A low-frequency suppression method for high-voltage and large-capacity disc motor wind power system based on MMC

Abstract

The present invention discloses a method for suppressing low frequency of a high voltage and large capacity disc motor wind power system based on MMC, and relates to the technical field of low frequency suppression of a high voltage and large capacity disc motor wind power system based on MMC, including respectively establishing mathematical models of modular multi-level converters and disc motors; analyzing the suppression of low frequency fluctuations of capacitor voltage in MMC by a double sine high frequency injection method; improving the hybrid injection method based on phase voltage harmonic injection, improving the phase voltage utilization rate through phase voltage harmonic injection, and reducing low frequency fluctuations of bridge arm power. The method of the present invention proposes an improved hybrid injection method based on phase voltage harmonic injection to address the problem of increased bridge arm current amplitude caused by high frequency injection, and reduces the bridge arm circulating current and circulating current peak-to-peak value through phase voltage harmonic injection and improved injected common mode voltage and circulating current waveforms.

Description

Technical Field

[0001] The invention relates to the technical field of disc motors, and in particular to a low-frequency suppression method for a high-voltage and large-capacity disc motor wind power system based on MMC. Background Art

[0002] Modular Multilevel Converter (MMC), compared with traditional multilevel converters, MMC is widely used in modern high-voltage direct current transmission due to its high reliability, high power quality, strong scalability and high voltage level. The application of MMC is not limited to the field of power transmission, but also has great development potential in the fields of offshore wind power grid connection, medium-voltage direct current distribution system and high-power motor drive.

[0003] With the advancement of power electronics technology, medium and high voltage high power motors have demonstrated a series of technical advantages in key areas such as industrial fans, machine tool traction and ship propulsion. Against this background, modular multilevel converters have become the first choice for high power motor drive technology with their innovative modular architecture, unlimited expansion capabilities and low harmonic content, promoting technological progress and application innovation in the entire industry.

[0004] When modular multilevel converters are applied to motor control, there is a problem of excessive capacitor voltage fluctuations during motor startup and low-speed conditions. Excessive capacitor voltage fluctuations can cause overvoltage in the submodule, deteriorate the output voltage waveform, affect the stability of motor control, and endanger the safety of power devices. In severe cases, it can also lead to loss of control of the control system. In summary, the low-frequency fluctuation problem in the MMC-AFPM drive system seriously affects the stability and efficient operation of the system. Therefore, the low-frequency suppression strategy proposed in this paper has great theoretical and practical significance for promoting the application of MMC in the field of high-power motor drives. Summary of the invention

[0005] The purpose of this section is to summarize some aspects of embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the specification abstract and the invention title of this application to avoid blurring the purpose of this section, the specification abstract and the invention title, and such simplifications or omissions cannot be used to limit the scope of the present invention.

[0006] In view of the above problems or problems existing in the prior art, the present invention is proposed.

[0007] Therefore, the technical problem solved by the present invention is: when the motor starts and in the low-speed state, there is a problem of excessive capacitor voltage fluctuation. Excessive capacitor voltage fluctuation will cause sub-module overvoltage, deteriorate the output voltage waveform, affect the stability of motor control, and endanger the safety of power devices. In severe cases, it will also cause the control system to lose control.

[0008] In order to solve the above technical problems, the present invention provides the following technical solutions: a low-frequency suppression method for a high-voltage and large-capacity disc-type motor wind power system based on MMC, which includes establishing mathematical models of a modular multilevel converter and a disc-type motor respectively;

[0009] The double-sine high-frequency injection method is used to suppress the low-frequency fluctuation of capacitor voltage in MMC.

[0010] The hybrid injection method is improved based on phase voltage harmonic injection. The phase voltage utilization rate is improved by phase voltage harmonic injection and the low-frequency fluctuation of bridge arm power is reduced.

[0011] As a preferred solution of the low-frequency suppression method of the high-voltage and large-capacity disc-type motor wind power system based on MMC of the present invention, wherein: the mathematical model of the multi-level converter includes:

[0012] Assume that the phase voltage is expressed as:

[0013]

[0014] Among them, U sm Represents the amplitude of the phase voltage, η sj Represents the reference phase of the AC side phase voltage, j = A, B, C, whose values ​​are 0, -2 / 3π and 2 / 3π respectively, and the angular frequency is ω; m is the voltage modulation ratio, and the specific expression is:

[0015]

[0016] Assume that the phase current is expressed as:

[0017]

[0018] Among them, I sm represents the amplitude of the phase current, Represents the power factor angle on the AC side;

[0019] During the normal operation of the three-phase MMC, current flows directly through the upper and lower bridge arms. This current is called circulating current i cirj According to Kirchhoff's current law, the upper and lower bridge arm currents of phase j are expressed as:

[0020]

[0021]

[0022] Subtract and sum the equations and simplify them again to get the relationship between phase current, bridge arm circulating current and upper and lower bridge arm currents:

[0023]

[0024] According to Kirchhoff's voltage law, the phase voltage, the upper and lower bridge arm voltages, and the upper and lower bridge arm currents satisfy the following relationship:

[0025]

[0026]

[0027] Ignoring the inductance, the mathematical equivalent models of the DC side and AC side of the MMC are:

[0028]

