A control method for neutral point potential balance of three-level converter

Abstract

The present invention discloses a control method for the midpoint potential balance of a three-level converter. The AC fluctuation signal in the midpoint potential is tracked and amplified by a harmonic extraction algorithm, and then the obtained signal is phase compensated to obtain the optimal zero-sequence component, which is then superimposed with the three-phase sinusoidal modulation signal obtained by adopting other traditional three-level converter control strategies to generate a saddle-shaped modulation wave for controlling the switch device of the circuit. The present invention is used in conjunction with the traditional grid-connected, off-grid and rectifier-related control strategies of the three-level converter to obtain a sinusoidal zero-sequence component with a frequency three times the fundamental frequency and an amplitude of 0.26 times the modulation ratio. This zero-sequence component is the optimal injection amount, which can suppress the low-frequency AC fluctuation component of the midpoint potential to the greatest extent, effectively control the midpoint potential fluctuation, and improve the working life of the circuit device, and realize the beneficial addition to the control effect of the traditional three-level converter control strategy, and enhance the ability of the system to operate stably.

Description

Technical Field

[0001] The invention relates to the technical field of power electronics, and in particular to a midpoint potential balance control method applied to a three-level converter. Background Art

[0002] Compared with the traditional two-level converter, the three-level converter has the advantages of effectively reducing the harmonic distortion rate of the AC side current and reducing the voltage stress of the switch tube. However, due to the non-ideal factors of the circuit and the individual differences of the power devices, the actual voltage values ​​of the upper and lower bus capacitors on the DC side of the three-level converter are biased, causing the midpoint potential to be unbalanced. The unbalanced DC side midpoint potential will cause the following hazards: (1) The AC side voltage and current contain low-order harmonics, which affects the efficiency and performance of the converter; (2) The voltage stress on the power switch device is inconsistent, which affects the normal operation of the switch device and even damages the switch device; (3) The service life of the DC side bus capacitor is reduced. Therefore, solving the midpoint potential imbalance is one of the key issues of the three-level converter.

[0003] At present, the control methods of the midpoint potential balance of the three-level topology structure are mainly divided into two categories: hardware and software. Most of the hardware methods are to add an independent DC voltage source to replace the DC side capacitor, which can effectively solve the problem of midpoint potential imbalance, but the addition of an independent voltage source will greatly increase the cost and volume of the system. Most of the software methods are to suppress the fluctuation of the DC side midpoint potential by optimizing the modulation technology without adding additional hardware structure. There are two common software methods: one is the zero-sequence component injection method, that is, injecting zero-sequence components into the three-phase modulation signal, changing the on and off time of the corresponding switch tube, and realizing the control of the midpoint current, thereby indirectly suppressing the AC fluctuation of the midpoint potential; the other is the space vector pulse width modulation algorithm based on redundant small vector adjustment, that is, by adjusting the action time of the redundant small vector in a single switching cycle, the fluctuation of the midpoint potential is suppressed. Both of the above methods can suppress the fluctuation of the midpoint potential to a certain extent, but there are also corresponding defects.

[0004] The traditional zero-sequence component injection algorithm offsets the AC component in the midpoint current by injecting the zero-sequence voltage component into the three-phase modulation wave. However, it is difficult to accurately control the extraction and injection amount of the zero-sequence voltage component, and it is difficult to obtain the optimal zero-sequence component, and the suppression effect on the midpoint potential fluctuation is limited. The space vector pulse width modulation algorithm based on redundant small vector adjustment can adjust the balance of the midpoint potential by appropriately adjusting the action time of the redundant small vector in a single modulation cycle. However, this method only qualitatively adjusts the midpoint potential deviation, lacks an essential understanding of the principle of midpoint potential fluctuation, and has a limited adjustment range for midpoint potential fluctuation, which cannot achieve the optimal control effect. At the same time, the traditional space vector pulse width modulation algorithm has many trigonometric operations and sector judgments, and requires a large number of data tables to be stored in advance, which is relatively complicated to implement. If other control algorithms are added, such as the midpoint potential balance algorithm, it will further increase the difficulty of algorithm implementation and limit the flexibility of system control design.

[0005] The midpoint current of the three-level converter mainly contains an AC fluctuation component three times the fundamental frequency, which causes a difference in the voltage between the upper and lower bus capacitors on the DC side. dif Available formulas Indicates that its amplitude is the same as the input / output current amplitude I on the AC side of the converter m , modulation index m, capacitance C of bus capacitor and angular frequency ω of input / output voltage on AC side.

