Method and system for suppressing frequency reduction effect of low-carrier ratio MMC-STATCOM under harmonic compensation

Abstract

The present invention discloses a method and system for suppressing the frequency reduction effect of a low-carrier ratio MMC-STATCOM under harmonic compensation. The method comprises: first, analyzing the modulation wave correction amount and the fifth harmonic brought by the carrier modulation strategy of harmonic compensation, defining the capacitor voltage imbalance, and dividing the nonlinear capacitor voltage harmonics into four types for simplified analysis. Secondly, determining the optimal carrier ratio value range in which the capacitor voltage hazard of MMC-STATCOM is suppressed under the influence of the fifth harmonic frequency reduction effect, and designing the decimal part of the optimal carrier ratio. By analyzing the association rules of different label data, the operating state of the MMC-STATCOM is optimized, taking into account the three evaluation labels of switching loss, harmonic THD and capacitor voltage imbalance, reducing the switching loss and THD, and suppressing the capacitor voltage imbalance phenomenon. Finally, the specific value of the optimal carrier ratio is selected through scoring evaluation. The present invention improves the operating efficiency of the MMC-STATCOM and enables it to operate stably under a low carrier ratio.

Description

Technical Field

[0001] The present invention belongs to the technical field of converter control, and relates to a method and system for suppressing the 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 scalability and has become a research hotspot in the field of ultra-high voltage transmission.

[0003] The carrier phase-shifted modulation strategy (CPS-PWM) offers advantages such as constant switching frequency, high consistency in submodule switching losses, and excellent harmonic characteristics. Currently, in high-power applications, the carrier ratio of CPS-PWM is often set to greater than 10. However, existing research has shown that CPS-PWM with a low carrier ratio (no greater than 5) can significantly reduce switching losses and cooling requirements, further improving the harmonic characteristics of MMC systems and having practical significance for MMC-STATCOM system optimization. However, there is no clear standard for determining the impact of low carrier ratios on MMC-STATCOM capacitor voltage.

[0004] The existing technical 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 enable the MMC-STATCOM to operate stably at a low carrier ratio. Summary of the Invention

[0005] To address 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 the influence of the fifth harmonic under harmonic compensation. The capacitor voltage of the low-carrier-ratio MMC-STATCOM under harmonic compensation is clustered, classified, predicted, and correlated, and labels are mined to form capacitor voltage fluctuations. The capacitor voltage balancing strategy is improved based on the labels to better promote the stable operation of the MMC-STATCOM, improve the operating efficiency of the MMC-STATCOM, and enable it to operate stably 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:

[0007] A low-carrier-to-carrier ratio carrier phase-shift modulation strategy is introduced into an MMC-STATCOM that performs fifth-order harmonic compensation. Taking into account the modulation wave correction caused by the fifth-order harmonic compensation, as well as the compensation current and bridge arm current correction, the analytical function of the nonlinear capacitor voltage harmonics is determined based on the relationship between the frequency reduction effect, low-carrier-to-carrier ratio, and capacitor voltage.

[0008] Based on the analytical function of nonlinear capacitor voltage harmonics, the influence of the carrier ratio of MMC-STATCOM on the operating state under the harmonic compensation frequency reduction effect is quantitatively analyzed. Based on the analysis results, the optimal value range of the carrier ratio in which the frequency reduction effect is suppressed is determined.

[0009] Using switching loss, harmonics THD, and capacitor-voltage imbalance as evaluation labels, we calculated label evaluation values for different carrier ratios within the optimal carrier ratio range. By comparing the label evaluation values at different carrier ratios, we determined the specific value of the optimal carrier ratio.

[0010] In order to solve 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.

[0011] Preferably, determining the analytical function of the nonlinear capacitor voltage harmonics according to the relationship between the frequency reduction effect, the low carrier ratio and the capacitor voltage includes:

[0012] Taking into account the fifth harmonic current and circulating current of MMC-STATCOM, determine the current of the upper and lower bridge arms of MMC-STATCOM;

[0013] 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);

[0014] 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) ;

[0015] Capacitor voltage harmonics Δu at different frequencies c_i(m5,n5) The harmonic components of simplification are carried out;

[0016] For the simplified capacitor voltage harmonic Δu c_i(m5,n5) Perform quantitative analysis and obtain the analytical function.

[0017] Preferably, the capacitor voltage harmonics Δu at different frequenciesc_i(m5,n5) The harmonic components are simplified to include:

[0018] The fundamental frequency-to-carrier ratio FR is constructed as a decimal expression and expressed as follows:

[0019]

[0020] Where, Round(FR) is the integer part of the baseband-to-carrier ratio FR, which is replaced by α; Dec(FR) is the decimal part of the baseband-to-carrier ratio FR, which is replaced by β, and its value range is [0,1).

[0021] 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;

[0022] 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 components of (m5, n5) are 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:

[0023]

[0024] According to the judgment criteria and the set (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 ;

[0025] 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 is used to determine the capacitor voltage nonlinear harmonics whose frequency can be reduced to greater than 0 and less than or equal to 250Hz.

[0026] Preferably, the simplified capacitor voltage harmonic Δu c_i(m5,n5) Quantification, determining analytical functions includes:

[0027] Capacitor voltage harmonic Δu c_i(m5,n5)|ω=0 The analytical function after quantitative analysis is expressed as follows:

[0028]

[0029] Where, I 0j and φ0 are the amplitude and phase of the fundamental frequency component of the compensation current respectively; I 5jand 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 i-th submodule.

[0030] Preferably, the Δu c_i(m5,n5)|0<ω<ω0 The analytical function after quantitative analysis is expressed as follows:

[0031]

[0032] Where, 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.

