A nonlinear harmonic suppression strategy for MMC and CCV cascade drive systems

Abstract

The present invention discloses a nonlinear harmonic suppression strategy applicable to MMC and CCV cascade drive systems, and belongs to the field of power electronic conversion technology. A nonlinear harmonic suppression strategy applicable to MMC and CCV cascade drive systems, the nonlinear harmonic suppression strategy comprises the following steps: Step 1: real-time monitoring of the output voltage of a modular multilevel converter, taking the output voltage as the output voltage reference value of the modular multilevel converter, and obtaining a capacitor current reference value through a voltage outer loop control link; Step 2: subtracting the capacitor current reference value from the actual value, and obtaining an AC control signal of the modular multilevel converter after adjusting the difference; The present invention can effectively ensure the voltage stability of the medium frequency AC link of the MMC and CCV cascade drive, and can suppress multiple nonlinear harmonics generated by the system with the operating conditions.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronic conversion, and in particular relates to a nonlinear harmonic suppression strategy suitable for MMC and CCV cascade drive systems. Background Art

[0002] In recent years, with the rapid development of power electronics technology, modular multilevel converters (MMC) have been widely used in medium voltage motor drive, rail transit, high voltage flexible DC transmission and other fields due to their advantages such as high modularity, low voltage devices, high energy transmission efficiency, low harmonic content of AC output voltage, and redundant control. However, MMC has an inherent disadvantage that cannot be ignored when applied to high power and low speed direct drive systems: the fluctuation of capacitor voltage of each submodule of MMC will increase significantly with the decrease of output frequency, and even affect the normal operation of MMC and motor in severe cases.

[0003] In order to solve the defects of the existing MMC motor drive system, an effective technical solution is to use a cascade drive system based on a modular multilevel converter and a cycloconverter (CCV), converting the DC side voltage into medium-frequency AC power through the MMC first, and then converting it into low-frequency AC power through the CCV to supply the motor, thereby effectively reducing the MMC energy storage capacitor capacitance, volume and weight. However, since this technical solution uses CCV as the motor-side converter, the thyristor commutation process will cause the appearance of nonlinear harmonics in the system, and the frequency of these harmonics will change with the speed of the motor, affecting the voltage stability of the medium-frequency AC link in the drive system, bringing hidden dangers to the stability and operational reliability of the system. Summary of the invention

[0004] The purpose of the present invention is to provide a nonlinear harmonic suppression strategy suitable for MMC and CCV cascade drive systems in view of the defects in the prior art.

[0005] The purpose of the present invention can be achieved through the following technical solutions:

[0006] A nonlinear harmonic suppression strategy applicable to MMC and CCV cascade drive systems, the nonlinear harmonic suppression strategy comprising the following steps: step 1: setting an output voltage reference value of a modular multilevel converter, and monitoring the output voltage of the modular multilevel converter in real time, and obtaining a capacitor current reference value through a voltage outer loop control link; step 2: subtracting the capacitor current reference value from an actual value, and obtaining an AC control signal of the modular multilevel converter after adjusting the obtained difference; step 3: obtaining a conduction signal of the modular multilevel converter after a modulation strategy and voltage balancing control according to the AC control signal of the modular multilevel converter, and controlling the opening or closing of a bridge arm submodule of the modular multilevel converter.

[0007] Furthermore, in the step 2, the adjustment of the difference is applied to an improved quasi-proportional resonant controller.

[0008] Furthermore, the specific control steps of the improved quasi-proportional resonant controller are as follows: the difference is adjusted through a proportional adjustment step to obtain a first control signal; the coordinate transformation angle is set to 3θ m The difference undergoes abc / dq coordinate transformation to obtain the control component in the dq coordinate system, and then passes through the quasi-resonance adjustment link, and then undergoes dq / abc reverse coordinate transformation to obtain the second control signal; the coordinate transformation angle is set to -3θ m , the difference is transformed through abc / dq coordinates to obtain a control component in a dq coordinate system, and then passes through a quasi-resonance adjustment link, and then undergoes a dq / abc reverse coordinate transformation to obtain a third control signal; finally, the first, second, and third control signals are added to the actual value of the output voltage of the modular multilevel converter to obtain an AC control signal of the modular multilevel converter.