[0029] Among them, the phase voltage is determined by the difference between the upper and lower bridge arm voltages, and the stability of the DC side voltage is determined by the sum of the upper and lower bridge arm voltages, while the upper and lower bridge arm voltages are determined by the corresponding switching function and the submodule capacitor voltage;

[0030] Assume that the switching functions of the N submodules of the upper and lower bridge arms of each phase are S pji , S nji (i=1,2……N), when the switch function is 1, it means the submodule is switched on, and when the switch function is 0, it means the submodule is switched off. The output voltage of the upper and lower bridge arms is:

[0031]

[0032] Substituting the formula into the formula, we can get the formula. It can be seen that in order to maintain the stability of the DC side voltage, the sum of the number of sub-modules put into the upper and lower bridge arms of each phase should be kept constant. While keeping the sum of the number of sub-modules put into operation unchanged, different phase voltages can be output by changing the difference in the number of sub-modules put into operation:

[0033]

[0034] As a preferred solution of the low-frequency suppression method of the high-voltage and large-capacity disc motor wind power system based on MMC of the present invention, wherein: the mathematical model of the disc motor includes:

[0035] The voltage equation in the three-phase stationary coordinate system is shown as follows:

[0036]

[0037] Among them, Ψa, Ψb, Ψc are the three-phase flux, and Rs is the stator resistance;

[0038] The three-phase flux equation is as follows:

[0039]

[0040] Among them, L aa , L bb, L cc is the stator three-phase winding self-inductance, L ab , L bc , L ac is the mutual inductance of the stator three-phase winding, Ψ f is the permanent magnet flux;

[0041] The torque equation is shown as:

[0042] Among them, T e is the electromagnetic torque, p n is the number of motor pole pairs, θ m is the mechanical angle;

[0043] Based on the coordinate transformation theory, the mathematical model of the disk motor in the rotating dq coordinate is obtained through Clark transformation and Park transformation.

[0044] Stator voltage equation:

[0045]

[0046] Among them, u d 、u q ;i d 、i q ; d , q are the components of the stator voltage, stator current and stator flux on the d and q axes respectively, ω is the rotor electrical angular velocity and ω=p n ω m ;ωm is the mechanical angular velocity;

[0047] Magnetic flux equation:

[0048]

[0049] Among them, L d , L q is the equivalent inductance of d and q axes;

[0050] Substituting the formula into, we get:

[0051]

[0052] Torque equation:

[0053]

[0054] Equations of motion:

[0055]

[0056] Among them, T L is the load torque, B m is the damping coefficient, and J is the moment of inertia.

[0057] As a preferred solution of the low-frequency suppression method of the high-voltage and large-capacity disc-type motor wind power system based on MMC of the present invention, the analysis of the double-sine high-frequency injection method to suppress the low-frequency fluctuation of the capacitor voltage in the MMC includes:

[0058] The high-frequency common-mode voltage and high-frequency circulating current expressions are:

[0059] u cm =U cm sin(ω h t)

[0060] i cm =I cm sin(ω h t)

[0061] Among them, U cm Represents the amplitude of the high-frequency common-mode voltage, I cm Represents the amplitude of the high-frequency circulating current, ω h represents the common mode voltage angular frequency, ω h =2πf h , f h Represents the common mode voltage frequency. The power of the upper and lower bridge arms after high frequency injection is expressed as:

[0062]

[0063]

[0064] Let the common mode component of the bridge arm power P j_com , power differential mode component P j_diff for:

[0065]

[0066]

[0067] The formula can be reformulated as:

[0068] p′ pj =p j_com +p j_diff

[0069] p′ pj =p j_com +p j_diff

[0070] The two phases in the common mode component are high frequency, and the resulting capacitor voltage fluctuation is negligible; at this time, only the differential mode component has low frequency, so we only need to consider the influence of the differential mode component on the capacitor voltage fluctuation. The low frequency p in the differential mode component of the bridge arm power is low for:

[0071]

[0072] According to the trigonometric function product and difference formula, we know u cm and i cm The product of appears low-frequency and high-frequency quantities:

[0073]

[0074] Make the power low frequency amount p low and u cm and i cm The low-frequency components in the product of cancel each other out, so that the bridge arm power only contains high-frequency components. Let:

[0075]

[0076] According to the above, the amplitude of the high-frequency circulating current that needs to be injected is:

[0077]

[0078] After the low-frequency components cancel each other out, the differential mode component P j_diff Re-expressed as:

[0079]

[0080] There is no low-frequency component in the bridge arm power, only the angular frequency ω h ,ω h ±ω、ω h ±2ω, 2ω h ±ω is the high-frequency component; since the voltage fluctuation of the submodule capacitor is inversely proportional to the angular frequency, the selected common-mode voltage angular frequency is greater than the fundamental frequency angular frequency. h When it is too large, the circulating current tracking effect will be poor. There will be amplitude and phase errors between the actual circulating current and the high-frequency circulating current given value. These errors will bring new voltage fluctuations of the low-frequency submodule. Therefore, ω h Must meet:

[0081] ω h ≥10*2πf s

[0082] Among them, f s Control frequency for the system;

[0083] The injection of high-frequency circulating current will cause the bridge arm current to increase, so U cm Increase to reduce the injected high-frequency circulating current amplitude, while U cm It is limited by the duty cycle and DC bus voltage. In order to prevent overmodulation, the following requirements must be met:

[0084]