[0033] Preferably, the quantitative analysis of the impact of harmonic compensation frequency reduction effect on MMC-STATCOM includes:

[0034] When the fundamental frequency 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 it to continuously increase or decrease, making the charge and discharge state of the capacitor unstable. At the same time, due to different phase shift angles, the change trends of the capacitor voltage harmonics of different submodules are different, causing capacitor voltage imbalance between submodules and the MMC-STATCOM harmonic compensation function to fail to function properly.

[0035] When the fundamental 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, which causes 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 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 of the line THD.

[0036] Preferably, determining the optimal value range of the carrier ratio in which the frequency reduction effect is suppressed based on the analysis result includes:

[0037] Ignore Δu c_i(m5,n5)|0<ω≤ω5，FR=5+β Phase angle and frequency, only Δu is retained c_i(m5,n5)|0<ω≤ω5，FR=5+β The sum of the absolute values of the amplitudes can be obtained as Δu c_i(m5,n5)|0<ω≤ω5，FR=5+β The amplitude coefficient H c_i (β); by drawing H c_iThe relationship curve between (β) and β is shown. The optimal β value range with the smallest capacitor voltage harmonic amplitude is selected. At this time, the impact of capacitor voltage harmonics on the MMC-STATCOM capacitor voltage is minimized, and the corresponding fundamental frequency carrier ratio FR value range is the optimal carrier ratio value range.

[0038] Preferably, determining the optimal carrier ratio includes:

[0039] Under the operating conditions of MMC-STATCOM with different carrier ratio values, the switching 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 ratio of different decimal parts and the weight with the corresponding evaluation value, the status of MMC-STATCOM under different carrier ratio values is obtained, and the corresponding value of the carrier ratio when the MMC-STATCOM is in the best operating state is taken as the optimal carrier ratio.

[0040] Preferably, the capacitor voltage balancing optimization includes:

[0041] 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 through the proportional regulator. The modulation wave is then 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 the sub-module is in the two working states. The capacitor voltage of the N sub-modules on the bridge arm tracks its reference value, suppressing the harmonic compensation frequency reduction effect under the optimal carrier ratio.

[0042] A 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 comprises:

[0043] Harmonic analysis module, used to analyze the hazards of harmonics;

[0044] A carrier ratio calculation module, used to determine an optimal carrier ratio interval in which the frequency reduction effect is limited;

[0045] A carrier ratio optimization module is used to determine the optimal carrier ratio value;

[0046] The voltage optimization module is used to optimize the capacitor voltage balance and suppress the frequency reduction effect.

[0047] Compared with the prior art, the beneficial effects of the present invention include at least:

[0048] The present invention introduces CPS-PWM with a low carrier ratio into an MMC-STATCOM, reduces the switching loss of the MMC-STATCOM, and improves the efficiency of the MMC-STATCOM. By introducing the concept of nonlinear capacitor voltage harmonics and reducing the carrier ratio of the MMC-STATCOM, the relationship between the low carrier ratio, the frequency reduction effect, and the capacitor voltage of the MMC-STATCOM is explored, and a capacitor voltage imbalance label is constructed under harmonic compensation to quantitatively analyze the operation status of the MMC-STATCOM, thereby determining the optimal carrier ratio for stable operation of the MMC-STATCOM. This can avoid damage to the capacitor voltage of the MMC-STATCOM caused by the low carrier ratio under harmonic compensation, reduce the damage caused by the frequency reduction effect, obtain a stable capacitor voltage waveform, and ensure the stable operation of the MMC-STATCOM with a low carrier ratio under harmonic compensation.

[0049] When clustering the capacitor voltage imbalance at different times under the optimal carrier ratio operation, the cluster center and K value are selected, the capacitor voltage imbalance label is set, and based on the clustering results, the capacitor voltage balancing strategy is improved by injecting the modified modulation wave with the direct bias component, which further reduces the interference of the frequency reduction effect, more intuitively describes the important operating characteristics of the MMC-STATCOM under low carrier ratio, completes the stable control of the MMC-STATCOM under low carrier ratio, and realizes the efficient operation of the MMC-STATCOM under low carrier ratio. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is a flowchart of the method of the present invention;

[0051] Figure 2 is the amplitude coefficient H of the present invention c_i (β) vs. β plot;

[0052] Figure 3 This is a diagram of the label association system of the present invention;

[0053] Figure 4 Comparison of states under different FR values;

[0054] Figure 5 It is a capacitor voltage balancing optimization strategy based on tag technology feedback regulation. DETAILED DESCRIPTION

[0055] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. The embodiments described in this application are only part of the embodiments of the present invention, not all of them. Based on the spirit of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0056] like Figure 1 As shown, Example 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:

[0057] S1: Introduce CPS-PWM with low carrier ratio into MMC-STATCOM, correct the carrier modulation strategy modulation wave and harmonic current caused by the 5th harmonic current compensation, and analyze the relationship between the frequency reduction effect, low carrier ratio and capacitor voltage under harmonic compensation. Further preferably, reduce the carrier ratio of the MMC-STATCOM carrier phase shift modulation strategy, calculate the nonlinear harmonics in the CPS-PWM modulation process, obtain the MMC-STATCOM capacitor voltage harmonic analytical function, change the carrier ratio value in the analytical function to quantitatively analyze the relationship between the three frequency reduction effect hazards of low-frequency fluctuation of capacitor voltage, unstable charging and discharging state of sub-module capacitor and unbalanced capacitor voltage between sub-modules and low carrier ratio. Divide the nonlinear capacitor voltage harmonics into four types according to the frequency size, simplify the four harmonics and analyze their capacitor voltage hazards.

