4mv modulation method for suppressing common-mode current of photovoltaic anpc type three-level inverter

Abstract

The application discloses a 4MV modulation method for inhibiting common-mode current of a photovoltaic ANPC type three-level inverter, and steps of the method comprise the following steps: 1. applying Kirchhoff's law to deduce a relationship formula of common-mode voltage and a parasitic capacitance of a photovoltaic array, and establishing a common-mode equivalent circuit; 2. deducing a functional relationship formula of instantaneous values of a neutral current, three-phase currents and a switch state; 3. according to a relationship between a basic vector diagram and a midpoint current expression and a principle that the common-mode voltage is 0, selecting six middle vectors as basic vectors, and using four middle vectors to synthesize a reference voltage vector in each sector, wherein two middle vectors are used for synthesizing the reference voltage vector, and the other two middle vectors are used as equivalent zero vectors and are used for introducing the neutral current to effectively reduce fluctuation of a midpoint potential. The application replaces the zero vectors with a pair of equal-amplitude and opposite-phase middle vectors, can reduce deviation of the midpoint potential, further reduces common-mode voltage caused by the deviation of the midpoint potential, and thus can reduce the common-mode current of the system.

Description

Technical Field

[0001] This invention relates to a 4MV modulation method for suppressing the common-mode current of a photovoltaic ANPC type three-level inverter, belonging to the field of photovoltaic power plant operation and maintenance and monitoring technology. Background Technology

[0002] Compared to two-level inverters, three-level inverters offer advantages such as higher withstand voltage and lower output voltage harmonic content. Non-isolated three-level inverters, by eliminating the isolation transformer, offer significant advantages in system structure simplification, size reduction, cost reduction, and efficiency improvement, leading to their increasingly widespread application. However, in actual operation, capacitor parameter mismatch and three-phase load asymmetry can easily cause neutral point potential imbalance, resulting in increased low-frequency harmonic content in the output voltage, elevated common-mode voltage, and even damage to switching devices. Furthermore, the absence of an isolation transformer creates an electrical connection between the photovoltaic (PV) and AC sides, and the parasitic capacitance between the PV array and ground allows the common-mode voltage in the loop to form a circuit through this parasitic capacitance, generating common-mode current and affecting the safe and stable operation of the system.

[0003] To suppress midpoint potential imbalance, related research from the perspective of PWM modulation includes carrier sinusoidal PWM with injected zero-sequence components, PWM based on specific harmonic elimination, space vector PWM, virtual space vector modulation (VSVM), and hybrid PWM. Although the above modulation methods can effectively control the midpoint voltage balance, the basic vector of the synthesized reference vector contains a vector with a large output common-mode voltage amplitude, resulting in a large common-mode voltage output and thus generating a large common-mode current in the photovoltaic power generation system.

[0004] There are roughly four methods for suppressing common-mode current. The first method is to shunt the current in parallel branches, for example, by directly connecting the common point n of the AC-side filter capacitor to the DC-side midpoint O, creating a high-frequency loop so that the high-frequency components of the common-mode current do not flow through the load. However, this approach affects the filter's performance, leading to increased harmonics in the output current at the switching frequency. The second method is to increase the common-mode loop impedance, for example, by adding a common-mode filter to reduce the amplitude of the common-mode current. However, this method increases the system size and R&D costs. Both of these methods involve changing the common-mode impedance to weaken the amplitude of the common-mode current. The third method is to use a series voltage source or parallel bridge arm to cancel the common-mode voltage, for example, by using an active common-mode noise suppressor. This involves connecting a common-mode voltage detection and tracking circuit to the primary side of the common-mode transformer, and connecting the secondary side in series between the inverter and the load. The voltage induced on the secondary side is equal in magnitude and opposite in direction to the common-mode voltage, effectively eliminating the common-mode voltage on the load side. These three methods are hardware suppression methods, all of which increase the system's hardware cost to varying degrees. The fourth method is to improve the modulation strategy, such as the zero common-mode vector modulation strategy, which uses seven zero common-mode vectors (six medium vectors and one zero vector) to form the basic vector. Currently, the three-level zero common-mode space vector modulation strategies reported in the literature are mainly 2M1Z (2 medium vectors and 1 zero vector) and 3MV (3 medium vectors). These two modulation strategies can theoretically completely eliminate common-mode voltage. However, due to the uncontrollable nature of the midpoint potential in the 2M1Z modulation strategy, common-mode voltage still exists at the inverter output, adversely affecting system performance. Therefore, researching zero common-mode voltage modulation strategies to suppress midpoint potential shift is of great significance. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, this invention proposes a 4MV modulation method for suppressing the common-mode current of a photovoltaic ANPC type three-level inverter, aiming to reduce the offset of the midpoint potential, further reduce the common-mode voltage caused by the offset of the midpoint potential, thereby reducing the common-mode current of the system and improving the working performance of the inverter.