[0006] Injecting zero-sequence components into the three-phase modulation wave can suppress the harmonic components of the DC side midpoint current, thereby indirectly suppressing the midpoint potential fluctuation. Ideally, when the modulation wave contains a sinusoidal zero-sequence component with a frequency three times the fundamental frequency and an amplitude of 0.26 times the modulation ratio, the AC fluctuation of the midpoint potential of the three-level converter will be suppressed to a minimum. Summary of the invention

[0007] The purpose of the present invention is to propose a mid-point potential balance control method based on optimal zero-sequence component injection in view of the defects of the prior art, which is used in conjunction with the traditional grid-connected, off-grid and rectifier-related control strategies of the three-level converter to obtain a sinusoidal zero-sequence component with a frequency 3 times the fundamental frequency and an amplitude of 0.26 times the modulation ratio. This zero-sequence component is the optimal injection amount, which can suppress the low-frequency AC fluctuation component of the mid-point potential to the greatest extent, effectively control the mid-point potential fluctuation, improve the working life of the circuit components, and achieve a beneficial addition to the control effect of the traditional three-level converter control strategy, thereby enhancing the system's ability to operate stably.

[0008] The technical solution adopted by the present invention is specifically as follows:

[0009] A control method for the midpoint potential balance of a three-level converter, the schematic diagram of the control strategy principle is as followsFigure 1 As shown, the method includes:

[0010] Step 1: Obtain the input / output voltage v on the AC side of the three-level converter according to the detection circuit a 、v b 、v c , input / output current i a 、i b 、i c , using the traditional three-level converter voltage / current control strategy, such as the grid-connected, off-grid and rectifier control strategies of the three-level converter, to obtain the three-phase sinusoidal modulation signal v ma 、v mb 、v mc .

[0011] Step 2: Obtain the voltage v of the upper and lower voltage divider capacitors on the DC side of the three-level converter according to the detection circuit c1 、v c2 , the voltage on the upper voltage divider capacitor v c1 The voltage of the lower voltage divider capacitor v c2 Difference, get the control quantity v dif The harmonic extraction algorithm is used to track and amplify the AC quantity and output the harmonic component v h .

[0012] Step 3: Use phase compensation algorithm to compensate the harmonic component v h Perform phase compensation.

[0013] Step 4: Compare the zero-sequence component obtained in step 3 with the three-phase sinusoidal modulation signal v obtained in step 1 ma 、v mb 、v mc Add together to get the modulated signal v' a 、v' b 、v' c , compared with the high-frequency triangular carrier signal, the corresponding driving signal for each switch tube is generated.

[0014] Furthermore, the principle of the harmonic extraction algorithm is as follows:

[0015]

[0016] In the formula, k p Represents the proportional coefficient of the controller; ω h Represents three times the fundamental angular frequency of the power frequency; K R Represents the resonance compensation coefficient of the harmonic extraction algorithm; ω c represents the control bandwidth of the harmonic extraction algorithm, and s represents the Laplace operator.

[0017] Furthermore, the phase compensation algorithm is as follows:

[0018] (1) According to the formula Calculate the phase difference between the harmonic component and the three-phase sinusoidal modulation signal; where ω is the angular frequency of the input / output voltage on the AC side; L is the value of the AC side filter inductance; I m is the input / output current amplitude on the AC side, V m is the amplitude of the input / output voltage on the AC side.

[0019] (2) Using the formula v m sin(3ωt-θ0)cos(θ0)+v m cos(3ωt-θ0)sin(θ0) for harmonic component v h Phase compensation is performed to obtain the zero-sequence component v0 with the same phase as the sinusoidal modulation wave, which is the optimal zero-sequence component proposed by the present invention, where v m is the harmonic component v h The amplitude of .

[0020] On the basis of the above, the present invention can also adopt the following beneficial improvements:

[0021] (1) Harmonic component v h With the three-phase sinusoidal modulation signal v ma 、v mb 、v mc The phase difference θ0 is less than In order to reduce the complexity of the algorithm and save software resources, the phase difference θ0 is set to zero, which does not affect the effect of the midpoint potential balance algorithm.

