Topological structure of three-port converter and control method thereof

Abstract

The invention discloses a topological structure of a three-port converter and a control method of the topological structure. The topological structure comprises a fuel cell port, a lithium battery port, a load port, a switching tube, a diode, an inductor and a capacitor, the first inductor, the first switch tube and the first diode form a booster circuit, so that the fuel cell provides average power for a load port; the second inductor, the third switch tube and the fourth switch tube form a bidirectional DC-DC converter, so that the lithium battery can provide high-frequency pulsating power for an output port or absorb feedback energy of a load port; and the first inductor, the first switch tube, the second switch tube and the second diode form a booster circuit, so that the fuel cell charges the lithium battery. According to the invention, an energy management strategy of optimal design is formulated according to the working mode of the three-port converter and the dynamic characteristics of the fuel cell and the lithium battery, communication between power supplies is not needed, and the proposed topology and control method can improve the power density, the dynamic response capability and the reliability of the system.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics, and in particular to a structure of a three-port converter and a control method thereof. Background Art

[0002] New energy vehicles are gradually changing people's transportation methods. Among them, fuel cell vehicles have attracted particular attention due to their superior endurance, efficient energy utilization and zero emission characteristics, and are considered to be the key direction of new energy vehicle development. Limited by the characteristics of slow dynamic response, low power density and irreversible energy of fuel cells themselves, it is necessary to introduce high power density auxiliary power sources (such as lithium batteries or supercapacitors) to form a fuel cell hybrid power system to jointly drive the load. However, due to the large difference in the output voltage variation range of fuel cells and lithium batteries, two independent port converters are required to achieve voltage conversion in order to achieve parallel connection on the output side of the port converter, which is bound to increase the volume, weight and cost of the system. In addition, in order to achieve the optimal allocation of load power under the dynamic response characteristics of different power sources and improve the performance of the power system, uninterrupted communication is required between different power sources. However, communication delays and packet loss deteriorate the reliability and control performance of the system. Summary of the invention

[0003] In view of this, an object of the present invention is to provide a topology structure of a three-port converter and a control method thereof to solve the technical problem of improving the power density, dynamic response capability and reliability of a fuel cell hybrid power system.

[0004] The three-port converter topology structure of the present invention includes a fuel cell port, a lithium battery port, a load port, a first switch tube, a second switch tube, a third switch tube, a fourth switch tube, a first diode, a second diode, a first inductor, a second inductor and a capacitor;

[0005] The negative electrode of the fuel cell port, the source electrode of the first switch tube, the negative electrode of the lithium battery port, the source electrode of the third switch tube, the negative electrode of the capacitor and the negative electrode of the load port are connected to each other, the positive electrode of the fuel cell port and the first end of the first inductor are connected, the second end of the first inductor, the drain electrode of the first switch tube, the drain electrode of the second switch tube and the anode of the first diode are connected to each other, the source electrode of the second switch tube and the anode of the second diode are connected to each other, the positive electrode of the lithium battery port, the cathode of the second diode and the first end of the second inductor are connected to each other, the second end of the second inductor, the drain electrode of the third switch tube and the source electrode of the fourth switch tube are connected to each other, and the cathode of the first diode, the drain electrode of the fourth switch tube and the positive electrode of the load port are connected to each other.

[0006] The present invention also discloses a control method for the three-port converter topology structure, which comprises the following steps:

[0007] 1) Setting an over-discharge threshold and an over-charge threshold of the lithium battery, and setting a first reference voltage value and a second reference voltage value of the load port, wherein the first reference voltage value is lower than the second reference voltage value;

[0008] 2) When the SOC of the lithium battery is greater than the set overcharge threshold, the first switch tube and the second switch tube are disconnected through logical judgment, so that the fuel cell end no longer outputs power; and a single voltage loop is used to control the third switch tube and the fourth switch tube: by comparing the second reference voltage value of the load port with the measured voltage value of the load port, and calculating the error, and sending the error to the PI4 regulator for processing, the system voltage steady-state error compensation is realized to maintain the stability of the load port voltage, so that the lithium battery alone supplies power to the load end;

