Isolated three-port DC-AC converter for energy storage system and modulation strategy

Abstract

The invention provides an isolated three-port DC-AC converter for an energy storage system and a modulation strategy, and belongs to the technical field of power conversion. The problem that a conventional modulation mode of a three-port converter is difficult to suppress large voltage fluctuation of ports of a hydrogen fuel cell and is difficult to realize stable grid connection is solved. The converter comprises a switch inductance unit, a full-bridge circuit, a high-frequency transformer, a matrix circuit, an LC filter circuit and three output ports. The modulation strategy comprises the following steps: calculating converter parameters based on the critical conduction state of the converter, detecting an input voltage V1, and setting an output voltage Vac of an AC output port and a rated voltage V2 of a DC bus according to the voltage of a converter system gain calculation port V2, and judging whether the voltage of the direct current bus is greater than the rated voltage V2 or not under the condition that the duty ratio D is equal to 0.5 based on the working state of the converter, entering a high-voltage mode if the voltage is greater than the rated voltage V2, entering a low-voltage mode if the voltage is smaller than the rated voltage V2, and regulating the voltage of the port V2 by regulating the duty ratio D so as to obtain the final output voltage.

Description

TECHNICAL FIELD

[0001] The application relates to an isolated three-port DC-AC converter and a modulation strategy for an energy storage system, and belongs to the technical field of power conversion. BACKGROUND

[0002] Hydrogen fuel cells have been widely concerned due to their advantages such as no pollution and long energy storage time. In the grid connection of hydrogen fuel cells, high-performance power converters are the core devices for improving the utilization efficiency of hydrogen fuel cells and the reliability of the system. Single-stage three-port DC-AC converters have high integration capability, can integrate various energy sources such as hydrogen fuel cells (power generation units) and lithium batteries (regulation units), realize efficient collaborative utilization of hydrogen fuel cells, and flexible operation in grid-connected and off-grid modes, and have broad application prospects in household energy storage systems. However, in the process of constructing a multi-source system to realize stable grid connection of household hydrogen fuel cell systems, there are still the following key challenges: 1) High voltage gain requirement: Due to the limitations of cost, safety and power, the direct current bus voltage of household fuel cell systems is usually low (for example, SMART FC-DC 48V / 5kw, PROMEP4S 65-90V / 25kw, etc.), and the output standard alternating current voltage (220V / 380V) is required, which requires the converter to have a high gain of more than 8-10 times; 2) Wide voltage adaptation capability: The capacity of household hydrogen fuel cell systems is small, and the state of charge of energy storage batteries and the output of fuel cells will cause a large fluctuation of the alternating current bus voltage. The inverter needs to have excellent wide voltage adaptation capability to maintain high-quality output when the input voltage fluctuates (meet the grid connection standard or independent power supply requirements).

[0003] Three-port converters can be divided into non-isolated and isolated types according to whether electrical isolation is required between ports. Non-isolated three-port converters do not have electrical isolation between ports, have the advantages of compact structure, high efficiency and low cost, and are commonly used in applications with high requirements for volume and efficiency. However, non-isolated inverters are limited by the through-time and modulation index, and the voltage regulation capability of such converters for DC ports is still limited, and the port voltage cannot meet the working requirements of lithium ion batteries in GB / T 36276-2023 standard. Isolated three-port converters achieve electrical isolation through transformers, which can make up for the lack of voltage adaptation capability of non-isolated topologies. However, in the DC-AC three-port system, the conventional modulation method will cause the energy backflow of the leakage inductance to be blocked, causing system oscillation. Although the existing methods can alleviate the above problems, they mostly need to increase components or improve the switching frequency, resulting in increased system loss. SUMMARY

[0004] The application discloses an isolated three-port DC-AC converter and a modulation strategy for an energy storage system.

[0005] The application discloses an isolated three-port DC-AC converter and a modulation strategy for an energy storage system. The switch inductance unit is connected with the fuel cell port and the lithium battery port at an input end, and is connected with the full-bridge circuit at an output end, and the input DC voltage is processed by controlling the series-parallel state of the inductances, and the processed DC voltage is transmitted to the full-bridge circuit. The full-bridge circuit is connected with the switch inductance unit and the lithium battery port at an input node, and is connected with the primary winding of the high-frequency transformer at an output node, and is used for voltage regulation of the input DC voltage. The matrix circuit is connected with the secondary winding of the high-frequency transformer at an input node, and is connected with the LC filter circuit at an output node, and is used for converting the voltage-regulated DC voltage into AC voltage. The LC filter circuit is connected with the matrix circuit at an input end, and is connected with the AC output port at an output end, and is used for eliminating the high-frequency harmonic wave in the AC voltage.

