A modulation method for a three-level inverter

Abstract

This invention discloses a modulation method for a three-level converter, mainly used to reduce switching losses and eliminate current zero-crossing distortion. Based on the three-phase normalization formula of the three-level converter SPWM modulation strategy, the normalized sine waves of phases A, B, and C are obtained as u... ma u mb u mc ; Determine the zero-crossing clamping angle and calculate the variable-interval clamping component u VAC Set the variable interval clamping coefficient k VAC The modified modulation wave is calculated based on u. mid Calculate the positive and negative zero-sequence voltage u z.m and the zero-sequence voltage u z.m By injecting a three-phase normalized sine wave, a three-level converter modulation wave u is obtained that reduces switching losses and eliminates current zero-crossing distortion in phases A, B, and C. ref_a u ref_b and u ref_c This method enables three-level converters to clamp near the current zero-crossing point at different modulation ratios, eliminating current zero-crossing distortion and significantly reducing switching losses. Furthermore, compared to traditional modulation methods, it is simpler to implement, requires less computation, and is easier to implement in practical engineering projects.

Description

Technical Field

[0001] This invention relates to the field of power electronics, and in particular to a modulation method for a three-level converter. Background Technology

[0002] Vienna rectifiers are similar to T-type three-level converters, offering the same current waveform quality but with higher efficiency. Due to their advantages such as high power density, high reliability, and low voltage stress, three-phase Vienna rectifiers are widely used in electric vehicle charging systems, communication power systems, wind power applications, aerospace systems, and other fields.

[0003] Although the Vienna rectifier is a three-level converter, it only achieves complete control of the neutral point connection through bidirectional switching. However, because the diodes are connected to the positive and negative voltage rails, the current direction determines the voltage of the upper and lower bus capacitors. When the Vienna rectifier operates outside of unity power factor, the reference voltage and current are out of phase near the current zero-crossing point. This will generate a terminal voltage error near the current zero-crossing point, causing current zero-crossing distortion, which in turn leads to low-frequency distortion.

[0004] The literature X. Zhang, Q. Wang, R. Burgos and D. Boroyevich, "Discontinuous pulsewidth modulation methods with neutral point voltage balancing for three-phase Vienna rectifiers," 2015 IEEE Energy Conversion Congress and Exposition (ECCE), 2015, pp. 225-232, proposes an discontinuous modulation method for three-phase Vienna rectifiers. This method can clamp the phase with larger current and reduce switching losses, but it suffers from severe current zero-crossing distortion.

[0005] The literature W. Zhu, C. Chen, S. Duan, T. Wang, and P. Liu, “A Carrier-Based Discontinuous PWM Method With Varying Clamped Area for Vienna Rectifier,” IEEE Trans. Ind. Electron., vol. 66, no. 9, pp. 7177-7188, Sep. 2019, proposes a variable interval carrier discontinuous modulation method for three-level converters. However, this method still uses continuous modulation in some intervals, which fails to reduce switching losses.

[0006] In view of the above-mentioned shortcomings, it is necessary to study a three-level converter modulation method that reduces switching losses and eliminates current zero-crossing distortion. Under different modulation ratios, it can simultaneously achieve the effects of reducing switching losses and eliminating current zero-crossing distortion, making it more valuable for industrial applications. Summary of the Invention

[0007] To overcome the shortcomings of traditional three-level converter modulation methods, this invention proposes a modulation method for three-level converters, primarily used to reduce switching losses and eliminate current zero-crossing distortion. This method enables the three-level converter to clamp near the current zero-crossing point at different modulation ratios, eliminating current zero-crossing distortion and clamping the phase with larger current, thus significantly reducing switching losses. Furthermore, compared to traditional space vector modulation methods, this invention eliminates the need to determine the size of the sector and calculate the vector action time, making engineering implementation simple and convenient.