[0022] (2) When the amplitude of the ninth and fifteenth harmonic components in the midpoint potential is not less than one tenth of the amplitude of the third harmonic component, the harmonic extraction algorithm described in step 1 can be used to obtain the corresponding harmonic components and inject them into the three-phase modulated signal to suppress the high-order AC fluctuation components in the midpoint potential. The algorithm principle is as follows:

[0023]

[0024] Among them, k p represents the proportional coefficient of the controller; variable i represents the harmonic order; ω h.i represents the angular frequency of the i-th harmonic; K R.i represents the resonance compensation coefficient of the harmonic extraction algorithm for the i-th harmonic; ω c.i represents the control bandwidth of the harmonic extraction algorithm for the i-th harmonic, and s represents the Laplace operator.

[0025] Beneficial effects of the present invention:

[0026] (1) The control strategy adopted by the present invention is universal and applicable to a variety of three-level converter topologies with neutral point potential balance problems, such as three-phase three-level neutral point clamped (NPC) inverter topology, three-phase T-type three-level inverter topology and Vienna rectifier topology, etc. It can be used in conjunction with the traditional three-level converter control strategy to achieve a beneficial addition to the control effect of the traditional three-level converter control strategy;

[0027] (2) The present invention can obtain the optimal zero-sequence component, which is superimposed on the three-phase modulation wave and can suppress the AC fluctuation of the midpoint potential to the greatest extent, solving the problem that the traditional zero-sequence voltage injection method can suppress the midpoint potential fluctuation to a certain extent, but cannot suppress it to the minimum;

[0028] (3) The present invention adopts a control strategy of zero-sequence voltage injection. The zero-sequence voltage is a zero-sequence component with the same phase as the sinusoidal modulation wave. The superposition of the two can obtain a saddle-shaped modulation wave. Compared with the traditional sinusoidal pulse width modulation algorithm, the modulation method can improve the utilization rate of the DC bus voltage.

[0029] (4) The present invention can realize tracking and amplification of AC harmonic components by adopting a harmonic extraction algorithm with the same resonant frequency as the frequency of the AC fluctuation component of the midpoint potential, which has better control performance than the traditional proportional-integral control;

[0030] (5) The control strategy of zero-sequence component injection adopted in the present invention is basically the same as space vector pulse width modulation in effect. However, compared with the traditional space vector pulse width modulation algorithm based on redundant small vector adjustment, the control strategy of the present invention does not require complex sector judgment and a large number of trigonometric operations, has a small amount of calculation, and is easier to implement digitally. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0032] Figure 1 It is a schematic diagram of the principle of the midpoint potential balance control strategy proposed by the present invention;

[0033] Figure 2 It is a schematic diagram of a process of applying an embodiment of the present invention to a three-phase three-level NPC inverter grid-connected system;

[0034] Figure 3 It is a schematic diagram of a process of applying an embodiment of the present invention to a three-phase three-level NPC type inverter off-grid system;

[0035] Figure 4 It is a flow chart of an embodiment of the present invention applied to a three-phase Vienna rectifier system. DETAILED DESCRIPTION

[0036] The technical solution of the present invention is further described below in conjunction with the accompanying drawings, but the present invention is not limited to these embodiments.

[0037] Embodiment 1: A midpoint potential balance control method applied to a three-phase three-level NPC inverter grid-connected system. The flow chart of the method is as follows: Figure 2 The three-phase three-level NPC inverter is a typical three-level structure converter, including a DC side power supply V dc , three identical bridge arms, two bus capacitors C1 and C2, each bridge arm contains 4 series-connected switching tubes and 2 series-connected clamping diodes, the three bridge arms are connected to the midpoint of the bus capacitor through the midpoint of the clamping diode, and the AC side is connected to the grid through a filtering circuit.

[0038] Specifically, the midpoint potential balance control method applied to the three-phase three-level NPC type grid-connected inverter system is combined with its corresponding grid-connected control strategy, and the implementation steps are as follows:

[0039] Step 1: Obtain the output voltage v of the AC side of the three-level converter according to the detection circuit a 、v b 、v c , output current i a 、i b 、i c , the traditional three-level inverter grid-connected control strategy is used to obtain the three-phase sinusoidal modulation signal v ma 、v mb 、v mc .

[0040] Step 2: Obtain the voltage v of the upper and lower voltage divider capacitors on the DC side of the three-level converter according to the detection circuit c1 、v c2 , the voltage on the upper voltage divider capacitor v c1 The voltage of the lower voltage divider capacitor v c2 Difference, get the control quantity v dif The harmonic extraction algorithm is used to track and amplify the AC quantity and output the harmonic component v h , the principle of the harmonic extraction algorithm is as follows:

[0041]

[0042] In the formula, k p Represents the proportional coefficient of the controller; ω h Represents three times the fundamental angular frequency of the power frequency; K R Represents the resonance compensation coefficient of the harmonic extraction algorithm; ω c represents the control bandwidth of the harmonic extraction algorithm, and s represents the Laplace operator.