[0006] Further, the positive pole of the lithium battery port is connected with one end of the capacitor C 2, the switch inductance unit and the full-bridge circuit, and the negative pole is connected with the other end of the capacitor C 2, the switch inductance unit and the full-bridge circuit. The positive pole of the fuel cell port is connected with one end of the capacitor C 1 and the switch inductance unit, and the negative pole is connected with the other end of the capacitor C 1.

[0007] Further, the switch inductance unit comprises a first switch inductance group and a second switch inductance group. The first switch inductance group comprises an inductor L 1, a diode L 2, a diode D 1, a diode D 2 and a diode D 3, an inductor L 1 and an inductor L 2 are designed as coupled inductances to reduce the number and size of the magnetic cores, and the inductor L 1 is connected with the fuel cell port V 1, the positive pole of the capacitor C 1, the diode D 1, the anode of the diode L 1 is connected with the diode DAnode of 2 and diode D Anode of 3; diode D Cathode connected inductor of 3 L One end of 2; diode D Cathode connected inductor of 2 L Another end of 2 and diode D Cathode of 1; Second switch inductor group includes inductor L 3, inductor L 4, diode D 4, diode D 5 and diode D 6, inductor L 3 and inductor L 4 is designed to couple inductor to reduce the number and size of magnetic core, inductor L One end of 3 connects first switch inductor group and diode D Anode of 4, another end connects diode D Anode of 5 and diode D Anode of 6; diode D Cathode connected inductor of 6 L One end of 4 and input node of full bridge circuit, and inductors are provided on the connecting circuit of full bridge circuit L k Diode D Cathode connected inductor of 5 L Another end of 4 and diode D Cathode of 4; Diode D 1 and diode D 3 is on, diode D 2 is off, enter series mode, inductor L 1 and inductor L 2 are in series, inductor L 3 and inductor L 4 are in series, two groups of series inductors work together to boost the input DC voltage; Diode D 2 is on, diode D 1 and diode D 3 is off, inductor L 1 and inductor L 2 are in parallel, inductor L 3 and inductor L 4 are in parallel, reduce the total inductance value of two groups of parallel inductors to adapt to different voltage scenarios.

[0008] Further, the full bridge circuit includes switch tubes S 1, switch tube S 2, switch tube S 3 and switch tube S4; switching transistor S 2 is connected to the positive pole of the lithium battery port, and the capacitor C 2 is connected to one end of the switching transistor S 4 is connected to the source pole of the switching transistor S 1 is connected to the drain pole of the switching transistor, and the input node is arranged on the connecting circuit S 1 is connected to the source pole of the switching transistor S 1 is connected to the negative pole of the lithium battery port, and the capacitor C 2 is connected to the other end of the switching transistor S 3 is connected to the source pole of the switching transistor S 3 is connected to the drain pole of the switching transistor S 4 is connected to the source pole of the switching transistor, and the output node is arranged on the connecting circuit S 4 is connected to the output node, and the output node is connected to the primary winding of the high-frequency transformer; The AC bus is arranged between the secondary winding and the primary winding of the high-frequency transformer; The matrix circuit comprises the switching transistor S n1 , the switching transistor S n2 , the switching transistor S n3 , and the switching transistor S n4 , and the switching transistor S p1 , the switching transistor S p2 , the switching transistor S p3 , and the switching transistor S p4 ; The switching transistor S n1 is connected to one end of the secondary winding of the high-frequency transformer, and the switching transistor S n3 is connected to the drain pole of the switching transistor S p1 is connected to the source pole of the switching transistor S p1 is connected to the drain pole of the switching transistor S n2 , and the input node is arranged on the connecting circuit; the switching transistor S n2 is connected to the source pole of the switching transistor S p2 is connected to the source pole of the switching transistor S p2 is connected to the other end of the secondary winding of the high-frequency transformer, and the switching transistor S p4 is connected to the drain pole of the switching transistorS p4 Source and switch S n4 Source connection; switching transistor S n4 Drain and switching transistor S p3 The drain is connected, and an output node is provided on the connection circuit; the switching transistor S p3 Source and switch S n3 The source connection.

[0009] Furthermore, the LC filter circuit includes an inductor. L and filter capacitor C ,inductance L One end is connected to the input node and AC output port of the matrix circuit, and the other end is connected to the filter capacitor. C Positive terminal; Filter capacitor C The negative terminal is connected to the output node of the matrix circuit and the AC output port.

[0010] Modulation strategies based on isolated three-port DC-AC converters include: Step 1: Under the critical conduction condition of the converter, calculate the inductance parameters and voltage gain function of the converter; Step 2: Set the output voltage of the AC output port V ac and the rated voltage of the DC bus V 2. Determine the DC bus voltage at a duty cycle D=0.5 based on the converter's operating state. Is it greater than the rated voltage? V 2; Step 3: If it is greater than the expected value, enter the high voltage mode, preset the voltage relationship, and calculate the output voltage based on the preset voltage. If the output voltage is not equal to the expected value, increase the rated voltage of the DC bus and recalculate the output voltage until the output voltage equals the expected value. If it is less than, then enter low voltage mode and turn off the switch. S 1 and switch S The duty cycle of 3 is set to the maximum allowed value, and the modulation index is calculated based on the set duty cycle. m And the phase shift angle α, based on the set duty cycle and modulation index m The output voltage is calculated using the phase shift angle α. If the output voltage is not equal to the expected value, the current input voltage is determined to be too low, the calculation is terminated, and an error is reported.

