Single-phase transformerless grid-connected solar micro-inverter with high voltage gain

Abstract

The application discloses a single-phase transformerless grid-connected solar micro-inverter with high voltage gain, which comprises switches T1, T2, T3, T4, T5, T6, capacitors C1 and C2, a photovoltaic power supply Vpv, a capacitor Cpv, inductors L1, L2 and Lg, diodes D1 and D2, and a triode D3; the positive pole of the photovoltaic power supply Vpv, the positive pole of the capacitor Cpv and the positive pole of the inductor L1 are connected; the positive pole of the capacitor C1 and the collector of the switch T1 are connected with the negative pole of the inductor L1; the negative pole of the capacitor C1 and the anode of the diode D1 are connected with the emitter of the switch T2; the collector of the switch T2 and the anode of the diode D2 are connected with the negative pole of the inductor L2; the cathode of the diode D2 and the collector of the switch T4 are connected with the collector of the switch T6; the collector of the switch T5 and the cathode of the diode D3 are connected; and the emitter of the switch T5 and the emitter of the switch T3 are connected with the positive pole of the inductor L2. The micro-inverter has high voltage gain.

Description

Technical Field

[0001] This invention relates to the field of inverter technology, and more particularly to a single-phase grid-connected transformerless solar micro-inverter with high voltage gain. Background Technology

[0002] Currently, renewable energy is favored over traditional fossil fuel-based energy sources because it is environmentally friendly and inexhaustible. Among these, the photovoltaic (PV) power generation industry has developed rapidly and has become an indispensable part of my country's energy sector. Solar microinverters are commonly used in residential PV systems for DC-to-AC power conversion of solar panels. They help maximize energy output and address issues associated with PV systems such as shading, dirt accumulation, or individual panel failures. Solar microinverters are becoming a viable solution because they offer modular, scalable, and plug-and-play solutions, and can extract maximum usable power from a single PV module. Microinverters can connect low-voltage, low-power PV modules to various power supply systems. Typically, the distribution voltage level is higher than the PV module voltage; therefore, one of the main challenges in designing microinverters is achieving high voltage gain. However, due to the high voltage gain requirements and low power ratings of microinverters, their efficiency is lower compared to cascade inverters and central inverters. Traditional solar microinverters also have the following disadvantages: (1) The active power decoupling circuit requires additional high-frequency switches and inductors, which reduces the overall efficiency of the system; (2) The negative terminal of the photovoltaic module is connected to the neutral point of the grid through a switch, which increases the possibility of leakage current flow; (3) The circuit operates in a continuous conduction mode, so the duty cycle of the boost stage is high and the required inductance value is high. Summary of the Invention

[0003] This invention aims to at least partially solve one of the technical problems in the related art. To this end, one object of this invention is to provide a single-phase grid-connected transformerless solar microinverter with high voltage gain, in which, of the six switches, T1 and T2 operate at high frequency, while T3 operates at high frequency during the negative half-cycle, and switches T4, T5, and T6 operate at low frequency.

[0004] According to the present invention, a single-phase grid-connected transformerless solar micro-inverter with high voltage gain includes switches T1, T2, T3, T4, T5, and T6; capacitors C1 and C2; a photovoltaic power supply Vpv; a capacitor Cpv; inductors L1, L2, and Lg; diodes D1 and D2; and a transistor D3. The positive terminals of the photovoltaic power supply Vpv, Cpv, and inductor L1 are connected together. The positive terminal of capacitor C1 and the collector of switch T1 are connected to the negative terminal of inductor L1. The negative terminal of capacitor C1 and the anode of diode D1 are connected to the emitter of switch T2. The collector of switch T2 and the anode of diode D2 are connected to... The negative terminals of inductor L2 are connected together. The cathode of diode D2 and the collector of switch T4 are connected to the collector of switch T6. The collector of switch T5 is connected to the cathode of diode D3. The emitters of switch T5 and switch T3 are connected to the positive terminals of inductor L2. The anode of diode D3, the positive terminal of capacitor C2, and the negative terminal of inductor Lg are connected to the emitter of switch T4. The positive terminal of inductor Lg is connected to the positive terminal of the power grid Vg. The negative terminals of power grid Vg, capacitor C2, photovoltaic power supply Vpv, capacitor Cpv, switch T1, diode D1, and switch T3 are connected to the emitter of switch T6.