[0058] In a preferred but non-limiting embodiment of the present invention, S1 specifically includes:

[0059] S1.1: Considering that the 5th current harmonic content in the six-pulse current generated by nonlinear loads can reach about 20%, take the 5th current harmonic compensation as an example and calculate the 5th carrier ratio FR5.

[0060] The 5th harmonic compensation of 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 value u1 through the bridge arm current inner loop controller and injects it into the modulation wave, and the modulation wave enters the CPS-PWM strategy to generate the sub-module drive signal, finally achieving i c Precise tracking L Complete grid current harmonic compensation.

[0061] At this time, the upper and lower arm currents of MMC-STATCOM i pj 、i nj The fifth harmonic compensation current appears, and the double frequency circulating current can be corrected to a sextuple frequency circulating current through the cyclic coupling of the capacitor voltage and the bridge arm current. Taking into account the circulating current of the MMC-STATCOM and the fifth harmonic compensation current, the upper and lower bridge arm currents of the MMC-STATCOM after the fifth harmonic compensation are modified. Therefore, the upper and lower bridge arm currents can be expressed by the following formula:

[0062]

[0063] Where ω0 is the fundamental angular frequency; I dc / 3 is the DC current input to each phase bridge arm from the DC side; I 0j and φ0 are the compensation current fundamental frequency components i 0j The amplitude and phase of 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 sextupled circulating current respectively.

[0064] 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), expressed as follows:

[0065]

[0066] Where, the four parts represent the DC bias, 50Hz fundamental component, 250Hz harmonic component, and the sideband harmonics and carrier harmonics generated by the 250Hz harmonic injection; M and M5 are the modulation ratios of the fundamental and fifth harmonic respectively; ΔS ji (m5,n5)(t) is the nonlinear harmonic after 250Hz injection, which consists of carrier harmonics and sideband harmonic components; ω0 is the fundamental angular frequency; ω5 is the fifth harmonic angular frequency; t represents time; θ i is the phase shift angle of the i-th submodule, which is equal to 2π(i-1) / N, where N represents the number of submodules. The coefficient H(m5,n5) is the harmonic amplitude related to the fundamental harmonic order m5 and the carrier harmonic order n5, which can be expressed as follows:

[0067]

[0068] Where, J n is a Bessel function.

[0069] The 5th subcarrier ratio FR5, the fundamental frequency carrier ratio FR and the relationship between the two can be expressed by the following formula:

[0070]

[0071] Where, f c and ω c are the switching frequency and switching angular frequency, f0 and ω0 are the fundamental frequency and fundamental angular frequency; FR5 can be replaced by FR / 5.

[0072] S1.2: Select any upper arm submodule SM of phase a of MMC-STATCOM api (i=1,2…N) as an example for analysis, the nonlinear switching function S jiSubstitute (t) and the bridge arm current into the capacitor voltage solution formula to obtain the MMC-STATCOM submodule capacitor voltage and capacitor voltage harmonics:

[0073]

[0074] Where i ci 、u c_i , Δu c_i are the capacitor current, capacitor voltage and capacitor voltage harmonics of the i-th submodule respectively; i j According to the submodule position, i pj 、i nj ; C is the submodule capacitance value.

[0075] Only the harmonic Δu of the capacitor voltage is retained c_i The nonlinear capacitor voltage harmonic Δu related to FR5 c_i(m5,n5) , Δu c_i(m5,n5) It can be expressed by the following formula:

[0076]

[0077] Δu c_i(m5,n5) Contains harmonics with a frequency of (m5FR5+n5±l / 5)f5. When the carrier ratio is reduced, different m5 and n5 values can be found to reduce the frequency to below five times the fundamental frequency ω5; and Δu c_i(m5,n5) The amplitude is inversely proportional to FR, and when FR decreases, it will cause Δu c_i(m5,n5) This results in excessive growth, leading to abnormal capacitor voltage overvoltage. In summary, the definition of the frequency reduction effect of the MMC-STATCOM under fifth-order harmonic compensation can be obtained: when the carrier ratio FR of the carrier phase-shift modulation strategy is reduced, and FR5, m5, and n5 are within the appropriate range, the harmonic frequency of the capacitor voltage of the MMC-STATCOM is significantly reduced to below 5 times the fundamental frequency, that is, the harmonic frequency is reduced to 250Hz and below.

[0078] S1.3: Δu under the frequency reduction effect based on harmonic compensation in S1.2 c_i(m5,n5) The variation rules of different m5, n5 and different FR, the capacitor voltage nonlinear harmonics can be divided into four harmonics, namely Δu c_i(m5,n5)|ω=0 , Δu c_i(m5,n5)|0<ω<ω0 , Δu c_i(m5,n5) |ω=ω0、Δu c_i(m5,n5)|ω0<ω<ω5 (ω is the harmonic angular frequency);

[0079] Δu c_i(m5,n5)|ω=0 Represents nonlinear harmonics with a frequency equal to 0. This part of the harmonic components is distorted into ramp harmonics, causing the capacitor voltage to continuously increase or decrease;

[0080] Δuc_i(m5,n5)|0<ω<ω0 Represents nonlinear harmonics below 50Hz. These harmonic components cause ultra-low frequency fluctuations in the capacitor voltage; Δu c_i(m5,n5) |ω=ω0 and Δu c_i(m5,n5)|ω0<ω<ω5 They represent the harmonic components with a frequency equal to 50Hz and the harmonic components with a frequency greater than 50Hz and less than 250H respectively.

