Neutral point voltage and carrier control method of T-type three-level converter based on zero common mode voltage

Abstract

The present invention discloses a method for controlling the midpoint voltage and carrier of a T-type three-level converter based on zero common-mode voltage, comprising: determining the common-mode voltage of each space vector according to a mathematical model of the T-type three-level converter. Selecting a vector with a common-mode voltage of 0, considering the constraint of the midpoint current, selecting three space vectors with a midpoint current sum of 0 to synthesize a new virtual vector, introducing a distribution coefficient, and realizing midpoint potential rebalancing by changing the distribution coefficient when the midpoint potential is offset; using a carrier modulation strategy to propose a modulation wave expression, and establishing a simulation model in MATLAB / Simulink to verify the effectiveness of the proposed strategy. The present invention realizes the balance of the midpoint potential while achieving zero common-mode voltage, and solves the trade-off problem between the common-mode voltage and the midpoint potential balance in the existing solution.

Description

Technical Field

[0001] The present invention relates to the field of converter modulation, and in particular to a method for controlling midpoint voltage and carrier of a T-type three-level converter based on zero common mode voltage. Background Art

[0002] As the proportion of new energy generation such as wind power and photovoltaic power generation in the power system continues to increase, the application of three-level inverters has become more and more extensive. Three-level inverters have the characteristics of low harmonics, high efficiency, low loss, and small voltage change rate. The pulse width modulation of three-level inverters mainly includes space vector pulse width modulation and carrier pulse width modulation. However, in practical applications, the high-frequency common-mode voltage of the three-level inverter will generate high-frequency coupling current, which will accelerate the aging of motor insulation. At the same time, the common-mode voltage will generate leakage current, which will eventually flow into the power grid through the conductor and generate electromagnetic interference. In addition, high-frequency leakage current may cause malfunction of current relay protection devices and reduce the stability of the power system. Moreover, the current generated by the common-mode voltage will consume power when flowing through the load, which is very unfavorable to the load and affects its long-term safe operation. Therefore, the optimization control of the common-mode voltage is of great significance.

[0003] At present, the research directions of common-mode voltage are: one is to suppress the common-mode voltage by hardware methods such as capacitor filtering and amplifiers with relatively high common-mode suppression. The other is to use a software method to achieve zero common-mode voltage using a zero common-mode voltage vector. The software method of eliminating common-mode voltage currently mainly uses space vector modulation to eliminate common-mode voltage. The software method can effectively reduce costs and has more research value in practical applications. However, when the midpoint potential is offset, the traditional control strategy cannot cope with the midpoint potential offset problem, which will cause uneven capacitor voltage distribution, thereby affecting the stable operation of the system. Summary of the invention

[0004] The purpose of the present invention is to provide a T-type three-level converter midpoint voltage and carrier control method based on zero common-mode voltage, which can achieve zero common-mode voltage and balance the midpoint potential, and solve the trade-off problem between common-mode voltage and midpoint potential balance in existing solutions.

[0005] The technical solution to achieve the purpose of the present invention is:

[0006] A method for controlling midpoint voltage and carrier of a T-type three-level converter based on zero common mode voltage, characterized by comprising:

[0007] Determine the common mode voltage of each space vector according to the mathematical model of the T-type three-level converter;

[0008] Select the vector with a common-mode voltage of 0. Considering the constraint condition of the neutral-point current, select three space vectors with the sum of neutral-point currents being 0 to synthesize a new virtual vector. At the same time, introduce a distribution coefficient, and when the neutral-point potential shifts, achieve the rebalancing of the neutral-point potential by changing the distribution coefficient;

[0009] Synthesize the target vector from the new virtual vector, and at the same time design the modulation wave expression based on the carrier modulation strategy.

[0010] Furthermore, the mathematical model includes the KVL equation and the KCL equation.

[0011] Furthermore, the space vectors are divided into zero vectors, small vectors, medium vectors, and large vectors according to the vector length. The common-mode voltages of each space vector are:

[0012]

[0013] where, (S a S b S c ) is the vector representation method of the switch state, S x is a single-phase switch, x = a, b, c, |V NO | is the common-mode voltage value, and V dc is the DC voltage.