[0006] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0007] The present invention provides a 4MV modulation method for suppressing common-mode current in a photovoltaic ANPC type three-level inverter, characterized by comprising the following steps:

[0008] Step 1: Based on Kirchhoff's laws, obtain the expression for the common-mode voltage of the three-level inverter, thereby establishing the relationship between the common-mode voltage and the common-mode current, in order to further construct the common-mode equivalent circuit, and obtain the factors affecting the common-mode current based on the common-mode equivalent circuit.

[0009] Step 2: Establish the relationship between the instantaneous values ​​of the neutral current and the three-phase current of the three-level inverter, and the three-phase switching function;

[0010] Step 3: Based on the expression for each space voltage vector of the three-level inverter and the midpoint current, obtain the neutral current corresponding to each space voltage vector;

[0011] Step 4: Based on the principle that the neutral current and common-mode voltage corresponding to each space voltage vector are zero, select six neutral vectors as basic vectors; and based on the reference voltage vector V of the three-level inverter... ref The α-β coordinate system is divided into six sectors by a clockwise angle θ with the positive half-axis of the α-axis. The action time of the four basic vectors in each sector is calculated, and the action time is output to the three-phase bridge arm of the three-level inverter to achieve point potential balance and common-mode voltage suppression in the three-level inverter.

[0012] The 4MV modulation method for suppressing common-mode current in photovoltaic ANPC type three-level inverters described in this invention is characterized in that, in step 1, the relationship between common-mode voltage and common-mode current is established using equation (1):

[0013]

[0014] In equation (1), R is the load resistance of the three-level inverter, and L f U is the AC side filter inductor of a three-level inverter. com This refers to the common-mode voltage output by the three-level inverter. The voltage across capacitor C2 on the DC side of the three-level inverter, C PV i is the parasitic capacitance of the photovoltaic array. com This is the common-mode current output by the three-level inverter.

[0015] In step 2, the midpoint current relationship is constructed using equation (2):

[0016] Define S k This represents the k-phase switching function of a three-level inverter, where k = A, B, C;

[0017] If S k =1 indicates that the k-phase voltage output by the three-level inverter is at a positive level P, and the amplitude of the k-phase voltage is Among them, U dc This refers to the voltage on the DC side of a three-level inverter.

[0018] If S k =0, which means that the k-phase voltage output by the three-level inverter is at zero level O, and the amplitude of the k-phase voltage is 0;

[0019] If S k=-1 indicates that the k-phase voltage output by the three-level inverter is negative level N, and the amplitude of the k-phase voltage is

[0020] i o =(1-|S A |)i A +(1-|S B |)i B +(1-|S C |)i C (2)

[0021] In equation (2), i A i B i C i represents the instantaneous value of the three-phase current output by the three-level inverter. o This is the neutral current of a three-level inverter.