[0011] Furthermore, the calculation of the converter's critical conduction condition in step 1 includes: Step 1.1: In the coupled case, the average current and ripple of inductor L 1 in the discontinuous conduction mode of the converter is calculated according to the energy conservation, and the energy of diode D 1 and diode D 3 is calculated according to the energy conservation based on the corresponding relationship between the average current and ripple, and the equivalent input current of the lithium battery port is obtained in the discontinuous conduction mode according to the energy conservation; Step 1.2: In the coupled case, the average value of inductor current and the ripple in the continuous conduction mode of the converter are calculated according to the energy conservation, the average value of inductor current in the matrix circuit is calculated according to the energy conservation in the continuous conduction mode, and the equivalent current of the lithium battery port is calculated according to the energy conservation theorem ; Step 1.3: The critical condition of discontinuous conduction mode and continuous conduction mode is obtained according to the equivalent input current of lithium battery port calculated in step 1.1 and step 1.2 respectively; Step 1.4: Replace step 1.1 and step 1.2 with uncoupled case, and repeat step 1.1- step 1.3 to obtain the critical condition of inductor in uncoupled case; The corresponding relationship between the average current and the ripple is: (1); The energy of diode D 1 and diode D 3 is: (2); In formulas (1) and (2), I 2 is the equivalent input current of lithium battery port V 2, is the equivalent average value of inductor L 1, D is the duty cycle of switch tube S 1 and switch tube S 3, is the current ripple of inductor L 1, is the input voltage of fuel cell port; The corresponding relationship between the average value of inductor current and the ripple is: (3); The equivalent current of lithium battery port is: (4); In formula (4), R is the equivalent resistance of the system; The critical condition of discontinuous conduction mode and continuous conduction mode is: (5); The critical condition for an inductor is: (6).

[0012] Furthermore, the converter's operating states in step 2 include: Mode Ⅰ, Mode ⅠⅠ, Mode ⅠⅠⅠ, and Mode Ⅳ. In Mode Ⅰ, the fuel cell port, lithium battery port, and AC output port are all operational. The fuel cell transfers energy to the lithium battery and the AC bus, the fuel cell and lithium battery transfer energy to the AC bus, and the lithium battery does not participate in energy transfer but only in power regulation. In Mode ⅠⅠ, the fuel cell port and lithium battery port are operational, and the fuel cell transfers energy to the lithium battery. In Mode ⅠⅢⅠ, the lithium battery port and the AC output port are operational, and the lithium battery transfers energy to the AC bus. In Mode Ⅳ, the lithium battery port and the AC output port are operational, and the AC bus transfers energy to the lithium battery port.

[0013] Furthermore, the calculation of the output voltage in high-voltage mode in step 3 includes: Under the condition of the converter critical conduction, the voltage of the DC bus at the preset maximum duty cycle =Rated voltage of DC bus V 2. Calculate the initial switching transistor based on the operating state of the switching inductor unit and the preset voltage relationship. S 1 and switching transistor S 3 duty cycle D Lowering the bus voltage enables the converter to boost voltage; the converter's maximum modulation ratio and port... V The relationship between the maximum input voltage and the input voltage is as follows: (7), n This is the ratio of the highest operating voltage of the lithium battery at port 2 to its rated voltage. Calculate the modulation index m Phase shift angle α, based on duty cycle D Modulation index m The output voltage is calculated using the phase shift angle α.

[0014] Furthermore, the operating states of the switching inductor unit include series operation and parallel operation, which are achieved by adjusting the switching transistor. S 1 and switching transistor S 3 duty cycle D To control the switching of the inductor state; In series operation, the total inductance value The inductor-voltage relationship satisfies ,in, V D1 , V D3 Diodes D 1, D 3. Voltage across the terminalsk is a coupling coefficient, M is a mutual inductance, is a diode D 1 is a voltage, is a diode D 3 is a voltage, is an inductance voltage; In a parallel operation state, the total inductance value , the inductance voltage relationship satisfies wherein, is a diode D 2 is a voltage.

