Multi-parameter collaborative optimization design method for modular multilevel converter

Abstract

The invention discloses a multi-parameter collaborative optimization design method for a modular multilevel converter, and belongs to the technical field of modular multilevel converter design. In order to solve the problem of reasonable design of main parameters of the modular multilevel converter, a mathematical model of the modular multilevel converter is constructed; designing a capacitor voltage fluctuation model, a sub-module capacitor capacitance value model and a fluctuation quantity model, and analyzing the relationship between the capacitor voltage fluctuation and the operation condition and the relationship between the sub-module capacitor capacitance value and the operation condition; determining an upper limit value of the bridge arm inductance based on consideration of a system power operation range, determining a design initial value of the bridge arm inductance based on consideration of ring current suppression, determining a lower limit value of the bridge arm inductance based on consideration of resonance avoidance and fault current rise rate limitation, and obtaining a design range of the bridge arm inductance; adjusting the bridge arm inductance to obtain a design value of the designed bridge arm inductance; and according to the capacitance value of the sub-module capacitor and the design value of the designed bridge arm inductor, analyzing the peak voltage and the peak current of the power device IGBT to determine the parameter selection of the power device.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of modular multilevel converter design, and particularly relates to a multi-parameter collaborative optimization design method of a modular multilevel converter. BACKGROUND

[0002] As an advanced power electronic conversion technology, the modular multilevel converter has been widely concerned in the fields of high-voltage direct current transmission, flexible direct current transmission, renewable energy grid connection and power electronic conversion. Due to its modular structure, low harmonic, high efficiency and good scalability, the MMC has become one of the mainstream converter topologies of the flexible direct current transmission system. The MMC adopts a plurality of sub-modules in cascade to form a multi-level topology, and each sub-module is usually composed of a half-bridge or full-bridge structure. Its core advantages are: (1) it can output a voltage waveform close to a sine wave, reducing the need for harmonic filtering; (2) it can reduce the voltage stress of the device and improve the system reliability by increasing the number of sub-modules; (3) it supports direct current fault self-clearing and direct current circuit breaker coordinated control, and improves the fault response capability of the system.

[0003] Due to the application scenarios of the modular multilevel converter, the number of sub-modules in the actual engineering is large, the system is large in size and expensive. In the main circuit topology of the system, the selection of sub-module capacitors, bridge arm inductors and power devices is an important part of engineering design. The selection of parameters is related to the system performance and economic effect. The capacitance of the sub-module capacitor and the price present a nonlinear factor. Excessive capacitance value will cause the cost of the system to rise, and the appropriate capacitance value needs to be selected in combination with the actual working condition demand. The bridge arm inductor is also an important part of the modular multilevel converter, which is related to the system bridge arm circulating current suppression, the power transmission range between systems, the fault current suppression when the system fails and other aspects, and is coupled with the sub-module capacitor, which may cause the system to resonate and affect the normal operation of the system. The parameter selection of the power device is also related to the normal operation of the system. Selecting a power device with a large margin can certainly ensure that the device is not burned out, but it will bring a large economic loss and unnecessary on-state loss, which will affect the steady-state operation performance of the system. The actual working condition needs to be selected to ensure that there is no large margin and the normal operation of the system can be ensured. SUMMARY

[0004] The problem to be solved by the present application is to realize the reasonable design of the main parameters of the modular multilevel converter, and a multi-parameter collaborative optimization design method of a modular multilevel converter is provided.

[0005] To achieve the above purpose, the technical scheme of the present application is as follows:

[0006] A multi-parameter collaborative optimization design method of a modular multilevel converter, comprising the following steps:

[0007] S1. determining a system voltage level, a power operating range of a modular multilevel converter to be designed, a number of bridge arm sub-modules, and a rated voltage of the bridge arm sub-modules;

[0008] S2. constructing a mathematical model of the modular multilevel converter;

[0009] S3. based on the mathematical model of the modular multilevel converter obtained in step S2, designing a capacitor voltage fluctuation model, a sub-module capacitor capacitance model, and a fluctuation amount model, analyzing the relationship between the capacitor voltage fluctuation and the sub-module capacitor capacitance and the operating condition, and obtaining a designed sub-module capacitor capacitance;

[0010] S4. determining an upper limit value of the bridge arm inductance based on the consideration of the system power operating range, determining a design initial value of the bridge arm inductance based on the consideration of the suppression of circulating current, determining a lower limit value of the bridge arm inductance based on the consideration of the avoidance of resonance and the limitation of the fault current rise rate, and obtaining a design range of the bridge arm inductance;

[0011] S5. judging the design initial value of the bridge arm inductance obtained in step S6 by using the design range of the bridge arm inductance obtained in step S6, verifying whether the design initial value of the bridge arm inductance meets the design range of the bridge arm inductance, changing the design amount in the design initial value of the bridge arm inductance to adjust the bridge arm inductance if it does not meet the design range of the bridge arm inductance, and obtaining a design value of the designed bridge arm inductance;

[0012] S6. according to the sub-module capacitor capacitance obtained in step S3 and the design value of the designed bridge arm inductance obtained in step S5, analyzing the peak voltage and the peak current of the power device IGBT, and determining the parameter selection of the power device IGBT.