[0081] S1.4: Due to the large variation range of FR, m5, and n5, Δu c_i(m5,n5) Contains many harmonic components, so it is necessary to c_i(m5,n5) Simplify, the simplification steps include:

[0082] (1) Due to the injection of the fifth harmonic, FR5 causes the frequency of the capacitor voltage nonlinear harmonics to decrease, resulting in more nonlinear harmonics. Therefore, FR is constructed as the sum of the integer part Round(FR) and the fractional part Dec[FR], expressed as follows:

[0083]

[0084] Where Round(FR) is equal to α; Dec[FR] is equal to β, and the value range is set to [0,1).

[0085] (2) In order to ensure that the MMC-STATCOM has sufficient power quality and reduce the distortion of the AC side compensation current, so that the MMC-STATCOM can operate stably, the value of α is set to be no less than 2; ensure that Δu c_i(m5,n5)|ω0<ω<ω5 It can cause large capacitor voltage fluctuations, so α is set to no more than 7. Finally, the value range of FR is 2 to 7.

[0086] (3) Simplify the harmonic content at different frequencies:

[0087] In order to ensure that the harmonic amplitude coefficient H(m5,n5) is large enough and the harmonic frequency can be lower than the fundamental frequency, m is set to 1 to 5 and n is set to -5 to 5. Simplify the harmonic content under different m5 and n5 values:

[0088] i. For Δu c_i(m5,n5)|ω=0 Simplifications include:

[0089] First, according to the numerical correspondence table of the harmonic amplitude coefficient H5(m5,n5) under different m5 and n5:

[0090] Table 1 H5(m5,n5) values under different m5 and n5 values

[0091]

[0092] According to Table 1, keep H5(m5,n5) greater than 1×10-9 The harmonic components of m5 and n5 are further simplified to 1 to 5 and 1 to 4.

[0093] Set the frequency (m5FR / 5+n5±l / 5)f5 equal to 0 and obtain Δu c_i(m5,n5)|ω=0 The judgment standard is expressed as follows:

[0094]

[0095] Where, It means rounding down (∓l-5n5) / m5.

[0096] When (m5,n5) is equal to (2,-1), (4,-3) and (5,-2), Δu c_i(m5,n5)|ω=0 The conditions for judging the harmonic components are shown in Table 2.

[0097] Table 2: When (m5, n5) is equal to (2, -1), (4, -3) and (5, -2), Δu c_i(m5,n5)|ω=0 Harmonic component judgment conditions

[0098]

[0099] According to the judgment criteria and the selected (m5, n5), the simplified harmonic component Δu with zero frequency under different FR settings can be obtained. c_i(m5,n5)|ω=0 , FR = α and Δu c_i(m5,n5)|ω=0 , FR=α+β.

[0100] ii. For Δu c_i(m5,n5)|0<ω≤ω5 Simplifications include:

[0101] For the component with frequency less than 5 times the fundamental frequency, in order to ensure the nonlinear harmonic Δu of capacitor voltage c_i(m5,n5)|0<ω≤ω5 It can produce sufficiently large fluctuations in the capacitor voltage, select harmonic components with large enough amplitudes, and keep H5(m5,n5) in Table 1 greater than 1×10 -4 The harmonic components of m5 and n5 are further simplified to 1 to 5 and 1 to 3.

[0102] In this value range, taking FR=5+β as an example, the frequency coefficient values of ω / ω5 under different (m5,n5) values are plotted.

[0103] Table 3 Frequency coefficient values of ω / ω5 under different (m5,n5) values when FR=5+β

[0104]

[0105] Substitute the value of β into Table 3, find the corresponding value of (m5,n5) when the frequency is lower than 250Hz, and determine the corresponding Δuc_i(m5,n5)|0<ω≤ω5 Harmonic components.

[0106] S1.5: For the simplified Δu c_i(m5,n5)|ω=0 , Δu c_i(m5,n5)|0<ω≤ω5 Perform quantitative analysis to determine Δu c_i(m5,n5)|ω=0 and Δu c_i(m5,n5)|0<ω≤ω5 The analytical function of .

[0107] (1) For Δu c_i(m5,n5)|ω=0 Quantitative analysis to obtain Δu c_i(m5,n5)|ω=0 The analytical function of is expressed as follows:

[0108]

[0109]

[0110] (2) For Δu c_i(m5,n5)|0<ω≤ω5 Quantitative analysis to obtain Δu c_i(m5,n5)|0<ω≤ω5 The analytical function of .

[0111] Considering the serious capacitor voltage harmonics when FR=5 and the increase in the frequency (m5FR5+n5±l / 5)f5 harmonic components when FR decreases, the Δu generated when FR=5+β is c_i(m5,n5)|0<ω≤ω5，FR=5+β Take Δu as an example for quantitative analysis. c_i(m5,n5)|0<ω≤ω5，FR=5+β The analytical function of can be expressed by the following formula:

[0112]

[0113] (3) According to the above formula, determine the ultra-low frequency harmonic Δu with a frequency below 50 Hz c_i(m5,n5)|0<ω<ω0 The analytical function of is expressed as follows:

[0114]

[0115] Where, 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.