[0009] 3) When the SOC of the lithium battery is between the over-discharge threshold and the over-charge threshold, the second switch tube is disconnected through logical judgment; the first switch tube adopts dual-loop control: the voltage outer loop calculates the error by comparing the first reference voltage value of the load port with the measured voltage value, and sends the error to the PI1 regulator for processing, so as to realize the system voltage steady-state error compensation and maintain the stability of the load port voltage; the set value of the current inner loop is provided by the output of the voltage outer loop, and the output current of the fuel cell is used as the feedback of the current inner loop; since the second reference voltage value of the load port is greater than the measured voltage value of the load port, PI4 is in a positive saturation state, at this time, the third switch tube and the fourth switch tube adopt current loop control, and the output current of the load port is compared with the output current of the lithium battery after high-pass filtering to obtain the error, and the error is sent to the PI5 regulator for processing, so that the lithium battery can provide pulsating power to the load;

[0010] 4) When the SOC of the lithium battery is lower than the over-discharge threshold, the first switch tube adopts dual-loop control. The voltage outer loop calculates the error by comparing the first reference voltage value of the load port with the measured voltage value, and sends the error to the PI1 regulator for processing to achieve system voltage steady-state error compensation and maintain the stability of the load port voltage; the set value of the current inner loop is provided by the output of the voltage outer loop, and the output current of the fuel cell is used as the feedback of the current inner loop; the second switch tube adopts current loop control, and the current loop compares the charging reference current of the lithium battery with the difference between the output current of the fuel cell and the low-pass value of the output current to calculate the error, and sends the error to the PI3 regulator for processing to achieve the purpose of charging the lithium battery with the fuel cell; since the second reference voltage value of the load port is greater than the measured voltage value of the load port, PI4 is in a positive saturation state. At this time, the third and fourth switch tubes adopt current loop control, and the output current i of the load port is o After high-pass filtering, the output current i of the lithium battery L2After comparison, the error is obtained and sent to the PI5 regulator for processing, so that the lithium battery can provide pulsating power to the load;

[0011] 5) When the measured voltage value of the load port rises suddenly, the first reference voltage value and the second reference voltage value of the load port are both less than the measured voltage value of the load port, so the PI1 regulator and the PI4 regulator become negative saturation states, making the dual-loop control output of the first switch tube 0, thereby disconnecting the first switch tube; at this time, the third switch and the fourth switch tube adopt current loop control, and the output current of the load port is used as the reference current and is high-pass filtered and compared with the output current i of the lithium battery. L2 After comparison, an error is obtained and sent to the PI5 regulator for processing, so that the load port can charge the lithium battery port.

[0012] Beneficial effects of the present invention:

[0013] The topological structure of the three-port converter and its control method in the present invention are as follows: the three-port topological structure efficiently integrates the fuel cell and the lithium battery, and an optimized energy management strategy is formulated according to the working mode of the three-port converter and the dynamic characteristics of the fuel cell and the lithium battery, without the need for communication between power sources, and the proposed topology and control method can improve the power density, dynamic response capability and reliability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 4 is a topological diagram of the three-port converter described in the embodiment.

[0015] Figure 2 This is the control strategy diagram of the three-port converter.

[0016] Figure 3 This is the constant power load simulation waveform of the lithium battery at 50% SOC.

[0017] Figure 4 This is the pulse power load simulation waveform of the lithium battery at 50% SOC.

[0018] Figure 5 This is the pulse power load simulation waveform of the lithium battery at 15% SOC.

[0019] Figure 6 This is the pulse power load simulation waveform of the lithium battery at 85% SOC. DETAILED DESCRIPTION

[0020] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0021] like Figure 1As shown, the three-port converter topology in this embodiment includes a fuel cell port FC, a lithium battery port BAT, a load port R, a first switch tube S1, a second switch tube S2, a third switch tube S3, a fourth switch tube S4, a first diode D1, a second diode D2, a first inductor L1, a second inductor L2 and a capacitor C.

