An isolated energy storage grid-connected inverter topology and control method

Abstract

This invention relates to an isolated energy storage grid-connected inverter topology and control method, comprising a battery, a grid positive half-cycle operating circuit, a grid negative half-cycle operating circuit, and an output capacitor. The grid positive half-cycle operating circuit and the grid negative half-cycle operating circuit are parallel flyback circuits connected between the battery and the output capacitor, and are used to convert the DC voltage of the battery into AC voltage and output it to the output port. Both the grid positive half-cycle operating circuit and the grid negative half-cycle operating circuit include an isolation transformer and a switching transistor. This invention uses an isolated topology, which provides better insulation and meets safety regulations with fewer relays. Furthermore, the energy storage inverter uses only a single-stage DC / AC circuit, requiring fewer power electronic devices (switching transistors), resulting in lower cost. It can simultaneously achieve electrical isolation, bidirectional energy flow, and a simple and efficient energy storage grid-connected inverter with a single-stage DC / AC topology.

Description

Technical Field

[0001] This invention relates to the field of energy storage technology, specifically to an isolated energy storage grid-connected inverter topology and control method. Background Technology

[0002] Grid-connected energy storage inverters are core power electronic devices that enable bidirectional energy exchange between energy storage batteries and the power grid. They typically operate in two modes: charging the battery from the grid and discharging the battery back to the grid. Based on whether they incorporate electrical isolation, they can be broadly classified into non-isolated and isolated types. Based on the number of power conversion stages, they can be further categorized into single-stage DC / AC and two-stage DC / DC plus DC / AC architectures.

[0003] Currently, most mainstream grid-connected energy storage inverters adopt a two-stage architecture of front-end DC / DC and back-end DC / AC. The front-end DC / DC is divided into isolated and non-isolated types: non-isolated topologies commonly use Boost / Buck, while isolated topologies commonly use LLC and dual active bridge DAB, which respectively achieve wide voltage adaptation of batteries and electrical isolation. The back-end DC / AC uses H-bridge inverters paired with LCL filters for grid connection to achieve precise control of grid-connected current.

[0004] See patent CN121055742A for an isolated inverter circuit whose topology includes a grid-side inductor; see patent CN113965064A for an isolated photovoltaic inverter, method, and grid-connected photovoltaic system, which uses LLC topology to achieve isolation; see patent CN108539983A for a small fast-charging and discharging bidirectional flyback circuit, which uses flyback circuit for DC-DC systems; see patent CN121770301A for a hybrid control method and system for an interleaved flyback photovoltaic micro-inverter, and patent CN120433340A for a flyback topology micro-inverter system and active power control method, which introduce flyback topology into micro-inverters. However, both of these solutions only support unidirectional power feeding to the grid and do not support charging from the grid, and both contain grid-connected H4 bridge circuits.

[0005] However, existing grid-connected inverter topologies for energy storage mainly suffer from the following drawbacks: First, they mostly adopt a two-stage topology of DC / DC plus DC / AC, resulting in a large number of power devices, superimposed losses, and low overall efficiency. At the same time, the use of more power electronic devices increases costs. Second, although common non-isolated grid-connected inverters have high efficiency, they lack electrical isolation, resulting in low system insulation safety and making it difficult to meet the mandatory electrical isolation requirements of scenarios such as residential energy storage and photovoltaic coupling. Summary of the Invention

[0006] One objective of this invention is to provide an isolated energy storage grid-connected inverter topology applicable to industries such as new energy power generation and grid-connected charging and discharging equipment for energy storage batteries. This topology enables the exchange of electrical energy from the energy storage battery with grid electrical energy using fewer power electronic devices, thereby improving the insulation safety of the entire battery and inverter.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: An isolated energy storage grid-connected inverter topology is characterized by comprising a battery, a grid positive half-cycle operating circuit, a grid negative half-cycle operating circuit, and an output capacitor. The grid positive half-cycle operating circuit and the grid negative half-cycle operating circuit are flyback circuits connected in parallel between the battery and the output capacitor and in reverse direction, used to convert the DC voltage of the battery into AC voltage and output it to the output port. The grid positive half-cycle operating circuit and the grid negative half-cycle operating circuit both include an isolation transformer and a switching transistor.