[0014] Furthermore, select the vector with a common-mode voltage of 0. Considering the constraint condition of the neutral-point current, the new virtual vector synthesized by selecting three space vectors with the sum of neutral-point currents being 0 is:

[0015]

[0016] where, k is the distribution coefficient, V 1M , V 2M , V 3M , V 4M , V 5M , V 6M are new virtual vectors, and V 1(10-1) , V 2(01-1) , V 3(-110) , V 4(-101) , V 5(0-11) , V 6(1-10) are medium vectors; for the new virtual vector V 1M , when the distribution coefficient k = 1 / 3, the neutral-point current is 0 and the neutral-point potential remains unchanged; when 0 < k < 1 / 3, the neutral-point current is (1 - 3k)i b / 2 and the neutral-point potential rises; when 1 / 3 < k < 1, the neutral-point current is -(3k - 1)i b / 2 and the neutral-point potential drops.

[0017] Furthermore, synthesizing the target vector from the new virtual vector specifically includes: dividing the entire vector plane into 6 sectors based on the virtual vector, designing a switching sequence for each sector, and synthesizing the target vector through the switching sequence.

[0018] Furthermore, the six sectors are: -30° to 30° for sector I, 30° to 90° for sector II, 90° to 150° for sector III, 150° to 210° for sector IV, 210° to 270° for sector V, and 270° to 330° for sector VI.

[0019] Furthermore, the switch timing of each sector includes:

[0020] Switching sequence of sector I:

[0021] V 2(01-1) |i a →V 1(10-1) |i b →V 0(000) |0→V 5(0-11) |i a →V 6(01-1) |i c

[0022] V 6(01-1) |i c →V 5(0-11) |i a →V 0(000) |0→V 1(10-1) |i b →V 2(01-1) |i a

[0023] Switching sequence of sector II:

[0024] V 3(-110) |i c →V 2(01-1) |i a →V 0(000) |0→V 6(01-1) |i c →V 1(10-1) |i b

[0025] V 1(10-1) |i b →V 6(01-1) |i c →V 0(000) |0→V 2(01-1) |i a →V 3(-110) |i c

[0026] Switching sequence of sector III:

[0027] V 4(-101) |i b →V 3(-110) |i c →V 0(000) |0→V 1(10-1) |i b →V 2(01-1) |i a

[0028] V 2(01-1) |i a →V 1(10-1) |i b →V 0(000) |0→V 3(-110) |i c →V 4(-101) |i b

[0029] Opening time of Ogiku IV:

[0030] V 5(0-11) |i a →V 4(-101) |i b →V 0(000) |0→V 2(01-1) |i a →V 3(-110) |i c

[0031] V 3(-110) |i c →V 2(01-1) |i a →V 0(000) |0→V 4(-101) |i b →V 5(0-11) |i a

[0032] Ogiku Ⅴ opening sequence:

[0033] V 6(01-1) |i c →V 5(0-11) |i a →V 0(000) |0→V 3(-110) |i c →V 4(-101) |i b

[0034] V 4(-101) |i b →V 3(-110) |i c →V 0(000) |0→V 5(0-11) |i a →V6(01-1) |i c

[0035] Switching sequence of sector VI:

[0036] V 1(10-1) |i b →V 6(01-1) |i c →V 0(000) |0→V 4(-101) |i b →V 5(0-11) |i a

[0037] V 5(0-11) |i a →V 4(-101) |i b →V 0(000) |0→V 6(01-1) |i c →V 1(10-1) |i b

[0038] Among them, i a 、i b 、i c is the midpoint current, V 1(*) ~V 6(*) represents the corresponding * vector.

[0039] Furthermore, the modulation wave expression is:

[0040]

[0041] Among them, k is the allocation coefficient, u1-u6 are the modulation wave signals, and d1 and d2 are the duty cycles corresponding to vectors V1 and V2.

[0042] Furthermore, the sectors I, III and V are further divided into two small sectors, namely sectors A and B, and the sectors II, IV and VI are further divided into two small sectors, namely sectors C and D. The relationship between the duty cycle and the three-phase voltage in sectors A, B, C and D is:

[0043]

[0044] Among them U max , U mid , U min are the maximum, middle and minimum values ​​of the three-phase voltage, and G1~G6 are the relationship parameters between the duty cycle of each sector and the three-phase voltage.