[0116] (4) Δu under different β values c_i(m5,n5) |ω0≤ω≤ω5, FR=5+β towards Δu c_i(m5,n5)|ω=ω0 , Δu c_i(m5,n5)|ω=ω5 Distortion; where β = 0.5 Δu c_i(m5,n5)|ω=ω0 , Δu c_i(m5,n5)|ω=ω5 The component amplitude is the most obvious, now take 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 Take Δu as an example to conduct quantitative analysis. c_i(m5,n5) |ω=ω 0，FR=5.5 and Δu c_i(m5,n5)|ω=ω5，FR=5.5The analytical functions can be expressed by the following formulas:

[0117]

[0118] S2: Quantitatively analyze the impact of the carrier ratio of the MMC-STATCOM on its operating state under the influence of harmonic compensation frequency reduction, and propose an optimal range of carrier ratio values to suppress the frequency reduction effect. Finally, set switching loss, harmonic THD, and capacitor voltage imbalance as three evaluation indicators for optimizing the operating state of the MMC-STATCOM under the influence of harmonic compensation frequency reduction.

[0119] In a preferred but non-limiting embodiment of the present invention, S2 specifically includes:

[0120] S2.1: Quantitatively analyze the impact of the MMC-STATCOM carrier ratio on the operating status of the MMC-STATCOM under the harmonic compensation frequency reduction effect, including:

[0121] (1) Quantitatively analyze the impact of capacitor voltage harmonics and THD.

[0122] According to the above formula, when FR is equal to the integer part or the decimal part has a special β value, Δu c_i(m5,n5)|ω=0，FR=α , Δu c_i(m5,n5)|ω=0，FR=α+β The distorted harmonics are proportional to the time t, which overlap with the DC component of the capacitor voltage, causing the capacitor voltage to continuously increase or decrease, and eventually causing the charge and discharge state of the capacitor to become unstable. At the same time, due to the different phase shift angles, the θ i changes, Δu c_i(m5,n5)|ω=0，FR=α , Δu c_i(m5,n5)|ω=0，FR=α+β The value of Δu changes, which means that the sub-modules are different, and the corresponding sub-module capacitor voltage change trends are also different, resulting in an imbalance in the capacitor voltage between different sub-modules; Δu c_i(m5,n5)|ω=0，FR=α , Δu c_i(m5,n5)|ω=0，FR=α+β with I 5j Positive correlation, in the case of lines with high harmonic pollution, harmonic compensation needs to control the generated I 5j As FR increases, the capacitor voltage becomes more unstable. ih (t) and i pj The capacitor voltage harmonics generated by the 5 times power frequency compensation current coupling overlap with the DC component (when the switching frequency is equal to 5 times the power frequency harmonic frequency), and the generated capacitor voltage harmonic amplitude is the largest. At this time, the capacitor voltage variation amplitude is the largest, and the harmonic compensation frequency reduction effect is the most serious. The harmonic compensation of the MMC-STATCOM cannot be performed normally, and the line THD cannot be suppressed.

[0123] According to the analytical function, Δu c_i(m5,n5)|0<ω<ω0The frequency is reduced to β*f0, 2β*f0 and 3β*f0, due to Δu c_i(m5,n5)|0<ω<ω0 The amplitude is inversely proportional to the frequency, so when FR changes with the fractional part β, Δu c_i(m5,n5)|0<ω<ω0 It can be distorted into ultra-low frequency harmonics with a frequency mainly of βf0, causing ultra-low frequency fluctuations Δu in the capacitor voltage c_i(m5,n5)|ω=0 ;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 , 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 of the line THD; at this time, the harmonic component with a frequency of β*f0 is in Δu c_i(m5,n5)|0<ω<ω0 The proportion is the largest. When β is too low, Δu c_i(m5,n5)|0<ω<ω0 It brings extremely low-frequency harmonics with extremely large amplitude to the capacitor voltage and even causes overvoltage hazards to the sub-module.

[0124] 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 capacitor voltage harmonics have a frequency equal to f0 or f5, which increases the fundamental frequency component and the five times power frequency compensation component in the compensation current, causing the harmonic compensation control performance to decline and worsening the line THD.

[0125] In addition, the switching loss of MMC-STATCOM decreases with the decrease of FR, and the system efficiency increases accordingly.

[0126] (2) Quantitatively analyze the impact of capacitor voltage imbalance.

[0127] The capacitor voltage imbalance ε combines the discrete degree of each submodule capacitor voltage and the average capacitor voltage, and is calculated by comparing it with the DC side voltage U dc The ratio of represents the degree of dispersion more realistically and can intuitively demonstrate the harm of capacitor voltage harmonics.

[0128] When FR is equal to the integer part or the decimal part has a special β value, Δu c_i(m5,n5)|ω=0，FR=α，∆uc_i(m5,n5)|ω=0，FR=α+β This causes the capacitor voltages of different submodules to continuously increase or decrease, and the capacitor voltage imbalance ε continues to increase, and the MMC-STATCOM cannot operate normally. When FR changes with the fractional part β, Δu c_i(m5,n5)|0<ω<ω0 This causes different submodules to have ultra-low frequency harmonics of different amplitudes, and the capacitor voltage imbalance ε is large.

[0129] The capacitor voltage imbalance can be expressed by the following formula:

[0130]

[0131] Where U dc is the DC side voltage of MMC-STATCOM;

[0132] N is the number of submodules;

[0133] u c_i is the capacitor voltage of the i-th submodule;

[0134] u c_avg is the average value of the submodule capacitor voltage.

[0135] (3) Quantitatively analyze the impact of switching losses.

[0136] The switching loss can be expressed as follows:

[0137]

[0138] Where, E SWT1_i , E SWT2_i , E SWD1_i , E SWD2_i They are the switching losses of IGBT1, IGBT2, VD1 and VD2 respectively;

[0139] U t 、U avg are the IGBT test voltage and the bridge arm average capacitance voltage respectively;

[0140] 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 IGBT1;

[0141] 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 IGBT2;

[0142] E r e ciD1_i (t j ) and E offiD2_i (t j ) are the reverse recovery energies of the first diode and the second diode respectively.