[0022] Step 3 includes:

[0023] Define the neutral current i o When the value is positive, the corresponding small vector is a positive small vector;

[0024] Define the neutral current i o When the value is negative, the corresponding small vector is a negative small vector;

[0025] The permutations and combinations of P, O, and N yield six positive small vectors, six negative small vectors, six medium vectors, six large vectors, and three zero vectors; and the amplitudes of the positive and negative small vectors are... The magnitude of the median vector is The magnitude of the large vector is The magnitude of the zero vector is 0;

[0026] The midline current corresponding to the positive small vector is:

[0027] When S A =0,S B =-1,S C When i = -1, then i o =i A ;

[0028] When S A =1,S B =1,S C When i = 0, then i o =i C ;

[0029] When S A =-1,S B =0,S C When i = -1, then i o =i B ;

[0030] When S A =0,S B =1,S C When i = 1, then i o =i A ;

[0031] When S A =-1,S B =-1,S C When i = 0, then i o =i C ;

[0032] When S A =1,S B =0,S C When i = 1, then i o =i B ;

[0033] The midline current corresponding to the negative small vector is:

[0034] When S A =1,S B =0,S C When i = 0, then i o =-i A ;

[0035] When S A =0,S B =0,S C When i = -1, then i o =-i C ;

[0036] When S A =0,S B =1,S C When i = 0, then i o =-i B ;

[0037] When S A =-1,S B =0,S C When i = 0, then i o =-i A ;

[0038] When S A =0,S B =0,S C When i = 1, then i o =-i C ;

[0039] When S A =0,S B =-1,SC When i = 0, then i o =-i B ;

[0040] The midline current corresponding to the mid-vector is:

[0041] When S A =0,S B =1,S C When i = -1, then i o =i A ;

[0042] When S A =-1,S B =1,S C When i = 0, then i o =i C ;

[0043] When S A =1,S B =0,S C When i = -1, then i o =i B ;

[0044] When S A =0,S B =-1,S C When i = 1, then i o =i A ;

[0045] When S A =1,S B =-1,S C When i = 0, then i o =i C ;

[0046] When S A =-1,S B =0,S C When i = 1, then i o =i B ;

[0047] The midline current corresponding to the large vector is:

[0048] When S A =1,S B =-1,S C When i = -1, then i o =0;

[0049] When S A =1,S B =1,S C When i = -1, then i o =0;

[0050] When S A =-1,S B =1,S C When i = -1, then i o =0;

[0051] When S A =-1,S B =1,S C When i = 1, then i o =0;

[0052] When S A =-1,S B =-1,S C When i = 1, then i o =0;

[0053] When S A =1,S B =-1,S C When i = 1, then i o =0;

[0054] The neutral current corresponding to the zero vector is:

[0055] When S A =S B =S C When i = 0, then i o =0;

[0056] When S A =S B =S C When i = -1, then i o =0;

[0057] When S A =S B =S C When i = 1, then i o =0.

[0058] Step 4 includes:

[0059] Step 4.1: Select six intermediate vectors as basic vectors, including:

[0060] The output is the median vector of the PON, denoted as...

[0061] The output is the median vector of the OPN, denoted as...

[0062] The output is the median vector of the NPO, denoted as .

[0063] The output is the median vector of the NOP, denoted as

[0064] The output is the median vector of the ONP, denoted as

[0065] The output is the median vector of PNO, denoted as

[0066] Step 4.2: According to V ref The period of the clockwise angle θ with the positive semi-axis α is 2π, and we have

[0067] when At that time, the reference voltage vector V ref Located in the first sector;

[0068] when At that time, the reference voltage vector V ref Located in the second sector;

[0069] when At that time, the reference voltage vector V ref Located in the third sector;

[0070] when At that time, the reference voltage vector V ref Located in the fourth sector;

[0071] when At that time, the reference voltage vector V ref Located in the fifth sector;

[0072] when At that time, the reference voltage vector V ref Located in the sixth sector;

[0073] Step 4.3: When the reference voltage vector V ref When located in the first sector, the duration T of the vector V1 is obtained using equation (6). a The duration of action of the vector V6 is T. b The duration of action of the vector V5 is T. ez1 The duration of action of the vector V2 is T ez2 :

[0074]

[0075] In equation (5), T s is the carrier period of the three-level inverter; m is the modulation index, and Among them, |V ref | is the reference output voltage vector V ref The amplitude;

