A modulation strategy for low common-mode voltage of three-level three-phase four-leg inverter

Abstract

The present invention discloses a low common-mode voltage modulation strategy for a three-level three-phase four-bridge-arm inverter, and belongs to the field of photovoltaic power generation. The modulation strategy comprises the following steps: sampling a three-phase modulation wave and a superimposed zero-sequence component; respectively giving the carriers used by the first three bridge arms and the fourth bridge arm; generating a PWM wave to drive the inverter to work. Aiming at the problem that the common-mode voltage of the three-level three-phase four-bridge-arm inverter is relatively high, the existing modulation algorithm is complex, the amount of calculation is large, and it is not conducive to promotion to engineering applications, the present invention proposes a low common-mode voltage modulation strategy for a three-level three-phase four-bridge-arm inverter without adding or changing any hardware. The modulation strategy can reduce the common-mode voltage amplitude of the three-level three-phase four-bridge-arm inverter and the number of common-mode voltage level changes within one carrier cycle by only changing the carrier phase.

Description

Technical Field

[0001] The present invention relates to the field of photovoltaic power generation, and in particular to a modulation strategy for low common mode voltage of a three-level three-phase four-bridge-arm inverter. Background Art

[0002] As a sustainable and clean energy, solar energy has many advantages. In recent years, photovoltaic power generation technology, as a method to effectively utilize solar energy resources, has received widespread attention and support.

[0003] In photovoltaic systems, three-level inverters have a series of advantages over traditional two-level inverters, including low output waveform harmonic content, reduced losses, increased switching frequency, reduced costs, low electromagnetic interference, good stability, and high reliability. The three-phase four-leg inverter topology is widely used in photovoltaic power generation systems because of its unbalanced load handling capabilities, flexible control, high DC voltage utilization, and small size and weight.

[0004] However, due to the lack of electrical isolation, the three-phase four-bridge-arm inverter will generate leakage current due to the common-mode voltage problem. The leakage current will cause unnecessary losses and even threaten personal safety in severe cases. Common-mode voltage will also cause common-mode interference and electromagnetic interference, affecting the normal operation of the equipment and reducing the reliability of the equipment. How to reduce the common-mode voltage of the inverter has always been a focus of research on photovoltaic power generation technology, but there are few literatures and patents that propose effective solutions for the common-mode voltage suppression of the three-phase four-bridge-arm inverter:

[0005] The Chinese invention patent disclosure specification "A common-mode voltage suppression method for three-level carrier reverse stacking SPWM modulation" (CN114629366A) realizes PWM wave generation by comparing the output with the SPWM modulation wave through the carrier reverse stacking technology. In theory, the common-mode voltage amplitude can be limited to within 1 / 6 of the DC bus voltage. However, the use of SPWM modulation makes the utilization rate of the DC voltage low, the use of the modulation strategy is relatively limited, and the amplitude of the common-mode voltage and the number of changes in one carrier cycle are still relatively large.

[0006] The Chinese invention patent disclosure specification "A three-level converter low common-mode voltage control method and system" (CN110247567A) provides a three-level converter low common-mode voltage control method and system based on optimal output current ripple, which can limit the common-mode voltage amplitude to within 1 / 6 of the DC bus voltage and significantly reduce the output current ripple. However, the calculation process is too complicated and cumbersome, wasting a lot of time and resources. The amplitude of the common-mode voltage is still relatively large, and corresponding computer equipment and computer programs need to be configured, which is costly and has a limited scope of application.

[0007] In summary, the prior art still has the following problems:

[0008] 1. The common mode voltage amplitude of the existing modulation method is still relatively high, which is 1 / 6 of the DC bus voltage;

[0009] 2. The existing modulation method has a cumbersome calculation process, a complex implementation algorithm, high cost, and is difficult to implement and apply to engineering; Summary of the invention

[0010] The technical problem to be solved by the present invention is the problem existing in the above-mentioned prior art. Specifically, based on the three-level three-phase four-bridge-arm inverter topology, a modulation strategy is proposed without adding or changing any hardware to suppress the common-mode voltage of the three-level three-phase four-bridge-arm inverter.