[0143] From the switching loss formula, we can see that the switching loss of the power device is related to the average capacitor voltage of the bridge arm and the frequency f of the triangular carrier. s Related to f s Positive correlation. Then: f sAs the switching loss increases, the efficiency of MMC-STATCOM decreases; f s The switching loss is reduced, and the efficiency of MMC-STATCOM is increased. Therefore, it is necessary to set a suitable carrier ratio to achieve efficient operation of MMC-STATCOM.

[0144] S2.2: Quantitatively analyze the optimal range of carrier ratio through the amplitude coefficient of capacitor voltage harmonics to suppress the frequency reduction effect.

[0145] According to S2.1, when FR is set to an integer or β is a set decimal part, it will cause the capacitor voltage to produce unstable charging and discharging states of the capacitor and unbalanced capacitor voltage, resulting in a large capacitor voltage imbalance variable ε. When β avoids these components, these large hazards can be avoided, and ε is reduced, but it will also cause low-frequency fluctuations with extremely large amplitudes in the capacitor voltage, which in turn increases 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 based on 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 ε is reduced, thus achieving harmonic compensation of MMC-STATCOM under low carrier ratio and suppressing the frequency reduction effect of harmonic compensation.

[0146] Considering FR=5, Δu c_i(m5,n5)|ω=0，FR=5 The capacitor voltage is the most harmful, ε rises faster, and the harmonic compensation frequency reduction effect is the most influential. c_i(m5,n5)|0<ω<ω5，FR=5+β For example, ignoring Δu c_i(m5,n5)|0<ω<ω5，FR=5+β Phase angle and frequency, only Δu is retained c_i(m5,n5)|0<ω<ω5，FR=5+β The absolute value of the amplitude of each low-frequency harmonic in is used to obtain the amplitude H of each low-frequency harmonic c_i (β), as shown in the following expression:

[0147]

[0148] Determine the I in actual operation of MMC-STATCOM under 5th harmonic compensation dc , I z6j , I 0j , I 5j , substitute these current data into H c_i (β) is operated and plotted as Figure 2 The amplitude coefficient H shown c_i The relationship diagram of (β) and β shows that the optimal β value range with the smallest capacitor voltage harmonic amplitude is selected. In this case, the impact of capacitor voltage harmonics on the MMC-STATCOM capacitor voltage is minimal, ε is small, and the THD deterioration caused by capacitor voltage harmonics can also be effectively suppressed. The corresponding FR value range is the optimal carrier ratio value range.

[0149] Specifically, the present invention sets I dc / 3 equals 0.2I 5j , I 0j =0.04I 5j , M5=0.09, considering the circulation suppression strategy setting I z6j Equal to 0.02I 5j , I 5j Set to 145.5A, the plotted amplitude coefficient H c_i The relationship diagram between (β) and β, such as Figure 2 As shown. Observe the graph to find H c_i (β) The optimal value interval of the fractional part of the minimum carrier ratio β. When FR=5+β, the optimal value interval of β is [0.82,0.9].

[0150] In summary, the relationship between the carrier ratio and voltage harmonics, total harmonic distortion (THD), and capacitor voltage imbalance under the harmonic compensation operation state of MMC-STATCOM is integrated. The switching loss, harmonic THD, and capacitor voltage imbalance labels are formed according to the improved K-means clustering method. The optimal carrier ratio is found through labeling technology and evaluation to optimize the operation state of MMC-STATCOM, such as Figure 3 shown.

[0151] S3: Taking into account the three evaluation tags of switching loss, harmonic THD, and capacitor voltage imbalance, the evaluation values of the operating tag results at different carrier ratios are calculated. By comparing the tag evaluation values of FR at different fractional values, the optimal low fractional value of the carrier ratio is determined, thereby determining the specific value of the optimal carrier ratio. To address the remaining capacitor voltage fluctuations, a capacitor voltage balancing strategy based on tag technology is proposed to further suppress the frequency reduction effect of the MMC-STATCOM under harmonic compensation.

[0152] In a preferred but non-limiting embodiment of the present invention, S3 specifically includes:

[0153] S3.1: Considering the three evaluation tags of switching loss, harmonics THD, and capacitor-voltage imbalance, the evaluation values are calculated for the operating tag results at different carrier ratios. By comparing the FR tag evaluation values at different fractions within the optimal carrier ratio range, the optimal low carrier ratio fraction is determined, thereby determining the specific value of the optimal carrier ratio.

[0154] (1) Optimize the carrier ratio value by combining multiple evaluation labels.

[0155] Preferably, in order to overcome the shortcomings of a single weighting method in the evaluation process, this design adopts a combined weighting method to determine the evaluation label weight. Specifically, the combined weight of the label is synthesized by the subjective weight and objective weight of the label, including:

[0156] i. Select the analytic hierarchy process to determine the subjective weight of the label.

[0157] Each label is compared pairwise to establish the following judgment matrix. By analyzing the judgment matrix using the following formula, the weight of each label can be obtained, that is, the importance of each label in the entire hierarchy.

[0158]

[0159] Where, is the arithmetic mean of the subjective weight 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.

[0160] ii. Select the entropy weight method to determine the objective weight of the label.

[0161] The entropy weighting method uses the concept of entropy to assign weights to labels. Entropy is used to measure uncertainty. The greater the dispersion of an indicator, the smaller the entropy value. This indicates that the indicator value provides more information, and therefore the weight of the indicator should be greater. The following formula non-dimensionalizes the raw data to make data with different labels comparable.