[0085] The coefficient K exists to leave a margin to prevent overmodulation caused by the low-frequency suppression voltage generated by the high-frequency circulating current injection being superimposed on the bridge arm voltage reference value; when U cm When the maximum value is taken, the amplitude of the injected high-frequency circulating current is:

[0086]

[0087] As a preferred solution of the low-frequency suppression method of the high-voltage and large-capacity disc-type motor wind power system based on MMC of the present invention, the mixed injection method includes:

[0088] I cm with U cm Inversely proportional, by increasing U cm The size of the injected circulating current is reduced by adjusting the size of the injected circulating current; injecting high-frequency square wave common-mode voltage and square wave high-frequency circulating current improves the waveform utilization rate. The expressions of injected voltage and circulating current are:

[0089]

[0090]

[0091] Perform Fourier series expansion on the above formula:

[0092]

[0093] According to the Fourier expansion form, the amplitude of the fundamental frequency sine wave in the square wave is 4 times the amplitude of the square wave, which is similar to the double sine wave injection method, making the fundamental frequency component in the upper and lower bridge arm power 0:

[0094]

[0095] The amplitude of the injected square wave high-frequency circulating current is:

[0096]

[0097] Compared with the formula, at the same m and u cm The peak value of the high-frequency circulating current using the double-wave injection method is reduced by 50% compared with the double-sine wave injection method; however, due to the rich harmonics of the high-frequency square wave circulating current, it is necessary to target ω h and 3ω h Two frequency points are tracked. Compared with the double sine injection method, the control bandwidth requirement is higher. At the same time, the voltage change rate is infinite when the square wave circulating current passes through the zero point. Due to the existence of the bridge arm inductance, MMC does not allow the circulating current to change drastically. In view of the difficulty of injecting the above square wave circulating current, a mixed injection method of square wave voltage and sine + third harmonic current is adopted. The injection voltage waveform expression is the same as the formula, and the injection circulating current expression is:

[0098]

[0099] Make the low-frequency components in the upper and lower bridge arm powers 0:

[0100]

[0101] The amplitude of the injected sinusoidal + third harmonic high-frequency circulating current is:

[0102]

[0103] The high-frequency common-mode voltage and high-frequency circulating current expressions of the hybrid injection method are:

[0104]

[0105]

[0106] As a preferred solution of the low-frequency suppression method of the high-voltage and large-capacity disc motor wind power system based on MMC of the present invention, the improvement of the hybrid injection method based on phase voltage harmonic injection includes:

[0107] Obtain three-phase sinusoidal modulation voltage u through vector control sA 、u sB and u sC , select the corresponding interval according to the positive and negative signs of the three-phase voltage, and compare the magnitude of the voltage with the same positive and negative signs in different intervals to obtain the injected voltage u z ;

[0108] Interval 1: If |u sA |≥|u sB |, injection voltage value u z =u sB / 2, if |u sA |<|u sB |,u z =u sA / 2;

[0109] Interval 2: If |u sA |≥|u sC |, injection voltage value u z =u sC / 2, if |u sA | sC |,u z =u sA / 2;

[0110] Interval 3: If |u sB |≥|u sC |, injection voltage value u z =u sC / 2, if |u​sB |<|u sC |,u z =u sB / 2;

[0111] Interval 4: If |u sB |≥|u sA |, injection voltage value u z =u sA / 2, if |u sB |<|u sA |,u z =u sB / 2;

[0112] Interval 5: If |u sC |≥|u sA |, injection voltage value u z =u sA / 2, if |u sC |<|u sA |,u z =u sC / 2;

[0113] Interval 6: If |u sC |≥|u sB |, injection voltage value u z =u sB / 2, if |u sC |<|u sB |,u z =u sC / 2;

[0114] The three-phase voltage after harmonic injection is:

[0115] u sj =u sj +u z

[0116] The Fourier series expansion of the modulated wave after harmonic injection is:

[0117]

[0118] Take ωt as the independent variable and take the derivative as 0 to obtain ωt = 60°. Substituting the solution into the equation, we get:

[0119] u sj =0.866U sm

[0120] It can be seen that the phase voltage amplitude after harmonic injection is 0.87 times the amplitude of its fundamental component, and its fundamental wave utilization rate is higher than that of a sine wave with the same amplitude. At this time, the value range of the high-frequency common-mode voltage is further increased:

[0121]

[0122] At the same time, u sj Substituting the equation into the formula, we get the bridge arm power expression after the third harmonic of the phase voltage is injected:

[0123]

[0124] After the third harmonic of the phase voltage is injected, the low-frequency component of the bridge arm power is obtained:

[0125]

[0126] Make the low-frequency components in the upper and lower bridge arm powers 0:

[0127]

[0128] At this time, the amplitude of the injected high-frequency circulating current is:

[0129]

[0130] The expressions of high-frequency circulating current and high-frequency common-mode voltage that need to be injected by the improved hybrid injection method based on phase voltage harmonic injection are:

[0131]

[0132]

[0133] Beneficial effects of the present invention: The present invention proposes an improved hybrid injection method based on phase voltage harmonic injection to address the problem of increased bridge arm current amplitude caused by high-frequency injection, and reduces the bridge arm circulating current and circulating current peak-to-peak value by injecting phase voltage harmonics and improving the waveforms of injected common-mode voltage and circulating current; finally, a comparative simulation study is conducted on the double sinusoidal injection method, the hybrid injection method and the improved hybrid injection method based on phase voltage harmonic injection in MATLAB / Simulink, and the bridge arm current and circulating current under the three injection methods are compared, verifying the superiority of the low-frequency suppression strategy based on phase voltage harmonic injection in reducing the bridge arm current and circulating current peak-to-peak value. BRIEF DESCRIPTION OF THE DRAWINGS

[0134] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative labor. Among them:

[0135] Figure 1 This is the basic control block diagram of MMC-AFPM.