[0005] Preferably, the set reflecting the on / off states of switches T1, T2, T3, T4, T5, and T6 is defined as [M1, M2, M3, M4, M5, M6], where switches T1, T2, T3, T4, T5, and T6 correspond one-to-one with M1, M2, M3, M4, M5, and M6. When any value of M1, M2, M3, M4, M5, or M6 is 1, it indicates that the corresponding switch is in the on / off state. When any value of M1, M2, M3, M4, M5, or M6 is 0, it indicates that the corresponding switch is in the off / off state. After the single-phase transformerless solar micro-inverter enters steady state, the half-cycle of the micro-inverter is Ts, which includes a boost stage and a buck-boost stage. The duty cycle of the boost stage is d1, and the duty cycle of the buck-boost stage is d2. There are six operating modes within one switching cycle:

[0006] Pattern 1: 0≤t<d2Ts, [M1,M2,M3,M4,M5,M6]=[1,1,1,1,0,0];

[0007] Pattern 2: d2Ts≤t<d1Ts, [M1,M2,M3,M4,M5,M6]=[1,0,1,1,0,0];

[0008] Pattern 3: d1Ts≤t<Ts, [M1,M2,M3,M4,M5,M6]=[0,0,1,1,0,0];

[0009] Pattern 4: Ts≤t<(1+d2)Ts, [M1,M2,M3,M4,M5,M6]=[1,1,1,0,1,1];

[0010] Pattern 5: (1+d2)Ts≤t<(1+d1)Ts, [M1,M2,M3,M4,M5,M6]=[1,0,0,0,1,1];

[0011] Pattern 6: (1+d1)Ts≤t<2Ts,[M1,M2,M3,M4,M5,M6]=[0,0,0,0,1,1].

[0012] Preferably, in Mode 1: An on signal is applied to switches T1, T2, T3, and T4; an off signal is applied to switches T5 and T6. Diode D2 blocks the current through switch T4, increasing the current through inductors L1 and L2. Capacitors C1 and C2 discharge. The current path is: photovoltaic power supply Vpv, inductor L1, switch T1, photovoltaic power supply Vpv and capacitor C1, switch T1, switch T3, inductor L2, switch T2 and capacitor C1. Power is supplied to the grid within this interval.

[0013] dI1 / dt=V pv / L1 (1)

[0014] dI² / dt = V c1 / L2 (2)

[0015] In the formula: L1 is the inductance of inductor L1, I1 is the current of inductor L1, L2 is the inductance of inductor L2, I2 is the current of inductor L2, V pv V represents the voltage value of the photovoltaic power source. c1 This is the voltage value of capacitor C1.

[0016] Preferably, in mode two: Turn-on signals are applied to switches T1, T3, and T4, and turn-off signals are applied to switches T2, T5, and T6. The current in inductor L1 continues to increase along the path of photovoltaic power supply Vpv, inductor L1, switch T1, photovoltaic power supply Vpv and capacitor C1, switch T1, switch T3, inductor L2, switch T2 and capacitor C1. The current in inductor L2 begins to decrease. The current path is inductor L2, diode D2, switch T4, power grid, switch T3 and inductor L2. If Δ2 ≥ (1-d1), the current is zero before or at the end of this mode. If Δ2 < (1-d1), iL2 maintains a finite value at the end of this mode, and no current flows through capacitor C1. During this interval, the voltage expression of inductor L1 is the same as in formula (1). For d2Ts ≤ t < (1-Δ2)Ts,