[0011] The object of the present invention is achieved in this way. The present invention provides a modulation strategy for low common mode voltage of a three-level three-phase four-bridge-arm inverter. The three-level three-phase four-bridge-arm inverter includes a DC voltage source V DC , two DC bus capacitors, a bridge arm system and a load system; the two DC bus capacitors are connected in series and then connected to a DC voltage source V DC Between the DC positive bus and the DC negative bus, and the connection point of the two DC bus capacitors is marked as point Q;

[0012] The bridge arm system includes 12 switch tubes with anti-parallel diodes, which are denoted as switch tubes S kj , where k represents the phase sequence, k = a, b, c, n; j represents the serial number of the switch tube, j = 1, 2, 3, 4; where the switch tube S k1 , switch tube S k2 , switch tube S k3 And switch tube S k4 The k-phase bridge arm is formed. Specifically, the switch tube S k1 The collector of is connected to the DC positive bus, and the switch tube S k1 The emitter and switch tube S k3 The collector of the switch tube S k4 The collector of k , where k represents the phase sequence, k = a, b, c, n; the switch tube S k3 The emitter and switch tube S k2 The emitter of the switch tube S K2 The collector of is connected to point Q, and the switch tube S k4 The emitter is connected to the DC negative bus;

[0013] The load system includes an inductor L a , L b , L c , L n , capacitor C a , Cb , C c and load Z a , Z b , Z c , where load Z a , Z b , Z c One end of the connection is connected together, and the connection point is marked as point N, and the other end is connected to the inductor L according to the phase sequence a , L b , L c , inductance L a , L b , L c The other end is connected to the point P of each phase sequence. k ; Inductance L n One end of the connection point P n , the other end is connected to point N; capacitor C a , C b , C c One end of each is connected together and connected to point N, and the other end is connected between the inductance and the load of each phase sequence;

[0014] The modulation strategy comprises the following steps:

[0015] Step 1: Sample the three-phase modulation wave and the superimposed zero-sequence component

[0016] The a-phase bridge arm, the b-phase bridge arm and the c-phase bridge arm are recorded as three-phase bridge arms, and the n-phase bridge arm is recorded as the fourth bridge arm; the three-phase modulation wave u is obtained by sampling from the three-phase bridge arm a ,u b ,u c , the zero-sequence component u is obtained by sampling from the fourth bridge arm n , and introduce the modulation wave u k , k = a, b, c, n;

[0017] Step 2: Give the carriers used by the first three-phase bridge arm and the fourth bridge arm respectively.

[0018] The carrier wave is two stacked triangular carrier waves;

[0019] The three-phase bridge arm uses the same set of carriers, while the fourth bridge arm uses another set of carriers; the upper carrier in the set of carriers used by the three-phase bridge arm is denoted as carrier T 1u 、Download wave is denoted as carrier wave T 1d , the upper carrier in another group of carriers used by the fourth bridge arm is recorded as carrier T 2u 、Download wave is denoted as carrier wave T 2d , where the carrier T 1u and carrier T 2u The range is [0, 1], and the carrier T 1d and carrier T 2dThe range is [-1, 0), the carrier T 1u and carrier T 1d The phase difference is 180 degrees, the carrier T 2u and carrier T 2d The phase difference is 180 degrees, and the carrier T 2u and carrier T 1d The phase of the carrier T 2d and carrier T 1u The phases are the same;

[0020] Introducing carrier T zh , where z is the carrier number, h is the carrier position, z = 1, 2; h = u, d;

[0021] Step 3: Generate PWM wave to drive the inverter

[0022] When ul≥0 and u k ≥T zu When the switch tube S k1 And switch tube S k2 The switch tube S is turned on. k3 And switch tube S k4 Shut down; when u k ≥0 and u k <T zu When the switch tube S k2 And switch tube S k3 The switch tube S is turned on. k1 And switch tube S k4 Turn off; when u k ≤0 and u k ≥T zd When the switch tube S k2 And switch tube S k3 The switch tube S is turned on. k1 And switch tube S k4 Shut down; when u k ≤0 and u k <T zd When the switch tube S k3 And switch tube S k4 The switch tube S is turned on. k1 And switch tube S k2 Shut down.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] 1. The modulation strategy of the present invention can reduce the amplitude of the common mode voltage to 1 / 8 of the DC bus voltage.