[0076] When the reference voltage vector V refWhen located in the second sector, the duration T of the action of the middle vector V2 can be obtained using equation (7). a The duration of action of the vector V1 is T b The duration of action of the vector V6 is T. ez1 The duration of action of the neutral vector V3 is T ez2 :

[0077]

[0078] When the reference voltage vector V ref When located in the third sector, the duration T of the vector V3 is obtained using equation (8). a The duration of action of the vector V2 is T. b The duration of action of the vector V1 is T ez1 The duration of action of the vector V4 is T ez2 :

[0079]

[0080] When the reference voltage vector V ref When located in the fourth sector, the duration T of the vector V4 is obtained using equation (9). a The duration of action of the vector V3 is T. b The duration of action of the vector V5 is T. ez1 The duration of action of the vector V2 is T ez2 :

[0081]

[0082] When the reference voltage vector V ref When located in the fifth sector, the duration T of the vector V5 is obtained using equation (7). a The duration of action of the vector V4 is T. b The duration of action of the vector V3 is T. ez1 The duration of action of the neutral vector V6 is T ez2 :

[0083]

[0084] When the reference voltage vector V ref When located in the sixth sector, the duration T of the vector V6 is obtained using equation (7). a The duration of action of the vector V5 is T. b The duration of action of the vector V4 is T. ez1 The duration of action of vector V1 is T ez2 :

[0085]

[0086] Compared with existing modulation strategies, the advantages of this invention are as follows:

[0087] 1. This invention abandons the high common-mode vector and uses the zero common-mode vector as the basic synthesis vector. By re-dividing the sector into 6 large sectors, and using four adjacent medium vectors in each sector for synthesis, the amplitude of the common-mode voltage is greatly reduced and the safety performance of the inverter is improved.

[0088] 2. This invention replaces the zero vector with a pair of equal-amplitude, opposite-phase center vectors. Compared with using the zero vector, this reduces the neutral current, thereby reducing the offset of the neutral point potential, further reducing the common-mode voltage caused by the offset of the neutral point potential, thus reducing the common-mode current of the system and improving the working performance of the inverter. Attached Figure Description

[0089] Figure 1 This is a topology diagram of a three-level ANPC photovoltaic inverter according to a specific embodiment of the present invention;

[0090] Figure 2 This is the common-mode equivalent circuit diagram of a three-level ANPC photovoltaic inverter;

[0091] Figure 3 A vector diagram of the output voltage of a three-level photovoltaic inverter;

[0092] Figure 4 This is a vector image of the present invention;

[0093] Figure 5 This is a composite image of the first sector of the present invention;

[0094] Figure 6a The DC-side midpoint potential fluctuation diagram for the 2M1Z modulation method;

[0095] Figure 6b This is a diagram showing the DC-side midpoint potential fluctuation of the modulation method of the present invention.

[0096] Figure 7a The common-mode voltage output of a three-level inverter using the 2M1Z modulation method is shown in the diagram.

[0097] Figure 7b This is a diagram of the common-mode voltage output of the three-level inverter using the modulation method of the present invention;

[0098] Figure 8a The common-mode current diagram of the output of a three-level inverter using the 2M1Z modulation method;

[0099] Figure 8b This is a diagram of the common-mode current output of the three-level inverter using the modulation method of the present invention. Detailed Implementation

[0100] In this embodiment, a 4MV modulation method for suppressing the common-mode current of a photovoltaic ANPC type three-level inverter is described, with the following specific steps:

[0101] Step 1: Apply Kirchhoff's laws to derive the expression for the common-mode voltage of the three-level inverter, analyze the relationship between the common-mode voltage and the common-mode current, establish the common-mode equivalent circuit, and reveal the factors affecting the common-mode current based on the equivalent circuit diagram.