[0017] dI² / dt = -V g / L2 (3)

[0018] In the formula: V g The voltage value of the power grid

[0019] Preferably, in mode three: an on signal is applied to switches T3 and T4, and an off signal is applied to switches T1, T2, T5, and T6. The energy stored in inductor L1 is transferred to capacitor C1 via the path of photovoltaic power source Vpv, inductor L1, capacitor C1, and diode D1. If Δ2 < (1-d1), the current iL2 continues to decrease and flows through the same path as in mode two. Before the end of this mode, both iL1 and iL2 become zero, and formula (3) remains valid until iL2 becomes zero within the interval d1 ≤ t < (1-Δ1)Ts.

[0020] dI1 / dt=(V pv -v c1 ) / L1 (4)

[0021] A volt-second balance is applied between inductors L1 and L2.

[0022] v c1 (1-d1-Δ1)=V pv (1-Δ1) (5)

[0023] V g (1-d2-Δ2)=V c1 d2 (6)

[0024] In the formula: d1 is the duty cycle of the boost stage, Δ1 is the percentage of the interval during which iL1 remains zero, d2 is the duty cycle of the buck-boost stage, and Δ2 is the percentage of the interval during which iL2 remains zero.

[0025] The positive half-cycle voltage gain of the inverter is obtained from formulas (5) and (6).

[0026] G1 = V out / V in =[(1-Δ1)d2] / [(1-d1-Δ1)(1-d2-Δ2)] (7)

[0027] Preferably, in mode four: turn-on signals are applied to switches T1, T2, T3, T5 and T6, and turn-off signals are applied to switch T4. Diodes D2 and D3 block the current through switches T6 and T5 respectively. The current flowing through inductors L1 and L2 increases as they flow through the photovoltaic power source Vpv, inductor L1, switch T1, photovoltaic power source Vpv and capacitor C1, switch T1, switch T3, inductor L2, switch T2 and capacitor C1. Capacitor C1 discharges during this interval, while capacitor C2 supplies power to the grid. In this case, both formula (1) and formula (2) are applicable.

[0028] Preferably, in mode five: An on signal is applied to switches T1, T5, and T6, and an off signal is applied to switches T2, T3, and T4. The current through inductor L1 continues to increase, while the current through inductor L2 begins to decrease, flowing through path inductor L2, diode D2, switch T6, the power grid, diode D3, switch T5, and inductor L2. If Δ2 ≥ (1-d1), the current iL2 is zero before or at the end of this mode; if Δ2 < (1-d1), iL2 maintains a finite value at the end of this mode, and the current flowing through C1 is zero. Formula (1) still applies to this interval, and...

[0029] dI² / dt = V g / L2 (8)

[0030] At this point, iL2 is decreasing.

[0031] Preferably, in mode six: an on signal is applied to switches T5 and T6, and an off signal is applied to switches T1, T2, T3, and T4. When current flows through the path of photovoltaic power supply Vpv, inductor L1, capacitor C1, diode D1, and photovoltaic power supply Vpv, the current through inductor L1 begins to decrease, thereby charging C1. Before the end of this interval, iL1 and iL2 will both become zero. If Δ2 < (1-d1), the current iL2 will continue to decrease and flow through the same path as in mode five. When both iL1 and iL2 are decreasing, formulas (4) and (8) are still valid. The voltage of switch T2 is zero during this interval. In the negative half-cycle,

[0032] v g (1-d2-Δ2)=Vc1 d2 (9)

[0033] Combining formulas (5) and (9), the voltage gain of the inverter in the negative half-cycle is obtained.