[0136] Figure 2is the injected voltage waveform.

[0137] Figure 3 This is the FFT analysis diagram of phase voltage after harmonic injection.

[0138] Figure 4 This is the control block diagram of the low-frequency suppression strategy.

[0139] Figure 5 The low-frequency control simulation model diagram is divided into a high-frequency injection module and a phase voltage harmonic injection module.

[0140] Figure 6 The speed waveforms are distinguished between those without high-frequency injection and those with double-sine high-frequency injection.

[0141] Figure 7 The capacitor voltage waveform is distinguished between no high-frequency injection and double-sinusoidal high-frequency injection.

[0142] Figure 8 The bridge arm current waveforms are distinguished between no high-frequency injection and double-sinusoidal high-frequency injection.

[0143] Fig. 9 The circulating current waveforms are distinguished between those without high-frequency injection and those with double-sinusoidal high-frequency injection.

[0144] Fig.10 Capacitor voltage waveforms to distinguish between hybrid injection and improved hybrid injection.

[0145] Fig.11 The mixed injection method is used to distinguish the bridge arm current and the circulating current.

[0146] Fig.12 A diagram showing the high-frequency injection method for distinguishing bridge arm current and circulating current. DETAILED DESCRIPTION

[0147] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the accompanying drawings.

[0148] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0149] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

[0150] Example 1

[0151] Reference Figures 1 to 4 , which is an embodiment of the present invention, provides a low-frequency suppression method for a high-voltage and large-capacity disc-type motor wind power system based on MMC, comprising:

[0152] S1: Establish mathematical models of modular multilevel converter and disc motor respectively.

[0153] Furthermore, the multilevel converter mathematical model includes:

[0154] The three-phase MMC system is in a symmetrical operation state, ignoring the circulating current imbalance caused by three-phase imbalance and ignoring the system impact caused by small fluctuations in DC bus voltage. Assume that the phase voltage is expressed as:

[0155]

[0156] Among them, U sm Represents the amplitude of the phase voltage, η sj Represents the reference phase of the AC side phase voltage, j = A, B, C, whose values ​​are 0, -2 / 3π and 2 / 3π respectively, and the angular frequency is ω; m is the voltage modulation ratio, and the specific expression is:

[0157]

[0158] Assume that the phase current is expressed as:

[0159]

[0160] Among them, I sm represents the amplitude of the phase current, Represents the power factor angle on the AC side;

[0161] Unlike a three-phase bridge inverter, a three-phase MMC has current flowing directly through the upper and lower bridge arms during normal operation. This current is generally referred to as circulating current. cirj According to Kirchhoff's current law, the upper and lower bridge arm currents of phase j are expressed as:

[0162]

[0163]

[0164] Subtract and sum the equations and simplify them again to get the relationship between phase current, bridge arm circulating current and upper and lower bridge arm currents:

[0165]

[0166] According to Kirchhoff's voltage law, the phase voltage, the upper and lower bridge arm voltages, and the upper and lower bridge arm currents satisfy the following relationship:

[0167]

[0168]

[0169] Ignoring the inductance, the mathematical equivalent models of the DC side and AC side of the MMC are:

[0170]

[0171] Among them, the phase voltage is determined by the difference between the upper and lower bridge arm voltages, and the stability of the DC side voltage is determined by the sum of the upper and lower bridge arm voltages, while the upper and lower bridge arm voltages are determined by the corresponding switching function and the submodule capacitor voltage;

[0172] Assume that the switching functions of the N submodules of the upper and lower bridge arms of each phase are S pji , S nji (i=1,2……N), when the switch function is 1, it means the submodule is switched on, and when the switch function is 0, it means the submodule is switched off. The output voltage of the upper and lower bridge arms is:

[0173]

[0174] Substituting the formula into the formula, we can get the formula. It can be seen that in order to maintain the stability of the DC side voltage, the sum of the number of sub-modules put into the upper and lower bridge arms of each phase should be kept constant. While keeping the sum of the number of sub-modules put into operation unchanged, different phase voltages can be output by changing the difference in the number of sub-modules put into operation:

[0175]

[0176] Furthermore, the disc motor mathematical model includes:

[0177] When establishing the mathematical model of the disc motor, it is assumed that the AFPM is an ideal motor with three-phase complete symmetry, and the saturation of the stator core, hysteresis loss, eddy current loss, and the influence of temperature and frequency on the motor parameters are ignored. The voltage equation in the three-phase stationary coordinate system is shown as follows:

[0178]

[0179] Among them, Ψa, Ψb, Ψc are the three-phase flux, and Rs is the stator resistance;

[0180] The three-phase flux equation is as follows:

[0181]

[0182] Among them, L aa, L bb , L cc is the stator three-phase winding self-inductance, L ab , L bc , L ac is the mutual inductance of the stator three-phase winding, Ψ f is the permanent magnet flux;

[0183] The torque equation is shown as:

[0184] Among them, T e is the electromagnetic torque, p n is the number of motor pole pairs, θ m is the mechanical angle;

[0185] Based on the coordinate transformation theory, the mathematical model of the disk motor in the rotating dq coordinate is obtained through Clark transformation and Park transformation.