[0045] Furthermore, the relationship parameters G1 to G6 are:

[0046] Sector S <![CDATA[G1]]> <![CDATA[G2]]> <![CDATA[G3]]> <![CDATA[G4]]> <![CDATA[G5]]> <![CDATA[G6]]> A 2 / 3 -4 / 3 2 / 3 2 / 3 2 / 3 -4 / 3 B 2 / 3 2 / 3 -4 / 3 2 / 3 -4 / 3 2 / 3 C -2 / 3 4 / 3 -2 / 3 -2 / 3 -2 / 3 -2 / 3 D -2 / 3 -2 / 3 4 / 3 -2 / 3 -2 / 3 -2 / 3

[0047] Compared with the prior art, the present invention has the following beneficial effects: (1) In view of the fact that the existing pulse width modulation algorithm makes a compromise between the common mode voltage and the midpoint potential, the present invention designs a new virtual vector and introduces a distribution coefficient, which can simultaneously achieve the balance of zero common mode voltage and midpoint potential; (2) The present invention proposes a carrier modulation implementation method, which further simplifies the carrier modulation implementation scheme while achieving the balance of zero common mode voltage and midpoint potential at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 This is the topology diagram of the T-type three-level inverter.

[0049] Figure 2 It is the space vector diagram of T-type three-level inverter.

[0050] Figure 3 This is the circuit diagram under the action of the middle vector.

[0051] Figure 4 For the new virtual vector and sector distribution map.

[0052] Figure 5 This is a composite diagram of the intermediate vectors.

[0053] Figure 6 It is the vector sequence of each phase and the midpoint current and voltage fluctuation diagram.

[0054] Figure 7 This is the modulation strategy control block diagram.

[0055] Figure 8 The three-phase output current diagram of the proposed modulation algorithm.

[0056] Fig. 9 This is the three-phase output current spectrum of the proposed modulation algorithm.

[0057] Fig.10 This is a comparison diagram of the midpoint potential when the capacitor voltage is unbalanced.

[0058] Fig.11 This is a comparison diagram of common-mode voltage when capacitor voltage is unbalanced. DETAILED DESCRIPTION

[0059] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. 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.

[0060] The topology of the T-type three-level inverter is as follows Figure 1 Each phase bridge arm of the inverter consists of a switch tube S with an anti-parallel diode.x1 , S x2 , S x3 , S x4 (x=a, b, c). Each phase bridge arm of the inverter can output 3 different voltages. x When (x=a, b, c) is 1, S a1 , S a2 The switch is turned on, S a3 , S a4 The switch tube is turned off, and the phase voltage V XO (x=A, B, C) is V dc / 2.

[0061] Table 1 is obtained based on different switch states and load voltages. It can be seen that there are 3 switch states in the T-type three-level circuit. 3 = 27 types. According to the vector length, it can be divided into zero vector, small vector, medium vector, and large vector, and further obtain the voltage vector under different switching states, such as Figure 2 .

[0062] Table 1 Switching state and output voltage of each phase of T-type three-level inverter

[0063]

[0064] according to Figure 1 Write out the KVL (Kirchhoff's Voltage Law) equation:

[0065]

[0066] Where V XO is the three-phase voltage; i a 、i b 、i c is the load current; V NO is the common mode voltage of the three-level circuit; R, L are the load resistance and inductance, e a 、e b 、e c is the three-phase grid voltage. According to the KCL (Kirchhoff's Current Law) equation, the inverter load side satisfies:

[0067]

[0068] According to equations (1) and (2), the common-mode voltage expression is obtained:

[0069]

[0070] The relationship between different switch states and common mode voltage is deduced from this, see Table 2. In Table 2, the vector representation is (S a S b Sc ). Therefore, the large vector (11-1) represents S a =1, S b =1, S c = -1. According to Figure 2 It can be seen that the common-mode voltage of the voltage vector is greater than zero. When SVPWM modulation is used, vectors with different common-mode voltages will be used, so high-frequency jumps in the common-mode voltage will occur.