[0034] G2 = V out / V in =[-(1-Δ1)d2] / [(1-d1-Δ1)(1-d2-Δ2)] (10)

[0035] For the voltage gain of a microinverter over one cycle:

[0036] From the above analysis, it can be deduced that the voltage gain expressions for the positive and negative half-cycles are the same, and the direction of the load current is opposite to the polarity of the output voltage. According to formulas (7) and (10), the overall expression for the voltage gain can be expressed as:

[0037] G = V out / V in =[sgn(v g )(1-Δ1)d2] / [(1-d1-Δ1)(1-d2-Δ2)] (11)

[0038] Preferably, the single-phase transformerless solar micro-inverter has a high-frequency switch of 50 kHz, a low-frequency switch of 50 Hz, and a switching time period Ts of 20 μs.

[0039] The beneficial effects of this invention are: 1) The micro inverter described in this invention has a high voltage gain; 2) The negative terminal of the photovoltaic module is directly connected to the neutral point of the grid, so the leakage current is zero; 3) Both the boost stage and the buck-boost stage operate in a discontinuous conduction mode, which can achieve the required voltage gain at a lower duty cycle. This reduces the size of the inductor and ensures that the high-frequency switch has a higher voltage gain and zero switching loss. Attached Figure Description

[0040] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings:

[0041] Figure 1 The circuit diagram is shown below for a single-phase grid-connected transformerless solar micro-inverter with high voltage gain proposed in this invention.

[0042] Figure 2 This is a circuit diagram of the first working mode of the solar micro-inverter proposed in this invention.

[0043] Figure 3 This is the circuit diagram of the second working mode of the solar micro-inverter proposed in this invention;

[0044] Figure 4 This is the circuit diagram of the third working mode of the solar micro-inverter proposed in this invention;

[0045] Figure 5 This is the circuit diagram of the fourth working mode of the solar micro-inverter proposed in this invention.

[0046] Figure 6 The circuit diagram for the fifth working mode of the solar micro-inverter proposed in this invention is shown.

[0047] Figure 7 This is a circuit diagram of the sixth working mode of the solar micro-inverter proposed in this invention.

[0048] Figure 8 The waveforms of the solar micro-inverter proposed in this invention are shown for different inverter parameters when Δ2>(1-d1).

[0049] Figure 9 The waveforms of the solar micro-inverter proposed in this invention are shown for different inverter parameters when Δ2 < (1-d1).

[0050] Figure 10 This is a schematic diagram of the switching pulse of the solar micro-inverter proposed in this invention. Detailed Implementation

[0051] Reference Figure 1A single-phase grid-connected transformerless solar micro-inverter with high voltage gain includes switches T1, T2, T3, T4, T5, and T6; capacitors C1 and C2; a photovoltaic power supply Vpv; a capacitor Cpv; inductors L1, L2, and Lg; diodes D1 and D2; and a transistor D3. The positive terminals of the photovoltaic power supply Vpv, Cpv, and inductor L1 are connected. The positive terminal of capacitor C1 and the collector of switch T1 are connected to the negative terminal of inductor L1. The negative terminal of capacitor C1 and the anode of diode D1 are connected to the emitter of switch T2. The collector of switch T2 and the anode of diode D2 are connected to the negative terminal of inductor L2. The cathode of diode D2 and the collector of switch T4 are connected to the collector of switch T6, respectively. The collector of switch T5 is connected to the cathode of diode D3. The emitters of switch T5 and switch T3 are connected to the anode of inductor L2, respectively. The anode of diode D3, the anode of capacitor C2, and the cathode of inductor Lg are connected to the emitter of switch T4, respectively. The anode of inductor Lg is connected to the anode of the power grid Vg. The cathodes of power grid Vg, capacitor C2, photovoltaic power supply Vpv, capacitor Cpv, switch T1, diode D1, and switch T3 are connected to the emitter of switch T6, respectively. The negative terminal of the photovoltaic module is directly connected to the neutral point of the power grid.