[0043] Step 3: Use phase compensation algorithm to compensate the harmonic component v h Perform phase compensation, the phase compensation algorithm is as follows:

[0044] (1) According to the formula Calculate the phase difference between the harmonic component and the three-phase sinusoidal modulation signal; where ω is the angular frequency of the input / output voltage on the AC side; L is the value of the AC side filter inductance; I m is the input / output current amplitude on the AC side, V m is the amplitude of the input / output voltage on the AC side.

[0045] (2) Using the formula v m sin(3ωt-θ0)cos(θ0)+v m cos(3ωt-θ0)sin(θ0) for harmonic component v h Phase compensation is performed to obtain the zero-sequence component v0 with the same phase as the sinusoidal modulation wave, which is the optimal zero-sequence component proposed by the present invention, where v m is the harmonic component v h The amplitude of .

[0046] Step 4: Compare the zero-sequence component obtained in step 3 with the three-phase sinusoidal modulation signal v obtained in step 1 ma 、v mb 、v mc Add together to get the modulated signal v' a 、v' b 、v' c , compared with the high-frequency triangular carrier signal, the corresponding driving signal for each switch tube is generated.

[0047] Embodiment 2: A midpoint potential balance control method applied to a three-phase three-level NPC inverter off-grid system. The flow chart of the method is as follows: Figure 3 The three-phase three-level NPC inverter is a typical three-level structure converter, including a DC side power supply V dc , three identical bridge arms, two bus capacitors C1 and C2, each bridge arm contains 4 series-connected switching tubes and 2 series-connected clamping diodes, the three bridge arms are connected to the midpoint of the bus capacitor through the midpoint of the clamping diode, and the AC side is connected to the three-phase load through a filtering circuit.

[0048] Specifically, the midpoint potential balance control method applied to the three-phase three-level NPC type off-grid inverter system is combined with its corresponding off-grid control strategy, and the implementation steps are as follows:

[0049] Step 1: Obtain the output voltage v of the AC side of the three-level converter according to the detection circuit a 、vb 、v c , output current i a 、i b 、i c , using the traditional three-level inverter off-grid control strategy to obtain the three-phase sinusoidal modulation signal v ma 、v mb 、v mc .

[0050] Step 2: Obtain the voltage v of the upper and lower voltage divider capacitors on the DC side of the three-level converter according to the detection circuit c1 、v c2 , the voltage on the upper voltage divider capacitor v c1 The voltage of the lower voltage divider capacitor v c2 Difference, get the control quantity v dif The harmonic extraction algorithm is used to track and amplify the AC quantity and output the harmonic component v h , the principle of the harmonic extraction algorithm is as follows:

[0051]

[0052] In the formula, k p Represents the proportional coefficient of the controller; ω h Represents three times the fundamental angular frequency of the power frequency; K R Represents the resonance compensation coefficient of the harmonic extraction algorithm; ω c represents the control bandwidth of the harmonic extraction algorithm, and s represents the Laplace operator.

[0053] Step 3: Use phase compensation algorithm to compensate the harmonic component v h Perform phase compensation, the phase compensation algorithm is as follows:

[0054] (1) According to the formula Calculate the phase difference between the harmonic component and the three-phase sinusoidal modulation signal; where ω is the angular frequency of the input / output voltage on the AC side; L is the value of the AC side filter inductance; I m is the input / output current amplitude on the AC side, V m is the amplitude of the input / output voltage on the AC side.

[0055] (2) Using the formula v m sin(3ωt-θ0)cos(θ0)+v m cos(3ωt-θ0)sin(θ0) for harmonic component v h Phase compensation is performed to obtain the zero-sequence component v0 with the same phase as the sinusoidal modulation wave, which is the optimal zero-sequence component proposed by the present invention, where v m is the harmonic component v h The amplitude of .

[0056] Step 4: Compare the zero-sequence component obtained in step 3 with the three-phase sinusoidal modulation signal v obtained in step 1 ma 、v mb 、v mc Add together to get the modulated signal v' a 、v' b 、v' c , compared with the high-frequency triangular carrier signal, the corresponding driving signal for each switch tube is generated.