[0013] Further, the specific implementation method of step S2 includes the following steps:

[0014] S2.1. based on the Kirchhoff voltage law, performing mathematical modeling on the modular multilevel converter for the j-phase circuit in the modular multilevel converter topology;

[0015] S2.2. defining the j-phase bridge arm circulating current i cirj ;

[0016] S2.3. defining the modulation ratio M of the modular multilevel converter as

[0017] S2.4. defining the upper bridge arm modulation function F p , the lower bridge arm modulation function F l of the modular multilevel converter;

[0018] S2.5. assuming that the modular multilevel converter does not have power loss, obtaining the relationship between the direct current I dc and the alternating current.

[0019] Further, the specific implementation method of step S3 includes the following steps:

[0020] S3.1. Taking U pj and U lj as the analysis objects, first assume that only DC component and fundamental frequency AC component are contained in the upper and lower bridge arm currents, and obtain the expression as follows:

[0021] (6)

[0022] Then, according to the working principle of the sub-module and the capacitance characteristic equation, obtain:

[0023]

[0024] Where, i SMjp represents the current of the j-phase upper bridge arm sub-module capacitor, U SMjp represents the voltage of the j-phase upper bridge arm sub-module capacitor; i SMjl represents the current of the j-phase lower bridge arm sub-module capacitor, U SMjl represents the voltage of the j-phase lower bridge arm sub-module capacitor, and C SM represents the capacitance value of the sub-module capacitor.

[0025] S3.2. Integrate the formula (5) obtained in step S2, and then obtain the capacitor voltage fluctuation model, and the specific expression of the sub-module capacitor voltage is as follows:

[0026] (8)

[0027] Where, U cref is the DC component of the sub-module capacitor voltage, ΔU c1 , ΔU c2 and φ1 represent the amplitude of the fundamental frequency fluctuation component of the sub-module capacitor voltage, the amplitude of the twice frequency fluctuation component of the sub-module capacitor voltage, and the phase angle difference between the sub-module capacitor voltage and the reference signal, respectively.

[0028] S3.3. Design the sub-module capacitor capacitance model and the sub-module capacitor fluctuation model.

[0029] First, the average current flowing through the sub-module capacitor is obtained, formula (5) is substituted into formula (7) to eliminate I vj , and the average current expression flowing through the sub-module capacitor is obtained as follows:

[0030]

[0031] Taking the average current flowing through the submodule capacitor as the analysis target, it is solved that the first zero crossing point Z1 and the second zero crossing point Z2 are -φ+arccos(-Mcos(φ) / 2) and 2p-φ+arccos(-Mcos(φ) / 2) respectively;

[0032] Then the average current of the submodule is integrated between the first zero crossing point Z1 and the second zero crossing point Z2, and the maximum fluctuation peak-to-peak value of the capacitor voltage of the submodule is obtained as:

[0033]

[0034] In order to facilitate the analysis of the offset of the capacitor voltage of the submodule relative to the rated value, the fluctuation rate ε of the capacitor voltage of the submodule is defined as: SM The ratio of the maximum fluctuation amplitude to the direct current U cref , and the following relationship is obtained:

[0035]

[0036] The capacitor value C SM of the submodule and the expression of the fluctuation rate are obtained by simultaneously solving equation (10) and equation (11) as:

[0037] (12)

[0038] S3.4. Based on the existence of the double-frequency circulating current in the bridge arm of the MMC, the relationship between the bridge arm voltage and the capacitor voltage of the submodule is obtained by combining equation (4) and equation (8):

[0039]

[0040] It is analyzed from equation (13) that there is a double-frequency component in the phase voltage of the modular multilevel converter, which will generate a double-frequency circulating current acting on the bridge arm inductor. After the correction of equation (6), the double-frequency component is added to obtain the corrected average current component of the submodule capacitor, and the expression for accurately estimating the capacitor current of the submodule is obtained as:

[0041]

[0042] Where, I nf represents the high-order harmonic component, and only the second-order harmonic component is considered, and the high-order harmonic component is ignored;

[0043] Since the circulating current suppression controller is added in the control link of the modular multilevel converter in actual operation, the double-frequency circulating current and higher circulating current are suppressed, so the parameters are designed based on equation (12) during parameter design, which meets the actual requirements.