[0052] The designed microinverter operates with a high-frequency switch at 50kHz and a low-frequency switch at 50Hz, with the high-frequency switch having a much higher switching frequency. The switching time period Ts is 20μs, which is very small compared to the grid voltage and current (20ms), and the output voltage and current can be considered constant within the switching time period. The intervals where iL1 and iL2 remain zero are Δ1Ts and Δ2Ts, respectively. The current and voltage waveforms within one switching cycle are as follows: Figure 8 and Figure 9 As shown, the switching pulse is as follows Figure 10 As shown.

[0053] The micro-inverter described in this invention can achieve high voltage gain; since the micro-inverter is inherently capable of power decoupling, there is no need to connect large capacitors or active power decoupling circuits to the photovoltaic terminals; the negative terminal of the photovoltaic module is directly connected to the grid neutral point, so the leakage current is zero; both the boost stage and the buck-boost stage operate in a discontinuous conduction mode, which can achieve the required voltage gain at a lower duty cycle, reducing the size of the inductor and ensuring that the high-frequency switch has higher voltage gain and zero switching loss.

[0054] In Mode 1: Turn-on signals are applied to switches T1, T2, T3, and T4, and turn-off signals are applied to switches T5 and T6. Diode D2 blocks the current through switch T4, increasing the current through inductors L1 and L2. Capacitors C1 and C2 discharge. The current path is: photovoltaic power source Vpv, inductor L1, switch T1, photovoltaic power source Vpv and capacitor C1, switch T1, switch T3, inductor L2, switch T2 and capacitor C1. Power is supplied to the grid within this interval.

[0055] dI1 / dt=V pv / L1 (1)

[0056] dI² / dt = V c1 / L2 (2)

[0057] In the formula: L1 is the inductance of inductor L1, I1 is the current of inductor L1, L2 is the inductance of inductor L2, I2 is the current of inductor L2, V pv V represents the voltage value of the photovoltaic power source. c1 This is the voltage value of capacitor C1.

[0058] In mode two: Turn-on signals are applied to switches T1, T3, and T4, and turn-off signals are applied to switches T2, T5, and T6. The current in inductor L1 continues to increase along the path of photovoltaic power source Vpv, inductor L1, switch T1, photovoltaic power source Vpv and capacitor C1, switch T1, switch T3, inductor L2, switch T2 and capacitor C1. The current in inductor L2 begins to decrease. The current path is inductor L2, diode D2, switch T4, power grid, switch T3 and inductor L2. If Δ2 ≥ (1-d1), the current is zero before or at the end of this mode. If Δ2 < (1-d1), iL2 maintains a finite value at the end of this mode. No current flows through capacitor C1. During this interval, the voltage expression of inductor L1 is the same as in formula (1). For d2Ts ≤ t < (1-Δ2)Ts,

[0059] dI² / dt = -V g / L2 (3)

[0060] In the formula: V g The voltage value of the power grid

[0061] In mode three: Turn-on signals are applied to switches T3 and T4, and turn-off signals are applied to switches T1, T2, T5, and T6. The energy stored in inductor L1 is transferred to capacitor C1 via the path of photovoltaic power source Vpv, inductor L1, capacitor C1, and diode D1. If Δ2 < (1-d1), the current iL2 continues to decrease and flows through the same path as in mode two. Before the end of this mode, both iL1 and iL2 become zero, and formula (3) remains valid until iL2 becomes zero within the interval d1 ≤ t < (1-Δ1)Ts.

[0062] dI1 / dt=(V pv -v c1 ) / L1 (4)

[0063] A volt-second balance is applied between inductors L1 and L2.

[0064] v c1 (1-d1-Δ1)=V pv (1-Δ1) (5)

[0065] V g (1-d2-Δ2)=V c1 d2 (6)

[0066] In the formula: d1 is the duty cycle of the boost stage, Δ1 is the percentage of the interval during which iL1 remains zero, d2 is the duty cycle of the buck-boost stage, and Δ2 is the percentage of the interval during which iL2 remains zero.

[0067] The positive half-cycle voltage gain of the inverter is obtained from formulas (5) and (6).