[0102] When a photovoltaic array has parasitic capacitance to ground, the common-mode current flow path is as follows: Figure 1 As shown, according to Kirchhoff's voltage law, we can obtain equation (1):

[0103]

[0104] In equation (1), i A i B i C The three-phase output phase current is given by R, where R is the load resistance and L is the voltage across the load. f For AC side filter inductance, U AO U BO U CO U is the output voltage of the three-phase bridge arm. NO This is the voltage difference between the load grounding point and the midpoint of the DC side.

[0105] Applying Kirchhoff's current law at node N, we can obtain equation (2):

[0106] i com =i A +i B +i C (2)

[0107] In equation (2), i com This is the common-mode current.

[0108] Combining equations (1) and (2), we can obtain equation (2):

[0109]

[0110] Define common-mode voltage U com for:

[0111]

[0112] Equation (3) can then be transformed into:

[0113]

[0114] according to Figure 1 From the voltage relationship, the voltage difference U between the load grounding point and the DC side midpoint can be obtained. NOWith the parasitic capacitance C of the photovoltaic array PV Voltage U on CPV and the voltage on the DC side capacitor C2 The relationship between them is given by equation (6):

[0115]

[0116] Substituting equation (6) into equation (5), the parasitic capacitance voltage U of the photovoltaic array is... CPV Using common-mode current, we can obtain equation (7):

[0117]

[0118] Draw the common-mode equivalent circuit of the three-level inverter according to equation (7) as follows: Figure 2 As shown.

[0119] analyze Figure 2 As shown in the common-mode equivalent circuit, there are two voltage sources in this circuit: one is the DC-side capacitor voltage of the three-level inverter. The second is the common-mode voltage U of the three-level inverter. com In this equivalent circuit, only the capacitor voltage needs to be satisfied. Stable and common-mode voltage U com Setting it to a constant value can suppress system leakage current.

[0120] Step 2: Establish the relationship between the instantaneous values ​​of the neutral current and the three-phase current of the three-level inverter, and the three-phase switching function.

[0121] Define S k This represents the k-phase switching function of a three-level inverter, where k = A, B, C;

[0122] If S k =1 indicates that the k-phase voltage output by the three-level inverter is at a positive level P, and the amplitude of the k-phase voltage is Among them, U dc This refers to the voltage on the DC side of a three-level inverter.

[0123] If S k =0, which means that the k-phase voltage output by the three-level inverter is at zero level O, and the amplitude of the k-phase voltage is 0;

[0124] If S k =-1 indicates that the k-phase voltage output by the three-level inverter is negative level N, and the amplitude of the k-phase voltage is

[0125] Depend on Figure 1It can be seen that when the three-level inverter is working, current will only flow through the neutral line in the O state. Therefore, according to equation (8), the relationship between the instantaneous values ​​of the neutral line current and the three-phase current, and the three-phase switching function of the three-level inverter can be established:

[0126] i o =(1-|S A |)i A +(1-|S B |)i B +(1-|S C |)i C (8)

[0127] In equation (8), i A i B i C i represents the instantaneous value of the three-phase current output by the three-level inverter. o This is the neutral current.

[0128] Step 3: Based on the expression for each space voltage vector of the three-level inverter and the midpoint current, obtain the neutral current corresponding to each space voltage vector.

[0129] Define the neutral current i o When the value is positive, the corresponding small vector is a positive small vector;

[0130] Define the neutral current i o When the value is negative, the corresponding small vector is a negative small vector;

[0131] The permutations and combinations of P, O, and N yield six positive small vectors, six negative small vectors, six medium vectors, six large vectors, and three zero vectors; and the amplitudes of the positive and negative small vectors are... The magnitude of the median vector is The magnitude of the large vector is The magnitude of the zero vector is 0;

[0132] Combination Figure 3 According to equations (4) and (8), the corresponding neutral current for each vector is shown in Table 1, and the corresponding common-mode voltage is shown in Table 2.

[0133] Table 1. Relationship between vector and midline current

[0134]

[0135] Among them, the zero vector and the large vector do not participate in the inflow and outflow of the neutral current, and both are 0.