[0008] Preferably, in the above technical solution, during the positive half-cycle of the power grid: the positive half-cycle working circuit converts the battery voltage into a positive voltage and outputs it to the output capacitor; during the negative half-cycle of the power grid: the negative half-cycle working circuit converts the battery voltage into a negative voltage and outputs it to the output capacitor.

[0009] Preferably, in the above technical solution, the positive half-cycle operating circuit of the power grid includes a first isolation transformer, a first switching transistor, a second switching transistor, and a third switching transistor. The positive terminal of the battery, the opposite terminal of the primary side of the first isolation transformer, the same terminal of the primary side of the first isolation transformer, the first switching transistor, and the negative terminal of the battery are connected in sequence. The positive terminal of the output capacitor, the third switching transistor, the second switching transistor, the same terminal of the secondary side of the first isolation transformer, the opposite terminal of the secondary side of the first isolation transformer, and the negative terminal of the output capacitor are connected in sequence. The negative half-cycle operating circuit of the power grid includes a second isolation transformer, a fourth switch, a fifth switch, and a sixth switch. The positive terminal of the battery, the fourth switch, the opposite terminal of the primary side of the second isolation transformer, the same terminal of the primary side of the second isolation transformer, and the negative terminal of the battery are connected in sequence. The positive terminal of the output capacitor, the sixth switch, the fifth switch, the opposite terminal of the secondary side of the second isolation transformer, the same terminal of the secondary side of the second isolation transformer, and the negative terminal of the output capacitor are connected in sequence.

[0010] More preferably, the topology is in battery discharge mode: During the positive half-cycle of the power grid, in the first timing sequence, the power grid working circuit operates as follows: the battery, the primary side of the first isolation transformer, and the first switching transistor form a loop; the inductor of the first isolation transformer stores energy; and the output capacitor outputs AC voltage to the output port. In the second timing sequence, the power grid working circuit operates as follows: the output capacitor, the secondary side of the first isolation transformer, the second switching transistor, and the third switching transistor form a loop; and the inductor of the first isolation transformer charges the output capacitor. During the negative half-cycle of the power grid, in the third timing sequence of the power grid negative half-cycle operating circuit: the battery, the fourth switch, and the primary side of the second isolation transformer form a loop, the inductor of the second isolation transformer stores energy, and the output capacitor outputs AC voltage to the output port; in the fourth timing sequence of the power grid negative half-cycle operating circuit: the output capacitor, the sixth switch, the fifth switch, and the secondary side of the second isolation transformer form a loop, and the inductor of the second isolation transformer charges the output capacitor.

[0011] More preferably, during the positive half-cycle of the power grid, the positive half-cycle operating circuit operates as follows: the output port voltage is positive and greater than the secondary voltage of the first isolation transformer, and the inductor of the first isolation transformer charges the battery; during the negative half-cycle of the power grid, the negative half-cycle operating circuit operates as follows: the output port voltage is negative and the absolute value of the output port voltage is greater than the secondary voltage of the second isolation transformer, and the inductor of the second isolation transformer charges the battery.

[0012] More preferably, the topology is in battery charging mode: During the positive half-cycle of the power grid, in the fifth timing sequence of the power grid positive half-cycle operating circuit: the output capacitor, the second switch, the third switch, and the secondary side of the first isolation transformer form a loop, and the output capacitor charges the inductance of the first isolation transformer; in the sixth timing sequence of the power grid positive half-cycle operating circuit: the battery, the primary side of the first isolation transformer, and the first switch form a loop, and the inductance of the first isolation transformer charges the battery; During the negative half-cycle of the power grid, in the seventh timing sequence of the power grid negative half-cycle operating circuit: the output capacitor, the sixth switch, the fifth switch, and the secondary side of the second isolation transformer form a loop, and the output capacitor charges the inductance of the second isolation transformer; in the eighth timing sequence of the power grid negative half-cycle operating circuit: the battery, the fourth switch, and the primary side of the second isolation transformer form a loop, and the inductance of the second isolation transformer charges the battery.

[0013] Preferably, in the above technical solution, the switching transistor is driven by a PWM signal with a frequency of not less than 200kHz.

[0014] Preferably, in the above technical solution, the switching transistor is an N-channel enhancement-mode MOSFET.

[0015] Another objective of this invention is to provide a control method for an isolated energy storage grid-connected inverter.