[0068] G1 = V out / V in =[(1-Δ1)d2] / [(1-d1-Δ1)(1-d2-Δ2)] (7)

[0069] In mode four: turn-on signals are applied to switches T1, T2, T3, T5 and T6, and turn-off signals are applied to switch T4. Diodes D2 and D3 block the current through switches T6 and T5 respectively. The current flowing through inductors L1 and L2 increases as they flow through the photovoltaic power source Vpv, inductor L1, switch T1, photovoltaic power source Vpv and capacitor C1, switch T1, switch T3, inductor L2, switch T2 and capacitor C1. Capacitor C1 discharges during this interval, while capacitor C2 supplies power to the grid. In this case, both formulas (1) and (2) are applicable.

[0070] For mode five: When turn-on signals are applied to switches T1, T5, and T6, and turn-off signals are applied to switches T2, T3, and T4, the current through inductor L1 continues to increase, while the current through inductor L2 begins to decrease. The current flows through inductor L2, diode D2, switch T6, the power grid, diode D3, switch T5, and inductor L2. If Δ2 ≥ (1-d1), the current iL2 is zero before or at the end of this mode. If Δ2 < (1-d1), iL2 maintains a finite value at the end of this mode, and the current flowing through C1 is zero. Formula (1) still applies to this interval, and...

[0071] dI² / dt = V g / L2 (8)

[0072] At this point, iL2 is decreasing.

[0073] For mode six: Turn-on signals are applied to switches T5 and T6, and turn-off signals are applied to switches T1, T2, T3, and T4. When current flows through the path of photovoltaic power source Vpv, inductor L1, capacitor C1, diode D1, and photovoltaic power source Vpv, the current through inductor L1 begins to decrease, thus charging C1. Before the end of this interval, both iL1 and iL2 will become zero. If Δ2 < (1-d1), current iL2 will continue to decrease and flow through the same path as in mode five. When both iL1 and iL2 are decreasing, equations (4) and (8) remain valid. The voltage of switch T2 is zero during this interval. In the negative half-cycle,

[0074] v g (1-d2-Δ2)=V c1 d2 (9) Combining formulas (5) and (9), the voltage gain of the inverter in the negative half-cycle is obtained.

[0075] G2 = V out / V in =[-(1-Δ1)d2] / [(1-d1-Δ1)(1-d2-Δ2)] (10)

[0076] For the voltage gain of a microinverter over one cycle:

[0077] From the above analysis, it can be deduced that the voltage gain expressions for the positive and negative half-cycles are the same, and the direction of the load current is opposite to the polarity of the output voltage. According to formulas (7) and (10), the overall expression for the voltage gain can be expressed as:

[0078] G = V out / V in =[sgn(v g)(1-Δ1)d2] / [(1-d1-Δ1)(1-d2-Δ2)] (11)。

Claims

1. A single-phase grid-connected transformerless solar micro-inverter with high voltage gain, characterized in that: This system includes switches T1, T2, T3, T4, T5, and T6; capacitors C1 and C2; a photovoltaic power supply Vpv; a capacitor Cpv; inductors L1, L2, and Lg; diodes D1 and D2; and a transistor D3. The positive terminals of the photovoltaic power supply Vpv, Cpv, and inductor L1 are connected. The positive terminal of capacitor C1 and the collector of switch T1 are connected to the negative terminal of inductor L1. The negative terminal of capacitor C1 and the anode of diode D1 are connected to the emitter of switch T2. The collector of switch T2 and the anode of diode D2 are connected to the negative terminal of inductor L2. The cathode of diode D2 and the collector of switch T4 are connected to the collector of switch T6. The collector of switch T5 is connected to the cathode of diode D3. The emitters of switch T5 and switch T3 are connected to the anode of inductor L2. The anode of diode D3, the anode of capacitor C2, and the cathode of inductor Lg are connected to the emitter of switch T4. The anode of inductor Lg is connected to the anode of the power grid Vg. The cathodes of power grid Vg, capacitor C2, photovoltaic power supply Vpv, capacitor Cpv, switch T1, diode D1, and switch T3 are connected to the emitter of switch T6.