[0136] Table 2 Common-mode voltage relationship for each space vector

[0137]

[0138] Step 4: Based on the principle that the neutral current and common-mode voltage corresponding to each space voltage vector are zero, select six neutral vectors as basic vectors; and based on the reference voltage vector V of the three-level inverter... ref The α-β coordinate system is divided into six sectors by a clockwise angle θ with the positive half-axis of the α-axis. The duration of action of the four basic vectors in each sector is then calculated. The calculated duration of action of each vector is applied to the three-phase bridge arm of the three-level inverter to achieve the midpoint potential balance and common-mode voltage suppression in the three-level inverter.

[0139] Step 4.1: Based on the principle that the neutral current and common-mode voltage corresponding to each space voltage vector are zero, six neutral vectors are selected as basic vectors:

[0140] Space voltage vector V * Defined as Equation (9):

[0141]

[0142] In equation (9), j is the imaginary part. and The output voltage u of phase b of the three-level inverter are respectively b and the output voltage u of the c-phase of the three-level inverter c The corresponding exponential expression in the complex plane.

[0143] Each fundamental voltage vector has one and only one fixed position in the space voltage vector diagram. The space voltage vector diagram composed of the six fundamental voltage vectors is as follows: Figure 4 As shown, it includes:

[0144] The output is the median vector of the PON, denoted as...

[0145] The output is the median vector of the OPN, denoted as...

[0146] The output is the median vector of the NPO, denoted as .

[0147] The output is the median vector of the NOP, denoted as

[0148] The output is the median vector of the ONP, denoted as

[0149] The output is the median vector of PNO, denoted as

[0150] Step 4.2: Based on the reference voltage vector V of the three-level inverter ref The clockwise angle θ between the α-axis and the positive semi-axis divides the α-β coordinate system into six sectors.

[0151] The reference output voltage vector is defined as:

[0152] V ref =|V ref |e jθ =|V ref |(cosθ+j sinθ) (10)

[0153] In equation (10), |V ref | represents the magnitude of the reference output voltage vector, and θ is the value of V. ref The clockwise angle with the positive semi-axis of α.

[0154] Because V ref The period of the clockwise angle θ with the positive semi-axis α is 2π, and we have

[0155] The sector determination rule is as follows:

[0156] when At that time, the reference voltage vector V ref Located in the first sector;

[0157] when At that time, the reference voltage vector V ref Located in the second sector;

[0158] when At that time, the reference voltage vector V ref Located in the third sector;

[0159] when At that time, the reference voltage vector V ref Located in the fourth sector;

[0160] when At that time, the reference voltage vector V ref Located in the fifth sector;

[0161] when At that time, the reference voltage vector V ref Located in the sixth sector;

[0162] Step 4.3: Use equation (11) to calculate the duration of action of the four basic vectors in each sector.

[0163] When the reference output voltage vector V ref When located in the first sector, such as Figure 5 As shown, according to the volt-second balance principle, we can obtain equation (11):

[0164]

[0165] When the reference voltage vector is located in the first sector, the duration of each vector is:

[0166]

[0167] In equation (12), T a T is the duration of action of vector V1. b This represents the duration of action of the V6 vector.

[0168] The effective time of the equivalent zero vector is T. ez1 T ez2 And T ez1 =T ez2 T s The carrier period is denoted by m, which represents the modulation index, and its value is: U dc This is the DC side voltage.

[0169] Similarly, the duration of each vector's action can be obtained when the reference voltage vector is located in other sectors. The specific durations of the vectors are shown in Table 3.

[0170] Table 3. Comparison of the action time and switching sequence of various quantities in this invention.

[0171]

[0172]

[0173]

[0174] Step 4.4: The three-phase bridge arms of the three-level inverter achieve midpoint potential balance and common-mode voltage suppression based on the sector base voltage vector action sequence and action time determined in Step 4.3.

[0175] To verify the effectiveness of this invention, simulations were performed using MATLAB / Simulink software. Figure 1 This is a specific implementation of the ANPC type three-level inverter main circuit. Simulation parameters are: DC side voltage U... dc =1500V, DC voltage divider capacitor C1=12mF, DC voltage divider capacitor C2=12mF, carrier period The fundamental frequency f = 50Hz, and the filter inductance L f =80uH, filter capacitor C f =600uF, three-phase load resistance R =0.2732Ω.