[0071] Table 2 Switching state and common mode voltage of T-type three-level inverter

[0072]

[0073] The vector with zero common mode voltage is called zero common mode voltage vector. When all selected vectors are zero common mode voltage vectors, the common mode voltage is guaranteed to be 0 at any time. Therefore, large vectors, small vectors, and some zero vectors are eliminated, and the middle vector and zero vector (000) are selected as the basic vectors for subsequent virtual vector synthesis. The midpoint voltage is:

[0074]

[0075] Where V O is the DC bus midpoint voltage offset, V C1 、V C2 is the upper and lower capacitor voltages on the load side. The reason for the change in capacitor voltage is the charging and discharging of the capacitor by the midpoint current. The midpoint current can be expressed as:

[0076]

[0077] From formula (5), we can see that when S x =1, the phase current i corresponding to phase x x Otherwise, when S x =1 or S x = -1, the phase current i corresponding to phase x x It has no effect on the midpoint voltage. Since the zero vector has no effect on the midpoint potential, we only need to consider the midpoint vector. Taking the midpoint vector of (10-1) as an example, we only need to consider the midpoint vector. Taking the midpoint vector of (10-1) as an example, the current flow direction is as follows: Figure 3 , the midpoint current flowing through is:

[0078] i o =i b =I m cos(ωt-2π / 3-θ) (6)

[0079] Where, θ = arctan(2πfL / R), I m is the phase current amplitude.

[0080] To achieve zero common-mode voltage, the selected vectors can only be zero vectors and medium vectors. Secondly, to balance the midpoint potential, the midpoint current synthesized by the three basic vectors should be ensured to be 0. When the midpoint potential deviates, the midpoint potential balance can be achieved by changing the distribution coefficients of each vector; when the midpoint potential is balanced, this method can achieve zero common-mode voltage and midpoint potential balance. The new virtual vectors and sector distributions are shown in Figure 4 .

[0081] Taking the right half-plane as an example, the virtual vector V 1M is synthesized by the three vectors V1, V2, and V6, and the virtual vector V 2M is synthesized by the three vectors V1, V2, and V3, and the zero vector remains unchanged. And so on, the virtual vectors can be expressed as:

[0082]

[0083] Taking V 1M as an example, when k = 1 / 3, the midpoint current is 0 and the midpoint potential remains unchanged; when 0 < k < 1 / 3, the midpoint current is (1 - 3k)i b / 2 and the midpoint potential rises; when 1 / 3 < k < 1, the midpoint current is -(3k - 1)i b / 2 and the midpoint potential drops.

[0084] Figure 4 Among them, the 6 medium vectors V 1M ~V 6M divide the space vector plane into six sectors. On the basis of the large sectors, each large sector is further divided into two small sectors, and the 1st and 2nd small sectors of the odd and even large sectors are combined into an overall sector, that is, the sectors I, III, and V are further divided into two small sectors, namely sector A and sector B, and the sectors II, IV, and VI are further divided into two small sectors, namely sector C and sector D, so as to simplify the calculation method of the duty cycle. Select different medium vectors and the zero vector V0 to synthesize the rotating vector V r , and write the equation according to the volt-second balance:

[0085]

[0086] In the formula, d1, d2, and d0 are the duty cycles of V1, V2, and V0 respectively. When determining the duty cycle of the non-zero vector, the duty cycle of the zero vector is also determined accordingly. Therefore, only the non-zero vectors of the new sector need to be analyzed. Combining the new sector division and Equation (9), the relationship between the duty cycle and the three-phase voltage can be summarized as:

[0087]

[0088] In the formula, U max , U mid , Umin are the maximum, middle and minimum values ​​of the three-phase voltage, G1~G6 are relational parameters, see Table 3 for details.

[0089] Table 3 Parameters of the relationship between duty cycle and three-phase voltage

[0090] Sector S <![CDATA[G1]]> <![CDATA[G2]]> <![CDATA[G3]]> <![CDATA[G4]]> <![CDATA[G5]]> <![CDATA[G6]]> A 2 / 3 -4 / 3 2 / 3 2 / 3 2 / 3 -4 / 3 B 2 / 3 2 / 3 -4 / 3 2 / 3 -4 / 3 2 / 3 C -2 / 3 4 / 3 -2 / 3 -2 / 3 -2 / 3 -2 / 3 D -2 / 3 -2 / 3 4 / 3 -2 / 3 -2 / 3 -2 / 3

[0091] like Figure 5 , taking the first sector as an example, dual-carrier six virtual modulation signals are used to synthesize intermediate vectors m1~m6. All virtual vectors are synthesized from intermediate vectors, expressed as:

[0092]

[0093] The midpoint current of a switching cycle can be analyzed by equation (5), and the voltage fluctuation can be analyzed by the current fluctuation. Figure 6 , is the sequence of each phase vector and the midpoint current and voltage fluctuations. According to the proportional relationship between the carrier and the modulation signal, we can get:

[0094]

[0095] By combining equation (12) with equation (7), the introduced distribution coefficient can control the midpoint current and achieve midpoint potential balance by changing the duty cycle of the synthetic vector. When the value of k is 1 / 3, it is guaranteed that the system continues to maintain the midpoint potential after balance. Table 4 shows the vector sequence and midpoint current of each sector.