2. The single-phase grid-connected transformerless solar micro-inverter with high voltage gain according to claim 1, characterized in that, The set reflecting the on / off states of switches T1, T2, T3, T4, T5, and T6 is defined as [M1, M2, M3, M4, M5, M6], where switches T1, T2, T3, T4, T5, and T6 correspond one-to-one with M1, M2, M3, M4, M5, and M6. When any value of M1, M2, M3, M4, M5, or M6 is 1, it indicates that the corresponding switch is in the on / off state. When any value of M1, M2, M3, M4, M5, or M6 is 0, it indicates that the corresponding switch is in the off / off state. After the single-phase transformerless solar micro-inverter enters steady state, the half-cycle of the micro-inverter is Ts, which includes a boost stage and a buck-boost stage. The duty cycle of the boost stage is d1, and the duty cycle of the buck-boost stage is d2. There are six operating modes within one switching cycle: Mode 1: 0≤t<d2Ts, [M1,M2,M3,M4,M5,M6]=[1,1,1,1,0,0]; Pattern 2: d2Ts≤t<d1Ts, [M1,M2,M3,M4,M5,M6]=[1,0,1,1,0,0]; Pattern 3: d1Ts≤t<Ts, [M1,M2,M3,M4,M5,M6]=[0,0,1,1,0,0]; Pattern 4: Ts≤t<(1+d2)Ts, [M1,M2,M3,M4,M5,M6]=[1,1,1,0,1,1]; Pattern 5: (1+d2)Ts≤t<(1+d1)Ts, [M1,M2,M3,M4,M5,M6]=[1,0,0,0,1,1]; Pattern 6: (1+d1)Ts≤t<2Ts,[M1,M2,M3,M4,M5,M6]=[0,0,0,0,1,1].

3. A single-phase grid-connected transformerless solar micro-inverter with high voltage gain according to claim 2, characterized in that, In Mode 1: Turn-on signals are applied to switches T1, T2, T3, and T4, and turn-off signals are applied to switches T5 and T6. Diode D2 blocks the current through switch T4, increasing the current through inductors L1 and L2. Capacitors C1 and C2 discharge. The current path is: photovoltaic power source Vpv, inductor L1, switch T1, photovoltaic power source Vpv and capacitor C1, switch T1, switch T3, inductor L2, switch T2 and capacitor C1. Power is supplied to the grid within this interval. ； ； In the formula: L1 is the inductance of inductor L1, I1 is the current of inductor L1, L2 is the inductance of inductor L2, I2 is the current of inductor L2, V pv V represents the voltage value of the photovoltaic power source. c1 This is the voltage value of capacitor C1.

4. A single-phase grid-connected transformerless solar micro-inverter with high voltage gain according to claim 3, characterized in that, In mode two: Turn-on signals are applied to switches T1, T3, and T4, and turn-off signals are applied to switches T2, T5, and T6. The current in inductor L1 continues to increase along the path of photovoltaic power supply Vpv, inductor L1, switch T1, photovoltaic power supply Vpv and capacitor C1, switch T1, switch T3, inductor L2, switch T2 and capacitor C1. The current in inductor L2 begins to decrease. The current path is inductor L2, diode D2, switch T4, power grid, switch T3 and inductor L2. Δ2 is the percentage of the interval where iL2 remains zero. If Δ2 ≥ (1-d1), it is zero before or at the end of this mode. If Δ2 < (1-d1), iL2 maintains a finite value at the end of this mode. No current flows through capacitor C1. During this interval, the voltage expression of inductor L1 is the same as in formula (1). For d2Ts ≤ t < (1-Δ2)Ts, ； In the formula: V g This represents the voltage value of the power grid.