[0057] Embodiment 3: A midpoint potential balance control method applied to a three-phase Vienna rectifier system. The flow chart of the method is as follows: Figure 4 The three-phase Vienna rectifier is a typical three-level structure converter. a 、v b 、v c is the three-phase grid voltage, i a 、i b 、i c is the input current on the AC side, C1 and C2 are DC bus capacitors, V dc is the DC bus voltage.

[0058] Specifically, the midpoint potential balance control method applied to the three-phase Vienna rectifier system is combined with its corresponding traditional voltage / current control strategy, and the implementation steps are as follows:

[0059] Step 1: Obtain the output voltage v of the AC side of the three-level converter according to the detection circuit a 、v b 、v c , output current i a 、i b 、i c , using the traditional three-level inverter off-grid control strategy to obtain the three-phase sinusoidal modulation signal v ma 、v mb 、v mc .

[0060] Step 2: Obtain the voltage v of the upper and lower voltage divider capacitors on the DC side of the three-level converter according to the detection circuit c1 、v c2 , the voltage on the upper voltage divider capacitor v c1 The voltage of the lower voltage divider capacitor v c2 Difference, get the control quantity v dif The harmonic extraction algorithm is used to track and amplify the AC quantity and output the harmonic component v h , the principle of the harmonic extraction algorithm is as follows:

[0061]

[0062] In the formula, k p Represents the proportional coefficient of the controller; ω h Represents three times the fundamental angular frequency of the power frequency; K R Represents the resonance compensation coefficient of the harmonic extraction algorithm; ω c represents the control bandwidth of the harmonic extraction algorithm, and s represents the Laplace operator.

[0063] Step 3: Use phase compensation algorithm to compensate the harmonic component v h Perform phase compensation, the phase compensation algorithm is as follows:

[0064] (1) According to the formula Calculate the phase difference between the harmonic component and the three-phase sinusoidal modulation signal; where ω is the angular frequency of the input / output voltage on the AC side; L is the value of the AC side filter inductance; I m is the input / output current amplitude on the AC side, V m is the amplitude of the input / output voltage on the AC side.

[0065] (2) Using the formula v m sin(3ωt-θ0)cos(θ0)+v m cos(3ωt-θ0)sin(θ0) for harmonic component v h Phase compensation is performed to obtain the zero-sequence component v0 with the same phase as the sinusoidal modulation wave, which is the optimal zero-sequence component proposed by the present invention, where v m is the harmonic component v h The amplitude of .

[0066] Step 4: Compare the zero-sequence component obtained in step 3 with the three-phase sinusoidal modulation signal v obtained in step 1 ma 、v mb 、v mc Add together to get the modulated signal v' a 、v' b 、v' c , compared with the high-frequency triangular carrier signal, the corresponding driving signal for each switch tube is generated.

[0067] The control method for midpoint potential balance of a three-level converter proposed in the present invention has the following advantages over the traditional control strategy:

[0068] (1) The present invention can obtain the optimal zero-sequence component, which is superimposed with the three-phase modulation wave obtained by other traditional three-level control methods, and can suppress the AC fluctuation of the mid-point potential to the greatest extent, solving the problem that the traditional zero-sequence voltage injection method can suppress the mid-point potential fluctuation to a certain extent, but cannot suppress it to the minimum.

[0069] (2) The present invention adopts a control strategy of zero-sequence voltage injection, wherein the zero-sequence voltage is a third harmonic with the same phase as the sinusoidal modulation wave. The superposition of the two can obtain an ideal saddle-shaped modulation wave. Compared with the traditional sinusoidal pulse width modulation algorithm, the use of a saddle-shaped modulation wave for modulation can improve the utilization rate of the DC bus voltage.

[0070] (3) The present invention can track and suppress AC harmonic components by adopting a harmonic extraction algorithm with the same resonant frequency as the frequency of the AC fluctuation component of the midpoint potential, and has better control performance than traditional proportional-integral control.

[0071] (4) The control strategy of zero-sequence component injection adopted in the present invention is basically the same as the space vector pulse width modulation algorithm in terms of effect. However, compared with the traditional space vector pulse width modulation algorithm based on redundant small vector adjustment, the control strategy of the present invention does not require complex sector judgment and a large number of trigonometric operations, has a small amount of calculation, and is easier to implement digitally.