[0016] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A control method for an isolated energy storage grid-connected inverter, comprising: Set the mains current reference, sample the mains current, obtain the AC output voltage reference value VolAcRef, and determine the sign of the voltage reference value VolAcRef. (1) When the voltage reference value VolAcRef > 0, the positive half-cycle working circuit of the power grid is started: Perform PWM modulation on the first switch and calculate its duty cycle: , Where: VBat is the DC battery terminal voltage, T1 is the turns ratio of the first isolation transformer, and Duty_Q1 ranges from 0 to 1. The PWM signal of the second switch is complementary to that of the first switch. The third switch remains constantly on. A positive voltage is obtained across the output capacitor Cout: Vout = VolAcRef; (2) When the voltage reference value VolAcRef < 0, start the negative half-cycle working circuit of the power grid: Perform PWM modulation on the fourth switch and calculate its duty cycle: , Where: VBat is the DC battery terminal voltage, T2 is the turns ratio of the second isolation transformer, and Duty_Q4 ranges from 0 to 1. The PWM signal of the sixth switch is complementary to that of the fourth switch. The fifth switch remains continuously on. A negative voltage is obtained across the output capacitor Cout: Vout = VolAcRef.

[0017] Preferably, the above technical solution involves using an MCU digital chip to calculate the drive signals for the first, second, third, fourth, fifth, and sixth switching transistors according to the above logic, and then outputting a PWM wave.

[0018] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art: This invention uses an isolated topology, which provides better insulation and meets safety requirements with fewer relays compared to non-isolated energy storage inverters. Furthermore, the energy storage inverter, which uses only a single-stage DC / AC circuit, requires fewer power electronic devices (switching transistors) and has a lower cost compared to a two-stage DC / DC+DC / AC topology. It can achieve electrical isolation, bidirectional energy flow, and a simple and efficient grid-connected energy storage inverter with a single-stage DC / AC topology. Attached Figure Description

[0019] Figure 1 This is a topology diagram of the isolated energy storage grid-connected inverter in this embodiment; Figure 2 This is the first timing diagram of the circuit operating during the positive half-cycle of the power grid when the battery is discharging in this embodiment; Figure 3 This is the second timing diagram of the circuit operating during the positive half-cycle of the power grid when the battery is discharging in this embodiment; Figure 4 This is the third timing diagram of the circuit operating during the negative half-cycle of the power grid when the battery is discharging in this embodiment; Figure 5 This is the fourth timing diagram of the circuit operating during the negative half-cycle of the power grid when the battery is discharging in this embodiment; Figure 6 This is the fifth timing diagram of the circuit operating during the positive half-cycle of the power grid when the battery is charging in this embodiment; Figure 7 This is the sixth timing diagram of the circuit operating during the positive half-cycle of the power grid when the battery is charging in this embodiment; Figure 8 This is the seventh timing diagram of the circuit operating during the negative half-cycle of the power grid when the battery is charging in this embodiment; Figure 9 This is the eighth timing diagram of the circuit operating during the negative half-cycle of the power grid when the battery is charging in this embodiment; Figure 10 This is a flowchart illustrating the control process of the AC output current in this embodiment; Figure 11 This is a timing diagram of the PWM waves of the first, second, and third switches, the output capacitor voltage, and the grid-side output current during the positive half-cycle of the power grid in this embodiment. Figure 12 This is a timing diagram showing the PWM waves of the fourth, fifth, and sixth switches, the output capacitor voltage, and the grid-side output current during the negative half-cycle of the power grid in this embodiment. Detailed Implementation

[0020] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0022] like Figure 1 As shown: An isolated energy storage grid-connected inverter topology includes: a battery DC, a grid positive half-cycle operating circuit, a grid negative half-cycle operating circuit, and an output capacitor Cout. The grid positive half-cycle operating circuit and the grid negative half-cycle operating circuit are flyback circuits connected in parallel between the battery and the output capacitor and in reverse direction, used to convert the DC voltage of the battery DC into AC voltage and output it to the output port AC_Grid.

[0023] In this embodiment, both the positive and negative half-cycle operating circuits of the power grid include isolation transformers and switching transistors. In other words, both circuits consist of isolation transformers and switching transistors; that is, they do not include common AC filter inductors except for the isolation transformers and switching transistors. The isolation transformer is a high-frequency isolation transformer (operating frequency typically exceeding 10kHz). The leakage inductance of the high-frequency isolation transformer serves as the current filter inductor. The switching transistor is driven by a PWM signal with a frequency greater than 200kHz, as shown in the figure, an N-channel enhancement-type MOSFET.