[0044] Further, the specific implementation method of step S4 includes the following steps:

[0045] S4.1. Considering the modular multilevel converter grid-connected voltage vector, setting u Leq , ivjd, ivjq are the voltage, the d-axis component, the q-axis component of the output current i eq on the equivalent inductance L vj , respectively;

[0046] In order to ensure that the modular multilevel converter operates in the four-quadrant of the system, the vector formed by the equivalent inductance is selected with o point as the center, then there is the following quantity relationship:

[0047]

[0048] Where, P, Q are the active power and reactive power output, respectively;

[0049] Assuming that the transformer leakage reactance, filter inductance, and line stray inductance are L j , then the expression of the design upper limit of the initial value L0 of the bridge arm inductance that satisfies the four-quadrant operation of the modular multilevel converter is obtained from equation (15):

[0050]

[0051] S4.2. Based on the fact that there is a double-frequency component in the phase unit of the modular multilevel converter, the current through the bridge arm inductance L0 is the bridge arm current, and the relationship between L0 and the amplitude I 2f of the double-frequency component is solved from the perspective of energy, taking the j-phase bridge arm as the analysis object, the energy stored is:

[0052]

[0053] Considering the double-frequency component, the average capacitor voltage of the sub-module is written as:

[0054]

[0055] The energy stored in the bridge arm is written from the perspective of the sub-module:

[0056]

[0057] S4.3. Comparing equation (17) and equation (19), both of which represent the energy stored in the bridge arm, so the coefficients before the double-frequency component should be the same, then we get:

[0058]

[0059] The expression of the amplitude I of the double-frequency component is obtained according to formula (20), and the design initial value L0 of the bridge arm inductance is represented as: 2f

[0060]

[0061] S4.4. Based on the fact that each phase bridge arm of the modular multilevel converter contains 2N sub-modules, according to the principle of constant capacitor energy storage, 2N capacitors are replaced by one capacitor, and the following formula is obtained:

[0062] (22)

[0063] The equivalent inductance of the bridge arm is 2L0, and the series resonant angular frequency ω1 of the phase unit of the modular multilevel converter is calculated as:

[0064]

[0065] S4.5. Since the value range of M is 0 to 1, the double-frequency resonant angular frequency ω cir is defined as:

[0066] (24)

[0067] S4.6. Based on formula (23), the resonant angular frequency ω1 of the phase unit is adjusted to be far away from the double-frequency resonance, and ω1 less than 1.55ω is selected, and the first lower limit of the design of L0 is obtained as:

[0068]

[0069] Considering that the transient component of the short-circuit current dominates at the moment of short-circuit fault, the transient current rise rate of the bridge arm current is defined as a, and the second lower limit of the design of L0 is obtained as:

[0070] (26).

[0071] Further, when the design initial value of the bridge arm inductance does not satisfy the design range of the bridge arm inductance, the method for changing the design amount in the design initial value of the bridge arm inductance to adjust the bridge arm inductance in step S5 is to adjust the amplitude of the double-frequency component in the bridge arm current.

[0072] ​Further, the specific implementation method of step S6 is that the peak value of the current flowing through the power device IGBT is the peak value of the bridge arm current, the maximum peak voltage of the power device is the peak-to-peak value of the sub-module capacitor voltage, and the selection principle of the power device IGBT is determined by formula (10) and formula (14):

[0073]

[0074] Wherein, i IGBTmax , U IGBTmax are the maximum peak current and the maximum peak voltage of the power device IGBT.

[0075] The beneficial effects of the application are:

[0076] The multi-parameter collaborative optimization design method of the modular multilevel converter disclosed by the application combines the multi-objective parameter design of the sub-module capacitor fluctuation value, the sub-module capacitor capacity value, the bridge arm inductance value, the power device withstand voltage and the peak current value. The application first analyzes the relationship between the sub-module capacitor capacity value, the fluctuation value and the actual operation condition of the modular multilevel converter. Then, the selection of the bridge arm inductance is limited from three aspects of the system power operation range, the suppression of circulating current and the avoidance of resonance and the limitation of fault current rise rate, and the selection principle of the bridge arm inductance is given. And based on this, the selection parameter principle of the power device switch is analyzed. The design method can select the key parameters of the system according to the actual operation condition of the modular multilevel converter, and complete the system design. BRIEF DESCRIPTION OF DRAWINGS

[0077] Figure 1 The flow chart of the multi-parameter collaborative optimization design method of the modular multilevel converter disclosed by the application;

[0078] Figure 2 The main topology circuit and the sub-module topology diagram of the modular multilevel converter of the application;

[0079] Figure 3 The working process diagram of the upper bridge arm sub-module of the modular multilevel converter of the application;

[0080] Figure 4 The sub-module capacitor voltage fluctuation analysis diagram of the modular multilevel converter of the application;

[0081] Figure 5 The grid-connected voltage vector analysis diagram of the modular multilevel converter of the application;

[0082] Figure 6 The sub-module capacitor voltage waveform diagram of the modular multilevel converter designed by the application;

[0083] Figure 7The current waveform diagram of the modular multilevel converter bridge arm designed for this invention;

[0084] Figure 8 Waveform diagram of the power device of the modular multilevel converter designed for this invention. Detailed Implementation

[0085] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention; that is, the described specific embodiments are merely a part of the embodiments of the invention, and not all of them. The components of the specific embodiments of the invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations, and the invention may also have other embodiments.

[0086] Therefore, the following detailed description of specific embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected specific embodiments of the invention. All other specific embodiments obtained by those skilled in the art based on these specific embodiments without inventive effort are within the scope of protection of this invention.