[0008] This invention obtains the normalized sine waves of phases A, B, and C as u based on the three-phase normalization formula of the SPWM modulation strategy of a three-level converter. ma u mb u mc Determine the zero-crossing clamp angle. Calculate the interval clamping component u VAC Set the variable interval clamping coefficient k VAC Calculate the corrected modulated wave as According to u mid Calculate the positive and negative zero-sequence voltage u z.m and the zero-sequence voltage u z.m By injecting a three-phase normalized sine wave, a three-level converter modulation wave u is obtained that reduces switching losses and eliminates current zero-crossing distortion in phases A, B, and C. ref_a u ref_b and u ref_c .

[0009] The three-level converter modulation method of the present invention for reducing switching losses and eliminating current zero-crossing distortion is as follows:

[0010] S1. Based on the three-phase normalization formula of the SPWM modulation strategy of the three-level converter, the normalized sine waves of phase A, phase B, and phase C are obtained as u ma u mb u mc The method for determining the three-phase normalized sine wave is as follows:

[0011]

[0012] In equation (1), f g Let m be the grid frequency, m be the modulation ratio, and m ∈ (0,1). The modulation ratio m can be expressed by the formula... We obtain U, where Um U represents the amplitude of the AC reference phase voltage. dc Indicates the DC-side bus voltage;

[0013] S2. Zero-crossing clamping angle Calculation: Define the zero-crossing clamping angle of the three-level converter modulation method to reduce switching losses and eliminate current zero-crossing distortion as: As shown in equation (2);

[0014]

[0015] Where, θ A The phase difference between the reference voltage and the alternating current can be obtained from equation (3). Meanwhile, under different modulation ratios, θ A Equation (4) needs to be satisfied.

[0016]

[0017]

[0018] S3. Calculate the clamping coefficient k for the variable interval. VAC A modulation method for three-level converters to reduce switching losses and eliminate current zero-crossing distortion, with the interval clamping coefficient k. VAC Calculate according to formula (5);

[0019]

[0020] In equation (V), m is the modulation ratio, and k VAC For interval clamping coefficients, 0≤k VAC ≤1, u VAC The clamping components of the variable interval are calculated according to equation (6);

[0021]

[0022] S4. Zero-sequence voltage calculation: Zero-sequence voltage u z.m in u mid When <0, calculate according to formula (7), in u mid When the value is >0, calculate according to formula (8):

[0023]

[0024]

[0025] In the above formula, for The maximum value, for The minimum value of the modulated wave is corrected. Calculated by equation (9), u z1Calculated using equation (10):

[0026]

[0027]

[0028] In equation (10), u max and u min Normalized sine waves u for phases A, B, and C respectively ma u mb u mc The maximum and minimum values.

[0029] S5. Obtain the modulation waveform of the three-level converter with reduced switching losses and elimination of current zero-crossing distortion:

[0030] Define the modulation wave of a three-level converter that reduces switching losses and eliminates current zero-crossing distortion as u. ref_a u ref_b and u ref_c This invention relates to u ref_a u ref_b and u ref_c The determination method is as follows:

[0031]

[0032] In the above formula, u ref_a u ref_b and u ref_c This represents the modulation wave of a three-level converter that reduces switching losses and eliminates current zero-crossing distortion. ma u mb and u mc These represent the normalized sine waves of phases A, B, and C, respectively. z.m This refers to the zero-sequence voltage injected by a three-level converter modulation method to reduce switching losses and eliminate current zero-crossing distortion.

[0033] The beneficial effects of adopting the above invention are as follows:

[0034] (1) The present invention enables the three-level converter to clamp near the current zero-crossing point under different modulation ratios, eliminate current zero-crossing distortion, reduce total harmonic distortion of current, and improve the quality of input current;

[0035] (2) Clamp the phase with larger current to significantly reduce switching losses.

[0036] (3) The implementation steps of the present invention are simple and the amount of calculation is small, which makes it easy to implement in actual engineering. Attached Figure Description

[0037] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show a certain embodiment of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 The circuit diagram of a three-level converter is shown below, illustrating the modulation method of a three-level converter according to the present invention.

[0039] Figure 2 This is a flowchart illustrating the implementation of a modulation method for a three-level converter according to the present invention.