[0024] The positive half-cycle operating circuit of the power grid includes a first isolation transformer T1, a first switching transistor Q1, a second switching transistor Q2, and a third switching transistor Q3. Specifically, the positive half-cycle operating circuit consists only of the first isolation transformer T1, the first switching transistor Q1, the second switching transistor Q2, and the third switching transistor Q3. The battery DC positive terminal, the opposite terminal of the primary winding of the first isolation transformer T1, the same terminal of the primary winding of the first isolation transformer T1, the first switching transistor Q1, and the battery DC negative terminal are connected in sequence. The output capacitor Cout positive terminal, the third switching transistor Q3, the second switching transistor Q2, the same terminal of the secondary winding of the first isolation transformer T1, the opposite terminal of the secondary winding of the first isolation transformer T1, and the output capacitor Cout negative terminal are connected in sequence. During the positive half-cycle of the power grid, the voltage across the output capacitor Cout is positive, and the positive half-cycle operating circuit converts the battery DC voltage into a positive voltage and outputs it to the output capacitor Cout.

[0025] The negative half-cycle operating circuit includes a second isolation transformer T2, a fourth switch Q4, a fifth switch Q5, and a sixth switch Q6. Specifically, the negative half-cycle operating circuit consists only of the second isolation transformer T2, the fourth switch Q4, the fifth switch Q5, and the sixth switch Q6. The battery DC positive terminal, the fourth switch Q4, the opposite terminal of the primary winding of the second isolation transformer T2, the same terminal of the primary winding of the second isolation transformer T2, and the battery DC negative terminal are connected in sequence. The output capacitor Cout positive terminal, the sixth switch Q6, the fifth switch Q5, the opposite terminal of the secondary winding of the second isolation transformer T2, the same terminal of the secondary winding of the second isolation transformer T2, and the output capacitor Cout negative terminal are connected in sequence. During the negative half-cycle of the grid, the voltage across the output capacitor Cout is negative, and the negative half-cycle operating circuit converts the battery DC voltage into a negative voltage and outputs it to the output capacitor Cout.

[0026] The following details the timing of the working circuits during the positive half-cycle and negative half-cycle of the power grid in this embodiment under battery discharge and charging modes.

[0027] 1. Battery discharge: During the positive half-cycle of the power grid, the operating circuit works as follows: like Figure 2 As shown: In the first timing sequence: the first switch Q1 is on, the second switch Q2 is off, and the third switch Q3 is continuously on. The battery DC, the primary side of the first isolation transformer T1, and the first switch Q1 form a circuit, through which current flows. The inductance of the first isolation transformer T1 stores energy, and the AC output voltage is supported by the output capacitor Cout. The output capacitor Cout, the secondary side of the first isolation transformer T1, the second switch Q2, and the third switch Q3 cannot form a circuit because the voltage on the secondary side of the first isolation transformer T1 is reversed and the second switch Q2 is off.

[0028] like Figure 3As shown: In the second timing sequence: the first switch Q1 is off, the second switch Q2 is on, and the third switch Q3 is continuously on. The circuit of the battery DC, the primary side of the first isolation transformer T1, and the first switch Q1 is disconnected, and no current flows through it. The output capacitor Cout, the secondary side of the first isolation transformer T1, the second switch Q2, and the third switch Q3 form a circuit. Since the inductance of the first isolation transformer T1 releases energy, the inductance of the first isolation transformer T1 charges the output capacitor Cout in the forward direction.

[0029] During the negative half-cycle of the power grid, the operating circuit works as follows: like Figure 4 As shown: In the third timing sequence: the fourth switch Q4 is on, the sixth switch Q6 is off, and the fifth switch Q5 is on continuously. The battery DC, the fourth switch Q4, and the primary side of the second isolation transformer T2 form a loop, through which current flows. The inductance of the second isolation transformer T2 stores energy, and the AC output voltage is supported by the output capacitor Cout. The output capacitor Cout, the secondary side of the second isolation transformer T2, the fifth switch Q5, and the sixth switch Q6 cannot form a loop because the voltage on the secondary side of the second isolation transformer T2 is reversed and the sixth switch Q6 is off.