[0186] Stator voltage equation:

[0187]

[0188] Among them, u d 、u q ;i d 、i q ; d , q are the components of the stator voltage, stator current and stator flux on the d and q axes respectively, ω is the rotor electrical angular velocity and ω=p n ω m ;ωm is the mechanical angular velocity;

[0189] Magnetic flux equation:

[0190]

[0191] Among them, L d , L q is the equivalent inductance of d and q axes;

[0192] Substituting the formula into, we get:

[0193]

[0194] Torque equation:

[0195]

[0196] Equations of motion:

[0197]

[0198] Among them, T L is the load torque, B mis the damping coefficient, and J is the moment of inertia. The above analysis and conversion result in a simplified equivalent model of AFPM, and the motor can be vector controlled according to the above formula.

[0199] S2: Analyze the dual-sine high-frequency injection method to suppress the low-frequency fluctuation of capacitor voltage in MMC.

[0200] Furthermore, the high-frequency injection method injects a high-frequency common-mode voltage u into the system. cm and high frequency circulation i cm To reduce the low-frequency fluctuation of capacitor voltage, this method uses the product and difference characteristics of trigonometric functions to generate low-frequency fluctuations in the bridge arm that are opposite to the original low-frequency fluctuations, and reconstructs the bridge arm power into a high-frequency fluctuation form. Since the capacitor voltage fluctuation caused by the high-frequency component is relatively small and negligible, the purpose of reducing the capacitor voltage fluctuation of the sub-module can be achieved.

[0201] Since the injection of common-mode voltage will not affect the three-phase line voltage on the AC side of the motor, and the circulating current only flows inside the modular multilevel converter and will not affect the current on the output side, the common-mode voltage and circulating current can be used as two independent control degrees of freedom. cm Injected into the AC output side, and at the same time i cm Injecting it into the circulating current of each phase not only maintains the stability of motor operation but also effectively reduces capacitor voltage fluctuations.

[0202] Furthermore, the analysis of the double-sine high-frequency injection method for suppressing low-frequency fluctuations of capacitor voltage in the MMC includes:

[0203] The high-frequency common-mode voltage and high-frequency circulating current expressions are:

[0204] u cm =U cm sin(ω h t)

[0205] i cm =I cm sin(ω h t)

[0206] Among them, U cm Represents the amplitude of the high-frequency common-mode voltage, I cm Represents the amplitude of the high-frequency circulating current, ω h represents the common mode voltage angular frequency, ω h =2πf h , f h Represents the common mode voltage frequency. The power of the upper and lower bridge arms after high frequency injection is expressed as:

[0207]

[0208]

[0209] Let the common mode component of the bridge arm power P j_com , power differential mode component P j_diff for:

[0210]

[0211]

[0212] The formula can be reformulated as:

[0213] p′ pj =p j_com +p j_diff

[0214] p′ pj =p j_com +p j_diff

[0215] The two phases in the common mode component are high frequency, and the resulting capacitor voltage fluctuation is negligible; at this time, only the differential mode component has low frequency, so we only need to consider the influence of the differential mode component on the capacitor voltage fluctuation. The low frequency p in the differential mode component of the bridge arm power is low for:

[0216]

[0217] According to the trigonometric function product and difference formula, we know u cm and i cm The product of appears low-frequency and high-frequency quantities:

[0218]

[0219] Make the power low frequency amount p low and u cm and i cm The low-frequency components in the product of cancel each other out, so that the bridge arm power only contains high-frequency components. Let:

[0220]

[0221] According to the above analysis, the amplitude of the high-frequency circulating current that needs to be injected is:

[0222]

[0223] After the low-frequency components cancel each other out, the differential mode component P j_diff Re-expressed as:

[0224]

[0225] There is no low-frequency component in the bridge arm power, only the angular frequency ω h ,ωh ±ω、ω h ±2ω, 2ω h ±ω is the high-frequency component; since the voltage fluctuation of the submodule capacitor is inversely proportional to the angular frequency, the selected common-mode voltage angular frequency is greater than the fundamental frequency angular frequency. h When it is too large, the circulating current tracking effect will be poor. There will be amplitude and phase errors between the actual circulating current and the high-frequency circulating current given value. These errors will bring new voltage fluctuations of the low-frequency submodule. Therefore, ω h Must meet:

[0226] ω h ≥10*2πf s

[0227] Among them, f s Control frequency for the system;

[0228] The injection of high-frequency circulating current will cause the bridge arm current to increase, so U cm Increase to reduce the injected high-frequency circulating current amplitude, while U cm It is limited by the duty cycle and DC bus voltage. In order to prevent overmodulation, the following requirements must be met:

[0229]

[0230] The coefficient K exists to leave a margin to prevent overmodulation caused by the low-frequency suppression voltage generated by high-frequency circulating current injection superimposed on the bridge arm voltage reference value. Generally, the value range of K is 1-1.2. cm When the maximum value is taken, the amplitude of the injected high-frequency circulating current is:

[0231]

[0232] S3: The hybrid injection method is improved based on phase voltage harmonic injection. The phase voltage utilization rate is improved through phase voltage harmonic injection, and the low-frequency fluctuation of the bridge arm power is reduced.