[0040] Figure 3 The above are simulation results of the modulation wave of the modulation method of the three-level converter of the present invention at a modulation ratio m = 0.4.

[0041] Figure 4 The modulation waveform simulation results of the modulation method of the three-level converter of the present invention at a modulation ratio m = 0.7 are shown.

[0042] Figure 5 The present invention provides a modulation method for a three-level converter in which m = 0.4, k VAC Simulation results of the A-phase modulation waveform, A-phase voltage and current, and A-phase bridge arm voltage when the voltage is 0.7.

[0043] Figure 6 The present invention provides a modulation method for a three-level converter in which m = 0.7, k VAC Simulation results of the A-phase modulation waveform, A-phase voltage and current, and A-phase bridge arm voltage when the voltage is 0.5. Detailed Implementation

[0044] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0045] Please see Figures 1 to 6 The figure shows a modulation method for a three-level converter according to the present invention.

[0046] like Figure 1 As shown, the main circuit topology diagram of this invention includes a voltage source e for phase A electricity. a The voltage source of phase B electricity e b The voltage source of phase C electricity e c First to third inductors L a L b L c Diodes D from the first to the sixth a+ D a- D b+ D b-D c+ D c- The first to sixth switching transistors Q1, Q2, Q3, Q4, Q5, and Q6; the first capacitor C1 and the second capacitor C2; the first resistor R1 and the second resistor R2; and the voltage source e for phase A. a The voltage source of phase B electricity e b The voltage source of phase C electricity e c One end is connected together, the first inductor L a Voltage source e connected in series with phase A a With the first diode D a+ Between the anodes, the second inductor L b Voltage source e connected in series with phase B b With the third diode D b+ Between the anodes, the third inductor L c Voltage source e connected in series with phase C c With the fifth diode D c+ Between the anodes, the second diode D a- Fourth diode D b- The sixth diode D c- The cathodes are connected together, denoted as point N, and the anodes are connected to the first diode D. a+ anode, third diode D b+ anode, fifth diode D c+ The anode connection is made, and one end of the second switch Q2, the fourth switch Q4, and the sixth switch Q6 are connected together, denoted as point O. The first switch Q1 and the second switch Q2 are connected in series and then connected to the first diode D. a+ Between the anode and point O, the third switch Q3 and the fourth switch Q4 are connected in series to the third diode D. b+ Between the anode and point O, the fifth switch Q5 and the sixth switch Q6 are connected in series and then connected to the fifth diode D. c+ Between the anode and point O, the first diode D a+ anode, third diode D b+ Fifth diode D c+ The cathodes are connected together, denoted as point P. The first capacitor C1 and the first resistor R1 are connected in parallel between point P and point O. The second capacitor C2 and the second resistor R2 are connected in parallel between point N and point O.

[0047] This invention obtains the normalized sine waves of phases A, B, and C as u based on the three-phase normalization formula of the SPWM modulation strategy of a three-level converter. ma u mb u mc Determine the zero-crossing clamp angle. Calculate the interval clamping component u VAC Set the variable interval clamping coefficient kVAC Calculate the corrected modulated wave as According to u mid Calculate the positive and negative zero-sequence voltage u z.m and the zero-sequence voltage u z.m By injecting a three-phase normalized sine wave, a three-level converter modulation wave u is obtained that reduces switching losses and eliminates current zero-crossing distortion in phases A, B, and C. ref_a u ref_b and u ref_c .

[0048] like Figure 2 As shown, the specific implementation process of the present invention to reduce switching losses and eliminate current zero-crossing distortion is as follows:

[0049] S1. Based on the three-phase normalization formula of the SPWM modulation strategy of the three-level converter, the normalized sine waves of phase A, phase B, and phase C are obtained as u ma u mb u mc The method for determining the three-phase normalized sine wave is as follows:

[0050]

[0051] In equation (1), f g Let m be the grid frequency, m be the modulation ratio, and m ∈ (0,1). The modulation ratio m can be expressed by the formula... We obtain U, where U m U represents the amplitude of the AC reference phase voltage. dc Indicates the DC-side bus voltage;

[0052] S2. Zero-crossing clamping angle Calculation: Define the zero-crossing clamping angle of the three-level converter modulation method to reduce switching losses and eliminate current zero-crossing distortion as: As shown in equation (2);

[0053]

[0054] Where, θ A The phase difference between the reference voltage and the alternating current can be obtained from equation (3). Meanwhile, under different modulation ratios, θ A Equation (4) needs to be satisfied.