[0162]

[0163] Where: X ij For a data matrix of m samples and n labels, the dimensionless data is substituted into the following formula to calculate the objective weight W of the j-th evaluation label: j-EWM .

[0164]

[0165] Where: P ij is the proportion of each indicator, e j is the entropy value of each of the n indicators.

[0166] iii. Determine the combined weight based on the subjective weight and the objective weight.

[0167] Substituting the calculated subjective and objective weights into the following combination weight calculation formula can obtain the combination weight of the i-th evaluation label.

[0168]

[0169] To comprehensively evaluate the impact of different carrier ratio values on harmonic compensation operating conditions for an MMC-STATCOM, this paper identifies three evaluation labels: switching loss, capacitor voltage imbalance, and line THD. Table 4 lists the label values for each evaluation label under different carrier ratio conditions. The labels are sorted according to actual operating results. Due to the nature of negative labels, the label with the lowest percentage is assigned a value of 100, the label with the second lowest percentage is assigned a value of 90, and so on. This provides the label values and corresponding scores for each evaluation label.

[0170] Table 4 Label values of each evaluation label

[0171]

[0172] Table 4 shows the label values of the three evaluation labels of the MMC-STATCOM system including 60 submodules under 5th harmonic compensation when the carrier ratio is different.

[0173] The subjective weights of the evaluation labels switching loss, capacitor voltage imbalance, and AC side current harmonics are 42.74%, 49.89%, and 7.37%, respectively. The corresponding objective weights are 34.69%, 64.42%, and 0.89%, respectively. The calculated combined weights of the evaluation labels are 0.869%, 91.629%, and 7.289%, respectively.

[0174] Combined with the above analysis and calculation of the label carrier ratio, we multiply the weights corresponding to the carrier ratios of different fractions by the label value to obtain the status of the MMC-STATCOM under different carrier ratio values. The specific results are as follows: Figure 4 As shown in the figure, it can be seen that when the carrier ratio is 5.85, the evaluation value calculated by the evaluation system integrating the three tags is the highest, that is, the MMC-STATCOM under the 5th harmonic compensation is in a good operating state.

[0175] As one of the outstanding substantive features of the present invention and a significant advancement over the prior art, the present invention takes into account the MMC-STATCOM switching function and bridge arm current changes under 5th harmonic compensation. In addition, the optimal value of the carrier ratio is determined by combining the weighting and evaluation of multiple evaluation tags, achieving a switching frequency of 282.5Hz during stable operation of the MMC-STATCOM, while significantly suppressing line THD suppression and low-frequency capacitor voltage harmonics.

[0176] S3.2 optimizes capacitor voltage balance based on tag technology feedback regulation, including:

[0177] The low carrier ratio value used to reduce switching loss will produce a frequency reduction effect in the capacitor voltage. By optimizing the value of the low carrier ratio, the impact of the frequency reduction effect can be minimized, but the imbalance degree of the capacitor voltage is still not zero. In order to further reduce the capacitor voltage imbalance on the basis of suppressing the frequency reduction effect, the present invention proposes the following Figure 5 The capacitor voltage balancing optimization strategy based on tag technology feedback regulation is shown.

[0178] The purpose of this strategy is to make the capacitor voltage of N sub-modules on the bridge arm track its reference value. The voltage balancing ring adopts a proportional regulator, which is selected according to the tag technology. The proportional regulator outputs the capacitor voltage balancing adjustment amount, and then adjusts the modulation wave according to the deviation between the sub-module capacitor voltage and the capacitor voltage reference value, thereby changing the time that the sub-module is in the two working states to adjust the capacitor voltage.

[0179] The specific process of capacitor voltage balancing optimization strategy based on tag technology feedback adjustment is as follows: Figure 5 shown.

[0180] Taking the upper bridge arm of phase A as an example, at this time, u up_i For the upper bridge arm capacitor voltage, set the upper bridge arm capacitor voltage average value u up_avg For u up_i The reference value is output through the proportional regulator to control the modulation amount. The capacitor voltage balance control is based on the direction of the bridge arm current. If u up_i up_avg , when i pj >0, the converter should absorb energy from the DC side to charge the submodule capacitor of the upper bridge arm. At this time, Δu ref1 Is a positive value, if this voltage condition i pj <0, then Δu ref1 Is a negative value; if u up_i >u up_avg , when i pj >0, the converter should provide energy to the DC side. At this time, the submodule capacitor of the upper bridge arm is discharged, Δu ref1 Is a negative value, if this voltage condition i pj <0, then Δu ref The above analysis can obtain the relationship between the relevant variables, which can be expressed by the following formula:

[0181]

[0182] Where i pj is the upper arm current of phase j(a,b,c), K p_uc The scaling factor for capacitor voltage balancing optimization strategy.

[0183] ​Similarly, the capacitor voltage regulation expression of the lower bridge arm can be obtained, which is expressed as follows:

[0184]

[0185] Where u c_avg is the average voltage of the lower bridge arm capacitor, u c_i is the lower bridge arm capacitor voltage.

[0186] Considering that the low carrier ratio used to reduce the switching frequency may cause the capacitor voltage to contain harmonic components, the proportional regulator K p_uc The value of must be appropriate. Otherwise, the harmonics in the capacitor voltage multiplied by a large proportional coefficient and superimposed on the modulated wave will distort the modulated wave, further threatening the stable operation of the STATCOM system. Therefore, the present invention utilizes the capacitor voltage imbalance tag to further clarify the range corresponding to other tags, and selects the proportional regulator based on the tag range.