[0015] Further, the calculation of the output voltage in step 3 in the low voltage mode includes: In the critical conduction state of the converter, the duty cycles S 1 and switch S 3 are set to the maximum allowed value, realizing the voltage rise of port D 2, the relationship between the maximum modulation ratio of the converter and the minimum voltage of port V 1 input is: V (8); The modulation index , the phase shift angle α are calculated, and the output voltage is calculated according to the duty cycle m , the modulation index D and the phase shift angle α. m

[0016] The beneficial effects of the present application are: The present application combines switch inductance and single-stage isolated inverter to realize energy transmission and efficient allocation between fuel cell, lithium battery and power grid. At the same time, the present application solves the industry problem that the large output voltage fluctuation of fuel cell leads to that the lithium battery port voltage does not meet the working requirements of lithium battery in GB / T 36276-2023, and provides key technical support for building high-reliability household fuel cell energy storage system. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is a structure diagram of an isolated three-port DC-AC converter for energy storage system; Figure 2 is a schematic diagram of converter energy transmission; Figure 3 is a series equivalent circuit diagram of switch inductance; Figure 4 is a parallel equivalent circuit diagram of switch inductance; Figure 5 is a flowchart of modulation strategy based on isolated three-port DC-AC converter; Figure 6 ​This is a waveform diagram of the converter in discontinuous mode; Figure 7 For fuel cells V 1. Lithium battery V 2 and duty cycle D A diagram illustrating the relationship between them. Detailed Implementation

[0018] Specific implementation method one: Combining Figures 1-4 This implementation method is described as follows: Figure 1 As shown, the structure of the isolated three-port DC-AC converter for energy storage systems described in this embodiment includes: The circuit consists of a switching inductor unit, a full-bridge circuit, a high-frequency transformer, a matrix circuit, an LC filter circuit, and three output ports. V 1 (Fuel Cell) V 2 (Lithium battery) V AC (AC output port), where the full English name of the matrix circuit is Matrix Converter, abbreviated as MC.

[0019] The input terminals of the switching inductor unit are connected to the fuel cell port and the lithium battery port, and the output terminal is connected to the full-bridge circuit. It processes the input DC voltage by controlling the series and parallel connection state between the inductors and then transmits the processed DC voltage to the full-bridge circuit. The switching inductor unit includes a first switching inductor group and a second switching inductor group; the first switching inductor group includes inductors... L 1. Inductor L 2. Diode D 1. Diode D 2 and diode D 3. Inductance L 1 and inductor L 2. The design incorporates coupled inductors to reduce the number and size of magnetic cores. L One end of 1 is connected to the fuel cell port. V 1. Positive terminal, capacitor C One end of 1, diode D The anode of 1 and the second switching inductor group, inductor L The other end of 1 is connected to a diode. D 2 Anode and Diode D 3's anode; diode D 3's cathode is connected to the inductor L One end of 2; diode D 2's cathode is connected to the inductor L The other end of 2 and the diode D The cathode of 1; the second switching inductor group includes an inductor. L 3. Inductance L 4. Diode D 4. DiodeD 5 and diode D 6, inductor L 3 and inductor L 4 is designed to reduce the number and size of the coupled inductor core, inductor L 3, one end of the first switch inductor group and diode D 4 anode, the other end of the diode D 5 anode and diode D 6 anode; diode D 6 cathode connected to the inductor L 4 one end and the input node of the full-bridge circuit, and the inductor is provided on the connection circuit of the full-bridge circuit L k ; diode D 5 cathode connected to the inductor L 4 the other end and diode D 4 cathode; In addition, the switch inductor unit is divided into parallel and series operation states in circuit operation. By analyzing the operation of the coupled inductor, the appropriate inductance value is selected. First, the inductor is in series operation, diode D 1 and diode D 3 is turned on, diode D 2 is off, and the inductor is in series mode, inductor L 1 and inductor L 2 in series, inductor L 3 and inductor L 4 in series, and the two groups of series inductors work together to step up the input DC voltage, and the working state is as shown in Figure 3 . The total inductance, inductor voltage, D 1, D 3 when in series are L , , , : (1); (2); In formulas (1) and (2), V D1 , V D3 are the voltages across diode D 1, D 3, respectively, k is the coupling coefficient, M is the mutual inductance, is the voltage of diode D 1, is the voltage of diode D 3, is the inductor voltage; Inductors are connected in parallel, and their working states are as shown in Figure 4 Fig. 2. D 2 is turned on, and the diode D 1 is turned off. D 3 is turned off, and the inductor L 1 is turned on. L 2 is connected in parallel with the inductor L 3. L 4 are connected in parallel, the total inductance of the two groups of parallel inductors is reduced to adapt to different voltage scenarios, the total inductance when connected in parallel, the inductor voltage, D 2 is L , , : (3). (4). In formula (3), is the voltage of the diode D 2.

[0020] One end of the positive electrode of the lithium battery port is connected with the capacitor C 2, the switch inductor unit and the full-bridge circuit, the other end of the negative electrode of the lithium battery port is connected with the capacitor C 2, the switch inductor unit and the full-bridge circuit; one end of the positive electrode of the fuel cell port is connected with the capacitor C 1 and the switch inductor unit, and the other end of the negative electrode of the fuel cell port is connected with the capacitor C 1.