[0233] Injecting high-frequency circulating current will significantly increase the current stress of the bridge arm device. Therefore, under the premise of effectively suppressing low-frequency fluctuations, studying how to reduce the amplitude of injected high-frequency circulating current has become an important research direction.

[0234] Furthermore, the mixed injection method includes:

[0235] I cm with U cm Inversely proportional, by increasing U cm The size of the injected circulating current is reduced by adjusting the size of the injected circulating current; injecting high-frequency square wave common-mode voltage and square wave high-frequency circulating current improves the waveform utilization rate. The expressions of injected voltage and circulating current are:

[0236]

[0237]

[0238] Perform Fourier series expansion on the above formula:

[0239]

[0240] According to the Fourier expansion form, the amplitude of the fundamental frequency sine wave in the square wave is 4 times the amplitude of the square wave, which is similar to the double sine wave injection method, making the fundamental frequency component in the upper and lower bridge arm power 0:

[0241]

[0242] The amplitude of the injected square wave high-frequency circulating current is:

[0243]

[0244] Compared with the formula, at the same m and u cm The peak value of the high-frequency circulating current using the double-wave injection method is reduced by 50% compared with the double-sine wave injection method; however, due to the rich harmonics of the high-frequency square wave circulating current, it is necessary to target ω h and 3ω h Two frequency points are tracked. Compared with the double sine injection method, the control bandwidth requirement is higher. At the same time, the voltage change rate is infinite when the square wave circulating current passes through the zero point. Due to the existence of the bridge arm inductance, MMC does not allow the circulating current to change drastically. In view of the difficulty of injecting the above square wave circulating current, a mixed injection method of square wave voltage and sine + third harmonic current is adopted. The injection voltage waveform expression is the same as the formula, and the injection circulating current expression is:

[0245]

[0246] Make the low-frequency components in the upper and lower bridge arm powers 0:

[0247]

[0248] The amplitude of the injected sinusoidal + third harmonic high-frequency circulating current is:

[0249]

[0250] The high-frequency common-mode voltage and high-frequency circulating current expressions of the hybrid injection method are:

[0251]

[0252]

[0253] It should be noted that, under the same m, the high-frequency circulating current peak value using the hybrid injection method is reduced by 26% compared with the double sine wave injection method. Compared with the double square wave injection method, although the hybrid injection method solves the problem of square wave circulating current injection, its ability to reduce the circulating current peak value is also weakened.

[0254] Furthermore, on the basis of the hybrid injection method, based on the principle of SVPWM modulation, an improved hybrid injection method of phase voltage harmonic injection is proposed. On the one hand, this method reduces the amplitude of the original sinusoidal modulation wave by improving the phase voltage utilization, thereby leaving a larger margin for the improvement of Ucm; on the other hand, the low-frequency fluctuation of the bridge arm power is reduced by injecting phase voltage harmonics.

[0255] In order to increase the speed of command input and reduce the time of calculation, the table lookup harmonic injection method is adopted. The specific implementation method is as follows:

[0256] The improvement of the hybrid injection method based on phase voltage harmonic injection includes:

[0257] Obtain three-phase sinusoidal modulation voltage u through vector control sA 、u sB and u sC According to the positive and negative signs of the three-phase voltage, select the corresponding interval according to Table 1, and compare the magnitude of the voltage with the same positive and negative sign in different intervals to obtain the injected voltage u z ;

[0258] Table 1 Interval selection table

[0259]

[0260] Interval 1: If |u sA |≥|u sB |, injection voltage value u z =u sB / 2, if |u sA |<|u sB |,u z =u sA / 2;

[0261] Interval 2: If |u sA |≥|u sC |, injection voltage value u z =u sC / 2, if |u sA | sC |,u z =u sA / 2;

[0262] Interval 3: If |u sB |≥|u sC |, injection voltage value u z =u​sC / 2, if |u sB |<|u sC |,u z =u sB / 2;

[0263] Interval 4: If |u sB |≥|u sA |, injection voltage value u z =u sA / 2, if |u sB |<|u sA |,u z =u sB / 2;

[0264] Interval 5: If |u sC |≥|u sA |, injection voltage value u z =u sA / 2, if |u sC |<|u sA |,u z =u sC / 2;

[0265] Interval 6: If |u sC |≥|u sB |, injection voltage value u z =u sB / 2, if |u sC |<|u sB |,u z =u sC / 2;

[0266] The three-phase voltage after harmonic injection is:

[0267] u sj =u sj +u z

[0268] The injected voltage u is calculated according to the table z Waveform Figure 2 As shown, the three-phase voltage waveform u after harmonic injection sj 'FFT analysis shows that u sj 'It is equivalent to superimposing a third harmonic on the original sinusoidal modulation wave. This method is to inject the third harmonic of the same phase at the phase voltage, and the three-phase symmetrical winding of the motor can cancel out the time harmonics of multiples of three. Therefore, the injection of phase voltage harmonics will not affect the line voltage and synthetic magnetomotive force, and the three-phase current of the motor is still a sine wave.