[0187] A 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, comprising:

[0188] Harmonic analysis module, used to analyze the hazards of harmonics;

[0189] A carrier ratio calculation module, used to determine an optimal carrier ratio interval in which the frequency reduction effect is limited;

[0190] The carrier ratio optimization module is used to determine the optimal carrier ratio value and suppress the frequency reduction effect;

[0191] The voltage optimization module is used to optimize the capacitor voltage balance and further unify the frequency reduction effect.

[0192] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection 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 carrier modulation strategy modulation wave and harmonic current caused by the 5th harmonic current compensation are corrected. According to the relationship between the frequency reduction effect, low carrier ratio and capacitor voltage, the 5th harmonic current and circulating current of the MMC-STATCOM are taken into account to determine the upper and lower arm currents of the MMC-STATCOM. Taking into account the 5th harmonic injection amount 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 to obtain the MMC-STATCOM submodule capacitor voltage, retaining only the capacitor voltage harmonic ∆u c_i Nonlinear capacitor voltage harmonics related to the 5th carrier ratio FR5 ,in m 5 is the fundamental harmonic order, n 5 is the number of carrier harmonics; Capacitor voltage harmonics at different frequencies Simplify the harmonic components of the capacitor voltage harmonics Perform quantitative analysis to obtain analytical functions; Based on the analytical function of nonlinear capacitor voltage harmonics, the influence of the carrier ratio of MMC-STATCOM on the operating state under the harmonic compensation frequency reduction effect is quantitatively analyzed. Based on the analysis results, the optimal value range of the carrier ratio in which the frequency reduction effect is suppressed is determined. Using switching loss, harmonics THD, and capacitor-voltage imbalance as evaluation labels, we calculated label evaluation values for different carrier ratios within the optimal carrier ratio range. By comparing the label evaluation values at different carrier ratios, we determined the specific value of the optimal carrier ratio. In order to solve 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, characterized in that: The capacitor voltage harmonics at different frequencies The harmonic components are simplified to include: The fundamental frequency-to-carrier ratio FR is constructed as a decimal expression and expressed as follows: Where, Round(FR) is the integer part of the baseband-to-carrier ratio FR, which is replaced by α; Dec(FR) is the decimal part of the baseband-to-carrier ratio FR, which is replaced by β, and its 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 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 components of (m5, n5) are further simplified by setting the frequency (m5FR / 5+n5±l / 5)f5 equal to 0, and we can obtain Criteria for determination: Where: According to the judgment criteria and the set (m5, n5), the simplified harmonic components with zero frequency under different fundamental frequency carrier ratio FR settings can be obtained. and ; 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 The frequency value table is used to determine the capacitor voltage nonlinear harmonics whose frequency can be reduced to greater than 0 and less than or equal to 250Hz.

3. The 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 simplified capacitor voltage harmonics Quantitative analysis is performed and the analytical functions obtained include: Capacitor voltage harmonics The analytical function after quantitative analysis is expressed as follows: Where, 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 i-th submodule.

4. The method for suppressing the frequency reduction effect of a low-carrier ratio MMC-STATCOM under harmonic compensation according to claim 3, characterized in that: described The analytical function after quantitative analysis is expressed as follows: Where, 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, and ω0 is the fundamental angular frequency.

5. The 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 impact of harmonic compensation frequency reduction effect on MMC-STATCOM under quantitative analysis includes: When the fundamental frequency to carrier ratio FR is equal to an integer or a set decimal, the ramp harmonic with zero frequency The DC component of the capacitor voltage overlaps, causing it to continuously increase or decrease, making the charge and discharge state of the capacitor unstable. At the same time, due to different phase shift angles, the change trends of the capacitor voltage harmonics of different submodules are different, causing capacitor voltage imbalance between submodules and the MMC-STATCOM harmonic compensation function to fail to function properly. When the fundamental frequency carrier ratio FR changes with the fractional part β, The distortion is mainly ultra-low frequency harmonics with a frequency of βf0, causing ultra-low frequency fluctuations in the capacitor voltage. ; Distortion 、 , 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 of the line THD; Where f0 and ω0 are the fundamental frequency and fundamental angular frequency.

6. The 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 determining, based on the analysis result, an optimal value range of the carrier ratio in which the frequency reduction effect is suppressed includes: neglect The phase angle and frequency of The sum of the absolute values of the amplitudes can be obtained The amplitude coefficient H c_i (β); by drawing H c_i The relationship curve between (β) and β is shown. The optimal β value range with the smallest capacitor voltage harmonic amplitude is selected. At this time, the impact of capacitor voltage harmonics on the MMC-STATCOM capacitor voltage is minimized, and the corresponding fundamental frequency carrier ratio FR value range is the optimal carrier ratio value range.

7. 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 ratio values, the switching 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 ratio of different decimal parts and the weight with the corresponding evaluation value, the status of MMC-STATCOM under different carrier ratio values is obtained, and the corresponding value of the carrier ratio when the MMC-STATCOM is in the best operating state is taken as the optimal carrier ratio.

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: The capacitor voltage balancing 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 through the proportional regulator. The modulation wave is then 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 the sub-module is in the two working states. The capacitor voltage of the N sub-modules on the bridge arm tracks its reference value, suppressing the harmonic compensation frequency reduction effect under the optimal carrier ratio.

9. 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 8, characterized in that: The frequency reduction effect suppression system includes: Harmonic analysis module, used to analyze the hazards of harmonics; A carrier ratio calculation module, used to determine an optimal carrier ratio interval in which the frequency reduction effect is limited; A carrier ratio optimization module is used to determine the optimal carrier ratio value; The voltage optimization module is used to optimize the capacitor voltage balance and suppress the frequency reduction effect.

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