[0021] The full-bridge circuit comprises switch tubes S 1, switch tubes S 2, switch tubes S 3 and switch tubes S 4; the drain electrode of the switch tube S 2 is connected with the positive electrode of the lithium battery port, one end of the capacitor C 2 and the drain electrode of the switch tube S 4, the source electrode is connected with the switch inductor unit and the drain electrode of the switch tube S 1, and an input node is arranged on the connection circuit of the switch tube S 1; the source electrode of the switch tube S 1 is connected with the negative electrode of the lithium battery port, the other end of the capacitor C 2 and the source electrode of the switch tube S 3; the drain electrode of the switch tube S 3 is connected with the source electrode of the switch tube S 4, and an output node is arranged on the connection circuit of the switch tube S 4, and the output node is connected with the primary winding of the high-frequency transformer; An AC bus is arranged between the secondary winding and the primary winding of the high-frequency transformer S n1 , the switch tube S n2 , the switch tube S n3 and the switch tube S n4 , the switch tube S p1 , the switch tube S p2 , the switch tube S p3 and the switch tube S p4 ; the drain of the switch tube S n1 is connected with one end of the secondary winding of the high-frequency transformer and the drain of the switch tube S n3 , the source of the switch tube S p1 is connected with the source of the switch tube S p1 , the drain of the switch tube S n2 is connected with the drain of the switch tube S n2 , the source of the switch tube S p2 is connected with the source of the switch tube S p2 , the drain of the switch tube S p4 is connected with the other end of the secondary winding of the high-frequency transformer and the drain of the switch tube S p4 , the source of the switch tube S n4 is connected with the source of the switch tube S n4 , the drain of the switch tube S p3 is connected with the drain of the switch tube S p3 , and the source of the switch tube S n3 is connected with the source of the switch tube

[0022] The LC filter circuit comprises an inductor L and a filter capacitor C , one end of the inductor L is connected with the input node of the matrix circuit and the AC output port, and the other end is connected with the positive electrode of the filter capacitor C ; the filter capacitor CThe negative terminal is connected to the output node of the matrix circuit and the AC output port.

[0023] Based on the working status of each port in the proposed topology, such as Figure 2 As shown, the converter mainly operates in four states: Mode Ⅰ) V 1. V 2. V The three AC ports operate as follows: the fuel cell transfers energy to the lithium battery and the AC bus; the fuel cell and lithium battery transfer energy to the AC bus; and the lithium battery does not participate in energy transfer but only in power regulation; Mode II) V 1. V 2. In operation, the fuel cell transfers energy to the lithium battery; Mode III) V 2. V ac In operation, the lithium battery transfers energy to the AC bus; Mode IV) V 2. V ac Working, AC bus V ac direction V 2. Transfer energy.

[0024] Specific Implementation Method Two: Combining Figures 5-7 This embodiment will be described. The gain of the converter in Discontinuous Conduction Mode (DCM) is affected by inductance and load resistance differently than in Continuous Conduction Mode (CCM). It is assumed that... L 1- L 4. With equal inductance values, the coupling coefficient is... k =1, this implementation uses an inductor L Taking 1 as an example, inductor L The voltage and current of 1 are as follows Figure 6 As shown, the converter's pass-through time is D T, the inductor current is D The connection will be interrupted after 1 terabyte.

[0025] First, based on the law of energy conservation, the critical conduction condition of the converter is calculated. When the system is critically conducting, the inductance flowing through the diode is... L The average current and ripple are related as follows: (5); Under critical conduction conditions, according to the law of conservation of energy, the diode... D 1 and diode D The energy of 3 is: (6); In formulas (5) and (6),I 2 is a lithium battery port V 2 is an equivalent input current, is an inductance L 1 is an equivalent average value, and D is a switch tube S 1 and a switch tube S 3 is a duty cycle, is an inductance L 1 is a current ripple, is an input voltage of a fuel cell port; According to Figure 3 the equivalent circuit when the coupling inductance is in series, under the critical condition, the relationship between the average value of the inductance current and the ripple is: (7); At the same time, according to the energy conservation calculation, the average value of the inductance current is: (8); According to the energy conservation theorem, the current flowing through the diode D 2 must be equal to the load current, considering that each switching inductance unit provides half of the energy for the converter, the equivalent current of V 2 is: (9); In formula (9), R is the equivalent resistance of the system; Therefore, the critical condition of DCM and CCM can be obtained as: (10); Similarly, the critical condition of the inductance in the uncoupling case is: (11).

[0026] Comparing formula (10) and (11), it can be obtained that the inductance value of the coupling inductance is half of the original inductance, which is beneficial to the design of the inductance.

[0027] The application proposes a hybrid modulation strategy of pulse width and sinusoidal pulse width modulation (SPWM), adopts an asymmetric pulse width to adjust the DC bus voltage, and adopts a two-dimensional scale to adjust the input voltage.