5. A single-phase grid-connected transformerless solar micro-inverter with high voltage gain according to claim 4, characterized in that, In mode three: Turn-on signals are applied to switches T3 and T4, and turn-off signals are applied to switches T1, T2, T5, and T6. The energy stored in inductor L1 is transferred to capacitor C1 via the path of photovoltaic power source Vpv, inductor L1, capacitor C1, and diode D1. If Δ2 < (1-d1), the current iL2 continues to decrease and flows through the same path as in mode two. Before the end of this mode, both iL1 and iL2 become zero, and formula (3) remains valid until iL2 becomes zero within the interval d1 ≤ t < (1-Δ1)Ts. ； A volt-second balance is applied between inductors L1 and L2. ； ； In the formula: d1 is the duty cycle of the boost stage, Δ1 is the percentage of the interval in which iL1 remains zero, and d2 is the duty cycle of the buck-boost stage. The positive half-cycle voltage gain of the inverter is obtained from formulas (5) and (6). （7）。 6. A single-phase grid-connected transformerless solar micro-inverter with high voltage gain according to claim 5, characterized in that, In mode four: turn-on signals are applied to switches T1, T2, T3, T5 and T6, and turn-off signals are applied to switch T4. Diodes D2 and D3 block the current through switches T6 and T5 respectively. The current flowing through inductors L1 and L2 increases as they flow through the photovoltaic power source Vpv, inductor L1, switch T1, photovoltaic power source Vpv and capacitor C1, switch T1, switch T3, inductor L2, switch T2 and capacitor C1. Capacitor C1 discharges during this interval, while capacitor C2 supplies power to the grid. In this case, both formula (1) and formula (2) are applicable.

7. A single-phase grid-connected transformerless solar micro-inverter with high voltage gain according to claim 6, characterized in that, In mode five: When turn-on signals are applied to switches T1, T5, and T6, and turn-off signals are applied to switches T2, T3, and T4, the current through inductor L1 continues to increase, while the current through inductor L2 begins to decrease. The current flows through inductor L2, diode D2, switch T6, the power grid, diode D3, switch T5, and inductor L2. If Δ2 ≥ (1-d1), the current iL2 is zero before or at the end of this mode. If Δ2 < (1-d1), iL2 maintains a finite value at the end of this mode, and the current flowing through C1 is zero. Formula (1) still applies to this interval, and... ； At this point, iL2 is decreasing.

8. A single-phase grid-connected transformerless solar micro-inverter with high voltage gain according to claim 7, characterized in that, For mode six: Turn-on signals are applied to switches T5 and T6, and turn-off signals are applied to switches T1, T2, T3, and T4. When current flows through the path of photovoltaic power source Vpv, inductor L1, capacitor C1, diode D1, and photovoltaic power source Vpv, the current through inductor L1 begins to decrease, thus charging C1. Before the end of this interval, both iL1 and iL2 will become zero. If Δ2 < (1-d1), current iL2 will continue to decrease and flow through the same path as in mode five. When both iL1 and iL2 are decreasing, formulas (4) and (8) remain valid. The voltage of switch T2 is zero during this interval. In the negative half-cycle, ； Combining formulas (5) and (9), the voltage gain of the inverter in the negative half-cycle is obtained. ； For the voltage gain of a microinverter over one cycle: From the above analysis, it can be deduced that the voltage gain expressions for the positive and negative half-cycles are the same, and the direction of the load current is opposite to the polarity of the output voltage. According to formulas (7) and (10), the overall expression for the voltage gain can be expressed as: （11）。 9. A single-phase grid-connected transformerless solar micro-inverter with high voltage gain according to claim 1, characterized in that: The single-phase transformerless solar micro-inverter has a high-frequency switch of 50 kHz, a low-frequency switch of 50 Hz, and a switching time period Ts of 20 µs.

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