[0096] Table 4 Vector sequence and midpoint current of each sector

[0097]

[0098] According to the above analysis, the Figure 7 The PI modulation strategy control block diagram is used to perform modulation wave control. This control strategy is well known in the art and will not be elaborated in detail here.

[0099] A simulation model is built in MATLAB / Simulink software for the proposed modulation algorithm. The main model parameters are shown in Table 5.

[0100] Table 5 Simulation model parameters

[0101] parameter Numeric <![CDATA[DC power supply V dc / V]]> 700 <![CDATA[Upper and lower capacitance values C1, C2 / μF]]> 2000 <![CDATA[Upper and lower capacitor voltages V c1 , V c2 / V]]> 350 Load inductance X / mH 10 Switching frequency f / kHz 10 Resistance R / Ω 10

[0102] After steady state, the three-phase output current of the proposed modulation algorithm is as follows: Figure 8 shown. Fig. 9 This is the spectrum diagram of the FFT analysis of the three-phase current. Assume that the capacitor V c1 =380V, lower capacitor voltage V c2=380V. At t=0.4, the modulation strategy is put into use. The midpoint potential comparison under the condition of capacitor voltage imbalance is as follows Fig.10 As shown in Figure 2, the common-mode voltage comparison under capacitor voltage imbalance is Fig.11 shown.

[0103] In view of the fact that the existing pulse width modulation algorithm makes a compromise between the common mode voltage and the midpoint potential, the present invention designs a new virtual vector and introduces a distribution coefficient, which can simultaneously achieve the balance of zero common mode voltage and midpoint potential; the present invention proposes a carrier modulation implementation method, which further simplifies the carrier modulation implementation scheme while achieving the balance of zero common mode voltage and midpoint potential at the same time.

[0104] Obviously, those skilled in the art can make various changes and modifications to the embodiments of the present invention without departing from the spirit and scope of the embodiments of the present invention. Thus, if these modifications and variations of the embodiments of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for controlling midpoint voltage and carrier of a T-type three-level converter based on zero common mode voltage, characterized in that: include: Determine the common mode voltage of each space vector according to the mathematical model of the T-type three-level converter; A space vector with a common mode voltage of 0 is selected, and the constraints of the midpoint current are considered. Three space vectors with a midpoint current sum of 0 are selected to synthesize a new virtual vector. At the same time, a distribution coefficient is introduced. When the midpoint potential is offset, the midpoint potential is rebalanced by changing the distribution coefficient. The target vector is synthesized from the new virtual vector, and the modulation wave expression is designed based on the carrier modulation strategy; The space vector is divided into a zero vector, a small vector, a medium vector and a large vector according to the vector length, and the common mode voltage of each space vector is: Among them, (S a S b S c ) is the vector representation of the switch state, S x For a single-phase switch, x = a, b, c, |V NO | is the common mode voltage value, V dc is the DC voltage; The new virtual vector is: where k is the distribution coefficient, V 1M , V 2M , V 3M , V 4M , V 5M , V 6M are new virtual vectors, and V 1(1 0 -1) , V 2(0 1 -1) , V 3(-1 1 0) , V 4(-1 0 1) , V 5(0 -1 1) , V 6(1 -1 0) are medium vectors; for the new virtual vector V 1M , when the distribution coefficient k = 1 / 3, the midpoint current is 0 and the midpoint potential remains unchanged; when 0 < k < 1 / 3, the midpoint current is (1 - 3k)i b / 2 and the midpoint potential rises; when 1 / 3 < k < 1, the midpoint current is -(3k - 1)i b / 2 and the midpoint potential drops.

2. The method for controlling midpoint voltage and carrier of a T-type three-level converter based on zero common mode voltage according to claim 1, characterized in that: The mathematical model includes the KVL equation and the KCL equation.

3. The method for controlling midpoint voltage and carrier of a T-type three-level converter based on zero common mode voltage according to claim 1, characterized in that: Synthesizing the target vector from the new virtual vector specifically includes: dividing the entire vector plane into 6 sectors based on the new virtual vector, designing the switching timing of each sector, and synthesizing the target vector through the switching timing.