[0025] 2. The modulation strategy of the present invention can suppress the common-mode voltage of the three-level three-phase four-leg inverter without adding or modifying any hardware.

[0026] 3. The algorithm of the modulation strategy of the present invention is simple, does not require additional configuration and complex calculation, and is easy to apply to engineering practice. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 4 is a topological diagram of a three-level three-phase four-bridge-arm inverter in an embodiment of the present invention.

[0028] Figure 2 Flow chart of the modulation strategy of the present invention.

[0029] Figure 3 This is a common-mode voltage simulation waveform diagram of the modulation strategy of the present invention in an embodiment of the present invention.

[0030] Figure 4 This is a common-mode voltage simulation waveform diagram of a traditional co-directional carrier modulation strategy adopted in an embodiment of the present invention.

[0031] Figure 5 It is an enlarged diagram of the common-mode voltage simulation waveform using the modulation strategy of the present invention in an embodiment of the present invention.

[0032] Figure 6 It is an enlarged diagram of the common-mode voltage simulation waveform using the traditional co-directional carrier modulation strategy in the embodiment of the present invention. DETAILED DESCRIPTION

[0033] The present invention will be further described below in conjunction with specific embodiments and drawings.

[0034] Figure 1 is a topology diagram of a three-level three-phase four-bridge-arm inverter in an embodiment of the present invention, Figure 1 It can be seen that the three-level three-phase four-bridge-arm inverter includes a DC voltage source V DC , two DC bus capacitors, a bridge arm system and a load system; the two DC bus capacitors are connected in series and then connected to a DC voltage source V DC Between the DC positive bus and the DC negative bus, and the connection point of the two DC bus capacitors is marked as point Q;

[0035] The bridge arm system includes 12 switch tubes with anti-parallel diodes, which are denoted as switch tubes S kj , where k represents the phase sequence, k = a, b, c, n; j represents the serial number of the switch tube, j = 1, 2, 3, 4; where the switch tube S k1 , switch tube S k2 , switch tube S k3 And switch tube S k4 The k-phase bridge arm is formed. Specifically, the switch tube S k1 The collector of is connected to the DC positive bus, and the switch tube S k1 The emitter and switch tube S k3The collector of the switch tube S k4 The collector of k , where k represents the phase sequence, k = a, b, c, n; the switch tube S k3 The emitter and switch tube S k2 The emitter of the switch tube S K2 The collector of is connected to point Q, and the switch tube S k4 The emitter is connected to the DC negative bus;

[0036] The load system includes an inductor L a , L b , L c , L n , capacitor C a , C b , C c and load Z a , Z b , Z c , where load Z a , Z b , Z c One end of the connection is connected together, and the connection point is marked as point N, and the other end is connected to the inductor L according to the phase sequence a , L b , L c , inductance L a , L b , L c The other end is connected to the point P of each phase sequence. k ; Inductance L n One end of the connection point P n , the other end is connected to point N; capacitor C a , C b , C c One end of each is connected together and connected to point N, and the other end is connected between the inductance and the load of each phase sequence;

[0037] exist Figure 1 In, C 1 is the first DC bus capacitor, C 2 is the second DC bus capacitor, ARM is the bridge arm system, and LOAD is the load system.