[0072] (5) The control strategy adopted by the present invention is theoretically applicable to a variety of three-level converter topologies with midpoint potential balance problems, such as a three-phase three-level NPC inverter topology, a three-phase T-type three-level inverter topology, a three-phase three-level ANPC inverter topology and a Vienna rectifier topology; and is also applicable to a variety of operating modes, such as a grid-connected operation mode of a three-phase three-level inverter and an off-grid operation mode of a three-phase three-level inverter.

[0073] The above descriptions are only some implementation cases of the present invention and are not intended to limit the present invention in other forms. Technicians in the relevant technical field can make various modifications or supplements to the specific embodiments described or replace them in a similar manner. However, any simple modifications and equivalent changes made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A method for controlling the midpoint potential balance of a three-level converter, characterized in that: The following steps are involved: Step 1: Obtain the input / output voltage v of the AC side of the three-level converter a 、v b 、v c , input / output current i a 、i b 、i c , a three-level converter voltage / current control strategy is used to obtain a three-phase sinusoidal modulation signal v ma 、v mb 、v mc ; Step 2: Obtain the voltage v of the upper and lower voltage divider capacitors on the DC side of the three-level converter c1 、v c2 , the voltage on the upper voltage divider capacitor v c1 The voltage of the lower voltage divider capacitor v c2 Difference, get the control quantity v dif , and the control quantity v is extracted by harmonic extraction algorithm dif Perform tracking amplification and output harmonic component v h ; Step 3: For the harmonic component v obtained in step 2 h Perform phase compensation and adjust the harmonic component v h The three-phase sinusoidal modulation signal v obtained in step 1 ma 、v mb 、v mc The phase difference between the harmonic components v h After phase compensation, the zero-sequence component v0 is obtained; Step 4: Compare the zero-sequence component v0 obtained in step 3 with the three-phase sinusoidal modulation signal v obtained in step 1 ma 、v mb 、v mc Add together to get the modulated signal v' a 、v' b 、v' c , compared with the high-frequency triangular carrier signal, the corresponding driving signal for each switch tube is generated.

2. A method for controlling midpoint potential balance of a three-level converter as claimed in claim 1, characterized in that: The harmonic extraction algorithm is as follows: In the formula, k p Represents the proportional coefficient of the controller; ω h Represents three times the fundamental angular frequency of the power frequency; K R Represents the resonance compensation coefficient of the harmonic extraction algorithm; ω c represents the control bandwidth of the harmonic extraction algorithm, and s represents the Laplace operator.

3. A method for controlling midpoint potential balance of a three-level converter as claimed in claim 1, characterized in that: The step 3 is specifically as follows: (1) According to the formula Calculate the phase difference between the harmonic component and the three-phase sinusoidal modulation signal; where, ω is the angular frequency of the input / output voltage on the AC side; L is the value of the AC side filter inductance; I m is the input / output current amplitude on the AC side, V m is the amplitude of the input / output voltage on the AC side; (2) Using the formula v m sin(3ωt-θ0)cos(θ0)+v m cos(3ωt-θ0)sin(θ0) for harmonic component v h Perform phase compensation to obtain the zero-sequence component v0 with the same phase as the sinusoidal modulation wave, where v m is the harmonic component v h The amplitude of .

4. A method for controlling midpoint potential balance of a three-level converter as claimed in claim 1, characterized in that: The optimal zero-sequence component is specifically expressed as a sine wave with a frequency three times the fundamental frequency and an amplitude of 0.26 times the modulation ratio.

5. The method for controlling the midpoint potential balance of a three-level converter according to claim 1, characterized in that: When the harmonic component v h With the three-phase sinusoidal modulation signal v ma 、v mb 、v mc The phase difference θ0 is less than When θ0 is zero, no phase compensation is required.

6. A method for controlling midpoint potential balance of a three-level converter as claimed in claim 1, characterized in that: When the amplitude of the ninth and fifteenth harmonic components in the midpoint potential is not less than one tenth of the amplitude of the third harmonic component, a harmonic extraction algorithm is used to obtain the corresponding harmonic components and inject them into the three-phase modulated signal. The principle of the harmonic extraction algorithm is as follows: Among them, k p represents the proportional coefficient of the controller; variable i represents the harmonic order; ω h.i represents the angular frequency of the i-th harmonic; K R.i represents the resonance compensation coefficient of the harmonic extraction algorithm for the i-th harmonic; ω c.i represents the control bandwidth of the harmonic extraction algorithm for the i-th harmonic, and s represents the Laplace operator.

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