[0030] like Figure 5 As shown: In the fourth timing sequence: the first switch Q4 is off, the sixth switch Q6 is on, and the fifth switch Q5 is continuously on. The circuit of battery DC, the primary side of the second isolation transformer T2, and the fourth switch Q4 is disconnected, and no current flows through it. The output capacitor Cout, the sixth switch Q6, the fifth switch Q5, and the secondary side of the second isolation transformer T2 form a circuit. Since the inductance of the second isolation transformer T2 releases energy, the inductance of the second isolation transformer T2 charges the output capacitor Cout in the negative direction.

[0031] II. Battery charging: During the positive half-cycle of the power grid, the operating circuit works as follows: The output port AC_Grid voltage VOut is positive, and the output port AC_Grid voltage VOut is greater than the secondary voltage V_T1s of the first isolation transformer T1. The same-name terminal of the secondary side of the first isolation transformer T1 is defined as positive voltage to realize battery charging.

[0032] like Figure 6 As shown: In the fifth timing sequence: the first switch Q1 is off, the second switch Q2 is on, and the third switch Q3 is continuously on. The circuit of the battery DC, the primary side of the first isolation transformer T1, and the first switch Q1 is disconnected, and no current flows through it. The output capacitor Cout, the second switch Q2, the third switch Q3, and the secondary side of the first isolation transformer T1 form a circuit, through which current flows. The output capacitor Cout is charged by the inductance of the first isolation transformer T1.

[0033] like Figure 7 As shown: In the sixth timing sequence: the first switch Q1 is on, the second switch Q2 is off, and the third switch Q3 is on continuously. The battery DC, the primary side of the first isolation transformer T1, and the first switch Q1 form a circuit, and current flows through them. The inductance of the first isolation transformer T1 charges the battery DC. The output capacitor Cout, the secondary side of the first isolation transformer T1, the second switch Q2, and the third switch Q3 cannot form a circuit because the second switch Q2 is off.

[0034] During the negative half-cycle of the power grid, the operating circuit works as follows: The output port AC_Grid voltage VOut is negative, and the absolute value of the output port AC_Grid voltage VOut is greater than the secondary voltage V_T2s of the second isolation transformer T2. The same-name terminal of the secondary side of the second isolation transformer T2 is defined as positive voltage, and the inductor of the second isolation transformer T2 charges the battery.

[0035] like Figure 8 As shown: In the seventh timing sequence: the fourth switch Q4 is off, the sixth switch Q6 is on, and the fifth switch Q5 is continuously on. The circuit of the battery DC, the primary side of the second isolation transformer T2, and the fourth switch Q4 is disconnected, and no current flows through it. The output capacitor Cout, the sixth switch Q6, the fifth switch Q5, and the secondary side of the second isolation transformer T2 form a circuit, and current flows through it. The output capacitor Cout is charged by the inductance of the second isolation transformer T2.

[0036] like Figure 9 As shown: In the eighth timing sequence: the fourth switch Q4 is on, the sixth switch Q6 is off, and the fifth switch Q5 is on continuously. The battery DC, the fourth switch Q4, and the primary side of the second isolation transformer T2 form a loop, and current flows through them. The inductance of the second isolation transformer T2 charges the battery DC. The output capacitor Cout, the secondary side of the second isolation transformer T2, the fifth switch Q5, and the sixth switch Q6 cannot form a loop because the sixth switch Q6 is off.

[0037] As mentioned above, when the grid positive half-cycle working circuit is working, the output capacitor Cout can output a positive voltage. When the grid negative half-cycle working circuit is working, the output capacitor Cout can output a negative voltage. Since the switching transistor is driven by a high-frequency PWM signal with a frequency of 200kHz or higher, which is much higher than the grid's 50Hz, as long as SPWM modulation is used on the PWM signal, the grid positive half-cycle working circuit outputs a sinusoidal voltage for the positive half-cycle, and the grid negative half-cycle working circuit outputs a sinusoidal voltage for the negative half-cycle. After combination, an AC sinusoidal voltage output can be obtained, thereby controlling the AC side grid-connected current.