[0269] The Fourier series expansion of the modulated wave after harmonic injection is:

[0270]

[0271] Take ωt as the independent variable and take the derivative as 0 to obtain ωt = 60°. Substituting the solution into the equation, we get:

[0272] u sj =0.866U sm

[0273] It can be seen that the phase voltage amplitude after harmonic injection is 0.87 times the amplitude of its fundamental component, which means that its fundamental wave utilization rate is higher than that of a sine wave with the same amplitude. At this time, the value range of the high-frequency common-mode voltage is further increased:

[0274]

[0275] At the same time, u sj Substituting the equation into the formula, we get the bridge arm power expression after the third harmonic of the phase voltage is injected:

[0276]

[0277] After the third harmonic of the phase voltage is injected, the low-frequency component of the bridge arm power is obtained:

[0278]

[0279] Make the low-frequency components in the upper and lower bridge arm powers 0:

[0280]

[0281] At this time, the amplitude of the injected high-frequency circulating current is:

[0282]

[0283] The expressions of high-frequency circulating current and high-frequency common-mode voltage that need to be injected by the improved hybrid injection method based on phase voltage harmonic injection are:

[0284]

[0285]

[0286] It should be noted that in order to better compare the injected high-frequency circulating current amplitudes under different injection waveforms, the I of the three high-frequency injection methods when m = 0.4 is calculated. cm Theoretical peak value, substitute m=0.4 into the corresponding formula to get the injection circulation peak value at this time:

[0287] I cm =1.67Ki sj [1-0.16sin 2 (ωt+η sj )]

[0288]

[0289]

[0290] Based on the circulating current amplitude of the double sine wave injection method, Table 2 gives the theoretical peak values ​​of the three injection methods. It can be seen from the table that when m=0.4, the peak value of the high-frequency circulating current injected by the hybrid injection method based on phase voltage harmonic injection modulation is 7% lower than that by the hybrid injection method.

[0291] Table 2 Comparison of three injection methods

[0292]

[0293] In summary, since the capacitor voltage fluctuation will decrease as the speed increases, the capacitor voltage fluctuation is very small after the motor speed reaches a certain value, which is acceptable for the normal operation of the motor. If the high-frequency injection method is still used in this case, the modulation ratio limitation will cause the injected high-frequency circulating current amplitude to be very large, which will outweigh the advantages. Therefore, the low-frequency suppression method based on phase voltage harmonic injection is only applicable to low-speed motor conditions. Different modular multilevel converter parameters will correspond to different high and low speed dividing points. Based on the calculation and analysis of the MMC parameters used in this article, the speed of the permanent magnet synchronous motor is divided into a low-speed segment of 0rpm-200rpm and a high-speed segment of 200rpm-900rpm.

[0294] In summary, the low-frequency suppression control block diagram of the motor at low speed is as follows: Figure 4 As shown in the figure, it can be seen that the low-frequency operation control strategy of the permanent magnet synchronous motor drive system based on the modular multilevel converter mainly includes given circulating current tracking, high-frequency common-mode motor injection and phase voltage harmonic injection.

[0295] Example 2

[0296] Reference Figures 1 to 12 An embodiment of the present invention provides a low-frequency suppression method for a high-voltage, large-capacity disc motor wind power system based on MMC. In order to verify the beneficial effects of the present invention, a scientific demonstration is carried out through simulation experiments.

[0297] In order to verify the superiority of the improved hybrid injection method based on phase voltage harmonic injection designed in this paper, a three-phase 5-level MMC-AFPM simulation model is built according to the simulation parameters shown in Table 3, where 5 levels refers to the number of levels of the output phase voltage on the AC side when the MMC modulation ratio is 1.

[0298] Table MMC-AFPM simulation parameters

[0299]

[0300]

[0301] according to Figure 1 and Figure 4 The MMC-AFPM simulation model was built in MATLAB / Simulink. The model mainly includes five parts: vector control module, closed-loop voltage balancing algorithm module, carrier phase shift modulation module, high-frequency injection module and phase voltage harmonic injection module. The low-frequency control system simulation was completed based on the above modules. The high-frequency injection module and phase voltage harmonic injection module are shown in Figure 2. Figure 5 shown.

[0302] In order to verify the effectiveness of the dual-sine high-frequency injection method in reducing capacitor voltage fluctuations, the motor speed was increased from 0 rpm to a given speed of 150 rpm under a load torque of 0.5 N·m. Figure 6 and Figure 7 They are the motor speed and capacitor voltage waveforms respectively. Figure 7 It can be seen that when there is no high-frequency injection, the capacitor voltage fluctuation amplitude is large during the motor startup phase, with a maximum value of 75.5V and a minimum value of 38V. After the motor reaches a stable state, the capacitor voltage fluctuation range is 43V-61.3V. When the double-sine wave high-frequency injection method is used, the maximum value of the capacitor voltage fluctuation amplitude during the motor startup phase is 57.5V, and the minimum value is 43.5V. When the motor is in a steady state, the capacitor voltage fluctuation range is 46.8V-53.8V. It can be seen that the double-sine high-frequency injection method reduces the capacitor voltage fluctuation range by 11.3V in the steady state.