[0087] To further understand the invention's content, features, and effects, the following specific embodiments are provided, along with accompanying drawings. Figure 1 -Appendix Figure 8 Detailed explanation is as follows:

[0088] Example 1:

[0089] A multi-parameter collaborative optimization design method for a modular multilevel converter includes the following steps:

[0090] S1. Determine the system voltage level and power operating range of the modular multilevel converter to be designed, and design the number of bridge arm sub-modules and the rated voltage of the bridge arm sub-modules;

[0091] Furthermore, such as Figure 2 The diagram shows the main topology of a modular multilevel converter. U dc It is the DC side voltage, U a U b U care three-phase grid side voltages. The main topology consists of three-phase six bridge arms, each phase contains two bridge arms, the upper bridge arm is analyzed as an example. The upper bridge arm contains N sub-modules and bridge arm inductance L0, R0 represents the parasitic resistance on the line and inductance. The sub-module is usually a half-bridge sub-module or a full-bridge sub-module, and the present application takes the half-bridge sub-module topology as an example, which contains two power switches IGBT and a supporting capacitor. The design of the main circuit topology parameters of the MMC needs to consider the power device IGBT parameters, the sub-module capacitance and rated voltage, the capacitor voltage fluctuation rate limit, the bridge arm inductance and the peak value of circulating current, etc. Multi-objective function, need to combine with the operation condition to design, and consider the economic factor.

[0092] S2. Constructing a mathematical model of the modular multilevel converter;

[0093] Further, the specific implementation method of step S2 includes the following steps:

[0094] S2.1. Based on Kirchhoff's voltage law, the j-phase circuit in the modular multilevel converter topology is modeled to obtain:

[0095] (1)

[0096] Where, U dc is the DC side voltage of the modular multilevel converter, U pj is the upper bridge arm voltage of the j-phase, U lj is the lower bridge arm voltage of the j-phase, U jp is the sum of the N sub-module voltages of the upper bridge arm of the j-phase, U jl is the sum of the N sub-module voltages of the lower bridge arm of the j-phase, i jp is the upper bridge arm current of the j-phase, i jl is the lower bridge arm current of the j-phase, R0 is the bridge arm parasitic resistance, L0 is the bridge arm inductance, L j is the j-phase AC side inductance, i vj is the j-phase AC side current, u j is the j-phase AC side voltage;

[0097] S2.2. Define the j-phase bridge arm circulating current i cirj as follows:

[0098]

[0099] S2.3. Define the modulation ratio M of the modular multilevel converter as:

[0100]

[0101] Where, Usj For the AC measurement output voltage vector of the modular multilevel converter

[0102] S2.4. Defining the upper arm modulation function F of the modular multilevel converter p , the lower arm modulation function F l respectively as:

[0103]

[0104] Where ω represents the fundamental angular frequency of the output AC power.

[0105] S2.5. Assuming that the modular multilevel converter has no power loss, the relationship between the DC side current I dc and the AC side current is obtained as:

[0106]

[0107] Where I vj is the amplitude of i vj , and φ is the phase angle difference between u j and i vj .

[0108] S3. Based on the mathematical model of the modular multilevel converter obtained in step S2, the capacitor voltage fluctuation model, the sub-module capacitor capacitance model and the fluctuation amount model are designed, the relationship between the capacitor voltage fluctuation and the sub-module capacitor capacitance and the operating condition is analyzed, and the designed sub-module capacitor capacitance is obtained.

[0109] In order to analyze the specific formula of the sub-module capacitor voltage, taking the half-bridge sub-module as an example, the specific working process detail diagram is shown in Figure 3 . Wherein S1, S2, C SM represent the upper power switch tube, the lower power switch tube and the sub-module capacitor of the half-bridge sub-module respectively.

[0110] Further, the specific implementation method of step S3 includes the following steps:

[0111] S3.1. Taking U pj and U lj as the analysis object, first assuming that the upper and lower arm currents only contain DC components and fundamental AC components, the expression is obtained as:

[0112]

[0113] Then according to the working principle of the sub-module and the capacitor characteristic equation, the following is obtained:

[0114]

[0115] wherein, i SMjp represents the current of the j-phase upper bridge arm submodule capacitor, U SMjp represents the voltage of the j-phase upper bridge arm submodule capacitor; i SMjl represents the current of the j-phase lower bridge arm submodule capacitor, U SMjl represents the voltage of the j-phase lower bridge arm submodule capacitor, C SM represents the submodule capacitor capacitance value;

[0116] S3.2. Integrating the formula (5) obtained in step S2, the capacitor voltage fluctuation model is obtained, and the specific expression of the submodule capacitor voltage is:

[0117]

[0118] wherein, U cref is the DC component of the submodule capacitor voltage, ΔU c1 , ΔU c2 , and φ1 represent the amplitude of the fundamental frequency fluctuation component of the submodule capacitor voltage, the amplitude of the twice frequency fluctuation component of the submodule capacitor voltage, and the phase angle difference between the submodule capacitor voltage and the reference signal, respectively. From the above formula, it can be seen that the fundamental frequency fluctuation in the upper and lower bridge arm submodule capacitor voltages is equal in size and opposite in phase. And the twice frequency fluctuation is the same in size and phase.