[0038] The control process of the AC output current in this embodiment is described in detail below: like Figure 10As shown: The grid current reference setting is used. After the grid current is sampled, it passes through the grid-connected current controller to obtain the AC output voltage reference value VolAcRef of the output capacitor Cout. Determine the sign of the voltage reference value VolAcRef: (1) When the voltage reference value VolAcRef > 0, the positive half-cycle working circuit of the power grid is started: Perform PWM modulation on the first switch and calculate its duty cycle: , Where: VBat is the DC battery terminal voltage, T1 is the turns ratio of the first isolation transformer, and Duty_Q1 ranges from 0 to 1. The PWM signal of the second switch is complementary to that of the first switch. The third switch remains constantly on. A positive voltage is obtained across the output capacitor Cout: Vout = VolAcRef; (2) When the voltage reference value VolAcRef < 0, start the negative half-cycle working circuit of the power grid: Perform PWM modulation on the fourth switch and calculate its duty cycle: , Where: VBat is the DC battery terminal voltage, T2 is the turns ratio of the second isolation transformer, and Duty_Q4 ranges from 0 to 1. The PWM signal of the sixth switch is complementary to that of the fourth switch. The fifth switch remains continuously on. A negative voltage is obtained across the output capacitor Cout: Vout = VolAcRef.

[0039] The MCU digital chip is used for control. Based on the calculated PWM duty cycle, PWM drive signals are output to the first switch Q1, the second switch Q2, the third switch Q3, the fourth switch Q4, the fifth switch Q5, and the sixth switch Q6.

[0040] like Figure 11 As shown: the first switch Q1 and the second switch Q2 are a pair of complementary PWM signals ( The switching frequency is 200 kHz; the third switch Q3 is always on during the positive half-cycle of the mains voltage and normally closed during the negative half-cycle, with a switching frequency of 50 Hz.

[0041] like Figure 12 As shown: the fourth switch Q4 and the sixth switch Q6 are a pair of complementary PWM signals. The switching frequency is 200kHz. The fifth switch Q5 is always on during the negative half-cycle of the mains voltage and normally closed during the positive half-cycle. The switching frequency is 50Hz.

[0042] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. An isolated energy storage grid-connected inverter topology, characterized in that: It includes a battery, a positive half-cycle grid operating circuit, a negative half-cycle grid operating circuit, and an output capacitor. The positive half-cycle grid operating circuit and the negative half-cycle grid operating circuit are flyback circuits connected in parallel between the battery and the output capacitor and in reverse direction. They are used to convert the DC voltage of the battery into AC voltage and output it to the output port. Both the positive half-cycle grid operating circuit and the negative half-cycle grid operating circuit include an isolation transformer and a switching transistor.

2. The isolated energy storage grid-connected inverter topology according to claim 1, characterized in that: During the positive half-cycle of the power grid: the circuit converts the battery voltage into a positive voltage and outputs it to the output capacitor; during the negative half-cycle of the power grid: the circuit converts the battery voltage into a negative voltage and outputs it to the output capacitor.

3. The isolated energy storage grid-connected inverter topology according to claim 1 or 2, characterized in that: The positive half-cycle operating circuit of the power grid includes a first isolation transformer, a first switching transistor, a second switching transistor, and a third switching transistor. The positive terminal of the battery, the opposite terminal of the primary side of the first isolation transformer, the same terminal of the primary side of the first isolation transformer, the first switching transistor, and the negative terminal of the battery are connected in sequence. The positive terminal of the output capacitor, the third switching transistor, the second switching transistor, the same terminal of the secondary side of the first isolation transformer, the opposite terminal of the secondary side of the first isolation transformer, and the negative terminal of the output capacitor are connected in sequence. The negative half-cycle operating circuit of the power grid includes a second isolation transformer, a fourth switch, a fifth switch, and a sixth switch. The positive terminal of the battery, the fourth switch, the opposite terminal of the primary side of the second isolation transformer, the same terminal of the primary side of the second isolation transformer, and the negative terminal of the battery are connected in sequence. The positive terminal of the output capacitor, the sixth switch, the fifth switch, the opposite terminal of the secondary side of the second isolation transformer, the same terminal of the secondary side of the second isolation transformer, and the negative terminal of the output capacitor are connected in sequence.