4. The method for controlling midpoint voltage and carrier of a T-type three-level converter based on zero common mode voltage according to claim 3, characterized in that: The six sectors are: -30° to 30° for sector I, 30° to 90° for sector II, 90° to 150° for sector III, 150° to 210° for sector IV, and 210° to 270° for sector V. 270°~330° is sector VI.

5. The method for controlling midpoint voltage and carrier of a T-type three-level converter based on zero common mode voltage according to claim 4, characterized in that: The switching timing of each sector includes: Switching sequence of sector I: V 2(0 1 -1) |i a →V 1(1 0 -1) |i b →V 0(0 0 0) |0→V 5(0 -1 1) |i a →V 6(1 -1 0) |i c V 6(1 -1 0) |i c →V 5(0 -1 1) |i a →V 0(0 0 0) |0→V 1(1 0 -1) |i b →V 2(0 1 -1) |i a Switching sequence of sector II: V 3(-1 1 0) |i c →V 2(0 1 -1) |i a →V 0(0 0 0) |0→V 6(1 -1 0) |i c →V 1(1 0 -1) |i b V 1(1 0 -1) |i b →V 6(1 -1 0) |i c →V 0(0 0 0) |0→V 2(0 1 -1) |i a →V 3(-1 1 0) |i c Switching sequence of sector III: V 4(-1 0 1) |i b →V 3(-1 1 0) |i c →V 0(0 0 0) |0→V 1(1 0 -1) |i b →V 2(0 1 -1) |i a V 2(0 1 -1) |i a →V 1(1 0 -1) |i b →V 0(0 0 0) |0→V 3(-1 1 0) |i c →V 4(-1 0 1) |i b Switching sequence of sector IV: V 5(0 -1 1) |i a →V 4(-1 0 1) |i b →V 0(0 0 0) |0→V 2(0 1 -1) |i a →V 3(-1 1 0) |i c V 3(-1 1 0) |i c →V 2(0 1 -1) |i a →V 0(0 0 0) |0→V 4(-1 0 1) |i b →V 5(0 -1 1) |i a Switching sequence of sector V: V 6(1 -1 0) |i c →V 5(0 -1 1) |i a →V 0(0 0 0) |0→V 3(-1 1 0) |i c →V 4(-1 0 1) |i b V 4(-1 0 1) |i b →V 3(-1 1 0) |i c →V 0(0 0 0) |0→V 5(0 -1 1) |i a →V 6(1 -1 0) |i c Switching sequence of sector VI: V 1(1 0 -1) |i b →V 6(1 -1 0) |i c →V 0(0 0 0) |0→V 4(-1 0 1) |i b →V 5(0 -1 1) |i a V 5(0 -1 1) |i a →V 4(-1 0 1) |i b →V 0(0 0 0) |0→V 6(1 -1 0) |i c →V 1(1 0 -1) |i b Among them, i a 、i b 、i c is the midpoint current, V 1(1 0 -1) 、V 2(0 1 -1) 、V 3(-1 1 0) 、V 4(-1 0 1) 、V 5(0 -1 1) 、V 6(1 -1 0) is the mean vector, V 0(0 0 0) is the zero vector.

6. The method for controlling midpoint voltage and carrier of a T-type three-level converter based on zero common mode voltage according to claim 4, characterized in that: The modulation wave expression is: Among them, k is the allocation coefficient, u1-u6 is the modulation wave signal, d1 and d2 are the duty cycles corresponding to vectors V1 and V2.

7. The method for controlling midpoint voltage and carrier of a T-type three-level converter based on zero common mode voltage according to claim 6, characterized in that: The sectors I, III and V are further divided into two small sectors, namely sectors A and B. The sectors II, IV and VI are further divided into two small sectors, namely sectors C and D. The relationship between the duty cycle and the three-phase voltage in sectors A, B, C and D is: Among them U max , U mid , U min are the maximum, middle and minimum values ​​of the three-phase voltage, and G1~G6 are the relationship parameters between the duty cycle of each sector and the three-phase voltage.

8. The method for controlling midpoint voltage and carrier of a T-type three-level converter based on zero common mode voltage according to claim 7, characterized in that: The relationship parameters G1 to G6 are:

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