[0028] As shown in Figure 5 , first, it is judged whether the voltage of the port is greater than the rated value V 2 selects a low voltage mode or a high voltage mode.

[0029] When the system detects that the voltage of the port is less than the rated value V2, low voltage mode is adopted. To minimize switching loss, S 1, S 3 duty cycle D is directly set to the maximum value allowed, when the above condition is unchanged, the input voltage is less than the rated voltage, the S 1, S 3 duty cycle D > 50%, to achieve the port V 2 voltage rise, at this time the port V 2 voltage is: (12); The maximum modulation ratio of the converter, the port V 1 input minimum voltage relationship is: (13); Then, the system calculates the modulation index m and the phase shift angle α, and calculates the actual output voltage V ac based on these parameters. The system will V ac Check with the target expected value: if they are the same, confirm and output the current calculation of the effective parameter combination (duty cycle D , modulation index m , phase shift angle α); if U ac Lower than expected, the current input voltage V 1 is too low to support the system to achieve the required output voltage, the calculation process is terminated and an error is reported.

[0030] Similarly, when the system detects that the voltage of the port is greater than the rated value V 2 under the maximum duty cycle, enter high voltage mode. The mode first presets a key condition: let = V 2. Based on this preset voltage relationship, calculate the initial duty cycle D . When the port V 1 input voltage is higher than the rated voltage, the S 1, S 3 duty cycle D < 50%, reduce the bus voltage to achieve system boost, the maximum modulation ratio of the converter, the port V 1 input maximum voltage relationship is: (14); In formula (14), n is the ratio of the highest working voltage of port 2 lithium battery to the rated voltage; Then, calculate the modulation index mCalculate the phase shift angle α and obtain the output voltage. V ac System verification V ac Does it meet the requirements? If so, output the final parameters. D , m , α; if not satisfied, the rated value will be actively increased. V The expected target value is 2. Subsequently, the system uses a new, higher... V 2. Expected value: Re-execute the entire buck mode calculation process to find a feasible solution that meets the output voltage requirements and ensure that the system operates in the optimal or feasible state.

[0031] This invention measures the input voltage under the condition that the AC output port voltage is 220VAC. V 1 (Rated 48V), Output Voltage V 2 (rated 138V) and duty cycle D The relationship between them is as follows Figure 7 As shown. When the input voltage V 1. When the voltage varies between 24V and 80V, and the duty cycle remains moderate, the lithium battery port voltage is precisely regulated within the range of 125V to 151V (i.e., rated voltage ±10%). This result meets the lithium battery operating voltage requirements specified in GB / T 36276-2023, achieving a 0.5V operating voltage for hydrogen fuel cells. V 1-1.67 V 1. Wide range of input voltage regulation.

[0032] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent substitutions, and improvements made to the above embodiments without departing from the scope of the present invention, based on the technical essence of the present invention and within the spirit and principles of the present invention, shall still fall within the protection scope of the present invention.

Claims

1. An isolated three-port DC-AC converter for energy storage systems, characterized by, The application relates to a fuel cell power supply system. The switching inductor unit is connected with a fuel cell port and a lithium battery port at an input end and connected with a full-bridge circuit at an output end, processes input direct-current voltage by controlling the series-parallel state of inductors, and transmits the processed direct-current voltage to the full-bridge circuit. The full-bridge circuit is connected with the switching inductor unit and the lithium battery port at an input node and connected with a primary winding of a high-frequency transformer at an output node, and is used for voltage regulation of input direct-current voltage. The matrix circuit is connected with a secondary winding of the high-frequency transformer at an input node and connected with an LC filter circuit at an output node, and is used for converting direct-current voltage into alternating-current voltage. The LC filter circuit is connected with the matrix circuit at an input end and connected with an alternating-current output port at an output end, and is used for eliminating high-frequency harmonics in alternating-current voltage.