4. The isolated energy storage grid-connected inverter topology according to claim 3, characterized in that: The topology in battery discharge mode: During the positive half-cycle of the power grid, in the first timing sequence, the power grid working circuit operates as follows: the battery, the primary side of the first isolation transformer, and the first switching transistor form a loop; the inductor of the first isolation transformer stores energy; and the output capacitor outputs AC voltage to the output port. In the second timing sequence, the power grid working circuit operates as follows: the output capacitor, the secondary side of the first isolation transformer, the second switching transistor, and the third switching transistor form a loop; and the inductor of the first isolation transformer charges the output capacitor. During the negative half-cycle of the power grid, in the third timing sequence of the power grid negative half-cycle operating circuit: the battery, the fourth switch, and the primary side of the second isolation transformer form a loop, the inductor of the second isolation transformer stores energy, and the output capacitor outputs AC voltage to the output port; in the fourth timing sequence of the power grid negative half-cycle operating circuit: the output capacitor, the sixth switch, the fifth switch, and the secondary side of the second isolation transformer form a loop, and the inductor of the second isolation transformer charges the output capacitor.

5. The isolated energy storage grid-connected inverter topology according to claim 3, characterized in that: During the positive half-cycle of the power grid, the positive half-cycle operating circuit operates as follows: the output port voltage is positive and greater than the secondary voltage of the first isolation transformer, and the inductor of the first isolation transformer charges the battery; during the negative half-cycle of the power grid, the negative half-cycle operating circuit operates as follows: the output port voltage is negative and the absolute value of the output port voltage is greater than the secondary voltage of the second isolation transformer, and the inductor of the second isolation transformer charges the battery.

6. The isolated energy storage grid-connected inverter topology according to claim 5, characterized in that: The topology in battery charging mode: During the positive half-cycle of the power grid, in the fifth timing sequence of the power grid positive half-cycle operating circuit: the output capacitor, the second switch, the third switch, and the secondary side of the first isolation transformer form a loop, and the output capacitor charges the inductance of the first isolation transformer; in the sixth timing sequence of the power grid positive half-cycle operating circuit: the battery, the primary side of the first isolation transformer, and the first switch form a loop, and the inductance of the first isolation transformer charges the battery; During the negative half-cycle of the power grid, in the seventh timing sequence of the power grid negative half-cycle operating circuit: the output capacitor, the sixth switch, the fifth switch, and the secondary side of the second isolation transformer form a loop, and the output capacitor charges the inductance of the second isolation transformer; in the eighth timing sequence of the power grid negative half-cycle operating circuit: the battery, the fourth switch, and the primary side of the second isolation transformer form a loop, and the inductance of the second isolation transformer charges the battery.

7. The isolated energy storage grid-connected inverter topology according to claim 1, characterized in that: The switching transistor is driven by a PWM signal with a frequency of not less than 200kHz.

8. The isolated energy storage grid-connected inverter topology according to claim 1, characterized in that: The switching transistor is an N-channel enhancement-mode MOSFET.

9. A control method for an isolated energy storage grid-connected inverter, characterized in that: The control method is implemented using the isolated energy storage grid-connected inverter topology described in any one of claims 3 to 8, and includes the following steps: Set the mains current reference, sample the mains current, obtain the AC output voltage reference value VolAcRef, and determine the sign of the voltage reference value VolAcRef. (1) When the voltage reference value VolAcRef > 0, the positive half-cycle working circuit of the power grid is started: Perform PWM modulation on the first switch and calculate its duty cycle: , Where: VBat is the DC battery terminal voltage, T1 is the turns ratio of the first isolation transformer, and Duty_Q1 ranges from 0 to 1. The PWM signal of the second switch is complementary to that of the first switch. The third switch remains constantly on. A positive voltage is obtained across the output capacitor Cout: Vout = VolAcRef; (2) When the voltage reference value VolAcRef < 0, start the negative half-cycle working circuit of the power grid: Perform PWM modulation on the fourth switch and calculate its duty cycle: , Where: VBat is the DC battery terminal voltage, T2 is the turns ratio of the second isolation transformer, and Duty_Q4 ranges from 0 to 1. The PWM signal of the sixth switch is complementary to that of the fourth switch. The fifth switch remains continuously on. A negative voltage is obtained across the output capacitor Cout: Vout = VolAcRef.

10. The control method for an isolated energy storage grid-connected inverter according to claim 9, characterized in that: The MCU digital chip calculates the drive signals for the first, second, third, fourth, fifth, and sixth switching transistors according to the above steps, and outputs PWM waves.

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