[0009] Furthermore, the θ m Obtained by the motor's rotor position detector.

[0010] Furthermore, the transfer function of the quasi-resonance adjustment link is:

[0011]

[0012] Among them, K 1 is the resonance coefficient of the first quasi-resonant regulator, ω 01 is the resonant angular frequency of the first quasi-resonant regulator, ω c1 is the cutoff frequency of the first quasi-resonant regulator, K 2 is the resonance coefficient of the second quasi-resonant regulator, ω 02 is the resonant angular frequency of the second quasi-resonant regulator, ω c2 is the cutoff angular frequency of the second quasi-resonant regulator, and s is the differential operator.

[0013] Furthermore, in the transfer function of the resonance adjustment link, the formula for selecting the resonance angular frequency is:

[0014]

[0015] Among them, ω mmc is the MMC output voltage angular frequency, f mmc is the output voltage frequency of the modular multilevel converter.

[0016] Beneficial effects of the present invention:

[0017] 1. The present invention can effectively ensure the voltage stability of the medium frequency AC link of the MMC and CCV cascade drive, and realize the reliable operation of the drive system;

[0018] 2. The present invention utilizes an improved quasi-proportional resonant controller to achieve high gain and zero static error control at harmonic frequency points, which can suppress multiple nonlinear harmonics generated by the system as the operating conditions change, and achieve tracking control when the operating conditions and harmonic frequency bands change.

[0019] 3. The nonlinear harmonic suppression strategy proposed in the present invention does not need to increase the system construction cost, the algorithm is easy to implement, the control is simple and direct, and it has strong practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The present invention will be further described below in conjunction with the accompanying drawings.

[0021] Figure 1 It is a control block diagram of the nonlinear harmonic suppression strategy applicable to the MMC and CCV cascade drive system of the present invention;

[0022] Figure 2 is a circuit diagram of the MMC in the cascade drive system of the present invention;

[0023] Figure 3 It is a circuit diagram of CCV in the cascade drive system of the present invention. DETAILED DESCRIPTION

[0024] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0025] like Figure 1As shown, the present invention is a nonlinear harmonic suppression strategy applicable to the MMC and CCV cascade drive system, and the circuit diagrams of the MMC and CCV in the cascade drive system are respectively as shown in Figure 2 and Figure 3 The control strategy of the present invention comprises the following steps:

[0026] Step 1: Use u j_ref It is the reference value of the MMC output voltage and monitors the output voltage u of the modular multilevel converter (MMC) in real time. j (j=a,b,c), after the voltage outer loop control link, the capacitor current reference value i is obtained cj_ref ;

[0027] Step 2: Set the capacitor current reference value i cj_ref With the actual value i cj Difference, the difference i error After the improved quasi-proportional resonant controller, the AC control signal u of the MMC is obtained. mmc ;

[0028] Step 3: According to the AC control signal u of MMC mmc After the modulation strategy and voltage balancing control, the MMC conduction signal is obtained to control the opening or closing of the MMC bridge arm submodule.

[0029] In step 2, the specific control steps of the improved quasi-proportional resonant controller are as follows:

[0030] The i error After the proportional adjustment link, the first control signal u is obtained 1 ;

[0031] Set the coordinate transformation angle to 3θ m ,i error After the abc / dq coordinate transformation, the control component in the dq coordinate system is obtained, and then through the quasi-resonance adjustment link, and then through the dq / abc reverse coordinate transformation, the second control signal u is obtained. 2 ;

[0032] Set the coordinate transformation angle to -3θ m ,i error After the abc / dq coordinate transformation, the control component in the dq coordinate system is obtained, and then through the quasi-resonance adjustment link, and then through the dq / abc reverse coordinate transformation, the third control signal u is obtained. 3 ;

[0033] The control signal u 1 、u 2 、u 3 The actual value of MMC output voltage uj Add together to get the AC control signal u of MMC mmc .

[0034] The θ m Obtained by the motor's rotor position detector.