[0022] The negative electrode of the fuel cell port, the source electrode of the first switch tube, the negative electrode of the lithium battery port, the source electrode of the third switch tube, the negative electrode of the capacitor and the negative electrode of the load port are connected to each other, the positive electrode of the fuel cell port and the first end of the first inductor are connected, the second end of the first inductor, the drain electrode of the first switch tube, the drain electrode of the second switch tube and the anode of the first diode are connected to each other, the source electrode of the second switch tube and the anode of the second diode are connected to each other, the positive electrode of the lithium battery port, the cathode of the second diode and the first end of the second inductor are connected to each other, the second end of the second inductor, the drain electrode of the third switch tube and the source electrode of the fourth switch tube are connected to each other, and the cathode of the first diode, the drain electrode of the fourth switch tube and the positive electrode of the load port are connected to each other.

[0023] It is determined that the first inductor L1, the first switch tube S1, and the first diode D1 form a boost circuit to enable the fuel cell to provide average power to the load port.

[0024] It is determined that the second inductor L2, the third switch tube S3, and the fourth switch tube S4 form a bidirectional DC-DC converter, so that the lithium battery can provide high-frequency pulsating power to the output port or absorb feedback energy from the load port.

[0025] It is determined that the first inductor L1, the first switch tube S1, the second switch tube S2, and the second diode D2 form a boost circuit to enable the fuel cell to charge the lithium battery.

[0026] The control method of the three-port converter topology structure in this embodiment includes the following steps:

[0027] 1) Set the over-discharge threshold SOC of the lithium battery min and overcharge threshold SOC max In this embodiment, the over-discharge threshold of the lithium battery is specifically set to SOC=20% ， The overcharge threshold is SOC = 80%. Set the first reference voltage value U of the load port o_ref1 and the second reference voltage U o_ref2 , where the first reference voltage value U o_ref1 Lower than the second reference voltage U o_ref2 In this embodiment, the first reference voltage value is 5V lower than the second reference voltage; the output voltage U of the fuel cell is set FC <The output voltage of lithium battery U BAT <Load port rated voltage U o .

[0028] 2) When the SOC of the lithium battery is greater than the set overcharge threshold, the first switch tube and the second switch tube are disconnected through logical judgment, so that the fuel cell end no longer outputs power; and a single voltage loop is used to control the third switch tube and the fourth switch tube: by comparing the second reference voltage value U of the load port o_ref2 The measured voltage value U at the load port o , and calculate the error, and send the error to the PI4 regulator for processing to achieve system voltage steady-state error compensation to maintain the stability of the load port voltage, so that the lithium battery can power the load end alone.

[0029] 3) When the lithium battery SOC is between the over-discharge threshold and the over-charge threshold, the second switch tube is disconnected through logical judgment; the first switch tube adopts dual-loop control: the voltage outer loop compares the first reference voltage value U of the load port o_ref1 The measured voltage value U o , calculate the error, and send the error to the PI1 regulator for processing to achieve system voltage steady-state error compensation and maintain the stability of the load port voltage; the set value of the current inner loop is provided by the output of the voltage outer loop, and the output current of the fuel cell is used as the feedback of the current inner loop; since the second reference voltage value U o_ref1 >Measured voltage value U at the load port o , so PI4 is in a positive saturation state. At this time, the third switch tube and the fourth switch tube adopt current loop control, and the output current i o After high-pass filtering, the output current i of the lithium battery L2 After comparison, the error is obtained and sent to the PI5 regulator for processing, so that the lithium battery can provide pulsating power to the load.

[0030] 4) When the lithium battery SOC is lower than the over-discharge threshold, the first switch tube adopts dual-loop control, and the voltage outer loop compares the first reference voltage value U of the load port o_ref1 The measured voltage value U o , calculate the error, and send the error to the PI1 regulator for processing to achieve system voltage steady-state error compensation and maintain the stability of the load port voltage; the set value of the current inner loop is provided by the output of the voltage outer loop, and the output current i of the fuel cell L1 As the current inner loop feedback quantity; the second switch tube adopts the current loop control, and the current loop converts the charging reference current i of the lithium battery into charge The output current i of the fuel cell L1 The difference between the low-pass value of the output current is compared to calculate the error, and the error is sent to the PI3 regulator for processing to achieve the purpose of charging the lithium battery with the fuel cell; since the second reference voltage value U o_ref1 >Measured voltage value U at the load porto , so PI4 is in a positive saturation state. At this time, the third switch tube and the fourth switch tube adopt current loop control, and the output current i o After high-pass filtering, the output current i of the lithium battery L2 After comparison, the error is obtained and sent to the PI5 regulator for processing, so that the lithium battery can provide pulsating power to the load.