[0055]

[0056]

[0057] S3. Calculate the clamping coefficient k for the variable interval. VAC A modulation method for three-level converters to reduce switching losses and eliminate current zero-crossing distortion, with the interval clamping coefficient k. VACCalculate according to formula (5);

[0058]

[0059] In equation (V), m is the modulation ratio, and k VAC For interval clamping coefficients, 0≤k VAC ≤1, u VAC The clamping components of the variable interval are calculated according to equation (6);

[0060]

[0061] S4. Zero-sequence voltage calculation: Zero-sequence voltage u z.m in u mid When <0, calculate according to formula (7), in u mid When the value is >0, calculate according to formula (8):

[0062]

[0063]

[0064] In the above formula, for The maximum value, for The minimum value of the modulated wave is corrected. Calculated by equation (9), u z1 Calculated using equation (10):

[0065]

[0066]

[0067] In equation (10), u max and u min Normalized sine waves u for phases A, B, and C respectively ma u mb u mc The maximum and minimum values.

[0068] S5. Obtain the modulation waveform of the three-level converter with reduced switching losses and elimination of current zero-crossing distortion:

[0069] Define the modulation wave of a three-level converter that reduces switching losses and eliminates current zero-crossing distortion as u. ref_a u ref_b and u ref_c This invention relates to u ref_a u ref_b and u ref_c The determination method is as follows:

[0070]

[0071] In the above formula, u ref_a u ref_b and u ref_c This represents the modulation wave of a three-level converter that reduces switching losses and eliminates current zero-crossing distortion. ma u mb and u mc This represents the normalized sine wave of phases A, B, and C, u z.m This refers to the zero-sequence voltage injected by a three-level converter modulation method to reduce switching losses and eliminate current zero-crossing distortion.

[0072] The three-level converter modulation method of this invention, which reduces switching losses and eliminates current zero-crossing distortion, shows the normalized sine wave of phase A, the injected zero-sequence component, and the generated modulation wave waveform at a modulation ratio m = 0.4 as follows: Figure 3 As shown, clamping can be performed near the zero-crossing point or on the phase with a larger current.

[0073] The three-level converter modulation method of this invention, which reduces switching losses and eliminates current zero-crossing distortion, shows the normalized sine wave of phase A, the injected zero-sequence component, and the generated modulation wave waveform at a modulation ratio m = 0.7 as follows: Figure 4 As shown, clamping can be performed near the zero-crossing point or on the phase with a larger current.

[0074] This invention utilizes PLECS software to build a simulation model of a three-level converter, and verifies the effectiveness of the three-level converter modulation method for reducing switching losses and eliminating current zero-crossing distortion. The simulation conditions are: simulation step size 2µs, AC voltage RMS value 115V, fundamental frequency 400Hz, and switching frequency 100kHz.

[0075] Figure 5 and Figure 6 The simulation waveforms of the three-level converter modulation method for reducing switching losses and eliminating current zero-crossing distortion, as invented, are shown for m=0.4 and m=0.7. z.m This is the zero-sequence voltage injected in this invention, u ref_a It is a phase A modulated wave, u a It is the AC side A-phase input voltage, i a It is the AC side A-phase input current, u AO This is the voltage of phase A bridge arm. (From...) Figure 5 and Figure 6 As can be seen, the three-level converter modulation method of this invention, which reduces switching losses and eliminates current zero-crossing distortion, can clamp the current to 0 near the zero-crossing point, eliminating current zero-crossing distortion, and the current waveform quality is good. Clamping the larger current phase reduces switching losses. This verifies the effectiveness of the three-level converter modulation method for reducing switching losses and eliminating current zero-crossing distortion.