[0303] Figure 8 and Fig. 9 Figure 2 shows the waveforms of the bridge arm current and circulating current when high-frequency injection is not used and when double-sinusoidal high-frequency injection is used. It can be seen from the figure that compared with the case without high-frequency injection, when double-sinusoidal high-frequency injection is used, the peak-to-peak value of the bridge arm current increases from 15.5A to 24A, and the peak-to-peak value of the circulating current increases from 7.3A to 18A during the motor startup phase; in the stable state of the motor, the peak-to-peak value of the bridge arm current increases from 6A to 12.4A, and the peak-to-peak value of the circulating current increases from 2.5A to 8.9A. It can be seen that the waveforms of the bridge arm current and circulating current become more complicated due to the injection of high-frequency circulating current.

[0304] In order to solve the problem of increased bridge arm current and circulating current caused by the injection method, a hybrid injection method and an improved hybrid injection method based on phase voltage harmonic injection are introduced. Fig.10 : The capacitor voltage waveforms when the above two injection methods are used. It can be seen from the figure that the capacitor voltage fluctuation is basically the same as when the double-sine injection method is used, indicating that the introduction of these two improved injection methods will not affect the capacitor voltage fluctuation.

[0305] Fig.11The bridge arm current and circulating current waveforms when the hybrid injection method is used. It can be seen from the figure that due to the improved utilization of common-mode voltage and high-frequency circulating current, the peak-to-peak value of the bridge arm current in steady state is 10.8A, which is 13% lower than that of the double-sine injection method; the peak-to-peak value of the circulating current is 6.9A, which is 22% lower than that of the double-sine injection method.

[0306] Fig.12 The bridge arm current and circulating current waveforms when the improved hybrid injection method based on phase voltage harmonic injection is adopted. This method can further reduce the bridge arm capacitance and the peak-to-peak value of the circulating current by injecting phase voltage harmonics. As shown in the figure, the peak-to-peak value of the bridge arm current is 9.7A, which is 22% lower than that of the double sinusoidal injection method and 8% lower than that of the hybrid injection method; the peak-to-peak value of the circulating current is 6A, which is 33% lower than that of the double sinusoidal injection method and 13% lower than that of the hybrid injection method.

[0307] The above simulation analysis shows that the low-frequency suppression strategy based on phase voltage harmonic injection can not only ensure the stable operation of the system, but also reduce the amplitude of the injected high-frequency circulating current and the peak-to-peak value of the bridge arm current without affecting the suppression of low-frequency capacitor voltage fluctuations, compared with the double-sine injection method and the mixed injection method.

[0308] If the function is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, or the part of the technical solution, can be embodied in the form of a software product. The computer software product is stored in a storage medium, including several instructions for a computer device (which can be a personal computer, server, or network device, etc.) to perform all or part of the steps of the methods of each embodiment of the present invention. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), disk or optical disk, etc. Various media that can store program codes.

[0309] The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as an ordered list of executable instructions for implementing logical functions, and can be embodied in any computer-readable medium for use by an instruction execution system, device or apparatus (such as a computer-based system, a system including a processor, or other system that can fetch instructions from an instruction execution system, device or apparatus and execute instructions), or in conjunction with such instruction execution systems, devices or apparatuses. For the purposes of this specification, "computer-readable medium" can be any device that can contain, store, communicate, propagate or transmit a program for use by an instruction execution system, device or apparatus, or in conjunction with such instruction execution systems, devices or apparatuses.

[0310] More specific examples of computer-readable media (a non-exhaustive list) include the following: an electrical connection with one or more wires (electronic device), a portable computer disk case (magnetic device), a random access memory (RAM), a read-only memory (ROM), an erasable and programmable read-only memory (EPROM or flash memory), an optical fiber device, and a portable compact disk read-only memory (CDROM). In addition, the computer-readable medium may even be a paper or other suitable medium on which the program is printed, since the program may be obtained electronically, for example, by optically scanning the paper or other medium, followed by editing, deciphering, or processing in another suitable manner as necessary, and then stored in a computer memory.

[0311] It should be understood that the various parts of the present invention can be implemented by hardware, software, firmware or a combination thereof. In the above-mentioned embodiments, multiple steps or methods can be implemented by software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented by hardware, as in another embodiment, it can be implemented by any one of the following technologies known in the art or their combination: a discrete logic circuit having a logic gate circuit for implementing a logic function for a data signal, a dedicated integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc. It should be noted that the above embodiments are only used to illustrate the technical solution of the present invention and are not limited. Although the present invention is described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solution of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

[0312] 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 preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A low-frequency suppression method for a high-voltage, large-capacity disc-type motor wind power system based on MMC, characterized by: include, The mathematical models of modular multilevel converter and disc motor are established respectively; The double-sine high-frequency injection method is used to suppress the low-frequency fluctuation of capacitor voltage in MMC. The hybrid injection method is improved based on phase voltage harmonic injection, which improves the phase voltage utilization rate and reduces the low-frequency fluctuation of bridge arm power through phase voltage harmonic injection. The expressions of high-frequency circulating current and high-frequency common-mode voltage that need to be injected by the improved hybrid injection method based on phase voltage harmonic injection are: Among them, I cm is the injected high-frequency circulating current; u cm is the injected common mode voltage; m is the voltage modulation ratio; K is the high frequency common mode voltage limiting parameter; i sj is a phase current; f h is the common mode voltage frequency; ω is the phase voltage angular frequency; t is the time; η sj is the reference phase of the AC side phase voltage; U dc is the DC voltage.

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