[0031] 5) When the measured voltage value of the load port is U o The first reference voltage value U of the load port increases suddenly. o_ref1 and the second reference voltage value U o_ref2 Both are smaller than the measured voltage value U at the load port o , so the PI1 regulator and PI4 regulator become negative saturation state, making the dual-loop control output of the first switch tube 0, and then the first switch tube is disconnected; at this time, the third switch and the fourth switch tube adopt current loop control, and the output current i o As the reference current, it is high-pass filtered and compared with the output current i of the lithium battery L2 After comparison, an error is obtained and sent to the PI5 regulator for processing, so that the load port can charge the lithium battery port.

[0032] By building a three-port converter topology simulation model and controlling it using the control method in the above embodiment, the following simulation results are obtained:

[0033] Figure 3 1 is a simulation waveform diagram of the control method based on the three-port converter described in this embodiment when the lithium battery is 50% and the load is a constant power load. o , P o , P BAT , P FC They represent the load port voltage, load power, lithium battery output power, and fuel cell output power respectively. In this simulation process, the load power alternates between 0kW and 10kW. Figure 3 It can be seen that the system remains stable throughout the operation, maintaining a constant load port voltage of 500 V. When the load power changes in a step-like manner, the fuel cell responds slowly to the load power change and eventually provides all the load power, while the lithium battery responds quickly to the sudden change in load power, and its steady-state output power is 0 kW.

[0034] Figure 4 The following is a simulation waveform diagram of the control method based on the three-port converter described in this embodiment when the lithium battery is 50% and the load is a pulse load. In this simulation, the pulsating load power is set to a sine wave with a frequency of 50Hz and an amplitude of 2kW, and the steady-state load power fluctuates between -5kW and 20kW. Figure 4 It can be seen that the system remains stable throughout the operation. Although the load power fluctuates continuously, the load port voltage remains at around 500V. The fuel cell only provides low-frequency, slow-changing power, and the lithium battery provides all high-frequency pulsating power. In the case of load braking energy feedback, the fuel cell output power is 0kW, and the lithium battery absorbs all regenerative power from the load.

[0035] Figure 5 The following is a simulation waveform diagram of the control method based on the three-port converter described in this embodiment when the lithium battery is 15% and the load is a pulse load. In this simulation, the pulsating load power is represented by a sine wave with a frequency of 50Hz and an amplitude of 2kW, while the steady-state load power fluctuates between 2.5kW and 10kW. Figure 5 As shown in the figure, the system remains stable throughout the operation, and the load port voltage remains at around 500V despite the load power fluctuations. In this state, the fuel cell not only provides all the low-frequency, slowly changing power, but also provides an additional 2.5kW to charge the lithium battery at a constant current. At the same time, the lithium battery provides all the high-frequency pulsating power. Once the lithium battery SOC recovers to 30%, the system will exit the multi-port coordinated power supply mode and return to the hybrid power supply mode.

[0036] Figure 6 The control method based on the three-port converter described in this embodiment is a simulation waveform diagram when the lithium battery is 85% and the load is a pulse load. In this simulation, the pulsating load power is represented by a sine wave with a frequency of 50Hz and an amplitude of 2kW, while the steady-state load power fluctuates between 2.5kW and 10kW. Figure 6 As shown in the figure, the system remains stable throughout the operation, and the load port voltage always remains around 500V despite the load power fluctuations. When operating in battery-only power mode, the fuel cell output power is 0kW and the lithium battery provides all the load power. Once the SOC of the lithium battery drops to 70%, the system exits battery-only power mode and returns to hybrid power mode.