[0119] The selection of the submodule capacitor voltage involves two key parameters. One is the withstand voltage value. In the case of determining the system transmission voltage level, the rated voltage level of each submodule is determined. A rough voltage level is selected based on this, and then the voltage fluctuation component under normal working conditions is considered, and a certain margin is left to determine the withstand voltage of the submodule. The other is the capacity. The capacity of the submodule capacitor is to ensure that the fluctuation of the capacitor voltage under normal working conditions is within the acceptable and reasonable range of the system. However, too large capacity will lead to economic loss, which needs to be weighed. The specific design of the submodule capacitor can be carried out by the following method.

[0120] S3.3. Designing the submodule capacitor capacitance model and the submodule capacitor fluctuation model;

[0121] First, the average current flowing through the submodule capacitor is obtained, and formula (5) is substituted into formula (7) to eliminate I vj to obtain the average current expression flowing through the submodule capacitor:

[0122]

[0123] Taking the average current flowing through the capacitor of the sub-module as the analysis target, the first zero-crossing point Z1 and the second zero-crossing point Z2 are solved as -φ+arccos(-Mcos(φ) / 2) and 2p-φ+arccos(-Mcos(φ) / 2) respectively;

[0124] Formula (8) is a specific expression of the capacitor voltage of the sub-module, but it is difficult to directly obtain the ripple value of the capacitor voltage of the sub-module from the formula. According to the capacitor characteristic equation, the zero-crossing points of the average current are found, and the peak-to-peak value of the capacitor voltage fluctuation of the sub-module can be obtained by integrating formula (7) between the two zero-crossing points. It can be obtained by analyzing formula (7) that, since the modulation ratio M is in the range of 0 and 1, the zero-crossing points of formula (7) are generated by the latter half. Taking the average current formula of the capacitor voltage of the upper bridge arm sub-module as the analysis target, two zero-crossing points Z1 and Z2 can be solved. The fluctuation analysis diagram of the capacitor voltage of the sub-module is shown in FIG. 2. Figure 4

[0125] Then, the average current of the sub-module is integrated between the first zero-crossing point Z1 and the second zero-crossing point Z2, and the maximum fluctuation peak-to-peak value of the capacitor voltage of the sub-module is obtained as:

[0126]

[0127] In order to facilitate the analysis of the offset of the capacitor voltage of the sub-module relative to the rated value, the fluctuation rate ε of the capacitor voltage of the sub-module is defined as the ratio of the maximum fluctuation amplitude to the direct current U SM cref , and the following relationship is obtained:

[0128]

[0129] By combining formula (10) and formula (11), the expression of the capacitor value C SM and the fluctuation rate of the sub-module are obtained as:

[0130]

[0131] Using the above two formulas, the selection of the capacitor value of the sub-module can be determined. The basic goal of selecting the capacitor value of the sub-module is to suppress the fluctuation of the capacitor voltage of the sub-module. In an ideal case, the capacitor voltage of the sub-module should be constant, but in the case of a finite value, the capacitor voltage will fluctuate. And considering the economic factor, the capacitor value and the price present a nonlinear relationship, and the growth of the capacitor value will bring a very high price increase. Therefore, it is necessary to select the smallest capacitor value to meet the capacitor voltage fluctuation rate ε SM ​​The requirement of the system. And from equation (12), it can be seen that when the power factor of the converter is 0, the fluctuation of the capacitor voltage of the sub-module is the largest, at this time, φ is 90 degrees, and the system works in the pure reactive power compensation mode. The sub-module capacitor selected under the working condition of the largest fluctuation can meet the operation requirements of the system, and can also meet the operation requirements under other working conditions.

[0132] S3.4. Based on the existence of the double-frequency circulating current in the bridge arm of the MMC, the relationship between the bridge arm voltage and the sub-module capacitor voltage is obtained by combining equation (4) and equation (8):

[0133]

[0134] From equation (13), it can be seen that there is a double-frequency component in the phase voltage of the modular multilevel converter, which will generate a double-frequency circulating current acting on the bridge arm inductance. After modifying equation (6) by adding the double-frequency component, the modified average current component of the sub-module capacitor is obtained, and the expression for accurately estimating the sub-module capacitor current is:

[0135]

[0136] where I nf represents the high-order harmonic component, generally only the second-order harmonic component is considered, and the high-order harmonic component is ignored;

[0137] Because in the control link of the actual operation of the modular multilevel, a circulating current suppression controller is added to suppress the double-frequency circulating current and higher circulating current, so the parameters are designed based on equation (12) to meet the actual requirements.

[0138] S4. Based on considering the power operating range of the system to determine the upper limit value of the bridge arm inductance, based on considering the suppression of circulating current to determine the initial value of the design of the bridge arm inductance, based on considering avoiding resonance and limiting the rising rate of fault current to determine the lower limit value of the bridge arm inductance, the design range of the bridge arm inductance is obtained;

[0139] The selection of the bridge arm inductance L0 involves the following three points. First, the bridge arm inductance, as a device for power transmission between the modular multilevel converter and the external AC system, can adjust the power transmission and suppress the fluctuation of the output current; second, the bridge arm inductance, as a circulating path, can be used to suppress circulating current, but the coupling relationship between the bridge arm inductance and the sub-module capacitor needs to be avoided to avoid resonance; third, when a DC side fault or an internal fault of the converter occurs, the rapid rise of the fault current will cause the burning of the converter, and the bridge arm inductance can limit the rising rate of the fault current to a certain extent. Figure 5 The voltage vector relationship diagram of the grid-connected side of the modular multilevel converter.