[0176] Figure 6a and Figure 6b The midpoint potential fluctuation diagrams for the ANPC three-level inverter operating at a modulation ratio m = 0.75, using the 2M1Z modulation method and the modulation method of this invention, are shown. Figure 6a This is a diagram showing the midpoint potential fluctuation of the 2M1Z modulation method. Figure 6b This is a diagram showing the midpoint potential fluctuation of the modulation method of the present invention. The horizontal axis represents time (t / s), and the vertical axis represents the terminal voltage U of the DC voltage divider capacitor C1. C1 The terminal voltage U of the DC voltage divider capacitor C2 C2 The deviation ΔU, ΔU=U C1 -U C2 The unit is V. According to Figure 6a It can be seen that the midpoint potential fluctuation amplitude of the 2M1Z modulation method is about 70V. Figure 6b The amplitude of the midpoint potential fluctuation in the modulation method of this invention is about 20V, which verifies that the modulation method of this invention can effectively suppress the midpoint potential shift.

[0177] Figure 7a and Figure 7b For the ANPC three-level inverter operating at a modulation ratio m = 0.75, the output common-mode voltage using the 2M1Z modulation method and the modulation method of this invention is... Figure 7a This refers to the output common-mode voltage of the 2M1Z modulation method. Figure 7b The output common-mode voltage of the modulation method of this invention is shown. The horizontal axis represents time (t / s), and the vertical axis represents the common-mode voltage amplitude in V. According to... Figure 7a It can be seen that the output common-mode voltage amplitude of the 2M1Z modulation method is approximately 23V. Figure 7b The common-mode voltage amplitude of the modulation method of the present invention is about 7V, which verifies that the modulation method of the present invention can suppress the common-mode voltage while suppressing the shift of the midpoint potential.

[0178] Figure 8a and Figure 8b For the ANPC three-level inverter operating at a modulation ratio m = 0.75, the output common-mode current using the 2M1Z modulation method and the modulation method of this invention is... Figure 8a The common-mode current is output using the 2M1Z modulation method. Figure 8b This represents the common-mode current of the modulation method of this invention. The horizontal axis represents time (t / s), and the vertical axis represents the output common-mode current amplitude, in A. According to... Figure 8a It can be seen that the output common-mode current amplitude of the 2M1Z modulation method is approximately 12.5A. Figure 8b The common-mode current amplitude of the modulation method of the present invention is only about 0.2A, which verifies that the modulation method of the present invention has a good effect on suppressing common-mode current.