2. The isolated three-port DC-AC converter for an energy storage system of claim 1, wherein, Positive connection capacitor of the lithium battery port C 2, switch inductor unit and full bridge circuit, negative connection capacitor C 2, switch inductor unit and full bridge circuit; Positive connection capacitor for fuel cell port C One end of the switch inductor unit, negative connection capacitor C The other end of the switch inductor unit The switching inductor unit comprises a first switching inductor group and a second switching inductor group. The first switched inductor group includes an inductor L 1. An inductor L 2. A diode D 1. A diode D 2 and a diode D 3. An inductor L 1 and an inductor L 2 is designed to couple inductor to reduce the number and size of the core, inductor L 1 is connected to one end of the fuel cell port V 1 of the positive electrode, capacitor C 1 of one end, diode D 1 of the anode and the second switched inductor group, inductor L 1 is connected to the other end of the diode D 2 of the anode and the diode D 3 of the anode; diode D 3 of the cathode is connected to the inductor L 2 of one end; diode D 2 of the cathode is connected to the inductor L 2 of the other end and diode D 1 of the cathode; The second switch inductor group comprises an inductor L 3. An inductor L 4. A diode D 4. A diode D 5. A diode D 6. An inductor L 3. An inductor L 4. is designed to reduce the number and size of the magnetic core coupled inductor L 3. One end is connected to the first switch inductor group and the anode of diode D 4. The other end is connected to the anode of diode D 5. and the anode of diode D 6. The cathode of diode D 6. is connected to the inductor L 4. One end and the input node of the full-bridge circuit, and an inductor L k is connected to the cathode of diode D 5. The other end is connected to the inductor L 4. The cathode of diode D 4. Diode D 1 and diode D 3 on, diode D 2 off, in series mode, inductor L 1 and inductor L 2 in series, inductor L 3 and inductor L 4 in series, two sets of inductors working in tandem to boost the input DC voltage Diode D 2 on, diode D 1 and diode D 3 off, inductor L 1 and inductor L 2 in parallel, inductor L 3 and inductor L 4 in parallel, reduce the total inductance of two sets of parallel inductors to adapt to different voltage scenarios.

3. The isolated three-port DC-AC converter for an energy storage system of claim 1, wherein, The full-bridge circuit comprises switching tubes S 1. switching tube S 2. switching tube S 3. and switching tube S 4; switching transistor S 2 is connected to the positive pole of the lithium battery port, and the capacitor C 2 is connected to one end of the switching transistor S 4 is connected to the source pole of the switching transistor S 1 is connected to the drain pole of the switching transistor S 1 is provided with an input node on the connection circuit; the switching transistor S 1 is connected to the negative pole of the lithium battery port, and the capacitor C 2 is connected to the other end of the switching transistor S 3 is connected to the source pole of the switching transistor S 3 is connected to the drain pole of the switching transistor S 4 is connected to the source pole of the switching transistor S 4 is provided with an output node on the connection circuit, and the output node is connected to the primary winding of the high-frequency transformer An alternating-current bus is arranged between the secondary winding and the primary winding of the high-frequency transformer. A matrix circuit includes a switching transistor S n1 , a switching transistor S n2 , a switching transistor S n3 and a switching transistor S n4 and a switching transistor S p1 , a switching transistor S p2 , a switching transistor S p3 and a switching transistor S p4 ; switching transistor S n1 the drain of the switching transistor S n3 is connected to one end of the secondary winding of the high-frequency transformer and the drain of the switching transistor S p1 the source of the switching transistor S p1 the drain of the switching transistor S n2 is connected to the drain of the switching transistor and an input node is provided on the connection circuit; the switching transistor S n2 the source of the switching transistor S p2 is connected to the source of the switching transistor; the switching transistor S p2 the drain of the switching transistor S p4 is connected to the other end of the secondary winding of the high-frequency transformer; the switching transistor S p4 the source of the switching transistor S n4 is connected to the source of the switching transistor; the switching transistor S n4 the drain of the switching transistor S p3 is connected to the drain of the switching transistor and an output node is provided on the connection circuit; the switching transistor S p3 the source of the switching transistor S n3 is connected to the source of the switching transistor.

4. The isolated three-port DC-AC converter for an energy storage system of claim 1, wherein, The LC filter circuit includes an inductor L and a filter capacitor C One end of the inductor L is connected to an input node of the matrix circuit and an AC output port, and the other end is connected to a positive electrode of the filter capacitor C A negative electrode of the filter capacitor C is connected to an output node of the matrix circuit and the AC output port.

5. Modulation strategy based on isolated three-port DC-AC converter, applied to the isolated three-port DC-AC converter for energy storage systems according to any one of claims 1-4, characterized in that, The application relates to a fuel cell power supply system. Step 1: under the critical conduction condition of the converter, the inductance parameters of the converter and the voltage gain function are calculated; Step 2: Set the output voltage of the AC output port V ac and the rated voltage of the DC bus V 2. Determine the DC bus voltage at a duty cycle D=0.5 based on the converter's operating state. Is it greater than the rated voltage? V 2; Step 3: if the output voltage is greater than the expected value, the high-voltage mode is entered, the preset voltage relationship is calculated, and the output voltage is calculated on the basis of the preset voltage; if the output voltage is not equal to the expected value, the rated voltage of the direct-current bus is increased and the output voltage is recalculated; and the output voltage is equal to the expected value. If less, enter low voltage mode and set duty cycle of switches S 1 and 3 to maximum allowed value and calculate modulation index S and phase shift angle a from set duty cycles, modulation index m and phase shift angle a, calculate output voltage from set duty cycles, modulation index m and phase shift angle a, if output voltage is not equal to desired value, determine that current input voltage is too low, terminate calculation and report error.