[0140] Further, the specific implementation method of step S4 includes the following steps:

[0141] S4.1. Considering the modular multilevel converter grid-connected voltage vector, setting u Leq , ivjd, ivjq are the voltage, the d-axis component, the q-axis component of the output current i eq on the equivalent inductance L vj , respectively;

[0142] In order to ensure that the modular multilevel converter operates in the four-quadrant of the system, the vector formed by the equivalent inductance is selected with o point as the center, then there is the following quantity relationship:

[0143]

[0144] Wherein, P, Q are the active power and reactive power output, respectively;

[0145] Assuming that the transformer leakage reactance, filter inductance, and line stray inductance are L j , then the expression of the design upper limit of the initial value L0 of the bridge arm inductance L0 that satisfies the four-quadrant operation of the modular multilevel converter is obtained from equation (15):

[0146]

[0147] S4.2. Based on the fact that there is a double-frequency component in the phase unit of the modular multilevel converter, the current through the bridge arm inductance L0 is the bridge arm current, and the relationship between L0 and the amplitude I 2f of the double-frequency component is solved from the perspective of energy, taking the j-phase bridge arm as the analysis object, and the energy stored is:

[0148]

[0149] Considering the double-frequency component, the average capacitor voltage of the sub-module is written as:

[0150]

[0151] The energy stored in the bridge arm is written from the perspective of the sub-module:

[0152]

[0153] S4.3. Comparing equation (17) and equation (19), both of which represent the energy stored in the bridge arm, so the coefficients before the double-frequency component should be the same, then we get:

[0154]

[0155] The expression of the amplitude I of the double frequency component is obtained according to formula (20), and the design initial value L0 of the bridge arm inductance is represented as: 2f

[0156]

[0157] S4.4. Based on the fact that each phase bridge arm of the modular multilevel converter contains 2N sub-modules, according to the principle of constant capacitor energy storage, 2N capacitors are replaced by one capacitor, and the following formula is obtained:

[0158]

[0159] The equivalent inductance of the bridge arm is 2L0, and the series resonance angular frequency ω1 of the phase unit of the modular multilevel converter is calculated as:

[0160]

[0161] S4.5. Since the value range of M is 0 to 1, the double frequency resonance angular frequency ω cir is defined as:

[0162]

[0163] S4.6. Based on formula (23), the resonance angular frequency ω1 of the phase unit is adjusted to be far away from the double frequency resonance, and ω1 less than 1.55ω is selected, and the first lower limit of the design of L0 is obtained as:

[0164]

[0165] Considering that the transient component of the short-circuit current dominates at the moment of short-circuit fault, the transient current rise rate of the bridge arm current is defined as a, and the second lower limit of the design of L0 is obtained as:

[0166] .

[0167] S5. The design range of the bridge arm inductance obtained by step S6 is used to judge the design initial value of the obtained bridge arm inductance, and whether the design initial value of the bridge arm inductance meets the design range of the bridge arm inductance is verified. If it does not meet, the design amount in the design initial value of the bridge arm inductance is changed to adjust the bridge arm inductance, and the design value of the designed bridge arm inductance is obtained;

[0168] ​Further, the step S5 is to change the design amount in the design initial value of the bridge arm inductor to adjust the bridge arm inductor when the design initial value of the bridge arm inductor does not meet the design range of the bridge arm inductor, and the method is to adjust the amplitude of the twice frequency component in the bridge arm current.

[0169] S6. According to the sub-module capacitance value obtained in the step S3 and the design value of the designed bridge arm inductor obtained in the step S5, the peak voltage and the peak current of the power device IGBT are analyzed to determine the parameter selection of the power device IGBT.

[0170] Further, the specific implementation method of the step S6 is to consider that the current peak value flowing through the power device IGBT is the peak value of the bridge arm current, and the maximum peak voltage of the power device is the peak-to-peak value of the sub-module capacitor voltage, and the selection principle of the power device IGBT is determined by the formula (10) and the formula (14) to obtain the selection principle of the power device IGBT:

[0171]

[0172] Wherein, i IGBTmax , U IGBTmax are the maximum peak current and the maximum peak voltage of the power device IGBT.

[0173] It should be noted that the terms such as "first" and "second" and the like are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between the entities or operations. Moreover, the terms "include", "contain" or any other variants thereof are intended to cover non-exclusive inclusion, so that the process, method, article or device including a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such process, method, article or device. Without more limitations, the element defined by the statement "including a" does not exclude the presence of another identical element in the process, method, article or device including the element.