[0038] Figure 2 It is a flow chart of the modulation strategy of the present invention. As can be seen from the figure, the modulation strategy includes the following steps:

[0039] Step 1: Sample the three-phase modulation wave and the superimposed zero-sequence component

[0040] The a-phase bridge arm, the b-phase bridge arm and the c-phase bridge arm are recorded as three-phase bridge arms, and the n-phase bridge arm is recorded as the fourth bridge arm; the three-phase modulation wave u is obtained by sampling from the three-phase bridge arm a ,ub ,u c , the zero-sequence component u is obtained by sampling from the fourth bridge arm n , and introduce the modulation wave u k , k = a, b, c, n;

[0041] Step 2: Give the carriers used by the first three-phase bridge arm and the fourth bridge arm respectively.

[0042] The carrier wave is two stacked triangular carrier waves;

[0043] The three-phase bridge arm uses the same set of carriers, while the fourth bridge arm uses another set of carriers; the upper carrier in the set of carriers used by the three-phase bridge arm is denoted as carrier T 1u 、Download wave is denoted as carrier wave T 1d , the upper carrier in another group of carriers used by the fourth bridge arm is recorded as carrier T 2u 、Download wave is denoted as carrier wave T 2d , where the carrier T 1u and carrier T 2u The range is [0, 1], and the carrier T 1d and carrier T 2d The range is [-1, 0), the carrier T 1u and carrier T 1d The phase difference is 180 degrees, the carrier T 2u and carrier T 2d The phase difference is 180 degrees, and the carrier T 2u and carrier T 1d The phase of the carrier T 2d and carrier T 1u The phases are the same;

[0044] Introducing carrier T zh , where z is the carrier number, h is the carrier position, z = 1, 2; h = u, d;

[0045] Step 3: Generate PWM wave to drive the inverter

[0046] When u k ≥0 and u k ≥T zu When the switch tube S k1 And switch tube S k2 The switch tube S is turned on. k3 And switch tube S k4 Shut down; when u k ≥0 and u k <T zu When the switch tube S k2 And switch tube S k3 The switch tube S is turned on. k1 And switch tube S k4 Shut down; when uk ≤0 and u k ≥T zd When the switch tube S k2 And switch tube S k3 The switch tube S is turned on. k1 And switch tube S k4 Shut down; when u k ≤0 and u k <T zd When the switch tube S k3 And switch tube S k4 The switch tube S is turned on. k1 And switch tube S k2 Shut down.

[0047] In order to verify the proposed modulation strategy's ability to suppress the common-mode voltage of a three-level three-phase four-bridge-arm inverter, the present invention builds a MATLAB / Simulink simulation model of a three-level three-phase four-bridge-arm inverter. The simulation parameters are as follows: DC voltage source V DC The voltage value is 650V, the inductance L a , L b , L c , L n The inductance value is 0.6mH, and the capacitor C a , C b , C c The capacitance value is 9.9uF, load Z a , Z b , Z c The load resistance is 10Ω and the PWM switching frequency is 5kHZ.

[0048] Figure 3 This is a common mode voltage simulation waveform diagram of the modulation strategy of the present invention in an embodiment of the present invention, wherein the common mode voltage amplitude is about 82V, which is about 1 / 8 of the DC voltage source V DC Voltage value, and Figure 4 This is a common mode voltage simulation waveform diagram of the conventional co-directional carrier modulation strategy in the embodiment of the present invention, wherein the common mode voltage amplitude is about 244V, which is about 3 / 8 of the DC voltage source V DC Voltage value, the modulation strategy of the present invention is adopted to greatly reduce the common mode voltage amplitude; Figure 5 This is an enlarged diagram of the common-mode voltage simulation waveform using the modulation strategy of the present invention in an embodiment of the present invention. It can be seen that within one carrier cycle, the common-mode voltage level of the three-level three-phase four-bridge-leg inverter changes twice. Figure 6 This is an enlarged diagram of the common-mode voltage simulation waveform using the traditional co-directional carrier modulation strategy in an embodiment of the present invention. Within one carrier cycle, the common-mode voltage level of the three-level three-phase four-bridge-arm inverter changes six times. The modulation strategy of the present invention is used to greatly reduce the number of common-mode voltage level changes.