Claims

1. A 4MV modulation method for suppressing common-mode current of a photovoltaic ANPC type three-level inverter, characterized in that, The method comprises the following steps: Step 1, obtaining an expression of a common-mode voltage of a three-level inverter based on a Kirchhoff law, thereby establishing a relationship between the common-mode voltage and a common-mode current by using formula (1), further constructing a common-mode equivalent circuit, and obtaining factors affecting the common-mode current according to the common-mode equivalent circuit; (1) In equation (1), This is the load resistor for a three-level inverter. This is the AC side filter inductor for a three-level inverter. This refers to the common-mode voltage output by the three-level inverter. DC-side capacitor of a three-level inverter voltage, The parasitic capacitance of the photovoltaic array. This refers to the common-mode current output by the three-level inverter. Step 2, establishing a relationship between a neutral current of the three-level inverter and instantaneous values of three-phase currents and three-phase switching functions by using formula (2); Definitions denotes the k-phase switching function of a three-level inverter, ; If , it means that the k-phase voltage outputted by the three-level inverter is positive level P, and the amplitude of the k-phase voltage is ; wherein, is the voltage of the direct current side of the three-level inverter. If , it means that the k-phase voltage outputted by the three-level inverter is zero level O, and the amplitude of the k-phase voltage is 0; If , it means that the k-phase voltage outputted by the three-level inverter is negative level N, and the amplitude of the k-phase voltage is ; (2) in formula (2), is the instantaneous value of the three-phase current output by the three-level inverter, is the neutral current of the three-level inverter. Step 3, obtaining the neutral current corresponding to each space voltage vector according to a relationship between each space voltage vector of the three-level inverter and the neutral current; Step 4, according to the principle of zero common-mode voltage and the corresponding neutral current of each space voltage vector, six middle vectors are selected as basic vectors; and according to the reference voltage vector of the three-level inverter With The clockwise angle of the positive half axis of the axis , the The coordinate system is divided into six sectors, so as to calculate the action time of the four basic vectors in each sector, and output the action time to the three-phase bridge arm of the three-level inverter, so as to realize the balance of the three-level inverter midpoint potential and the suppression of the common-mode voltage. Step 4.1: selecting six middle vectors as basic vectors, including: The output is the middle vector of PON, denoted as ; The output is the middle vector of OPN, denoted as ; The output is the middle vector of NPO, denoted as ; The output is the middle vector of NOP, denoted as ; The output is the middle vector of the ONP, denoted as ; The output is the middle vector of PNO, denoted as ; Step 4.2: According to With The clockwise angle of the axis with the positive half-axis The period of , there are ; When the reference voltage vector is located in the first sector; When the reference voltage vector is located in the second sector; When the reference voltage vector is located in the third sector; When the reference voltage vector is located in the fourth sector; When the reference voltage vector is located in the fifth sector; When the reference voltage vector is located in the sixth sector; Step 4.3: When the reference voltage vector The time of action of the mid- vector V1 is obtained using equation (6) when the first sector The time of action of the mid- vector V6 The time of action of the mid- vector V5 And the time of action of the mid- vector V2 : (6) In formula (6), is the carrier period of the three-level inverter; is the modulation index, and ; wherein, is the amplitude of the reference voltage vector . When the reference voltage vector The time of action of the mid-vector V2 is obtained using equation (7) when located in the second sector The time of action of the mid-vector V1 The time of action of the mid-vector V6 The time of action of the mid-vector V3 : (7) When the reference voltage vector is located in the third sector, the time of action of the middle vector V3 is obtained using equation (8) :​​​ (8) When the reference voltage vector The time of action of the mid- vector V4 is obtained using equation (9) when located in the fourth sector The time of action of the mid- vector V3 The time of action of the mid- vector V5 And the time of action of the mid- vector V2 : (9) When the reference voltage vector The time of action of the middle vector V5 is obtained using equation (10) when in the fifth sector The time of action of the middle vector V4 The time of action of the middle vector V3 And the time of action of the middle vector V6 : (10) When the reference voltage vector The time of action of the mid-vector V6 is obtained using equation (11) when in the sixth sector The time of action of the mid-vector V5 The time of action of the mid-vector V4 And the time of action of the mid-vector V1 : (11)。 2. The 4MV modulation method of suppressing common-mode current of a photovoltaic ANPC type three-level inverter according to claim 1, characterized in that, The step 3 comprises: Defining the midline current The small vector corresponding to the positive timing is a positive small vector; Defining the midline current A small vector corresponding to a negative time is a negative small vector. Six positive small vectors, six negative small vectors, six middle vectors, six large vectors and three zero vectors are obtained by permutation and combination of P, O and N; the amplitude of the positive and negative small vectors is the amplitude of the middle vectors is the amplitude of the large vectors is the amplitude of the zero vectors is 0. The neutral current corresponding to the positive small vector is: When then ; When then ; When then ; When then ; When then ; When then ; The neutral current corresponding to the negative small vector is: When then ; When then ; When then ; When then ; When then ; When then ; The neutral current corresponding to the middle vector is: When then ; When then ; When then ; When then ; When then ; When then ; The neutral current corresponding to the large vector is: When then ; When then ; When then ; When then ; When then ; When then ; The neutral current corresponding to the zero vector is: When then ; When then ; When then .

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