[0174] Although the present application has been described above with reference to specific embodiments, various modifications can be made thereto and equivalents can be substituted for elements thereof without departing from the scope of the present application. In particular, each feature disclosed in the specific embodiments of the present application can be combined with any other feature disclosed in the present specification, unless there is structural conflict. The combinations of these features are not exhaustively described in the present specification, which is only for the purpose of omitting the length and saving resources. Therefore, the present application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A method for multi-parameter collaborative optimization design of a modular multilevel converter, characterized in that, The method comprises the following steps: S1. determining the system voltage level and power operating range of the modular multilevel converter to be designed, designing the number of bridge arm sub-modules and the rated voltage of the bridge arm sub-modules; S2. constructing a mathematical model of the modular multilevel converter; S3. based on the mathematical model of the modular multilevel converter obtained in step S2, designing a capacitor voltage fluctuation model, a sub-module capacitor capacitance model and a fluctuation model, analyzing the relationship between the capacitor voltage fluctuation and the sub-module capacitor capacitance and the operating condition, and obtaining the designed sub-module capacitor capacitance; S4. determining the upper limit value of the bridge arm inductance based on the consideration of the system power operating range, determining the design initial value of the bridge arm inductance based on the consideration of suppressing circulating current, and determining the lower limit value of the bridge arm inductance based on the consideration of avoiding resonance and limiting the fault current rise rate, to obtain the design range of the bridge arm inductance; S5. judging the design initial value of the bridge arm inductance obtained in step S6 by using the design range of the bridge arm inductance, verifying whether the design initial value of the bridge arm inductance meets the design range of the bridge arm inductance, changing the design amount in the design initial value of the bridge arm inductance to adjust the bridge arm inductance, and obtaining the design value of the designed bridge arm inductance; S6. according to the sub-module capacitor capacitance obtained in step S3 and the design value of the designed bridge arm inductance obtained in step S5, analyzing the peak voltage and peak current of the power device IGBT, and determining the parameter selection of the power device IGBT.

2. The multi-parameter collaborative optimization design method of a modular multilevel converter according to claim 1, characterized in that, The specific implementation method of step S2 comprises the following steps: S2.

1. based on Kirchhoff's voltage law, a mathematical model of the modular multilevel converter is constructed for the j-phase circuit in the modular multilevel converter topology, and the following formula is obtained: （1）； wherein U dc is the DC side voltage of the modular multilevel converter, U pj is the upper bridge arm voltage of phase j, U lj is the lower bridge arm voltage of phase j, U jp is the sum of the N sub-module voltages of the upper bridge arm of phase j, U jl is the sum of the N sub-module voltages of the lower bridge arm of phase j, i jp is the upper bridge arm current of phase j, i jl is the lower bridge arm current of phase j, R0 is the bridge arm parasitic resistance, L0 is the bridge arm inductance, L j is the AC side inductance of phase j, i vj is the AC side current of phase j, u j is the AC side voltage of phase j; S2.

2. Defining the j-phase bridge leg circulating current i cirj is: ； S2.

3. the modulation ratio M of the modular multilevel converter is defined as: ； wherein U sj is the AC measurement output voltage vector of the modular multilevel converter; S2.

4. Defining the upper bridge arm modulation function F of the modular multilevel converter p , the lower bridge arm modulation function F l respectively: ； wherein ω represents the fundamental angular frequency of the output alternating current; S2.

5. Assuming that there is no power loss in the modular multilevel converter, the relationship between the DC-side current I dc and the AC-side current is given by ； where I is the magnitude of i vj u vj and φ is the phase angle difference between i j and i vj .

3. The multi-parameter collaborative optimization design method of a modular multilevel converter according to claim 1 or 2, characterized in that, The specific implementation method of step S3 comprises the following steps: S3.

1. With U pj and U lj As the analysis object, first assume that only DC component and fundamental frequency AC component are contained in the upper and lower bridge arm currents, and the expression is obtained as follows: ； Then, according to the working principle of the sub-module and the capacitor characteristic equation, the following formula is obtained: ； wherein, i SMjp represents the current of the jth phase upper bridge arm submodule capacitor, U SMjp represents the voltage of the jth phase upper bridge arm submodule capacitor; i SMjl represents the current of the jth phase lower bridge arm submodule capacitor, U SMjl represents the voltage of the jth phase lower bridge arm submodule capacitor, C SM represents the submodule capacitor capacitance value; S3.

2. integrating formula (5) obtained in step S2, the capacitor voltage fluctuation model and the specific expression of the sub-module capacitor voltage are obtained: ； Wherein, U cref is the DC component of the submodule capacitor voltage, ΔU c1 , ΔU c2 , φ1 represent the amplitude of the fundamental frequency fluctuation component of the submodule capacitor voltage, the amplitude of the twice frequency fluctuation component of the submodule capacitor voltage, and the phase angle difference between the submodule capacitor voltage and the reference signal, respectively. S3.

