, and the value range is [0,1); Set the value range of α to 2 to 7, the value range of m5 to 1 to 5, and the value range of n5 to -5 to 5; Capacitor voltage harmonics Δu c_i(m5,n5)|ω=0 Simplify and draw a corresponding table of the values ​​of the harmonic amplitude coefficient H5(m5,n5) under different m5 and n5, and keep H5(m5,n5) greater than 1×10 -9 The harmonic component of (m5, n5) is further simplified by setting the frequency (m5FR / 5+n5±l / 5)f5 equal to 0, and obtaining Δu c_i(m5,n5)|ω=0 Criteria for determination: According to the judgment criteria and the setting (m5, n5), the simplified harmonic component Δu with a frequency equal to zero under different fundamental frequency carrier ratio FR settings can be obtained. c_i(m5,n5)|ω=0，FR=α and Δu c_i(m5,n5)|ω=0，FR=α+β ; For harmonic components with frequencies less than 5 times the fundamental frequency, keep the amplitude H5(m5,n5) greater than 1×10 -4 The harmonic components of Δu are plotted. c_i(m5,n5)|0<ω≤ω5 The frequency value table determines the capacitor voltage nonlinear harmonics whose frequency can be reduced to greater than 0 and less than or equal to 250Hz.

4. The method for suppressing the frequency reduction effect of a low-carrier ratio MMC-STATCOM under harmonic compensation according to claim 1 is characterized in that: The simplified capacitor voltage harmonic Δu c_i(m5,n5) Quantification, determining analytical functions include: Capacitor voltage harmonic Δu c_i(m5,n5)|ω=0 The analytical function after quantitative analysis is expressed as follows: In the formula, I 0j and φ0 are the amplitude and phase of the fundamental frequency component of the compensation current respectively; I 5j and φ5 are the amplitude and phase of the five-fold frequency compensation current respectively; I z6j and φ6 are the amplitude and phase of the six-fold frequency circulating current respectively; C is the submodule capacitance value; t represents time; θ i is the phase shift angle of the ith submodule.

5. The method for suppressing the frequency reduction effect of a low-carrier ratio MMC-STATCOM under harmonic compensation according to claim 4 is characterized in that: The Δu c_i(m5,n5)|0<ω<ω0 The analytical function after quantitative analysis is expressed as follows: In the formula, I 5j / 4ω5C is the amplitude of the capacitor voltage 5 times the power frequency component when the frequency reduction effect is not taken into account, ω5 is five times the fundamental frequency, I 5j is the amplitude of the five-fold frequency compensation current.

6. The method for suppressing the frequency reduction effect of a low-carrier ratio MMC-STATCOM under harmonic compensation according to claim 1 is characterized in that: The impact of harmonic compensation frequency reduction effect on MMC-STATCOM under quantitative analysis includes: When the baseband-to-carrier ratio FR is equal to an integer or a set decimal, the ramp harmonic Δu with zero frequency c_i(m5,n5)|ꞷ=0 The DC component of the capacitor voltage overlaps, causing the DC component of the capacitor voltage to continue to increase or decrease, and the charging and discharging state of the capacitor becomes unstable. At the same time, due to different phase shift angles, the change trends of the capacitor voltage harmonics of different sub-modules are different, resulting in an unbalanced capacitor voltage between sub-modules, and the harmonic compensation function of the MMC-STATCOM cannot be performed normally; When the base frequency carrier ratio FR changes with the fractional part β, Δu c_i(m5,n5)|0<ω<ω0 The distortion is mainly ultra-low frequency harmonics with a frequency of βf0, causing ultra-low frequency fluctuations in the capacitor voltage, Δu c_i(m5,n5)|ꞷ=0 ; Δu c_i(m5,n5)|0<ω<ω0 Distortion becomes Δu c_i(m5,n5)|ω=ω0 , Δu c_i(m5,n5)|ω=ω5 , the fundamental component of the capacitor voltage and the fifth harmonic component of the capacitor voltage overlap, causing the fundamental component and the fifth harmonic component in the compensation current to increase, resulting in a decrease in the harmonic control effect of the MMC-STATCOM and a deterioration in the line THD.

7. The method for suppressing the frequency reduction effect of a low-carrier ratio MMC-STATCOM under harmonic compensation according to claim 1 is characterized in that: The determining, based on the analysis result, an optimal value interval of the carrier ratio in which the frequency reduction effect is suppressed comprises: Ignore Δu c_i(m5,n5)|0<ω≤ω5，FR=5+β The phase angle and frequency of the c_i(m5,n5)|0<ω≤ω5，FR=5+β The absolute value of the amplitude can be summed to obtain Δu c_i(m5,n5)|0<ω≤ω5，FR=5+β The amplitude coefficient H c_i (β); By drawing H c_i The relationship curve between (β) and β is used to select the optimal β value interval with the smallest capacitor voltage harmonic amplitude. At this time, the impact of capacitor voltage harmonics on the MMC-STATCOM capacitor voltage is the smallest, and the corresponding fundamental frequency carrier ratio FR value interval is the optimal carrier ratio value interval.

8. A method for suppressing the frequency reduction effect of a low-carrier ratio MMC-STATCOM under harmonic compensation according to claim 1, characterized in that: Determining the optimal carrier ratio includes: Under the operating conditions of MMC-STATCOM with different carrier ratios, the switch loss, capacitor voltage imbalance and line-side THD evaluation labels are sorted according to the actual operating results to obtain the label value and corresponding evaluation value of each evaluation label; by multiplying the carrier ratios of different decimal parts and the weights with the corresponding evaluation values, the status of MMC-STATCOM under different carrier ratios is obtained, and the corresponding value of the carrier ratio when MMC-STATCOM is in the best operating state is taken as the optimal carrier ratio.

9. A method for suppressing the frequency reduction effect of a low-carrier ratio MMC-STATCOM under harmonic compensation according to claim 1, characterized in that: The capacitor voltage equalization optimization includes: A proportional regulator is used to design a voltage balancing ring. The proportional regulator is selected according to the evaluation label. The capacitor voltage balancing adjustment amount is output by the proportional regulator. The modulation wave is adjusted according to the deviation between the sub-module capacitor voltage and the capacitor voltage reference value. The capacitor voltage is then adjusted by changing the time when the sub-module is in two working states. The capacitor voltage of N sub-modules on the bridge arm tracks its reference value, and the harmonic compensation frequency reduction effect under the optimal carrier ratio is suppressed.

10. A system for suppressing the frequency reduction effect of a low-carrier ratio MMC-STATCOM under harmonic compensation, which runs the method for suppressing the frequency reduction effect of a low-carrier ratio MMC-STATCOM according to any one of claims 1 to 9, characterized in that: The frequency reduction effect suppression system comprises: Harmonic analysis module, used to analyze the harm of harmonics; A carrier ratio calculation module, used to determine an optimal carrier ratio interval for limiting the frequency reduction effect; A carrier ratio optimization module, used to determine the value of the optimal carrier ratio; The voltage optimization module is used to optimize the capacitor voltage balance and suppress the frequency reduction effect.

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