[0037] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A three-port converter topology, characterized in that: It includes a fuel cell port, a lithium battery port, a load port, a first switch tube, a second switch tube, a third switch tube, a fourth switch tube, a first diode, a second diode, a first inductor, a second inductor and a capacitor; The negative electrode of the fuel cell port, the source electrode of the first switch tube, the negative electrode of the lithium battery port, the source electrode of the third switch tube, the negative electrode of the capacitor and the negative electrode of the load port are connected to each other, the positive electrode of the fuel cell port and the first end of the first inductor are connected, the second end of the first inductor, the drain electrode of the first switch tube, the drain electrode of the second switch tube and the anode of the first diode are connected to each other, the source electrode of the second switch tube and the anode of the second diode are connected to each other, the positive electrode of the lithium battery port, the cathode of the second diode and the first end of the second inductor are connected to each other, the second end of the second inductor, the drain electrode of the third switch tube and the source electrode of the fourth switch tube are connected to each other, and the cathode of the first diode, the drain electrode of the fourth switch tube and the positive electrode of the load port are connected to each other.

2. A control method for the three-port converter topology structure according to claim 1, characterized in that: The following steps are involved: 1) Setting an over-discharge threshold and an over-charge threshold of the lithium battery, and setting a first reference voltage value and a second reference voltage value of the load port, wherein the set first reference voltage value is lower than the second reference voltage; 2) When the SOC of the lithium battery is greater than the set overcharge threshold, the first switch tube and the second switch tube are disconnected through logical judgment, so that the fuel cell end no longer outputs power; and a single voltage loop is used to control the third switch tube and the fourth switch tube: by comparing the second reference voltage value of the load port with the measured voltage value of the load port, and calculating the error, and sending the error to the PI4 regulator for processing, the system voltage steady-state error compensation is realized to maintain the stability of the load port voltage, so that the lithium battery alone supplies power to the load end; 3) When the lithium battery SOC is between the over-discharge threshold and the over-charge threshold, the second switch tube is disconnected through logical judgment; the first switch tube adopts dual-loop control: the voltage outer loop calculates the error by comparing the first reference voltage value of the load port with the measured voltage value, and sends the error to the PI1 regulator for processing to achieve system voltage steady-state error compensation and maintain the stability of the load port voltage; the set value of the current inner loop is provided by the output of the voltage outer loop, and the output current of the fuel cell is used as the feedback of the current inner loop; since the second reference voltage value of the load port is greater than the measured voltage value of the load port, PI4 is in a positive saturation state. At this time, the third and fourth switch tubes adopt current loop control, and the output current i of the load port is o After high-pass filtering, the output current i of the lithium battery L2 After comparison, the error is obtained and sent to the PI5 regulator for processing, so that the lithium battery can provide pulsating power to the load; 4) When the SOC of the lithium battery is lower than the over-discharge threshold, the first switch tube adopts dual-loop control. The voltage outer loop calculates the error by comparing the first reference voltage value of the load port with the measured voltage value, and sends the error to the PI1 regulator for processing to achieve system voltage steady-state error compensation and maintain the stability of the load port voltage; the set value of the current inner loop is provided by the output of the voltage outer loop, and the output current of the fuel cell is used as the feedback of the current inner loop; The second switch tube adopts current loop control, which compares the charging reference current of the lithium battery with the output current of the fuel cell and the difference between the low-pass value of the output current, calculates the error, and sends the error to the PI3 regulator for processing, so as to achieve the purpose of charging the lithium battery with the fuel cell; since the second reference voltage value of the load port is greater than the measured voltage value of the load port, PI4 is in a positive saturation state, at this time, the third switch tube and the fourth switch tube adopt current loop control, and the output current of the load port is compared with the output current i of the lithium battery after high-pass filtering. L2 After comparison, the error is obtained and sent to the PI5 regulator for processing, so that the lithium battery can provide pulsating power to the load; 5) When the system is in braking state, the measured voltage value U of the load port o The voltage at the load port rises suddenly, causing the first reference voltage value and the second reference voltage value of the load port to be less than the measured voltage value of the load port. Therefore, the PI1 regulator and the PI4 regulator become negative saturation states, making the dual-loop control output of the first switch tube 0, thereby disconnecting the first switch tube. At this time, the third switch and the fourth switch tube adopt current loop control, and the output current of the load port is used as the reference current and is high-pass filtered and then compared with the output current i of the lithium battery. L2 After comparison, an error is obtained and sent to the PI5 regulator for processing, so that the load port can charge the lithium battery port.

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