6. The modulation strategy based on the isolated three-port DC-AC converter according to claim 5, characterized in that, The calculation of the critical conduction condition of the converter in step 1 comprises the following steps: Step 1.1: In coupled case, average current and ripple of transformer in discontinuous conduction mode are calculated based on energy conservation L 1 and the corresponding relationship between average current and ripple, based on the corresponding relationship between average current and ripple, the energy of diode D 1 and diode D 3 in discontinuous conduction mode is calculated according to energy conservation and the equivalent input current of lithium battery port is obtained; Step 1.2: In the coupled case, the relationship between the average value of the inductor current and the ripple in the continuous conduction mode of the converter is calculated according to the law of conservation of energy. In the continuous conduction mode, the average value of the inductor current in the matrix circuit is calculated according to the law of conservation of energy, and the equivalent current of the lithium battery port is calculated according to the law of conservation of energy ; Step 1.3: the critical conditions of the discontinuous conduction mode and the continuous conduction mode are obtained according to the equivalent input current of the lithium battery port calculated in steps 1.1 and 1.2 respectively; Step 1.4: steps 1.1 and 1.2 are replaced by the uncoupling case, and steps 1.1-1.3 are repeated to obtain the critical condition of the inductance under the uncoupling case; The corresponding relationship between the average current and the ripple is as follows: （1）； Diode D 1 and diode D 3 is: （2）； In equations (1) and (2), I 2 is a lithium battery port V 2 is an equivalent input current, is an inductance L 1 is an equivalent average value, and D is a switch S 1 and a switch S 3 is a duty cycle, is an inductance L 1 is a current ripple, is an input voltage for a fuel cell port; The corresponding relationship between the average value of the inductance current and the ripple is as follows: （3）； Equivalent current of lithium battery port is: （4）； In formula (4), R is the equivalent resistance of the system; The critical conditions of the discontinuous conduction mode and the continuous conduction mode are as follows: （5）； The critical condition of the inductance is as follows: （6）。 7. The modulation strategy based on isolated three-port DC-AC converter according to claim 5, characterized in that, The working states of the converter in step 2 comprise Mode I, Mode II, Mode III and Mode IV; under Mode I, the fuel cell port, the lithium battery port and the alternating-current output port work, the fuel cell transmits energy to the lithium battery and the alternating-current bus, the fuel cell and the lithium battery transmit energy to the alternating-current bus, and the lithium battery does not participate in energy transmission but participates in power regulation; under Mode II, the fuel cell port and the lithium battery port work, and the fuel cell transmits energy to the lithium battery; under Mode III, the lithium battery port and the alternating-current output port work, and the lithium battery transmits energy to the alternating-current bus; and under Mode IV, the lithium battery port and the alternating-current output port work, and the alternating-current bus transmits energy to the lithium battery port.

8. The modulation strategy based on isolated three-port DC-AC converter according to claim 5, characterized in that, The calculation of the output voltage under the high-voltage mode in step 3 comprises the following steps: Under the condition of the converter critical conduction, the voltage of the DC bus at the preset maximum duty cycle =Rated voltage of DC bus V 2. Calculate the initial switching transistor based on the operating state of the switching inductor unit and the preset voltage relationship. S 1 and switching transistor S 3 duty cycle D Lowering the bus voltage enables the converter to boost voltage; the converter's maximum modulation ratio and port... V The relationship between the maximum input voltage and the input voltage is as follows: (7), n This is the ratio of the highest operating voltage of the lithium battery at port 2 to its rated voltage. calculating a modulation index m , a phase shift angle a, from the duty cycle D , the modulation index m and the phase shift angle a to calculate the output voltage.

9. The modulation strategy based on the isolated three-port DC-AC converter according to claim 8, characterized in that, The operating state of the switch inductor unit includes a series operating state and a parallel operating state, and the switching of the inductor state is controlled by adjusting the duty cycle of the switch tubes S 1 and the switch tube S 3. D ​ In series operation state, total inductance value , inductance voltage relationship satisfies , wherein, V D1 , V D3 Respectively, diode D 1, D 3 two end voltage, k Coupling coefficient, M Mutual inductance, Diode D 1 voltage, Diode D 3 voltage, Inductance voltage; In parallel operation state, total inductance value , inductance voltage relationship satisfies , wherein, is the voltage of diode D 2.

10. The modulation strategy based on the isolated three-port DC-AC converter according to claim 9, characterized in that, The calculation of the output voltage under the low-voltage mode in step 3 comprises the following steps: When the converter is critically turned on, the switch will be turned on. S 1 and switch S 3 duty cycle D Set to the maximum allowed value to implement the port. V 2. Voltage rise, converter maximum modulation ratio, port V The relationship between the minimum input voltage and the input voltage is as follows: (8); Calculating the modulation index m , the phase shift angle a, from the duty cycle D , the modulation index m and the phase shift angle a to calculate the output voltage.

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