3. designing the sub-module capacitor capacitance model and the sub-module capacitor fluctuation model; First, the average current through the sub-module capacitor is obtained, and formula (5) is substituted into formula (7) to eliminate I vj The average current expression through the sub-module capacitor is obtained: ； Taking the average current flowing through the sub-module capacitor as the analysis target, the first zero crossing point Z1 and the second zero crossing point Z2 are-φ+arccos(-Mcos(φ) / 2) and 2p-φ+arccos(-Mcos(φ) / 2) respectively; Then, the average current of the sub-module is integrated between the first zero crossing point Z1 and the second zero crossing point Z2, and the maximum peak-to-peak value of the sub-module capacitor voltage is obtained as: ； In order to facilitate the analysis of the sub-module capacitor voltage relative to the rated value of the offset, the fluctuation rate ε of the sub-module capacitor voltage is defined SM The ratio of the maximum fluctuation amplitude to the direct current U cref is, then, the following relationship: ； Substitute equations (10) and (11) to get the sub-module capacitance value C SM And the expression of volatility is: ； S3.

4. based on the existence of the double-frequency circulating current in the bridge arm of the MMC, the relationship between the bridge arm voltage and the sub-module capacitor voltage is obtained by combining formula (4) and formula (8): ； It is analyzed from formula (13) that there is a double-frequency component in the phase voltage of the modular multilevel converter, which acts on the bridge arm inductance to generate a double-frequency circulating current. After the formula of formula (6) is modified and the double-frequency component is added, the modified sub-module capacitor average current component is obtained, and the accurate expression for estimating the sub-module capacitor current is: ； where I nf The high harmonic component is represented by, generally only considering the second harmonic component, ignoring the high harmonic component; due to the actual operation in the modular multilevel control link, the circulating current suppression controller is added, thereby suppressing the double frequency circulating current and higher circulating current, so the parameters are designed based on formula (12) during parameter design, which can meet the actual requirements.

4. The multi-parameter collaborative optimization design method of a modular multilevel converter according to claim 3, characterized in that, The specific implementation method of step S4 comprises the following steps: S4.

1. Considering the modular multilevel converter grid-connected voltage vector, set u Leq , ivjd, ivjq are respectively the voltage, the d-axis component, the q-axis component of the output current i eq on the equivalent inductance L vj ​ In order to ensure the modular multilevel converter in the four quadrant operation of the system, the vector formed by the equivalent inductance is selected with the o point as the center, and the following quantity relationship is obtained: ； Wherein, P and Q are the output active power and reactive power respectively; Assuming that the leakage reactance of transformer, filter inductance and line stray inductance are L j , then the expression of the design upper limit of the initial value L0 of the bridge arm inductance satisfying the four-quadrant operation of the modular multilevel converter is obtained from equation (15) as follows: ； S4.

2. There is a double-frequency component in the phase unit based on the modular multilevel converter, and the current through the bridge arm inductor L0 is the bridge arm current. The relationship between L0 and the amplitude I of the double-frequency component is solved from the perspective of energy, taking the j-phase bridge arm as the analysis object. The energy stored is: 2f The energy stored is: ； Considering the double frequency component, the average capacitor voltage of the sub-module is written as: ； The energy stored in the bridge arm is written from the perspective of the sub-module as: ； S4.

3. Comparing formula (17) and formula (19) represents the energy stored in the bridge arm, so the coefficient before the double frequency component should be the same, and then formula (20) is obtained: ； The amplitude of the second harmonic component I is obtained according to the expression of formula (20) 2f The design initial value L0 of the bridge arm inductance is represented as: ； S4.

4. Based on the fact that each phase bridge arm of the modular multilevel converter contains 2N sub-modules, according to the principle of constant capacitor energy storage, 2N capacitors are replaced by one capacitor, and the following formula is obtained: ； The equivalent inductance of the bridge arm is 2L0, and the resonant angular frequency ω1 of the modular multilevel converter phase unit in series is calculated as: ； S4.

5. Since M has a value ranging from 0 to 1, define the double frequency resonance angular frequency ω cir of: （24）； S4.

6. Based on formula (23), adjust the resonant angular frequency ω1 of the phase unit to be far away from the double frequency resonance, and select ω1 less than 1.55ω, then the first lower limit of L0 design is obtained as: ； Considering that the transient component of the short-circuit current dominates at the moment of short-circuit fault, the transient current rise rate of the bridge arm current is defined as a, and the second lower limit of L0 design is obtained as: 。 5. The multi-parameter collaborative optimization design method of a modular multilevel converter according to claim 4, characterized in that, Step S5 When the initial value of the bridge arm inductance design does not meet the design range of the bridge arm inductance, the method for adjusting the initial value of the bridge arm inductance design includes adjusting the amplitude of the double frequency component in the bridge arm current.

6. The multi-parameter collaborative optimization design method of a modular multilevel converter according to claim 5, characterized in that, The specific implementation method of step S6 is to consider that the peak current flowing through the power device IGBT is the peak value of the bridge arm current, and the maximum peak voltage of the power device is the peak-to-peak value of the sub-module capacitor voltage, and the selection principle of the power device IGBT is determined by formula (10) and formula (14), and the selection principle of the power device IGBT is obtained as: ； where i IGBTmax , U IGBTmax are the maximum peak current and the maximum peak voltage of the power device IGBT, respectively.

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