[0035] The transfer function of the quasi-resonant regulation link is:

[0036]

[0037] Among them, K 1 is the resonance coefficient of the first quasi-resonant regulator, ω 01 is the resonant angular frequency of the first quasi-resonant regulator, ω c1 is the cutoff frequency of the first quasi-resonant regulator, K 2 is the resonance coefficient of the second quasi-resonant regulator, ω 02 is the resonant angular frequency of the second quasi-resonant regulator, ω c2 is the cutoff angular frequency of the second quasi-resonant regulator, and s is the differential operator.

[0038] In the transfer function of the resonance adjustment link, the formula for selecting the resonance angular frequency is:

[0039]

[0040] Among them, ω mmc is the MMC output voltage angular frequency, f mmc is the MMC output voltage frequency.

[0041] The above nonlinear harmonic suppression strategy applicable to MMC and CCV cascade drive systems can effectively ensure the voltage stability of the medium frequency AC link of MMC and CCV cascade drives, realize the reliable operation of the drive system, and realize high gain and zero static error control at the nonlinear harmonic frequency point generated by the system, and suppress the multiple nonlinear harmonics generated by the system with the operating conditions. In addition, the nonlinear harmonic suppression strategy proposed by the present invention does not need to increase the system construction cost and has strong practicality.

[0042] In the description of this specification, the description with reference to the terms "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0043] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, and these changes and improvements all fall within the scope of the present invention to be protected.

Claims

1. A nonlinear harmonic suppression method suitable for MMC and CCV cascade drive systems, characterized in that: The nonlinear harmonic suppression method comprises the following steps: Step 1: setting an output voltage reference value of the modular multilevel converter, and monitoring the output voltage of the modular multilevel converter in real time, and obtaining a capacitor current reference value through a voltage outer loop control link; Step 2: Subtract the capacitor current reference value from the actual value, and adjust the difference to obtain an AC control signal of the modular multilevel converter; Step 3: According to the AC control signal of the modular multilevel converter, a conduction signal of the modular multilevel converter is obtained after modulation strategy and voltage balancing control, and the opening or closing of the bridge arm submodule of the modular multilevel converter is controlled; In the step 2, an improved quasi-proportional resonant controller is used to adjust the difference; The specific control steps of the improved quasi-proportional resonant controller are as follows: the difference is adjusted by the proportional adjustment step to obtain a first control signal; the coordinate transformation angle is set to 3θ m The difference undergoes abc / dq coordinate transformation to obtain the control component in the dq coordinate system, and then passes through the quasi-resonance adjustment link, and then undergoes dq / abc reverse coordinate transformation to obtain the second control signal; the coordinate transformation angle is set to -3θ m The difference undergoes abc / dq coordinate transformation to obtain a control component in a dq coordinate system, and then passes through a quasi-resonance adjustment link, and then undergoes a dq / abc reverse coordinate transformation to obtain a third control signal; finally, the first, second, and third control signals are added to the actual value of the output voltage of the modular multilevel converter to obtain an AC control signal of the modular multilevel converter.

2. The nonlinear harmonic suppression method applicable to MMC and CCV cascade drive systems according to claim 1, characterized in that: The θ m Obtained by the motor's rotor position detector.

3. The nonlinear harmonic suppression method applicable to MMC and CCV cascade drive systems according to claim 1, characterized in that: The transfer function of the quasi-resonant regulation link is: Among them, K1 is the resonance coefficient of the first quasi-resonant regulator, ω 01 is the resonant angular frequency of the first quasi-resonant regulator, ω c1 is the cutoff angular frequency of the first quasi-resonant regulator, K2 is the resonance coefficient of the second quasi-resonant regulator, ω 02 is the resonant angular frequency of the second quasi-resonant regulator, ω c2 is the cutoff angular frequency of the second quasi-resonant regulator, and s is the differential operator.

4. The nonlinear harmonic suppression method applicable to MMC and CCV cascade drive systems according to claim 3 is characterized in that: In the transfer function of the resonance adjustment link, the formula for selecting the resonance angular frequency is: Among them, ω mmc is the MMC output voltage angular frequency, f mmc is the output voltage frequency of the modular multilevel converter.

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