[0076] The working principle and process of this invention are as follows:

[0077] Based on the three-phase normalization formula of the SPWM modulation strategy of the three-level converter, the normalized sine waves of phases A, B, and C are obtained as u... ma u mb u mc Determine the zero-crossing clamp angle. Calculate the interval clamping component u VAC Set the variable interval clamping coefficient k VAC Calculate the corrected modulated wave as According to u mid Calculate the positive and negative zero-sequence voltage u z.m and the zero-sequence voltage u z.m By injecting a three-phase normalized sine wave, a three-level converter modulation wave u is obtained that reduces switching losses and eliminates current zero-crossing distortion in phases A, B, and C. ref_a u ref_b and u ref_c Then, a three-level converter simulation model was built using PLECS software, and the effectiveness of the three-level converter modulation method of the present invention, which reduces switching losses and eliminates current zero-crossing distortion, was verified by simulation.

Claims

1. A modulation method for a three-level converter, characterized in that, Based on the three-phase normalization formula of the SPWM modulation strategy of the three-level converter, the normalized sine waves of phases A, B, and C are respectively obtained. , , ; Determine the zero-crossing clamp angle Calculate the clamping components of the variable interval. Set the variable interval clamping coefficient Calculate the corrected modulated wave as According to u mid Calculate the positive and negative zero-sequence voltage and zero-sequence voltage By injecting a three-phase normalized sine wave, a three-level converter modulation wave with reduced switching losses and elimination of current zero-crossing distortion is obtained for phases A, B, and C. , and .

2. The modulation method for a three-level converter according to claim 1, characterized in that, The specific modulation method for reducing switching losses and eliminating current zero-crossing distortion in a three-level converter is as follows: S1. Based on the three-phase normalization formula of the SPWM modulation strategy of the three-level converter, the normalized sine waves of phases A, B, and C are respectively obtained. , , The method for determining the three-phase normalized sine wave is as follows: （1） In equation (1), For the power grid frequency, The modulation ratio, The modulation ratio It can be derived from the formula We obtain, where Indicates the amplitude of the AC reference phase voltage. Indicates the DC-side bus voltage; S2. Zero-crossing clamping angle Calculation: Define the zero-crossing clamping angle of the three-level converter modulation method to reduce switching losses and eliminate current zero-crossing distortion as: As shown in equation (2); （2） in, The phase difference between the reference voltage and the alternating current can be obtained from equation (3). Meanwhile, under different modulation ratios, It needs to satisfy equation (4). （3） （4） S3. Calculate the clamping coefficient for the variable interval. A modulation method for three-level converters to reduce switching losses and eliminate current zero-crossing distortion, and a frequency range clamping coefficient. Calculate according to formula (5); （5） In equation (5), The modulation ratio, For the variable interval clamping coefficient, , The clamping components of the variable interval are calculated according to equation (6); （6） S4. Zero-sequence voltage calculation: Zero-sequence voltage in u mid When < 0, calculate according to formula (7), in u mid > Calculate according to formula (8) when the value is 0: （7） （8） In equation (8), for The maximum value, for The minimum value of the modulated wave is corrected. x = a, b, c is calculated using equation (9), u z1 Calculated using equation (10): （9） （10） In equation (10), and Normalized sine waves for phases A, B, and C, respectively. , , The maximum and minimum values; S5. Obtain the modulation waveform of the three-level converter with reduced switching losses and elimination of current zero-crossing distortion: Define the modulation waveforms of a three-level converter to reduce switching losses and eliminate current zero-crossing distortion as follows: , and ,right , and The determination method is as follows: （11） In the above formula, , and This represents the modulation waveform of a three-level converter to reduce switching losses and eliminate current zero-crossing distortion. , and These represent the normalized sine waves of phases A, B, and C, respectively. This refers to the zero-sequence voltage injected by a three-level converter modulation method to reduce switching losses and eliminate current zero-crossing distortion.

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