Claims

1. A modulation strategy for low common mode voltage of a three-level three-phase four-bridge-arm inverter, wherein the three-level three-phase four-bridge-arm inverter comprises a DC voltage source V DC , two DC bus capacitors, a bridge arm system and a load system; the two DC bus capacitors are connected in series and then connected to a DC voltage source V DC Between the DC positive bus and the DC negative bus, and the connection point of the two DC bus capacitors is marked as point Q; The bridge arm system includes 12 switch tubes with anti-parallel diodes, which are denoted as switch tubes S kj , where k represents the phase sequence, k = a, b, c, n; j represents the serial number of the switch tube, j = 1, 2, 3, 4; where, Switching tube S k1 , switch tube S k2 , switch tube S k3 And switch tube S k4 The k-phase bridge arm is formed. Specifically, the switch tube S k1 The collector of is connected to the DC positive bus, and the switch tube S k1 The emitter and switch tube S k3 The collector of the switch tube S k4 The collector of k , where k represents the phase sequence, k = a, b, c, n; the switch tube S k3 The emitter and switch tube S k2 The emitter of the switch tube S K2 The collector of is connected to point Q, and the switch tube S k4 The emitter is connected to the DC negative bus; The load system includes an inductor L a , L b , L c , L n , capacitor C a , C b , C c and load Z a , Z b , Z c , where load Z a , Z b , Z c One end of the connection is connected together, and the connection point is marked as point N, and the other end is connected to the inductor L according to the phase sequence a , L b , L c , inductance L a , L b , L c The other end is connected to the point P of each phase sequence. k ; Inductance L n One end point P n , the other end is connected to point N; capacitor C a , C b , C c One end of each is connected together and connected to point N, and the other end is connected between the inductance and the load of each phase sequence; Characterized in that the modulation strategy comprises the following steps: Step 1: Sample the three-phase modulation wave and the superimposed zero-sequence component The a-phase bridge arm, the b-phase bridge arm and the c-phase bridge arm are recorded as three-phase bridge arms, and the n-phase bridge arm is recorded as the fourth bridge arm; the three-phase modulation wave u is obtained by sampling from the three-phase bridge arm a ,u b ,u c , the zero-sequence component u is obtained by sampling from the fourth bridge arm n , and introduce the modulation wave u k , k = a, b, c, n; Step 2: Give the carriers used by the first three-phase bridge arm and the fourth bridge arm respectively. The carrier wave is two stacked triangular carrier waves; The three-phase bridge arm uses the same set of carriers, while the fourth bridge arm uses another set of carriers; the upper carrier in the set of carriers used by the three-phase bridge arm is denoted as carrier T 1u 、Download wave is denoted as carrier wave T 1d , the upper carrier in another group of carriers used by the fourth bridge arm is recorded as carrier T 2u 、Download wave is denoted as carrier wave T 2d , where the carrier T 1u and carrier T 2u The range is [0, 1], and the carrier T 1d and carrier T 2d The range is [-1, 0), the carrier T 1u and carrier T 1d The phase difference is 180 degrees, the carrier T 2u and carrier T 2d The phase difference is 180 degrees, and the carrier T 2u and carrier T 1d The phase of the carrier T 2d and carrier T 1u The phases are the same; Introducing carrier T zh , where z is the carrier number, h is the carrier position, z = 1, 2; h = u, d; Step 3: Generate PWM wave to drive the inverter When u k ≥0 and u k ≥T zu When the switch tube S k1 And switch tube S k2 The switch tube S is turned on. k3 And switch tube S k4 Shut down; when u k ≥0 and u k <T zu When the switch tube S k2 And switch tube S k3 The switch tube S is turned on. k1 And switch tube S k4 Shut down; when u k ≤0 and u k ≥T zd When the switch tube S k2 And switch tube S k3 The switch tube S is turned on. k1 And switch tube S k4 Shut down; when u k ≤0 and u k <T zd When the switch tube S k3 And switch tube S k4 The switch tube S is turned on. k1 And switch tube S k2 Shut down.

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